OCR Text |
Show applied to burners to achieve a reduction in NOx without comprotnJSlng the thermal performance of the process. Process industries are becoming increasingly interested in what oxy/fuel combustion has to offer. This is because there is the potential for increased heat transfer rates, reduced wasted gas volumes, increased productivity, improved product quality and a reduction in pollutant emissions. In most oxy-fuel combustion situations, the oxygen supplied is in the form of a cryogenic liquid, the purity of which is very high. The most likely source of nitrogen ingress therefore, comes from the natural-gas supply. Natural-gas can contain up to 15% nitrogen depending on its place of origin and several percent are common. Thermal-NO production, which constitutes the majority of the total-NO produced, is strongly dependent on the flame temperature. Oxy/natural gas combustion can result in very high flame temperatures (=3020K). Therefore the small amount of nitrogen, inherent in the natural gas, plays a crucial role in the overall NO concentrations obtained. Prompt-NO is also significant in the case of air-staged air/fuel combustion as a result of the fuel rich zones present in the primary and secondary air-staging positions. Oxy/fuel combustion involves the production of very high temperatures and consequently there is a large amount of dissociation of the products. This must be accounted for when considering the numerical calculations. The Four-Step global reaction scheme includes two competing fuel breakdown reactions and the dissociation of lLO to lli and 02 as well as the slow oxidation of CO to C02. The result is accurate flame temperature and characteristic predictions. Theory Air/Fuel Combustion fhe Two-Step Reaction Mechanism: Adiabatic temperatures for an air/natural-gas flame are approximately 2100K. At this temperature there are substantial amounts of CO and lL in equilibrium with the C02 and lLO in the combustion ptoducts. Other unstable species in equilibrium are H, 0 and OH. In the case of air/natural gas combustion therefore, a more accurate flame temperature in the flame zone can be obtained using a two-step global reaction mechanism. CH4 + 1.5 O2 ~ CO + 2 H20 (Rl) CO + 0.5 O2 (R2) 2 |