Smart Grid and Renewable Energy, 2012, 3, 56-61 Published Online February 2012 (
Lowering Gasifier Tars and Particulates Using Heated
Dololmite Catalyst and a Particulate Filter
Christopher O. Akudo, Beatrice G. Terigar, Chandra S. Theegala*
Department of Biological and Agricultural Engineering, Louisiana State University, Baton Rouge, USA.
Email: *
Received December 21st, 2011; revised January 31st, 2012; accepted February 7th, 2012
For this study, a fixed-bed, down-draft gasifier was designed to investigate the effect of a dolomite catalytic bedon tar
removal. Pine pellets and wood chips (cypress) were used to produce syngas from the down-draft gasifier. For the gas
conditioning, a combination of a heated dolomite (bed temperature at 850˚C for catalytic cracking of tars) and a par-
ticulate filter (for particulate removal) was used. Investigation of temperature effects on dolomite activity between
650˚C and 950˚C bed temperatures, showed optimum catalytic efficiency at approximately 850˚C. At the optimum con-
ditions, gravimetric tar and particulate concentrations in syngas produced from pine pellets were 0.85 g/Nm3 (±0.16)
and 4.75 g/Nm3 (±0.07), respectively before gas condition ing, and 0.09 g/Nm3 (±0.02) and 2.01 g/Nm3 (±0.13), respec-
tively after gas conditioning . Syngas fro m wood ch ips con tained 1.63 g/Nm 3 (± 0.45) and 3 .84 g/Nm3 (±1.16) of tars and
particulates, respectively before gas cleaning and 0.19 g/Nm3 (±0.02) and 2.27 g/Nm3 (±0.27) tars and particulates, re-
spectively after gas conditioning. The combustible portion of the gas constituted carbon monoxide (12% - 14%), hy-
drogen (11% - 12%), and methane (~2%). These results suggest that syngas produced from gasification of pine pellets
and wood chips in a down-draft biomass gasifier can be effectively cleaned using a heated catalyst bed and a particulate
filter. However, the benefits of gas conditioning will be offset by the need to maintain a heated catalyst bed for tar
Keywords: Gasification; Dolomite; Tars; Particulates; Syngas
1. Introduction
Biomass gasification is a thermo-chemical process that
produces relatively clean and combustible gases through
pyrolytic reaction. Depending on the gasifying ag ent (air,
steam, oxygen) the raw gas produced at the gasifier exit
has different compositions. The most commonly used
agent at demonstration and commercial scale is air, with
equivalence ratios of 0.2 - 0.3 [1]. By use of air, the flue
exit gas contains, on a volumetric basis, about 50% N2,
8% - 20% H2and CO (each) and 2% - 4% CH4, with the
remainders being CO2 and H2O [1]. This gas composition
is mostly useful only for heat generation and electricity
production. The exact gas composition at the gasifier exit
in the process depends on other operating variables, and
it has been well studied by different authors, at different
operating scales and with different gasifiers [2-4]. In ad-
dition to these gases, a condensable mixture of organic
compounds referred to as tars are formed [5]. Tars are
undesirable because of the problems associated with
condensation in the downstream processes where they
form tar aerosols, and polymerization to form more com-
plex structures which can damage internal combustion
engines, gas turbines, and other machinery. Therefore,
the syngas needs to be cleaned of tars and other solid
particles before it can be used in any fuel conversion
device. The final gas quality dictates the end use or ap-
plication suitabilit y of the generated syngas.
There has been extensive research on tar elimination
using catalysts that aid in the decomposition of the hy-
drocarbons found in tar. The most widely used catalysts
are non-metallic oxides such as dolomite, olivine, mag-
nesite, and zeolite [6]. Dolomite is a calcium magn esium
ore with the general chemical formula (CaMg)(CO3)2
that contains approximately 20% MgO, 30% CaO, and
45% CO2 on a weight basis [6]. The use of calcined dolo-
mites in biomass gasification for tar cracking and re-
moval has been the subject of interest in hot gas cleaning.
Delgado and co-workers [7] studied the use of calcined
dolomites in biomass gasification with steam. The cata-
lytic decomposition of biomass tars using calcined dolo-
mites was also reported by Devi and co-investigators [8].
Calcination of dolomite involves decomposition of the
carbonate mineral, eliminating CO2 to form MgO-CaO.
*Corresponding a uthor.
