catalytic conditioning of organic volatile products produced by peat pyrolysis

Biomass and Bioenergy 23 (2002) 209–216

Catalytic conditioning of organic volatile products producedby peat pyrolysis

David Sutton ∗, Brian Kelleher, Julian R. H RossMaterials and Surface Science Institute, University of Limerick, Limerick, Ireland

Received 29 January 2001; received in revised form 15 March 2002; accepted 20 March 2002

Abstract

A series of metal oxides were investigated for up-grading the quality of the product gas from peat pyrolysis. The designedtest rig pyrolysed brown peat in a nitrogen atmosphere at 550

◦C. This resulted in 35:5 wt% of the carbon present in the

peat being converted to a volatile fraction. The volatile fraction was transferred to a secondary catalytic reforming reactor,at 800

◦C, which was located downstream from the gasi3er. The thermal e4ect alone of the second reactor resulted in an

increase in the CO, CO2 and CH4 content of the volatile fraction, a synthesis gas ratio of 0.75 and a HHV of 26:5 MJ kg−1.A selection of metal oxides (Al2O3; SiO2;ZrO2;TiO2) and MOR1 investigated in the secondary reactor at 800

◦C showed an

increase in the quality of the product gas. Notably TiO2 increased the conversion of the hydrocarbons present in the gasstream to 79.5%, resulting in a synthesis gas ratio of 2.22 and increased the HHV by 71:5 MJ kg−1. ? 2002 Elsevier ScienceLtd. All rights reserved.

Keywords: Biomass; Tar; Hot gas; Synthesis gas; Catalyst

1. Introduction

Gasi3cation is an established technology for con-verting coal and biomass fuels into a gas. Coal gasi3-cation was commonly used in the 3rst half of the 20thcentury to supply gas for cooking and lighting in largecities. Gasi3cation is a complex process consisting ofmany reactions. Some of these reactions are exother-mic but the overall process is slightly endothermic.The main objective of gasi3cation is to transfer the

maximum usable chemical energy from the feedstockto the gaseous fraction. Solid, liquid and gas areby-products of gasi3cation The products of biomassare a solid, a liquid and a gas. The most useful

∗ Corresponding author. Tel.: +353-86-88-21933.E-mail address: [email protected] (D. Sutton).

products of gasi3cation are H2 and CO, so-called syn-thesis gas. The synthesis gas may be burned to gener-ate electricity, while the waste heat generated by com-bustion may be used to raise steam to power a steamturbine or in district heating schemes. An IntegratedGasi3cation Combined Cycle plant (IGCC) involvinga turbine can have energy conversion eDciencies ofwell over 40% compared with 34% for coal burningpower plants [1,2]. During o4-peak electrical demand,the synthesis gas may be used for the manufacture ofchemicals such as methanol [3].Remarkable progress has been achieved in recent

years in the design of gasi3ers. However, gas clean-ing is still the bottleneck in advanced gas utilisationthat limits the deployment of the use of biomass forelectricity generation. The continual build up of con-densable organic compounds (often referred to as tars)

0961-9534/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S 0961 -9534(02)00041 -7

210 D. Sutton et al. / Biomass and Bioenergy 23 (2002) 209–216

present in the product gas can cause blockages andcorrosion and also reduce overall eDciency. In addi-tion, the presence of impurities can a4ect the end us-age of the synthesis gas and the techniques involvedin the removal of the impurities in such processes arecostly.Hot gas cleaning includes the catalytic decompo-

sition of the unwanted hydrocarbons. The catalystsemployed in this process are responsible both forpuri3cation and bringing about compositional ad-justment of the product gas. Hot gas conditioning isachieved by passing the raw gasi3er product gas overa solid catalyst in a 3xed-bed (or a Juidised-bed)under temperature and pressure conditions that es-sentially match those of the gasi3er. As the raw gaspasses over the catalyst, the hydrocarbons may bereformed on a catalyst surface with either steam (Eq.(1)) or carbon dioxide (Eq. (2)) or both to produceadditional carbon monoxide and hydrogen:

