Torrefaction Conditions Research Articles

Catalytic microwave-assisted torrefaction of sugarcane bagasse with calcium oxide optimized via Taguchi approach: Product characterization and energy analysis

The production of biochar, bio-oil, and biogas from sugarcane bagasse (SB) is investigated via catalytic microwave-assisted torrefaction where calcium oxide is employed as a microwave absorber. Using the Taguchi approach, three parameters of microwave power, holding time, and catalyst concentration are considered. The optimal combination of the three parameters provides the highest energy yield of 77.64%. The highest calorific value is recorded at 24.66 MJ⋅kg−1, demonstrating a 46.96% improvement compared to the dry-basis raw feedstock. The results from ANOVA indicate that microwave power significantly affects the energy yield, followed by the holding time and catalyst concentration. Scanning electron microscopy shows severe degradation of SB surfaces while XRD curves demonstrate the retention of the crystalline structures of the torrefied SB samples. The main compounds identified in the bio-oil are phenolic compounds. FTIR results show no significant changes in the functional groups after acid-washing and catalyst impregnation. The upgrading energy index profiles portray that light torrefaction conditions lead to higher energy efficiencies, rendering higher energy yields while consuming less energy inputs. Overall, catalytic microwave-assisted torrefaction with calcium oxide is a feasible technique to transform SB into bioenergy products such as biochar, bio-oil, and biogas. More importantly, attaining a balance between product quality and energy efficiency is crucial for the feasible production of fuels that are ready for industrialization.

Elemental Loss, Enrichment, and Transformation as Well as Life Cycle Assessment of Torrefied Corncob

Elemental loss, enrichment, and transformation are crucial to evaluate the fuel properties of torrefied biomass, and life cycle assessment (LCA) can reflect the environmental impact of torrefaction operation. Herein, corncob is selected as the feedstock for the exploration of fuel properties variation during the torrefaction process. The obtained result implies that torrefaction operation obviously changes the macroscopic and microscopic morphologies of torrefied corncob, and leads to a higher calorific value and bigger surface area. With the development of the torrefaction process, the loss of weight percentage of elemental C, H, and O occurs and is in the order of O > C > H. The relative content of elemental C gradually increases and leads to better fuel properties. The forms of elemental N and O are stable and do not change during the torrefaction process, but the form of C gradually changes from a combined state to an elemental state. These results are confirmed by Fourier transform infrared (FT-IR) spectra, contact angle detection, and thermogravimetric and derivative thermogravimetric analysis. The LCA result infers that slight torrefaction possesses the lowest environmental impact, whereas the mild torrefaction condition of 250 °C appears to be the optimal choice for corncob upgrading.

Process simulation development of a clean waste-to-energy conversion power plant: Thermodynamic and environmental assessment

Waste-to-energy (WTE) conversion technologies for generating renewable energy and solving the environmental problems have an important role in the development of sustainable circular economy. This paper presents a novel high-efficiency WTE power plant using refuse-derived fuel (RDF) as feedstock by integrating torrefaction (T) pretreatment with plasma gasifier (PG), solid oxide fuel cell (SOFC), and combined heat and power (CHP) system. The combined impacts of torrefaction conditions (i.e. temperature and residence time) and steam-to-fuel (S/F) ratio on the energy and environmental performances of the proposed T-PG-SOFC-CHP power plant without CO2 capture (System I) is first evaluated. Results show that torrefaction of RDF prior to plasma gasification provides better syngas quality and therefore the system electrical efficiency (SEE) and CHP efficiency (CHPE) of System I can be markedly boosted compared to that of untreated RDF. However, the integration of torrefaction unit shows a negative effect on the energy return on investment (EROI) due to high energy demands for torrefaction and plasma gasification. Overall, the values of CHPE of System I range from 47.25% to 55.39% when the torrefaction temperatures of 200 and 250 °C are adopted. In contrast, the torrefaction of RDF at 300 °C is not a recommended condition for operation in the T-PG-SOFC-CHP power plant because of noticeably negative energy and environmental impacts. Moreover, to prevent the risk of carbon deposition on the SOFC anode, a recirculation ratio (RR) of the anode off-gas of 30% is required. Finally, the introduction of oxy-fuel combustion technology into the T-PG-SOFC-CHP system for CO2 capture (System II) allows to achieve a zero direct CO2 emission WTE power plant. However, this results in an energy penalty of about 5.40–6.77% associated with the CO2 capture and compression process.

