Conventional Torrefaction Research Articles

Study on molten salt on torrefaction and subsequent pyrolysis of elm branches

Torrefaction was a promising pretreatment for enhancing biomass properties, and molten salt torrefaction could achieve superior torrefaction benefits at equivalent temperatures. To explore the difference between molten salt torrefaction (MT) and conventional torrefaction (CT) and the effect of which on the subsequent pyrolysis of biomass, torrefaction experiments at 260 ℃ and pyrolysis experiments were conducted in a horizontal furnace, a thermogravimetric mass spectrometry (TG-MS) and a pyrolysis gas-chromatography mass-spectrometry (Py-GC-MS), respectively. The results showed that the O/C of washed MT sample (MTw) decreased by 13.89 % and the volatile content decreased by 14.56 % compared with washed CT sample (CTw). Concurrently, the MTw sample demonstrated the highest calorific value and largest specific surface area among the samples. The maximum weight loss rate of MT sample during subsequent pyrolysis increased. In comparison with CT, MT could decrease the pyrolysis activation energy. Furthermore, MTw sample produced more H2 and CH4, while simultaneously less CO2 and CO during pyrolysis compared with CTw. The bio-oil derived from the MTw sample exhibited the lowest relative content of acidic compounds and the highest relative content of aromatic compounds, signifying its enhanced quality.

Relative fuel property variation of gas-pressurized and conventional torrefaction for biochar performance assessment

Exploring biochar from gas-pressurized torrefaction for solid biofuel production is of great concern for the process performance assessment. This study conducts several relative indexes of biochars produced from gas-pressurized and conventional torrefaction, identifying the merits of the former. The results suggest that a higher pressure is more feasible for reducing biochar volume and enhancing energy efficiency. The results of the relative higher heating value (1.01–1.07) and relative enhancement factor (1.00–1.06) indicate that gas-pressurized torrefaction is more efficient for energy density improvement. Furthermore, gas-pressurized torrefaction dramatically facilitates biochar grindability, with the relative Hardgrove grindability index value ranging from 1.20 to 2.40. The proximate and elemental analysis results suggest that a higher pressure facilitates biochar carbonization and deoxygenation with a higher carbonized component. The energy consumption, efficiency, and expense calculation results imply that gas-pressurized torrefaction is more energy-efficient and cost-saving, with the relative upgrading energy index range of 1.00–1.12.

The fate of organic compounds in organic waste during torrefaction and implications for its valorization

Torrefaction is commonly used to improve biomass properties, applications, and economy. The characteristics and subsequent applications of torrefied biomass are largely contingent on the organic compounds in parent biomass and their evolution during torrefaction. Yet, the evolution of organic compounds in biomass particularly minor components (e.g., polyphenols) is far less investigated for torrefaction. To address such issues, a superheated steam (SHS) boosted torrefaction process at different temperatures (200, 250, and 300 °C) and residence times (15, 30, and 60 min) was performed on spent coffee ground (SCG), which is an emerging biowaste and is rich in various organic compounds. Results found that both temperature and residence time determine SHS torrefaction performance. SHS torrefaction could effectively remove volatile matters to upgrade SCG for solid fuel. The relatively high content of N and S in torrefied SCG may negatively affect fuel quality but may benefit its adsorption of environmental pollutants. SHS boosted torrefaction could facilitate cellulose and lipids degradation compared to conventional torrefaction. Efficient reduction/removal of labile carbon and ecotoxic chemicals (e.g., phenols and caffeine) in SCG was successfully achieved with SHS torrefaction. As a result, SHS-torrefied SCG with higher biostability and lower phytotoxicity was evaluated as soil amendments and additives to soilless growing substrate. Implications for subsequent application by revealing the evolution of organic compounds during SHS torrefaction were discussed. This study highlighted the potentiality of SHS torrefaction as a pretreatment of biomass for versatile applications.

Comparative advantages of gas-pressurized torrefaction for corn stalk conversion to achieve solid biofuel production

Torrefaction is a feasible approach to produce biochar. Several new torrefaction methods have been conducted for biochar production. Among them, gas-pressurized torrefaction is considered a new technology to produce solid biofuel with better fuel performance, without carrier gas, and higher deoxygenation. This study conducts a comparative analysis between conventional torrefaction and gas-pressurized torrefaction to identify the potential applications of the latter. The results indicate that increasing reaction pressure can achieve biochar volume reduction and HHV improvement, thus leading to a higher energy density. The conventional torrefied biochar yield is in the range of 54.13–90.11%, and it is 48.59–80.13% for gas-pressurized one. Moreover, a more extensive surface area, higher carbonization degree, and more stable pyrolysis characteristics are obtained compared to conventional torrefaction. The carbonization index values of conventional and gas-pressurized conditions are 1.04–1.42 and 1.06–1.48. The correlation analysis results suggest that biochar grindability is highly correlated to carbonaceous properties, regardless of torrefaction methods. The expense and environmental impact analysis indicates that gas-pressurized torrefaction possesses lower cost and environmental pollution potential for biochar production.

