Tingting Jiao,Huiling Fan,Shoujun Liu,Song Yang,Wenguang Du,Pengzheng Shi,Chao Yang,Yeshuang Wang,Ju Shangguan,*

1 State Key Laboratory of Clean and Efficient Coal Utilization,Taiyuan University of Technology,Taiyuan 030024,China

2 College of Chemistry and Chemical Engineering,Taiyuan University of Technology,Taiyuan 030024,China

3 Shanxi Engineering Central of Civil Clean Fuel,Taiyuan University of Technology,Taiyuan 030000,China

4 Taiyuan Green Coke Energy Co.,Ltd.,Taiyuan 030006,China

Keywords:Coal combustion Pyrolysis Environment Nitrogen oxides Quantum chemical calculation

ABSTRACT The emission of NOx during coal combustion contributes to the formation of acid rain and photochemical smog,which would seriously affect the quality of atmospheric environment.Therefore,the decrease of NOx is of great importance for improving the efficient utilization of coal.The present review comprehensively summarized the influence factors and mechanisms of migration and transformation of nitrogen during the coal pyrolysis and combustion based on experimental study and quantum chemical calculation.Firstly,in the process of pyrolysis:the occurrence state and transformation of nitrogen were concluded.The influence of temperature,atmosphere,heating rate and catalyst on formation of NOx precursor and nitrogen migration path at the molecular level were summarized;Secondly,during the process of combustion:the influence of temperature,ambient oxygen concentration,physical structure of coal char,catalyst on heterogeneous oxidation of char (N) were summarized;The effects of char surface properties,catalyst and ambient atmosphere on heterogeneous reduction of NOx were also concluded.Based on the quantum chemical calculation,the reaction path of heterogeneous oxidation of char-N and heterogeneous reduction of NOx were described in detail.Current studies focus more on the generation of HCN and NH3,but in order to reduce the pollution of NOx from the source,it is necessary to further improve the process conditions and the optimal formula of producing more N2 during pyrolysis,as well as clarify the path of the generation of N2.Experiments study and quantum chemistry calculation should be combined to complete the research of directional nitrogen reduction during pyrolysis and denitration during combustion.

1.Introduction

Coal,as China’s basic energy and an important raw material,is related to the economic lifeline of national and energy security.However,the utilization of coal will cause serious environmental problems[1–3].Therefore,clean and efficient utilization coal technology is of great significance.It is known that the emission of NOxduring coal combustion can cause acid rain and photochemical smog [4–7].How to control NOxproduction and release during and after combustion has drawn attention of the Chinese government and people[8–10].Many countries have promulgated a stringent limit for nitrogen oxides emission.According to the formation mechanism,NOxcan be divided into three types[2,4,11]:thermal-NOx,prompt-NOxand coal-NOx.And it has been announced that the source of NOxmainly comes from fuel nitrogen from coal combustion.Therefore,the key to reducing NOxgeneration is to control the generation of fuel NOx.

There are three main ways to affect the final emission of coal-NOx[11]:(i) Generation of NOxprecursor during coal pyrolysis;(ii) NOxformation from volatile-N and char-N during combustion;(iii) Reduction of NOx.Therein,coal pyrolysis plays a fundamental role in the scheme of fuel nitrogen conversion.During the process of pyrolysis,coal nitrogen will migrate to gas (HCN,NH3),solid(char-N) phases.The distribution of coal nitrogen during pyrolysis is the key factor to affect the formation of NOxdirectly in the subsequent combustion process.The formation of NOxdepends largely on the types of precursors.During the combustion process.Char-N would convert to NOx,which is a very complex process.During this procedure,NOxreduction occurs simultaneously on the surface of the inner porous and the outer surface of the particles.The complete process includes the adsorption,diffusion and desorption of the reactants and products.The production of NO mainly originates mainly from the oxidation of fuel-N during coal combustion [12].The NO-char reaction has been considered to be one of the most important reactions during coal combustion[2,13].Thus,it is necessary to understand the transformation rule of nitrogen in the combustion process.

With the development of quantum chemistry theory and computer technology,quantum chemistry software has been used to explore the relationship between the coal structure and the reaction.Recent studies have shown that quantum chemistry calculation can provide a systematic and reliable explanation for the structure,physical properties,reaction mechanism and reaction activity of organic molecules.It can also make some practical predictions and facilitate the discussion of chemical reactions into the level of reaction mechanism at the molecular level.From this point of view,on the basis of screening the coal structure model,the macromolecular structure model of coal has been calculated by quantum chemistry method.Some relationships between coal structure and reactivity have been predicted and explained from the perspective of molecular orbital theory.Quantum chemistry calculation can provide substantially accurate information that cannot be detected in the experiment and reproduce the experimental phenomena observed during combustion [14,15].

The complexity of coal combustion has made more researchers divide the combustion process into two stages:pyrolysis and late combustion.Up to now,many studies have been done on transformation behavior of nitrogen during coal pyrolysis [16,17] and occurrence state and content of nitrogen in coal [18].But little attention has been paid to the research of nitrogen conversion in combustion.In particular,the research on nitrogen migration and transformation in pyrolysis and combustion stages based on experiment and quantum chemical calculation has not been paid much attention.A comprehensive review of pyrolysis and combustion is helpful to better understand the situation of NOxevolution in coal combustion.The uncertainty of the conditions in the experimental process and the complexity of the atmosphere surrounding the heterogeneous reaction have made it difficult to explain the path of the combustion process at the molecular level.It is far from enough to explain it only from an experimental point of view.The emphasis of this work is to review the understanding of nitrogen conversion in the process of coal pyrolysis and combustion based on experiment and quantum chemical calculation.The influence factors,including temperature,atmosphere,heating rates and catalyst,on nitrogen migration and the mechanism of precursor formation during pyrolysis,NOxformation and NOxreduction during combustion were summarized in detail.At the same time,different models were selected to explain the formation mechanism of precursor,the path of oxidation of nitrogen and reduction of NOxat the molecular level which made corresponding supplement to the experimental part.

2.Release of NOx Precursor during Coal Pyrolysis

Coal pyrolysis is a good way to achieve efficient utilization of coal.Coal pyrolysis,as an efficient and clean utilization technology,has been regarded as an important step in most coal conversion processes and has an important impact on subsequent stages such as coal liquefaction,gasification and combustion [19].Fuel-N is partitioned to volatile-N and char-N during pyrolysis.Volatile-N includes tar-N,HCN,NH3and N2,among which HCN and NH3are considered as two important precursors of NOxduring coal combustion.The pyrolysis process can be divided into two stages,the initial and the second stage.In the primary devolatilization of any coal,nitrogen species 85%–100% mass content is found in tar and oils as aromatic compounds [20,21],The proportion of light gas released is so small that it is negligible.Formation of HCN and NH3occurred at higher temperatures than that required for maximum tar production.As the temperature rises,volatile-N released from the nitrogen-containing ring breaking.That is,the release of nitrogen-containing volatiles generally occurs in the second stage of pyrolysis process[22].HCN and NH3are partly derived from tar cracking but mainly from coke nitrogen decomposition[21,23].

