Catalytic destruction of tar in biomass derived pr

Ruiqin Zhang a ,Robert C.Brown

b,*,Andrew Suby b ,Keith Cummer b a

Department of Chemistry,Zhengzhou University,Zhengzhou 450052,China b Center for Sustainable Environmental Technologies,Iowa State University,286Metals Development Building,

Ames,IA 50011-3020,USA

Received 25April 2003;accepted 19August 2003

Abstract

The purpose of this study is to investigate catalytic destruction of tar formed during gasification of

biomass,with the goal of improving the quality of the producer gas.This work focuses on nickel based catalysts treated with alkali in an effort to promote steam gasification of the coke that deposits on catalyst surfaces.A tar conversion system consisting of a guard bed and catalytic reactor was designed to treat the producer gas from an air blown,fluidized bed biomass gasifier.The guard bed used dolomite to crack the heavy tars.The catalytic reactor was used to evaluate three commercial steam reforming catalysts.These were the ICI46-1catalyst from Imperial Chemical Industry and Z409and RZ409catalysts from Qilu Petrochemical Corp.in China.A 0.5–3l/min slipstream from a 5tpd biomass gasifier was used to test the tar conversion system.Gas and tar were sampled before and after the tar conversion system to evaluate the effectiveness of the system.Changes in gas composition as functions of catalytic bed temperature,space velocity and steam/TOC (total organic carbon)ratio are presented.Structural changes in the catalysts during the tests are also described.

Ó2003Elsevier Ltd.All rights reserved.

Keywords:Biomass gasification;Steam reforming;Tar;Hydrogen

1.Introduction

Gasification of biomass produces a dirty raw gas mixture composed of hydrogen (H 2),carbon monoxide (CO),carbon dioxide (CO 2),water (H 2O),methane (CH 4)and various light hydro-carbons along with undesirable dust (ash and char),tar,ammonia (NH 3),alkali (mostly Energy Conversion and Management 45(2004)995–1014

www.elsevier.com/locate/enconman

*Corresponding author.Tel.:+1-515-294-3759;fax:+1-515-294-3091.

E-mail address:rcbrown@iastate.edu (R.C.Brown).

0196-04/$-see front matter Ó2003Elsevier Ltd.All rights reserved.

doi:10.1016/j.enconman.2003.08.016

potassium)and some other trace contaminants.Most applications require removal of at least part of the dust and tar before the gas can be used.

It is widely recognized that tar in the producer gas presents a significant impediment to the use of biomass gasification systems.Tar can deposit on surfaces infilters,heat exchangers and engines where they reduce component performance and increase maintenance requirements.These effects must be mitigated if biomass gasification is to become a viable option for energy producers. The purpose of this study is to investigate catalytic destruction of the tar formed during gasi-fication of biomass.This work focuses on nickel based catalysts treated with alkali in an effort to promote steam gasification of the coke that deposits on the catalyst surface.Three metal catalysts were tested:ICI46-1,Z409and RZ409.

2.Background

Methods to remove tars from producer gas can generally be classified into one of three cate-gories:physical processes,thermal processes or catalytic processes.Physical processes,such as filters or wet scrubbers,remove the tar from the producer gas through gas/solid or gas/liquid interactions.While these methods are effective and relatively easy to maintain,they do not truly alleviate the problem,as the tar is not destroyed and environmentally responsible disposal of the resulting tar ladenfilter material is difficult.

Thermal processes raise the temperature of the producer gas to levels that‘‘crack’’the heavy aromatic tar species into lighter and less problematic species,such as hydrogen,carbon monoxide and methane.For thermal cracking of tars,it is suggested that temperatures exceed1000°C[1]in order to reduce tars effectively.High temperatures require the cracking system be constructed of expensive alloys.

Catalytic processes can operate at much lower temperatures(600–800°C)than thermal pro-cesses,alleviating the need for expensive alloys for reactor construction.Depending on the cata-lytic process,this temperature range may eliminate the need to heat and/or cool the producer gas as it leaves the gasifier.Most physical processes usually require the producer gas temperature to be lowered to150°C or less.Gas cooling substantially reduces the thermodynamic efficiency of the gasification process,impairing the performance as well as the economics of the system.Also, unlike physical processes,catalytic cleaning destroys the tar,eliminating waste disposal problems. Potentially,catalytic cracking processes provide the simplest and most effective means of re-moving tars while retaining the sensible heat required for efficient use of the producer gas in close coupled applications.

