a review of the primary measures for tar elimination in biomass gasification processes

Biomass and Bioenergy 24 (2003) 125140

A review of the primary measures for tar eliminationin biomass gasi cation processes

Lopamudra Devi, Krzysztof J. Ptasinski, Frans J.J.G. JanssenEnvironmental Technology Group, Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology,

Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, Netherlands

Received 1 February 2002; received in revised form 11 July 2002; accepted 15 July 2002

Abstract

Tar formation is one of the major problems to deal with during biomass gasi cation. Tar condenses at reduced temperature,thus blocking and fouling process equipments such as engines and turbines. Considerable e3orts have been directed on tarremoval from fuel gas. Tar removal technologies can broadly be divided into two approaches; hot gas cleaning after thegasi er (secondary methods), and treatments inside the gasi er (primary methods). Although secondary methods are provento be e3ective, treatments inside the gasi er are gaining much attention as these may eliminate the need for downstreamcleanup. In primary treatment, the gasi er is optimized to produce a fuel gas with minimum tar concentration. The di3erentapproaches of primary treatment are (a) proper selection of operating parameters, (b) use of bed additive/catalyst, and(c) gasi er modi cations. The operating parameters such as temperature, gasifying agent, equivalence ratio, residence time,etc. play an important role in formation and decomposition of tar. There is a potential of using some active bed additivessuch as dolomite, olivine, char, etc. inside the gasi er. Ni-based catalyst are reported to be very e3ective not only for tarreduction, but also for decreasing the amount of nitrogenous compounds such as ammonia. Also, reactor modi cation canimprove the quality of the product gas. The concepts of two-stage gasi cation and secondary air injection in the gasi er areof prime importance. Some aspects of primary methods and the research and development in this area are reviewed and citedin the present paper.? 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Tar removal; Gasi cation; Bed additive; Gasi er design; Catalyst

1. Introduction

With respect to global issues of sustainable energyand reduction in greenhouse gases, biomass is gettingincreased attention as a potential source of renewableenergy. Biomass is not yet competitive with fossil fu-els. Fossil fuel contributes to the major part of worlds

Corresponding author. Tel.: +31-40-2473734; fax: +31-40-2446653.

E-mail address: [email protected] (L. Devi).

total energy consumption. According to the World En-ergy Assessment report, 80% of the worlds primaryenergy consumption is contributed by fossil fuel, 14%by renewable (out of which biomass contributes 9.5%)and 6% by nuclear energy sources [1]. A sustainableenergy future requires combination of factors such asrenewable resources and advanced energy technology.Biomass refers to all organic materials that are

originated from plants. Biomass is being traditionallyused as energy source especially for cooking andheating particularly in the developing countries.

0961-9534/03/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S0961 -9534(02)00102 -2

126 L. Devi et al. / Biomass and Bioenergy 24 (2003) 125140

Nomenclature

Daf dry, ash freeER equivalence ratio: O2 content of

air supply=O2 required for completecombustion

FCC Iuid catalytic crackingGR gasifying ratio: (H2O + O2)=Biomass

[(kg h1) (kg daf h1)1]

LHC light hydrocarbonsm3o normal cubic meterPAH polycyclic aromatic hydrocarbonRDF refuse derived fuelSB steambiomass ratio; H2O=biomass

[(kg h1) (kg daf h1)1]

Di3erent biomass conversion processes produce heat,electricity and fuels. Biomass integrated gasi ca-tion/combined cycle systems are of prime importanceas modern technologies [2]. Among all biomass con-version processes, gasi cation is one of the promisingones. The energy eKciency in case of gasi cationis higher than that of combustion. One of the majorissues in biomass gasi cation is to deal with the tarformed during the process. Tar is a complex mixtureof condensable hydrocarbons, which includes singlering to 5-ring aromatic compounds along with otheroxygen-containing hydrocarbons and complex PAH.Various research groups are de ning tar di3erently.In the EU/IEA/US-DOE meeting on tar measurementprotocol held in Brussels in the year 1998, it wasagreed by a number of experts to de ne tar as allorganic contaminants with a molecular weight largerthan benzene [3]. Tar is undesirable because of vari-ous problems associated with condensation, formationof tar aerosols and polymerization to form more com-plex structures, which cause problems in the processequipment as well as the engines and turbines used inapplication of the producer gas. However, the mini-mum allowable limit for tar is highly dependent on thekind of process and the end user application. Bui etal. [4] mentioned that the preferable tar and dust loadsin gases for engines must be lower than 10 mg m3o .Milne et al. [5] tabulated the tar tolerance limitfor various end use devices, suggested by di3erentresearchers.The present paper discusses the strategies for tar re-

moval and aims to elaborate more on the importance ofprimary measures. This paper provides an overview ofprimary methods used for tar removal during biomassgasi cation. A direct comparison of the data avail-

able for tar in the literature, is very diKcult becauseof the following reasons; use of di3erent operatingconditions, di3erent type of gasi er used, di3erent tarcapturing, sampling and analysing method, and mostimportantly the non-consistency in de ning tar usedby various researchers. Detailed discussion regardingthe tar de nition, tar sampling and analysing methodsare out of the scope of this review, those can be foundin the corresponding references.

2. Tar removal methods

Several approaches for tar reduction have been re-ported in the literature. A major part of ongoing re-search deals with the development of eKcient meth-ods for tar removal in an economical and optimizedway. There are some sophisticated options available,which have claimed to reduce tar amount signi cantly.However, the method must be eKcient in terms of tarremoval, economically feasible, but more importantly,it should not a3ect the formation of useful gaseousproducts. All the methods available can be categorizedin two types depending on the location where tar is re-moved; either in the gasi er itself (known as primarymethod) or outside the gasi er (known as secondarymethod). The following sections describe both meth-ods with emphasis on the primary method.