2.1.The occurrence state of nitrogen in coal/char and its variation during pyrolysis

Future utilization of coal and ecological safety require an accurate knowledge of its structure and properties.The distribution and release rate of volatile-N were directly affected by the type and composition of nitrogen.Almost all the nitrogen in coal is believed to occur principally in heterocyclic moieties.There is also a small part of nitrogen that exists in the form of ammonia ion in highrank coal [24].It is universally believed that nitrogen in coal existed almost entirely in four main types,including pyrrolic nitrogen(N-5),pyridinic nitrogen(N-6),quaternary nitrogen(N-Q)and oxidized nitrogen (N-X).N-5 was the predominant form of organically bound nitrogen,followed by N-6 and N-Q [25–28];In pyrolyzed chars,N-Q was the dominant form followed by N-6 and N-5 respectively.It seems that the HCl/HF/HCl sequential demineralizing process had no significant effect on the nitrogen functional morphology of raw coals and the chars [29].

N-5 is relatively stable at temperatures as high as 600 °C[30,31].However,the nitrogen contained in N-5 is less stable than that in N-6.As the temperature continues to rise,N-5 was completely transformed into N-6 and N-Q [32–34];Higher pyrolysis temperatures are required to release HCN and NH3for N-6 pyrolysis[22].Interestingly,N-6 is converted to N-Q,N-5 for some chars at higher temperature[31];With the pyrolysis temperature further increasing,N-Q could be converted into N-6 under more severe pyrolysis conditions (1200 °C for 2 h,in N2).It may be due to the fact that a breaking up of the graphene structures to smaller fragments.A part of N-Q originally incorporated in the graphene layers was present at the edges of these layers,i.e.,N-Q was converted into N-6.

So far,the majority of related research has been done to focus on the stability of N-Q.It is generally believed that N-Q is stable.In pyrolyzed chars,N-Q was the dominant form followed by N-6,N-5 and protonated/oxidized heterocyclic nitrogen forms respectively [29].Liu [30] and Kelemen [35] reported that N-Q showed a decreasing trend in the mild pyrolysis condition (400–500 °C),corresponding to the increase of N-6 and N-5;With the temperature continuing to rise,it enters the stage of carbon condensation.During the condensation process,nitrogen atoms are incorporated into the graphene layers replacing carbon atoms.After severe pyrolysis,all N atoms appear as N-6 in the six-membered rings at the edge of the graphene layer or as N-Q inside the graphene layer[31].In other words,there was a sharp decrease in N-5 while in favor of the increment in N-Q and N-6 with raising the temperature.N-Q drops sharply at low temperature,which seems to contradict the stability conclusion.However,it is not.The presence of N-Q in raw coal is related to the pyridine form containing hydroxyl functional groups.And the reduction of N-Q is related to the loss of hydroxyl oxygen in mild pyrolysis [31].The N-Q formed during high-temperature pyrolysis is fundamentally different from most of the N-Q(hydroxy-bound nitrogen)originally present in the parent coal.

The work on N-X has been less studied.The existence of N-X may be resulted from oxidation of the surface upon being exposed to the ambient.And N-X would be released as volatiles during the severe pyrolysis [30].

In conclusion,there are four main types of nitrogen in coal,including N-5,N-6,N-Q and N-X.N-5 was the predominant form of organically bound nitrogen.And in pyrolyzed chars,N-Q was the dominant form followed by N-6,N-5 respectively.The different nitrogen forms in coal and char and the migration rules of nitrogen as shown in Fig.1 N-Q1is the N-Q in raw coal and N-Q2is the N-Q in the char.

2.2.Influencing factors of NOx precursor release during pyrolysis

So far,researchers have done a lot of research to reveal the conversion mechanism of fuel nitrogen to NOxprecursors,such as HCN and NH3,during coal pyrolysis [36–42].Experiments showed the formation and distribution of HCN and NH3are very complicated.It would be influenced by coal structure,and also be affected by many external factors [11,37,43–48],such as temperature [37],heating rates [45,47,49,50],free radical [39,40,51],atmosphere[11,36,45,52],and catalyst [53–59].

2.2.1.Effect of temperature on the migration of nitrogen during pyrolysis

Temperature is one of the most important factors greatly affecting pyrolysate distribution.which has been paid much attention by researchers.Duanet al.[45] and Xieet al.[37] indicated that the yield of HCN increased with the increase of pyrolysis temperature,and the delay release time of HCN shortened to nearly zero [11].This may be attributed to the higher temperature can exactly increase the activity of fuel nitrogen systems in coal samples,especially the less stable N-5 bonds.As temperature increases,the peak of NH3release curves tends to increase first,and then decreases with a maximum at 800 °C [45].This is because N-Q is easy to decompose into NH3during the coal pyrolysis at low temperature.When temperature is high,NH3can react with the quartz reactor,stainless steel materials to form N2[47,60],and with the increase of reaction driving force,NH3would decompose into N2and H2[61],Or the catalysis of Fe and Ca promotes the decomposition of NH3[62].On the other hand,compared with the tendencies of HCN,the release of NH3is more slow and for a longer time.Even at the end of the analyzing time,the release of NH3was still measurable [11].This is because the relatively more stable Ncontaining hetero-aromatic ring,such as N-Q,were the main sources of NH3[40].Kambaraet al.[63]studied the relation between the formation of NOxprecursor and nitrogen functionality at different temperatures.It was found that the relationship between observed NH3yields and calculated NH3yields based on the evolution of N-Q shows a good consistency at each temperature.So does the HCN and N-6,N-5.It is further proved that the N-Q is finally converted into NH3,N-5 and N-6 is converted to HCN.The yields of HCN,NH3can be estimated by determination of N-6,N-5 and N-Q using XPS.Some studies[64]also showed that the yield of HCN increases first and then decreases with the increase of temperature.This is because there are two opposite processes of HCN during coal pyrolysis [55].At low temperature,the formation of HCN is dominant,while at high temperature,the secondary decomposition reaction of HCN is more significant[55].In summary,the migration fates of HCN and NH3under different temperatures can be obtained,as shown in Fig.2.

2.2.2.Effect of atmosphere on the migration of nitrogen during pyrolysis

The gas atmosphere surrounding coal/char particles has a great influence on the formation of NH3and HCN during coal pyrolysis.It is well known that the majority of coal-N exists in N-containing heterocyclic systems [26,65].Therefore,the generation of NH3and HCN requires ring-opening to expose the active sites of nitrogen during pyrolysis.The formation of NH3and HCN begins with the opening of nitrogen-containing heteroaromatic rings by radicals,particularly H-radicals.The thermally unstable N-containing structures are the main source of HCN formation,the thermally stable N-containing structures may be hydrogenated to NH3slowly by the H-radicals[39,40,51].Therefore,the existence of H-radicals is of great importance.Any factors affecting the formation of Hradicals would affect the formation of NH3and HCN.

Fig.1.The occurrence state of nitrogen in coal/char and its variation during pyrolysis.

Fig.2.Effect of temperature on the migration of nitrogen during pyrolysis.

The atmosphere influences the generation of precursor by affecting the formation of N-sites and H-radicals during the process of pyrolysis [51].Results show that CO2has two opposite influences on the release of nitrogen compounds.One is that CO2plays an inhibitory role.The CO2would react with H-radicals or block the contact of the N-sites and H-radicals by absorbed on the coal matrix [45,51].Or CO2can inhibit the polymerization of aliphatic chains,and more H remains as-CH2or-CH3in the char[66],thus affecting the final formation of NH3and HCN;It was also reported that CO2has no obvious promoting effect on the formation of HCN at high temperature[45],even inhibited the formation of HCN,which may be due to the consumption of HCN by CO2at this temperature[66].The other is that CO2plays a promoting role.CO2gasification may break stable -CN or C-C bonds and make more N-sites and H-radicals exposed on the char surface[11,45,66],to promote the formation of NH3and HCN.For different coals,the effect of CO2atmosphere on the yields of HCN and NH3is different,which depends on the properties of coal.

