Hydrogen-Rich Syngas Production through Synergistic Methane-Activated Catalytic Biomass GasificationAmoolya Lalsare,† Yuxin Wang,† Qingyuan Li,† Ali Sivri,‡ Roman J. Vukmanovich,†

Cosmin E. Dumitrescu,‡ and Jianli Hu*,†

†Department of Chemical and Biomedical Engineering and ‡Department of Mechanical and Aerospace Engineering, West VirginiaUniversity, 395 Evansdale Drive, Morgantown, West Virginia 26505, United States

*S Supporting Information

ABSTRACT: The abundance of natural gas and biomass in the U.S. was themotivation to investigate the effect of adding methane to catalytic nonoxidativehigh-temperature biomass gasification. The catalyst used in this study was Fe−Mo/ZSM-5. Methane concentration was varied from 5 to 15 vol %, and thereaction was performed at 850 and 950 °C. While biomass gasification withoutmethane on the same catalysts produced ∼60 mol % methane in the total gasyield, methane addition had a strong effect on the biomass gasification, withmore than 80 mol % hydrogen in the product gas. This indicates that thereverse steam methane reformation (SMR) reaction is favored in the absenceof additional methane in the gas feed as the formation of H2 and CO shifts theequilibrium to the left. Results showed that 5 mol % additional methane in thefeed gas allowed for SMR due to formation of steam adsorbates from oxygen inthe functional groups of aromatic lignin being liberated on the oxophilictransition metals like Mo and Fe. This oxygen was then available for the SMRreaction with methane to form H2, CO, and CO2. This study was not a detailed catalytic activity evaluation, but it wasexploratory research to ascertain the synergy presented in the co-gasification of biomass and natural gas.

KEYWORDS: Biomass gasification, Co-gasification of biomass and methane, FeMo/ZSM-5 catalyst

■ INTRODUCTION

Per capita energy demand increased sharply over the lastcentury, which caused irreversible changes in the weatherpatterns across the globe.1 Climate change, which has longbeen perceived as a futuristic phenomenon, is alreadyproducing adverse climatic changes around the globe. Theaverage annual temperature of the planet has already risen by1.5 °C, and the current efforts in reducing emissions arefocused on curbing this temperature rise to 2 °C.2,3 At thecurrent rate of emissions, the planet may see a recordtemperature rise of 2 °C by 2050. According to theIntergovernmental Panel on Climate Change (IPCC), thecarbon dioxide (CO2) concentration in Earth’s atmosphere hascrossed the unprecedented mark of 400 ppm.4 The worldcommunity is diligently working toward moving to cleaner,green, and sustainable energy sources like solar, wind, andbiomass to meet energy needs especially in the power andtransportation sectors.5 Biomass has been a major source ofenergy for humankind since the discovery of fire in theprehistoric era and was the predominant source of fuel forheating and cooking applications until fossil fuels likepetroleum, coal, and natural gas were successfully harnessedin the 19th century. Fossil-fuel-based technologies enabled thehuman population to increase sharply from less than 1 billionbefore the 18th century to more than 7 billion by 2020, a spanof just three centuries. Fossil fuels account for nearly 80% of

the world energy needs and the shift to renewable energysources is not an easy task, owing to the high costs andunreliability of energy sources like solar and wind. Biomass,which accounts for 10−14% of the global energy supply, canreplace fossil fuels as a reliable, sustainable, and green source ofenergy, contingent on the development of technologies thatcan harness its energy in clean, efficient, and economical (i.e.,cost-effective) means.6 Even today, biomass accounts for mostof the energy utilized in the remotest, underdeveloped, anddeveloping regions globally. Biomass is available in differentforms such as agricultural and forestry waste, animal waste,biological materials byproducts, wood, and municipal waste.These sources of biomass have vastly different moisturecontent and elemental compositions, thus making it difficult todevelop a cost-effective technology which utilizes most types ofbiomass.7,8 Although woody biomass such as dry firewood isused in combustion for heating and power generation,9 theenergy efficiency is low, and air emission control becomes anissue. In contrast, both heat and power generation require-ments using gasification can be met in an efficient, effective,and clean way.10 Energy utilization by converting solidfeedstock to high-heating-value syngas is one of the cleanest

Received: May 17, 2019Revised: August 22, 2019Published: September 4, 2019

Research Article

pubs.acs.org/journal/ascecgCite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Dow

nloa

ded

via

WE

ST V

IRG

INIA

UN

IV o

n Se

ptem

ber

18, 2

019

at 2

0:33

:19

(UT

C).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

ways to harness renewable energy source from woody biomass.Biomass fast pyrolysis and gasification holds good potential forthe production of hydrocarbon fuels and value-addedchemicals.11,12 Pyrolysis has been extensively researched andapplied for producing biofuels for direct use in transportationand related applications.13−21 However, the decomposition ofthe highly aromatic lignin structure in lignocellulosic biomassleads to formation of bio-oil at temperatures below 700°C.22−24 Bio-oil is high in oxygen content and has limiteddirect application as fuel owing to high acidity, poor resistanceto extreme weather conditions, and instability.11,25 Theselimitations are the result of the highly oxygenated aromaticstructure of biomass. The thermochemical conversion ofbiomass to energy and fuels have been extensively studiedfor the past few decades.26,27 Biomass decomposes rapidlybetween 400 and 600 °C giving devolatilization componentslike carbon monoxide (CO), carbon dioxide (CO2), methane(CH4), bio-oil, aerosols, and biochar.9,11,12,22

There have been numerous efforts to upgrade the yieldsfrom biomass to value-added chemicals such as benzene,toluene, ethylbenzene, and xylene (BTEX), and producehydrogen-rich syngas.28,29 In situ tar cracking, hydrodeoxyge-nation (HDO), and rearrangement to form ketonic inter-mediates leading to alkanes are typical reaction steps necessaryfor conversion of biomass to valuable chemicals.28,30,31

Transition metals on catalytic surfaces like zeolite (HZSM-5), SiO2, and γ-Al2O3 have been studied to initiate biomass-upgrading reactions like dehydration, rearrangement, decar-boxylation, decarbonylation, and hydrodeoxygenation.7,28,32

However, the complexity of reaction chemistry, the require-ment of high-pressure hydrogen, and the rapid catalystdeactivation due to coke deposition and poisoning increasesthe cost of the process, rendering it economically nonviable.Methane-promoted catalytic biomass gasification can achievein situ tar reformation and cracking, thus producing highsyngas yields with an enhanced H2/CO ratio, suitable forchemical synthesis via a conventional Fischer−Tropsch (FT)synthesis process. Air−steam catalytic and noncatalytic gas-ification were reported on Ni/CeO2/Al2O3 with catalystloadings of 20−40% in a fluidized bed reactor at 725, 825,and 900 °C.33 Nishikawa et al. reported that at 900 °C highcatalyst-to-biomass loading of 40% was effective in tar crackingand high-purity hydrogen production.33 Ni nanoparticles onMCM-41 supports were utilized for biomass gasification toenhance hydrogen production.34,35 Use of a suitable catalystfor biomass gasification increased the H2/CO ratio in syngasfrom 1.15/2.15 to 1.87/4.45.36,37 Fe−Ni/CeO-Al2O3 andnoble metals like Pt, Pd, and Rh were synthesized and usedfor biomass steam gasification which showed that Fe-promotedcatalysts produce higher gas yields.33 Coconut-shell gasificationwas performed with steam on Pt-, Fe-, and Co-promotedcatalysts, and it was reported that the use of Fe-promotedcatalysts increased hydrogen and carbon monoxide composi-tion by improving the water−gas shift (WGS) reaction.38

