Comparison of steam-gasification characteristics of coal char and petroleum coke char in drop tube furnace☆

Lu Ding,Zhijie Zhou*,Wei Huo,Guangsuo Yu*

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education,Shanghai Engineering Research Center of Coal Gasification,East China University of Science and Technology(ECUST),Shanghai 200237,China

Keywords:Drop tube furnace Steam/char ratio Thermodynamic equilibrium Reactivity

ABSTRACT The steam-gasification reaction characteristics of coal and petroleum coke(PC)were studied in the drop tube furnace(DTF).The effects of various factors such as types of carbonaceous material,gasification temperature(1100–1400°C)and mass ratio of steam to char(0.4:1,0.6:1 and 1:1 separately)on gasification gas or solid products were investigated.The results showed that for all carbonaceous materials studied,H2 content exhibited the largest part of gasification gaseous products and CH4 had the smallest part.For the two petroleum cokes,CO2 content was higher than CO,which was similar to Zun-yi char.When the steam/char ratio was constant,the carbon conversion of both Shen-fu and PC chars increased with increasing temperature.When the gasification temperature was constant,the carbon conversions of all char samples increased with increasing steam/char ratio.For all the steam/char ratios,compared to water gas shift reaction,char-H2O and char-CO2 reaction were further from the thermodynamic equilibrium due to a much lower char gasification rate than that of water gas shift reaction rate.Therefore,kinetic effects may play a more important role in a char gasification step than thermodynamic effects when the gasification reaction of char was held in DTF.The calculating method for the equilibrium shift in this study willbe a worth reference for analysis of the gaseous components in industrial gasifier.The reactivity of residual cokes decreased and the crystal layer(L002/d002)numbers of residual cokes increased with increasing gasification temperature.Therefore,L002/d002,the carbon crystallite structure parameter,can be used to evaluate the reactivity of residual cokes.

1.Introduction

Gasification reaction of coal and petroleum coke(PC)is important for fundamental research in gasification technology.Since the middle of the twentieth century,researchers have deeply and systematically studied on the characteristics of coal gasification reaction,and they have designed and built several methods to study coal gasification.A lot of valuable information has been obtained in experiments which provided an important direction of the industrialization of coal gasification.Moreover,coal,which is a main raw material of gasification,belongs to the non-renewable energy.Nevertheless,petroleum coke,as a carbonaceous material abundant in carbon,which comes from the delay-coke plant,is still going up in its production.The gasification of petroleum coke for the production of steam,power,and hydrogen has widely generated interests due to the broad application perspective[1].Therefore,it is necessary to study the structure and reaction characteristics of petroleum coke for full use in industrial gasification in the future.

The gasification reaction characteristics of carbonaceous material were widely studied with the usage of thermogravimetric analyzers(TGA),self-made fluidized bed reactors and fixed bed reactors[2–5].Zhan et al.[2]studied the effects of blending methods on the cogasification of petroleum coke and lignite from experiments performed at atmospheric pressure and 1000°C with a heating rate of 25°C·min-1by TGA.Kim et al.[4]used a horizontal fixed bed reactor in the char-CO2gasification experiments and studied the effects of coal type and particle size on char-CO2gasification via gas analysis.Their experiments were carried out at different reaction temperatures(1050–1400 °C)with a slow heating rate(20 °C·min-1).Tay et al.[5]performed the gasification of a Victorian brown coal at 800°C in a novel fluidised-bed/ fixed-bed reactor and the changes in char reactivity and structure were compared under two different gasifying agents(O2and CO2).

Most of the previous studies were carried out at a low temperature(≤1100°C)ora slow heating rate(≤100°C·s-1).With the development of entrained- flow gasification technology,a great many gasification reactions data obtained in the conditions of high temperature,rapid heating and short residence time are needed for the design and operation of gasifiers.The drop tube furnace(DTF),with characteristics of rapid heating and high gasification temperature,is close to industrial entrained-bed gasifier.

