The role and potential of attapulgite in catalytic pyrolysis of refinery waste activated sludge

Qinghong Wng ,Knghong Ho ,Chelse Benlly ,Yue Kou ,Zhexun An ,Mohmed Gml El-Din ,Chunmo Chen ,*

a State Key Laboratory of Heavy Oil Processing,Beijing Key Laboratory of Oil and Gas Pollution Control,China University of Petroleum-Beijing,Beijing,102249,China

b Department of Civil and Environmental Engineering,University of Alberta,Edmonton,Alberta,T6G 1H9,Canada

Keywords:Catalytic pyrolysis Attapulgite Kinetics Product characteristics

1.Introduction

A great amount of refinery waste activated sludge (rWAS) is generated from biological treatment processes in refinery wastewater treatment plants(WWTPs).rWAS is mainly biomass but also contain high contents of organic contaminants,heavy metals and inorganic impurities.Conventional methods of disposal for rWAS usually includes landfill disposal,composting,anaerobic digestion,and incineration (Liu et al.,2009;Yang et al.,2015).Landfills are a final disposal method for many hazardous wastes,however this method is not preferred due to the associated environmental risks that can result from the leakage of hazardous chemicals into the vadose zone and the underlying aquifer (Mishra et al.,2017).Composting can convert the nitrogen,phosphorus,and organic matters in biological sludge to nutrients and humus for agricultural uses (Raheem et al.,2018).However,the high content of heavy metals and oils in rWAS hinders the use of composting.Anaerobic digestion is effective in the recovery of value-added energies (e.g.CH4and H2) from biological sludge.However,the poisonous and harmful substances in rWAS often inhibit digestion efficiency and reduce the production of energies (Nges and Liu,2010).Lastly,incineration can completely decompose the rWAS.The energy from organic matters can only partially supplement the required heat for incineration.As a result,secondary treatment for hazardous residual ash and flue gas are often needed post incineration (Gong et al.,2018).

Pyrolysis is a thermal-chemical process that can convert organic matter in sludge into gaseous,liquid and solid products in an inert environment.Pyrolysis exhibits great potential in rWAS treatment with the advantages of sludge reduction,production of valueadded energies,as well as immobilization of hazardous chemicals(Fonts et al.,2012).Pyrolysis conditions and properties of sludge are related to the pyrolysis mechanisms and characteristics of products.During municipal sludge pyrolysis,an increase of termination temperature can dramatically reduce the residue yield while significantly increasing the total yields of bio-oil and gases (Zhou et al.,2019).At 850°C,the highest conversion for municipal sludge was obtained and 74%of energy was recovered by pyrolytic syngas and tar(Karaca et al.,2018).Furthermore,high termination temperature may decrease the ratio of bio-oil to syngas,and promote the dehydrogenation and polymerization reactions of shortchain hydrocarbons,enhancing the generation of H2,aromatic and polyaromatic compounds(Lin et al.,2019).Thus,the H2content in pyrolytic gases and aromatics in bio-oil both increase.The high temperature also promotes the gasification of residue and decomposition of primary vapor,increasing the production of pyrolytic gases as well as the content of H2,CO and CH4(Shen and Yoshikawa,2013;Han et al.,2015).