2.1. Secondary methods

Secondary methods are conventionally used astreatments to the hot product gas from the gasi er.The concept of secondary methods is given in Fig. 1.These methods can be chemical or physical treatment

L. Devi et al. / Biomass and Bioenergy 24 (2003) 125140 127

ApplicationGasifierSyn. gas

+ TarBiomass Gasifier

Air/Steam/O2

Tar Removal

Gas Cleanup

Downstream Cleaning(Tar; Dust; N, S, halogen compounds)

Tar free gasApplicationGasifier

Syn. gas

+ TarBiomass Gasifier

Air/Steam/O2

Tar Removal

Gas Cleanup

Downstream Cleaning(Tar; Dust; N, S, halogen compounds)

Tar free gas

Fig. 1. Tar reduction concept by secondary methods.

Biomass

Gasifier

+

Tar Removal

Air/Steam/O2

Application

Gasifier

+

Tar Removal

Tar free gasGas cleanup

Dust N, S, halogen Compounds

Biomass

Gasifier

+

Tar Removal

Air/Steam/O2

Application

Gasifier

+

Tar Removal

Tar free gasGas cleanup

Dust N, S, halogen Compounds

Fig. 2. Tar reduction concept by primary method.

as follows

tar cracking downstream the gasi er either ther-mally or catalytically,

mechanical methods such as use of cyclone, baOe lter, ceramic lter, fabric lter, rotating particleseparator, electrostatic lter and scrubber.

Although, downstream gas cleaning methods are re-ported to be very e3ective in tar reduction, in somecases they are not economically viable. In addition,several researchers reported secondary methods to bevery e3ective in terms of ammonia reduction [6,7]. Itis also observed that production of very clean gas witha considerably low tar content requires a complex pro-cess. NarvPaez et al. [8] reported a three-step processthat could produce a very clean gas (with very low tarconcentration).The secondary methods are widely being studied

and are well understood. Since they are out of thescope of this paper, interested readers are advised tolook into excellent reports byMilne et al. [5] and Neeftet al. [3].

2.2. Primary methods

Primary methods can be de ned as all the mea-sures taken in the gasi cation step itself to preventor convert tar formed in the gasi er. An ideal pri-mary method concept eliminates the use of secondarytreatments as shown in Fig. 2. Primary methods arenot yet fully understood and yet to be implementedcommercially.To get the best-quality exit gas, the gasi er per-

formance has to be optimized. For an optimizedperformance of the gasi er, the main attractive factorsare the design and the operation of the gasi er. Corellaet al. [9] mentioned that in-bed use of dolomite resultsin a similar tar content as using it in downstream cat-alytic reactor. If the gasi er bed is well designed andoperated, the measures taken during the gasi cationstep can produce a very clean gas with respect to theend user application, eliminating the use of down-stream secondary steps. The primary issues include(a) the proper selection of the operating conditions,(b) the use of a proper bed additives or a catalystduring gasi cation, and (c) a proper gasi er design.


2.2.1. Gasi5cation conditionsThe operating conditions play a very important

role during biomass gasi cation in all respects, suchas carbon conversion, product gas composition, tarformation and tar reduction. The most important in-Iuencing parameters include temperature, pressure,gasifying medium, catalyst and additives (explainedin the next section), equivalence ratio (ER), residencetime, etc. The selection of these parameters also de-pends on the type of gasi er used. A homogeneousbed temperature pro le and a well-functioning bedIuidization are of utmost importance to avoid distur-bances in the operation of a Iuidized-bed gasi er.To achieve a high carbon conversion of the biomass

and low tar content in the resultant product gas, ahigh operating temperature (above 800C) in thegasi er is preferred. To produce a relatively cleangas by increased temperature, several temperatureranges are reported in the literature. The present pa-per cites only a few of these works. Temperature notonly a3ects the amount of tar formed, but also thecomposition of tar by inIuencing the chemical re-actions involved in the whole gasi cation network.Kinoshita et al. [10] observed during sawdust gasi- cation in a xed bed gasi er that the total numberof detectable tar species decreased with increasingtemperature. Oxygen-containing compounds such asphenol, cresol and benzofuran exist in signi cantquantities only at temperature below 800C. They alsocon rmed that higher temperature favour the forma-tion of fewer aromatic tar species without substituentgroups such as benzene, naphthalene, phenanthrene,etc. Destruction of these aromatic hydrocarbons oc-curs only at temperatures above 850C. Yu et al. [11]performed pyrolysis experiments of birch wood in afree-fall reactor to observe the temperature e3ect onthe process and found that an increasing temperaturepromotes the formation of gaseous products at theexpense of total tar. More than 40% reduction in taryield was reported when the temperature was raisedfrom 700C to 900C. With increase in temperature,the amount of total oxygen-containing componentsdrastically goes down, the amount of substituted1-ring and 2-ring aromatics also decrease, but for-mation of 3- and 4-ring aromatics increases rapidly.Almost 40% increase in naphthalene content wasreported at 900C. Similar results were reported byBrage et al. [12] for gasi cation of birch wood. Dur-

ing the same experimental time they observed almostcomplete reduction of phenol content, a 50% decreasein toluene content, but a considerable increase inbenzene (from 14 to 24 mg l1) and in naphthalenecontents (from 2 to 8 mg l1), when the tempera-ture was raised from 700C to 900C. By changingthe bed temperature of the bubbling Iuidized bedfrom 700C to 850C, NarvPaez et al. [13] observedan increase in H2 content from 5 to 10 vol%, COcontent from 12 to 18 vol%, slight decrease in CO2content from 16 to 13 vol% and almost no changein the amount of CH4 and C2H2. They observed adrastic decrease in tar content (about 74% less); thetar amounts being 19 g m3o at 700

