Torrefaction Process Research Articles

Modelling of a Torrefaction Process Using Thermal Model Object

So far, the torrefaction process has been merely discussed based on the physical and chemical characteristics of the final product, but the mechanism has not been yet pondered and investigated. Thus far, microwave torrefaction has been predominately used for thermal pre-treatment; therefore, a paradigm shift in the methodology has been introduced by using a Joule heating system. The article mainly focuses on the thermal engineering aspect of the torrefaction process. The densified black pine underwent thermal pre-treatment at a temperature of 523 K. The furnace used for torrefaction was initially improvised to carry out thermal degradation at quasi-static/dynamic conditions. A 3D PDE thermal model was developed to determine the numerical solution and temperature distribution across a black pine pellet. To compare the effect of the linear ramping profile, time-dependent as well as fixed Dirichlet conditions were applied to the proposed model. The mass distribution, duration of the torrefaction process, the effect of Nusselt and Reynold’s number of inert gas, and thermal history are some of the factors whose influence on the numerical solution was investigated. The simulation of thermal pre-treatment and its effect on the heat transfer characteristic was examined with help of a PDE thermal model, whereas the numerical solution of diffusion of the product of reactions was determined by solving the partial differential equations with the help of the discretisation method (PDEPE). The densification of black pine was performed in a ring die, whereas initial milling of biomass was carried out using a 1.5 mm sieve size. The system was found to have a homogeneous distribution in energy and temperature with time, whereas the amplitude of heat flux along the radial direction was reduced by 15% if the same pellet underwent torrefaction for a duration of 5 min in dynamic mode. Similarly, a 64.46% drop in amplitude of heat flux along the azimuth plane could be seen while performing torrefaction in a time-dependent thermal history. However, the relative amplitude of the heat flux at the centre of the pellet was estimated to be lowered by 98.41% along the vertical axis for heating a pellet in a quasi-static condition. The net change in the mass fraction of carbon dioxide across the boundary film was seen to be 40% higher than that of carbon monoxide. The rate of change of mass fraction of carbon monoxide across the boundary film was increased by 7–11% with the increase in torrefaction time. A 6.8% rise in the evaporation of water was noticed during the first half interval of torrefaction (from 5 min to 10 min). In the second half, from 10 min to 15 min, it was merely increased by 5.8%. A relative drop of 17.24% in water evaporation was estimated in the dynamic state of the system.

Multiple drivers, interaction effects, and trade-offs of efficient and cleaner combustion of torrefied water hyacinth

Developing cleaner and affordable alternatives to the sole reliance on fossil fuels has intensified efforts to improve the thermochemical conversion property of the second-generation lignocellulosic biomass. This study aimed to explore the effects of the two torrefaction temperatures (200 and 300 °C), the two reaction atmospheres (N2/O2 and CO2/O2), and the three heating rates (5, 10, and 15 °C/min) on the combustion regime of water hyacinth (WH). Decomposition behaviors, reaction kinetics, thermodynamics, and mechanisms, evolved emissions and functional groups, and fuel microstructure properties were quantified. The deoxygenation and dehydration reactions acted as the main drivers of the torrefaction process, with the peak degree of deoxygenation of 86.21% for WH torrefied at 300 °C (WH300). WH300 significantly reduced the quantity of oxygen-containing functional groups and altered the fuel microstructure properties. The order of the decomposition rates of the pseudo-components were hemicellulose > cellulose > lignin for both WH and WH torrefied at 200 °C (WH200) and cellulose > lignin > hemicellulose for WH300. The average activation energy fell from 197.71 to 195.71 kJ/mol for WH, 287.90 to 195.97 kJ/mol for WH200, and 226.92 to 184.94 kJ/mol for WH300 when the atmosphere changed from N2/O2 to CO2/O2. The heating rate exerted a stronger control on their combustion behaviors than did the reaction atmosphere. CO2, NO, and NO2 emissions dropped by 46.0, 53.1, and 65.9% for WH200 and 29.6, 42.8, and 62.5% for WH300, respectively, when compared to WH. 473.7 °C, 5 °C/min, and the CO2/O2 atmosphere were the optimal settings for the maximized combustion efficiency. 717.1 °C was determined as the optimal setting for the minimized combustion emissions. Our study can yield new insights into the large-scale and cleaner combustion of the torrefied water hyacinth.

