Enhanced NOx Reduction in Staged Combustion: Technical Application of Premix Technology in Boilers

This paper describes a new approach to the reduct ion of the NOx emissions of oil-fired furnaces (oil no.2 and no.6). The potential of air-staged combust ion was investigated in an experimental and theoretical research program showing a clear advantage for strong air preheat and the use of a premix burner in the first fuel-rich stage instead of a conventional diffusion burner. Using a premix burner (100kWth), the optimum emissions were less than 60% of those from the best diffusion flame in the presence of fuel bound nitrogen. Optimum residence times under substoichiometric conditions were found to be only 100-l50ms. Based on the positive results, this novel technique was developed on an industrial scale using a 2-stage-process for medium size boilers, while large scale power plant boilers will require a 3-stage combustion system. In a cooperative project between a power utility and ABB, a pilot plant of 10MWth was built and intensively tested. The influence of air preheat , oil quality, load variations, primary and secondary equivalence ratios were investigated in a parametric study. Optimum emissions of 27-47ppm (3% 02) for oil no.2 and 04-119ppm NOx for oil no.6 were measured in a load range of 3-10.5MWth while CO and UHC emissions were negligible. A residual 02 content of 0.5-1 % in the stack could be established while maintaining low levels of soot emissions. Based on these results, the potential of the large scale 3-stage process could be estimated to be <=75ppm NOx for a heavy fuel oil with 0.5%N.