Catalytic fast pyrolysis (CFP) on inexpensive Mo−Ag/ZSM-5catalysts with methane in a single reactor system operated atatmospheric pressure showed that the catalyst deactivation ratedecreased with an increasing H/Ceff ratio.This ratio can be increased by fast pyrolysis/gasification of

biomass with a hydrogen-rich source like methane. H/Ceff ratioof biomass is about 0.3, which is not suitable for producinghydrogen-rich syngas for downstream production of value-added chemicals. Methane that comes from an inexpensive and

abundant source like natural gas has a very high H/Ceff ratio of4.39 In recent years, there have been efforts to utilize methaneand biomass in a single reactor, but mainly for the purpose offast pyrolysis. In one of the recent studies, pine saw dustbiomass was impregnated with Ni and gasified with steam at600 °C to produce H2-rich syngas with maximum H2 yield of60% in the outlet gas. Steam was replaced by methane toperform catalytic methane decomposition (CMD) for 10 h at850 °C on Ni/carbon catalyst to further increase H2. About90% methane conversion was reported with the process.35 Thepresent study focuses on nonoxidative catalytic gasification in asingle stage using co-feeding of methane (5−15 mol %) withbiomass. Although renewable energy sources have startedcontributing significantly to power generation in the US,sources like solar and wind face severe limitations to beconsidered as mainstream source for power generation. Thus,natural gas biomass synergy is of key importance given thepower and energy requirements of the US in coming decades.With the abundance of natural gas in the US and especially theAppalachian region, efficient and cost-effective utilization ofnatural gas for fuels and conversion to valuable chemicals alongwith biomass offers great potential for meeting US energyneeds in the near future. Methane is a major constituent innatural gas (>80 mol %), which makes it an inexpensive sourceof hydrogen for tar reforming in the biomass pyrolysis andgasification process. Methane can be activated with the use ofsynthetic transition metal catalysts like Ni, Fe, Co.33,40

This study chose Fe−Mo/ZSM-5 for the co-gasification ofmethane and biomass at 750−950 °C in a fixed-bed reactorsystem. In general, zeolites such as ZSM-5 with acidic functioncan be utilized to crack large molecules such as biomass.Adding metals to zeolite creates bifunctional propertiesfacilitating dehydrogenation and hydrogen transfer.41−43 Feon ZSM-5 was shown to assist methane decomposition withless energy intensive cleavage of the C−H bond.39,40,44 Fe-impregnated ZSM-5 catalyst has been subjected to high-temperature methane decomposition to produce hydrogen andcarbon nanotubes (CNTs). Fe−Mo/ZSM-5 catalyst has alsobeen utilized for methane and ethane dehydroaromatizationstudies. Iron active sites supported on ZSM-5 have been shownto cause 3D coke deposition on the catalyst in the form ofCNTs. Coke characterization on the iron-based catalystshowed that crystalline coke formation is pronounced thanthat of amorphous coke. Amorphous coke deposition on thecatalyst leads to the blocking of active metal sites thus causingcatalyst deactivation. Crystalline coke deposition in the form ofCNTs delays Fe−Mo/ZSM-5 catalyst deactivation.40 Iron isknown to be conducive for high-temperature methaneactivation, and atomic molybdenum impregnated on ZSM-5is highly oxophilic (i.e., having good oxygen-binding affinity).Oxophilicity of Mo on ZSM-5 is key for hydrodeoxygenationand steam methane reforming (SMR) in the methane-activatedbiomass gasification reaction system. This was the primaryreason we selected the Fe−Mo/ZSM-5 catalyst. It is postulatedthat intermediate hydrogen species participate in cracking theoxygenated aromatic structure of lignocellulosic biomass byhydrodeoxygenation (HDO), decarbonylation, and decarbox-ylation reactions. Fe and FeOx active sites are directlyresponsible for high-temperature WGS (HT-WGS) and SMRreactions.41,45−47 Although there is little evidence that acidsites on ZSM-5 directly facilitate WGS and SMR reactions, theacidity of the ZSM-5 support affects the conversion of CO (inWGS) and CH4 (in SMR). Meanwhile, Bronsted sites and

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

B

some Lewis acidic sites on ZSM-5 are known to be effective inconversion of highly oxygenated lignin to value-addedchemicals like BTEX.31,34,48,49 This study investigated thepossible synergy between co-gasification of hardwood biomassand methane. Iron (Fe) and molybdenum (Mo) were used aspromoters on ZSM-5 zeolite as they are inexpensive andprovide active sites for C−H bond activation and convertinglignin oxygen, thus minimizing tar formation. The catalystpreparation method, type of biomass and its elementalcomposition, type of reactors used, and product analysistechnique are described in the Experimental Section. Biomassgasification in TGA, biomass gasification tests in a fixed bedwithout methane, methane−biomass gasification tests, cokeformation, and the synergistic effect of methane addition onH2/CO ratio have been discussed in detail in the “Results andDiscussion” section.

■ EXPERIMENTAL SECTIONCatalyst Preparation. The catalyst (Zeolyst, Inc.) was NH4−

ZSM-5 zeolite with a silica/alumina (SAR) of 23. Ammoniummolybdate (VI) tetrahydrate and iron(III) nitrate nonahydrate werepurchased from Acros Organics. The zeolite catalyst was first calcinedat 500 °C in air for 3 h to convert NH4−ZSM-5 to H−ZSM-5. Theconventional incipient wetness technique was used to prepare Mo−Fe/ZSM-5. After drying the catalysts at 105 °C to remove the waterovernight, the dry powder catalyst sample was further calcined in airat 550 °C for 4 h. The chemical composition of the synthesizedcatalysts are shown in Table 1. The elemental composition ofhardwood biomass subjected to all experimental tests presented in themanuscript is shown in Table 2.

Reaction Conditions. Catalytic hardwood-pellet biomass gas-ification was performed in a downdraft fixed-bed reactor (12.7 mmdiameter, 915 mm long) stainless steel (316SS) reactor tube(Charleston Valve and Fitting Co.). In a typical experimental test,0.75 g of Mo−Fe/ZSM-5 catalyst was premixed with 1 g oflignocellulose hardwood-pellet biomass. Biomass was ground andscreened to a mean particle diameter of 432 μm. This particle size wasselected to maintain consistency with the 432 μm particles used for abench-scale continuous-bubbling fluidized-bed setup that is currentlybeing tested under different parametric conditions of temperature,gasifying agent, feed/bed material ratio, and premixed catalyst.Reactor bed temperature was measured using a K-type thermocouple(Omega). Prior to loading into the reactor, the catalyst was subjectedto reduction with 10 vol % hydrogen (H2) with nitrogen (N2) at 600°C for 3 h. Catalyst and biomass were mechanically mixed in a ratio of0.75:1 respectively. A total nitrogen flow of 300 sccm was maintainedas the temperature was raised from room temperature to 100 °C.After moisture and air removal from the reactor, a constant flow of300 sccm with the desired methane concentration was maintaineduntil the reactor pressure reached 50 psig. Fixed-bed temperature was

ramped from 100 °C to the reaction temperature, initiated while thereactor valves were shut until the temperature ramp was completewith a ramp rate of 20 °C min−1. This method was adopted forreplication of continuous feeding of biomass in a fluidized bed ormoving bed reactor at the reaction temperature.

It was observed through elemental composition analysis ofhardwood lignocellulosic biomass that oxygen accounts of 49 wt %of the biomass on dry basis. Therefore, for hydrogen-rich syngasproduction, an external source of oxygen is not required for the typicalbiomass. With the use of external oxygen as gasifying agent, like CO2or H2O, it was observed that when >5 vol % carbon dioxide wasadded along with 5 vol % methane in the catalyst−biomass system,high concentrations of CO and CO2 were observed in the productgas. However, using 1% CO2 and 5% CH4 with biomass on Fe−Mo/ZSM-5 and Fe−Mo/CNF(carbon nanofiber) catalysts produced aH2/CO ratio = 2. CO2 thermal activation occurs in neighborhood ofatomic hydrogen (from methane and biomass) on the catalyst activesites. This finding is being published as an independent study.Moreover, external steam addition would lead to direct SMR, thushindering the possible synergy between natural gas and biomass.

