Kechang Xie

State Key Laboratory of Clean and Efficient Coal Utilization,Key Laboratory of Coal Science and Technology (Taiyuan University of Technology),Ministry of Education,Taiyuan 030024,China

Keywords:Advanced coal to chemicals industry trategic understanding Clean energy

ABSTRACT It is important to develop the advanced coal to chemicals industry (ACCI) against a backdrop of coal-based energy structures,excessive imported oil and natural gas,and strict environmental constraints in China.In this study,the technology and industry of China’s ACCI are reviewed to explain the effect of using coal to replace oil and natural gas,and the corresponding resource and environmental burdens that this will create.Development trends in technology and industry are also proposed to explore future scenarios.The review shows that although excellent progress has been made on an industrial scale,demonstrative level,and in terms of technology and equipment,the lack of strategic understanding,severe external constraints,partly underdeveloped technologies,and weak foundations must be immediately addressed.Therefore,it is necessary to clarify the importance that the ACCI has on the energy revolution and energy system.Based on technological innovation,a variety of external factors should be considered as a whole with emphasis on filling the knowledge gap of theoretical foundations and industry standards to support high-quality development for ACCI.

1.Introduction

Energy is a critical basis for economic growth and social development.As the largest energy producer and consumer globally,China has built an excellent energy sector with coal as the main source,electricity as the center,and highly developed oil,natural gas,and renewables [1].Nevertheless,China’s reliance on fossil fuel energy has led to increased resource constraints,prominent ecological and environmental problems,a sharp rise in pressure on energy security,and a series of new challenges for energy development [2–4].

To address these issues,a revolution associated with the production and consumption of energy has been proposed,which aims to build an energy sector that is clean,low carbon,safe,and highly efficient.According to a study by the Chinese Academy of Engineering,the energy production and consumption revolution in China will be a long-term process and shall be completed in three steps [5].The first step was expected to take place from 2016 to 2020,in which the energy structure would be optimized,and the consumption proportion of coal,oil and natural gas,and other energy sources was predicted to reach 6:2.5:1.5.Actually in 2020,coal,oil and natural gas,and other primary energy sources in China accounted for 57.7%,27.0%,and 15.3%,respectively [6],which is very close to the expectation of 2020.The second step will take place from 2020 to 2030,and shall be a transformation period.The consumption proportion of coal,oil and natural gas,and other energy sources in this period is predicted to reach 5:3:2.The third step will take place from 2030 to 2050,and is known as the stabilization period,with the consumption proportion of coal,oil and natural gas,and other energy sources reaching 4:3:3.In the future,coal will still dominate the energy sector despite its decreasing proportion of the energy structure.With the increasing import proportions of oil and natural gas [7],coal presents a great strategic significance for stabilizing and ensuring the safety of the energy sector.The increasing use of renewables,such as wind and solar power,also puts forward a greater requirement on the peak load regulation capacity by coal power [8,9].Therefore,promoting the exploitation and utilization of coal in a clean and highly efficient manner should be considered a major foothold and the primary goal of energy transformation and development in China,based on national basic conditions,development stage,and energy trends.In addition,China’s commitments to the international community regarding carbon peak and neutral targets set higher requirements for the low-carbon utilization of coal.

As an important approach to using coal in a clean,highly efficient,and low carbon manner in China,the advanced coal to chemicals industry (ACCI) has rapidly increased in scale with gradually maturing engineering and has constantly improved its technology and equipment since the 12th Five-Year Plan.However,it is currently facing several challenges,such as insufficient strategic understanding,severe external constraints,partly underdeveloped technology,and a weak foundation.This study aims to clarify the progress and deficiencies in China’s ACCI,thus providing a valuable reference for future high-quality and sustainable development.

