OCR Text |
Show 1.0 INTRODUCTION The United States is projected to rely heavily on the use of domestic coal and shale in order to reduce its reliance upon foreign sources of petroleum. The availability and use of alternative fuels will depend upon economic and environmental factors. This paper is concerned with the impact on pollutant emissions associated with the use of coal or shale-derived liquid fuels as replacement for petroleum-derived fuels in boilers and process furnaces. The pollutants of major interest are nitrogen oxides (NO ) and particulate matter, since both are related to fuel properties. Fuel-bound nitrogen can be readily oxidized in the heat release zone to produce nitric oxide, and the hydrogen-to-carbon ratio of liquid fuels influences their smoking tendencies. Alternative liquid fuels can be broadly classified as those synthesized from the products of coal gasification, and those derived directly as liquids. The fuels in the first category tend to be clean, low boiling point fuels such as alcohols, and are essentially free from nitrogen and sulfur; thus their impact upon pollutant emissions is minimal. The liquids in the second category may be compared to crude petroleum oils, containing a wide range of hydrocarbon compounds with boiling points from 300°K to greater than 900°K. The bound nitrogen content of crude synfuels is generally higher than petroleum crudes and for many applications it might be necessary to upgrade the fuel by removing the nitrogen. Recognizing that alternative liquid fuels contain more bound nitrogen than the petroleum fuels that they would be replacing, one key factor in their production is to what extent combustion modification will allow control of NO emissions and reduce the necessity for substantial denitrification, thereby reducing the cost of synfuels. Nitrogen oxides produced during combustion emanate from two sources. Thermal NO is formed by the fixation of molecular nitrogen and its formation rate is strongly dependent upon temperature. Fuel NO is formed by the oxidation of chemically-bound nitrogen in the fuel by reactions with a weak temperature dependence but a strong dependence upon oxygen availability. Thus, those emission control techniques which minimize peak 7-2 |