Preliminary biomass gasification studies were also performed usinga thermogravimetric analysis (TGA) instrument (TA Instruments,Waters LLC, model SDT 650). The product line from this instrumentwas connected to a mass spectrometry (MS) instrument (Quantach-rome) as described in the following section. Reaction conditions inTGA instrument were similar to those in the fixed-bed reactor. Thereaction was performed at several temperatures ranging from 750 to950 °C. Helium (He) was used as the carrier gas for these tests.Continuous product composition analysis was carried out with theMS instrument connected downstream. Sample size for a typical testvaried from 20 mg for a biomass without catalyst system to 70 mg fora biomass−ZSM-5 system.

Product Analysis. Product gases collected in sampling bags(SKC) from the fixed-bed reactor tests were analyzed by a four-column gas chromatograph (Inficon Fusion micro-GC). The fourcolumns consisted of a molecular sieve with a 3 m long PLOT Uprecolumn, an 8 m long RT−PLOT U with a 1 m long PLOT Qprecolumn, an 8 m long aluminum column, and a 20 m long RTX−1column. The four columns allowed for calibrated (ppm level)detection of hydrogen, methane, carbon monoxide, carbon dioxide(complete syngas profile), ethylene, ethane, acetylene, water, nitrogen,and ammonia. All gases used for calibration were ultrahigh purity(UHP)-grade (AirGas).

■ RESULTS AND DISCUSSION

Conventional Biomass Gasification and Devolatiliza-tion Studies in TGA. TGA-MS was used for the initialbiomass gasification screening tests to identify a temperaturerange for hydrogen and carbon monoxide evolution. Figure 1shows the concentration profile versus time of the four majorsyngas components obtained from the TGA-MS studies. Fordevolatilization and gasification at 650 °C, only 7% H2 yieldwas obtained. CO and CO2 made up most of the gas yieldobtained at temperatures less than 300 °C with methaneevolving beyond 300 °C possibly indicating thermal crackingof the array of oxygenated aromatic rings in the lignin structureof biomass.As methane started appearing in the gas products, carbon

dioxide yield began to fall, and only 6% carbon dioxideremained at 515 °C. This suggested that the dry H2 reformingreaction dominated between 335 and 515 °C, where all thehydrogen that was bonded to saturated/unsaturated carbon inbiomass reacted with the CO2 from devolatilization andthermal cracking forming CO. Higher CO concentrations weredetected at higher temperatures as char oxidation reactionkicks in. Hydrogen started to appear as a predominant speciesin the normalized concentrations obtained from the mass

Table 1. Mo−Fe/ZSM-5 Metal Loading of Mo and Fe onZSM-5 Catalyst Synthesized by Wet Incipient Method

catalyst Mo Fe

FeMo1 4 wt % 0.5 wt %FeMo2 4 wt % 1.5 wt %

Table 2. Elemental Composition, Moisture, and AshContent (wt %) of Hardwood Biomass Used in the Studya

carbon(C)

hydrogen(H)

oxygen(O) moisture ash

hardwoodbiomass

45.25 4.65 49.2 7.16 0.32

aPerformed by National Research Center for Coal and Energy.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

C

spectrometry ionization signals only after a threshold temper-ature of about 650 °C. Figures 2 and 3 show that between 650

and 950 °C hydrogen and carbon monoxide constituted mostof the gas yield. This indicated that at a lower temperature andwithout an oxidative atmosphere like steam or air thedevolatilization and biochar formation occurred between 200and 600 °C.50−52 In the TGA tests, the biomass sample sizewas too small (∼20 mg) to trace any tar formation.53 In thealumina crucible used for the tests, only ash residue wasobserved with the end weight of the tared crucible measured asaround ∼0.5 mg. The isothermal reaction time at the testtemperatures was sufficient to completely gasify biochar. Chargasification reactions were observed at higher temperatures,evident by the high H2/CO and CO/CO2 ratios attemperatures beyond 650 or 700 °C (Table 3). Surprisingly,no methane formation was observed at char gasificationtemperatures (650−950 °C) possibly indicating the Bou-

douard reaction (eq 1), which converted carbon to CO due toits highly endothermic nature:9

+ Δ ° = −HC CO 2CO 172 kJ mol2 2981F (1)

Similarly, a sharp increase in H2 and improvement in theH2/CO ratio may also be due to continuous heat beingsupplied driving the endothermic gasification reactionsforward. Between 650 and 950 °C, methane was not observedin the product probably because of reforming with water orCO2, which is evidenced by the increase in CO and H2concentration shown in Figure 3.

Steam reforming:

+ + Δ ° = −HCH H O CO H 206 kJ mol4 2 2 2981F (2)

Dry reforming:

+ + Δ ° = −HCH CO 2CO H 247 kJ mol4 2 2 2981F

(3)

Typically, methane decomposition occurs at temperatureshigher than 1000 °C in the absence of a catalyst. Due to thepresence of mineral materials in biomass, the methanedecomposition reaction cannot be ruled out here. Theproportion of tar in dry woody biomass falls in the range of5−12 mg/g (0.5−1.2 wt % on a dry basis) of dry biomass.10

With only 20 mg of biomass in the TGA tests, the possibility oftraceable tar formation is very low.34,54−56 Endothermicequilibrium reactions in the regime of char gasification andhomogeneous volatile reactions may have pronounced effectson the H2/CO and CO/CO2 ratios.

57

Figure 4 illustrates the results of a TGA test where biomasswas mixed with ZSM-5 and gasified under identical reactionconditions as used in the previous TGA tests. It was observed

Figure 1. Biomass gasification (hardwood pellet; heating rate of 20°C/min, then 30 min at 650 °C; 100 sccm He flow).

Figure 2. Biomass gasification (hardwood pellet; heating rate of 20°C/min, then 30 min at 850 °C; 100 sccm He flow).

Figure 3. Biomass gasification (hardwood pellet; heating rate of 20°C/min, then 30 min at 950 °C; 100 sccm He flow).

Table 3. H2/CO and CO/CO2 Ratios of TGA BiomassGasification Tests Observed at Temperatures in the Rangeof 750−950 °C

temperature (°C) H2/CO CO/CO2

750 1.87 16.2850 1.1 N/A950 1.1 23

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

D

that the H2/CO ratio improved significantly at 750−800 °C.The presence of ZSM-5 may have enabled cracking reactionsat higher temperatures. Notably, a high concentration ofmethane was observed between 450 and 550 °C, indicatinghydrogenation of higher aromatics and some aliphatic chainsforming CH4 almost as much as CO at ∼400 °C. More COand H2 was observed as temperature in the reactor increased.This indicated that the presence of a catalyst like zeolite helpedto improve the syngas quality with in situ tar reforming andcracking. The fixed-bed reactor biomass and methane−biomassgasification had a significantly different reaction environment,heat flux, and heat flux direction than those in the TGA samplecrucible. A fixed-bed reactor was operated as a downdraftgasifier, whereas gas flow was horizontal in the case of TGA.58

The interaction between catalyst active sites, char, and gaseousproducts was different in the case of the fixed-bed reactor ascompared to the TGA. Having said this, TGA provided abetter study of the evolution and profile of possible gasificationreactions occurring in a real reactor like the fixed-bed studiespresented in following sections.Biomass Catalytic Gasification in the Absence of

Methane in a Fixed-Bed Reactor. Figure 5 shows that inthe absence of methane the biomass gasification yields in afixed-bed reactor were (on average) 55 and 58 mol % methanefor the reaction at 850 and 950 °C, respectively. A H2/COratio between 0.5 and 1.0 was obtained, which was roughly inthe range of conventional noncatalytic gasification. However,with almost 55−60% methane in the gas yield and a very lowCO2 yield of 2−13%, the results suggested that the reverseSMR and dry reforming (eq 2) was dominant at hightemperature as both of these reactions were endothermic.Reverse SMR leading to conversion of hydrogen and carbonmonoxide into methane could be due to the presence of acidicactive sites on Fe and Mo metals.59 In the case of biomassgasification on ZSM-5, the methane yield was 29 mol %, whichis more than 3 times that seen in typical noncatalytic biomassgasification. The high methane yield in the product wasattributed to the thermal cracking of the aromatic array ofbiomass. Moreover, in the absence of an acidic oxophilictransition metal like Fe or Mo, the oxygenated carbon in thebiomass was converted to CO, CO2, and H2 which reacted onthe surface. Also, without additional methane in the gas feed,

the surface was deficient in methane, which favored theformation of methane and water vapor as the adsorbed COand H2 underwent reverse SMR at high temperature.The results suggest a synergy between methane and biomass,

which shifted the SMR equilibrium to the right and formshydrogen-rich syngas. Detailed discussion on the results ofmethane−biomass gasification studies are discussed in the nextsection. Without an external methane feed, typical gasificationreactions only yield about 1:1 H2/CO ratio as is the molarratio of biomass. Biomass and natural gas are both abundantand largely untapped sources of clean and efficient energywhich can help meet more than 80% of the total energy needsin the US by 2030. Utilization of natural gas with thedevelopment of new processes will also help revive theAppalachian economy especially that of West Virginia whichstrategically lies in the Marcellus shale gas basin.60 Thus, thesynergy between biomass and methane has been explored inthe following sections.

