Yanfeng Shen ,Yongfeng Hu ,Meijun Wang, *,Weiren Bao ,Liping Chang, *,Kechang Xie

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

2 Key Laboratory of Coal Science and Technology,Ministry of Education and Shanxi Province,Taiyuan University of Technology,Taiyuan 030024,China

3 Canadian Light Source,44 Innovation Boulevard,Saskatoon,SK S7N 2V3,Canada

Keywords:High-sulfur coal Sulfur forms Coal blend Pyrolysis Coking Mass transfer

ABSTRACT The utilization of high-sulfur coal is becoming more urgent due to the excessive utilization of low-sulfur,high-quality coal resources,and sulfur removal from high-sulfur coal is the most important issue.This paper reviews the speciation,forms and distribution of sulfur in coal,the sulfur removal from raw coal,the thermal transformation of sulfur during coal pyrolysis,and the sulfur regulation during coal-blending coking of high organic-sulfur coals.It was suggested that the proper characterization of sulfur in coal cannot be obtained only by either chemical method or instrumental characterization,which raises the need of a combination of current or newly adopted characterization methods.Different from the removal of inorganic sulfur from coal,the organic sulfur can only be partly removed by chemical technologies;and the coal structure and property,particularly high-sulfur coking coals which have caking ability,may be altered and affected by the pretreatment processes.Based on the interactions among the sulfur radicals,sulfur-containing and hydrogen-containing fragments during coal pyrolysis and the reactions with minerals or nascent char,regulating the sulfur transformation behavior in the process of thermal conversion is the most effective way to utilize high organic-sulfur coals in the coke-making industry.An in-situ regulation approach of sulfur transformation during coal-blending coking has been suggested.That is,the high volatile coals with an appropriate releasing temperature range of CH4 overlapping well with that of H2S from high organic-sulfur coals is blended with high organic-sulfur coals,and the C–S/C–C bonds in some sulfur forms are catalytically broken and immediately hydrogenated by the hydrogencontaining radicals generated from high volatile coals.Wherein,the effect of mass transfer on sulfur regulation during the coking process should be considered for the larger-scale coking tests through optimizing the ratios of different coals in the coal blend.

1.Introduction

Coal,as an important energy resource,has made a significant contribution to social development and economic growth for decades.With the excessive utilization of high-quality coal resources,the relative amount of inferior coals in total coal reserves,such as high-sulfur coals,has been increasing.In the coke-making process,a higher ratio of high-sulfur coal in the coal blend will not only increase the sulfur content in resultant coke,leading to a change of blast furnace operation and unqualified iron,but also increase the load of desulfurization process.Thus,it is important to remove sulfur from high-sulfur coal before its utilization or during its conversion for the industrial utilization of the relatively more abundant high-sulfur coal.

Coals with different metamorphic degrees show great differences in structure,even within a similar rank,the characteristics also can be quite distinctive.The transformation and conversion of sulfur in the entire coalification process determine the forms and distribution of sulfur in coal,which thus dominate the difficulty in removing different sulfur forms [1–3].Physically-related sulfur removal technologies [4–16] have been widely used in the industry,which are mostly used to remove inorganic sulfur from coal,and the desulfurization rate varies greatly depending on the coal types and process conditions,etc.Chemically-related technologies [17–43] can remove both inorganic and organic sulfurs from coal,but the efficiency is dependent on the sulfur forms in coal,the method or chemical reagents,and the conditions used.The structure and property of coal can be generally altered by chemical processes,and most of them have not been applied in the industrial process commercially.Different sulfur forms in coal will undergo various transformations during conversion processes.For the coal pyrolysis process,unstable sulfur compounds will decompose,generate sulfur radicals and release as sulfurcontaining gases,this becomes the main pathway for sulfur removal.Coal properties,minerals and sulfur forms in coal,and pyrolysis conditions such as temperature,atmosphere,and the mass transfer,are the main factors that significantly influence the sulfur transformation behavior during coal pyrolysis.For high-sulfur coals used in the coke-making industry,their structure and property are quite important characteristics to differentiate these coals from other types of coals,which is the caking and coking ability.To avoid the effect of sulfur removal processes on coal structure,protect and maintain this unique property,only physical processes are used and most inorganic forms of sulfur are removed.Thus organic sulfur becomes the primary sulfur form,the timely release of sulfur-containing gases induced by the decomposition of organic sulfur is critical to the sulfur removal during the coking of the obtained high organic-sulfur coals.Therefore,regulating the sulfur transformation behavior during coal pyrolysis,promoting more sulfur release with volatiles,and reducing the sulfur retention in coke,becomes the most feasible way to the utilization of high organic-sulfur coals in the coking process.

This paper reviews the research progress of the speciation and transformation of sulfur in coal,current relevant desulfurization technologies,and factors that affect the sulfur transformation behavior during coal pyrolysis.In particular,the regulation of sulfur during coal-blending coking of high organic-sulfur coking coal is proposed.The knowledge gap between current studies is summarized and the outlook for future utilization of high-sulfur coal is also proposed.

2.Sulfur Forms in Coal

Sulfur in coal exists in three types,which are inorganic sulfur,organic sulfur,and elemental sulfur.Elemental sulfur is generally formed by the weathering effect and its percentage in total sulfur is quite low,thus it is neglected in the most sulfur-related analysis.Pyrite is the most abundant sulfur mineral in coal,with some other sulfide minerals that exist as marcasite (FeS2),pyrrhotite (Fe1–xS),sphalerite (ZnS),galena (PbS),and chalcopyrite (CuFeS2) [1,44].Sulfate,usually with lower content in coal,includes gypsum(CaSO4·2H2O),barite (BaSO4),anhydrite (CaSO4),and some iron sulfate minerals which are formed from the weathering of pyrite.Organic sulfur is directly associated with the coal matrix and part of the macromolecular structure of coal.The forms of organic sulfur in coal mainly include thiols,sulfides and disulfides,thiophene,and its derivatives [1].

