Applied Energy 92 (2012) 778–782

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Applied Energy

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Steam gasification of oil palm trunk waste for clean syngas production

Nimit Nipattummakul a,b, Islam I. Ahmed a, Somrat Kerdsuwan b, Ashwani K. Gupta a,⇑a The Combustion Laboratory, Dept. of Mechanical Engineering, University of Maryland, College Park, MD, USAb The Waste Incineration Research Center, Dept. of Mechanical and Aerospace Engineering, King Mongkut’s University of Technology North Bangkok, Thailand

a r t i c l e i n f o

Article history:Received 11 March 2011Received in revised form 12 August 2011Accepted 16 August 2011Available online 15 September 2011

Keywords:High temperature steam gasificationOil palm trunk wasteBiomass to clean syngasHydrogen generationAgricultural wastesFuel reforming

0306-2619/$ - see front matter Crown Copyright � 2doi:10.1016/j.apenergy.2011.08.026

⇑ Corresponding author. Tel.: +1 301 405 5276.E-mail address: akgupta@umd.edu (A.K. Gupta).

a b s t r a c t

Waste and agricultural residues offer significant potential for harvesting chemical energy withsimultaneous reduction of environmental pollution, providing carbon neutral (or even carbon negative)sustained energy production, energy security and alleviating social concerns associated with the wastes.Steam gasification is now recognized as one of the most efficient approaches for waste to clean energyconversion. Syngas generated during the gasification process can be utilized for electric power genera-tion, heat generation and for other industrial and domestic uses. In this paper results obtained fromthe steam assisted gasification of oil palm trunk waste are presented. A batch type gasifier has been usedto examine the syngas characteristics from gasification of palm trunk waste using steam as the gasifyingagent. Reactor temperature was fixed at 800 �C. Results show initial high values of syngas flow rate,which is attributed to rapid devolatilization of the sample. Approximately over 50% of the total syngasgenerated was obtained during the first five minutes of the process. An increase in steam flow rateaccelerated the gasification reactions and resulted in reduced gasification time. The effect of steam flowrate on the apparent thermal efficiency has also been investigated. Variation in steam flow rate slightlyaffected the apparent thermal efficiency and was found to be very high. Properties of the syngas obtainedfrom the gasification of oil palm trunk waste have been compared to other samples under similaroperating conditions. Oil palm trunk waste yielded more syngas, energy and hydrogen than that fromother types of biomass such as mangrove wood, paper and food waste.

Crown Copyright � 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Oil palm (scientific name Elaeis guineensis) is an agro-industrialcommodity that is used to produce edible oil [1]. Some oil palm isalso used as fuel via direct combustion. In 2007, oil palm accountedfor some 25% of edible oil in the world and this represents about38.5 million tons of oil palm produced [2]. This large amount ofpalm oil produced has resulted in large amounts of biomassresidues during the process. The residues consisted of 30.5% ofempty fruit brunches (oil palm trunk), 17.23% of fibers, 10.62% ofshells, 37.86% of fronds and trunks, and 3.79% of palm kernels[2]. A hectare of cultivated oil palm results in approximately50–70 tons of biomass residues [3]. In order to seek benefits fromsuch biomass residues, several researches have examined thepotential of energy generation from these agricultural wastes. Hightemperature steam gasification has been considered as one of themost effective and efficient approaches waste to clean chemicalenergy conversion without any environmental degradation. Thefuel thus produced can then be utilized for electric power

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generation, heat generation and in other industrial, transportationand domestic sectors.

The objective of this paper is to examine the thermo-chemicaltransformation of palm trunk waste to chemical energy usingsteam as the gasifying agent. Note that air gasification results inlower heating value of the syngas as compared to steam assistedgasification. The results are compared with the baseline case ofpyrolysis under the same temperature condition. A batch typegasifier, maintained at a fixed temperature of 800 �C, was usedfor the results presented here. The effect of steam flow rate onthe amounts of syngas produced and its characteristics from oilpalm trunk are presented here. The experimental results arepresented on the effect of steam flow rate on the evolutionarybehavior of the resulting syngas flow rate, chemical compositionof the syngas, hydrogen flow rate in the syngas as well as theoverall syngas yield. Results of oil palm wastes under pyrolysisconditions are presented for evaluating the role of gasificationon the syngas comparison. The emphasis in this investigation ison determining H2 content, hydrogen/CO ratio and apparentthermal efficiency at different steam flow rates at a constantreactor temperature. These characteristics help identify the mostsuitable conditions of gasifying agent flow rate for efficientgasification of such biomass wastes.

ights reserved.

Table 1Proximate and ultimate analysis of oil palm trunk.