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Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter 57
Complete dolomite calcination occurs at fairly high tem-
peratures and is usually performed at 800˚C - 900˚C [6]
and restricts its effective use to these relatively high tem-
peratures. Aznar and co-authors [9] performed experi-
ments involving a bed of calcined dolomite placed after a
biomass fluidized bed gasifier in which gasification was
made with steam-oxygen mixtures to clean the raw syn-
An alternative to the dolomite is the naturally occur-
ring particles of olivine, which are a mineral containing
magnesium, iron oxide and silica. Rapagna and co-work-
ers [10] have indicated that the tar reforming activity of
olivine was comparable to calcined dolomite. Olivine is
advantageous in terms of its ability to withstand friction
and does not easily break [8]. However, there is still am-
biguity on the prospective use of olivine as a tar decom-
posing catalyst. Nickel based catalysts have been found
to almost completely remove the tar and are also very
effective for NH3 removal at temperatures above 800˚C
[11]. The main limitation of using nickel based catalysts
is the tendency to lose their catalytic effect because of
carbon fouling. This fouling effect occurs mainly when
the catalyst is placed right after the gasifier. High tar
concentration is one good example of the devastating
effect on catalyst activity [12]. The use of a catalytic re-
actor downstream of the gasification reactor has proven
to be a more effective approach to tar destruction [13].
From an energy balance point-of-view, the energy con-
tained in the tars is fractional when compared to the en-
ergy content in the exiting gases. Due to these reasons,
tar cracking is not aimed at enhancing the energy content
of the gas, but primarily to improve the suitability of the
generated syngas and to minimize problems with the
downstream equipment. This fact was corroborated by
Corella and co-workers [14], who concluded that there is
almost no difference in the heating value of the gas pro-
duced af ter tar cracking. They indicated that the increase
in the hydrogen production is compensated by a decrease
in carbon monoxide, and there is hardly any change in
methane production .
This study presents the results of further investigation
of the use of dolomite for tar removal. The effect of cat-
alytic bed temperature on tar removal was investigated.
Also, since previous research focused on the use of do-
lomite in a fluidized bed system, its use in a fixed bed
downdraft biomass gasifier is being explored in this pa-
2. Experimental Section
2.1. Gasifier System Operation
The fixed bed gasifier system used in this project con-
sisted of a feeder unit, down-draft gasifier, gas flare
chamber, temperature monitoring system, catalytic bed
and a in house developed protocol [15,16] for impurity
sampling as shown in Figure 1. The gasifier feeder unit
was designed to allow for b atch feeding of biomass. Two
15 cm diameter knife-gate valves (with 68 kg rating)
attached 45 cm apart on the 15 cm diameter metal feed-
ing pipe was used as an airlock. This airlock allowed
periodic feeding of biomass without the loss of internal
pressure. The raw materials used were cypress mulch
woodchips (less than 5 cm long, bought at local hardware
stores) and pine pellets (Tractor Supply Co., Zachary, LA)
that were less than 1.3 cm long and 0.6 cm diameter. The
moisture con tent of the woodch ips varied between 11% -
17% during storage, while the pellets maintained fixed
moisture content of 3.2%. After the feed was weighed, it
was manually loaded into the system. Using the gate
valves and feed level detector, materials were reloaded
during gasification. Six K-type thermocouples were
connected to the wall of the furnace from top to bottom
with approximately 7 cm distance between adjacent
thermocouples. The majority of the gas that was pro-
duced from the gasifer (6 - 8 CFM) was flared in a flar-
ing-unit. A small slip stream was drawn from the exiting
Figure 1. Gasification experimental setup consisted of a gasifier, flaring unit (for disposing the majority of the syngas), a slip-
stream connected to a catalyst bed (in a muffle furnace), a filter assembly (in a heated oven) and a tar quantification assem-
ly (series of acetone bottles). b
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Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter
syngas and was used for tar-cracking and particulate
cleaning experiments as discussed below.
2.2. Maintaining the Integrity of the
Specifications Evaluation of the Gas
Cleaning Component
A small slip-stream of 94.4 cm3/s (0.2 CFM) was used
for all gas cleaning experiments. The first component of
the gas cleaning assembly consisted in a dolomite bed,
set inside an electrically heated muffle furnace. The
temperature of the dolomite bed was varied from 650˚C
to 950˚C by altering the thermostat setting of the muffle
furnace. A fixed-bed reactor (2.36 mm diameter) was
used for all catalyst experiments. The fixed-bed had an
internal diameter of 19 mm and length of 152.4 mm
made from a steel pipe and was placed inside a muffle
furnace. Catalyst depth in the pipe was 101.6 mm
supported by wire mesh at both ends of the pipe. Prior to
the experimentation involving dolomite, calcination of
the dolomite was done at 850˚C in a furnace for 2 hours.
