| dc.description.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. | en_US |
