Empirical Studies on Biomass Briquette Production: A Literature Review

Abstract

The densification of raw material into fuel briquettes is one of the routes to convert biomass into energy. This method provides uniformity to the solid fuel, better physical and energy properties, facilitating its storage and transport, in addition to more homogeneous combustion. Given the importance of these characteristics, this work presents a literature review, emphasizing the experimental levels of the variables of the briquetting process, as well as on the most relevant quality parameters for obtaining briquettes. We also carry out a survey of the main technologies used in the production of briquettes, as well as the experimental methodologies and statistical analysis used in the planning and validation of processes. It was observed among the studies that the raw material granulometry, followed by pressure, initial moisture, compaction time and binder are the most used process variables for the production of briquettes. Other factors, such as the proportion of biomass, process temperature and thermal pre-treatments are used to obtain greater energetic and physical responses. Among the works, divergences were observed regarding the relevance and interaction of some process variables on the quality variables of the briquettes, indicating the need for the experiments to be mathematically modeled.

1. Introduction

Humanity is known for its ability to transform environments to ensure ideal conditions for its survival and development, and thanks to this, it has been thriving over the years. However, with modernization and increasingly extravagant consumption habits, what was previously a matter of adaptation, gave way to a lifestyle based on well-being at all costs and material growth. For this, society has been using natural resources in an unruly way, disregarding its recovery time and limitations [1].

Research in crude oil exporting countries estimates that the use of fossil fuels has a significant positive effect on long-term environmental degradation. This perspective indicates that current energy transition policies are insufficient to meet the demands for environmental sustainability imposed on these countries [2].

Thus, in order to simultaneously deal with the growth in energy demand and mitigate problems, such as the emission of CO₂ and polluting gases, a new energy system is needed [1]. The use of renewable energies presents itself as an ally to sustainable development, based on technologies that enable energy efficiency and development [3].

One of the best-known and longest-used sources of renewable energy is biomass. It is the only renewable source with fixed carbon, which is essential to meet various fuel and consumer goods requirements [4]. Biomass is a relatively cleaner source than coal, which can be obtained from inputs of organic origin, such as wood, annual crops and agricultural residues. If used sustainably, it can supply much of the world’s energy needs [5].

Biomass is the only renewable resource that can be converted directly into fuel through combustion processes [5]. Despite this, in natura resources are not recommended as an energy source due to their precarious characteristics, such as low bulk density, high moisture content and low energy density, causing high transportation, storage and handling costs [6].

The improvement of these characteristics results in a solid fuel that is easier to store, handle and transport. For this, biomass densification is an efficient method of guaranteeing desirable characteristics for solid biofuels, such as higher apparent and energetic density, hardness and resistance. The biomass densification can be done in the form of briquettes, pellets, cubes or disks [7].

Briquettes are fuels whose energy conversion occurs through direct burning and are used for heating and cooking, for domestic, or industrial use, best used in fixed bed chambers, due to their physical structure. Pellets have the same applicability, but can be used in reactors with different geometries and different feeding systems [8,9].

The use of densified biomass in the form of solid fuel has gained relevance in developing and developed countries [10]. In Europe, countries like Sweden, France, Austria, Switzerland and Germany use biomass heating systems on a small scale, which use pellets for combustion [11,12]. In Brazil, briquettes are used as substitutes for firewood in bakeries, pizzerias, food establishments and factories with wood ovens, such as red brick factories. However, the use of biomass briquettes for energy purposes in the country has not yet fully spread, since this sector depends on a briquette market, adequate technologies and availability of waste [6,13].

Adequate selection and setup of processing variables are fundamental for briquetting optimization. Navalta et al. [14] state that parameters, such as pressure, temperature, particle size, type of binder and characteristics of the chosen biomass have a great influence on the compression strength, density and energy potential of the briquettes. Works, such as Kpalo et al. [15] carried out bibliographic surveys to assess the economic and technical viability of the production of briquettes, and for that, it was necessary to review the types of biomass used, the types of briquetting equipment, the skills of human resources and the investment capital. Other review work, such as that of Faizal, Rahman and Latiff [16] and Tumuluru et al. [17] also emphasized the machines used to produce briquettes and some production variables, since these affect the physical and energetic characteristics of the briquettes. Dinesha et al. [18] provided a comprehensive review on transforming agro and industrial biomass residues into briquettes as alternative fuels. They explored the effects of process parameters, types of feedstock and processing technologies on combustion characteristics. Oladeji [19] reviewed biomass briquetting technologies and their fundamental principles. The authors also showed which factors influence biomass briquetting as well as some advantages and limitations of the briquetting process.

Differently from the aforementioned works, this research aims to provide guidelines on how to design experiments in order to optimize not only energy and combustion briquette characteristics but also physical-mechanical properties. Comprehensive data analyses summarizing the range of process parameters, as well as their final setup, were deeply discussed. Elucidative graphics highlighting the observed energy, combustion and physical-mechanical characteristics were contrasted against their international standard requirements. Additionally, the experimental design, modeling techniques and statistical software were reviewed.

The searches were made in the Web of Science databases in order to select indexed articles for a more detailed analysis. Therefore, after a series of searches, a set of scientific works were defined that understood the variables of input and response in the production of briquettes, as well as experimental methods and production of these solid fuels. The selected articles were the result of an advanced search using Boolean operators and field labels along with keywords. The search was carried out as follows: (TI = ((briquette) NOT (pellet *)) AND TS = ((production briquette AND biomass AND optimization) OR (biomass AND production briquette AND properties AND characterization))). Twenty-nine results have been found, without using filters or delimiting the publication period of the works. Among these, seven were discarded, as they are not experimental works for the production and characterization of briquettes, or due to unavailability of access, including in other research bases. The searches, therefore, reached 22 experimental studies that associate mathematical modeling and statistical analysis to the production processes of briquettes and their variables.

2. Briquetting

2.1. Biomass and Pre-Treatments Used in the Production of Briquettes

Biofuels are subdivided into four generations and classified according to raw material and conversion process [20]. The first generation concerns biofuel obtained from food crops, such as oil seeds and sugarcane for the production of biodiesel and bioethanol [21,22]. The second generation is associated with biomass residues, such as stems, leaves and bark from the agroforestry and municipal waste sectors. Such raw materials are of vegetable origin and their primary composition is lignin, hemicellulose and cellulose [20,23]. The third generation covers algae and microalgae with high levels of total sugars for the production of bioethanol and biodiesel [21]. The fourth generation benefits from bioengineering to modify the cellular metabolism of algae and cyanobacteria to increase the production of biofuels [24].

The production of briquettes is commonly associated with second-generation biofuels, promoting the reuse of agricultural and forestry residues, as shown in Table 1. Most of the works made use of waste material, such as stem, bark, leaves, straw, wood or sawdust. Waste from cut wood [25] and from species, such as eucalyptus (Eucalyptus spp. and Eucalyptus grandis) [26] and pine (Pinus spp.) [27] were used. Corn husks, cobs and straw were mixed with other biomass, such as cassava husk [28] and oil palm husk [29]. In addition, some works also incorporated plastic waste associated with other materials in the composition of their briquettes, such as sawdust, date palm trunk and even sub-bituminous coal (non-renewable coal) [30,31].

The mixture of more than one type of biomass in different proportions was evaluated by several of the works presented in Table 1. This process seeks to obtain a fuel with the combination of the best physical and chemical characteristics that each component has [32,33]. Masullo et al. [34] for example, used sugarcane bagasse and straw, whose moisture content before briquetting was 12% and observed that the briquettes with a higher proportion of straw showed an increase in the content of moisture after densification, reaching 13.3%. This result indicated the need to reduce the percentage of sugarcane straw in the final composition of the briquettes.

The mixing of raw materials and the briquetting process itself are artifices used to overcome the natural characteristics of biomass, such as low density, high moisture, low energy density, and irregular size and shape. These properties hinder the use of biomass as biofuel, besides making its handling, storage and transportation costly and inefficient [6,35].

The process of densification of biomass into briquettes was able to raise the basic density of Phyllostachys aurea (bamboo) biomass from 0.48 g·cm⁻³ to an average bulk density of 1.16 g·cm⁻³. Consequently, the energy density went from about 8.8 GJ·m⁻³ to 21.478 GJ·m⁻³ [36]. Something similar was observed for Lantana câmara biomass, whose initial density was 0.51 g·cm⁻³ and after briquetting reached 1.20 g·cm⁻³ [37]. The main results found in each survey are registered in Table 1 and some of them were analyzed in more detail in the topic that deals with the response variables of the briquetting process.

In order for the biomass to be efficiently converted into solid biofuel, it goes through some pre-treatment processes, which can vary according to the natural characteristics of the material and the availability of technology. Table 1 shows the treatments used on the raw material before the production of briquettes.

The thermal pre-treatments of carbonization and torrefaction are employed in order to increase the energy characteristics of the fuel since the process results in increasing the amount of fixed carbon of biomass [38]. The torrefaction also presents increments in the physical quality of the briquettes, regarding their durability, density and compression strength [28]. Despite the advantages, such procedures require an extra expense of energy, since they require heating at temperatures that can range from 180 to 500 °C [26,31].

Commonly, drying processes are performed to reduce the moisture of the biomass, this process can be done in the sun and in ambient conditions [29,31,39] or in a greenhouse [13,28]. Another common procedure is the grinding or crushing of the biomass in machines, such as hammer and knife mills [13,40] before particle size classification through sieves. The reduction and homogenization of the particles allow for better compaction of the feedstock, influencing its physical and mechanical characteristics [41].

Depending on the biomass, it is not necessary to perform such processes, as occurred with the in-natura rice husk used by Oliveira Maia et al. [42] whose moisture content was already within the appropriate for briquetting (10%), as well as its initial granulometry. It is worth noting in Table 1 that, regardless of the machine used to produce the briquettes, it was necessary to carry out processes to reduce the initial size of the biomass and its initial moisture through drying. The importance of both processes is discussed in the topics dealing with process variables in briquette production.

Table 1. Main briquette production machines.

