Ming Liu,Zhongjie Shen,Qinfeng Liang,Jianliang Xu,Haifeng Liu,*

1 Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education,East China University of Science and Technology,Shanghai 200237,China

2 Shanghai Engineering Research Center of Coal Gasification,East China University of Science and Technology,Shanghai 200237,China

Keywords:Petcoke particle Reaction kinetics Pore growth Reaction mechanism High temperature gasification

ABSTRACT Particle concentration significantly affected the gasification of petcoke particles according to our previous studies.In this work,gasification characteristics and morphological evolution of single petcoke particle were investigated using a high temperature stage microscope experimental setup.The results showed that the reaction temperature significantly affected the reactivity of petcoke in the temperature range of 1200-1300 °C.While the promoting effect on gasification reactivity decreased with further increasing the reaction temperature,the SEM analysis demonstrated the pore development during the gasification process,which attributed to the increase of reaction rate with conversion.The Raman analysis,HRTEM and SEM-EDX analysis showed that the heterogeneous graphitization of petcoke and non-uniform distribution of catalytic elements in petcoke attributed to the development of surface pores with limited depth.The gasification mechanism of petcoke particle can be briefly described as the reaction rate mainly contributed from the fast-reaction area.Besides,the pore development in fast-reaction area also enlarged the surface area of petcoke particle.

1.Introduction

The increasing demands of petroleum and the deep processing of crude oil in worldwide supply leads to the large amounts of petroleum coke(petcoke).Particularly,the petcoke is characterized by its lower price,high calorific value and lower ash content(＜4%),which makes it a very promising raw material for chemical products,power generation,graphite electrodes for steel production and so on[1,2].However,the high sulfur content of some petcoke cannot be directly used for combustion.Besides,the direct combustion of petcoke also leads to high greenhouse gases emissions and the gasification technology is a good choice for producing raw chemical materials[3,4].Moreover,the gasification of petcoke decreased the emissions of sulfur and carbon dioxide.The entrained flow gasifier was operated with high temperature and pressure and showed a high efficiency and capacity.But the low porosity[5-7]and low alkali contents of petcoke lead to its low reactivity,which are technical problems for entrained flow gasification[8].

The gasification reactivity of char samples(coal char,petcoke char and biomass char)was related to the reaction temperature,gasifying agent pressure,physical structure characteristics and ash composition[9-15].Many studies have reported the factors affecting the CO2gasification rate and the morphological changes during the reaction process.Ren et al.[16]found that the catalytic effects and high temperatures was favorable to the CO2gasification of petcoke.The diffusion conditions also had effects on the reactivity of petcoke[5],and the reactivity was positively correlated with the total pore volume and the BET surface area.And the average pore sizes of petcoke showed an increasing trend[17].There were cracks in the surface of partially gasified petcoke and a small fraction of swollen,thin-shelled particles appeared during the pyrolysis process[18].The cracks in the surface of petcoke decreased the diffusion difficulty of gasifying agent.The particle size also affected the diffusion of gasifying agent,while pore diffusion effects were not observed until particle size＞250 μm because of the lower reactivity of petcoke[19].Besides,an increased development of microand macropores was observed as particle size increased[20].Meanwhile,Runstedtler et al.[21]found that the low pressure led to the decrease of conversion as a result of the poor flow dynamics.And it was the CO2partial pressure that significantly affected the reactivity instead of the total pressure[22].Studies showed that the addition of oxygen and a higher recirculation ratio led to higher conversions[23,24].