The total sulfur in coal is widely tested based on the ASTM method D3177-02(2007) [45],where sulfur is dissolved in hot water and precipitated from the resultant solution as BaSO4,then filtered,ashed,and weighed based on the Eschka method,or precipitated as BaSO4from oxygen-bomb calorimeter washings,then filtered,ashed,and weighed based on the Bomb Washing method.For the determination of sulfur forms in coal,the ASTM method D2492-02(2012) is generally used [46].Sulfate sulfur is determined by extraction with hydrochloric acid gravimetrically,pyritic sulfur is calculated as a stoichiometric combination with iron which is extracted by nitric acid or determined by atomic absorption techniques,then the organic sulfur can be obtained by the difference between total sulfur,sulfate,and pyritic sulfur.However,it also has been reported that the extraction of non-pyritic iron would result in a significant error for the determination of pyritic sulfur [47],which ultimately affects the value of organic sulfur.

Solvent extraction could extract some soluble sulfur forms from coal.The extraction of different coals using 1-methyl naphthalene by Sugawaraet al.[26]showed that most pyritic sulfur remained in residue,and sulfur in soluble was organic sulfur and exclusively thiophenic sulfur.The aromatic and sulfur components in Homestead coal detected through extraction with benzene in a soxhlet using gas chromatograph (GC) and gas chromatograph coupled with mass spectrometer (GC–MS) showed that the coal extract contains elemental sulfur and reactions between hydrocarbons and element sulfur or pyrite might be the origin of some sulfurcontaining aromatic compounds [27].In a later study by Whiteet al.[28],many individual thiophenes were positively identified in a Bevier coal extract,while mercaptans,sulfides,and disulfides were tentatively identified by a low-voltage high-resolution mass spectrometry.The study from Gryglewiczet al.[29],for the sulfur compounds in supercritical fluid extraction by toluene,toluene/2-propanol,and toluene/tetrahydrofuran using GC–MS,showed that diphenyl sulfide,diphenyl disulfide,thiophene,benzothiophene,dibenzothiophene and benzonaphto thiophene and their C1–C4alkyl derivatives were present in flame coal,while ortho coking coal only contains 2–5 rings of polycyclic sulfur compounds and their alkyl derivatives.Although certain sulfur-containing compounds could be obtained and determined by solvent extraction,it should be noted that there exists a large difference with the original coal structure,which leads to the deviation of real sulfur forms in coal.Sulfur in coal will be transferred into gas,tar,and char/coke during the pyrolysis process,and the sulfur forms in coal directly determine its transformation,this suggests that it can be qualified or quantified through the analysis of sulfur-containing compounds in the pyrolysis products.A detailed review of this,including the factors that affect sulfur transformation,will be discussed in the later section.

With the development of the modern analytical instrument,direct analyses of sulfur forms in coal have been investigated.Schultzet al.[48] showed that various types of sulfur bonding could be determined from the chemical shift data by electron emission spectroscopy.Calkins[49]studied the sulfur-containing components in coals of various ranks by using Pyroprobe GC–MS and continuous isothermal flash pyrolysis conditions,and suggested the presence of aliphatic sulfur-containing side chains(thioethers)and heteroatoms in single-ring or multi-ring aromatic clusters in coal.Buckleyet al.[50] found that the sulfur was predominantly in thiophene form in Guiding coal by X-ray photoelectron spectroscopy(XPS).The sulfur XPS study for oxidized coals with different sulfur content by Grzybeket al.[51] showed that the sulfur forms such as alkylsulfide,arylsulfide,thiophene,sulfoxide,sulfone,and sulfate presented on the coal surface.Although some sulfur forms could be determined by XPS,it is a technique that could only detect the sulfur information on the coal surface.Therefore,sulfur K edge X-ray absorption spectroscopy(XANES),which could detect the sulfur species in both the surface and bulk of coal,was introduced and investigated by some researchers.The study from Huffmanet al.[52–54] to determine sulfur forms in coal or coal macerals by XANES showed that it is a direct,nondestructive method of investigating the molecular structure of organic sulfur in coal.A range of S-containing model compounds were chosen to ascertain the forms of sulfur in coal by Wanget al.[55](Fig.1),and the major sulfur forms in coal,including pyrite,organic sulfide,thiophene,sulfoxide,sulfone,and sulfate,could be determined based on the least-squares analysis of the XANES spectra.In-situXANES study was also carried out to investigate the sulfur forms in coal during pyrolysis and oxidation,and the quantitative determination of thermal reactions of organic and inorganic sulfur forms in coal could be further obtained [54].In addition to the least-squares analysis of XANES spectra,other methods,such as third-derivative analysis [56,57],linear combination fit [58–61],were also used.Recently,new studies on sulfur forms in coals with different rank and sulfur transformation during pyrolysis of highsulfur coal have also been reported[17,55,62–69],and the relevant results provide some guidance for the understanding of sulfur forms in coal and for the utilization of high-sulfur coals.

Fig.1.Sulfur K-edge XANES spectra of different S-containing model compounds[55].

3.Removal of Sulfur from Coal

Sulfur in coal could be transferred into the gas phase and causes environmental problems,or transferred into the liquid phase and solid phase and imposes adverse effects on the tar upgrading process and blast furnace operation,respectively.Thus,removing sulfur before the utilization of coal should be paid with special attention.Three types of pre-desulfurization technologies of high-sulfur coal are mostly studied,including physically-related process,chemically-related process,and sulfur removal during the coal conversion process.The former two types of processes are also generally known as direct sulfur removal technology,which could remove different forms and content of sulfur from coal by various process conditions and chemical reagents.Physical technologies mainly remove the inorganic sulfur from coal,and chemical technologies are more often used to remove organic sulfur.Nevertheless,the combination of different types of processes has also been proposed to achieve better desulfurization efficiency.