2.The Status of ACCI in China

2.1.Technology status

The industrialized technology routes of ACCI include preparing gas and liquid fuels and chemicals.They use coal as the feedstock for preparing synthetic natural gas and conventional oil-based products,such as gasoline,diesel,olefins,aromatics,and ethylene glycol.This is conductedviapyrolysis,gasification,liquefaction,and the corresponding downstream processes,and thus partly replaces oil and natural gas [10].Compared with the processes used in the traditional coal chemical industry,they utilize advanced conversion technology,have long and complex processes.Fig.1 shows that the gasification approach starts from coal gasification technology,then uses syngas as an intermediate product to produce methanol,F-T (Fischer-Tropsch) oil (e.g.,naphtha,gasoline,diesel oil,liquefied petroleum gas,etc.),synthetic natural gas,and ethylene glycol.Methanol can be further converted into olefins,gasoline,and aromatics,etc.The liquefaction approach refers to the direct coal liquefaction process,of which,the main products are diesel and jet fuel.The pyrolysis approach is the deep processing of coal tar producedviamedium-and low-temperature pyrolysis.

Coal to gas fuel is the process of converting coal to synthetic natural gas(CTSNG).Four CTSNG projects have been put into operation by the end of 2019:a 2 billion cubic meters CTSNG plant of Yili Xintian Coal Chemical Industry Corporation Ltd.,a 1.38 billion cubic meters CTSNG plant of China Kingho Energy Group Corporation Ltd.,a 1.33 billion cubic meters CTSNG plant of China Datang Corporation Ltd.,and a 0.4 billion cubic meters CTSNG plant of Huineng Group.These projects use Topsoe Corporation’s and Davy Corporation’s methanation technologies.According to an investigation,the CTSNG projects that are currently under construction mostly employ Davy Corporation’s methanation technology [11].

Coal to liquid fuel includes direct coal liquefaction (DCL),indirect coal liquefaction (ICL),coal to methanol to gasoline (CTMTG),and coal tar hydrogenation (CTH).In 2008,China Energy Investment Corporation’s DCL project was put into trial operation.After a one-year technology upgrade,the project was commercialized in 2010.It is the largest and most advanced plant in the world,which indicates that China’s DCL technology has now reached a world-leading level [12].At present,self-developed ICL technologies in China include Shandong Energy Group’s F-T synthesis in a high-temperature fixed fluidized bed and a low-temperature slurry bed,and the Chinese Academy of Sciences’ high-temperature slurry bed[13].For CTMTG technology,Exxon Mobil Corporation’s fixed bed process and a new one-step process developed by the Chinese Academy of Sciences are now being used in an industrial setting in China.The Chinese one-step process has several advantages,including mild reaction conditions,high gasoline selectivity,long one-way catalyst life,low catalyst consumption,high gasoline octane number,and inadequate equipment investment costs.Lowgrade coal resources are abundant in China,and coal tar produced via pyrolysis and gasification processes using lignite and longflame coal as the feedstock is increasing annually,especially in Yulin[14].China’s coal tar hydrogenation process usually employs a fixed bed,suspension bed,or ebullated bed to produce fuel oil.The specific technologies that are used are full-range hydrogenation,delayed coking hydrogenation,hydrocracking and hydroupgrading,deasphalting hydrogenation,and dephenolization hydrogenation.

Coal to chemicals refers to the coal to methanol to olefin(CTMTO),coal to ethylene glycol (CTEG),and coal to methanol to aromatics (CTMTA) processes.At present,the Chinese Academy of Sciences’ dimethyl ether/methanol to olefins (DMTO) and Sinopec’s methanol to olefin processes are the most successful CTMTO technologies and have been widely used in Chinese demonstration projects.The basic idea of CTEG is to firstly convert coal into syngas and then synthesize ethylene glycol directly from that;alternatively,ethylene glycol can be indirectly produced through the transformation of intermediate compounds.The Chinese Academy of Sciences’oxalate synthesis technology is the first in the world to achieve industrial applications.Aromatics can be preparedviamethanol aromatization and toluene methanol alkylation.The main non-Chinese technologies are from Mobil Corporation and Saudi Basic Industries Corporation,and the Chinese ones are from Tsinghua University and the Chinese Academy of Sciences.