Research on the pyrolysis mechanism of tobacco based on low temperature torrefaction

The temperature range of 100–220 °C is known to cause the volatilization of some volatile components and the pyrolysis of some unstable components in tobacco. However, there is a lack of research on the pyrolysis mechanism of tobacco at this temperature range. To address this gap, the present study employed an innovative cascade pyrolysis method based on stepwise heating to analyze the composition of tobacco's rapid pyrolysis products within this temperature range. The analysis was conducted at intervals of 30 °C. The study summarized the release characteristics of organics at different temperatures and analyzed the representative substances of various organic families. In addition, considering that tobacco has a unique composition compared to most biomass, the same five temperature points as they do in the cascade pyrolysis process were selected to torrefy the tobacco samples. These temperatures are lower than the conventional torrefaction temperatures typically used for biomass pretreatment. Multiple techniques were used to systematically analyze the composition, functional group structure, and thermal degradation characteristics of tobacco torrefied at different temperatures. Subsequently, the effect of torrefaction temperature on the release performance of products from tobacco pyrolysis at 650 °C was also studied. The results of this study not only contribute to a more comprehensive understanding of the pyrolysis mechanism of tobacco but also provide theoretical support for cigarette processing and tobacco waste utilization.

Economic feasibility analysis and environmental impact assessment for the comparison of conventional and microwave torrefaction of spent coffee grounds

Biomass torrefaction is a promising method for carbon-neutral solid biofuel production. The economic feasibility and environmental pollution potential of the torrefaction operation are of great concern to developing a sustainable energy system. In this study, spent coffee grounds are selected as the feedstocks for comparing the economic feasibility of different torrefaction methods and evaluating the environmental pollution potential of conventional torrefaction and microwave torrefaction. The result shows that microwave torrefaction possesses a more economically efficient approach for biomass upgrading, as a result of lower energy input and carrier gas consumption. For environmental impact assessment, several different indicators are adopted for evaluating the torrefaction process, which is based on openLCA software of CML 2001 method. The life cycle assessment results indicate that the environmental impact of microwave torrefaction is lower than conventional torrefaction, especially when torrefied at light conditions. Thus, microwave light torrefaction is the optimal condition for spent coffee grounds upgrading. However, the growth rate of the environmental impact of microwave torrefaction is greater than conventional torrefaction, including resource depletion and terrestrial ecotoxicity. Moreover, the environmental impact of the torrefaction process is strongly correlated to the torrefaction severity.

A comparison of conventional and oxidative torrefaction of microalga Nannochloropsis Oceanica through energy efficiency analysis and life cycle assessment

Torrefaction is a promising method for producing microalgal solid biofuel. This study compared conventional torrefaction and oxidative torrefaction for microalgal solid biofuel production. Particular attention was paid to evaluating energy efficiency analysis and life cycle assessment, and several parameters and indexes were adopted and calculated. The results showed that mild oxidative torrefaction was the most energy-efficient operation, with the maximum value of relative upgrading energy index and relative energy return on investment. The life cycle assessment indicates that oxidative torrefaction had a lower environmental impact than conventional torrefaction, mainly due to its lower carrier gas consumption and electricity input. The environmental impact positively correlated with the torrefaction severity. The obtained results in this study can provide new insights on improving the cognition of energy efficiency evaluation and environmental impact assessment of the torrefaction process, which helps select optimal torrefaction conditions.

Torrefaction of Oil Palm Frond using Dry Flue Gas

This study aimed to experimentally investigate the effects of dry flue gas on biomass torrefaction. In this work, an oil palm frond (OPF) was torrefied in a laboratory-scale horizontal torrefaction reactor at temperatures of 240, 260, and 280 °C, with a residence time of 20 and 40 min, using dry flue gas with variable compositions (N2/CO2/O2: 76/16/8, 80/14/6, and 74/12/4). In addition, the experiments of conventional torrefaction were conducted for comparison. The effects of operating parameters on mass yield, the proximate and ultimate analysis, and the heating value of the fuel were investigated. The proposed method improved the fuel property of OPF substantially. With dry flue gas, OPF had lower moisture and volatile, and higher fixed carbon. The heating value of the selected biomass increased from 14.6 MJ/kg to 25.8 MJ/kg. Therefore, torrefaction using dry flue gas exhibited a potential for practical use for the biomass fuel pre-treatment method. However, this method resulted in a decreased mass yield and, consequently, the energy yield of the sample compared to conventional torrefaction.

