Torrefaction Temperature Research Articles

Effect of Biomass Particle Size on the Torrefaction Characteristics in a Fixed-Bed Reactor

The aim of this study is to investigate the influence of biomass particle size on the torrefaction characteristics under different torrefaction temperatures and times. Paulownia wood with particle sizes ranging from 12 to <0.3 mm was selected. It was torrefied at 260 and 290 °C in a fixed-bed reactor for 30–90 min. The results showed that biomass particle size did affect the product’s evolution during biomass torrefaction. With the decrease in particle size from 12 to <0.3 mm, the yield of the solid product decreased by 5.41 and 3.54 wt.%, the yield of the liquid product increased by 5.87 and 3.25 wt.%, and the yield of the gas product changed insignificantly, at 260 and 290 °C, respectively. Comparatively, torrefaction temperature had a more significant effect on the composition of gas products than particle size and torrefaction time. At lower temperatures, decarboxylation reactions dominated in the torrefaction process with more CO2 produced. However, at higher temperatures, decarbonylation reactions were significantly strengthened with more CO generated. The contents of CO2 and CO could account for more than 98 vol% of the product gas. The influence of particle size on the chemical composition of the solid product was much smaller than that of torrefaction temperature and time, but the energy yield of the solid product decreased with the decrease in particle size. The increase in torrefaction temperature and time could significantly increase the C content in the solid product while reducing its O content. It is recommended to use a relatively higher temperature (e.g., 290 °C) for the torrefaction of large particle biomass, as it could significantly reduce the impact of particle size on the torrefaction process and reduce the torrefaction time.

The influence of torrefaction temperature and reaction time on the properties of torrefied sun‐dried millet and sorghum straws from the arid and semi‐arid zones of western Africa

AbstractThere are increasing demands for syngas production. Fossil‐based fuels are expensive and environmentally unfriendly. Affordable, sustainable, and environmentally healthy alternatives and sustainable fuel sources are needed. This study focused on producing tor‐chars from two different tropical biomasses (from Nigeria, West Africa), which were torrefied and characterized to improve their properties for potential use as sustainable feedstocks in an entrained flow gasifier for syngas production. Torrefaction temperatures of 230, 240, 250, and 270 °C and residence times of 30 and 60 min were used to torrefy sundried sorghum straw (SS) and millet straw (MS) at particle size ≤5 mm. The properties of torrefied MS and SS improved significantly due to torrefaction. The optimum conditions for pretreatment of sundried MS and SS were 240 °C and 60 min, at which increased energy density significantly compensated for the severe mass loss and significant degradation of hemicellulose and cellulose. Under these conditions, the respective mass and energy yields were 53 wt% and 75% for millet straw and 49 wt% and 75% for sorghum straw. The O:C atomic ratio decreased to 0.3 in MS tor‐chars and 0.2 in SS tor‐chars resulting in higher heating values of 25 MJ kg−1 and 27 MJ kg−1, respectively. Under optimum conditions, the ash content increased by 107% to be approximately 7 wt% in MS tor‐char, and by 118% to about 7 wt% in SS tor‐char. These properties are promising for potential feedstocks in co‐firing plants or gasification systems. © 2022 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley & Sons Ltd.

Multi-objective operation optimization of spent coffee ground torrefaction for carbon–neutral biochar production

Many energy-intensive processes are employed to enhance biomass fuel properties to overcome the difficulties in utilizing biomass as fuel. Therefore, energy conservation during these processes is crucial for realizing a circular bioeconomy. This study develops a newly devised method to evaluate SCG biochars’ higher heating value (HHV) and predict moisture content from power consumption. It is found that the increasing rates of HHV immediately follow decreases in power consumption, which could be used to determine the pretreatment time for energy conservation. The non-dominated sorting genetic algorithm II (NSGA-II) maximizes SCG biochar’s HHV while minimizing energy consumption. The results show that producing SCG biochar with 23.98 MJ∙kg−1 HHV requires 20.042 MJ∙kg−1, using a torrefaction temperature of 244 °C and torrefaction time of 27 min and 43 sec. Every kilogram of biochar with an energy yield of 85.93 % is estimated to cost NT$ 12.21.

