Measurement of nitrogen evolution in a staged oxy-combustion coal flame

Oxy-combustion is a technology attracting a significant amount of attention because it enables CO2 capture from the products of coal combustion. Understanding the potential for NOX reduction is a key factor in the economic analysis and design of oxy-combustion systems. In order to better understand NOX formation and destruction in oxy-combustion, measurements of major gas species and nitrogen containing species, NH3 and HCN, were obtained in a down-fired, premixed, staged, laboratory reactor. The near-burner region was fuel-rich and reducing followed by the addition of burnout oxidizer in order to simulate the staged combustion process of a full-scale boiler. In the vicinity of the burner, gas species measurements were obtained at a resolution of 25 mm. The data show that nitrogen from the coal is rapidly converted to NOX in the first 50-100 mm of the reactor followed by a reduction of NOX in the fuel-rich, near-burner region. NOX increases at the point where burnout oxidizer is added and then remains constant. A slight increase in NOX which is potentially the result of the thermal mechanism in air combustion does not occur in oxy-combustion. In the fuel-rich, near-burner region, stoichiometric ratio (S.R.) = 0.75, oxy-fuel combustion produces more hydrocarbons, NH3, and HCN suggesting reburning reactions are more prevalent. Air combustion can be made to produce a similar NOX profile to oxy-combustion by lowering the S. R. (deeper staging) of the fuel-rich region. The minimum NOX achievable is approximately the same for air and oxy-combustion, but the oxy-combustion minimum NOX occurs at a higher S. R. in the fuel-rich zone which allows oxy-combustion to achieve higher carbon conversion for the same NOX reduction. The higher concentrations of NH3 and HCN seen at a given fuel-rich zone equivalence ratio also suggest recycled NOX may be reduced more efficiently in oxy-combustion than would be expected for air combustion. (C) 2011 Elsevier Ltd. All rights reserved.