The presence of H2O also has a certain influence on the types of products during pyrolysis.Parket al.[48]found that the formation of NH3can be promoted by increasing concentrations of H2O.Changet al.[51] also proved that the presence of H2O greatly enhances the formation of NH3and HCN due to greatly enhanced availability of H-radicals.Lin[36],McKenzie[52]and Tian[67]also emphasized the importance of H-radicals on the formation of NH3and HCN.The availability of H-radicals is the critical key to controlling the conversion of char-N into NH3and HCN.H-radicals could be enriched by the introduction of H2O,thus promoting the char-N converted to NH3and HCN,especially for the conversion of NH3.

Stated thus,the effects of CO2and H2O on product distribution were mainly through the formation of N-sites and H-radicals.The research progress is presented in Table 1.

The comparison of HCN and NH3production under different atmospheres is summarized in Table 2.It can be seen from the table that the amount of HCN and NH3varies greatly with different types of coal.The minimum yield of HCN is 4 mg·kg-1and the maximum amount can reach 1300 mg·kg-1.The minimum yield of NH3is 12 mg·kg-1and the maximum amount can reach 1230 mg·kg-1.It indicates that the amount of HCN and NH3is closely related to the properties of coal.For different kinds of coal,the effect of CO2atmosphere is not the same.These differences can be explained by the competition between CO2inhibition effect and its gasification effect.The final effect depends on which role is dominant.This is also decided on the coal property.

Table 1Effect of atmosphere on the migration of nitrogen during pyrolysis

Table 2Comparison of maximum production of HCN and NH3 in different atmospheres

2.2.3.Effect of heating rate on the migration of nitrogen during pyrolysis

Furthermore,the heating rates could be one of the primary factors affecting the behavior of nitrogen release as well.The yields ofNH3and HCN from the fast pyrolysis were usually much higher than that from the slow pyrolysis.It seems that the fast heating rate is conducive to the formation of N-6 while the content of NQ in char is relatively high in the process of the slow heating rate.That is to say,carbonization and polycondensation can make the N-containing heterocyclic system more stable and produce fewer H-radicals in the slow heating process [49].The lack of radicals which is to open nitrogen-containing rings leads to lower yields of HCN and NH3during slow pyrolysis.The distribution of nitrogen functional forms can also be affected by the heating rate.Kidenaet al.[50] indicated that more N2is produced under the condition of slow pyrolysis,while the yield of HCN is higher in the fast heating process.That is because the secondary reaction occurred to form N2in the pyrolysis at lower heating rates.Bassilakiset al.[68]observed HCN is the dominant product in the fast heating process.NH3is the dominant product at low heating due to the secondary reaction of HCN and coal hydrogen in the char pores to generate NH3can be completed with enough residence time within the char pores.But under the condition of hydrogenation,it’s going to be different.Xuet al.[55]reported NH3is the main product,only a small amount of HCN is generated,that is because most of HCN is converted into NH3through the secondary reaction.

2.2.4.Effect of catalyst on the migration of nitrogen during pyrolysis

Both mineral substances contained in coal and additives have great influence on nitrogen conversion in coal [69–71].The distribution of nitrogen-containing compounds and the distribution of nitrogen-containing products would be affected by alkali metals,alkaline earth metals and transition metals which are important minerals in coal during the pyrolysis process.

The existing state of Fe-containing minerals in coal and the added Fe catalyst play an important catalytic role in nitrogen migration during coal pyrolysis.It is found that the addition of Fe remarkably promotes the formation of N2[54,56].Fe has no effect on high-sulfur coal,because FeS2and/or FeS formed by Fe and S are catalytically inactive for the formation N2from the solid phase.And it is proved that α-Fe is the catalytic active species for the generation of N2from char-N[54].The existing state and dispersion of Fe are important for the catalytic formation of N2[56,64,72].The different states of Fe in coals with different ranks play different roles in the formation of N2.Thus the catalytic effect of Fe depends on the coal type.Fe in ionexchangeable forms which only exist in low rank coals would be essential for the catalytic N2formation from coal nitrogen during pyrolysis.That is to say the Fe promotes N2formation only for low rank coals but has a much smaller effect on high rank.Highly dispersed Fe can promote the formation of HCN and N2[56,64].The presence of Fe also affected the formation of NH3and HCN.Xuet al.[64]found that Fe can promote the formation of HCN.Yiet al.[57] have studied that the iron effect is a remarkable property that enables the significant reduction of NH3.This was probably because NH3was consumed by Fe compounds to generate FeNx.On the other hand,Fe compounds promoted thermal cracking of heterocyclic N with the release of HCN.

To minimize the emission of NOxprecursors,the calcium salts were also applied during pyrolysis.Tsubouchi [54,58,73] showed that N2increased by highly dispersed Ca mainly from reaction of heterocyclic nitrogen at solid phase.Ohtsukaet al.[59] indicated that the introduction of Ca could promote considerable NH3formation between 450°C and 600°C.But in contrast inhibit HCN formation in this region.It could be that the NH3increased by Ca addition arises partly from HCN.Yiet al.[57] have studied that Ca can promote the conversion of HCN to NH3and fixed HCN to generate CaCxNysimultaneously.A higher emission of N2was found with mixed Fe/Ca additives than with either Fe or Ca individually.That is because the formation of Ca2Fe2O5enhanced the transformation of intermediates to N2.

It would be valuable to study the influence of the inherent mineral matter on the distribution of Nitrogen during coal pyrolysis.But it’s worth noting that the primary mineral matter in coal is different from those catalysts intentionally added onto the carbon supports.The former is very complex which consists of many active and inert compositions.Therefore,an integrated experiment is needed to study the catalytic effect of mineral matter in coal.The mineral matters in coal promote the evolution of volatile nitrogen from char to N2apparently[55].It is Fe and Ca that have the main effect,because Fe and Ca are the main demineralizing components[56].Tsubouchiet al.[61]has studied the catalytic roles of inherent Ca and Fe ions in conversion of char-N during pyrolysis.It can be suggested that fine particles of Fe and CaO can promote the formation of N2from char-N through solid–solid interactions with heterocyclic nitrogen in char matrix and catalyze volatile-N to tar-N,HCN,NH3through gas–solid interactions.

Comparison of catalytic effect on N2generation is summarized in Table 3.The effect of inherent mineral content of six different coals on N2formation during pyrolysis at 1300 °C was examined in Ref.[54].The formation content of N2increases with the increase of the content of inherent minerals (Ca or Fe) in coal.The conversion of the char-N to N2reached a level of about 50%,and N2was found to be the dominant N-containing product.It can be seen that naturally occurring Ca and Fe can effectively promote the formation of N2.The effect of demineralization of four kinds of coal and addition of 0.5% (mass) Fe on the formation of N2during coal pyrolysis was studied in Ref.[56].It can be seen from the data in the table that the addition of Fe and Ca contributes to the formation of N2,up to 57.21%.However,the amount of N2produced by different deashing coals and the promotive effect of additives during pyrolysis is different,indicating that the amount of N2produced and catalytic effect depend on the coal type.The effects of Ca catalysts on nitrogen release during pyrolysis of low rank coals was discussed in Ref.[58].For Zalainuoer coal,the addition of 3% Ca can increase the production of N2from 47% to 73%.For Adaro coal,the addition of 3% Ca can increase the productionof N2from 47% to 65%.It can be seen that the addition of Ca can significantly increase the production of N2.The increase of N2corresponds to the decrease of char-N,and it is concluded that the generation of N2mainly comes from char-N.However,it is not clear which form of nitrogen is responsible for the formation of N2.As a green and harmless gas,the amount of N2generated increases during pyrolysis,which is conducive to NOxemission reduction.It is necessary to further improve the process conditions and the optimal formula of producing more N2during pyrolysis,as well as clarify the path of the generation of N2.