References	Raw-Material	Pre-Treatment	Briquetting Machine	Variables of Briquetting Equipment	Results and Conclusions of the Studies
[40]	Cassava waste	Drying, grinding and sieving	Hydraulic Press	Pressure	Briquettes produced without heating or binder achieved good results: relaxed density of 918 kg·m−3, compressive strength of 1.29 MPa, durability of 94.1%, expansion of 8.68 and energy density of 15.7 GJ·m−3
[29]	Corncobs (CC) and oil palm trunk bark (OPTB)	Drying, grinding and sieving	Hydraulic Piston Press	Temperature and pressure	They were obtained: MC of 9.24–10.00%, density of 0.38–0.40 g·cm−3, water resistance of 87.60–92.00%, mechanical strength of 98.28–99.08%, compressive strength of 18.47 to 21.75 MPa and calorific value of 16.54 to 16.91 MJ·kg−1
[8]	Olive mil solid waste (OMSW)	Drying, grinding and sieving	Hydraulic Press	Temperature and pressure	Briquettes with unit density: 2.04–3.03 g·cm−3 and bulk density: 0.84–1.20 g·cm−3. With 15% of binder, greater compressive strength (4581 kN), HHV (16.92 MJ·kg−1) and lower AC (6.72%) were obtained
[43]	Cashew nut shells (CNS) and areca nut shells (ANS)	Torrefaction (CNS), drying (ANS), grinding and sieving	Compacted screw motor-driven	Compressed screw speed	Average values: 18.9–21 MJ·kg−1 of calorific value, hardness 103–123 HB, porosity 52–65%, TU 3.5–4.5%, VM 71.2–73.2%, TC 3.3–5.2% and CF 18.8–20%. The “A” mixture is suitable for the production of briquettes, with a speed of 90 rpm and small CNS
[37]	Lantana camara e Prosopis juliflora	Drying, breaking, grinding, pulverizing and sieving	Industrial machine. Piston and ram	Not specified	The briquettes reached a density of 1200 kg·m−3, an energy density of 23.05 GJ·m−3. Ideal MC of 10–12% and lower AC of ≤2%
[38]	Palm Stem	Chopping, cleaning, carbonization and sieving	Hydraulic Press	Pressure	Through optimization, HHV of 306.704 MJ·kg−1 and compressive strength of 10.0608 kg·cm−2 were obtained, with the parameters of 120 mesh, ≈47% binder and ≈9.32 MPa
[27]	Wood Waste of Pinus spp.	Collects and sieving *	Manual mechanical device	Not specified	The biomass had an apparent density of 160 and 170 kg·m−3, TC of 0.38–1.78%, VM of 65.3–95.01% and FC of 4.49–34.3%. The briquettes had MC of 11.52–13.26%, HHV of 17.0–18.3 MJ·kg−1 and density of 930–1240 kg·m−3
[28]	Corn husk (CH) e cassava peel (CP)	Drying, milling, torrefaction and humidification	Hydraulic piston press machine	Pressure	Briquettes torrefied at 300 °C with a mixture of 10/90 (CH/CP) showed good combustible characteristics. These showed increases of briquettes without thermal treatment of 18.19%; 2.98% and 32.25% in density, durability and compressive strength, respectively
[44]	Coal dust (C), sawdust (S) and post-consumer plastics (P)	Washing, drying, grinding and sieving	Manual Press	Not specified	They were obtained HHV from 26.5 to 33.8 MJ·kg−1. Impact resistance/break rate ≥ 90% in all briquettes and density from 1100 to 1300 kg·m−3. However, it had low resistance to compression, below 1 MPa
[13]	Coffee shrub residues and pinewood	Drying, grinding and sieving	Hydraulic Piston Press	Temperature and pressure	The coffee residue mixed with 75% of the pine wood yielded an apparent density of 1107–1163 kg·m−3, energy density of 19.13–19.89 GJ·m−3, tensile strength of 415–569 kgf and equilibrium moisture content of 9–11% by weight
[42]	Banana leaves and pseudostem, and rice husk	Chopping, milling, sieving, pressing and drying	Hydraulic Press	Pressure	The rice husk briquettes showed the highest compressive strength (19.8 MPa) and the highest HHV (18.1 MJ·kg−1), however they had a high AC value (25.4%)
[34]	Sugar bagasse and straw	Drying, grinding and sieving	Manual Hydraulic Press	Load measured in tonnes	Briquettes with a greater amount of sugarcane bagasse had greater resistance to compression, less generation of fines, moisture content of 11 to 12%. The use of straw is only feasible when associated with sugarcane bagasse
[45]	Palm kernel shell biochars (PKSB)	Uninformed	Compression cylindrical ram machine	Compaction speed and pressure	Average calorific value of 31.29 ± 1.44 MJ·kg−1. Impact resistant, preserving 95% of its initial weight. Poor performance for water resistance. Depending on the processing conditions, tensile strength > 800 kN·m−2 can be achieved.
[36]	Phyllostachys aurea	Drying, grinding, sieving and humidification	Laboratory press	Temperature and pressure	Biomass, briquettes produced from it and coal were evaluated. The briquettes showed higher energy density (21.47 GJ·m−3) than coal (11.38 GJ·m−3), but both have potential for energy applications
[30]	Sawdust (S) and date palm trunk (PT), plastic waste	Grinding and sieving	Laboratory hydraulic piston press	Temperature and pressure	Although the S70-W30 briquettes produced at 130 °C have a higher density (≈1190 kg·m−3) and durability (100%), almost all PT briquettes produced at room temperature showed high densities and durability, and the largest LHV with PT90-ASR10
[39]	Charcoal thin waste and sanitary sewage sludge	Drying, grinding and sieving	Conical screw extruder	Pressure	The 50:50 mixture of charcoal and sludge was ideal for energy production with HHV of 17.47 MJ·kg−1, breaking strength of 41.19 MPa and apparent density of 913 kg·m−3
[31]	Plastic waste, sawdust, maize husk and carbonized sub-bituminous coal	Drying, chopping, carbonization, grinding and sieving	Hydraulic Press	Load of up to 10 tons	HHV for briquettes with 0% and 90% coke was 13.8 and 21.45 MJ·kg−1, respectively. The raw material of 100% coke produces HHV of 23.86 MJ·kg−1. The highest VM (31.74%) and compressive strength (4.61 N·mm−3) were obtained in the briquette with 100% biomass and 0% coke
[46]	Coconut Fiber (FC) and sugarcane straw (PC)	Drying, grinding and sieving	Manual Hydraulic Press	Pressure	The briquettes made of PC e FC can be manufactured without heating or adding binder. The highest amount of FC was favorable for the compressive strength (0.37 MPa), HHV (18.24 MJ·kg−1), fixed carbon (22.28%) and AC (3.71%)
[26]	Wood of Eucalyptus spp. and Eucalyptus grandis	Drying, cutting, torrefaction, grinding and sieving	Hydraulic Piston Press	Temperature, pressing time and pressure	Values for E. spp. and E. grandis, respectively. Bulk density: 1.14 and 1.06 g·cm−3; energy density (Treatment at 200 °C): 24.79 and 21.70 GJ·m−3; lowest MC: 9.9 and 9.6%; lower hygroscopy: E. grandis at 200 °C and 10 MPa; greater compressive strength: relative
[47]	Banana Leave Waste	Milling and sizing	Hydraulic Press	Pressure	The compaction time of 0.6 and 1 s was negligible for density, with values of 999 and 1000 kg·m−3, respectively. Higher compressive strength (5.3 MPa) was achieved with 1 s. The calculation of the energy density was based on the density of 999 kg·m−3
[48]	Corn stover	Drying, chopping and Milling	Laboratory piston press (Plunger)	Pressure	Given the different experiments, with different interactions of the input variables, better dry density and bulk density of 470 and 190 kg·m−3 are observed, respectively
[25]	Woody (timber) cutting waste (WCW)	Drying and sieving	Automatic hydraulic piston press	Clamping force and temperature	The 10:90 mass ratio of crude glycerol and WCW are suitable for forming briquettes. Its density is 798 kg·m−3, durability of 86.7%; 0.91 kN of stress resistance, HHV of 17.1 MJ·kg−1 and AC of 0.9%

MC: Moisture content; VM: Volatile matter; AC: Ash content; FC: Fixed carbon; HHV: High heating value; LHV: Lower heating value. * It was not clear whether there were grinding and drying processes.

Table 1. Main briquette production machines.

References	Raw-Material	Pre-Treatment	Briquetting Machine	Variables of Briquetting Equipment	Results and Conclusions of the Studies
[40]	Cassava waste	Drying, grinding and sieving	Hydraulic Press	Pressure	Briquettes produced without heating or binder achieved good results: relaxed density of 918 kg·m−3, compressive strength of 1.29 MPa, durability of 94.1%, expansion of 8.68 and energy density of 15.7 GJ·m−3
[29]	Corncobs (CC) and oil palm trunk bark (OPTB)	Drying, grinding and sieving	Hydraulic Piston Press	Temperature and pressure	They were obtained: MC of 9.24–10.00%, density of 0.38–0.40 g·cm−3, water resistance of 87.60–92.00%, mechanical strength of 98.28–99.08%, compressive strength of 18.47 to 21.75 MPa and calorific value of 16.54 to 16.91 MJ·kg−1
[8]	Olive mil solid waste (OMSW)	Drying, grinding and sieving	Hydraulic Press	Temperature and pressure	Briquettes with unit density: 2.04–3.03 g·cm−3 and bulk density: 0.84–1.20 g·cm−3. With 15% of binder, greater compressive strength (4581 kN), HHV (16.92 MJ·kg−1) and lower AC (6.72%) were obtained
[43]	Cashew nut shells (CNS) and areca nut shells (ANS)	Torrefaction (CNS), drying (ANS), grinding and sieving	Compacted screw motor-driven	Compressed screw speed	Average values: 18.9–21 MJ·kg−1 of calorific value, hardness 103–123 HB, porosity 52–65%, TU 3.5–4.5%, VM 71.2–73.2%, TC 3.3–5.2% and CF 18.8–20%. The “A” mixture is suitable for the production of briquettes, with a speed of 90 rpm and small CNS
[37]	Lantana camara e Prosopis juliflora	Drying, breaking, grinding, pulverizing and sieving	Industrial machine. Piston and ram	Not specified	The briquettes reached a density of 1200 kg·m−3, an energy density of 23.05 GJ·m−3. Ideal MC of 10–12% and lower AC of ≤2%
[38]	Palm Stem	Chopping, cleaning, carbonization and sieving	Hydraulic Press	Pressure	Through optimization, HHV of 306.704 MJ·kg−1 and compressive strength of 10.0608 kg·cm−2 were obtained, with the parameters of 120 mesh, ≈47% binder and ≈9.32 MPa
[27]	Wood Waste of Pinus spp.	Collects and sieving *	Manual mechanical device	Not specified	The biomass had an apparent density of 160 and 170 kg·m−3, TC of 0.38–1.78%, VM of 65.3–95.01% and FC of 4.49–34.3%. The briquettes had MC of 11.52–13.26%, HHV of 17.0–18.3 MJ·kg−1 and density of 930–1240 kg·m−3
[28]	Corn husk (CH) e cassava peel (CP)	Drying, milling, torrefaction and humidification	Hydraulic piston press machine	Pressure	Briquettes torrefied at 300 °C with a mixture of 10/90 (CH/CP) showed good combustible characteristics. These showed increases of briquettes without thermal treatment of 18.19%; 2.98% and 32.25% in density, durability and compressive strength, respectively
[44]	Coal dust (C), sawdust (S) and post-consumer plastics (P)	Washing, drying, grinding and sieving	Manual Press	Not specified	They were obtained HHV from 26.5 to 33.8 MJ·kg−1. Impact resistance/break rate ≥ 90% in all briquettes and density from 1100 to 1300 kg·m−3. However, it had low resistance to compression, below 1 MPa
[13]	Coffee shrub residues and pinewood	Drying, grinding and sieving	Hydraulic Piston Press	Temperature and pressure	The coffee residue mixed with 75% of the pine wood yielded an apparent density of 1107–1163 kg·m−3, energy density of 19.13–19.89 GJ·m−3, tensile strength of 415–569 kgf and equilibrium moisture content of 9–11% by weight
[42]	Banana leaves and pseudostem, and rice husk	Chopping, milling, sieving, pressing and drying	Hydraulic Press	Pressure	The rice husk briquettes showed the highest compressive strength (19.8 MPa) and the highest HHV (18.1 MJ·kg−1), however they had a high AC value (25.4%)
[34]	Sugar bagasse and straw	Drying, grinding and sieving	Manual Hydraulic Press	Load measured in tonnes	Briquettes with a greater amount of sugarcane bagasse had greater resistance to compression, less generation of fines, moisture content of 11 to 12%. The use of straw is only feasible when associated with sugarcane bagasse
[45]	Palm kernel shell biochars (PKSB)	Uninformed	Compression cylindrical ram machine	Compaction speed and pressure	Average calorific value of 31.29 ± 1.44 MJ·kg−1. Impact resistant, preserving 95% of its initial weight. Poor performance for water resistance. Depending on the processing conditions, tensile strength > 800 kN·m−2 can be achieved.
[36]	Phyllostachys aurea	Drying, grinding, sieving and humidification	Laboratory press	Temperature and pressure	Biomass, briquettes produced from it and coal were evaluated. The briquettes showed higher energy density (21.47 GJ·m−3) than coal (11.38 GJ·m−3), but both have potential for energy applications
[30]	Sawdust (S) and date palm trunk (PT), plastic waste	Grinding and sieving	Laboratory hydraulic piston press	Temperature and pressure	Although the S70-W30 briquettes produced at 130 °C have a higher density (≈1190 kg·m−3) and durability (100%), almost all PT briquettes produced at room temperature showed high densities and durability, and the largest LHV with PT90-ASR10
[39]	Charcoal thin waste and sanitary sewage sludge	Drying, grinding and sieving	Conical screw extruder	Pressure	The 50:50 mixture of charcoal and sludge was ideal for energy production with HHV of 17.47 MJ·kg−1, breaking strength of 41.19 MPa and apparent density of 913 kg·m−3
[31]	Plastic waste, sawdust, maize husk and carbonized sub-bituminous coal	Drying, chopping, carbonization, grinding and sieving	Hydraulic Press	Load of up to 10 tons	HHV for briquettes with 0% and 90% coke was 13.8 and 21.45 MJ·kg−1, respectively. The raw material of 100% coke produces HHV of 23.86 MJ·kg−1. The highest VM (31.74%) and compressive strength (4.61 N·mm−3) were obtained in the briquette with 100% biomass and 0% coke
[46]	Coconut Fiber (FC) and sugarcane straw (PC)	Drying, grinding and sieving	Manual Hydraulic Press	Pressure	The briquettes made of PC e FC can be manufactured without heating or adding binder. The highest amount of FC was favorable for the compressive strength (0.37 MPa), HHV (18.24 MJ·kg−1), fixed carbon (22.28%) and AC (3.71%)
[26]	Wood of Eucalyptus spp. and Eucalyptus grandis	Drying, cutting, torrefaction, grinding and sieving	Hydraulic Piston Press	Temperature, pressing time and pressure	Values for E. spp. and E. grandis, respectively. Bulk density: 1.14 and 1.06 g·cm−3; energy density (Treatment at 200 °C): 24.79 and 21.70 GJ·m−3; lowest MC: 9.9 and 9.6%; lower hygroscopy: E. grandis at 200 °C and 10 MPa; greater compressive strength: relative
[47]	Banana Leave Waste	Milling and sizing	Hydraulic Press	Pressure	The compaction time of 0.6 and 1 s was negligible for density, with values of 999 and 1000 kg·m−3, respectively. Higher compressive strength (5.3 MPa) was achieved with 1 s. The calculation of the energy density was based on the density of 999 kg·m−3
[48]	Corn stover	Drying, chopping and Milling	Laboratory piston press (Plunger)	Pressure	Given the different experiments, with different interactions of the input variables, better dry density and bulk density of 470 and 190 kg·m−3 are observed, respectively
[25]	Woody (timber) cutting waste (WCW)	Drying and sieving	Automatic hydraulic piston press	Clamping force and temperature	The 10:90 mass ratio of crude glycerol and WCW are suitable for forming briquettes. Its density is 798 kg·m−3, durability of 86.7%; 0.91 kN of stress resistance, HHV of 17.1 MJ·kg−1 and AC of 0.9%

