Characteristics of reactivity and structures of palm kernel shell(PKS)biochar du

Guozhang Chang ,Ximin Yan ,Pengyu Qi,Mei An ,Xiude Hu ,Qingjie Guo ,*

1 State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering,College of Chemistry and Chemical Engineering,Ningxia University,Yinchuan 750021,China

2 Key Laboratory of Clean Chemical Processing of Shandong Province,College of Chemical Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

Keywords:Palm kernel shell Reactivity Pore structure Carbon ordering degree CO2/intermittent H2O gasification

ABSTRACT Palm kernel shell(PKS)biochars with different levels of carbon conversion were initially prepared using a tube furnace,after which the reactivity of each sample was assessed with a thermogravimetric analyzer under a CO2 atmosphere.The pore structure and carbon ordering of each biochar also examined,employing a surface area analyzer and a Raman spectroscopy.Thermogravimetric results showed that the gasification index R s of the PKS biochar decreased from 0.0305 min-1 at carbon conversion(x)=20%to 0.0278 min-1 at x=40%.The expansion of micropores was the dominant process during the pore structure evolution,ad mesopores with sizes ranging from 6 to 20,48 to 50 nm were primarily generated during gasification under a CO2/H2O mixture.The proportion of amorphous carbon in the PKS biochar decreased significantly as x increased,suggesting that the proportion of ordered carbon was increased during the CO2/H2O mixed gasification.A significantly reduced total reaction time was observed when employing a CO2/intermittent H2O process along with an 83.46%reduction in the steam feed,compared with the amount required using a CO2/H2O atmosphere.

1.Introduction

Oilpalm is an importantcrop thatis widely grown throughout South Asia.A mature oil palm tree is about 20-m tall and produces abundant bunches of fruit.Palm kernel shell(PKS)is a solid waste that is generated during the extraction of palm oil from this fruit,and from 7 to 10 t of PKS is produced per hectare of this crop[1].The oil palm was successfully introduced to China by the Chinese Academy of Tropical Agricultural Sciences in 2012,and it is predicted that PKS will become a significant solid waste in the future[2].

The utilization of PKS as a resource has become a research focus in recent years.Many studies have focused on the pyrolysis of PKS to generate hydrogen and bio-oil,as well as co-combustion and activation to obtain activated carbon[3-6],with pyrolysis being the most widely research technology.Yang et al.[7]examined the thermal decomposition of various palm wastes,including PKS,empty fruit bunches and palm mesocarp fiber.Ma et al.[8]reported that two mass loss peaks are observed during PKS pyrolysis,in contrast to results obtained with other biomass samples[9,10],due to the high lignin content.In general,the characteristics of PKS pyrolysis are significantly differentfrom those of other biomasses,and the pyrolysis of PKS also yields more bio-oils than are obtained from other lignocellulosic materials[11,12].

Currently,the goal of most research into PKS pyrolysis is to generate high quality bio-oils.Chan et al.[13]studied the liquefaction of PKS using both subcritical and supercritical water,and obtained a maximum bio-oil yield of 38.53 wt%at 390°C and 25 MPa.Jeong et al.[14]investigated the effect of activated carbon on the preparation of PKS bio-oils containing high levels of acetic acid and phenol,which agrees with the results from Chang et al.[15],while Oh et al.[16]improved the yield ofphenolusing a two-stage pyrolyzer.To date,otherstudies concerning the gasification of PKS have been focused on the production ofbiomassbased activated carbons.Lua et al.[17]obtained a PKS biochar with a Brunauer,Emmett and Teller(BET)specific surface area(SSA)of 519 m2·g-1following pyrolysis under optimum conditions consisting of 600 °C,a 10 °C·min-1temperature ramp and a hold time of 2 h.A PKS biochar with a higher BET SSA value of 762 m2·g-1was achieved by Rahman et al.[18]through steam gasification.Various additives,including ZnCl2and KOH,have also proven to be effective for the chemical activation of PKS biochar[19,20],and the formation of PKS biochar composed of hollow carbon nano fibers with tubular or bamboo-shaped structures has been studied[21].Biomass gasification is a promising technology,especially as an effective method for the utilization of distributed biomass energy,such as the PKS produced by palm oil processing plants.However,there have been very few studies regarding the utilization ofPKS in conjunction with gasification technology.In a previous project,we found that PKS biochar exhibited signi ficantly lower reactivity during CO2or H2O gasification than wheat straw or pine sawdust biochars[22].This slower gasification could lead to lower processing efficiencies,accompanied by the incremental agglomeration of the biochar with minerals in an actual gasifier.