Table 3Comparison of catalytic effects on N2 generation

In summary,both Fe and Ca can promote the formation of N2and change the distribution of nitrogen-containing phase products.And catalytic effect depends on the coal type.Fe mainly promotes the release of HCN and inhibits the release of NH3,while Ca has the opposite catalytic effect.The catalytic mechanism and influence research progress are shown in Table 4.

Table 4Mechanism and influence of catalysts on the migration of nitrogen during pyrolysis

2.3.Quantum chemical study on the release of NOx precursor during coal pyrolysis

To establish a reasonable and simplified model of coal char containing nitrogen is of great importance for quantum chemistry.It is an important factor for the accuracy of calculation results.The structure of coke is formed by small graphite microcrystals stacked irregularly[74,75].At present,the model of polycyclic carbon is the most widely used.And it was first proposed by Kyotaniet al.[76]who selected the coal char model composed of 4–9 aromatic rings for calculation.Sendtet al.[77–79] calculated the reaction of O2and O with char surface by using 6-membered and 9-membered ring.It is also found that the aromatic cluster model composed of 5–7 benzene rings can reproduce the experimental results well[14,80–84].There are four main types nitrogen in coal,including N-5,N-6,N-Q and N-X.Therefore,in addition to the above polycyclic carbon model structure,some simple nitrogen-containing heterocyclic rings as model compounds of coal for quantum chemical analysis have also been recognized,such as pyrrole [85–87],indoles [88,89],etc.

2.3.1.Mechanism of the migration path of nitrogen in pyrolysis

The difficulty of the ring-opening method directly affects the reaction during the process of pyrolysis.Pyrrole can undergo ring scission at the C-N bond to promote the subsequent reaction[90,91].But it is generally believed that pyrrole is more aromatic in character than furan.The dissociation energy of C-N bond in pyrrole is as high as 376.73 mol–1C-N which is much higher than the actual energy required (313.94 kJ·mol-1) for the overall disappearance of pyrrole in the reaction.This difference in thermochemistry makes it difficult to get experimental verification.They proposed the hydrogen shift method of pyrrole during pyrolysis[91].They suggest that the first step in pyrrole pyrolysis is a 1,2-hydrogen migration from nitrogen to C to form 2H-pyrrole(pyrrolenine),followed by the cleavage of C-N bond to yield the biradical intermediate.The latter can rearrange to form HCN and propyne[85–87].Bacskay [86] proposed three distinct hydrogen transfer pathways for pyrrole.The lowest energy pathway includes the isomerization of pyrrole to a cyclic carbene,and the formation of allenic imine type intermediate on ring opening [87].This result is consistent with experimental activation[90].Zhai[87]also considered the reaction of continuous hydrogen transfer in pyrrole ring,which broadened the pyrolysis path of pyrrole and enriched the pyrolysis mechanism.Unfortunately,only the continuous transfer of hydrogen on the N-site of pyrrole ring was studied.

Pyrrole does not exist independently in coal [92].It is usually bound to aromatic rings in the form of carbazole,indoles,or naphthalazole [93].Therefore,merely studying the mechanism of pyrrole pyrolysis cannot be comprehensive to reveal the formation mechanism of NOxprecursor.It is worth noting that indole has a stable structure composed of benzene and pyrrole.Therefore,indole is difficult to decompose during coal pyrolysis.Liuet al.[88] investigated possible pathways for the formation of HCN and NH3from indole.Calculation results show that there are two possible initial reactions in the pyrolysis of indole which are internal hydrogen transfer and hydrogen homolysis reaction,respectively.While the comparison calculated results showed that it is more likely to initiate the subsequent reaction by internal hydrogen transfer.And indole pyrolysis can produce two nitrogencontaining products,i.e.HCN and NH3.HCN is easier to generate than NH3by comparing all calculated indole pyrolysis mechanisms.Liuet al.[89] investigated the influence and mechanism of Ca2+on the pyrolysis of indole to form HCN.The results suggest that Ca2+could alter the distribution of original electron density in the pyrrole ring and the configurations of pyrrole derivatives without changing the HCN formation pathway.And Ca2+could greatly reduce the energy barriers of the rate determining steps for HCN formation which lead to Ca2+has a stronger influence on the formation of HCN during indole pyrolysis.

To sum up,the reaction of internal hydrogen transfer is preferred in the pyrolysis for both pyrrole and indoles.Pyrrole products only HCN,while indoles can produce both HCN and NH3during pyrolysis,and HCN is more easily generated.Ca2+could promote HCN by reducing energy barrier of hydrogen transfer reaction and changing the distribution of original electron density in the pyrrole ring.The pyrolysis path of pyrrole and indole is shown in Table 5.

2.3.2.Quantum chemical study on the effect of radicals on the migration of nitrogen during pyrolysis

Free radicals are crucial to opening of nitrogen-containing heteroaromatic rings,especially H-radicals.Espinalet al.[94]used zigzag and armchair configurations to model the structure of carbonaceous materials and proposed the reaction mechanisms of consecutive hydrogenation steps and rearrangements for the production of NH3.The results show that the existence of H-radicals can promote the formation of NH3.On the other hand,from the point of view of thermodynamics and kinetics,it is more likely to form NH3from armchair edges than from zigzag edges.Xinet al.[95]elucidated the evolution mechanisms of NH3during the reaction of H2by using zigzag and armchair configurations which containing 2-pyridone and got the same results as those in [94].Liuet al.[96] investigated the formation mechanism of HCN during pyrolysis of pyrrole in the presence of H-radicals.The results further indicated that the hydrogen radicals can significantly lower the energy barrier in the pyrrole pyrolysis and promote the formation of HCN.It can be seen that the effect of H-radicals on the formation of products is closely related to the nitrogen-containing structure.The effect of radicals on the migration of nitrogen during pyrolysis is shown in Table 6.

According to Ref.[96],the energy of each pathway for generating HCN under the high temperature of coal pyrolysis is relatively low,with the maximum energy is 189 kJ·mol-1.In addition,it was summarized in the previous section that HCN was the only product of pyrrole pyrolysis[85–87,90,91].NH3can be released from indole pyrolysis,but HCN is more easily formed [88].Therefore,pyrrole plays an important role in the formation of HCN during coal pyrolysis.And the result is consistent with experimental results[45,63].That is to say,with the increase of temperature,the delayed release time of HCN is shortened,and HCN is mainly derived from N-5 with poor stability in the pyrolysis process.

