Physical and Energy Properties of Fuel Pellets Produced from Sawdust with Potato Pulp Addition

Abstract

This paper presents the findings of a study of the pelleting process of pine sawdust with the addition of waste in the form of potato pulp (as a natural binder), in the context of producing fuel pellets. The process of pelleting was carried out for sawdust and for a mixture of sawdust and potato pulp (10, 15, 20, and 25%). The highest moisture content was obtained in the case of pellets produced from a mixture of straw with a 25% potato pulp content, i.e., 26.54% (with a potato pulp moisture content of 85.08%). Increasing the potato pulp content in a mixture with sawdust from 10 to 25% reduced the power demand of the pelletizer by approx. 20% (from 7.35 to 5.92 kW). The obtained density values for pellets made from a mixture of sawdust and potato pulp (over 1000 kg∙m⁻³) with a potato pulp content of 10% make it possible to conclude that the obtained pellets meet the requirements of the ISO 17225-2:2021-11 standard. Increasing the potato pulp content from 0 to 25% caused a slight decrease in the heat of combustion, i.e., from 20.45 to 20.32 MJ∙kg⁻¹, as well as in the calorific value, from 19.02 to 18.83 MJ∙kg⁻¹ (both for dry sawdust matter and the mixture). The results of the laboratory tests were used to verify the densification process of mixtures of sawdust and potato pulp under industrial conditions at the PANBAH plant, using pelleting mixtures with a 5%, 10%, and 25% content of potato pulp. Industrial research also confirmed that the use of the addition of potato pulp in a mixture with sawdust significantly reduces the power demand of the pelletizer, and it also increases the kinetic strength of the obtained pellets.

1. Introduction

The current economic situation, especially in the field of energy policy, clearly shows how important it is to develop methods of using energy from renewable sources. As part of the amendment to the strategy included in Poland’s Energy Policy until 2040, it is assumed that by 2030, Poland should achieve up to a 50% share of green energy in its gross domestic energy consumption. Implementation of this assumption requires increasing the share of renewable energy to more than three times the current level. According to the Central Statistical Office, in 2022, the share of energy from renewable sources in gross final energy consumption was 16.8% [1]. Hence, it is necessary to use the entire range of renewable energy resources, including the increasingly intensively developing branch of solid biofuels produced on the basis of waste wood raw materials.

Among the various types of biomass, wood is the primary raw material utilized for pellet production [2]. Due to the fact that even small impurities are eliminated by removing the bark and washing the logs before the wood-cutting (sawing) process, sawdust is an ideal raw material for producing clean pellets [3]. However, as the demand for wood pellets rises and pellet fuels gain popularity, the supply of sawmill residue has also grown. Therefore, there has been a growing interest in the production and use of pellets from materials other than sawdust [4].

According to Azargohar et al. [5], one approach to enhancing pellet quality involves co-pelletization, where two or more raw materials are mixed. When producing pellets from waste materials, the common practice to improve quality is to incorporate wood materials such as sawdust as additives [5]. This increases the lignin content in the pellets, thereby enhancing their calorific value, bulk density, and durability, and reducing the ash content [5,6,7]. Introducing binder additives or other biomass wastes typically increases the kinetic strength of the pellets and lowers the energy demand during pelleting [8,9]. Harun et al. [10] highlight co-pelletization as a frequently researched method in fuel pellet production. Their study involved producing pellets from mixtures of canary grass, timothy hay, and switchgrass combined with spruce and pine biomasses, demonstrating the potential to achieve high-quality pellets from these blends. They also observed that mixing pine sawdust with other biomass types reduces the energy required for densification, thus lowering pellet production costs [10]. Gheorghe and Neacsu [11] presented the possibilities of pelletizing agricultural residue from grape pomace and corn cobs with the addition of starch, sawdust, and rapeseed oil. Jekayinfa et al. [12] produced pellets from rice bran with the addition of a starch binder and examined the influence of the degree of compression and matrix configuration on the properties of pellets. Kumar et al. [13] made fuel pellets from eucalyptus sawdust with the addition of pearl millet cob and corn cob. They found that this combination enabled the extension of the combustion time by increasing the values of parameters such as bulk density, kinetic strength, compressive strength, and resistance to impacts, abrasion, and water, which improved the storage and handling properties of the produced pellets. Kamga et al. [14] studied the physical and energy properties of pellets produced from agricultural and forest biomass, i.e., corn spathes, Movingui sawdust, coconut shells, and mixtures of these materials. In another study, the authors produced five types of pellets from mixtures with different proportions of wood sawdust and peanut shells. Their results show that, compared to bulk samples, pelletizing makes ignition more difficult and prolongs the combustion time [15]. Woo et al. [16] produced fuel pellets from a mixture of waste coffee grounds and pine sawdust. During the research, it was found that increasing the share of coffee grounds in the pellets resulted in an increase in its calorific value.

Abedi et al. [17], in their studies focused on pelleting, determined the impact of lignin, lignosulfonate, proline, starch, and torrefied oat husk to sawdust on the quality (density, mechanical strength, porosity, and water resistance) and the exhaust emissions generated during the gasification of pellets produced from these mixtures under various conditions. Stulpinaite et al. [18] investigated the possibilities of producing fuel pellets from hemp biomass and improving their quality by using the addition of lignin and oak sawdust. Elniski et al. [19] examined the effect of using two types of lignin as an additive on the properties of willow pellets (ash content, energy value, bulk density, durability, pellet length, moisture absorption, and carbon monoxide emissions). In another study, the authors determined the influence of binders (lignosulfonate, wheat flour, and starch: 2 and 15 wt%) on the mechanical (hardness, mechanical resistance, and wettability) and energetic properties of pellets made from meadow hay, before and after torrefaction [20]. Kulig [21] examined the effect of pressure (from 45 to 113 MPa) on the compaction parameters of milk thistle straw with the addition of a binder in the form of calcium lignosulfonate. Mack et al. [22] produced pellets from eight kinds of wood without bark, with the addition of three types of starch and one type of kaolin (starch: 0.5 w-% and 1.8 w-%, kaolin: 0.17 and 0.34 w-%). According to Zhang et al. [23], adding 0.3 to 1.5% starch to wood sawdust has been demonstrated to extend the life of the matrix, lower the temperature of the pelleting process by 15%, and reduce its energy demand. Dobrowolska [24] examined the impact of adding 10, 15, and 20% wheat bran to pine and spruce sawdust on the kinetic strength, bulk density, and specific density of pellets. Lykidis et al. [25] investigated the effect of black pine bark content (0–100%) and densification temperature on the properties (density, moisture content, ash content, and surface roughness) of black pine wood pellets produced using a single pellet matrix.

One of the waste raw materials that can have a positive impact on the quality of the produced pellets is potato pulp, which is post-production waste generated in potato processing plants. Research reported by Obidziński [26] showed that potato pulp is a material characterized by very high moisture content (over 88%). This content poses a serious problem as far as using the potato pulp directly as solid fuel is concerned. The annual production of potato pulp in Europe exceeds one million tons [27]. According to Trabert et al. [28], the authors claim that thermomechanical processing (extrusion) of potato pulp changes both the structure of dietary fiber polysaccharides and their technofunctionality. As a result, a significant portion of the waste remains untreated, leading to additional disposal costs for the company and increasing the biological load of wastewater [29]. Potato waste, such as pulp and peels, is mainly used as fertilizer [30,31] and animal feed [32,33] in the meat industry [34], and in bakeries [35]. Kurnik et al. [36] showed that potato pulp could serve as a potential source of enzymes for biotechnological applications. Patelski et al. [37] indicated that potato pulp waste is a promising raw material for producing yeast biomass. Potato waste obtained from various industries is also used for the production of bioplastics, biocomposites [38,39], nanocrystals [40,41], or lactic acid [42]. In another study, the authors proposed that potato pulp might be used as a bioadsorbent [43]. Mayer [44] developed approaches for dried potato pulp pellets to be used for the production of certain products in the furniture and paper industries. Another promising way of processing potato waste is to produce biogas [45,46,47], bioethanol, biobutanol, and bio-oil [48,49,50], hydrogen [35], electricity [51], and heating fuels [9,52]. According to Kurnik et al. [36], potato pulp, i.e., a starch industry by-product, has the potential for applications in wastewater treatment.

Obidziński et al. [9] examined the impact of adding potato pulp (10, 15, and 20%) to a mixture with hemp waste and the effect of the mass flow rate of the raw material (10, 30, and 50 kg·h⁻¹) on the power demand of the pelletizer and the density and kinetic strength of the resulting pellets. Additionally, they investigated the agglomeration of straw and hard coal with the addition of potato pulp.

