| Global warming is being aggravated by the emission of the greenhouse gas CO2,which comes mainly from the burning of fossil fuels like coal.O2/H2O?oxy-steam?combustion,as a new generation of oxy-fuel combustion technology,is perceived as a feasible and promising technology to reduce CO2 emission from the coal-fired power plants,which has attracted high attention all over the world in recent years.In O2/H2O combustion,a mixture of pure oxygen and steam are mixed for coal combustion instead of the recycled flue gas compared with O2/CO2 combustion.The role of the recycled steam is to moderate the flame temperature.In this way,the concentration of CO2 in the exhaust gas is high,facilitating the following separation and storage of CO2.Such a high concentration of steam in O2/H2O combustion may have an effect on the sulfur self-retention of coal ash,thus influencing the SO2 emission characteristics during coal combustion.Therefore,this thesis mainly works on the characteristics of sulfur transformation of coal under O2/H2O combustion.SO2 emission and sulfur self-retention by coal ashes were first investigated under O2/H2O,O2/N2 and O2/CO2 combustion on a laboratory-scale drop tube furnace?DTF?,respectively,aiming to ascertain the effects of combustion atmosphere,sulfur content and Ca/S molar ratio on the sulfur transformation characteristics of coals.The results showed that SO2 emissions and sulfur self-retention efficiencies under O2/H2O combustion had no remarkable difference compared with those under O2/N2 and O2/CO2 combustion.SO2 emissions were mainly affected by sulfur contents of coal.Coals with higher sulfur contents discharged more SO2during combustion process.The sulfur self-retention efficiencies of coal ash were not always improved with the increase of Ca/S molar ratios and they were also influenced by the reactivity of calcium in the coal.Sulfur self-retention efficiencies of SH,HLH and PDS coals under O2/H2O?24/76?atmosphere were 25.29%,35.43%and 19.6%at 950 oC,respectively.The sulfur self-retention efficiencies of coal ashes were lower than 36%and sulfur in coal was mainly released in the form of SO2 during combustion.Then,experiments were carried out on a horizontal tube furnace?HTF?and a thermogravimetric analyzer?TGA?to compare the indirect sulfation of limestone in O2/H2O,O2/N2 and O2/CO2 combustion atmosphere.It focused on the effects of combustion atmosphere and temperature on the indirect sulfation of limestone.Besides,the sulfated products were collected for X-ray fluorescence?XRF?,X-ray powder diffraction?XRD?,pore structure characteristics and scanning electron microscopy?SEM?analysis to investigate the mechanism of the positive effect of steam on the indirect sulfation of limestone.The results showed that the indirect sulfation reaction exhibited a negligible difference at the kinetically-controlled stage under different atmospheres but it was significantly promoted at the diffusion-controlled stage under O2/H2O atmosphere compared with O2/N2 and O2/CO2 atmosphere.A plausible explanation was that steam promoted the solid-state ionic diffusion through product layer at the diffusion-controlled stage.BET surface area and pore volume of sulfated products at 240min were much lower under O2/H2O atmosphere compared with O2/N2 and O2/CO2atmosphere and the aggregation of crystals became more serious.Besides,temperature had little effect on calcium conversion ratios at the beginning of the indirect sulfation reaction.However,after the reaction entered diffusion-controlled stage,the calcium conversion ratios at 950 oC were higher than those at 850 oC. |
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