Effects of oxidative torrefaction on the physicochemical properties and pyrolysis products of hemicellulose in bamboo processing residues

Torrefaction pretreatment is applied to improve the fuel quality of biomass. However, in industrial production, oxygen can be unintentionally involved in the torrefaction process due to insufficient airtightness of the torrefaction reaction equipment or the presence of voids in the biomass particles. In this study, hemicellulose was extracted from the bamboo processing residue and was used as raw material for torrefaction experiment. We mainly studied the effects of oxidative torrefaction on physical and chemical properties of torrefied hemicellulose and the distribution of pyrolysis products under different oxygen concentrations and torrefaction temperatures. Results showed that a lower torrefaction temperature (200 °C) yielded solid products with higher HHV even when the oxygen concentration was increased to 15%. With a 5% oxygen concentration, a higher torrefaction temperature can apparently reduce the oxygen and hydrogen contents, and decrease atomic O/C and H/C ratios in torrefied hemicellulose. In terms of the pyrolysis products, at torrefaction temperature of 230 °C and 5% oxygen concentration, pyrolysis products had the lowest acid (7.85%) and the highest ester (6.32%) contents, indicating the highest quality of chemical compounds. A higher oxygen concentration of 5% was beneficial for the formation of alcohols in pyrolysis products. The highest content of furans (34.75%) in the pyrolysis products was achieved after torrefaction pretreatment at 260 °C and 5% oxygen concentration. The production of phenolics compounds favored lower temperature of 200 °C and 0% oxygen concentration in torrefaction pretreatment. This work provides a new avenue to improve the quality of pyrolysis products of biomass through oxidative torrefaction. Effects of oxidative torrefaction on the physicochemical properties and pyrolysis products distributions of hemicellulose derived from bamboo processing residues. • Effects of oxidative torrefaction on hemicellulose properties were investigated. • Hemicellulose underwent dehydrogenation, and deoxygenation, and less extent decarbonization during torrefaction. • Torrefaction pretreatment of hemicellulose at 230 °C and 5% oxygen concentration favored high-value pyrolysis products.

Elucidating on Time and Temperature Effects on Torrified Moldy Bread

Waste-to-energy is the preferred solution, according to the waste management hierarchy considering landfill waste disposal may not be the most effective method of waste usage. Torrefaction of kitchen waste to produce higher-quality solid fuels is an effective option with lower temperature requirements than pyrolysis and gasification. By addressing the problems, the fuel quality in terms of high heating value can be investigated. Also, the torrefaction parameters, temperature and time, can be examined on the fuel performance. The moldy bread undergoes torrefaction by torrefying it in the furnace with temperatures of 200, 250 and 300°C, respectively, with 15, 30, 45 and 60 mins of processing times. With increased torrefaction temperature, the mass dropped while the higher heating value (HHV) increased. The rise in carbon content also enhanced the torrefied moldy bread's fuel properties. Also, this is because the primary components of the moldy bread, particularly hemicellulose, have significantly decomposed. Therefore, processed temperature of 300°C at elevation time of 45 min produced tremendous gain than other parameters observed.

Influence of Torrefaction on Sewage Sludge

Sewage sludge (SS) from sewage treatment plants has been seen as a waste for decades and little attention to investigate the potential and its beneficial product. SS contain a large amount of recoverable energy; however, it has high moisture, ash, heavy metals, and organic contents. Thus, the goal of this research is to use torrefaction to produce upgraded or torrefied SS. The proximate and ultimate analysis was used to determine the physical and chemical characteristic of raw SS. Torrefaction was performed through inert atmosphere in fixed bed reactor. According to the findings, at torrefaction temperature of 300 °C, torrefaction had improved the quality of raw SS, increasing its fixed carbon content by 47.8%, and its moisture content by 61.3%. The optimum temperature for torrefaction was at 300 °C with holding time of 60 minutes. This study improved the quality of sewage sludge to be utilised in subsequent thermochemical conversion.