The aim of the study was to analyze the usefulness of potato pulp as an additive in the production of high-quality wood pellets. The specific objective of this study was to assess the effect of different contents of potato pulp on the pelleting process of pine sawdust and to assess the quality of the obtained pellets. This study, for the first time as a novelty effect, enables the determination of the optimal potato pulp content in fuel pellets made from wood sawdust waste to maintain their physical (mechanical) and energy properties, while also reducing electricity production costs.

2. Materials and Methods

2.1. Materials

The basic raw material used in the research was pine sawdust (Figure 1A) obtained from the Wood Production Plant PANBAH, located in Trzonki, Warmian-Masurian Voivodeship, Poland. During the tests, pine sawdust was added to the potato pulp (Figure 1B) obtained from a potato processing plant, i.e., PEPEES S.A., located in Łomża, Podlaskie Voivodeship, Poland. The tested potato pulp was produced as post-production waste during the production of potato starch.

2.2. Determination of Moisture Content

The moisture contents of the raw materials, i.e., sawdust, potato pulp, and mixtures of sawdust and potato pulp, before pelleting were determined according to the PN-EN ISO 18134-1:2015-11 [53] standard, using a WPE 300S moisture analyzer (Radom, Poland) with a measurement accuracy of ±0.01%. During the tests, the moisture content was determined for five samples. A 5 g sample was taken for measurement and dried at a temperature of 105 °C. The average values of the determinations were considered as the final moisture content results.

2.3. Determination of Bulk Density

To determine the bulk density of the raw materials and pellets according to the PN-EN ISO 17828:2016-02 [54] standard, a metal container with a volume of 407.5 cm³, a WPS 360 laboratory balance, and a steel scraper were used. The bulk density measurements consisted of filling the measuring container completely with the material and then scraping off any excess from the top of the cylinder. Bulk density was defined as the ratio of the material’s weight to the volume of the container. The OHAUS AX224M analytical balance (Nänikon, Switzerland) was used to measure the weight of the material.

2.4. Determination of Particle Size Distribution

The sieve analysis process was carried out using the programmable LPz-2e shaker from Multiserv Morek (Marcyporęba, Poland) and the OHAUS AX224M analytical balance, according to the PN-R-64798:2009 [55] standard. During the analysis, a set of sieves with square apertures of 6.0 mm, 4.0 mm, 2.0 mm, 1.0 mm, 0.5 mm, 0.25 mm, 0.125 mm, and 0.063 mm was used. The analysis was performed in five repetitions using 100 g of sawdust. The final result was determined by taking the average of the obtained measurements.

2.5. Elemental Composition Analysis

The contents of carbon, nitrogen, hydrogen, and sulfur were determined according to the PN-EN ISO 16948:2015-07 [56] and PN-EN ISO 16994:2016-10 [57] standards using the LECO CHN628 analyzer (St. Joseph, MI, USA), and the methodology described in publication [58].

2.6. Heavy Metals Analysis

The contents of heavy metals were determined according to the PN-EN ISO 17225-2:2021-11 [59] standard using the S2 PICOFOX spectrometer (Billerica, MA, USA).

2.7. Determination of Calorific Value and Heat of Combustion

The calorific value and the heat of combustion were determined according to the PN-EN ISO 1928:2002 [60] standard, using the KL-12Mn calorimeter (Bydgoszcz, Poland) from Precyzja-Bit, and the methodology described in publication [61].

The calorific value and the heat of combustion were determined using Equation (1):

$$\mathrm{L}\mathrm{H}\mathrm{V}=\mathrm{H}\mathrm{H}\mathrm{V}-24.43\left(\mathrm{w}+8.94{\mathrm{H}}^{\mathrm{a}}\right) [\mathrm{M}\mathrm{J}·{\mathrm{k}\mathrm{g}}^{-1}]$$

(1)

where

• LHV—calorific value (Lower Heating Value) [MJ·kg⁻¹];
• HHV—heat of combustion (Higher Heating Value) [MJ·kg⁻¹];
• w—moisture content of the sample [%];
• H^a—hydrogen content of the sample [%];
• 24.43—coefficient accounting for the heat of water vaporization at 25 °C in pellets with a 1% water content;
• 8.94—coefficient accounting for the stoichiometry of the hydrogen combustion reaction (quantitative changes).

2.8. Investigations of the Compaction Process under Laboratory Conditions

The research of the pelleting process of sawdust and potato pulp mixtures was carried out on a prototype pelleting and briquetting device with a flat stationary matrix shown in Figure 2.

The drive system of pelletizer 6 consists of electric motor 8 (proMOTOR). The motor transmits torque to pelletizer shaft 6 through a system comprising two clutches 9 (with torque meter 10 mounted between them) and bevel gear 11. To control the rotational speed of pelletizer shaft 6, frequency converter 13 (ABB ACS) is connected to electric motor 8.

A uniform supply of compacted raw material to the working system of pelletizer 6 is ensured by the use of screw feeder 14 driven by the electric motor (NORD) 15, connected to the reducer. Screw feeder 14 is also equipped with a frequency converter, which makes it possible to change the amount of the raw material fed to the working system of pelletizer 6. From screw feeder 14, the raw material passes through hopper 3 to the interior of mixing–pelleting–dosing device 1. The drive system of mixing–pelleting–dosing device 1 consists of electric motor 2 (proMOTOR (Radom, Poland)), from which the drive is transmitted via a chain transmission to the shaft of mixing–pelleting–dosing device 1. From mixing–pelleting–dosing device 1, the raw material goes to the working system of pelletizer 6 (between the flat stationary matrix, cooperating with a rotary system of three bearing-mounted compaction rollers, forcing the compacted raw material into the 12 mm matrix holes). The pellets leave the working system of the pelletizer through discharge 7.

The station is equipped with a universal meter for measuring the device’s power demand 17 (METROL (Zielona Góra, Poland)), torque and force indicator 16, and recorder 18 (Spider 8, Hottinger Baldwin Messtechnik (Darmstadt, Germany)) coupled with computer 19. Signals from the device’s power demand meter 17 and from torque and force indicator 16 are fed to recorder 18 in the form of binary files, which are further processed using Microsoft Excel 2010 and Statistica 13.3 software. The purpose of the tests was to determine the effect of various potato pulp contents in a mixture with sawdust. The constant values during the tests were as follows:

• w_m = 14—mixture humidity [%];
• d_o = 12—diameter of holes in the matrix [mm];
• Q_m = 50—mass flow rate of the mixture [kg·h⁻¹];
• n_r = 170—rotational speed of the compaction roller system [rpm];
• h_r = 0.4—gap between the rollers and the matrix [mm].

2.9. Investigations of the Compaction Process under Industrial Conditions

Research verifying the course of the pelleting process of mixtures of sawdust and potato pulp under industrial conditions has been carried out on a stand operating at Wood Production Plant PANBAH Marek Jankowski.

The tests were carried out on a pelletizer with the use of a rotating ring matrix with a hole diameter of 6 mm, cooperating with a system of two compression rollers.

The constant values during the tests were as follows:

• d_o = 6—diameter of the holes in the matrix of the pelletizer [mm],
• Q = 350—raw material mass flow rate [kg·h⁻¹],
• n_r = 170—rotational speed of the compaction roller system [rpm],
• h_r = 0.2—gap between the rollers and the matrix [mm].

The set of output quantities (dependent variables) included the following:

• power demand of the pelletizer N_g,
• strength of the obtained pellets P_dx,
• density of the obtained pellets ρ_g,
• bulk density of the obtained pellets ρ_ug.

2.10. Density and Bulk Density of Pellets

The determination of the density of the obtained pellet was carried out 24 h after they had left the compaction chamber. As part of the tests, the height and diameter of 15 randomly sampled pellets were measured with an accuracy of ±0.02 mm, and the results were determined using a WPS 360 laboratory scale (Radom, Poland) with an accuracy of ±0.001 g. The density of the pellets was calculated as the ratio of their mass to the sum of their volume.

The bulk density of the raw materials and the obtained pellets were determined according to the PN–EN ISO 17828:2016-02 [54] standard. The measurement involved filling the measuring cylinder (with a diameter of 167 mm and a height of 228 mm) completely around its circumference and then scraping off any excess material from the top of the cylinder. Bulk density was defined as the weight of the material (measured using the OHAUS AX224M analytical balance, with a precision of ±0.1 mg) into the volume of the container.

