Co-pyrolysis of torrefied biomass and coal: Product evaluation and reaction modelling
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Résumé
The co-utilization of biomass and coal in existing coal-based conversion technologies has received increasing interest for its ability to reduce the dependence on fossil fuels and produce high quality products. However, the fuel properties of biomass as a feedstock (high moisture content, low energy density and high O/C ratio) lead to various operational challenges. Torrefaction is considered to be one of the most promising pre-treatment methods for upgrading biomass fuel properties to become more similar to coal. During this process, the physical and chemical properties of biomass as fuel are improved by increasing its energy density, lowering O/C and H/C ratios and inverting its hydrophilic nature. Co-pyrolysis of torrefied biomass and coal has the potential to be an efficient route for converting biomass to bio-energy and bio-refinery products. For example, as a first step in the gasifiers of coal-to-liquid (CTL) industrial plants for the production of Fischer Tropsch fuels. However, to stimulate the transition to bioenergy-based technologies, a reduction in process costs is required. This may be achieved by improving the efficiency of these technologies through a better understanding of the chemistry and physics of this process. While the practical aspects of the pyrolysis process with the successful design, optimization and operation are the principal economic drivers, the control of fundamental chemistry and physics rationalized by effective and accurate reaction modelling is critical. Due to the course of primary and secondary pyrolysis reactions being interwoven, clear limitations of knowledge still exist in the field of modelling, specifically for torrefied biomass. The aim of this thesis was to investigate the yields and composition of products derived during co-pyrolysis of torrefied biomass and coal in order to develop a mechanistic lumped kinetic model (including primary and secondary reactions) applicable to a wide range of operating conditions. To achieve this aim, torrefied biomass was produced at 280 °C in a pilot rotary kiln and subsequently co-pyrolysis was conducted in a fixed bed reactor at variable mix ratios (25-75 wt.%), temperatures (400-600 °C) and pressures (1-30 bar) to investigate the extent of secondary- and synergistic reactions. The results showed that during co-pyrolysis, condensation and dehydration reactions of depolymerized fractions were inhibited in favour of synergistic reactions between the fragments. Furthermore, the presence of coal vapours lead to the significant enhancement of methoxyphenols (guaiacols) and furanics yields, whilst phenol yields were inhibited, suggesting the inhibition of demethoxylation reactions in the presence of coal fragments. These synergistic reactions occurred predominantly in the molten/liquid-phase and the reactor pressure significantly affected these reaction pathways by controlling the evaporation rate of the molten-phase (limiting it at high pressures), thereby favouring molten-phase recombination reactions. The amount of inherent inorganics in the blends also significantly affected the extent of the synergistic reactions by catalysing solid/gas and solid/liquid free radical reactions. Moreover, the pressure promoted the adsorption of volatiles onto the surface of these catalytic sites (primary inorganics) and favoured the production of secondary light oxygenates/char. From these experimental observations, new reaction pathways for the molten-phase reactions were proposed. After concluding on the significant effect of molten-phase synergistic and secondary reactions under slow pyrolysis conditions, an analytical Py-GC-MS/FID study was performed to obtain further insight into the composition of hot pyrolytic volatiles from raw and torrefied biomass, allowing the development of a lumped kinetic model to describe primary reactions of three reference biopolymers (cellulose, hemicelluloses and lignin) in a total of 13 consecutive primary first order reactions. The primary reaction scheme was validated by comparing the model outputs of the reaction scheme to the results of the Py-GC-MS/FID analysis as well as to comprehensive experimental data of bio-oil derived from raw pine sawdust in a fast pyrolysis micropyrolyzer. The model accurately predicted the yields of major pyrolysis products (volatiles and char) as well as the yields of various chemical condensate groups in the bio-oil (maximum deviation < 4 wt.%). The reaction scheme was then further extended to include secondary reactions, which were described in 6 consecutive first order reactions. These reactions included 3 liquid/molten-phase secondary reactions for conversion of high molecular weight depolymerized fragments and 3 gas-phase secondary reactions for conversion of primary volatiles. The secondary scheme was validated by comparing the model outputs to the experimental results derived in the fixed bed reactor at various temperatures and pressures. The comparison of the model outputs and the experimental results were satisfactory and the model predicted the correct trends in product distribution for an increase in pressure: bio-oil yields significantly decreased, whereas gas yields increased. Moreover, the composition of the bio-oil and gas was changed: sugar groups were absent at higher pressures, whereas CO2 and H2 yields increased. The proposed kinetic model was therefore shown to provide adequate descriptions of the yields and composition of pyrolysis products derived from raw/torrefied biomass in different reactor systems (μg- and mg-scale micropyrolyzers and a g-scale fixed bed reactor) for a wide range of process conditions (heating rate, temperature, volatile residence time and pressure) and could easily be incorporated into reactor models during co-pyrolysis reactor design.
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