Review of hydrogen-enriched gas production from steam gasicationof biomass: The prospect of CaO-based chemical looping gasication

Jakkapong Udomsirichakorn, P. Abdul Salam n

Energy Field of Study, School of Environment, Resources and Development, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120,Thailand

a r t i c l e i n f o

Article history:Received 1 December 2012Received in revised form26 September 2013Accepted 19 October 2013Available online 20 November 2013

Keywords:Hydrogen productionBiomass steam gasicationCO2 captureCatalytic tar reformingCaOChemical looping gasication

a b s t r a c t

Global warming, climate change and energy security issues are the forces driving the fossil fuel basedenergy system towards renewable and sustainable energy. Hydrogen as a clean energy carrier is believedto be the most promising source to replace fossil fuel. Biomass gasication with the presence of steamoffers a feasible, sustainable, and environment-friendly option as well as a favorable alternative forhigher hydrogen yields and for large-scale hydrogen production which can satisfy the need of hydrogenin the future. However, the process suffers from the problem of undesirable CO2 and tar formation.Calcium oxide (CaO) has been acknowledged as a catalyst to produce hydrogen-rich gas and has currentlygained broad attention due to its cheapness and abundance. Nevertheless, the deactivation of CaO aftercarbonation reaction is challenging for continuous hydrogen production and economical perspective. Toconquer such challenge, the concept of CaO-based chemical looping gasication (CaO-based CLG) hasemerged recently. Additionally, due to its energy-efcient and environment-friendly aspects, the CaO-based CLG using biomass as feedstock is gaining more attention in recent years. This study rst presentsa review on conventional steam gasication of biomass without catalysts for producing hydrogen-richproduct gas. The effects of key variables, such as biomass characteristics, gasier temperature, steam-to-biomass ratio (S/B) and equivalence ratio (ER), on hydrogen-enriched gas production are discussed basedon recent researches and developments. Then the use of CaO in biomass steam gasication for hydrogenproduction with in situ CO2 capture and tar reduction is described. The prospect of CaO-based CLG usingbiomass fuel is also discussed as a promising process for renewable, sustainable and environment-friendly hydrogen production.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5662. Routes for biomass to hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5663. Hydrogen-enriched gas production from biomass gasication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567

3.1. Mechanism of biomass gasication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5673.2. Types of gasication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568

4. Hydrogen production costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5695. Parametric study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

5.1. Effect of biomass characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5695.2. Effect of gasier temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5705.3. Effect of steam-to-biomass ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5715.4. Effect of equivalence ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573

6. Challenges and prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5736.1. Problematic CO2 and tar production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5736.2. CaO-based in situ CO2 and tar reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5736.3. CaO-based chemical looping gasication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575

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n Corresponding author. Tel.: 66 2 524 5420; fax: 66 2 524 5439.E-mail address: salam@ait.ac.th (P.A. Salam).

Renewable and Sustainable Energy Reviews 30 (2014) 565579

7. Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577

1. Introduction

Global warming, climate change and energy security have beengaining more attention worldwide. These are the major forcesdriving the dependence on fossil energy towards renewable andsustainable energy. Hydrogen as a clean energy carrier has beenprojected as the most promising source of energy that can be usedin internal combustion engines as well as fuel cells with lesspollution on the environment, especially without CO2 emission[13]. Currently, there are a number of energy sources andtechnologies to produce hydrogen but about 99% of hydrogencomes from fossil fuels, mainly by steam reforming of natural gaswhich is the fossil fuel based process that could not address aboveserious concerns [4]. To meet the renewable and sustainablehydrogen production, biomass is considered as the ideal primaryenergy source for gradually replacing the depleting fossil fuels [5].It is also accepted as the greatest promise due to its availabilityeverywhere in the world [2].

Though hydrogen can be sustainably produced from biomass, thetechnologies for this are not yet fully developed [6]. The technologiesavailable for biomass conversion into hydrogen-rich gas can begenerally classied into biological and thermo-chemical routes [69].Among them, the latter is concluded to be more economically viableand likely to become competitive with the conventional natural gasreforming method [10,11]. Gasication as one of the thermo-chemicalprocesses is agreed to be favorable for hydrogen production andproved to be an effective and attractive method for converting biomassto energy [10,12]. Gasication is also suitable for large scale hydrogenproduction [9]. However, until today, all conventional process systemsrequired for hydrogen production is well established in commercialuse, except the biomass gasiers [6,13]. Thus, the development ofbiomass gasication technology for hydrogen production needs urgentattention.

The process of gasication takes place by converting a feed-stock such as biomass into combustible gas mixture, calledsynthesis gas or syngas, under the controlled conditions of limitedamount of oxygen [14,15]. Apart from oxygen, air, steam and theirmixtures can be used as the gasifying agent as well. However,gasifying biomass with steam as the gasication agent favors theproduction of syngas rich in hydrogen [1621]. In addition, it hasbeen recommended to be the most favorable option for enhancingboth hydrogen concentration and yield in the syngas produced[9,22,23]. Nevertheless, biomass steam gasication is inevitablyproblematic with undesirable CO2 and tar formed during theprocess. This may consequently incur a major cost penalty incommercial applications, if separation of hydrogen fromhydrogen-rich gas stream contained with impurities, e.g., CO2,CH4, CO and tar is needed [16,24]. Thus, if CO2 and tar producedduring steam gasication of biomass can be simultaneouslycaptured and cracked within the process, this technology is likelyto be much more attractive and cost-effective in the eld ofhydrogen production.

Calcium oxide (CaO) has been acknowledged as a catalyst toproduce hydrogen-rich gas [1921,25,26]. Utilization of calciumoxide (CaO) as a CO2 sorbent in biomass gasication is gainingmore interest due to its cheapness and abundance [3,9]. CaOadded to the process can play the important roles of not only CO2sorbent as mentioned above, but also tar cracking catalyst [9]. Thiscan demonstrate an economically feasible aspect of using CaO to

eliminate undesirable CO2 and tar within the process, instead ofthe need for expensive and complex downstream cleanup. None-theless, the deactivation of CaO after capturing CO2 can be achallenge in continuous hydrogen production. Frequent relling offresh CaO in the process will make the process less attractiveeconomically [27].

To overcome these challenges, the concept of CaO-basedchemical looping gasication (CaO-based CLG), which is basicallyaimed for continuous hydrogen production with in-process CO2capture, is thus emerged. Within this looping process, the usedCaO can be regenerated and then reused within the cyclic process.Additionally, due to its energy-efcient and environment-friendlyaspects, the CaO-based CLG process using biomass fuel, which isgaining more attention in recent years, can play an important roleas a promising technology for renewable, sustainable andenvironment-friendly hydrogen production in near future.

This study rst focuses on conventional steam gasication ofbiomass without catalysts for hydrogen-enriched gas production.A brief review on other technologies for hydrogen production frombiomass is also presented to point out why steam gasication ofbiomass is a promising pathway for sustainable hydrogen produc-tion. The effect of process parameters on hydrogen productionfrom steam gasication of biomass is also reviewed. Then the useof CaO in the process to eliminate undesirable CO2 and tarformation is described. The concept of CaO-based CLG of biomassis discussed as a promising process for renewable, sustainable andenvironment-friendly hydrogen production.

2. Routes for biomass to hydrogen

The current pathways for hydrogen production from biomass canbe classied into biological and thermo-chemical methods [69].

In biological methods, the specic processes can be classied asfollows: (i) biophotolysis of water using green algae and blue-green algae (cyanobacteria), (ii) photodecomposition, (iii) dark-fermentation, and (iv) hybrid system [28]. All biological processesof hydrogen production basically depend on the use of ahydrogen-producing microorganism. Biophotolysis is the methodto use light energy to split water molecules into oxygen andhydrogen ion in the presence of micro-algae or cyanobacteria. Anattractive and more favored possibility of using this route is theuse of solar radiation to directly split water to produce hydrogen[29]. Among all the processes of biological hydrogen production,photo-fermentation is favored because of its relatively higherhydrogen yields, its ability to utilize wide spectral light and to con-sume organic substrates derived from various waste sources [30].Nevertheless, some drawbacks of using this process reported byKirtay [31] are: (i) high-energy demand for nitrogenase enzyme;(ii) low solar energy conversion efciency; and (iii) demand foranaerobic photobioreactors covering large areas. Dark-fermen-tation is a ubiquitous reaction under anoxic conditions. Thisanaerobic reaction enables the mass production of hydrogen viarelatively simple processes from a variety of potentially usablesubstrates without using light. An important advantage of fermen-tation is fast degradation of solids and other complex organics foundin wastes and agricultural products [32]. However, while fermentationis fast, it is not yet efcient for capturing the energy value of biomassto hydrogen [32]. Moreover, even though the fermentative method has

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579566

been demonstrated in laboratory and accepted to be a promising routeof biological hydrogen production due to its high rate of hydrogenevolution, yields remain low and it is uncertain whether this technol-ogy can be developed to produce high yields of hydrogen and becomeeconomically competitive with gasoline or with other alternativehydrogen production routes [32]. Hybrid system is a combination ofphotodecomposition and dark-fermentation coupled with each otherin two subsequent stages to enhance the overall yield of hydrogen[32]. In such a system, the anaerobic fermentation of organic wastesproduces intermediates, such as low molecular weight organic acids,which are then converted into hydrogen by photosynthetic bacteria inthe second step in a photo-bioreactor. Tao et al. [33] reported that theyield of hydrogen produced by this hybrid system was increased two-fold as compared to that utilizing only dark-fermentation.

