Research on prediction model of formation temperature of ammonium bisulfate in air preheater of coal-fired power plant

Kai Zhang,Fangming Xue,Zhiqiang Wang,*,Xingxing Cheng,*

1 School of Energy and Power Engineering,Shandong University,Jinan 250061,China

2 Huadian Electric Power Research Institute Co.,Ltd.,Hangzhou 310030,China

Keywords:Air preheater Ammonium bisulfate ABS formation temperature Reaction kinetics

ABSTRACT Ammonium bisulfate(ABS)is a viscous compound produced by the escape NH3 in the NO reduction process and SO3 in the flue gas at a certain temperature,which can cause the ash corrosion of the air preheater in coal-fired power plants.Therefore,it is essential to study the formation temperature of ABS to prevent the deposition of ABS in air preheaters.In this paper,the SO3 reaction kinetic model is used to analyze the SO3 generation process from coal combustion to the selective catalytic reduction(SCR)exit stage,and the kinetic model of NO reduction is used to analyze the NH3 escape process.A prediction model for calculating the ABS formation temperature based on the S content in coal and NO reduction parameters of the SCR is proposed,solving the difficulty of measuring SO3 concentration and NH3 concentration in the previous calculation equation of ABS formation temperature.And the reliability of the model is verified by the actual data of the power plant.Then the influence of S content in coal,NH3/NOx molar ratio,different NOx concentrations at SCR inlet,and NO removal efficiency on the formation temperature of ABS are analyzed.

1.Introduction

The selective catalytic reduction (SCR) technology is widely used in the treatment of power plant tail gas due to its high efficiency of NOxremoval [1,2].The escaped NH3generated during the NO reduction process and the SO3in the flue gas will generate ammonium bisulfate(ABS),which sticks to the air preheater,causing the pressure difference of the air preheater to rise.

Saleemet al.[3] and Burkeet al.[4] conducted a theoretical analysis on the formation temperature of ABS and found that the formation temperature of ABS is related to the concentration of reactants NH3and SO3.Ammonium sulfate (AS) is the preferred product of thermodynamic analysis,and the resulting product is also affected by NH3/SO3molar ratio.Saleemet al.and Burkeet al.predict the formation temperature of ABS is relatively low,between 150-190 °C and 180-215 °C respectively.Matsudaet al.[5] first carried out the experimental research on the formation temperature of ABS.And the reactant concentration in this experiment deviated from the actual power plant reactant concentration,and the ABS formation temperature obtained was higher,between 260 °C and 330 °C.Weiet al.[6] used acid dew point measurement to measure the formation temperature of ABS.Menashaet al.[7] designed an experiment to simulate the formation of ABS with a single-channel air preheater and found that ABS was first appeared in the form of aerosol and then condensed on the wall.The formation temperatures of ABS predicted by Weiet al.and Menashaet al.are relatively close and are between those predicted by Saleemet al.and Matsudaet al.Although the formation temperature range of ABS obtained by different researchers is different under different experimental conditions,the formation temperature of ABS is positively correlated with the product of NH3and SO3concentrations.

According to many previous studies,the formation temperature of ABS is mainly measured by experimental methods,and it is mainly related to the concentration of escaped NH3and the concentration of SO3in the flue gas.Whether in the experiment or the actual operation of the power plant,it is difficult to accurately measure the concentration of escaped NH3and SO3,which makes it difficult to measure the formation temperature of ABS.To solve the thorny problem,a kinetic model of SO3generated is built,which can get the concentration change of SO3from the boiler to the SCR outlet according to the sulfur content in coal.Then the kinetic model of NO reduction is built,and the NH3escape concentration can be obtained by some available parameters such as NOxconcentration at the inlet of the SCR and NO removal efficiency,etc.Finally,the experimental data of Menashaet al,which is the most precise experimental process of ABS formation temperature at home and abroad,is selected as an important basis,and the Clausius-Clapeyron relation is established to fit his experimental data to obtain the basic equation of ABS formation temperature,which is related to the concentration of SO3and NH3.Substituting the SO3and NH3in the equations with easily available coal sulfur content and parameters of NO reduction,respectively,a model can be obtained that predicts the formation temperature of ABS in flue gas by parameters such as coal sulfur content and parameters of NO reduction.This model cleverly solves the problem of the difficulty of measuring SO3and NH3to obtain the formation temperature of ABS,and the detailed flow chart is shown in Fig.1.

