| OCR Text |
Show INTRODUCTION A constraint in substituting coal for presently used relatively "clean" oil and gas fuels is the increase in gaseous and solid emissions that will accompany their use in existing combustors. In particular, total U.S. N0X emissions will increase substantially if pulverized coal is substituted for natural gas and distillate oil fuels in existing technology combustors. This is due to the large amounts (~1 percent) of fuel-bound nitrogen contained in the coal relative to the small amounts of nitrogen found in petroleum distillate fuels and the absence of bound nitrogen in natural gas. This nitrogen is partially converted to N0X when the coal is burned conventionally, resulting in high emissions. To prevent degradation of the environment by N0X emissions from burning pulverized coal, the DOE, EPA, and private groups have funded the development of N0X control techniques, primarily for utility boiler applications. These studies showed that coal combustion N0X emissions were sensitive to combustion condition modification, and substantial N0X reductions could be achieved with techniques such as controlled-mixing low-NOx burners, overfire air (OFA), and reduced volumetric firing rate (reference 1). Additional N0X reduction can be achieved by passing the products from the initial combustion through a second-stage flame. This "fuel staging" process could provide combustor designers with an additional option to control N0X in pulverized-coal-fired boilers. This paper describes an experimental study to define the pulverized-coal N0X emission control benefits of fuel staging. Several laboratory studies (references 2 and 3) have shown that non-nitrogen-bearing fuels introduced into NOx-laden combustion products lead to reburning and subsequent N0X reduction of up to 76 percent. Both flat-flame burners (reference 2) and tubular reactors (reference 3) were used in these studies, and reductions were observed to occur in less than 1 sec. In these studies, the effectiveness of the reduction increased when the oxygen in the combustion gases was depleted by secondary fuel injection. Maximum reduction was observed when the 8-2 |