Operational processes of thermo-chemical method are fundamen-tally dependent on the environment of controlled operating para-meters in a reactor. The thermo-chemical routes for hydrogenproduction generally consist of pyrolysis and gasication. Pyrolysis isconversion of biomass to liquid, solid and gaseous fractions by heatingthe biomass in the absence of air or oxygen at around 500 1C [7].Although pyrolysis can produce solid, liquid, and gaseous products,most of them are designed only for production of char or liquidhydrocarbons as primary products of interest. However, reforming ofthese primary products can also generate hydrogen-rich gas but thismakes the whole system inefcient [34]. Biomass gasication is theprocess taking place at high temperature (typically 800900 1C) bypartial oxidation to convert biomass into combustible gas mixturesincluding hydrogen. In general, as compared to pyrolysis, the tem-perature of gasication is higher and the yield of hydrogen from thegasication is also higher [31].

Among the different technologies for producing hydrogen frombiomass, biological methods are found to be less energy-intensive andmore environment-friendly compared to thermo-chemical methods[35]. However, low rate of hydrogen production and yield is likely tobe a major challenge for biological methods. Due to these constraints,biomass-based hydrogen production using biological methods had sofar not been considered as important in most scenarios of futurehydrogen economy. Ni et al. [11] analyzed alternative biological andthermo-chemical technologies for biomass-based hydrogen produc-tion and concluded that the thermo-chemical hydrogen productionroutes (pyrolysis and gasication) are economically viable and likely tobecome competitive with the conventional natural gas reformingmethod. Biomass gasication has been identied as a possible systemfor producing renewable hydrogen by developing highly efcient andclean large-scale hydrogen production plants and to lessen depen-dence on fossil energy sources [6,9,31,36].

Considering the different technologies for hydrogen productionfrom biomass, gasication has been accepted as an economicallyviable and environment-friendly option as well as a favorable alter-native for higher hydrogen yields and large-scale hydrogen production.As a result, this technology is likely to play a major role in the futureneed of green environment and hydrogen based energy economy.

3. Hydrogen-enriched gas production from biomassgasication

3.1. Mechanism of biomass gasication

Biomass gasication reaction processes generally refer tothermo-chemical conversion of biomass, using a gasicationagent, into gaseous products typically including H2, CO, CH4, CO2,H2O and other gaseous hydrocarbons (CHs). In general, a typicalbiomass gasication process consists of drying, pyrolysis, combus-tion and reduction. The mechanistic steps of gasication areexplained below and the important reactions are summarized inTable 1.

Drying: The moisture in the biomass is driven out and con-verted into vapor. The feedstock in this zone is not decomposedbecause the temperature is not high enough to cause anychemical reaction.

Pyrolysis or devolatilization: Dried biomass feedstock from theprevious reaction is decomposed into low to high molecularweight volatiles including tar and solid char, as presented inEq. (1), in the absence of oxygen. The reactions in this zone areendothermic, thus the heat needed for this zone is suppliedfrom the combustion of biomass in the adjacent zone.

Combustion or oxidation: The products of the pyrolysis reac-tions are partially oxidized with oxygen contained in the airsupplied and then form carbon monoxide, carbon dioxide andwater as shown in Eqs. (2)(4). As combustion reactions areexothermic but other reactions in gasication are endothermic,the overall heat required for endothermic reactions is suppliedby this process.

Gasication or reduction: The chemical reactions in this zonetake place in the absence of oxygen because oxygen is con-sumed in the combustion reactions. The nal products fromthis reaction are mainly gas mixtures including carbon mon-oxide, carbon dioxide, hydrogen and methane. The formationreactions of these gases take place as shown in Eqs. (5)(10).

Table 1Important reactions in biomass gasication.

Name of reaction Chemical equation (equation no.)

Pyrolysis Biomass heat- gases (H2 CO CO2 H2O CH4 CnHm) tar char(1)Combustion 2C O2 - 2CO (2)

C O2 - CO2 (3)2H2 O2- 2H2O (4)

Boudouard C CO2- 2CO (5)Water gas (primary) C H2O- CO H2 (6)Water gas (secondary) C 2H2O - CO2 2H2 (7)Water-gas shift CO H2O- CO2 H2 (8)Methanation C 2H2 - CH4 (9)Methane reforming CH4 H2O- CO 3H2 (10)Tars reforming Tars H2O- H2 CO2 CO hydrocarbons (11)Hydrocarbon reforming Hydrocarbons H2O- H2 CO2 CO (12)

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579 567

3.2. Types of gasication

In gasication reactions, one of the factors which has signi-cant inuence on quantity and quality of the product gas is thegasifying agent. Air, oxygen, steam as well as mixtures of these canbe generally utilized as a gasifying agent. The choice of whichdepends on the desired product gas composition and energyconsiderations. Air gasication of biomass seems to have a feasibleapplication and has been developed actively for industrial applica-tions. However, this technology produces low caloric value gasesof 46 MJ/Nm3 and an 814 vol% H2 [37,38]. Oxygen gasicationcould produce a better quality of product gas with mediumcaloric value, but it needs a pure oxygen supply which maycause simultaneous penalty of cost and safety [10,11,14]. Biomasssteam gasication could produce medium caloric value gases(1016 MJ/Nm3) and especially the product gas rich in hydrogencontent (3060 vol%) [38,39].

In the open literature, there are many research works usingthese kinds of gasifying agents. This study mainly emphasizes theimportant aspects of the biomass gasication process relevant tohydrogen-enriched gas production.

Zhang et al. [40] conducted an air-blown uidized-bed gasica-tion experiment aimed to produce product gas rich in hydrogenfrom seed corn. The experiment resulted in product gas withrelatively low concentrations of hydrogen (about 8 vol%). Oncesteam was supplied, hydrogen content in the product gasincreased to almost 30 vol% due to steam reforming of tars andlight hydrocarbons and reacting steam with carbon monoxide viathe water-gas shift reaction (Eq. (8)). Chang et al. [41] experimen-tally studied gasication of commercial cellulose and agriculturalwastes, i.e., bagasse and mushroom, with air and mixtures of airand steam in a uidized bed for hydrogen production. They alsoinvestigated the inuence of varied steam-to-biomass ratio (S/B).When S/B of 0.0 (i.e., no steam or only-air gasication), reactiontemperature 800 1C and equivalence ratio (ER) 0.27, hydrogencontent was found at 13.5 vol%. Once partial amount of steamwas introduced to the same process and operating conditions byvarying S/B to 1, hydrogen concentration raised to almost 20 vol%.The effect of using different gasifying agents on hydrogen compo-sition in the product gas is compared in Fig. 1.

Fig. 1 shows the effect of different gasication agents, i.e. air,oxygen and steam, on the hydrogen concentration. Air gasicationstudies conducted by Mohammed et al. [14], Chang et al. [41], andSalleh et al. [42] are considered here as a meaningful baseline tocompare the enhanced concentration of hydrogen produced byintroducing both partial [38,43] and pure [44] amount of steam tothe gasication process. Moreover, oxygen gasication conductedby Zhou et al. [45] is also compared. Compared to air gasication,using pure oxygen as a gasifying agent could produce the productgas with better caloric value (due to no dilution effect from

nitrogen) and hydrogen content. This can be seen from Fig. 1 inwhich the range of hydrogen content from oxygen gasication ishigher than that of air gasication. In case of partial amount ofsteam diluted in air or oxygen gasication, the resulting productgas is relatively richer in hydrogen composition. On the otherhand, in the absence of both air and oxygen, pure steam gasica-tion experiment conducted by Gao et al. [44] showed higherhydrogen concentration.

Gil et al. [23] examined the effect of air, steam-oxygen mixturesand pure steam as gasifying agent. The results of Gil et al. [23]agreed with the previous studies as summarized in Fig. 2. It can beseen that the range of hydrogen content is higher in case ofgasifying with pure steam and the next is the gasication with themixture of steam and oxygen. The phenomenon of higher hydro-gen content results from the decomposition of water (in the formof steam) added to the thermo-chemical conversion of biomass.As water-gas reaction (Eq. (6)) and water-gas shift reaction (Eq.(8)) take a key role in the steam gasication process, hydrogenproduction increased [44]. More detailed parametric study ofbiomass steam gasication reactions relevant to hydrogen produc-tion is discussed in Section 5.

Fig. 1. Effect of using different gasifying agents on hydrogen concentration.

Fig. 2. Comparative study of hydrogen production from different gasifying agents[97,98].

Fig. 3. Effect of using different gasifying agents on hydrogen yield.

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579568

A gasication agent inuences not only the quality of productgas but also its quantity. Turn et al. [43] examined the effect ofgasifying agents on the amount of hydrogen in the product gasusing a bench-scale uidized-bed gasier and sawdust with themixtures of oxygen and steam as a gasifying agent. The resultinghydrogen yield at S/B1.7 varied from 62 (at ER of 0.37) to 128 gH2/kg biomass (at ER of 0.0, i.e., no oxygen or only steam waspresent). Gonzalez et al. [46] investigated the reactions inuencingair and steam/air gasication of biomass (olive oil waste) forproduction of hydrogen-rich gas. Their experimental works wereconducted with three experimental series in the same reactor.First was air gasication and the second was steam/air gasicationusing steam diluted in air in the rst case. The last was catalyzedsteam gasication. The different range of hydrogen yield obtainedusing different gasifying agents is shown in Fig. 3. As compared toair gasication, adding steam to the process helps to enhance theproduction of hydrogen. The result of hydrogen yields obtained byGao et al. [44] using pure steam also added to Fig. 3 forcomparison.

Nipattummakul et al. [47] produced hydrogen and syngas fromsewage sludge via air and steam gasication and found thathydrogen yield increased three-fold in the case of steam gasica-tion as compared to air gasication. Other authors also havereported higher hydrogen yields due to the introduction of steamto gasication process [16,17].

Although a direct comparison of hydrogen concentration andyield due to different gasifying agents is not possible due tovariation of many different operating conditions, such as biomassfeedstock, reaction temperature, reactor design, equivalence ratio,and steam-to-biomass ratio, the study results compared aboveprovide a general insight into the inuence of different gasifyingagents. As evidenced in the literature, adding steam to gasicationprocess leads to the production of syngas rich in hydrogen.Especially, pure-steam gasication has a considerable inuenceon hydrogen composition and yield in the product gas.

