Study on sodium emissions during the combustion of blends of biomass and high-ash content coal in an oxyfuel environment

Abstract

Biomass combustion power generation has received significant attention as it is a carbon-neutral fuel. Countries with coal as the primary resources for power generation are adopting means of co-firing locally available biomass with coal to reduce carbon emissions. Depending upon the source of biomass particles, there are unique challenges associated with the combustion of biomass particles. However, emissions of alkali metals, chlorine, and sulfur-based gases during biomass combustion poses serious challenges in terms of the operability of power plants. In this work experimental investigations have been carried out to study the effect of co-firing different blends of high-ash content coal with biomass on sodium (Na) emissions in oxyfuel and non-oxyfuel environments. Two types of biomass have been studied: beechwood, a woody type of biomass, and paddy straw, an agro-residue-based biomass. Experiments on pellets composed of different blends of biomass and high-ash content coal have been conducted in an environment maintained at approximately 1110 K and 30% O₂/N₂/H₂O or 30% O₂/CO₂/H₂O. Temporal emission of Na has been measured quantitatively using the laser-induced breakdown spectroscopy technique. The effect of blending high-ash content coal with biomass, along with the impact of an oxyfuel environment in reducing Na emissions has been studied. A profound reduction in emissions of Na was found by blending high-ash content coal with biomass. High ash content coal was more effective in reducing Na emissions from coal-beechwood blends when compared with coal-paddy straw blends. Additionally, replacing N₂ with CO₂ in the combustion environment further reduces Na emissions from the coal–biomass blend pellets. As the concentration of high ash coal increased in the blend, the effective reduction in Na emissions due to CO₂ was observed to decrease. The effect of grain size used to make the fuel pellets of different blending ratios of paddy straw and high-ash content coal has also been explored. A finer grain size of paddy straw in the blend was more effective in reducing Na emissions. A mathematical model has been developed to identify the dependence of peak Na emission on different parameters during the combustion of coal–biomass blended pellets of different blending ratios of biomass and high-ash content coal.