Coke Formation and H2 from CH4 Decomposition.When using either 0.5 or 1.5% Fe-promoted Mo/ZSM-5surface, coke formation was observed during co-feeding ofbiomass and methane. However, in the absence of methane,biomass gasification coking was not observed on either catalyst.This indicates that some of the methane in the gas feed isdecomposed at high temperatures to form coke and producehydrogen (H2) due to the presence of Fe active sites on ZSM-5. A stoichiometric amount of hydrogen formed from methanedecomposition was accounted for based on coke formed frommethane decomposition.61 The calculation for hydrogenobtained from biomass−methane synergy was calculated inmol % as follows:

= −H H H2(prod) 2(total) 2(coke) (4)

H2/CO ratios obtained for various concentrations ofmethane and reaction temperatures were calculated afteraccounting for hydrogen obtained possibly from methanedecomposition. Very little or no tar was recovered from thegas−liquid separator at the bottom of the vertical tubularreactor. However, small concentrations of ethane, ethylene,and acetylene were seen in the product analysis on the micro-GC.

Figure 4. Biomass gasification on ZSM-5 (hardwood pellet; heatingrate of 20 °C/min, then 30 min at 950 °C; 100 sccm He flow).

Figure 5. Biomass gasification in the absence of methane operated ina fixed-bed reactor ZSM-5 support and Mo−Fe/ZSM-5 at 850 and950 °C.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

E

Gas Yield, Tar Formation, and Char Deposition onCatalyst. Overall carbon balance for the methane-activatedbiomass gasification was calculated using the inert gas andmethane moles before the temperature ramp and the molarvolume occupied by the gas inside the tubular fixed-bed reactorbased on reaction temperature and pressure. All of the biomasscarbon was recovered using TGA studies of spent catalyst forcoke and char characterization. Amount of condensed tar wasestimated using hexane solvent wash of the reactor tube andcondenser. The mole percent and yield of individual productswere calculated based on gas moles obtained in the product.However, we chose to present mole percent and product yieldbased on syngas moles as it would be more accurate to do so inthis case. This is because there was coke deposition on thecatalyst as presented in the manuscript, and hydrogen frommethane decomposition was not considered as hydrogenproduced from methane−biomass synergy. To be consistentwith the objective of this work, which is to show natural gas−biomass synergy, mol % and yield was calculated on the basisof product gas mole. To be fair, if one was to calculate molepercent on the basis of starting moles of biomass, then themole percent of all the species would be smaller by about 10−20% depending on the gas, tar, and char yield.

=Yield (mol %)

number of moles of species/total product gas moles (5)

It was also observed that normalized product gascomposition and mole percent calculated from the yield ofindividual species based on gas moles were similar and thuswere used interchangeably in discussing the results. It wasobserved that all the experimental tests produced greater thanor equal to 80 mol % product gas. Tar mole percent rangedbetween 10 and 15%, while char was only about 2−8 mol %.Product gas yield from methane-activated biomass gasificationcan be termed as biomass conversion obtained through thereaction. Gas yield, tar, and char composition of the biomasscarbon balance are presented in Figure 6. It was observed thatproduct gas yield was 88 mol % for the reaction at 950 °C and

84 mol % for the reaction at 850 °C. Gas yield substantiallyincreased from 37.5 mol % at 750 °C (not shown here) to 84mol % at 850 °C and further to 88 mol % at 950 °C. However,gas yield decreased slightly from 84 to 80 mol % at 850 °Cwhen the gas feed methane concentration was increased from 5to 15 vol %. A similar trend was also observed at 950 °C as gasyield decreased from 88 to 82 mol % when methaneconcentration was increased from 5 to 15 vol %. A marginalincrease in char yield from 3 to 8 mol % at 850 °C and from 2to 7 mol % at 950 °C was also observed when methaneconcentration was increased from 5 to 15 vol %. This could beexplained based on the catalyst deactivation phenomenonpossibly occurring due to the high methane coverage on Feand Mo active sites leading to a paucity of sites available formethane−biomass reaction. Moreover, without methane in thegas feed, biomass conversion was much higher on the FeMo1catalyst (87.5% at 850 °C and 90% at 950 °C) compared tothat in methane-activated biomass gasification tests. However,>80 mol % product gas yield is an indication that low methaneconcentration leads to high methane and biomass conversiondue to Fe and Mo being conducive to methane activation andbiomass hydrodeoxygenation. Gas yield, char, and tarcomponents in biomass gasification without methane andmethane-activated biomass gasification on FeMo1 catalyst aregiven in Table 4 and Figure 6.

As described in Table 4, coking on the catalyst was between10−30 wt % of the original weight of catalyst after reduction.Under identical conditions, biomass gasification in the absenceof methane in the gas feed yielded almost no recoverable cokeon the FeMo1 catalyst. No weight loss was seen in the catalysteither before or after calcination. It can be concluded that cokedeposition on both FeMo1 and FeMo2 catalysts was due tomethane decomposition when 5−15% was used in the gas feed.The amount of coke from each test using methane is shown inTable 4. No considerable coke formation on the catalyst wasobserved for biomass gasification without methane, indicatingthat the aromatic components of lignin with branchedfunctional groups like carbonyl carbon (CO) and hydroxylcarbon (C−OH) undergo surface reactions by latching ontothe ZSM-5 acidic sites. Most of the oxygen from the biomasspossibly reacted on the surface with the available hydrogen toundergo reverse SMR without external methane (eq 2). Thequantifications of moles of hydrogen, carbon monoxide, carbondioxide, and methane are provided in Table S1.

Synergistic Methane-Activated Biomass Gasification.Synergistic gasification of hardwood biomass with methanewas studied in the fixed-bed reactor on FeMo1 and FeMo2catalysts for methane concentrations ranging from 5−15 mol

Figure 6. Gas, tar, and char yield obtained in biomass gasificationwithout methane and in methane-activated synergistic catalyticbiomass gasification at 850 and 950 °C and CH4 gas feedconcentrations 0, 5, 10, and 15 vol % performed on FeMo1 catalyst.