Influence of pretreated coconut shell on gasification product yield

Gasification of untreated and pretreated coconut shell (CS) was carried out in a fixed-bed reactor to assess the effect of temperature (600, 650, 700, 750, and 800 °C) and holding time (30 and 40 min) on gases composition. The untreated CS was first torrefied in a fixed-bed reactor at different temperatures (200 – 300 °C) and holding times (30 min, 60 min and 90 min). Pretreated CS at the optimal torrefaction temperature (275 °C and 60 min) was used for gasification. Under optimal conditions of 750 °C and 30 min holding time, gasification contributed the most gas production. At this optimum condition, the gas composition of pretreated CS was 35.03 % of CH4, 24.43 % of CO2, and 40.54 % of H2 + CO. Untreated CS contains 37.63 % of CH4, 24.03 % of CO2, and 38.34 % of H2 + CO gases. The production of CH4 gas was higher when untreated CS was used for gasification rather than pretreated CS. Moreover, when untreated CS was used for gasification, the amount of CO2, H2, and CO produced was minimal. Therefore, for high H2 production, pretreatment prior to gasification is appropriate.

Performance Evaluation of Torrefaction Coupled with a Chemical Looping Gasification Process under Autothermal Conditions: Flexible Syngas Production from Biomass

Herein, a coupled biomass torrefaction and chemical looping gasification (BTCLG) process was proposed, and process simulations were performed under autothermal conditions. The effects of operational parameters on product distribution with torrefaction temperatures from 240 to 300 °C, gasification temperatures from 700 to 880 °C, and steam/biomass (S/B) ratios from 0.5 to 1.5 were explored. The results showed that the process could stably operate under autothermal conditions. Torrefaction increased the syngas yield, heating value, and cold gas efficiency by 12.44, 5.11, and 36.73%, respectively. Moreover, it reduced the bed material circulation rate required for autothermal operation by 59.03%. At the torrefaction temperature of 240 °C, the syngas yield reached a maximum value of 0.78 Nm3/kg. Increases in the gasification temperature also positively impacted syngas production. However, the optimal gasification temperature was 810 °C based on the bed material circulation rate constraint. The syngas H2/CO ratio could be flexibly adjusted from 1.62 to 3.34 by changing the S/B ratio to meet downstream demands. Overall, the simulation results support the technical viability and demonstrate the excellent performance of the BTCLG process compared to the biomass CLG process.

Properties of Biochar Derived from Tea Waste as an Alternative Fuel and Its Effect on Phytotoxicity of Seed Germination for Soil Applications.

Tea waste as a potential biofuel and bio fertilizer was analyzed. Samples were collected from various tea species and torrefied to five different temperatures. All samples were analyzed for their proximal composition and calorific value. From the results, stoichiometric properties were calculated. A phytotoxicity test was performed, and the germination index was measured. Tea waste torrefied at 350 °C may be suitable biofuel reaching the calorific value of 25-27 MJ kg-1, but with quite a high share of ash, up to 10%, which makes its use technically challenging and may lead to operating issues in a combustion chamber. The same biochar may be a suitable fertilizer for increasing the germination index, therefore, applicable to the soil. The non-torrefied sample and the sample treated at 250 °C are not suitable as fertilizers for being toxic. The total phenolic content in waste black tea was reduced from 41.26 to 0.21 mg g-1, depending on the torrefaction temperature. The total flavonoid content was also reduced from 60.49 to 0.5 mg g-1. The total antioxidant activity in the non-torrefied sample was 144 mg g-1, and after torrefaction at 550 °C, it was 0.82 mg g-1. The results showed that black tea waste residues have the potential for further use, for example, in agriculture as a soil amendment or as a potential biofuel.