4. Hydrogen production costs

One of the problems in the use of hydrogen as a fuel is itsunavailability in nature that consequently leads to the need forinexpensive production methods [6]. The production of hydrogencan be achieved by various primary energy sources and differentmethods, some of which are shown in Table 2 with comparison ofrelated production costs [6].

Table 2 presents an IEA [48]'s long-term scenario of hydrogenproduction cost from different primary energy sources and tech-nologies. The majority of hydrogen produced today comes fromreforming of natural gas and petroleum and coal gasication[49,50]. Hydrogen production using steam methane reforming(SMR) process is the most economical method among the currentcommercial processes [51]. Rothwell and Williams [52] reported

that the cost of hydrogen using natural gas (6 US$/GJ natural gasprice) via SMR process is about 12 US$/GJ. However, in fossil basedprocess, capturing CO2 produced during the process adds about2530% to the original cost of hydrogen production [53]. Thisresults in higher cost for hydrogen production from natural gas viaSMR process as appeared 1218 US$/GJ, similar to use of coalreported 1318 US$/GJ (Table 2). The production of hydrogen frombiomass gasication is not expected to develop in the near termdue to costs, lack of demonstrated technology, and lack of wide-spread hydrogen market and infrastructure [6]. However, for long-term view point, it is forecasted to be cost-competitive, if thevariability in feedstock prices and the likely use of CO2 capture andstorage (CCS) are considered in the use of fossil fuel based process[54]. This cost-competitiveness with fossil based processes is alsoevidenced in total cost (1425 US$/GJ) of hydrogen productionfrom biomass gasication as shown in Table 2. The cost ofproducing hydrogen from supercritical water gasication of wetbiomass was several times higher than that from SMR process [51].Another alternative method for hydrogen production from bio-mass is pyrolysis, the hydrogen production cost of which wasreported in the range of 8.8615.52 US$/GJ [55]. Besides biomassgasication, Table 2 also reports the hydrogen production costsfrom other primary energy sources and technologies but theirproduction costs are still uneconomical, when compared to fossilbased processes and biomass gasication. Although biomassgasication is expected to become a cost-effective process, theeconomies of scale of the process's facilities also have inuence onthe hydrogen production cost. It was reported that low costs ofproducing hydrogen are attained with facilities larger than 100 t/day of hydrogen [51].

5. Parametric study

Several operating conditions inuence the amount of hydrogenin the product gas from steam gasication of biomass. Inuence ofthe important operating conditions are analyzed and summarizedin this section.

5.1. Effect of biomass characteristics

Many researchers carried out gasication of different types ofbiomass for the production of hydrogen. Florin and Harris [16]have characterized biomass based on: (i) the chemical constituent(cellulose, hemicelluloses and lignin); (ii) elemental composition;(iii) inherent mineral content; (iv) amount of volatile matter;(v) moisture content; and (vi) physical properties (particle size,shape and density). All these parameters have been recognized toinuence the product gas composition and yield during biomasssteam gasication [19,41,56]. Table 3 shows the characterization ofdifferent biomass by Franco et al. [19]. They investigated the effectof different biomass types on the steam gasication process in an

Table 2Hydrogen production cost for long-term scenario.

Method Feedstock price Feedstockcost ($/GJ H2)

Other prod.cost ($/GJ H2)

Transportcost ($/GJ H2)

Refuelingcost ($/GJ H2)

Total cost atfuel pump ($/GJ H2)

Natural gas with CCS 35 $/GJ 3.86.3 1.22.7 2 57 1218Coal with CCS 12 $/GJ 1.32.7 4.76.3 2 57 1318Biomass gasication 25 $/GJ 2.97.1 56 25 57 1425Onshore wind 34 cents/kWh 9.813.1 5 25 57 2230Offshore wind 45.5 cents/kWh 13.118.0 5 25 57 2737Solar thermal elec. 68 cents/kWh 19.626.1 5 25 57 3242Solar PV 1220 cents/kWh 39.265.4 5 25 57 5258Nuclear 2.53.5 cents/kWh 8.211.4 5 2 57 2027

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579 569

atmospheric-pressure uidized-bed gasier and correlated theproduct gas composition and yields with the type of biomassspecies. The nature of biomass species appeared to inuence thevariations in the gas composition as evidenced from the experi-mental study of Franco et al. [19] and shown in Fig. 4.

Chang et al. [41] investigated the biomass gasication toproduce the hydrogen and syngas in a uidized bed. They usedthree different species of biomass (i.e., commercial cellulose,bagasse and mushroom waste) at different reaction temperatures(700900 1C), ER (0.270.34) and S/B (00.5). The hydrogenproduction was found to be highest in case of using commercialcellulose corresponding with its higher carbon-hydrate concentra-tion than the other two species. In addition, for the biomass withrelatively high ash contents like mushroom waste, it was noticedthat the deposition of ash and char produced in the gasier mayblock the manifold which may lead to the lower gas productioncapacity.

Other examples of the inuence of biomass species on theproduct gas can be seen in Nipattummakul et al. [17], Ahmed andGupta [5759], who have studied steam gasication using sewagesludge, paper, food wastes, and plastics, respectively. The com-parative results show that syngas and hydrogen evolution fromsteam gasication of sewage sludge is slower than that of paper,food wastes and plastics. On the other hand sewage sludge yieldedmore hydrogen than that from paper and food wastes.

Ahmed and Gupta [60] also investigated the characteristic ofsyngas and hydrogen evolution from pyrolysis and gasication ofrubber. Rubber consisting of relatively low hydrogen-to-carbonratio yielded less hydrogen than wood.

Several authors have studied the effect of biomass particle sizeon the steam gasication reactions. Rapagn and Latif [61]investigated the inuence of biomass particle size and tempera-ture on product yield and distribution using almond shells withsteam in a continuous bench-scale uidized-bed reactor. The meansize of almond shell particle was varied between nearly 300 mand over 1 mm under the bed temperature ranging from 600 to800 1C. The experimental result found that the yield of hydrogenand total gas decreased with increasing particle size in the entirerange of operating temperature. However, at higher temperature,the inuence of smaller particle sizes was practically found nosignicant differences in product yield and distribution. Theseresults were similar to those obtained by Herguido et al. [62].

Herguido et al. [62] also studied the effect of different types ofbiomass feedstock on the yields of gas, char and tar as well as thecomposition and heating value of the product gas. Pine wood

chips, pine sawdust, thistles, and cereal straw were gasied withsteam in a uidized-bed reactor operated at the temperature rangeof 650780 1C. The differences in size of pine sawdust and pinewood chips inuenced total gas yield and composition. The gasyield of 1.2 kg, and 0.7 kg per kg of pine sawdust and kg of pinewood chips obtained, respectively. This shows that the smaller theparticle size, the greater the gas yields. Nevertheless, the differ-ences in product yield and distribution resulting from differentparticle size were not signicant when the operating temperatureincreased to a maximum of 780 1C.

Recently, Luo et al. [63] also studied the effects of biomassparticle size at different bed temperatures on product yield andcomposition. Municipal solid waste (MSW) with different particlesize fractions (i.e., below 5 mm, 510 mm and above 10 mm) wasused to investigate the inuence of particle size on gasication andpyrolysis performance. The result showed that total gas yieldincreased with the decreasing particle size at the same tempera-ture. Furthermore, smaller particle sizes allowed higher hydrogenand carbon monoxide and less carbon dioxide contents. The majordifference in product yield and composition is due to differentparticle size practically disappeared at the highest temperatures.The results of Luo et al. [63] strongly agree with the study resultsof Rapagn and Latif [61] and Herguido et al. [62]. In addition, thesimilar result is observed from many other studies such as Lv et al.[38] and Luo et al. [56].

It is generally found that smaller particles can produce highergas yield. Di Blasi [64] reported that higher heating rates producemore light gases as well as less char and condensate. Since smallerparticles contribute to a larger surface area and hence fasterheating rates, it can be expected that the size of biomass particlehas a signicant inuence on the product gas yield. Moreover, it isgenerally reported that the smaller particles produce more hydro-gen and carbon monoxide and less carbon dioxide contents thanthe larger ones [56,63]. As mentioned by Lv et al. [38], this result isrelated to the fact that the fractions of the carbon monoxide,carbon dioxide and hydrogen are linked together by the equili-brium of the water-gas shift reaction (Eq. (8)) under test condi-tions. Besides, water-gas reaction (Eqs. (6) and (7)) and Boudouardreaction (Eq. (5)) have an important role to contribute these gasspecies in the initial phase of biomass gasication reaction.

In order to improve the quality and quantity of product gas oreven increasing hydrogen and carbon monoxide, minimizing thesize of feedstock seems to be an alternative method suitable forthe gasication process.

5.2. Effect of gasier temperatures

Since gasication is known as one of the thermo-chemicalprocesses using heat to convert the feedstock into product gas,temperature at which heat is supplied has a major inuence on the

Fig. 4. Hydrogen content derived from steam gasication of different biomassspecies.

Table 3Analysis of different biomass species [19].