Table 4. Co-Gasification of Biomass and MethaneGasification over Fe−Mo/ZSM-5a

parameters 850 °C 950 °C

gas yield in biomass, no methane (mol %) 87.5 90.1

coking FeMo1 (mg, % CH4)75.6, 5 68.2, 5135, 10 136, 10246, 15 204.5, 15

coking FeMo2 (mg, % CH4)51.5, 5

N/A169.2, 10267.3, 15

aMeasured amount of coke on catalyst and gas yield for no-methanebiomass gasification.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

F

%. Figures 7 and 8 show the concentration of typical syngascomponents obtained from methane-activated biomass gas-

ification. Hydrogen was the dominant species in the productgas with hydrogen to carbon monoxide (H2/CO) ratio as highas 7.5 when reaction was performed at 950 °C with 5% CH4 inthe gas feed along with 95% carrier gas N2. As the methaneconcentration is further increased to 10 and 15%, the H2/COratio dropped sharply at 950 °C to 3.8 and 3.7, respectively.When the same reaction is performed at 850 °C with 5, 10, and15% CH4 in the feed, H2/CO drops steadily from 6 to 4.2. Inthe absence of a catalyst, typical hardwood biomass gasificationproduced a H2/CO ratio of 0.3−0.5. This sharp increase inhydrogen gained by adding methane is an interesting aspect ofthis study, which likely shows the synergy between methaneand biomass at high temperatures on a catalyst surface. Thesame gasification reaction when performed under identicalconditions on the same catalyst without methane in the gasfeed shows contrasting results. As high as 70 mol % methanewas seen in the product gas with hydrogen yield averaging at17% at 850 and 950 °C. CO and CO2 yield averaged at 19 and

11%, respectively, at 850 °C as seen in Figure 5. At 950 °C,CO yield increased to 25%, and CO2 decreased to 4%.However, the average methane yield was more than 50% of thetotal moles obtained in the product gas at both 850 and 950°C. Hydrogen mole percent in biomass that was used for thestudy is almost equal to the carbon mole percent (see Figure5). Biomass gasification typically yields H2/CO less than orclose to 1.54 The synergy between methane and biomassreacting together on an active catalyst like FeMo/ZSM-5 wasapparent from the multifold increase in the hydrogen yield.The presence of methane in the gas feed suggested anequilibrium shift in some of the typical gasification reactionswith SMR possibly occurring on the active sites of Mo and Fe.Both molybdenum (Mo) and iron (Fe) are known to bemoderately oxophilic: They have a higher binding affinity tooxygen as compared to those of metals like Zn, Ni, and Cu, butthey have a lower oxygen binding affinity as compared to thoseof Ti, V, and Sc among the transition state or d block metals.Mo is slightly more oxophilic than Fe (0.6 and 0.4), thushaving the capability to activate C−OH and CO type ofbonds abundantly present in the complex array of aromatics inlignin component of the hardwood biomass.62 Hydrodeoxyge-nation (HDO) reactions studied on PdZn surfaces have shownthat Zn, being an oxophilic metal, latches the oxygen present inthe functional groups thus activating C−OH bonds.62

Figure 9 shows a representative structure of aromatic andfurfural chains and −OH functional groups in lignin. The

loosely bonded oxygen and hydroxyl groups on the surfacepotentially react with the hydrogen obtained from devolatiliza-tion gases of biomass to produce H2O adsorbates on the activesites of Mo and Fe. Methane in the feed reacts with the H2Oadsorbates on the surface sites undergoing high-temperatureSMR to produce hydrogen and carbon monoxide. H2Oadosrbates would evolve by selective adsorption of lignocellu-losic oxygenates like alcohols, phenols, furfurals, ethers, andacids. High-temperature thermal cracking of lignoceullosiccomponents to single-chain phenols, furfurals, and alcoholsallows for passage through microporous ZSM-5 channels andselective adsorption of iron and molybdenum active sites in theZSM-5 framework. The presence of Fe and Mo on acidicZSM-5 in metallic form helps shift the SMR reaction forwardthus producing more H2 and CO. Reverse SMR is one of themain reactions grouped in biomass gasification reactionchemistry. Although reverse SMR is a highly exothermicreaction with ΔHrxn,298 K of −206 kJ mol−1, in the presence ofmethane as a gas feed, the reaction equilibrium seems to shiftto the right to form hydrogen and carbon monoxide. Thislikely scenario explains the more than 85% yield of hydrogen inthe gas feed with a very high H2/CO ratio of 7.5 in one of thecases.The effect of temperature and methane concentration on

biomass gasification was compared at 750, 850, and 950 °C inthe presence of 5 mol % CH4. As seen in Figure10, methane-

Figure 7. Methane-activated biomass gasification at 850 °C formethane concentrations of 5, 10, and 15 mol % using FeMo1 catalyst.

Figure 8. Methane activated biomass gasification at 950 °C formethane concentration of 5, 10, and 15 mol % using FeMo1 catalyst.

Figure 9. Representation of the type of C−O bonds in lignin. Rrepresents a type of aromatic structure similar to that in the rest of theproposed description of the lignin molecule.64

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

G

activated biomass gasification reaction produces a H2/CO ratioof 1.18 which is higher than that obtained from fixed-bedbiomass gasification but not suitable for hydrogen-rich syngasproduction. The product gas composition obtained frommethane-activated biomass gasification reaction at 850 and 950°C with 10 vol % CH4 in the gas feed on FeMo1 catalyst isshown in Figure 11. As shown in Figure 10, the H2/CO ratio

drops sharply when the methane concentration is increased to10 mol % at 950 °C, whereas the decrease in H2/CO ratio isnot as steep at 850 °C. A decrease in the H2/CO ratio from 7.4at 950 °C to 3.7 when methane concentration was increasedfrom 5 to 15 vol % is possibly due to partial catalystdeactivation and reaction kinetics. The Fe−Mo/ZSM-5catalyst has a tendency to form CNTs due to carbondeposition. It appears from the H2/CO trend that theoptimum methane concentration in the gas feed giveshydrogen-rich syngas. When excess methane is available inthe reaction atmosphere, high-temperature methane decom-position leads to coke deposition on the catalyst. However, we

note that the catalyst subjected to higher methane concen-trations is not completely deactivated since the H2/CO rangesfrom 3.7 to 5 for 10−15 vol % CH4 at 850 and 950 °C.Without catalyst and methane in the gas feed, the H2/CO ratioreached a maximum of 0.9 which is typical for the givenbiomass gasification. Another possibility behind decrease inH2/CO is an adverse effect of high concentrations of methanein the gas feed on other hydrogen producing reactions likeWGS. This may be due to gradual increase in Fe/O ratio at thewustite−Fe interface when the temperature increases from 700and 950 °C, leading to the loss of active sites on the surfaceand consequently leading to decrease in hydrogen productionand a slight increase in carbon monoxide production.63

Standard deviations and 95% confidence interval for biomassgasification without methane and methane-activated biomassgasification are shown in Table 5.

Proposed Reaction Mechanism of Synergistic Meth-ane−Biomass Gasification. The following elementaryreaction pathways are proposed to illustrate the interactionof methane with water molecules produced from biomass overthe catalyst surface. Reactant molecules are associativelyadsorbed on a single metal site (Fe or Mo) forsimplification.65−69 The Langmuir−Hinshelwood adsorption

Figure 10. Effect of temperature on biomass gasification on FeMo1catalyst in the presence of 5 vol % methane at 750, 850, and 950 °C.

Figure 11. Effect of temperature on biomass gasification on FeMo1catalyst in the presence of 5 vol % methane at 850 and 950 °C.

Table 5. Statistical Analysis for Product Composition ofSyngas Components for Different Reaction Temperatureswithout CH4 and with 5−15 vol % CH4

test conditionssyngasspecies average

standarddeviation

confidenceinterval

850 °C, no CH4

H2 16.47 0.11 ±0.15CO 18.63 4.64 ±6.43CO2 10.81 2.51 ±3.48CH4 54.09 7.26 ±10.06

950 °C, no CH4

H2 16.35 1.26 ±1.43CO 23.78 8.11 ±9.17CO2 4.60 3.29 ±3.73CH4 55.27 11.93 ±13.50

850 °C, 5% CH4

H2 72.64 15.33 ±21.2CO 12.29 2.96 ±4.1CO2 0.90 0.32 ±0.45CH4 0.81 0.29 ±0.4

850 °C, 10%CH4

H2 81.58 0.78 ±0.88CO 16.42 0.44 ±0.49CO2 1.05 0.61 ±0.69CH4 0.94 0.55 ±0.62

850 °C, 15%CH4

H2 68.26 13.28 ±18.41CO 16.14 0.79 ±1.10CO2 2.77 0.29 ±0.40CH4 2.49 0.26 ±0.36

950 °C, 5% CH4

H2 74.90 13.98 ±19.37CO 10.04 1.94 ±2.69CO2 4.73 3.86 ±5.35CH4 4.26 3.48 ±4.82

950 °C, 10%CH4

H2 63.30 20.41 ±28.29CO 16.83 6.64 ±9.21CO2 0.82 0.61 ±0.85CH4 0.74 0.55 ±0.76

950 °C, 15%CH4

H2 70.84 1.57 ±2.18CO 18.93 1.45 ±2.00CO2 2.13 3.01 ±4.17CH4 3.83 0.00 0

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

H

mechanism is considered in our analysis (Table 6, where ‘*’represents an active site):[R−C−OH] is the type of bond present in the lignin