2.11. Kinetic Durability of Pellets

After 24 h, from the moment the pellets left the working system, their kinetic durability was determined using a Holmen tester (Norfolk, UK). Durability tests were conducted according to the PN–EN ISO 17831-1:2016-02 standard [62]. The detailed examination was repeated 5 times. For each 100 g sample of the pellets, air was introduced into the test chamber causing the pellets to circulate and hit the perforated metal walls of the tester. The pellets remained in the chamber for 60 s. After this time, the remaining pellets were placed on the sieve in the test chamber, sieved, and weighed. Kinetic durability was calculated according to Equation (2):

$${\mathrm{P}}_{\mathrm{d}\mathrm{x}}={\displaystyle \frac{{\mathrm{m}}_2}{{\mathrm{m}}_1}}·100\mathrm{\%} \left[\mathrm{\%}\right]$$

(2)

where

• P_dx—kinetic durability of the pellets [%];
• m₁—mass delivered before the test [kg];
• m₂—mass delivered after the test [kg].

2.12. Energy Indicators of Pellet Production under Industrial Conditions

Considering the energy input required for pellet production, the energy yield was estimated using Equation (3):

$$\mathrm{E}\mathrm{Y}=\mathrm{L}\mathrm{H}\mathrm{V}-\mathrm{E}\mathrm{C}\mathrm{U} [\mathrm{Wh}·{\mathrm{kg}}^{-1}],$$

(3)

where

• EY—energy yield of pellets [Wh·kg⁻¹],
• LHV—Lower Heating Value (calorific value) [Wh·kg⁻¹],
• ECU—Energy Consumption Unit [Wh·kg⁻¹].

The energy efficiency of pellet production was calculated using Equation (4).

$$\mathrm{E}\mathrm{E}={\displaystyle \frac{\mathrm{IE} - \mathrm{ECU}}{\mathrm{IE}}}·100\% [\mathrm{\%}]$$

(4)

where

• EE—energy efficiency [%],
• IE—Initial Energy Content of Biomass [Wh·kg⁻¹],
• ECU—Energy Consumption Unit [Wh·kg⁻¹].

Equation (5) was used to determine the energy density of the pellets, which is an indicator of the efficiency of the processes of the transport and storage of the pellets.

$$\mathrm{E}\mathrm{D}=\mathrm{B}\mathrm{D}·\mathrm{L}\mathrm{H}\mathrm{V} [\mathrm{GJ}·{\mathrm{m}}^{-3}],$$

(5)

where

• ED—energy density [GJ·m⁻³],
• BD—bulk density [kg·m⁻³].
• LHV—Lower Heating Value (calorific value) [GJ·kg⁻¹].

2.13. Thermogravimetric Analysis

Thermogravimetric analysis was performed using the Libra 209 F1 NETZSCH TG thermobalance (Selb, Gemrany), by placing the prepared sample in a ceramic crucible on a highly sensitive c-DTA^® sample carrier with a SiO₂ coating.

2.13.1. Analysis of the Content of Analytical Moisture, Volatile Matter, Fixed Carbon, and Ash in Biomass

The moisture content volatile matter, fixed carbon, and ash in biomass tests were carried out in three successive stages. At each stage of the analysis, the sample was subjected to different thermal and atmospheric conditions, controlled by the apparatus, in the following manner:

• Stage I—determination of moisture content according to ISO 18134-3:2023-12 [63].
• Stage II—determination of volatile matter content according to ISO 18123:2023-10 [64].
• Stage III—determination of ash content according to ISO 18122:2023-05 [65].

The fixed carbon content, FC, was determined using Equation (6):

$$\mathrm{F}\mathrm{C}=100-\mathrm{M}\mathrm{C}-\mathrm{V}\mathrm{M}-\mathrm{A}\mathrm{C} \left[\mathrm{\%}\right]$$

(6)

where

• FC—fixed carbon content [%],
• MC—analytical moisture value [%],
• VM—volatile matter value [%],
• AC—ash content [%].

2.13.2. Ignition Characteristics

The test was carried out in an atmosphere of synthetic air (80% nitrogen and 20% oxygen). The ignition point was determined on the basis of the TG curve as the first significant inflection point after the moisture had evaporated.

The combustion characteristics of biomass fuels are assessed using the S-index of the comprehensive combustion characteristics, i.e., the comprehensive combustion index [66]. The value of the combustion characteristics index indicates combustion efficiency; the higher it is, the greater the efficiency. The S-index of the comprehensive combustion characteristics can be calculated using Equation (7):

$$S={\displaystyle \frac{(dX·{dt}^{-1}{)}_{max}·(dX·{dt}^{-1}{)}_{mean}}{T_i^2·T_f}}$$

(7)

where

• (dX·dt⁻¹)_max—maximum combustion rate (%·min⁻¹),
• (dX·dt⁻¹)_mean—average combustion rate (%·min⁻¹),
• T_i—ignition temperature (K),
• T_f—burn-out temperature (K).

The average combustion rate was calculated using Equation (8):

$$W_a=\beta ·{\displaystyle \frac{{\mathrm{A}}_{\mathrm{I}}-{\mathrm{A}}_{\mathrm{F}}}{{\mathrm{T}}_{\mathrm{f}}-{\mathrm{T}}_{\mathrm{i}}}}[\mathrm{\%}·{\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}]$$

(8)

where

• β—heating rate [K·min⁻¹],
• A_I—sample residue during ignition [%],
• A_F—sample residue during firing [%].

2.14. Emissions from Pellet Combustion

The emissivity test performed during the combustion of the produced pellets was conducted on a laboratory stand (Figure 3). The stand was equipped with an SKP BIO 20 boiler (PHU Krzaczek, Góra Puławska, Poland) with automatic fuel loading and a power of 20 kW. It was also equipped with a burner featuring a movable grate that cleans automatically. The burner is designed to burn A1-, A2-, and B-class pellets and agropellets. The boiler’s operation is controlled by means of an electronic controller.

The concentrations of carbon dioxide CO₂, nitrogen oxides NO_x, sulfur dioxide SO₂, and oxide O₂ were measured using a Testo 350 flue gas analyzer (Testo SE & Co. KGaA Baden-Württemberg, Deutschland). The TSP mass was measured using a Testo 380 particulate matter measurement system combined with Testo 330-2 LL (Testo SE & Co. KGaA, Baden-Württemberg, Germany).

The combustion tests were carried out under pre-established operating conditions of the boiler, at rated settings. The test time was 1 h and did not include the additional time needed to stabilize the boiler’s operation. Fuel consumption was determined by weighing the amount of fuel fed into the hopper before the test and the amount remaining after the test. This procedure was performed for each of the tested fuels.

The obtained results were normalized to an oxygen (O₂) content of 10%, according to Equation (9):

$$Z_{s2}={\displaystyle \frac{21-O_2^{\prime }}{(21-O_2^{″})}}·Z_{s1}[\mathrm{\%},\mathrm{m}\mathrm{g}·{\mathrm{m}}^{-3}]$$

(9)

where

• Z_s1—the actual content of a chemical compound in exhaust gases [%, mg·m⁻³];
• Z_s2—the content of a chemical compound in exhaust gases for a given oxygen content [%, mg·m⁻³];
• ${\mathrm{O}}_2^{\prime }$—the assumed oxygen content in exhaust gases [%];
• ${\mathrm{O}}_2^{″}$—the actual oxygen content in exhaust gases [%].

3. Results

3.1. Moisture of the Tested Raw Materials

Table 1. Results of moisture measurements of raw materials used for pelleting.

Raw Material	Moisture ± SD [%]
Potato pulp	85.08 ± 0.34
Pine sawdust	6.88 ± 0.22

presents the results of testing the moisture content of pine sawdust and fresh potato pulp dried in a WPE 300S moisture analyzer.

Based on the performed measurements, it was found that the potato pulp used for the tests has an average moisture content of approx. 85%, while the pine sawdust has an average moisture content of approx. 6.9%. From the point of view of the pelleting process, the moisture content of sawdust was too low. To ensure proper processing and obtain high-quality pellets, water or another additive with a higher moisture content should be added to sawdust, which is characterized by a low moisture content. Therefore, potato pulp characterized by a high moisture content and a small amount of potato starch may be suitable. Changes in the moisture content of the mixture of raw materials depending on the addition of potato pulp are presented in

Table 2. Moisture content of mixtures of sawdust and potato pulp.

Potato Pulp Content [%]	Moisture Content of the Mixture [%]
10	11.72 ± 0.20
15	17.90 ± 0.22
20	21.72 ± 0.18
25	26.54 ± 0.24

.