A comprehensive study on torrefaction of empty fruit bunches: Characterization of solid, liquid and gas products

Empty fruit bunches (EFB), after fruits-bunches separation, are highly potential as an energy source. However, due to some inherited inferior fuel properties such as bulk density, hydrophobicity, moisture content and calorific value (CV), pretreatment is required before EFB can be utilized as a solid fuel. Torrefaction is promising to upgrade EFB fuel properties, and thorough products characterization is essential to reveal process mechanism and potential application. In this study, the effects of temperature (225–300 °C) and retention time (20–60 min) on EFB torrefaction were investigated in a fixed bed reactor. While an increased temperature and retention time had led to a lower energy and mass of solid yields, the liquid and gaseous yields showed otherwise. The oppositely reduced O (37%) and increased C (45%) contents of torrefied EFB with increasing temperature, and the resulting decrease in O/C ratio, and H and O contents, had greatly improved the CV of the torrefied solid up to 46% compared to its raw form. In addition, the chlorine content was significantly reduced (21–54%) after torrefaction. Both the raw and torrefied solid EFB showed a strong thermal degradation of hemicellulose with lower decomposition rate of the latter due to mass depletion during torrefaction. Oxygen removal from EFB during torrefaction migrated to the liquid and gaseous products, most dominant as water and CO2, followed by CO and oxygen-containing organic compounds. The liquid product consisted of condensable components; water (main) and functionalised compounds, among which were acids (as relatively higher concentration), furans, ketones, alcohols and phenols. The gaseous product was dominated by CO2 and CO in all torrefaction conditions. The findings contribute partly as biomass resource database for future bioenergy upscaling and deployment.

Char production with high-energy value and standardized properties from two types of biomass

In this paper, a new approach to the torrefaction process is proposed. The approach consists of obtaining a single torrefied product with an energy value and standardized physicochemical properties in the range of sub-bituminous coal from two types of waste: the cotton stalk and vine pruning. A multifactorial design was used to investigate the effect of temperature, time, and reaction atmosphere on the properties of the torrefied product (char). The process was optimized according to the mass and energy yield and the increase in the higher heating value (HHV). A series of physicochemical, kinetic, and microscopic analyses were carried out on the raw and torrefied biomass to evaluate the combustible properties. Char was obtained with an energy value in the range of sub-bituminous coal and with more uniform fuel properties concerning raw biomass. The optimal torrefaction conditions for cotton stalk were 6% O2–257.8°C–60 min and 0% O2–275°C–20 min for vine pruning. The HHV of the cotton stalk and vine pruning increased from 16.88 to 22.21 MJ kg−1 and from 17.01 to 22.43 MJ kg−1, respectively. The char from both residues presented similar O/C and H/C atomic ratios, and its value decreased compared with the raw biomass. Char is hydrophobic and easy to grind. Oxygen concentration played an essential role in reducing the time and temperature of torrefaction.

Identification of Suitable Biomass Torrefaction Operation Envelops for Auto-Thermal Operation

Auto-thermal operation of biomass torrefaction can help avoid additional heat investment and the associated costs to the system. This work provides a general method for relating the feedstock-specific parameters to the energy balance and pre-diagnosing the potential of auto-thermal for different biomass torrefaction and pyrolysis systems. Both solid and gas thermal properties under various torrefaction conditions and their influences to the torrefaction system energy balances are considered. Key parameters that influence the process auto-thermal operation are analyzed, which include torrefaction reaction heat, torrefaction conditions, drying method, biomass species, and inert N2 flowrate. Equations of torgas and biomass higher heating values (HHVs), as well as the torrefaction reaction heat at different operating conditions are developed. It is found that torgas and biomass HHVs increase with torrefaction temperature and biomass weight loss. Torrefaction reaction heat has a linear relationship with the biomass weight loss, with a positive slope at 250–260°C, and a negative slope at 270–300°C, which indicates that torrefaction shifts from endothermic to exothermic at ∼270°C. Applying advanced drying technology and avoiding the use of N2 can help the system achieve auto-thermal operation at lower torrefaction temperature and residence time, thus leading to a higher process energy efficiency and product yield. This is the first work to relate the micro level element changes of biomass to the macro level process energy balances of the torrefaction system. This work is important in design and operation of the torrefaction system in both pilot and industrial scales to improve process efficiency and predict product quality in a reliable and economic manner.

