V Conferência Nacional de Mecânica dos Fluidos, Termodinâmica e Energia
MEFTE 2014, 11–12 Setembro 2014, Porto, Portugal
© APMTAC, 2014
Destruction of the Tar Present in Syngas by Combustion in Porous
Media
T. Carvalho1, M. Costa1, C. Casaca1,2, R. C. Catapan3, A. A. M. Oliveira4
1
IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal
Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, Lisboa, Portugal
3
Center for Mobility Engineering, Federal University of Santa Catarina, Joinville, Brazil
4
Mechanical Engineering Department, Federal University of Santa Catarina, Florianópolis, Brazil
email: [email protected], [email protected], [email protected], [email protected],
[email protected]
2
ABSTRACT: The main objective of this article is to evaluate the destruction of the tar present in the
syngas from biomass gasification by combustion in porous media. A gas mixture was used to emulate the
syngas, which included toluene as a tar surrogate. Initially, the CHEMKIN was used to assess the
potential of the proposed solution. The simulations revealed the complete destruction of the tar surrogate
for a wide range of operating conditions. Subsequently, experimental tests were performed in a porous
burner fired with pure methane and syngas. In these tests the toluene concentration in the syngas varied
from 50 to 200 g/Nm3. In line with the CHEMKIN predictions, the results revealed that the toluene was
almost completely destroyed for all tested conditions and that the process did not affected the
performance of the porous burner regarding the emissions of CO and NOx.
KEY-WORDS: Experimental; Kinetic modeling; Porous burner; Tar destruction.
1
INTRODUCTION
Biomass includes woody plants, herbaceous plants/grasses, aquatic plants, and manures [1,2], and is
commonly converted in fuels and/or heat through either thermo-chemical or biological processes. Within
the thermo-chemical conversion processes, the gasification of biomass is a vey promising technology.
The gasification of biomass originates a gaseous fuel that can be used directly for power and/or heat
generation, to fabricate fuels for transportation and as chemical feedstock [3]. The gaseous fuel is known
as syngas and is composed by H2, CO, CO2, CH4, C2H4 and other gases, and impurities such as alkali
compounds and tars. The fuel mixture has usually a low heating value, between 4 and 6 MJ/Nm3, if air is
used as gasifying agent [4,5]. Basu [4] and others investigators [5-7] discussed in detail the design,
operation, and formation and composition of impurities formed in gasifiers. The impurity levels depend
on the gasifier design and gasification conditions. Woolcock and Brown [8] presented a useful review on
the main techniques available for impurities removal, which are very expensive systems that can generate
waste products that require sfurther treatment.
A major concern associated with the cleaning systems for tars is that they can condensate or polymerize
before arriving to the removal or reduction system, and thus can block or damage upstream equipment.
Milne et al. [9] defined tars as the organics produced under thermal or partial-oxidation regimes
(gasification) of any organic material that are generally assumed to be largely aromatic, while Moersch et
al. [10] considered tars as all aromatic and polyaromatic hydrocarbons present in the syngas. Valuable
reviews on technologies for reduction, removal and destruction of tars in the syngas through catalytic
processes can be found in references [11-16]. These methods (thermal and catalytic cracking, and partial
combustion) for tar destruction are very attractive since they increase the gasification efficiency through
the conversion of the tar to useful gases, such as light hydrocarbons, without the need for separating the
tar from the syngas. In particular, the catalytic cracking of tar is a valuable method since it allows high
conversion rates at moderate temperatures, but the high associated costs and need for regeneration are
major disadvantages. The syngas partial combustion also allows the cracking of the tar into lighter
hydrocarbons, but a relative high amount of useful compounds can be found in the flue gas [16].
In this context, tar removal through the integral combustion of the syngas might be an interesting
alternative. A suitable technology to achieve this might be the combustion of the syngas in porous media.
Useful reviews on the various aspects of the combustion in porous media are provided in references [1719], and specific studies on porous burners may be found in references [20-29]. The present study
evaluates the destruction of the tar present in the syngas from biomass gasification by combustion in
porous media. A gas mixture was used to emulate the syngas, which included toluene as a tar surrogate.