2.Model Development

Firstly,based on the kinetic theory of S-SO2-SO3conversion,a model for obtaining the SO3concentration in flue gas according to the sulfur content in coal is established.Then,based on the kinetic theory of NO reduction in the process of SCR,a model for calculating the NH3escape concentration after SCR denitration is established based on the parameters of the NO reduction process.Finally,the established SO3generation model and NH3escape model are applied to the equation obtained by Menasha’s experiment to explore the ABS formation temperature,and the prediction model of the formation temperature of ABS is obtained.

2.1.SO2 produced by combustion of S in coal

There are more than 30 chemical elements in coal,and coal combustion is mainly a chemical reaction between combustible elements C,H and S in coal and oxygen in the air,and the flue gas produced mainly includes N2,CO2,SO2,and water vapor.The SO2concentration in the flue gas is related to the theoretical air supply,excess air coefficient,and theoretical flue gas volume.Assuming that the air required for fuel combustion and flue gas are ideal gases,the volume of SO2produced by the combustion of 1 kg received base fuel is as shown in Eq.(1)and the SO2concentration in the flue gas is as shown in Eq.(2) [8].

where,VSO2represents the volume of SO2produced by the combustion of 1 kg received base fuel,m3·kg-1；Sarrepresents the sulfur content in coal,kg;CSO2represents the SO2concentration in the flue gas,mg·m-3；Vyrepresents the actual amount of gas,m3·kg-1;0.7 represents 0.7 m3SO3produced by the complete combustion of 1 kg sulfur.

2.2.Generation model of SO3

Almost all S contained in coal is converted into SO2after combustion[9],and part of SO2is converted into SO3by homogeneous and heterogeneous reactions.The homogeneous reaction is mainly the gas phase reaction in the furnace flame combustion,and the heterogeneous reaction is mainly the gas-solid catalytic reaction of fly ash and the SCR catalyst in the flue gas.The chemical reactions involved in the model are included in R(1) — R(4) [10].

2.2.1.Homogeneous formation of SO3

Some scholars have analyzed the elementary reactions in the process of SO3homogeneous reaction under boiler combustion conditions by CHEMIKIN software,and the main reactions involved are R(1) — R(3) [11].The H2O and O2in the flue gas will increase the amount of OH and O and promote the formation of SO3.The detailed elementary reactions involved are more complicated and difficult to use and calculate.To simplify the calculation process of homogeneous and heterogeneous reaction of SO3,the oxidation process of SO2is simplified as R(4).The research of Grahamet al.[12] found that the reaction order of SO2in the R(4) is 1,and the reaction order of O2is 0.5.Therefore,the reaction rate Eqs.(3)and (4) of SO3formation can be obtained based on the kinetics of the chemical reaction and the conversion rate of SO3in the process of SO2oxidation in the furnace is shown in Eq.(5).

where,A1represents the pre-exponential factor,(m3·mg-1)0.5·s-1;Ea1represents the activation energy,kJ·mol-1;Rrepresents the gas constant,8.3145 J·mol-1·K-1;Trepresents the absolute temperature,K;andCO2represent the mass concentration of SO2and O2,mg·m-3;represent the mass concentration of SO3,mg·m-3.

Studies [13,14] show that the homogeneous oxidation rate of SO2decreases when the temperature is lower than 900 °C .Liu and Fleiget al.[15,16] studied the relationship between SO3conversion rate and temperature and found that the reaction rate increased with the increase of temperature when the temperature was between 800 and 1100 °C,and decreased when the temperature was higher than 1100 °C.This is because R(2) and R(3) play a major role in the formation of SO3at 800 °C — 1100 °C.With the increase of temperature,the OH content produced by the decomposition of H2O increases,which promotes the positive reaction of R(2),and then promotes the formation of SO3in R(3).When the temperature is higher than 1100°C,the main effect on the formation of SO3is R(1).When the temperature is higher,the decomposition of HOSO2occurs,reducing the SO3generated in R(2)and R(3),and the decomposition reaction of SO3is accelerated.

