Limiting NO x and SO 2 emissions from an industrial-size fluidised-bed combustor

Abstract

For the reported series of tests, the largest reductions in NO x emissions were achieved (together with maximum combustion efficiencies) by using a primary-to-secondary-air ratio of 30 70 for the fluidised bed (FB). Under such an operating condition, high freeboard-temperatures could be attained, e.g. up to 200° C above that of the bed, so promoting the combustion of unburnt carbon-fines in the freeboard zone, i.e. secondary combustion ensued. This tended to counteract the effect of the reduced rate of carbon combustion in the bed, i.e. as a result of the increased rate of oxidation occurring in the freeboard. Reductions in both the NO x and SO 2 emissions could be achieved by using such a two-stage combustion process, without affecting adversely the overall combustion efficiency. If a primary-to-secondary-air ratio of 70 30 (rather than 30 70 ) was employed, the rate of SO 2 emissions fell, but the temperature uplift in the freeboard zone was less pronounced and a reduction in combustion efficiency of 2% was observed. The rates of emissions of NO x and SO 2 could also be reduced by the adoption of wise choices for the design and operation procedure of the fluidised-bed combustor (FBC). For example, a 45% decrease in the rate of NO x emissions was achieved in one test merely by halving the bed's depth. The lower the available oxygen-concentration (including that provided by recycled flue-gases) within the bed, the smaller was this decrease. On the other hand, a doubling of the bed's depth led to a reduction in the SO 2 emission of approximately 4%, provided that the in-bed oxygen concentration was maintained sufficiently high, so as to promote sulphation of the ash and any limestone present in the bed. The location of the secondary-air nozzle (i.e. in the freeboard zone) also had significant influences on the emission rates of the NO x and SO 2. A 13% reduction in the rate of NO x emissions was achieved by increasing the height (above the sparge pipes) of the secondary-air's nozzle from 1·5 to 2·6 m. The larger this height, the greater the opportunity for char-NO x-reducing reactions (for which high-temperature conditions are preferred within the freeboard) to ensue, prior to the secondary-air injection. However, the rates of SO 2 emission fell by ∼ 10% when the height of the secondary-air's nozzle was reduced from 2·6 to 1·5 m above the sparge pipes. This was due presumably to the increased residence times of the ash and limestone sorbents (for facilitating sulphation) in the freeboard, under the oxidising condition (following secondary-air injection), which favours the complete sulphation of the sulphidated ash/lime to CaSO 4. In the presently reported series of tests, the rate of NO x emissions could be reduced by up to 83% merely by adjusting the bed's depth and the secondary-air's nozzle-height, although these alterations led simultaneously to a 14% increase in the rate of SO 2 emissions, and vice versa. Thus, the emissions of NO x and SO x could be controlled by the use of an appropriate design of FBC. The choice made, in practice, usually depends on which of these emission rates is critical with respect to complying with the environmental-pollution directives. Great care must be taken when comparing emission rates from the FBs, either of different sizes or incorporating alternative methods of air distribution, in order to take account of scaling effects. It is unwise, for instance, to use observations from a laboratory-size FBC to predict the quantitative behaviour of an industrial-size (i.e. large) FBC unit. This arises because of the variations in the oxygen concentration, resulting from the usually non-homogeneous, relatively less effective fluidisation achieved in the large bed; the rates of NO x emissions from the large FBC tend to be smaller (as a result of the existence of pockets of relatively low concentration oxygen in the bed) and simultaneously the rates of SO 2 emission are invariably higher. In the presently reported tests, under otherwise nominally similar conditions, an approximate halving of the rate of NO x emissions resulted when using the larger FBC (rather than the small one) together with a doubling of the SO 2-emission rates, even though the average percentages of oxygen present in the flue gases remained identical for both the small and large FBC units. The recommended bed-depth depends upon what emission one seeks to reduce. However, where feasible, for an industrial-size combustor, it is wise to employ a shallow bed (∼ 340 mm depth, when static) in order to incur relatively low operating costs.