Torrefaction Process Research Articles

Determination of the marker for automation of torrefaction and slow pyrolysis processes – A case study of spherical wood particles

Abstract Torrefaction and slow pyrolysis are processes that can be used for improving the fuel properties of low-quality biomass, which could be effectively used for improving fuel logistics and making biomass a fully tradeable commodity. Automation of these processes could prove vital for their economic viability and carbon footprint, giving the possibility to reduce manpower requirement and raining time for operators. Automation based on online control of the quality of biocoal is not possible. However, finding a suitable marker in torgas could make automation possible. Results presented in the study show that formaldehyde can be considered as a viable marker. Obtained results are limited to torrefaction and slow pyrolysis of lignocellulosic biomass at a maximum reactor temperature of 430 °C. Temperature measurements show that endothermal evaporation of the bound moisture delays the time necessary to reach the point where exothermal reactions start taking a leading role in the thermal decomposition process, which is extremely important in the context of thermal run-away of such installation. Artificial neural network proved to be an effective tool for determining of correlation between the heating value of pyrolysis gas and its concentration of formaldehyde. This could be used in advanced control solutions for pyrolysis gas combustion systems.

Torrefied Biomass as an Alternative in Coal-Fueled Power Plants: A Case Study on Grindability of Agroforestry Waste Forms

The use of biomass as a renewable energy source is currently a reality, mainly due to the role it can play in replacing fossil energy sources. Within this possibility, coal substitution in the production of electric energy presents itself as a strong alternative with high potential, mostly due to the possibility of contributing to the decarbonization of energy production while, at the same time, contributing to the circularization of energy generation processes. This can be achieved through the use of biomass waste forms, which have undergone a process of improving their properties, such as torrefaction. However, for this to be viable, it is necessary that the biomass has a set of characteristics similar to those of coal, such that its use may occur in previously installed systems. In particular, with respect to grindability, which is associated with one of the core equipment technologies of coal-fired power plants—the coal mill. The objective of the present study is to determine the potential of certain residues with agroforestry origins as a replacement for coal in power generation by using empirical methods. Selected materials—namely, almond shells, kiwifruit pruning, vine pruning, olive pomace, pine woodchips, and eucalyptus woodchips—are characterized in this regard. The materials were characterized in the laboratory and submitted to a torrefaction process at 300 °C. Then, the Statistical Grindability Index and the Hardgrove Grindability Index were determined, using empirical methods derived from coal analysis. The results obtained indicate the good potential of the studied biomasses for use in large-scale torrefaction processes and as replacements for coal in the generation of electrical energy. However, further tests are still needed, particularly relating to the definition of the ideal parameters of the torrefaction process, in order to optimize the grindability of the materials.

The effect of particle size, temperature and residence time on the yields and reactivity of olive stones from torrefaction

Abstract Olive stones obtained as a by-product from olive oil extraction in combination with the favourable climate in Mediterranean countries are value-added feedstocks for the energy sector due to low moisture content ( 20 wt. %), suitable calorific value ( > 18.7 MJ kg−1 as received) and high bulk density (about 750 kg m−3). The torrefaction process at Arigna Fuels with high energy efficiency of (above 90%) improves biomass properties for conversion to a high-value fuel for use in solid fuel stoves. This study reports the effect of moisture content, organic composition, inorganic matter, particle size, heat treatment temperature and residence time on product yields, O2/CO2 reactivity, calorific value, composition and thermal conductivity value of torrefied olive stones. Results showed that both lignocellulosic content and ash composition equally influenced the reactivity of torrefied material. For the first time, time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that the structure of torrefied material from small olive stone particles contains more cellulose than lignin when compared to large grains. Importantly from a technological standpoint, the lower heating values of torrefied olive stones (21.8 MJ kg−1) [1] from a small scale reactor were within the range of values for torrefied woodchip briquettes containing high starch binder content which was an energy increase of ≈ 15% when compared to the raw feedstock. The results showed that olive stones of particle size ≤ 2 mm produced during torrefaction at 270 °C for 30 min are the most suitable material and conditions for briquetting due to high solid yield, low reactivity and low thermal conductivity values. These conditions are recommended for the pilot plant operation using olive stones from the Mediterranean region.