CnHm + nH2O ↔ nCO +(n+

m2

)H2; (1)

CnHm + nCO2 ↔ 2nCO +(m2

)H2: (2)

Additional steam or carbon dioxide may be added forthe reforming processes [4].The use of a catalyst to reform condensable organic

compounds and methane can increase the overall ef-3ciency of the biomass conversion process by 10%.Lindman [5], investigating air gasi3cation, reportedhigher eDciency being achieved by lower oxygen con-sumption, better heat recovery and higher carbon con-version compared to a process based on non-catalytictechniques. Thermal cracking of the hydrocarbons isalso possible; however, this method is not consid-ered a feasible option as it requires high temperatures(¿ 1100◦C) to achieve high cleaning eDciency andit also produces soot. Reforming does not reduce thetotal chemical energy content of the fuel gas, as heat-ing of the gas to the higher temperatures for thermalcracking is not required.In this paper, we report results for catalytic condi-

tion of the products of peat pyrolysis. While the pro-cess is not gasi3cation (due to the inert atmosphere)the products are similar to that of gasi3cation. Thelow temperature slow pyrolysis of peat was favouredas it produces a high wt% of hydrocarbons and also

Table 1Selection of metal oxides calcined at 800

◦C for 6 h in a Jow of

10 mls min−1 air and resultant BET surface areas

Catalyst BET surface area, m2 g−1

Al2O3 212SiO2 259ZrO2 26TiO2 22MOR1 14

H2CO; CH4 and CO2 all in a volatile fraction whichwould be passed over a catalyst bed. This high con-centration of hydrocarbons in the product gas whilenot typical of an industrial process provides a goodbase for the study of activity for selected catalysts un-der extreme conditions. The pyrolysis process resultedin 35:5 wt% of the carbon in the peat being convertedto carbon in the volatile fraction. The percentageof carbon as carbon gases (CO, CH4; CO2) in thevolatile fraction was 29:4 wt% while the remainder(70:6 wt%) were hydrocarbons (C1+n). The inJuenceof a series of metal oxides on the conditioning of theproduct gas was investigated in a secondary reactorlocated down stream. This bed was kept at 800◦C,which is the typical operating temperature of industrialgasi3ers and hot gas catalytic conditioners. Table 1shows the selection of calcined metal oxides andtheir surface areas, investigated for the up gradingof the product gas. The metal oxides selected aretypical oxides used in supported metal catalysts forthe reforming of methane. MOR1 was also investi-gated, a recently developed proprietary tar destructioncatalyst, which had an iron concentration in excessof 45 wt%.

2. Experimental

Due to the complex nature and numerous prod-ucts of peat pyrolysis, a complete quantitative andqualitative analysis of the process would be virtuallyimpossible and inordinately time consuming. Consid-ering these limitations the evaluation of the catalystsis given by the following criteria. Several authors[6–9], in reporting the activity of a catalyst for theup grading of gasi3cation product gas, refer to one or

D. Sutton et al. / Biomass and Bioenergy 23 (2002) 209–216 211

Fig. 1. Test rig for the evaluation of catalysts for catalytic conditioning.

more of the following criteria.

(1) The conversion of carbon present in the volatilefraction to carbon as CO, CO2 and CH4.

(2) The selectivity of the system to convert carbon inthe volatile fraction to CO, CO2 and CH4.

(3) The molar percentage composition of the perma-nent gases of the gas stream.

(4) The e4ect of the catalyst on the higher heatingvalue of the dry gas stream.