component of hardwood biomass. C−OH bonds are possiblyactivated on either Fe or Mo active sites which interact withthe surface and methane as described in the equations above.On the basis of the discussion of the experimental results ofmethane−biomass gasification, the common conditions for allmethane concentrations and iron loadings were a high H2/COratio, 1−5 mol % CO2, CH4 yield 50−90% less thanconventional biomass gasification, and a high hydrogen molepercent in the gas yield. This pathway can explain the majordistinction between biomass gasification, methane dissociation,methane SMR, and synergistic methane−biomass gasification.On the basis of the experimental data, the followingassumptions can be made for the above mechanistic model.Rate constant k18≫ k-18, which implies that desorption of H2in gaseous form from the surface is fast step. On the contrary,based on the product composition, it could be argued that rateconstant k-16 ≫ k16 and rate constant k-15 ≫ k15, implyingthat both CO2 and CO are highly stable on the surface,respectively. This could be due to the oxygen in CO and CO2binding strongly with oxophilic Fe/Mo metals on the surface.This could also be the reason behind the 50−90% less yieldobtained for CO and CO2. Equation 17 shows two surfacehydrogen atoms forming H2*. Desorption of the surface H2appears to be relatively easy if the assumption for eq 17 wastrue. Among the fast, nonequilibrium reactions eqs 11 and 14,the rates of formation of CHO*, H2, and surface-adsorbed H*must be higher than that of CO2* and H2* formation (k11 ≫k14), as apparent from the very low CO2 yield. Experimentalstudies indicate that water−gas shift does not contributesignificantly to the gas yield apparent from low CO2 yield in

the gas phase. Surface-adsorbed atomic hydrogen (H*) ispossibly in the neighborhood with other H* adsorbates, thuscombining to form more hydrogen (H2) in the gas phase.Based on high hydrogen yield in the gas phase, it can be saidthat k17 ≫ k-17. The remaining eqs 7, 9, 10, 12, and 13 couldbe competent in being the rate-limiting steps. However, amongthese five equations, reactions from eqs 9 and 13 could be therate-limiting steps depending on the stability and bindingenergy of [R−C+−O−*H] on the given surface. Themechanistic pathway proposed above for a unique SMRreaction explains the high H2 yield (>80 mol %) in the productgas.

Comparison of Catalytic and Noncatalytic BiomassGasification. Figures 12 and 13 compare the productcomposition and H2/CO ratios for hardwood biomassgasification without catalyst and external methane, biomassgasification on ZSM-5 catalytic support without methane, andmethane-activated biomass gasification with 5 and 10 vol %CH4 on FeMo1 and FeMo2 catalysts. Biomass-only gas-ification was performed noncatalytically and on ZSM-5. OnFeMo1 and FeMo2 catalysts, biomass−methane experimentswere performed with methane concentrations of 5 and 10%.The H2/CO ratio on ZSM-5 more than doubled compared tothat in a typical noncatalytic test. This indicated that thepresence of acid sites on ZSM-5 along with Fe active sitesprovide favorable atmosphere for WGS and SMR reactions.This is evident from the decrease in methane concentrationand increase in carbon dioxide formation. The addition of Feand Mo to ZSM-5 leads to the synergistic interaction betweenmethane and biomass to produce hydrogen-rich syngas. TheH2/CO ratio increases significantly when methane−biomassgasification was performed on FeMo1 and FeMo2 catalyst.With 5% CH4 in the gas feed, a H2/CO ratio of ∼7 was

Table 6. Proposed Reaction Pathway for Synergistic Methane−Biomass Gasification

+ * *‐

CH CHk

k4(g)

6

64H Ioo eq 6: methane adsorption on the active site

[ − − ] + * [ − − * ]‐

R C OH R C O Hk

k(g)

7

7H Ioo eq 7: selective adsorption of functional group oxygen atom of the lignocellulosic biomass on Fe or Mo

+ * *‐

H 2Hk

k2(g)

7

7H Ioo eq 8: gas-phase hydrogen dissociation on catalyst surface

[ − − * ] + * [ − − * ]‐

+ −R C O H H R C O Hk

k

9

92H Ioo eq 9: coordination of adsorbed oxygen with H2 present in the gas phase

[ − − * ] ′ + *+ −

‐R C O H R H O

k

k2

10

102H Iooo eq 10: H2O molecule breaks off from the phenolic carbon and forms steam adsorbate

* + * ⎯→⎯ * + + *CH H O CHO 2H Hk

4 211

2(g) eq 11: first equation of methane reforming with steam adsorbate from step 4

* + * * + *‐

CHO CO Hk

k

12

12H Iooo eq 12: intermediate CHO* converting to CO* and H* adsorbates

[ − − * ] + * [ − − * ]‐

+ −R C O H H R C O Hk

k

13

132H Iooo eq 13: further phenolic oxygens coordinate with H* atom adsorbate and follows similar kinetics as steps 3−6

* + * ⎯→⎯ * + *CO H O CO Hk

214

2 2 eq 14: water−gas shift reaction

* + *‐

CO COk

k

15

15(g)H Iooo eq 15: CO* desorption

* + *‐

CO COk

k2

16

162(g)H Iooo eq 16: CO2* desorption

* + *‐

2H Hk

k

17

172(g)H Iooo eq 17: H* atom adsorbates combine to form gas phase H2

* + *‐

H Hk

k2

18

182(g)H Iooo eq 18:H2* desorption

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

I

obtained on FeMo1 which increased further to 10 on theFeMo2 catalyst. With 10% CH4 in the feed, the H2/CO ratio islower than that for 5% CH4, 5 for FeMo1 and ∼8 for FeMo2catalyst, respectively. The lower H2/CO ratios observed forhigher methane concentrations are possibly due to rapidcatalyst deactivation due to coke deposition and hightemperature. The comparison of catalytic and noncatalyticbiomass and methane-activated biomass gasification was intune with the proposed reaction mechanism in Table 6.

■ CONCLUSIONSCo-gasification of methane−biomass was conducted on 0.5%Fe−4% Mo/ZSM-5 and 1.5%Fe−-4%Mo/ZSM-5 catalysts.The results showed a synergy between additional methane andbiomass that produced hydrogen-rich syngas with more than80% hydrogen in the gas yield. The high hydrogen productionon the Fe−Mo based catalyst was probably due to the

oxophilic nature of both iron (Fe) and molybdenum (Mo),which converted the oxygen in the various functional groups oflignin to steam that in turn reacted with the available methaneto form more hydrogen. Catalyst surface and active sites on themetals were also favorable to a HT-WGS reaction, as suggestedby the multifold increase in the H2/CO ratio. However, theH2/CO ratio dropped by more than 50% when the methaneconcentration increased from 5−15 vol %, possibly due topartial catalyst deactivation and adverse reaction kinetics. Thesynergistic biomass−methane nonoxidative gasification pro-vided a good motivation for the catalytic conversion of biomassto fuels with natural gas utilization. The synergistic relationshipbetween methane and hardwood biomass will be investigatedin detail using transition metals like Ni and Ru along with DFTcalculations for getting information on the reaction inter-mediates, activation barriers, transition state pathway, and highselectivity toward hydrogen.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.9b02663.

Moles of gas species, SEM characterization of spentFeMo1 catalyst (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: john.hu@mail.wvu.edu.

