Effect of torrefaction on rice straw physicochemical characteristics and particulate matter emission behavior during combustion

Enhancing fuel characteristics and combustion performance of cellulose-rich straws through CO2-assisted torrefaction

Cellulose-rich straws of corn and rice were torrefied under carbon dioxide, and the fuel characteristics and combustion performance of the obtained biochar were investigated. A high severity resulted in surface collapse, greater pore volume, elimination of oxygen, elevated calorific value, and improved hydrophobicity in biochar. Following carbon dioxide torrefaction, the cellulose content in solid biochar experienced a slight decrease when the temperature was raised to 220 °C for longer residence durations. At 300 °C, the cellulose content in the biochar was nearly eliminated, while the relative proportion of non-sugar organic matter in corn stover and rice straw increased to 87.40 % and 77.27 %, respectively. The maximum calorific values for biochar from corn and rice straws were 22.38 ± 0.03 MJ/kg and 18.72 ± 0.05 MJ/kg. The comprehensive combustion indexes of rice and corn straw samples decreased to 1.06 × 10-7 and 1.31 × 10-7 after torrefaction at 300 °C, respectively. In addition, the initial decomposition temperatures increased by 38 °C and 45 °C, while the ultimate combustion temperatures rose by 13 °C and 16 °C for corn and rice straws, respectively. These results imply an extended combustion timeframe for the torrefied samples.

Effects of different low temperature pretreatments on properties of corn stover biochar for precursors of sulfonated solid acid catalysts

In this study, the effects of different pretreatment methods including phosphoric acid (PA), freeze drying (FD) and phosphoric acid-freeze drying combined (PA-FD) pretreatment on corn stover characteristics and pyrolysis of corn stover samples was investigated. The results demonstrated that the physiochemical properties of biochars varied significantly. In comparison, PA pretreatment could effectively remove a large portion of inorganics and improve the fuel characteristics. PA-CSB-600 had a greater HHV, lower O/C and H/C ratios, and a lower biochar energy yield (Y_e), indicating the possibility for an attractive fuel source. PA-FD pretreatment would significantly affected cell volume and caused mechanical damage to corn stover structure. As a sulfonated solid acid catalyst precursor, the results of cellulose catalytic hydrolysis indicated that the density of -SO₃H in FD-CSA was much higher than PA-FD-CSA, but lower surface special area. Specifically, PA-FD-CSB prepared at 600 °C resulted in the maximum increase of cellulose conversion by 34.7-81.3%.

A Review of the Sustainable Utilization of Rice Residues for Bioenergy Conversion Using Different Valorization Techniques, Their Challenges, and Techno-Economic Assessment

The impetus to predicting future biomass consumption focuses on sustainable energy, which concerns the non-renewable nature of fossil fuels and the environmental challenges associated with fossil fuel burning. However, the production of rice residue in the form of rice husk (RH) and rice straw (RS) has brought an array of benefits, including its utilization as biofuel to augment or replace fossil fuel. Rice residue characterization, valorization, and techno-economic analysis require a comprehensive review to maximize its inherent energy conversion potential. Therefore, the focus of this review is on the assessment of rice residue characterization, valorization approaches, pre-treatment limitations, and techno–economic analyses that yield a better biofuel to adapt to current and future energy demand. The pre-treatment methods are also discussed through torrefaction, briquetting, pelletization and hydrothermal carbonization. The review also covers the limitations of rice residue utilization, as well as the phase structure of thermochemical and biochemical processes. The paper concludes that rice residue is a preferable sustainable biomass option for both economic and environmental growth.

Recent advances of thermochemical conversion processes for biorefinery

Lignocellulosic biomass is one of the most promising renewable resources and can replace fossil fuels via various biorefinery processes. Through this study, we addressed and analyzed recent advances in the thermochemical conversion of various lignocellulosic biomasses. We summarized the operation conditions and results related to each thermochemical conversion processes such as pyrolysis (torrefaction), hydrothermal treatment, gasification and combustion. This review indicates that using thermochemical conversion processes in biorefineries is techno-economically feasible, easy, and effective compared with biological processes. The challenges experienced in thermochemical conversion processes are also presented in this study for better understanding the future of thermochemical conversion processes for biorefinery. With the aid of artificial intelligence and machine learning, we can reduce time-consumption and experimental work for bio-oil production and syngas production processes.