Moisture (% as received) 8.34

Proximate analysisAsh (% at dry basis) 6.87Volatile matter (% at dry basis) 79.82Fixed carbon (% at dry basis) 13.31

Ultimate analysisCarbon (% at dry basis) 43.80Hydrogen (% at dry basis) 6.20Nitrogen (% at dry basis) 0.44Oxygen (% at dry basis) 42.65Sulfur (% at dry basis) 0.09Higher heating value (MJ/kg) 19.257

Fig. 1. Photograph of oil palm sample, ash and char.

Fig. 2. Schematic diagram of

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2. Experimental facility and conditions

2.1. Experimental facility

Fig. 2 shows a schematic diagram of the laboratory-scale experi-mental facility used for the gasification and pyrolysis experiments.Steam was generated from the stoichiometric combustion ofhydrogen and oxygen in a specially designed burner. The steamgenerated was then introduced into a gasifying agent conditioner.The temperature of the gasifying agent conditioner was kept atsame temperature as that desired in the main reactor wheregasification of the sample material occurs. Steam is thenintroduced into the main reaction chamber containing the biomassfeedstock hydrocarbon sample (oil palm trunk). The syngas flowingout from the reactor is allowed to flow into two sections; onepasses to the gas sampling line for gas analysis while the remainingsyngas is vented to the environment via the exhaust system. Thebypass line incorporated a no-return valve and a flow meter tomonitor the flow rate and to ensure the desired unidirectional flowout from the gasification reactor. The syngas sample is thenintroduced into a condenser followed by a filter and a moistureabsorber (anhydrous calcium sulfate) so that the sample ismoisture free prior to its introduction into the GC or other gasanalyzers. The flow of syngas is then directed to a three way valve.This three way valve allows sampling by two means. First is fromcollection of the syngas sample in the sampling bottles. The secondmeans involved introducing the syngas directly into the micro gaschromatograph (GC). Sampling bottles were used only when shortsampling intervals were required (in the range of 0.5–11 min) inbetween the sampling. This procedure allowed determination ofevolutionary behavior of syngas from the sample. However, directsampling and analysis were carried out by the GC when longersampling time intervals were desired. A constant flow rate of aninert gas (nitrogen) was introduced with the oxygen flow to thereactor. The nitrogen is detected by the GC and is then used to

the experimental setup.

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determine the flow rate of different syngas species producedduring gasification and pyrolysis.

2.2. Oil palm trunk feedstock material

The proximate and ultimate analysis of oil palm trunk used asthe feedstock sample material for gasification and pyrolysis isshown in Table 1. A photograph of the oil palm trunk sample isshown in Fig. 1. The physical size of the sample was controlled tobe approximately 25 mm. The mass of the oil palm trunk sampleused for the gasification test was fixed at 35 g. The sample was welldistributed in a stainless steel mesh before placing it in the mainreactor. Temperature of the steam flow as well as the gasificationreactor was well controlled before each experiment. The reactortemperature was set at 800 �C. The steam flow rate was varied as3.10, 4.12, and 7.75 g/min, which changed the steam to sampleratio.

3. Results and discussion

The results obtained from steam gasification of oil palm trunkusing the laboratory scale reactor facility are presented in thissection. Evolutionary behavior of syngas characteristics has beenmonitored for both pyrolysis and gasification conditions. Syngascharacteristics were determined in terms of syngas flow rate,hydrogen flow rate, syngas chemical composition and hydrogento carbon monoxide ratio. The effect of steam flow rate on overallsyngas yield, hydrogen yield and apparent thermal efficiency isalso presented. The results obtained help quantify the role of eachoperational condition on the gasification process.

3.1. Evolution of syngas flow

Results obtained on the evolution of syngas from pyrolysis andsteam gasification of oil palm trunk are shown in Fig. 3. The resultsalso show the effect of steam flow rate on syngas evolution. Syngasevolved from pyrolysis is also included in order to provide thedirect role of gasification in pyrolysis. Examination of the processof steam gasification and pyrolysis reveals that steam gasificationconsists of two distinct regimes. The first is the pyrolysis stage,which starts from the beginning of the experiment until aboutthe seventh minute. The role of steam as the gasifying agent occursafter initial pyrolysis of the sample. The second stage is the chargasification stage, which starts after approximately the seventhminute into gasification (i.e., after initial pyrolysis of the sample).In the first stage, a high yield of volatile matter is observed dueto rapid decomposition of the sample. This is because the oil palmtrunk contains 79.81% of volatile matter. This is significantly higherthan that from other types of biomass, such as paper, cardboardand wood chips [4–9]. At a high reactor temperature of 800 �Cmuch of the volatile matter rapidly evolves from the sample. Under

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Fig. 3. Syngas generation rate with time.

gasification conditions, the presence of high temperature steampromotes steam reforming reactions to provide excess evolutionof syngas at the initial stages of gasification. The role of steam inthe process can be understood by considering the followingreactions (reactions (1)–(6))