The second component of the gas cleaning assembly
included a glass fiber filter (Whatman: GF/F Cat No
1825 090) and a filter holder assembly placed inside an
electrically heated oven for particulates removal and
quantification. The filter assembly was heated to 250˚C
to prevent tar condensation on the filter paper. The
particulates collected on the filter paper were quantified
2.3. Sampling and Analysis
The syngas sampling was initiated after steady state
temperatures were reached inside the gasifier. The steady-
state point was also indicated by a self-sustainable flame
in the flaring unit.As indicated earlier, the bulk portion of
the produced syngas (6 - 8 CFM) was flared and all the
gas conditioning experiments were conducted on a small
94.4 cm3/s (0.2 CFM) slip-stream of the generated syn-
gas. The sampled gas was passed through insulated 0.32
cm copper tubing to the filter assembly, where the par-
ticulates were collected. The filter assembly in the oven
was maintained at 250˚C, which prevented tars from
condensing on the filter. The gas exiting the filter oven
was passed through a series of four impinger bottles
containing acetone, which collected the tars by dissolu-
tion (Figure 2). Passing the gas through a series of bot-
tles ensured total collection of tars and number of bottles
to be used in series was decided based on the amount of
tars collected in the various impinger bottles during the
trial runs.
Due to restrictions and depositions on the filter paper,
the sample gas flow rate dr opped from 94.4 cm3/s to 78 .7
cm3/s after 30 minutes of sampling. The lowering of air
flow was anticipated prior to running the experiments
and appropriate adjustments were made during the quan-
tification of tar and particulate concentrations.
The syngas exiting the acetone bottles was drawn
through a volatile organic carbon absorber and a flow
regulator before entering the vacuum pump. The gas ex-
iting the vacuum pump was tested for syngas composi-
tion. The gas samples were drawn using a 10 ml syringe
with a 0.3 µm syringe filter. The collected sample was
then analyzed using a gas chromatograph (SRI, MG#1)
equipped with a 0.32 cm stainless steel silica gel-pack
column and a thermal conductivity detector. An amount
of 1 mL of gas sample was injected into the gas chroma-
tography and ultra-high purity helium at a flow rate of 10
mL/min was used a carrier gas [15,16].
3. Results and Discussion
3.1. Gasifier Temperature Profile
The temperature of the gasifier during operation was
continuously measured and recorded by a PC based data
acquisition system. Temperature averages were calcu-
lated within a 30 seconds interval and average tempera-
ture profiles were created. Figure 3 shows a typical
steady-state gasification temperature profile within the
gasifier at various locations from the bottom grate. Tem-
perature 1 represents the thermocouple immediately
above the grate, and Temperature 2 to Temperature 6
represents the temperatures from thermocouples that
Figure 2. An in-house built particulate and tar quantification system was used for the present study. The particulate
quantification unit consited of an stainless steel assembly placed inside an oven maintaiend at 250˚C. A series of four
impinger bottles containing acetone collected the tars, and the syngas exiting the acetone bottles was drawn through a volatile
organic carbon absorber and a flow regulator before entering the vacuum pump and tested for composition.
Copyright © 2012 SciRes. SGRE
Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter 59
Figure 3. Temperature profile as gasification approaches
steady state. Temperature 1 represents the thermocouple
immediately above the grate, and Temperature 2 to Tem-
perature 6 represents the temperatures from thermocouples
that were placed approximately 7 cm higher than the pre-
ceding thermocouple.
were placed approximately 7 cm higher than the preced-
ing thermocouple.
Based on the construction of the gasifier and the inlet
ports (which were place between thermocouples 3 and 4),
the highest steady-state temperatures (or oxidation zone)
will be experienced by Temperature 2. All temperatures
measured were relatively constant. However, Tempera-
ture 1 had a sharp decrease at about 35 min due to a
malfunction of the grate which lead to a small accumula-
tion of ash close to the thermocouple. This problem was
promptly fixed resulting in a more accurate temperature
Based on experimental observations, true gasification,
as indicated by a self-sustainable flame in the flaring unit,
occurs when the Temperature 2 temperatures exceed
700˚C [17]. The high reactor temperature obtained also
reduces the producti on of de tectable tar species [18].
