Nitrogen Species Measurements and Modeling in Staged Oxy-Fuel Combustion

摘要

Oxy-fuel combustion is a technology attracting a significant amount of attention because it enables CO_2 capture from the products of coal combustion. Previous studies, including pilot scale measurements, have demonstrated not only the feasibility of oxy-fuel combustion for carbon capture but the reduction of NO_x emissions. Understanding the potential for NO_x reduction is a key factor in the economic analysis of oxy-fuel combustion. In order to understand NO_x formation and destruction in oxy-fuel combustion more clearly, measurements of major gas species and nitrogen containing species, NH_3 and HCN, were obtained in a down-fired, premixed, staged, laboratory reactor. In the vicinity of the burner, gas species measurements were obtained at a resolution of 25 mm. In addition to gas species, wall temperature, carbon burnout, and ash composition measurements were obtained. Measurements were obtained using a sub-bituminous coal. A one-dimensional model containing coal devolatilization, gas phase kinetics, and char oxidation was developed for oxy-fuel conditions to help explain trends in the data. The data show that nitrogen from the coal is rapidly converted to NO_x in the first 50-100 mm of the reactor followed by a reduction of NO_x presumed to be caused by reburning reactions in the fuel rich staged portion of the reactor. NO_x increases at the point where burnout air is added and then remains constant. In oxy-fuel combustion, the slight increase in NO_x caused by thermal processes in air combustion does not occur. Instead, at the same rich zone equivalence ratio, oxy-fuel combustion produces more hydrocarbons, NH_3, and HCN in the fuel rich zone suggesting reburning reactions are more prevalent. Air combustion can be made to produce a similar NO_x profile as oxy-fuel combustion by increasing the equivalence ratio (deeper staging) of the fuel rich region. The minimum NO_x achievable is approximately the same for air and oxy-fuel combustion, but the oxy-fuel combustion minimum NO_x occurs at a higher stoichiometric ratio in the fuel rich zone which allows oxy-fuel combustion to achieve higher burnout for the same NO_x reduction. The higher concentrations of NH_3 and HCN seen at a given fuel rich zone equivalence ratio also suggest recycled NO_x will be reduced more efficiently in oxy-fuel combustion than would be expected in air. The model was found to capture most of the trends seen in the data and was useful at estimating the contributions of NO_x formed through various processes.