Fuchen Wang*,Guangsuo Yu,Haifeng Liu,Weifeng Li,Qinghua Guo,Jianliang Xu,Yan Gong,Hui Zhao,Haifeng Lu,Zhongjie Shen

Institute of Clean Coal Technology,East China University of Science and Technology,Shanghai 200237,China

National Engineering Research Center of CWS Gasification and Coal Chemical Industry (Shanghai),East China University of Science and Technology,Shanghai 200237,China

Keywords:Opposed multi-burner Gasification Multiscale Numerical simulation Industrial application

ABSTRACT Opposed multi-burner (OMB) gasification technology is the first large-scale gasification technology developed in China with completely independent intellectual property rights.It has been widely used around the world,involving synthetic ammonia,methanol,ethylene glycol,coal liquefaction,hydrogen production and other fields.This paper summarizes the research and development process of OMB gasification technology from the perspective of the cold model experiment and process simulation,pilotscale study and industrial demonstration.The latest progress of fundamental research in nozzle atomization and dispersion,mixing enhancement of impinging flow,multiscale reaction of different carbonaceous feedstocks,spectral characteristic of impinging flame and particle characteristics inside gasifier,and comprehensive gasification model are reviewed.The latest industrial application progress of ultralarge-scale OMB gasifier and radiant syngas cooler(RSC)combined with quenching chamber OMB gasifier are introduced,and the prospects for the future technical development are proposed as well.

1.Introduction

Coal gasification is the key technology for the clean and efficient utilization of coal,and has been widely used in the synthesis of coal-based bulk chemicals (synthetic ammonia),synthesis of coal-based liquid fuel,coal to olefin,coal to natural gas,integrated gasification combined cycle (IGCC),hydrogen production and direct-reduction ironmaking,etc.Due to the advantages of large single-gasifier operating load,high carbon conversion rate and wide feedstock adaptability,the entrained-flow gasification technology has become the mainstream of research,development and industrial application.Opposed multi-burner (OMB) gasification technology is the first large-scale gasification technology developed in China with completely independent intellectual property rights.Due to its unique structure of gasifier body and burner,the residence time distribution is optimized and the mixing process is significantly strengthened.Due to its obvious advantages of high efficiency and large-scale,OMB gasification technology has become the most widely used technology [1].

The fundamental research,technology development and engineering amplification of OMB gasifier have been supported by the National Key Science and Technology Project of China,the National High Technology Research and Development Program of China,the National Key Basic Research Program of China and the National Natural Science Foundation of China,which highlights the leading and supporting role of fundamental research on the technology development and the engineering amplification (as shown in Fig.1).This paper gives a brief overview of the research and development process,pilot test and the first demonstration device of OMB gasification technology.The important fundamental research and the latest industrial application in the recent decade are reviewed.Moreover,the prospects for the future technical development of large-scale coal gasification are proposed.

2.Review of Research and Development of OMB Gasification

2.1.Cold model experiment and process simulation

Fig.1.R&D history of OMB gasification technology.

In the middle and late 1980s,China imported Texaco coal water slurry (CWS) gasification technology,which had been applied in Lu’nan(Shandong Province),Weihe(Shaanxi Province)and Wujing(Shanghai City).These gasifiers have been operated since 1992.However,a series of engineering problems occurred,such as the short life of burner and firebricks at the bottom of gasifier,the ablation of quenching ring and downcomer,the syngas from quenching chamber entraining water and ash,the ash blockage of syngas washing system,fine ash content over standard,low carbon conversion (approximately 95%),etc.These problems restricted the long-term and stable operation of gasifier [2–8],and thus need to be urgently solved.Under the support of China Petrochemical Corporation and the Ministry of Chemical Industry of China,East China University of Science and Technology(ECUST)had successively established cold mold test platforms for entrained-flow gasification with diameters of 300 mm and 1000 mm(maximum in China,adjustable height)in order to reveal the scientific mechanism of engineering problems.The flow field characteristics and residence time distribution in entrained-flow gasifier were studied[9–11],the numerical simulation of flow processes were carried out [12].Based on these work,the regional model and mathematical model of CWS gasifier were established,and the simulation of CWS gasification process was carried out[13,14].In 1991,under the support of China Petrochemical Corporation,ECUST built the largest cold mold test platform for entrained-flow gasifier in China (with diameter of 1000 mm and adjustable height).The hierarchical mechanism model of entrained-flow gasification was proposed [15],the mathematical models for the concentration distribution and residence time distribution in gasifier were established [16,17],and the shortcircuit mixing model of gasifier based on the time scale of micromixing and macro-mixing was proposed.Moreover,the optimal process parameters were obtained based on the simulation calculation of three CWS gasifiers with different scales [18].These results provided important theoretical guidance for the optimized operation and long-term stable operation of the imported CWS gasification plants.

Based on numerous fundamental research,the coal gasification research team of ECUST firstly proposed a new OMB scheme for CWS gasification in 1995,with comprehensively innovating the CWS gasification process system,which laid a foundation for developing large-scale coal gasification technology with independent intellectual property rights.In 1997,a large-scale cold mold platform for OMB gasifier was built.The flow and mixing process in the gasifier were systematically studied[19–24],and the mathematical models of gasifier and gasification system had been established [25].These work laid a foundation for designing software package for pilot and industrial demonstration plant.

2.2.Pilot-scale study

At the beginning of 2000,the pilot plant of OMB CWS gasifier with the single-gasifier capability of 22 t?d–1and design pressure of 4.0 MPa was built in Lu’nan Chemical Industry Company of Yankuang Group Co.,Ltd.In September 2000,it passed the 72h continuous operation performance test organized by China Petroleum and Chemical Industry Association (named as China Petroleum and Chemical Industry Federation (CPCIF) now).In October 2000,it passed the expert appraisal organized by China Petroleum and Chemical Industry Association with all the process indexes completely exceeding the imported technologies [26–28].