In previous research,DTF was often used to study carbonaceous material combustion or char-CO2gasification characteristics[6–10].Chen et al.[6]simulated the process of pulverized coal injection(PCI)in a blast furnace with the usage of DTF and investigated the characteristics of volatiles release,particle formation and reactivities of unburned char and soot of two different coals.Ahn et al.[7]and Kajitani et al.[8]carried out experiments on char-CO2gasification in DTF.The effect of pore diffusion on gasification reaction at high temperature was studied and the corresponding kinetic models were established.Milenkova et al.[9]studied the behavior of three fuel-grade petroleum cokes of different origination under pulverized fuel combustion conditions in DTF.However,to the best of our knowledge,there are rare reports about steam gasification characteristics of coal or petroleum coke in DTF[11].In the present,water-coal(coke)slurry gasification technology has been extensively applied.Therefore,it is necessary to study char-steam gasification reaction characteristics for process parameter optimization of water–coal(coke)slurry gasification unit.

The present work aims to study the characteristics of steam gasification reaction of coal char and PC char under high temperature(≥1100 °C)and rapid heating rate(103–104°C · s-1)in DTF.Effects of various factors such as types of carbonaceous material,gasification temperature and concentration of gasification agent(steam/char ratio)on gasification gas production,carbon conversion,structure and gasification reactivity of residual cokes were investigated.Meanwhile,the main reaction balances in DTF were calculated based on the concept of equilibrium temperature interval so as to provide a worth reference for the analysis of product gas composition from the industrial gasifier.

2.Experimental

2.1.Samples

Nei-meng(NM)coal,Shen-fu(SF)coal,Zun-yi(ZY)coal,Jin-ling(PC1)petroleum coke and Hui-zhou(PC2)petroleum coke were used for this study.The coals and petroleum cokes were crushed and sieved,and the size fraction of 80–120 μm was chosen as sample.The proximate and ultimate analyses and ash fusion temperatures of samples are summarized in Table 1.

2.2.Apparatus

Fig.1.Schematic diagram of drop tube furnace setup.

As shown in Fig.1,a drop tube furnace(0.054 m.i.d.×1.5 m high)was designed and built for the present study.In this work,coals or chars were fed into the upper surface of the isothermal zone of the furnace with a water-cooling nozzle.The particles flowed through a screw feeder from a hopper with a feeding capacity of0.3–0.4 g·min-1,and Ar was used as a carrier gas during the pyrolysis process.The stability of the feeding rates for different samples was measured in the preliminary testing.The feeding amount in 5 min of each sample was recorded for several tests.Standard deviation in population(STDEVP)was adopted to evaluate the stability of the feeding rates.As shown in Table 2,the values of STDEVP are small enough to ensure the feeding stability for different carbonaceous materials.When char-steam gasification experiments were carried out,N2was used as a carrier gas(also gas tracer),and water was pumped into a vaporizer by a double-type axial piston pump for steam generation.The steam was mixed with N2and heated to 220°C for injection into the reactor.During the char-steam gasification process,the char particles were carried by small gas flow rates(around 1 L·min-1)to ensure a laminar flow,and the residence time of particles was nearly 2 s in the main reaction zone.The furnace was heated by three compartments of silicon and molybdenum rod high temperature electric heating elements which could heat the reaction zone to 1400°C.Each heating compartment was controlled separately to maintain a constant temperature over the reactor length.The product gases and residues were quenched and separated in a char collector.The heating rate in DTF is 103–104°C·s-1,which is similar to that in the industrial coal gasifier.The temperature profiles in DTF were measured by a K-type thermocouple.Fig.2 shows that the isothermal zone is about 600 mm along the tube,and the maximum temperature difference between the external surface of reaction wall and gas in the internal isothermal zone of the tube is less than 20°C.Thus,the practical gas temperature in the main reaction zone of the alumina tube was approximately represented with the corresponding temperature of the external surface of reaction wall(1100–1400 °C).

Table 1 Proximate and ultimate analyses and ash fusion temperature of coals and petroleum coke samples

Table 2 Feeding rate of different samples

Fig.2.Temperature distribution profiles in DTF reactor.

2.3.High temperature steam gasification of coal and petroleum coke in DTF

Coal gasification reactions in a gasifier include two processes:coal pyrolysis and char gasification.Char gasification is usually the ratelimiting step during gasification of solid fuels.So before char-steam gasification experiments,the raw coal and petroleum cokes experienced rapid pyrolysis at 1200°C in DTF in order to remove most of volatile matter.The average diameters of coal char and PC char were in the range of 100–120 μm,and the characteristic analysis data of char samples are summarized in Table 3.The produced char then reacted with steam at different conditions which were mainly divided into thefollowing two parts.Gasification conditions in the first part were as follows:reactor temperature(1100–1400 °C),steam/char ratio(nearly 1:1),and samples(NM,SF,ZY,PC1 and PC2 char).Gasification conditions in the second part were as follows:reactor temperature(1100–1400°C),steam/char ratio(0.4:1,0.6:1 and 1:1,respectively),and samples(SF char,PC1 and PC2 char).The first part studied the characteristics of gasification products including gas products and solid residues of different carbonaceous materials.The second part investigated the effect of gasification agent concentration on the gasification products and the equilibrium movement of the main gasification reaction.