Introduction of catalysts into the sludge pyrolysis can greatly accelerate the reactions as well as improve yields and qualities of pyrolysis products.A variety of catalysts,such as metallic oxides,natural minerals,zeolites and even solid wastes,have been used in sludge pyrolysis (Shao et al.,2010;Zhao et al.,2013).γ-Al2O3may promote the aromatization reaction and then facilitate the formation of aromatic and polycyclic compounds.γ-Al2O3also assists the deoxygenation reaction of fatty acids,upgrading the quality of the organic phases which affect the resulting LHV,viscosity,steadiness,and chemical composition (Azuara et al.,2015).Dolomite can promote the conversion of aromatics and heavy hydrocarbons into light aliphatic hydrocarbons(Lin et al.,2019).The catalytic cracking performance of natural minerals can be enhanced by loading metal oxides such as Fe,Mg,Ni,etc.Mg-supported natural aluminosilicate clay removes 94% of alkanes from the pyrolytic gases (Zabłocka-Malicka et al.,2016).Fe/Ni-based palygorskite enhances the cracking reaction of tar vapor and more gaseous products (H2and CH4) (Liu et al.,2010).The attapulgite,with a hydrated Mg-Al-Si crystal,has an exceptionally low market price due to earthabundant reserve (Cui et al.,2013).The potential use of attapulgite or metals loaded attapulgite in catalytic pyrolysis of organic matters and biomass has been previously demonstrated (Li et al.,2015).To our knowledge,it has not been utilized as a catalyst during the pyrolysis treatment of rWAS.

This study aims to:(1) investigate the effect of natural attapulgite as a catalyst to pyrolyze rWAS,(2)characterize the pyrolytic products and determine the pyrolytic products yields,(3) investigate the rWAS pyrolysis mechanisms involved with attapulgite catalytic.

2.Materials and methods

2.1.Materials preparation

The rWAS was obtained from the WWTP in the Liaohe Petrochemical Company of China.Attapulgite was purchased from Gansu Haodi Mining Co.,Ltd.Both of the rWAS and attapulgite were dried for 24 h at 105°C to remove the moisture before use and sieved into fine particles(＜75 μm).However,9 wt%of bound water were still in the attapulgite.

2.2.Pyrolysis experiments

In each experiment,attapulgite with a designed ratio (rWAS:attapulgite=1:0,1:0.5,1:0.75 and 1:1) was mixed with 20 g of rWAS and then put into the pyrolysis reactor.The reactor was heated by a STGK-100-12 horizontal tube furnace(SanTe,Luoyang,China).During the pyrolysis process,N2(1 L/min) was used to purge air to ensure there was an inert environment.Then heating each sample to 650,750 and 850°C for 60 min with an uninterrupted rate of 0.5°C/s,respectively.And the pyrolytic vapors were condensed to collect the liquid products including bio-oil and water.The non-condensable gases were collected in collection bags.Pyrolytic residue was taken out from the pyrolysis reactor not until cooling to the room temperature.Each experiment was performed in duplicate.

2.3.The calculation of pyrolytic products yields

At a high temperature,the bound water will separate from the attapulgite and amplify the yield of pyrolytic liquids.The actual yields of pyrolytic gases,liquids and residue are related to rWAS in this study.Therefore,the influence of the bound water in attapulgite on yield of pyrolytic liquids should be eliminated.The pyrolytic residue(MT,g)include the attapulgite added(MA,g)and the biochar (MBc,g) from rWAS.The yield of biochar (CBc,100%) and liquid (CL,100%) were defined as the ratio of biochar (MBc,g) to rWAS(MS,g)(Formula 1),and pyrolytic liquid(ML,g)to rWAS(MS,g)(Formula 2),respectively.Then the non-condensable gases yield(CG,100%) was calculated by Formula 3.The reduction rate (CR,100%) of rWAS was calculated by Formula 4.

2.4.Characterization and analysis

2.4.1.Analysis of attapulgite and rWAS

The elemental contents of rWAS and bio-oil were measured by an elemental analyzer (Vario EL III,Elementar Analysen Systeme GmbH,Germany).Proximate analysis of rWAS was carried out according to Test method No.E871,E872 and D1102 of ASTM standard,and the fixed carbon content in rWAS was calculated according to Liu et al.(2015).The oil content of pyrolytic residue and raw rWAS were determined according to the HJ 637-2018 standard.XRF Spectrometer (Panalytical BV,Almelo,Netherland)was used for determination of metal compositions of attapulgite and rWAS.