C + H2O! CO + H2 ð1Þ

CO + H2O! CO2 + H2 ð2Þ

C + 2H2O! CO2 + 2H2 ð3Þ

CH4 + H2O! CO + 3H2 ð4Þ

CnHm + 2nH2O! nCO2 + [2n + (m/2)]H2 ð5Þ

CnHm + nH2O! nCO + [n + (m/2)]H2 ð6Þ

From the above reactions one can observe that there are tworeasons why steam causes an increase in syngas flow rate ascompared to syngas generated from pyrolysis. The added syngasflow is due to the reaction between steam and char or fixed carbon(via reactions (1) and (3)) that remains after the pyrolysis process.The additional syngas flow is attributed to steam cracking of tarand condensable hydrocarbons via reactions (5) and (6). Somelow molecular weight hydrocarbons (e.g., methane) are alsoreformed to produce CO and hydrogen via reaction (4).

The second stage of syngas production, primarily from chargasification, is distinctly different from the pyrolysis process. Inthis stage, the reaction time depends on the amounts of steam flowrate or the ratio of sample/steam flow rate in the reactor. At anincreased steam flow rate, a reduction in char gasification timeoccurs. The presence of steam clearly reveals increased crackingof the residual char and heavy carbonaceous materials that remainor are produced during the initial pyrolysis process. Note that thecharacteristic amounts of char and tar formed during pyrolysiscan be as much as 30% so that more energy is available in theresidual char and tar after the pyrolysis process. Therefore,gasification plays a pivotal role in additional chemical energyrecovery from the feedstock material.

3.2. Evolution of hydrogen flow

Fig. 4 shows the evolution of hydrogen flow rate which can bedistinguished into two stages similar to that observed for the syn-gas flow. In the first range, hydrogen is produced by pyrolysis ofvolatile matter in the sample. In the gasification stage, steam–charreforming reactions ((1) and (3)) and tar reforming reactions ((2),(4)–(6)) play important role in the production of hydrogen. During

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7.75 g/min 4.12 g/min 3.10 g/min Pyrolysis

Fig. 4. Hydrogen generation rate with time.

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gasification, some of the condensable hydrocarbons/tar producedduring the pyrolysis process are also transformed into hydrogen.Consequently, H2 yield from gasification is significantly higherthan that from pyrolysis, even during the initial/pyrolysis stage.In addition, an increase in steam flow rate provided a reductionin the gasification time as evidenced from the hydrogen flow rateevolution data shown in Fig. 4.

3.3. Syngas composition

Figs. 5–7 show the syngas composition at different steam flowrates of 7.75, 4.12, and 3.10 g/min, respectively. Although a

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H2 CH4 CO CO2 CxHy

Fig. 5. Variation of syngas composition with time at steam flow rates of 7.75 g/min.

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H2 CH4 CO CO2 CxHy

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Fig. 6. Variation of syngas composition with time at steam flow rates of 4.12 g/min.

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Fig. 7. Variation of syngas composition with time at steam flow rates of 3.10 g/min.

comparison of the three figures shows similar trends, a closeexamination reveals that an increase in the steam to sample ratioaffects the syngas composition. An increase in hydrogen content isobserved and is attributed to steam reforming reactions (5) and (6).

The results show a decrease in CO, CO2, CH4, and CxHy

concentrations during the first three minutes. In contrast the molefraction hydrogen is observed to increase rapidly during the firstthree minutes. This is then followed by a mild increase in thehydrogen concentration after about the third minute. A monotonicincrease in CO2 yield can be observed after the third minute aswell. The initial decrease in CO2 is possibly due to its consumptionduring the gasification process (CO2 produced in the process acts asa gasifying agent) [9]. Results show that over 60% increase in H2

yield can be obtained from gasification as compared to that frompyrolysis.

In the first interval, the sample is heated from the ambienttemperature (�25 �C) to the gasification temperature of 800 �C inthe process [4]. Therefore, the steam involved reactions arepossibly hampered during the initial stage. Almost all of the syngasevolved in the form of non-condensable gases is produced frompyrolysis with major gas phase components being CO, H2 andCO2. The sample temperature is then progressively increased tohigh values, in excess of 600 �C. At this temperature the steamreforming reactions show a gradual acceleration, resulting in agradual increase in the hydrogen content during the first threeminutes. These results are in agreement with those reported inthe literature for waste paper and cardboard samples [4,5]. Afterthe third minute, the CO2 mole fraction increased while the COmole fraction decreased as a result of the water gas shift reactiongiven below:

Water gas shift reaction : COþH2O() CO2 þH2O

3.4. H2/CO ratio

The variation of the H2/CO ratio with time is shown in Fig. 8. Theresults show that H2/CO ratio starts at a relatively low value andthen increases with an increase in the gasification time. At the startof steam gasification, the dominant process is pyrolysis. Duringpyrolysis a significant amount of CO and a smaller amount of H2

are generated in the syngas. The percentage of H2 increases whilethe percentage of CO gradually decreases as shown in Figs. 5–7.The steam reforming reactions significantly increase hydrogenproduction during the first two minutes to result in an increaseof the H2/CO ratio. Such high values of H2/CO are favorable for itsconversion to liquid fuels, using for example, the Fisher–Tropsch(F–T) process.