Properties Pine Holm-oak Eucalyptus

Proximate analysis (wt%)Volatile 71.5 70.2 74.8Fixed carbon 16.0 17.8 13.9Ash 0.5 2.4 0.7Moisture 12.0 9.5 10.6Elemental analysis (wt% d.a.f.)Carbon 51.6 51.1 52.8Hydrogen 4.9 5.3 6.4Nitrogen 0.9 0.9 0.4Sulfur Not detected Not detected Not detectedOxygen 42.6 42.7 40.4HHV (kJ/kg d.a.f.) 20,189 19,448 21,250Chemical composition (wt%)Celluloses 39.4 33.9 43.1Hemicelluloses 27.8 21.8 28.8Lignin 32.8 44.3 28.1

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reaction and consequently on the amount and composition of theproduct gas generated. The yield of product gas and hydrogen issubjected to reactor temperature [61]. Furthermore, the relation-ship between reaction temperature and gas yield has been wellstudied by many researchers [43,62,65]. Higher gasication tem-perature produces a product gas rich in H2 and CO but with smallamounts of CH4 and higher hydrocarbons [7]. Nipattummakulet al. [47] and Shen et al. [66] found that with the increase ofreaction temperature, H2 and CO increased, while CO2 and CH4decreased. Similar trends were observed by Chang et al. [41], Turnet al. [43], Gao et al. [44,67], although there were some differencesin operating conditions. The comparison developed in the presentstudy provides a general insight into the inuence of reactiontemperature on the H2 output.

Fig. 5 shows the effect of temperature on hydrogen concentra-tion in the product gas produced from biomass steam gasication.The results obtained from all studies have a good agreement witheach other, i.e., an upward trend of H2 content related to elevatedtemperature. It can be concluded that an increase in reactiontemperature tends to enhance the extent of conversion of abiomass fuel to a gas product. The possible explanation supportingthis statement can refer to the experimental study of Nipattum-makul et al. [17] and Ahmed and Gupta [59] on solid residuesinuenced by reactor temperature. They reported that thedecreasing residual solid leftover after gasication was subjectedto an increment of reactor temperature. This indicates a betterconversion of biomass into product gas due to higher heating rateresulting from a rise of reactor temperature, which consequentlyresults in higher gas yields and enriched H2 and CO contents. Theincrease in gas yields and H2 as explained by Franco et al. [19] andGao et al. [44] could be due to various reasons, such as: (i) fastpyrolysis at the commencement of biomass introduced intoreactor, (ii) endothermic char gasication reactions, and (iii) steamreforming and cracking of heavier hydrocarbons and tars.Moreover, referred to Le Chatelier's principle, higher temperaturesfavor the reactants in exothermic reactions and the productsin endothermic reactions. Therefore, the endothermic reaction(Boudouard reaction (Eq. (5)), water-gas reaction (Eq. (6)) andsteam reforming reaction (Eq. (10))) were strengthened withincreasing temperature, which causes an increase of H2 and COconcentration and a decrease of CH4 and CO2 concentration. Onthe contrary, there have been some studies, such as Franco et al.[19] and Li et al. [68], having the trends of CO and CO2 opposite tothe trends mentioned in earlier studies. The downward and upwardtrends of CO and CO2, respectively, are possibly attributed to water-gas shift reaction (Eq. (8)) and higher reaction rate for CO2 produc-tion caused by increasing temperature. The water-gas shift reaction isinuenced by the presence of steam favorable for the reactionleading to an increment of H2 and CO2 as well as a decrease of COcontents with elevating temperature. More details on the inuenceof steam on the gasication reaction are provided in Section 5.3.

Besides the effect on gas yield and composition, the variation ofreactor temperature also inuences the evolution of the hydrogenow rate and the time of hydrogen release [17,47]. The increase ofreactor temperature in the steam gasication process enhanceshydrogen ow rate and shortens the time of hydrogen release[17,47]. This is attributed to the endothermicity of hydrogenrelease for the gasication process and increment in reaction rateswith increasing reactor temperature [59]. Ahmed and Gupta [60]also found similar result.

As seen in the literature, the reactor temperature exhibits thedominant experimental parameter, inuencing both the amountand concentration of H2 in the product gas as well as the total gasyield. The favorable range of reactor temperature observed byresearchers is between 650 and 1000 1C. Higher reactor tempera-ture affects higher heating rate which leads to a better conversionof biomass into the product gas and consequently allows the gasrich in H2. Moreover, the enriched H2 in the product gas ispromoted by the endothermic reactions (Boudouard reaction (Eq.(5)), water-gas reaction (Eq. (6)) and steam reforming reaction (Eq.(10))), which are strengthened with the rise of temperature.However, the H2 derived from steam gasication depends notonly on the reaction temperature but also the amount of steam fedto the reactor. The H2 gas enriched with the presence of steam inthe process can be explained by water-gas shift reaction (Eq. (8)).Increasing temperature also inuences higher mass ow rate ofhydrogen produced and shorter time of hydrogen release.

5.3. Effect of steam-to-biomass ratio

Steam-to-biomass ratio denoted by S/B refers to the feed ofsteam per the feed of biomass [41]. It could practically be variedeither by changing the biomass feed rate while holding the steamow rate constant or vice versa. The S/B is an important parameter,like temperature having a strong inuence on both the yield andconcentration of H2 in the product gas as well as the total gas yield[69].

Many researchers have found that increasing the amount ofsteam fed to the gasier leads to a rise of H2 production. This factcan be evidenced from the study results of Turn et al. [43], Gaoet al. [44] and Herguido et al. [62]. Figs. 6 and 7 show tendency ofH2 concentration and yield, respectively, corresponding to higherS/B. The higher production of H2 due to the inuence of steam isprobably attributed to water-gas shift reaction (Eq. (8)) and steamreforming reaction (Eq. (10)). Apart from an increase in H2, thewater-gas shift reaction (Eq. (8)) promoted by the greater amountof steam (higher S/B) would be accompanied by an increase in CO2concentration in the product gas. These trends have been reportedby several researchers [19,38,43,66]. Additionally, the researchersalso found that CO concentration decreased with higher S/B, whichis the consequent effect of promoted water-gas shift reaction. Theundesirable increase in CO2 concentration is inevitably

Fig. 5. Effect of temperature on H2 concentration from biomass steam gasication. Fig. 6. Effect of S/B on H2 concentration from biomass steam gasication.

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problematic in hydrogen-enriched gas production from conven-tional biomass steam gasication due to water-gas shift reaction.However, one promising option to eliminate the problem is usingCaO-based CO2 sorbent and this option is discussed in detail inSection 6.2.

Besides the enhancement of H2 production, increasing S/B hasan inuence on decreasing char and tar yields [62,67]. Thisphenomenon is likely attributed to char gasication, steamreforming and tar cracking reactions, which lead to higher con-version as well as higher gas production. Reed [70] stated that thepresence of steam enhances participation of tar in steam gasica-tion. This phenomenon results in the decrease in tar content athigher S/B. As a result, the yields of H2 and total gas can increase asproducts of tar reforming reaction (Eq. (11)).

In practice, excess steam is often used to drive the cracking andreforming reactions, according to water-gas reaction (Eqs. (6) and(7)). Nevertheless, use of excess steam may cause some disadvan-tages including high water quantity in the product gas, whichconsequently leads to more energy consumption in separation of

steam out of the product gas via downstream process of con-densation and dryness. In addition, the lower yield of H2 andproduct gas as well as lower H2 content with higher S/B due toexcess steam fed to the reactor was observed in many studies[12,41,71]. Some obvious cases of excess steam that adverselyinuences H2 production are found by Gao et al. [67] in Fig. 6 andGao et al. [44] in Fig. 7. The trend of H2 increased rst and thendeclined with continued increase of S/B. The declination isprobably attributed to a decrease in temperature of gasicationreaction due to the heat absorbed by excess steam. The reducedtemperature affected by excess steam can also result in theincrease in tar content. Tar formation is one of the major issuesthat occur during biomass gasication [11]. The unwanted tar maycause the formation of tar aerosols and polymerization to a morecomplex structure that are not favorable for hydrogen productionthrough steam reforming [11]. Currently, there are several meth-ods to minimize tar formation, such as proper design of gasier,proper control and operation, and catalysts [11]. In the presentstudy, only the last method is discussed in Section 6.2 focusing oncalcium oxide (CaO).

On the other hand, use of insufcient steam may lead toincomplete reaction with the biomass during the gasicationprocess. Consequently, water-gas reaction (Eqs. (6) and (7)), steamreforming reaction (Eq. (10)) and mainly water-gas shift reaction(Eq. (8)) would not appear to reach a state of completion. There-fore, to maximize H2 production, the optimal S/B should properlybe selected according to different operating conditions includingbiomass characteristics, reactor temperature as well as the typeand design of the reactor. The differences in optimal level of S/Bhave been reported by many authors for H2 production frombiomass steam gasication. Table 4 summarizes the importantinvestigations found in this area. Even though the optimal S/Bwhich has been investigated by several researchers is differentfrom each other, the increasing trend of H2 output with anincrease of S/B up to some extent shows a strong agreement.

Table 4Summary of important investigations on steam gasication of biomass without catalysts.a

Reactor Biomass Gasifyingagent

Temperature(1C)

S/B ER H2 yield (g/kg d.a.f.) H2 content (Vol%) References Remarks

Fluidizedbedgasier

Pine,eucalyptusand holm-oak

Steam 700900 0.40.85 0 N/A 2145 (Maximumfound at gasication ofholm-oak at 900 1C, S/Bof 0.8 and ER of 0)

Francoet al. [19]

Fluidizedbedgasier

Sawdust Steam andoxygen

750950 1.14.7 00.37 2383 (Maximumfound at 800 1C, S/B of4.7 and ER of 0)

27.657.4 (Maximumfound at 800oC, S/B of4.7 and ER of 0)

Turn et al.[43]

Fluidizedbedgasier

-Cellulose Steam andair

800 01.5 0.27 N/A 13.5018.56(Maximum found at8001C, S/B of 1 and ER of0.27)

Changet al. [41]

Values of H2 yield in theinterested range were notreported

Fluidizedbedgasier

Pine sawdustand woodchip, andcereal straw

Steam 650780 0.43.0 0 N/A 59 (Maximum foundat750oC, S/B of 0.8 andER of 0)

Herguidoet al. [62]

Fluidizedbedgasier

Rice hull Steam 700800 N/A 0 N/A 32.8042.62(Maximum found at800oC, S/B of 5.2 and ERof 0)

Boatenget al. [96]

Updraftgasier

Pine sawdust Steam andoxygen

850 1.053.47 0 75.4186.08(Maximum found at850oC, S/B of 2.53 andER of 0)

4243.37(Maximum found at850oC, S/B of 3.47 and ERof 0)

Gao et al.[44]

Only xed temperature of850oC and varied S/B wasconducted on steamgasication withoutcatalyst.