2.3.3.Quantum chemical study on the influence of atmosphere on the migration of nitrogen during pyrolysis

The existence of H2O can facilitate the formation of NH3and HCN by providing massive H-radicals.The availability of Hradicals was the key to controlling the conversion of char-N to NH3and HCN [36,51,52,67].Liuet al.[97] investigated the effects of H2O on the formation of HCN and NH3from pyrrole pyrolysis by using the density functional theory (DFT) method.The calculation results indicate that only HCN is formed in the absence of H2O[85–87].The addition of H2O changed the pyrolysis mechanism and path,and the type of products was determined by the initial interactions between pyrrole and H2O.In addition,the DFT calculations results clearly show that the formation of NH3will be promoted,whereas the formation of HCN is inhibited.For NH3and HCN,the formation of NH3needs to overcome lower energy barriers than those of HCN.Therefore,it can be concluded that in the presence of H2O,the generation of NH3is easier than that of HCN during the pyrrole pyrolysis process.The calculations results are consistent with the previous experimental results [48],that is,the formation of NH3can be promoted by increasing concentrations of H2O.The presence of H2O greatly facilitated NH3production due to greatly enhanced availability of H-radicals[36,51,52,67].During the process of oxy-fuel combustion of coal,CO2is an important atmosphere gas,which may act as an oxidizing agent.The reaction of CO2with char-N is important for reducing the emission of NOx.At present,the research[98,99]on CO2atmosphere was limited to the initial reactions between char and CO2,little literature [100] investigated NO formation and conversion during char(N)–CO2interaction.However,the effect of CO2on the formation of NOxprecursors has not been studied.In view of the important influence of CO2during pyrolysis,it is suggested that the effect of CO2on nitrogen migration should be further studied in the future to further clarify the mechanism of directional migration of nitrogen.The influence of the atmosphere on the migration of nitrogen during pyrolysis is summarized in Table 6.

Table 5The pyrolysis path of pyrrole and indole

Table 6Quantum chemical study on the influence of radicals and atmosphere on the migration of nitrogen during pyrolysis

2.3.4.Quantum chemical study on the effect of catalyst on the migration of nitrogen during pyrolysis

Metal ions have a significant influence on the coal pyrolysis process[59,101].The addition of catalysts not only affects the pyrolysis path,but also affects the distribution of products,but all of them remain in the experimental stage.Liuet al.[102] studied the influence of Na+and K+on the formation mechanism of HCN during pyrrole pyrolysis by using density functional theory (DFT).The results show that the presence of K+and Na+can affect the formation of HCN and NH3by changing the reaction energy barrier of each process.While it cannot change the reaction path.Zhanget al.[103] elucidated the mechanism of effect of Fe on the heterogeneous formation of HCN from nitrogen-containing char.Calculation results show that chemisorption of Fe atoms on seven different hollow sites were exothermic spontaneous reactions with forming the interstitial iron carbides and iron nitrides.It is found that the participation of Fe inhibited the heterogeneous formation of HCN except the direct binding of Fe to N.

Calcium-based compounds are the main inorganic components in coal and are also widely used as additives in the coal pyrolysis process.They play an important role in the decomposition of nitrogen compounds into NOx.Liuet al.[89] investigated the influence and mechanism of Ca2+on the pyrolysis of indole to form HCN.The results show that Ca2+could alter the distribution of original electron density in the pyrrole ring and the configurations of pyrrole derivatives.And Ca2+could greatly reduce the energy barriers of the rate determining steps for HCN formation which lead to Ca2+has a stronger influence on the formation of HCN during indole pyrolysis.The catalytic effect of Ca2+is much stronger than that of Na+,and the main reason for the difference in catalytic performances is determined by their electronic properties.Chenet al.[104] selected the structure of a seven-membered ring containing pyridine and with Ca adsorption on coal surface as a coal model.The theoretical calculation results make clear that Ca can improve the surface activity of coal,and lower the energy barrier value of the rate-determining step of NH3formation by 274.74 kJ·mol-1.So the existence of Ca can obviously promote the formation of NH3.While the presence of Ca inhibits HCN formation.Because Ca significantly increases the interatomic bonding force between N and C in the pyridine ring,so that during the formation of HCN,the removal of N atom from benzene ring in the ratedetermining step requires higher activation energy.The calculated results are consistent with the experimental results in Refs.[57,59],but inconsistent with those calculated results in literature[89].It shows that the generation of products is related to models.Different models lead to different product types and difficulty level of product formation.Quantum chemical study on the effect of catalyst on the migration of nitrogen during pyrolysis is summarized in Table 7.

2.4.Comparison of influencing factors of NOx precursors

Advantages and disadvantages of influencing factors of NOxprecursors are listed in Table 8.It can be concluded from the table that current studies focus more on the generation of HCN and NH3.While as a green and harmless gas,N2is rarely studied.Actually more fuel nitrogen should be converted into N2during pyrolysis.So it is necessary to further study the main source of N2,as well as clarify the mechanism and path of the generation of N2.

Table 7Quantum chemical study on the effect of catalyst on the migration of nitrogen during pyrolysis

Table 8Advantages and disadvantages of influencing factors of NOx precursors

According to the comparison results in the table,we should also further discuss the influence of atmosphere and catalyst on nitrogen forms to explore the mechanism of nitrogen migration and transformation in depth.In terms of quantum chemistry.A more realistic structure of nitrogen-containing functional groups should be established and the range of catalyst selection should be expanded to improve the effects of sulfur and metal elements on nitrogen migration and transformation during pyrolysis and combustion.

2.5.Mechanism of nitrogen transformation and conversion during coal pyrolysis

In general,coal pyrolysis plays a fundamental role in the scheme of fuel nitrogen conversion.The products,HCN and NH3,formed during pyrolysis are considered as two important precursors of NOxduring coal combustion.Generally speaking,the formation of HCN and NH3is related to coal structure and many external factors.The formation of NH3and HCN begins with the opening of nitrogen-containing heteroaromatic rings by radicals,particularly the H-radicals.The thermally unstable N-containing structures(N-5) are the main source of HCN formation,the thermally stable N-containing structures(N-Q)may be hydrogenated to NH3slowly by the H-radicals.As a result,with the increase of temperature,HCN released rapidly while NH3released slowly and for a long time.The atmosphere and the heating rates could affect the formation of HCN and NH3by affecting the formation of H-radicals,Nsites and the stability of N-containing structure.At low tempera-tures,the CO2would block the contact of the N-sites and Hradicals,thus affecting the final formation of NH3and HCN.While at high temperature,CO2gasification may make more N-sites and H-radicals to promote the formation of NH3and HCN.The presence of H2O could greatly enhance the formation of NH3and HCN due to greatly enhanced availability of H-radicals.The yields of NH3and HCN from the fast pyrolysis were usually much higher than that from the slow pyrolysis.That is because the slow heating rate can make the nitrogen heterocyclic system more stable and produce fewer H-radicals.Catalysts affect the formation of products by affecting the formation of active substances.

The quantum chemistry study describes the formation path of HCN and NH3in detail.The reaction of internal hydrogen transfer is preferred in the pyrolysis for both pyrrole and indoles,followed by the cleavage of C-N and the latter rearrange to form HCN.The formation of NH3is greatly influenced by free radicals.And it is produced by continuous hydrogenation and rearrangement.It also proves that pyrrole plays an important role in the formation of HCN during coal pyrolysis.While the main source of NH3and N2generation has not been reported.In view of the importance of NH3and N2in pyrolysis products.Therefore,these details are suggested to be provided in future studies.The quantum chemistry calculation also shows that catalysts could inhibit or promote the reaction by changing the energy barrier.The results above are consistent with experimental results.

3.Nitrogen Conversion during Coal Combustion

NOxcan be produced in both the fast homogeneous volatile combustion of coal and the relatively slow heterogeneous combustion of char.As is known to all that the reduction of NOxemission from volatile-N can be realized by controlling the concentration of O2in the process of devolatilization and combustion [12].However,the formation of NOxfrom char-N is more complicated to be controlled due to the whole process including the adsorption,diffusion and desorption of reactants and products.In particular,char-N is the main contributor to the overall NOxemissions [12].The formation mechanism of NOxhas been studied extensively,and the results indicated that the reaction between NOxand char is one of the important reactions which has significant influence on the emission of NOxduring char combustion [2,13].Therefore,it is of great significance to study the heterogeneous oxidation and NO heterogeneous reduction of char-N during coal combustion for controlling the generation of NOx.