Fig.1.Technology routes for China’s ACCI.

2.2.Industrial status

After years of development,China’s ACCI has made great progress and has reached the largest industrial scale in the world.By the end of 2019,one DCL project,seven ICL projects,16 CTMTG projects,35 CTH projects,four CTSNG projects,25 CTMTO projects,28 CTEG projects,and four CTMTA projects have been put into production,with annual production capacities of 1.08 million tons,8.65 million tons,1.88 million tons,8.59 million tons,5.1 billion cubic meters,16.65 million tons,7.63 million tons,and 370 kilotons,respectively [15].Fig.2 illustrates that these demonstration projects are mainly distributed across the north and northwest of China,especially in the four typical coal chemical bases.CTMTG,CTH,CTMTO,and CTEG projects are more widely distributed than the other four types of projects.

In 2019,China’s ACCI produced 13.32 million tons of refined oil,3.94 billion cubic meters of synthetic natural gas,11.94 million tons of olefins,3.33 million tons of ethylene glycol,and 280 kilotons of aromatics [15].These products substituted 40.24 million tons of imported oil and 3.94 billion cubic meters of imported natural gas,which is equivalent to reducing oil and natural gas dependence proportions by 1.7 percentage points and 1.3 percentage points,respectively.

Benefitting from the technological development and facility popularization of energy conversion and emission reduction,China’s ACCI achieved a preliminary effect by substituting imported oil and natural gas,albeit with slight resource and environmental costs.In 2019,a total of 109 million tons of coal and 370.82 million tons of water are consumed by ACCI,accounting for only 2.7%and 0.6‰of the country’s total consumption,as shown in Table 1 [15].Correspondingly,a total of 132.27 million tons of CO2,28.04 kilotons of SO2,and 27.19 kilotons of NOXwere emitted,which contributed only 1.4%,3.4‰,and 2.3‰ to the total national emissions.Nearly zero wastewater was discharged,and almost all solid waste was treated or recycled[16].In short,ACCI is an important approach to achieving the efficient and clean utilization of coal.

Table 1Resource consumption and emissions of China’s ACCI in 2019

3.The Trend of ACCI in China

3.1.Technology trend

Coal gasification is a key conversion technology that leads coal utilization towards cleanliness and high efficiency.It can be integrated with various downstream processes such as hydrogen or natural gas conversion from coal,liquefaction,and syngas conversion,which are important ways to produce special oils and chemicals.In the future,coal gasification technology is expected to be flexible and reliable,large capacity,low investment,wide coal adaptability,high efficiency,highly water-conserving,and intelligent in its direction.High-reliability gasification technologies and equipment suitable for coal with high ash and ash melting points will be developed.At present,the coal to methanol industrial technology has reached a mature point.In the future,a new process for the efficient directional conversion of methanol and syngas,largescale efficient catalyst preparation,large-scale reactor design,and optimization and upgrading key equipment and processes will be developed.High-performance catalysts for methanol synthesis are the current focus of development.

A great deal of research has been carried out on CTSNG technology in China,which mainly focused on the development of the methanation process and catalyst,but has failed to be applied to large-scale projects.In the future,an efficient and low-cost catalyst for CTSNG and corresponding processes will be developed.An optimization process for integrated energy and material for CTSNG technology will be investigated and then industrialized to improve conversion efficiency and reduce energy and water consumption.

Fig.2.Spatial distribution of China’s ACCI in 2019.