Effects of torrefaction conditions on the hygroscopicity of biochars

Agroforestry biomass involving cedar wood (CW), polar wood (PW), corn stalk (CS) and wheat straw (WS) was torrefied in a fixed-bed reactor under the torrefaction temperature (T) of 200–300 °C and the holding time (τ) of 10–60 min to prepared straw biochars (WSC, CSC) and woody biochars (PWC and CWC). The effects of biomass species, torrefaction temperature and holding time on the hygroscopicity of biochars were investigated under constant temperature and humidity. Combined with the results of N2 adsorption/desorption and FTIR of raw biomass and its biochars, the mechanism of torrefaction conditions on the hygroscopicity of biochar was discussed. The obtained results indicated that water absorption process of biochar can be divided into two stages of initial rapid water absorption and the later slow water absorption. The equilibrium moisture content (EMC) decreases, the hygroscopicity reduction extent (HRE) increases gradually, and hydrophobicity of biochar is enhanced with increasing torrefaction temperature. However, too high torrefaction temperatures (T ≥ 275 °C) has little or even adverse effect on the hydrophobicity of biochars. The longer holding time has a certain influence on the hydrophobicity of biochar, but the overall effect is not significant. The hydrophobicity of woody biochars (PWC and CWC) is better than that of straw biochars (WSC and CSC) at the same torrefaction conditions. The severe torrefaction of T ≥ 275 °C and τ ≥ 40 min are able to improve the hydrophobicity of woody biochars, however it has negative effect on straw biochars. The hydrophobicity of four biochars is ordered as PW > CW > WS ≈ CS at the same torrefaction conditions. The water adsorption mechanism of biochars is influenced by the coupling effects of physical structure evolution, release of hydrophilic oxygen-containing functional groups and pyrolytic tar condensation, and the severer the torrefaction, the more significant the influence is.

Torrefaction of Almond and Walnut Byproducts

While the US nut industry is growing, markets for nut by-products, particularly nutshells and tree prunings, have not kept pace. Torrefaction is a thermochemical process used to improve physicochemical properties of biomass for energy and other applications. The goal of the paper was to characterize the effects of a range of torrefaction conditions on the properties of nut by-product feedstock. The process consists of thermal treatment of biomass at a temperature between 200 and 300°C in the absence of oxygen, where final material properties of the torrefied biomass depend on the temperature, heating rate, and residence time. In general, torrefied biomass exhibits higher hydrophobicity and calorific value with reduced moisture absorption compared to untreated biomass, making it an ideal fuel source for energy applications compared to raw biomass. In this study, almond shells of soft, semi-soft, and hardshell varieties, as well as walnut shells and almond wood, were torrefied at two different temperatures (230 and 290°C) and three different residence times (20, 40, and 60 min) in order to characterize the physicochemical properties. The thermal behavior of raw and heat-treated biomass was investigated by TGA analysis, elemental analysis, pH, helium pycnometry, FTIR spectroscopy, and dynamic vapor sorption analysis.

A Review of Torrefaction Technology for Upgrading Lignocellulosic Biomass to Solid Biofuels

The utilization of lignocellulosic biomass is closely related to one of the renewable sources of energy. However, a few inherent properties of lignocellulosic biomass such as high moisture content, low density, high volume, and heterogeneous composition make it unfavorable for bulk transportation, storage, handling, and conversion. The pretreatment of biomass is found to resolve a few of the aforementioned limitations while increasing the conversion efficiency of lignocellulosic biomass into biofuels. This article reviews the opportunities, challenges, and state-of-art research on torrefaction as a widely used biomass upgrading technique for solid fuel production and thermochemical biomass conversion. Some notable applications of such pretreatments in high-value solid biofuel production, densification, combustion, co-firing, gasification, and metallurgy have been reviewed. The broad changes in physicochemical characteristics and structural chemistry of lignocellulosic biomass because of torrefaction have been thoroughly described. This review also comprehends the effects of different process parameters and operating conditions of torrefaction of lignocellulosic biomass to produce high-value solid fuel. This article attempts to highlight some recent advancements in biomass torrefaction technology concerning the fundamental characteristics of biomass and process operation and optimization as well as the evolution of physicochemical features of torrefied biomass. Lastly, the value-added industrial applications of torrefaction technology and torrefied biomass are also elucidated.

