NOx reduction using advanced techniques in a 175MWth multi-fuel corner-fired boiler

Abstract

The techniques used to reduce emissions in a typical coal boiler are well established, and projects concerning wall-fired, tangentially-fired and down-fired boilers have been widely published in literature. This article discusses a recent project concerning the reduction of NOx and unburnt losses through advanced primary measures in a Czech corner-fired furnace using swirl-stabilised burners, usually typically installed on wall fired units. The 200t/h boiler is part of a large steel manufacturing complex. The firing system has been designed to fire coking coal as the primary fuel, with blast furnace gas and coke oven gas as secondary fuels. Relying on an indirect firing system and a single mill while attempting to burn a suite of difficult fuels presents a number of challenges, especially when targeting high-performance combustion as required for IED compliant emissions performance. A site assessment and a baseline testing programme were performed to obtain detailed process data. The 2.2 fuel ratio coal proved to be difficult to burn, and resulted in baseline carbon-in-ash levels over 20%, with NOx emissions above 470mg/Nm3. The experimental results were replicated in computational fluid dynamics and used to calibrate the full-furnace combustion CFD model used to confirm the design of the final solution. Bespoke burners were designed and installed for each fuel and an advanced over fire air system was installed to allow heavy staging of the combustion air. Low NOx performance was achieved through a combination of the ultra-low NOx burner design and the use of vapour injection, which, in conjunction with an efficient overfire air system, allowed the plant to achieve a NOx reduction of up to 39%, while carbon-in-ash was reduced by up to 88%.