Jezerska et al. [67] examined the impact of adding modified wheat starch on the pelleting process of spruce sawdust. In their tests, sawdust mixtures were pelletized with starch additions of 0%, 5%, 10%, 15%, and 20%. The results showed that while the pellets had slightly lower density, they exhibited greater durability, hardness, and moisture resistance. Specifically, adding starch in amounts ranging from 5% to 20% decreased the pellet density from 1260 to 1230 kg·m⁻³, compared to 1290 kg·m⁻³ for pellets made from sawdust alone. However, this addition significantly improved the kinetic strength of the pellets, increasing it from 80.9% to 99.2%, compared to 70.3% for pellets made from sawdust alone.

Based on the performed measurements, it was found that increasing the content of potato pulp in a mixture with sawdust from 10 to 25% resulted in an increase in the moisture content of the mixture from 11.72 to 26.54%.

3.2. Granulometric Distribution of Tested Raw Materials

The granulometric distribution of sawdust particles, with a mass of 50 g, subjected to sieve analysis, is shown in Figure 4.

The conducted research shows that the largest proportion of the tested sawdust was in the 0.25 mm fraction (26.84%), followed by the 0.125 mm fraction (20.88%) and the 1 mm fraction (19.34%). The smallest proportion was the extra-sieve fraction <0.063 mm, accounting for 0.1%. Fine fractions below 0.5 mm constitute approx. 50% of the mass share in the tested raw material. According to Obidzinski et al. [68], from the point of view of the pelleting process, this is an unfavorable situation due to its increased energy demand, the shorter life of the working system, and the risk of spontaneous combustion of the fine fraction. Therefore, the use of potato pulp as an additive to sawdust results in partial pressureless pelletization of the dusty fraction (by enveloping the potato pulp particles and combining with them during the mixing process in the conditioner). This eliminates parts of the ultrafine fraction and reduces the energy demand of the subsequent pelletization process.

3.3. Bulk Density of the Tested Raw Materials

Table 3. Results of bulk density tests of sawdust.

Raw Materials	Medium Bulk Density ± SD [kg·m−3]
Pine sawdust	105.17 ± 1.39
Potato pulp	534.92 ± 4.27

presents the results of bulk density tests of sawdust and potato pulp.

The performed analyses show that sawdust has a low bulk density of approx. 105 kg·m⁻³, while potato pulp has a higher—but still low—bulk density of approx. 535 kg·m⁻³. The addition of potato pulp and partial pressureless pelleting of the dusty fraction increases the bulk density of the pelleted mixture entering the working system of the pelletizer. An increase in the density of the densified raw material reduces the energy demand of the pelletization process.

3.4. Elemental Composition of Tested Raw Materials

Table 4. Elemental composition of tested raw materials.

Raw Material	C ± SD [%]	H ± SD [%]	N ± SD [%]	S ± SD [%]	Cl ± SD [%]
Pine sawdust	47.54 ± 0.13	6.85 ± 0.04	0.11 ± 0.01	0.01 ± 0.001	0.0039 ± 0.0004
Potato pulp	40.82 ± 0.109	5.27 ± 0.03	0.12 ± 0.02	0.42 ± 0.001	0.0191 ± 0.0002

shows the results of the tests of the elemental composition of the tested raw materials.

The carbon content varied depending on the type of raw material. Pine sawdust contained over 47% of carbon; potato pulp contained approx. 41%. According to the literature data, the carbon content in sawdust ranges from 45 to 55% [67,69]. Ward et al. [70] found that pine chips contain approx. 69% of carbon. However, according to Dąbrowska et al. [71], the value is over 49%.

The hydrogen content in the tested raw materials was 6.85% for pine sawdust and 5.27% for potato pulp. The literature indicates that hydrogen content in sawdust is typically over 5% [72], while the nitrogen content in woody residues and pine sawdust can range from 0.1% to as high as 6.5% [73,74]. In this study, the nitrogen content was relatively low, at 0.11% for pine sawdust and 0.12% for potato pulp. This is significant because the EN Plus A2 certificate [59] specifies a maximum allowable nitrogen content of 0.5% in pellets, while the EN Plus B certificate allows up to 1%. Based on the nitrogen content in the tested raw materials, it can be concluded that the produced pellets, regardless of the amount of potato pulp added, meet the criteria for the EN Plus A2 certificate [59].

In the analyzed raw materials, the sulfur content was 0.01% for sawdust and 0.42% for potato pulp. The sulfur contained in the raw materials may be of a natural origin or come from pollutants absorbed by plants. Additionally, a high sulfur content in raw materials may indicate high SO₂ air pollution, for example, in regions where the trees from which the samples were taken grew. A high sulfur content in fuel material is unfavorable because it may accelerate the corrosion of boilers during pellet combustion. The tested pine sawdust contained 0.01% sulfur, whereas in a study conducted by Khalili et al. [75], the content of this element in pine cones was 1.21%. However, research performed by Kot et al. [76] indicated that potato pulp contained 0.87% sulfur, which is twice as much as in the samples tested in this study.

Moreover, chlorine content in the analyzed raw materials was also low and amounted to approx. 0.004% in the case of sawdust and 0.02% for potato pulp. Hence, fuel materials produced from these raw materials would also have low contents of this element, significantly below the limit established in the ISO 17225-2:2021-11 standard [59].

3.5. Heavy Metal Content

The use of various types of post-production waste to create biofuels in pellet form is widely recognized as an eco-friendly practice with a beneficial environmental impact. To assess whether a particular type of waste is suitable as a biofuel, it is essential to analyze its chemical composition. The ISO 17225-2:2021-11 [59] standard governs the levels of heavy and alkali metals in wood pellets, specifying the maximum allowable concentrations of these elements. Monitoring heavy metal content is crucial for environmental protection, ensuring boiler durability, and evaluating the potential use of ash as fertilizer for soil improvement or reclamation.

The tested pine sawdust exhibited low levels of heavy metals. Pine sawdust contained the highest amount of zinc, i.e., 39.71 mg·kg_d.w.⁻¹, and the lowest contents of cadmium and lead, i.e., below 0.05 mg·kg_d.w.⁻¹ (

Table 5. Content of heavy metals in tested raw materials and pellets.

Material	Content of Heavy Metals ± SD [mg∙kgd.m.−1]
	Cr	Ni	Cu	Zn	As	Cd	Pb
Pine sawdust	2.45 ± 0.18	2.99 ± 0.04	5.42 ± 0.38	39.71 ± 1.40	0.11 ± 0.03	x ≤ 0.05	x ≤ 0.05
Potato pulp	0.00	1.33 ± 0.11	8.96 ± 0.32	28.57 ± 1.02	0.16 ± 0.04	1.08 ± 0.02	8.21 ± 0.44
Pellets containing 10% Potato pulp	2.21 ± 0.12	2.82 ± 0.12	5.77 ± 0.39	38.60 ± 1.19	0.12 ± 0.02	0.15 ± 0.01	0.87 ± 0.11
Pellets containing 15% Potato pulp	2.08 ± 0.11	2.74 ± 0.09	5.95 ± 0.33	38.04 ± 1.23	0.12 ± 0.03	0.20 ± 0.02	1.27 ± 0.06
Pellets containing 20% Potato pulp	1.96 ± 0.14	2.66 ± 0.10	6.13 ± 0.41	37.48 ± 1.28	0.12 ± 0.03	0.26 ± 0.02	1.68 ± 0.07
Pellets containing 25% Potato pulp	1.84 ± 0.13	2.58 ± 0.11	6.31 ± 0.38	36.93 ± 1.15	0.12 ± 0.03	0.31 ± 0.03	2.09 ± 0.05

), which are the elements that are particularly harmful to the environment. According to Grzybek [77], the zinc content in coniferous tree wood is much higher than that of any other heavy metal and amounts to 37.6 mg·kg_d.w.⁻¹. In the research conducted by Kraszkiewicz et al. [78], low cadmium and lead contents were found, i.e., 0.0004 and 0.03 mg·kg_d.w.⁻¹, respectively. When examining wood produced from coniferous trees, Grzybek [77] obtained slightly higher results; namely, the cadmium content was 0.61 mg·kg_d.w.⁻¹, while the lead content was 2.38 mg·kg_d.w.⁻¹.

The analyzed pine sawdust had similar chromium and nickel contents (2.46 mg·kg_d.w.⁻¹ Cr and 2.99 mg·kg_d.w.⁻¹ Ni). In the case of the samples tested by Grzybek [77], the amounts of these elements were twice as high, while Kraszkiewicz et al. [78] recorded much lower contents of chromium (0.04 mg·kg_d.w.⁻¹) and nickel (0.0 mg kg_d.w.⁻¹).

In the case of potato pulp, no chromium was detected. Similarly to the case of pine sawdust, the highest content was recorded for zinc, i.e., 28.57 mg·kg_d.w.⁻¹. The contents of copper and lead were three times lower, with similar levels, i.e., 8.96 and 8.21 mg·kg_d.w.⁻¹, respectively. For nickel and cadmium, the values were 1.33 and 1.08 mg·kg_d.w.⁻¹, respectively.