Fig. 1 shows the test rig developed at the Universityof Limerick for production of the volatile gas streamwhich was passed over the catalysts investigated. Two3xed bed, plug Jow quartz reactors were connectedin series. Reactor A was charged 10 g of peat (200–500 �m particle size) and held in the temperature zoneby quartz wool. Reactor B contained the catalyst. Thepeat was pyrolysed in a Jow of 4:47 ml min−1 of ni-trogen at a heating rate of 5◦C min−1 to 550◦C. Thevolatile fraction was transferred to reactor B through aheated zone at 550◦C. The catalyst bed was at 800◦C,which was monitored by a K-type thermocouple. TwoCarbolite Eurotherms controlled the temperatures ofthe reactors, held in two separate furnaces. Gas Jow

was regulated by a Brooks mass Jow controller (5850TR series). The gas stream was analysed by a Varian3200 CX gas chromatograph equipped with a ThermalConductivity Detector (TCD). A packed Carboxen1000 column separated the gases N2; H2, CO, CO2

and CH4, using argon as the carrier gas. Online sam-pling of the gas stream was carried out every 10 min.A cold trap was located after the catalytic reactor toremove condensable products in the gas stream priorto analysis. Test for each catalyst was repeated a to-tal of 10 times and the results averaged, the standarddeviation for each was ±10%.For uncatalysed experiments, the second reactor and

the heating zone were removed from the experimentalsystem. The products were passed through a cold trapin which the condensable products were removed fromthe gas stream and were then analysed by online GCas above.

3. Calculations

From elemental analysis results, the mass of car-bon in the peat and in the char may be calculated; by


subtracting these two values the mass of carbonpresent in the volatile fraction may be determined.

Mcp −Mcc =Mcvf ; (3)

where Mcp is the mass of carbon in peat, Mcc the massof carbon in char and Mcvf the mass of carbon involatile fraction.Analysis of the gas stream for CO, CO2 and CH4

throughout the process yielded quantitatively theamount of carbon present as CO, CO2 and CH4 inthe volatile fraction. Subtraction of this value fromthe mass of carbon in the volatile fraction yieldedthe mass of carbon in condensable hydrocarbons (Eq.(4)).

Mcvf −Mcg =Mcon ; (4)

where Mcg is the mass of carbon in CO, CO2 andCH4; Mcvf the mass of carbon in volatile fraction andMcon the mass of carbon in condensable hydrocarbons.Furthermore, the extent of converstion of the peat

can be determined using

%Gasi3ed =Mcvf

Mcp× 100: (5)

The activity of a catalyst was evaluated by the in-crease in carbon as CO, CO2 and CH4 relative to theavailable carbon in the volatile fraction according to

%Conversion =Mcg

Mcvf× 100; (6)

where Mcvf is the mass of carbon in volatile fraction(uncatalysed test) and Mcg the mass of carbon in car-bon gases.The selectivity of catalysts to produce CO, CO2 or

CH4 was calculated according to

%Selectivity =Mi

cg

Mcvf× 100; (7)

where Micg is the mass of carbon in gas component i

(i=CO;CO2 or CH4) and Mcvf the mass of carbon involatile fraction (uncatalysed test).

4. Results and discussion

4.1. Peat characterisation

Table 2 shows the results for the characterisationtests carried-out on the peat. The average moisture

Table 2Analysis results of the brown peat employed (Results were asreceived peat)

Analysis Weight (%)

Moisture 16.5Ash Content 2.99Elemental analysisC 47.56H 5.93N 1.84

Table 3Elemental analysis results of char after gasi3cation of peat

Elemental analysis Weight (%)

C 75.67H 2.95N 2.01

content was 16:5 wt% (±1 wt%) which is quite lowfor bog peat. The moisture content of freshly cut peatmay be up to 70 wt% [11], and, when air-dried thisfalls to approximately 50 wt%. The low moisture con-tent observed is as a result of the storage conditionsof the peat prior to receipt. Ideally, biomass for gasi-3cation should have a low moisture content (below25 wt%) to avoid the additional step of drying whichincreases process costs [12].The ash content (2.99%) of the selected peat is

typical for slightly decomposed peat. Doyle [11] hasreported that the ashes content for brown and blackpeat samples from Ireland were 1.72 and 2:95 wt%,respectively.The elemental analysis of the peat was in the range

expected for slightly decomposed peat [13], with47:56 wt% carbon, 5:93 wt% hydrogen and 1:84 wt%nitrogen.Table 3 shows the elemental analysis results for the

peat char. The results are the averaged analysis of thechar remains of 3ve separate experiments. The charhad a high carbon content (75:67 wt%), which was asa result of the low temperatures of gasi3cation and theinert gas used in the process.

