Yuan Li,Hao Zhou*,Ning Li,Runchao Qiu,Kefa Cen

State Key Laboratory of Clean Energy Utilization,Institute for Thermal Power Engineering,Zhejiang University,Hangzhou 310027,China

Keywords:NO N2O Single coal particle O2/N2 O2/H2O Low temperature

A B S T R A C T Oxy-steamcombustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N,and char-N to NO and N2O during combustion of a single coal particle in O2/N2and O2/H2O were studied in a tube reactor at low temperature.In O2/N2,NO reaches the maximum value in the devolatilization stage and N2O reaches the maximum value in the char combustion stage.In O2/H2O,both NO and N2O reach the maximum values in the char combustion stage.The total conversion ratios of fuel-N to NO and N2O in O2/N2are obviously higher than those in O2/H2O,due to the reduction of H2O on NO and N2O.Temperature changes the trade-off between NO and N2O.In O2/N2and O2/H2O,the conversion ratios of fuel-N,volatile-N,and char-N to NO increase with increasing temperature,and those to N2O show the opposite trends.The conversion ratios of fuel-N,volatile-N,and char-N to NO reach the maximum values at＜O2＞ =30 vol%in O2/N2.In O2/H2O,the conversion ratios of fuel-N and char-N to NO reach the maximum values at＜O2＞ =30 vol%,and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration.The conversion ratios of fuel-N,volatile-N,and char-N to N2O decrease with increasing oxygen concentration in both atmospheres.A higher coal rank has higher conversion ratios of fuel-N to NO and N2O.Anthracite coal exhibits the highest conversion ratios of fuel-N,volatile-N,and char-N to NO and N2O in both atmospheres.This work is to develop efficient ways to understand and control NO and N2O emissions for a clean and sustainable atmosphere.

1.Introduction

Coal is the primary source of energy for power generation in China and accounts for 70%of the power generation[1].Coal- fired power generation technologies cause high CO2emissions,which is one of the major environmental problems.Carbon capture and storage(CCS),including pre-combustion capture[2],post-combustion capture[3],and oxy-fuel combustion [4], is regarded to be technically feasible to capture CO2emissions from coal- fired power plants[5].Oxy-fuel combustion is considered to be an economically feasible CCS technology for coal- fired power plants[4,6],which includes oxy-CO2combustion and oxy-steam combustion depending on different diluents[6,7].Oxy-steam combustion,using H2O as the diluent instead of CO2,was proposed by Salvador et al.[8]and Seepana and Jayanti[9].Compared with oxy-CO2combustion,oxy-steam combustion is simpler and easier to start up/shut down,due to the deletion of the recycled evolved gas[10,11].In addition,NOxand SOxemissions are suppressed,and the power consumption of the recycling evolved gas is reduced[9,12].Thus,oxy-steam combustion is regarded to be a promising next-generation combustion technology.In oxy-steam combustion,H2O vapor will accelerate the reactions as follows[13]:

where Reaction(1)represents the gasification reaction of carbon by H2O vapor,Reaction(2)represents the steam shift reaction,and Reaction(3)represents the combustion reaction of H2with O2.

Oxy-fuel combustion in fluidized-bed(FB)has the advantages of fuel flexibility,high boiler efficiency,moderate combustion temperature,and low NOxand SOxemissions[14–16].At FB temperature,N2O is formed which can destruct the ozone layer[17],and the thermal NOxand the prompt NOxcan be negligible[18].NOxand N2O are formed through homogeneous and heterogeneous reactions [19–21]. Coal combustion can be divided into the devolatilization and char combustion stages,and fuel-N in coal includes volatile-N and char-N accordingly.NH3and HCN derived from volatile-N are the NOxprecursors,and HCN is also the N2O precursor through the reaction path of HCN→NCO→ N2O[19–22].Moreover,NOxand N2O are converted from char-Nthrough complex reaction mechanisms[19–21,23–26].In conventional and oxy-CO2FB combustion,NOxand N2O emissions have been studied by many researchers[26–32].Although the reduction effect of H2O vapor addition on NOxduring coal combustion has been widely investigated[32–38],only a few studies have focused on NOxemission in oxysteam combustion.Tu et al.[13]numerically studied NO emission in oxy-steam combustion of coal and indicated that the presence of H2O vapor could accelerate NO reduction by H2.H2O vapor addition promotes the decomposition of N2O[39].However,there are few studies on N2O emission in oxy-steam combustion.