Evaluation of Pyrolysis Reactivity, Kinetics, and Gasification Reactivity of Corn Cobs after Torrefaction Pretreatment

To reveal the effect of torrefaction pretreatment on pyrolysis and gasification reactivity of biomass, corn cob was first subjected to torrefaction pretreatment in a fixed-bed reactor at various reaction temperatures. The pyrolysis reactivity, kinetics, and gasification reactivity of torrefied corn cob were systematically assessed by various methods, proving that torrefaction pretreatment has a substantial influence on the physicochemical properties of corn cobs. The O/C and H/C molar ratios of corn cobs considerably drop with the increasing torrefaction temperature, and their higher heat-ing value (HHV) and energy density rise as well. It is found that torrefaction improves the pyrolysis reactivity of corn cobs because hemicellulose degradation is more severe than cellulose degradation during torrefaction, resulting in an increase in the percentage of cellulose in torrefied corn cobs. However, the severe depolymerization, polycondensation, and carbonization reaction during torre-faction of corn cobs at 280–300 °C can lead to a significant decline in the pyrolysis reactivity of corn cobs. Torrefaction pretreatment increases the pyrolysis activation energy of corn cobs, in addition to decreasing the char gasification reactivity of corn cob. The average char gasification reactivity of corn cobs drops when torrefaction severity increases. The passivation of active sites on the char surface may cause condensation and carbonation reactions of corn cobs during torrefaction. These findings provide new sights into the reasonable design of efficient torrefaction methods for appli-cation prior to pyrolysis and gasification of biomass.

The combined effect of solar assisted torrefaction and pyrolysis on the production of valuable chemicals obtained from water hyacinth biomass

In this study, both the torrefaction and pyrolysis of water hyacinth biomass samples were accomplished by using concentrated solar radiation. The first attempt was to prepare torrefied samples and then used those in the following pyrolysis process. The aim of preparing pretreated torrefied samples was to reduce moisture from the feedstock and make it perfect for the pyrolysis process besides improving product quality. Three different torrefied samples were prepared by pretreating at 200, 250, and 300℃. The pyrolysis operation was done by maintaining a reactor temperature of 440±10℃. Torrefaction severity had a greater influence on char yields. The char yield was found to be 27.4 wt.% for the non-torrefied sample (ST-Raw) while 42.3, 51.2, and 59.4 wt.% for torrefied samples of ST-200, ST-250, and ST-300 respectively. However, an increase in torrefaction temperature had negative consequences on bio-oil and syngas yields. The acidity of the bio-oils was greatly reduced with an increase in torrefaction temperature. The lowest acid content in ST-300 bio-oil sample was confirmed by the FTIR, and GC-MS analysis. This ensures that the torrefied bio-oils are more non-corrosive than non-torrefied bio-oils. The heating values of ST-300 bio-oils were found to be about 75% of diesel fuel. The char obtained in the pyrolysis process for all torrefied samples was of high quality and graded as peat or brown coal. Considering the characteristics of biomass, products, and method used in this study, solar pyrolysis is a promising technique to produce high-quality chemical intermediates. • The combined effect of solar torrefaction and pyrolysis on the products was investigated. • Torrefaction severity significantly increased char (27.4 to 59.4 wt.%) content. • Torrefaction severity improved the product's heating values. • Torrefaction reduced the acidity of bio-oils obtained in solar pyrolysis.

Enhancing the potential of sugarcane bagasse for the production of ENplus quality fuel pellets by torrefaction: an economic feasibility study

When fossil fuel substitution with biomass is viewed as a potential solution to global warming caused by greenhouse gas emissions, the demand for biomass fuel pellets has increased worldwide. Although agricultural waste is an attractive potential feedstock for fuel pellet production due to its relatively high calorific value and low cost, its excessive ash content is a major drawback. This research investigates the properties of sugarcane bagasse fuel pellets treated by dry and wet torrefaction and evaluates the economic value of selling the fuel pellets, which were priced based on their quality. It was found that the wet torrefaction could significantly reduce the ash content in the product (1% ash content at a torrefaction temperature of above 180°C), resulting in higher quality and more marketable fuel pellets. Consequently, the yield and the net present value of the production of wet torrefied fuel pellets were greater than those of dry torrefied pellets. Nevertheless, the production of fuel pellets from sugarcane bagasse treated by either process is shown to be economically viable.

