Fuel 84 (2005) 817–824
www.fuelfirst.com
Tar reduction through partial combustion of fuel gas
M.P. Houben, H.C. de Lange*, A.A. van Steenhoven
Technische Universiteit Eindhoven, Department of Mechanical Engineering, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Received 6 September 2004; received in revised form 8 December 2004; accepted 8 December 2004
Available online 28 January 2005
Abstract
A partial combustion burner is introduced as a cleaning system for the tar content of gaseous (bio) fuel. The results of experiments, using a
synthetic low calorific gas mixture, demonstrate the effectivity of the proposed process. In these experiments naphthalene is added as a model
tar component. The effect of partial combustion of the fuel gasmixture on the naphthalene is examined for different air/fuel ratios (l) and
varying hydrogen-methane fuel concentrations. For a fuel gasmixture with high methane concentrations or for higher l-values the total
tarcontent slightly decreases. In this case the naphthalene polymerises, i.e. forms higher ring components and sometimes even turn into soot.
At lower l’s and higher hydrogen concentrations the tarcontent strongly decreases. Moreover, the naphthalene is now cracked, i.e. converted
into lighter tars and permanent gases. It is found that, for fuel gases representative for biogasification products and at a l of 0.2, the presented
burner reduces the tar content of the gas with over 90% by cracking. The paper ends with a short discussion on the conditions that may
determine the cracking/polymerisation mechanism.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Tar; Biomass; Gascleaning; Partial oxidation; Tarcracking
1. Introduction
For the introduction of small-scale biomass gasifiers the
production of tars in this process is one of the major
problems. Apart from causing environmental hazards, tar is
known to create process-related problems in the end use
devices, such as fouling, corrosion, erosion and abrasion.
Before the gas can be introduced into the gas engine, the tar
content has to be reduced to low values. In literature,
various overviews can be found of the existing types of
gasifiers and cleaning methods (e.g. [5,22]).
Several methods for tar removal are possible [24]: tar
removal by physical processes (e.g. filters), thermal methods
and catalytic methods are the options that are most often
used. Most of these cleaning systems nowadays are too
expensive or complex to be used in small-scale applications.
In this case thermal methods seem to be the most appropriate.
Thermal treatment of the fuel gasmixtures can be realised
either by external heating or by partial combustion of
the fuel gasses. Until now little attention is paid to partial
* Corresponding author. Tel.: C31 402472129.
E-mail address: [email protected] (H.C. de Lange).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2004.12.013
combustion as a way to remove tars. Although, there have
been studies in which air is added separately, i.e. combined
with external heating. In [2,3] external heating of pyrolysis
gas is combined with air addition in a reactor at 800, 900 and
1000 8C. Experiments are performed with an excess air ratio
varying from 0 to 0.7. The minimum tar content was
measured at 900 8C together with an excess air ratio of 0.5.
The results show that the temperature in the reactor only had
an influence at small excess air ratios. In [13] it was shown
that tar reduction is a function of temperature and oxygen
content. Tar-reduction seems to take place when raising the
temperature from 500 to 900 8C. Furthermore, adding
oxygen above 700 8C also results in a considerable
reduction of the tar content. The tar reduction at 500 8C is
88%. This increases to almost to 99% by raising the
temperature to 900 8C and adding oxygen. In [18] the same
tendency was found using partial oxidation of naphthalene
in an artificial biomass producer gas.
In more practical studies different gasifier concepts have
been developed. These concepts also show that the tarcontent can influenced by carefully controlling the combustion zone. For example an internal pyrolysis recycle loop
with a separate internal combustion chamber in a fixed bed
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M.P. Houben et al. / Fuel 84 (2005) 817–824
gasifier [25] leads to a synthesis gas with a very low tar
content. This configuration resembles the Delacott gasifier
used for char production [16]. The use of a recycle loop (the
‘recycle gasifier’) is also tested in e.g. [8]. Another
comparable concept is the so-called ‘two stage’ gasifier
(see e.g. [1,9]), which uses staged addition of air.