The gasification reactivity of chars can be evidently improved by the catalytic effects of alkali metal,alkaline earth metal and transition metal[25-29].Li et al.[30]found that the catalysts obviously promoted the steam gasification and the decreasing order of their effects was:alkali metal(K)＞alkaline earth metal(Ca)＞transition metal(Fe).The high content of K and Ca in biomass ash was in favor of the reactivity of petcoke[31].While the catalytic effect would be decreased when the potassium and calcium elements in biomass reacted with aluminosilicate minerals in petcoke[32,33].There existed a significant synergistic effect between calcium hydroxide and iron species during petcoke gasification[34].Besides,the catalysts also would affect the structure of petcoke and further influence its reactivity.While Ren et al.[7]found that the limited AAEMs(alkali and alkaline earth metals)lead to the low CO2gasification reactivity of petcoke.And the catalyst was less effective when loaded onto the char than onto the raw petcoke[35].The low ash content in petcoke limited the reactivity of gasification,thus many studies investigated the co-gasification of petcoke and char with high ash content[6,16,36,37].The weight and distribution of AAEMs in petcoke significantly affected the gasification kinetics.

The factors that affected the gasification kinetics of petcoke were widely studied in present works as mentioned above.It also was generally acknowledged that the diffusion difficulty increased with the reaction temperature(e.g.gasification reaction and pyrolysis process)[38,39].The gasification characteristics of petcoke were significantly affected by the amount of petcoke particles in crucible according to the previous study[40].However,gasification characteristics of single petcoke particle were scarcely studied in present studies.The diffusion resistance of gasifying agent in high temperature stage microscope experiments was greatly decreased compared with ordinary fixed bed reactors(e.g.TGA).Meanwhile,the diffusion hindrance was limited in entrained flow gasifier.And studies on the reaction kinetics of single particle were scare[41,42].Thus,gasification characteristics and kinetics of single petcoke particle were reevaluated in this study.The experimental data obtained from the high temperature stage microscope experiments was reliable[40,43-45].In this study,a high temperature stage microscope was applied to investigate the gasification characteristics of single petcoke particle at the temperature range of 1200-1400°C.The microstructure evolution and reaction kinetics of single petcoke particle at high temperatures also were analyzed.

2.Experimental Method

2.1.Samples

The petroleum coke was from Yangzi Petrochemical Company,China,and the sample was crushed and screened to the size between 80 and 150 μm.Meanwhile,the proximate analysis and ultimate analysis of the sample are carried out according to the Chinese standards of GB/T212-2008 and GB/T476-2008 and summarized in Table 1.It can be found that the ash content of the sample was only 2.11%,while the sulfur content was up to 2.28%.And the ultimate analysis showed that the carbon content was up to 90.85%,which was favorable for the application of gasification.The ash composition of petcoke is shown in Table 2,it can be found that the main composition of the ash are SiO2,V2O5and Al2O3,and the catalytic composition in the ash was limited.

2.2.In situ gasification of petcoke

In this study,a high temperature stage microscope(HTSM)was applied to investigate the gasification characteristics of petcoke at high temperatures.The schematic diagram of the HTSM system,which consists of a TS 1500 heating stage(Linkam,U.K.),DM 4500P microscope(Leica,Germany),is shown in Fig.1.And the experimental setup also contains a gas supply system,recycled cooling water systemand a computer.Detailed introduction of the experimental method can be referred in our previous study[44].In this study,the petcoke particles were spread on the sapphire slip in a ceramic crucible.The particles in the ceramic crucible were heated to 100 °C by copper heating element,and it was held for a minute in argon atmosphere to preheat the apparatus.The temperature then rose to reaction temperature at the rate of 100 °C·min-1and it was held for a minute at the reaction temperature to stabilize the temperature of samples.After a minute,the gas flow was switched from Ar to CO2at the flow rate of 0.4 L·min-1.And the microscope camera system began to record the reaction process at different temperatures.After that,the projecting area of petcoke particles during the gasification process was measured by ImageJ software[46].The evolution of particle size was obtained according to the projecting area analyzed by ImageJ software.