Table 3 Proximate and ultimate analysis of chars

2.4.Measurements of isothermal H2O gasification reactivity of residual cokes

Gasification reactivity experiments of residual cokes were carried out with NETZSCH TGA.Typical 8 mg residual coke was placed in an alumina pan and heated at 25 °C·min-1to the prescribed temperature(900 °C)under a continuous nitrogen flow of 80 ml·min-1.After thermal balance,the gas atmosphere was switched to steam(80 ml·min-1).The reactivity index[12,13]of residual cokes was taken as the quantitative index system of gasification reactivity.

where R0.5is the reactivity index of residual coke(min-1),and t0.5is the time required to reach a fixed carbon conversion of 50%(by mass)(min).

2.5.Property tests of samples

Pore structure parameters of samples were measured with ASAP-2020 physisorption apparatus by the method of N2/CO2gas adsorption.N2gas adsorption was operated at-196°C,and it can measure the structure of pore range from 2 nm to 200 nm.CO2gas adsorption was operated at 0°C,and it could measure the structure of micropore range from 0 nm to 2 nm.The calculation model of CO2gas adsorption was DFT model,and that of N2gas adsorption was BET/BJH model.

Carbon crystallite structures of samples were measured by X-ray diffraction(XRD)spectrometer with monochromatic Cu Kαradiation(40 kV,100 mA)and a step size of 0.1°over the angular 2θ range 10–80°and a scan rate of 2(°)·min-1.The techniques described in the literature[14]were used.The spectrum was corrected for polarization,and then converted to reduced intensity.Microcrystalline structure parameters were calculated through Eqs.(2)and(3).where d002is the spacing between graphitic sheets,λ is the wavelength of the incident X-rays(nm),θ002is the peak position,B002is the width of(002)peak at half-maximum intensity and L002is the average stacking height.

Gas products(H2,CO,CH4,CO2and so on)were determined with analytical equipments including Agilent 7890A gas chromatograph and on-line FT-IR.

2.6.Calculation of gas analysis and carbon conversion in DTF

Exhausted gas compositions including H2,CO,CH4,CO2and N2were measured to determine the global gasification conversion.Based on the volumetric percentages of each species from the gas analyzer,the yields of H2,CO,CH4and CO2were calculated as gas generation(STP)per unit mass of char(g).In calculation,nitrogen was assumed as a tracer gas,that is,the total volume of nitrogen contained in exhaust gas was assumed to be the same as the total volume of nitrogen supplied to the reactor.As shown in Table 1,due to all samples containing nitrogen less than 1.5%,it is reasonable to use nitrogen as the tracer gas.Based on these assumptions,the product gas yield was defined by literature[15]

where niis the product gas yield of component i(mol·g-1),QN2is the flow of N2(L·min-1),VN2is the volume percent of N2in exit gas(%),Viis the volume percent of component i in exit gas(%),m is the feed rate of char(g·min-1)and Vmis the molar volume of gas(L·mol-1).

Carbon conversion was calculated as follows:

where nCO,nCO2and nCH4denote the amount of gas productions of CO,CO2and CH4respectively(mol·g-1),MCis the molar mass of carbon,12 g·mol-1and xCis the content of carbon in the char(%).

3.Results and Discussion

3.1.Gas analysis of char-steam gasification at different temperature in DTF

Fig.3 shows the gas products of char-steam gasification at different temperature.H2exhibits the largest part of gasification gas products and CH4shows the least part.However,CO content is always higher than H2in syn gas in the industrial gasification,which is different from the results obtained in this research.The reason might be that the energy needed for gasification reaction in this study was produced by electric heating elements and steam was used as the gasification agent.These conditions were different from the heating method and the components of gasification agents in the industrial gasifier.What is more,the pyrolysis char was used as the gasification carbonaceous materials instead of raw coal in order to facilitate the pyrolysis and gasification process research separately.These factors above led to CO and H2contents obtained in DTF varying largely from those done in the industrial gasifier.