Initially, the CHEMKIN V4.1 was used to assess the potential of the proposed solution. Subsequently,
experimental tests were performed in a porous burner fired with pure methane and syngas. In these tests
the toluene concentration in the syngas varied from 50 to 200 g/Nm3.
2
MATERIALS AND METHODS
Figure 1 presents a schematic of the experimental setup. It includes devices for controlling the flow rates
of air, fuel and air for the tar saturator, for measuring the temperatures and the concentration of the
chemical gas species, for tar control and tar sampling. The air supply was provided through a bottle of
pressurized industrial air and a reducing-control valve. The fuel supply system was composed of bottles
of pressurized gases and a reducing-control valve. The gases used include CH4 (99.8% pure), CO
(99.997% pure), CO2 (99.995% pure), H2 (99.999% pure) and N2 (99.999% pure). Electronic flow meters
connected to a control valve were used for measuring and controlling the CH4 flow rate. The others gases
were controlled through precision manual valves. The main air was introduced in a stainless steel pipe
with the air saturated by toluene, providing a homogeneous mixture before reaching the fuel injection
zone. The mass flow of toluene injected was carefully monitored before being injected in the main air.
The porous burner used in this work is described in detail elsewhere [26,27,29]. In this study the
reactants feeding system was modified to accommodate the tar injection. Figure 2 presents a schematic of
the porous burner used. The burner is made with four layers of alumina (Al2O3) and zirconium (ZrO2)
porous foams, with 80% of volumetric porosity, diameter of 70 mm and thickness of 20 mm. The porous
burner is composed by two different regions of foams, the preheating region that is made of two layers of
40 porous per inch (ppi) and the stable-burning region composed by two layers of 10 ppi. A single
injection hole with 16 mm orifice is made in all the insulation plates and in a steel plate. These plates,
insulation and steel, were placed upstream of the preheating region. The steel plate has the objective to
avoid the toluene absorption by the insulation plates through direct contact due to the low flow velocity.
Figure 1: Schematic of the experimental setup.
Figure 2: Schematic of the porous burner.
The temperatures within the porous burner were measured by 8 thermocouples Pt/Pt:13% Rd made of
250 µm diameter wires placed inside alumina double-holed tubes with diameter of 1.59 mm. The
thermocouples were positioned in the center of the porous foams and were connected to a data acquisition
system interfaced with a computer. The hot junctions of the thermocouples enter into thermal equilibrium
with the gas and the solid through convection, radiation and conduction. Thus, the measurement provided
by these sensors should be understood as an average temperature between the gas and the solid at each
position.
A chimney was placed at the top of the burner to homogenize the flue gas and to facilitate the sampling
(see Figure 1). Combustion products were sampled with the aid of a water-cooled stainless steel probe for
the measurement of O2, CO, CO2, NOx and hydrocarbons concentrations. Before reaching the analyzers,
the sample was cleaned and dried. The analyzers included a magnetic pressure analyzer for O2
measurements, non-dispersive infrared gas analyzers for CO and CO2 measurements, a flame ionization
detector for HC measurements and a chemiluminescent analyzer for NOx measurements. Zero and span
calibrations with the standard mixtures were performed before and after each daily session. In the post-
flame region, probe effects are negligible and errors arise mainly from quenching of chemical reactions,
sample handling and analysis. Repeatability of the post-flame gas species concentration data was, on
average, within 2% of the mean value.
There two main methods for the measurement of tars: the cold solvent trapping (CST) and the solidphase adsorption (SPA) methods [30-33]. In this study, the tar measurements were made using the SPA
method. A SKC pump with a flow rate of 100 mL/min was used for sampling. Before reaching the
adsorbent, the sample was dried with silica gel. After adsorption, the Tenax adsorbent was analyzed in a
gas chromatograph following the ATD-GC-MS method according to the standard ISO 16000-6:2011.