Updraftgasier

Pine sawdust Steam 800950 1.052.53 0 39.0379.91(Maximum found at950oC, S/B of 1.4 and ERof 0)

34.541 (Maximumfound at 850oC, S/B of1.4 and ER of 0)

Gao et al.[67]

a Compiled from previous experimental study of steam gasication of biomass without catalysts.

Fig. 7. Effect of S/B on H2 yield from biomass steam gasication.

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5.4. Effect of equivalence ratio

The equivalence ratio (ER) is another important factor in thebiomass steam gasication when the air or oxygen is partially usedto mix with steam as a gasifying agent in the processes. However,since the presence of nitrogen in the air dilutes the product gasand decreases the gas quality, oxygen has been introduced as agasifying agent in many cases. ER is dened as the ratio of theamount of air supplied relative to the amount of air needed forstoichiometric combustion [44,72]. The variation of ER means thechange in the amount of oxygen introduced into the gasier,which affects the gasication temperature and reactions andnally inuences the product gas quality. It has been reported byseveral authors that there exist two contradictory effects of ER[38,72]. On one hand, higher ER would cause a degradation of gasquality because more combustible gases including H2 are con-sumed due to more oxidization reactions. On the other hand,higher ER could lead to a higher gasication temperature, whichcan accelerate the gasication and improve the product gas qualityto some extent. The effect of ER on hydrogen production has beenreported in literatures extensively [43,44,73].

Figs. 8 and 9 present the effect of ER on hydrogen concentrationand yield respectively in the product gas produced from biomasssteam gasication. From both gures, the experimental result of Lvet al. [73] shows a good agreement with the theory of twocontradictory effects of ER. ER was varied in the range of0.220.3 with xing the oxygen ow rate at 1.5 Nm3/h and S/Bat 0.17, and varying the feeding rate to explore the impact of ER onhydrogen production from downdraft gasier. Change of ERresulted in the variation of temperature in oxidation zone from798 to 905 1C, which led to a variation in oxidation reactions. Thetest result can be explained based on two contradictory effects ofER by dividing the phenomenon into two stages, upward anddownward trends of H2 output with increasing ER. In the rststage, the positive effect of ER played a more important role, sothat the H2 output increased to some extent. In the second stagewith still continuously increasing ER, the H2 output appeared adecreasing trend because oxidation reaction became more impor-tant than steam gasication reaction due to the increased oxygenquantity. As a result, more combustible gases including H2 wereconsumed to produce more CO2 and H2O. Turn et al. [43] carriedout experiment in uidized-bed gasier with xing the tempera-ture and S/B at 850 1C and 1.7 respectively and varying ER from0 to 0.37 at a constant feed rate. Gao et al. [44] tested on updraftxed-bed gasier by varying ER in the range of 00.3 with steadytemperature and S/B at 850 1C and 1.4, respectively. However, thetest results of both researches have a good agreement on H2production related to varying ER as displayed in Figs. 8 and 9.Hydrogen concentration and yield tend to decrease while increas-ing ER. But, their highest values over the range of varied ER arefound when ER is zero. This is because of when oxygen was notintroduced into reactor (ER0) and only steam as a gasicationagent, the reactions of water gas (Eq. (6)) and water-gas shift (Eq.(8)) would play an important role as primary reactions in the

gasication process. Nevertheless, once oxygen is added to thereactor (ER increased), the oxidation reactions (Eqs. (2)(4)) wouldtake part in the gasication process. This also results in more H2consumed in the oxidation reaction, which leads to a declinationof H2 produced.

Overall, ER has complex effects on test results. Through theanalysis on the experimental data of varying ER, it can be under-stood that it is unfeasible to apply too low or too high ER inbiomass air-steam gasication. Too low ER will lower reactiontemperature, which is unfavorable for biomass air-steam gasica-tion for hydrogen production. Conversely, too high ER will enhancemore oxidation reactions, which then consume more H2 and othercombustible gases. So, there exists an optimal value for ER, whichvaries according to different operating parameters.

6. Challenges and prospects

6.1. Problematic CO2 and tar production

Even though steam gasication of biomass has been perceivedas the attractive pathway for sustainable hydrogen production, theprocess is inevitably problematic with undesirable CO2 generationdue to water-gas shift reaction. Another serious issue in develop-ing hydrogen production from this process is the formation ofunwanted tar which may restrict the quick and widespread use ofhydrogen [9]. Tars dened as condensable organic compoundscould be produced during gasication and become entrained inthe syngas [74,75]. Use of the syngas contaminated with tars cancause fouling and blocking of downstream pipelines and equip-ments as tars can condense at reduced temperature of the gas.Most post gasication applications therefore require removal oftars to some extent using various techniques before applying thegas [76].

6.2. CaO-based in situ CO2 and tar reduction

With respect to commercial applications, separating hydrogenfrom hydrogen-rich gas stream contained with impurities, e.g.,CO2, CH4, CO and tar incurs a major cost penalty [16,24]. Thus, ifCO2 and tar produced during steam gasication of biomass can besimultaneously captured and cracked within the process, this canbe more attractive and cost-effective in the eld of hydrogenproduction. In order to achieve this, some catalysts have been usedeither inside of the gasier (called the primary method) ordownstream (called the secondary method) [9,77]. Although thelatter has been proven to be effective, treatments inside thegasier as the former one are gaining much attention as thesemay eliminate the need for complex downstream cleanup [78].Acknowledged as a catalyst to produce hydrogen-rich gas [1921,25,26], use of CaO as a CO2 sorbent in biomass gasication forhydrogen production has currently gained a broad attention due toits cheapness and abundance [3,9,16]. The introduced CaO can playFig. 8. Effect of ER on H2 concentration from biomass steam gasication.

Fig. 9. Effect of ER on H2 yield from biomass steam gasication.

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the vital roles of not only CO2 sorbent, but also tar cracking catalystand bed material based heat carrier in a uidized-bed reactor [9].Removing CO2 from the gasication reaction as soon as it isformed alters the equilibrium composition of the produced gasand promotes the production of gas rich in hydrogen [3]. Similarly,the catalytic reforming of tar not only reduces the tar amount inthe product gas but also enhances the yields of total gas andhydrogen as well as conversion efciency [1,36]. Hence, in order toenhance hydrogen production with in-process CO2 capture and tarreduction, the concept of steam gasication of biomass in thepresence of CaO can play an important role as a promising optionfor sustainable hydrogen production.

Many researchers have recently paid attention to the role ofCaO in capturing CO2 only. Table 5 summarizes the importantinvestigations on the role of CaO as a CO2 sorbent and tarreforming catalyst [12,7983]. Fundamentally, CO2 produced dur-ing steam gasication in the presence of CaO would be captured byCaO via a carbonation reaction (Eq. (13)) and then form calciumcarbonate (CaCO3). As a result, this reduces CO2 partial pressure in

water-gas shift reaction (Eq. (8)) and then enhances the reaction tomove in forward direction to produce more H2 [9,12,82]. However,the capability of the reaction also depends on other operatingconditions, such as S/B, temperature, pressure and CaO content,details of which are described by Florin and Harris [16].

Carbonation : CaOCO2-CaCO3 13

Calcination : CaCO3-CaOCO2 14Besides CO2 capture, the introduced CaO could also reform tarsproduced during the gasication process and form gaseous pro-ducts which result in higher yields of total gas and H2. Thisphenomenon could be explained by tar reforming reaction(Eq. (11)) and hydrocarbon reforming reaction (Eq. (12)). Someprevious studies in this eld are also included in Table 5[9,26,56,8487]. Typically, more information in the literature hasbeen focused on total tar mass production. Studying only tarcontent or tar yield is not enough to fully describe the tar problem[23,88]. In downstream processes, only some specic tar

Table 5Summary of important investigations on the role of CaO as a CO2 sorbent and tar reforming catalyst.

Reactor Biomass Gasifyingagent

Temp.(oC)

S/B CaO CaO/B Tar studyn Gas concentration (Vol%) References

H2 CO2 CH4 CO

Fixed bed Sawdust Steam 670 0.83 Calcinedlimestone

2 N/A 54.43 1.56 21.58 21.92 Acharya et al.[12]

Bubblinguidizedbed

Sawdust Steam 580 1.5 Calcinedlimestone

1 N/A 71 1 21 7 Acharya et al.[79]

Fixed bed Japanese oak Steam 700 N/A N/A 2 N/A 880 ml/g 390 ml/g 180 ml/g N/A Hanaoka et al.[80]

Fixed bed Pine sawdust Steam 800 0.42 Calcinedlimestone

20 N/A 72 mol% 10 mol% 9 mol% 9 mol% Wei et al. [81]

Quartztube

Pine sawdust Steam 650 0.9 Ca(OH)2powder

0.5 N/A 57 6 14 23 Guoxin and Hao[82]

Fixed bed Six energycrops inPoland

Steam 650 N/A Calcinedlimestone

4.04.7 N/A 7378 N/A N/A N/A Howaniec andSmolinski [83]

Bubblinguidizedbed

Sawdust Steam 740 2.18 CaOreagent

1 Tar was studied considering thecolorchange of the condensed tarsample.The color turned from deep redin theabsence of CaO into light yellowat CaO/C of 1.

62 9 11 16 Han et al. [9]

Bubblinguidizedbed

Larch Steam 650 N/A Calcinedlimestone

In-bedCaO

Total tar yield was found31.24 wt% of carbondistribution.

