Experimental Study On The Evolution Of SO₃ In Coal-Fired Flue Gas And Its Removal Characteristics

With the gradual acceleration of China’s modernisation process and the increasing demand for energy,coal remains at the core of China’s energy mix type.The popularisation of selective catalytic reduction denitrification(SCR)technology in coal-fired power plants has brought about some problems such as SO3 generation while achieving ultra-low emissions of nitrogen oxides.NH3 in coal-fired flue gas is highly susceptible to the formation of ammonium bisulphate(ABS)and ammonium sulphate(AS)with the highly reactive SO3,causing blockages in the air preheater and jeopardising the safe and economic operation of coal-fired power plants.In addition,the production and emission of SO3 can cause a series of hazards.The search for efficient and economical methods for SO3 removal has become the focus of research at home and abroad.This paper focuses on the generation,deposition and decomposition characteristics of ABS on fly ash under different conditions and the use of alkali-based absorbers for SO3 removal to provide theoretical support for effective SO3 prevention and control in coal-fired power plants.The deposition rates and temperatures of the products were investigated in a fixed-bed reactor simulating the temperature field system of an air preheater in a real power plant under different SO3 and NH3 concentrations,reaction times and reaction temperatures,and the deposition characteristics of ABS on fly ash were recorded.The initial deposition temperature of ABS was in the range of 202-290℃,with the most likely occurrence between 230-250℃.Under different reaction conditions,the high temperature section reacted with no obvious phenomenon in both the high temperature state and the room temperature state,and the initial generation bit generation was continuously shifted back with the concentration or reaction time reduction,and the white mist was more obvious in both cases in the low temperature section than in the initial generation temperature section,and the trailing low temperature section changed from small liquid droplets to a small amount of small liquid droplets or even not obvious with the concentration or reaction time reduction,and the properties presented by the fly ash were gradually changed from caking The properties of the fly ash also change gradually from lumpy to partially granular or even inconspicuous.Based on the study of the generation and deposition characteristics of ABS on fly ash,the fly ash samples were experimentally characterised after ABS deposition.The decomposition characteristics,chemical and physical properties of the deposited fly ash samples were analysed by Fourier transform infrared spectroscopy(FT-IR),scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDX),X-ray diffraction testing(XRD)and thermogravimetric analysis(TG).The chemical composition of the sediment was essentially the same under different reaction conditions.increasing concentrations of SO3 and NH3 favoured the deposition of ABS and increasing concentrations of NH3 mainly promoted the deposition of ABS.Increases in both reaction time and reactant concentration promoted ABS deposition and conversion.The main product of the experiments was ABS,and as the experimental reaction time increased ABS deposition increased with a corresponding increase in its S content,ABS deposition would lead to the accumulation and adhesion of fly ash particles and would react with the fly ash and convert to various ammonium salts and sulphates with higher decomposition temperatures.The sulphates formed by ABS conversion have a lower relative decomposition temperature than those formed by sulphate drop deposition.The SO2 and SO3 in the coal-fired flue gas will have a competitive relationship,and SO3 selective removal experiments are performed on a fixed bed reactor.First,the SO3 removal performance of the sodium-based,calcium and magnesium-based six base absorbents is compared.The selective removal effect of the absorbent to SO3 is obtained by screening trials from high to low order:Na2CO3>NaHCO3>Mg(OH)2>MgO>Ca(OH)2>CaO.The three absorbents of Na2CO3,Ca(OH)2 and CaO were selected,and the effects of reaction temperature,chemical equivalent ratio,reaction time,H2O concentration and SO3 initial concentration on SO3 removal properties were selected.After the absorbent calcination pretreatment,the absorbent pore structure is increased,which is advantageous to react to the SO3 into the absorbent,which can effectively improve the absorption rate of SO3.The reaction temperature,the chemical equivalent,the reaction time,and the initial concentration of the SO3 can promote the removal of the absorbent to SO3,and increase the H2O concentration in an appropriate range facilitates the removal of the absorbent to SO3,and the range will reduce SO3 Removal rate.The SO3 removal experiment was carried out,and the SO3 removal of the gas-solid response model was constructed.As the SO3 removal reaction progresses,the sulfate cover formed on the surface of the absorbent can affect the diffusion of SO3 to the unreacted interface,resulting in a decrease in reaction rate.The Na2CO3 removal SO3 reaction rate was mainly guided by the external diffusion,and the pre-reactivity rate of Ca(OH)2 and CaO was mainly guided by the external diffusion,and the reaction rate was diffused from the external reactivity rate.Commonly dominated.