4.2. Uncatalysed results

Tables 4 and 5 show the results for the pyrolysisof the peat and the e4ect of the secondary reactor at


Table 4Conversion of the carbon contained in the volatile fraction tocarbon as CO, CO2 and CH4 and the selectivity of catalysts toCO, CO2 and CH4 in the volatile fraction

↓ Catalyst % Conversion % Selectivity to carbon gases(%) (%)

Gas→ CO CH4 CO2 CxHyBlank+ 29.4 5.4 2.9 21.2 70.6Qz 56.4 13.2 11.3 31.9 43.6Al2O3 70.6 23.5 14.1 33.0 29.4SiO2 77.3 18.3 11.7 47.3 22.7ZrO2 78.4 19.6 19.6 39.2 21.6TiO2 79.5 23.8 13.2 42.4 20.5MOR1 83.4 17.1 10.3 56.0 16.6

Table 5Mol% composition, synthesis gas ratio and heating value of theresultant gas stream after second reactor at 800

◦C, containing

catalysts

Catalyst Mol% composition of gas H2 : CO HHVa

streamb

H2 CO CH4 CO2Blank 18 15 8 59 1.20 23.9Qz 15 20 17 48 0.75 26.5Al2O3 40 20 12 28 2.0 76.4SiO2 29 17 10 44 1.70 44.3ZrO2 36 16 16 32 2.25 67.9TiO2 40 18 10 32 2.22 71.8MOR1 27 15 9 49 1.8 38.9

aHHV calculated on a N2 free basis units MJ kg−1.bThe mol% compositions are integrated over the evolution of

the gases with respect to the gasi3cation of the biomass.

800◦C. The table is presented showing 3ve criteria;

(1) The conversion of carbon in the peat to carbonin the volatile fraction, which is as a result of theexperimental conditions.

(2) The percentage of carbon in the volatile fractionas CO, CO2 and CH4, which is inJuenced by boththe peat pyrolysis conditions and the secondaryreactor.

(3) The selectivity to CO, CO2 and CH4, which isinJuenced by both the peat pyrolysis conditionsand the secondary reactor.

(4) Mol% composition of gas stream on exiting theprimary and the secondary reactor.

(5) The higher heating value of the gas stream exitingthe primary reactor and the secondary reactor.

Table 6Most active catalyst material for the individual criterion investi-gated for the up grading of the product gas of gasi3cation undergiven conditions

Criterion Catalyst Result

Highest conversion MOR1 83.4%of volatile fraction tocarbon gases

Highest selectivity to TiO2 23.9%carbon monoxide

Suitable H2 : CO Al2O3, TiO2, ZrO2 2, 2.22, 2.25Highest heating value Al2O3 76:4 MJ kg−1

The percentage conversion of carbon in the peat tocarbon in the volatile fraction for the primary reactorwas 35:5 wt%. Published results for carbon conver-sion [6–10] are in the order of 70–80%, however, theconditions used were signi3cantly di4erent to thoseemployed in this study and this would account for thelower conversion observed here. Since the primary re-actor conditions are unchanged during the screeningof the catalysts, the percentage conversion of the car-bon of the peat to carbon in the volatile fraction isuna4ected.The percentage of carbon present in the volatile

fraction exiting the primary reactor, as CO, CO2 andCH4 is 29:4 wt%. After the secondary reactor at 800

◦Cthis value increases to 56:4 wt%. The increase is dueto the thermal cracking reaction of the hydrocarbonspresent in the volatile fraction.The conditions employed in the absence of catalyst

resulted in poor selectivity to CO (5.4%) and CH4

(2.9%), while there was a high selectivity to CO2

(21.2%). The high selectivity to condensable hydro-carbons (70.6%) was probably due to the low temper-ature and inert atmosphere [13]. The e4ect of thermalcracking increases the selectivity to CO (13.2%), CH4

(11.3%) and CO2 (31.9%) while reducing the selec-tivity towards condensable hydrocarbons (43.6%).Also shown in Tables 5 and 6 are the molar com-

positions of the permanent gases which were evolved.The largest fraction of the gas was CO2 (59 mol%),while CH4 is present at 8 mol% and the H2: CO ra-tio was 1.2. In addition to the poor quality of the gasstream, a high content of hydrocarbons (70.6%) is alsopresent.


