Role of a hot coflow on establishment of MILD combustion of biomass gasified gas

Abstract

Biomass gasification is a promising clean technology. However, biomass gasified gas (BGG) suffers from problems related to actual utilization. BGG contains approximately 65% dilution components, that cause ignition difficulty, combustion instability, and high pollutant emissions. Within this context, Moderate or intense low-oxygen dilution (MILD) combustion is a clean and efficient combustion mode under dilution conditions, which is naturally suitable for BGG; however, the mechanism of establishing BGG MILD combustion remains unclear. Here, we investigated the MILD combustion of BGG with a hot coflow generated by a non-premixed CH4-air flat flame. Chemiluminescence and OH planar laser-induced fluorescence (PLIF) imaging were performed to characterize flame structures. Further, NOx and CO were detected in flue gas. Results show that CO emission can be reduced by 43–70% and NOx emission by 63–81% under MILD combustion. Decreasing the equivalence ratio (φ) of the hot coflow can reduce NOx emissions and increase CO emissions. The BGG MILD combustion can also reduce NOx from the hot coflow, which supports the hot coflow to enter industrial or civil burner applications. Introduction of the hot coflow causes local self-ignition; therefore, the combustion zone of the BGG flame moves upstream. This effect becomes more significant with an increase in φ. Further, BGG MILD combustion can be established by a coflow with a φ of 1.1; O2 in the hot coflow was no longer necessary to establish the MILD combustion. The limitations of BGG MILD combustion applications can be reduced considerably, and the mixed combustion of natural gas and BGG based on this jet in a hot coflow configuration can easily establish MILD combustion.