Pyrolysis and Gasification of Biomass and Waste

1. Introduction

The use of renewable solid fuels, including biomass, is of great importance in today’s society. At the same time, the generated amount and variety of solid waste is increasing, and such waste should be reused. Both biomass and waste can be utilized in an efficient and environmentally friendly manner with the use of thermochemical processes, such as pyrolysis and gasification. These processes enable the conversion of biomass and waste into useful solid, liquid and gaseous products. Simultaneously, thermochemical conversion processes significantly reduce their negative impact on the environment and the emission of toxic compounds into the atmosphere.

The Special Issue “Pyrolysis and Gasification of Biomass and Waste” presents a selection of studies on the course of pyrolysis and the gasification of biomass and waste, an assessment of the used feedstock material, and provides information on the mechanism of these processes, optimization and intensification of the used pyrolysis and gasification technologies, and the modeling of these processes.

2. A Short Review of the Contributions in This Issue

The Special Issue contains 12 papers (11 research and 1 review article), which are briefly introduced below.

Safarian et al. [1] developed two simulation models of biomass pyrolysis based on the thermodynamic equilibrium and kinetic approaches. The results obtained from the simulation models were compared with modeling and experimental data from the literature. The performance of the thermodynamic equilibrium model is not satisfactory, whereas the kinetic approach model results suggest that it can be used for biomass pyrolysis processes modeling. The analysis of the deviation (mean square error) experiment and kinetic model results corroborated experimental data for temperature (400–600 °C) and various feedstocks (beech wood, pine spruce sawdust, bagasse, paddy straw and wood bark) in comparison with other kinetic models from the literature.

Butnaru and Brebu [2] present the thermochemical conversion (by torrefaction and pyrolysis) of forestry residues. Silver fir residues (cones, needles and bark) were thermally processed using separated and combined torrefaction (250 °C) and pyrolysis (550 °C). During the torrefaction process, the humidity and extractives were removed, the hemicelluloses were degraded, the oxygen content significantly decreased, the carbon content increased, and the calorific value increased to 32 MJ/kg. On the other hand, after the pyrolysis process, chars had high fixed carbon and heating value of about 29 MJ/kg. The combination of torrefaction and pyrolysis caused an increase in the yield of energy, decreased the H/C and O/C atomic ratios and led to more homogeneously distributed compounds in condensable products. Chemical compounds in the condensable products were confirmed, depending on the feedstock type and thermal treatment. Differences between the thermal procedures and the origin of the samples were shown, indicating the presence different types of lignin in the three analyzed materials.

A slow pyrolysis application to convert waste plastics derived from a compost-reject stream into py-char is presented in the study by Iwanek and Kirk [3]. The authors identified and quantified the types of polymers in the compost-reject stream, and the main components are polyethylene (72 wt.%), polypropylene (14 wt.%) and polyethylene terephthalate (12 wt.%) with minor contributions of polyvinyl alcohol (PVA), polybutylene adipate terephthalate (PBAT), polystyrene (PS), poly methyl methacrylate (PMMA), nylon and nitrile rubber. Whether a compost-reject stream could be successfully converted into char without the separation of components was determined. As-received waste containing plastics and fibrous material samples was subjected to a slow pyrolysis process. The char yield depends on the process conditions, and a synergistic effect was noted in the co-pyrolysis of the plastic and fibrous materials. The variable pyrolysis conditions were examined, and the study of the mechanism of char formation with use of DTA-TGA-MS indicates that this positive effect is due to the interaction between plastics and the air during the initial/pre-treatment step in the presence of fibrous material.

Yang et al. [4] analyzed four typical industrial polymeric wastes (plastic, rubber, cloth and leather), combustion behavior (using thermogravimetric analysis), and thermochemical structure evolution (using TG-FTIR, 2D-PCIS, ICP and TEM). The combustibility of cloth and leather is better than rubber and plastic, which have a wider temperature range of combustion for a higher content of C-H bonds and oxidation process of the intermediate and the stubborn cracking process of C=C bonds. For rubber and plastic, the surface reaction was considered to be the main reaction, while both cloth and leather experienced a complex multiple decomposition. The devolatilization products are gases (e.g., CO₂, CH₄) and small molecules (e.g., H₂O). The high contents of basic metals in the industrial waste could cause serious slagging and fouling tendencies, which have a serious adverse influence on waste incineration plant operations.