The e4ect of thermal cracking of condensable hy-drocarbons on the gas composition is also notable withan increase in the CH4 content from 8 to 17 mol%. Thereduction in the amount of CO2 from 59 to 48 mol%may be as a result of the dry reforming of the hydro-carbons. The H2 : CO ratio was also reduced from 1.2to 0.75 which may be accounted for by the formationof CH4 and other CxHy products. The increase in CH4

is due to the reaction:

Volatile matter ↔ CH4 + C; (8)

which is the more favourable product of gasi3cationat high temperatures.The HHV of the gas increased from 23.8 to

26:5 MkJ kg−1 after the secondary reactor. Thesevalues are less than half that of natural gas which hasa HHV of 53:1 MJ kg−1 (20◦C and 1 atm) [14,15].The heating value of the product gas from biomassgasi3cation depends on the C, H and O content ofthe biomass fuel and the extent of gasi3cation. Theobserved increase in the HHV of the product gas isdue to a higher CH4 and CO content of the dry gasstream after thermal cracking.Aznar et al. [4] has also reported the “thermal ef-

fect” of a second reactor for the steam gasi3cationof biomass at 750◦C, when the second reactor was at840◦C, the tar content of the gas stream was reducedby 50%. The reduction by 50% is higher than thatfound in this work and the di4erence may be due tothe presence of steam and the higher temperature ofthe second reactor. It has also been shown by AldTenet al. [16] that the temperature has a distinct e4ect ontar decomposition with an increase in temperature re-ducing tar content.Aznar et al. [4] reported that for the steam gasi3ca-

tion of biomass at 750◦C with a secondary reactor at840◦C there was an increase in the H2 : CO ratio from1.34 to 2.08. This increase in ratio was due to furthersteam reforming of the hydrocarbons occurring in thesecondary reactor at higher temperature than the gasi-3cation unit. Several authors [7,8,17] have reportedsimilar e4ects.If the products of the gasi3cation process are to be

used in an IGCC plant a synthesis gas ratio of approx-imately 2 would be preferred since it would produce amaximum methanol yield based on the methanol syn-thesis reaction [15]. The HHV of the gas stream indi-cates the extent to which the heating value of the gas

stream has been a4ected by the catalyst, as an increasein the H2, CO or CH4 fraction causes an increase inthe heating value. In evaluating the catalysts in thisstudy, a high selectivity to H2 and CO with zero se-lectivity to CH4 is desirable.

4.3. Catalytic reforming of the volatile fractionin the second reactor at 800◦C

Table 4 shows the results for the percentage conver-sion of the carbon in the volatile fraction to carbon asCO, CO2 and CH4 by the metal oxides and MOR1 at800◦C and the selectivity towards CO, CO2, CH4 andother hydrocarbons. Included, for comparison, are theresults for the gasi3cation of the peat with no secondreactor (“Blank”) and the thermal e4ect of the secondreactor at 800◦C (“Qz”).The order for the conversion of the volatile fraction

to carbon gases was: quartz¡Al2O3¡SiO2¡ZrO2

¡TiO2¡MOR1. The MOR1 gave the greatest con-version of the hydrocarbons in the volatile fraction tocarbon as CO, CO2 and CH4.The selectivity of each oxide investigated is also