2.4.2.Analysis of bio-oil

Bio-oil was separated from the pyrolytic liquids after settlement.The organic compositions were analyzed by an Agilent 7890B-5977A GC-MS spectrometer(Agilent,Palo Alto,America)according to Ye et al.(2020).The saturates,aromatics,resins and asphaltenes(SARA)fractions were measured according to the standard method of SY/T 5119-2008 (Lin et al.,2017)using an AcceleSep accelerated separation system(Agela,Tianjin,China).The lower heating values(LHV) were measured by a calorific analyzer (HY-A9,HENGYA,China)using the ASTM bomb calorimeter method.The mobility(M)of O element was defined as follows (Cheng et al.,2017).

whereWsrepresents the weight percentage of O in the rWAS,andWois the weight percentage of O in the bio-oil.

2.4.3.Analysis of non-condensable gases

An Agilent 6890A gas chromatograph (Agilent Palo Alto,America)was used to analyze the collected non-condensable gases.LHV calculation of the gas followed formula(6)(Gil-Lalaguna et al.,2014):

whereCiandLHViare the molar percentages and lower calorific values of CO,H2,CH4,and C2Hn,respectively,LHVgasis the lower heating value of the gases.

2.4.4.Characterizations of pyrolytic residue

Pyrolytic residue includes the biochar from the rWAS and the added attapulgite.The surface functional groups were investigated by a FT-IR spectrometer (Magna-IR 560 ESP,Nicolet,America).Heavy metals in pyrolytic residues were extracted according to Xu et al.(2017)using Tessier's method,and then measured by an ICPOES (OPTIMA 7300V,Perkin Elmer,America).LHVof pyrolytic residues were measured using an HY-A9 calorific meter.

2.5.Thermogravimetric analysis and kinetics

The rWAS samples were analyzed by thermogravimetric (TG)and derivative TG (DTG) using a thermal analyzer (STA449 F3,Netzsch,Germany).Total weight of 10 mg mixture in the ratios of rWAS with attapulgite as 1:1,1:0.75,1:0.5 and 1:0 were heated to 900°C in a nitrogen environment.The pre-exponential factor (A),activation energy (E) and kinetic parameters of pyrolysis process were investigated according to the DTG curves using the Coats-Redfern method.And the Coats-Redfern equations followed formula (7)and (8) (Gao et al.,2014):

where α is the conversion rate of rWAS,α=(m0-mi)/(m0-mf),wherem0,mi,andmfindicate the initial,present,and final residual mass of samples,respectively.R is the universal gas constant(R=1.986 cal/(mol·K));Tis the temperature,K;and β is the constant heating rate,K/min.

Plots of ln[(1-(1-α)1-n)/(T2(1-n))]versus1/Tfor samples were graphed.AandEvalues were calculated based on the intercept and slope in the Arrhenius plot at each experimental data with different ratios of rWAS to attapulgite.

3.Results and discussion

3.1.Properties and compositions of rWAS and attapulgite

The rWAS has high contents of ash (32.9 wt%),fixed carbon(11.4 wt%) and volatile (49.2 wt%) (Table S1).The rWAS showed a high oil content(7.0 wt%),as well as high C(37.6 wt%),H(5.4 wt%),S(2.6 wt%) and O (17.4 wt%) contents in comparison with sewage sludge(C=17.7 wt%,O=13.1 wt%,H=3.2 wt% and S=0.4 wt%)(Hu et al.,2018).The high content of organic matter in rWAS benefits the generation of value-added pyrolytic product (such as oils and gases).The ash in rWAS is mainly SiO2,Al2O3,Fe2O3,CaO,and Na2O (Table S2).The ash may catalyze the pyrolysis (Cheng et al.,2017).The main chemical compositions of attapulgite were SiO2,Al2O3,MgO,Fe2O3,CaO and K2O.