A principle question which needs to be answered is
whether the tars dissapear through polymerisation or
cracking. In [11], it is shown that external heating of a tar
containing gas (in a range of 900–1150 8C) results in
polymerisation of small tar components, which produces
heavier hydrocarbons. This polymerisation finally leads to
the formation of soot. In practice, this soot can be removed
by means of a filter. Therefore, this process can be used for
gascleaning. It is, however, more desirable to reverse the
process: to crack the tars into lighter components instead of
polymerise them. If this process could be optimised it could
lead to complete cracking of the tars into permanent gasses.
There are indications (e.g. [6]) that the presence of
(H–)radicals in the heated zone indeed reverses the process.
A combustion chamber in which the fuel-gasmixture is
partially burned supplies both features (heating and radical
production) at the same time.
Therefore, this study focuses on the effect of a
combustion chamber where the tar-contamined gasses are
partially burned. When compared to the thermal treatment,
the temperature in this chamber is increased moderately (till
about 500 8C) by burning only a small amount of the lowcalorific fuel gas from the gasifier. This small amount of the
gas is burned by adding little air. The paper concentrates on
one main question: under which conditions does thermal
treatment/partial oxidation (fuel-rich combustion) polymerise the tars into heavier hydrocarbons or crack them into
lighter components like carbon monoxide and hydrogen.
First, the burner geometry and the experimental setup will
be described. Two sets of experiments will be performed. In
the first set, the air/fuel ratio is varied to determine the effect
of partial combustion on the tars. In the second set of
experiments, the behaviour of the tars will be studied as a
function of the mole fraction of hydrogen. In this way the
effect of different concentration ratios on the tar can be
determined. The results of the experiments will proof that
under certain conditions the desired cracking indeed occurs.
The paper ends with a short discussion on the mechanism
responsible for the occurence of polymerisation vs. cracking.
It is worthwile to notice that in the experiments a model
tar is chosen: naphthalene. There are a number of practical
reasons for this choice. First, it is denoted as relatively
harmless, compared with more carcinogeneous components
like benzene. Second, higher ring components compared to
naphthalene are difficult to process due to their condensation
behaviour: the whole set-up should be heated to higher
temperatures, and inserting the component in the gas flow
would be more difficult. But there are also more fundamental considerations. For example, in downdraft gassification tertiary aromatics (a.o napthalene) are predominant.
Furthermore, the aim of the experiments is to determine
whether the tar components will polymerise or crack into
lighter particles. Since naphthalene is a 2-ring aromatic
hydrocarbon, it can show both reaction paths.
2. Experimental set-up
In Fig. 1 a schematic overview of the whole set-up is
shown. A saturator is used to saturate a small nitrogen flow
with naphthalene. Solid naphthalene is placed inside the
saturator. Hot air from an electrical blower is used to heat
the wall of the saturator. The nitrogen flow is injected into
the naphthalene, where it is saturated. The set point of the
blower leads to a steady state temperature of 200 8C inside
of the saturator. The piping downstream the saturator is also
heated by the hot air from the blowers. The cold fuel
gasmixture and the nitrogen/naphthalene flow are mixed in
the mixing unit, also shown in Fig. 1 in the center of the
figure. The fuel gasmixture is controlled by mass flow
controllers (MFC’s).
To prevent cold spots in the mixing unit, which might
cause condensation during the mixing of fuelgas and
naphthalene, the fuel gasmixture is preheated using copper
piping (depicted as the black left to right line around the unit).
Again, the unit is heated with hot air coming from an
electrical air blower. After leaving the mixing unit the
fuel/naphthalene mixture is fed into the burner where primary
air can be added through the injection nozzles. As shown in
the figure at the left side of the burner at the bottom of the
glass bell, a small secondary air flow is added. This coflow is
used to stabilise the flow structures above the burner. It has no
effect on the combustion/cracking processes.
The burner geometry is described in detail in [10]. It is
based on a central tube for the fuel gasflow with seven
nozzles on the circumference through which air is injected.