A number of catalytic processes have been previously investigated.Early on,it was discovered that in situ catalysis,in which the catalysts are placed directly in the gasification reactor,is not effective.Nickel based catalysts showed rapid deactivation when employed influidized bed gasi-fiers[2].In tests with non-metallic catalysts,the catalysts eroded and were elutriated from the bed [3,4].Tar destruction was verified for each type of catalyst,but the short lifetimes of these cata-lysts precluded the continued use of in situ catalysts.

Adding steam and/or oxygen to the catalytic reactor can enhance catalytic tar conversion.The addition of oxygen at600–700°C accelerates the destruction of primary products and inhibits the formation of aromatics.However,once benzene rings,the primary component of aromatics,areR.Zhang et al./Energy Conversion and Management45(2004)995–1014997 formed,they cannot be easily destroyed by oxygen.The addition of steam has been reported to produce fewer refractory tars,enhance phenol formation,reduce the concentration of other oxy-genates,convert few of the aromatics and produce tars that are easier to reform catalytically[1]. The addition of steam also facilitates the water/gas shift reaction:

COþH2O!CO2þH2ð1ÞAs greater amounts of steam are introduced into the system,the H2and CO2concentrations increase,while the CO concentration decreases[5].This reaction is extremely beneficial for methanol production applications,as methanol production occurs most efficiently when the H2/ CO ratio is2.The H2/CO ratio for unprocessed producer gas is usually less than1,and steam addition to a catalytic tar conversion system has demonstrated the ability to adjust the H2/CO ratio to levels as high as13[6].

The use of a catalytic reactor downstream of the gasification reactor has proven to be an ef-fective approach to catalytic tar destruction[7].A variety of catalysts have shown significant ability to destroy tar in gasifier streams.These catalysts include dolomite[8,9],nickel and alumina based catalysts[10,11]and various proprietary catalysts[6,12,13].System variables,such as biomass composition,residence time and reactor temperature,play important roles in the suc-cessful application of these catalysts.

The use of a guard bed of inexpensive catalytic material upstream of a metallic catalyst bed has been demonstrated to improve the life of the metallic catalysts[14].The inexpensive mineral catalyst converts many of the heavy tars,while the metallic catalysts serve to‘‘polish’’the gas stream,reducing tar concentrations to very low levels.Lifetime tests have not been reported for catalysts protected by guard beds,but Milne et al.[1]recommend this approach to catalytic tar destruction.

Nickel based catalysts almost completely remove the tar,but they are gradually deactivated by the deposition of coke on the catalysts.Coke formation on nickel results from a balance between coke formation and gasification.In industrial operations,coke gasification is accelerated by the use of alkali or alkali containing supports.Magnesium and potassium based materials are mainly used[15–17].A complex of potassium alumina silicate and calcium magnesia silicate is used in the ICI nickel based catalyst.The potassium is liberated slowly as non-volatile K2CO3,which is hydrolyzed to the hydroxide.Mobility on the surface ensures good coke-alkali contact and rapid gasification[15].Recent research results[18,19]also verify that alkali,by itself or added to commercial catalysts,promotes carbon or coke gasification.

3.Experimental methods

The major subsystems,consisting of a gasifier,catalytic tar conversion system and tar and gas analysis systems,are described in the following paragraphs.

3.1.Gasifier

Tests were conducted at the Biomass Energy Conversion Facility(BECON)in Nevada,IA, which is operated by the Iowa Energy Center.A pilot scalefluidized bed reactor was used toperform the experiments.The system is rated800kW(2.8MMBtu/h)thermal input,which corresponds to an average throughput of180kg/h(400lb/h)of solid biomass fuel at a heating value of16,000kJ/kg(7000Btu/lb).The major components of the plant include thefluidized bed reactor,fluidization gas system,fuel delivery system,data acquisition system and gas sampling system.

The current experiments employed discard seed corn as fuel,which is a waste stream of interest to one segment of the agricultural processing industry.The proximate and ultimate analyses of the waste seed corn are given in Table1.A variable speed auger metered the fuel into a rotary airlock where it fell into a constant speed injection auger.The high speed auger injected the fuel into the bottom of thefluidized bed.A small amount of air,introduced immediately below the airlock, prevented backflow of the fuel gas into the fuel hopper.