3.2.Pyrolytic products yields

The increase of termination temperature promoted the pyrolytic gas yield (Fig.1),which increased from 16.1 wt% to 19.7 wt% with the termination temperature raised from 650°C to 850°C for the pyrolysis of rWAS alone.The increased yield of pyrolytic gas at high temperatures is mainly attributed to the secondary cracking of pyrolytic vapors(Syed-Hassan et al.,2017).Metallic oxides in rWAS,which consist of Al2O3,MgO,Fe2O3,CaO,ZnO,Na2O,K2O and so on,may favor the cracking of pyrolytic vapors(Shen et al.,2016).High temperature can enhance the gasification of biochar and decomposition of macromolecular substances (Zhou et al.,2019),decreasing the biochar yield.At 750°C,the highest pyrolytic liquid yield (36.6 wt%) was obtained.

The addition of attapulgite increased the yield of pyrolytic gas but decreased the yield of biochar and pyrolytic liquids.The yield of biochar,liquid and pyrolytic gas were 40.0 wt%,34.3 wt% and 25.7 wt%,respectively,in the ratio of rWAS and attapulgite at 1:1 and temperature of 750°C.The Al2O3and Fe2O3in attapulgite may promote the cracking reaction of organics in rWAS (Song et al.,2016;Yang et al.,2018).CaO and MgO can promote the reforming reaction of tar with CO2and H2O,reducing the liquid yield while promoting the pyrolytic gas production (Zhou et al.,2014;Tang et al.,2018).Alkali metals (Na2O,K2O) in attapulgite may have affected the pyrolysis reactions,such as dehydroxylation and decarboxylation,increasing the non-condensable gases yield and reducing the biochar yield (Shimada et al.,2008).

3.3.Characteristics of liquid

The bio-oil and water in the pyrolytic liquid were clearly layered.With the catalysis of attapulgite,theMvalue reduced and the O/C molar ratio of bio-oil decreased (Table 1).A high O content can negatively affect the fuel properties of bio-oil and may result in instability (Shen et al.,2016).The lowestMvalue (12.8%) and O/C molar ratio (0.0265) of bio-oil was obtained at 1:1 of the ratio of rWAS to attapulgite and the termination temperature of 750°C.Al2O3in attapulgite promotes the deoxygenation reaction of Ocontaining compounds in vapor leading to more O element into non-condensable gases (Azuara et al.,2015).

Table 1 Elemental compositions and LHVs of bio-oils resulting from different rWAS:attapulgite ratios.

The highest LHV(8.5 kcal/kg)of the bio-oil was detected at the rWAS:attapulgite of 1:0.75.The LHVs are related to the organic compositions.According to the GC-MS results,the organics in biooil can be divided into three groups:＜C13(gasoline component),C13-C18(diesel component)and ＞C18(heavy oil component)(Wang et al.,2007).The increase of attapulgite dosages significantly decreased the carbon numbers of organic compounds in bio-oil(Fig.2b).When rWAS:attapulgite ratio at 1:1,the content of＜C13(28.7%) fraction in bio-oil doubled compared to without attapulgite addition (13.5%).Consequently,the content of C13-C18fraction in bio-oil dramatically decreased.Fe2O3in attapulgite may promote the generation of light fractions of bio-oil by enhancing the cracking reaction of C-C bonds in vapor during pyrolysis(Yang et al.,2018).At high temperature,the large molecules tend to break,resulting in the decrease of C13-C18fraction.Al2O3in attapulgite may further promote the polymerization of the C12-C18fraction in vapor,increasing the content of ＞C18heavy oil component(Azuara et al.,2015).Meanwhile,some newly generated small molecules may form aromatic substances,contributing to an increase of content of ＜C13fraction (Hu et al.,2018).

Fig.1.Yields of products from pyrolysis of rWAS alone at different termination temperatures(a);yields of products from pyrolysis of rWAS under different rWAS:attapulgite ratios at termination temperature of 750 °C (b).

For bio-oil from pyrolysis of rWAS alone,the O-containing compounds(aldehydes,esters,alcohols and so on)dominated.The contents of aromatic hydrocarbons and alkanes were only 6.0%and 8.5%,respectively (Table 2).The attapulgite may promote the dehydroxylation and cracking reaction of acids and esters,increasing the content of alcohols and aldehydes(Fonts et al.,2012).Attapulgite mediated deoxygenation reaction may decrease the content of some O-containing compounds in bio-oil (Zhang et al.,2014).As shown in Table 2,there is not a clear trend with respect to all O-containing compounds.Some O-containing compounds may increase,and others will decrease.