3.1.Physically-related sulfur removal technology

Sulfur removal by the physically-related processes has been used commercially in the industrial coal cleaning process,including magnetic separation,gravity separation,oil agglomeration,and froth flotation.A summary of different physically-related processes and the corresponding sulfur removal efficiencies is listed in Table 1.

Table 1Representative physically-related and chemically-related sulfur removal processes

Magnetic separation is based on the principle that the magnetization coefficient of coal and sulfate is negative,showing reverse magnetism,while that of pyrite and other minerals is positive and paramagnetic.Thus pyritic sulfur could be separated and removed due to this difference in magnetization property.Zhouet al.[4] found that under the optimum conditions of gravityenhanced high gradient magnetic separation,up to 72% of pyritic sulfur could be cleaned.Zhanget al.[5]studied the desulfurization of pulverized coal by high-gradient magnetic separation coupled with microwave treatment,the maximum sulfur removal was 52%at a microwave pretreatment time of 4 min,a background field strength of 0.5 T,and a raw coal particle size of–0.074 mm.

Gravity separation makes use of the different density and centrifugal forces of coal and pyrite,which can be divided into wet separation desulfurization and dry separation desulfurization,both have the characteristics of high precision,large amount,and low labor cost.Wet separation desulfurization,including heavy medium,jigging,etc.,has the following drawbacks such as large investment in water washing system,the coal slime production with higher content of water and ash and lower caloric value,higher treatment cost,and difficult storage and transportation of coal slime.Dry separation desulfurization uses air and pulverized coal as composite medium,and air flow and mechanical vibration as the power,the materials are loosed and separated according to the density of coal.Pneumatic separation desulfurization and air heavy medium desulfurization are the two main processes,and Mozley Multi-Gravity Separator,Knelson concentrator,and Falcon concentrator are the three recognized enhanced gravity separators[6].The study of Ibrahimet al.[7]showed that the total sulfur and pyritic sulfur contents in Egyptian coal were decreased by Falcon concentrator from 2.90% and 1.60% to 1.72% and 0.48%,respectively.

Oil agglomeration is used in coal beneficiation based on the difference of surface property between coal and minerals.The hydrophobicity of coal will be enhanced if non-polar oil is added to the coal.Non-polar oil and hydrophobic oil conglutinate and spread on the particle surface mainly improve the hydrophobic surface of the particle,then enhance the hydrophobic agglomeration behavior of the particle suspension.Non-polar oil could form an oil bridge between two hydrophobic particles,which links them closely and improves the crushing resistance and continuous growth of agglomerates [8].The efficiency of oil agglomeration separation is affected by pH,particle size,oil dosage,agitation rate and time,oil type,and pulp density.The study of fine coal recovery from washery tailings by Yasaret al.[9]showed that 57.60%of the sulfur could be removed,and Kenet al.[10]showed that 60.51%of total sulfur and 58.70% of pyritic sulfur could be effectively removed by oil-agglomeration for Indian high-sulfur coal using linseed oil.

Froth flotation based on the difference in hydrophobicity between minerals and coal has been widely used to remove pyrite from coal.Oxygen-containing functional groups in the coal matrix,such as carbonyl and hydroxyl,make the surface hydrophobic,while the initial oxidation products(elemental sulfur and polysulfide)on the surface of pyrite are hydrophilic.Since the flotation efficiency is limited due to the natural flotability of pyrite in practical application,auxiliary technologies such as electrolytic reduction and ultrasonic pretreatment are used to enhance the difference between the surface properties of pyrite and coal.Qiet al.[11]studied the flotation behaviors of coal-pyrite and high-sulfur coal with dodecane as the collector,the results showed that the sulfur content of clean coal is higher than the organic sulfur content of raw coal,which indicates the mix of a large amount of inorganic sulfur in clean coal.Molecular dynamic simulation(MDS)showed that the pyrite in coal could not be collected by dodecane.Jiaet al.[12]found that a very small addition of tetrahydrofuran(THF)reagent before the dodecane addition could greatly enhance the flotation of coal.The research by Ayhanet al.[13]indicated that pH and frother types were the most effective parameters in pyrite removal from Hazro coal.The study by Sahinoglu[14]suggested that the separation efficiencies of ash,pyritic sulfur,and sulfate sulfur varied with the coal particle size.Maximumpyritic sulfur and sulfate sulfur rejections were obtained at a particle size of–0.125 mm and oil dosage of 250 g·t-1,while maximum pyritic sulfur separation efficiency of 57.80%was observed at coal particle size of–0.500 mm and oil dosage of 750 g·t-1.Zhanget al.[15]found that the enhanced desulfurizing flotation of high-sulfur coal by sonoelectrochemical method could reach the sulfur reduction rate of 75.40%,and ash was also partially removed.

The physical desulfurization method has the advantages of simple process,low investment,and low operating costs.To improve the desulfurization efficiency,desulfurization methods by combining multiple processes,which are selected according to the characteristics of different coal types,were also proposed and studied[16].

3.2.Chemically-related sulfur removal technology

Chemically-related sulfur removal technology generally uses chemical reagents to perform chemical reaction on coal,so that sulfur-containing compounds could be converted into non-solid form,and then separated by conventional means.Chemical leaching has been widely studied to remove sulfur from coal,among which acid and alkali leachings are mostly investigated.HCl,HF,and HNO3have been used to remove minerals from coal,during which process some inorganic sulfur will also be removed.Transition metallic and alkaline metallic minerals,aluminosilicates,and sulfate could be removed by HCl and HF,while pyrite and some organic sulfur can be oxidized by HNO3and removed by subsequent leaching processes[17–21].Karacaet al.[22]used hydrogen peroxide to remove minerals and sulfur from coal,it showed that the maximum reductions of pyritic sulfur and total sulfur were from 70%to 95%,and 42%to 58%,respectively.However,the reduction of organic sulfur was relatively small(a maximum of 25%).The study by Mukherjeeet al.[23] showed that hydrogen peroxide alone removed over 76% pyritic sulfur,70% sulfate sulfur,and around 5%organic sulfur,while almost complete removal of pyritic and sulfate sulfur and over 26% organic sulfur could be reached under the combined process with dilute sulfuric acid.Pyrite and organic sulfur could also be removed during the alkali leaching process,the removal rate generally depends on the solution used.Baruah and Khare [24] reported that under sodium hydroxide treatment,complete removal of inorganic and a maximum of 33%organic sulfur from oxidized Baragolai coal was achieved.Saydutet al.[25] investigated the desulfurization of Hazro coal by combined froth flotation and sodium hydroxide treatment,88.06% of total sulfur and 59.27% of organic sulfur were removed.