Existing DCL plants will be further optimized and upgraded to extend their operating cycle.The efficiency of DCL technology will be improved by developing new catalysts and processes to achieve high efficiency and water conservation,mild liquefaction,and cotreatment of coal and coal tar or heavy oil.New catalysts can further increase product selectivity to improve energy efficiency and reduce water consumption.By developing technology for producing special fuels,high-value solvent oils,high-end lubricating oils,ultra-clean oils,and aromatic hydrocarbons,the structure of DCL products can be optimized.The existing million-ton ICL projects will be also optimized to improve the operating cycle and reduce resource consumption.The technology that is used to produce high-end lubricating oils,high-end waxes,PAOs,olefins,clean oxygen-containing fuels,and high-end fine chemicals will be developed.New processes and cobalt-or iron-based catalysts with high efficiency and selectivity will also be developed.DCL and ICL integrated coupling technology will be investigated and engineering demonstrations will be carried out to test it.

To further meet demand,methanol to olefin technology needs to improve the selectivity of ethylene and propylene and achieve a widely adjustable ratio of products.Methanol to propylene technology requires the localization of catalysts and further optimization to reduce material and energy consumption and economic costs.The future focus of CTEG technology is the large-scale preparation of highly selective catalysts,amplification of high-efficiency reactors,and system optimization and integration,with the goal being large-scale industrialization.There are already widespread pilot schemes for Chinese CTMTA technology with fluidized beds,but these have not yet been demonstrated on an industrial scale.In the future,high-efficiency catalysts and aromatic synthesis achieved directly from syngas should be the focus of attention.

In China,rapid progress has been made in terms of system analysis,key process development,and small-scale demonstrations for coal-based polygeneration technology.In the future,key technologies such as directional control of products,cascade utilization of energy in co-production systems,and optimization and control of integrated systems will be developed.In addition,key unit technologies,and subsystem and multi-production system simulations will be optimized,and the whole process package for co-production systems will be developed.The development of typical coal-based multi-production integrated processes based on coal pyrolysis and gasification should be emphasized.The concept of co-generation of syngas from coke oven gas and gasification gas put forward by the Taiyuan University of Technology and the related basic research has laid the foundation for this kind of process[17].

In the coupling technology of coal conversion and hydrogen production from renewable energy,researches have gradually emerged in recent years.This technology is in its infancy,which needs to be strengthened in matching system scale,coordinating stable operation,and optimizing the process.It provides a win–win opportunity for large-scale consumption of renewable energy and reducing CO2reduction in the coal conversion process.

ACCI uses syngas as an intermediate material.Therefore,the concentration of CO2discharged from the syngas purification process is high,which greatly reduces the economic costs of CO2capture.This advantage provides an opportunity for deploying carbon capture and storage (CCS) or carbon capture,utilization,and storage (CCUS) technologies.Focusing on the development of CCS or CCUS technology in ACCI can greatly reduce the consequences of greenhouse gas emissions.The key to deep decarburization lies in economical and feasible CCS and large-scale CCUS technologies.A large amount of CO2emitted by advanced coal to chemicals enterprises can be timely captured,transported,and utilized,which is a win–win scenario in terms of economic and environmental benefits.As novel and forward-looking technologies,CCS and CCUS are important technological approaches for reducing total CO2emissions in the medium and long term and can be utilized to achieve China’s carbon mitigation commitments,which should be strongly adhered to.At present,CCS and CCUS technologies are still in the research and demonstration stage;the main barriers to success are high resource consumption and unacceptable economic costs.Investing in science and technology and international exchanges and cooperation will be improved to grasp the development trends of international frontiers.The introduction,integration,and re-innovation of international advanced technologies should be focused on.

3.2.Industrial trend

In the future,China’s ACCI will focus on upgrading the promotion of demonstration and commercialization.According to our investigation,the future production capacities of DCL,ICL,CTMTG,CTH,CTSNG,CTMTO,CTEG,and CTMTA are expected to reach 5 million tons,27.65 million tons,15.21 million tons,12.64 million tons,133.3 billion cubic meters,52.5 million tons,33 million tons,and 5.8 million tons,respectively,based on the assumption that all of the current construction and planning projects will be put into production [18,19].