Thermal decomposition behavior and sulfur release characteristics for torrefied wheat straw during pyrolysis process

Understanding the release characteristics of S for pyrolysis process is crucial to the development of biomass thermochemical conversion. The thermal decomposition behavior and S release characteristics for torrefaction and pyrolysis process as well as the impact of torrefaction on the S release during subsequent pyrolysis process of wheat straw were evaluated. In the case of torrefaction, high reaction temperature promoted the increase in S release percentage, which was linearly proportional to mass loss. For pyrolysis process, the release percentage of S increased rapidly up to 70.50% at 500 ℃, whereas the release percentage curve showed an unchanged trend for further increase of the pyrolysis temperature. Additionally, torrefaction pretreatment enhanced the pore properties of char, which promoted the physical resistance of released S during the diffusion process. Thus, torrefaction pretreatment effectively inhibited the release of S into the gas phase, and has a promoting effect on retaining more S in char.

Kinetic Study of the Isothermal Degradation of Pine Sawdust during Torrefaction Process.

The reaction kinetics of solid fuel is a critical aspect of energy production because its energy component is determined during the process. The overall fuel quality is also evaluated to account for a defined energy need. In this study, a two-step first-order reaction mechanism was used to model the rapid mass loss of pine sawdust (PSD) during torrefaction using a thermogravimetric analyzer (Q600 SDT). The kinetic analysis was carried in a MATLAB environment using MATLAB R2020b software. Five temperature regimes including 220, 240, 260, 280, and 300 °C and a retention time of 2 h were used to study the mechanism of the solid fuel reaction. Similarly, a combined demarcation time (i.e., estimating the time that demarcates the first stage and the second stage) and iteration technique was used to determine the actual kinetic parameters describing the fuel’s mass loss during the torrefaction process. The fuel’s kinetic parameters were estimated, while the developed kinetic model for the process was validated using the experimental data. The solid and gas distributions of the components in the reaction mechanism were also reported. The first stage of the degradation process was characterized by the rapid mass loss evident at the start of the torrefaction process. In contrast, the second stage was characterized by the slower mass loss phase, which follows the first stage. The activation energies for the first and second stages were 10.29 and 141.28 kJ/mol, respectively, to form the solids. The developed model was reliable in predicting the mass loss of the PSD. The biochar produced from the torrefaction process contained high amounts of the intermediate product that may benefit energy production. However, the final biochar formed at the end of the process increased with the increase in torrefaction severity (i.e., increase in temperature and time).

Torrefaction influence on combustion kinetics of Malaysian oil palm wastes

The use of renewable energy has increased in the last decades including the consumption of biofuels, so the research focused on improving these energetic vectors has increased too, developing techniques that enhance energetic properties of these fuels. In this context, torrefaction is a thermal pre-treatment that diminishes the main disadvantages present in biomass such as hygroscopicity, low calorific value per unit mass and high susceptibility to self-ignition. In this study raw and torrefied palm oil wastes samples are tested using thermogravimetric analysis (TGA) in order to study the influence of torrefaction on combustion kinetic parameters using Coats-Redfern and Freeman-Carroll methods, showing that torrefaction increases activation energy as it enhances thermal stability of the samples. The composition of the samples is also estimated using TGA through Fraser-Suzuki deconvolution showing the changes of lignin, cellulose, and hemicellulose due to torrefaction process.

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.

Upgrade the torrefaction process of bamboo based on autohydrolysis pretreatment

Torrefaction is a routine and effective method for recovering energy from biomass. In this study, autohydrolysis pretreatment was proposed to upgrade the torrefaction process of moso bamboo, and their relationship was evaluated. Results showed that about 80.24 % of bamboo hemicellulose were dissolved during autohydrolysis pretreatment. Compared with main chain, arabinan side chain of hemicellulose was much easier to be degraded, and more uniform hemicellulose could be obtained at 160 °C. The O/C values of autohydrolysis solid residues and the mass yield of torrefied bamboo decreased gradually with the enhancement of reaction severity (autohydrolysis and/or torrefaction), while the HHV values increased. The higher torrefaction temperature or autohydrolysis pretreatment severity was, the better thermostability of torrefied bamboo could be observed. 2D-HSQC of milled wood lignin revealed that autohydrolysis pretreatment can effectively accelerate the degradation of β-O-4 linkage in lignin and increase G-type lignin content during torrefaction process. Compared with direct torrefaction, prehydrolysis can greatly facilitate the torrefaction process and enhance the quality of final products.