Investigation of kinetic compensation effect in lignocellulosic biomass torrefaction: Kinetic and thermodynamic analyses

Abstract The kinetic compensation effect between the activation energy and the pre-exponential factor has extensively existed in the thermochemical conversion processes of lignocellulosic biomass. The research on the kinetic compensation effect in lignocellulosic biomass torrefaction has been insufficient yet. The torrefaction of the pinewood sample was experimentally investigated by thermogravimetric analysis (TGA) at five isothermal temperatures of 220, 250, 265, 280 and 295 °C. The reaction order model was used to analyze the isothermal torrefaction kinetics of lignocellulosic biomass, and the results showed that many sets of activation energy and pre-exponential factor could describe the experimental data at each temperature equally well and they excellently satisfied the kinetic compensation effect relationship. The linear regression lines of the kinetic compensation effect points at different temperatures intersected at one point, whose values corresponded to the obtained optimal kinetic parameters. A kinetic-compensation-effect-based method was developed and verified to determine the kinetic parameters of isothermal biomass torrefaction. Based on the optimal kinetic parameters, the thermodynamic parameters (including Gibbs free energy, enthalpy, and entropy) of biomass torrefaction processes at various temperatures were calculated and analyzed.

Thermal Effects during Biomass Torrefaction

A study of the thermal effects observed during the low-temperature pyrolysis of various types of pelletized biomass (sunflower husk, coniferous wood, litter-litter mass) is presented. It is shown that during sunflower husk torrefaction the greatest exothermic effect is observed and the possible reasons for its occurrence are explained. The influence of a small oxygen content in the heating medium on the exothermic effects during biomass torrefaction is analyzed. It has been suggested that the use of internal energy resources of the process (waste heat of exhaust gases and internal energy released in the process of biomass torrefaction) significantly increases the torrefaction process energy efficiency and allows to consider secondary biomass as a real competitor to traditional fuels, a substitute for fossil coal.

Assessing the Effectiveness of the Hydrothermal Carbonization Method to Produce Bio-Coal from Wet Organic Wastes

The study is devoted to the assessment of the thermodynamic efficiency of the hydrothermal carbonization (HTC) method to produce bio-coal from wet organic waste (biomass). The HTC method is a process of thermochemical conversion of biomass into solid biofuel (bio-coal) as a result of its heating up to 150–280°С in the presence of water. The competitive advantages of the HTC method and the properties of bio-coal produced by this method from various organic waste are analyzed. The energy consumption values for implementation of the HTC process and the conventional torrefaction method are compared. It is demonstrated that the energy consumption in the production of bio-coal by the HTC method is several times lower than in the torrefaction process since water heated at a high pressure remains a liquid. This, in turn, offers a high potential for recovery of thermal energy (approximately 87%) in the HTC process. The heating value of the produce bio-coal is as high as 27 MJ/kg, which is comparable with that of commercial power coals. An analysis of the properties of biochar produced in the HTC process suggests that, in principle, it may be burned at coal-fired power stations together with conventional coal (if proper gas treatment equipment is available). The HTC method can be used for pretreatment of water-rich organic wastes and biomass to produce a quality fuel and a water solution that can be used as a base for production of fertilizers.

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.