The properties of bio-oil were further analyzed based on SARA results (Fig.2c).The increase of attapulgite dosages significantly decrease the content of aromatics in bio-oil.Fe2O3in attapulgite promotes the cracking of aromatic compounds in pyrolytic vapor(Yu et al.,2018).Al2O3in attapulgite may further promote the formation of polycyclic compounds (Azuara et al.,2015).A low content of aromatics (10.2 wt%) and a high content of saturate(47.2 wt%)were obtained when the rWAS:attapulgite ratio was 1:1.Meanwhile,the content of resins and asphaltenes were very high and reached 42.6 wt%collectively.High temperature promotes the polymerization of polycyclic compounds and increases the content of resins and asphaltenes in bio-oils (Hu et al.,2018).The addition of attapulgite improved the bio-oil quality based on the higher content of light fraction,the low O mobility,as well as the lower O/C molar ratio.

3.4.Characteristics of pyrolytic gas

CH4,H2,CO and CO2are the main components in pyrolytic gas(Fig.3).The aromatization reaction of alkanes in vapor may increase the H2content in pyrolytic gas.However,Al2O3in attapulgite can crack oxygen functional groups derivatives to CO and CO2during the secondary decarboxylation reactions (Huo et al.,2018).Therefore,the total contents of CO and CO2increased,resulting in the decrease of H2content.The highest total content of CO2and CO reached 60.7 vol% and the lowest LHV of non-condensable gases was 3.1 kcal/m3when the rWAS:attapulgite ratio was set at 1:1.The non-condensable gases from pyrolysis of rWAS alone had the highest LHV (4.2 kcal/m3) (Table S3).

Table 2 Chemical compositions of bio-oils resulting from different rWAS:attapulgite ratios.

Fe2O3in attapulgite may intensify the cracking reaction of C-C bonds and reforming reaction of alkanes (Yang et al.,2018),increasing the CH4content (12.6 vol %) of non-condensable gases when the rWAS:attapulgite ratio was 1:0.5,compared to pyrolysis of rWAS alone (10.4 vol%).Further addition of attapulgite resulted in the production of less CH4.This can be explained by the following.With the increase of attapulgite dosage,more H2O and CO2were released by dehydroxylation and decarboxylation (Fonts et al.,2012).The catalytic reforming reaction of steam may occur(Formulas 9,10 and 11) (Mei et al.2020).These reactions increase the content of H2,CO and CO2in non-condensable gases.The highest content of H2(28.6 vol%) occurred at rWAS:attapulgite of 1:0.75.Alkali metal in attapulgite may reduce the generation of alkanes from rWAS and decrease the CH4content in noncondensable gases (Tang et al.,2018).Meanwhile,CH4may react with CO2and promote the formation of CO and H2(Formula 12)(Yu et al.,2018).The metal oxides in attapulgite can catalyze coke to form CO and CO2(Formula 13) (Cheng et al.,2017).

Fig.2.Total ion chromatograms (a),carbon distribution (b),and SARA results (c) for bio-oils resulting from different rWAS:attapulgite ratios.

High CO and CO2contents in pyrolytic gas are consistent with the low O/C molar ratio andMvalue in bio-oil.Therefore,attapulgite does not improve the quality of non-condensable gases according to the low LHV and content of H2and CH4.

3.5.Characteristics of pyrolytic residues

The attapulgite catalyzed pyrolysis significantly reduced the rWAS.The reduction rate of rWAS reached 60.0 wt% when the rWAS:attapulgite ratio was 1:1.Addition of attapulgite led to a higher reduction relative to rWAS alone (54.5 wt%).More biomass and oils in rWAS were converted to non-condensable gases and bio-oil.Moreover,the attapulgite can hinder the coalescence of coke particles and promote the conversion of coke(Formula 11 and 13) (Bi et al.,2007).Less coke remained in the biochar,which resulted in the lowered LHV (1.1 kcal/kg) (Table 3).