Process Modeling of Drying and Torrefaction of Oil Palm Trunk (OPT)

This paper concerns on process modeling of drying and torrefaction of oil palm trunk (OPT), abundant biomass resources coming from replantation of oil palm. The processes are very crucial in treating of OPT before utilization for energy source due to high level of water content and volatile matters in the biomass. Modeling with Aspen Plus is performed to predict the composition of product in mass in fastest way without doing experiment. The study uses Flash Dryer Model and Reactor Model available in Aspen as tool box for modeling the drying process and torrefaction process. Objective of the paper is to validate the process model with experimental results from that of Prasetyo et al.(2016) in identifying final mass and calorific value of torrefied OPT. The process modeling of OPT drying and torrefaction gives the charcoal yield of 0.2972 compared to experiment yield of 0.24. The calorific value (HCV) of torrefied OPT with RGibbs is 6,901.0 kcal/kg that is similar with experiment result of 6,939.2 kcal/kg.

Biochar from Oil Palm Frond to Reduce Fe Ions in Artificial Solution and Peat Water

Raw oil palm fronds (OPF-R) converted into biochar (OPF-B) by torrefaction processes at temperatures range of 200 °C to 300 °C. This study aims to evaluate OPF-B performance to reduce iron ions in artificial solutions and peat water. Batch adsorption carried out at room temperature by stirring (175 rpm) using an orbital shaker. Some parameters observed included optimum torrefaction temperature, optimum conditions of Fe ions adsorption (optimum pH and contact time) and sorption isotherm. Metal content measured using atomic absorption spectroscopy (AAS). Functional groups on material surface characterized using Fourier Transform Infrared (FT-IR). Torrefaction at 260 °C produces biochar with the highest efficiency in removing iron ions. Both functional groups C=O and C-O attributed to lignocellulose on OPF-B were diminished and absorption band of aromatic (C=C) increased which caused by torrefaction. The optimum conditions for removal of Fe ions at pH 5 and 30 minutes contact time. Adsorption process follows the Langmuir Isotherm which indicates the formation of a monolayer with qmax 111.11 mg/g. Amount of 0.025 g OPF-B able to reduce iron concentration from 13.960 mg/L to 0.746 mg/L in 100 mL peat water (has complied with Indonesian clean water standard for iron concentration).

Comparative life cycle energy and greenhouse gas footprints of dry and wet torrefaction processes of various biomass feedstocks

This study compares the life cycle energy consumption and greenhouse gas (GHG) emissions of electricity generation from bio-coals produced from various biomass feedstocks via dry torrefaction (DT) and wet torrefaction (WT) processes. Wheat straw, pine woodchips, grape pomace, manure and algae are the main feedstocks evaluated. The energy consumption and the associated GHG emissions at each life cycle stage were calculated. The main stages included are in-field preparation, feedstocks transportation to torrefaction plant, torrefaction process, bio-coal transportation to power plant, and power plant operations. The results show that all pathways are competitive with coal-based electricity in terms of GHG emissions except pathways with algae as feedstock and manure biochar-based electricity generation. Among the pathways, electricity generation from pine woodchips biochar appears to be the best option, with an 82% GHG emissions reduction compared to coal-derived electricity, followed by wheat starw biochar and grape pomace hydrochar with 80% and 75% emission reductions, respectively. Electricity generation from wheat straw biochar, pine woodchips biochar, and grape pomace hydrochar lead to the highest net energy ratios (NERs) of 4.19, 3.58, and 2.59, respectively. The results also show that the combustion of the biomass for supplying electricity used in the plant decreases GHG emissions in all pathways particularly in WT ones. Sensitivity analysis of using different emissions allocation methods, transportation distances to torrefaction plant and to power plant, emission factor of electricity consumed, and quantity of natural gas consumed were aslo conducted. The developed information is novel and can be used for investment decisions and policy formulation around the world.

Energy Utilization of Torrefied Residue from Wine Production.