C and 5 g m3oat 800C. It is also possible to decrease the amountof tar to a considerably lower value by increasing thefreeboard temperature in case of Iuidized-bed gasi- ers [13,14]. Besides tar, temperature also inIuencesthe formation of NH3 and N2. The level of these twospecies in the product gas largely depend on the ther-mochemical reactions occurring in the gasi er andthese reactions are directly related with temperature.Zhou et al. [15] reported almost 58% decrease in NH3content when the temperature was raised from 700Cto 900C for sawdust gasi cation. Over the sametemperature range, NO and HCN were also detected,but at a much lower level than NH3. For leucaenagasi cation, an 80% decrease in NH3 content is re-ported when the gasi er temperature was increasedfrom 750C to 900C. Increases in molecular nitrogenwere observed in the product gas which is basicallydue to the thermochemical conversion of NH3 [15].However, there are several other factors that limit

the operating temperature. Hallagren [16] mentioneda typical temperature range for di3erent feed materi-als with respect to various critical factors that largelyinIuences the entire gasi cation system. Besides thetar content, these factors, as shown in Fig. 3, arethe gas heating value, char conversion and the risk ofsintering.Several researchers have investigated pressurized

biomass gasi cation. Knight [17] investigated the ef-fect of system pressure for biomass gasi cation. Whenthe pressure was increased to 21:4 bar, almost com-plete elimination of phenols was observed for wis-consin whole tree chips. Although the amount of totaltar decreased, the fraction of PAH increased with in-creasing pressure. Moilanen et al. [18] studied the


700 OC 00 O C 1000 O C800 O C

Agrofuels

RDF WoodyBiomass

Coal

Higher Gas heating value Lower

Higher Tar content Lower

Lower Char conversion Higher

Decreasing risk Sintering Increasing risk

700 OC 00 O C 1000 O C800 O C

Agrofuels

RDF WoodyBiomass

Coal

700 OC 00 O C 1000 O C800 O C

Agrofuels

RDF WoodyBiomass

Coal









9

Fig. 3. Typical gasi cation temperature for various feedstock and inIuence of temperature change on some critical factors as reported byHallgren [16].

dependence of pressure on wood char gasi cation. Thepressure was raised from 1 to 15 bar of CO2 and H2Oat temperatures of 750C and 850C. Increase in CO2pressure slightly reduced char gasi cation whereasthis increased with increasing H2O pressure. Pressur-ized gasi cation (520 bar) is also being investigatedin the Lund University [19,20]. Wang et al. [19] ob-served a decrease in the amount of light hydrocarbons(LHC, lower than naphthalene) as well as that of tarin the fuel gas with an increasing ER for pressurizedgasi cation with 100% carbon conversion.Di3erent gasifying agents such as air, steam,

steamoxygen and carbon dioxide have been reportedin the literature. Selectivity of the gasi cation re-actions varies with di3erent gasifying media, thusa3ecting the product gas composition as well as theheating value. Heating value of the product gas withair as gasifying medium is lower because of dilutionof the gas by nitrogen. NarvPaez et al. [13] reporteda gas composition of 10% H2, 14% CO, 15% CO2(vol%) from a gasi er with gasi cation temperatureof 800C with an ER of 0.35. But the tar contentdecreases sharply as the ER was increased, being aslow as 2 g m3o . The ER strongly inIuences the typeof gasi cation products. According to Kinoshita et al.[10], tar yield and tar concentration decreases as theER increases because of more availability of oxygento react with volatiles in the Iaming pyrolysis zone.This e3ect of ER is more signi cant at higher tem-

perature. The ER is very crucial because its highervalue results in lower concentration of H2, CO andhigher CO2 content in the product gas, thus decreas-ing the heating value of the gas. Although the total tarconcentration decreased by almost 30% when the ERwas increased from 0.22 to 0.32 for a temperature of700C, the fraction of PAH increased in the total tar.The decrease in total tar concentration could be muchhigher at higher temperature. Almost all phenols wereconverted at an ER of 0.27. Increase in contents ofbenzene, naphthalene, 3- and 4-ring compounds werereported. NarvPaez et al. [13] reported a similar trendwith increasing ER for gasi cation of pine sawdustat 800C. A tar content of about 27 g m3o was re-ported when the ER was increased until 0.45. Also,they gave a comparison of various experiments withvarying ER by other researchers. According to Zhouet al. [15], ER does not signi cantly inIuence theconcentration of nitrogenous products during biomassgasi cation. A slight increase in NH3 (from 310 to350 ppm) was observed when the ER was increasedfrom 0.25 to 0.37 at 800C for sawdust gasi cation.In view of lower heating value as well as poor gas

composition, several researchers used pure steam andthey observed a completely di3erent product gas dis-tribution. The steam gasi cation product is more orless free from N2 and more than 50% H2 in the prod-uct gas. Herguido et al. [21] reported steam gasi ca-tion and the e3ect of steam/biomass ratio (SB) on the


gasi cation products. They observed an increase inH2 (as high as 60%) and CO2 (from 10 to 30%) con-tents, a sharp decrease in CO (from 35 to 10%) con-tent, a slight decrease in CH4 content and relativelyno change of C2-fractions (C2H2;C2H4;C2H6) whenthe SB ratio was increased from 0.5 to 2.5. In spite ofsharp reduction in tar (8% yield at 0.5 SB decreasedto almost nil at 2.5) there was a sharp decrease in thelower heating value that was attributed by the decreasein CO. GarcPSa et al. [22,23] used steam as gasifyingagent for pine sawdust using an Ni/Al coprecipitatedcatalyst. The catalyst improves the reaction rate ofsteam with char and also can participate in secondaryreactions, thus leading to decrease in tar content of thegas. The high H2 production during steam gasi cationcan be attributed by the following chemical reactions:

CnHx + nH2O (n+ x=2)H2 + nCO; (1)

CH4 + H2O 3H2 + CO;

TH0298 = +205:81 kJ mol1; (2)

C2H4 + 2H2O 4H2 + 2CO;

TH0298 = +210:07 kJ mol1; (3)

CO + H2O H2 + CO2;

