NO_x Control And Efficiency Optimization Of Oxy-fuel Combustion System

Carbon dioxide (CO2) released from combustion of fossil fuels is the most important reason for global warming. The main primary energy of China is coal. Nitrogen oxides (NOx) formed during utilization of coal is one of the most important environmental pollutants. As an important carbon capture technology, oxy-fuel combustion performs great potential in reducing NOx emission. Thus it is necessary to understand the NOx mechanisms during oxy-fuel combustion and control its formation.NOx formation and control during oxy-fuel combustion were investigated through a series of studies including:homogeneous reaction mechanism of oxy-fuel infinite-stage combustion, NOx formation from coal combustion in O2/CO2atmosphere in bench-scale batch test, control of NOx via staged oxy-fuel combustion in a drop tube furnace, pilot test of staged oxy-fuel combustion and combustion/process simulation of industrial boiler retrofitting to oxy-fuel combustion. Ways to reduce energy penalty induced from carbon capture in oxy-fuel combustion and other CCS systems were explored.A one-dimensional finite element model was setup to study the theory of NOx control by infinite-stage combustion and the influence of oxy-fuel atmosphere on it. Distribution of oxidant and environmental parameters such as temperature and residence time are key factors that yield best performance of NOx control. Compared to combustion in air, NOx formation in infinite oxy-fuel combustion was inhibited when oxygen was not sufficient in the primary stream; while it was facilitated when oxygen level was relatively higher in the primary stream. An unbranched-chain-reaction mechanism induced by CO2was proposed to explain this phenomenon. No thermal NOx was formed in O2/CO2atmosphere at high temperatures due to the absence of nitrogen. Formation of NOx during coal oxy-fuel combustion was studied in a horizontal batch-test oven. Increasing of oxygen concentration reduced duration of reactions, increased NO concentration until level off when the bulk oxygen volume fraction was greater than70%, and reduced the impact of temperature on NOx formation. Coal with higher rank may yield higher conversion ratio of fuel nitrogen to NOX. NOx control through staged coal oxy-fuel combustion was studied in a drop tube furnace and analyzed via a heterogeneous one-dimensional model. Under non-stage conditions, NOx formation in30%O2/CO2was77%-80%of that in air. It was attributed to longer duration of flue gas in the furnace in30%O2/CO2. Under staged conditions, longer residence time of primary stream in the reductive zone led to25%-29%lower NOx production. Gasification of coal char by CO2during oxy-fuel combustion facilitates reduction of NO on char surface and inhibits NOx formation, and it was more efficient in fuel rich atmosphere.The tendency of NOx control yielded in the pilot scale test with oxygen concentration of74%matched with the results from drop-tube furnace the one-dimensional heterogeneous model. Total NOx production rate was decreased by reducing primary oxygen and increasing tertiary oxygen. Large amount of thermal NOx may form in the high temperature condition with fuel conveying by air. The feasibility of retrofitting boiler in thermal power plant was verified through3-D CFD model and thermodynamic calculation. The more operable criteria of "accordance of adiabatic flame temperature" was proposed for determining the total oxygen level for oxy-fuel retrofitting.Method of NOx control after oxy-fuel retrofitting was studied numerically. NOx formation can be47.3%lower. NOx production was reduced by1/3by prompt the rate of over fire air (OFA) from0to20%. It was attributed to more reductive atmosphere, smaller flow rate of flue gas and longer residence time in primary zone. The net production of NOx was reduced by72%by reburn of NO in recycled flue gas.Process simulations were carried out for oxy-fuel combustion and other main CCS technologies. The electric energy penalties of oxy-fuel, IGCC-CCS and post amine absorption systems were around1.30MJ/kgCO2. Pressurized oxy-fuel technology can reduce this penalty by25.6%. The liquid oxygen self-pressurized process proposed in this paper can increase the net efficiency of pressurized oxy-fuel combustion power system by2.5%.