Experimental Study Of The SO₂ And NO₂ Absorption By Biomass Ash

With the rapid development of biomass power plant,a lot of biomass ash is produced.In the meantime,SO₂,NOx and other gaseous pollutants are released.As the requirements for air pollution control are becoming increasingly strict,the ultra low emission transformation of biomass power plants has been put on the agenda.In view of the strong alkalinity of biomass ash,our research group proposed the technical idea of using biomass ash to absorb SOZ and NO₂ in flue gas,which can achieve ultra low emission of sulfur and nitrogen pollutants at low cost.In this paper,the characteristics of SO₂ and NO₂ absorption of simulated flue gas by typical biomass ash are studied,and the mechanism of desulfurization and denitrification is revealed through the characterization of reaction products.In this paper,we select five kinds of typical biomass in the north of China?wheat straw,cotton stalk,poplar bark,corn straw,corn cob?,and the ash is prepared after drying and the composition and properties are analyzed.The particle of biomass ash is different and irregular in size and morphology,which is often attached with alkali metals and alkaline-earth metals.K,Cl,Si and Ca are the main inorganic elements in the biomass ash,and the content of inorganic elements is very different.The content of calcium in the poplar bark ash is obviously higher than that of other ash samples,and wheat straw ash and corn stalk ash contain more silicon,and corncob ash contains a lot of potassium.XRD anlysis shows that the biomass ash mainly contains KCl and SiO₂,and the Ca and Mg minerals are detected in the cotton stalk ash.Ca and Mg carbonates are detected in the bark ash.The effects of temperature,water content,water vapor content and other parameters on desulfurization and denitrification characteristics were investigated through the fixed bed experiment of SO₂ and NO₂ absorption.The results showed that at the beginning of the experiment,SO₂ was fully absorbed and then the absorption rate decreased rapidly,and the absorption rate was stable finally.With addition of water,the time of break-through was prolonged,and the decrease rate slowed down,which obviously promoted the absorption of SO₂.When the vapor content in simulated flue gas increased,the drying time was prolonged,and the absorption capacity increased.The lower reaction temperature is beneficial to the absorption of SO₂.There was no NO released in the initial period of NO₂ absorption,and then the concentrations of NO gradually increased,which was basically stable after 50 minutes.The mixed water of biomass is beneficial to the absorption of NO₂,and the increase of temperature makes the absorption of NO₂ worse.The absorption of SO₂ and NO₂ by biomass ash is neutralization reaction,and NO₂ and SO₂ are competitive relationship in simultaneous absorption.When the biomass ash is mixed with water,there is a liquid film formed in the particle surface,and the gas-solid reaction turns into the ionic reaction in the liquid phase,so the rate of reaction is accelerated.Along with the time,the moisture in the absorbent particles evaporates,and the products cover the particle surface,which lead to the gas diffusion resistance increases and the reaction rate decreases.Then,the absorption products were analyzed.SEM anlysis showed that after experi-ment,corn straw ash and cotton stalk ash particles were covered in smooth and irregular shape products,and wheat straw ash particles produced some burr shape products,and poplar bark ash and corncob ash surface were uneven attachments.Analysis of products shows that SO₂ absorption by biomass ash is associated with K,Ca.Desulfurzation pro-ducts are mainly composed of S,O,Cl,K,Ca.The reaction products are mainly K and Ca sulfate.Finally,the analysis of fly ash in typical biomass power plant and SO₂ absorption experiment show that fly ash is heterogeneous material with various appearance,mainly including irregular lumbar,strip,globule and floccus.The floccus contains a lot of CaO and has the potential of absorption of SO₂ in low temperature.The addition of water in raw material ash plays an important role in the humidification and activation.The CaO of biomass ash is converted into Ca?OH?2,and the water film is formed on the surface of biomass ash which improve the efficiency of desulfurization.After desulfurization the content of S element in the ash increases obviously,and the calcium sulfate is produced on the surface of ash,which is a flake crystalline substance.Through the experimental study on the absorption of SO₂ and NO₂ in simulated flue gas by biomass ash,the key reaction conditions for the desulfurization and denitrification are obtained.The reaction path of desulfurization and denitrification is described,which provides reliable basic data and theoretical reference for the popularization and application of the technology.