Improving Lignocellulosic and Non-Lignocellulosic Biomass Characteristics through Torrefaction Process

In this study, three locally available biomasses, namely miscanthus, hops, sewage sludge, and additionally, their mixtures, were subjected to the torrefaction process to improve their fuel properties. The torrefaction process was conducted at 250–350 °C and 10–60 min in a nitrogen (N2) environment. The torrefaction temperature and time were studied to evaluate the selected biomass materials; furthermore, heating values, mass and energy yields, enhancement factors, torrefaction severity indexes (TSI), and energy-mass co-benefit indexes (EMCI) were calculated. In addition, thermogravimetric (TGA) and Fourier transform infrared analyses (FTIR) were performed to characterize raw and torrefied biomass under the most stringent conditions (350 °C and 60 min). The results showed that with increasing torrefaction temperature and duration, mass and energy yields decreased, and heating values (HHVs) increased for all studied biomasses. The results of the TSI and EMCI indexes showed that the optimum torrefaction conditions were as follows: 260 °C and 10 min for pure miscanthus and hops, whilst this could not be confirmed for the sewage sludge. Furthermore, the combination of sewage sludge and the above-mentioned types of lignocellulosic biomass exhibited better fuel properties than sewage sludge alone.

Impact of salt composition and temperature on low-temperature torrefaction of pine in molten nitrate salts

An experimental investigation of torrefaction of pine in molten, nitrate salt blends was conducted. Five blends, consisting of varying amounts of LiNO3 (20–40 wt%), NaNO3 (0–20 wt%), and KNO3 (50–70 wt%), and torrefaction temperatures from 210 to 240 °C were investigated and compared to torrefaction in nitrogen. The effect of temperature and salt blend composition on the appearance, higher heating value (HHV), proximate analysis (volatiles, fixed carbon, and ash), and mass yields are investigated. The results show that the molten salts effectively catalyze the reaction. At low temperatures, there is significant increase in the fixed carbon content and HHV compared to the raw pine. For example, at 225 °C and depending on the salt composition, the fixed carbon content increases by 51–166% and the HHV by 6–20% relative to raw. Compared to nitrogen torrefaction at equivalent torrefaction temperatures, salt-torrefied pine is more severely torrefied and is more sensitive to temperature. LiNO3 has the strongest the catalytic effect. Blends with higher LiNO3 content result in more severely torrefied biomass at a given temperature than other blends regardless of changes in sodium and potassium content. NaNO3 content is the next most influential component salt followed by KNO3.

In-situ measurement of combustion characteristics and potassium release concentration during torrefied biomass burning based on spontaneous emission spectroscopy

The orthogonal experiment designed in this paper investigated the effect of changes in experimental parameters such as biomass species, torrefaction temperature, torrefaction time, torrefaction oxygen concentration on the physicochemical properties and combustion characteristics of the torrefaction products. Two optical fiber spectrometers were used to capture the spontaneous emission spectra of the combustion flame of compressed pellets of torrefied biomass, and the in-situ measurement of pellet combustion temperature and K release concentration at high heating rate was realized by multi-spectral analysis algorithm. The experimental results indicate that for biomass, torrefaction will lead to a decrease of volatile and moisture content and an increase of fixed carbon and ash content in components, a decrease of O and H and an increase of C content in elements, a certain loss of biomass quality, an increase of high heat value, and each indicator move closer to coal. The effects of various experimental parameters on the volatile content, mass yield, high heat value increase rate and energy yield in biomass torrefaction products have similar laws, i.e., biomass species / torrefaction oxygen concentration > torrefaction temperature > torrefaction time. The mild torrefaction product pellets have a shorter ignition delay, increased volatile combustion flame height and peak temperature, longer volatile and char combustion duration, but lower peak char combustion temperature, among which the selected torrefaction products of rice straw and pine wood showed an increase in peak temperature of 0.56 % (11 K) and 0.61 % (12 K) for the volatile stage respectively, and a decrease of and 1.29 % (20 K) and 1.29 % (21 K) for the char stage respectively. In addition, the peak K release concentration during combustion of the mild torrefaction product pellets was reduced, among which the peak K release concentrations during the volatile and char stage were reduced by 43.22 % (3.89 ppm) and 52.93 % (51.55 ppm) of the rice straw torrefaction product respectively, and by 73.14 % (1.28 ppm) and 62.08 % (15.01 ppm) for the pine wood torrefaction product respectively. Biomass torrefaction products have fuel properties closer to coal, less K released from the combustion process, which have the potential to become an alternative to coal.