Since 1998,ECUST has started to study the pulverized coal pressurized gasification technology,and established the pulverized coal transportation test device and lab-scale research platform[29–31].In August 2004,the pilot plant for OMB pulverized coal pressurized gasification was built in Lu’nan Chemical Industry Company of Yankuang Group Co.,Ltd.with refractory lining structure and the gasification pressure of 4.0 MPa.This gasifier was firstly operated in September 2004,and passed the 72 h continuous operation performance test and expert appraisal organized by the Ministry of Science and Technology of China in December 2004 [32,33].In June 2005,the first pulverized coal pressurized gasification test using CO2as carrier gas was completed in China[34].In July 2007,the pilot plant of membrane-wall coal gasifier with capacity of 30 t?d–1was built and started up.In November 2007,it passed the 72 h continuous operation performance test organized by China Petroleum and Chemical Industry Association[35].

2.3.Industrial demonstration

In 2001,under the support of the National High Technology Research and Development Program of China,two sets of OMB CWS gasifier with the single-gasifier capability of 1150 t?d–1were built in Guotai Company of Yankuang Group Co.,Ltd,with production of 240,000 t?d–1methanol and 80 MW IGCC power generation unit.This gasification unit was built and successfully started up for the first time in July 2005,and was officially put into operation in October 2005.It passed the 72 h continuous and stable operation performance test and the expert appraisal organized by China Petroleum and Chemical Industry Association in December 2005 and January 2006,respectively.The operation results of the industrial demonstration plant showed that OMB gasification technology exhibited better technical indexes and the key equipment working life than the similar imported technologies,and achieved safe,stable,long-term,full load and optimal operation [36–38].

Frankly speaking,in the early stage of technological development,a series of engineering problems or shortcomings were encountered,such as the short life of burner and firebricks at the dome of gasifier,the ablation of quenching ring and downcomer,the syngas from quenching chamber entraining water and ash,the ash blockage of syngas washing system,the fine ash content over standard,low carbon conversion,etc.Fortunately,the Ministry of Chemical Industry of China,China Petrochemical Corporation and Yankuang Group Co.,Ltd provided continuous supports.On this basis,ECUST gasification research team has conducted continuous research on the internal flow,mixing,complex reactions,flame structure,temperature field,process simulation of the entrained flow gasifier,etc.,to overcome these shortcomings,and OMB gasification technology has also experienced a process from bench-scale to pilot-scale and to industrialization.Now,OMB CWS gasification technology has great innovation in gasifier structure,burner structure,flow and reaction coupling,hightemperature syngas washing,etc.,and its successful industrialization is a milestone in the development history of China’s coal gasification technology with marking that China had a large-scale coal gasification technology with completely independent intellectual property rights,which broken the technological monopoly of foreign multinational corporations and strongly supported the rapid development of modern coal chemical industry in China.

3.Latest Progress of Fundamental Research

In the past 30 years,ECUST gasification research team has conducted continuous fundamental research on the OMB gasification technology.The relevant research progress and results before 2012 are summarized in literature [39].This paper will focus on the main progress of the related fundamental research on OMB coal gasification technology in the past 10 years.

3.1.Nozzle atomization and dispersion process

The nozzle is the core device of the entrained-flow gasifier with two crucial functions.One is to strengthen the mixing between coal and gasification agent by atomizing the slurry or dispersing the powder,and the other one is to match the flow field of gasifier,which can affect the concentration distribution,temperature distribution and chemical reaction process in the gasifier.The nozzle performance and different combinations are important technical methods to strengthen mixing in the gasifier.

3.1.1.Atomization mechanism of CWS nozzle

The residence time of raw materials in the entrained-flow gasifier is very short,so the nozzle should make sure the mix of coal particle,oxygen,etc.quickly and efficiently.In order to achieve CWS atomization,the help of the high-speed gas flow is needed.At present,the suitable nozzle for CWS gasification is the prefilming air-blast nozzle,whose key structure parameters are the exit diameter,exit wall thickness,exit area ratio,channel angle,etc.These parameters can generates the suitable flow field,concentration distribution and temperature field by controlling the structure of nozzle,which are determined by a large number of theoretical analysis,experimental test and engineering applications.

Based on the study on the structural parameters(such as nozzle diameter and exit area ratio) and the process parameters (such as pressure,velocity and momentum ratio),the airflow atomization model is established,which laid the foundation for the development of efficient and long-life CWS gasification burners.Experimental results showed the phenomenon that the drop size of airflow atomization changes non-monotonously with the gas velocity,and revealed the influence law of the gas distribution ratio on the drop size of liquid atomization.The finite stochastic breakup model (FSBM) of atomization process has been proposed according to the self-similarity of droplet breakup.FSBM can reliably predict the nonlinear relationship between the mean droplet diameters and droplet size distribution of the air-blast atomization process with considering co-effects of breakup and aggregation and taking the critical breakup condition of droplet as the criterion.

Based on morphology,the breakup regimes of CWS can be classified into three different modes,i.e.,Rayleigh-type breakup,fibertype breakup and atomization as shown in Fig.2 [40,41].Compared with water or other Newtonian fluids,the atomization of CWS has no obvious film-like breakup process because during the process of CWS deforming into a film-like structure under the action of airflow,the coal particles will quickly rupture the film-like structure and fail to form an obvious film-like structure when the thickness of slurry film is close to that diameter of coal particles.When the CWS viscosity is low,the CWS breakup mode is Rayleigh-type breakup at low air velocity;and fiber type breakup at high air velocity.Meanwhile,high-viscosity CWS will oscillate under the action of airflow as shown in Fig.3.

The Rayleigh–Taylor wavenumber is proposed as the theoretical criterion for the secondary breakup of CWS droplet based on the interface instability,and it is found that the secondary breakup modes of CWS droplets include deformation,multi-mode breakup(including two sub-modes,hole breakup and tensile breakup),and shear breakup [42,43].Among them,the deformation and shear breakup mode are similar to the corresponding breakup modes of Newtonian fluid.When the yield stress of CWS is small,the hole breakup mode will appear as shown in Fig.4,which is similar to the bag breakup mode of Newtonian fluid:The central part of the droplet breaks firstly and then the edge part breaks,the breakup mechanism of which is mainly attributed to the effect of Rayleigh–Taylor instability.The difference between the hole breakup mode of CWS and the bag breakup mode of Newtonian fluid is no membranous structure in the former.When the yield stress of the CWS is very large,the hole breakup mode will be replaced by the tensile breakup mode.Due to the rheological characteristics of shear thinning,the viscosity of CWS droplet will be different during the deformation process,causing that CWS droplets with low viscosity will breakup first.