On these nozzles separate local diffusion flames are
formed. As shown in Fig. 2, the burner consists of two
concentric tubes. The fuel/tar gasmixture enters the central
inner tube at the bottom of the set-up. The air enters the
outer tube at two sides symmetrically and then passes one
of the seven injection nozzles into the inner tube. The
flames stabilise at the injection nozzles in the inner tube.
Note that, the injection of air into the crossflow by means
of swirling jets is not very common for air design systems.
The Babcock and Wilcox Company has currently adopted
this type of air introduction system, based on numerical
modeling studies. They state that uniform air distribution
by swirling jets allows the burner to achieve the lowest
possible emissions [19]. The swirling air introduces
recirculation zones near the wall, whereas the inner jet
provides mixing in the core of the main flow. The system
allows thorough mixing in the core of the flow as well as in
the near wall zone.
M.P. Houben et al. / Fuel 84 (2005) 817–824
819
Fig. 1. A schematic overview of the set-up.
Downstream, the partly burnt gas mixture enters the
diverging part. The diverging outlet decreases the flow rate,
which helps to stabilise the combustion process.
In all experiments the base inlet gasmixture is:
fuel gascomposition: [H2]: 22.4%, [CH4]: 5% and [N2]:
72.6%
lower heating value (LHV): 4.2 MJ/Nm3;
added naphthalene (C10H8): 2.6 mg/Nm3;
fuel flow: 65!10K3 Nm3 which is equivalent to 2.8 kW.
The air added is set to give a l of 0.2, unless mentioned
otherwise.
The condensation behaviour of the tar components is of
special concern in the design of the set-up; the tar
components could clog the tubing, the burner or the
measuring system. Therefore, electrically traced tubing
(heated at 200 8C) is used to transport the mixture of the
gases and the naphthalene to the burner. Also for the
transport of the gas-sample to the GC heated tubing is used
(heated also at 210 8C). The inside diameter of this tubing is
1/8 inch, which is small for traced tubing, therefore the
tubing used is especially made for this application. For this
sampling, two tubes are used: one for the sample taken
before the burning and one for the sample taken downstream
of the burner. To measure the amount of naphthalene added,
samples are also taken from the sampling point downstream
of the burner without igniting the flame.
A gas chromatograph (Interscience) is used to analyse
the gases and tars. The permanent gases are measured using
the thermal conductivity detector (TCD). All hydrocarbons
are measured using a flame ionisation detector (FID). For
the analysis of the hydrocarbons a capillary column is
placed in a programmable ultra-fast oven (UFO). Use of the
UFO reduces the analysis time of the polyaromatic
hydrocarbons (PAHs) and the benzene, toluene and xylene
(BTX) to about three minutes. In our analysis the tars will be
seperated only by the amount of rings. When concentrations
in parts per million (ppm) are converted to grams per cubic
metre the following (average) mole masses are used: for
one-ring 78 g/mol, two-ring 128 g/mol, three-ring
178 g/mol, four-ring 228 g/mol and five-ring 278 g/mol.
3. Primary air
In this section the effect of the air/fuel ratio on the
naphthalene concentration is studied The air/fuel ratio is
820
M.P. Houben et al. / Fuel 84 (2005) 817–824
Fig. 2. The partial combustion burner in 2D and 3D.
expressed in l:
lh
m_ fuel
m_ air
j
j
m_ air exp m_ fuel stoi
in which m_ fuel and m_ air are the mass flow of fuel and air,
respectively. The index exp indicates the present experimental conditions, while the index stoi indicates the
flowratio in the stoichiometric case.
Fig. 3 shows the tar components classified by the number
of rings. Since benzene (C6H6) is generally not defined as a
tar component, the amount of benzene is plotted separately,
and excluded from the one-ring group.
The increase in l results in an increase of the total tar
concentration for the l range 0.2–0.65. Similar results (the
increase of the tar concentration when increasing the l) can
be found in literature ([15,17,21,28]). These studies state
that there is an optimum in the addition of oxygen with
regard to the reduction of the tar concentration: no oxygen
Fig. 3. Ring-grouped tar components in the outlet gas as a function of l.