Solvent extraction,as a technology that takes advantage of the physical and chemical interaction between solvent molecules and sulfur-containing functional groups to extract sulfur from coal by mixing coal and solvent in a certain proportion and heating,pressurizing (or atmospheric pressure) under an inert atmosphere,is also used to remove sulfur from coal.The coal particle size,solvent,reaction time,reaction temperature,and slurry concentration will greatly influence the experimental effect[30–34].Due to the complexity of the macromolecular structure of coal and the variety of organic sulfur forms,only with a comprehensive understanding of the types and quantities of organic sulfur,can the appropriate solvent be chosen to achieve the maximum degree of desulfurization.Organic desulfurization of Assam coal by successive sequential extractions of morpholine,anthracene oil,andN-methyl-2-pyrrolidone showed that more than 80% organic sulfur was removed [34].Tanget al.[35] studied the desulfurization of highsulfur coal by using the potassiumtert-butanol/hydrosilane system,and found that more than 60% organic sulfur was removed,but more sulfonates were retained in the coal matrix due to side reactions.The change of coal properties could be mainly attributed to the fracture of fat branch chain and the reduction of oxygencontaining functional groups.The study by Lianget al.[36] for high-sulfur coal treated with four imidazole ionic liquids showed that imidazole ionic liquid had high selectivity for thiophenic sulfur,ionic liquid cations could form π–π bond with thiophene rings,and organic sulfur structure combined with coal matrix in the form of hydrogen bond could be destroyed by the anion.At the same time,organic sulfur connected to coal macromolecular structure with weak covalent bond,ionic bond,and van der Waals force could be destroyed by ionic liquids,thus the content of sulfur compounds was reduced.

Electrochemical reduction,microwave radiation,ultrasonic radiation,and the combination of these radiation processes and other treatments have also been used to remove sulfur from coal.It was found that the removal of both inorganic sulfur and organic sulfur could reach a high degree,but oxygen functional groups in coal increased,which might have some effects on the coal property[17,37–43].In general,chemical desulfurization can remove organic and inorganic sulfur in coal efficiently,but most chemical methods are conducted under high temperature and pressure,or other severe conditions,some still need to use the oxidant.Besides,operation and investment costs are generally high,and the reaction condition is relatively severe,which changes the coal quality at the same time,such as the caloric value,coking properties,and swelling can be altered or destroyed.Also,the use of the product after purification is restricted and it is difficult to be adopted in the industry on a large scale.

3.3.Sulfur removal during the process of coal utilization

From the above review of direct sulfur removal technology,it can be concluded that the removal of most inorganic sulfur by the physical process has been widely applied,while those chemical processes,which have a better removal rate of organic sulfur,are rarely used in the industrial process.This could be mainly attributed to the comprehensive consideration of process efficiency,investment cost,and the effect on coal properties.Also,since some pre-desulfurization processes may affect the coal structure and cause changes of coking property,especially,for high-sulfur coals used in the coke-making industry,metallurgical coke is the main target product and except for the sulfur content in coke,other indices of coke quality,such as mechanical strength,reactivity,coke strength after reaction,are also quite important.Therefore,the selection of pre-desulfurization process for high-sulfur coking coals should be cautious.Actually,the combination of predesulfurization process and sulfur removal during coal conversion is generally used to obtain a better desulfurization rate in the industrial process.Unstable sulfur compounds will decompose,generate sulfur radicals and release sulfur-containing gases during coal pyrolysis,which is the main pathway for sulfur removal during coal conversion.Although researchers have conducted adjustments of process conditions and pretreatment processes of certain coals,they all come down to induce more decomposition of sulfur forms and improve the sulfur transformation.Also,alkaline compounds,which could react with sulfur-containing components during coal pyrolysis and form corresponding sulfates or sulfides,have been widely used to retain sulfur in the solid phase and reduce its emission.Since coal itself contains some alkaline minerals,these minerals will also participate in the transformation of sulfur[70–75].However,for the pyrolysis process of high-sulfur coal that is used in coke-coking,the target product,metallurgical coke,has a strict requirement for sulfur content,which dominates the iron reduction process,iron quality,and the production capacity in the blast furnace.Thus the utilization of alkaline compounds during the coal coking process should be avoided.In addition,since the removal of sulfur during coal pyrolysis is closely related to its transformation behavior,the detailed review of sulfur removal and transformation and the affecting factors is stated in the later section.

4.Transformation of Sulfur During Coal Pyrolysis

Coals with different ranks have various properties and sulfur existing forms.Even within a similar coal rank,they also can be quite distinctive,which directly determines the sulfur transformation behavior during coal pyrolysis.Pyrite and unstable sulfur forms will firstly decompose at a lower temperature,stable sulfur compounds like thiophene and its derivatives are generally indecomposable even at higher temperature,but the pretreatment of coal and change of pyrolysis conditions may alter this.In general,coal properties,sulfur forms and minerals in coal,the pyrolysis temperature and atmosphere,the mass transfer,are the main factors influencing the sulfur transformation behavior during coal pyrolysis.