The slurry presents the characteristics of homogeneous fluid macroscopically,and thus can be simplified based on model establishment and numerical calculations.However,the liquid–solid mixture will show strong heterogeneity on the microscopic scale of the last stage of slurry breakup as shown in Fig.5 [44–46].CWS droplet will go through the suspension stage,transition stage and liquid stage in sequence during breakup process.In the transition stage where the characteristic scale is 0.5–2 times greater than the particle diameter,the particle size is already close to the smallest diameter of the slurry,and the breakup characteristics will be changed from slurry to liquid.This phenomenon verified that the usage of the traditional continuum model in the middle and last stages of slurry breakup is not valid.

3.1.2.Particle entrainment and thick-wall effect of pulverized coal nozzle

During the operation of entrained-flow pulverized coal gasification,severe abrasion would occur at the nozzle outlet and thus significantly reduces the lifetime of nozzle.Based on the study of dense gas–solid two-phase jet in pulverized coal gasification [39],we found the main reason for nozzle abrasion is the strong localized particle backflow at the nozzle outlet,which is caused by gas jet entrainment due to nozzle thickness.This is named as thick-wall effect,which clarifies the abrasion mechanism of the pulverized coal nozzle outlet [47,48].

Fig.6 shows the reverse entrainment of particles in coaxial parallel jets at different annulus gas velocities.When the annulus gas velocity is very small,the central particle jet is hardly affected by the reverse entrainment of the coaxial airflow,and thus can maintain the original flow pattern.When annulus gas velocity reaches～10 m?s-1,the particles on the jet surface begin to be affected by the annulus backflow gas and move in the opposite direction,which is named as the critical annulus gas velocityugc.ugcis the criterion for distinguishing whether there is reverse entrainment for the central particle jet under the thick-wall effect has.Therefore,the particle jet can be divided into the non-reverse entrainment area and the reverse entrainment area with the change of the annulus air velocity,and the increase of the annulus gas velocity will promote the entrainment effect of the gas backflow on the particles.It is worth noting that with the increase of the annulus air velocity,the number of particles carried into the recirculation zone due to the thick-wall effect also increases,causing that the entire nozzle wall is basically covered.Essentially,the reverse entrainment of particles in the central dense particle jet mainly depends on the momentum transfer between the particles and the return gas.Therefore,the annulus gas velocity is the most fundamental and important influencing factor.

Fig.2.Primary breakup modes of CWS [40].

Fig.3.Oscillation of CWS jet (time interval is 2 ms) [40].

3.2.Impinging jets

Based on the in-depth study on the radial jet,the stagnation point offsets and the impingement plane oscillations of two impinging jets and four impinging jets [39],the effects of the jet velocity distribution and the turbulence intensity on the stagnation point offsets were investigated and the regulation method was proposed [49,50].The deflecting oscillation of free planar opposed jets was studied,and the results revealed that the deflecting oscillation was self-sustained by the periodic changes of the velocity field and the pressure field.The planar laser induced fluorescence(PLIF) was used to study the flow and mixing in the confined impinging jets reactor(CIJR),and the mixing enhancement by excitation under different flow patterns was considered [51].The half deflecting oscillation of planar jets in T-jet reactors was observed for the first time,which revealed that the positive pressure of the top chamber was the main reason for the half deflecting oscillation[52].By continuously switching the dyed and undyed fluids,the recirculation region where the fluids were trapped infinitely was observed in cross-jets reactors.Moreover,the particle trapping in these regions was further explored [53].

The flow patterns of two dense granular impinging jets were investigated using two high-speed cameras from the side and front views simultaneously.The results show that as the solid fraction of the granular jet (xp) increased,three patterns,i.e.,the penetrating pattern,diffuse pattern and granular sheet,displayed in turns[54,55],as shown in Fig.7.For the diffuse pattern at lowerxp,better mixing performance between two granular jets occurred as a result of interparticle collisions and penetrations.For the granular sheet at higherxp,mixing was hardly observed because the particles were too dense to penetrate from one side into another[56,57].

Fig.4.Breakup of CWS droplet [42].

Fig.5.Microscopic breakup of slurry [44].

The liquid-like granular sheet differs qualitatively from the penetrating and diffuse patterns produced by the impingement of granular jets with low solid fractions.Fig.8 presents the side and front views of granular and liquid sheets.It can be seen in Fig.8(a) and (b) that although granular materials are cohesionless,thin and vertical granular sheets normal to the plane of two jets are formed for 2θ=60°and 120°,which are similar to the liquid sheets presented in Fig.8(c) and (d).It should be noted that the liquid sheet displays a leaf-like shape due to the equilibrium between the inertia and capillarity,while the granular sheet always broadens during its downward spreading process because interparticle cohesive force is nearly zero[58].For 2θ=120°,the arch-like granular sheets bounded by the rims,like liquid,which is mainly attributed to that the particles above the impingement point propagate downwards.

The increase of the granular jet velocity or the decrease of the solid fraction may cause the asymmetric oscillation of the granular sheet,as illustrated in Fig.9.The time-average oscillation frequencies of granular sheets showed liner increase relationship with the jet velocities.The impingement of two granular jets is a compromise in competition as displayed in Fig.10.When the momentum(M) of one granular jet is much larger than another one,the impinging granular flow is biased toward the side of the smaller momentum one.The instability of granular jet induced by the enhanced gas–solid interaction will lead to the momentum imbalance of the impinging jets at the impact point,and consequently results in the asymmetric oscillation of the granular sheet.Increasing the upright length of the nozzle or embedding a screen into the nozzle could regulate the instability of the granular jet,and further enhance the particle dispersion effectively.