Table 1 Proximate and ultimate analysis of petcoke

Table 2 The ash composition of petcoke

2.3.Microstructural analysis of petcoke

The microstructure of raw petcoke and partially gasified petcoke was analyzed by a SU1510 scanning electron microscopy(Hitachi,Japan).And the elements on the surface of samples were analyzed by coupling with Energy Dispersive X-Ray Spectroscopy(EDX).The carbon structure characteristics of petcoke were analyzed by Raman spectrum(Renishaw in Via Reflex).The characteristics of ash distribution in petcoke were analyzed by JEM-2100 high-resolution transmission electron microscopy(HRTEM).

3.Results and Discussion

3.1.Gasification process of petcoke at different temperatures

The gasification process of petcoke particle at the temperature of 1400°C is shown in Fig.2.It can be found that the diameter of petcoke particles with irregular shape was about 100 μm.The distance between petcoke particles was about 400 μm,which was about 4 times higher than that of particle size.Compared with fixed bed reactor experiments(e.g.TGA),the inter-particle diffusion resistance could be significantly decreased.The diameter of particles was not evidently changed at the first 130 s.And then the petcoke particles dramatically shrank during the time of 130-220 s.It was inferred that the gasification rate was slower in the early reaction stage compared with that in the later stage.Besides,the change of particle size was less resulting from the initial large particle diameter.During the reaction process,the structure of petcoke changed at the time of 180 s as shown in Fig.2,and the edge of the particle showed a fragmentation marked in Fig.2.The fragmentation of petcoke particles increased the reaction area and the reaction rate thus was promoted.

Fig.1.Schematic diagram of high temperature stage microscope.

Fig.2.Photographs of in-situ gasification process of petcoke at 1400°C.

In this in-situ gasification experiments,the projecting area of petcoke particles during the gasification process was measured by ImageJ software.It was assumed that the petcoke particles were spherical.The pore volume of petcoke particles was not significantly changed according to relevant references[34].Thus,it was reasonable to suppose that the density of petcoke particles was a constant during the reaction process.The conversion x was defined as Eq.(1).

where A is the projection area of petcoke particles during the reaction process and A0is the initial projection area of petcoke particles.

The conversion of single petcoke particle at different temperatures is shown in Fig.3a-e.The diameter of single petcoke particle was in the range of 85-110 μm.It can be found that the conversion of different petcoke particles showed an agreement at the same reaction temperature.The complete reaction time of petcoke particles was about 1000,640,350,270 and 230 s at the reaction temperatures of 1200,1250,1300,1350 and 1400°C,respectively.The reaction time of small sized particles was slightly more than that of larger sized particles.It was because that the gasification reactivity of the petcoke is related to the particle size,volatile component,surface structure,and carbon structure and so on.The petcoke particles showed a nearly linear shrank with the reaction time at lower temperatures(1200-1250°C),while it showed a convex function curve at higher temperatures(1300-1400°C).It was noted that the difference of reaction time between the temperature of 1200°C and 1300°C was more evident than that between the temperature of 1300°C and 1400°C.It can be inferred that the reaction characteristics were different at different reaction temperatures.

Fig.3.The conversion of single petcoke particle at(a)1200°C,(b)1250°C,(c)1300°C,(d)1350°C and(e)1400°C and the average carbon conversion of petcoke.

To contrast the general gasification characteristics of petcoke particles at different reaction temperatures,the average conversion xawas calculated by Eq.(2).

And the average conversion of petcoke particles at different reaction temperatures is shown in Fig.3f.The reaction time of petcoke was about 17 times higher than that of bituminous char at the same reaction conditions[44].It can be found that the gasification reactivity of petcoke is extremely slow compared with bituminous char.Besides,it was noted that the gasification rate increased with the reaction temperature.Meanwhile,the reaction time decreased 700 s when the temperature increased from 1200 to 1300°C.While the reaction time decreased only 100 s when the reaction temperature increased 100 °C at the temperature of 1300°C.It was concluded that the differences of reaction time at different reaction temperatures decreased with the increase of temperature.