Table 1Proximate and ultimate analyses of coal samples

To understand in-depth the NOxand N2O emission characteristics in oxy-steam combustion, it is necessary to distinguish the devolatilization and char combustion stages of coal combustion and achieve the conversions of volatile-N and char-N to NOxand N2O.A lot of research studied the NOxand N2O emissions from volatiles and chars separately[22–26].Bai et al.[40]used carbon conversion to distinguish the two stages of biomass combustion.Zhou et al.[41,42]applied isothermal thermogravimetric analysis(TGA)to distinguish the two stages of coal/biomass combustion.Moreover,the combustion image sequences were used to distinguish the two stages of fuel combustion by many researchers[43–45].

The purpose of this work is to study the conversions of fuel-N,volatile-N,and char-N to NOxand N2O in oxy-steam combustion at low temperature.The combustion image sequences of the coal particle were applied to distinguish the two stages of coal combustion.The experiments were carried out in a tube reactor with single coal particles in O2/N2and O2/H2O at 700–900 °C.Three different coal ranks and oxygen concentrations of 21 vol%–50 vol%were considered.

2.Experimental Section

2.1.Coal samples

Three different coal ranks were employed as the research objects, including lignite coal (LC), bituminous coal (BC), and anthracite coal(AC).The original coal particles with irregular shapes were shaped to spherical particles.The diameters of the spherical particles were set to 7 mm uniformly.The proximate and ultimate analyses of the coal samples are given in Table 1.All the coal samples were dried in a vacuum dry box at 60°C for 10 h before tests.

2.2.Nitrogen distributions in samples

Preparing and characterizing coal chars are necessary to determine the nitrogen distributions in coals.As presented in Fig.1,coal chars were produced at 700,800 and 900°C in a horizontal flow tube reactor located in an electric furnace.The reactor is a silica tube with a length of 800 mm and an inner diameter of 50 mm.After the reactor was heated to the desired temperature,the high-purity Ar(99.999%)of 1 L·min-1was used to purge the air in the reactor.Coal sample in an alumina ceramic boat was rapidly inserted into the constant-temperature zone of the reactor with the desired temperature,to avoid air leaking into the reactor.Pyrolysis was conducted at the desired temperature with the Ar flow of 1 L·min-1for 1 h.After pyrolysis,coal char was removed from the reactor and cooled to room temperature in an Ar atmosphere.The properties of the coal chars are listed in Table 2.

Fuel-N in coal can be divided into volatile-N and char-N.Based on the method of ash balance,the ratio of char-N content in fuel-N content for a raw coal(Rchar-N)is calculated as follows:

where Ncoalis the nitrogen content of the raw coal,%;Acoalis the ash content of the raw coal,%;Ncharis the nitrogen content of the corresponding coal char,%;and Acharis the ash content of the corresponding coal char,%.The ratio of volatile-N content in fuel-N content for the raw coal(Rvolatile-N)is calculated as follows:

For the three coals,the nitrogen distributions in the coal samples at different temperatures are shown in Fig.2.The reaction temperature slightly affects the nitrogen distribution in coal at the range of 700–900 °C,and the effect of temperature on nitrogen distribution is greater for a lower coal rank.

2.3.Experimental system and procedure

Fig.1.Schematic of the coal char preparation system.

Table 2Properties of coal chars

Fig.2.Nitrogen distributions in samples at different temperatures.