2.2.2.Formation of SO3 catalyzed by fly ash

The flue gas fly ash contains SiO2,Al3O4,Fe2O3,and other compounds,which have a certain catalytic effect on the SO2contained in the flue gas and Fe2O3has a more obvious catalytic effect on SO2.Kimet al.[17] studied the catalytic reaction mechanism of fly ash and found that the reaction order of SO2is 1,and the reaction order of O2is 0.5.The reaction rate equation of SO2catalytic oxidation is proposed as follows:

Xiaoet al.[18] studied the fly ash formed at different furnace temperatures and found that the fly ash in the high temperature furnace (above 1500 °C) contains complex iron oxides with more oxygen vacancies,which has a more obvious catalytic effect on SO2than Fe2O3.The catalytic effect of fly ash containing composite iron oxide on SO2first increases and then decreases with the increase of temperature and the catalytic effect is most obvious at 900 °C,which may be related to the participation of other substances such as CaO contained in fly ash in chemical reactions.Therefore,the reaction rate constantK2in Eq.(6) needs to be discussed in different temperature ranges.

2.2.3.Formation of SO3 catalyzed by catalyst during NO reduction

In the SCR process,V2O5-WO3/TiO2is the most commonly used catalyst,containing Ti,W,V,and other elements,among which V2O5has the most significant catalytic effect on SO2.Svachulaet al.[19] systematically studied the effects of experimental operating conditions,catalyst design parameters,and gas composition(O2,SO2,H2O,NOxand NH3concentration) on the catalytic oxidation of SO2to SO3on honeycomb commercial catalysts.Combined with the catalytic reaction mechanism of SO2,a kinetic model of SO2catalytic oxidation reaction is proposed [20]:

where,CSO2,CSO3,CNO,CH2O,CNH3,CO2respectively represent the molar concentration of SO2,SO3,NO,H2O,NH3,O2in the flue gas,mol·cm-3；k1,k2,k3,k4,k5,k6andbrespectively represent the influence coefficients of different substances.

The kinetic equation can explain the following phenomena in related experimental studies [20]: When the SO2concentration is low,the SO2reaction order is about 1,but with the increase of SO2concentration,the reaction order of SO2decreases;When the concentration of O2is high,the effect on the catalytic reaction of SO2is almost zero;H2O and NH3have a certain promotion effect;NO has a certain promotion effect on the catalytic oxidation of SO2.

2.3.NH3 escape model

NH3is one of the important reactants of ABS.Based on SCR reaction kinetics,this section combines SCR reactor performance index reactor potential,the generation of escaped NH3is explored[21,22].Relevant studies[23]have shown that the E-R mechanism is suitable for reactions above 200 ℃,and the L-H mechanism is suitable for reactions below 200 °C.The temperature range of NO reduction is generally 300— 400 °C in the SCR,so the E-R mechanism is dominant.Therefore,the reaction kinetics of NO reduction is built based on the E-R mechanism[24,25],such as Eqs.(9)-(11).Combined with the reaction rate of NO reduction and the time of flue gas passes through the SCR catalyst,the NOxconcentration at the outlet of the SCR and its removal efficiency can be obtained as Eq.(12).

where,RNOrepresents the reaction rate of NO,mol·m-3·s-1;CNOandrepresent the molar concentration of NO and NH3,mol·m-3;arepresents the adsorption rate constant of NH3,s-1;η represents the NO removal efficiency,%;CNOxinandCNOxoutrespectively represent the NO inlet and outlet concentration of the SCR reactor,mg·m-3.

After conducting a large number of experiments and field tests,Songet al.proposed the potentialPof the SCR reactor to measure the performance of the SCR catalyst under actual flue gas conditions [21].The relationship between reactor potential and NO removal efficiency,NOxconcentration,NH3/NOxmolar ratio,and NH3escape concentration is as follows Eqs.(13) and (14).The Eq.(15) of NH3escape concentration can be obtained by combining Eqs.(9)-(14).

where,Prepresents the potential of the reactor;MRrepresents the molar ratio of NH3/NOx;η represents the NO removal efficiency;represents the concentration of escape NH3,mg·m-3;represents the NOxconcentration at the SCR inlet,mg·m-3.