TH0298 =41:16 kJ mol1; (4)

C + H2O H2 + CO;

TH0298 = +131:29 kJ mol1: (5)

The rst reaction represents tar reforming reactionwhich contributes to increment in the content of H2and CO gases.Steam gasi cation is endothermic and hence some-

times requires complex design for heat supply inthe process. Use of some amount of oxygen in thegasifying medium can provide the necessary heat forgasi cation and then the gasi er works as an auto-thermal reactor. In view of that, many researchersused steamoxygen mixtures for biomass gasi cation.Aznar et al. [24] reported more than 85% reduction intotal tar when they increased the (steam+O2)/biomassratio termed as gasifying ratio (GR) from 0.7 to 1.2.The researchers also reported that for low GR val-ues in the gasi er produces light tars which can beeasily destroyed using catalyst [25]. Gil et al. [26]recommended H2O=O2 ratio of around 3.0 (mol/mol)

for steamO2 mixture for gasi cation. When the GRwas increased from 0.6 to 1.7, they observed the fol-lowing changes in gas composition, decrease in H2content from 29% to 13%, decrease in CO contentfrom 50% to 30%, increase in CO2 content from 14%to 37%, slight decrease in CH4 content from 7% to5% and a change in C2 hydrocarbons from 3.5% to2.3% (all percentages are expressed in volume on drybasis). Tar content of the raw gas sharply decreaseswith GR, less than 5 g m3o at a GR of 1.2. The sameresearchers also observed that the e3ect of gasi erbed temperature on tar content is signi cant at lowerGR values ( 1:0). They recommended a gasi erbed temperature between 800C and 860C with GRof 0.81.2 and a gas residence time of 2 s [26]. Theyalso compared their work with that of NarvPaez et al.[13] and Herguido et al. [21], which involved gasi -cation with air and steam, respectively. A relationshipbetween ER, SB and GR values is mentioned forthe purpose of comparison [27] by the authors of thepaper. Under selected conditions, more tar is formedwith pure steam, than that with steamO2 mixtureand less with air as gasifying agent. It should bementioned at this point that the operating parametersof the works compared were not exactly the same,but similar; also the gasi ers used were di3erent indesign. Hofbauer et al. [28] and Fercher et al. [29]reported use of a combination of steam and air in afast internally Iuidized bed biomass gasi er whichis explained in the following section of this paper.Air combustion provided the heat required for steamgasi cation.The use of CO2 as gasifying medium is promising

because of its presence in the gasi cation atmosphere.Tar reduction is also enhanced by dry reforming reac-tions of CO2, which is a gasi cation product. Accord-ing to Minkova et al. [30], a mixture of steamCO2gives highest degree of carbonization for pyrolysis andgasi cation of biomass in a horizontal rotating reactor.They also mentioned that a steamCO2 mixture pro-duced the highest activity char, which resulted in highash content. GarcPSa et al. [31] investigated CO2 gasi -cation in the presence of Ni/Al coprecipitated catalystand compared the results with those of steam gasi ca-tion [22,23]. CO2 gasi cation in the presence of a cat-alyst transformed tars and also causes a decrease of theamounts of CH4 and C2-fraction (C2H2;C2H4;C2H6)as well as an increase in H2 and CO yields. Also, a


signi cant decrease in the CO2 content was observedwith a CO2/biomass ratio of 1.16 indicating that CO2itself converts to other products. Deposition of carbonin the catalyst particles can be avoided to a certainextent by feeding CO2 in excess. The main chemicalreactions with CO2 as gasifying medium are listedbelow:

CnHx + nCO2 (x=2)H2 + 2nCO; (6)

CH4 + CO2 2H2 + 2CO;

TH0298 = +246:98 kJ mol1; (7)

C2H4 + 2CO2 2H2 + 4CO;

TH0298 = +292:41 kJ mol1; (8)

CO2 + H2 CO + H2O;

TH0298 = +41:16 kJ mol1; (9)

CO2 + C 2CO;

TH0298 = +172:46 kJ mol1: (10)

Reaction (6) represents dry reforming reaction of tar.According to Kinoshita et al. [10] residence time

has little inIuence on the tar yield, but it signi cantlyinIuences the tar composition. The residence timethey reported was based on the super cial velocityof the wet product gases in the gasi er. Amounts ofO2-containing compounds tend to decrease with in-creasing residence time. Yields of 1- and 2-ring com-pounds (except benzene and naphthalene) decreasewhereas that of 3- and 4-ring compounds increasesin the total tar fraction. Corella et al. [9] observed adecrease in the total tar content when the space timewas increased for biomass gasi cation with in-beduse of dolomite. The space time was expressed as(kg calcined dolomite (kg biomass daf h1)1). Thetar amounted 2 g m3o at space time 1.0 which was6 g m3o at a space time of 0.1.In addition to the optimized operational conditions,

presence of some active materials in the gasi er canlargely improve the product gas distribution. The fol-lowing section covers some of the reported literatureon use of bed additives.

2.2.2. Bed additivesCatalytic tar reduction has been extensively re-

ported in the literature [32]. These catalysts include

Ni-based catalysts, calcined dolomites and magne-sites, zeolites, olivine and iron catalysts. Among allthese only few have been tried as active bed additiveinside the gasi er itself during gasi cation. There isa great potential of in-bed additives in terms of tarreduction and thus avoiding complex downstream tarremoval methods. These bed additives act as in situcatalysts promoting several chemical reactions in thesame gasi er. The presence of additives not only inIu-ences the gas composition, but also the heating valueof the product gas. The use of catalytically active ma-terials during biomass gasi cation promotes the chargasi cation, changes the product gas composition andreduces the tar yield. Besides these, addition of activebed materials also prevents the solid agglomerationtendencies and subsequent choking of the bed.Limestone was one of the rst additives used in the