3.6. Calorific Value and Heat of Combustion

Table 6. Results of tests on influence of potato pulp addition to mixture with sawdust on its calorific value and heat of combustion.

Content of Potato Pulp [%]	Moisture Content of Mixture before Combustion [%]	HHVar ± SD	HHVdry ± SD	LHVar ± SD	LHVdry ± SD
		[MJ·kg−1]
0	4.78	19.471 ± 0.05	20.449 ± 0.04	17.919 ± 0.05	19.022 ± 0.04
25	9.46	18.398 ± 0.06	20.320 ± 0.03	16.813 ± 0.07	18.825 ± 0.08

presents the results of tests on the influence of potato pulp addition to a mixture with pine sawdust on its calorific value and heat of combustion.

Based on tests performed for sawdust alone (0% potato pulp addition) and for the mixture of sawdust with a 25% potato pulp content, the influence of potato pulp addition on the heat of combustion (HHV) during combustion was determined and described by Equation (10):

HHV = −0.005z_p + 20.45 [MJ·kg⁻¹]

(10)

where

• HHV—heat of combustion [MJ·kg⁻¹],
• z_p—potato pulp content [%].

The influence of pulp addition (z_p) on calorific value (LHV) was determined from Equation (11):

LHV= −0.008z_p + 19.02 [MJ·kg⁻¹]

(11)

where

• LHV—calorific value [MJ·kg⁻¹],
• z_p—potato pulp content [%].

The observed relationships indicate that the potato pulp content has minimal impact on both the heat of combustion (HHV) and the calorific value (LHV) of the tested waste. The heat of combustion (HHV) of pine sawdust is 19.47 MJ·kg⁻¹, while the calorific value (LHV) is 17.92 MJ·kg⁻¹. Slightly lower values of the heat of combustion (19.00 MJ·kg⁻¹) and calorific value (16.96 MJ·kg⁻¹) were obtained by Kraszkiewicz et al. [78] for pellets produced from pine sawdust. The addition of potato pulp to sawdust causes a slight decrease in the heat of combustion (HHV) and the calorific value (LHV) of the pellets. Based on the obtained test results and the calculations based on the obtained relationships in (3) and (4), it was found that increasing the potato pulp content from 0 to 25% causes a slight decrease in the heat of combustion (HHV), from 20.45 to 20.32 MJ·kg⁻¹, for the dry mass of sawdust and the mixture, respectively. The calorific value (LHV) also decreases, from 19.02 to 18.83 MJ·kg⁻¹, for the dry mass of sawdust and the mixture, respectively. Based on the obtained relationships, the heat of combustion (HHV) and the calorific value (LHV) of the tested pellets were determined.

Table 7. Influence of the addition of potato pulp to a mixture with sawdust on its calorific value (LHV) and heat of combustion (HHV).

Content of Potato Pulp [%]	HHV [MJ·kg−1]	LHV [MJ∙kg−1]
	For Dry Matter	For Dry Matter
0	20.449	19.022
5	20.424	18.983
10	20.398	18.943
15	20.373	18.904
20	20.347	18.864
25	20.320	18.825

shows the results of tests examining the effect of adding potato pulp to a mixture with pine sawdust on its calorific value (LHV) and heat of combustion (HHV).

The obtained values of heat of combustion (HHV) and the calorific value (LHV) (Table 6 and Table 7) prove that the addition of potato pulp maintains the good energy properties of the produced pellets. From this point of view, there are no contraindications to using potato pulp as an additive to sawdust in the production of fuel pellets.

3.7. Pelleting Process under Laboratory Conditions and Pellet Characteristics

During the pelleting process of mixtures produced under laboratory conditions, the demand for active power from the pelletizer for the mixture containing 10% potato pulp was 7.35 kW (Figure 5). Increasing the potato pulp content in the pelletized mixtures from 10 to 25% resulted in a decrease in the power demand of the pelletizer by approx. 20% (from 7.35 kW to 5.92 kW).

The influence of potato pulp addition on the active power demand of the pelletizer (N_g) during the compaction of pine sawdust in the working system of a pelletizer with a flat matrix is described by Equation (12):

N_g = −0.45z_p + 7.72 [kW]

(12)

where

• z_p—potato pulp content [%];
• N_g—power demand of the pelletizer [kW].

Figure 6 shows pellets obtained with various potato pulp contents in a mixture with sawdust.

Pellets obtained with a 10% addition of ground residue are characterized by high strength and smooth surfaces (Figure 6A). With an increase in the amount of ground middlings in the mixture up to 15%, several cracks appear on the surface of the obtained pellets (Figure 6B). A further increase in the amount of ground middlings up to 20% results in an uneven, jagged surface with visible indentations and cracks (Figure 6C). The addition of 25% of the wiper results in the produced pellets spilling out (Figure 6D).

3.8. Pelleting Process under Industrial Conditions and Pellet Characteristics

After laboratory tests were conducted on a stand with a prototype pelletizing and briquetting device (Figure 2), verification tests were performed on a technical scale (in industrial conditions) at the PANBAH Wood Production Plant.

Table 8. Selected physical properties of tested pellets.

Potato Pulp Content [%]	Pellet Density [kg·m−3]		Bulk Density [kg·m−3]		Kinetic Strength [%]
	Laboratory Conditions	Industrial Conditions	Laboratory Conditions	Industrial Conditions	Laboratory Conditions	Industrial Conditions
0	-	1212.24	-	709.12	-	85.62
5	-	1240.73	-	724.75	-	96.20
10	1191.03	1161.72	567.79	523.09	98.01	80.17
15	1109.63	535.04	501.28	329.00	93.28	30.74
20	627.03	-	339.52	-	34.50	-
25	428.08	-	275.95	-	24.29	-

presents the results of the research on the influence of the addition of potato pulp to a mixture with sawdust used by the PANBAH Wood Production Plant on the pelleting process. The table includes data on the power demand of the pelletizer, kinetic strength, density, and bulk density of the pellets obtained during the pelletization of the sawdust and potato pulp mixture under industrial conditions at the PANBAH Wood Production Plant pelleting station.

Based on the test results obtained (Figure 7, Table 8), it was concluded that the content of potato pulp in a mixture with sawdust significantly impacts both the pelletization process (including the power demand of the pelletizer) and the kinetic strength of the produced pellets.

Increasing the residue content in the sawdust mixture from 0 to 5% results in a significant decrease in the power demand of the pelletizer, by approx. 13% (from 26.16 to 22.71 kW). A further increase in the amount of residue in the sawdust mixture from 5 to 10% results in a further decrease in the power demand of the pelletizer by approx. 16% (from 22.71 to 18.98 kW). Increasing the residue content in the sawdust mixture from 10 to 15% results in a further decrease in the power demand of the pelletizer by approx. 24% (from 18.98 to 14.38 kW). However, the addition of 15% of potato pulp does not result in a satisfactory level of kinetic strength of the produced pellets, as the obtained value of 31.74% cannot be regarded as sufficient.

The values of the power demand of the pelletizer obtained during the compaction of a mixture of sawdust and potato pulp in the working system of a pelletizer with a flat matrix at the PANBAH Wood Production Plant confirm the results of laboratory tests and show that the addition of potato pulp has a pronounced beneficial effect on power demand reduction, which is thus much lower than the demand obtained in the case of pelleting sawdust alone.

Obidziński et al. [9] conducted research on the effect of the addition of potato pulp on the pelletization process of shredded hemp shives. The results showed that the addition of potato pulp had a huge impact on the decrease in the power demand of the pelletizer. When a mixture containing 10% of the additive (at a mixture mass flow rate of 30 kg·h⁻¹) was pelleted, the power demand of the pelletizer was 1.7 kW, while when a mixture containing 20% was pelleted, the power demand decreased by 50%, i.e., down to 0.85 kW.

The beneficial impact of maize starch or lignosulphonate addition on the pelleting process was demonstrated in a study by Mediavilla et al. [79]. They found that adding 2.5%, 5.0%, and 7.0% (w/w) of dry additive improved process stability and reduced power demand during poplar pelletization. Kaliyan and Morey [80] also reported a positive effect of lignosulphonate binder on the power demand of the pelletizer.

The effect of potato pulp content (z_p) in the sawdust mixture on the values of the power demand of the pelletizer (N_g) under industrial conditions (at the PANBAH Wood Production Plant) is described by Equation (13):

N_g = −3.91z_p + 30.33 [kW]

(13)

where

• z_p—potato pulp moisture [%],
• N_g—power demand of the pelletizer [kW].