Fig.3.Compositions of non-condensable gases resulting from different rWAS:attapulgite ratios.

The oils in rWAS were completely converted by pyrolysis.The oil content of residue is only 6.4 mg/kg when the ratio of rWAS to attapulgite was 1:1.The organics were converted to noncondensable gases,bio-oil,and biochar during high temperature catalytic pyrolysis.The content of residual state heavy metals in pyrolytic residues were higher than that of exchangeable state heavy metals (Table 3),showing the residue was relatively stable and with low toxicity to the environment (Zhai et al.,2014).The attapulgite addition decreased the contents of residual state Ni,Cr and Cu.Heavy metals in residual state have low biohazard and are relatively stable (Gao et al.,2020).With the increasing dosage of attapulgite,the contents of Co,Cr,Cu and Mn in residual state significantly increased.The attapulgite greatly promotes the immobilization of heavy metals when the rWAS:attapulgite ratio was 1:1,suggesting the low toxicity of pyrolytic residues(Xu et al.,2017).Overall,attapulgite catalyzed rWAS pyrolysis reduced environmental risks caused by organics and heavy metals.Exchangeable Mn in residue was mainly from attapulgite rather than rWAS.The addition of attapulgite likely increased the environmental risks of pyrolytic residue.The control of Mn release from attapulgite during pyrolysis should be investigated in future studies.

Typical adsorption peaks of C-H,O-H,C-O and C=O functional groups were detected in both of rWAS and pyrolytic residues,shown in Fig.4 (Alvarez et al.,2016;Pentr′ak et al.,2018).C-H functional groups of hydrocarbons in FT-IR spectra generally include the aliphatic C-H stretching band (3100-2800 cm-1),aromatic and olefinic C-H skeletal vibrations (1600-1700 cm-1),aliphatic-CH2and-CH3deformation vibrations(1490-1365 cm-1)and aliphatic C-(CH2)n-C (n=1-3) rocking vibrations(700-900 cm-1) (Gao et al.,2020).Compared to raw rWAS,the pyrolytic residue from rWAS alone showed low adsorption peaks of C-H stretching at 2960-2850 cm-1and C=O at 1520-1750 cm-1,as well as a high adsorption peak of C-O stretching at 800-1210 cm-1.Aliphatic chains,acids,and esters in raw rWAS are decomposed by pyrolysis and possibly converted to the alcohol,ethers and aldehydes (Song et al.,2016;Naqvi et al.,2019).The addition of attapulgite reduced the intensity of adsorption peaks of C-H deformation in the regions of 1500-1300 cm-1and C-H stretching at 2960-2850 cm-1for the residue.Attapulgite may promote the breaking of aliphatic chains which may go on to form light hydrocarbons in the bio-oil,and it may also promote the production of CH4(Zhang et al.,2011).Adsorption peaks of C-O stretching found at 800-1210 cm-1and O-H stretching at 3459 cm-1significantly decreased by attapulgite addition.The attapulgite promotes deoxygenation,dehydroxylation and dehydrogenation reactions,producing more CO,CO2and H2O (Naqvi et al.,2019).The H2O may participate in the reforming reaction (Formula 9,10 and 11),increasing CO and CO2in non-condensable gases and reducing the O/C of bio-oil.

3.6.Catalytic pyrolysis mechanism of attapulgite

The catalytic pyrolysis of rWAS with different dosages of attapulgite occurs in three stages from TG and DTG curves(Fig.5).Stage I (230-400°C) corresponds to the decomposition and reforming reaction of light components (Naqvi et al.,2019).Stage II(400-500°C) is mainly related to the decomposition of bacterial matter(e.g.protein and polysaccharide)and some macromolecular aliphatic compounds (Liu et al.,2015;Prashanth et al.,2021).The decomposition and devolatilization of light char both contribute to a small weight loss that occurs in Stage III(620-720°C)(Dai et al.,2015).The decomposition of rWAS mainly occurred during Stage I and II.The addition of attapulgite significantly promoted the weight loss of rWAS relative to rWAS alone.