The experimental system is shown in Fig.3,including a vertical tube reactor,a steam generator,a CCD camera,and a gas analyzer.The tube reactor placed in an electric furnace is a quartz tube with a height of 500 mm and an inner diameter of 60 mm.The reaction temperature was monitored by a thermocouple located in the center of the reactor close to the location of the sample.A temperature controller was used to control the reaction temperature with±1°C inaccuracy.A sample particle placed in a platinum wire basket was rapidly fed into the tube reactor.The combustion process of the sample particle was recorded by the CCD camera with a rate of 45 frames·s-1.Two ALICAT mass flow controllers(MFCs)(ALICAT SCIENTIFIC,AMERICA)were used to control the flow rates of O2and N2with±0.5%inaccuracy of full scale.gas could be also pre-heated by the steam generator before it was supplied into the reactor.The flow rate of the ‘O2+H2O’or ‘O2+N2’gas was set at 8 L·min-1to make sure the uniform boundary condition.The evolved gas from the reactor was cooled using a condenser,and a beaker placed in an ice bath was used to receive the condensed water from the evolved gas.By further drying and filtering,the evolved gas was measured using the gas analyzer which was a GASMET FT-IR Dx4000(GASMET TECHNOLOGIES,FINLAND)with the measurement time interval of 2 s.The evolved gas was diluted by N2to make sure that the oxygen concentration of the mixture gas was less than 25 vol%,considering that the oxygen concentration of the evolved gas might be over 25 vol%(especially for the O2/H2O atmosphere in which H2O vapor was removed from the evolved gas)and the oxygen range of the gas analyzer was 0–25 vol%.Two MFCs were used to control the dilution ratio precisely.The reaction temperature in the tube reactor was set at 700,800 and 900°C in this work.The temperature profiles obtained at different reaction temperatures in O2/N2and O2/H2O are shown in Fig.4.The temperature profile in O2/H2O is almost the same as that in O2/N2at a certain reaction temperature.

In this work,the tests of each case were duplicated over 2 times to make sure that the relative standard deviation of the experimental results was within 10%.

2.4.Data processing

Fig.3.Schematic of the experimental system.

Fig.5.NO and N2O emission characteristics and conversions of fuel-N to NO and N2O during BC particle combustion(a)in O2/N2and(b)in O2/H2O(＜O2＞ =30 vol%and T=800°C).

Fig.4.Temperature profiles inside the vertical tube reactor.

In the tests,the NO2concentration of the evolved gas was below 1 ppmv,and NO2was ignored.This work focuses on NO and N2O emissions which are proposed to be converted from fuel-N merely at low temperature.The conversion ratios of fuel-N,volatile-N,and char-N to NO and N2O were calculated as follows:A pre-calibrated peristaltic pump was used to control the flow rate of distilled water.Distilled water was carried by O2into the steam generator whose temperature was set at 600°C to make sure the complete evaporation of distilled water.In addition,the ‘O2+H2O’or ‘O2+N2’where x-N represents fuel-N,volatile-N,or char-N;Nx-Nis the x-N content of the sample(Nfuel-Nis the fuel-N content of the sample),%;Rx-Nis the ratio of Nx-Nin Nfuel-Nof the sample,%;ηx-N,NOis the conversion ratio of x-N to NO,%;ηx-N,N2Ois the conversion ratio of x-N to N2O,%;minitis the initial mass of the sample particle,g;Q is the flow rate of the evolved gas in the reactor,L·s-1;MNis the nitrogen molar mass,g·mol-1;Vmis the molar volume of gas,L·mol-1;CNOis the NO concentration of the evolved gas in the reactor,m3·m-3;CN2Ois the N2O concentration of the evolved gas in the reactor,m3·m-3;tx-Nrepresents the entire combustion(fuel-N),devolatilization(volatile-N),or char combustion(char-N)period,s;and t is the time,s.Given the dilution of the evolved gas by N2,the NO and N2O concentrations in Eqs.(7)and(8)were corrected to the actual concentrations in the reactor rather than the values directly measured by the gas analyzer.