3.2.Microstructure evolution of partially gasified petcoke

The microstructure of partially gasified petcoke particles at the temperature of 1400°C is shown in Fig.4.The structure of petcoke is compact and there is no visible pores or cracks in the particles as shown in Fig.4a,which decided that the gasification reaction would mainly occur on the external surface of petcoke.Fig.4b shows the microstructure of petcoke particles at the conversion of 0.16.It can be found that there exist surface pores with limited depth that showed up on the surface of particles.Furthermore,the pore size increased with the conversion as shown in Fig.4c.The developed pores on the surface of particles increased the contacting area with gasifying agent.The reaction area of petcoke particles was increased compared with the original particle.Thus,the gasification rate of petcoke would be promoted.And the visible pore size in the surface of petcoke was measured at the conversion of 0.16 and 0.35 as shown in Table 3.It can be found that the pore size at the conversion of 0.16 was in the range of 0.88-1.04 μm,and the pore size increased to 1.6-2.25 μm at the conversion of 0.35.Moreover,different spots of petcoke particle were analyzed at the conversion of 0.64 and the results are shown in Table 4.The carbon content of different spots on partially gasified petcoke particle varied from 28.23%to 87.28%.It was found that the accumulated ash appeared on the surface of petcoke particles at the conversion of 0.64 as shown in Fig.4d.The catalytic elements in the ash promoted the gasification of carbon material,thus,the gasification rate would increase with the ash concentration on the surface of petcoke particles[47].

Fig.4.Micrographs of partially gasified petcoke particles at the reaction temperature of 1400°C.(The pores marked in red circles were measured as shown in Table 3 and the EDX analysis is shown in Table 4).

Table 3 The distribution of individual pore size in partially gasified petcoke particles at different conversions(measured pores were red marked in SEM micrograph Fig.4b-c)

Table 4 The EDX analysis of petcoke particle at the conversion of 0.64(measured spots were red marked in SEM micrograph Fig.4d)

Furthermore,the microstructure of partially gasified petcoke particles at the temperature of 1200°C is shown in Fig.5.It was found that the pore development at the temperature of 1200°C was slower than that at the temperature of 1400 °C.The developed pores in petcoke particle were evident at high conversions as shown in Fig.5.The difference of the reaction rate at different temperatures accounted for the different pore development,which also agree with relevant studies[48].Combing the experimental results at the reaction temperature of 1400°C,it was concluded that the porosity of petcoke increased with the conversion at high temperatures,and the development of porosity promoted the reaction in return.

Fig.5.Micrographs of partially gasified petcoke particles at the reaction temperature of 1200°C.

Fig.6.Raman spectrum analysis(a)(b)and Variations of band area ratios(c)(d)of different micro-area on petcoke particle.

Fig.6a shows the Raman spectrum analysis of petcoke particles,and it was found that the graphitization of petcoke was evident[49,50].The intensity was different for different micro-area on petcoke particle.The curve fits for Raman spectra of petcoke particle were shown in Fig.6b.The intensity of D1 band was higher than other D bands.The results reflected that the disordered carbon mainly existed as graphitic lattice(graphene layer edges,A1gsymmetry).Meanwhile,the graphitization degree differed in different micro regions.The polyenes and ionic impurities in petcoke were less as a result of the lower intensity of D4 band.Furthermore,the band area ratios ID1/IGand IG/IAllof different micro-area was analyzed as shown in Fig.6.The values of ID1/IGreflected the average planar size of the graphitic micro-crystallites.It can be found that the values of ID1/IGwere in the range of 2.5 to 3.6.The planar size of graphitic micro-crystallites differs in different micro-areas.The band ratio of IG/IAllrepresented the weight of organized carbon in samples.The values of IG/IAllwere in the range of 0.18 to 0.24,which demonstrated the different graphitization in different micro-areas of petcoke.The gasification reactivity of organized carbon and disordered carbon was significantly different.Thus,the different planar size of graphitic micro-crystallites and graphitization in different micro-areas attributed to the pore development of petcoke.