63.56 24.75 5.75 4.85 Weerachanchaiet al. [26]

Fixed bed Pine sawdust(o0.075 mm)

Steam 900 1.2 Calcineddolomite

N/A Tar amount was not foundat this operating condition.

52 14 6 23 Luo et al. [56]

Bubblinguidizedbed

Crushedalmond shells

Steam 770 1 Calcineddolomite

In-bedCaO

The tar content of 0.6, 43 and2.4 g/Nm3 was obtained fromthe experimental tests withCaO, sand and olivine,respectively.

55.5 14.1 6.4 24 Rapagn et al.[84]

Fixed bed Pine sawdust Steam 900 2.1 Calcineddolomite

N/A Tar amount was not found atthis operating condition.

52.7 23.9 2.7 17.2 Luo et al. [85]

Fixed bed Pine bark Steam 600 N/A CaOreagent

1 Tar was studied consideringparticulates and suspendedmatter.The sample from thegasicationwithout CaO had moreparticulatesand suspended matter ascomparedto the one with CaO.

83 9.2 1.5 6.3 Mahishi andGoswami [86]

Dualuidizedbed

Wood chips Steam 675 N/A Calcinedlimestone

Tar content was found about1 g/Nm3.

50.6 12.3 12.9 16.5 Koppatz et al.[87]

n Tar studies reported in this table were focused only on total tar mass formation.

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compounds may be problematic and others may be harmless oreven benecial [89]. Thus, addressing the relation between tarconcentration and composition and gasication conditions in asystematic way is fundamental to the efcient operation ofcommercial biomass gasication systems [74]. This kind of studyrelated to the inuence of CaO is still lacking in the literature. Theinformation available in this regard is summarized below.

Pfeifer et al. [90] studied in situ CO2 absorption in a dualuidized bed biomass steam gasier to produce a hydrogen-richsyngas. The results obtained from the Adsorption EnhancedReforming (AER) process using CaO as a bed material in theirstudy were compared to those obtained from the conventionalprocess without CaO. The former process allowed higher H2production of 75 vol%, compared to the latter case which producedonly 40 vol% of H2 in product gas. Tar content as low as 0.5 g/Nm3

obtained from CaO-based process at 600700 1C is less than that of25 g/Nm3 obtained from the process without CaO at 850 1C. Thiscan be implied that no signicant increase in tar formation occursin the process with CaO despite the low gasication temperatures.This consequently results in higher efciency of energy conversionwithout compromising the hydrogen-rich gas production and tarreduction. Apart from tar content, tar composition was alsostudied but it was reported as the inuence of solid circulationrate, which could not point out the effect of CaO being focused inthis section.

Pfeifer et al. [91] studied the inuence of CaO on gas and tarproduction in the same experimental unit as the one used byearlier workers [90]. The concentration of H2 and CO2 in theproduct gas was 74 vol% and 6 vol% respectively in CaO-basedprocess. Meanwhile, 40 vol% of H2 and 18 vol% of CO2 wereobtained from the process without CaO. Tar content of 1.4 g/Nm3

was obtained from CaO-based process at 645 1C, whereas that of3.53 g/Nm3 was found in the case without CaO at 841 1C. Despitethe signicant lower gasication temperature, much lower tarcontent in the CaO based experiments have been contributed tothe higher catalytic activity of CaO on cracking the tar. Thiscatalytic effect could be also noticed from the change in tarcomposition. It was found that heavier tars in polycyclic aromatichydrocarbon (PAH) group were catalytically reformed to lighter tarcompounds like phenol and styrene in the presence of CaO, ascompared to the process without CaO. Similar effect of CaO on tarcomposition was observed by Jordan and Akay [75].

Jordan and Akay [75] studied the impact of CaO on tarconcentration and composition as well as syngas yield by mixingCaO at 2, 3 and 6 wt% with bagasse pellets and then feeding intoan air-blown downdraft gasier. As compared to the baseline casewithout using CaO, the use of 2, 3 and 6 wt% in-bed CaO promotedthe conversion of Class 1, 4 and 5 tars (higher ring tar species) toClass 3 tars (1 ring tar species), which consequently resulted in adecrease in tar yield ranging 1635%, a decrease in tar concentra-tion of 4480%, and an increase in syngas yield of 1737%. As aresult of a reduction in higher ring tar compounds in the presenceof CaO, tar dew point decreased in the range of 3760 1C, ascompared to the baseline case without CaO. Knowing the tar dewpoint of the product gas is very useful in commercial applications,

as it helps determine the operating conditions of such a system toavoid undesirable tar condensation and deposition due to reducedtemperature of the product gas. Besides, it can facilitate anevaluation of the potential impact on downstream equipment.However, the knowledge in this area is still limited in theliterature.

6.3. CaO-based chemical looping gasication

From the previous experimental studies summarized inTables 5 and 6, one can nd that the major disadvantage of thoseprocesses is a discontinuous hydrogen production due to thedeactivation of used CO2 sorbent. Despite attractiveness in itscombined roles as a CO2 sorbent and tar reforming catalyst, use ofCaO may not be economically viable, if fresh CaO is frequentlyneeded to rell to the process after capturing CO2 via carbonationreaction and then the used CaO in the form of calcium carbonate(CaCO3) cannot be reused in the gasication system. This makesthe process less attractive from an economical perspective [92]. Toovercome such a challenge, the concept of chemical loopinggasication (CLG), which is fundamentally aimed for continuoushydrogen production with in situ CO2 capture using uidized-bedtechnology, is thus emerged. Within this looping system, the usedCaO can be regenerated and then reused within the cyclic processbetween two major reactors, i.e., the regenerator and the gasier.

Chemical looping is one of several emerging technologyoptions that can facilitate the uptake of low-emission energytechnologies and help in a diverse range of applications forproductions of fuels, chemicals and electricity [93]. The conceptof chemical looping has been found in various applications, such aschemical looping gasication, chemical looping reforming, chemi-cal looping combustion, sorbent chemical looping for post com-bustion capture of CO2 and chemical looping air separation. Theseapplications of chemical looping concept have different purposeof main production, details of which are well discussed byMoghtaderi [93]. However, as the present work is focusing onhydrogen production with in situ CO2 capture, thus only theconcept and the past applications and development of CaO-based chemical looping gasication are discussed here.

The applications of the CaO-based chemical looping gasication(CaO-based CLG) process can be traced to early times. Theprocesses for CaO-based CLG include the CO2 Acceptor Processthat was developed in the 1970 s and ended in 1977 after beingtested successfully in a pilot plant and proved technically feasible[94]. This is a notable application and forms the basis of recentdevelopment in CaO-based CLG processes [94]. Other CaO-basedCLG processes that are being developed or have currently beendeveloped include the HyPr-Ring Process, the Zero Emission CoalAlliance (ZECA) Process, the ALSTOM Hybrid Combustion-Gasication Process, and the Fuel-Flexible Advanced Gasication-Combustion (AGC) Process [94]. Some important information ofthese processes is summarized in Table 6, details of which arehowever described by Fan [94].

From Table 6, one can see that all CaO-based CLG processes aremainly aimed to produce the product gas rich in hydrogen with

Table 6CaO-based chemical looping gasication.

Parameters CO2 acceptor HyPr-Ring ZECA ALSTOM AGC

Product CH4 and ue gas H2 and CO2 H2 and CO2 H2 , N2 and CO2 H2 , hot air and CO2CO2 capture No Yes Yes Yes YesGasier operating condition 800850 1C, 10 atm 650 1C, 30 atm 880980 1C, 6 atm 750850 1C, 1720 atmH2% 65.6% 76% 80%Fuel Coal Coal Coal Coal CoalReactor type Fluidized bed Pressurized reactors Pressurized reactors Fast uidized bed Fluidized bed

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579 575

integrated CO2 capture, except only CO2 Acceptor process. How-ever, these processes are not considered as sustainable andenvironmental hydrogen production due to the dependence onfossil fuel. Moreover, the processes were designed to operate athigh pressure and/or temperature and some of them were com-plexly designed with multiple-loop conguration for the loopingsystem. These aspects possibly cause high operational complexityand cost as well as high capital cost. Due to these troublesomeissues, the current status of the CaO-based CLG is still in the R&Dstage, with no commercial plants in operation [93].

To overcome these challenges, the recent researches anddevelopments simplify the conguration of looping system withthe single-loop and atmospheric operation. More sustainable andenvironmental concerns are taken in to account with utilization ofbiomass feedstock. Fig. 10 presents the schematic principle ofrecent CaO-basd CLG. This looping system mainly consists of tworeactors, i.e., the regenerator and the gasier. In the gasier, thebiomass fuel is gasied in the presence of CaO and steam toproduce hydrogen-rich gas, whereas CO2 generated during thegasication is instantaneously captured by CaO and then formsCaCO3 via a carbonation reaction (Eq. (13)). Afterwards, the usedCaO particles in the form of CaCO3 are circulated to the regeneratorto get calcined and then release a highly concentrated stream ofCO2 via a calcination reaction (Eq. (14)). The hot CaO regeneratedin the regenerator is then cycled back to the gasier as a repeatedprocess and also provides additional heat for the gasicationprocess. In addition, heat released by the exothermic carbonationreactions would compensate for the endothermic gasicationreactions [79]. Due to these energy-efcient and environment-friendly aspects, a CaO-based CLG process is gaining more interestfor sustainable hydrogen production with in situ CO2 capture inrecent years.

With this concept of CaO-based CLG, Acharya et al. [79]developed a single-loop process for atmospheric biomass steamgasication for hydrogen-enriched gas production with in-processCO2 capture using calcined limestone as a CO2 sorbent. This single-loop conguration is much simpler than previous CaO-based CLGunits, where two or more loops are used. The main components inthe process consist of a bubbling uidized-bed gasier, a circulat-ing uidized-bed regenerator, and a cyclone. In theoretical analysisof the process, it was reported that the system efciency as high as87.49% can be received at an ideal scenario with 100% CO2 capture.A volumetric concentration of 71% for H2 and nearly 0% for CO2 inthe product gas was received from the experiment conducted in abatch-type uidized-bed gasier.