MC: Moisture content; VM: Volatile matter; AC: Ash content; FC: Fixed carbon; HHV: High heating value; LHV: Lower heating value. * It was not clear whether there were grinding and drying processes.

2.2. Main Briquette Production Processes

The production of briquettes can be done on an industrial scale and greater control of process conditions in automated machines, as was done by Kumar and Chandrashekar [37] and Raslavičius [25]. These processes normally aim to serve the solid biofuels market, requiring effective production and the best cost-benefit ratio. Briquettes can also be produced manually, using hydraulic presses, or using manual mechanical devices (Figure 1) [27,49,50]. Manual briquetters are generally simpler to operate. They operate at lower pressures and may have heating components, however, produce briquettes with lower density [29,51].

According to Tumuluru et al. [17], the production of briquettes is usually done on hydraulic, mechanical, or roll presses. Kpalo et al. [15] also included briquetting using a screw press, piston press (mechanical or hydraulic) and manual press. In addition to these technologies, it is possible to find biomass briquettes produced using a cuber, agglomerator and tabletizer [17].

Mendoza Martinez et al. [13] and Seco et al. [52] carried out experimental studies to evaluate the briquetting using machines at the laboratory level of the piston press. As previously stated, such equipment can be hydraulically or mechanically driven. Mechanics generally exert a greater compressive force than hydraulics, delivering briquettes with a higher unit density. On the other hand, hydraulic briquetters admit moisture contents higher than the 15% tolerated by their similar and have unit densities ranging from 800 to 1000 kg·m⁻³ [17]. In Figure 2, it is possible to observe a specific experimental rig for the production of briquettes that has a mold, punch and pressing plate [53].

Screw press machines (screw extruders) are popular for the production of briquettes in developing countries, such as Brazil, Africa and India, being a suitable technology for small scale uses [10]. The briquettes produced on these machines, unlike those produced on piston presses, are suitable for carbonization, gasification and torrefaction [6]. However, both technologies produce briquettes hard enough to support their transport in trucks without degrading [51].

The schematic and photo of a Metalchem S45 screw extruder, which produces solid fuel in the form of briquettes, can be seen in Figure 3a,b). Its operation does not allow the briquetting pressure to be determined directly, being necessary to control the rotation speed of the compression screw, or to vary the internal diameter of the die, in order to change the operating pressure [54].

In piston press machines, usually, the biomass contained in a matrix receives a high-pressure load through an alternative cylinder. In the screw extruder, which requires a heated matrix to reduce friction, the biomass is continuously extruded by a screw through a conical matrix. Although piston presses consume less energy in their execution and cause less wear on the contact parts, screw press machines deliver briquettes of superior quality [55,56].

Some of the main briquette production machines can be seen in Table 1. It also shows the feedstock, the pre-treatments used, the input variables associated with the briquetting equipment used, and the results achieved. Despite the diversity of materials composing the briquettes, most of the research made use of hydraulic presses (manual, automated, bench and experimental). As the model in Figure 4, used in at least three of the works [34,40,46]. Piston presses, screw extruders and others were also used.

According to Table 1, it is possible to observe that the authors who used hydraulic presses had pressure as a common input variable. When it was not possible to measure the pressure, the load in tons responsible for compressing the briquettes was given, as reported Nwabue, Unah and Itumoh [31] and Masullo et al. [34]. Despite these patterns, the relationship between process variables, briquetting equipment, raw material and other elements of briquette production is not obvious, as will be discussed throughout this literature review.

3. Main Parameters of Briquette Production in Academic Works

Several briquetting parameters are used to achieve the desired characteristics for the briquettes, which also vary according to the production process applied.

Table 2. Main input variables for briquette production.

Researchers	Temperature	Compaction Time	Cooling Time	Pressure	Granulometry	% Biomass	Type of Biomass	Binder	Compaction Speed	Moisture Content
[40]		X		X	X					X
[29]	X			X	X	X		X		X
[8]	X	X		X	X			X
[43]					X	X		X	X	X
[37]					X		X			X
[38]		X		X	X			X
[27]					X			X
[28]		X		X	X	X		X
[44]					X	X		X
[13]	X	X	X	X	X	X	X			X
[42]		X		X	X		X			X
[34]		X			X	X				X
[45]		X		X				X	X
[36]	X	X	X	X	X					X
[30]	X	X		X	X	X
[39]				X	X	X		X		X
[31]					X	X		X		X
[46]		X		X	X	X				X
[26]	X	X	X	X	X		X			X
[47]		X		X	X					X
[48]				X	X					X
[25]	X			X	X			X		X
Occurrence	7	13	3	16	21	10	4	11	2	15
Percentage	32%	59%	14%	73%	95%	45%	18%	50%	9%	68%

presents some works found in the literature, whose content is the briquetting parameters used and their respective authors.

It is possible to observe that of the parameters for the production of the briquettes, the granulometry was the most frequent process (or input) variable among the researchers analyzed. It was present in 21 of the 22 studies, equivalent to 95% of the studies. The granulometry refers to the size of the biomass particles to compose the briquettes, giving it uniformity.

The pressure applied in the production of the briquettes was also widely explored, being one of the main parameters associated with the briquetting equipment. This variable was studied in 16 studies, corresponding to 73% of these. Subsequently, the initial moisture content of the raw material was the third input variable with the highest occurrence among the surveys, being present in 15 of them.

The pressing time to which the briquette is subjected to being compacted was studied by 13 works, equivalent to 59% of them. The fraction (or percentage) of binder incorporated in the briquettes was present in 11 of the studies analyzed. It is worth mentioning that moisture in the production of briquettes is important to activate some binders, such as starch, helping in the densification process [37,52].

Table 2 also shows that the proportion of different types of biomass composing the briquettes was studied in 10 studies. Meanwhile, the briquetting temperature was applied in seven of the studies. The type of biomass of the briquettes was observed in four studies. Unlike the biomass proportion parameter, this variable assesses different types of raw materials to compose the briquettes, comparing them separately.

Finally, the compaction speed and the cooling time of the briquettes after their production were the parameters with the lowest number of occurrences among the studies explored. Its frequency was in two and three works, respectively. This does not mean that these variables are less important for the briquetting process, or for the final properties of solid fuel. For Chungcharoen and Srisang [43], speed was the variable with the greatest effect on the rate of production of their briquettes. While the cooling time was necessary for processes involving heating in the production of briquettes [13,26,36].

Based on the occurrence between the works, it is observed that the most used input parameters for the production of briquettes are the briquetting temperature, the proportion of biomass mixture, presence of binder, pressing time of the briquettes during its production, moisture content of raw material before briquetting, compaction pressure and material granulometry.

Since the production of briquettes consists of the densification of loose particles of solid material in a high-density fuel [10,56], it is necessary that operational factors, such as pressure, temperature and compaction time are defined for the production of the briquettes. As the definition itself suggests, biomass must initially be in the form of loose particles, preferably classified according to their size and distribution, being one of the most important material parameters for the densification process [13,41]. This variable can influence the energy spent during briquetting, as well as favor the strength of the briquettes [57,58].

Another characteristic of the material that must be controlled before briquetting is its moisture content. This parameter must have a value that allows smooth densification, without compromising the physical and energetic properties of the briquettes [55].

The presence of binder is a relevant factor for the adhesion of the biomass particles to each other, and it can be added to the material or its own structure [14]. The binder can also be activated through external variables, such as the addition of heat and pressure during the production of briquettes [14,59]. It can be seen that there is an interaction between operational variables and the initial characteristics of the material. Pressure, for example, allows the densification of biomass, influencing the increase in density and strength of the briquette [40].

All these parameters are extremely important for the biomass densification process in the form of briquettes, so that those already mentioned as the most common among the works will be discussed in detail in individual topics.

3.1. Levels of Briquette Production Parameters

In order to find the best production results for biomass briquettes, several studies vary the process parameters at some levels, in order to assess the behavior of the densified material.

Table 3. Temperature levels of the briquetting process.

Researchers	28	30	38	60	90	110	120	130	200
	(°C)
[29]
[8]
[13]
[36]
[30]							120	130
[26]
[25]

,

Table 4. Variation of the levels of the proportion of biomass to compose the briquettes.

Researchers	0	10	22	25	46.66	50	65	70	75	80	90	100
	(%)
[29]												100
[43]							65
[28]		10 *									90
[44]		10
[13]									75			100
[34]									75
[30]											90
[39]						50 *						100
[31]											90
[46]												100

* Optimal values indicated by the authors, [29] Corncobs (CC), [43] Cashew nut shells (CNS) and contain 10 to 20% of cassava flour, [28] Cornhusk (CH), [44] Sawdust (S), [13] Pine wood, [34] Sugar cane bagasse, [30] Date palm trunk biomass, [39] Charcoal fines, [31] Coke, [46] Coconut fiber (FC).