Applicability of municipal solid waste incineration (MSWI) system integrated with pre-drying or torrefaction for flue gas waste heat recovery

A system based on pre-drying and torrefaction of raw municipal solid waste (MSW) is proposed in this study to minimize the amount of pollutants and improve the efficiency of the system by recovering flue gas waste heat as heat source for the pre-drying or torrefaction stage. A steady-state model integrated with the mass residual rate and energy coefficient of this novel system is established to evaluate the effect of pretreating solid waste on the performance of the municipal solid waste incineration (MSWI) system comprehensively. Thermodynamic calculation results show that the low heating value (LHV) of the MSW is increased to approximately 9000 kJ/kg at a torrefaction temperature of 533 K for 30 min; the combustion flue gas temperature is approximately 1150 K, thereby satisfying the incineration temperature requirement. Further, pre-drying is conducive for improving the energy utilisation rate; for torrefaction, when the mass residual rate of MSW is greater than 39.25%, the energy coefficient of the pretreatment exceeds 1. For the pre-processing method involving torrefaction, the mass loss and LHV improvement after torrefaction should be considered to select the appropriate torrefaction conditions. This study supplies effective and feasible reference for developing higher performance MSWI system with pre-drying or torrefaction as the pretreatment.

Upgraded pongamia pod via torrefaction for the production of bioenergy

Torrefaction can be used to reduce the oxygen content of biomass and improve the feedstock properties for thermochemical conversion. Pongamia (Millettia pinnata), a leguminous, oil-seed bearing tree, is a potential resource for sustainable aviation fuel production due to the high oil content of its seeds. The present work investigates thermochemical pretreatment of pongamia processing residues, i.e. pods. Torrefaction tests were performed with both a fixed bed reactor and a macro thermogravimetric analyzer (TGA) under nitrogen atmospheres. The effects of process conditions on feedstock properties relevant to thermochemical conversion technologies, proximate and ultimate composition, heating value, and Hardgrove grindability index (HGI), were measured. The chemical structure, reactivity, and changes in elemental composition of the torrefied materials were also investigated. The mass and energy yields decreased 43% and 25%, respectively, from the mildest (165 °C) to the most severe (281 °C) torrefaction conditions, while the energy densification index increased from 1.15 to 1.68. The HGIs of pods torrefied at temperatures >215 °C were found to equal or exceed the HGI of a reference bituminous coal sample. A LECO model TGA801 macro-TGA with a sample loading capacity of ~95 g was also used to torrefy pongamia pods. Products from the LECO and the fixed bed reactor were comparable, and the macro-TGA was demonstrated to be a useful fast screening tool to study effects of process parameters.

Characterization and quality analysis of wood pellets: effect of pelletization and torrefaction process variables on quality of pellets

A lab-scale single-channel press was employed for producing pellets and the effect of pressure, die temperature, moisture content (MC), particle size, and binding agent on pellet quality was investigated. Meanwhile, torrefaction subsequent to pelletization was performed and the quality of pellets was evaluated under different torrefaction conditions. From the analysis of variance (at p < 0.05), temperature (70–200 °C), pressure (70–160 MPa), and MC (5.5–28%) were found most significant factors for density, strength, and compression energy of pellets. Particle density and strength of raw wood pellets were significantly affected by temperature and pressure. The highest particle density and strength (1.307 g/cm3, 11.8 N/mm2) for particles of 0.25–0.5 mm and (1.300 g/cm3 and 10.7 N/mm2) for particles of 0.5–1.41 mm were found at a temperature of 200 °C (pressure ≥ 100 MPa and MC 5.5%), indicating that pelletization at a temperature of 200 °C was beneficial. The relaxed density of pellets remained the same as of particle initial density after storage time of 2 weeks. The expansion ratio of the pellet was found lower. Use of synthetic resin as a binder in the proportion of 1:9 was found optimum, the particle density increased from 1.19 to 1.24 g/cm3 (0.25–0.5 mm) and from 1.17 to 1.22 g/cm3 (0.5–1.41 mm) and energy consumption reduced by 9.39% (0.25–0.5 mm) and 8.7% (0.5–1.41 mm). Pellets made at a temperature of 200 °C were found water-resistive as compared to those made at other temperatures. The highest lower heating value, i.e., 26.76 MJ/kg of torrefied pellets (TOPs) was achieved at 300 °C and 120 min. Torrefaction process parameters adversely affected the particle density, volumetric energy density, strength, and durability of TOPs. The highest true density (1.85 g/cm3) and porosity (65 v %) for TOPs were achieved at 300 °C and 120 min, much higher than those of raw pellets. Moisture uptake of TOPs at 300 °C was 2.0–2.8 wt.%, showing strong water-resistant ability. From the results of FTIR, O–H bond was destroyed after torrefaction.