A numerical simulation of venturi scrubber performance during gasification gas cleaning is presented by Khadara et al. [5]. These authors developed a mass transfer two-dimensional simulation using the volume of fluid (VOF) model and CFD software for air inlet velocities of 10–20 m/s and a water inlet mass flow of 0.02–0.06 kg/s. The authors analyzed pressure and velocity contours, profiles, the mass fraction and the mass transfer probability density function (PDF). The obtained results show how the venturi scrubber removal efficiency increases with an increase in the air velocity and a decrease in the water flow rate. Therefore, they determined the optimal operation conditions for the correct removal efficiency.

Kajda-Szcześniak and Czop [6] present a comparison of pyrolysis and combustion processes of vinyl floor panels. They analyzed the thermal degradation of the main component of PVC waste vinyl panels in an oxidative and inert atmosphere using a coupled thermogravimetry–mass spectrometry analysis (TG-MS) combined with Fourier transform infrared spectrometry (TG-FTIR). During the TGA tests, which were carried out in the temperature range of 40–1000 °C, and for two heating rates of 5 and 20 K/min, mass losses were determined, and products from thermal degradation were identified. During the pyrolysis and incineration of waste vinyl panels, several physical and chemical phenomena occurred, indicating that both processes are difficult, complex and multifaceted. The results show the decomposition of individual components at different temperatures, depending on the type of atmosphere and heating rate.

Dyjakon et al. [7] studied the effect of torrefaction temperature (200–300 °C) on the physical–chemical properties of residual exotic fruit seeds. Torrefaction was proposed by these authors as a method for the valorization of mango, lychee, avocado seeds. The obtained results show that the torreficates are characterized by a higher heating value, higher fixed carbon content, improved hydrophobic properties and significant mass loss, by 50–60%. Thermal treatment also caused an increase (approx. 2–3 times) in ash content; however, the torreficates from residual exotic fruit seeds are competitive with those of coal.

The influence of the atmosphere during pyrolysis on the CO₂ gasification of waste tire char was analyzed by Grzywacz et al. [8]. These authors used two approaches during the non-isothermal gasification examinations in the temperature range of 20–1100 °C and for three heating rates: 5; 10; 15 K/min: pyrolysis during gasification in an inert atmosphere of argon and in a carbon dioxide atmosphere. Thermogravimetry (TG) and derivative thermogravimetry (DTG) curves were developed based on the results, and the KAS (Kissinger–Akahira–Sunose) and FWO (Flynn–Wall–Ozawa) isoconversion methods were used to calculate the kinetic parameters. A variability in the activation energy values was observed with the progress of the gasification reaction. At the beginning of the CO₂ gasification process, the highest activation energy values were observed, and the lowest values for the conversion degree were 0.5–0.7. In addition, this was assessed using the Coats and Redfern method reaction order, and the kinetic parameters were also calculated. For tire waste char formed in an argon atmosphere, DTG and conversion curves shifted to higher temperatures and higher mean values of activation energy, proving that they had a lower reactivity compared to char formed in CO₂ atmosphere.

Śpiewak et al. [9] present the effect of temperature and pressure conditions on the gasification of alternative refuse-derived fuel (RDF) in an atmosphere of carbon dioxide and steam. For selected RDF samples, proximate and ultimate analyses, ash composition and mercury content analyses, and finally, gasification examinations were performed. Isothermal, thermovolumetric gasification measurements were carried out using a fixed-bed reactor under various temperatures (700–900 °C) and pressures (0.5–1.5 MPa), with carbon dioxide and steam acting as gasifying agents. The increase in temperature for the entire analyzed range positively affected both the CO₂ and steam gasification of RDF, i.e., the formation rates and yields of the main gas products (H₂ and CO) and maximum carbon conversion degrees increased. The aforementioned effect of temperature was higher, with a lower pressure. The influence of pressure on the gasification process was more complex. In the case of carbon dioxide gasification of RDF, an increase in pressure had a positive impact on the process only at low temperatures (700–750 °C), while for steam gasification, pressure had a negative effect on the entire temperature range.

