PARTⅡ GASIFICATIONPRODUCTION OF CLEAN PRODUCER GAS WITH HIGHHEATING FROM BIOMASS BY AIR GASIFICATIONUSING TWO-STAGE GASIFIER
Joo-Sik Kim*, Tae-Young Mun, Bo-Sung Kang
Faculty of Environ. Eng., Univ. of Seoul, 90 Jeonnong-Dong, Dongdaemun-Gu,
Seoul 130-743, Korea
E-mail: joosik@uos.ac.kr
Abstract: A great interest in environment-friendly alternative energy resources that can
reduce dependency on fossil fuels has been growing. In particular, among a number ofalternative energy resources, biomass is seen to play an important role both as chemical
feedstock and as an alternative to fossil fuels. The conversion of biomass to chemicals usuallytakes place via thermo-chemical and bio-chemical technologies. Among thermo-chemical
conversion, gasification converts biomass into combustible gases, such as H2, CO, CH4 thatcan be used in boilers and in internal combustion engine or turbine to produce electricity
generation [1]. During gasification of biomass, however, a large amount of tar is also formed.
An average of 5,000 tons/day of woody wastes has been generated in Korea recently, andmajority of them have been simply treated through incineration. Sewage sludge, usuallydisposed via ocean dumping in Korea, also as a biomass resource can be energetically utilized
through gasification. The production of sewage sludge in Korea is continually increasing
every year, amounting to 2.7 million ton/yr in 2006.
This paper reports experimental results on the air gasification of biomass in a newly
developed two-stage gasifier (Fig.1). In the experiments activated carbon was applied toreduce tar components in producer gas. In experiments, the total amount of tar in producer gas
was reduced sixfold when activated carbon was used in the upper reactor of the gasifier. The
producer gases (inclusive of N2) that were obtained with the application of activated carbon
had high hydrogen contents (24 vol%), and their lower heating values (LHVs) amounted to11.6 MJ/Nm3.
1. Introduction
The conversion of biomass to chemicals usually takes place via thermo‐chemical andbio-chemical technologies. Among thermo‐chemical conversion, gasification converts
biomass into combustible gases, such as H2, CO and CH4 that can be used in boilers and ininternal combustion engine or turbine to produce electricity generation [1]. During
gasification, however, tar formation is one of the major problems to deal with because of- 123 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
blocking and fouling process equipments such as pipes and valves in gas engine and turbines[2] [3]. Tar can be defined in various ways. In the EU/IEA/US‐DOS meeting on tar
measurement protocol held in Brussels in the year 1998, it was agreed by a number of experts
to define tar as all organic contaminants with a molecular weight larger than benzene [4]. Tar
removal technologies can be divided mainly into two methods; primary methods andsecondary methods. Primary methods are treatments inside the gasifier. These are proper
selection of operation parameters, use of bed additive or catalyst and gasifier modifications.
Secondary methods are a measure downstream of the gasifier. These are tar‐cracking either
thermally or catalytically and mechanical methods such as use of cyclone, ceramic filter,electrostatic precipitator and scrubber. Secondary methods have been well studied, but till
now these seem to be not effective enough and too expensive. An average of 5,000 tons/dayof woody wastes has been generated in Korea recently, and majority of them have been
simply treated through incineration. In this study, a fraction of woody waste supplied by awood pellet production facility was gasified in a newly developed two‐stage gasifier. The
aim of the research was to reduce tar content directly in the gasifier and to produce a producer
gas with a high caloric value. For the purpose, activated carbon was applied in the upper
reactor of the gasifier. In accordance with purpose, the paper reports the development ofproducer gas composition according to reaction conditions, such as reactor temperature andequivalence ratio, and LHV, etc.
2. Experiments
2.1 Feed material
A fraction of woody waste was supplied by a wood pellet production facility. The
fraction was grinded, and was sieved by 20~30 mesh. Fine particles (600~850 µm) of themwere used as feed material. The specification of the feed material is listed in the Tab. 1.
Tab. 1 Specifications of feed material
Proximate analysis

wt%

Element analysisa

wt%

Moisture

7.1

C

48.0

Volatile matter

92.3

H6.4

Fixed carbon

0.2

N0.1

Ash0.4O

42.3

Lower Heating Value (MJ/kg)

16.3

S-- 124 -PARTⅡ GASIFICATION
2.2 Gasifier and experimental procedure
Air gasification experiments were carried out in a two‐stage fluidized bed. A schematic
diagram of the plant is given in Fig. 1.
Cyclone Hot filter

Condensers

Activatedcarbon

Thermocouples
Distributor

SiloChiller(Ethanol)

Fluidizedbedreactor

sf

Screw sffeederThermocouples
Distributor
Electrostatic
Precipitator

Water
Pre-heaterff

Gas sample

Gas meterf

Flare stackAirFig. 1 Schematic diagram of the gasification plant
The plant has two reactors in series, which were heated indirectly with electricity andwere made of STS‐316. The bottom reactor has a height of 380 mm and an inner diameter of110 mm. It has three thermocouples for checking fluidization stability in the reactor and wasfilled with silica sand. The upper reactor has a height of 340 mm and an inner diameter of 160mm. It was filled with activated carbon for tar‐cracking. The two reactors were separated bya distributor. Air, a fluidizing medium, was heated by a pre‐heater to reduce heat loss ofbottom reactor and then entered in the bottom reactor. Immediately after the gasification,
producer gas passes through a char separation system consisting of a cyclone and a hot filter.
After the char separation system, liquid was collected in a series of glass condensers that werecooled up to a temperature of 10 ˚C using water as a cooling solvent. Finally, electrostatic
precipitator (EP) captured particles and aerosols in producer gas. The remaining producer gas
was either burned in flare stack or was sampled using teflon gas bags at 3 min. intervals toanalyze their composition using a GC-TCD and GC-FID.
2.3 Reaction Conditions
Experiments were carried out to investigate the influence of the main reaction conditions,such as bottom reactor temperature, upper reactor temperature, and equivalence ratio. The
influence of activated carbon on the development of producer gas and tar reduction was also
estimated, while experiments without activated carbon in the upper reactor were performed.
Tab.2 shows the reaction conditions in the experiments.