A significant amount of waste is generated in the food industry, which is both an environmental and an economic problem. The recycling of this waste has become an important area of research. The processing of grapes produces 20–30% of the waste in the form of grape pomace and stalks. This article assesses the fuel values of these materials before and after torrefaction. The input materials were grape pomace samples from the varieties Riesling (Vitis vinifera “Welschriesling”) and Cabernet Sauvignon (Vitis vinifera “Cabernet Sauvignon”) from the South Moravia region and stalks from the variety Welschriesling. The torrefaction process was performed using a LECO TGA 701 thermogravimetric analyzer under nitrogen atmosphere at set temperatures of 225 °C, 250 °C, and 275 °C. The residence time was 30 min. Elemental analysis, calorific value, and gross calorific value were determined for all samples. The analyses show a positive effect of torrefaction on fuel properties in the samples. Between temperatures 250 °C and 275 °C, the carbon content increased by 4.29 wt.%, and the calorific value increased with the increase in temperature reaching a value of 25.84 MJ·kg−1 at a peak temperature of 275 °C in the sample grape pomace from blue grapevine.

Poplar wood torrefaction: Kinetics, thermochemistry and implications

The kinetic and thermochemical models for poplar wood torrefaction were developed in the present work. The torrefaction kinetic model satisfactorily fitted the experimental thermogravimetric analysis (TGA) data of poplar wood torrefaction and provided a coherent description of the evolution of torrefaction volatile and solid products in terms of a set of identifiable chemical components and elemental compositions. The torrefaction thermochemical model described the thermochemical performance of poplar wood torrefaction processes. The results from the kinetic and thermochemical models for poplar wood torrefaction showed that (1) high temperature increases the evolution rate of torrefaction products, and favors the formation of torrefaction volatiles; (2) the heating rate has a slight effect on evolution for torrefaction process; (3) the mass and energy yields of torrefaction products are significantly influenced by both torrefaction temperature and residence time; (4) the heat of torrefaction reaction is mostly endothermic with a relatively small amount (less than 10% of the raw material energy content); (5) for the overall torrefaction processes, the sensible and latent energy of torrefaction products accounts for 5–18% of the total energy input and the remaining energy input transfers into the energy contents of torrefaction products. This work provides a theoretical guidance for future evaluation and optimization of woody biomass torrefaction systems/processes, and thereafter for the industrial application of woody biomass thermochemical conversion.

Kitchen waste torrefaction with the aid of charcoal microwave absorber agent

The disposal of kitchen waste (KW) from households associated with the increasing population is an alarming environmental concern with the release of greenhouse gases additional to the large landfill spaces requirement. An experimental rig was developed to characterize the torrefaction process of banana peels (BP) and stale bread (SB) in a microwave-heated reactor. The new approach to enhance the microwave irradiation absorption was investigated by adding charcoal bed. The KW materials were upgraded successfully into torrefied product fuel with up to 31% elevation in the heating value compared to the raw materials. However, the significant reduction in mass and the amount of energy consumed during the torrefaction have a direct effect on the economic evaluation of the product. For domestic production of solid fuels, 0.058 $/kg production cost was achieved for both types of KW which is significantly lower than the current market prices for charcoal in Malaysian markets.

The Effect of Lifters, Inclination Angle and Rotational Speed on Residence Time of Rotary Kiln for Torrefaction Process

Torrefaction process is the innovation to improve the properties of biomass. Residence time is one of the parameters that affects the properties of torrefied biomass. The residence time of a rotary kiln is the time of biomass drop into the cylinder until getting out of the cylinder. So, the propose of this study is the effect of lifters, inclination angle and rotational speed on the residence time of a rotary kiln for the torrefaction process. Palm kernel shell was used in this research. Palm kernel was chopped and minced to reduce size. The rotary kiln (0.3 meters diameter 6 meters in length) was used in this research. The number of lifters in this research were 0, 1 and 2. The rotational speed were 1, 2 and 3 rpm. The inclination angle was 1, 3 and 5 degrees. The hopper was used for the feeding system. The time was collected from biomass drop into the cylinder until getting out of the cylinder. The results reveal that the number of lifters, rotational speed and inclination angle affects the residence time of the rotary kiln. The residence time of rotary kiln was an increase when the number of lifters increase. The residence time of the rotary kiln was decreased when the rotational speed and inclination angle increase. It can be concluded that the least residence time of rotary kiln was 86.94 minute at 0 lifters, 3 rpm and 5 degrees. The most residence time was 9.22 minute at 2 lifters, 1 rpm and 1 degree.