M.P. Houben et al. / Fuel 84 (2005) 817–824
added leads to the formation of polyaromatic hydrocarbons
and soot, but ‘too much’ oxygen does the same. In
between, there is an optimum for the tar removal. The
present results show that if there is indeed a minimum, it
must be below l is 0.2.
Note that the result presented in Fig. 3 means that the
total mass of tar at the inlet differs from that at the outlet (in
all cases this value is 2.6 mg/Nm3). Therefore, not all
naphthalene is converted to measurable tar components in
the outlet gases. It is turned either into permanent gases or
into soot. To show whether partial combustion leads to
cracking or polymerisation, it is interesting to see what
happens to the naphthalene in more detail. The results show
that the increase of l leads to the formation of higher ring
aromatic components. As shown here, the benzene
follows the same trend as the small tar components:
when higher ring components evolve the benzene disappears. Another indication for polymerisation at higher l’s is
the flame colour. When increasing l a red/yellow flame
appears, which generally indicates that there is soot
formation in the flame. On the other hand, for very lean
air combustion (at lZ0.2), the tar concentration is low: only
7.5% of the initial value. Furthermore, the remaining tars
are mostly single ring. Clearly, in this situation the tars are
indeed cracked.
In [28] the behaviour of tar components is studied for
different gasification conditions. One of the parameters
studied is the equivalence ratio (ER); which is similar to
varying the l as used in this paper. Their results show that
increasing the ER leads to heavier tar components (an
increasing amount of multi-ring tarcomponents). Similar
gasification results are found in [15], where it is shown that
the tar concentration decreases when adding some oxygen
and increases again when the amount of oxygen is increased
more.
4. Inlet gas composition
To gain more insight in the parameters that influence
the tar conversion process, the hydrogen-methane content
of the fuel is varied. To assure that the outlet temperture
remains more or less constant both l (0.2) and the LHV of
the fuel (4.2 MJ/Nm 3) are kept constant in these
experiments. Therefore, a decrease of the hydrogen
concentration is directly coupled to an increase of the
fuel methane content.
In Fig. 4 several grouped tar components are shown as a
function of the hydrogen fraction in the inlet fuel. The
amount naphthalene at the inlet is about 2.6 mg/Nm3.
This figure shows a decrease in the tar concentration as
the hydrogen content of the fuelgas increases. Again,
a difference is found between the total amount of tar at the
inlet and outlet. So, in all experiments the naphthalene
converts to unmeasurable components. This amount of
821
Fig. 4. Ring-grouped tars components in the outlet gas as a function of
hydrogen fraction.
unmeasurable components strongly increases for gasmixtures with a hydrogen fraction larger than 25%.
For increasing the hydrogen fraction the higher ring
components decrease. At high hydrogen fractions almost all
naphthalene is converted to smaller components; benzene
increases while the other aromatic components all decrease.
For fuel mixtures with a higher methane concentration,
higher ring components are observed. Apparently, a sooting
tendency is present in these situations. As mentioned, the
naphthalene converts to unmeasurable components in all
situations. However, in the pure methane case these
unmeasurable components are likely to be soot particles,
whereas in the pure hydrogen case these components are
more likely to be small components like benzene and
permanent gases.
In literature similar results can be found in e.g. [14,23,26,
27]. In [6] it is concluded that hydrogen, with its large
diffusivity, can be quite effective at suppressing soot
inception, despite a corresponding increase in flame
temperature. It is also known that once soot formation has
started, there is an acceleration in the soot particle growth
even at low hydrocarbon concentrations [26]. This acceleration is caused by bonding of hydrocarbon radicals on the
growing surface [27].