Improvement of wheat (T. aestivum) straw catalytic fast pyrolysis for valuable chemicals production by coupling pretreatment of acid washing and torrefaction

The present work investigated the coupling pretreatment of acid washing and torrefaction to address secondary cracking of target products and condensation of intermediates and improved bio-oil quality and hydrocarbon selectivity of the catalytic fast pyrolysis (CFP) process. The total hydrocarbon yields in CFP increased from 28.06 wt% to 55.31 wt% by torrefaction and 46.29 wt% by coupling pretreatment. The acid washing resulted in fewer acids and phenols and more sugars in bio-oil by decreasing the AAEMs. The removal rate of the potassium was up to 96.7 %, and other elements were above 83 %. Torrefaction elevated the C/O ratio and the higher calorific value of biomass. It could also decrease the acid content in obtained bio-oil in the CFP process. In addition, the TG/TGA results showed that higher torrefaction temperature decreased the total weight loss of biomass. Meanwhile, the acid washing pretreatment improved the resolution of the two pyrolysis peaks of the DTG curve. These results showed that the AAEMs removal and intermediates inhibition could elevate the selectivity of the valuable chemical. • Acid washing removed 83 wt% of AAEMs to facilitate valuable chemicals conversion; • Torrefaction elevated the C/O ratio and the higher calorific value of wheat straw; • The coupling pretreatment improved bio-oil quality and hydrocarbons selection; • The CFP with pretreated biomass reduced phenols, acids, and ketones production.

Influence of Temperature and Residence Time on Torrefaction Coupled to Fast Pyrolysis for Valorizing Agricultural Waste

Torrefaction is a promising pretreatment technology for valorizing biomass and upgrading pyrolysis products. This study sets out an original procedure consisting of subjecting the biomass to torrefaction before fast pyrolysis to increased value-added compounds based on agricultural waste biomasses production. This study uses a combined biomass treatment consisting of torrefaction (280–320 °C) and subsequent fast pyrolysis (500 °C) using the same reactor. Under different torrefaction temperatures and residence times, olive pomace (OP) and almond shell (AS) have been evaluated. The study demonstrated OP rather than AS was thermally unstable. The highest total yield of carboxylic acids (mainly acetic acid) was obtained by means of torrefaction at 280 °C with a residence time of 20 s for OP, and at 300 °C and 20 s for AS. Higher torrefaction temperature and residence time promoted phenolic compounds production for OP. However, OP had a higher lignin content and inherent metals that promoted a catalytic reaction during the procedure. The highest yield (47.7%) was obtained using torrefaction at 320 °C with a residence time of 240 s. Overall, the torrefaction of biomass combined with fast pyrolysis constituted a very simple and efficient strategy for valorizing the conversion of agricultural waste biomass into value-added chemicals.