The mass of main elements(C,Si,V,S,K,Ca,Fe,Na,Mg and Al)at different micro-regions on the surface of petcoke was analyzed by SEM-EDX as shown in Fig.7.The alkali metal(K,Na),alkaline earth metal(Ca,Mg)and transition metal(Fe)have significant catalytic effects on the CO2gasification of petcoke[30,34].Therefore,the mass of K,Na,Ca,Mg and Fe on the surface of petcoke particles was listed as shown in Fig.7a-e.The results showed that the mass of the catalytic elements differed in different micro-regions.The mass of Mg was the least and Fe was the most on the surface of petcoke,which also was corresponding to the ash composition.Meanwhile,the total concentration of catalytic elements on the surface of petcoke also showed a heterogeneous distribution as shown in Fig.7f.The different distribution of catalytic compositions in petcoke differently promoted the gasification reactivity of micro-regions,which also resulted in the pore development.

Furthermore,the heterogeneous distribution of ash in petcoke was analyzed by HRTEM as shown in Fig.8.The accumulation of petcoke powder can be observed,enclosed in the square marked in blue color as shown in Fig.8.It was evident that the high gray area showed a nonuniform distribution.Meanwhile,uniformly distributed spots with high gray can be observed in the area enclosed in red square.The high gray area was a result from the higher atomic weight of ash compared with carbon in petcoke particles.This result also demonstrated the heterogeneous distribution of catalytic elements in petcoke[51].The graphitization of petcoke decided its low reactivity,while the different weight of the catalytic elements in various micro-regions differently promoted the gasification of carbon material.This result demonstrated that the non-uniform distribution of catalytic elements contributed to the pore development.

3.3.Gasification kinetics of petcoke

The reaction rate r was defined as Eq.(3):

Furthermore,the equation can be rewritten as:

The average reaction rate ra,which was assumed as a constant during the gasification process,was obtained after the integration of Eq.(4)as shown in Eq.(5):

Fig.7.Distribution of catalytic elements on the petcoke surface.(a)K,(b)Na,(c)Mg,(d)Ca,(e)Fe,(f)the sum of K,Na,Mg,Ca,and Fe.

where τ was the complete reaction time.

Fig.8.HRTEM analysis of petcoke powder.(Heterogeneous distribution of ash marked in red square).

Fig.9 shows the gasification rate and average reaction rate of petcoke particles at different reaction temperatures.It was evident that the reaction rate at different temperatures differed.Below the temperature of 1250 °C,the reaction rate slightly increased and then decreased during the gasification process.While the reaction rate of petcoke particles continuously increased with the reaction time when the temperature was above 1300°C.Meanwhile,there is a sharp increase in the reaction rate curve at the conversion of 0.3-0.4 when the reaction temperature was above 1300°C.The SEM analysis of partially gasified particles showed that the pores in petcoke particles were developed with the conversion,which account for the increase of reaction rate during the gasification process.The different pore development at diverse temperatures attributed to the difference of reaction rate as shown in Fig.9a.The average reaction rate of petcoke at different gasification temperatures is shown in Fig.9b.It can be found that the average reaction rate increased from 0.001 to 0.004 s-1when the gasification temperature increased from 1200 to 1400°C.

According to the Arrhenius's equation,the gasification rate of petcoke is given as Eq.(6),and the equation can be rewritten as Eq.(7).

Fig.9.The gasification rate of petcoke at different temperatures.

Fig.10.Arrhenius plots of gasification rate with temperature.