3
CHEMICAL KINETIC MODEL
In order to assess the potential of the proposed solution for tar destruction, a preliminary chemical kinetic
study was performed with the CHEMKIN 4.1, using the PREMIX model, developed by Kee et al. [34].
Previous related studies using the CHEMKIN with the PREMIX model in porous burners include those of
Rortveit et al. [21], Barra and Ellzey [22] and Gauthier et al. [35]. The chemical kinetic mechanism
employed in this work involves 119 species and 612 elementary reactions [36-38].
4
RESULTS AND DISCUSSION
Table 1 presents the fuel compositions and equivalence ratios used in this study. The burner power used
was 510 W for the ɸ = 0.5 and 420 W for the ɸ = 0.4. The mean flow velocity considered for all
compositions and equivalence ratios was 8 cm/s.
Table 1. Fuel composition and equivalence ratios used.
Fuel
1
Composition
100% CH4
ɸ
0.5
2
20% CH4; 8.9% H2; 8.9% CO2; 26.7% CO; 35.6% N2
0.5 and 0.4
The literature [9] reveals that the highest tar concentration that can be found in real applications is close
to 200 g/Nm3. In this work, toluene (C6H5CH3) was used as a tar surrogate with three different
concentrations in the syngas, namely 50, 100 and 200 g/Nm3, for each condition presented in Table 1.
Note, however, that the mass flow rate of toluene was different for each case, ranging from 41 mg/min for
fuel 1 at ɸ = 0.5 to 320 mg/min for fuel 2 at ɸ = 0.5.
4.1
Chemical kinetics results
In the simulations using the CHEMKIN with the PREMIX model, the measured axial temperature profile
was prescribed for each condition. Figures 3 and 4 show predictions of gas species concentrations along
the center line of the porous burner for fuel 2 at ɸ = 0.4, and ɸ = 0.5, respectively. For ɸ = 0.5 the flame
front is located at an axial distance of around 20 mm, while for ɸ = 0.5 is located at an axial distance of
around 30 mm. The differences in the flue gas concentrations of O2 and CO2 reflect the different
equivalence ratios used. The predictions indicate that total toluene destruction is attained in both cases.
Figure 3: Predictions of gas species concentrations
along the center line of the porous burner for fuel 2
at ɸ = 0.4.
Figure 4: Predictions of gas species
concentrations along the center line of the porous
burner for fuel 2 at ɸ = 0.5.
Figure 5 shows predictions of toluene concentration along the center line of the porous burner for fuel 2,
with three concentrations of toluene, at ɸ = 0.5. The figure reveals that total toluene destruction is
accomplished regardless of the concentration of toluene in the syngas. Similar results (not shown here)
were obtained for the remaining test conditions. All cases examined showed that the toluene addition to
the syngas had no impact on the HC, CO and NOx emissions.
Figure 5: Predictions of toluene concentration along the center line of the porous burner for fuel 2, with
three concentrations of toluene, at ɸ = 0.5.
4.2
Experimental results
Figure 6 shows the measured temperatures along the center line of the porous burner for fuels 1 and 2 at
ɸ = 0.5. The flame region can be identified as the position where the temperatures reach the highest
values. In both cases, the flame front was found in the axial position of 20 mm. The flame behavior was
very similar for both fuels, despite the different flow mass rate of toluene used. Specifically, the mass
flow rate of toluene was 40.8 mg/min for fuel 1 with ɸ = 0.5 and 113.9 mg/min for fuel 2 with ɸ = 0.5.
Figure 7 shows the toluene flue gas concentration as a function of the concentration of toluene in the
fuel for fuels 1 and 2 at ɸ = 0.5. The figure reveals generally higher destruction rates of toluene for the
porous burner fired with fuel 1 than with fuel 2, most likely because of the slightly higher temperatures
within the porous media for the former case. Note, however, that the average toluene destruction was over
99.8% regardless of the case studied.