3.Results and Discussion

3.1.NO and N2O emission characteristics

The turning point of the sample particle from the devolatilization stage to the char combustion stage can be obtained from the combustion image sequences recorded by the CCD camera.The disappearance of the volatile flame is assumed to be the turning point between the two stages.Fig.5(a)and(b)display the NO and N2O released characteristics during BC particle combustion in O2/N2and O2/H2O(＜O2＞ =30 vol%and T=800 °C).The NO and N2O concentrations have been corrected to the actual concentrations in the reactor and normalized to a unit mass of the sample particle,i.e.ppmv·g-1fuel.The NO emission in O2/N2is significantly higher than that in O2/H2O.The maximum value of NO is located in the devolatilization stage in O2/N2,while it is delayed to the char combustion stage in O2/H2O.The N2O emission in O2/N2is higher than that in O2/H2O.In O2/N2and O2/H2O,the N2O release profiles are both bimodal distributions,and the maximum values of N2O are located in the char combustion stage.

The conversions of fuel-N to NO and N2O during BC particle combustion in O2/N2and O2/H2O are also presented in Fig.5(a)and(b)(＜O2＞ =30 vol%and T=800 °C).The conversions of fuel-N to NO during the two stages in O2/N2(7.27%and 10.73%)are significantly higher than those in O2/H2O(0.44%and 5.00%).It suggests that the presence of H2O vapor is conducive to the NO reduction.The amounts of H/OH radicals and CO are increased in a H2O atmosphere[12],and the presence of H2O vapor accelerates the reduction of NO via the reactions as follows[35,37]:

H/OH radicals can accelerate the reduction of NO by NHivia the reactions as follows[46]:

Moreover,H radicals can promote NO reduction through the reaction path of NO→HNO→NH→N2O→N2in the presence of H2O vapor[35].The NO reduction by NHimay be accelerated via Reactions(15)and(16)in O2/H2O,resulting in the more obvious NO reduction in the devolatilization stage than that in the char combustion stage.In O2/N2and O2/H2O,N2O maintains at a low level in the devolatilization stage.The conversions of fuel-N to N2O during the devolatilization stage in O2/N2(0.27%)is slightly lower than that in O2/H2O(0.36%),whereas the conversion of fuel-N to N2O during the char combustion stage in O2/N2(5.58%)is obviously higher than that in O2/H2O(3.75%).L?ffler et al.[39]indicated that the presence of H2O vapor was conducive to the decomposition of N2O via the reactions as follows:

N2O can be destructed by H/OH radicals[Reactions(17)and(18)]and CO[Reaction(19)].Moreover,N2O can also be destructed by O radicals[Reactions(20)and(21)]and by the thermal decomposition with a third body M[Reaction(22)]as follows[21]:

Zou et al.[47]suggested that the temperature profiles of oxy-steam combustion were consistent with air combustion when the oxygen concentration was about 30 vol%.The volatile combustion temperature in 30%O2/70%H2O may be lower than that in 30%O2/70%N2,due to the higher heat capacity of H2O than that of N2.The moderation effect of H2O vapor on the volatile combustion temperature may result in the slightly higher conversion of fuel-N to N2O during the devolatilization stage in O2/H2O than that in O2/N2.The improvement of N2O reduction in a H2O atmosphere via Reactions(17)–(19)leads to the lower conversion of fuel-N to N2O during the char combustion in O2/H2O than that in O2/N2.The total conversions of fuel-N to NO in O2/N2and O2/H2O are 18.01%and 5.44%,respectively.Moreover,the total conversions of fuel-N to N2O in O2/N2and O2/H2O are 5.85%and 4.11%,respectively.Thus,H2O vapor has the reduction effects on both NO and N2O,and the reduction effect on NO is more intense than that on N2O.In addition,the char combustion stage takes a dominant part in the NO and N2O emissions compared with the devolatilization stage in both atmospheres.