4.1.Effect of coal properties

With the proceeding of coal maturation,the carbon content increases,while that of oxygen,hydrogen,nitrogen decreases.The coal structure also becomes more ordered,the aromaticity increases,and the macromolecular skeleton structure is tightened [76,77].Also,the compositions of coal macerals undergo significant changes during coal maturation,which as well as affects the sulfur forms in coal and transformation behavior during pyrolysis.Vitrinite is affected by the original plants and coalification environment,the volatile matters,oxygen content,H/C,and O/C in vitrinite decrease with increasing coal rank.Inertinite has the lowest volatile matters,hydrogen content,and H/C,while liptinite shows the highest volatile matters and hydrogen content.In lower rank coal,more active disulfide and sulfide can be decomposed and released as gas products below 500 °C,while a more complex thiophenic structure in higher rank coal is difficult to decompose even at 1000 °C [63].High temperature pyrolysis experiments of Polish hard coals with different ranks carried out by Gryglewicz [78] indicated that the degree of sulfur removal had a decreasing trend with increasing coal rank(Fig.2).Sunet al.[79] studied the release behavior of sulfurcontaining gases from pyrolysis of coal macerals.It showed that the vitrinite had a higher amount of organic sulfur which would form more H2S and carbonyl sulfide (COS) at around 480 °C,while inertinite had more pyrite and the release of H2S and COS occurred at 600 °C.Besides,more release of C4H4S and C8H6S from vitrinite was detected,which suggested that vitrinite had more thiophene and benzo thiophene structural units directly connected with macromolecular structure of coal by weak bridge bonds than inertinite.Wanget al.[55] reported that the maximal release temperature of sulfur-containing gases for the pyrolysis of inertinite-rich coal was higher than that of a coal with significantly different property.The organic sulfur compounds in inertinite-rich coal could be oxidized to sulf oxide species due to the decomposition of oxygen-containing functional groups in the coal matrix.During flash pyrolysis of coal and coal macerals,Chouet al.[80] reported that the maximum evolution of volatile organic sulfur compounds occurred at around 700 °C,the evolution of sulfur compounds observed for separate coal macerals was similar to that of original coal samples,and it varied as a function of the amount of organic sulfur in the samples.

Fig.2.Total sulfur removal from coal by high temperature pyrolysis vs.coal rank[78].

4.2.Effect of sulfur forms and minerals

For sulfur forms in coal,the percentage of aliphatic sulfur decreases while aromatic sulfur such as thiophene and its derivatives increases during coal maturation,this is also the main reason that the sulfur removal of high-rank coal is generally limited under conventional pyrolysis condition.The decomposition of pyrite occurs at the interface of the gas phase and solid phase,and the corresponding temperature is at around 550 °C under inert atmosphere,but it will be moved forward if it is under reductive atmosphere.Gryglewiczet al.[81,82] reported that the conversion of pyrite to ferrous sulfide during the pyrolysis of a Polish coal occurred in the range of 360–700 °C.A comparison between the decomposition of pyrite in coal and pure pyrite indicated that the former could be markedly affected by the presence of the organic coal matrix.Chenet al.[83]also found that the indigenous hydrocarbon with hydrogen donor ability in coal could promote the reduction of pyrite.For unstable organic sulfur forms in coal,such as thiols,sulfides,disulfides,they are generally subjected to decompose at a lower temperature.Some of the generated sulfur radials,sulfur-containing components,will react with each other,or with hydrogen radicals and minerals in coal,ultimately result in the release of low-molecular sulfur-containing compounds with volatiles,or retain in the coke as sulfides,sulfates,and stable organic sulfur-containing compounds such as thiophene and its derivatives.Wanget al.[63] found that the inter-conversions among sulfur species in solid played a dominant role in the transformation of sulfur forms above 600°C.Wanget al.[84]studied the sulfur transformation during coking coal pyrolysis and found that thioether and disulfides could decompose completely at 650 and 850°C,respectively,and the inter-conversions between disulfides,sulfur oxides,and Ar–S type sulfides occurred in the range of 550–850°C.The pyrolysis of demineralized coking coals conducted by Liet al.[85] showed that organic sulfides decomposed completely at 700°C,which evolved as sulfur-containing gases under lower temperature and transferred into thiophenic compounds under higher temperature.

Gryglewicz[81]found that the total sulfur in the char increased in the range of 700–1000°C,this was associated with the probable fixation of hydrogen sulfide by alkali carbonates,mainly calcite and siderite,present in the coal mineral matter.Chenet al.[86]reported that mineral matter could not only fix H2S to form chars with higher sulfur content,but also catalyze the desulfurization reactions to form lower sulfur content tars in hydropyrolysis.Guanet al.[70] studied the effect of Ca-based additives on desulfurization during coal pyrolysis,it showed that the sulfur content of the tar decreased with the addition of Ca-additives,CaO and Ca(OH)2had better sulfur removal effect than that of CaCO3,and CaS was the main product of sulfur retained by Ca-additives.Liuet al.[62] carried out the XANES study of sulfur transformations during co-pyrolysis of calcium-rich lignite and high-sulfur bituminous coal,the results indicated that the inherent calcium in lignite likely facilitated the decomposition of sulfide,the co-pyrolysis process synergistically promoted the decomposition of pyrite and thioether,and the retention of sulfur in char was promoted as calcium sulfide and calcium sulfate.In another study [87],it was found that although the effect of absorbing sulfur by the alkaline mineral under argon atmosphere was obvious,the catalyzed effect of some mineral was also evident under CO2atmosphere.In the study of sulfur transformation during pyrolysis and raw coal and pre-treated coal samples[17],it was found that the sulfur removal was enhanced after different amounts of minerals were removed(Fig.3).In general,the existence of alkali minerals in coal hindered the release of sulfur-containing gases from pyrolysis under inert atmosphere [88–91].