2.4.Prediction model of ABS formation temperature

Comparing with several existing experiments about ABS formation temperature,it is found that Menashaet al’s experimental process is more precise and the physical model was established to simulate the single-channel air preheater to study the formation temperature of ABS.The advantages of this experiment are outstanding,and the measurement accuracy is extremely high and its innovation of this experiment lies in not only the visual observation and electrode measurement of generation of ABS,but also the design of a control experiment without thermocouple and electrode interference to avoid the influence of thermocouple and electrode,which is not found in other studies about formation temperature of ABS.In addition,the NH3and SO3concentrations of the typical SCR outlet selected in this experiment are also more in line with the actual process,and the actual operating conditions of the air preheater are simulated as accurately as possible in every detail.The experimental data show that the formation temperature of ABS is related to the concentration of reactants.The higher the concentration product of reactants,the higher the formation temperature of ABS.There is two equilibrium in the reaction process of ABS formation: the first is the chemical reaction equilibrium in which NH3reacts with SO3(or H2SO4) to form gas phase ABS,and the second is the condensation equilibrium in which gas phase ABS condenses into liquid ABS when it is cold,as follows R(5):

The relationship between temperature and pressure in the process of ABS condensation equilibrium can be expressed by the Clausius-Clapeyron relation.Due to technical reasons,it is difficult to measure the partial pressure of ABS.Because the concentration of the reactant is related to the partial pressure of the reactant,the concentration product of NH3and SO3is used to represent the partial pressure of ABS.Therefore,the concentration of ABS can be expressed as the relationship between the NH3and SO3concentration product and the temperature,as shown in Fig.2.

According to the Clausius-Clapeyron relation,the relationship between NH3concentration,SO3concentration,and the formation temperature of ABS is obtained as Eq.(16).Combined with Eqs.(3),(6),(8),(15),and(16),the prediction model of ABS formation temperature is obtained as Eqs.(17)-(19).

3.Results and Discussion

The prediction model of ABS formation temperature established in Section 2 is based on the coupling of the SO3prediction model and the NH3escape model.Subsequently,the SO3model and NH3escape model are verified by the actual measurement data of the power plant,and then the correctness of the formation temperature prediction model of ABS is indirectly verified.Then,the effects of S content,NH3/NOxmolar ratio,NOxconcentration at different SCR inlet,and different NO removal efficiency on the formation temperature of ABS are analyzed.

3.1.Model validation

To verify the correctness of the SO3prediction model and the NH3escape model built in this paper,firstly,the test data of more than 40 units of the power plant are used to analyze,and the results show that the potential of the reactor decreases with the increase of operation time of units,but the degradation trend slows down as shown in Fig.3,The reactor potential degradation Eq.(20)is obtained by fitting its data.The potential of the reactor increases with the decrease of unit operating load rate,as shown in Fig.4.Based on the field test data,the relationship between the potentialP0of the reactor and the operating load rate of the unit is as shown in Eq.(21).

where,trepresents the cumulative operating time,h;P0represents the reactor potential at a fixed unit operating load rate;Lrepresents unit operating load rate,%.

Fig.2.Experimental data fitting of Menasha et al. [7].

Fig.3.Derioration trend of reactor potential over time.

Fig.4.The relationship between reactor potential and load rate.

Under the quality parameters of as-received coal as shown in Table 1,the amount of SO3generated by the homogeneous reaction of SO2,fly ash catalysis,and the catalytic oxidation of V2O5in the SCR catalyst is calculated,and the parameters used in the model calculation are shown in Table 2 and Table 3.The amount of SO3generated in the three ways as shown in Fig.5.Among them,the amount of SO3generated by the catalytic oxidation of V2O5is the largest,which is 56.46 mg·m-3;the second is generated by the homogeneous phase,which is 31.9 mg·m-3;the amount of SO3generated by fly ash is the least,which is 6.1 mg·m-3,of which the proportion of SO3generated by the SCR catalyst accounts for the largest,which is 60%.Therefore,reducing catalytic rate of the SO2in the SCR catalyst is the key to reducing the SO3in the flue gas.