3.2.Effect of temperature on nitrogen conversion

Pels et al.[19]demonstrated that NO increased with increasing temperature and N2O exhibited the opposite trend during conventional FB combustion of coals.In Fig.6(a)–(c),the effects of temperature on the conversions of fuel-N,volatile-N,and char-N to NO during BC particle combustion in O2/N2and O2/H2O(＜O2＞ =30 vol%)are shown.In O2/N2and O2/H2O,ηfuel-N,NO,ηvolatile-N,NOand ηchar-N,NOall increase with increasing temperature.The combustion rates and free radicals are increased at a higher temperature,which are conducive to NO formation.Moreover,the NH3and HCN yields from coal pyrolysis are also increased at a higher temperature[48].It should be noted that NH3(including its related NHiradicals)can act as a reducing agent for NO,similar to the selective non-catalytic reduction(SNCR)process,referring to the reaction as follows[49]:

There is a temperature window for the NO reduction by NH3[Reaction(23)]in the SNCR process[50].Moreover,the temperature window is controlled by free radicals[50,51].At＜O2＞ =30 vol%,the optical temperature window is about 850–900 °C in O2/N2[51].In O2/N2,the slightly lower increasing rate of ηvolatile-N,NOwhen the temperature exceeds 800°C may be attributed to the enhancement of the NO reduction by NH3.In O2/H2O,the temperature window may shift to a lower temperature,due to the higher levels of free radicals than those in O2/N2.When the temperature exceeds 800°C in O2/H2O,the reaction temperature may be away from the optical temperature window,leading to the slightly higher increasing rate of ηvolatile-N,NO.The NO reduction in a H2O atmosphere leads to the significantly lower ηvolatile-N,NOin O2/H2O than that in O2/N2at T=700–900 °C.A higher temperature results in a higher conversion ratio of char-N to NO in conventional FB combustion[20].Meanwhile,the NO reduction over charsurface can be accelerated at a higher temperature via the reactions as follows[52]:

Fig.7.Effects of temperature on the conversion ratios of(a)fuel-N,(b)volatile-N,and(c)char-N to N2O during BC particle combustion(＜O2＞ =30 vol%).

where(–C)represents active carbon and(–CO)represents active radical in char.When the temperature exceeds 680°C,Reaction(24)occurs and forms CO.Reaction(25)respects the NO reduction over char surface by CO.In O2/N2and O2/H2O,a lower increasing rate of ηchar-N,NOis observed when the temperature exceeds 800°C.In O2/H2O,the behavior is more obvious due to more CO formed.Given the NO reduction in a H2O atmosphere,ηchar-N,NOin O2/H2O is obviously lower than that in O2/N2O at T=700–900 °C.The conversion ratios of fuel-N to NO in O2/H2O decrease by 10.44,3.31,and 2.76 times compared with those in O2/N2at T=700,800,and 900°C,respectively.

In Fig.7(a)–(c),the effects of temperature on the conversions of fuel-N,volatile-N,and char-N to N2O during BC particle combustion in O2/N2and O2/H2O(＜O2＞ =30 vol%)are illustrated.In O2/N2and O2/H2O, ηfuel-N,N2O, ηvolatile-N,N2Oand ηchar-N,N2Oall decrease with increasing temperature.Due to the higher levels of free radicals(mainly O/H radicals)at a higher temperature,the fast destruction reactions of N2O[mainly Reactions(17)and(18)]are accelerated.Moreover,the thermal destruction reaction of N2O[Reaction(22)]is also accelerated at a higher temperature.Although the HCN yield from coal pyrolysis increases with increasing temperature,HCN tends to form NOr ather than N2O,and NCO(from HCN)is rapidly removed by free radicals mainly via the reactions NCO+H→NH+CO and NCO+O→NO+CO[53].These may lead to the lower ηvolatile-N,N2Oat a higher temperature in both atmospheres.Given the moderation of the volatile combustion temperature in a H2O atmosphere,the N2O destruction in O2/H2O may be weaker than that in O2/N2,leading to the higher ηvolatile-N,N2Oin O2/H2O than that in O2/N2at T=700–900 °C.N2O can be reduced over char surface during the char combustion stage via the reactions as follows[26]:

where(–C)represents active carbon and(–CO)represents active radical in char.The N2O reduction over char surface is accelerated at a higher temperature[26],resulting in the lower ηchar-N,N2Oat a higher temperature in O2/N2and O2/H2O.In both atmospheres,the sharp decreases of ηvolatile-N,N2Oand ηchar-N,N2Oat T=700–900 °C are mainly attributed to the enhancement of the fast and thermal destruction of N2O[Reactions(17),(18),and(22)].Given the destruction of N2O in a H2O atmosphere,ηfuel-N,N2Oin O2/H2O is lower than that in O2/N2at T=700–900 °C.The conversion ratios of fuel-N to N2O in O2/H2O decrease by 1.07,1.42,and 3.36 times compared with those in O2/N2at T=700,800,and 900°C,respectively.

3.3.Effect of oxygen concentration on nitrogen conversion

The effects of oxygen concentration on the conversion ratios of fuel-N,volatile-N,and char-N to NO during BC particle combustion in O2/N2and O2/H2O(T=800 °C)are shown in Fig.8(a)–(c).In O2/N2and O2/H2O,ηfuel-N,NOachieves the maximum value at＜O2＞ =30 vol%.In O2/N2,ηvolatile-N,NOachieves the maximum value at＜O2＞ =30 vol%,and it slightly increases with increasing oxygen concentration in O2/H2O.Similar to the effect of temperature,a higher oxygen concentration results in higher combustion rates,higher free radical concentrations,and more NH3and HCN yields,which promote the NO formation.Thus,ηvoltile-N,NOincreases with increasing oxygen concentration at＜O2＞ =21 vol%–30 vol%in O2/N2.However,higher oxygen and free radical concentrations shift the temperature window to a lower temperature in the SNCR process[51].When the oxygen concentration exceeds 30 vol%in O2/N2,the NO reduction by NH3/HCN may be enhanced,leading to the decrease of ηvoltile-N,NOat＜O2＞ =30 vol%–50 vol%in O2/N2.The temperature window may shift to a lower temperature than that in O2/N2,due to the higher free radical concentrations in O2/H2O.At a higher oxygen concentration in O2/H2O,the optical temperature window may be below 800°C,which is away from the reaction temperature.Thus,the NO reduction by NH3may be weakened at a higher oxygen concentration in O2/H2O.This may result in the slight increase of ηvolatile-N,NOat＜O2＞ =21 vol%-50 vol%in O2/H2O.Given the reduction effect of H2O on NO,ηvolatile-N,N2Oin O2/H2O is lower than that in O2/N2at＜O2＞ =21 vol%-50 vol%.In both atmospheres,ηchar-N,NOachieves the maximum value at＜O2＞ =30vol%.A higher oxygen concentration promotes the NO formation, resulting in the increase ofηchar-N,NOwith increasing oxygen concentration at＜O2＞=21vol%-30vol%inO2/N2andO2/H2O.However,the NO reduction over char surface is also accelerated at a higher oxygen concentration,resulting in the decrease of ηchar-N,NO when the oxygen concentration exceeds 30 vol%in both atmospheres.Given the reduction effect of H2O on NO,ηchar-N,NOin O2/H2O is lower than that in O2/N2at＜O2＞ =21vol%–50vol%.The conversion ratios of fuel-N to NO in O2/H2O decrease by a factor of 2.42,3.31,4.15,and 3.78 compared with those in O2/N2at＜O2＞ =21 vol%,30 vol%,40 vol%and 50 vol%,respectively.

The effects of oxygen concentration on the conversion ratios of fuel-N,volatile-N,and char-N to N2O during BC particle combustion in O2/N2and O2/H2O(T=800 °C)are illustrated in Fig.9(a)–(c).In O2/N2and O2/H2O,ηfuel-N,N2O,ηvolatile-N,N2Oand ηchar-N,N2O all decrease with increasing oxygen