Upgrading Bagasse Quality by Torrefaction for a Biomass Power Plant

Torrefaction process was employed to upgrade bagasse as a better fuel compared with raw bagasse in a biomass power plant. Torrefaction is a process that unnecessary organic and inorganic matters are removed from the biomass structure. The remaining solid components can then be used as fuel. This preliminary work was a set of experimental investigations of bagasse torrefaction. The process temperatures were varied as 250, 280 and 320 °C during a fixed reaction time of 30 minutes. Moisture content, mass and heating value of untreated bagasse (before) and torrified bagasse (after) were determined and compared for the 3 different temperatures. Then, energy yield for each case was calculated based on the experimental results to find the optimal condition of bagasse torrefaction. At 320 °C, moisture content was reduced by 92.4% and the torrified bagasse has 4 times higher heating value compared to raw bagasse. While at 250 °C, the reduction of moisture was only 79.7% with approximately 2 times higher in heating value. It was found that the higher reaction temperature was, the better physical structure and higher heating value were. However, higher temperature resulted in lower remaining mass and less energy yield. Therefore, the optimum case was selected based on the energy yield. It was shown that the highest energy yield of 176% was obtained in the case of 250 °C. On the other hand, for the case of 320 °C, energy yield was only 135%. It was thus evident that bagasse should be upgraded by torrefaction process with a temperature as low as 250 °C for a better energy yield rather than better heating value.

Isothermal torrefaction kinetics for sewage sludge pretreatment

Torrefaction of sewage sludge, a by-product from waste water treatment plants, has been receiving increased attentions in terms of sustainable sludge treatment, energy recovery and possibility to mitigate the environmental impacts. Torrefied sewage sludge is recognized as a better solid fuel than the dry sewage sludge, and it is also beneficial for subsequent thermochemical processes. In this study, sewage sludge was subjected to different torrefaction conditions in a thermogravimetric analyzer. The thermogravimetric data were then simulated using a two-consecutive reaction model to reproduce the mass losses during torrefaction. The obtained results from the torrefaction of the non-lignocellulosic biomass are also compared with other lignocellulosic biomass including a softwood and a hardwood. The findings from this study reveal that the non-lignocellulosic biomass is less thermally resistant and degrades much faster than the lignocellulosic biomass during the first stage of torrefaction. Moreover, the solid yield from the sewage sludge torrefaction at temperatures less than 280 °C is lower than that from the woody biomass torrefaction. However, this trend is inverted when the torrefaction temperature attains 280 °C and higher. The results obtained are useful for new design or retrofit of the sewage sludge torrefaction processes.

Gas-pressurized torrefaction of biomass wastes: The effect of varied pressure on pyrolysis kinetics and mechanism of torrefied biomass

A novel gas-pressurized (GP) torrefaction of biomass waste was proposed in our recent work to deeply upgrade the biomass under the pressure of 5 MPa prior to their subsequent use in thermo-conversion technologies such as pyrolysis, combustion and gasification. However, the effect of varying pressure on the GP torrefaction process and the thermochemical behaviour of the torrefied biomass were not clear. In this work, the effect of varying torrefaction pressure on the pyrolysis kinetics and mechanism of GP torrefied biomass was studied in detail. It was revealed that the activation energy (Ea) of the GP torrefied biomass ranged from 195 to 198 kJ mol−1 increasing with torrefaction pressure, and their activation energy distributions were invariably wider compared to raw and torrefied biomass produced by atmospheric pressure torrefaction. The carbon and fixed carbon content, aromaticity, and crystallinity of the GP torrefied biomass were promoted by increasing the torrefaction pressure. On the other hand, the porosity of the GP torrefied biomass was reduced by elevated pressure. These are the reasons for the Ea increase of the GP torrefied biomass under elevated pressure. The presented kinetics and mechanism study provide a deep understanding of the novel GP torrefaction method.

Evaluation of hydrogen production with iron-based chemical looping fed by different biomass

In this study, the iron-based chemical looping process driven by various biomasses for hydrogen production purposes is studied and evaluated thermodynamically through energy and exergy approaches. The overall system consists of some key units (combustor, reducers and oxidizer) a torrefier, a drying chamber, an air separation unit, a heat exchanger, and auxiliary units as well. The biomasses considered are first dried and torrified in the drying chamber and sent to reactors to produce hydrogen. The exergy and energy efficiencies of the iron based chemical looping facility are investigated comparatively for performance evaluation. The maximum exergy destruction and entropy production rates are calculated for the torrefaction process as 123.15 MW and 4926 kW/K respectively. Under the steady–state conditions, a total of 8 kg/s hydrogen is produced via chemical looping process. The highest energy efficiency is obtained in the looping of rice husk with 86% while the highest exergy efficiency is obtained in the looping using sugarcane bagasse with 91%, respectively.