3.1.Heterogeneous oxidation of char-N during coal combustion

The emission of NOxduring combustion mainly comes char-N which accounts for 60%–95% of the total fuel-NOx[105],and it is difficult to control the generation of this part of NOx.

3.1.1.Experimental study on char-N heterophase oxidation in coal combustion

The heterogeneous oxidation of char-N cannot be controlled simply by reducing the concentration of O2[106].The conversion of NO has been proved that it would be influenced by temperature and porous structure of char.Jensenet al.[106] found that the intrinsic formation of NO selectivity from char-N during coal char combustion is close to 100% at temperatures in the range of 1050–1150°C and about 65%at 850°C.Kleinet al.[107]In the process of combustion,the applied temperatures range from low temperature to high temperature may change from kinetic control to diffusion control.The different reaction systems will significantly affect the diffusion of O2into the pores of chars,which directly influence the formation of NO and subsequent reduction of NO on the internal pore surface[107].Xuet al.[8]found that the conversion of char-N/NO decreased with an increasing O2concentration at low temperatures (700–900 °C).That is because at low temperature range more accessible pore surface area and increased reduction time of NO result in more oxygen getting into the pores and less NO formed.While the opposite result was found at high temperatures.Because the reaction was mainly dominated by the external surface of particles,and with the increase concentration of O2,more NO was released due to the shortening of reaction time and the more rapid formation of NO.It is also pointed out that lowranked coal chars showed lower conversion of char-N to NO due to their higher reactivity,which is because of their larger pore and higher number of active sites.This result is consistent with Thomas[12].That is,the more developed the pore structure,the less NOxemissions.

As an important part of coal,it is of great significance to study the influence of minerals on NO emission during coal combustion.Zhaoet al.[2] found that the emission of NO during char combustion is greatly influenced by the minerals in chars.Active components in minerals reduced the conversion of char-N to NO (In the case of Longkou char,the conversion of char-N to NO decreased from 6.3%to 2.7%),while the inert components increased the emission level of NO(For Wulong char,the conversion of char-N to NO increased from 8.5% to 10.9%).The combination catalytic effect of minerals on NO emission depends on the relative contributions of different components in mineral matter.Zhaoet al.[25]investigated the catalytic effects of Na,Ca and Fe on the formation of NOxduring the char combustion.The results showed that the effect of catalyst is greatly affected by combustion temperature.At low temperature,catalysts enhance the emission of NOxbut inhibit NOxformation at high temperature due to the reduction effect of the catalysts increasing (For Fe additive,the conversions of fuel-N to NOxincreased from 20% to 40% at 700 °C;While decreased from 14%to 8%).Thus optimized the combustion conditions is possible for catalysts to inhibit the formation and emission of NOx.

3.1.2.Quantum chemistry study on the heterogeneous oxidation of char-N

The formation of NOxmainly comes from the in-phase oxidation of volatile-N and the out-of-phase oxidation of char-N [108,109].However,it is difficult to clarify the combustion pathways of char-N converted to NO by experiments due to the uncertainty of experimental conditions and the complexity of the gaseous environment around the coal char during combustion.Quantum chemical calculation is increasingly being called upon to make up for this gap in both homogeneous and heterogeneous systems to achieve information that cannot be detected experimentally,especially in the field of coal combustion.The aromatic ring cluster model has been widely used in the simulation of gasification reaction of coal char.The simulated bond length,bond angle and bond energy are in good agreement with the experimental results.Frankcombeet al.[110]found that the unsaturated carbon atoms at the edge of char were the most active and the edge attack played the most important role in gasification and other reactions.Zhanget al.[14]used a simplified nitrogen-containing char model to clarify the path and desorption mechanisms for CO and NO production and desorption in the reaction between molecular oxygen and nitrogen-containing char.The calculated result shows that the nitrogen in char could be preferentially oxidized due to its lower energy barriers.Zhanget al.[83]investigated reaction mechanisms of the evolution of NO during the reaction of O2with carbonaceous materials containing nitrogen by using density functional theory.It is obtained that O2is adsorbed on the char surface to form -NO and-CO,and NO and CO are generated through the pathway consisting of a series of atomic rearrangement reactions.It is also concluded that O2has no selectivity for the oxidation of nitrogen and carbon during char combustion at high temperature.

3.2.Study on NOx reduction in coal combustion

NO can be reduced by volatile matter and char.Both reduction methods play an important role in the reduction of NO.The heterogeneous reduction of NO by char has attracted extensive attention[6,8,9,111,112] due to its contribution rate to NO heterogeneous reduction is as high as 40%–79% [113,114].

3.2.1.Experimental study on NOx reduction in coal combustion

There are a large number of free radicals,active sites and organic functional groups on the surface of coke,which are highly active and play a determining role in the conversion reaction of and the formation of air pollutants.The chemical properties of char surface had the original promoting effect on the reduction of NO[115].The formation of reactive C(O) intermediates on the surface of char can promote the formation of free carbon sites.These active sites can react directly with NO or act as acceptors for oxygen species generated on the catalyst surface.Pevidaet al.[116] emphasized that the presence of C(O) determines the chemical adsorption of NO on the char surface,thus affecting the NO-C gasification reaction at higher temperature.Pevidaet al.[117] implied a direct attack of NO on the surface of char to form surface complexes(C(N)and C(O)),especially C(O),which can promote the formation of new active sites and promote NO reduction.The reduction of NO on char surface promoted by metal and metal compounds is also closely related to the compound on char surface[118].Wuet al.[119]proposed the catalytic mechanism of intrinsic K on the reaction of char-NO.The results showed that K could promote the formation of the oxygen complex and the reduction of NOx.The first step of the catalytic mechanism of K was the chemisorption of NO at K active sites and to convert to C-K(ON).And the C-K(ON)will react with NO to form N2and C-K(O)or dissociates into C-K(N) then react with NO to C-K(O) and N2.The oxygen in C-K(O)would quickly pass to carbon active sites to form C(O)which desorbs to form CO or CO2.It is well known that CO can enhance the reaction of char-NO,because NO can be directly reduced by CO through the reaction CO+NO →CO2+1/2N2(NO+C →C-O+1/2N2;CO+C-O →CO2+C [2].The presence of O2can significantly enhance the reduction of NO by raw coal char and deashing char [2].That is because more C(O) complexes of higher thermal stability can be generated by char-O2reaction.And the C(O) complexes play an important role in the increase of the reaction rate by activating the adjacent carbon atoms [120].Suzukiet al.[121] found that surface oxygen complexes formed by the C-O2reaction are essential for the C-NO reaction.Effects of the surface properties of char on NO heterogeneous reduction is shown in Table 9.

The presence of OH-radicals and H-radicals can also promote the reduction of NO.Xuet al.[122] and Javedet al.[123] emphasized the important effect of OH-radicals on NO reduction.Studies have shown that the formation of OH-radicals is controlled by temperature.At low temperature (<800 °C),-NH2cannot be formeddue to the OH-radicals is formed slowly and thus inhibiting the reduction of NO.When the temperature is higher than 1200 °C,a large amount of OH-radicals formed to promote the formation of NH2which can participate in the reduction reaction of NO and promote the reduction of NO.Liet al.[124] analyzed the behaviors of alkali metal hydroxides and chlorides on NO reduction.Nacontaining species and K-containing species can effectively inhibit the conversion of HCN into NH,HNO and avoid NO formation by controlling the formation of H-radicals and OH-radicals.Luet al.[5] found that Na/K-additives and water produce plenty of OHradicals,leading to the reduction of NO.And the catalytic sequence of alkali metals was NaCl >NaOH–Na2CO3>KCl.Excessive water vapor would lower the reduction efficiency due to the nitrogen free radical was oxidized to NO.Effects of OH-radicals and H-radicals on NO heterogeneous reduction is shown in Table 9.Cfand Cacare the C-sites on the surface of char.

