The reduction of NOx emissions from a fluidized bed combustor by staged combustion combined with ammonia addition

Abstract

The European Economic Community is proposing legislation aimed at limiting NO x , SO x and particulate emissions from large combustion plants. The limit values proposed indicate that NO x emissions from the present generation of atmospheric fluidized bed combustors, burning large coals (6–25 mm) in shallow beds, may need to be reduced by up to 50%. This paper reports on an investigation in which air-staging is combined with gaseous ammonia injection, for the reduction of NO x emissions. The experimental rig consisted of a stainless steel combustor, 2 m high and 0.3 m square in section. Fluidzing air was supplied through a tuyure distributor and secondary air was introduced into the freeboard via an injector whose height could be adjusted. Ammonia was injected over the bed surface. The experiments were carried out at an overall fluidizing velocity of 2 m/s, bed temperature 870 C and 40% excess air. Combustion gases were sampled by probes located at various locations in the combustor and also by a vertical probe that could scan the total length of the combustor. A 50% reduction in NO x emissions was achieved using 35% air staging combined with ammonia injection at a NH 3 /NO mole ratio of 0.6:1. The measured ammonia emissions in the flue gas were less than 20 ppm. The present investigation has shown that staged combustion combined with ammonia injection has potential for reducing NO x , emissions, but technical issues such as fouling, corrosion, excess ammonia slippage and the costing implications for this technology remain to be addressed on commercial sized fluidized be combustors.