The pyrolysis of rWAS and attapulgite aided rWAS both followed the second-order reaction kinetics(Fig.S1,Table 4).The addition of attapulgite significantly decreased theEvalues of Stage I and II in comparison with sewage sludge(82.3 kJ/mol and 48.3 kJ/mol)(Gao et al.,2014).Compared to rWAS alone,theEvalues decreased from 21.7 kJ/mol to 13.8 kJ/mol and 46.9 kJ/mol to 23.6 kJ/mol in Stage I and II,respectively,for rWAS:attapulgite ratio of 1:1.TheEvalue of Stage III,however,increased with the addition of attapulgite.The decomposition and devolatilization of light char may require a higher temperature(Liu et al.,2015).The pre-exponential factor(A)increased with the increase of termination temperature and the addition of attapulgite.Usually,the value ofAreflects the effective contact between reactant molecules in a reaction.Therefore,a highvalue ofAindicates that the reaction rate of organic matter in rWAS is promoted by attapulgite during the thermal decomposition reaction (Gao et al.,2014).

Table 3 Exchangeable state(F1) and residual state(F5) of heavy metals in attapulgite and pyrolytic residues.

Table 4 Pyrolysis dynamic parameters of rWAS with different dosages of attapulgite.

Fig.4.FTIR spectra of raw rWAS,attapulgite and pyrolytic residues.

The catalytic effects of attapulgite on rWAS mainly act on enhancing the devolatilization of rWAS in Stage I and the reformation of primary vapors in Stage II (Fig.6) (Lin et al.,2019).Attapulgite also promotes the decomposition of remaining heavy organic matter,such as bacterial matter and some macromolecular aliphatic compounds,reducing the yield of biochar.The secondary decomposition of the pyrolytic vapor was further catalyzed by Fe2O3in attapulgite and rWAS,increasing the content of ＜C13components (Yang et al.,2018).The dehydrogenation,dehydroxylation and deoxygenation reactions of the pyrolytic vapor were also catalyzed by metallic oxides to release H2O,CO2and CO(Zhang et al.,2014;Ozbay et al.,2019).This results in the increased content of alcohols and aldehydes,while decreased the O mobility towards bio-oil.Steam(H2O)aided reforming reactions increase the content of H2,CO and CO2in non-condensable gases(Formula 9,10,11 and 12).The metallic oxides in attapulgite may react with coke,reducing the amount of biochar and producing CO and CO2at high temperature in Stage III (Formula 13) (Cheng et al.,2017).

Fig.5.TG (a) and DTG (b) curves of rWAS with different dosages of attapulgite.

Fig.6.Proposed reaction process of attapulgite catalyzed pyrolysis of rWAS.

4.Conclusions

Attapulgite decreased the activation energy of rWAS pyrolysis,reduced the biochar production and enhanced the quality of bio-oil.The metallic oxides in attapulgite promoted a series of catalytically thermal reactions of biomass and oils,including cracking,dehydrogenation,dehydroxylation and deoxygenation.Compared to rWAS alone,attapulgite catalyzed pyrolysis increased LHVs,the contents of ＜C13fractions and saturates,decreased the O mobility and the O/C molar ratio in bio-oil,and reduced the environmental risks caused by organics and heavy metals in pyrolytic residue.This research showed the potential application of natural attapulgite in the catalytic pyrolysis of rWAS.

Acknowledgments

This study was supported by National Key R&D Program of China (2018YFC1801903-01 and 2019YFC1806201-01),National Natural Science Foundation of China (No.21776307),and Science Foundation of China University of Petroleum-Beijing (No.2462018BJB001 and 2462020YXZZ035).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.petsci.2021.09.043.

Conflicts of interest

The authors declare no conflict of interest.