Three reactions of char-steam gasification mainly take place in DTF:

Eq.(6)is the main reaction of char-steam gasification.Eq.(7)is the water gas shift reaction and Eq.(8)is the basic reaction of char-CO2gasification.

Note that CO content was higher than CO2in the gas products of NM and SF char.However,for ZY anthracite char,CO2content was higher than CO.Some researchers explored the mechanism of char-CO2or char-steam gasification[16,17].Dutta et al.[16]proposed that the char-CO2gasification reaction took place only on the surface of the pores which were greater than 1.5 nm.Koba et al.[17]reported that steam had a higher reactivity with coal char than CO2because the smaller size of steam may be favorable for the diffusion in the micropores of the semicoke.Therefore,compared to CO2,steam could react with char samples on the surface of smaller micropore.In this study,the micropore specific surface area(112.6 m2·g-1)of ZYchar was notably higher than mesopore specific surface area(5.17 m2·g-1).This indicates that reaction surface for char-H2O was larger than that for char-CO2.Thus,for ZY char,Eq.(6)mainly processed in DTF,the extent of Eq.(8)was low.Table 4 presents the gas product quantitative data of ZY and PC char-steam gasification at the steam/char ratio of 1:1 and 1400°C.It should be noted that carbon conversion of ZY char is about 10.7%.There was a small amount of fixed carbon participating in gasification reaction due to the low activity of anthracite.CO was continuously consumed to produce CO2under the condition of excess steam,leading to CO2content higher than CO for ZY char gasification.Similarly,the two petroleum cokes also exhibited a low carbon conversion in the gasification process.Their specific surface area of micropore was richer than that of mesopore and CO2contents were higher than CO.

3.2.Effect of steam/char ratio on gasification gaseous products

Fig.4 shows the output of gaseous products of SF char at the steam/char of 0.4:1,0.6:1 and 1:1,respectively.It demonstrates that CO and H2contents generally increase with increasing gasification temperature.When temperature increases from 1100 to 1200°C,the yields of CO and H2increase rapidly,but the growth rate gradually slows down with further increasing temperature.Xu et al.[18]found that ash fusion phenomenon in char could change the pore structure of char,which might lead to gasification reactivity of char decreasing.The SEM graphs(Fig.5)of SF gasification residual cokes obtained at different temperature show that the melting ball shape of minerals can be observed on the surface of gasification residual cokes obtained at 1300 and 1400°C.As shown in Table 1,the ash fusion point of SF coal is 1175°C.Therefore,the partial melting of ash might block part of the pore,and then hindered the gasification reaction when the gasification temperature reached 1200°C.CO2content generally decreases with increasing gasification temperature and is slightly less than CO at 1100°C,but far below that when the gasification temperature came to 1400°C.

Fig.6 shows that the variation trend of gasification gaseous production of PC2 char is similar to that of PC1 char.The yields of H2,CO and CO2increase gradually with increasing reaction temperature,while CH4yield exhibits an opposite trend.CO2yield is higher than CO.Compared to SF char,the gaseous yields of PC1 and PC2 char are obviously lower.

3.3.Carbon conversion

The effects of gasification temperature and steam/char ratio on carbon conversion of three char samples are shown in Fig.7.Due to the poor activity of petroleum coke,the whole conversion of PC1 and PC2 chars is lower than that of SF char at the steam/char ratio of 0.4:1,0.6:1 and 1:1,respectively.The carbon conversion of PC2 char is even less than 1%at the steam/char ratio of 0.4:1 and 1100°C,which means that char-steam gasification reaction barely occurs at this condition.When keeping the steam/char ratio constant,gasification reaction rate accelerates and conversion continuously increases with increasing gasification temperature.When the gasification temperature is constant,carbon conversion of all char samples could be improved with increasing steam/char ratio.However,Lee et al.[19]considered that steam did not have significant effect on carbon conversion but had obvious effect on the composition of the product gas.These can be attributed that in Lee's research,coal was directly fed into the reactor without experiencing rapid pyrolysis,which was different from char gasification in this study.

Fig.3.Gas products of char-steam gasification at different temperatures.

Table 4 Gas products of ZY char and PC char gasified at 1400°C at the steam/char ratio of 1:1

3.4.Equilibrium temperature interval

Fig.4.Yields of gas products of SF char gasified at different steam/char ratios.