Microwave torrefaction integrated with gasification: Energy and exergy analyses based on Aspen Plus modeling

The serious environmental issues are risen by the high-moisture herb residue (HR). Thermochemical conversion of HR shows the potential for not only clean treatment, but also utilizing its energy. In this study, microwave torrefaction (MT) and conventional torrefaction (CT) were integrated with gasification respectively, for the HR treatment. The systems were simulated and comparatively investigated by Aspen Plus. The modeling of MT was achieved for the first time, and it was validated by the experimental data. The effects of torrefaction on gasification performance were evaluated by exergy and energy analyses. The results showed that the exergy of MT-HR was 17.68 MJ/kg, which was higher than the raw HR and CT-HR, indicating the significant improvement for HR by MT. Meanwhile, the exergy of gasification syngas reached the highest as 7.93 MJ/h with MT, while and the exergy of tar was the lowest as 3.13 MJ/h. It was proved by energy analysis that MT could achieve self-powered in the integrated process, although MT consumed some energy. MT-gasification showed the best performance for electricity yield, due to MT could not only improved the gasification and combustion efficiency, but also reduce the negative influence of tar. This study provides a comprehensive evaluation for the application of MT, and it demonstrates the potential of MT as a promising pretreatment for thermal conversion of biomass waste.

Integration of microwave co-torrefaction with helical lift for pellet fuel production

Abstract The heating performance of empty fruit bunch pellets (EFBPs) has been limited by its low energy density, high moisture, and ash content. Hence, microwave co-torrefaction (MCT) was performed with microwave heating unto waste oil mixed EFBP to produce high-energy biofuel. However, the non-homogeneous electromagnetic fields distribution in the microwave cavity results in an uneven heating behavior, producing the hot and cold spots. Hence, MCT coupled with helical lift was examined for its potential to improve heat distribution. The effect of temperature and types of waste oil on the proximate analysis and surface properties were studied. In comparison to the conventional torrefaction using a furnace (>30 min), MCT provided rapid heating (50–80°C·min−1) and a shorter process time (10 min). The use of helical lift with 2-dimensional movement – rotational (24 rpm·min−1) and vertical motion (5 cm·min−1) simultaneously, distributed microwave radiation uniformly for rapid heating. The proximate analysis demonstrated that the ash content was reduced from 8 to 3 wt%, and the highest fuel ratio of 2.0 was achieved. Additionally, the highly porous structure of EFBP biochar can act as an activated carbon precursor. MCT coupled with helical lift represents a promising approach to prevent hot spots during microwave heating.

Fast torrefaction of large biomass particles by superheated steam: Enhanced solid products for multipurpose production

Conventional torrefaction technology upgrade biomass for improved storage or as pretreatment prior to combustion or pyrolysis, and are generally performed on small-scale particles. Superheated steam (SHS) as the carrier gas and sole heat source was proposed to heat biomass rapidly and evenly while generating intensively torrefied biomass. This work revealed the potential of SHS torrefaction to upgrade large wood particles (Φ = 30 mm, H = 60 mm), to improve the feedstock suitability and expand products diversity in torrefaction technology. The effects of SHS torrefaction at different depths in large particles were investigated. A torrefaction time of 30 min was deemed capable of upgrading even the inner sections of large particle to have appropriate fuel properties. The HHV of the torrefied biomass enhanced by 50% reaching 28.76 MJ kg−1. Combustion and pyrolysis behaviour of torrefied biomass demonstrated their properties as benign fuel and pyrolysis feedstock, respectively. Moisture absorption (from 13.52 to 3.42 wt%) and grindability results (386.96–10.52 kWh ton−1) indicated that large particles from SHS torrefaction were easy for handling, transportation, and storage. The investigation of physicochemical properties at different depths of the torrefied biomass helped to understand SHS torrefaction behaviour in large biomass particles, hence, to control the reaction extent in the desired end product.