3.3.Multiscale reaction of different carbonaceous feedstocks

The systematic researches on gasification characteristics of typical coals and other carbonaceous feedstocks[39,59–68]have been reported in detail by researchers in last 20 years,thus traditional kinetic studies for coal gasification were not extended for discussion herein.Furthermore,multiscale reactions,including the effect of ash melting on gasification,particle group reaction inside gasifier,particle reaction on the wall of gasifier and multiscale phenomena during gasification process were comprehensively studied and discussed in detail,which were not considered in the past research work.The multiscale reaction includes listed types,but not limited to,

Fig.6.Reverse entrainment of particles at different annular gas velocities [48].

These researches provided an important guidance for the deep understanding of the reaction mechanism of carbonaceous feedstock under conditions of high temperature,high pressure and multiple flow.

3.3.1.Effect of coal ash melting on gasification

Fig.7.Images from the side view of granular impinging jets at various xp (2θ=60°, d=82 μm, D=3 mm, u0～2.1 m?s-1) [54].

Fig.8.Images from the side and front views of two granular and liquid impinging jets (D=3 mm) [54].

Krishnamoorhyet al.[69]found a coal ash melting phenomenon on the particle surface when the gasification temperature was higher than the ash flow temperature during the high-temperature Pittsburg No.8 coal gasification process and this melting behavior started at the carbon conversion of 30%.The char-to-ash transition mechanism during the coal gasification process is recently studied by our research team[70].The results showed that the evolution of coal char particle would be performed as a shrinking mode at the early and middle stage and as a shrinking core mode at the later stage.When the gasification temperature was higher than the ash flow temperature,the formed liquid slag would block the pores of coal char and thus hinder the gasification reaction.Thus,reaction (I) can be considered as the following mechanism reaction form:

With a further research on the evolution characteristics of the coal ash melting layer on the coal char surface at the later reaction stage[71,72],we found the melting layer covered the reaction surface of the coal char and the residual carbon floated on the liquid slag,acted as an ‘‘ice mountain” mode.Thus,it is speculated that the difference in coal char gasification reactivity caused by the coal type is not the key factor under the high-temperature operating condition in the entrained-flow gasifier,which also verified the correctness of the conclusions reported in previous studies [39].Otherwise,the interaction between char and ash melting during coal gasification was studied by Bai’s group [73–75],including the effect of chemical compositions (e.g.Fe2O3and SiO2/Al2O3),graphitization degree of char.The valence state of Fe affected by char reduction caused different ash fusion temperatures.Besides,the influence by residual char is more obvious when the mass fraction exceeds 5%.The increase of graphitization degree of residual char impedes the mineral reaction and the carbothermal reaction.

The gasification characteristics of petroleum coke on the molten slag surface was also studied[76],and the results showed that the alkali and alkali earth metals(AAEMs)in the molten slag have catalytic effects on the gasification of petroleum coke on the slag interface,as shown in Fig.11.The gasification of petroleum coke on the molten slag surface was influenced by both the catalytic action of metals in the slag and the covering action of liquid slag,which also prolonged the residence time of the petroleum coke in gasifier and thus was conducive to the petroleum coke conversion during the coal and petroleum coke co-gasification process in the entrained-flow gasifier.The multiscale reaction(VI)is transferred to

Fig.9.Side views of the granular sheets at different jet velocities and solid fractions (2θ=60°, d=82 μm) [55].

Fig.10.Patterns of imbalance impingement of two granular jets [55].

However,we found different phenomenon when studied the combustion characteristics of coal char particles on the molten slag surface (as shown in Fig.12).

Fig.11.Gasification of petroleum coke on the molten slag surface [76].

The combustion rate and burnout time of coal char on the molten slag surface were 20%–25% lower than those of original coal char combustion[77].Based on the analysis of heat transfer model,the heat transfer direction during the particle combustion on molten slag layer was from coal char to slag,which was opposite to that during gasification.Part of the heat from the combustion was transferred to molten slag,resulting in lower combustion temperature on the particle surface and the reduction of combustion rate,which will be more obvious with the increase of the particle size and thus is not conducive to carbon conversion.

Fig.12.Comparison between model prediction and experimental data for the lifetime at carbon conversion of 0.9 [77].

3.3.2.Particle group reaction characteristics

Based on comparing the coal char gasification kinetics with thermogravimetric analyzer(TGA)and high-temperature hot stage microscope (HTHSM),the gasification reaction characteristics of different coal char particles and petroleum coke particles were systematically studied [78],and the reaction characteristics of dilute and dense particles were compared (as shown in Fig.13).The mechanism reactions can be given as:

The results showed that the increase of the particle number in the group led to the decrease of the gasification agent concentration in the group,which reduced the overall gasification reaction rate and carbon conversion rate.In addition,the reaction rates of dilute phase and dense phase were different.For lignite and bituminous coal char particles with high reactivity,the effect of particle concentration on gasification reactivity was limited,and the effects of increasing particle temperature and decreasing gasification agent concentration on reactivity was opposite.However,for the anthracite coal char and petroleum coke particles with low reactivity,the reaction rate decreased significantly with the increase of particle concentration due to the slight decrease of particle temperature and the significant decrease of gasification agent concentration.

The influence of particle group on the combustion reaction characteristics of coal char and petroleum coke particles was also studied,as shown in Fig.14 [79],and the mechanism reactions are including

The results showed that the particle group had the similar effect on the combustion delay of bituminous coal particles with high reactivity and petroleum coke particles with low reactivity under the dense phase condition in the furnace.The increase of the particle group concentration prolonged the burnout time.With the increase of particle combustion temperature,the burnout time can be extended by 20%–80%.The results also showed that the combustion process of the external particles in the group can inhibit that of the internal particles.The combustion time of the bituminous coal char particles can be delayed up to 4 times,while that of the petroleum char particles with low reactivity was extended～3 times.Particle fluctuation phenomenon was observed in particle reaction[80],which was mainly attributed to the irregular microstructure on the particle surface and uneven active sites distribution.Additionally,it was found that the release rate of gaseous products generated in different micro-reaction zones was different,which led to the unbalanced force of particles and directly resulted in particle pulsation.