concentration.Because of the higher free radical concentrations and the higher combustion temperature at a higher oxygen concentration,the fast destruction of N2O[Reactions(17)and(18)]and the thermal destruction of N2O[Reaction(22)]are accelerated,similar to the effect of temperature.Moreover,the N2O destruction by O radicals[via Reactions(20)and(21)]is also accelerated at a higher oxygen concentration.NCO can be rapidly removed by free radicals via the reactions NCO+H→NH+CO and NCO+O→NO+CO[53],which is also conducive to the decrease of ηvolatile-N,N2Owith increasing oxygen concentration.Given the moderation of H2O on the volatile combustion temperature,ηvolatile-N,N2Oin O2/H2O is higher than that in O2/N2at＜O2＞ =21 vol%–40 vol%.At＜O2＞ =50 vol%,the moderation of H2O on the volatile combustion temperature becomes weaker and the reduction effect of H2O on N2O takes a dominant part,leading to the lower ηvolatile-N,N2Oin O2/H2O than that in O2/N2.The N2O reduction over char surface is accelerated at a higher oxygen concentration,leading to the decrease of ηchar-N,N2Owith increasing oxygen concentration.Given the reduction effect of H2O on N2O,ηchar-N,N2Oin O2/H2O is lower than that in O2/N2at＜O2＞ =21 vol%-50 vol%.The conversion ratios of fuel-N to N2O in O2/H2O decrease by a factor of 1.33,1.42,4.03,and 3.94 compared with those in O2/N2at＜O2＞ =21 vol%,30 vol%,40 vol%and 50 vol%,respectively.

3.4.Effect of coal rank on nitrogen conversion

Fixed carbon and fuel-N contents take dominant parts in NO and N2O emissions[54].Pels et al.[55]demonstrated that a higher coalrank had higher conversion ratios of fuel-N to NO and N2O in conventional FB combustion.Fig.10(a)–(c)show the effects of coal rank on the conversion ratios of fuel-N,volatile-N,and char-N to NO in O2/N2and O2/H2O(＜O2＞ =30 vol%and T=800 °C).In O2/N2and O2/H2O,ηfuel-N,NO,ηvolatile-N,NOand ηchar-N,NOall increase with increasing coal rank.Among the three coal ranks,AC has the lowest volatile-N content and the highest char-N content,and BC has the highest volatile-N content.More NH3released from the lower rank coal with higher volatile-N content is the reducing agent for NO,which may account for the highest ηvolatile-N,NOof AC among the three coal ranks.The NH3/HCN ratio duringpyrolysis is strongly affected by temperature and decreases with increasing temperature[56,57].Bu et al.[58]indicated that the volatile combustion temperature of LC was higher than that of BC.Thus,the NH3/HCN ratio of LC may be lower than that of BC,resulting in the lower ηvolatile-N,N2Oof LC than that of BC. Given the reduction effect of H2O on NO,ηvolatile-N,NOin O2/H2O is lower than that in O2/N2for each coal tested.A lower coal rank with higher volatile content can produce a higher porous char with greater reactivity due to the rapid release of volatile.Thus,the NO reduction over char surface is more intense for a lower coal rank, leading to the increase ηchar-N,NOwith increasing coal rank.Given the reduction effect of H2O on NO,ηchar-N,NOin O2/H2O is lower than that in O2/N2for each coal tested.The conversion ratios of fuel-N to NO in O2/H2O decrease by 2.52,3.31,and 2.66 times compared to those in O2/N2for LC,BC,and AC,respectively.

Fig.8.Effects of oxygen concentration on the conversion ratios of(a)fuel-N,(b)volatile-N,and(c)char-N of NO during BC particle combustion(T=800°C).

Fig.11.Effects of coal rank on the conversion ratios of(a)fuel-N,(b)volatile-N,and(c)char-N to N2O(＜O2＞ =30 vol%and T=800 °C).