Moghtaderi [95] also adopted this single-loop CaO-based CLGconcept to produce high-purity combustible gases (e.g., hydrogen)from carbon-based fuel including biomass. Similar to Acharya et al.[79], Moghtaderi [95] found that using CaO from naturally occur-ring CaO-based material like calcined rocks (limestone or

dolomite) suffers from particle attrition (due to weak mechanicalstrength) and deactivation. This results in greater demands forfresh CaO-based sorbent that would consequently lead to higheroperational costs. To avoid these problems, concrete and demoli-tion waste (CDW) was used as a CO2 sorbent in the loopingprocess. The experimental result shows that the bed of CDWexhibits a very similar behavior to limestone to allow high H2 andlow CO2 in the product gas. In addition, it was reported that CDWparticles do not suffer from attrition issues and are not severelyeroded with a repeated number of carbonationcalcination cycles,unlike calcined limestone used by Acharya et al. [79]. Due to theseadvantages, CDW can also be an option for further research.

Similar to CaO-based CLG concept, an Absorption EnhancedReforming (AER) process, operated in a dual uidized bed gasier(DFBG) has been developed at the Vienna University of Technologywith the aim for hydrogen-rich gas production with in situ CO2capture. The operational principle of the AER process can beexplained with the same Fig. 10 as that of CaO-based CLG detailedearlier. However, there is one major difference applied to aregeneration reactor, details of which are further described below.The experimental investigations in this AER process were con-ducted by Pfeifer et al. [90,91]. Some important details on theinuence of the process on H2 production, tar reduction and CO2capture are previously discussed in Section 6.2. In addition,another test investigation on the same AER setup conducted byKoppatz et al. [87] is also summarized in Table 5.

In contradiction to the CaO-based CLG developed by Acharyaet al. [79], the AER process developed at the Vienna University ofTechnology used direct burning of fuel (residual char circulatedwith bed material from the gasier) in the regeneration reactor(so called combustor) rather than using external heaters. This rendersthe process more energy-efcient. However, since air supplied forcombustion causes a production of ue gas (mainly containing N2and CO2), a complex and expensive post-combustion separationsystem may be required, if one wishes to capture CO2. To avoid thisconcern, Acharya et al. [79] later developed the CaO-based CLG, asdescribed earlier, using CO2 or steam as a regeneration medium inthe regenerator heated using external heaters. In this process, pureCO2 stream was produced from calcination reaction in the regen-erator. The production of pure CO2 stream eliminates the need fordownstream CO2 separation system which consequently lowersoperational complexity and cost as well as capital plant cost. More-over, CO2 stream produced from calcination reaction can be partiallyreturned to use as a regeneration medium in the regenerator.Nonetheless, the major disadvantage of this process is energy-intensive due to externally heating the regenerator. Both thedescribed processes have different merits. Selection should bebalanced with the main purpose of application and the considerationof energy/exergy efciency and economic feasibility study of theprocesses. Further research is encouraged as the knowledge in thisarea remains scarce.

7. Conclusions and recommendations

This work rst focuses on conventional steam gasication ofbiomass without catalysts for hydrogen-enriched gas production.The obvious challenge of undesirable CO2 and tar formation duringthe process can be overcome by adding CaO to the process withoutcompromising hydrogen production. The deactivation of CaO aftercarbonation reaction is also challenging for continuous hydrogenproduction and long-term stability of the gasication process,which consequently makes the process less attractive both intechnical and economical perspectives. Due to these challenges,the concept of CaO-based chemical looping gasication (CaO-based CLG), which is principally aimed for continuous hydrogen

Regenerator850-900oC

Gasifier600-700oC

Steam

BiomassHeat

Steam/CO2/Air

H2-rich gas stream

CO2-rich gas stream Heat

CaO

CaCO3

Char

Fig. 10. Schematic principle of CaO-based chemical looping gasication.

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579576

production with in situ CO2 capture, is thus emerged and gainingmore attention in recent years.

Important conclusions drawn from the present study are asfollows:

Several aspects of biomass characteristics have signicantinuence on hydrogen production. Biomass containing highercellulose and hydrogen-to-carbon ratio is found to inuencehigher hydrogen production. Smaller biomass particles whichcontribute to a larger surface area and hence faster heatingrates can allow higher hydrogen production in biomass steamgasication.

The increase of gasication temperature favors the hydrogenproduction because higher reaction temperature allows higherheating rate which improves the conversion of biomass into theproduct gas and consequently provides the gas rich in hydro-gen. Increasing temperature also enhances hydrogen ow rateand shortens the time of hydrogen release.

Increasing steam-to-biomass (S/B) ratio increases the concen-tration and yield of hydrogen in the product gas since theinuence of steam affects the chemical reaction of steamreforming and water-gas shift reactions. However, too muchsteam can also adversely cause temperature drop in thegasier, which leads to lower hydrogen production and highertar formation.

The variation of equivalence ratio (ER) affects the change ingasication temperature and reactions. Applying higher ER cancause a reduction of hydrogen in product gas due to moreoxidation reactions which consume more combustible gases.Conversely, higher ER can also improve hydrogen productionsince higher gasication temperature driven by higher ER canenhance the gasication reactions and consequently improvethe product gas quality.

One of the challenges for hydrogen production from conven-tional biomass steam gasication without catalysts is undesir-able formation of CO2 and tar. Use of CaO in the process canabsorb CO2 and lower its partial pressure, therefore movingwater-gas shift reaction in forward direction to enhance hydro-gen output. Besides, CaO can simultaneously play the role ofreforming tar into gas, which consequently results in lower tarcontent and higher yield of hydrogen and total gas. CaO cancatalytically reform tar compounds from higher ring species inpolycyclic aromatic hydrocarbon (PAH) group to fewer ringspecies. Moreover, shifting the tar compounds from heavierPAHs to the lighter species also results in reduction of tardew point.

The biggest challenge of using CaO in biomass steam gasica-tion that makes the process less attractive is the deactivation ofCaO after carbonation reaction. This directly interrupts contin-uous hydrogen production and may not be economically viable,if fresh CaO is frequently needed to rell to the process. Use ofCaO-based CLG can thus overcome such a challenge and allowcontinuous hydrogen production with in situ CO2 capture. Based on previous studies discussed in the present work, thefollowing suggestions for future studies and developments tomove the present technology towards the commercial use ofsustainable hydrogen production from biomass steam gasica-tion are made:

Use of CaO in the process for hydrogen-rich gas productionwith in situ CO2 capture and tar reforming is recommended. In-depth study on tar reforming is encouraged to analyze theeffect of CaO on tar compounds. Knowledge of tar compoundscan link to tar dew point which is the important indicator thatcan help to determine the operating conditions of gasicationas well as its downstream systems to avoid undesirable tarcondensation and deposition due to reduced temperature of

the product gas. The information in this area remains limited inliterature.

More development of the CaO-based CLG system is encouraged,as the process is very useful for continuous hydrogen produc-tion with in situ CO2 capture. Optimization study on thisprocess is also needed as this knowledge is still lacking in theliterature. Comparative study on different CaO-based bedmaterial can be an option for further research.

Acknowledgments

One of the authors, Jakkapong Udomsirichakorn, thanks EnergyPolicy & Planning Ofce (EPPO) of Ministry of Energy, Thailand forproviding the King HRD scholarship for his PhD study at AsianInstitute of Technology. The author also acknowledges the supportprovided by PTT Public Company Limited, Thailand.

The authors greatly appreciate the information provided by Dr.Bishnu Acharya as well as his scientic suggestions and commentson this work.

References

[1] Tanksale A, Beltramini JN, Lu GQM. A review of catalytic hydrogen productionprocesses from biomass. Renew Sustain Energy Rev 2010;14:16682.

[2] Asadullah M, Ito S, Kunimori K, Yamada M, Tomishige K. Biomass gasicationto hydrogen and syngas at low temperature: novel catalytic system usinguidized-bed reactor. J Catal 2002;208:2559.

[3] Chen S, Wang D, Xue Z, Sun X, Xiang W. Calcium looping gasication for high-concentration hydrogen production with CO2 capture in a novel compactuidized bed: simulation and operation requirements. Int J Hydrogen Energy2011;36:488799.

[4] Mirza UK, Ahmad N, Harijan K, Majeed T. A vision for hydrogen economy inPakistan. Renew Sustain Energy Rev 2009;13:11115.

[5] Yolcular S. Hydrogen production for energy use in European Union countriesand Turkey. Energy Sour A 2009;31:132937.

[6] Balat H, Kirtay E. Hydrogen from biomass - present scenario and futureprospects. Int J Hydrogen Energy 2010;35:741626.

[7] Kalinci Y, Hepbasli A, Dincer I. Biomass-based hydrogen production: a reviewand analysis. Int J Hydrogen Energy 2009;34:8799817.

[8] Guan J, Wang Q, Li X, Luo Z, Cen K. Thermodynamic analysis of a biomassanaerobic gasication process for hydrogen production with sufcient CaO.Renew Energy 2007;32:250215.

[9] Han L, Wang Q, Yang Y, Yu C, Fang M, Luo Z. Hydrogen production via CaOsorption enhanced anaerobic gasication of sawdust in a bubbling uidizedbed. Int J Hydrogen Energy 2011;36:48209.

[10] Saxena RC, Seal D, Kumar S, Goyal HB. Thermo-chemical routes for hydrogenrich gas from biomass: a review. Renew Sustain Energy Rev 2008;12:190927.

[11] Ni M, Leung DYC, Leng MKH, Sumathy K. An overview of hydrogen productionfrom biomass. Fuel Process Technol 2006;87:46172.

[12] Acharya B, Dutta A, Basu P. An investigation into steam gasication of biomassfor hydrogen enriched gas production in presence of CaO. Int J HydrogenEnergy 2010;35:15829.