Torrefaction and Thermochemical Properties of Agriculture Residues

In this study, the densification of three agriculture waste biomasses (corn cobs, cotton stalks, and sunflower) is investigated using the torrefaction technique. The samples were pyrolyzed under mild temperature conditions (200–320 °C) and at different residence times (10 min–60 min). The thermal properties of the obtained bio-char samples were analyzed via thermo-gravimetric analysis (TGA). Compositional analysis of the torrefied samples was also carried out to determine the presence of hemicellulose, cellulose, and lignin contents. According to the results of this study, optimum temperature conditions were found to be 260 °C–300 °C along with a residence time of 20 min–30 min. Based on the composition analysis, it was found that biochar contains more lignin and celluloses and lower hemicellulose contents than do the original samples. The removal of volatile hemicelluloses broke the interlocking of biomass building blocks, rendering biochar brittle, grindable, and less reactive. The results of this study would be helpful in bettering our understanding of the conversion of agricultural waste residues into valuable, solid biofuels for use in energy recovery schemes. The optimum temperature condition, residence time, and GCV for torrefied corn cobs were found to be 290 °C, 20 min, and 5444 kcal/kg, respectively. The optimum temperature condition, residence time, and GCV for torrefied cotton balls were found to be 270 °C, 30 min, and 4481 Kcal/kg, respectively. In the case of sunflower samples, the mass yield of the torrefied sample decreased from 85% to 71% by increasing the residence time from 10 min to 60 min, respectively.

An integrated study on the pyrolysis mecanism of peanut shell based on the kinetic analysis and solid/gas characterization

An in-depth understanding of peanut shell pyrolysis reaction is essential for its efficient utilization. Detailed analysis of thermodynamics, kinetics, and reaction products can provide valuable information about pyrolysis reaction. In this work, pyrolytic reaction mechanism was elucidated with the analysis of thermogravimetric-mass spectrometry and the structural characterization of the derived biochar. The thermodynamic and kinetic parameters of three sub-stages were matched well in different model-free methods. The positive ΔH and ΔG values indicated that the pyrolysis reactions for three stages were endothermic and nonspontaneous. The reaction mechanism predicted by integral master-plots were F3 (f(α) = (1-α)³), F1 (f(α) = (1-α), and F3 (f(α) = (1-α)³) for the three sub-stages, respectively. The negative ΔS in the third stage was related to the reduced releasing of low-molecular weight gases and ordered graphite-like carbon structure. This study provides a prospective approach to understand the pyrolysis mechanism of biomass.

Combustion performance and emissions from torrefied and water washed biomass using a kg-scale burner

This study investigates wastes and biomass as alternative fuels in a kg-scale burner in terms of combustion characteristics and emissions. Water washing, torrefaction, and their combination are used to improve the properties of the wastes and biomass. The air pollutants in the exhaust of the burner are also analyzed. It could be concluded that the reactivity and average heat supply from the pretreatment are improved significantly. The improvement ratio of average heat supply can be up to 103.5 %, stemming from water-soluble ash removal during water washing. Torrefaction can lift the average heat supply due to the increment of fixed carbon content in the fuels, but it reduces the reactivity owing to the decrement of volatile matters. Most of the raw or pretreated materials can be directly combusted, as a result of lower regulated air pollutants (e.g., NO_x, SO₂, CO) from them than from coal. Water washing can successfully remove chlorine in the wastes by dissolution since most of the chlorine in the wastes are in salt form. The chlorine reduction significantly reduces the HCl concentration (55-58 % reduction efficiency) and the toxicity concentration of polychlorinated dibenzo-p-dioxins and dibenzofurans (78-84 %), while torrefaction increases the toxicity concentration owing to the de novo synthesis.

Components and Persistent Free Radicals in the Volatiles during Pyrolysis of Lignocellulose Biomass

Persistent free radicals (PFRs) may cause negative impacts to human health and the environment because of the induced reactive oxygen species. We expect that PFRs could be generated in the condensable volatiles formed during lignocellulose biomass pyrolysis. Elucidating the structural origin and the formation mechanism of PFRs is important for an in-depth understanding of air pollutants from the pyrolysis or combustion of lignocellulose biomass. This work selected rice straw and pine sawdust to represent agricultural and forest biomass residues. The pyrolysis mechanism, volatile components, and PFR generation were discussed based on the analysis of thermogravimetry-Fourier transform infrared spectroscopy-mass spectrometry (MS), pyrolysis-gas chromatography/MS, and electron spin resonance (ESR). Levoglucosan, furans, and 2-methoxyphenols were the main pyrolytic compounds for cellulose (CL), hemicellulose (HC), and lignin (LG), respectively. Obvious ESR signals were detected in the condensable volatiles of LG, while no ESR signals were detected for those of CL and HC. Higher ESR signals were detected in lignocellulose with a higher content of LG. Therefore, LG was the main structural basis to generate PFRs in lignocellulose condensable volatiles, mostly attributed to the methoxyphenol components. This study provides useful information regarding the generation mechanisms of and the structures related to PFRs, which is essential to understand the risks of lignocellulose pyrolytic volatiles.