In real-world gasifiers,steamis used to increase the reactivity ofbiochar with CO2,and so the biochar reacts simultaneously with CO2and H2O,and many studies have investigated the mechanisms and interactions of these two reactions[23,24].The textural properties of the biochar,including carbon structure and pore structure,have been found to greatly affect reactivity during gasification.The reaction with either CO2or H2Ohas been reported to produce a moderately porous structure[25],while the simultaneous use of CO2and H2O yields biochars with greater pore volumes at activation times in excess of 1 h[26].Briesemeister et al.[27]investigated variations in particle size,surface area and char density during biochar gasification using an air-blown entrained flow reactor in conjunction with a CO2/H2O atmosphere.They observed a reduced surface area following the application of longer residence times at1100-1300°C,which was attributed to ash fusion effects.Although there has been much research concerning the reactions of biochar with CO2and H2O[28],few studies have focused on the evolution of carbon and/or pore structures during the combined CO2/H2O gasification of biochar,even though such research could provide valuable information regarding the reactivity of biochar with a CO2/H2O atmosphere.

Based on the above,itwould be helpfulto examine the reactivity and pore structure of PKS biochar during CO2/H2O-based gasification,with the aim of increasing the efficiency of this process.Therefore,the key objective of the present study was to investigate changes in the pore structure and the degree of carbon ordering in PKS biochar during gasification in a CO2/H2O atmosphere,employing various carbon conversions.The reactivities of PKS biochars using different gasification conditions were also evaluated under a CO2atmosphere.Finally,the effectofthe intermittentintroduction ofsteam on the reactivity ofbiochar with CO2was studied at various CO2flow rates,based on analyses of reactivity and pore structure.The aim of the present study was to increase the reactivity of PKS biochar during gasification with CO2and to improve the gasification efficiency.

2.Experimental

2.1.Biomass

PKS obtained from a palm oilprocessing plant in Indonesia was used as the feedstock for this study.First,the PKS was air dried after washing treatment.The PKS was then ground by a mechanical grinder to obtain particles with sizes of less than 0.18 μm.The chosen particle sizes were to eliminate the diffusion effect during the former biochar gasification.The PKS samples were finally dried at 105°C for 6 h prior to the tests.The physical analysis of PKS is shown in Table 1.

2.2.Biochar preparation

PKS biochar was prepared in a bench-scale tube furnace reactor shown in Fig.1.This tube furnace reactor comprised a gas supply unit,an electric heating furnace with a quartz tube reactorand a bio-oiltreatment unit.In each trial,～5 g of PKS biomass was put into a porcelain crucible situated in one side of the quartz reactor,and then the reactor was heated from the ambient temperature to 850°C.N2(99.999%,200 ml·min-1)was used as the carrier gas to remove the air frominside of the reactor.As the temperature of the reactor reached and stabilized at 850°C,the porcelain crucible with PKS was immediately pushed into the center of the high temperature zone.After the pyrolysis lasting for 10 min,～1.2 g of PKS biochar was obtained.Then the N2was swift to CO2(100,200 ml·min-1),H2O(1.04,1.5 g·min-1),or the mixture of CO2(100 ml·min-1)/H2O(1.04 g·min-1).During the last stage,PKS biochars with different carbon conversions were obtained by controlling the reaction time,respectively.