Thefluidized reactor was a46cm(18in.)diameter cylindrical steel vessel standing2.44m(8ft) tall.The reactor wall was lined with castable ceramic to insulate the vessel.Fluidization air en-tered the reactor through an array of perforated pipes that evenly distributed the air to the bottom of the bed.The bed media were sand mixed with a small quantity of limestone,making up about 5%of the total bed weight,to prevent agglomeration of the bed material arising from alkali in the biomass feed.The particulate laden fuel gas exited the reactor through the freeboard and passed through a cyclone that removed much of the particulate matter larger than10l m in size.Details on the operation of the biomass gasifier can be found in Ref.[20].

The gasifier wasfluidized with air at an equivalence ratio between0.25and0.35,which maintained the reactor in the temperature range of700–760°C.The feed rate of seed corn during these trials was in the range of350–450lb/h.

3.2.Catalytic tar conversion system

The catalytic tar conversion system is illustrated schematically in Fig.1.A slipstream was drawn immediately downstream of the cyclone at a rate of0.5–3.0l/min.The slipstream passed through a heated particulatefilter before entering the tar conversion system.Both the slip-stream line and the particulatefilter were maintained at450°C to prevent condensation of the tars.

The tar conversion system consisted of a guard bed reactor of dolomite stone in series with a tar-cracking reactor containing a metallic catalyst.The guard bed was designed to capturefine particulates as well as steam reform the heavy tars and absorb hydrogen sulfide.The metallic Table1

Chemical characterization of obsolete seed corn used as fuel

Seed corn Proximate analysis Ultimate analysis a

Moisture Volatile

matter Fixed

carbon

Ash C H N S O

As Rec.9.077.911.7 1.441.70 6.43 1.100.1349.24 Dry0.085.612.9 1.545.82 5.96 1.210.1445.37 a Oxygen determined by difference.

998R.Zhang et al./Energy Conversion and Management45(2004)995–1014

catalyst bed,which is susceptible to coking during destruction of the heavy tars and poisoning by hydrogen sulfide,was designed to convert the lighter tars into carbon monoxide and hydrogen.The two reactors,which were identical in construction,were operated as fixed beds.

Fig.2illustrates the construction of the reactors.They have internal diameters of 22mm and can be filled to various depths to give space velocities between 1500and 6000h À1.Each was installed in an electrically heated oven to maintain the desired temperature for each experiment.Each reactor had two thermocouples,as indicated in Fig.2:one at the center of the fixed bed,which was moveable for obtaining longitudinal temperature profiles and the other at the perimeter of the bed.

The catalysts evaluated in our tests included three kinds of commercial steam reforming Ni based catalysts:ICI46-1was produced by the Imperial Chemical Industry,while Z409and RZ409are products of Qilu Petrochemical Corp.,P.R.China.The compositions of the catalysts are listed in Table 2.All three catalysts contained alkali additives,such as potassium,calcium and mag-nesium oxides,which promote the elimination of coke formed on the catalyst by reactions of the type:

C þH 2O !CO þH 2ð2

Þ

R.Zhang et al./Energy Conversion and Management 45(2004)995–1014999

Although the potassium promoter might be expected to diffuse readily out of the catalyst,it is in the form of potassium aluminosilicate,which releases the potassium very slowly,resulting in long service life.

Catalysts are usually activated before use by exposure to a reducing environment,typically a mixture of N 2and H 2at 750–850°C for several hours.However,in our experiments,we tested ICI46-1and Z409without reduction.RZ409is a reduced form of Z409prepared by the manu-facturer.The as-received catalysts were in the form of 15mm rings.For use in our reactor,these rings were crushed and sifted to obtain 0.9–2.0mm diameter particles.The pore size distributions of the crushed and sieved catalyst particles were obtained by mercury porosimetry.Typical characteristics for catalysts used in steam reforming are:specific surface area of 16–23m 2/g,total pore volume of 0.14–0.18cm 3/g and average pore diameter of 200–500 A.