,

Table 5. Binder levels added to densified biomass.

Researchers	0	0.5	1	4	8	10	15	20	25	30	40	47
	(%)
[38]											40	≈47 *
[29]
[8]							15
[43]						10
[27]
[28]
[44]									25
[45]				3–5
[39]
[31]						10
[25]						10

Source: Authors. * optimization value.

,

Table 6. Pressing time levels for the production of briquettes.

Researchers	0	0.6	1	2	10	30	60	120	240	300	420	900
	(s)
[40]								120
[8]
[38]
[28]
[13]
[42]
[45]	0
[34]
[36]
[30]
[46]
[26]
[47]			1

,

Table 7. Biomass moisture levels for the briquetting process.

Researchers	4	5	8	9	10	11	12	14	55
	(%)
[40]
[29]				9.27
[43]
[37]					10–12
[13]
[42]
[34]
[36]
[39]
[31]					10.62
[46]
[26]					10.54–12.86
[47]
[48]			8.3
[25]

,

Table 8. Pressure levels applied during the briquetting process.

Researchers	7	8.27	10	12	14	15	18	22	27.6	40	50	60	67	80	100	125	150	204
	(MPa)
[40]															102			204
[29]
[8]																	150
[38]			≈ 9.3 *	≈11–12
[28]
[13]
[42]
[45]										40		60
[36]					13.73
[30]										44
[39]
[46]					14											122.31
[26]	Not significant
[47]
[25]

* Optimization value.

and

Table 9. Granulometry levels of the biomass used for the production of briquettes.

Researchers	0.10	0.125	0.149	0.25	0.297	0.31	0.5	0.84	1	1.19	2	2.38	3	5	6	19	40	42
	(mm)
[40]										>1.19
[29]
[8]
[43]													4.76
[37]									Mixture
[38]		0.125 *	0.149
[27]							Mixture
[28]										<1.18
[44]
[13]						Mixture
[42]												Mixture
[34]			Mixture
[36]
[30]
[39]					Mixture
[31]
[46]			Mixture
[26]							0.42
[47]												2.5
[48]																40–42
[25]

* Optimization value.

show the levels of the briquetting variables found in the studies. It is worth mentioning that it is common among researchers to keep some of their production variables fixed, while other parameters have varied in levels to assess their influence on the final properties of the briquettes. This can be seen in the work of Araújo et al. [26]; Kpalo et al. [29]; Mendoza Martinez et al. [13] and Masullo et al. [34].

As stated earlier, certain process variables had a small frequency among the analyzed surveys. Thus, only the levels of the variables with the highest occurrence among the studies were represented in tables. The variables were briquetting temperature, percentage of biomass, binder, compaction time, moisture content, pressure and particle size.

3.1.1. Effect of Temperature on the Production of Briquettes

Table 3 shows the briquetting temperature levels. In it are the seven works that made use of this variable in their production process. The values highlighted in the table indicate with which temperature levels the greatest results were obtained for the studied physical and mechanical properties. The studies that kept this parameter fixed did not have their values highlighted in Table 3.

Among the works cited, only Garrido, Conesa and Garcia [30] explored this variable at different levels. Heating at 120 and 130 °C was carried out only on briquettes that did not compact at room temperature. This rise in the briquetting temperature provided durability of approximately 100% and density values around 1.2–1.4 times greater than those obtained at room temperature. Some of the briquettes produced without heating, also showed good results of density and durability, indicating the influence of other factors, such as compaction pressure.

Khlifi et al. [8], despite having used a lower temperature level, also achieved good physical properties, with high unitary and bulk density. On the other hand, high compaction pressure and low granulometry were used, which according to Okot, Bilsborrow and Phan [60], is necessary when reducing the briquetting temperature.

Analyzing Table 3, it is observed that the temperature level of 120 °C was the most studied. The densities achieved in these conditions were high and above 1000 kg/m³. For low moisture biomass, rising temperatures can harden the briquettes and make them denser [61]. In biomass briquettes produced under high temperatures (150–250 °C), there is also an increase in the properties of calorific value, volatile matter, density, water-resistance and relaxation rate of solid fuel [16,62].

The work of Araújo et al. [26] was one of those that produced his briquettes at 120 °C. Despite applying a relatively high temperature, the torrefaction of the biomass for some briquettes was carried out in order to improve the properties of the solid fuel.

The briquetting temperature, however, must not exceed 300 °C to avoid the risk that the biomass components will decompose [55]. This value can also vary when the biomass is mixed with other types of materials. One of the plastic residues (printed circuit board—PCB) mixed with the biomass of the briquettes produced by Garrido, Conesa and Garcia [30], started to decompose at temperatures of 120 and 130 °C. The event, therefore, prevents the production of briquettes in these conditions.

The heat generated with the briquetting temperature is able to activate the natural biomass binders, such as lipids, lignin, starch and protein, or the binders added to the biomass, by means of attraction forces between the particles [14,35] At high temperatures, plastic deformations of thermoplastic particles occur, enabling the formation of permanent bonds in the material [35].

As noted, the heat applied in the production of the briquettes is capable of altering its physical, mechanical and energetic properties. Despite the influence that the increase in temperature has on the final characteristics of solid fuels, this variable is directly related to the resources that the briquetting equipment has. For this reason, the type of machine used in biomass densification may be a limitation for the exploration of this variable.

3.1.2. Variation in the Proportion of Biomass Mixed in the Production of Briquettes

The mixture of more than one type of biomass for the production of solid fuels has been the target of research that seeks to find the appropriate relationship between quality, availability of inputs and cost-benefit [33,63]. This is because, the association of different raw materials implies different physical characteristics, such as density, granulometry and humidity [33]. The same happens with the chemical characteristics and composition of cellulose, hemicellulose and lignin of the biomass [17,32].

The combination of materials also promotes the energy reuse of excess waste from some cultures or processes, bringing environmental and socioeconomic benefits [29]. However, given the possible differences that may exist between the raw materials, it is necessary to apply appropriate processing techniques, since they influence the handling, the final quality of the product and the costs with energy consumption [33]. In this way, it is possible to extract the best that each material has to offer for solid fuel.

Table 4 shows 10 of the 22 studies that evaluated the effects of combining more than one type of biomass on the composition of briquettes. The biomass levels are presented in terms of the proportion of one of its constituents. So, Kpalo et al. [29] varied the composition of corncobs briquettes (CC) and oil palm trunk bark (OPTB) in the following proportions: 100:0, 75:25, 50:50, 25:75, 0:100. The same was done for the other four works shown in the table, which analyzed the range from 0 to 100% of the waste combination. Varying the composition of the briquettes in the quoted quantities allows for levels formed by the pure raw material of each component of the briquette, as well as their interactions in different fractions.

The criterion for selecting the biomasses used by Iftikhar et al. [32] was about the greater calorific power that each raw material could contribute. Table 4 shows the levels of biomass that delivered the highest calorific value per briquette. Only the search for Masullo et al. [34] did not assess her briquettes for calorific value, but for physical-mechanical characteristics. They came to the conclusion that the 75:25 ratio of bagasse and sugarcane straw, respectively, was the best mixing ratio for their briquettes.

Mendoza Martinez et al. [13] measured the HHV only for the biomass individually, since it was necessary to calculate the energy density of the briquettes. Thus, with 100% of the pine wood biomass, the largest HHV (20.7 MJ·kg⁻¹) was obtained. Already the briquettes made up of 75% pine wood and 25% of other residues resulted in excellent properties of equilibrium moisture content (9–11% by weight), tensile strength (415–569 kgf), apparent density (1107–1163 kg·m⁻³) and energy density (19,133–19,899 MJ·m⁻³).

The study by Chungcharoen and Srisang [43] reinforced the concept that the final calorific value of briquettes depends on the initial calorific value of individual raw materials. Thus, from Table 4, it is observed that the briquettes whose largest fraction corresponds to the biomass with the highest calorific value will also have the highest final calorific value. This pattern was repeated for the other studies that evaluated this variable. The exception was given only to the research carried out by Gwenzi, Ncube and Rukuni [44], who reached the highest calorific value, of 33.8 MJ·kg⁻¹ with a mixture of 50% coal dust, 40% plastic waste and 10% sawdust. Such proportions achieved the best results of water absorption and resistance properties for the briquettes.

For the work carried out by Waheed and Akogun [28] and de Oliveira et al. [39], two different levels for each were highlighted in the table. One of them concerns the level with which the greatest calorific value was obtained, and the other corresponds to the optimal value for a set of output variables, indicated by the authors.

In their research, de Oliveira et al. [39] obtained the highest calorific value (23.10 MJ·kg⁻¹) with 100% of the briquette made from fine charcoal. This result is due to the greater amount of fixed carbon present in charcoal and the strong bonds between its carbon atoms. On the other hand, when using 50% of sewage sludge and 50% of charcoal, a high HHV (17.47 MJ·kg⁻¹) is also obtained, in addition to achieving the highest mechanical strength and apparent density for the briquettes. This effect may be due to the existence of functional groups present in the sewage sludge that influence the strength, given the formation of hydrogen interactions.

As for the briquettes produced by Waheed and Akogun [28], the optimal proportion of biomass to compose the briquettes was 10:90 of corn husks (CH) and cassava husks (CP). This combination resulted in fuel with better density, durability and mechanical resistance, in addition to a calorific value of 17.89 MJ·kg⁻¹. On the other hand, the greater calorific value (19.31 MJ·kg⁻¹) was due to the higher concentration of CH, in the composition of 90:10 of CH and CP, respectively. It is worth mentioning that in both proportions, the authors attributed their results, in part, to the biomass torrefaction process.

Like most authors in Table 4, Kpalo et al. [29] identified that more than one combination for the biomass proportions of the briquettes serve well for the energy purposes they propose. Thus, it is necessary to find the combination that results in good energy and physical properties simultaneously.

3.1.3. Effect of the Addition of Binders on Biomass for the Production of Briquettes

The use of binders for the production of solid fuels, such as pellets and briquettes gives them better physical properties and optimizes operational processes [32]. Once subjected to process variables, such as pressure and/or temperature, such binders, which may be part of the biomass structure, or be added to it, are rearranged in the structure of the densified material, or form bonds between the particles [14].

One of the most important connections for the production of densified material is solid bridges. They can be formed due to chemical reactions, hardening of ligands, solidification of fused compounds, or by the crystallization of some components, such as lignin and proteins. Solid bridges are formed preferentially during the drying/cooling of densified products [64].

Chung [59] pointed out industrial techniques to promote adhesion and increase the molecular contact between a set of molecules, such as the supply of heat at temperatures above the glass transition point, the use of pressure, and the use of solvents, such as water. When applying heat to temperature in the glass transition band, associated with humidity, it is possible to activate (soften) the natural binders in order to achieve a durable bond between particles [64]. Manufacturing processes that employ high pressures and temperatures generally do not require the addition of artificial binders, and this technique is more associated with low-pressure compressions [15].

Several binders, such as molasses, corn and cassava starch, crude glycerol and paper pulp have been used in previous studies as solid fuel binders [27,28,29,38,44]. Some of these studies can be found in Table 5, where various levels of binders have been incorporated into the briquettes. The table highlights the fractions of binder that were responsible for improving some of the briquettes’ output properties, such as compressive strength and calorific value.

The binder used most often among the works studied was starch, which is often mixed with heated water to produce a paste with a gelatinous consistency [27,45]. The research by Khlifi et al. [8] used corn starch and analyzed the briquettes produced with 0, 10, 15 and 30% of the binder. The conclusion reached was that the addition of 15% of corn starch provides better resistance to compression, an increase in the higher calorific value and a reduction in the ash content.