Gas-pressurized torrefaction of biomass wastes: Self-promoted deoxygenation of rice straw at low temperature

Gas-pressurized (GP) torrefaction of biomass, proposed in our previous work, has a much higher oxygen removal efficiency compared to conventional torrefaction, but how the pressure promoted the deoxygenation remains unclear. In this work, the GP torrefactions under different partial pressures of volatiles (Pvol) and total pressures (Ptot) were conducted. The results revealed that the Pvol was the root cause rather than the Ptot of the acceleration of biomass decomposition and deoxygenation for the first time. This process can be regarded as a self-promoted deoxygenation process, and consisted of three stages. Stage 1 was the volatiles formation stage, where hemicellulose decomposed, volatiles formed and the Pvol increased. Stage 2 was self-promoted dehydration stage, where the hydroxyl in biomass was dramatically removed. Stage 3 was structure rearrangement stage, where the aromatic structure formed. The stage 2 and stage 3 were highly correlated with the Pvol, and resulted in the deep deoxygenation of biomass. This study should be not only suitable for the GP torrefaction, but can also be extended to pressurized pyrolysis and gasification of biomass.

Microwave co-torrefaction of waste oil and biomass pellets for simultaneous recovery of waste and co-firing fuel

The low bulk density and heating value of biomass pellets limit their application as a co-firing fuel and a partial substitute fuel for high-efficiency coal boilers. In this study, we develop an innovative microwave co-torrefaction (MCT) process by combining microwave heating and torrefaction to convert palm waste fruit bunch pellets and used cooking oil (UCO) into torrefied biomass pellets as an alternative fuel. Microwave heating requires a shorter torrefaction duration of only 6–8 min compared with a conventional furnace (20 min) to attain the desired high process temperatures ranging from 200 °C to 300 °C. Torrefied biomass pellets produced from MCT yield less volatile matter (33–49 wt%) and lower oxygen contents (23.1–40.1 wt%) compared with those produced from conventional torrefaction. By conventional torrefaction at 300 °C, the highest fuel ratio of torrefied biomass pellets obtained is 2.0. MCT yields the desired energy yield of 98.1% at 250 °C, the highest fuel ratio of 1.9, and a heating value of 26.4 MJ/kg at 300 °C. MCT exhibits a lower activation energy of 12.0 kJ/mol compared with that of conventional co-torrefaction (13.6 kJ/mol), indicating that MCT can be performed at a relatively low operating temperature and with low energy consumption. These results indicate the potential of microwave heating for performing the co-torrefaction of biomass pellets using UCO as an economical approach to produce desirable pellet fuel from waste and biomass materials.

Microwave Torrefaction of Oat Hull: Effect of Temperature and Residence Time

Microwave torrefaction of oat hull was conducted to enhance its physicochemical properties. A bench-top reactor with an internal stirrer was used for oat hull pretreatment at temperatures of 225 °C, 255 °C, and 285 °C, and residence times of 3, 6, and 9 min, respectively. Results showed that a high temperature level at 3 min residence time or severe torrefaction increased calorific values by up to 35% of its original value, while decreasing mass yield down to 60.77%. Severe torrefaction further decreased moisture absorption, moisture content, and grinding energy consumption but decreased energy yield and bulk density. Residence time had no significant effect on biomass physicochemical changes; however, production cost may be significantly increased by longer residence times. It was also concluded that increased microwave power levels from 400 to 650 W decreased energy consumption by shortening processing times, resulting in a positive economic impact of the process. Moderate and severe torrefaction significantly enhanced biomass fuel properties, and short residence times are recommended in order to decrease electricity consumption. In addition, microwave pretreatment enhances biomass in a similar way to conventional torrefaction, but at a faster processing time. Moreover, the liquid fraction as a by-product may represent a valuable product for the food industry.

Investigation on microwave torrefaction: Parametric influence, TG-MS-FTIR analysis, and gasification performance

Microwave torrefaction (MT) was proposed for pre-treating herb residue (HR), which is a typical high-moisture biomass waste. A series of comparative experiments between MT and conventional torrefaction (CT) on HR were conducted. The effects of temperature, time, moisture content of HR, and microwave power on torrefaction performance and microwave energy consumptions were comprehensively investigated. With the response surface methodology (RSM), the combined effects between each two parameters were analyzed, and the moisture content was evaluated as the most influential factor for torrefaction and energy consumption. The effects of microwave on torrefaction were comparatively evaluated by TG-MS-FTIR. MT could remove bound water efficiently. With the rupture of HR structure and the high-degree decomposition of hemicellulose and lignin, MT positively influenced the yields of the permanent gases and organic volatiles. Moreover, the gasification performance of different HR samples were evaluated in a fixed bed gasifer. MT-HR showed the highest gas yield (0.86 Nm3/kg) and heating value of gaseous products (13.70 MJ/Nm3), with the lowest tar generation (12.72 wt%). This study provides an in-depth understanding of MT process, and the MT integrated with steam gasification can be regarded as a feasible approach for high-moisture biomass waste treatment.