Figure 8 shows pellets obtained under industrial conditions (at the PANBAH Wood Production Plant) with various contents of potato pulp in a mixture with sawdust.

Pellets obtained without the addition of potato pulp have smooth and shiny surfaces (Figure 8A). However, due to the insufficient moisture content of the sawdust from which it has been made, they are very short. They have larger amounts of dividing surfaces, which is important as far as the kinetic strength test is concerned because, as kinetic strength increases, the amount of fine fraction generated during the test increases, which reduces the kinetic strength of the pellets. This is an important factor regarding the storage and transport of the obtained pellets as the above coefficient determines the resistance of the obtained pellets to storage and transport conditions, during which the pellets are subjected to repeated impacts and pressures through their subsequent layers during storage. This is the result of insufficient moisture and thus the lack of a binder in the pelleted raw material. This situation can be remedied through the addition of water (or preferably steam) during the conditioning process or through the addition of a binder in the form of, for example, potato pulp. Compared to water, the advantage of adding potato pulp is that, in addition to moisture, it also introduces potato starch into the pelleted raw material. When combined with water at a high temperature—as it is formed in the pelletization process—this creates a sticky gel (binder). This binder causes better bonding between sawdust particles and—after drying—forms a durable agglomerate.

With the use of potato pulp contents of up to 5% in the mixture, the length of the pellets being formed increases (through a reduction in the pellet dividing surface). At the same time, they still have smooth and shiny surfaces (Figure 8B). A further increase in the potato pulp content in the sawdust mixture, from 5% up to 10%, causes the appearance of several cracks on the surface of the pellets (Figure 8C), which reduces their strength and density. Increasing the addition of potato pulp up to 15% causes the surfaces of the pellets to become uneven and ragged, with visible depressions and cracks (Figure 8D). In addition, a significant proportion of the pellets fall apart due to low pressure.

3.9. Physical Properties of the Produced Pellets

Table 8 presents the results of tests on the effect of adding various potato pulp contents to the sawdust mixture—used at the PANBAH Wood Production Plant—on selected physical properties of the pellets, including kinetic strength, pellet density, and bulk density.

Based on laboratory test results (Table 8), increasing the potato pulp content in the sawdust mixture from 10 to 25% leads to a significant reduction in the kinetic strength of the pellets. Specifically, raising the potato pulp content from 10 to 15% results in a slight decrease in kinetic strength, by about 5% (from 98.01 to 93.28%). Further increasing the potato pulp content to 20% reduces the kinetic strength to approx. 35%, and at 25% potato pulp, it drops to around 24%. In the research conducted by Dobrowolska [24], pellets made from pine sawdust were characterized by a much higher kinetic strength, i.e., 97.63%. However, research conducted by Obidziński et al. [9] showed that hemp shives pellets containing 10% potato pulp had a kinetic strength of 98.1%, while increasing the amount of potato pulp added resulted in a minimal decrease in kinetic strength, i.e., down to 96.42%.

Tests conducted under industrial conditions confirmed the results obtained under laboratory conditions. An addition of 5% of potato pulp caused an increase in the kinetic strength by approx. 12% (from 85.62 to 96.20%). However, compared to the results obtained under laboratory conditions, increasing the contents of the additive up to 10% and 15% had the opposite effect as the kinetic strength of the pellets began to decrease, up to 80.17% and 30.74%, respectively. According to the ISO 17225-1:2021:11 standard [59], fuel pellets should have a minimum kinetic strength of 97.5%. Based on the obtained results (Table 8), it can be concluded that only pellets containing 10% potato pulp produced under laboratory conditions meet these requirements.

The influence of potato pulp addition to a mixture with sawdust (z_p) on the kinetic strength of (P_dxg) pellets is described by Equation (14):

P_dxg = −27.99z_p + 132.51 [%]

(14)

where

• z_p—potato pulp content [%],
• P_dxg—kinetic strength of pellets [%].

The influence of potato pulp addition to a mixture with sawdust (z_p) on the kinetic strength of pellets (P_dxg) obtained under industrial conditions (at the PANBAH Wood Production Plant) is described by Equation (15):

P_dxg = −15.003z²_p + 56.95z_p + 43.34 [%]

(15)

where

• z_p—potato pulp content [%],
• P_dxg—kinetic strength of pellets [%].

The relationships between pellet density, the bulk density of the pellets, and the content of potato pulp in the sawdust mixture are similar to the relationship observed for the kinetic strength of the pellets (Table 8).

Based on the laboratory tests, it was found that increasing the potato pulp content in the sawdust mixture from 10 to 25% causes a significant decrease in the density of the pellets. In the case of compaction under laboratory conditions, increasing the potato pulp content in the sawdust mixture from 10 to 15% causes a decrease in the density of the pellets by approx. 7% (from 1191.03 to 1109.63 kg·m⁻³), which corresponds to a decrease in the bulk density of the pellets by approx. 12% (from 567.79 to 501.628 kg·m⁻³). A further increase in the potato pulp content by up to 20% causes a decrease in the density of the pellets by approx. 44% (from 1109.63 to 627.03 kg·m⁻³), which corresponds to a decrease in the bulk density of the pellets by approx. 32% (from 501.628 up to 339.52 kg·m⁻³). Increasing the potato pulp content from 20 to 25% causes a further decrease in the density of the pellets by approx. 32% (from 627.03 to 428.08 kg·m⁻³), which corresponds to a decrease in the bulk density of the pellets by approx. 19% (from 339.52 to 275.95 kg·m⁻³). In the research conducted by Núñez-Retana [81], the bulk density of the produced pellets ranged from 557 to 703 kg·m⁻³, while according to the PN-EN ISO 17225-1:2021-11 standard [59], the highest quality fuel pellets should have a bulk density of at least 600 kg·m⁻³.

Industrial research conducted at the PANBAH Wood Production Plant showed a positive effect of the addition of potato pulp, which resulted in an increased density of the pellets. An addition of 5% of potato pulp caused an increase in the density of the pellets by approx. 3% (from 1212.24 to 1240.73 kg·m⁻³), which corresponds to an increase in the bulk density of the pellets by approx. 2% (from 709.12 to 724.75 kg·m⁻³). However, further increasing the potato pulp content up to 10 and 15% had a negative impact on the density of the pellets, causing a decrease of 6% (1161.72 kg·m⁻³) and 57% (535.04 kg·m⁻³), respectively, which corresponds to a decrease in the bulk density of the pellets by 28% (523.03 kg·m⁻³) and 55% (329.0 kg·m⁻³), respectively. A study by Soucek and Jasinskas [82], who investigated the impact of adding potatoes to heating pellets made from flax stalks, found that pellets with the potato addition exhibited improved specific gravity (by 1092.28 kg·m⁻³) and increased durability (by 0.714%).

A study by Cui et al. [83] on the production of fuel pellets from waste wood biomass and microalgae found that adding microalgae in quantities ranging from 15% to 50% of the total mass can significantly enhance the mechanical strength and bulk density of the produced pellets by 0.7–1.6% and 9–36%, respectively. Additionally, this addition notably reduces the energy demand of the process by 23.5–40.4%. With 50% microalgae, the maximum bulk density achieved was 1580.2 kg·m⁻³.

The influence of the addition of potato pulp to a mixture with sawdust (z_p) on the density (ρ_g) and the bulk density (ρ_ug) of the pellets under laboratory conditions are described by Equations (16) and (17):

ρ_g = −277.15z_p + 1531.8 [kg·m⁻³]

(16)

ρ_ug = −103.73z_p + 680.45 [kg·m⁻³]

(17)

where

• z_p—potato pulp content [%],
• ρ_g—pellet density [kg·m⁻³],
• ρ_ug—bulk density of pellets [kg·m⁻³].

The influence of the addition of potato pulp to a mixture with sawdust (z_p) on the density (ρ_g) and bulk density (ρ_ug) of the pellets obtained under industrial conditions (at the PANBAH Wood Production Plant) are described by Equations (18) and (19):

ρ_g = −163.79z²_p + 607.9z_p +746.12 [kg·m⁻³]

(18)

ρ_ug = −52.43z²_w + 127.95 z_p +644.85 [kg·m⁻³]

(19)

where

• z_p—potato pulp content [%],
• ρ_g—pellet density [kg·m⁻³],
• ρ_ug—bulk density of pellets [kg·m⁻³].

3.10. Energy Indicators of Pellet Production

Table 9. Energy indicators of pellet production.