Table 1 Elemental analysis of coal samples (%)

Table 2 Kinetic parameters for reaction mentioned: part I

Table 3 Kinetic parameters for reaction mentioned: part II

It can be seen from Fig.6 that the value predicted of SO3and the mean value measured at the SCR inlet of the power plant are 38 mg·m-3and 36.36 mg·m-3,respectively,with a relative error of 4.32%.the value predicted of SO3and the mean value measured at the exit of SCR are 94.46 mg·m-3and 83.34 mg·m-3,respectively,with a relative error of 11.77%.The predicted value of NH3escape is 2.15 mg·m-3,the actual measured average value is 2.39 mg·m-3,and the relative error is 10.04%.The predicted value of SO3is consistent with the actual value at the SCR inlet,the deviation of SO3concentration is slightly larger at the SCR outlet,and the predicted value of SO3concentration is in good agreement with the actual value as a whole.Similarly,the prediction model of NH3escape is more accurate,as shown in Fig.7.The model has high accuracy and good universality and the prediction errors of SO3and NH3are within the acceptable range The ABS prediction model is a coupling of the SO3generation model and the NH3escape model,which indirectly proves the accuracy of the ABS prediction model.

Fig.7.The trend of concentration of escape NH3.

3.2.The effect of S in coal on the formation temperature of ABS

Fig.8 shows the effect of different sulfur content in coal on the formation temperature of ABS.As the S content in coal increases,the SO3content in the flue gas increases almost linearly,while the escaped NH3volume hardly changes,and the formation temperature of ABS increases,but the growth rate slows down.When the concentration of NH3in flue gas is 3.9 mg·m-3and the content of S in the coal changes from 0.1%to 3%,the formation temperature of ABS increases from 209 °C to 243.6 °C,which increases by 34.6°C.Especially when the S content is within 1%,the formation temperature of ABS increases rapidly with the increase of S content.

With the increase of sulfur content in coal,the concentration product of SO3and NH3increases,which makes the formation temperature of ABS increase.To reduce the ABS formation temperature,the power plant burns low-sulfur coal as much as possible so that ABS is deposited as much as possible on the cold end of the air preheater instead of the middle section,so that the air preheater can use soot blowing and hot air circulation to reduce the deposited ABS.When the S content in the coal exceeds 2%,the temperature of ABS formation changes little with the increase of the content of S in the coal.

3.3.Effect of NH3/NOx molar ratio on the temperature of ABS formation

To study the influence of NH3/NOxsingle factor on ABS formation temperature,NO reduction efficiency is fixed at 90%.Fig.9 shows the effect of different NH3/NOxmolar ratios on the formation temperature of ABS.It can be seen from Fig.9 that with the increase of NH3/NOxmolar ratio from 0.7 to 1.2,the concentration of escaped NH3increased from 3.18 mg·m-3to 4.79 mg·m-3,with an increase of 50.6%.While the concentration of SO3gradually decreased from 85.29 mg·m-3to 64.06 mg·m-3,with a decrease of 24.9%;and the formation temperature of ABS changed little,only increased by 1.4 °C.The concentration of NH3in the flue gas increases but SO3decreases and the concentration product of NH3and SO3has little change,so the formation temperature of ABS has little change.

The increase of NH3/NOxmolar ratio will seriously increase the concentration of escape NH3.The main reason is that the ideal ratio of NH3to NO reaction is 1:1.When NH3/NOxis excessively increased,the amount of NH3that is not involved in the reaction in the flue gas is increased severely,resulting in serious NH3escaping.On the contrary,the SO3concentration in the flue gas decreases,because the escape NH3increased with the increase of NH3/NOxduring NO reduction,which inhibits the conversion of SO2to SO3,thereby reducing the concentration of SO3,which is consistent with the results of Svachulaet al[19].In the actual process,adding ammonia injection grids and baffles in the SCR flue to reduce the unevenness of the local NH3/NOxmolar ratio can reduce the escape of NH3,thereby reducing the formation temperature of ABS.