The activation energy was obtained when the conversion was chosen as a constant at different temperatures.The reaction rate in Eq.(7)was chosen as an average reaction rate.And the relationship between ln(ra)and 10000/T is shown in Fig.10.The activation energy was calculated according to Eq.(7).The activation energy of petcoke gasification was 106.7 kJ·mol-1.The gasification activation energy of petcoke was lower than that in literatures[7].The activation energy in literatures was in the range of 176.5-260 kJ·mol-1[5-7,21,22].The values of calculating activation energy were relevant to the adopted reaction model and the fitting quality of reaction model(shrinking core model or random pore model).The diffusion hindrance in sample bed of fixed bed reactors also was proved[40].The intrinsic reaction kinetics of petcoke obtained from fixed bed reactors thus should be reevaluated in previous studies.What's more,the reaction characteristics of petcoke at different reaction temperatures were different.Thus,the activation energy obtained in different temperature ranges should be different.The reaction rate in this study was evidently faster than that in literatures.It was proved that the activation energy of petcoke in this study should be lower than that in literatures.

3.4.Reaction mechanism analysis

The gasification rate of graphite was extremely slow and the mineral matter in ash promoted the reactivity.According to the Raman spectrum analysis and SEM-EDX analysis,the petcoke particle showed a high graphitization and heterogeneous distribution of catalytic elements.Thus,the gasification reaction area on petcoke particle surface can be divided into two parts:slow reaction region and fast reaction region.The total reaction area s consists of slow-reaction area ssand fast-reaction area sf,and the relationship between ssand sfwas shown in Eq.(8).And the total reaction rate was mainly contributed from the fast reaction.Thus,the total reaction rate r of petcoke particles can be written as Eq.(9).

Fig.11.Pore evolution of coal char and petcoke particles.

where rpwas the average reaction rate per unit area,rswas the reaction rate per unit area of slow reaction,rfwas the reaction rate per unit area of fast reaction,respectively.

For petcoke particles,the value of rfwas higher than that of rs.The gasification process of petcoke mainly occurred in fast reaction regions.Therefore,surface pores with limited depth on particle surface came into being as a result of the different reactivity in different microregions.Fig.11 showed the pore development of single petcoke particle.The pore size in petcoke particles increased with the conversion as shown in Fig.11.The fast reaction area sfthus also increased with conversion,which resulting to the increase of total reaction rate r.

The pore evolution of petcoke was divided into two steps:the generation of pores in petcoke was the first step.The second step was the enlargement of pore size occurring after a critical conversion xcas shown in Fig.11.The reaction time at the critical conversion tcand the complete reaction time τ was compared in this study.The ratios of tc/τ reflected the weight of the first step in the complete reaction process.Differences of xcand tc/τ between the reaction temperatures of 1200 and 1400°C are shown in Fig.12.In this study,the critical conversion of petcoke particles was about 0.33 and 0.16 at the reaction temperatures of 1200 and 1400°C,respectively.The ratio of critical conversion time and complete reaction time was 0.49 and 0.29,respectively.It can be found that the critical reaction time of the first step accounted for a high proportion and the conversion was lower at the critical reaction time.Meanwhile,the initial porosity of coal char was higher than that of petcoke.The pore evolution for coal char thus was mainly in the form of enlargement of pore size[52](Step 2).

4.Conclusions

Fig.12.Differences of xcand tc/τ between the reaction temperatures of 1200 and 1400°C.

In this study,the gasification characteristics,morphological evolution and reaction kinetics of single petcoke particle were investigated using high temperature stage microscope experimental setup.The experimental results showed that the reactivity of petcoke was significantly affected by the reaction temperature in the temperature range of 1200-1300°C.While the promoting effects were limited with further increasing the reaction temperature(1300-1400°C),the SEM analysis demonstrated the surface pore development during the gasification process,which result to the increase of reaction rate with conversion at high temperatures.The Raman analysis,HRTEM and SEM-EDX analysis demonstrated that the graphitization of petcoke and non-uniform distribution of catalytic elements in petcoke led to the development of surface pores in particles.And the development of pores in return increased the reaction surface area.The apparent activation energy obtained in this study was lower than that in literatures.The gasification rate of petcoke particle mainly contributed from the fast-reaction area.Besides,the pore development in fast-reaction area also enlarged the surface area of petcoke particle.The gasification mechanism of petcoke particles was analyzed in detail based on the microstructure analysis in this study.