Enhanced Oil Palm Waste Properties through Torrefaction and Pelletization

Torrefaction is a thermal pre-treatment process to treat biomass at temperature range of 200-300°C under an inert atmosphere. It was known that torrefaction process strongly depended on the decomposition of the lignocellulosic constituents in oil palm waste namely hemicellulose, cellulose and lignin. The objective of this research is to study the effect of torrefaction on the characteristics of pellet formed from palm mesocarp fiber (PMF), oil palm frond (OPF) and palm kernel shell (PKS) as a solid fuel. The samples were torrefied at 270°C for 30 minutes. The torrefied samples were then densified into pellets through hot press machine at different temperatures (110 °C, 130 °C, 150 °C) and different pressure (10 MPa, 12 MPa, 14 MPa) for 20 minutes to find the optimum pelletization condition. The pellets were analyzed through mass yield, high heating value, fourier-transform infrared spectroscopy (FTIR) analysis, moisture absorption and compression test. Compression test is to study the strength of pellet by identifying the force required to break the pellet. The result shows that the mass yield of PKS was higher than PMF and OPF. The high heating value of torrefied oil palm waste improved as compared to the raw oil palm waste. Through FTIR, the changes of chemical bonds in oil palm waste after torrefaction have been identified. From the analysis of the pellet, the optimum conditions for pelletization were obtained, which are 130 °C and 12 MPa. The moisture absorption rate of torrefied pellet was lesser than raw pellets. Lastly, the torrefaction and pelletization process produced better quality pellets in term of high heating value, durability and more hydrophobic compared to raw pellets which can used as biofuel to replace the coal.

Comparison of different woody biomass gasification behavior in an entrained flow gasifier

The behavior of pulverized woody biomass gasification in an entrained flow gasifier (EFG) is investigated by utilizing an analogy to a coal gasifier. A computational fluid dynamics (CFD) model of a laboratory-scale coal gasifier was developed intended to extend a tool for further investigation of woody biomass gasification. The model is based on a Eulerian-Lagrangian approach that incorporates sub-models of heat and mass transfer, devolatilization, radiation, homogeneous and heterogeneous reactions, as well as the interactions between the gas and particle phases. The developed model is primarily validated with available experimental data and two similar published works. The model is developed for three types of woody biomass, which include typical wood, raw bamboo, and torrefied bamboo, to predict the gas compositions, temperature, and heating value of the produced syngas. The results indicate that the torrefaction process significantly enhances the gasification performance. The heating value of the produced syngas from the torrefied bamboo is increased by more than 35% compared to that of raw bamboo. The maximum predicted temperature for the typical wood is 1940 K while it is 2010 K and 2160 K for the raw and torrefied bamboos, respectively. Moreover, the predicted results suggest that while the range of the volatile matter is between 50 to 60 wt%, the fixed-carbon range is between 30 and 40 wt%, and the moisture content is under 10 wt%, different woody biomasses could be replaced with each other without significant reduction in the gasification performance.

Aquaculture wastewater treatment through microalgal. Biomass potential applications on animal feed, agriculture, and energy

The use of microalgae to remediate raw effluent from brown crab aquaculture was evaluated by performing batch mode growth tests using separately the microalgae Chlorella vulgaris (Cv), Scenedesmus obliquus (Sc), Isochrysis galbana (Ig), Nannocloropsis salina (Ns), and Spirulina major (Sp). Removal efficiencies in batch growth were 100% for total nitrogen and total phosphorus for all microalgae. Chemical oxygen demand (COD) remediations were all above 72%. Biomass productivity varied from 20.9 mg L−1 day−1 (N. salina) to 146.4 mg L−1 day−1 (C. vulgaris). The two best performing algae were C. vulgaris and S. obliquus and they were tested in semi-continuous growth, reaching productivities of 879.8 mg L−1 day−1 and 811.7 mg L−1 day−1, respectively. The bioremediation of the effluent was tested with a transfer system consisting of three independent containers and compared with the use of a single container. The single container had the same capacity and received weekly the same volume of effluent as the three containers together. The remediation capacity of the 3 containers was much higher than the single one. The supplementation with NaNO3 was tested to improve the nutrient removal microalgae’ capacity, with positive results. The removal efficiencies were 100% for total nitrogen and total phosphorus and higher than 96% for COD. The obtained C. vulgaris and S. obliquus biomass were composed of 31 and 35% proteins, 6 and 8% lipids, 39 and 30% carbohydrates, respectively. The composition of these biomass suggest that it can be used as novel and sustainable ingredients in aquaculture feeds. The algal biomass of Cv and Sc were used as biostimulants in the germination of wheat and watercress, and very promising results were attained, with increases in the germination index for Cv and Sc of 175% and 48% in watercress and 84% and 98% in wheat, respectively. The biomasses of Cv and Sc were also subjected to a torrefaction process with 72.5 ± 1.7% char yields. The obtained biochars were tested as biostimulants for germination seeds (wheat and watercress) and as bio-adsorbent of dye solutions.