Bazargan, Rough and Mckay [45] also used starch as a binding agent. They observed that resistant briquettes can be made with the addition of 3 or 5% of starch, provided that the pressure used for compacting the briquettes is 60 or 40 MPa, respectively. In this way, it is up to the manufacturer to choose the process condition that best suits him.

Kpalo et al. [29] observed that the biomass used for their briquettes had a low content of lignin, water-soluble carbohydrates, low apparent density and a low amount of proteins. The increase in the concentration of these constituents is essential for the formation of solid bridges during the production of briquettes. These characteristics led them to use paper pulp as a binder to provide greater adhesion between the particles. As the research did not intend to evaluate the variation of binder to produce the briquettes, this parameter was kept fixed at 10%. The same was true in studies carried out by Morales-Máximo et al. [27], Waheed and Akogun [28] and de Oliveira et al. [39]. Therefore, the proportion of binder in these studies was not highlighted in Table 5.

Helwani et al. [38] applied the highest percentage of binder studied, but it is worth mentioning that the briquetting occurred under low pressure (max. 12 MPa) and without heating. This work used crude glycerol and evaluated it at three levels (20, 30 and 40%), also performing optimization of the response surface methodology, of CCD (Central Composite Design) arrangement, so that other levels were studied. Among the three levels mentioned, the greatest resistance to compression and a high calorific value were obtained with 40% of the binder. However, the optimization value that simultaneously generated the greatest calorific value (30.67 MJ·kg⁻¹) and the greatest compressive strength (10.06 kg·cm⁻²) was approximately 47% of crude glycerol. This binder provides firmness to the briquettes as well as having the ability to increase the calorific value.

Another study that used crude glycerol was that of Raslavičius [25]. This time it was tested at levels below 0, 10 and 20%, allowing the reuse of this binder, which is a by-product of large biodiesel plants. The ideal level of crude glycerol for the process was limited by the technology used in the research. The value of 10% delivered greater physical benefits, allowing the transport and handling of the briquettes, as well as adequate combustion with little ash generation. Additionally, with 10% binders, Nwabue, Unah and Itumoh [31] achieved the best responses to compressive strength and calorific value. However, the authors used random proportions of more than one binder. The greatest calorific value was obtained with the binder in the proportions of 5% of starch and 5% of bodywork with briquettes made of 90% of coke. The binder that resulted in the greatest compressive strength was composed of 5% limestone and 5% bodywork, with briquettes made of 80% carbonized biomass and 10% of carbonized plastic.

In general, the binder helped in the physical and energetic properties of the briquettes, functioning as a glue, especially among briquettes produced under low pressure and temperature. The only authors in Table 5 who fled this trend were de Oliveira et al. [39], who used a pressure of 100 MPa, and Raslavičius [25], who preheated his pressing chamber to 200 °C, and still used binder.

3.1.4. Effect of Pressing Time on the Production of Briquettes

Table 6 addresses the 13 works that had the pressing time as a process variable. This parameter concerns the time that the biomass will remain in the mold while it is compacted by the equipment used for the briquetting. Among the research analyzed, only those of Granado et al. [40], Bazargan, Rough and Mckay [45] and de Oliveira Maia et al. [47] evaluated different levels for this parameter. For this reason, the levels of compaction time with which the greatest results were achieved for physical properties were highlighted in Table 6. The works that kept the parameter in question fixed, do not have their values highlighted in the table.

The compaction time when associated with other variables, such as the presence of binder and the conditions of compaction, pressure and temperature, determine the density and compressive strength of the briquettes [15]. Similarly, other authors have reached statistical conclusions (p-value < 0.05) that the pressing time is a significant process variable to achieve appropriate results of density and modulus of elasticity for the briquettes [65].

On the other hand, Bazargan, Rough and Mckay [45] observed that the pressing time (or retention time) had no significant influence on the tensile strength, since, under higher pressures, this effect is negligible. In this way, as long as the quality of the briquettes is preserved, the retention time can be reduced and provide a higher production rate. This scenario led the authors to disregard this parameter (0 s) and achieve greater production of briquettes. Granado et al. [40] also evaluated the level of 0 s for the variable, however, they observed the need to determine a waiting time to obtain denser briquettes. The 120 s time associated with 204 MPa pressure of pressure gave the best results of relaxed and energetic densities, durability and resistance to compression. However, the compaction time showed significant differences only when a lower pressure (102 MPa) was applied during briquetting.

For de Oliveira Maia et al. [47], the compression time had little influence on the physical properties of the briquettes. The sensitive variation in the pressing time, from 0.6 to 1 s, resulted in a small increase in the compressive strength of the briquettes, going from 3.6 to 5.3 MPa. As for the apparent density, this change was practically nil.

Other works, in turn, used long pressing times, in order to obtain more stable briquettes. Araújo et al. [26] applied a pressing time of 420 s (7 min) and 360 s (6 min) of rest time to the briquettes in order to avoid the formation of cracks in the compacted material. Khlifi et al. [8] applied the optimal value of residence time, according to the literature they researched, of 900 s (15 min). This was the longest compaction time among the 13 works in Table 6, even though it used high levels of pressure.

Table 6 also allows us to observe that there is no common sense as to the ideal pressing time for all densification processes. De Oliveira Maia et al. [42], reported that operating conditions, such as pressure and compaction time, are chosen according to the configuration and availability of the briquetting equipment. In this way, some works, such as that of Sette Júnior et al. [36] carry out preliminary tests to determine the ideal level for compaction time. As can be seen, the divergences regarding the pressing time and its impact on the final product indicate how particular each briquetting process is and justify a more detailed investigation for this process variable.

3.1.5. Effect of Biomass Moisture on the Production of Briquettes

The moisture content is an important parameter both for the production of the briquettes and for the final characteristics of the solid fuel. In the briquetting process, it assists in smoother densification of the biomass, reducing the risks of cracking and breaking of the briquettes [37,55].

Naturally, biomass in natura has a high humidity that makes it difficult to use as an energy source. Some residues, such as cut wood and sewage sludge can have a moisture content of approximately 40% [25,39]. Thus, it is common to carry out drying processes of the raw material in the sun, in ovens or greenhouses with the intention of reducing humidity [13,36,39].

Some authors report that the ideal moisture content for briquetting ranges from 10 to 12% [13,37]. Others claim that biomass must have 8% to 15% for densification to occur smoothly [34].

Moisture acts as a binder in materials composed of organic and cellular products. It promotes connections of van de Walls forces, which in ideal amounts promote an increase in the contact area between the biomass particles [55,64]. Very dry materials demonstrate a weak interaction between the particles. While a high amount of water (TU > 15%) causes fragile internal connections and less durability of the briquettes [61].

For the production of briquettes in roller press machines, it is recommended that the biomass has 10 to 20% (wet basis) of moisture [35]. In extruder machines, this parameter is more critical, varying from 4 to 8% according to Tumuluru et al. [17] and 8 to 10%, according to Grover and Mishra [55]. In piston press machines, on the other hand, there is a tolerance of 10 to 15%. A value greater than 15% for this variable may be permissible in hydraulic piston presses [55]. Such divergences as to the ideal moisture content show the need to adapt it to the type of briquetting machine.

As can be seen in Table 7, most works produced their briquettes with the moisture content of the biomass varying from 8 to 12%. The ideal initial humidity levels for the research by Kumar and Chandrashekar [37] and Thoreson et al. [48] are highlighted in the table. Those with only one level of moisture content, kept this variable fixed, and so they were not highlighted in the table.

Chungcharoen and Srisang [43] produced briquettes composed of two biomasses, each with different moisture content. In the same study, the cashew shell (CNS) after carbonization showed a moisture content of 8%, while the areca nut (ANS), after drying, reached 11 to 12%. Such humidity levels, although different, were not highlighted in Table 7 because they are fixed for the biomasses composing the briquettes. The same happened for the work of de Oliveira et al. [39] and Nwabue, Unah and Itumoh [31]. For Araújo et al. [26] which produced briquettes with different raw materials, but without mixing them, the range of moisture content found for the Eucalyptus spp. and Eucalyptus grandis wood was presented. For each of them, moisture content was obtained under conditions without thermal pre-treatment of the biomass and with thermal pre-treatment at different temperatures.

Kumar and Chandrashekar [37] produced denser, stronger and more stable briquettes with the moisture content of the biomass between 10 and 12%. At these levels, it was possible to perform the briquetting with less energy consumption and with a minimum of cracks and blockages. Thoreson et al. [48] carried out a three-way interaction experiment, with an explored moisture content of 8.3 and 54.5%. In this scenario, the briquettes produced with the highest moisture content suffered a negative influence on the average dry particle density and the elasticity of the briquettes.

The moisture content of the biomasses composing the briquettes made by de Oliveira et al. [39] was 4% for charcoal fines and 14.11% for sewage sludge. Such results were obtained after exposing the raw materials to the sun while drying in the open air. However, the moisture contained in the dry biomass was not sufficient to perform the briquetting, requiring the addition of 8% moisture and 10% binder together with the biomass mixture.

3.1.6. Effect of Pressure on the Production of Biomass Briquettes

One of the most important factors in the biomass densification process is the application of a given pressure to obtain solid fuels. According to Nunes, Andrade and Dias Júnior [66], this process variable does not normally alter the characteristics of the material’s calorific value, however, it has a great influence on the physical-mechanical properties of briquettes, as well as on the moisture content and ashes. Helwani et al. [38] in turn, claim that pressure also has an influence on the calorific value of briquettes, as it allows rawer material to come into contact with the binder made from crude glycerol, causing an increase in calorific value.

The increase in pressure also influences the resistance to compressive strength, since it acts at the molecular level on the natural constituents of biomass, such as lipids, lignin, starch and protein. Such components are redistributed on the compacted biomass while they are expelled from their original matrix and start to fill the empty spaces, forming a new matrix of briquettes [14]. It is also known that, at a given moment, the effect of increased pressure does not increase the durability of the briquettes. This happens when the briquetting process reaches the compaction phase, where the biomass particles deform plastically in the direction of the applied stress and become entangled with the adjacent particles, making the biomass more compact [67].

The works presented in Table 8, applied pressure to produce their briquettes as an input variable. Some of them even set out to evaluate it at different levels, which had a wide range between their values, whose extremes were from 7 to 204 MPa. In these surveys, the pressure levels that optimize the characteristics of the briquettes, making their production processes more suitable are highlighted in Table 8. Some studies did not indicate optimal values, or maintain the pressure as a fixed variable and therefore, did not have values highlighted in the table. Works that did not measure pressure as a densification element used other means to carry out compaction. Like the work of Nwabue, Unah and Itumoh [31] and Masullo et al. [34] who used loads measured in tons to compress their briquettes and did not convert the values into pressure units. Others did not specify how the process was done or had its compression driven by an engine [27,43].

Despite the relevance of pressure in the production of solid fuels, Araújo et al. [26] observed that the input variable did not cause major differences in the characteristics of their briquettes. The effects of pressure were evaluated at the levels of 7, 10 and 14 MPa. It is worth mentioning that the briquetting was carried out under heating of 120 °C, with small particles (0.42 mm) and at relatively low pressures, factors that may have contributed to this result.

As for the study by Helwani et al. [38], both the compressive strength and the calorific value are influenced by the pressing pressure. Both properties are optimized at a pressure of 93.1821 bar (≈9.3 MPa). Apart from the optimization study, the greatest compressive strength and calorific value were obtained with pressures of 11.77 MPa (≈12 MPa) and 10.78 MPa (≈11 MPa), respectively. The rounded values were highlighted in Table 8, as well as the value of the optimization pressure.

Mendoza Martinez et al. [13], indicated the pressure of 8.27 MPa as ideal for the production of their paper-based briquettes. Similarly, Kpalo et al. [29] applied 7 MPa of pressure, however, they kept this parameter constant throughout the briquetting process.

