Study On The Characteristics Of Alkali Metals Sulfation Reaction During High-alakali Fuels Oxy-fuel Combustion

Ash-related problems such as slagging,fouling,and corrosion arise during high-alkali fuels combustion,which severely limits the large-scale utilization of high-alkali fuels.Sulfation reaction can convert alkali chloride into more refractory and less corrosive sulfate,which can alleviate slagging,fouling and corrosion problems.Oxy-fuel circulating fluidized bed?CFB?combustion is one of the most promising Carbon Capture,Utilization and Storage technologies,of which the relatively lower operating temperature?850-950??is also conducive to alleviating the ash-related problems mentioned above.Previous researches indicated that due to the accumulation action of recycled flue gas and strong oxidization in the combustion atmosphere,oxy-fuel combustion atmosphere contains high concentrations of SO₂ and SO₃,which is beneficial to the alkali chloride sulfate reaction.Therefore,it is of great academic significance and application value to perform researches on the characteristics of sulfation reaction during oxy-fuel CFB combustion.The present thesis focuses on the sulfation reaction of alkali metal and its effect on the migration and transformation of Na?K?/S/Cl/N during typical high-alkali fuels oxy-fuel CFB combustion.Based on a horizontal tube furnace and a 50 kW oxy-fuel CFB combustion test rig,the mechanism of heterogeneous alkali metal sulfation reaction,the effect of sulfation reaction on Na?K?/S/Cl/N migration and transformation during high-alkali coal and biomass oxy-fuel combustion,the importance of SO₂ and SO₃ to sulfation reaction and the effect of operating parameters on sulfation of alkali metals by SO₃ were investigated.The major research contents are as follows:1.The kinetics of alkali metal sulfation reaction was investigated,and the effect of the common minerals in deposits on the sulfation reaction was also revealed.The results show that compared with air combustion atmosphere,the heterogeneous KCl sulfation reaction would be inhibited under oxy-fuel combustion atmosphere.Under oxy-fuel combustion atmosphere,the sulfation reaction is a kinetically controlled process.Fe₂O₃ can catalyze the heterogeneous KCl sulfation reaction,while CaO will inhibit the heterogeneous KCl sulfation reaction through a competitive mechanism.2.The effect of sulfation reaction on migration and transformation of Na/S/Cl among ashes during high-alkali coals oxy-fuel combustion was investigated,and the importance of SO₂ and SO₃ to the sulfation reaction was analyzed.The results show that compared with air combustion atmosphere,the migration and transformation of Na towards bottom ash and circulating ash is enhanced,mainly in the form of Na₂Al₂SiO₆,under oxy-fuel combustion atmosphere at identical inlet oxygen concentration.Elevating the inlet oxygen concentration can promote the homogeneous sulfation of NaCl and decrease the Cl content in the deposits significantly during high-sodium coal oxy-fuel combustion.Compared with SO₂,SO₃ is more prone to the sulfation reaction.The sulfation reaction can enhance the migration and transformation of Na towards the bottom ash and the circulating ash and enhance the migration and transformation of Cl towards the flue gas.3.The effect of sulfation reaction on the formation of NO and N₂O during oxy-fuel combustion was investigated.The results show that SO₂ in the recycled flue gas can significantly enhance the NO reduction reaction.The alkali chloride sulfation reaction can reduce NO concentration but will increase CO and N₂O concentrations in the flue gas.Moreover,NO in the recycled flue gas can be reduced in the riser,and the reduction ratio of the recycled NO can reach up to 45.6%.However,there are hardly any interactions between the recycled NO and alkali metals.4.The effect of sulfation reaction on the migration and transformation of K/S/Cl/N during biomass oxy-fuel combustion was investigated,and the effect of operating parameters on the promotion of alkali metal sulfation by ammonium sulfate was revealed.Also,a strategy to synergistically control Cl corrosion and achieve NO_x ultra-low emission by ammonium sulfate during biomass combustion was proposed.The results show that ammonium sulfate can enhance the migration and transformation of K towards bottom ash and circulating ash and enhance the migration and transformation of Cl towards the gas phase via promoting the KCl sulfation reaction.Therefore,the K and Cl contents in the deposits can be decreased.At the same time,NH₃ produced by the decomposition of ammonium sulfate can reduce NO and enhance the transformation of N towards N₂.CaO in the bottom ash can greatly inhibit the sulfation reaction when ammonium sulfate is added in the lower part of the riser,while the shortened reaction time and decreased reaction temperature can also inhibit the sulfation reaction rate when ammonium sulfate is added in the upper part of the riser.Hence,the best position to add ammonium sulfate is in the middle part of the riser.Compared with the condition without ammonium sulfate added,the Cl content in the deposits can decrease by at most73.3%when ammonium sulfate is added in the middle part of riser in the temperature range of 850-900?.At the same time,the NO concentration in the flue gas can decrease by 74.0%.