Fig.13.Reactivity index (R0.5) of particle groups of coal char and petroleum coke [78].

Fig.14.The delayed degree of the burnout time due to the effect of particle group[76]:(a) petroleum coke and (b) bituminous char [79].

3.3.3.Particle–wall reaction characteristics

Based on the numerical simulation study of the entrained-flow gasifier,it was found that 20%of the total coal particles in the furnace would adhere to the slag surface on the wall of the gasifier and react with the near-wall gas [81].Due to the hightemperature and high-pressure operating conditions,it was hard to study the particle reaction on the slag surface in gasifier by conventional instruments.Accordingly,the gasification/combustion reaction characteristics of coal char particles on the hightemperature molten slag surface were studied using HTHSM[77,82].The results showed that when the particles adhered to the molten slag surface,the contact surface between the particle and the gas reactant decreased but the gasification reaction rate and carbon conversion rate increased (～1 time higher than those of the conventional gasification of the coal char),as shown in Fig.15.

Fig.15.Relationship between carbon conversion and time due to effects of particle size [82].

Through heat transfer analysis,it is found that the ‘‘hot bath effect”from the molten slag provided heat for the particle gasification reaction,and thus the gasification reaction rate accelerated with the increase of particle temperature.In addition,the gasification characteristics of different coal char and petroleum coke particles on the molten slag surface were studied,and it was found in Fig.16 that the fragmentation phenomenon at the later reaction stage could lead to the significant increase of conversion,resulting in higher overall reaction rate.The particle fragmentation degree showed direct relationships with the evolution degree of gasification feedstocks:lignite>bituminous coal>anthracite coal>petroleum coke [83].The mechanism model is given as

3.3.4.Multiple-scale reaction phenomena

Fig.16.Gasification processes of lignite coal char,bituminous coal char,anthracite coal char and petroleum coke on the molten slag surface.LC:lignite coal char,BC:bituminous coal char,AC:anthracite coal char,and PC:petroleum coke [83].

Fig.17.Bubble formation phenomenon during the gasification process of coal char on the molten slag surface [84].

The high-temperature molten slag layer flows along the wall of the entrained-flow gasifier.Coal char particles adhere to the liquid slag surface and continue to react with gas near the wall [77,82].During the particle–wall reaction,gas–liquid–solid multiscale phenomena,such as particle sinking,slag encapsulation and particle breakage,also occur at this gas–liquid–solid interface [76,83].The bubble phenomenon was also found in the study on the wall reaction of coal char particles,as shown in Fig.17 [84].Bubbles were formed at the slag-char particle interface when the gasification reaction was proceeded to a certain extent.The bubble volume increased gradually during the reaction process,and the bubble number appeared randomly.With the increase of particle size,the formation time of bubbles gradually increased and the ratio of cumulative volume of bubbles to total gas products decreased.Therefore,more bubbles would be formed in the reaction process for smaller particle size.Based on the diffusion reaction theory of porous media,it is proposed that the formation mechanism of bubbles was that the carbon dioxide diffused into the particle through the coal char pores and reacted with the carbonaceous substance,and then the gaseous product (CO) diffused to the slag-char particle interface with the formation of bubbles floating on the liquid surface.That is,the formation mechanism of bubbles was proved to be related to the initial particle size of coal char.

3.4.Spectral of impinging flame and particle characteristics in gasifier

Based on the in-depth study of the structure,temperature field and flame stability of the impinging stream flame,ECUST gasification research team further revealed the flame spectral radiation characteristics,the particle behavior inside gasifier,the formation and microstructure evolution of ash particles,etc.

3.4.1.Spectral emission characteristics of flame

Based on the lab-scale experimental platform of OMBCWS gasification and the diffusion flame chemiluminescence detection platform (combined with different visual imaging systems),the spectral emission characteristics of diesel,CWS and methane flames are deeply explored.Impact flame can be divided into three zones:the jet zone in impacting plane,the impacting zone and the jet zone in vertical direction,and the vertical jet zone includes an upward stream and a downward stream.The OH*emission intensities at different equivalence ratios are all the maximum on the impacting plane,indicating that the impacting zone is the core reaction zone.The OH* intensity in the impacting zone of the two-burner flame is lower than half of the four-burner flame.The impacting effect of the four-burner flame led to the reuniting of fuel and oxygen in the impacting zone,and thus made the reaction more concentrated[85].The deduction method of background radiation of diesel and CWS flame in the CH* chemiluminescence is determined by the flame spectrum emission curve,as shown in Fig.18.According to the two-dimensional distribution of CH* chemiluminescence,it is found that in the impacting center of four-burner flame,the four jet flames inhibit each other,and the formation of the restricted flow field and the turbulence intensity change in the impact zone are the two main reasons for the generation of the new reaction center.In addition,the syngas concentration in the furnace can be evaluated by the peak intensity of CH*[86].

By exploring the relationship between the emission intensity and equivalence ratio,temperature and gas composition of the flame,it is found that the OH* and H* distributions can evaluate the area of the redox reaction zone,and the intensity ratio changes linearly with the equivalence ratio.Moreover,OH* chemiluminescence intensity can predict the change of CO2concentration and Na* and K* can reflect the high temperature area in gasifier [87].As the equivalence ratio increases,OH*/C2* changes relatively irregularly,CH*/C2* remains almost unchanged,and OH*/CH*showed an exponential relationship with the equivalence ratio[88].The intensities of OH* and CH* of turbulent flame is lower than those of laminar flame.When the oxygen concentration increases,the OH* intensity of decreases but the intensity of CH*gradually increases [89].The OH* distribution can be divided into two main chemical reaction zones(Zone I and Zone II).In the Zone I,the OH* are close to the methane side with high concentration but narrow distribution mainly concentrated at the burner outlet region.In the zone II,the OH*are located near the flame front with relatively wide distribution region as the same as the distribution of OH* obtained in the measured image,as shown in Fig.19.