Table 1 The results of physical properties of PKS

2.3.Gasification reactivity of PKS biochar

A thermogravimetric analyzer(Netzsch STA449 F3,Germany)was used to determine the reactivity of the PKS biochars during CO2/H2O mixture gasification.In each test,～10 mg of PKS biochar was loaded into an alumina crucible,and N2(99.999%,120 ml·min-1)was used as carrier gas for 30 min to ensure an inert atmosphere for the heating stage.Then the alumina crucible with biochar was heated from 25 to 850°C and held there for 5 min.As the temperature was stabilized at 850 °C,the N2was swift to the CO2(99.999%,120 ml·min-1).The reaction of biochar with CO2lasted for 1.5 h.Last,the N2was used again to protect the TG instrument.

The carbon conversion ratio(x)and gasification rate(R)were used to qualitatively assess the reactivity of PKS biochars obtained from Section 2.2.The gasification reactivity index(Rs)was adopted to quantitatively access the reactivity of PKS biochars.All the three indexes were well defined elsewhere in the literatures[29,30].

2.4.Pore structure

A surface analyzer(Quantachrome autosorb-iQ,USA)was employed to determine the evolution of the PKS biochars.The N2at 77 K was adopted as the adsorbed gas.The biochar was outgassed at 200°C in vacuum for 12 h before each N2adsorption-desorption experiment.The specialsurface area ofbiochars was calculated by the BETequation,the BETof micropores and external pores were determined by the t-plot method,and the pore size distribution was obtained by the QSDFT method.

2.5.Degree of carbon structure ordering

Scanning electron microscope(SEM)was carried out on a Hitachi S4800 SEM analyzer.The resolution was 1 nm and the magnification was from 2 to 800 thousands times[31].

2.6.Degree of carbon structure ordering

A Raman spectrometer(Horiba HR800-LS55,Japan)was used to assess the carbon structure of the PKS biochars.The aromatic structure of PKS biocharwastested using an excitation laserat532 nm.The PKS biochar was first mixed with spectroscopic-grade KBr,and then ground to limit thermal emissions.Raman spectra were recorded in the range of 800-2000 cm-1,and each biochar were determined fifth to analyze the heterogeneity of the PKS biochars.

3.Results and Discussion

3.1.The reactivities of PKS biochars under a CO2 atmosphere

The reaction of biochar with CO2exhibits lower reactivity than reactions with air or steam.For this reason,a CO2atmosphere was used for comparison purposesto assess the reactivity ofPKS biocharsduring gasification with differentagents.The results ofthe reaction ofPKSbiochars with differentcarbon conversion(x)values with CO2,H2O and H2O/CO2atmospheres are summarized in Figs.2-4,respectively,while the associated Rsvalues are provided in Table 2.

Fig.1.Experimental setup for PKS biochar preparation.

These data demonstrate that the CO2reactivity index of the biochar during the reaction with CO2first decreased and then increased,to 0.0877 min-1,at a flow rate of 100 ml·min-1.The lowest Rsvalue of 0.0342 min-1was observed at x=35%,indicating thatthis PKS biochar was the least reactive during the CO2gasification process.The Rsdata show a similar trend at a flow rate of 200 ml·min-1.These data appear to correlate with the evolution of the PKS biochar pore structure during CO2gasification.As reported in the literature,closed pores on the biochar surface are initially opened,followed by the formation of mesopores and macropores in the initial stage of CO2gasification[30,32].These mesopores and macropores slow the reaction of the biochar with CO2,such that the reactivity of the material drops to a minimum value before increasing as more micropores are generated at x above approximately 16%.

Fig.2.Conversion vs.time plots obtained from biochars gasified in CO2(a):100 ml·min-1 of CO2;(b):200 ml·min-1 of CO2.

Fig.3.Conversion vs.time plots obtained from biochars gasified in H2O(a):1.04 g·min-1 of H2O;(b):1.5 g·min-1 of H2O.

Fig.4.Conversion vs.time plots obtained from biochars gasified in CO2/H2O.