3.2. Gas Composition and Heating Value
The average yield of measured syngas components from
wood chips and pine pellets are shown in Figure 4. The
average concentrations of the combustible gases namely:
carbon monoxide (CO), hydrogen (H2), and methane
(CH4) obtained were within the limits reported in previ-
ous work [3]. However, the high percentage of nitrogen
(N2) present in the gas mixture could be explained by the
fact that air was used as the gasification medium. The use
of pure oxygen will result in higher percentages of the
combustible gases; however, no additional efforts were
made to lower the nitrogen content for this study. The
attempt to minimize the air flow from the blower did not
seem to have any noticeable effect on the gas composi-
Figure 4. Volumetric concentration of combustible gases in
the syngas from wood chips and pine pellets after gasifica-
tion (data not shown) but rather reduced the velocity of
the syngas flowing from the gasification chamber.
The heating value of a gas cannot be measured directly,
but only with respect to a reference state. The most widely
used is th e Lower Heating Value (LHV), parameter that-
defines the potential energy available per unit volume of
the biomass. This uses water vapor as its reference state.
The heating value of the syngas was calculated from the
concentration of combustible gases in the mixture.
Lower Heating Value (LHV) = %H2 × LHVH2 + %CH4
4 + %CO × LHVCO
The heating values obtained were within the range of
previous work using woodchips as raw materials. A com-
parison with data published on gasification with similar
systems is presented in Table 1.
3.3. Gravimetric Tar and Particulates
The process of tar and particulate sampling was initiated
only after the temperatures in the gasifier approached
steady state and there was evidence of combustible gases
produced, indicated by a self-sustaining flame. Figure 5
Table 1. Comparison of wood chips and pine pellets syngas
composition from experimental results with literature pub-
lished data.
Parameters Published
Data [3]Experimental Results
Wood Chips Experimental Results
Pine Pellets
H2 (% Vol.)15 - 2111.56 11.44
CO (% Vol.)10 - 2212.09 13.49
CO2 (% Vol.)11 - 1312.50 12.98
CH4 (% Vol.)1 - 5 1.70 2.05
N2 (% Vol.)39 - 6362.16 59.73
LHV(MJ/Nm3)4.0 - 5.63.38 3.67
Copyright © 2012 SciRes. SGRE
Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter
Figure 5. Concentration of tar and particulates in the wood chip and pine pellet syngas before and after the gas cle aning pro-
shows the concentrations of tars and particulates both
before and after gas conditio ning in the syngas generated
from wood chips and pine pellets.
A comparison of tar concentration in the syngas before
and after treatment with dolomite at 850˚C showed a
significant decrease (p < 0.05) of 90% and 92% for the
wood chips and pine pellets respectively, after passing
the syngas through the dolomite bed.
Similarly, significant reduction (p < 0.05) was observed
in the particulates concen tration as well. A 41% and 57%
reduction in particulates concentration was achieved for
wood chips and pine pellets, respectively.
3.4. Effect of Temperature on Dolomite
The catalytic bed temperature was varied in 100˚C
increments from 650˚C to 950˚C to investigate its effect
on tar removal, expressed as the concentration left in the
gas. The results presented in Figure 6 shows an increase
in the performance of the dolomite as bed temperature
increased from 650˚C to 950˚C, for both pine pellets and
wood chips. However, after increasing the temperature
beyond 850˚C, the rise in temperature did not produce
any improvement on the catalyst performance. This indi-
cates that temperatures beyond 850˚C yield diminishing
returns (with respect to tar cracking), th ereby, making th e
overall process economically impractical.
The incremental cost of maintaining a catalyst at a
higher temperature also goes up significantly due to
higher heat losses at elevated temperatures. Furthermore,
at temperatures higher than 1000˚C, thermal cracking of
tars can take place to a certain degree purely due to the
high temperatures. Considering the heating costs and
cracking efficiency, catalyst bed temperatures of 800˚C -
850˚C appear to be the ideal temperature range for the
dolomite bed.
Figure 6. Performance of dolomite as bed temperature increases from 650˚C to 950˚C on tar removal for both pine pellets
nd wood chips, expressed asthe concentration left in the gas. a
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Lowering Gasifier Tars and Particulates Using Heated Dololmite Catalyst and a Particulate Filter 61
4. Conclusion
Gasification of biomass for energy production requires
effective gas cleaning techniques that removes the tars
and particulates formed during process. In this study,
dolomite was investigated for its tar cracking capability
in a downdraft gasification system. Temperature studies
indicate an optimum catalytic bed performance at 800˚C
- 850˚C. The gravimetric analysis presented in this p aper
is an attempt to overcome the tar and particulate prob-
lems associated with biomass gasification. Additional
work is needed in order to investigate the tar cracking
potential of proprietary low temperature catalysts and a
possible combination of different catalytic bed reactors.
A chemical analysis of the tar components and deposits
on the spent catalyst (if any) is needed to better under-
stand and design a gasification system.
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