The formation mechanism of OH* in the zone I and zone II are different.CH+O2=OH*+CO (R1) and H+O+M=OH*+M (R2) are the main paths for OH* formation in Zone 1 and Zone 2,respectively.The dilution level can lead to the change the OH*formation mechanisms.For undiluted flames,the reaction R1 plays a leading role.When the diluent content further increases,the free oxygen in the flame gradually decreases and the oxygen mainly exists in the form of molecule oxygen.Consequently,R2 becomes the dominant reaction of OH* formation [90].

3.4.2.Particle type inside furnace

Based on a large number of experimental data accumulated by industrial high-speed camera,image processing technology and temperature reconstruction method,the particle temperature distribution in different space regions and the particle reaction characteristics were studied,and the particle in the gasifier were classified.

Fig.18.Post-processing of CH* chemiluminescence image [86].

Fig.19.Flame images:(a) measured OH* emission distribution,(b) calculated OH* concentration distribution,(c) actual visible image [90].

The particles in gasifier are mainly classified into five types (as shown in Fig.20):high temperature particle without wake (HTP),high temperature particle with high temperature wake (HTPHTW),low temperature particle with high temperature wake(LTP-HTW),low temperature particle with low temperature wake(LTP-LTW) and low temperature particle without wake (LTP).LTP would be transformed into LTP-HTW when contacted with high temperature flame,then transformed into LTP as reactions terminate and the particles become non-reactive.LTP-LTW would be transformed into LTP-HTW when move to high temperature regions.As LTP-HTW adhere to the refractory wall,their wakes vanish and particles would be transformed into HTP.HTP-HTW would be finally transformed into HTP when reaction completed[91].

Particles reaction characteristics are greatly affected by temperature,gas composition and flow field at different zones,which are the underlying driving force of particle transformation.In particle dense area of gasifier,such as burner plane,temperature is greatly affected by particles size in the way of combustion/gasification reaction rate.Atomization process is directly related to the particle size distribution,thus the change of temperature distribution is basically caused by atomization.In particle sparse area,the temperature is mainly affected by hot gas flow.Particles reaction characteristics are greatly affected by temperature,gas composition and flow field at different zones.The reaction rate increases as particle entrance to high temperature area,while the atmosphere determines the reaction type from gasification to combustion as particle move to oxygen-rich areas,thus the underlying driving force of transformation from LTP-LTW to HTP is the temperature and gas phase composition.(1)When LTP particles with reactivity and small particle size drop from dome to impinging plane,they could transform to LTP-HTW in very short time as they contact with high temperature flame.As the combustion reactions terminate,wake would no longer be generated and the LTP-HTW particles would transform to LTP particles without reactivity.(2) LTPLTW particles are in unsteady state.Once contact with high temperature region or flame,they would transform to LTP-HTW and the combustion reaction starts due to the higher concentration of oxygen.The combustion proceeds so fast that the reaction of carbon in particles would terminate in very short time and particles get stable,thus the inverse transformation is hardly to be observed.(3)LTP-HTW particles are in relatively steady state and frequently contacting with refractory wall.Some of the LTP-HTW particles could not adhere to the refractory wall after impinging to it,and the particles rebound.The other LTP-HTW particles adhere to refractory wall after impingement and melt into slag.The temperature of particles rises to the temperature of refractory wall while the wakes would no longer generate and particles transform to HTP particles.(4)HTP-HTW particles with large particle size would not generate wakes when the reactions terminate,and the particles transform to HTP particles which disperse around burner plane.HTP-HTW particles with small particle size would captured by high temperature slag on wall when they move downwards along the axis of gasifier affected by flow field and the temperature would decrease as the reactions terminate.

Fig.20.Classification of different kinds of particles in gasifier [91]:(a) low temperature particle with low temperature wake,(b) low temperature particle with high temperature wake,(c) high temperature particle with high temperature wake,(d) high temperature particle without wake,(e) low temperature particle without wake.

3.4.3.Ash/slag particle behavior characteristics

High-temperature endoscopy combined with high-speed photography is applied to obtain images inside the gasifier,and image processing techniques are used to distinguish the object from the high brightness background.The results show that an ash/slag deposition layer is formed under lower operating temperature,as shown in Fig.21.

Fig.21.The formation process of low temperature ash/slag deposition layer [92].

HTP with medium size would slide on ash/slag layer after impacting,and low temperature ash/slag on the deposition layer adheres on the particles and detach from the deposition layer together.HTPs with large size would attract low temperature ash while travelling in gasifier space and ash/slag surface after impact,then form a LTP group with high temperature core and detach from the basic ash/slag layer.LTPs with large size and high space speed would impact and embed into the ash/slag deposition layer,then particle temperature rises up while particle volume decreases.The embedded particles hardly detach from the basic layer.The low temperature ash/slag deposition layer would detach from the old slag layer on refractory wall,and the detachment of ash/slag layer is mainly classified into three typical patterns according to the detach process:detach after sliding pattern,detach after tilting and shaking pattern,detach after warping and fracturing pattern[92].

The deposition process of the droplet and particle and the detachment of adhered particles behavior in gasifier are detailedly studied.The results show that the deposition behavior can be divided into five types:impact,deformation,breakage,rebound and adhesion.The behavior characteristics of droplet and particle deposition inside the gasifier,as well as the detaching after adhesion are statistically analyzed.The results show that the droplet deposition behaviors are divided into three types:deform and rebound without adherence,breakup and rebound without adherence and breakup with partial rebound and partial adherence,among which 14.1%–15.4%of the droplets adhere to the refractory wall.The particle deposition behaviors were divided into four types:rebound completely without breakup and adherence,adhere completely without breakup and rebound,breakup and rebound completely without adherence and breakup with partial rebound and partial adherence.The detachment behaviors were divided into four types:complete detach,breakup and detach,detach after impact and multi-mode detach [93].

3.4.4.Formation and microstructure evolution of ash particles

The morphology of ash particles with different particle sizes in the gasifier is quite different.The coarse particles are mainly hollow and flake particles,the fine particles are,the fine particles are mainly flocculent and spherical at the burner plane and the gasification chamber outlet,respectively.The formation mechanism of fine ash particles is different at different reaction stages.Particle reaction behavior is presented as particle breakup and mineral release at the early stage and late stage,respectively.The particles in the gasifier mainly include five types:hollow coal cell type,porous framework type,fragmented,flocculent and spherical particles.Different shapes of particles have different carbon content and surface element distribution.

