INVESTIGATION OF COAL COMBUSTION BY OPPOSING JETS

Abstract

The common impingement of opposing/premixed flames and heat transmission from triple flames effectively extended the firing capacity and reduced the NOx emissions from coal combustion while the premixed flame NOx was limited by losing some energy to preheat the coal. At proper mixture conditions, the gaseous fuel combustion accelerates coal radiation prior to the NOx formation region at the outer flame envelope and stimulates favorable interaction between the early generated NOx and the enhanced HCN release from coal. Increasing the staging height or the heat input ratio decreased the overall NOx formation rates for higher opposing/cross-flow momentum flux ratios. Decreasing the mixture fraction gradient and increasing the overall equivalence ratio experimentally increased the rates of radiation heat transfer to yield less NOx emissions. While it was computationally found that smaller coal particles with a higher number of opposing jets and larger diameter to separation ratios ensured higher mixing rates for minimum levels of pollutants. A future work may be commissioned to investigate other hydrocarbon fuels of higher carbon to hydrogen ratios to address the interaction with the higher prompt NOx contribution. Another potential direction is to enrich the flow turbulent kinetic energy by using sharp corners' jets whose extra flow strain induced at each corner (where two boundary layers interact) is beneficial for enhancing the combustion performance. More detailed flow visualization utilizing non-intrusive techniques is needed to accurately depict the flow structure under both non-reactive and reactive conditions. In the light of such enriched flow detection, more path lines can be possibly included in the temperature history analysis which can be further enhanced by extending the large eddy simulation into the hot flow conditions. Some other issues should be addressed such as the flame lift-off from the coal/gas concentric ports and the magnitude of discrepancy in the co-flow velocities. Future efforts may be devoted to minimize the errors currently involved during the combustion gas temperature measurements due to accumulation of soot on the thermocouple bead. For this purpose, an infrared thermometer can be used as a non-intrusive instrument to support the thermocouple measurements by locally indicating the temperature across the combustion zone relying on a char emissivity value of unity. In conjunction, the radiation spectrum in the infrared wavelength range can be measured using an infrared spectrum analyzer. As comprehensive information is thus established for the burner currently developed, the burner use can be extended to applications such as the boilers pertaining to both power production and other industrial needs. In such manner, the firing capacities can be appreciably extended and the NOx environmental impact can be reduced while the radiation output can be flexibly controlled.