Fig.5.SEM photographs of SF gasification residual cokes obtained at different temperature.a 1100 °C;b 1200 °C;c 1300 °C;d 1400 °C.

Fig.6.Yields of gas products of PC1 and PC2 char gasified at different steam/char ratios.

Gasification of carbonaceous materials in DTF includes two reaction types,namely the heterogeneous gas–solid reaction[Eqs.(6)and(8)],and the gas phase homogeneous reaction[Eq.(7)].These three reactions can be affected by the chemical equilibrium.There is always a gap between the practical composition of the product gas obtained at the reactor outlet and that of the product gas which is in equilibrium for the corresponding gasification temperature.The concept of equilibrium temperature interval was adopted to describe the degree of three reactions and variation of the product gas composition.Assuming that reactions have reached equilibrium,equilibrium temperature interval quantified the gap between the calculated equilibrium temperature and the practical gasification temperature in DTF.That is

where ΔT is the equilibrium temperature interval(°C),Tpis the equilibrium temperature corresponding to the product gas obtained at the reactor outlet(°C)and TRis the practical gasification temperature(°C).

In this work,the gasification experiments were operated at ambient pressure,so the reaction equilibrium constant satisfied the following formula[20]:

where νBis the stoichiometric coefficient of gas composition,PB,eis the partial pressure of gas composition,is the standard equilibrium constant,Kfis the equilibrium constant denoted by fugacity and p?is the standard atmospheric pressure.

When reaction temperature changed in a small range,the linear interpolation method could be adopted to calculate the equilibrium temperature corresponding to the equilibrium constant[20].

Fig.7.Effect of steam/char ratio on carbon conversion of char gasification.

For the endothermic reaction,ΔT＜0 means that reaction has not yet reached the thermodynamic equilibrium and it goes on toward the forward direction,ΔT＞0 means that reaction goes on toward the reverse direction and ΔT=0 means that reaction has already reached the thermodynamic equilibrium.For the exothermic reaction,the tendency of reaction direction with ΔT is reverse to the endothermic reaction.

Eqs.(6)and(8)belong to the endothermic reaction,and Eq.(7)to the exothermic reaction.It is known that the rate of Eq.(7)is much faster than that of the char-gas reaction[Eqs.(6)and(8)].As there are only a few seconds of residence time for char-gas reaction in DTF,kinetics may play a more important role in a char gasification step than thermodynamics.It is reasonable to speculate that compared to Eq.(7),Eqs.(6)and(8)might be further from the thermodynamic equilibrium due to a much lower char gasification rate than the water gas shift reaction rate.As observed in Table 5,for SF char,the equilibrium temperature intervals of Eqs.(6)and(8)are all between-750°C and-450°C,which means that these two reactions are indeed far from the thermodynamic equilibrium and they go on toward the forward direction in DTF.Table 5 shows that the absolute values of equilibrium temperature intervals of Eqs.(6)and(8)gradually increase with increasing gasification temperature at different steam/char ratios.That means Eqs.(6)and(8)are further from reaching the thermodynamic equilibrium with the rise of gasification temperature.For Eq.(7),the absolute values of equilibrium temperature intervals at different gasification temperature are all within 100°C at the steam/char ratio of 1:1,which implies that Eq.(7)is close to the thermodynamic equilibrium in these conditions.

Table 5 Equilibrium temperature interval of SF char gasification at the steam/char ratio of 0.4:1,0.6:1 and 1:1

Eqs.(6)and(8)ofPC1 and PC2 chars are far from the thermodynamic equilibrium and go on toward the forward direction,and these trends are also observed for SF char.For PC1 char,equilibrium temperature intervals of Eqs.(6)and(8)are all between-910°C and-580 °C,and for PC2 char between-960 °C and-670 °C,respectively.Tables 6 and 7 show that the absolute values of equilibrium temperature intervals of Eqs.(6)and(8)have a generally increasing trend with the rise of gasification temperature at different steam/char ratios for these two chars.As shown in Table 7,for Eq.(7),the absolute values of equilibrium temperature intervals of PC2 char are all higher than 2000°C at different steam/char ratios with the gasification temperature of 1100°C,which proves that the char-steam gasification reaction barely occurs in these conditions for PC2 char.