Energy recovery from sewage sludge: Product characteristics, heating value prediction and reaction kinetics

Energy recovery from sewage sludge was carried out by using microwave and conventional torrefaction. The microwave torrefaction was carried out by using a laboratory-scale microwave oven that provides single-mode microwave irradiation at 2.45 GHz, and the amount of sewage sludge for each experiment was approximately 20 g. The efficiency of microwave heating can be substantially promoted at higher power level, resulting in higher heating rate and maximum temperature. According to higher energy yield and heating value of torrefied sewage sludge, the optimum power level for bioenergy produced by microwave torrefaction of sewage sludge should be 200 W. Because of lower mass yield and temperature required to obtain the same yield, microwave heating can be more effective than conventional heating for sewage sludge torrefaction. The elemental composition of torrefied sewage sludge at 400 W was similar to that of anthracite, and its low hydrogen and oxygen contents could prevent excessive formation of smoke. Two correlations were obtained to predict the HHV of SS based on proximate and elemental compositions. With the recovery of liquid and gas products as bioenergy, the energy return on investment for microwave torrefaction of sewage sludge can be up to 16.4, much higher than the minimum value required for a sustainable society. Because of lower activation energy but higher pre-exponential factor, microwave heating can be approximately five times faster than conventional heating.

Gas-pressurized torrefaction of biomass wastes: The optimization of pressurization condition and the pyrolysis of torrefied biomass

A novel gas-pressurized (GP) torrefaction with high oxygen removal efficiency at mild temperature was proposed in our previous work. However, the optimal condition of the GP torrefaction and subsequent pyrolysis of the torrefied biomass were not clear. In this work, the effect of pressure on the GP torrefaction and pyrolysis product properties of the torrefied biomass were studied in detail. The results show that the pressure increasing from 1.7 MPa to 5.0 MPa just slightly contributed to further oxygen removal, and 1.7 MPa was thus selected as the optimum pressure. The GP torrefaction significantly improved the product property of biomass pyrolysis compared to the conventional torrefaction (AP torrefaction). The acids content in bio-oil was reduced from 15–20% to less than 5%, and the calorific value of biogas increased to as high as 16.57–19.31 MJ/Nm3. Furthermore, an overall conversion mechanism of combined GP torrefaction and subsequent pyrolysis of biomass was proposed.

Proof-of-Concept of High-Pressure Torrefaction for Improvement of Pelletized Biomass Fuel Properties and Process Cost Reduction

This paper provides a comprehensive description of the new approach to biomass torrefaction under high-pressure conditions. A new type of laboratory-scale high-pressure reactor was designed and built. The aim of the study was to compare the high-pressure torrefaction with conventional near atmospheric pressure torrefaction. Specifically, we investigated the torrefaction process influence on the fuel properties of wooden-pellet for two different pressure regimes up to 15 bar. All torrefaction processes were conducted at 300 °C, at 30 min of residence time. The initial analysis of the increased pressure impact on the torrefaction parameters: mass yields, energy densification ratio, energy yield, process energy consumption, the proximate analysis, high heating value, and energy needed to grind torrefied pellets was completed. The results show that high-pressure torrefaction needed up to six percent less energy, whereas energy densification in the pellet was ~12% higher compared to conventional torrefaction. The presence of pressure during torrefaction did not have an impact on the energy required for pellet grinding (p < 0.05).

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.

An Oxyfuel Combustion-Based Torrefaction Process: Technoeconomic Analysis

Abstract A torrefaction process operating on oxyfuel combustion concept is introduced. The working fluid employed in the process is carbon dioxide captured from the combustion products. The thermodynamic modeling of the process is carried out to determine the energy requirement, recycled CO2 flow rate, energy yield, and CO2 production rate at various torrefaction conditions. The total capital investment and production costs of the new system are also estimated. By increasing the torrefaction severity, the CO2 production rate, recycled gas mass, and the process thermal energy requirement increase, whereas the grinding energy and the energy yield decrease. A comparison made between the performance and economic parameters of the new and a conventional torrefaction processes shows that the proposed process is expected to produce torrefied wood pellet of compatible fuel quality and overall efficiency while eliminating CO2 and NOx emissions at the expense of 11.5% and 9% increase in the capital and production costs, respectively. The proposed torrefaction process requires 91.8 M$ capital investment and 113.2 $ to produce 1 ton of torrefied wood pellets with 91% energy yield and 88% overall plant efficiency. Sensitivity analysis shows that the reactor type and raw biomass costs have significant impact on cost structures.