3.4.The effect of different NOx inlet concentrations on the formation temperature of ABS

It can be seen from Fig.10 that with the increase of the NOxconcentration at the SCR inlet the ABS formation temperature gradually increases,which is consistent with the research results of Siet al[26].The main reason is that when the rate of NH3/NOxis fixed at 0.908,the concentration of escape NH3increases with the increase of NOxconcentration at the SCR inlet,thereby reducing the SO3concentration.Generally speaking,the formation tempera-ture of ABS increases with the increase of the concentration product of NH3and SO3.With the increase of the NOxconcentration at the SCR inlet,the NO removal efficiency decreases,resulting in an increase of the amount of escaped NH3;The reason for the decrease of SO3concentration may be that excessive NH3occupies more V+5vacancies,which inhibits the oxidation process of SO2on V2O5.

Fig.5.The production of different forms of SO3 in flue gas.

Therefore,the rational use of technologies such as low-nitrogen combustion in power plants not only reduces the concentration of NOxemissions but also helps reduce the formation temperature of ABS,thereby reducing the condensation of ABS in the middle temperature section of the air preheater.

3.5.The effect of NO removal efficiency on ABS formation temperature

To study the effect of NO removal efficiency on the formation temperature of ABS,the molar ratio of NH3/NOxis fixed as 1.The influence of different NO removal efficiency on the formation temperature of ABS can be seen in Fig.11.With the increase of no removal efficiency,the formation temperature of ABS increases gradually,and the growth rate increases,while the SO3concentration in the flue gas hardly changes,but the concentration of NH3escape increases gradually.Especially when the NO removal efficiency exceeds 90%,if the NO removal efficiency continues to increase,the concentration of NH3escape will increase rapidly,and the formation temperature of ABS will increase rapidly,which causes ABS to be produced at a higher temperature,causing more serious clogging and corrosion problems to the SCR catalyst and air preheater.

The interaction between NH3/NOxmolar ratio and NO removal efficiency can be seen in Fig.12.With the increase of NH3/NOxmolar ratio,the NO removal efficiency increases rapidly at first,but when the NH3/NOxmolar ratio exceeds 0.9,the NO removal efficiency increases very little with the increase of NH3/NOxmolar ratio,but the concentration of escape NH3increases very fast.The synergistic effect between NH3/NOxmolar ratio and no removal efficiency on ABS formation temperature is shown in Fig.13.When NH3/NOxmolar ratio exceeds 0.9,if the NH3/NOxmolar ratio continues to increase,the ABS formation temperature will increase rapidly.The comprehensive effect between NH3/NOxmolar ratio and NO removal efficiency on the formation temperature of ABS is shown in Fig.13.When NH3/NOxmolar ratio exceeds 0.9,if the NH3/NOxmolar ratio continues to increase,the formation temperature of ABS will increase rapidly.Therefore,to reduce the formation temperature of ABS,it is necessary to rationally balance the relationship between the NH3/NOxmolar ratio and the NO removal efficiency,and try to control the NH3/NOxmolar ratio at about 0.9,which can ensure the higher NO removal efficiency at 85%-90%,and control the lower concentration of escaped NH3.

4.Conclusions

Fig.6.Trend of total SO3 generation.

Fig.8.Relationship between S content in coal and ABS generation temperature.

Fig.9.The influence of NH3/NOx molar ratio on ABS formation temperature and escaped NH3.

Fig.10.The influence of different NOx inlet concentrations on ABS formation temperature and escaped NH3.

Fig.11.The effect of NO removal efficiency on ABS formation temperature.

Fig.12.The relationship between NH3/NOx molar ratio,NO removal efficiency,and the concentration of escaped NH3.

Fig.13.The comprehensive effect between NH3/NOx molar ratio and NO removal efficiency on the formation temperature of ABS.

In this study,a prediction model of ABS formation temperature in air preheater of the coal-fired power plant was established by combining the reaction kinetics of SO3and kinetics of NO reduction.The amount of SO3generated from the combustion of S in coal to the inlet of the air preheater was calculated by the reaction kinetics of SO3,and the concentration of escaped NH3was calculated by the reaction kinetics of NO reduction,and finally,the model of predicting the ABS formation temperature was obtained.Considering the whole process,the prediction model of ABS formation temperature obtained by using S content in coal and parameters of NO reduction overcomes the difficulty of calculating the formation temperature of ABS by measuring the SO3concentration and NH3concentration in the past,and the prediction results of the model are in good agreement with the actual data,which has good applicability.