As shown in Fig.3 and Table 2,the CO2reactivity index of the PKS biochar during the reaction with H2O was gradually decreased from 0.0345 min-1at x=6%to 0.0171 min-1at x=46%,at a H2O flow rate of 1.04 g·min-1.These changes are closely related to the evolution of the pore structure in the biochar during H2O gasification.Just as during CO2gasification,the closed pores on the biochar surface formed during pyrolysis were opened in the initial stage[33].In addition,pore expansion dominated the changes in pore structures during the H2O gasification[30,34].As a result of these processes,the reactivity of the PKS biochar was gradually decreased as x increased during H2O gasification.

Combined Figs.2-3 and Table 2,the Rsof PKS biochar gasified with CO2was accordingly decreased as the increasing flow rate of CO2,while the Rsof that with H2O was increased as the increasing flow rate of H2O.It demonstrates that the increment of flow rate presented different effects on the distributions of pore structures on the surface of the PKS biochar.It is notable that the value of Rsof biochar gasified with CO2atthe rate of 100 ml·min-1was close to the value of Rsof biochar gasified with H2O at the rate of 1.04 g·min-1,indicating that the two agents generated the closer reactivity of PKS biochar.

Table 2 The reactivity index of PKS biochars obtained from different gasification conditions

Fig.5.Adsorption-desorption isotherms of PKS biochars during CO2/H2O gasification.

The data in Fig.4 and Table 2 demonstrate that the CO2reactivity index of PKS biochar during the reaction with CO2/H2O was decreased from 0.0305 min-1at x=22%to 0.0278 min-1at x=40%.Rather than equaling the sum of the individual values obtained from the reactions with CO2and H2O,these values are lower than those for the biochar reacted with CO2but higher than those for the samples reacted with H2O.This result suggests an inhibitive effect during the reactions of biochar with H2O and CO2in this study.The evolution of the pore structure in the biochar gasified under a CO2/H2O mixture was subsequently investigated to examine the reaction mechanism.

3.2.Evolution of pore structure during CO2/H2O gasification of PKS biochar

3.2.1.Evolution of surface area

The N2adsorption/desorption isotherms(at 77 K)for PKS biochars during CO2/H2O gasification were shown in Fig.5.According to the IUPAC standard classification system,the shapes of these isotherms took on a mixture of type I and IV isotherms,and type H4 hysteresis loops were present.It was observed that the pore structures of PKS biochars during CO2/H2O gasification were continuous distributed of micropores and mesopores.As the carbon conversion increased from 9%to 71%,the adsorption data especially at P/P0values of 0.01 to 1 increased gradually,indicating thatthe pore structures especially the mesopores were increasingly formed on the surface of PKS biochar during CO2/H2O gasification.It is suggested that more mesopores could be produced during the reaction of PKS biochar gasified with the CO2/H2O mixture.

The parameters used to assess the biochar pore structure are presented in Table 3.These data demonstrate that the BET SSA,BETmicropore area,and micropore volume in the PKS biochar reacted with a CO2/H2Omixture were allinitially increased butthen decreased.The BET SSA and micropore volume reached maximum values of 766.89 m2·g-1and 0.214 cm3·g-1,respectively,at x=60%,while the maximum BET micropore area was 431.72 m2·g-1at x=50%.The ratio of the external SSA to the BET SSA gradually increased,from 24.5%at x=9%to 49.5%at x=71%,demonstrating that numerous mesopores were formed during the CO2/H2O gasification.The total pore volume of the material also gradually increased along with x,with a maximum value of 0.748 cm3·g-1at x=71%.

Table 3 Typical parameters of pore structure of PKS biochars during CO2/H2O mixture gasification

Fig.6.Pore size distribution of PKS chars during CO2/H2O gasification.