Table 9Effects of char surface properties and OH-radicals and H-radicals on NO heterogeneous reduction

For the practical coal combustion processes,the existence of oxygen and carbon monoxide are inevitable,and the presence of these gases would greatly modify the reduction rate and the reaction mechanism of NO,especially in the case of catalysts such as mineral matter.The reduction effect of NO is obviously improved with the presence of CO,but it is not suitable for the reduction of NO in deashing coal char[2].This means that the minerals inherent in coal play a more important role in the reaction than coal char itself.It is of great significance to predict and control NO emissions to fully understand the influence of minerals and additives in coal on NO reduction during coke combustion.A comprehensive understanding of the influence of mineral matter in coal on NO formation and reduction during char combustion would be of great significance to predict and the control of NO emission.Zhaoet al.[2]studied the influence of mineral matter on NO emission during char combustion.The results show that mineral matter has an influence on the enhancement of CO and O2on NO reduction and resulting in the decrease of NO emission.Zhaoet al.[13,25,118,125] also studied the effect of additional catalyst on the char-NO reaction,and it was found that Na-Fe compound has better reduction effect on the reduction of NO than single additive [13].At low temperature,catalysts of Na,Ca,Fe can enhance the emission of NOxbut inhibit NOxformation at high temperature due to the reduction effect of the catalysts is increased[25].Ca and Fe can also promote the NO char reaction,and the Fe has stronger catalytic effect[125].This is because Fe can inhibit the sintering of char and increase the oxygen content on the surface of coal char[126].Zhaoet al.[125] also proved that NO-char reaction can be promoted by Fe in the presence of O2and CO.

Among the rare earth metal catalysts commonly used during catalytic combustion,the oxides and salts of lanthanide metals are of great value in catalytic activity.These rare earth metal compounds have strong oxygen storage or release function.As a result,they can accelerate the oxygen transfer rate during combustion process.CeO2is a promising catalyst,due to its catalytic activity can not only promote coke combustion but also NOxreduction[127–129].The NO can be reduced to N2by the oxygen anion defect center in CeO2which could be deoxidized.That is to say,the oxygen defects in a reduced ceria were confirmed to be the catalytic sites for NO to N2.Gonget al.[130] analyzed the process of catalytic combustion and catalytic denitration based on the oxygen transfer theory,and put forward the combination of catalytic combustion and catalytic denitration and synergistic catalytic effect of ferric oxide and cerium oxide.The results show that the addition of Fe2O3and CeO2can greatly improve the combustion efficiency and reduce the total emissions of NO and CO.

The Comparison of results of heterogeneous oxidation is summarized in Table 10.It can be seen from the table that there are different methods in the literature for evaluating the reduction of NOx.The conversion rate of NO,the conversion rate of char-N toNO and the reduction rate of NOxwere respectively used.It can be seen from the data of Ref.[116],for the C/NO reaction,there was little difference between Cu/char and Non/char.The Cu catalyst is very effective in promoting the formation of C(O) species during the C-NO-O2and C-O2reactions that the conversion of NOxcan be increased from 1% to 61%.Ref.[13] shows the conversions of char-N to NOxduring pure char and catalyst-loaded chars.The order of char-N to NO is as follows:Na-Fe-char

Table 10Comparison of results of heterogeneous oxidation

Different kinds of coal and different methods to evaluate NOxemission make the experimental results unable to be compared.So the trend of NOxreduction cannot be given.While the purpose of reducing the emission of NOxis the same.And we shoud combine the experimental and quantum mechanics research together,focusing on mechanism research.So as to better provide theoretical support for NOxemission reduction.

3.2.2.Quantum chemistry study on NOx heterogeneous reduction during coal combustion

In the process of coal combustion,the in-phase reduction mechanism of NO has been relatively clear,while the out-of-phase reduction mechanism is not.The heterogeneous reduction contribution of char to NO accounts for 40%–79% of the total no reduction 40%–79% [111,114].Based on the importance of heterogeneous reduction of NO,the mechanism of this reaction deserves further systematic study.There are two mutually related aspects about the mechanism of the C-NO reaction:(i)Adsorption of C-NO;(ii) The importance of the formation of surface complexes and C-O surface complexes.The first reaction step for heterogeneous reduction is the chemisorption of NO.The way of adsorption directly affects the formation of products.Oyarzúnet al.[131] showed that N-down adsorption of the monomer is more thermodynamic.The number of continuous adsorption sites determines the reaction path.For continuous adsorption of two monomers,both N-down and O-down configurations provide a feasible path to form CO and N2.Kyotaniet al.[76] attempted to analyze the reaction between carbon and NO or N2O by using an ab initio molecular orbital theory.The results showed that the adsorption of NO was more stable at the edge of the carbon model by N-down method,while the O-down mode was more stable for the adsorption of N2O.And the former process can release a N2molecule to form a surface oxygen complex.

Doping metal atoms into a simplified char model is a common method to study the influence of metal elements on different reactions.Zhanget al.[132] studied adsorption of alkali metal atoms and its effect on subsequent NO adsorption.The results indicated that the addition of alkali metal can enhance the chemical adsorption of NO.And the catalytic activity was K>Na>Li.Gas molecular concentration or coverage often plays a decisive role in the adsorption of gas [133,134].Liuet al.[135] investigated the influence of calcium and concentration of NO molecules on the heterogeneous adsorption nitric of oxide (NO) with carbon.The results showed that van der Waals interaction among the NO molecules improved the adsorption of NO on the pure graphene surface.And with the addition of Ca,the electron transfers from the 4s and 3d orbitals of calcium to 2p orbitals of nitrogen and oxygen atoms which leads to hybridization of the orbitals and promotes the adsorption of NO.

The reduction of NO by char plays a crucial role in NOxemission reduction during coal combustion.In addition to experimental methods,quantum chemical calculation is also helpful to understand the reaction mechanism.Zhouet al.[84] investigate the mechanisms of the desorption of N2from NO during heterogeneous reduction on char surface by density functional theory.Two different N2desorption pathways were obtained by calculations,and the two different mechanisms can be expressed by the following reactions:R1,R2 and R3 are presented.The calculation result of the energy barrier of the rate-limiting step and the rate constant of the rate-determining step showed that pathway 1(R1,R2) is easier to do than pathway 2 (R1,R3).

Zhanget al.[82]investigated the effect mechanism of Ca on the heterogeneous reduction of NO by char.Electron spin density and dual descriptor of zigzag configuration were analyzed to predict the adsorption sites of the NO and Ca.The result shows that the addition of Ca could enhance the heterogeneous reduction of NO by lowering the fitting activation energy of reduction reaction and shortening the reaction path of the heterogeneous reduction of NO.Montoyaet al.[136] selected the absence and presence of preadsorbed oxygen for calculation.It found that CO and N2were formed between NO and char-N in the absence of oxygen.While in the presence of adsorbed oxygen on the char promote the reduction of the NO to N2as a dominant product and a small amount of N2O.Chenet al.[137] investigate the effect mechanism of oxygen concentrations on NO heterogeneous reduction by char.The results reveal that the oxygen concentration on the char surface has a significant effect on the activation energy of NO decomposition.And low concentration of oxygen is more conducive to NO reduction.This is consistent with the experimental results [126] that oxygen can promote the reduction of NO and the production of CO by promoting the reaction rate of coal char.It is widely known that CO can enhance the reaction of char-NO.Chenet al.[138] found that CO can synergize with char to promote the reduction of N2O,and can significantly reduce the activation energy of CO2desorption which is beneficial to CO2release.However,not all the reaction between char-NO could be promoted by CO.Zhuet al.[139] concluded that CO promotes the reduction of NO by zigzag configuration char and does not promote the reduction of NO by armchair configuration char.