Wądrzyk et al. [10] present a study of the catalytic pyrolysis of biocrude obtained from fruit pomace hydrothermal liquefaction (HTL). The authors proposed a new two-stage processing of blackcurrant pomace, i.e., the thermochemical liquefaction of a wet-type organic matter into liquid biocrude followed by thermal and catalytic pyrolysis in order to obtain a value-added, hydrocarbon-rich biocrude fraction. A binary solvent system of water and isopropanol was used to obtain biocrude from raw materials by liquefaction. The effect of selected catalysts (SM-5 and HY zeolite) on the composition of volatiles released during the pyrolysis of the biocrude was investigated. The biocrude pyrolysis study was performed by microscale coupled pyrolysis–gas chromatography with mass spectrometry technique (Py-GC-MS). In the volatiles from catalytic pyrolysis, unsaturated hydrocarbons (both aliphatic and cyclic ones) were dominant components and, to a lesser extent, so were oxygen and nitrogen compounds. The addition of catalysts increased the total share of hydrocarbons in the volatiles; therefore, catalytic pyrolysis with use of zeolites seems to be effective for the promotion of deoxygenation reactions.

The influence of torrefaction on the mechanical durability and grindability of pellets [11] was analyzed by Dyjakon et al. [11]. These authors examined five various types of pellets (sunflower husk, pine tree, beetroot pomace, grass, wheat straw), and their torreficates formed at a temperature of 200 and 300 °C. The mechanical durability index and grindability of untreated and torrefied pellets were determined. The obtained results indicated that the mechanical durability of the untorrefied pellets is significantly greater than pellets after torrefaction. There were no significant differences between torrefied pellets at 200 and 300 °C regarding mechanical durability.

Finally, the review paper by Rasaq et al. [12] presents the opportunities and challenges of the high-pressure fast pyrolysis of biomass. The authors described the following high-pressure fast pyrolysis issues pyrolysis classification and reactors used in a fast process; heat transfer in pyrolysis feedstock; the parameters of fast pyrolysis; fast pyrolysis products properties and yields; the influence of high pressure on the pyrolysis process; catalysts and their applications; and problems to overcome in the fast pyrolysis process. They demonstrate the speed at which pyrolysis under high pressure can bring about the high-quality conversion of biomass into new products. They showed that fluidized bed reactors (circulating and bubbling) are very profitable and suitable in terms of product yields. The use of high pressure, especially in combination with a high heating rate, could be more beneficial and efficient than operating under ambient pressure. However, there are challenges associated with obtaining high product yield and quality on a technical scale. The authors conclude that future research should focus on the development of reactors and types of material in a high-pressure process that might have greater promise for using biomass, as well as understanding the impact of pyrolysis technology on the various output products, especially those with lower energy demands.

3. Conclusions

Both biomass and waste are potential feedstocks for thermal conversion processes, i.e., pyrolysis and gasification. Currently, several studies are being conducted on their utilization, including those presented in this Special Issue. Biomass, and especially waste, encompasses very diverse materials, as evidenced by the variety of materials investigated within the framework of this SI. In the case of biomass, the following materials were analyzed: sunflower husk, pine tree, beetroot pomace, grass, and wheat straw; needles, cones and bark from silver fir; mango, lychee, and avocado seeds; and blackcurrant pomace, whereas the following types of waste were examined: waste plastics from a compost-reject; rubber, leather, plastic and cloth; vinyl floor panels; waste tire char; and refuse-derived fuel (RDF). In their papers, the authors presented the characteristics of several materials; an assessment of the pyrolysis, torrefaction and gasification processes; the influence of the process conditions; and the quality and composition of the obtained products. Two papers focused on the modelling of biomass pyrolysis and gas cleaning processes. Finally, the opportunities and challenges of biomass high-pressure fast pyrolysis are presented.

I would like to thank all authors for their contributions of interesting studies, making the Special Issue “Pyrolysis and Gasification of Biomass and Waste“ a valuable publication for the scientific community. I would also like to thank the staff and reviewers for their efforts and input.