Figure 8 shows the measured temperatures along the center line of the porous burner for fuel 2 at ɸ =
0.4 and 0.5. In the case of ɸ = 0.4 the measured on-axis temperatures are consistently lower than those
measured for ɸ = 0.5. Moreover, the highest temperature for the lower equivalence ratio occurs further
downstream revealing a shorter pre-heating region with lower temperatures close to the injection point of
the fuel. Despite this, the region available for the cracking of toluene is larger at ɸ = 0.4 than at ɸ = 0.5.
Figure 9 shows the toluene flue gas concentration as a function of the concentration of toluene in the
fuel for fuel 2 at ɸ = 0.4 and 0.5. The relatively high toluene flue gas concentration for 50 g/Nm3 of
toluene in the fuel at ɸ = 0.4 can be attributed to the lower temperatures measured in the preheating zone.
Apart from this data point, both curves reveal the same behavior in regard to the toluene destruction by
the porous burner.
Figure 10 shows the toluene destruction percentage as a function of the toluene concentration in the fuel
for fuel 2 at ɸ = 0.4 and 0.5. It is seen that the toluene destruction percentage is always lower with the
porous burner operating with a ɸ = 0.4 than a ɸ = 0.5, which is attributed to the lower temperatures within
the porous medium for the former case. Moreover, in this case there is a larger region with lower
temperature close to the fuel inlet, which contributes to the lower toluene conversion observed, despite
the larger preheating region. Figure 10 also reveals that the toluene conversion increases for higher mass
flow rates of toluene in the syngas probably due to the contribution of the toluene oxidation to the
increase in the temperature within the porous burner.
5
CONCLUSIONS



The CHEMKIN results revealed the complete destruction of the tar surrogate for a wide range of
operating conditions.
The porous burner proved to be an efficient way for toluene destruction; the results indicated
destruction percentages over 99.8% regardless of the fuel used to fire the burner.
The toluene destruction rate is always lower with the porous burner operating with a
ɸ = 0.4 than a ɸ = 0.5, which is attributed to the lower temperatures within the porous medium
for the former case.

1400
800
1200
700
Fuel 1
600
Fuel 2
[C6H5CH3]flue gas (µg/Nm3)
Temperature (ºC)
The toluene conversion rate increases for higher mass flow rates of toluene in the fuel mixture
probably due to the contribution of the toluene oxidation to the increase in the temperature
within the porous burner.
1000
800
600
400
Fuel 1
200
Fuel 2
500
400
300
200
100
0
0
0
10
20
30
40
50
Axial position (mm)
60
0
70
50
100
150
200
250
[C6H5CH3]fuel (g/Nm3)
Figure 6: Measured temperatures along the center
line of the porous burner for fuels 1 and 2 at
ɸ = 0.5.
Figure 7: Toluene flue gas concentration as a
function of the toluene concentration in the fuel
for fuels 1 and 2 at ɸ = 0.5
[C6H5CH3]flue gas (µg/Nm3)
1400
Temperature (ºC)
1200
1000
800
600
400
ɸ = 0.5
200
ɸ = 0.4
ɸ = 0.5
ɸ = 0.4
0
0
10
20
30
40
50
Axial position (mm)
60
0
70
50
100
150
200
250
[C6H5CH3]fuel (g/Nm3)
Figure 8: Measured temperatures along the center
line of the porous burner for fuel 2 at ɸ = 0.4 and
0.5.
Figure 9: Toluene flue gas concentration as a
function of the toluene concentration in the fuel
for fuel 2 at ɸ = 0.4 and 0.5.
C6H5CH3 destruction (%)
100
99.5
99
ɸ = 0.5
98.5
ɸ = 0.4
98
0
50
100
150
200
250
[C6H5CH3]fuel (g/Nm3)
Figure 10: Toluene destruction as a function of the toluene concentration in the fuel for fuel 2 at ɸ = 0.4
and 0.5.
ACKNOWLEDGMENTS
This work was supported by Fundação para a Ciência e a Tecnologia (FCT), through IDMEC, under
LAETA Pest-OE/EME/LA0022 and under PTDC/EME-MFE/116832/2010.
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