The particle size distribution(PSD)at the burner plane shows a three-peak distribution,i.e.,a bimodal distribution at the gasification chamber outlet,and a unimodal distribution near the dome.From the burner plane to the outlet of the gasification chamber,the conversion rate gradually increases,along with the breakup of the coarse particles,the release of minerals,the formation of fine ash particles with spherical shape and the decrease of the average particle size.The O/C ratio has a greater influence on the particle size.Particle size decreases as O/C ratio increases.In addition,fine particle agglomeration occurred at the gasification chamber outlet and near the dome.Both specific surface area and pore volume of the particles in the gasifier increase by at least one order of magnitude than that of the raw coal.When the O/C ratio is 1.0,as the conversion rate increases from the burner plane to the of the gasification chamber outlet,the specific surface area and pore volume gradually increase But the pore diameter gradually decreases.As the O/C ratio increases,the specific surface area of the particles at the burner plane gradually increases,and the specific surface area of the particles at the gasification chamber outlet firstly increases and then decreases.

3.4.5.Thermal atomization characteristics of the coal-water slurry

An advanced visualization system combined with image postprocessing technology was applied to investigate the thermal atomization (including the primary and secondary atomization)of CWS in the gasifier,and to acquire the oscillation behavior characteristics of the atomization process.Based on statistical method,the atomization modes distribution,the effectiveness changes and the atomization stability versus operating conditions were investigated.Different from the atomization under cold conditions,the high temperature environment and high-speed oxygen jet in the gasifier led to that the bag breakup and ring-piercing breakup mode hardly exist in the gasifier,and the shear breakup mode is always accompanied with deflagration in the gasifier.Additionally,a special kind of secondary breakup mode exists in the gasifier,which was noted as synergistic atomization mode and only occurred when the droplets from different burners collide with another one,as shown in Fig.22.

When the O/C ratio increases,the average droplet concentration,average droplet size in each moment and their oscillation scopes decreases.In the three operating conditions (O/C ratio of 0.9,1.0,and 1.1),compared with the oscillation scope in 0–100%,the oscillation scope of droplet concentration in 10%–90%decreases to 22.2%,24.8%and 32.1%,respectively,and that of droplet size in 10%–90% decreases to 26.4%,33.6% and 31.8%,respectively.These results demonstrated that the increase of O/C ratio can improve the stability of the atomization process.

The oscillation can be divided into two aspects:oscillation of the atomization process and oscillation of the CWS flame.The small droplets generated by atomization could react with gasification agent quickly,and the large droplets generated by atomization continue to be atomized into small droplets containing only single or multiple coal particles in the posterior segment of the jet zone and impinging zone.Most of the particle trajectories in the jet zone show fan-shaped rays starting from the burner,and a small part of the particle trajectories shows that the space above the jet zone moves downward [94].

3.5.Comprehensive gasification model

Mixed model[18]and degradation model[95]have been developed successively for the complex gasification reaction process,and have played important roles in the simulation of optimization and engineering scale-up of gasifier.As mentioned above,the molten slag surface on the wall of gasifier is an important chemical reaction zone.Therefore,a comprehensive model of gasifier has been developed combined with the experimental study of slag interface reaction.

3.5.1.Molten slag surface reaction model

In the previous research,the simulation of entrained-flow gasifier mainly focuses on the multiphase flow reaction in spatial space,and molten slag flow and heat transfer in wall.The experimental study shows that the particles deposited on the molten slag surface will continue to react with the gasification agent in the gasifier.In addition,the endothermic reaction rate of char is promoted (twice than that of the particles in the spatial space) due to the heat transfer from the molten slag on the wall to the deposited char particles[79].We propose a molten slag surface reaction model for CFD modeling [81],as shown in Fig.23.

The reaction model of char deposited on molten slag surface can be expressed as below:

whereRjis the reaction rate of normal char particle in the spatial space andC0is the reaction promotion factor.The random pore model is employed to model the char gasification with CO2,H2O and O2.tw，pis the wall reaction time,which is expressed as the duration time when the particles deposited on theslag surface are covered by the slag,and can be modelled as:

Fig.22.The synergistic atomization process of multi-burners [94].

Fig.23.Molten slag surface reaction mode [81].

wheredpis the char particle diameter,mdr(kg·m-2·s-1) is the particle deposition rate on the gasifier wall,is the mean trapped particle density on the wall.The diameter and density of trapped particle are related to the reaction mechanism and conversion rate of particle[96].In the pyrolysis stage,the diameter of coal particles will be extended and can be expressed as:

wheredp，0is the diameter of raw coal particles from the nozzle.mp，0andmpare the mass of raw coal and particle in pyrolysis process,respectively.fw，0andfv，0are the moisture and volatile mass fraction of raw coal particle.In the stage of coal char combustion and gasification,the density and diameter of char can be determined according to CBK model.

3.5.2.Comprehensive gasification model

Fig.24.Illustration of the three regions in the CGM model [81,97].

Fig.25.Velocity distribution,temperature distribution and CO mole fraction distribution in SECWS gasifier.

Based on the molten slag surface reaction model,we proposed a more comprehensive gasification model (CGM) [97].In CGM,the multiphase turbulent reaction process in entrained-flow gasifier is divided into three regions:gas/particle flow and reaction region,particle deposition and wall reaction region,slag flow and heat transfer region,as shown in Fig.24(a).For the gas particle two phase flow and reaction process in the spatial space is based on the CFD code of Fluent 6.2.The time-averaged steady-state Navier–Stokes equations as well as the energy conservation,gas species transport equations are solved.The realizablek-ε model is adopted to model the turbulence.The DPM model is use to describe the particle motion.The two competing rates model and the eddy dissipative concept(EDC)are employed to model the coal devolatilization process and interaction between the turbulent flow and reaction.The reaction model of the char gasification in the spatial space [98],wall reaction model of the adhere particle[97],ash deposition model [99] and slag flow and heat transfer[81] are added to Fluentviauser defined functions.The iterative process of three regional submodels in the CFD code is shown in Fig.24(b).CFD method combined with CGM model is used to simulate the entrained-flow gasifier,and it is found that about 20% of the char in the entrained-flow gasifier is converted in the wall reaction zone,indicating that molten slag surface on the gasifier wall is an important reaction zone,and thus provides an important reference for the structural design and development of the gasifier.

