The Experimental Investigation On Emission And Control Of Multi-pollutant By Ca-based Sorbent During Oxy-coal Combustion

Oxy-fuel combustion is recognized as one of the most promising technologies forcoal-fired power plants to mitigate CO₂emission from combustion. Furthermore, it is alsopossible to realize additional benefits such as the low NOx emission and high in-furnacedesulfurization efficiency. Due to the flue gas recirculation and the change of sulphationmechanism, higher system desulfurization efficiency and better calcium utilization areachieved through limestone in-furnace injection during oxy-fuel combustion. Therefore,in-furnace injection of limestone is a promising option for flue gas cleanup during oxy-fuelcombustion, which further compensates for the reduction of power efficiency. The directsulphation of limestone is proposed in oxy-fuel combustion with high CO₂partial pressure,which is different from CaO-SO₂sulphation in conventional coal combustion. However,the contribution of direct sulphation mechanism to the total desulfurization efficiency hasnot been clearly quantified by now. Moreover, as one of the most important constituents ofPM1derived from coal combustion, S captured by limestone should play an important rolein PM1emission. And it’s still not clear if the change of reaction atmosphere duringoxy-fuel combustion is preponderant for PM1control by sorbents. Besides, Ca-basedsorbents also have an important influence on the species of trace metals. It is far fromunderstanding of the transformation behavior and control of trace metals during coalcombustion.The objective of this dissertation is to investigate the sulphation mechanism oflimestone during oxy-fuel combustion. Furthermore, the influence of Ca-based sorbents onPM1emission and Cr oxidation are also to be studied. Through systematic experiments andtheoretical studies, the emission and control mechanisms of coal-derived multi-pollutantwere investigated. Finally, effective methods were proposed for the control ofmulti-pollutant including SO₂, PM1and trace metal Cr during oxy-fuel combustion. Thekey findings of this dissertation are as follows:Mechanisms for calcination and sulphation of limestone during oxy-fuel combustionwere studied through drop tube furnace experiments. The contribution of limestone directsulphation reaction to in-furnace desulfurization efficiency was quantitatively investigatedfor the first time. And the influence factors on the contribution of limestone direct sulphation were also discussed. In oxy-fuel combustion with high CO₂partial pressure, theCaCO₃decomposition is inhibited and direct sulphation occurs. It is found that whenCa/S=4.4, the system desulfurization efficiency was36.2%for27%O₂/CO₂combustion at1273K, and more than1/3was through direct sulphation reaction. The contribution ofdirect sulphation reaction to desulfurization efficiency was increased with the increase oflimestone addition. When Ca/S=16, the system desulfurization efficiency was68.1%,which was completely through direct sulphation of limestone. Moreover, the inherent Si/Alin coal had more important influence on limestone behaviour in oxy-fuel combustion thanin air due to the interaction between Si/Al and limestone. The formed molten eutecticsdeposited on limestone surface inhibited the calcination and sulphation of limestone byincreasing diffusion resistance.Based on the formation mechanism of sub-micron and central mode particles duringcoal combustion, limestone and kaolin were proposed to control the emission of PM1. It isfound that oxy-fuel combustion provided a better condition for the control of PM1by kaolinor limestone than air combustion due to higher surface area in oxy-fuel combustion. Kaolincaptured metal vapors through surface reactions, and shifted them from PM1to coarseparticles. While limestone reacted with SO₂by competing with alkali and alkaline-earthmetals in coal, to reduce sub-micron sulphate formed by SO₂reaction with metals in coal.The results show that the optimum temperature for PM1reduction by kaolin or limestonewas1373K during oxy-fuel combustion. At this temperature, adding kaolin when（2SiO₂*Al₂O）/（Na+K）=1.6, PM1was reduced by （56⁷4）%. Adding limestone whenCa/S=3.1, PM1was reduced by （70⁷8）%.However, the using of limestone for SO₂and PM1control leads to the formation oftoxic Cr（Ⅵ）. Extensive efforts have been made to clarify the Cr mode of occurrence inresulting fly ashes. The vaporization of organically bound Cr and the toxic Cr（Ⅵ）formation during air and oxy-fuel combustion were investigated in this dissertation. Theinfluence factors of Cr（Ⅵ） formation were also revealed. Furthermore, Different additiveswere employed to investigate their ability on capture Cr vapors and control Cr（Ⅵ）formation during coal combustion. It is found that the vaporization of organically bound Crin coal during air combustion was overwhelmingly dominated by the initial pyrolysis step.With the same O₂concentration, the abundant CO₂in oxy-fuel combustion inhibited the decomposition of organically bound Cr, whereas promoted the formation of Cr（Ⅵ）.Increasing the O₂partial pressure facilitated the vaporization of organically bound Cr, andfavored the oxidation of Cr（Ⅲ） by a certain extent. HCl promoted the vaporization of Crvia chlorination to form CrCl3and CrO₂Cl2vapors. SO₂increased Cr retention in the ash byreaction with Cr vapors to form Cr₂（SO₄）₃. The results show that Na2SO4, MgO, Fe₂O₃andCaO captured Cr vapors effectively during coal combustion. However, the addition of CaOresulted in toxic Cr（Ⅵ） formation in the resulting fly ashes. A new pathway of Cr（Ⅲ）oxidation by CaO was proposed. It is found that CaO assisted the electron transfer fromCr（Ⅲ） to O₂through the redox between Ca^（2+）and Ca^（0）. Thereby Cr（Ⅲ） was oxided toCr（Ⅵ）. It was proposed that Cr（Ⅵ） formation was correlated positively with the standardreduction potential of the metals. However, Fe₂O₃and MgO played an important role ininhibiting Cr（Ⅵ） formation in the ash during combustion. Therefore, adding Fe₂O₃or MgOwith limestone together for combustion can not only capture Cr vapors effectively, but alsorealize inhibition of Cr（Ⅵ） formation in the ash.