From the perspective of the spatial distribution of ACCI,a largescale coal-to-fuel base lead by CTSNG and ICL projects will be built in Xinjiang;a chemicals and materials base dominated by CTMTO projects will be built in the junction region of Inner Mongolia,Shaanxi,and Ningxia;intensive CTMTO,CTSNG,CTEG,and CTH projects will be based in the east of Inner Mongolia;ICL and CTGH projects will be given priority in Shanxi and Henan;and plenty of CTMTO projects will be deployed in coastal regions,benefitting from convenient methanol imports [20].At that time,China’s oil and natural gas dependence proportions are expected to be kept below 70% and 40%,respectively [21].

With the progress of clean coal conversion technology,the resource consumption and pollution emission coefficient of products in China’s ACCI will be further reduced.However,the rapid expansion in production capacity will inevitably lead to more resource consumption and pollutant emissions.It is estimated that the theoretical coal and water consumption(calculations based on production capacity) in the future will reach 780 million tons and 2.10 billion tons,and the emissions of CO2,SO2,and NOXwill reach 850 million tons,180 kilotons,and 170 kilotons,increasing by 477%,394%,424%,426%,and 411%,respectively,compared with the theoretical values in 2019.

4.Progress and Challenge of ACCI in China

4.1.Progress

4.1.1.Industrial scale expansion

The 12th and 13th Five-Year Plans were a period of growth in the scale of China’s ACCI,as shown in Table 2[15].During the period of the 12th Five-Year Plan,CTSNG and CTMTA were fully industrialized,and the industrial scale of ICL,CTMTG,CTH,CTMTO,and CTEG grew rapidly,especially over the last two years.When comparing 2015 with 2010,the industrial scale of these processes expanded 3.3 times,16.8 times,4.4 times,6.1 times,and 10.8 times,respectively.During the period of the 13th Five-Year Plan,ICL,CTH,CTMTO,and CTEG continued to maintain strong growth momentum,with growth rates of 424%,110%,150%,and 225%,respectively.However,the development of other technologies entered a stable period with relatively insignificant growth on an industrial scale.

Table 2Scale expansion of China’s ACCI from 2010 to 2019

4.1.2.Project demonstration effect

The operation of production plants in demonstration projects is constantly improving.The world’s only commercial DCL project that has an annual production capacity of 1.08 million tons,and the globe’s largest ICL project that has an annual production capacity of 4 million tons,both belonging to China Energy Investment Corporation,have been operated with a long-term and stable status.Shandong Energy Group’s ICL project with one million tons of annual production capacity has been put into operation.A cobalt-based slurry-bed F-T synthesis plant with a capacity of 150 kilotons and a CTEG plant with a capacity of 500 kilotons are currently operating at full capacity.In addition,several CTMTO projects that use DMTO technology have been commercialized,each with a capacity of 500–600 kilotons.

A high-temperature F-T synthesis device with a capacity of 100 kilotons has also been successfully demonstrated;a cobalt-based fixed-bed F-T synthetic plant with a capacity of 200 kilotons has been fully commissioned;a 10,000-ton demonstration project has been completed for the production of diesel and mixed alcohol from syngas;a 50-kiloton industrial demonstration plant for the coupling of syngas and ethylene to propanal,and then ton-propanolviahydrogenation,has been started up one time;an industrial plant for dimethyl carbonate production from methanol has completed construction;the world’s first industrialized coal to ethanol project with a capacity of 500 kilotons is currently under construction.

The resource consumption and emissions of demonstration projects have been continuously reduced,as shown in Fig.3 [15].The benefits of reducing resource consumption and emissions were significant from 2013 to 2015 and were stable during the 13th Five-Year Plan.The reduction rates of raw coal consumption,fuel coal consumption,water consumption,CO2emissions,SO2emissions,and NOXemissions from 2013 to 2019 were almost within 15%–40%,5%–10%,30%–50%,20%–40%,40%–60%,and 40%–60%,respectively.CTMTG,CTMTO,and CTEG have greater reduction benefits for resource consumption and emissions than other technical routes.

