Torrefaction Of Wood Research Articles

Influence of Elevated Pressure on the Torrefaction of Wood

The use of pressurized reactors in industrial processes can improve efficiency and economics. Torrefaction is a partial pyrolysis of lignocellulosic biomass designed to result in a solid product with improved fuel properties for utilization in combustion and gasification. In this work, the influence of elevated pressure on the torrefaction of wood has been investigated. Wood samples were torrefied using a pressurized thermogravimetric reactor (PTGR) with pressures of 0.1 to 2.1 MPa. The results indicate that reactor pressure, particle size of feedstock, and wood species are all factors in torrefaction yield improvements. Torrefaction at 2.1 MPa pressure improved the higher heating value (calculated) of single-particle beech cylinders from 20.4 to 22.2 MJ kg–1, the increase ranging from 7.5 to 19% from the untreated heating value. Decomposition reactions were accelerated with pressure so that a given mass yield was realized in a shorter time. At 2.1 MPa pressure and 280 °C the time was reduced by over 60% ...

Exothermicity in wood torrefaction and its impact on product mass yields: From micro to pilot scale

This paper focuses on the effects of exothermic reactions during torrefaction, a mild heat treatment process in the temperature range 200 to 300 °C. Three different scales are considered, the micro‐particle (powder scale), the macro‐particle (woodchips or larger) and the thick fixed bed (pilot reactor) together with three wood types, spruce, beech and locust. At the powder scale, TGA‐DSC tests indicate that exothermic reactions are noticeable principally during the first stages of torrefaction. The mass loss kinetics are used to evaluate parameters for a DAEM (Distributed Activation Energy Method) model. At the macro‐particle scale, temperature measurements within wood planks heated in an oven depict the presence of temperature overshoots due to the exothermic reactions that lead to unevenly treated particles. At the reactor scale, a large fixed bed of wood chips is heated in the same oven by an up‐flowing recirculated mixture of inert gas and volatiles. The exothermic reactions are found to generate a heat wave that propagates up the bed. Total mass losses are found to largely exceed those predicted with the DAEM model based on recorded bed temperatures. This means that, in order to reach the measured mass yields, the inner core temperatures of the wood chips must be higher than those of their outer surface and of the gas flow. Multi‐scale modelling approaches are therefore required to take into account the combined exothemicity and diffusional limitations within the wood chips and at their exchange surfaces.

Effect of temperature during wood torrefaction on the formation of lignin liquid intermediates

Torrefaction enhances physical properties of lignocellulosic biomass and improves its grindability. Energy densification, via fuel pellets production, is one of the most promising uses of torrefaction. Lignin contributes to self-bonding of wood particles during pelletization. In biomass thermal pretreatment, part of lignin (in the form of lignin liquid intermediates – LLI) migrates from the cell wall and middle lamella and deposits on the fibers’ surfaces and/or inner surface of the secondary cell wall. This material can play an important role on bonding particles during wood pelletization as well as production of wood composites. The objective of this paper was to investigate the influence of torrefaction conditions on amount, composition, molecular weight, and pattern of deposition of LLI on wood cells. Torrefaction of extracted ponderosa pine in the range of temperatures 225–350°C was conducted in a tube furnace reactor and the torrefied wood was extracted with dichloromethane (DCM) to isolate the lignin-rich soluble material. A maximum yield of DCM-soluble material was observed in wood torrefied at approximately 300°C for 30min. ESI/MS revealed that the molecular weight of the removed material is less than 1200gmol−1 and decreases as torrefaction temperature augments. Semiquantitative Py-GC/MS of the DCM-soluble material suggests that this lignin-rich material migrates and deposits on cells’ surfaces in amounts that depend on the torrefaction conditions. Py-GC/MS of the solid fraction after the DCM process showed a progressive reduction of products of the pyrolysis of lignin and levoglucosan as torrefaction temperature increased, revealing that lignin content in the solid decreased due in part to migration. SEM of torrefied particles helped to show the apparent formation of LLI during torrefaction. Results suggest that it is possible to control the thermal pretreatment conditions to increase or reduce the amount of lignin-rich material on fiber surfaces as required for downstream processes (e.g., fuel pellets or wood composites manufacture).

Volatile species release during torrefaction of wood and its macromolecular constituents: Part 1 – Experimental study

Torrefaction tests were carried out in a thermobalance and in a lab-scale device on beechwood and on its constituents – cellulose, lignin and xylan. The main volatile species measured were water, formaldehyde, acetic acid and carbon dioxide. Thanks to measurement before volatiles condensation, the yield of formaldehyde was shown to be higher than usually observed in literature. Smaller amounts of methanol, carbon monoxide, formic acid and furfural were also quantified. Each constituent did not produce all species. Beech torrefaction could be described by the summative contribution of its three constituents up to 250 °C. At 280 °C and 300 °C, tests performed with constituents mixtures showed that there were interactions between cellulose and the two other constituents. A hypothetical mechanism was proposed to explain these interactions.

Thermal Decomposition Kinetics of Woods with an Emphasis on Torrefaction

The pyrolysis kinetics of Norwegian spruce and birch wood was studied to obtain information on the kinetics of torrefaction. Thermogravimetry (TGA) was employed with nine different heating programs, including linear, stepwise, modulated and constant reaction rate (CRR) experiments. The 18 experiments on the 2 feedstocks were evaluated simultaneously via the method of least-squares. Part of the kinetic parameters could be assumed common for both woods without a considerable worsening of the fit quality. This process results in better defined parameters and emphasizes the similarities between the woods. Three pseudo-components were assumed. Two of them were described by distributed activation energy models (DAEMs), while the decomposition of the cellulose pseudo-component was described by a self-accelerating kinetics. In another approach, the three pseudo-components were described by n-order reactions. Both approaches resulted in nearly the same fit quality, but the physical meaning of the model, based on three n-order reactions, was found to be problematic. The reliability of the models was tested by checking how well the experiments with higher heating rates can be described by the kinetic parameters obtained from the evaluation of a narrower subset of 10 experiments with slower heating. A table of data was calculated that may provide guidance about the extent of devolatilization at various temperature–residence time values during wood torrefaction.