Effect of Torrefaction Treatment on Physical and Fuel Properties of Caragana (Caragana korshinskii) Pellets

Caragana is potentially an alternative biomass feedstock in China. The influences of different torrefaction conditions on the physical characteristics and fuel properties of pellets made from Caragana were investigated. The Caragana pellets were torrefied under the temperature of 225, 250, and 275 °C with the residence time of 10, 15, and 20 min, respectively. The results showed that the density of torrefied pellet reduced by 24.07 to 35.20%. Compression strength decreased to 7.62 N/mm2 for treated pellets under the condition of 275 °C and 20 min with the maximum weight loss of 29.93%, and the higher heating value increased to 20.95~22.93 MJ/kg. The equilibrium moisture content of the torrefied pellets decreased by 46.17% compared with raw materials. The mass yield and energy yield of torrefied pellet were 70.07 to 88.54% and 70.93 to 90.79%, respectively. With the increase of the torrefaction severity, the mechanical and physical properties of the pellets decreased, while the storage performance and fuel properties enhanced. Simultaneously, the torrefaction temperature had more influence on the fuel properties of Caragana pellets than the residence time. The Caragana pellets torrefied at 250 °C for 20 min maintained not only almost the same mechanical and physical properties as the untreated pellets but performed better storage and fuel properties.

Impact of torrefaction conditions on the physicochemical properties of mustard crop residue

Abstract Torrefaction process is related to pyrolysis treatment in an inert atmosphere which is employed to improve the chemical and physical properties of biomass/solid fuels for energy generation. In present study, mustard crop residue (MCR) which is an abundantly available lignocellulosic waste was torrefied at three different temperatures (200 °C, 250 °C, and 300 °C) for three different residence time (30, 45, and 60 min). Mass yield, energy yield, high heating value, elemental composition, ash content, and volatile matter content of raw and torrefied MCR are computed. The functional behavior and combustion characteristics of raw and torrefied MCR was studied by Fourier Transform Infra-Red Spectrometry (FTIR). The mass and energy yield of torrefied MCR were found to decrease with an increase in torrefaction temperature and residence time. Elemental analysis of torrefied MCR showed an increase in weight percentage of carbon with increase in temperature and residence time of torrefaction due to release of oxygen congaing volatiles. The high heating value was found to upgrade from 16.92 MJ kg−1 for raw MCR to 21.93 MJ kg−1 of torrefied MCR at higher torrefaction temperature and time. FTIR analysis confirmed that hydroxyl group content decreased due to release of H2O, CO, and CO2. Therefore, torrefaction is found to be an effective treatment process for improving physicochemical properties of MCR.

Impact of torrefaction on entrained-flow gasification of pine sawdust: An experimental investigation

Entrained-flow gasification of raw and torrefied pine sawdust (PS) was conducted at 1300 °C and different equivalence ratios (ERs). The effects of torrefaction conditions on gasification performance as well as the relationship between soot yield and the volatile content of feedstocks were investigated. An additional burning procedure after gasification was used to quantify the carbon deposits (CD) inside the reactor tube, which completed the mass balance calculation and thus enabled reliable conclusions to be made. The results of the entrained-flow gasification tests indicated that PS torrefied at 280 °C exhibited a better gasification performance than raw PS and PS torrefied at other temperatures. Moreover, it was found that torrefaction pretreatment also reduced soot yield. These results indicated that the total yield of soot and CD is closely related to the content of volatile matter in feedstocks.

Machine learning application to predict yields of solid products from biomass torrefaction

Machine learning was used to develop a model that had the capability to predict yields of solid products from biomass torrefaction using input features of biomass properties and torrefaction conditions. With ten-fold cross-validation, several machine learning algorithms were evaluated, and their hyper-parameters were optimized by a full-factor grid search. Gradient tree boosting algorithm was found to have the highest prediction accuracy with R2 of about 0.90 and an average error of 0.07 w/w. Six highly important features on making predictions of the model were torrefaction temperature, residence time, and O2 concentration in the reacting gas for torrefaction conditions, as well as volatile matter, carbon content, and oxygen content for biomass properties. Unlike the carbon content, the other features were found to have a negative effect on the yields of torrefied biomass. The biomass property features contributed to the solid yields for about 30%, with approximately one-third accounted by the volatile matter.