ORCIDCosmin E. Dumitrescu: 0000-0003-1797-4584Jianli Hu: 0000-0003-3857-861XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We are pleased to acknowledge U.S. Department of Energyand National Energy Technology Laboratory, Morgantown,along with Leidos Research Support Team for the continuoussupport and cooperation for the specific work and broadly forthe NETL Gasifier Support Stand Project (Project No.10024037). This work was funded by the Department ofEnergy, National Energy Technology Laboratory, an agency ofthe United States Government, through a support contractwith Leidos Research Support Team (LRST). Neither theUnited States Government nor any agency thereof, nor any oftheir employees, nor LRTS, nor any of their employees, makesany warranty, expressed or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by theUnited States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarilystate or reflect those of the United States Government or anyagency thereof.

Figure 12. Hardwood biomass gasification without catalyst andexternal methane, biomass gasification on ZSM-5 catalytic supportwithout methane, and methane-activated biomass gasification with 5vol % CH4 on FeMo1 and FeMo2 catalysts.

Figure 13. Hardwood biomass gasification without catalyst andexternal methane, biomass gasification on ZSM-5 catalytic supportwithout methane, and methane-activated biomass gasification with 10vol % CH4 on FeMo1 and FeMo2 catalysts.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

J

■ REFERENCES(1) International Energy Agency. World Energy Statistical Outlook,2018. https://www.iea.org/weo2018/.(2) Summary for Policymakers. In Global Warming of 1.5°C;Intergovernmental Panel on Climate Change, 2018.(3) Hansen, J.; Ruedy, R.; Sato, M.; Lo, K. Global surfacetemperature change. Rev. Geophys. 2010, 48, 345.(4) Lindsey, R.; Dlugokencky, E. Climate change − atmosphericcarbon dioxide. 2019. https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide(5) Intergovernmental Energy Agency. Renewables 2018. https://www.iea.org/renewables2018/.(6) Annual Energy Outlook 2016; Report No. DOE/EIA-0383(2016); U.S. Energy Information Agency Administration, 2016.(7) Fantini, M. Biomass Availability, Potential and Characteristics.Biorefineries 2017, 57, 21−52.(8) Molino, A.; Larocca, V.; Chianese, S.; Musmarra, D. BiofuelsProduction by Biomass Gasification: A Review. Energies 2018, 11,811.(9) Qian, K.; Kumar, A.; Patil, K.; Bellmer, D.; Wang, D.; Yuan, W.;Huhnke, R. L. Effects of Biomass Feedstocks and GasificationConditions on the Physiochemical Properties of Char. Energies 2013,6, 3972−3986.(10) Uzi, A.; Shen, Y.; Kawi, S.; Levy, A.; Wang, C. Permittivity andchemical characterization of woody biomass during pyrolysis andgasification. Chem. Eng. J. 2019, 355, 255−268.(11) Fatih Demirbas, M.; Balat, M. Biomass pyrolysis for liquid fuelsand chemicals: A review. J. Sci. Ind. Res. 2007, 66, 797−804.(12) Kumar, A.; Jones, D. D.; Hanna, M. A. ThermochemicalBiomass Gasification: A Review of the Current Status of theTechnology. Energies 2009, 2, 556−581.(13) Czernik, S.; Bridgwater, A. V. Overview of Applications ofBiomass Fast Pyrolysis Oil. Energy Fuels 2004, 18 (2), 590−598.(14) Wang, D.; Czernik, S.; Chornet, E. Production of Hydrogenfrom Biomass by Catalytic Steam Reforming of Fast Pyrolysis Oils.Energy Fuels 1998, 12 (1), 19−24.(15) Bridgwater, A. V. Review of fast pyrolysis of biomass andproduct upgrading. Biomass Bioenergy 2012, 38, 68−94.(16) Chen, N. Y.; Walsh, D. E.; Koenig, L. R. Fluidized-BedUpgrading of Wood Pyrolysis Liquids and Related Compounds In:Pyrolysis Oils f rom Biomass. ACS Symp. Ser. 1988, 376, 277−289.(17) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Bilbao, J.Transformation of Oxygenate Components of Biomass Pyrolysis Oilon a HZSM-5 Zeolite. I. Alcohols and Phenols. Ind. Eng. Chem. Res.2004, 43, 2610−2618.(18) Czernik, S.; Bridgwater, A. V. Overview of Applications ofBiomass Fast Pyrolysis Oil. Energy Fuels 2004, 18 (2), 590−598.(19) Sharma, R. K.; Bakhshi, N. N. Catalytic Upgrading of PyrolysisOil. Energy Fuels 1993, 7, 306−314.(20) Carlson, T. R.; Vispute, T. P.; Huber, G. W. Green Gasoline byCatalytic Fast Pyrolysis of Solid Biomass Derived Compounds.ChemSusChem 2008, 1, 397−400.(21) French, R.; Czernik, S. Catalytic pyrolysis of biomass forbiofuels production. Fuel Process. Technol. 2010, 91, 25−32.(22) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy Fuels 2006, 20, 848−889.(23) Huber, G. W.; Corma, A. Synergies between Bio- and OilRefineries for the Production of Fuels from Biomass. Angew. Chem.,Int. Ed. 2007, 46, 7184−7201.(24) Wright, M. M.; Brown, R. C.; Boateng, A. A. Distributedprocessing of biomass to bio-oil for subsequent production ofFischer−Tropsch liquids. Biofuels, Bioprod. Biorefin. 2008, 2, 229−238.(25) Mettler, M. S.; Vlachos, G.; Dauenhauer, P. J. Top tenfundamental challenges of biomass pyrolysis for biofuels. EnergyEnviron. Sci. 2012, 5, 7797−7809.(26) Evans, R. J.; Milne, T. A. Chemistry of Tar Formation andMaturation in the Thermochemical Conversion of Biomass. In

