Low-NOx coal combustion via combining decoupling combustion and gas reburning

Abstract

Through separating coal pyrolysis and char combustion and letting the pyrolysis gas burn in its passing through the combusting char bed, the so-called decoupling combustion has been proven to be effective for lowering NOx emission of coal and biomass combustion. The original design of the decoupling combustion was based on coal pyrolysis incurring with the heat transferring from the char combustion zone, thus causing the difficulty of scale-up in matching the reactions of slow coal pyrolysis and quick char combustion. An idea of using partial gasification to replace such a pyrolysis was proposed to allow the combination of reburning part of the gasification gas over the combusting char layer and the decoupling combustion of the left gasification gas by letting it pass through the char bed. This article compared first the effectiveness of reducing NOx emission by a few different combustion methods including reburning of gasification gas and the decoupling combustion based on coal pyrolysis in a laboratory scale dual-stage reactor for five different kinds of coal. It was shown that both the decoupling combustion and reburning of gasification gas represented the most effective low-NOx combustion methods, which can reduce the NOx emission by 30–40% in comparison with the traditional combustion without air-staging and reburning. The fuel ratio of fixed carbon to volatile matters of the coal also affected the effectiveness of NOx reduction, the lower the fuel ratio, the higher the NOx reduction effect was. The actual running test of a 1.4MW industrial boiler based on combining the decoupling combustion and reburning part of the partial gasification gas reduced the NOx emission by 33% in comparison with the traditional industrial boiler. The result showed a successful scale-up of the technology and verified the idea of reducing NOx emission by combining the decoupling combustion and the reburning of coal gasification gas.