Co-processing textile sludge and lignocellulose biowaste for biofuel production through microwave-assisted wet torrefaction

Microwave-assisted wet torrefaction was adopted in this study for co-processing textile sludge (TS) and biowaste. Key operating parameters, including different temperatures (120–180 °C) and reaction times (10–30 min), for the torrefaction of legally recyclable TS (R-0906 in Taiwan) were evaluated. Furthermore, TS was blended with six widely produced types of lignocellulose biowaste at different blending ratios for improving the biochar quality and combustion characteristics. The results indicated that the optimal temperature and reaction time for TS torrefaction were 180 °C and 20 min, respectively, beyond which the fixed carbon content and higher heating values (HHVs) of biochar increased from 1.7% to 5.1% and 13.0–18.7 MJ/kg, respectively. However, biochar produced through the torrefaction of TS and macadamia husk with a blending ratio of 30/70 (db%) exhibited the highest fixed carbon content (29.8%) and HHV (19.7 MJ/kg) among the six tested lignocellulose biowaste samples with the atomic H/C and O/C ratios close to those of peat. The thermal stability, activation energy, frequency factor, and ignition and combustion temperatures of the biochar produced through microwave-assisted wet torrefaction co-processing were higher than those produced using raw TS or biowaste. The energy return on investment of the biochar system reached 10.9. Greenhouse gas emissions can be decreased by 57.5% if biochar is used as a bituminous coal substitute. Therefore, microwave-assisted wet torrefaction is a sustainable approach for mitigating the disposal problems of TS and lignocellulose biowaste as well as improving the energy performance and environmental friendliness of biochar-to-energy systems. Besides, the protein and carbohydrate contents in the supernatant of TS co-torrefied with biowaste at different blending ratios increased by 1.0–4.1 and 1.2–5.9 times, respectively. These contents can be recycled back to the activated sludge unit as a nutrient source. Overall, the microwave-assisted wet torrefaction process is green and economically beneficial.

Alternative Fuels from Forestry Biomass Residue: Torrefaction Process of Horse Chestnuts, Oak Acorns, and Spruce Cones

The global energy system needs new, environmentally friendly, alternative fuels. Biomass is a good source of energy with global potential. Forestry biomass (especially wood, bark, or trees fruit) can be used in the energy process. However, the direct use of raw biomass in the combustion process (heating or electricity generation) is not recommended due to its unstable and low energetic properties. Raw biomass is characterized by high moisture content, low heating value, and hydrophilic propensities. The initial thermal processing and valorization of biomass improves its properties. One of these processes is torrefaction. In this study, forestry biomass residues such as horse chestnuts, oak acorns, and spruce cones were investigated. The torrefaction process was carried out in temperatures ranging from 200 °C to 320 °C in a non-oxidative atmosphere. The raw and torrefied materials were subjected to a wide range of tests including proximate analysis, fixed carbon content, hydrophobicity, density, and energy yield. The analyses indicated that the torrefaction process improves the fuel properties of horse chestnuts, oak acorns, and spruce cones. The properties of torrefied biomass at 320 °C were very similar to hard coal. In the case of horse chestnuts, an increase in fixed carbon content from 18.1% to 44.7%, and a decrease in volatiles from 82.9% to 59.8% were determined. Additionally, torrefied materials were characterized by their hydrophobic properties. In terms of energy yield, the highest value was achieved for oak acorns torrefied at 280 °C and amounted to 1.25. Moreover, higher heating value for the investigated forestry fruit residues ranged from 24.5 MJ·kg−1 to almost 27.0 MJ·kg−1 (at a torrefaction temperature of 320 °C).