Developments in Thermochemical Biomass Conversion; Bridgwater, A.V., Boocock, D. G. B., Eds.; Springer: Dordrecht, 1997.(27) Demirbas, A. Yields of oil products from thermochemicalbiomass conversion processes. Energy Convers. Manage. 1998, 39 (7),685−690.(28) Besson, M.; Gallezot, P.; Pinel, C. Conversion of Biomass intoChemicals over Metal Catalysts. Chem. Rev. 2014, 114, 1827−1870.(29) Mullen, C. A.; Boateng, A. A.; Dadson, R. B.; Hashem, F. M.Biological Mineral Range Effects on Biomass Conversion to AromaticHydrocarbons via Catalytic Fast Pyrolysis over HZSM-5. Energy Fuels2014, 28, 7014−7024.(30) Xie, Y.; Su, Y.; Wang, P.; Zhang, S.; Xiong, Y. In-situ catalyticconversion of tar from biomass gasification over carbon nanofibers-supported Fe-Ni bimetallic catalysts. Fuel Process. Technol. 2018, 182,77−87.(31) Wang, A.; Song, H. Maximizing the production of aromatichydrocarbons from lignin conversion by coupling methane activation.Bioresour. Technol. 2018, 268 (July), 505−513.(32) Balagurumurthy, B.; Singh, R.; Bhaskar, T. Catalysts forThermochemical Conversion of Biomass. Recent Advances in Thermo-Chemical Conversion of Biomass 2015, 109−132.(33) Nishikawa, J.; Nakamura, K.; Asadullah, M.; Miyazawa, T.;Kunimori, K.; Tomishige, K. Catalytic performance of Ni/CeO2/Al2O3 modified with noble metals in steam gasification of biomass.Catal. Today 2008, 131, 146−155.(34) Ye, M.; Tao, Y.; Jin, F.; Ling, H.; Wu, C.; Williams, P. T.;Huang, J. Enhancing hydrogen production from the pyrolysis-gasification of biomass by size-confined Ni catalysts on acidicMCM-41 supports. Catal. Today 2018, 307, 154−161.(35) Zhang, J.; Li, X.; Xie, W.; Hao, Q.; Chen, H.; Ma, X. Handysynthesis of robust Ni/carbon catalysts for methane decomposition byselective gasification of pine sawdust. Int. J. Hydrogen Energy 2018, 43,19414−19419.(36) Shahbaz, M.; yusup, S.; Inayat, A.; Patrick, D. O.; Ammar, M.The influence of catalysts in biomass steam gasification and catalyticpotential of coal bottom ash in biomass steam gasification: A review.Renewable Sustainable Energy Rev. 2017, 73, 468−476.(37) Sutton, D.; Kelleher, B.; Ross, J. R. H. Review of literature oncatalysts for biomass gasification. Fuel Process. Technol. 2001, 73,155−173.(38) Chaiprasert, P.; Vitidsant, T. Effects of promoters on biomassgasification using nickel/dolomite catalyst. Korean J. Chem. Eng. 2009,26, 1545−1549.(39) Kumar, A.; Yang, Z.; Apblett, A. W. Synergistic co-pyrolysis ofbiomass and methane for hydrobarbon fuels and chemicalsproduction, WO patent no. 2016/196517 Al, 2016.(40) Robinson, B.; Bai, X.; Samanta, A.; Abdelsayed, V.; Shekhawat,D.; Hu, J. Stability of Fe- and Zn-Promoted Mo/ZSM-5 Catalysts forEthane Dehydroaromatization in Cyclic Operation Mode. EnergyFuels 2018, 32, 7810−7819.(41) Galvita, V.; Sundmacher, K. Hydrogen production frommethane by steam reforming in a periodically operated two-layercatalytic reactor. Appl. Catal., A 2005, 289, 121−127.(42) Blasco, T.; Corma, A.; Martínez-Triguero, J. Hydrothermalstabilization ofZSM-5 catalytic-cracking additives by phosphorusaddition. J. Catal. 2006, 237, 267−277.(43) Ogura, M.; Shinomiya, S. Y.; Tateno, J.; Nara, Y.; Nomura, M.;Kikuchi, E.; Matsukata, M. Alkali-treatment technique newmethod for modification of structural and acid-catalytic propertiesof ZSM-5 zeolites. Appl. Catal., A 2001, 219, 33−43.(44) Ayillath Kutteri, D.; Wang, I-W.; Samanta, A.; Li, L.; Hu, J.Methane decomposition to tip and base grown carbon nanotubes andCOx-free H2 over mono- and bimetallic 3d transition metal catalysts.Catal. Sci. Technol. 2018, 8, 858−869.(45) Zhang, L.; Wang, X.; Millet, J. M. M.; Matter, P. H.; Ozkan, U.S. Investigation of highly active Fe-Al-Cu catalysts for water-gas shiftreaction. Appl. Catal., A 2008, 351, 1−8.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

K

(46) Wang, X.; Gorte, R. J. The effect of Fe and other promoters onthe activity of Pd/ceria for the water-gas shift reaction. Appl. Catal., A2003, 247, 157−162.(47) Andreeva, D.; Tabakova, T.; Idakiev, V.; Christov, P.;Giovanoli, R. Au/α-Fe2O3 catalyst for water ± gas shift reactionprepared by deposition ± precipitation. Appl. Catal., A 1998, 169, 9−14.(48) Wang, A.; Austin, D.; Qian, H.; Zeng, H.; Song, H. CatalyticValorization of Furfural under Methane Environment. ACSSustainable Chem. Eng. 2018, 6, 8891−8903.(49) Pineda, D. I. Modeling Biomass Gasification Surface Reactions:The Effect of Hydrogen Inhibition. Thesis. University of California,2014.(50) Biagini, E.; Lippi, F.; Petarca, L.; Tognotti, L. Devolatilizationrate of biomasses and coal-biomass blends: an experimentalinvestigation. Fuel 2002, 81, 1041−1050.(51) Albadri, S.; Jiang, Y.; Wagland, S. Reduce toxic emissions of As,Cr, and Cu phases during woody biomass gasification: Athermodynamic equilibrium study. In Proc. 5th Int. Conf. Environ.Renewable Energy (ICERE 2018), May 18−19, 2018, Vienna, Austria,2018.(52) Nikoo, M. B.; Mahinpey, N. Simulation of biomass gasificationin fluidized bed reactor using ASPEN PLUS. Biomass Bioenergy 2008,32, 1245−1254.(53) Gebhard, S. C.; Wang, D.; Overend, R. P.; Paisley, M. A.Catalytic Conditioning of synthesis gas produced by biomassgasification. Biomass Bioenergy 1994, 7, 307−313.(54) Zhou, B.; Dichiara, A.; Zhang, Y.; Zhang, Q.; Zhou, J. Tarformation and evolution during biomass gasification: An experimentaland theoretical study. Fuel 2018, 234, 944−953.(55) Rakesh N; Dasappa, S. A critical assessment of tar generatedduring biomass gasification - Formation, evaluation, issues andmitigation strategies. Renewable Sustainable Energy Rev. 2018, 91,1045−1064.(56) Ferrin, P.; Simonetti, D.; Kandoi, S.; Kunkes, E.; Dumesic, J. A.;Nørskov, J. K.; Mavrikakis, M. Mavrikakis. Modeling EthanolDecomposition on Transition Metals: A Combined Application ofScaling and Brønsted-Evans-Polanyi Relations. J. Am. Chem. Soc.2009, 131, 5809−5815.(57) Valderrama Rios, M. L.; Gonzalez, A. M.; Lora, E. E. S.;Almazan del Olmo, O. A. Reduction of tar generated during biomassgasification: A review. Biomass Bioenergy 2018, 108, 345−370.(58) Perez, J. F.; Melgar, A.; Benjumea, P. N. Effect of operating anddesign parameters on the gasification/combustion process of wastebiomass in fixed bed downdraft reactors: An experimental study. Fuel2012, 96, 487−496.(59) Li, Y.; Wang, Y.; Zhang, X.; Mi, Z. Thermodynamic analysis ofautothermal steam and CO2 reforming of methane. Int. J. HydrogenEnergy 2008, 33 (10), 2507−2514.(60) Kargbo, D. M.; Wilhelm, R. G.; Campbell, D. J. Natural GasPlays in the Marcellus Shale: Challenges and Potential Opportunities.Environ. Sci. Technol. 2010, 44 (15), 5679−5684.(61) Wang, D.; Lunsford, J. H.; Rosynek, M. P. Characterization of aMo/ZSM-5 Catalyst for the Conversion of Methane to Benzene. J.Catal. 1997, 169, 347−358.(62) Kepp, K. P. A Quantitative Scale of Oxophilicity andThiophilicity. Inorg. Chem. 2016, 55, 9461−9470.(63) Chen, R. Y.; Yuen, W. Y. D. Review of the High-TemperatureOxidation of Iron and Carbon Steels in Air or Oxygen. Oxid. Met.2003, 59, 433−467.(64) Bajpai, P. Structure of lignocellulosic biomass. SpringerBriefs inMolecular Science 2016, 7.(65) Arcotumapathy, V.; Alenazey, F. S.; Al-Otaibi, R. L.; Vo, D.-V.N.; Alotaibi, F. M.; Adesina, A. A. Mechanistic investigation ofmethane steam reforming over Ce-promoted Ni/SBA-15 catalyst.Appl. Petrochem. Res. 2015, 5, 393−404.(66) Hou, K.; Hughes, R. The kinetics of methane steam reformingover a Ni/α-Al2O catalyst. Chem. Eng. J. 2001, 82, 311−328.

(67) Froment, G. F. Production of synthesis gas by steam- and CO2-reforming of natural gas. J. Mol. Catal. A: Chem. 2000, 163, 147−156.(68) Maier, L.; Schadel, B.; Herrera Delgado, K.; Tischer, S.;Deutschmann, O. Methane Over Nickel: Development of a Multi-Step Surface Reaction Mechanism. Top. Catal. 2011, 54, 845−858.(69) Maestri, M.; Vlachos, D. G.; Beretta, A.; Groppi, G.; Tronconi,E. Steam and dry reforming of methane on Rh: Microkinetic analysisand hierarchy of kinetic models. J. Catal. 2008, 259, 211−222.

ACS Sustainable Chemistry & Engineering Research Article

DOI: 10.1021/acssuschemeng.9b02663ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

L