Using high pressures, Khlifi et al. [8], who used pressures of 100, 125 and 150 MPa, observed that for briquettes with a proportion of 85% biomass and 15% binder, the pressure of 150 MPa guaranteed greater resistance to compression. The increase in compaction pressure also directly influenced the density and compressive strength of cassava residue briquettes produced in the absence of heating or binder [40]. The application of 204 MPa in a hydraulic press, resulted in the highest values of relaxed density, energy, durability and resistance to compression [40]. The pressure is considered high when it is equal to or greater than 100 MPa, it is intermediate for values between 5 and 100 MPa and is considered low when it is less than 5 MPa [15].

By using the compaction pressure for intermediate values of 40 and 60 MPa (depending on the binder content), Bazargan, Rough and Mckay [45] were able to produce briquettes with adequate strength and greater than their reference value, 375 kN·m⁻². With lower pressures, it is possible to reduce operating costs with energy, and with maintenance, since the equipment is more susceptible to wear and tear when using high pressures.

Also with pressures at intermediate levels, Garrido, Conesa and Garcia [30] reached the highest density and durability when densification occurred in the middle of heating and with pressures of 22 and 44 MPa. The pressure influence was more relevant for the briquettes produced at room temperature, but in both temperature conditions, there was an increase in density with the increase in pressure. In general, the highest durability and density were obtained with 44 MPa, however, for the process with high temperatures, dense briquettes with good durability at 22 MPa are also obtained.

In general, most works applied pressures classified as intermediate (5–100 MPa), allowing densification with less energy expenditure. It was also observed that the greatest pressure does not always provide the highest results for the output variables, and there may be interactions with other input factors in addition to technical limitations.

3.1.7. Effect of Particle Size on the Production of Biomass Briquettes

The particle size refers to the size of the particles to be densified in the production of the briquettes, giving them greater uniformity. The size of the particles and their distribution make up some of the main determinants of the physical and mechanical properties of the briquettes [41]. It is known that the larger the size of the particles, the greater the energy required to densify the biomass, requiring that greater pressure be applied during densification [58,68].

The particle size also has an effect on energy conversion, storage and emissions, as it affects calorific power, drying and the durability of briquettes [69,70]. In addition, the distribution of particles, in their various sizes, is one of the most important attributes of the input material in the densification process [13]. Thus, all the works previously presented in Table 3 used granulometry. Sixteen of them presented it as a variable of fixed input or evaluated in levels. Another five carried out a granulometric study to determine it before briquetting, and only one did not specify it anywhere. As in 21 studies (equivalent to 95% of them) the raw material underwent a process of determining the particle size to perform the briquetting, all of these were presented in Table 9.

Table 9 presents the levels at which some works varied the size of the particles for the production of their briquettes. In it, only [43], Helwani et al. [38], Thoreson et al. [48] evaluated the influence of different particle sizes on the final characteristics of biofuels. These authors achieved the best characteristics for the briquettes at the levels highlighted in Table 9. The other works kept the parameter fixed at only one level, or produced their briquettes with a heterogeneous granulometry, being listed in the table as “mixture”. This last granulometric configuration occurs when agitating sieves with holes of different dimensions, resulting in the collection of particles of varying sizes [39]. Even so, there is a predominance of one size of granulometry over the others inside the briquettes. For de Oliveira Maia et al. [42], 60% of the biomass particles in its briquettes had a particle size of 2.36 mm, while the remaining 40% had varying sizes from 0.5 to 5 mm. Kumar and Chandrashekar [37] used large (5 to 6 mm) and small (≤1 mm) particles in the proportion of 1: 4. D. Padilla et al. [46] used granulometry smaller than 0.84 mm.

Just as the particle size may not be uniform, its distribution within the solid biofuel is also not [71]. Usually, the smaller particles agglomerate at the bottom of the briquettes, while the larger ones are in the middle. The promotion of a homogeneous distribution of these particles during densification could guarantee the greatest contact between particles of different sizes bringing greater mechanical resistance and less abrasion during the handling and transport of the briquettes [57]. This is because during the compaction of biomass there is a significant difference between the volumetric expansion of larger and smaller particles. The spaces between the larger particles are occupied by the smaller ones, causing greater cohesion between them and a lower expansion rate, having an effect on the quality of the briquettes [72].

When working with smaller particles there is also an increase in the contact area by decreasing the distance between them, promoting the strong connection of mechanical interlocking forces, chemical forces, solid bridge connection and van der Waals force, which consequently has an influence on the mechanical properties of briquettes [73]. This increase in contact points also facilitates heat transfer, strengthening the bonds between particles, which in turn can give solid fuels a higher apparent density, energy density, durability, axial compression resistance, impact resistance and modulus of elasticity [73,74,75].

In this same perspective, Chungcharoen and Srisang [43] observed that by reducing the size of the particles, a greater hardness of the briquettes was obtained, since the smaller particles allow them to have a more compact aggregation. In addition, an increase in the fixed carbon content and ash content was observed, as well as a decrease in the volatile matter content.

Helwani et al. [38] observed a strong relationship between granulometry, pressing pressure and composition of the briquette matrix with resistance to compression. Among the three levels (60, 80 and 100 mesh) of granulometry evaluated by the authors, the largest of them, which corresponds to the smallest particle size (0.149 mm) allowed the achievement of the greatest resistance to compression. The authors also carried out a study to optimize the results, reaching the conclusion that the greatest compressive strength is obtained with a particle size equal to 0.125 mm. The optimization value is also highlighted in Table 9.

In contrast to the use of small particles to optimize the physical characteristics of briquettes, Thoreson et al. [48] reached qualitative conclusions that larger particles resulted in smaller flakes divisions in the raw corn straw briquettes. Despite this finding, no consistent evidence of the effect of particle size on the density of dry particles has been found. The authors concluded that the corn stover from the industrial harvest should not be subjected to additional size reduction processes. This allows densification to occur at the harvest site, reducing transportation costs. The use of larger particles also reduces the production costs of briquettes, common in very fine particle grinding [57].

Given the above, it is clear that although there is a propensity for smaller particles to promote stronger bonds between particles, there are factors that cause deviations in this pattern. In this way, the ideal granulometry for biomass densification varies with the type of raw material used and the production process employed [58]. It is important to know the agglomeration process of the raw material during densification, allowing greater control of the briquetting, as well as the final quality of the briquettes [57].

4. Main Response Parameters of Briquette Production

After the densification of raw material into solid fuel, it is important to know its thermal and energetic characteristics through parameters, such as high heating value and ash content [53]. With the same importance, the physical characteristics can determine the durability and resistance of the densified material through the knowledge of its density, durability, resistance to compression and resistance to water [16]. As previously mentioned, these parameters can vary according to several factors, including the production process, type of raw material used, process temperature and pressure [16]. In view of so many elements that can influence the final characteristics of fuel, it is necessary to standardize them in order to guarantee an adequate level of quality for each application. For this, the use of solid biofuels is certified by standards, which standardize the quality of densified fuels and specify criteria for use in commercial and residential applications [17].

In order to assist in the understanding of the results obtained with densification in the form of briquettes, information on the values obtained for energy and physical-mechanical output variables of some works are treated in

Table 10. Main energy and combustion output variables to qualify the briquettes.

Researchers	Heating Value (MJ/kg)		Ash Content (%)	Moisture Content (%)	Volatile Material (%)	Fixed Carbon (%)	Energy Density (GJ·m−3)	Heat Pretreatment	Principle of Briquetting Drive
	Higher	Lower
[40]							15.7		Hydraulic
[29]	17.78			9.24					Hydraulic
[8]	16.92		6.72	9.88	61.86	18.75			Hydraulic
[43]	21.78		2.40	3.10	70.00	20.62		Torrefaction of CNS (300°)	Motor driven screw
[37]	19.60	17.80	0.70	2.50	73.20	18.60	23.3		NS
[38]	30.67		0.45	5.50	19.73	71.40		Carbonization (400 °C and 2 h)	Hydraulic
[27]	18.30			11.52					Manual mechanic
[28]	19.31		3.30	3.60	40.10	36.80		Torrefaction of CH and CP (200, 250 and 300 °C)	Hydraulic
[44]	33.80								Manual mechanic
[13]				9.50			19.90		Hydraulic
[42]	18.10		9.85	6.68	69.40	18.70	17.52		Hydraulic
[34]				10.40					Hydraulic
[45]	31.29								NS
[36]							21.47		NS
[30]		20.80							Hydraulic
[39]	23.10								Screw extruder
[31]	21.45		27.93	3.23	17.23	42.41		Carbonization: biomass (300–500 °C) and plastic waste (200 e 250 °C)	Hydraulic
[26]			0.42	9.60	83.39	15.92	24.79	Torrefaction (180, 200 and 220 °C, 1 h)	Hydraulic
[47]	17.70		10.70	7.20	75.30	14.00	17.52		Hydraulic
[25]	17.50		0.40						Hydraulic
Arithmetic Average	21.95	19.30	6.29	7.07	56.69	28.58	20.03
SD	5.52	1.50	8.12	3.06	23.32	17.71	3.09
ENplus A1		≥16.50	≤0.70	≤10.0
ENplus A2		≥16.50	≤1.20	≤10.0
ENplus A3		≥16.50	≤2.00	≤10.0
EN 14961-2 [76]	≥18.82
DIN 51731 [77]	≥17.50

NS—Not specified; SD—Standard deviation.

and

Table 11. Main physical-mechanical output variables to qualify the briquettes.

Researchers	Densities (Kg·m−3)		Compressive Strength (MPa)	Impact Resistance (%)	Water Resistance (%)	Tensile Strength (kN·m−2)	Durability (%)	Generation of Fines (%)	Longitudinal Expansion (%)	Water Absorption (%)	Principle of Briquetting Drive
	Density [m/v]	Bulk
[40]	918.00		1.29				94.10		8.68		Hydraulic
[29]	430.00		22.33	99.20	93.20						Hydraulic
[8]	3030.00	1200.00									Hydraulic
[37]	1250.00										NS
[38]	1060.00		0.99								Hydraulic
[27]	1240.00										Manual mechanic
[28]	590.00		1.23				99.30				Hydraulic
[44]	1300.00		0.86	99.00						3.00	Manual mechanic
[13]		1163.00									Hydraulic
[42]		990.00	19.80								Hydraulic
[34]		920.00	0.37					4.00	15.00		Hydraulic
[45]				95.00	50.00	830.00					NS
[36]		1170.00				2070.00	99.89				NS
[30]	1190.00						100.00				Hydraulic
[39]		913.00	41.19								Screw extruder
[31]	1524.00										Hydraulic
[46]			0.37					6.62	13.47		Hydraulic
[26]		1140.00	16.00						3.00	1.00	Hydraulic
[47]		1000.00	5.30								Hydraulic
[48]	500.00	190.00									NS
[25]	861.00						95.60				Hydraulic
Arithmetic average	1157.75	965.11	9.98	97.73	71.60	1450.00	97.78	5.31	10.49	2.00
SD	652.92	293.31	12.73	1.93	21.60	620.00	2.45	1.31	4.68	1.00
ENplus A1		600 ≤ BD ≤ 750					≥98.00	<1.00
ENplus A2		601 ≤ BD ≤ 750					≥97.50	<1.00
ENplus A3		602 ≤ BD ≤ 750					≥97.50	<1.00

NS—Not specified; SD—Standard deviation; BD—Bulk density.

. Factors, such as the principles of driving the machines used for the production of briquettes and the use or not of thermal treatments prior to densification were also included in the tables, given the possible influence on the results achieved. In addition, the means and standard deviations for each response variable were calculated and some of them were related to international standardization norms.

The standardization standards for solid biofuels used were ENplus, EN 14961-2 and DIN 51731. Despite referring to the characteristics of densified biomass in the form of pellets, several studies use them as a reference standard for briquettes, as was done by Chungcharoen and Srisang [43]; Khlifi et al. [8]; Kpalo et al. [29] and Niño et al. [65].