gasi er to improve the gasi cation. Walawender et al.[33,34] performed series of experiments using lime-stone as bed additive in a Iuidized bed gasi er. Theyused a mixture of 25 wt% limestone and 75 wt% silicasand as a bed material for steam gasi cation of alphacellulose with an intention of predicting the behaviourof the gasi er with increase in temperature [33]. Theresearchers also applied the same mixture of bed ma-terial for steam gasi cation of manure [34]. Althoughno attempt was made to observe the tar formation,the authors reported that the gas composition, heatingvalue and yield were all inIuenced by the presenceof 25 wt% limestone in the bed. The most importantoutcome of their experiments was that the addition oflimestone to the bed of silica sand could prevent ag-glomeration of the bed [34].Among all the active materials, dolomite is the most

popular and mostly studied in-bed additive. A lot ofresearch has been done using this catalyst with regardto tar cracking in bed as well as in a secondary reactor.Karlsson et al. [35] reported the successful demon-stration of biomass IGCC process (VEGA Gasi ca-tion with combined cycle) which involved dolomiteas bed material. The tar content observed was about12 g m3o of light tars (excluding benzene) and 100300 mg m3o of heavy tars. RapagnVa et al. [36] alsomentioned the use of calcined dolomite directly inthe gasi er and observed improvements in the gasyield. Corella et al. [37] reported that the use of cal-cined dolomite inside the gasi er could decrease thetar amount from 6.5 (without dolomite) to 1:3 wt%.


The two research groups in Spain (University of Com-plutense and University of Saragossa) have done sev-eral experiments with in-bed use of dolomite. NarvPaezet al. [13] suggested that addition of calcined dolomite(15 wt% of the biomass feed) improves the qualityof the product gas. Their experiments resulted in atar reduction of about 40% with addition of 3% cal-cined dolomite to biomass feed. A 10 wt% of calcineddolomite is reported to be suKcient by Olivares et al.[38] for signi cant improvement of the gas qualityas well as tar reduction. The elimination of tar overcalcined dolomite is mostly due to steam and dryreforming reactions.Corella et al. [9] observed almost no di3erence in

the lower heating value of the gas produced by us-ing in-bed dolomite and that of downstream use ofdolomite. The increase in H2 production compensatedthe CO decrease; also there was hardly any variationin the CH4 and C2Hn amounts in the product gas. Thetar content in the product gas with the use of in-beddolomite was signi cantly reduced and this reduc-tion is more or less comparable to using downstreamdolomite. Besides the use of active additive in thebed, the researchers also observed that the gasifyingmedium plays a very important role in tar reduction,which has already been discussed in the previous sec-tion of this paper. An amount of 2030 wt% dolomite(rest being silica sand) in the gasi er was reported toreduce the tar content as low as 1 g m3o at an ERof 0.3 [14]. The authors also studied the inIuence ofseveral operating parameters in combination with useof in-bed dolomite and found that a proper selectionof parameters in addition to dolomite can signi cantlyenhance production of a clean gas. Although dolomitehas been proven to be a very e3ective bed additive interms of tar reduction, it has some critical limitations.Dolomite is softer and thus gets eroded by the silicasand particles. Also, some dolomite particles breakduring the calcination and give rise to a large produc-tion of nes. So, there is a great problem of carry overof solids from the bed.An alternative of dolomite can be naturally occur-

ring particles of olivine, which is a mineral containingmagnesium, iron oxide and silica. Olivine is advanta-geous in terms of its attrition resistance over that ofdolomite. RapagnVa et al. [39] investigated the catalyticactivity of olivine and observed that it has a good per-formance in terms of tar reduction and the activity

is comparable to calcined dolomite. They reportedmore than 90% reduction in average tar content, the taramounted 2:4 g m3o compared to 43 g m

3o with only

sand. The authors also performed experiments witholivine as the bed material and a LaNiFe trimetallicperovskite catalyst in a secondary reactor. The com-bined action of the materials was very promising; agas with around 0:3 g m3o of tar was produced [40].RosPen et al. [41] reported the use of olivine as a bedmaterial for pressurized gasi cation (0.41:0 MPa) ofbirch.Mudge et al. [42] studied the catalytic steam

gasi cation of biomass using alkali carbonate andNi-based catalyst at the Paci c Northwest Laboratory(PNL). Supported Ni catalysts were found to be verye3ective in producing high yields of synthesis gas[42,43]. Baker et al. [44] studied a number of cat-alysts (Ni=Al2O3;K2O3;NiCuMo=SiO2Al2O3) forpressurized steam gasi cation of bagasse and woodto produce synthesis gas for methanol and ammoniaproduction in a laboratory-scale gasi er. The catalyticgasi cation resulted in increase in the gas yield atthe expense of tar and char. They also carried outgasi cation tests in a 1-t d1 Iuidized-bed processdevelopment unit (PDU) using the same catalysts.They observed severe deactivation of the Ni-basedcatalyst due to carbon deposition [44].A number of other metal oxide catalysts such as

V2O5, Cr2O3;Mn2O3;Fe2O3, CoO, NiO, CuO, MoO3on Al2O3 were tested by Yoshinori et al. [45] forwood gasi cation to produce methanol synthesis gas.All these metal oxides produced high yields of gas,but the gas composition strongly varied depending onthe type of metal oxide used. MoO3, CoO and V2O5produced higher amount of CO2, whereas NiO, Cr2O3and Fe2O3 produced relatively low amounts of CO2.The most suggested favourable gasi cation catalystwas NiO=Al2O3, which could produce a gas with aH2=CO ratio close to 2.0, also observed to produceonly traces of total ole ns. Although these authors didnot quantitatively measure the tar amount, their resultsshowed that the additives greatly improve the overallproduct distribution and the gas heating value. Sev-eral other researchers reported in-bed use of Ni-basedcatalysts. Bilbao et al. [46] observed that addition of50 wt% of the NiAl catalyst to bed material (sand),increases hydrogen production up to 62% with con-siderable decrease in the methane content. The same


