578 / 2019-01-18 16:09:35

Experimental and numerical study on fuel-NOx formation in oxy-fuel processes in a jet stirred reactor

oxy-fuel, fuel-NOx, NH3, jet stirred reactor

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CO2 is formed from combustion processes and is the main source of greenhouse gas. The greenhouse effect has caused significant effects on global climate change. To meet this challenge and as an important step of carbon capture and storage, oxy-fuel combustion adopts O2/CO2, not air, as a diluent. Thus, the CO2 concentration in flue gas can be increased such that the efficiency of CO2 capture is increased and the cost of CO2 capture is decreased. Oxy-fuel combustion can reduce NOx formation from N2 because N2 is separated. However, elemental N exists in solid fuel, such as coal and biomass. Elemental N in solid fuel is the main source for the combustion process. CH4 and most elemental N are gasified in the devolatilization process. HCN and NH3 in volatiles are the main source of fuel-NOx. NH3 is the primary fuel NO precursor in low rank coal volatiles.
New experimental results on fuel-NOx formation in oxy-fuel processes are obtained in a jet stirred reactor. In the present work, effects of CO2 (0~100%), temperature (873~1323 K) and equivalence ratio (0.56~1.61) on fuel-NOx in oxy-fuel processes are experimentally and numerically researched. NH3 is selected as the N element source. The predicted results by the PG2018 mechanism (Progress in Energy and Combustion Science, 67, 31-68, 2018) are consistent with the experimental results. Compared with N2 diluted conditions, CO2 increases in diluents can reduce NO formation in fuel-lean conditions and HCN formation in fuel-rich conditions. Under fuel-lean conditions (Φ≤1), as the CO2 concentration increases from 0 to 100%, the formation of NO is decreased by approximately 30%. NH2+O<=>HNO+H is reduced as CO2 increases. Therefore, the main path of NO formation, namely, NH3→NH2→HNO→NO, is reduced. In addition, increases in CO2 enhance NH2+NO<=>N2+H2O and NH2+NO<=>NNH+OH. More NO is reduced through NH3→NH2→N2. Under fuel-rich conditions, NH2 reacts with CH3, CH2, and CH to form CH3NH2, CH2NH2 and CH3NH, which are transformed to H2CN and HCNH. HCN is ultimately formed. Under Φ=1.61 at N2 dilution conditions, greater than 25% NH3 is transformed to HCN. As CO2 increases, considerable NH2 is transformed to NH3. The path of transformation to HCN is decreased by 16%. Under fuel-rich at oxy-fuel conditions, HCN formation increases at first and then decreases. The peak occurs at Φ=1.4. This finding is attributable to NH2 reaching the peak at Φ=1.4 under oxy-fuel fuel-rich conditions.