4.1. Thermal, Energetic and Moisture Content Response Variables

Table 10 shows the thermal and energy output variables and the moisture content of the briquettes studied in 20 of the 22 studies analyzed during this bibliographic review.

As indicated by Mendoza Martinez et al. [13], the raw material and the final briquettes must have high energy and fixed carbon content, and low ash and volatile matter content. In this way, the maximum values of calorific value, energy density and fixed carbon found in the research are shown in the table. On the other hand, the minimum results of volatile matter, ash content and moisture content were presented. The minimum moisture content in the table was motivated by the poor combustion characteristics obtained when this parameter is high, compromising the heat production and the residence time in the combustion chamber [29].

From Table 10, Figure 5, Figure 6 and Figure 7 were generated for the high heating value, moisture content and ash content, respectively. In the Figures, reference lines were informed for these output variables based on the standardization standards ENplus, EN 14961-2 and DIN 51731. Given the variety of machines used to carry out densification and the influence that the briquetting process has on the final quality of briquettes [15], the principle of operation of these machines was also included in the graphics.

4.1.1. Volatile Matter and Fixed Carbon

An important variable for the combustion of briquettes is their volatile matter content, which is proportional to the energy released during combustion [42]. It represents the fraction of combustible and non-combustible gases, with the exception of moisture, which is released when the fuel is heated to high temperatures. The greater the volatility of a fuel, the more reactive and easily flammable it will be, resulting in faster burning [78]. However, mainly in domestic use, it is preferred that the burning occurs more slowly [13]. The high content of volatile material is also responsible for causing large amounts of smoke at the start of combustion, resulting in unburned gaseous fuel [26].

Biomass, in general, has about 65 to 85% by weight of volatile matter, while woody biomass has 76 to 86% by weight [13]. According to Table 10, the average of this variable among the studies analyzed was 56.69%, with a SD of 23.32%. The high SD is justified by the low VM values for the works by Helwani et al. [38] and Nwabue, Unah and Itumoh [31], which were 19.73% and 17.23%, well below the results found for other surveys. These two works used the raw material carbonization process at high temperatures before the densification process, which contributed to the volatilization of gases and reduction of VM.

In addition to releasing part of the volatiles and moisture present in the raw material, it is also known that thermal pre-treatments, such as torrefaction and carbonization are capable of promoting an increase in the fixed carbon content present in the fuel [38,79]. This is because the fixed carbon content is inversely proportional to the volatile matter content, with the remaining part after the release of the gases, moisture and excluding the ashes [13,78]. In this way, the high FC allows for slower and more stable combustion [13], with less heat being spent during the decomposition of the volatile material and more energy being generated through the greater volume of fixed carbon that will be consumed [26].

According to Fernandes et al. [78], the fixed carbon content is around 7 to 20%, however, in Table 10 it is possible to observe much higher values. As expected, the highest values of fixed carbon of 71.40% and 42.41% were obtained in the same briquettes where the smallest VM were achieved, in the works by Helwani et al. [38] and Nwabue, Unah and Itumoh [31] respectively. Once again, there was the influence of the heat treatment carried out, generating a positive direct correlation between higher energy production, higher calorific value and the percentage of fixed carbon [31].

4.1.2. High Heating Value

The calorific value of a fuel is obtained through combustion under standard conditions of temperature and pressure of a fuel, being quantified in heat generated by the combustion [79]. The high heating value (HHV) indicates how much of that heat is produced in the presence of oxygen [8]. The lower heating value (LHV), on the other hand, is obtained by means of a calculation based on the HHV minus the moisture and H₂ contents of the analyzed material [79]. The two studies that studied this parameter showed results above 16.6 MJ·kg⁻¹ required by the ENplus standard, indicating a good potential for energy generation.

When analyzing Figure 5, which concerns the HHV, it can be seen that a large part of the works meets at least one of the standardization norms. The work done by Gwenzi, Ncube and Rukuni [44] stands out for its higher, high heating value of 33.8 MJ·kg⁻¹, with its briquettes produced in a machine with a principle of manual mechanical operation. This high HHV can be justified by the composition of briquettes that are made from unconventional waste, such as coal dust (C), sawdust (S) and post-consumer plastics (P).

The other two jobs that had high HHV in their briquettes were produced on machines with different driving principles. Helwani et al. [38], like most other researchers, used a hydraulic press to obtain 30.67 MK·kg⁻¹ in charred palm stem briquettes. While Bazargan, Rough and Mckay [45] reached 31.29 MJ·kg⁻¹ of HHV in briquettes made from Palm kernel shell biochars (PKSB), whose machine with a cylindrical compression ram did not have its driving principle specified. What the three jobs with the largest HHV have in common are the raw materials of the briquettes that have high initial HHV, either due to their own nature or thanks to heat pretreatment processes.

Analyzing Table 10 and Figure 5 together, it can be seen that the average of 21.95 MJ·kg⁻¹ for the PCS of the works is well above that required by the standards EN 14961-2 and DIN 51731. The lowest HHV of 16.92 MJ·kg⁻¹ is slightly lower than the 17.5 MJ·kg⁻¹ standardized by DIN 51731, indicating a good energy potential for all the studies analyzed.

The calorific value is also used to calculate the energy density of the briquettes, which indicates how much energy there is per unit volume of the fuel, directly affecting its efficiency [13]. This variable is given by the product of the calorific value with the density of the fuel, so that the greater these two variables are, the greater the energy density [37].

4.1.3. Moisture Content

According to the ENplus standard in its A1, A2 and A3 classes, the moisture content (MC) of the briquettes must be ≤10% [80]. As can be seen in Figure 6, most works had their lowest moisture content within what the standard requires. Of the two studies that did not meet the standard, one obtained a value of 10.40% humidity, a result very close to that standard. A common fact between the two works that did not meet ENplus’s humidity requirement is that both were produced on manual machines, being hydraulically driven for research by Masullo et al. [34] and mechanical for Morales-Máximo et al. [27].

The low humidity ranges for solid fuels are desirable since high MC can result in fragile, low density briquettes, which would cause problems in their handling, in addition to problems associated with boiler shutdown [67,80]. The limits for the moisture content may vary according to the standard used as a reference standard. In conformity with Kz Ku Ahmada, Khaziq Sazalia and Kamarolzaman [81]. the standard of quality standardization of wood as a solid fuel, ISO 17225, recommends that the moisture content be from 2.2 to 15.9%, so that the briquettes produced are not too dry, causing a very rapid burning, nor too humid.

4.1.4. Ash Content

The ash content (AC) of a fuel is a good parameter to assess its quality because it is related to other properties [82]. According to Table 10, the average for ash content in the studies is 6.29%, which is higher than the highest value allowed by the ENplus standards, which is 2%. The increase in this average is due to the high ash content found by Nwabue, Unah and Itumoh [31] which, as can be seen in Figure 7, is in an isolated point from other studies, with an AC of 27.93%. This result is related to the composition of the briquette, which in addition to biomass, is made from coke, limestone dust and laterite that contains a large amount of non-combustible material contributing to the increase in ash content.

For herbaceous biomass the ash content is usually 10% [83], approaching the values found by de Oliveira Maia et al. [42] (9.85%) and de Oliveira Maia et al. (10.70%) who used banana residues as leaves and stems, in addition to rice husks. However, for the production of briquettes, it is recommended that the ash content be less than 4%, avoiding corrosion of the equipment [46]. In addition, briquettes classified according to quality standards must have low ash contents, in order to avoid high dust emissions and damage to combustion [83].

4.2. Physical-Mechanical Output Variables

One of the main advantages of biomass densification is to increase the low density of raw materials in natura, facilitating the storage, handling and transportation of solid fuels [17,63,84]. The process also benefits properties, such as mechanical strength, tensile strength and durability [30,39,45]. These and other physical and mechanical properties are shown in Table 11.

The output variables compressive strength, impact resistance, tensile strength, water resistance, durability and densities are shown in the table according to their highest values achieved in each job. Maximizing these properties favors less brittle and more resistant briquettes, reducing transportation and storage costs [54,85].

The other variables present in Table 11 are the generation of fines, longitudinal expansion and water absorption. As will be discussed in this topic, these properties must be minimized to contribute to obtaining suitable briquettes for transportation, storage and handling. That said, the values shown in Table 11 are the minimum for each study.

Among the physical and mechanical properties explored in Table 11, it turns out that density and compressive strength were studied more frequently to evaluate and classify a solid fuel. In view of this, both variables were analyzed graphically. Through Figure 8 the individual apparent density of each job was compared with the reference value given by the ENplus standard and classified according to the principle of operation of the briquetting machine. In a similar way, Figure 9 was elaborated, which deals with the individual compression resistance of the briquettes. However, this variable was not compared with the standard, but with a reference value in the literature.

4.2.1. Longitudinal Expansion

Longitudinal expansion refers to how much a material tends to expand after it has become denser and to achieve dimensional stability. This property is also related to the hygroscopicity of the material, which concerns the water absorption capacity over time [34]. Thus, the expansion of a briquette may be due to excess moisture absorbed, causing weakening of the bonds between particles and decreased durability [35]. Therefore, the dimensional stability of the briquettes refers to their resistance, reflecting on the conditions of transportation and storage of the fuel [26]. It is preferable that the densified material undergoes less expansion.

For a period of 21 days, at 20 °C and 95% relative humidity, an expansion below 20% is ideal [51]. In Table 11, it can be seen that the longitudinal expansion of the works is smaller than this range, varying from 3 to 15%. The briquette produced in the presence of heat by Araújo et al. [26], obtained the smallest expansion (3%). While the briquettes produced by D. Padilla et al. [46] and Masullo et al. [34], densified without heating, reached higher values of 13.47 and 15%, respectively. This behavior can be explained by the softening of lignin in the presence of heat, acting as a binder between the particles, which in turn causes less expansion of the densified material [86].

4.2.2. Generation of Fines

Another important variable that should be minimized is the generation of fines, measured by the friability test. Its measurement is given as to the degree of friability of the briquettes, so that the more friable, the greater the disintegration of the fuel during the test [34]. According to the standard ENplus [80], fuels must have a fines generation of up to 1%, which would classify them as very little friable (fines generation < 10%) [34]. In Table 11, although none of the studies met the standard, they can be classified as very little friable, which makes them resistant briquettes, according to D. Padilla et al. [46].

4.2.3. Water Absorption

The resistance of the briquettes is also assessed through water absorption. As with the other variables, it also affects the storage and transport of briquettes, in addition to the calorific value [26]. Because it is a property that relates to the characteristics of the fuel raw material, the use of thermal pre-treatments, such as torrefaction is able to reduce it, causing beneficial changes to the quality of the densified material [26,44]. According to Gwenzi, Ncube and Rukuni [44] less water absorption also implies briquettes with greater resistance to compression. Among the studies evaluated in Table 11, only two studied this response parameter, corresponding to 1 and 3% of water absorption.

4.2.4. Durability

Durability, despite being an important variable and having a quality value standardized by the ENplus standard, was addressed only five times among the works analyzed in Table 11 and for this reason, it was not represented graphically. This variable measures the resistance of the briquettes to vibrations, falls, abrasions and other actions resulting from their handling and transportation [28,36].

According to ENplus [80], durability must be at least 97.5% for classes A2 and A3 of the standard, or 98% for class A1. The study by Raslavičius [25] showed durability of 95.6%, slightly less than the standardized value. However, the general average found among the authors of Table 11 was 97.78%, meeting the reference values, implying resistant briquettes. It was also observed that the briquettes of greater durability tend to be denser [25,28] and have a strong logarithmic correlation with the compressive strength [54].