3.3.Mechanism of nitrogen conversion during coal combustion

It is well-known that NOxcan be produced in both the fast homogeneous volatile combustion of coal and the relatively slow heterogeneous combustion of char.Moreover,char-N is the main contributor to NOxgeneration.The conversion of NO would be influenced by temperature and porous structure of char.The conversion of char-N/NO would increase as rising in temperature and decrease with the development of pore structure.The quantum chemistry study introduces the generation path of NO.The O2is adsorbed on the char surface to form -NO and -CO,and NO and CO are generated through the pathway consisting of a series of atomic rearrangement reactions.

The reaction between NOxand char can greatly reduce the emission of NOxduring char combustion.The formation of surface complexes (C(N) and C(O)) can promote the formation of free carbon sites which can react directly with NO,thus promoting NO reduction.CO can enhance the reaction of char-NO,because NO can be directly reduced by CO.Moreover,minerals can promote the reduction of NO by CO.The presence of O2can significantly enhance the reduction of NO because more C(O) complexes can be generated by char-O2reaction.The presence of OH-radicals and H-radicals can also promote the reduction of NO.By controlling the formation of H-radicals and OH-radicals,the conversion of HCN to NH,HNO and N can be effectively inhibited to avoid the formation of NO.The addition of Fe2O3and CeO2can greatly improve the combustion efficiency and reduce the total emissions of NO.

The quantum chemistry study summarized the adsorption ways of NO.It showed that N-down adsorption of the monomer is more thermodynamic.For continuous adsorption of two monomers,both N-down and O-down configurations provide a feasible path to form CO and N2.And the addition of K,Na,Li and Ca can enhance the chemical adsorption of NO.The quantum chemistry study also shows that the complexes on the surface of coal char and the presence of adsorbed oxygen on the char can promote the reduction of NO.Not all the reaction between char-NO could be promoted by CO.CO promotes the reduction of NO by zigzag configuration char and does not promote the reduction of NO by armchair configuration char.

4.Comparison of Quantum Chemical Calculation and Experimental Study

The present review comprehensively summarized the effects of the migration and transformation mechanisms of N during the coal pyrolysis and combustion based on experimental study and quantum chemical calculation.The research progress of coal pyrolysis and combustion process is summarized in Table 11.

Table 11Research progress of coal pyrolysis and combustion process

During pyrolysis,the effects of temperature,atmosphere,free radical,heating rate and catalyst on the formation of NOxprecursor were summarized;Temperature affects the formation of products mainly by affecting the activity of fuel-N and the secondary reaction of pyrolysis products;atmosphere and heating rate could influence the formation of products by affecting free radicals(especially H-radicals),active sites and the stability of N-containing structure;catalysts affected the formation of products by affecting the formation of active substances.Calculation results show that the reaction of internal hydrogen transfer is preferred in the pyrolysis for both pyrrole and indoles;atmosphere could change the reaction process by providing free radicals,and catalysts could inhibit or promote the reaction by changing the energy barrier.

For the formation of HCN,the consistent conclusion is obtained by experiment and calculation,that is,HCN mainly comes from N-5,which has poor stability during coal pyrolysis.For NH3,the experimental results showed that the relatively more stable Ncontaining hetero-aromatic rings,such as N-Q,were the main sources of NH3.While in terms of calculation,it is only mentioned that NH3can be generated under the action of free radical,H2O and catalyst,but the main source of NH3generation has not been reported.In view of the importance of NH3in pyrolysis products.Therefore,these details are suggested to be provided in future studies.Current studies focus more on the generation of HCN and NH3.While as a green and harmless gas,N2is rarely studied.So it is necessary to further study the main source of N2during pyrolysis,as well as clarify the path of the generation of N2.Similarly,the experimental and the quantum chemical analysis method have a good consistency in explaining the importance of free radicals.However,the effect of CO2on the formation of NOxprecursors has not been studied.It is suggested that the effect of CO2on nitrogen migration could be further studied in the future to further clarify the mechanism of directional migration of fuel-N.For catalysts,the same conclusion is obtained from different ways:Fe can promote the formation of HCN,while Ca can promote the formation of NH3.Therefore,in order to get the best catalyst,it can be predicted by calculation first,then verified by experiments,which can greatly shorten the test time and reduce the number of experiments.

During the combustion process,the effects of temperature,ambient oxygen concentration,physical structure of coal char,catalyst and the path of NO formation on heterogeneous oxidation of char(N) were summarized;and the effects of coke surface properties,catalyst and ambient atmosphere on heterogeneous reduction of NOxwere also concluded.The formation of surface complexes,C(O) and C(N),are beneficial to NOxreduction.The presence of O2and CO can also promote NOxreduction.However,not all the reaction between char-NO could be promoted by CO.Catalysts promoted NOxreduction by improving combustion efficiency and reducing reaction activation energy of the adsorption of NO.In this process,the consistency between the experimental and the quantum chemistry results is once again confirmed that quantum chemistry calculation can predict the structure and properties ofkey solids which cannot be detected by experiments at the molecular level,and can predict the experimental results.

5.Summary and Prospect

To sum up,the experimental and quantum computing methods reach the following consistent conclusions.The release of HCN mainly comes from the N-5 structure with poor stability during coal pyrolysis;The atmosphere influences the generation of precursor by affecting the formation of N-sites and H-radicals during the process of pyrolysis;The promotion effect of H-radicals is closely related to the structure of nitrogen in coal.It can promote the formation of NH3obviously,and it can also promote the formation of HCN for some structures.In the combustion process,the surface complexes(C(N)and C(O))on the surface of char can promote NOxheterogeneous reduction.It can be seen that quantum chemistry can explain and predict experimental results to a certain extent.

However,there are still many scientific problems to be further explored.In the aspect of experiment:It is necessary to further improve the process conditions and the optimal formula of producing more N2during pyrolysis,as well as clarify the path of the generation of N2.Further discussed the influence of atmosphere and catalyst on nitrogen forms to explore the mechanism of nitrogen migration and transformation in depth.In terms of quantum chemistry:The main sources of NH3and N2should be further clarified.A more realistic structure of nitrogen-containing functional groups should be established to improve the effects of sulfur and metal elements on nitrogen migration and transformation during pyrolysis and combustion.Expand the range of catalyst selection.Calculate to predict the catalytic effect to find the best catalyst.So as to shorten the time and reduce the number of experiments.By combining experiments with quantum chemistry,the research on directional nitrogen reduction during pyrolysis and denitration during combustion could be better completed,and the source control of coal pollution could be realized.

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

This study was supported by National Natural Science Foundation of China(21878210),Shanxi‘‘1331”Civil Clean Fuel Engineering Research Center,Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2019L0313),Patent Promotion and implementation in Shanxi Province(20200719) and sponsored by Taiyuan Green Coke Energy Co.,Ltd.(China).