The operating conditions for the catalysts are given in Table 3.In addition to the type of catalyst,operating variables include temperature of the guard bed (T GB ),temperature of the catalytic bed (T CR ),space velocity (SV)calculated on a dry gas basis and the ratio of steam to

total Table 2

Chemical composition of tested catalysts

Catalyst

Active component Promoter Carrier Preparation ICI46-1

NiO CaO,K 2O SiO 2,Al 2O 3Not reduced Z409

NiO MgO,K 2O,FeO x

SiO 2,Al 2O 3Not reduced RZ409NiO MgO,K 2O,FeO x SiO 2,Al 2O 3Reduced

1000R.Zhang et al./Energy Conversion and Management 45(2004)995–1014

organic carbon ratio (steam/TOC).Total organic carbon represents the amount of carbon in the organic compounds that are susceptible to steam reforming.3.3.Sampling and analysis of gas and tar

Tar is a very difficult substance to sample and analyze with the result that many research groups have developed their own protocols,which makes it difficult to compare results.To avoid this difficulty,we employed the Provisional Protocol for the Sampling and Analysis of Tar and Particulates in the Gas From Large Scale Biomass Gasifiers (Version 1998)prepared by the Working Group of the Biomass Gasification Task of the IEA Bioenergy Agreement [21].We only briefly outline this procedure;details can be found in the literature.

Gas drawn from the slipstream is passed through a heated particulate filter followed by a series of six impinger bottles placed in cooling baths (Fig.1).The first four bottles were immersed in an ice bath while the last two bottles were immersed in an acetone/dry ice bath.The first and sixth bottles were filled with glass beads,while the second,third and fourth bottles were filled with dichloromethane.The fifth bottle was filled with both glass beads and dichloromethane.Gas leaving the impinger train is passed through a vacuum pump before exiting through a wet test meter to determine accurately the total (dry)gas volume sampled.

Gas samples were taken before the guard bed and after the catalytic reactor to provide in-formation about overall system performance.Gas sampling was done every half hour after steady operation of the gasifier and catalytic reactors was achieved.Gas samples were analyzed offline by gas chromatography using a Varian Micro-GC CP-2003Quad equipped with Molsieve 5A BF,Poroplot Q and CP-Sils CB columns and a thermal conductivity detector with argon as carrier gas

Table 3

Operating parameters for catalytic reactors Parameter

ICI46-1test Z409test RZ409test Amount of calcined dolomite (guard bed reactor)

120ml (132.16g)120ml (132.10g)120ml (132.10g)Amount of Ni based catalyst (metal catalyst reactor)

20ml (22.30g)20ml (23.24g)20ml (23.10g)Amount of inert material (metal catalyst reactor)

20ml (15.30g)20ml (15.20g)20ml (15.40g)

Pretreatment of catalyst

No reduction No reduction Pre-reduced by manu-facturer Guard bed temperature,T GB (±5°C)

650°C

650°C

650°C

Catalytic reactor temperature,T CR

(±3°C)

740,760,780,800and 820°C

740,760,780,800and 820°C

740,760,780,800and 820°C

Space velocity––dry gas basis (h À1)

1500,3000,4500and 6000

1500,3000,4500and 6000

1500,3000,4500and 6000

Operating time without steam injection

12h (Steam/C ¼2.8)12h (steam/C ¼2.8)12h (steam/C ¼2.8)Operating time with steam injection

0h

6h (steam/C ¼4.5,5.5,6.5)

6h (steam/C ¼4.5,5.5,6.5)

At the completion of a test,dichloromethane was rinsed through gas lines connected to the impingers to remove any tar condensed in them.This rinse liquid and impinger liquid were combined and refrigerated until the tar analysis was performed.

Two types of analysis were performed on these tar samples:evaporation at105°C and dis-tillation at75°C.In either case,analysis began byfiltering out solids from the sample mixture and decanting the water from it.Evaporative analysis was the simpler of the two analyses performed and yielded tar values in good agreement with traditional methods of measuring‘‘heavy tar’’[1]. This analysis consists of pouring50ml of DCM/tar mixture in a ceramic dish,letting it stand in a fume hood overnight,moving it to a heating chamber at105°C for1h and recording the weight of the remaining residue.From knowledge of the total gasflow through the sampling system,the tar concentration in the producer gas can be obtained,which we shall refer to as‘‘heavy tar.’’The second method of analysis is based on distilling50ml of the DCM/tar mixture in a water bath maintained at75°C for30min.This distillation produces two fractions of hydrocarbons: light hydrocarbons(still dissolved in the distilled dichloromethane)and the distillation residue.In addition,the decanted water contains a third fraction of dissolved hydrocarbons referred to as water soluble hydrocarbons.These three fractions were sent out for total organic carbon(TOC) analysis,which is useful in estimating the amount of steam required to convert the hydrocarbons in the tar to carbon monoxide and hydrogen.