Ni/Al coprecipitated catalyst was also observed to bevery e3ective for CO2 gasi cation of biomass [23] Theratio of catalyst weight and biomass Iow rate signi -cantly inIuences the product gas distribution.The major problem with Ni-based catalysts is fast

deactivation due to carbon deposition on the catalystand poisoning due to the presence of H2S. These prob-lems can be avoided to some extent by increasing thetemperature [7,47]. In view with the experiences ofPNL, in bed use of Ni catalyst is not an attractive op-tion. Baker et al. [48] and Mudge et al. [49] reportedthat the lifetime of the Ni catalysts could be extendedby placing it in a secondary bed instead of using it inthe gasi er. Placing the NiCuMo on SiAl2O3 and Nion -alumina catalysts in a secondary Iuidized-bedreactor downstream the gasi er, they observed no de-activation for 3040 h tests.Ni-based catalysts are also very e3ective for NH3

reduction. Wang et al. [47] reported that NH3 canbe decomposed e3ectively with an Ni-based catalystabove 800C. Wang and his coworkers [50] reported95% conversion of NH3 along with 89% conversionof light hydrocarbons, which they de ned as C2H6,C6H6; C7H8 and C9H8, with Ni-based catalyst in asecondary reactor at a temperature of 874C, pressureof 12 bar and a space time of 3 s. Their observationwas that the amounts of light hydrocarbons have anegative e3ect on NH3 decomposition. Mojtahedi etal. [51] investigated Ni/alumina and Ru/alumina cat-alyst for NH3 decomposition in a synthesis gas mix-ture. The same catalysts were studied for ammoniaand tar decomposition in an actual biomass gasi ca-tion system [6]. These catalysts are less e3ective interms of NH3 reduction in the presence of tars. Simellet al. [7] also observed that Ni/alumina catalyst is ef-fective for NH3 as well as tar reduction. The authorsobserved that NH3 decomposition could be enhancedwith iron-containing materials over a temperature of900C. It should be noted that all above-mentionedstudies on catalytic NH3 decomposition were carriedout in external reactors as secondary treatment of thehot gas from the gasi er.Douglas et al. [52] used 8 wt% potassium carbon-

ate as bed additive impregnated on wood for steamgasi cation in a Iuidized bed gasi er at operatingtemperature of 750C. They observed a reduction ofphenolic tar compounds by a factor of 5 and PAH bya factor of about 10. It is observed that alkali metals,

especially potassium, act as a promoter in unzippingthe cellulose chains during the thermal decompositionof woody biomass, thus e3ecting the product gasdistribution.Corella et al. [37] also reported the use of

in-equilibrium FCC catalyst as bed additive for steamgasi cation of pine sawdust. This catalyst containedzeolite in a silicaalumina matrix, and was once usedfrom oil re neries. The experiment was done by feed-ing the catalyst to biomass feed (biomass=FCC= 10)and 20% reduction in tar content was reported. How-ever, this catalyst was rapidly deactivated by cokedeposition and the catalyst particles were entrainedout of the bed just in a few seconds. The authorsperformed some more experiments with in-bed useof the same catalyst at a higher temperature range(800820C) and observed almost a 60% decrease intar content with the in-bed use of 5% in-equilibriumFCC catalyst [14]. Besides cracking, hydrocrackingmight be a possible reason of tar decomposition.Another inexpensive material can be char and it has

also been widely reported in the literature. Char isnot only a cheap and available material, but also it isindigenously produced inside the gasi er itself. Charhas been reported to be used in secondary tar crackingreactors by several researchers [53,54]. Chembuku-lam et al. [55] reported that cracking over a char bedat a temperature of 950C resulted in almost com-plete decomposition of tar and pyroligneceous liquorinto gases of low calori c value. It should be notedthat char itself gets converted during the gasi cationprocess, and hence there may be need of externalcontinuous supply of char into the gasi er. Severalresearchers reported use of char in two-stage gasi- er which is explained in the next section of thispaper.Based on the reported literature, the following gen-

eral observations can be made with the addition ofactive bed material during gasi cation

1. a change in product gas distribution,2. a decrease in tar amount,3. an increase in hydrogen production,4. a slight decrease in the amount of CO and increase

in the amount of CO2,5. an almost no variation in the amount of CH4,6. problems regarding catalyst deactivation and

carryover of nes were severe,


Table 1In-bed additives used by researchers under di3erent operating conditions

Feed Feed properties Operating conditions Additive Reference

Moisture (%) Size (mm) Gasif. agent Temp (C) Time (s)

Cellulose 1.02.0 Steam 600800 1.261.54 Limestone Walawender et al. [33]Wood Steam 750 K2CO3 Douglas et al. [52]Pine sawdust 8.5 1.0 Steam 750 Dolomite Corella et al. [37]

FCCPine sawdust 1025 4.00.8 Air 800 0.6 Dolomite NarvPaez et al. [13]Pine chips 1012 5.01.0 Steam=O2 795835 Dolomite Olivares et al. [38]Pine sawdust 10 Steam 700 0:4 NiAl Bilbao et al. [46]Pine sawdust 10 CO2 700 NiAl GarcPSa et al. [23]Pine chips Air;steam/O2 800850 Dolomite Corella et al. [9]Pine chips 1015 Air 800845 Dolomite Gil et al. [14]

FCCAlmond shell 7.9 1.1 Steam 770820 Olivine RapagnVa et al. [39]

DolomiteBirch 68 1.03.0 O2N2 700900 Silver sand RosPen et al. [41]

OlivinePine/bagasse Steam 750 30 Ni-based Baker et al. [44]

7. catalytic tar reduction depends on gasi cationconditions.

A summary of some of the research work done usingbed additives is given in Table 1.