Plasma gasification performances of various raw and torrefied biomass materials using different gasifying agents

Plasma gasification of raw and torrefied woody, non-woody, and algal biomass using three different gasifying agents (air, steam, and CO₂) is conducted through a thermodynamic analysis. The impacts of feedstock and reaction atmosphere on various performance indices such as syngas yield, pollutant emissions, plasma energy to syngas production ratio (PSR), and plasma gasification efficiency (PGE) are studied. Results show that CO₂ plasma gasification gives the lowest PSR, thereby leading to the highest PGE among the three reaction atmospheres. Torrefied biomass displays increased syngas yield and PGE, but is more likely to have a negative environmental impact of N/S pollutants in comparison with raw one, especially for rice straw. However, the exception is for torrefied grape marc and macroalgae which produce lower amounts of S-species under steam and CO₂ atmospheres. Overall, torrefied pine wood has the best performance for producing high quality syngas containing low impurities among the investigated feedstocks.

Gas-pressurized torrefaction of biomass wastes: Roles of pressure and secondary reactions

A gas-pressurized (GP) torrefaction method, proposed in our resent work, can significantly promote the upgrading and oxygen removal of biomass wastes, compared to the traditional torrefaction (AP). However, the mechanism of the GP torrefaction process is not clear. In present work, semi-closed (SC) torrefaction, GP torrefaction, and AP torrefaction were conducted to reveal the roles of pressure and secondary reactions during GP torrefaction quantitatively. The results showed that the pressure significantly promoted the upgrading of biomass during GP torrefaction at 200 °C. The contribution of pressure on the oxygen removal of GP torrefaction at 200 °C was 63.87%. At relatively high temperature of around 250 °C, the promotions were caused by the synergistic effect of pressure and secondary reactions. The contribution of secondary reactions on the oxygen removal was 53.99%. Thus, the process of the GP torrefaction of biomass wastes was preliminarily understood.

Biochar production and applications in agro and forestry systems: A review

Biochar is a product of biomass thermochemical conversion. Its yield and quality vary significantly with the production technology and process parameters, which also affect its performance in agro and forestry systems. In this review, biochar production technologies including slow pyrolysis, fast pyrolysis, gasification, and torrefaction were compared. The yield of biochar was found to decrease with faster heating rate or more oxygen available. The benefits of biochar application to agro and forestry systems were discussed. Improvements in soil health, plant growth, carbon sequestration, and greenhouse gas mitigation are apparent in many cases, but opposite results do exist, indicating that the beneficial aspect of biochar are limited to particular conditions such as the type of biochar used, the rate of application, soil type, climate, and crop species. Limitations of current studies and future research needed on biochar are also discussed. Specifically, the relationships among biochar production technologies, biochar properties, and biochar performance in agro and forestry systems must be better understood.

Biomass torrefaction: An overview on process parameters, economic and environmental aspects and recent advancements

Torrefaction is one of the pretreatment processes used to overcome the disadvantages of using biomass as a fuel such as low energy density, high moisture, and oxygen contents. The torrefaction increases energy density, hydrophobicity, and reduces grinding energy requirement of biomass. This paper provides a review of the recent advancements in the torrefaction process. The discussion will cover the environmental and economic aspects of the torrefaction process and torrefied pellets, and various applications of torrefaction products. The cost competitiveness of torrefied pellets is one of the major concern of the torrefaction process. Integrating the torrefaction with other processes makes it economically more viable than as a standalone process.

The effect of microwave drying pretreatment on dry torrefaction of agricultural biomasses

This paper examines the effect of microwave drying on biomass characteristics and subsequent dry pyrolysis and characteristics of produced biochar from rice straw, sugarcane bagasse, rice husk and cotton stalk compared to oven drying at 105 °C. Dried samples from both methods are torrefied at 250 and 300 °C with 30-minutes residence time. Drying time reached 60 times faster with microwave. The fast and violent microwave drying ruptured the biomasses' surface, releasing more volatiles and having lower crystallinity; these lowered the heating value, energy yield and elemental carbon compared to oven drying except for cotton stalk only due to its woody nature which reduced devolatilization. Sugarcane, rice husk and cotton stalk have the most promising values of elemental carbon, energy yield and heating value reaching that of the bituminous coal. Torrefied rice straw showed high crystallinity of 50.7% while sugarcane and rice husk were completely amorphous.