Table 4 The QSDFT model coefficient of fitting error(%)for pore structure types of PKS biochars-CO2/H2O

It can be concluded that the closed pores on the biochar surfaces were initially opened during the reaction with the CO2/H2O mixture,and that the more polar H2O molecules were the first to react.Micropores and mesopores(see Fig.6)were formed during the reaction of the biochar with H2O,while this effect was not as prominent during the reaction with CO2,due to the low polarity and larger diameter of CO2compared with H2O[35,36].A greater concentration of micropores on the biochar surface promoted the reactions with both H2O and CO2.Thus,the BET SSA gradually increased.At x N 50%,the rate at which micropores were consumed by the CO2/H2O gasification exceeded the rate at which they were produced by CO2gasification.Thus,the pore expansion effect dominated the evolution of the pore structure.As a result,the BET micropore area slowly decreased,leading to a lower BET SSA value for the biochar during the CO2/H2O gasification.

3.2.2.Evolution of pore sizes distribution

The types of pore structure of PKS biochar during CO2/H2O mixture gasification was analyzed using the QSDFT model,and the coefficient of fitting errors for pore structure types were shown in Table 4.The coefficient of fitting errors were obtained as the lowest value when using the slit/cylindrical/spherical model,indicating that the pore structures of PKS biochar-CO2/H2O were composed of the mixture of slit pores,cylindrical pores,and spherical pores.

The pore size distributions of the PKS biochar-CO2/H2O were presented in Fig.6.It is evident that with the development of the gasification of PKS biochar with the CO2/H2O,the most probable pore size of the PKS biochar-CO2/H2O exhibited the tendency of micropores with sizes of 0.8 to 1.5 nm to mesopores with sizes of 5 to 20 nm.

Fig.7.SEM images for PKS biochar during CO2,H2O or CO2/H2O atmospheres.

As shown in Fig.6,the closed pores with sizes of0.8 to 1.5,2 to 30,and 48 to 50 nm were opened during the initial stage of gasification(carbon conversion x≤9%).As the x increased to 20%,the volume ofsuch pores increased obviously,and the sizes ranging from 48 to 50 nm were the most probable pore sizes for PKS biochar at this stage.As the x reached to 30%,abundant 0.7 nm micropores were formed,representing the most common pore size on the surface of PKS biochar;which is similar to the pore size distributions of biochar reacted with H2O near x of 30%[30].It is indicated that the reaction of PKS biochar-CO2was inhibited by the reaction of biochar-H2O to a certain extent.The pore structure of PKS biochar at x of 40%presented a mesoporous characteristic,which is the main reason that a lower CO2gasification reactivity was observed in Fig.4.The cylindrical mesopores with size of 5 nm favors the process of diffusion rather than the reaction with carbons.The mesopores with sizes ranging from 48 to 50 nm gradually dominated the pore structures of PKS biochars when the x exceeded 50%.

3.2.3.SEM images of the PKS biochars

SEMobservationswere employed to furtherexplore the evolution of pore structures in the PKS biochars under different atmospheres,and the resulting images are shown in Fig.7.The pore structures in the biochars were mainly within the nanometer and micrometer size ranges,and that similar variations in the pore structures occurred under all three atmospheres.Cylindrical and spherical pores dominated the surface pore structures during the early stage of gasification,with slit pores emerging during the later stage(defined by x N 40%),due to the pore expansion effect.These findings are in good agreement with the results in Fig.6.

3.3.Evolution of carbon structure ordering during CO2/H2O gasification of PKS biochar

The Raman spectra were employed to investigate the ordering of carbon structure for better understanding the reactivities of biochars during the CO2/H2O mixture gasification.Based on test observations and studies in the literatures[37], five bands corresponding to different carbon structures were used to analyze the spectra of the PKS biochar-CO2/H2O,D3 bond at～1500 cm-1representing the amorphous carbon contentand Gband at～1580 cm-1associating with the graphitic lattice contentwere selected to accountfor the carbon structure ofthe PKSbiochars.Fig.8 presented the typical Raman spectra and the obtained fitting results of the PKS biochars by the Gaussian method.

