Low-temperature natural-gas-fired combustors and NOx formation

Abstract

This paper discusses the characteristics of low-temperature cooled combustors as currently being developed and utilized in situations where very low NOx emissions are required with natural gas firing. Cyclone or swirl-type combustors are commonly used owing to their very wide blow-off which when translated to a cooled situation results in a system with acceptable blow-off limits for flame front temperatures as low as 1000 °C with only 20 per cent excess air. This depends on the careful integration of the heat transfer surface with the flame front region such that the products of combustion are directed away, inwards from the cooled surface, thus permitting complete fuel burn-out to occur. The combustion aerodynamics of a multi-inlet pre-mixed natural gas-air cyclone combustor has been characterized using laser Doppler anemometry together with detailed mapping of NOx, temperature and gas species concentration. This has been undertaken in 32 different operational cases involving five different multi-inlet cyclone combustors operated over the equivalence ratio range 0.45 ≤ φ ≤ 1.25. The NOx emission data has been examined in light of the prompt NO model of De Soete and modified in light of the known combustion temperatures, NO and species concentration levels. A correlation was derived of similar form to that of De Soete and found to give a reasonable data fit over the temperature range from 900 to 1500 °C, especially for lean equivalence ratios. The constants in this correlation are quite different from those of De Soete; this is felt to reflect the much lower temperature levels over which the data were gathered. Conclusions are then drawn as to the configuration of very low NOx emission combustors.