[13] Kong L, Li G, Zhang B, He W, Wang H. Hydrogen production from biomasswastes by hydrothermal gasication. Energy Sour A 2008;30:116678.

[14] Mohammed MAA, Salmiaton A, Wan Azlina WAKG, Mohammad Amran MS,Fakhru'l-Razi A, Tauq-Yap YH. Hydrogen rich gas from oil palm biomass as apotential source of renewable energy in Malaysia. Renew Sustain Energy Rev2011;15:125870.

[15] Mohammed MAA, Salmiaton A, Wan Azlina WAKG, Mohammad Amran MS,Fakhru'l-Razi A. Air gasication of empty fruit bunch for hydrogen-rich gasproduction in a uidized-bed reactor. Energy Convers Manage 2011;52:155561.

[16] Florin NH, Harris AT. Enhanced hydrogen production from biomass within situ carbon dioxide capture using calcium oxide sorbents. Chem Eng Sci2008;63:287316.

[17] Nipattummakul N, Ahmed II, Gupta AK, Kerdsuwan S. Hydrogen and syngasyield from residual branches of oil palm tree using steam gasication. Int JHydrogen Energy 2011;36:383543.

[18] Baratieri M, Baggio P, Fiori L, Grigiante M. Biomass as an energy source:thermodynamic constraints on the performance of the conversion process.Bioresour Technol 2008;99:706373.

[19] Franco C, Pinto F, Gulyurtlu I, Cabrita I. The study of reactions inuencing thebiomass steam gasication process. Fuel 2003;82:83542.

[20] Rapagn S, Provendier H, Petit C, Kiennemann A, Foscolo P. Development ofcatalysts suitable for hydrogen or syn-gas production from biomass gasica-tion. Biomass Bioenergy 2002;22:37788.

[21] Ross D, Noda R, Horio M, Kosminski A, Ashman P, Mullinger P. Axial gasproles in a bubbling uidized bed biomass gasier. Fuel 2007;86:141729.

J. Udomsirichakorn, P.A. Salam / Renewable and Sustainable Energy Reviews 30 (2014) 565579 577

[22] Moghtaderi B. Effects of controlling parameters on production of hydrogen bycatalytic steam gasication of biomass at low temperatures. Fuel 2007;86:242230.

[23] Gil J, Corella J, Aznar MP, Caballero MA. Biomass gasication in atmosphericand bubbling uidized bed: effect of the type of gasifying agent on theproduct distribution. Biomass Bioenergy 1999;17:389403.

[24] McKendry P. Energy production from biomass (part 3): gasication technol-ogies. Biosour Technol 2002;83:5563.

[25] Florin NH, Harris AT. Hydrogen production from biomass coupled with carbondioxide capture: the implications of thermodynamic equilibrium. Int JHydrogen Energy 2006;31:94552.

[26] Weerachanchai P, Horio M, Tangsathitkulchai C. Effects of gasifying conditionsand bed materials on uidized bed steam gasication of wood biomass.Bioresour Technol 2009;100:141927.

[27] Manovic V, Lu D, Anthony EJ. Steam hydration of sorbents from a dualuidized bed CO2 looping cycle reactor. Fuel 2008;87:334452.

[28] Das D, Veziroglu TN. Advances in biological hydrogen production processes.Int J Hydrogen Energy 2008;33:604657.

[29] Momirlan M, Veziroglu TN. Current status of hydrogen energy. Renew SustainEnergy Rev 2002;6:14179.

[30] Gadhamshetty V, Sukumaran A, Nirmalakhandan N, Myint MT. Photofermen-tation of malate for biohydrogen production a modeling approach. Int JHydrogen Energy 2008;33:213846.

[31] Krtay E. Recent advances in production of hydrogen from biomass. EnergyConvers Manage 2011;52:177889.

[32] Kotay SM, Das D. Biohydrogen as a renewable energy resource prospectsand potentials. Int J Hydrogen Energy 2008;33:25863.

[33] Tao Y, Chen Y, Wu Y, He Y, Zhou Z. High hydrogen yield from a two-stepprocess of dark- and photo-fermentation of sucrose. Int J Hydrogen Energy2007;32:2006.

[34] Acharya B. Chemical looping gasication of biomass for hydrogen-enrichedgas production [Doctoral thesis] Department of Mechanical Engineering,Dalhousie University, Canada; 2011.

[35] Das D, Veziroglu TN. Hydrogen production by biological processes: a survey ofliterature. Int J Hydrogen Energy 2001;26:1328.

[36] Balat M, Balat M, Kirtay E, Balat H. Main routes for the thermo-conversion ofbiomass into fuels and chemicals. part 2: gasication systems. Energy ConversManage 2009;50:315868.

[37] Delgado J, Aznar MP. Biomass gasication with steam in uidized bed:effectiveness of CaO, MgO, and CaO-MgO for hot raw gas cleaning. Ind EngChem Res 1997;36:153543.

[38] Lv P, Xiong Z, Chang J, Wu C, Chen Y, Zhu J. An experimental study onbiomass air-steam gasication in a uidized bed. Bioresour Technol2004;95:95101.

[39] Xiao X, Meng X, Le DD, Takarada T. Two-stage steam gasication of wastebiomass in uidized bed at low temperature: parametric investigations andperformance optimization. Bioresour Technol 2011;102:197581.

[40] Zhang R, Cummer K, Suby A, Brown RC. Biomass-derived hydrogen from anair-blown gasier. Fuel Process Technol 2005;86:86174.

[41] Chang ACC, Chang HF, Lin FJ, Lin KH, Chen CH. Biomass gasication forhydrogen production. Int J Hydrogen Energy 2011;36:1425260.

[42] Salleh MAM, Kisiki NH, Yusuf HM, Ghani WAWAK. Gasication of biochar fromempty fruit bunch in a uidized bed reactor. Energies 2010;3:134452.

[43] Turn S, Kinoshita C, Zhang Z, Ishimura D, Zhou J. An experimental investigation ofhydrogen production from biomass gasication. Int J Hydrogen Energy 1998;23:6418.

[44] Gao N, Li A, Quan C, Gao F. Hydrogen-rich gas production from biomass steamgasication in an updraft xed-bed gasier combined with a porous ceramicreformer. Int J Hydrogen Energy 2008;33:54308.

[45] Zhou J, Chen Q, Zhao H, Cao X, Mei Q, Luo Z, et al. Biomass-oxygen gasicationin a high-temperature entrained-ow gasier. Biotechnol Adv 2009;27:60611.

[46] Gonzalez JF, Roman S, Bragado D, Calderon M. Investigation on the reactionsinuencing biomass air and air/steam gasication for hydrogen production.Fuel Process Technol 2008;89:76472.

[47] Nipattummakul N, Ahmed II, Kerdsuwan S, Gupta AK. Hydrogen and syngasproduction from sewage sludge via steam gasication. Int J Hydrogen Energy2010;35:1173845.

[48] Gielen D, Simbolotti G. Prospects for hydrogen and fuel cells. IEA energytechnology analysis series, vol. 2. Paris: OECD/IEA; 2005.

[49] Cohce MK, Dincer I, Rosen MA. Thermodynamic analysis of hydrogen produc-tion from biomass gasication. Int J Hydrogen Energy 2010;35:497080.

[50] Cohce MK, Rosen MA, Dincer I. Efciency evaluation of a biomass gasication-based hydrogen production. Int J Hydrogen Energy 2011;36:1138898.

[51] Balat M. Potential importance of hydrogen as future solution to environmentaland transportation problems. Int J Hydrogen Energy 2008;33:401329.

[52] Rothwell G, Williams K. Is nuclear power more competitive producingelectricity or hydrogen? Int J Nucl Hydrogen Prod Appl 2006;1:15469.

[53] Tseng P, Lee J, Friley P. A hydrogen economy: opportunities and challenges.Energy 2005;30:270320.

[54] International Energy Agency, Prospects for Hydrogen from Biomass, IEAhydrogen implementing agreement, Annex 16, Subtask B, Final Report, June2006.

[55] Padro CEG, Putsche V. Survey of the economics of hydrogen technologies.Technical report, NREL/TP-57027079. Colorado: National Renewable EnergyLaboratory; September, 1999.

[56] Luo S, Xiao B, Guo X, Hu Z, Liu S, He M. Hydrogen-rich gas from catalytic steamgasication of biomass in a xed bed reactor: inuence of particle size ongasication performance. Int J Hydrogen Energy 2009;34:12604.

[57] Ahmed I, Gupta AK. Syngas yield during pyrolysis and steam gasication ofpaper. Appl Energy 2009;86:181321.

[58] Ahmed I, Gupta AK. Pyrolysis and gasication of food waste: syngas char-acteristics and char gasication kinetics. Appl Energy 2010;87:1018.

[59] Ahmed I, Gupta AK. Hydrogen production from polystyrene pyrolysis andgasication: characteristics and kinetics. Int J Hydrogen Energy 2009;34:625364.

[60] Ahmed I, Gupta AK. Characteristic of hydrogen and syngas evolution fromgasication and pyrolysis of rubber. Int J Hydrogen Energy 2011;36:43407.

[61] Rapagn S, Latif A. Steam gasication of almond shells in a uidized bedreactor: the inuence of temperature and particle size on product yield anddistribution. Biomass Bioenergy 1997;12:2818.

[62] Herguido J, Corella J, Gonzalez-Saiz J. Steam gasication of lignocellulosicresidues in a uidized bed at small pilot scale. Effect of the type of feedstock.Ind Eng Chem Res 1992;31:127482.

[63] Luo S, Xiao B, Hu Z, Liu S, Guan Y, Cai L. Inuence of particle size on pyrolysisand gasication performance of municipal solid waste in a xed bed reactor.Bioresour Technol 2010;101:651720.

[64] Di Blasi C. Kinetic and