Indicator	Unit	Pellets Containing 0% Potato Pulp	Pellets Containing 5% Potato Pulp	Pellets Containing 10% Potato Pulp	Pellets Containing 15% Potato Pulp
HHV	[Wh·kg−1]	5680	5673	5666	5659
LHV	[Wh·kg−1]	5284	5273	5262	5251
Energy Consumption Unit (EU)	[Wh·kg−1]	74.74	64.89	54.23	41.09
Energy Yield of Pellets (EY)	[Wh·kg−1]	5209	5208	5208	5210
Energy Efficiency (EE)	[%]	98.68	98.86	99.04	99.27
Energy Density (ED)	[GJ·m−3]	13.49	13.76	9.91	6.22

presents the results of the calculations of energy indicators for pellet production, obtained using Formulas (3)–(5).

Among the tested pellets, pine sawdust pellets had the highest calorific value (i.e., the highest initial energy content) (Table 9). The addition of potato pulp caused this parameter to decrease. In the case of pellets containing 15% of this additive, it was lower by 33 MWh·kg⁻¹ compared to pellets produced from pine sawdust alone. At the same time, the process of densification of pine sawdust was the most energy-intensive (74.74 Wh·kg⁻¹). The addition of potato pulp caused a reduction in the power demand of the pelletizer, which then decreased with the increase in the content of the additive, reaching the lowest value for pellets containing 15% potato pulp (41.09 Wh·kg⁻¹). This resulted in the energy yield (EY) values for all the tested pellets being very similar. The differences found were of the order of 2 Wh·kg⁻¹ (Table 9). Figure 9 illustrates the relationship between the energy yield of pellets and the characteristics and energy parameters of the pelleting process. The energy efficiency of the pressure agglomeration process was lowest when only pine sawdust was used. However, the parameter increased with the addition of potato pulp, proportional to its amount. Thus, using potato pulp as an additive to pine sawdust reduced energy consumption and improved process efficiency while maintaining similar energy yield levels.

Figure 10 presents the relationship between the kinetic strength of pellets, their density, and the energy yield (EY), allowing for the analysis of their quality parameters in terms of the potential energy benefits related to their production and use for energy purposes. Pellets with an addition of 15% potato pulp were characterized by low kinetic strength (30.7%) and low pellet grain density (<600 kg·m⁻³). At similar values of energy yield (EY), this disqualifies them from being used as fuel for boilers with automatic fuel-feeding systems. Pellets containing 5% potato pulp were characterized by the highest strength and grain density. At the same time, it should be noted that the tested pellets produced from pine sawdust alone as well as pellets containing 5% and 10% potato pulp had grain densities ranging from 1161 to 1240 kg·m⁻³.

Considering the energy density (ED) of the obtained pellets (Table 9), it was the least beneficial for pellets containing 15% of this additive. Research on the process of the densification of wood waste with waste from the agri-food industry used as an additive [84] showed that the value of this parameter ranged from 7.7 to 12.0 GJ·m⁻³. In this study, the energy density (ED) indicators were found to be higher or fell within the quoted range—except for pellets containing 15% potato pulp, for which the value of the indicator was 6.22 GJ·m⁻³. However, as previously stated, due to their low kinetic strength and low pellet grain density these pellets were rated poorly. Moreover, their combustion was accompanied by the highest dust emissions (see Section 3.12).

All the tested pellets were characterized by similar energy yield (EY) values. The highest energy density values were observed in the case of pellets containing 5% potato pulp whereas significantly lower values were recorded for pellets containing 10% of the additive (almost 4 GJ·m⁻³). At the same time, the strength of all the produced pellets was below 96.5%, which is the threshold value for class B pellets. However, in the case of pellets containing 5% potato pulp, this parameter differed slightly from the requirement specified in the applicable wood pellet standards [59].

The literature data confirm that adding waste from the agricultural and food industries positively affects the pressure densification process and the quality of pellets produced from pine sawdust [61,85], straw [86], and miscanthus biomass [87]. Additionally, pellets made from biomass mixtures exhibit better energy and mechanical properties compared to those produced from a single type of biomass [88,89].

3.11. Thermogravimetric Analysis

The primary components of biomass are cellulose, hemicellulose, and lignin, along with moisture, ash, and non-structural components such as extracts [90]. The proportions of these ingredients vary depending on the material.

Table 10. The results of the technical analysis of the produced pellets.

Potato Pulp Content [%]	Analytical Moisture Content MC [%]	Volatile Matter Content VM [%]	Fixed Carbon Content FC [%]	Ash Content AC [%]
0	2.57	80.2	12.2	5.03
5	2.45	79.8	12.71	5.04
10	1.95	78.86	13.55	5.64
15	2.02	77.83	14.17	5.98

presents the results of the technical analyses of the produced pellets.

When evaluating the energy suitability of solid fuels, the quantity of volatile matter is a crucial parameter. Fuels with high volatile matter content (>60%) generate a long flame during combustion and need additional air to achieve complete smokeless combustion.

The content of volatile matter decreased with the increase in the addition of potato pulp. For pellets without potato pulp addition, the value was 80.2%, while for the addition of 15%, the content of volatile matter was 77.83%. In the study conducted by Jia [66], the volatile matter content for the tested energy pellets ranged from 62.65 (in the case of Masson pine) to 78.86% (for Chinese fir).

The content of volatile matter decreases with the increase in the content of fixed carbon. For pellets containing 0% potato pulp, the value was 12.2% and increased up to 14.17% in the case of pellets containing 15% potato pulp.

The ash content also increased with the addition of potato pulp, from 5.03% to 5.98%. In a study performed by Jia [66], the ash content in the tested pine pellets was 1.36%. However, in the research conducted by Nath et al. [91], who performed thermogravimetric tests of wheat straw, the ash content in the tested samples was 7.09%.

The materials subjected to thermogravimetric analyses were completely dried biomass, i.e., with a moisture content below 10%.

Thermal decomposition of all the tested pellets began at a temperature of approx. 220 °C (Figure 11). As the temperature increases, the individual components of pellets, i.e., hemicellulose (220–320 °C), cellulose (320–370 °C), and lignin (320–500 °C) start to decompose. The higher share of volatile matter in biomass during thermal processing means that the biomass in question poses a lower environmental risk compared to coal.

Pellets made without the addition of potato pulp are characterized by an ignition temperature of 249.4 °C and a combustion temperature of 325.2 °C (Figure 12). The addition of potato pulp causes a minimal reduction in these temperatures. In the case of pellets containing 5% potato pulp, these are 249.2 °C and 319.2 °C (

Table 11. Comprehensive combustion performance indicators.

Combustion Characteristics	Pellets Containing 0% Potato Pulp	Pellets Containing 5% Potato Pulp	Pellets Containing 10% Potato Pulp	Pellets Containing 15% Potato Pulp
Ignition point Ti (°C)	249.40	249.20	244.70	244.30
Combustion temperature Tf (°C)	325.20	319.20	317.70	316.70
Maximum combustion rate (%·min−1)	92.74	92.00	94.1	94.46
Average combustion rate (%·min−1)	24.32	26.43	25.31	25.41
Comprehensive combustion index S (10−5)	1.38	1.50	1.50	1.52

), respectively. According to Jia [66], the ignition temperature for pellets produced from Masson pine was 241 °C, while the combustion temperature was 318 °C. However, according to Liu et al. [92], who tested corn straw briquettes, the ignition temperature for the tested samples, at a heating rate of 20 °C·min⁻¹, was 248 °C, while the firing temperature was 393 °C.

The comprehensive combustion index S was the lowest for pellets produced without the addition of potato pulp, at 1.38 × 10⁻⁵, and increased minimally with the addition of potato pulp, which for pellets containing 15% of the additive was 1.52 × 10⁻⁵. Guo et al. [93], who analyzed the possibility of co-combustion of biomass with hard and brown coal, achieved a comprehensive combustion index at a level above 0.1 × 10⁻⁷, depending on the process parameters. However, corn straw briquettes had a comprehensive combustion index of 3.6 × 10⁻⁸ [92].

3.12. Exhaust Composition during Combustion

The varied characteristics of the energy parameters of the produced pellets influenced the average volume share of exhaust gas emissions during their combustion (

Table 12. The composition of the exhaust gases from the combustion of the tested pellets (10% O₂ in the exhaust).

Potato Pulp Content [%]	CO [mg·m−3]	NO [mg·m−3]	CO2 [%]	λ [-]	O₂ [%]	TS [°C]	PM [mg·m−3]
0	311.86	86.55	5.71	3.34	15.06	108.1	17.5
5	298.47	66.76	4.94	3.81	15.85	112.2	16.7
10	282.49	69.82	4.69	3.99	16.11	112.4	16.8
15	273.95	66.92	4.14	4.46	16.68	112.2	19.2

).

