Investigation of the oxyfuel fluidized bed combustion characteristics of high ash Indian coal

Abstract

Coal continues to be an important source of energy in developing countries like India. However, Indian coal has high ash content, which causes it to have high burnout time and, hence, lower combustion efficiency in pulverized coal combustors. Moreover, coal combustion is also one of the largest sources of anthropogenic emissions of CO2 , which contributes to global warming. Therefore, research on Carbon Capture Utilization and Storage (CCUS) technologies that can sustainably utilize poor fuels like high ash coal becomes imperative. Oxyfuel fluidized bed combustion is a powerful CCUS technology for the combustion of poor fuels like high ash coal for power generation purposes. This technology utilizes a mixture of pure oxygen and recirculated flue gas for the combustion of fuels to generate an exhaust rich in CO2 , which can be utilized for industrial purposes or sequestration. Oxyfuel co-combustion of biomass and coal can help mitigate carbon emissions. In this research, combustion characteristics of high ash coal have been studied in different oxyfuel environments. In the first part of the study, basic visualization experiments were conducted on a flat flame burner with a flame temperature of approximately 1110 K, which corresponded to the temperature in fluidized bed combustors. Single spherical particle combustion experiments were conducted for 6 mm and 9 mm particle sizes, 10%, 20%, and 30% O2 concentration, and oxyfuel and non-oxyfuel environments. Combustion characteristics such as particle temperature, burnout time, volatile burnout time, and ash effect have been studied. The ash layer was found to drastically affect the conversion time during the char combustion phase, especially for 9 mm particles. The second part of the study consisted of experiments conducted in a lab-scale fluidized bed combustor. Batches of in-situ prepared coal char from coal of sieved sizes 1.2 mm and 3 mm were used for the experiments to solve for particle-to-particle variability. The experiments were conducted with a batch size of 0.5 g of coal at 1073, 1173, and 1273 K in environments consisting of varying concentrations of O2 , N2 , CO2 , and H2 O. The burnout time of char was found to be comparable for 3-mm particles at 10% O2 /90% H2 O and 20% O2 /80% N2 at bed temperatures of 1173 and 1273 K, making oxy-combustion of coal with steam addition a feasible option. In the third part of the study, a transient one-dimensional mathematical model for the prediction of combustion characteristics of a burning char particle was formulated to understand the char conversion process. The model was validated against experiments. The contributions of different chemical reactions toward the conversion of char were studied. Biomass-coal cofiring can be an important strategy to reduce CO2 emissions. In the fourth part of the study, the alkali emission characteristics of biomass were investigated. Alkali metal emissions during biomass combustion could have detrimental effects on the operation of power plants due to challenges like corrosion of heat transfer surfaces, ash fouling, and agglomeration in fluidized beds. The Laser-Induced Breakdown Spectroscopy (LIBS) technique was used to measure sodium (Na) emissions from a spherical biomass particle burning on a flat flame. The O2 concentration was identified as the most important factor affecting Na emissions. It was also noticed that the presence of CO2 reduced Na emissions by inhibiting the decomposition of carboxylates and carbonates. In the fifth and last part of the study, blending high-ash coal with biomass was explored as a method to reduce alkali emissions from biomass. An agro-residue-based biomass (paddy straw) and woody biomass (beechwood) were blended in different proportions with high ash coal. The effectiveness of adding high ash coal with biomass was found to be higher in the case of beech wood when compared with paddy straw. A mathematical model was formulated to understand the chemical effect of coal and CO2 in reducing Na emissions. The chemical effect of coal was attributed to the difference in alumino-silicate sites present in biomass and coal, with a higher difference indicating higher effectiveness. The current study has led to an improved understanding of the strategies that can be used for the effective combustion of high-ash coal in fluidized beds.