4.3.Effect of pyrolysis conditions

In addition to the effect of coal property and sulfur forms in coal on sulfur transformation during pyrolysis,temperature is another important factor that determines the decomposition of major sulfur forms.With increasing temperature,the release of volatile matter is intensified,which improves the release of some sulfurcontaining gases,and increases the overall degree of sulfur removal.It has been shown that the primary release range of sulfur-containing gases is 350–800 °C,but large differences exist for coals with different ranks,sulfur content and forms.Pyrite is firstly decomposed to elemental sulfur and ferrous sulfide with increasing temperature,but the ferrous sulfide is quite stable and it could not decompose at an even higher temperature.Since the amount of hydrogen generated from the decomposition of coal organic matrix under low temperature is unmatched with the sulfur radicals,the interactions between elemental sulfur,different sulfur forms,generated iron-compounds from pyrite,and coal matrix limit the sulfur removal.With increasing temperature,the ratio of surface S to bulk S increases,which promotes the sulfur transfer from the bulk to the char surface during pyrolysis[65,92].

Coal pyrolysis is generally conducted under inert atmosphere,and the atmosphere itself does not participate in the coal pyrolysis process.The decomposition of pyrite and unstable sulfur forms generates the primary sulfur radicals during pyrolysis,but only part of these sulfur radicals could react with the hydrogen radicals generated from the decomposition of aliphatic side chains.These all lead to limited sulfur removal during pyrolysis.Therefore,to promote the decomposition of more sulfur forms and increase the release of sulfur-containing gases,studies on pyrolysis under reductive or oxidative atmosphere have been carried out.Yanet al.[93]found that SH radicals were the key intermediate in sulfur transformation during coal pyrolysis,H2could stabilize the SH radicals and weaken the interactions between SH radicals and coal char(Fig.4).Chenet al.[86]studied the sulfur transformation during pyrolysis and hydropyrolysis of coal,and their results indicated that more sulfur was removed under hydropyrolysis,during which process partial thiophenic sulfur could be hydrogenated and removed.Liuet al.[92] found that the sulfur on the char surface under H2atmosphere was much lower than that under N2,and sulfidic,thiophenic,and sulfoxide sulfur on the char surface of lower rank coal disappeared.Zhouet al.[94] compared the sulfur transformation during coal pyrolysis under different atmospheres and found that CO could promote the formation of COS,CO2inhibited the evolution of sulfur-containing gases below 600°C,H2improved the release of H2S and generated the most effective degree of sulfur removal.Similar results were also obtained by Guoet al.[95,96]by blowing additional gas into the coking chamber during pyrolysis process.However,the reductive atmosphere is generally highcost for large-scale industrial pyrolysis and the compact structure of coke in the coking chamber is not conducive to the gas diffusion,so the economic efficiency of this technology and limited interactions between reductive gas and sulfur species should be taken into consideration.Lianget al.[97]used ReaxFF molecular dynamics simulation to study the mechanism of sulfur transformation and desulfurization during coal hydropyrolysis,it was thought that the introduction of hydrogen could weaken the C–S bonds in thiophene,thiophenol,phenyl sulfide,reduce the bond dissociation energy and improve the desulfurization rate.A diagram of organic S-atom transforming cycle in lignite was proposed (Fig.5),which stated that S-containing radicals were involved in the reactions as intermediates,and H radicals or molecules was critical for the formation of hydrosulfuryl structure of S atom,which could further react with H2and produce H2S molecule.Liuet al.[98]studied the sulfur behavior during pyrolysis of high-sulfur coals under different atmosphere,it showed that both C–S and C–C bond could be broken in pyrolysis of a higher rank coal under 2% O2–He atmosphere.The pyrolysis of sulfur-containing model compounds under oxidative atmosphere showed that oxygen could break C–S bonds more easily than C–C bonds [99].Wanget al.[100] studied the effect of steam on sulfur transformation during pyrolysis of demineralized coal and found that the formation of H2S was promoted,the organic sulfur removal rate was increased,which could be attributed to the steam promoting the decomposition of thiophene sulfur above 500 °C.

From the above review,it can be concluded that the sulfur transformation behavior during coal pyrolysis is extremely complicated.A diagram of the path of sulfur transformation during coal pyrolysis is proposed as shown in Fig.6.With the increasing pyrolysis temperature,the decomposition of pyrite and unstable organic sulfur forms in coal produces the important sulfur radicals,sulfurcontaining,and hydrogen-containing fragments in the pyrolysis system.With the proceeding of pyrolysis process,interactions between these radicals and components to form sulfurcontaining gases and release with volatiles,or reaction with minerals and nascent char for sulfur retention in the solid phase occurred,which further determine the final sulfur distribution in the gas,liquid,and solid phase.Previous studies [65,92,95] have indicated that the interactions between volatiles and char greatly affected the sulfur transformation.With the increasing temperature,sulfur was transferred from the bulk to the char surface,more sulfur could be promoted into the gas phase at lower temperatures of below 600°C,while more sulfur remained in char at higher temperatures of above 600 °C.The evolution of CH4,H2,and other hydrogen-containing gases had an impact on H2S release,while CO,CO2,and other oxygen-containing gases affected the release of COS.

Fig.3.Accumulated amount of H2S and COS during pyrolysis of raw coal and pre-treated coals (YZR:raw coal;YZC:flotation treated coal;YZD:HCl–HF treated coal;YZN:HCl–HF–HNO3 treated coal;YZU:ultrasonication treated coal;YZ-D:flotation–HCl–HF treated coal;YZ-N:flotation–HCl–HF–HNO3 treated coal;YZ-U:flotation–HCl–HF–HNO3–ultrasonication treated coal) [17].

Fig.4.Sulfur transformation in decomposition of 2-naphthalenethiol on the char [93].

Fig.5.Diagram of organic S-atom transforming cycle in lignite,where R=alkyl/aryl and red arrows represent reactions with assistance of H2 or H atoms [97].