During the combustion of pine sawdust pellets (without the addition of potato pulp), the highest CO₂ emission was 5.71%, accompanied by the highest CO emissions of 311.86 mg·m⁻³ and NO emissions of 86.55 mg·m⁻³. Pine sawdust was characterized by higher carbon and hydrogen contents compared to potato pulp (Table 4), which resulted in its greater potential for heat release as well as higher CO and CO₂ emissions during combustion. Combustion of the pellets with the addition of potato pulp resulted in reduced emissions of the aforementioned gases. In the case of the combustion of pellets containing 15% potato pulp, the CO₂ emission was the lowest and amounted to 4.14%. CO emissions were similarly reduced to 273.95 mg·m⁻³. NO emission was lower compared to the case of the combustion of wood pellets alone. A decrease in NO emissions was observed, ranging from 16.73 to 19.63 mg·m⁻³, compared to the combustion of wood pellets alone. Pellets containing a larger amount of potato pulp did not show a significant reduction in NO emissions, which remained at a similar level (66.76–69.82 mg·m⁻³). The nitrogen content in both pine sawdust and potato pulp was similar, at 0.11% and 0.12%, respectively (Table 4). Hence, it can be concluded that the observed differences in NO emissions were not caused by the formation of fuel nitrogen oxides, but rather by thermal reactions involving atmospheric nitrogen and oxygen. This is confirmed by the higher volume fraction of oxygen during the combustion of pellets with the addition of potato pulp, which increased with the increase in the share of potato pulp in the pellets. Changes in oxygen levels are a critical factor influencing CO, CO₂, and NO emissions. This indicates a lower flame temperature and, therefore, lower NO production from air nitrogen.

The sulfur content was higher in potato pulp (0.42%) than in pine sawdust (0.01%) (Table 4). In this respect, pine sawdust pellets met the requirements for A1 class pellets in accordance with the ISO 17225-1:2021-11 standard (S < 0.04%). Pellets produced with the addition of potato pulp did not contain excessive levels of this element. During the combustion tests of pellets, no SO₂ content was found in the exhaust gases, i.e., it was below the device’s detection level (<5 ppm) [59].

The air–fuel ratios were relatively high across all the tests, with average values ranging from 3.4 to 6.8. The differences in oxygen concentrations in the exhaust gases raise the question of whether the emission differences were due to variations in oxygen standardization. However, the oxygen concentration range is quite narrow (15.5–18%), suggesting that normalization of the results cannot account for the observed differences.

Similarly, PM emissions were also varied (Table 12). The lowest dust emissions were found during the combustion of pellets containing 5% and 10% potato pulp, with the highest emissions found during the combustion of pellets containing 15% potato pulp. This was due to the very low durability of pellets with a 15% addition of potato leachate (30.74%) (Table 8). It should also be noted that the emission of particulate matter could have been influenced by the high ash content (5–6%), both in the case of pellets made from pine sawdust and pellets produced with the addition of potato pulp (Table 9). This content increased with the increase in the share of potato pulp in the pellets.

Taking into account the environmental aspects, pine sawdust pellets containing 5% and 10% potato pulp can be considered low-emission biofuels. Variations in the gas and dust emission characteristics of the processes of combustion of the pellets in question do not affect this overall positive assessment. However, due to higher PM dust emissions when burning pellets contain 15% potato pulp, it should be noted that such fuel should not be used in low-power boilers without a dust removal system typically found in individual households, but rather in higher-power installations equipped with suitable PM filters and precise combustion control systems. Compared to low-power devices in households, they are characterized by higher energy efficiency and lower pollutant emissions.

Performing a comparative analysis of the obtained test results in terms of the emissions of toxic exhaust gas components is difficult because the studies available in the literature concern boilers of different furnace designs. The environmental burden of gaseous combustion products is significantly influenced by the combustion conditions and the technology used [78]. In study [94], it was found that the fuel ash content is a factor influencing the overall emission. Emissions of all the tested exhaust gas components—except CO₂—increased with the increase in the ash content in the fuel. This resulted both from an increased release of inorganic matter and incomplete combustion. In general, when the wood is burned, carbon monoxide emissions from fireplaces range from 100 to 1000 mg·m⁻³; however, under unfavorable combustion conditions, the share of CO in the exhaust gases may reach values as high as several percent. NO_x emissions during wood combustion range from 170 to 920 mg·m⁻³. On the other hand, due to the low sulfur content in wood, no SO₂ emissions are observed [95,96]. Nevertheless, compared to burning wood logs, the combustion of wood pellets is notably less emissive, owing to the superior characteristics of the combustion process. Moreover, advanced technologies such as automatic fuel delivery systems and lambda probes help ensure more complete combustion [97]. During the tests of the combustion of wood pellets in an automatic low-power boiler [98], the CO and SO₂ emissions were 70.16 mg·m⁻³ and SO₂ 0.0655 mg·m⁻³, respectively, while NO emissions were 271.73 mg·m⁻³. The emission characteristics during the combustion of agrobiomass pellets were much more unfavorable. Much lower CO emissions were recorded during the combustion of wood pellets in study [99], with an average value of 47.2 mg·m⁻³. However, CO₂ emissions were higher, amounting to 10.57, and the NO_x emission of 519.4 mg·m⁻³ can be considered very high. Similarly to the authors’ own research, no SO₂ emissions were detected. Slightly lower but comparable dust emissions (15.45 mg·m⁻³) were found during the combustion of wood pellets [100], whereas NO_x emissions were significantly lower compared to those reported in study [101], which amounted to 421.7 mg·m⁻³.

When assessing new solid biofuels intended for direct combustion, qualitative and quantitative knowledge about emissions from various materials is crucial. When various fuels and combustion techniques are used, detailed characterization and quantification of the emissions of toxic exhaust gas components are needed [102]. Research shows the existence of a correlation between the chemical properties of a given fuel and emission characteristics, which can be used to predict emissions from new pelleted feedstocks [103].

4. Conclusions

Based on the research conducted, the following was found:

• The use of potato pulp as an additive to wood sawdust, characterized by a granulometric composition with fine fractions below 0.5 mm and a mass fraction exceeding 50%, improves the conditions for the pelletization process. This reduces its energy demand by causing partial pressureless pelletization of the dusty fraction. It also reduces the risk of spontaneous combustion of this fraction.
• The addition of potato pulp has a very beneficial effect on reducing the power demand of the pelletizer during the pelleting, which is significantly lower than the demand in the case of pelleting sawdust alone. Increasing the potato pulp content in the sawdust mixture from 10 to 25% reduces the power demand of the pelletizer by approx. 20% (from 7.35 kW to 5.92 kW).
• The density values of pellets produced from a mixture of sawdust and potato pulp (over 1000 kg·m⁻³) with a 10% potato pulp content indicate that these pellets are of good market quality and represent a high-quality solid fuel. However, increasing the potato pulp content from 10% to 15% results in a decrease in pellet density from 1191.03 kg·m⁻³ (with a bulk density of 567.79 kg·m⁻³) to 428.08 kg·m⁻³ (with a bulk density of 275.95 kg·m⁻³).
• Increasing the potato pulp content in the sawdust mixture from 10 to 25% causes a significant decrease in the kinetic strength of the pellets, from 98.01% to approx. 24% for a potato pulp content of 25%.
• Increasing the potato pulp content from 0 to 25% causes a slight decrease in the heat of combustion (HHV), from 20.45 to 20.32 MJ·kg⁻¹ (for the dry mass of sawdust and for the mixture, respectively), and the calorific value (LHV), from 19.02 to 18.83 MJ·kg⁻¹ (for the dry mass of sawdust and for the mixture, respectively). This allows for the pellets to be used as an additive to sawdust in the production of fuel pellets.
• During the tests, a high content of volatile matter was found, i.e., above 77%. Fuels containing significant amounts of volatile matter (>60%) produce a long flame during combustion and require additional amounts of air for completely smokeless combustion.
• Thermogravimetric analysis showed a positive effect of potato pulp on the quality of the pellets, reducing their ignition temperature from 249.4 °C to 244.3 °C and the combustion temperature from 325.2 °C to 316.7 °C. The addition of potato pulp also resulted in an increase in the comprehensive combustion index by 0.14 × 10⁻⁵.
• The study on the combustion of pine sawdust pellets with the addition of potato pulp showed that an addition of up to 10% of potato pulp did not increase CO, CO₂, and NO_x. During the combustion of pellets containing 5 and 10% potato pulp, PM emissions also decreased; however, the increased emissions of this compound were identified during the combustion of pellets containing 15% potato pulp. This is directly related to the low kinetic durability of such pellets and their overall high ash content. A beneficial feature of the tested pellets is also low sulfur content, which translated into low SO₂ emissions, below the detection level of the device.