| CO2emission from coal combustion is harmful to the environment, which has been greatly concerned in many countries. As one of the feasible technology for CO2mitigation in the short term, oxy-fuel combustion is beneficial to the capture, storage, and utilization of CO2from coal combustion, and can be applied to the existent combustion equipments. The technology of oxy-fuel combustion in circulating fluidized bed (CFB) takes the advantages of circulating fluidized bed combustion (CFBC) and oxy-fuel combustion. However, the research on oxy-fuel CFBC starts late and there are many problems to be solved, and the low concentration of oxygen is usually focused on in the present study, In this paper, three test platforms are used to carry out a series of experiments:thermo gravimetric and mass spectrometric (TG-MS) analyzer, bubbling fluidized bed (BFB) and circulating fluidized bed (CFB). In this paper, the feasibility of oxy-fuel CFBC at high oxygen concentration is explored, and the effects of combustion temperature and atmosphere, oxygen concentration, and operating parameters on the characteristics of coal combustion, gas pollutant formation, heat transfer are investigated.The oxy-fuel combustion experiments were conducted at O2/N2and O2/CO2atmosphere in a5kW oxy-fuel BFBC test platform and a thermo gravimetric analyzer respectively, in order to study the combustion characteristics of Shuozhou bituminous coal. It is found that the increase in the oxygen concentration and combustion temperature can improve the combustion characteristics of bituminous coal, which are slightly better at O2/CO2atmosphere than that at O2/N2atmosphere.Meanwhile, oxy-fuel combustion tests were also conducted at a BFBC test platform and TG-MS analyzer to study the gaseous pollutant generation characteristics of Shuozhou bituminous coal. The results of TG-MS tests show that, compared with O2/N2atmosphere, the emission of SO2, NO, NO2, NH3and HCN reduces at O2/CO2atmosphere. With the increase in the oxygen concentration, all the gases precipitate quickly significantly, and the intensity of precipitation peak increases. NO presents a double peak precipitation, and NO2shows an unimodal precipitation, while SO2shifts from a double peak precipitation to single peak. The results of tests in the BFBC platform reveal that NO and N2O emission decreases with the increase in the oxygen concentration; and when the combustion temperature increases, NO emission first decreases and then increases while N2O emission decreases. At O2/CO2atmosphere, SO2emission is inhibited, and the effect of calcium based sorbent on SO2removal is obvious. With the addition of calcium based sorbent, N2O emission decreases at both atmospheres, and NO emission remains unchanged at O2/CO2atmosphere while increases greatly at O2/N2atmosphere.In order to study the effects on the heat transfer characteristics of the furnace, experimental tests at high oxygen concentration were conducted in a0.15MW4oxy-fuel CFBC test platform. The results demonstrate that the total heat transfer coefficient of0.15MWth oxy-fuel CFBC test platform is about27W/(m2K); The fluidization velocity, the primary air ratio, and the combustion atmosphere has little influence on the total heat transfer coefficient of the test platform. There is little difference of heat transfer characteristics between oxy-fuel CFBC and air CFBC at high oxygen concentration.The CFB O2/CO2combustion tests at high oxygen concentration were carried out in the reformed0.1MWth oxy-fuel CFBC test platform to investigate the effects of the combustion temperature, overall oxygen concentration, excess oxygen ratio, secondary flow ratio and the oxygen concentration in primary flow on the combustion and pollutant emission characteristics. The research results show that the increase in the combustion temperature and overall oxygen concentration is benefit to improve coal combustion in CFB, the emission of CO, SO2and N2O decreases, while NO emission increases slightly. The effect of excess oxygen ratio is noticeable. With its increase, CO emission decreases significantly, SO2emission decreases slightly, while NO and N2O emission increases gradually. When the oxygen concentration in primary flow increases, SO2emission decreases, while CO and N2O emission increases at first and then decreases, NO emission decreases at first and then increases. But when the oxygen concentration in primary flow is over45%, there is little change of in N2O and NO emission. In oxy-fuel CFBC at high oxygen concentration, flow staging and oxygen staging is still an effective method to control the gas pollutant emission.The0.1MWth oxy-fuel CFBC test platform is reformed by adding the flue gas recycle system. The start-up and switch procedures of the O2/recycled flue gas (O2/RFG) combustion at high oxygen concentration in the0.1MWth oxy-fuel CFBC test platform are explored, and the effects of combustion temperature, combustion atmosphere, calcium based sorbent on combustion and pollutant emission characteristics was investigated. The results show that the0.1MWth oxy-fuel CFBC test platform can be used for O2/RFG combustion and operated stably. It takes about40minutes to switch from O2/N2atmosphere to O2/RFG atmosphere, and about9hours from ignition to stable operation of O2/RFG combustion. When the oxygen concentration is in the range of49.6%-55.2%and45.3%-51.7%in the primary flow and secondary flow respectively, CO2concentration in the dry flue gas can be higher than90%. Compared with O2/CO2combustion, NO emission basically remains unchanged, CO and SO2emission increases to some extent, and N2O emission decreases. Under O2/RFG atmosphere, the carbon content of the fly ash is26.1%-34.1%, the SO2emission in flue gas is87-197mg/MJ, N2O emission is48-78mg/MJ, and NO emission is19-44mg/MJ. |
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