Fig.11(a)–(c)illustrates the effects of coal rank on the conversion ratios of fuel-N,volatile-N,and char-N to N2O in O2/N2and O2/H2O(＜O2＞ =30 vol%and T=800 °C).In O2/N2and O2/H2O,ηfuel-N,N2Oincreases with increasing coal rank.A higher degree of N2O destruction exists for the coal type with higher volatile matter content[59],leading to the highest ηvolatlie-N,N2Oof AC among the three coal ranks.Since the NH3/HCN ratio of LC is lower than that of BC,ηvolatile-N,N2Oof LC is higher than that of BC.Given the reduction effect of H2O on N2O,ηvolatile-N,N2Oin O2/H2O is lower than that in O2/N2for each coal tested.For a lower coal rank,the N2O reduction over char surface is accelerated due to the higher porous char derived from a lower coal rank,which accounts for the increase of ηchar-N,N2Owith increasing coal rank.Given the reduction effect of H2O on N2O,ηchar-N,N2Oin O2/H2O is lower than that in O2/N2for each coal tested.The conversion ratios of fuel-N to N2O in O2/H2O decrease by 1.40,1.42,and 1.93 times compared with those in O2/N2for LC,BC,and AC,respectively.

4.Conclusions

The conversions of fuel-N,volatile-N,and char-N to NO and N2O were studied in the tube reactor using a single coal particle in O2/N2and O2/H2O at low temperature.The following conclusions can be drawn:

(1)At T=800 °C and ＜O2＞ =30 vol%,NO reaches the maximum value in the devolatilization stage in O2/N2,whereas the maximum value of NO is delayed to the char combustion stage in O2/H2O.In O2/N2and O2/H2O,N2O reaches the maximum value in the char combustion stage.Char-N takes a dominant part in NO and N2O emissions in both atmospheres.The presence of H2O vapor promotes the reduction of NO and N2O.The conversions of fuel-N to NO during the two stages in O2/N2are significantly higher than those in O2/H2O.Although the conversions of fuel-N to N2O during the devolatilization stage in O2/N2is slightly lower than that in O2/H2O,the conversion of fuel-N to N2O during the char combustion stage is obviously higher than that in O2/H2O.

(2)Similar to that in conventional combustion,temperature changes the trade-off between NO and N2O in O2/N2and O2/H2O.In both atmospheres,the conversion ratios of fuel-N,volatile-N,and char-N to NO increase with increasing temperature,whereas those to N2O decrease with increasing temperature.

(3)At T=800°C,the conversion ratios of fuel-N and char-N to NO reach the maximum values at＜O2＞ =30 vol%in O2/N2and O2/H2O.The conversion ratios of volatile-N to NO reaches the maximum value at＜O2＞ =30 vol%in O2/N2,and it slightly increases with increasing oxygen concentration in O2/H2O.The conversion ratios of fuel-N,volatile-N,and char-N to N2O show decreasing trends with increasing oxygen concentration in both atmospheres.

(4)Similar to that in conventional combustion,a higher coal rank has higher conversion ratios of fuel-N to NO and N2O in O2/N2and O2/H2O.In both atmospheres,the conversion ratios of fuel-N,volatile-N,and char-N to NO increase with increasing coal rank.The conversion ratios of fuel-N and char-N to N2O increase with increasing coal rank in both atmospheres.Although the conversion ratio of volatile-N to N2O of BC is slightly lower than that of LC,the conversion ratio of char-N to N2O of BC is obviously higher than that of LC,resulting in the higher conversion ratio of fuel-N to N2O of BC in both atmospheres.

Nomenclature

Acharash content of char,%

Acoalash content of coal,%

AC anthracite coal

ar air dry basis

BC bituminous coal

CNON2O concentration,m3·m-3

CN2Oconcentration,m3·m-3

CCS carbon capture and storage

db dry basis

FB fluidized-bed

LC lignite coal

MNnitrogen molar mass,g·mol-1

minitinitial partial mass,g

Ncharnitrogen content of char,%

Ncoalnitrogen content of coal,%

Nx-Nx-N content,%

Q flow rate,L·s-1

Rx-Nratio of x-N content in fuel-N content,%

TGA thermogravimetric analysis

t time,s

tx-Nperiod of x-N conversion,s

Vmmolar volume of gas,L·mol-1

ηx-N,N2Oconversion ratio of x-N to N2O,%

ηx-N,NOconversion ratio of x-N to NO,%

Subscripts

x-N fuel-N,volatile-N or char-N