A comprehensive dual-scale wood torrefaction model: Application to the analysis of thermal run-away in industrial heat treatment processes

A dual-scale model of the torrefaction of wood was developed and used to study industrial configurations. At the local scale, the computational code solves the coupled heat and mass transfer and the thermal degradation mechanisms of the wood components. At the global scale, the two-way coupling between the boards and the stack channels is treated as an integral component of the process. This model is used to investigate the effect of the stack configuration on the heat treatment of the boards. The simulations highlight that the exothermic reactions occurring in each single board can be accumulated along the stack. This phenomenon may result in a dramatic heterogeneity of the process and poses a serious risk of thermal runaway, which is often observed in industrial plants. The model is used to explain how thermal runaway can be lowered by increasing the airflow velocity, the sticker thickness or by gas flow reversal.

More efficient biomass gasification via torrefaction

Wood torrefaction is a mild pyrolysis process that improves the fuel properties of wood. At temperatures between 230 and 300 °C, the hemicellulose fraction of the wood decomposes, so that torrefied wood and volatiles are formed. Mass and energy balances for torrefaction experiments at 250 and 300 °C are presented. Advantages of torrefaction as a pre-treatment prior to gasification are demonstrated. Three concepts are compared: air-blown gasification of wood, air-blown gasification of torrefied wood (both at a temperature of 950 °C in a circulating fluidized bed) and oxygen-blown gasification of torrefied wood (at a temperature of 1200 °C in an entrained flow gasifier), all at atmospheric pressure. The overall exergetic efficiency of air-blown gasification of torrefied wood was found to be lower than that of wood, because the volatiles produced in the torrefaction step are not utilized. For the entrained flow gasifier, the volatiles can be introduced into the hot product gas stream as a ‘chemical quench’. The overall efficiency of such a process scheme is comparable to direct gasification of wood, but more exergy is conserved in as chemical exergy in the product gas (72.6% versus 68.6%). This novel method to improve the efficiency of biomass gasification is promising; therefore, practical demonstration is recommended.

Torrefaction of wood: Part 1. Weight loss kinetics

Torrefaction is a thermal treatment step in the relatively low temperature range of 225–300 °C, which aims to produce a fuel with increased energy density by decomposing the reactive hemicellulose fraction. The weight loss kinetics for torrefaction of willow, a deciduous wood type, was studied by isothermal thermogravimetry. A two-step reaction in series model was found to give an accurate description. For the two steps, activation energies of 76.0 and 151.7 kJ/mol, respectively, and pre-exponential factors of 2.48 × 10 4 and 1.10 × 10 10 kg kg −1 s −1, respectively, were found. The first reaction step has a high solid yield (70–88 wt%, decreasing with temperature), whereas less mass is conserved in the second step (41 wt%). The fast initial step may be representative for hemicellulose decomposition, whereas the slower subsequent reaction represents cellulose decomposition and secondary charring of hemicellulose fragments. The kinetic model is applied to give recommendations for industrial torrefaction process conditions, notably operating temperature, residence time and particle size.

Torrefaction of wood: Part 2. Analysis of products

Torrefaction is a mild pyrolysis process carried out at temperatures ranging from 225 to 300 °C, in which hemicellulose, the most reactive fraction of wood, is decomposed. Dehydration and decarboxylation reactions cause a mass loss of the wood, whereas the lower heating value of the wood is largely conserved. Deciduous wood types (beech and willow) and straw were found to produce more volatiles than coniferous wood (larch), especially more methanol and acetic acid. These originate from acetoxy- and methoxy-groups present as side chains in xylose units present in the xylan-containing hemicellulose fraction. The torrefied wood product has a brown/black color, reduced volatile content and increased energy density: 20.7 MJ/kg (after 15 min reaction time at 270 °C) versus 17.7 MJ/kg for untreated willow. It has favorable properties for application as a fuel for gasification and/or (co-)combustion.

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Wood torrefaction is a mild pyrolysis process that improves the fuel properties of wood. At temperatures between 230 and 300 °C, the hemicellulose fraction of the wood decomposes, so that torrefied wood and volatiles are formed. Mass and energy balances for torrefaction experiments at 250 and 300 °C are presented. Advantages of torrefaction as a pre-treatment prior to gasification are demonstrated. Three concepts are compared: air-blown gasification of wood, air-blown gasification of torrefied wood (both at a temperature of 950 °C in a circulating fluidized bed) and oxygen-blown gasification of torrefied wood (at a temperature of 1200 °C in an entrained flow gasifier), all at atmospheric pressure. The overall exergetic efficiency of air-blown gasification of torrefied wood was found to be lower than that of wood, because the volatiles produced in the torrefaction step are not utilized. For the entrained flow gasifier, the volatiles can be introduced into the hot product gas stream as a ‘chemical quench’. The overall efficiency of such a process scheme is comparable to direct gasification of wood, but more exergy is conserved in as chemical exergy in the product gas (72.6% versus 68.6%). This novel method to improve the efficiency of biomass gasification is promising; therefore, practical demonstration is recommended.