5.Regulation of Sulfur Forms During Coal-Blending Coking of High Organic-Sulfur Coking Coals

It can be concluded from the above that most of the inorganic sulfur can be removed from coal by physical process,and the organic sulfur associated with coal matrix can also be partially removed by some chemical processes.But the structure and property of treated coal are usually changed or altered during the chemical process,in particular for coking coals used in the cokemaking industry.Caking and coking property are the most important characteristics that differentiate coking coals from other types of coals,which determine the plastic layer property of a mixed phase of gas,liquid,and solid during coking,and the quality of resultant coke.Therefore,coking coals are generally treated with physical process and organic sulfur becomes the primary sulfur source for sulfur transformation,in which the timely release of sulfur-containing gases induced by the decomposition of organic sulfur is critical to the sulfur removal during coking of the obtained clean coking coals.The blending of different kinds of coking coals is commonly used in the coke-making industry,and the layer-bylayer coking process proceeds in a relatively closed chamber.The gases generated during the coking process usually have two release pathways:one is from the dry-coal layer and part of the plastic layer,which releases from the inside and upper part of the plastic layer and flows through the headspace of the coking chamber;the other one is from the semi-coke and most of the plastic layer,which flows through the cranny in the coke and passes the gap between coke and oven wall to the headspace of the coking chamber (Fig.7).This unique coking method and gas release pathway will result in the complex interactions between different coking coals,volatiles in the plastic layer and the semi-coke and/or coke.Consequently,the sulfur transformation behavior during the coking process is further affected.Based on this,directionally regulating the sulfur transformation behavior during the coking process and promoting more sulfur release to the gas phase or liquid phase under the premise of without changing other coking conditions were considered as the most feasible way to increase the utilization of high organic-sulfur coking coals in coal-blending coking.

It can be known from the previous review that various factors and interactions affect the sulfur transformation behavior during coal pyrolysis,but primarily,three essential conditions are needed to achieve the regulation of sulfur:(1)more decomposition of sulfur forms in coal;(2)sufficient active hydrogen radicals,acting as a hydrogen donor to catalyze the cleavage of C–S/C–C bonds and bond with the formed sulfur radicals;(3) timely formation of sulfur-containing gases and the release with volatiles,which avoids the secondary reactions with coke.When increasing the ratio of high organic-sulfur coking coal in coal blend,it is likely to result in the insufficient amount of hydrogen-donor radicals(to catalyze the cleavage of some sulfur bonds and react with sulfur radicals) in the relatively closed coking system.Thus more interconversions of sulfur radicals,interactions between sulfur radicals and nascent coke occur,finally,the retention of sulfur in the coke is increased,which leads to the higher sulfur content in coke.A large amount of active hydrogen radicals in the volatile matters will be generated during the pyrolysis of high volatile coal,thus it is feasible to blend high volatile coal with high organic-sulfur coking coal and utilize these generated hydrogen radicals as hydrogen donor forin-situregulation of the sulfur transformation behavior of high organic-sulfur coking coal,promoting more sulfur release to the gas phase (which can be removed by the current commercial and high efficient gas desulfurization technology) and decreasing the sulfur content in coke to meet the coke quality criterion.

Fig.6.Path of sulfur transformation during coal pyrolysis (revised based on [101]).

Fig.7.Release pathway of pyrolysis gas in a coking chamber [95].

In the authors’previous work[65],a high organic-sulfur fat coal was selected to replace low-sulfur fat coal in an industrial coking coal blend to study the effect of external volatile matters on sulfur transformation during pyrolysis.When high organic-sulfur fat coal was added into the coal blend at different ratios,more evident sulfur transformation occurred on the coke surface due to the interactions between volatile matters and nascent coke.By comparing the release behaviors of CH4and H2for high volatile coals and H2S for coal blend with high organic-sulfur coking coal,it showed that the temperature range of H2S release was more overlapped with that of CH4than H2(Fig.8),which indicated that the hydrogen radicals and low-molecular hydrogen-containing fragments,while generating CH4,affected the formation and release of H2S.The experimental results also confirmed that the more overlapping of temperature range of volatile matters (in particular CH4) released from high volatile coal with that of sulfur release in coal blend,the stronger interactions between volatile matters and nascent coke were promoted,and the lower sulfur content in coke was obtained.

Fig.8.Correlation between release ranges of CH4 and H2 from high volatile coals and H2S from coal blend with high organic-sulfur coal (HVC1:long flame coal;HVC2:gas coal;BC2:coal blend with high organic-sulfur coal) [65].

In a subsequent study [66],prime coking coal with high organic-sulfur,and gas coals with appropriate volatile content but different ash contents and alkaline indexes were used to investigate the regulation of sulfur transformation behavior during the coal-blending coking process.By comparing the sulfur content and distribution in coke obtained from various coal blends,it was found that the decrease of sulfur content in coke was not in a linear relationship with the content of gas coal in coal blend,and there was an optimal ratio of gas coal and coking coal in the coal blend.Mineral matters in clean gas coal,in particular those alkaline compounds,which are authigenic minerals and more evenly distributed,imposed a major effect on the sulfur retention in coke.A mechanism of directional regulation of sulfur by gas coal as shown in Fig.9 illustrated that besides the generation and interactions of sulfur-containing,oxygen-containing,and hydrogencontaining radicals from the pyrolysis of coal blend,the sulfur regulation by the introduced gas coal primarily occurred on the coke surface or interspace,the interactions between sulfur radicals and alkaline minerals were also reduced under an optimal ratio of gas coal and more sulfur was released with volatiles,while it imposed a slight change on sulfur in the bulk coke due to the restriction of mass transfer,and the sulfur regulation would be inhibited by the alkaline minerals under an excessive ratio of gas coal.When the ratio of gas coal in coal blend was excessive,the secondary reactions between sulfur radicals and coke and the effect of mineral matters from gas coal could deteriorate the sulfur regulation,which increased the sulfur retention in coke.The 10 kgscale coke oven test results (Table 2) showed that the quality of coke,in terms of the sulfur content (Sd),crushing strength (M40),abrasive strength(M10),reactivity(CRI),and strength after reaction(CSR),from the optimal blending ratio of high organic-sulfur coking coal and high volatile gas coal,was close to that of the basic coke.This also proved the feasibility of sulfur regulation by the proposed method.