4.Results and discussion

4.1.Properties of raw producer gas

When operating in the equivalence ratio range of0.25–0.35and at gasification temperature of 700–760°C,the average composition of the producer gas was(dry,volumetric basis):51.2% nitrogen,15.7%carbon monoxide,14.2%carbon dioxide,6.5%hydrogen,4.8%methane and4% higher hydrocarbons.The concentration of tar determined from evaporation at105°C was10.4g/ nm3.

From the total organic carbon analysis of the three tar sampling fractions,the carbon con-centrations associated with hydrocarbons recovered in the tar impinger train are:27.8g/nm3from the distillation residue,13.6g/nm3from the light hydrocarbons and5.7g/nm3from the water soluble hydrocarbons.Thus,the total carbon concentration arising from hydrocarbons recovered from the impinger train was47.1g/nm3.The carbon concentration arising from CH4and C2H4in the producer gas(that is,hydrocarbons not recovered in the impinger train)was58.9g/nm3. Taken together,the steam/TOC ratio in the producer gas is estimated to be2.8.

4.2.Tar destruction

For all catalysts and operating conditions tested,no visible tar was observed in the lines after the catalytic reactor or in the impingers.The dichloromethane mixtures recovered after these testswere clear with no hint of color.Analysis by evaporation at105°C found no detectable tar at the exit of the catalytic reactor for any of the tests,indicating heavy tar reduction in excess of99%. Analysis by distillation at75°C was performed for only one test:the ICI46-1catalyst operated at 800°C with a space velocity(SV)of3000hÀ1and a steam/TOC ratio of2.8.Carbon from the light hydrocarbon fraction was present in the amount of5.8g/nm3,while carbon in the form of soluble hydrocarbons was0.6g/nm3.Although6.4g/nm3of carbon associated with hydrocarbons re-covered in the impinger train may appear to be a relatively large concentration,it includes organic compounds that are not considered‘‘tar’’in many applications since they would not normally condense out.Nevertheless,it represents a carbon conversion efficiency of86%for hydrocarbons collected from the raw gas by the impinger train operated at)70°C.

4.3.Effect of catalytic reactor operating conditions on gas composition

The effects of space velocity,catalytic bed temperature,and steam/C ratio on gas composition (H2,CO,CO2,CH4and C2H4)for each of the three catalysts are presented in Figs.3–11(in all tests,the inlet temperature to the tar destruction system was650°C).In thesefigures,‘‘GB Inlet’’refers to the concentration of a gas species at the guard bed inlet(upstream of the tar destruction system),and‘‘CR Outlet’’refers to the concentration of a gas species at the catalytic reactor outlet (downstream of the tar destruction system).In general,H2and CO2increase,while CO decreases in the producer gas as it passes through the tar destruction system,as expected for steam re-forming reactions acting in tandem with the water–gas shift reaction.Concentrations of CH4and C2H4decrease in the producer gas.The decrease in CH4was about0.2–1.0vol%,while the de-crease in C2H4was about0.5–1.5%.Although these low molecular weight hydrocarbon can be products of steam reforming of tar,they are also susceptible to further steam reforming to CO and H2.The ICI46-1catalyst showed no deactivation during12h of testing,while the Z409and RZ409catalysts showed no deactivation during18h of testing.

The effect of space velocity on hydrogen concentration in the producer gas is illustrated in Fig. 3for catalysts ICI46-1,Z409and RZ409(T CR¼800°C;Steam/TOC¼2.8).There was little ev-idence that decreasing space velocity significantly increases hydrogen production(observed variations are within the uncertainty of hydrogen measurements).The effects of space velocity on CO and CO2concentrations in the producer gas are illustrated in Fig.4for catalysts ICI46-1, Z409and RZ409.For space velocities less than4500hÀ1there is no effect(in excess of uncer-tainty)on CO concentration.The concentration of CO2is not substantially influenced by space velocity in the range of1500–6000hÀ1.The effects of space velocity on CH4and C2H4concen-trations in the producer gas are illustrated in Fig.5for catalysts ICI46-1,Z409and RZ409.No definitive trends(in excess of uncertainty)are evident for CH4,while C2H4clearly decreases as space velocity decreases.These observations indicate that tar destruction is not mass transfer limited in the present experimental system.