4.1.3.Progress in technology &equipment

A considerable portion of the technology is at an internationally advanced or world-leading level.With a breakthrough in the key aromatization technology of tail gas from F-T synthesis,a novel route for the production of coal-based aromatics has been developed.The technology that directly converts syngas to olefin has been verified to have a CO conversion rate of more than 30%,and an olefin selectivity of over 70%.A corresponding slurry bed reactor has also been developed.Third-generation DMTO technology has also succeeded,it reduced the coal consumption per ton of methanol to less than 2.7 tons of standard coal.It is currently world-leading,the same as DCL and CTH technologies.In addition,Chinese equipment tends to be large.The daily coal consumption of a single multi-nozzle opposed-type coal water slurry gasifier has reached up to 4000 tons[22].The 2 kilotons of large-scale F-T synthesis reactor,air separation equipment with an oxygen generating capacity of 100 kilometers per hour,gas purification and separation device with a processing capacity of 400 kilometers per hour,a large reciprocating compressor for combined gas,and wear-resistant control valve with large-scale high differential pressure and coal slurry pump with high temperature and high solid that is suitable for harsh process conditions.In China,the market share of Chinese coal gasification technology is over 60%,and the equipment used in ACCI that is selfproduced makes up over 90%of all equipment.

4.2.Challenge

4.2.1.Lack of strategic understanding

Unfortunately,there are still several factors restricting the development of China’s ACCI.A key factor is the unclear strategicpositioning of industrial development.Coal dominates the energy structure in China;however,the recognition that ACCI can promote the usage of coal in a clean,highly efficient,and low carbon manner,and partly replace the petroleum chemical industry is lacking across the country.Cases of indiscriminate cutting of coal that contrary to basic conditions and development stage in China make the strategic position of the coal chemical industry a lot less clear,leading to inconsistent policies and uncertain industry prospects [23].Given the present day’s volatile global landscape,it is urgent to clarify the strategic position of scientific development in ACCI and to take the clean and efficient development and utilization of coal as the foothold and primary goal of China’s energy transition and development.

Fig.3.Industrial levels of China’s ACCI from 2013 to 2019.

4.2.2.Severe external constraints

The current external environment heavily restricts industrial development.Due to the same products,ACCI becomes a potential competitor to the petrochemical industry.In recent years,the high uncertainty of oil prices has posed huge challenges to the economic benefits of ACCI,especially for the coal to liquid fuel route that levies the consumption tax[24].With the launch of a large number of ACCI and petrochemical projects,the competition is expected to become increasingly fierce.Especially for CTMTO and CTEG,the risk of overcapacity appears [25].The geographical mismatch between coal and water resources hinders the sustainable development of ACCI [26].The ecological environment of large coal bases in China is generally fragile,which leads to a relatively small environmental capacity [27].To deploy ACCI projects in regions without environmental capacity,it is necessary to offer the available environmental capacity by equal or reduced replacement.China recently stated that it will increase its Intended Nationally Determined Contributions that adopting more powerful policies and measures to reach its peak carbon emissions as soon as possible before 2030,and to strive to achieve carbon neutrality by 2060 China’s carbon neutrality goal requires controlling the total coal consumption in the future if no disruptive technology appears.In view of the limited coal consumption scope and the continuously expanding scale of ACCI,CCS or CCUS may be a priority technology for large-scale decarbonization [28].Overall,to address external constraints,the clean,efficient,and sustainable use of coal must be adhered to and further be strengthened based on the development strategy proposed in the past[27,29].Clean and efficient utilization of coal technology needs to reach the international advanced level as soon as possible to form a sustainable development in an all-round way.