The following conclusions can be noted:

(1)The prediction model of SO3concentration shows that in the process of coal combustion to the SCR outlet,the SO3generated by the catalysis of V2O5accounted for 60%,by the homogeneously generated SO3accounted for 33.8%,and the fly ash catalyzed SO3accounted for 6.2%.Reducing the catalytic oxidation of SO2on the SCR catalyst is the key to reducing the concentration of SO3,which is beneficial to lower the formation temperature of ABS,thereby slowing down the deposition of ABS on the air preheater.

(2)The formation temperature of ABS is directly related to the S content in coal.The increase of the sulfur content in coal leads to the increase of SO3concentration in the flue gas,which makes the formation temperature of ABS increase rapidly.To reduce the influence of ABS from the source,low sulfur coal should be burned as far as possible or desulfurization treatment should be carried out before combustion.

(3) Although the change of the NH3/NOxmolar ratio has little effect on the ABS formation temperature when the NH3/NOxdeviates from the design value seriously,it will cause serious NH3escape.Therefore,in practical,adding ammonia injection grid and guide plate in SCR Flue can reduce the non-uniformity of local NH3/NOxmolar ratio,which can not only improve the NO removal efficiency but also effectively reduce NH3escape,thus reducing the formation temperature of ABS.When the NH3/NOxmolar ratio is fixed,the formation temperature of ABS increases with the increase of the emission concentration of NOxat the SCR inlet.Therefore,the use of low-nitrogen combustion technology not only reduces NOxemissions concentration,but also reduces the formation temperature of ABS.

(4)Although the improvement of NO removal efficiency has little effect on the SO3concentration,it has a greater impact on the concentration of NH3escape.As the NO removal efficiency exceeds 90%,continuing to increase the NH3/NOxmolar ratio will not significantly improve the NO removal efficiency,but will increase the NH3concentration rapidly,resulting in the rapid increase of ABS formation temperature.Therefore,to control the formation temperature of ABS,adjusting the NH3/NOxmolar ratio at about 0.9 can not only ensure the NO removal efficiency at 85%-90%but also control the concentration of NH3escape at a low level.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the Key Research and Development Plan of Shandong Province (2019GSF109004) and Natural Science Foundation of Shandong Province (ZR2020ME190) for funding and supporting this work.

Nomenclature

Athe adsorption rate constant of NH3,s-1

A1the pre-exponential factor,(m3·mg-1)0.5·s-1

A2the pre-exponential factor,(m3·mg-1)0.5·s-1

bsimplify the factor

CH2Othe molar concentration of H2O,mol·cm-3

CNOthe molar concentration of NO,mol·cm-3

Ea1the activation energy,kJ·mol-1

Ea2the activation energy,kJ·mol-1

K1the homogeneous reaction rate constant

K2fly ash catalytic reaction rate constant

K3the reaction rate constant of NO reduction

kithe influence coefficients of different substances,i=1,...,6

Lunit operating load rate,%

MRthe molar ratio of NH3/NOx

[NH3] escape NH3concentration,mg·m-3

Pthe potential of the reactor

P0initial reactor potential

Rthe gas constant,J·mol-1·K-1

Rf1homogeneous SO3formation rate,mol·L-1·s-1

Rf2fly ash catalytic SO3generation rate,mol·L-1·s-1

Rf3De-NOxcatalytic SO3generation rate,mol·L-1·s-1

RNOthe reaction rate of NO,mol·m-3·s-1

Ssulphur content in coal,%

Sarthe sulfur content in coal,kg

[SO3] SO3concentration,mg·m-3

Tthe absolute temperature,K

TIFTthe ABS formation temperature,℃

Vythe actual amount of gas,m3·kg-1

ηNO removal efficiency,%

Subscript

ar as-received basis

in the NO at the SCR inlet

out the NO at the SCR outlet