Performance of Swirl-Stabilized Distributed Combustion With Hydrogen-Enriched Methane: Stability, Blowoff and Emissions

Abstract

Swirl-assisted distributed combustion was investigated with hydrogen-enriched methane. Distributed reaction zones were fostered from a conventional swirl-flame at a heat release intensity of 5.72 MW/m3-atm by diluting the main airstream with either carbon dioxide or nitrogen. The effect of hydrogen addition to the fuel mixture on the performance of distributed combustion was studied for reaction zone stability, variation of blowoff equivalence ratio, and emissions of nitrogen oxide, carbon monoxide, and carbon dioxide. High-speed imaging of reaction zone chemiluminescence was performed for different cases without any spectral filtering. Gradual increase of %H2 in the fuel mixture increased the chemiluminescence intensity in both the swirl and distributed combustion cases. The standoff distance was gradually reduced with hydrogen enrichment along with the appearance of a narrow flame shape from increased reactivity in the flame brush. Fluctuation of pressure (p′) and heat release (q′) was qualitatively measured from the microphone and photomultiplier (fitted with CH* filter) signals at different %H2 enrichments. The amplitude of fluctuation of p′ and q′ showed the existence of a common peak in swirl combustion indicating the possibility of thermo-acoustic coupling. This peak diminished in distributed combustion for H2 enrichment between 0–20% providing enhanced stability compared to swirl combustion. However, a small peak common to p′ and q′ appeared at 40% H2-enrichment indicating the departure of this reaction zone from its distributed nature. Such fluctuations of reaction zones were further investigated with the proper orthogonal decomposition to verify if the vortex shedding influenced these fluctuations. The appearance of vortex shedding characteristics for the distributed combustion with 40% H2-enrichment was found to be responsible for the fluctuations of reaction zones resulting in a departure from the purely distributed behavior. Measurement of lean blowoff equivalence ratios (ϕLBO) at different combustion conditions showed extension of ϕLBO in distributed combustion indicating wider operational limits in distributed combustion. The performance of distributed reaction zones was analyzed from the exhaust emission characteristics of NO, CO, and CO2. The NO levels (ppm) gradually increased in conventional swirl combustion while it consistently decreased in distributed combustion with the increase of %H2. The increase in NO in normal swirl combustion was attributed to the increase in flame temperature. The overall exhaust CO (ppm) decreased with hydrogen enrichment. The exhaust CO2 gradually decreased with %H2-enrichment for both swirl and distributed reaction zones. The higher CO2 observed with CO2 dilution (compared to N2 dilution) is attributed to the usage of CO2 as the diluent. Emission characteristics were also investigated with preheating of inlet airstream (in the range 373–573 K) to study the performance of distributed combustion relevant to actual gas turbines. The results of reduced pollutant emission with hydrogen enrichment at any preheats temperature were consistent with the non-preheated case. However, some increase in pollutants concentration was observed with gradual preheating that was attributed to higher flame temperature and high-temperature dissociation of CO2. The decreased CO2 emission observed in this research further signifies the favorable potential of distributed combustion with hydrogen-enriched methane to support the decarbonization goal worldwide.