Experimental Study on Fast Pyrolysis of Raw and Torrefied Woody Biomass

Herein, the conversion times of biomass pyrolysis are measured at temperatures between 450 and 550 °C and particle size fractions between 0 and 400 μm using a cyclonic thermogravimetric analyzer (TGA) device. Beech, spruce, and ash wood are investigated, as well as torrefied samples. The conversion time decreases with the increasing temperature from 7 to less than 1 s. The influence of the type of biomass on the conversion time is limited and only significant for low temperatures, when kinetics dominates the conversion process. A clear influence of the torrefaction process on the pyrolysis conversion time is noted and can be explained by the modified porosity of the biomass particles during the torrefaction process. Kinetic data are extracted from the conversion times, and a model is developed for a better interpretation of the measured results. The model shows good comparison with the experimental data and may also be used for design purposes of fast pyrolysis reactors.

Biomass Torrefaction for the Production of High-Grade Solid Biofuels: a Review

Torrefaction of biomass materials has received a tremendous attention over the years due to its ability to produce a high-grade solid biofuel with enhanced durability, excellent grindability, higher bulk density and calorific value, and greater energy density, as compared to the original untreated biomass material. It is a mild pyrolysis treatment technology under inert atmosphere which can improve the chemical and physical properties of raw biomass through the elimination of oxygen, reduction of moisture content, and change of chemical compositions. When raw biomass is mildly pyrolyzed in a default-oxygen or N2 atmosphere at moderate temperatures, the properties of raw biomass including low calorific value, hydrogen-carbon ratio, hygroscopicity, and grindability can be significantly enhanced. In the present review, the operating mechanism of different torrefaction processes including wet, dry, and ionic-liquid-assisted torrefaction is analyzed and discussed. More importantly, the reactor design for commercialization purpose, reaction kinetics and mechanism, economics, and sustainability of biomass torrefaction is discussed in detail. This review is extended to the torrefaction of agro-residue biomass since torrefied agro-residue-based pellets can be produced from agro-residues. The various technological applications of biomass torrefaction are also reviewed and the prospects in ensuring the continuous production of high-grade fuels are summarized.

Effect of Leaching and Fungal Attacks During Storage on Chemical Properties of Raw and Torrefied Biomasses

Coffee husk, eucalyptus, and pine residues were torrefied at 290 °C in a screw reactor, during 5, 10, 15 or 20 min. The effects of feedstock type and torrefaction process parameters (holding time) on their energy characteristics were investigated. Raw and torrefied biomasses were then submitted successively to leaching and to white and brown rot fungi, to mimic storage conditions. Mass loss after leaching step, water content and weight loss due to fungal deterioration after 2, 4, 8, 12, 16 weeks were recorded. The chemical composition and high heating value (HHV) of the torrefied samples were measured to determine the alterations compared to raw biomass during their storage. Increasing torrefaction residence time improves the decay resistance of the biomasses. Variation of carbon content (%wt., dry basis) and HHV (kJ/kg, dry basis) were observed during native and torrefied biomasses fungal degradations. Carbon contents and HHV values of raw and torrefied biomasses decreased during Trametes versicolor exposure [49.65% > C > 44.07% and 19.71 kJ/kg > HHV > 17.19 kJ/kg, results from results from all tests combined.], whereas they increased during exposure to Coniophora puteana [46.15% < C < 52.70% and 17.43 kJ/kg < HHV < 20.74 kJ/kg]. Severe torrefaction is therefore a good way to improve coffee husk, eucalyptus, and pine energy properties while limiting loss of their energy properties during storage.

