Study And Simulation On Combustion Characteristics And Noxemission Of Pulverized Coal In O₂/CO₂Atmosphere

Oxy-fuel combustion, also known as O2/CO2flue gas recirculationcombustion, is a new combustion technology which can realize cooperativecontrol of CO2, SO2, NOxand other pollutants. Research on the effects ofCO2atmosphere on NOxemissions in coal combustion is of greatsignificance.First, the work uses the chemical kinetics software CHEMKIN tosimulate CH4oxidation process in the presence of CO2or N2, which is usedas dilution gas, and to study the effects of CO2with high concentration onCH4combustion. And then imitating the fuel-N with NH3, CH4combustionprocess with or without (one-through) flue gas recirculation under differentatmosphere (O2/CO2or O2/N2) is simulated, to further explore theperformance of NH3and NOxformation under O2/CO2atmosphere. Resultsshow that high concentration of CO2involved in reaction CO2+H=OH+COchanged the balance of O/H, consequently affect the NH3decomposing andNOxemission characteristics. NOxemission is higher than that of airatmosphere for fuel-rich condition, while it is lower than that of airatmosphere for fuel-lean condition. When considering the flue gasrecirculation, NOxemission is lower than that of one-through simulation by95%for fuel-rich condition and18%for fuel-lean condition respectively, inthe presence of CO2.Second, a model for nitrogen chemistry of pulverized coal whichincludes the mixing between primary and secondary oxidant, devolatilization, gas-phase reactions and heterogeneous reactions of soot and char is built andused to simulate the fuel-N performance in O2/CO2pulverized coalcombustion with CHEMKIN. The effect of CO2chemistry, temperature andthe mixing rate between primary and secondary oxidant on NOxemission isinvestigated. And to further explore the NOxemission characteristics inO2/CO2pulverized coal combustion, a flue gas recirculation is thenconsidered. The effect of CO2chemistry in the process of pulverized coalcombustion on fuel-N to NO conversion ratio (CRNO) is negligible, while themixing rate between primary and secondary oxidant, and temperature haveconsiderable influences on CRNO. When considering the flue gas recirculation,NO concentration at the reactor outlet is about twice higher than that of airatmosphere, but the final CRNOis about half that of air combustion due to thesignificantly reduced flue gas flow rate. The reaction NO with soot plays amain role in the NO reduction in the recirculating flue gas, and the reactionNO with char has a certain contribution, especially for low excess aircoefficient conditions. According to the present study, the low NOxemissiondetected in a actual oxy pulverized coal combustion process, is a integratedresult of a changed combustion temperature, mixing rate and flue gasrecirculation, comparing to conventional air combustion.Finally, reaction kinetics parameters (activation energy E and formerfactor A) of char in different atmosphere are determined experimentally, on adrop-tube reactor. The results show that the char surface oxidation reactionactivation energy E under air atmosphere is higher than that of O2/CO2atmosphere, and with the increase of O2concentration, char surface reactionactivation energy E under O2/CO2atmosphere gradually goes downindicating that char surface is reactive towards higher O2concentration.