Sustainable Drying and Torrefaction Processes of Miscanthus for Use as a Pelletized Solid Biofuel and Biocarbon-Carrier for Fertilizers

Miscanthus is resistant to dry, frosty winters in Poland and most European Union countries. Miscanthus gives higher yields compared to native species. Farmers can produce Miscanthus pellets after drying it for their own heating purposes. From the third year, the most efficient plant development begins, resulting in a yield of 25–30 tons of dry matter from an area of 1 hectare. Laboratory scale tests were carried out on the processes of drying, compacting, and torrefaction of this biomass type. The analysis of the drying process was conducted at three temperature levels of the drying agent (60, 100, and 140 °C). Compaction on a hydraulic press was carried out in the pressure range characteristic of a pressure agglomeration (130.8–457.8 MPa) at different moisture contents of the raw material (0.5% and 10%). The main interest in this part was to assess the influence of drying temperature, moisture content, and compaction pressure on the specific densities (DE) and the mechanical durability of the pellets (DU). In the next step, laboratory analyses of the torrefaction process were carried out, initially using the Thermogravimetric Analysis TGA and Differential Scaning Calorimeter DSC techniques (to assess activation energy (EA)), followed by a flow reactor operating at five temperature levels (225, 250, 275, 300, and 525 °C). A SEM analysis of Miscanthus after torrefaction processes at three different temperatures was performed. Both the parameters of biochar (proximate and ultimate analysis) and the quality of the torgas (volatile organic content (VOC)) were analyzed. The results show that both drying temperature and moisture level will affect the quality of the pellets. Analysis of the torrefaction process shows clearly that the optimum process temperature would be around 300–340 °C from a mass loss ratio and economical perspective.

Enhancing the fuel properties of rubberwood biomass by moving bed torrefaction process for further applications

Rubberwood biomass is widely available in the southern region of Thailand and it has high potential for biofuel applications. Applications of this biomass still have some limitations, however, such as low energy properties and high biological decomposition. Therefore, the aim of this study was to enhance the fuel properties of rubberwood biomass by a torrefaction process. Rubberwood sawdust (RWS) was torrefied at different temperatures (200, 250, and 300 °C) and for various times (20, 40, and 60 min) in a moving bed reactor. The product yield and characteristics of torrefied RWS were investigated. Results showed that the solid yield of torrefied RWS was in the range of 39.07–88.69%, depending on temperature and time. The fuel atomic ratios of torrefied RWS were better than of raw RWS. The energy content and energy density of torrefied RWS were clearly enhanced (19.78–27.17 MJ/kg and 4.94–6.59 GJ/m3). ICP-OES results revealed a variation of inorganic elements in torrefied RWS, which was consistent with the ash components given by XRF. Ash fusion temperature of raw RWS and torrefied RWS ashes was stable at 1458 °C. The slagging index of torrefied RWS was decreased, while its fouling index was elevated.

Simultaneous implementation of sludge dewatering and solid biofuel production by microwave torrefaction

Sludge dewatering is a matter of great concern to reduce the volume of sludge, stabilize its organic components, and achieve resource utilization. This study investigates sludge dewatering by microwave torrefaction along with the production of sludge solid biofuel at 480–800 W combined with durations of 5–25 min. Proximate analysis, calorific value analysis, thermogravimetric analysis, and scanning electron microscopy observations are employed to evaluate the dewatering degree, fuel properties, and energy efficiency of the torrefaction process. The independent parallel reaction (IPR) model and particle swarm optimization (PSO) analysis are also adopted for sludge pyrolysis kinetics calculation. The results show that microwave torrefaction is efficient for sludge dewatering with a short duration. The produced sludge solid biofuel is similar to stone-like coal, and can be used for civil or industrial boilers after flotation or just co-firing with briquette. The ash content of sludge solid biofuel shows a declining trend and the surface characteristics change from smooth to rough and fluffy with increasing the torrefaction severity. The bio-oil is mainly composed of phenols, siloxanes, and cholesterol. In addition, hydrogen is detected in the torgas. Furthermore, it is found that lower torrefaction power with a shorter duration yields a higher energy efficiency of the torrefaction process.