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Tab.2. Reaction conditions
Parameter

Run1Run2Run3Run4Run5Run6Run7

Run8Upper reactor (˚C)

799

794

805

675

732

796

796

796

Bottom reactor (˚C)

763

820

873

823

818

829

829

816

Feed rate (g/min)

1213

13

13

14

17

1014

Equivalence ratio

0.37

0.34

0.34

0.33

0.31

0.25

0.46

0.32

Activated carbon

OOOOOOOX

Run1 to Run3 were conducted to find out the effect of bottom reactor temperature on theproducer gas composition. Run2, Run4 and Run5 have similar bottom reactor temperatures,but different upper reactor temperatures. Experiments Run2, Run6 and Run7 were carried out
to investigate the effect of equivalence ratio. Run8 was conducted without activated carbon tofind out the effect of activated carbon on tar reduction and producer gas composition. Theresults can be compared with the results of Run2. In each experiment, the flow rate of air wasapproximately 20.2 ℓ/min. 2.5 kg of silica sand (SiO2 99.9 wt%), the average particle size ofwhich was 325 µm, was used as the bed material.
3. Results and Discussions
The composition of producer gases according to upper reactor temperature andequivalence ratio is shown in Fig. 2.
50
H2CH4C2H6
C2H430C3+C4+C5Benzene
2010

Gas composition (vol.%)40

Gas composition (vol.%)

H250

COCO2COCO2CH440

C2H6
C2H4
C3+C4+C530

Benzene201000675732

Upper temperature (0C)

794

0.25

0.34

0.46

Equivalence ratioFig. 2 Gas development with upper reactor temperature and equivalence ratio
It can be seen in Fig. 2 that H2 and CO2 increased with temperature. The maximum valueof H2 reached up to 16.6 vol.% in Run2. The higher amount of H2 at elevated temperatures
can be explained on one hand by the enhanced tar-cracking at these temperatures. In theexperiments with equivalence ratio (ER), the increase in the concentration of CO2 with ER
was observed. On the other hand, CO, H2, CH4 and hydrocarbons decreased with ER.

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PARTⅡ GASIFICATION30

Tar yield during gasification (g)50

Gas composition (vol.%)

Run2 (with activated carbon)
Run12 (without activated carbon)
40302010Tar captured by an electrostatic precipitator
Tar captured by condensers

25201510500
Hydrogen Carbonmonoixide Methane

Carbondioxide

EtheneThe others

Benzene

Run2Run8Fig. 3 Producer gas composition and tar yield with and without activated carbon
Fig. 3 shows the comparison of producer gas composition and tar yield obtained withand without activated carbon. The producer gas with activated carbon contains higher H2 andless other hydrocarbons including methane than without activated carbon. During gasification,total collected tar with activated carbon was much less than that without activated carbon. The
strong tar reduction with activated carbon may result from tar adsorption itself by activatedcarbon, and from the enhanced possibility of tar cracking with the help of adsorption of tar on
the surface of activated carbon.
4. Conclusions
The gasification of waste woody biomass was performed in a newly developed
two‐stage fluidized bed reactor equipped with a char separation system. The producer gas
was mainly composed of CO2, H2, CO, CH4 and C2H4 as in typical gasification. At first, theinfluence on the upper reaction temperature, on the hydrogen production was significant. As
the reactor temperature increased, H2 concentration strongly increased up to 16.6 vol.%. With
respect to the influence on equivalence ratio, it was found that increase in equivalence ratio
led to the higher production of CO2, whereas the concentration of H2, CH4 and hydrocarbons
decreased. Therefore at higher equivalence ratio, a producer gas with a low heating value wasobtained. In an experiment that was carried out at an ER of 0.25, a producer gas with LHVaround 9.5 MJ/Nm3 was obtained. This value was much higher than the typical LHV values(4~6 MJ/Nm3) obtained in general air biomass gasification.
Activated carbon played a significant role in tar‐cracking and H2 production. Total tar
content consisting of tar on apparatus and in condensate liquid reduced from 24 g to 4 g, when
activated carbon is used. As a result, a producer gas with a high caloric value and low amountof tar could be obtained by applying activated carbon in the upper reactor in the two‐stage
fluidized bed gasifier. The producer gas obtained in the experiments seemed to be appropriate
for fuel gas in internal combustion engines.

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Acknowledgement
This work was supported by the Korea Science and Engineering Foundation(KOSEF) thegrant funded by the Korea government(MEST) (No. R01-2008-000-20705-0)
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[2] J. J. Manya, J. L. Sanchez, J. Abrego, A. Gonzalo, J. Arauzo, Fuel ,2006, 85, 2027-2033.
[3] L. Devi, K. J, Ptasinski, F. J .J. G. Janssen, Biomass and Bioenergy, 2003, 24,125-140.
[4] J. P. A. Neeft, H. A. M. Knoef, P. Onaji, Behaviour of tar in biomass gasification systems.
Tar related problems and their solutions. Novem. Report No.9919. Energy from Wasteand Biomass(EWAB), The Netherlands, 1999.

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