Table 6 Equilibrium temperature interval of PC1 char gasification at the steam/char ratio of 0.4:1,0.6:1 and 1:1

Table 7 Equilibrium temperature interval of PC2 char gasification at the steam/char ratio of 0.4:1,0.6:1 and 1:1

3.5.Structure and activity analysis of steam-gasification residual cokes

In this study,N2and CO2were used to measure pore structure parameters of gasification residual cokes obtained at the stream/char ratio of 1:1.As shown in Table 8,specific surface area of micropore is close to that of mesopore for NM gasification residual cokes,bothof which gradually decrease with increasing gasification temperature.For SF,ZY,PC1 and PC2 chars,specific surface area of micropore is markedly richer than that of mesopore.The specific surface area and volume of micropore for the four char samples also gradually decrease with increasing gasification temperature,while mesopore structure parameters do not show the same trend.The results indicate that the consumption of micropore specific surface area increases with the rise of gasification temperature and carbon conversion for both coal char and PC char.Table 7 also shows that PC char has a much less specific surface area of micropore or mesopore compared to coal char,which indicates that petroleum coke may have a low reaction activity due to its poor developed pore structure.

Table 8 Pore structure parameters of samples measured by CO2 and N2 gas adsorption analysis separately

Fig.8 is X-ray diffraction spectra of residual cokes gasified with steam.Compared to SF gasification residual cokes,ZY,PC1 and PC2 gasification residual cokes show relatively narrower diffraction peaks.It should be noted that the(002)peaks of PC2 gasification residual cokes are smooth and sharp,which exhibit high graphitization.In order to quantitatively analyze the graphitization degree of these samples,numbers of crystal layers(L002/d002)were calculated.Table 9 indicates that for each char sample,values of L002/d002and L002of gasification residual cokes increase with increasing gasification temperature.

Gasification reactivity experiments of residual cokes obtained at the steam/char ratio of 1:1 were measured in TGA at 900°C in steam atmosphere.Table 10 shows that for each char sample,gasification reactivity of residual cokes gradually decreases with the rise of gasification temperature.It should be noted that gasification reactivities of all residual cokes show the reverse variation trend to crystal layer(L002/d002)numbers of residual cokes with increasing gasification temperature.Therefore,Carbon crystallite structure parameter of L002/d002can be used to evaluate the gasification reactivity of residual cokes.

4.Conclusions

The steam-gasification reaction characteristics of coal and petroleum coke have been studied in the temperature range from 1100 to 1400°C at the stream/char ratio of 0.4:1,0.6:1 and 1:1 respectively.

(1)Gas products releasing characteristics of char-steam gasification reaction in DTF vary with types of carbonaceous materials.The carbon conversion of both SF char and PC char shows an increasing tendency as the gasification temperature increases.In this study,when the gasification temperature is constant,carbon conversion of all char samples could be improved with increasing steam/char ratio.Due to the low activity of PC char,the total conversions of PC1 and PC2 chars are much lower than that of SF char,which might indicate that the method of co-gasification of petroleum coke and coal could be adopted in order to make full use of petroleum coke for industrial gasification in the future.The co-gasification characteristic of these two carbonaceous materials in DTF for the following work will be studied.

(2)Compared to Eq.(7),Eqs.(6)and(8)are further from the thermodynamic equilibrium due to a much lower char gasification rate than the water gas shift reaction rate.Therefore,kinetic effects may play a more important role in a char gasification step than thermodynamic effects in DTF.Char-H2O and char-CO2reactions are further from the thermodynamic equilibrium with the rise of gasification temperature,and they go on toward the forward direction for both coal char and PC char.While the equilibrium shift of the water gas shift reaction is sensitive to the influencial factors of gasification reaction including reactivity of carbonaceous materials,steam/char ratio,gasification temperature etc.,and further research are needed to explore these phenomena.The method for calculating the equilibrium shift in this study may be a worth reference for the analysis of product gas composition from the industrial gasifier.

Fig.8.XRD spectra of residual cokes gasified with steam at the stream/char ratio of 1:1.

Table 9 Crystallite parameters of residual coke samples gasified with steam

(3)The order of gasification reactivity of residual cokes obtained in DTF is 1100°C＞1200°C＞1300°C＞1400°C,which shows the reverse variation trend to crystal layer(L002/d002)numbersof residual cokes with increasing gasification temperature.Therefore,the carbon crystallite structure parameter of L002/d002can be used to evaluate gasification reactivity of residual cokes.

Table 10 Reaction index(R0.5)of gasification residual coke obtained at different temperatures