The effect of catalytic bed temperature on hydrogen concentration in the producer gas is illustrated in Fig.6for catalysts ICI46-1,Z409and RZ409(SV¼3000hÀ1;Steam/TOC¼2.8). As expected,hydrogen production increases with increasing reaction temperature although the increase is less than25%in going from740to820°C.The effect of catalytic bed temperature on CO and CO2concentrations in the producer gas are illustrated in Fig.7for catalysts ICI46-1, Z409and RZ409.Carbon monoxide increases,while CO2decreases with increasing temperature.

The strongest effect is observed for catalyst Z409(CO increases40%in going from740to820°C)and weakest for ICI46-1.The effects of catalytic bed temperature on CH4and C2H4con-centrations in the producer gas are illustrated in Fig.8for catalysts ICI46-1,Z409and RZ409. No definitive trends(in excess of uncertainty)are evident for CH4,while C2H4clearly decreases, especially for the Z409and RZ409catalyst(reduction greater than85%in going from740to 820°C).These observations indicate that the rate of tar destruction is controlled by chemical kinetics.

The effect of steam/TOC ratio on hydrogen concentration in the producer gas is illustrated in Fig.9for catalysts Z409and RZ409(T CR¼800°C;SV¼3000hÀ1).As expected,hydrogen production increases with increasing steam/TOC ratio,although the increase is less than30% in going from a steam/TOC ratio of2.8–6.5.The effects of steam/TOC ratio on CO and CO2concentrations in the producer gas are illustrated in Fig.10for catalysts Z409and RZ409.Carbon monoxide decreases by50%,while CO2increases by50%in going from a steam/TOC ratio of2.8–6.5for both catalysts,indicating a strong water–gas shift reaction.The effects of steam/TOC ratio on CH4and C2H4concentrations in the producer gas are illustrated in Fig.11for catalysts Z409 and RZ409.No definitive trends are evident for either CH4or C2H4.

One reason for evaluating both the Z409and RZ409catalysts was to determine whether re-ducing the catalyst prior to use on gasification streams was important to catalytic activity(RZ409 catalyst is pre-reduced Z409).During the tests,we observed that hydrogen concentrations exiting the tar destruction system were2.0–3.0vol%higher for RZ409than for Z409during thefirst2–3 h.However,for longer times,the difference between them disappeared.Thus,it appears that the producer gas is able to quickly reduce the metallic catalysts,making unnecessary a separate re-ducing step before using the catalyst.

4.4.Mercury porosimetry analysis

The catalysts were analyzed by mercury porosimetry to compare surface areas,pore sizes and pore size distributions before and after use in the tar destruction system(fresh and used catalyst, respectively).The results are shown in Table4.In all cases,the pore structure of the used catalysts changed.

The ICI46-1catalyst showed an insignificant change in surface area while the Z409and RZ409 catalysts showed surface area reductions of30–35%.Furthermore,all three catalysts showed shifts away from small pores(R<100 A)and micro-pores(100–500 A)to medium pores(500–2000 A)and large pores(R>2000 A).Although this could result from coke blocking the smaller pores,the fact that pore volume increased suggests the conversion of small pores and micro-pores

into larger pores during high temperature operation.If this transformation were to continue,the catalytic activity would eventually degrade.

4.5.Carbon and sulfur analysis of catalysts and dolomite

Carbon and sulfur analyses were performed on each of the three metallic catalysts and the dolomite catalyst both before and after the catalysis tests.Since all of the catalysts are inorganic, the appearance of carbon is an indication of coking.Likewise,the accumulation of sulfur on the metal catalyst indicates the breakthrough of hydrogen sulfide from the guard bed.The results are listed in Table5.

Although the metallic catalysts were selected for their high resistance to carbon deposition, both the metallic and mineral catalysts accumulated carbon.However,the accumulation on the

dolomite bed was6–20times greater than on the metallic catalysts,suggesting that the guard bed was doing its job of cracking the heaviest tar compounds,which are most likely to produce coking.