The effect of torrefaction temperature and catalyst loading in Microwave-Assisted in-situ catalytic Co-Pyrolysis of torrefied biomass and plastic wastes

In the current study, the effect of torrefaction temperatures (125–175 °C) and catalyst quantity (5–15 g) on co-pyrolysis of torrefied sawdust (TSD) and polystyrene (PS) are investigated to obtain value-added products. The role of torrefaction in co-pyrolysis of TSD: PS was analyzed to understand the product yields, synergy, and energy consumption . As the torrefaction temperature increases, oil yield (48.3–59.6 wt%) and char yield (24.3–29 wt%) increase while gas yield (27.4–11.4 wt%) decreases. Catalytic co-pyrolysis showed a significant level of synergy when compared to non-catalytic co-pyrolysis. For the conversion (%), a positive synergy maximum (-2.6) exists at a torrefaction temperature of 175 °C and 15 g of KOH catalyst. To develop the model, polynomial regression-based machine learning was used to predict pyrolysisproduct yields and energy usage variables. The developed models showed significant prediction accuracy (R2 > 0.98), suggesting the experimental values and the predicted values matched well.

Impact of Biochar Addition in Microwave Torrefaction of Camelina Straw and Switchgrass for Biofuel Production

The possibility of applying biochar in mild torrefaction treatment to improve the thermochemical characteristics of ground biomass was the focus of the study. Camelina straw and switchgrass were torrefied in a reactor using microwave irradiation at torrefaction temperatures of 250 °C and 300 °C with residence times 10, 15 and 20 min, under nitrogen-activated inert conditions. Both biochar addition of more than 10% and residence time significantly affected the product yields, as MW torrefaction temperatures shifted from 250 °C to 300 °C. Overall, the results indicated a slight increase in ash content, mass loss percentage intensification, heating values, and fixed carbon, while moisture content and volatile matter decreased in camelina straw and switchgrass, with or without biochar. Biochar addition with a long residence time (20 min) at 250 °C reduced energy requirement during the microwave torrefaction process. The combustion index values showed that torrefied camelina straw or switchgrass with biochar addition suits co-combustion with coal in a coal-fired plant and is a potential biomaterial for biofuel pellets.

In situ formation of fatty acid methyl esters via thermally assisted methylation by lignin during torrefaction of oil palm biomass

Torrefaction of oil palm empty fruit bunch (EFB) biomass was performed at six temperatures between 210 and 360 °C. The composition of the evolved and trapped condensates was analyzed by GC/MS and the torrefied biomass samples were characterized by pyrolysis-GC/MS (Py-GC/MS). The analyses showed that the thermal treatment of the biomass produced significant amounts of fatty acid methyl esters (FAMEs). The in situ FAME formation was explained by the reaction of the lipid and lignin contents of the biomass; i.e., the fatty acid moieties of triglycerides were methylated by the methoxy side groups of lignin during the thermal treatment. The reaction was verified by the pyrolysis of a model mixture, using Py-GC/MS and thermogravimetry/mass spectrometry, which consisted of the EFB lipid extract and an isolated beech wood lignin. Demethylation of lignin resulted changes in its thermal decomposition; smaller amounts of 4-methylguaiacol and 4-methylsyringol were evolved and the char yield was increased. The GC/MS analysis of the EFB condensates showed that FAME conversion was the highest and the sources of fatty acid moieties were depleted if the torrefaction temperature had been set at and above 330 °C. The free fatty acid content of EFB was relatively high, which evaporated at lower temperatures before the rate of FAME formation increased between 270 and 300 °C. In comparison with torrefaction, only a small portion of FAMEs formed during the flash pyrolysis of raw EFB at 450 °C indicating that the prolonged heating of the oily biomass favors the FAME formation. Pyrolysis-GC/MS measurements of the torrefied samples showed that the triglyceride residue could also be utilized if the torrefaction as pretreatment was performed at or below 300 °C. In addition, torrefaction increased the accessibility of the biomass components for FAME formation. Some other fatty acid derivatives (i.e., ketone, hydrocarbon, amide, and nitrile) were also detected in smaller amounts. The phenomenon of in situ FAME formation during pyrolysis might be a useful process to enhance pyrolytic biomass utilization by producing FAMEs as additional fuel products and increasing the char yield simultaneously.