4.2.5. Bulk Density

Given the coverage of the density variable, the researchers evaluated their briquettes and raw material particles using more than one type of density. Those in most frequent use are shown in Table 11, being the bulk density and simple density, which is measured by the ratio of the mass of the briquette to its volume [29]. There is also the granular density or true density, which is given disregarding the voids in the material. The bulk density is the density that takes into account porosity between the particles of the material [87].

Bulk density is one of the main properties of evaluating biomass as a solid fuel, which is normally increased with densification through processes, such as briquetting [13]. The elevation of the variable makes it economically feasible to use biomass as solid fuel by reducing the costs of transportation, storage and handling [36,63,84]. In addition, it promotes a higher energy density given the increase in the energy/volume ratio in pellets and briquettes [29].

In Table 11 the bulk density of the briquettes is compared with the reference value of the ENplus standard and is based on tests carried out according to the ISO 17828 standard. However, not all works have measured the density of the briquettes according to this standard, which can also be used for non-compacted biomass. It consists of pouring the briquettes into a container of known volume and measuring their weight to determine the mass/volume ratio, similar to what was done in the works carried out by Khlifi et al. [8] and Thoreson et al. [48]. Some of the research even determined the apparent density by the method of immersion of the briquettes in mercury, to determine the displaced volume and calculate the ratio between mass and volume [13,26]. Others used calipers to measure the dimensions of the briquettes and thus, calculate their volume [34,39,42]. Despite their differences, they all evaluate the relationship between mass and volume, taking into account the presence of pores or voids in the material, which is what defines the bulk density.

Analyzing Figure 8, it can be seen that only one of the works had a bulk density of less than 600 kg·m⁻³, the minimum stipulated by the ENplus standard. The low density of 190 kg·m⁻³ was attributed to the large amount of voids inside the briquettes [48]. The briquetting took place on an unspecified driving principle machine, at a pressure of 10.5 MPa and without reducing the size of the particles, which may have caused expansion of the briquettes after ejection.

The highest bulk density of 1200 kg·m⁻³ was obtained in a hydraulic-driven press, at a pressure of 150 MPa and heating at a temperature of 38 °C [8]. The other three studies that achieved high results, above 1000 kg/m³, used pressures from 8.27 to 14 MPa and a temperature of 120 °C [13,26,36]. Two of these works used hydraulic-driven machines, while the other did not specify this information. Mendoza Martinez et al. [13] attributed the higher density of their briquettes to the high compaction rate during briquetting. They suggest that briquettes produced with raw materials of lower initial density are more easily densified and require less energy to do so.

Araújo et al. [26], state that the torrefaction of the biomass had a greater influence on the increase in the apparent density of the briquettes, also being related to the lower hygroscopicity acquired after the heat treatment. This behavior is in agreement with Waheed and Akogun [28], who observed the influence of torrefaction and water preconditioning to increase the simple density of briquettes. Other authors, such as Kpalo et al. [29]; Kumar and Chandrashekar [37] and Thoreson et al. [48] pointed out the increase in humidity as one of the factors that reduce the density of briquettes.

Depending on the machine used to produce the briquettes and the parameters applied, it is possible to increase the density of the raw material by 10 to 20 times [10]. Sugarcane straw biomass went from a density of 280 kg·m⁻³ to approximately 920 kg·m⁻³ of apparent density after 48 h of densification [34]. The banana leaf residues, whose apparent density is 46 kg·m⁻³, reached the result of 990 kg/m³ after densification [42]. Both works used hydraulic operating principle presses. Figure 8 reinforces that all the briquettes produced in machines driven by hydraulic components reached high values of bulk density, even exceeding the 750 kg·m⁻³ limited by the ENplus standard. The same happened in the study by de Oliveira et al. [39] producing the briquettes in a screw extruder.

4.2.6. Compressive Strength

Compressive strength is defined as the maximum crushing load that briquettes or pellets support until the moment before failure (cracking or breaking). The fracture load is recorded on a stress-strain curve, reporting resistance to compressive forces. This test is intended to reproduce the compressive stress suffered during transport and storage in silos or boxes, where the briquettes or pellets are piled up [13,35]. Thus, it is understood that the greater the compressive strength of a briquette, the better its quality for storage and transportation.

Given its relevance, the variable was studied in most of the studies shown in Table 11, ranging from 0.37 MPa to 41.19 MPa, resulting in an average of 9.98 MPa. To ensure that there is a minimum of breakage in the handling, storage and transport of briquettes, the literature recommends a minimum compressive strength of 2.56 MPa [85]. Although it is not a value determined by a quality standard, it was used as a reference point to graphically analyze the other results.

As can be seen in Figure 9, the lowest results of compressive strength (0.37 MPa) were obtained in manual machines with hydraulic functioning principles. Additionally, in a manual machine, with mechanical operation, the second-lowest value was obtained, equal to 0.86 MPa.

Also according to Figure 9 of the works that used hydraulic driven briquetters exceeded the stipulated 2.56 MPa. The briquette that showed the highest value (41.19 MPa) for the response variable was obtained in a screw extruder machine. Despite the significant increase in compressive strength obtained in the briquette produced in a screw extruder machine, it is necessary that more work using the same working principle for briquetting be analyzed. This would allow us to assess whether this factor is really differential in order to achieve the highest values for compressive strength.

5. Modeling and Statistical Analysis of Experiments

Throughout this work, it is possible to notice that some input and response variables are more studied than others. However, divergences are still found between the surveys regarding the influence of certain process variables on the chosen responses. This is related to several factors, such as the type of machine used for briquetting, the way the experiments were conducted, the raw materials chosen and others.

Such variations indicate that there is no consensus on the ideal path for the production of briquettes. One way to identify which are the process parameters that result in the best qualities of briquettes is to use a design of experiments (DOE), delimited by mathematical models and statistical analysis to evaluate the variables.

Table 12. Studies conducted following a DOE, mathematical models and statistical analyzes to assess process variables.

Researchers	DOE	Statistical Analysis	Data Analyzed	Statistical Softwares
[40]	Uninformed	Analysis of variance (ANOVA) and Tukey’s test	The effects of densification pressure and pressing time on properties were evaluated. The data analyzed statistically were relaxed density, energy density, resistance to compression and durability.	R statistical software
[29]	Random sampling method	Analysis of Variance (ANOVA) and Fisher’s Least Significance Difference (LSD)	The 5 different briquette compositions and the properties of HHV, MC, density, compressive strength, water resistance and break index were randomly tested and replicated four times.	Uninformed
[43]	Full factorial design (2 × 6 × 2)	Analysis of variance (ANOVA) and Tukey’s HSD test	The influence of operational parameters (SNC sizes, mixed proportions and speed) and their interactions on mechanical and fuel properties and the production rate.	SPSS v.14
[38]	Response Surface Methods (RSM) with Central Composite Design (CCD)	Analysis of variance (ANOVA), regression model and lack of fit.	The effect of the input variables (particle size, binder composition and pressing pressure) on the calorific value responses and the compressive strength of the briquettes.	Uninformed
[28]	D-optimal crossed design	Analysis of variance (ANOVA), regression analysis, R2, R2 adj, pareto analysis	The physical and combustion indices of the briquettes, namely density, durability, resistance to compression, calorific value, MC, VM, FC and AC, in addition to the fuel ratio, combustibility index and volatile flammability.	Design Expert v.6.0.8
[44]	Factorial design	Kolmogorov-Smirnov e Levene tests; nonparametric Kruskal–Wallis test, Analysis of variance (ANOVA), least significant difference (LSD) and t test.	The breakage index/impact resistance, density, compressive strength, water absorption and calorific value were analyzed.	SPSS v.16
[13]	Factorial design (5 × 5)	Analysis of variance (ANOVA) and Tukey’s test	Proportions of waste mix, five mixtures of biomass on the answers: equilibrium moisture content, tensile strength, apparent density and energy density.	Statistica v.8.0 and R program
[34]	Completely Randomized design (CRD)	Tests of Shapiro-Wilk and Box-cox. Analysis of variance (ANOVA) and Tukey’s test	The chemical characterization data of the biomasses (MC, VM, FC, AC) and the physical analysis data of the briquettes (MC and compressive strength) were analyzed.	R statistical software
[36]	Completely Randomized design (CRD)	Analysis of variance (ANOVA)	The means, standard deviation and coefficients of variation were made for all variables. ANOVA was used in the data of the immediate analysis and HHV, energy density of biomass and coal.	Uninformed
[46]	Uninformed	Bartlett’s test, analysis of variance (ANOVA) and Tukey’s test	HHV, dimensional stability (longitudinal expansion), resistance to compression and generation of fines (friability) were analyzed.	Statgraphics v.15.2.05 and Excel 2013
[48]	Full factorial design	Analysis of variance (ANOVA)	Evaluate the main effects and interactions of input variables on dry particle density.	Minitab

shows some works that used these devices to perform their experiments and analyze their data.

Table 12 shows that 100% of the studies used the analysis of variance (ANOVA) of their data. Tukey’s test was also performed in five of the studies, which are important tools for the analysis of statistical data. The experimental planning of the works followed models, such as the factorial models and response surface methodology (RSM). The use of these methodologies applied to results optimization can also assist in the ideal choice of process conditions, reducing capital, maintenance and operating costs [44].

6. Conclusions

Biomass has a high potential for energy production when subjected to densification techniques, such as briquetting, promoting a noble destination for various materials including forest, agricultural or urban waste. In order to expand the knowledge about the briquetting process, this work presented the main input parameters of the process. In addition, the physical and energetic properties were also analyzed to qualify the briquettes. Through this review of the literature and the analysis of the input and output variables of the briquetting process, it was possible to notice that:

• The input variables that were studied most often were: particle size of biomass (95%), compaction pressure (73%), moisture content of the raw material (68%), compaction time (59%), proportion of binder (50%), proportion of biomass to compose the briquettes (45%) and temperature (32%);
• The response variables most used to classify the briquettes were: higher calorific value, moisture content, ash content, density and resistance to compression.

From the analysis of the levels of each process variable, associated with its results, it appears that:

• The initial moisture content of the raw materials ranged from 8 to 12%;
• The most used briquetting temperature among the briquettes was 120 °C;
• Most works produce their briquettes at intermediate pressures. It is also noticed that higher pressure values are not necessarily ideal for maximizing the physical quality of briquettes;
• In order to obtain a higher final calorific value of the briquettes, it is preferable that they are composed mostly of the raw material with the greatest initial calorific value.;
• Generally speaking, particles of less granulometry promote stronger bonds between particles, resulting in more resistant briquettes. However, the ideal granulometry varies according to the raw material used and the briquetting process;
• The presence of a binder has an influence on the physical-mechanical and energetic properties of the briquettes. Its use is related to processes carried out at low temperatures and pressure. The most used binder was starch;
• Briquettes with apparent density above the limit of 750 kg·m⁻³, stipulated by the ENplus quality standard, can be produced in hydraulic driven machines;
• The lowest values of compressive strength were obtained in manual machines (hydraulic or mechanical).

For data analysis, ANOVA and Tukey’s test are the most used statistical tools among the works. As for the design of the experiments, factorial models are used more frequently.

The conditions of the briquetting parameters, as well as the composition of the briquettes, varied according to several factors, such as the available technology and the studied raw materials. The same was observed regarding the physical and energetic properties that qualify the briquettes. In this way, it is understood that the characteristics, parameters and process levels ideal for briquette production vary between the research, making it necessary to plan experiments and mathematical models that enable a better investigation of the results.

As a suggestion for future work, it is recommended to analyze a greater number of works with different principles for driving briquetting machines. This would allow a clearer comparison between the results obtained for the different types of technologies. From an experimental point of view, it is clear that few studies have used process optimization tools. Therefore, the authors suggest that the experiments follow experimental plans to deal with multiple variables and carry out optimization studies.