2.2.3. Gasi5er design modi5cationReactor design is very crucial for gasi cation in

terms of eKciency, heating value of the product gasand also for tar formation. Modi cation of the gasi erdesign can be very e3ective in producing clean gas.Some attempts have been reported in this regard andare observed to be successful in tar reduction.Secondary air injection to the gasi er results in a

signi cant tar reduction. A higher temperature couldbe attained due to the secondary air injection in thegasi er. Pan et al. [56] injected secondary air justabove the biomass feeding point in the Iuidized bed.They reported that an optimal secondary to primaryair ratio of about 20% is suKcient to reduce 88:7 wt%of the total tar for a gasi cation temperature range of840880C. NarvPaez et al. [13] performed few exper-iments with secondary air injection in the freeboardof Iuidized-bed gasi er. They observed a tempera-ture rise of about 70C and a tar reduction from 28 to16 g m3o . They expected that with longer size gasi-

ers the temperature rise in the freeboard due to airinjection could be higher, thus a higher possibility oflower tar content in the gas.Two-stage gasi er design has been reported to

be very e3ective in producing clean gas. The basicconcept of this design is to separate the pyrolysiszone from the reduction zone. A two-stage gasi er isequivalent to two single-stage gasi ers. Tars formedduring the pyrolysis ( rst stage) are decomposed inthe reduction zone (second stage). A two-stage gasi- er has been studied in the Asian Institute of Tech-nology (AIT), Thailand, and it resulted in a gas of tarcontent about 50 mg m3o , about 40 times less than asingle-stage reactor under similar operating conditions[4]. This concept involves two levels of air intakes.The high temperature achieved in the second zone dueto the addition of a secondary air helps in reducing thetar level to a considerably lower value. Fig. 4 repre-sents the two-stage gasi cation concept applied in theAIT. It was observed that most of the tars were formedduring the warm-up period. This could be avoided by lling the gasi er with a bed of char just above theprimary air inlet. The gasi er was loaded with charbefore ignition. It was reported that char lling al-most totally eliminates tar formation during start-upin the reactor. Further modi cation was done by


Pyroly(1 st stage)

Reduction zone(2nd stage)

Biomass Feed

Tar free gas

Secondary air

Primary air

Pyrolysis gas ,Tar

Gas

ifier

Pyroly(1 st stage)


Biomass Feed

Secondary air

Primary air

Pyrolysis gas ,Tar

Pyroly(1 st stage)


Biomass Feed

Tar free gas

Secondary air

Primary air

Pyrolysis gas ,Tar

Gas

ifier

Pyrolytic zone (1 st stage)


Biomass Feed

Secondary air

Primary air

Pyrolysis gas ,Tar

Fig. 4. Two-stage gasi cation concept (Asian Institute of Tech-nology, Thailand).

Bhattacharya et al. [57] in such a way that char wasproduced inside the gasi er itself, thus avoiding inputof external charcoal for subsequent experiments. Forthis purpose, an extra primary air inlet above the origi-nal air supply was added. The researchers coupled thechar gasi er with the original two-stage gasi er andreported that this design could produce considerablylow tar content (19 mg m3o ) producer gas.Successful operation of this type of gasi er largely

depends on how stable the pyrolysis zone is [4]. Thestabilization of this zone is dependent on the balancebetween downward solid movement and upward Iamepropagation. If the wood particles move faster thanthe Iame propagation, the pyrolysis zone reaches thesecond air intake thus making the whole system to actlike single-stage gasi er. If Iame propagation upwardexceeded wood consumption, both stages remained instable operation. The Iame propagation control wasdone by adjustments of the airIow by changing thewidth of the second air inlet.Another two-stage gasi er designed at the Tech-

nical University of Denmark is a combination ofpyrolysis of the biomass feed with subsequent partialoxidation of the volatile products in presence of acharcoal bed as shown in Fig. 5 [58,59]. Henriksen

Fig. 5. Two-stage gasi er (Technical University of Denmark).

et al. [58] reported the successful demonstration ofthis type of gasi er (50 kW) for straw gasi cation.Brandt et al. [59] also mention the same reactor design(100 kW) for gasi cation of wood chips. The charand the volatile pyrolysis products from the pyrolysisunit entered the top of the gasi cation unit where thegases were mixed with the preheated steam and airstarting partial combustion. The char is transported tothe char gasi cation unit. Allowing the gases to passthrough the bed of char, signi cant tar reduction inthe total tar content (15 mg m3o ) has been reported.Lower amounts of tar are attributed to the partialcombustion of the pyrolytic gases as well as the cat-alytic e3ect of the charcoal bed. The char conversionwas measured to be 7090%.Susanto et al. [60] developed a cocurrent moving

bed gasi er with internal recycle and separate com-bustion zone of pyrolysis gas. Their aim was to pro-duce a design suitable for scaling up a downdraft gasi- er maintaining a low tar content in the product gas.This system was able to produce a clean gas with tarcontent as low as 0:1 g m3o . This gasi er concept isrepresented in Fig. 6. In this design, biomass is rstpyrolysed and the produced char enters the reductionzone. The volatile pyrolysis gases are mixed with thegasifying air and burnt in an internal separate pre-combustion chamber. The Iue gas from this cham-ber then acts as the gasifying medium for the gasi er.


CombustorCounter-currentReduction zone

Co-currentReduction zone

Drying + Pyrolysis zone Injector

Gas

ifier

Biomass Air

Product gas

Char

Injec

ted ga

sFl

ue

gas

1Fl

ue

gas

2

Rem

aini

ng

(Fro

m

redu

ctio

nzo

ne)

Recyclegas

Flue gasCha

r

Recycle gas=Volatile +Injected gas

CombustorCounter-currentReduction zone

Co-currentReduction zone

Drying + Pyrolysis zone Injector

Gas

ifier

Biomass Air

Product gas

Char

Injec

ted ga

sFl

ue

gas

1Fl

ue

gas

2

Rem

aini

ng

(Fro

m

redu

ctio

nzo

ne)

Recyclegas

Flue gasCha

r

Recycle gas=Volatile +Injected gas

Fig. 6. Moving-bed gasi er with internal recycle.