The no-methane flame is of special interest, because the
naphthalene that is added is the only carbon source in this
flame. Therefore, it is possible to determine what happens to
the naphthalene in detail. Table 1 shows the distribution of
the carbon containing components in the hydrogennaphthalene-nitrogen flame. The permanent gases are
shown in the first three rows of the table. As shown, the
total amount at the outlet agrees well with the amount
measured at the inlet. Methane, carbon monoxide and
carbon dioxide are the components that are most formed in
the flame. This is probably due to the fact that the flame is
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M.P. Houben et al. / Fuel 84 (2005) 817–824
Table 1
The carbon balance in the no-methane experiment
CO
CO2
CH4
C2Hx
Benzene (C6H6)
Toluene (C7H8)
Xylene (C8H10)
Naphthalene (C10H8)
Higher-rings
Total
C-input (mg/h)
C-output (mg/h)
–
–
–
–
–
–
–
2.4919
–
2.4919
0.4361
0.6853
0.7164
0.1246
0.3925
0.0011
0.1221
0.0050
2.4914
a (very) fuel-rich flame. There is a lot of fuel (hydrogen) and
only little oxygen present. The gases are formed out of the
naphthalene, after most of the oxygen has been consumed.
The 2-ring components are really low and even the 3-, 4and 5-ring components (not shown) are all zero. Clearly, the
naphthalene added at the inlet is converted to smaller
components in the outlet gas.
Increasing the methane content of the gas leads to the
formation of larger polyaromatic hydrocarbons (for [H2]
smaller than about 20%). In the no-hydrogen case the total
tar content at the in- and outlet are almost equal. Almost
50% of the tars are now turned into 5-ring PAH’s. This
indicates that at least part of the tars are converted into even
higher ring-components. It is, therefore, clear that the
low/no-hydrogen combustion leads to polymerisation.
Remarkebly, for the same experimental conditions without
naphthalene added to the inlet (described in [10]) the
combustion process hardly produces any tars or soot. This
resembles the results presented in [23]. They suggest that
methane is not an actual soot promoter in flame situations.
However, methane does interact with other fuelcomponents
to produce more polyaromatic hydrocarbons and soot than
would otherwise have been expected. They state that the
synergy of methane with other hydrocarbons to produce
polyaromatic hydrocarbons may be attributed to the ability
of methane to produce methyl radicals. These radicals will
then promote the production of aromatics. Benzene,
naphthalene and pyrene show the strongest sensitivity to
the presence of methane: this synergy trickles down to soot
via enhanced inception and surface growth. This effect was
found to be the strongest in fuel-rich diffusion flames, i.e.
conditions, which resemble the conditions used in the
experiments described in this paper.
5. Conclusion
The effect of partial combustion on tar is studied in
the burner geometry Naphthalene is used as a model
component in these experiments. It is assumed that
the polymerisation/cracking process of naphthalene will
be similar to that of longer (more complex) tarcomponents
as they will be present in the productgas of a gasifier.
At very low primary airrates (lZ0.2), the partial
combustion process reduces the total tarcontent with over
90%. It is interesting that when more air (lR0.4) is added to
the burner, the same sooting tendency is found as in the case
of thermal treatment only [11]. By changing the amount of
hydrogen in the inlet gas, the tar concentration in the outlet
is considerably affected. For very low hydrogen concentrations (methane-rich fuels) polymerisation/sooting is
found. However, for a fuel gas without methane (with
40% hydrogen) almost no napthalene is found in the outlet
gas; all product components are lighter hydrocarbons or
even permanent gases. Therefore, in this case the naphthalene is cracked. Hydrogen appears to be an inhibitor for soot
formation: hydrogen in the inlet gas transforms the
polymerisation/sooting process into cracking. This process
starts to take effect at low concentrations (at about 5%). For
a fuel gasmixture with a LHV of 4.2 MJ/Nm3 the optimal
hydrogen concentration seems to be about 20%. This type of
fuel gas is representative for the product gas of biomass
gasification. Therefore, the process created by the burner
geometry might be a promising method for tar-removal in
(small-scale) biomass gasifiers. Testing this will be the next
step in this research.
6. Discussion
In biomass literature, little is known of the effect of
hydrogen on thermal cracking in combination with partial
oxidation From hydropyrolysis and gasification of coal, it
appears that methane might be formed by hydrogenation of
one of the double bonds of an aromatic ring (e.g. [12,20]).
This is probably the process that takes place in the cases
when the fuel is hydrogen rich. For the results presented,
this methane forming process is evident for the pure
hydrogen-nitrogen flame situation.