Torrefaction of Empty Fruit Bunches: Evaluation of Fuel Characteristics Using Response Surface Methodology

Empty Fruit Bunches (EFB) can be used as an alternative energy source by torrefaction process. Torrefaction is a treatment process of biomass into solid fuel within temperature range of 200-300 °C in an inert condition. This research aims to study the effect of process variable interaction process and to study the characteristic of solid fuel resulted. Torrefaction of EFB was performed by using horizontal fixed bed reactor at three different temperatures, times and particle sizes, that are 225, 250 and 275 °C, for 15, 30 and 45 minutes, and particle size 2, 4 and 6 cm, respectively. Response surface methodology (RSM) was used for to optimization optimize of torrefaction conditions so that biofuel of with high energy density, maximum energy properties, and minimum weight loss could be manufactured. The analysis showed that increase in heating values was affected by treatment severity (cumulated effect of temperature, time and particle size). Our results clearly demonstrated an increased degradation of the material due to the combined effects of temperature and treatment time. While the reaction time had less impact on the energy density of torrefied biomass, the effect of reaction temperature was considerably stronger under the torrefaction conditions used in this study. The most significant factor affecting all responses was temperature of torrefaction, then followed by time and particle size. The optimized torrefaction conditions were verified empirically and the applicability of the model was confirmed. The torrefied biomass occurred more suitable than raw biomass in terms of calorific value, physical and chemical properties.

Torrefaction as a Pretreatment Technology for Chlorine Elimination from Biomass: A Case Study Using Eucalyptus globulus Labill

The recognition of the effects of fossil fuel consumption resulted in several agreements, legislation, and projects focusing on the minimization of impacts caused. Biomass is a versatile energy source. Eucalyptus is a fast-growing crop, mainly used by the pulp and paper industry. Torrefaction is a thermochemical conversion process that can improve biomass fuel properties, enabling its use in the energy sector. However, correct management of biomass is crucial for the sustainability of this process. Torrefaction can also be used to eliminate some elements that can hinder subsequent conversion processes. One example is chlorine, which, during combustion or gasification processes, can form hydrochloric acid that leads to corrosion of metal surfaces. In this context, this research aimed to determine the temperature at which chlorine is eliminated during torrefaction process. For this purpose, several tests were performed at different temperatures and residence times. All samples were analyzed before and after the process, and were characterized by proximate and elemental analysis, calorimetry, and chlorine titration. The analysis showed that, even for the lowest torrefaction temperature used, chlorine content was already below the detection value, showing that, even at lower temperatures, thermal treatment is an efficient technique for the elimination of chlorine from biomass.

Gasification of raw and torrefied empty fruit bunch using bubbling fluidized bed

An experimental study of bubbling fluidized bed gasification process using empty fruit bunch (EFB) as a feedstock has been performed in order to study the effect of gasification temperature on gasification performance. Two types of EFB have been used, which are raw EFB and torrefied EFB at 300 °C. The composition of the synthesis gas, gas yield, low heating value (LHV), cold gas efficiency (CGE), and carbon conversion (CC) was determined and analyzed. In terms of synthesis gas, the composition of hydrogen and carbon monoxide gases were increased while methane and carbon monoxide gases were decreased for raw and torrefied EFB when the temperature is increased from 650 to 900 °C. Meanwhile, the synthesis gas yield was significantly enhanced and reached the maximum value of 1.21 and 1.45 MJ/Nm3 for raw and torrefied EFB respectively, which indicates that the torrefaction process improves the synthesis gas yield. Besides, the LHV for the torrefied EFB at highest gasification temperature is 11.08 MJ/Nm3 which is higher than raw EFB (10.81 MJ/Nm3). However, the value of CGE and CC of torrefied EFB are lower than raw EFB with the different about 3% and 5% respectively.

Treatment of granulated organic manure based on bird droppings by hydrothermal carbonization and torrefaction methods

Hydrothermal carbonization (HTC) and torrefaction are modern low-temperature methods of converting different types of biomass into biocoil. In both processes temperature significantly affects the product distribution and character of solid product. The heating values of HTC biocoil was much higher than torrefacted biochar and 40% higher compared to the initial raw materials (15.68 MJ/kg for raw materials, 22.3 MJ/kg for HTC 210 °C). Biomass has increased carbon content. The maximun one has biochar obtained at 210 °C (55% higher compared to the initial raw materials). The energy densification ratio and energy yield was also higher in HTC process compared to torrefaction process. Hydrochar has less ash that biochar obtained by torrefaction due to washing part of inorganic into the liquid phase.