The Iue gas is divided into two parts, one part be-ing fed to rst reduction zone. This recycle gas Iowscountercurrently with the solid feed and char, thus en-abling a more complete pyrolysis of the solids uponentering the reduction zone. The second part is sent

CombustionZone

Gasification Zone

Biomass

SteamAir

Product gasFlue gas

Heat

Bed material, char

Bed material

Gasifier

CombustionZone

Gasification Zone

Biomass

SteamAir

Product gasFlue gas

Heat

Bed material, char

Bed material

Gasifier

Fig. 7. Fast internally circulating Iuidized-bed gasi cation concept.

to gasify the remaining char in the second reductionzone, thus providing more complete gasi cation. Themain cause reported by the researchers for tar reduc-tion in such design is due to the fact that the tar Iowis premixed with air prior to combustion and all tarsmust pass through hot Iame. Also they mentioned thatany tar that might have escaped from the hot combus-tion chamber is subsequently converted over the charin the combustion zone. A recycle ratio of 0.60.9 issuggested to be optimal with regard to clean gas pro-duction as well as good gasi cation eKciency.Several attempts have been made to develop new

Iuidized gasi cation techniques in view of bettereKciency as well as quality fuel gas. A two-stageIuidized gasi er which is a fast internally circulat-ing Iuidized-bed gasi er (FICFB), was reported andobserved to produce a gas with higher calori c valueand nearly free of N2 [28,29,61]. This gasi er conceptis shown in Fig. 7. The Iuidized-bed gasi er was di-vided into two zones, a gasi cation zone with steamas the Iuidizing medium and a combustion zone withair. Between these two zones, a circulation of bedmaterial is created and those bed material acts as heatcarrier from combustion to gasi cation zone. Thebiomass is fed into the gasi cation zone and the bedmaterial together with char produced circulates to thecombustion zone. In the combustion zone the charcoal


Biomass

Tar free

gasSecondary

Method

Optional

ApplicationPre

Treatments

Milling

Drying

Gasification

+

Primary

Methods

Gas

Cleanup

Primary methods:

Gasification conditions

Additives/Catalysts

Gasifier design

Secondary methods:

Mechanical Separation

Thermal Cracking

Catalytic Cracking

Separators for:

Particles, N, S and

halogen compounds

Prime Movers

Fuel Cells

Liquid Fuels

Chemicals

Clean

gasBiomass

Tar free

gasSecondary

Method

Optional

ApplicationPre

Treatments

Milling

Drying

Gasification

+

Primary

Methods

Gas

Cleanup

Primary methods:

Gasification conditions

Additives/Catalysts

Gasifier design

Secondary methods:

Mechanical Separation

Thermal Cracking

Catalytic Cracking

Separators for:

Particles, N, S and

halogen compounds

Prime Movers

Fuel Cells

Liquid Fuels

Chemicals

Clean

gas

Fig. 8. Tar removal approach in biomass gasi cation.

is burnt. The exothermic reaction in the combustionzone provides the energy for the endothermic gasi -cation reactions with steam. They observed that totaltar amounted about 1 g m3o that can be reduced to alower value with the addition of some catalyst as bedadditives. Fercher and his coauthors [29] performedseveral experiments to nd the e3ect of olivine parti-cles in the same gasi er. The addition of olivine de-creases the tar content of the product gas, but also itdepends on the gasi cation temperature. A total taramount of about 0:5 g m3o was observed for woodgasi cation with olivine at a temperature of 800C.

3. Remarks and conclusion

An optimized gasi cation process can produce aconsiderably clean gas, thus eliminating the need ofdownstream hot gas cleaning. This approach is termedas ideal primary method by the authors, and shownin Fig. 2. According to this concept all tars should beprevented or eliminated in the gasi er itself and thisway no additional after treatments for tar removal isrequired. The following factors are most important asprimary measures:1. A proper selection of operating parameters (spe-

cially temperature, gasifying medium, ER and resi-dence time) can simplify the entire gasi cation chain.2. Addition of some active bed materials has been

proven to be e3ective. But there are a number of fac-tors that have to be taken into account when selectingthe bed materials. These bed additives should be eco-

nomically available, attrition resistant, and of courseactive and selective in terms of tar reduction. Amongall additives used so far dolomite, olivine and char areimportant and there is need for further research todevelop other cheap additives.3. The modi cations made in the gasi er design

must be easily implementable. Most of the attemptsmade to modify the gasi er design claimed to producea clean gas. Moreover, few factors should be keptin mind while designing the entire gasi cation chain:it should be environmental friendly, should be ableto produce a gas of high heating value with low tarcontent, and it should be economically feasible.However, the type of treatments that have to be

used in the gasi cation chain highly depends on theend-use applications. The rst step for process selec-tion for biomass gasi cation must be to evaluate thefuture application. Depending on the application of thegas produced, it must be decided whether a primary orsecondary tar removal method or a combinationof both should be applied. This approach is shown inFig. 8.Although primary measures are of potential impor-

tance for the gasi cation chain as a whole, but theycannot solve the purpose of tar reduction without ef-fecting the useful gas composition and heating value.Combination of proper primary measures with down-stream methods is observed to be very e3ective in allrespect. More research is required for developing aneKcient primary technique (combination of optimaloperational parameters, bed additives and reactor de-sign) which would be suKcient to produce a clean


gas and eliminating the need of downstream tar re-moval techniques. Also, more attention must be paidto reduce other contaminants such as NH3, HCN, HCl,H2S, etc. in combination with tar reduction.

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A review of the primary measures for tar eliminationin biomass gasification processesIntroductionTar removal methodsSecondary methodsPrimary methodsGasification conditionsBed additivesGasifier design modification

Remarks and conclusionReferences

a review of the primary measures for tar elimination in biomass gasification processes

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