Table 2Evaluation indices of coal blend cokes from 10 kg-scale coke oven [66]

From the above,it can be concluded that for the use of high volatile coals in sulfur regulation during coal-blending coking of high organic-sulfur coking coals,low content of sulfur and minerals,in particular alkaline minerals,appropriate release temperature range of CH4that overlaps well with that of H2S from high organic-sulfur coals,should be firstly considered.This further increases the possibility of using high organic-sulfur coals that has high volatiles and low minerals.

6.Knowledge Gap and Outlook

The use of high-sulfur coal becomes more urgent due to the excessive depletion of low-sulfur high-quality coal,while addressing the sulfur problem is the most important issue for efficient utilization of the high-sulfur coal.Coal property and the forms and distribution of sulfur in coal are the main factors that directly determine the subsequent transformation behavior of sulfur during coal conversion.From the above review on speciation and thermal transformation behavior of sulfur,the main knowledge gap of the current research and the prospect for future study can be summarized as below:

(1) There are continuous transformation and inter-conversion for the organic sulfur during the whole process of coalification of high-sulfur coal.The determination of sulfur forms in coal is commonly conducted by the chemical method,where the organic sulfur is obtained by the difference between total sulfur,pyritic and sulfate sulfur.Although some modern instruments such as XPS and XANES have been used to characterize the sulfur forms in coal,in particular organic sulfur,the results obtained are generally semiquantified.The forms and distribution of sulfur in coal are quite complex,either by chemical method or instrumental characterization,the accurate information of sulfur in coal cannot be obtained.However,the principle of removing sulfur before the utilization of high-sulfur coal must be based on the understanding of the speciation of sulfur in coal.Thus the development and exploration of new determining methods and improvement of the current methods of sulfur speciation in coal are necessary and urgent.Also,since sulfur forms in coal have different transformation behavior during pyrolysis and the decomposition of sulfur forms occurs only at certain temperature,a combination of chemical,instrumental,and sulfur transformation behavior during coal conversion,such asin-situstudy by XANES,should be further studied and developed to more accurately identify the sulfur content and forms in coal.

Fig.9.Mechanism of directional regulation of sulfur during pyrolysis of coal blend with high organic-sulfur coking coal and high volatile gas coal [66].

(2) Removing sulfur before coal conversion is the most direct way before the utilization of high-sulfur coal.Physicallyrelated technologies have been widely used in the industrial process to more effectively remove inorganic sulfur,but removing the pyrite that is embedded with ultra-fine particles and symbiotic in the coal matrix is restricted.For chemically-related technologies,the extent of the removal of both inorganic and organic sulfur is quite different due to the various treatments used.In this case,a proper combination of physical and chemical methods should be an effective sulfur removal process for high-sulfur coal.But it has not been widely used due to the comprehensive consideration of desulfurization efficiency,operating conditions,investment,handling of side products,regeneration of expensive reagents,and industrial feasibility of chemical process.More importantly,current studies rarely investigate the effect of pre-desulfurization process on the structure of high-sulfur coal,which is quite important and necessary.Particularly,since organic sulfur is associated with the coal matrix,the relationship between sulfur and the caking ability for high organic-sulfur coking coals,which has been seldomly studied,is critical for the desulfurization and following utilization of these coals.Therefore,on one hand,there should be more effort on the development of new pre-desulfurization process,and the degree of the effect of pre-desulfurization on the change of structure and property of coal should be emphasized.On the other hand,a more fundamental and deeper understanding of the relationship between sulfur and property of high organic-sulfur coking coals should be carried out,which is critical to reveal the intrinsic property of these coals,and assess the development and feasibility of the relevant pre-desulfurization process.

(3) Part of the sulfur in coal is transformed into sulfurcontaining gases and released with the volatiles during pyrolysis,which becomes the main pathway for sulfur removal during coal utilization.Coal property,forms and distribution of sulfur in coal,minerals in coal,pyrolysis conditions,and inter-conversions between various components during coal pyrolysis directly determine the sulfur transformation behavior and the final sulfur distribution in the gas,liquid,and solid phases.For the high organic-sulfur coals used in the coke-making industry,it has been proven that the sulfur transformation behavior can be regulated by selecting some high volatile coals.The hydrogencontaining radicals generated from high volatile coals can act as anin-situhydrogen donor to catalyze the breakage of C–S/C–C bonds in some sulfur forms,and bond with the sulfur-containing radicals to release with volatiles.Minerals in high volatile coals,in particular alkaline compounds,also affect the sulfur regulation.Thus,to promote more sulfur release to the gas phase,on one hand,the interactions between minerals and sulfur-containing radicals,fragments,or gases on the coke surface should be further restricted;on the other hand,pretreatment of high volatile coals becomes necessary and minerals should be further removed to alleviate their adverse impact on sulfur regulation,which raises the optimization of current coal beneficiation technology and development of new combined technology.Also,although the effect of mass transfer on sulfur regulation is quite important,relevant studies on this have not been carried out yet.Actually,in the industrial relatively closed coking chamber,the mass transfer between different phases is intensified during the layer-by-layer coking process.Therefore,the study of the mechanism of the effect of mass transfer on sulfur regulation during the coking process should be brought to the attention.Coking tests on a larger scale coke oven are also necessary to optimize the ratios of different coals in the coal blend.This is quite critical to inhibit the surface reactions between sulfur-containing components and coke,and achieve the better utilization of high organicsulfur coking coals in the coke-making industry.

Declaration of Competing Interest

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

Acknowledgements

The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (U1910201,21878208),Transformation of Scientific and Technological Achievements Programs of Higher Education Institutions in Shanxi(TSTAP),Shanxi Province Science Foundation for Key Program(201901D111001(ZD)).