For hydrogen concentrations lower than 20% a strong
increase in the total tar concentration is found. Examination of the composition of the tars shows that higher ring
compounds are formed. Therefore, the effect of
methane and naphthalene might be dominant in this
situation. The hypothesis for these situations is that the
combination of methane and a little naphthalene results in
a sooting tendency. This result agrees with the findings
presented in [23]. They find that (in diffusion flames)
methane together with a small amount of naphthalene
interact synergistically to produce polyaromatic hydrocarbons and soot.
There are a number of mechanisms available to explain
the polymerisation of tars. In [7] a H-abstraction/C2H2addition (HACA) reaction mechanism is proposed. Recent
literature shows that other species than acethylene can also
play an important role. For example, it has been shown [4]
that for non-premixed flames, propargyl addition to benzyl
M.P. Houben et al. / Fuel 84 (2005) 817–824
radicals is one of the key components in the formation of
naphthalene. In [23] the recombination of cyclopentadienyl
radicals and the addition of benzyl and propargyl is stated to
be the mechanism that leads to soot in methane-air diffusion
flames doped with a small amount of hydrocarbons. They
conclude that the HACA mechanism is less important for
this flame type.
For a basic understanding of the influence of the
hydrogen concentration, it is interesting to take a closer
look on the HACA mechanism, which states that aromatic
rings grow by H abstraction, which activates the aromatic
molecules, and acetylene addition, which propagates
molecular growth by cyclization. For the HACA mechanism
three regimes exist, in which hydrogen plays different roles:
I½C2 H2 % ½H2 ;
II½C2 H2 [ ½H2 ;
823
hydrocarbons can be broken into permanent gasses. This
hydrogen inhibition process is confirmed by the results at
increased l’s. At higher l’s one would maybe expect an
increase of the cracking process through the increased
temperature. However, the experiments show a decreased
cracking. This would then be due to the fact that the
increased l (increased air injection) induces a decreased
availability of H2 and thus stops the inhibition of the cyclic
hydrocarbons.
It is noteworthy to recall that the presented results are
based on experiments using naphthalene (and not the
combinations of tars as they are found in gasification
productgas). However, it would seem that a reaction
mechanism based on hydrogen addition will also work on
longer (more complex) tarcomponents. The next step in this
research will, therefore, be the application of the partial
combustion burner on real gasification productgasses.
III½C2 H2 / ½H2 :
Because of the hydrogen variations performed, these
regimes are of special interest for the explanation of the
results found. Note that, contrary to the present results, these
regimes in [7] are connected to temperature regimes. The
regimes II and III are of particular interest to the present
paper. Roughly, regime (II) resembles the situations with a
low hydrogen fraction (lower then 20%), while regime (III)
resembles the higher hydrogen fractions.
For regime (II), the growth of aromats to higher
molecular compounds is explained by a mechanism
consisting of two reaction pathways:
(1) direct combination of intact aromatic rings, e.g. the
combination of two benzene rings leads to biphenyl,
which reacts further towards PAH compounds
(2) a sequence of H-abstraction/C2H2 addition (HACA).
For regime (III), the growth rates of PAH vary with
[H]/[H2]. The inverse dependence on [H2] is due to the
reverse reaction AiCH20AiHCH. Here AiH denotes an
aromatic molecule containing i aromatic rings and Ai
denotes an aromatic radical. In this way, chemical
suppression of soot formation due to addition of hydrogen
to the fuel seems likely. The aromatic radicals are
neutralised before they can combine together or with
C2H2. Therefore, the reaction paths of regime II are closed.
The present results appear to take this mechanism one
step further. The growth of the aromats is not only stopped,
but even reversed. At high enough concentrations the H2
and H appear to add to the hydrocarbon rings, while
acetylene addition no longer plays a part. Possibly, the thus
induced cracking is a combination of the influence of the
moderately high temperature and the available hydrogen. It
seems that the moderate temperatures are sufficient for the
hydrogen atoms to inhibit the naphthalene ringstructure
(and form two benzene rings). Even more so, the cyclic
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