Steam/TOC ratios of4–6are typically used in Ni based catalytic steam reforming of naphtha. In our tests,thefirst several hours of testing for all the catalysts were performed without steam injection,that is,only steam arising from biomass gasification was present.This resulted in steam/ TOC ratios of only2.8.In an effort to remove coke accumulated after18h of testing,the steam/ TOC ratio of the producer gas was increased to4–6for the last6h of testing of the Z409and RZ409catalysts.Although higher steam levels may enhance destruction of hydrocarbons ab-

sorbed on the catalysts,we saw no evidence that coke already deposited was readily removed by the steam/carbon reaction of Eq.(2).

We hoped that the calcined dolomite in the guard bed would absorb most of the hydrogen sulfide existing in the producer gas.However,the appearance of sulfur in all the samples of used metallic catalysts and the relatively low concentration of sulfur in the used dolomite indicates significant breakthrough of hydrogen sulfide from the guard bed.In fact,the high concentration of sulfur in the used ICI46-1catalyst(0.4wt.%after12h without steam injection)indicates a very serious problem.However,relatively little sulfur accumulated on the used Z409and RZ409 catalysts,which were subjected to steam injection for the last6h of testing.This observation

suggests that steam injection can regenerate metallic catalysts that have been poisoned by sulfur. The regenerative process may consist of the following three reactions:

NiSþH2O!NiOþH2Sð3ÞNiOþH2!NiþH2Oð4ÞNiOþCO!NiþCO2ð5ÞAfter as little as6–8h of testing,a white powder was found in the tar sampling line after the catalytic reactor.This proved to be dolomite that had attrited in the guard bed and blown through the slipstream line.Clearly,the strength of catalytic material for the guard bed needs to be im-proved.

5.Conclusions

A tar conversion system consisting of a guard bed and catalytic reactor was designed for the purpose of improving the quality of producer gas from an air blown,fluidized bed biomass gasifier.All three metal catalysts(ICI46-1,Z409and RZ409)proved effective in eliminating heavy tars(>99%destruction efficiency)and in increasing hydrogen concentration by6–11vol%(dry basis).Space velocity had little effect on gas composition while increasing temperature boosted hydrogen yield and reduced light hydrocarbons(CH4and C2H4),thus suggesting tar destruction is controlled by chemical kinetics.

Table4

Pore volume,specific surface and pore size distribution of catalyst samples(by mercury porosimetry)

Sample no.Pore volume

(cm3/g)Specific surface

(m2/g)

Distribution of pore radius(%)

R<100A100–500A500–2000A R>2000A

Fresh ICI46-10.1716.4613322629 Used ICI46-10.2116.2210183735

Fresh Z4090.1422.923438226 Used Z4090.2315.999284914

Fresh RZ4090.1823.322834344 Used RZ4090.2114.7811244322

R.Zhang et al./Energy Conversion and Management45(2004)995–10141013 Table5

Carbon and sulfur analysis of metallic catalysts and dolomite

Sample S(wt.%)C(wt.%)Condition

Fresh ICI46-10.016$0Fresh

Used ICI46-10.40.3612h run(no injected

steam)

Fresh Z4090.013$0Fresh catalyst

Used Z4090.0210.8018+6h with injected steam Fresh RZ4090.018$0Fresh catalyst

Used RZ4090.019 1.0418+6h with injected steam Fresh dolomite0.0084$0Fresh

Used dolomite10.0147.2612h(no injected steam) Used dolomite20.012 6.4618h(no injected steam) Used dolomite30.013 6.7618h(no injected steam) Although the reactivity of the tar conversion system did not diminish during the12–18h of testing,measurements of surface area and pore size distribution indicated the conversion of small pores into larger pores during high temperature operation.If this transformation were to con-tinue,the catalytic activity would eventually degrade.Furthermore,coke accumulated on both the dolomite and metallic catalysts,although this might have been mitigated if higher steam/TOC ratios had been employed from the beginning of the tests.

Significant breakthrough of hydrogen sulfide from the guard bed occurred.However,relatively little sulfur accumulated on the Z409and RZ409catalysts,which were subjected to steam in-jection for the last6h of testing.This observation suggests that steam injection can regenerate metallic catalysts that have been poisoned by sulfur.

Acknowledgements

The authors wish to acknowledge the support of the Iowa Energy Center,the US DOE Hydrogen program under contract no.DE-FC36-01GO11091,and the Institute for Physical Research and Technology at Iowa State University.The authors appreciate the contributions of Jim Pollard and Jerod Smeenk in performing gasification tests.

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