Based on the discovery of the wall reaction zone,the structure optimization and design of the traditional top-mounted-singlenozzle-gasifier were carried out,and an high-efficiency SE CWS gasifier was developed.Fig.25 shows the velocity distribution,temperature distribution and CO mole fraction distribution in SE gasifier.It can be observed that there are three zones,i.e.,jet zone,reflux zone and plug flow zone,in the gasifier.With the increase of height-diameter-ratio of gasifier,the residence time of fly ash particles increases and the residual carbon content decreases.For the wall reaction region,the deposition amount per unit wall particle is reduced with the increase of height-diameter-ratio.Consequently,the wall reaction time of trapped particles is prolonged,the carbon conversion rate increased and the residual carbon content is reduced.When the height-diameter-ratio of straight section is greater than 4.3,the residual carbon content of slag is less than 5% [100].

4.Latest Progress of Industrial Application

Continuous fundamental research has supported the large-scale and wide application of OMB gasification technology,and industrial applications have in turn promoted the deepening of related fundamental research.Namely,identify the problems from engineering practices,explore the scientific issues behind the problem through fundamental research,develop the key technology by revealing the scientific issues,and solve the problems during industrial application eventually.This is the Research and Development principle that ECUST gasification research team has been practicing for more than 30 years,and also the important reason that the OMB coal gasification technology can continue to dominate the market among domestic and abroad gasification technologies.

By the end of 2020,the OMB gasification technology has been applied in 63 enterprises with 187 gasifiers under construction or operation around the world,covering many fields such as synthetic ammonia,methanol,ethylene glycol,and hydrogen production from refineries (as shown in Fig.26).Till now,OMB gasification technology still ranks the first place in Chinese market share.

Fig.26.Application of the OMB CWS gasification technology.

Fig.27.OMB gasification unit with single-gasifier capability of 4000 t?d–1.

In October 2019,the gasifier with the largest single-gasifier capacity in the world (4000 t?d–1) was put into operation in Rongxin Co.,Ltd,Inner Mongolia,China(as shown in Fig.27).In December 2020,China’s first plant of radiant syngas cooler (RSC)combined with quenching chamber OMB gasifier with capability of 2000 t?d–1was firstly started up in Yulin,Yankuang Co.,Ltd,which improved the system energy efficiency of large-scale coal gasification unit.

5.Prospects of Development

The overall development of China’s coal gasification technology has been analyzed and predicted in our previous study[101],some of which can be also adopted in OMB gasification technology:

(1) Improve the unit volume capacity of gasifier unit,and reduce the investment of device through the process intensification

The outside diameter of 4000 t?d–1OMB gasifier (Rongxin Co.,Ltd) has reached 4.0 m.The increase of the gasifier capacity through the volume enlargement would be inevitably restricted by the size of the equipment.Therefore,improving the unit volume capacity of the gasifier through process intensification is the fundamental way for the gasifier scale-up and the investment reduction.Can the reaction time of coal particles in gasifier be reduced to 1 s or even lower?We believe it is possible in theory.However,extremely challenging scientific and technical problems will exist for this idea,and it is necessary to carry out systematic innovation from the aspects of raw material preparation,transportation,gasifier structure,etc.,which is an important direction for the future research.

(2) Optimize the technological process,reduce the system material consumption,and improve the whole system efficiency

Due to the rapid development of coal chemical industry and technology demand in the past 20 years,the engineering companies pay insufficient efforts on the process optimization and system energy consumption and there are few changes in the overall process.Therefore,the technological process of coal gasification system should be improved and optimized according to the specific conditions of downstream including the water–gas shift,purification,synthesis and separation units.

(3)Increase CWS concentration,reduce the material and energy consumption,and improve the efficiency of the whole system

The water content in CWS is an important factor affecting the overall efficiency of gasification system.The increasing of CWS concentration can significantly reduce the specific oxygen consumption and specific coal consumption.Therefore,developing the preparation,transportation and atomization technology of high-concentration CWS is an effective way to improve the efficiency of CWS gasification system and reduce the consumption of gasification feedstock.

(4)Develop environment-friendly technologies to achieve nearzero emissions

In the process of coal gasification,hazardous elements in coal will be migrated and transformed into syngas,circulating water,waste water and ash/slag.There have been mature technologies for the migration,transformation and control of hazardous elements such as sulfur and nitrogen.The great progress has been also achieved on the mechanism study of chlorine and trace heavy metals migration.However,the specific technologies for chlorine and trace heavy metals migration should be further developed.

(5) Develop the simple ‘‘gasification island” to ‘‘gasification+heat+environmental protection island”

In essence,gasification is a high-temperature thermochemical conversion process,which determines that it can treat almost all solid waste carbonaceous and organic waste liquid besides coal.On the other hand,the steam production through sensible heat recovery of high-temperature syngas is an important measure to improve the system energy efficiency.Thus,it is important to change the simple ‘‘gasification island” into ‘‘gasification+heat+environmental protection island” for the Research and Development of coal gasification technology.

(6) Ensure the safe,stable,long-term,full-load and optimal operation of coal gasification unit based on big data and information technology

The plant failure rate could be significantly reduced and the plant operation rate could be significantly improved by relying on the technical convenience from the information technology revolution,changing the management and operation mode of plants and workshops,and realizing early warning and treatment of accidents through big data monitoring.In the future,it is important to establish the mechanism model,realize the dynamic optimization control of the whole system,improve the system efficiency and build the smart plant eventually based on the combination of big data and process principle.

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 was supported by the National Natural Science Foundation of China (21776086,21761132034).