Prediction of high-temperature rapid combustion behaviour of woody biomass particles

Jun Li,Manosh C Paul,Paul L Younger,Ian Watson,Mamdud Hossain,Stephen Welch

doi:10.1016/j.fuel.2015.10.061

Jun Li, Manosh C Paul + Show 4 more

Open Access

https://doi.org/10.1016/j.fuel.2015.10.061

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Abstract

Biomass energy is becoming a promising option to reduce CO2 emissions, due to its renewability and carbon neutrality. Normally, biomass has high moisture and volatile contents, and thus its combustion behaviour is significantly different from that of coal, resulting in difficulties for large percentage biomass co-firing in coal-fired boilers. The biomass combustion behaviour at high temperatures and high heating rates is evaluated based on an updated single particle combustion model, considering the particle size changes and temperature gradients inside particle. And also the apparent kinetics determined by high temperature and high heating rate tests is employed to predict accurate biomass devolatilization and combustion performances. The time-scales of heating up, drying, devolatilization, and char oxidation at varying temperatures, oxygen concentrations, and particle sizes are studied. In addition, the uncertainties of swelling coefficient and heat fractions of volatile combustion absorbed by solid on the devolatilization time and total combustion time are discussed. And the characterised devolatilization time and total combustion time are finally employed to predict the biomass combustion behaviour. At the last, a biomass combustion/co-firing approach is recommended to achieve a better combustion performance towards large biomass substitution ratios in existing coal-fired boilers.

Highlights

Biomass is a potential fuel that can deliver a significant reduction in net carbon emissions when compared with fossil fuels, and additional environmental and social benefits could be expected [1]
The devolatilization and char oxidation models have been validated by the experimental data characterised at high temperatures and high heating rates, as reported in previous work [15,16]
In both devolatilization and char oxidation studies, the biomass particles were collected and analysed to determine the mass conversion according to the ash tracer method, which assumes ashes to be inert and thermally stable

Summary

Introduction

Biomass is a potential fuel that can deliver a significant reduction in net carbon emissions when compared with fossil fuels, and additional environmental and social benefits could be expected [1]. The combustion properties of biomass are significantly different to those of coals, due to its high volatile contents and low energy densities. The challenges of the large percentage biomass co-firing (over 20% on energy basis) in existing pulverized coal boilers are keeping the same steam parameters and having a high boiler efficiency and a stable operation. When co-firing biomass e.g. in an existing coal-fired boiler, drag forces acting upon the biomass particles are more important than that acting on coal particles, which can cause unexpected flame patterns compared to coal flames. Elucidation of the combustion mechanism is key to understanding biomass combustion per se, as well as in co-firing behaviours.

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