shown in Table 4. All exhibited an increase in the se-lectivity to CO, CO2 and CH4 at the expense of selec-tivity to hydrocarbons (excluding CH4). The MOR1gave the highest selectivity to CO2 at 56.0% and thelowest selectivity to CO and CH4 (17.1% and 10.3%,respectively). The SiO2 gave the second highest selec-tivity to CO2 at 47.28% and analogously to the MOR1,the second lowest selectivity towards CO and CH4.ZrO2 exhibited the highest selectivity to CH4, a poorselectivity to CO2 and a moderate selectivity to CO.Al2O3 and TiO2 exhibited the highest selectivity toCO at 23.5% and 23.8%, respectively. Both catalystsgave moderate selectivity to CH4 (13.2% and 14.1%).Table 5 shows the mol% gas composition of the

permanent gases, the higher heating value for eachresultant gas composition and the synthesis gas ratiofor the resultant gas composition, after the reactionof the volatile fraction over the various catalysts at800◦C.Almost all of the oxides investigated show a good

H2 : CO ratio; 2 Al2O3, 2.22 TiO2 and 2.25 ZrO2,which are suitable synthesis gas ratios for an IGCCplant [3,15,18]. However, the mol% methane presentin the gas stream for all of the catalysts is unac-ceptably high for the IGCC process [16]. For all the


oxides investigated (with the exception of MOR1which has the highest selectivity to CO2) the car-bon dioxide content of the gas stream has been re-duced compared to that of the thermal gas streamcomposition.The e4ect on the higher heating value of the resul-

tant gas composition is also shown in Table 6. Thereis a notable increase in the heating value and amountof gas composition for all of the oxides investigated.This is due to the increase in the H2 content of thegas compositions, (Table 4), since both the CH4 andthe CO content have been reduced. The alumina gavethe largest increase by a factor of almost 3 over thehigher heating value of the thermal e4ect. The TiO2

and ZrO2 as catalysts increased the heating value ofthe gas by a factor of 2.5, while the SiO2 and MOR1increased the HHV by a factor of 1.5.Table 6 shows the most active individual oxide for

each of the criterion investigated. If the combined cri-terion for the performance of a metal oxide is consid-ered the most active is TiO2. The H2 : CO ratio of TiO2

is 2.22, which is ideal for an IGCC plant; this catalysthas the second highest conversion at 79.5%. MOR1has the highest conversion but shows poor selectivityto CO and the lowest heating value of the dry prod-uct gas. The TiO2 gave the second highest increase inheating value of the dry product gas. However, Al2O3

resulted in the highest increase in heating value butgave the lowest conversion. Finally, the TiO2 gave thehighest selectivity to carbon monoxide.

5. Conclusions

The low temperature pyrolysis of brown peat in aJow of nitrogen resulted in 35.5% of the carbon in thepeat being converted to carbon in the volatile fraction.The percentage of carbon as carbon gases (CO, CH4,CO2) in the volatile fraction was 29.4% while the re-mainder (70.6%) were condensable hydrocarbons. Onpassing the volatile fraction through an empty secondreactor at 800◦C the carbon content (of carbon as CO,CH4 and CO2) was increased to 56.4% and the HHVincreased from 23.9 to 26:5 MJ kg−1. The synthesisgas ratio was reduced and the methane content of thegas stream doubled.A series of metal oxides and MOR1 were inves-

tigated in the secondary reactor at 800◦C and were

found to increase the conversion of carbon in thevolatile fraction to carbon as CO, CH4 and CO2

above the thermal e4ect alone. All of the materialsinvestigated increased the synthesis gas ratio withAl2O3; ZrO2 and TiO2 being suitable for methanolsynthesis. The resultant gas streams showed a re-duction in the methane content however; none weresuitable for use in an IGCC plant. The heating valueof the resultant gas streams after each catalyst in-creased, Al2O3 being the greatest increasing the valueby a factor of almost 3. The MOR1 resulted in thehighest conversion of the carbon to CO, CH4 andCO2 indicating its suitability for use as an eDcientguard bed material for a IGCC plant.

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catalytic conditioning of organic volatile products produced by peat pyrolysis

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