Fig.8 showed the most pronounced differences were obtained for the relative intensity(area ratio of D3 bond to G bond,AD3/AG)of the D3 bond,which was that one time higher for biochar at 20%than that at 40%.Meanwhile,the peak area of the G3 bond was decreased from 65,732.90 at 20%to 38,743.93 at 40%.All the data suggested that the proportion of amorphous carbon in biochar at the x of 20%was obviously higher than that at 40%.In other words,the relative content of orderly carbon was increased as x increased.It is an important reason that the reactivity of PKS biochar at x of 40%was lower than that at 20%,which was shown in Fig.4.

Fig.9.The TG results of the PKS biochars during CO2 gasification at different rates.

As the x of the PKS biochar increased from 20%to 40%,the peak area value ofGbond was increased from 16,531.42 to 19,323.12,accompanying with the value of full width at half-maximum(FWHM)of G bond was increased from 63.35 to 68.30.It is proved that the higher degree of carbon structure order was achieved at x of 40%,while its degree of graphitization was weaken.

3.4.CO2/intermittent H2O gasification technique

From the analysis above,the evolution of carbon structure is negative for the reactivity ofthe PKSbiochar,and the evolution ofpore structure could be positive for the reactivity of the PKS biochar during the CO2/H2O mixture gasification.Therefore,the CO2/intermittent H2O gasification technique was employed to improve the reactivity of the PKS biochar.In the present study,the CO2/intermittent H2O gasification tests were performed during the gasification in the chemical reaction regime and the diffusion reaction regime by a thermogravimetric analyzer.The flow rates ofCO2were selected as 50,120,200,240,300 ml·min-1in the TG test to initially differentiate the gasification conditions under the chemical reaction and diffusion reaction regime,respectively.The result of the pretest was shown in Fig.9.The evolutions of carbon conversion of PKS biochars under different conditions were presented in Fig.10,and the results of total reaction time t100%were listed in Table 5.

As shown in Fig.9,it is clear that the reaction of biochar-CO2was controlled by the diffusion regime when the flow rate was less than 200 ml·min-1.The external diffusion of the reaction of biochar-CO2could be ignored under CO2atmosphere at a rate of 200 ml·min-1.It is accepted that the internal diffusion could not be considered during gasification of biochar with a particle sizes of less than 178 μm[38].Therefore,it could be concluded that the reaction of biochar-CO2was controlled by the chemical regime.

Fig.8.The Raman spectra and its fitting results of PKS biochar-CO2/H2O.

Fig.10.The TG results of the PKS biochars during gasification under different conditions.

Table 5 The total reaction time of the PKS biochar during gasification under different conditions

The steamwas added into the reaction ofbiochar-CO2,only during the initial stage of 10 min in the process of biochar-CO2,which is named as CO2/intermittent H2O gasification.Fig.10 and Table 5 demonstrated that a significant lower t100%was obtained during the gasification of PKS biocharby the CO2/intermittentH2O technique,especially during the gasification in the diffusion regime,compared to those by CO2and CO2/H2O mixture technique.During the gasification in the diffusion regime,the t100%of 40.3 min was achieved by the CO2/intermittent H2O technique,only equals 60.69%of the t100%by the CO2/H2O mixed gasification.In addition,the addition ofH2Owas reduced by 4.75 g,equivalentto an 83.46%reduction.The shortening of the t100%could significantly alleviate the risk of slagging in a practical biomass gasifier.

4.Conclusions

The evolutions of pore structure and carbon ordering degree of PKS biochars during the CO2/H2O mixed gasification were investigated by a BET analyzer and a Raman spectrometer.The reactivity of the PKS biochar was evaluated by a thermogravimetric analyzer under CO2atmosphere.The Rsof the PKS biochar decreased as the increased carbon conversion,which was mainly attributed to the micropores expansion effect and the increment of carbon ordering degree during the CO2/H2O mixed gasification.The CO2/intermittent H2O gasification technique was employed to improve the reactivity of the PKS biochar,based on the analyses of reactivity and pore structure,carbon ordering degree.With the employment of this technology,the t100%and H2O feed were reduced by 39.31%and 83.46%,respectively,than using the CO2/H2O mixed technology.