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.

Comparative study of the fuel quality and torrefaction performance of biomass and its molded pellets: effects of temperature and residence time

ABSTRACT Torrefaction is a promising pretreatment for upgrading biomass. Biomass pellets have been widely used in heating and power plants, but the pellets torrefaction has not been fully studied. The torrefaction performance of raw biomass samples and molded pellets of those biomass samples were compared in this study. The torrefaction was performed using a horizontal sliding tube furnace with different torrefaction severities (210°C, 240°C, 270°C, and 300°C; 15 min, 30 min, and 45 min). Three key results were obtained from this work. (i) The temperature and residence time had an obvious influence on biomass torrefaction performance. Compared to the untreated samples (rice husk particles, rice husk pellets, pine wood sawdust, and pine wood pellets), the oxygen content of the samples torrefied under the most severe torrefaction conditions (300°C and 45 min) decreased by 43.1%, 27.4%, 34.2%, and 25.3%, respectively. In contrast, the high heating value and hydrophobicity of the torrefied samples increased. (ii) The biomass source had important effects on the torrefaction results. Under the same torrefaction conditions, the mass yields and energy yields were highest for pine wood sawdust, followed by rice husk particles, pine wood pellets, and rice husk pellets. This is related to the composition and thermal decomposition of the different biomass samples. (iii) The loose raw biomass released more volatiles during torrefaction compared to biomass pellets, which were limited by heat and mass transfer, leading to complete thermal decomposition of hemicellulose and other components. Overall, the torrefaction performance of biomass varied with the temperature and residence time, and torrefaction had a greater effect on the raw biomass than on the biomass pellets.

Torrefaction of ponkan peel waste in tubular fixed-bed reactor: In-depth bioenergetic evaluation of torrefaction products

The objective of this study is to evaluate the optimum torrefaction conditions needed to improve the energetic characteristics of Ponkan peel waste (a citrus-processing industry by-product). Torrefaction was performed in a tubular fixed-bed reactor at 200–300 °C for 15–60 min. The non-condensable and condensable gases were analyzed by a gas analyzer and GC-MS, while the energetic and physicochemical characteristic of torrefied solids were evaluated with several techniques and indexes. The energetic characteristics of the torrefied solid are better for torrefaction performed at 300 °C, however, the energy yield is lower than those produced at 200 °C. The torrefied solid produced at 200 °C had heating values similar to those of biomasses widely used in combustion and gasification systems. CO2 and CO were the main compounds found in non-condensable gases, and their content increases according to the torrefaction temperature. The condensable gases are composed of several compounds whose distribution changes according to the torrefaction temperature. The temperature of 200 °C was the most recommended for torrefaction of Ponkan peel waste since the condensable gases included products with market value, lower CO2 release, and higher solid and energy yield, and improved energetic characteristics of the solid.

Determination of the Optimal Operative Conditions for the Torrefaction of Olive Waste Biomass

The need for new energy sources and the problems associated with waste in the agroforestry industry are an opportunity for the recovery of this waste. For the use of this agricultural waste as energy, different pretreatments, such as torrefaction, can be carried out. Torrefaction is a thermochemical treatment involving energetic densification of biomass at temperatures ranging from 200 to 300 °C under an inert and anaerobic environment. This study developed a numerical model to evaluate the effect of temperature and residence time of torrefaction on biomass from olive tree waste to determine optimum operative conditions for the process. Four temperatures and four residence times, in the operation range of the process, were tested to determine the weight loss and the higher heating values (HHVs) of the torrefied sample. From these data, a numerical model was developed to infer the complete behavior of the process in the temperature range between 200 and 300 °C and in the residence time range of a few minutes to 2 h. The HHV of the torrefied sample increased at a temperature between 200 and 275 °C. However, from 275 to 300 °C, there was an HHV decrease. The effect of the residence time depended on the torrefaction temperature. At low temperatures, there were no statistically significant differences, although an increase of HHV was detected under 120 min. However, at 250 °C this effect was reversed, and statistically significant differences were not observed between 30 and 120 min. Overall, the increase of temperature in the torrefaction process reduces the residence time needed to achieve the maximum HHV. As a result, the optimum conditions of torrefaction for this biomass were, approximately, 275 °C and 30 min of residence time. This reaction yielded an optimum 5830 cal/g HHV.