3.5. Apparent thermal efficiency

Apparent thermal efficiency (ATE) provides a measure ofefficient energy conversion. This was determined in order to

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Fig. 8. Variation of H2/CO ratio with time.

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Fig. 9. Variation of apparent thermal efficiency value with steam flow rate.

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Fig. 10. Syngas flow rate at 900 �C from steam gasification of oil palm, mangrove,paper and food waste.

Table 2Chemical composition and properties of syngas from different samples.

Syngasyield (g)

Hydrogenyield (g)

Energyyield (kJ)

Apparent thermalefficiency (–)

Oil palm 52.4 2.86 685 1.11Mangrove 47.6 2.48 668 0.91Paper 38 1.64 411 0.85Food waste 29.3 1.56 477 0.9Polystyrene 36.8 3.03 677 0.47

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evaluate the potential of energy conversion from the solid fuel togaseous fuel. The ATE is defined as:

Apparent thermal efficiency ðATEÞ

¼ Syngas energy yieldSolid fuel energy yield

� 100 ð%Þ ð7Þ

It is to be noted that the ATE determined here does not consider

the energy consumed to heat-up the reactor, steam, andsample. The chemical energy present in the syngas was calculatedfrom the heating value of chemical species detected (H2, CO, CO2,CH4, C2H4, C2H6, C3H6, and C3H8). Calculated values of apparentthermal efficiency (ATE) are shown in Fig. 9. Results suggest thatvariation in steam flow rate has a slight effect on the ATE. Theaverage value of ATE was found to be approximately 110%.

3.6. Comparison of oil palm waste with other samples

Fig. 10 shows the evolution of syngas flow rate from thegasification of oil palm waste and its direct comparison withseveral other biomass samples, such as paper, food waste andmangrove wood. Note that mangrove wood grows in shallow seawaters and is used for the production of high quality charcoal.The charcoal produced from mangrove biomass is much superiorto other types of wood. In general, syngas evolution from allsamples followed a similar trend. Oil palm waste resulted in a highflow rate of syngas than that by paper and food waste in the initialstage. However, mangrove wood provided a lower syngas flow rateas compared to oil palm waste during gasification at the examinedtemperature of 800 �C. This is attributed to the high volatilecontent in both oil palm waste and mangrove wood. Note thatthe volatile matter in mangrove sample is somewhat lower thanin oil palm waste. Gasification of oil palm lasted for a shorter timeas compared to that of mangrove and food waste. The shorter timeduration of oil palm gasification indicates a higher reactivity of oilpalm char than those indicated by the food waste and mangrove.Gasification duration of paper was similar to that of oil palm waste.

Table 2 shows the properties of syngas yield from the gasificationof oil palm waste and other samples.

Oil palm waste yielded the highest energy and apparentthermal efficiency and yielded a comparable amount of hydrogenas that from polystyrene.

4. Conclusions

Gasification of oil palm trunk has been examined in a batchreactor using steam as the gasifying agent at a reactor temperatureof 800 �C. The results are also obtained under pyrolysis conditions.The effect of steam flow rate on syngas composition for a fixedamount of material has been examined in detailed. The resultsshowed that the high initial syngas flow rate is mainly attributedto the pyrolysis of volatile matter from the oil palm sample. Almost50% of the syngas is produced during the first five minutes. Theresults showed that there is over 60% increase in hydrogenproduction with steam gasification as compared to that withpyrolysis. The increase in steam flow rate reduced the timeduration of gasification, and promoted steam reforming reactionsto result in increased hydrogen yield. Increase in steam flow rateprovided negligible effect on the apparent thermal efficiency. Incomparison to other biomass samples, oil palm waste yielded moresyngas, energy and hydrogen than paper, food waste and mangrovesamples under identical gasification conditions.

Acknowledgments

The research was partly supported by the ONR and is gratefullyacknowledged. The authors (N.N. and S.K.) would like to expresstheir gratitude to the Combustion Laboratory, Department ofMechanical Engineering, University of Maryland for providing allthe test and diagnostics facilities used here. The financial supportfrom the Thailand Research Fund through the Royal Golden JubileePh.D. Program (Grant No. PHD/0146/2549) to Assoc. Prof. SomratKerdsuwan and Mr. Nimit Nipattummakul is gratefullyacknowledged.

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