4.2.3.Partly underdeveloped technology

Internal deficiencies also reduce industrial competitiveness.For China’s ACCI,energy utilization and resource conversion efficiencies are relatively low due to the many processes still being optimized;the environmental pollution caused by gaseous,liquid,and solid waste,especially wastewater,are very prominent.Therefore,clean technologies must be vigorously developed in the future.The value chains of China’s ACCI are short,because a large number of primary products are lacking refined,differentiated,and specialized downstream product development,the comparative industrial advantages are not very significant.Owing to deficiencies in technology integration and production management,the economic cost is high,and the whole production efficiency needs to be improved.Additionally,some key core technologies,equipment,parts are still relying on imports,which increases the security risks of technology.

4.2.4.Weak foundation

The theoretical basis and industry standards of coal conversion technology need to be improved.The industrial applications of coal conversion technology are ahead of its theoretical development.The lack of theoretical understanding of the structure and reactivity of coal,coal-based chemical engineering systems,and catalyst designs has caused a huge waste of material and human resources throughout the industrialization process.Meanwhile,as a conventional high-carbon energy source,the material nature of coal is greatly underestimated.The evolution mechanism of the original carbon structure during the chemical conversion process remains unnoticed.As new low-carbon coal conversion routes become available,the scientific and technical basis of coal to functional materials and fine chemicals is yet to be noticed and supported by the country.According to statistics,there are currently over 200 national and industrial standards in action and undergoing research.Nevertheless,the design work for the standard system lacks a systematic process.In particular,the standards about the industrial base,management,crossing field,and difference from petrochemical products are insufficient and have a huge gap with the current demand.

5.Concluding Remarks and Thoughts for Future Work

At a time when COVID-19 is spreading across the world and global political and economic trends are uncertain,the importance of an independent energy supply must be appreciated.The development of ACCI is not only a beneficial supplement to the petrochemical industry,but is also an important part of the energy revolution and energy system.Therefore,based on a strategic demand orientation of the national energy security,China’s ACCI should be developed in an orderly and reasonable manner and should receive support in the form of reasonable financial policies,overall planning and layout,and macroscopic readjustment and control.

In terms of the layout of ACCI,constraints such as coal and water resources,environmental and ecological capacity,carbon tax,and market demand should be fully considered to avoid blind development and homogenized competition,which may potentially affect the industry’s sustainable development.

Technology is the foundation of the development of ACCI.Therefore,encouraging innovation,improving cutting-edge technologies,and developing disruptive technologies should be heavily focused on.Chemical products should be improved for the sake of premiumization and differentiation.The industrial chain should also be extended for the value increase,but the near-zero wastewater discharge,solid waste reduction,and resource utilization throughout the industrial chain should be considered together.Technical-economic performance should also be improved and impacts on the environment and climate should be mitigated to achieve high-quality development in ACCI.For example,converting coal into low-cost,high-value functional materials and fine chemicals are of the characters of low-carbon and high-value,it is suggested to develop technologies for preparing battery electrode materials,carbon supercapacitor materials,and other functional materials with coal as raw material.These are expected to open new approaches for coal conversion.

The theoretical basis and standards of coal chemistry and coal chemical engineering are the foundations for guiding the clean and efficient conversion of coal.In view of the complexity of coal’s structure and immature basics of chemical reactions and conversion processes,major basic disciplines and research subjects should be set up to guide the scientific community.It is suggested to promote the fundamental research about coal reactivity and coal structure,carbon structure evolution and regulation,industrial classification of low-grade coal,and so on.It is necessary to improve the research and exploration of engineering foundations,develop the research and development of generic technology and equipment,and establish a clean production standard and system classified as the basic general,product,method,and management,to guide the clean,low-carbon,and efficient development of ACCI.

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 work wad supported by the Foundation of the Chinese Academy of Engineering,China (CKCEST-2021-1-15 and 2020NXZD3).