Modeling of secondary reactions of tar (srt) using a functional group model

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME
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ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online) IJMET
Volume 3, Issue 3, Septmebr - December (2012), pp. 123-136
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MODELING OF SECONDARY REACTIONS OF TAR (SRT) USING A
FUNCTIONAL GROUP MODEL

Ashraf Elfasakhany*

Department of Mechanical Engineering, Faculty of Engineering, Taif University, Box 888, Al-Haweiah,
Taif, Saudi Arabia
*
Corresponding author Tel.: +966 (02) 7272020; Fax: +966 (02)7274299
E-mail address: ashr12000@yahoo.com

ABSTRACT

During flash pyrolysis/gasification of biomass, tar is the major product, which reaches up to 70%.
This tar causes a lot of problems that present significant impediments to the application of biomass
gasification systems. The reduction or decomposition of tar at the gasification process becomes one of
the most necessary and urgent problems. As a result, a great need exists for a comprehensive model
that helps to understand the mechanism of tar decomposition/cracking (second reactions of tar),
which will possibly improve biomass gasifiers and furnaces design. The purpose of this study was at
detail modelling of the second reactions of tar (SRT) using a functional group model. Types of
gasses, yields and kinetic rates during the SRT within temperature range of 500-1200 oC are modelled
and validated with experimental measurement from literature. The model discusses the scenario of tar
cracking mechanism, the quantitatively most important product from the SRT, and the best indicator
for the tar conversion process. The model also helps to classify tar compounds into different
categories with identifying quantitatively the most important compound from each category as well as
the proper method to crack each one. The model can explain the complicated behavior of parallel,
series and overlapping reactions of different tar compounds at the SRT conditions.

Keywords: Functional group, Tar cracking, Numerical modelling, Gasification.

1. INTRODUCTION

During fast biomass pyrolysis/gasification, relatively high amount of tar is produced. This tar,
which may reach up to 70%, is an extremely complex mixture [1]. The term ’tar’ describes a lump
involving thousands of single substances. Because of its complexity, there is no consistent and
generally accepted definition for tar [2]. However, the tar could be classified, as a result of thermal
biomass pyrolysis/gasification, into three major classes: primary, secondary and tertiary tars [3].
Primary tar is formed due to the presence of oxygen compounds in a temperature range of 400-700oC.
Secondary tar is formed in a temperature range of 700-850 oC, and it includes phenolics and olefins.

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Tertiary tar products appear in the temperature regime of 850-1000 oC and are characterized by
aromatics. Sometimes, these three main classes are divided into sub-classes as well.
During thermal biomass pyrolysis/gasification, the tar classes are formed and cracked hereafter.
However, some of such tar classes and in particular their compounds are not fully cracked at
pyrolysis/ gasification process where some of which are left and they are so called non-reactive tars.
Their values are mainly attributed to the structure of biomass and type of pyrolysis/gasification
process (slow or fast) [4]. For example, tar left after the gasification of beech wood and sweet gum
hardwood is respectively about 22 and 11% [5-6].
Tar disposal during pyrolysis/gasification becomes one of the most necessary and urgent problems
[7]. Tar in the product gases will condense in low temperature, and lead to clog or blockage in fuel
lines and corrosion in engines and turbines [8-9]. Tar also represents a not negligible energy loss and,
in turn, that lowers the overall efficiency of the process. Therefore, the reduction or decomposition of
tar in gasifiers is one of the biggest obstacles in its utilization for power generation and synthesis gas
applications [7, 10-11].
Attempts to remove the tar can mainly be divided into two main methods; treatments inside
gasifiers (primary methods) and hot gas cleaning after the gasifiers (secondary methods). Primary
methods require the appropriate selection of operating parameters, the use of a bed additive or
catalyst, as well as modifications upon the design of gasifiers. These primary methods only remove or
capture the tar from product gases, while the energy in tar is lost [12]. Besides, bed additives or
catalysts have technical shortcomings, such as inactivation by carbon, soot and H2S [13-15].
Secondary methods involve tar-cracking after gasifiers, either thermally or catalytically using of a
ceramic filter, electrostatic precipitator and scrubber. Secondary methods have been extensively used
but until now have not appeared to be sufficiently effective. Besides, the high costs associated with
the downstream equipment and their operation [12, 15-17]. Accordingly, simple, effective and cheap
methods to reach high tar conversion within and/or after gasifiers are needed [9, 18-19]. To achieve
such goal, a great need exists for further understanding the mechanism of tar decomposition/cracking
(second reactions of tar) using modelling approach.
There are different approaches found in the literature to model the second reactions of tar (SRT).
The simplest approach has been the treatment of the numerous reactions taking place as a single
reaction following first order kinetics [20-21]. Nevertheless, this approach did not consider the
complex nature of tar. In another approach, the SRT has been treated using a multiple reaction model
based on the assumption of many parallel reactions following first order kinetics. This model is rather
complicated to solve and it requires at least four experimental parameters; the results are uncertain as
they turn out that the different sets of parameters with different values are often found to fit equally
the data [22]. In a different approach, the tar compounds are lumped according to their chemical
structures and pyrolysis behaviors. This approach leads to very complex reaction mechanisms with a
large number of parameters, whose determination is uncertain at best [23-25]. In summary, there is a
lack in a simple, accurate and general modelling for the SRT mechanism [5, 6, 26]. Besides, the
inaccuracy of the SRT modelling is one of the main reasons for the discrepancy in the overall biomass
gasification/combustion modelling [27]. The difficulties of the SRT modelling may be attributed to
that tar compounds under thermal cracking process interact to each other and, in turn, they are
changed rapidly to other compounds [28-29].
The objective of this work was at investigating a simple and accurate modelling tool for the SRT
mechanism at high heating rate condition. This model, which uses several different functional groups,
provides information regarding the change in quantity and composition during the thermal tar
cracking process. The model also helps to explain the complicated behavior of parallel, series and
overlapping reactions of different tar compounds under the SRT condition.

2. NUMERICAL MODEL

There has not been an agreement in literature concerning the compounds used to represent tar on
modeling condition [30]. Some researchers considered tar is modeled using toluene and naphthalene

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compounds at high temperature and atmospheric pressure conditions [31]. Others considered tar is
modeled using benzene, naphthalene and heavy aromatic [32]. Simell et al. [33-35] used benzene and
naphthalene for a tar modeling in number of studies. Dou et al. [30] used Methylnaphthalene as tar
modeling. However, other researchers argued that naphthalene and phenol are the best compounds
used for tar modelling [2, 35-39].
In order to commonly characterize compound(s) used for tar modelling, it is needed to analyze tar
compounds firstly. Tar consists of 53 compounds [28-29]; but they might be classified into four main
groups under cracking conditions: non-condensable gasses (CO, CO2, CH4, H2, etc.), primary tar
(Acetol, Acetic acid, Guaiacol, etc.), secondary tar (Phenol, Cresol, Ethyltoluene, etc.) and tertiary tar
(Naphthalene, Benzopyrene, etc.). Typical representatives of primary, secondary and tertiary tars are
presented in detail in Milne et al. [2]. Yields of tar compounds are varied significantly, which depend
on the working conditions, biomass source, etc.; hence some of tar compounds might be neglected
due to their very small yields.
In the current study, the four main groups of tar compounds are modelled as eleven different
functional groups. We considered only these eleven groups since the rest of tar compound are in very
small quantities (< 0.002) under current working conditions and, in turn, they are neglected. Table 1
summarizes the eleven functional groups, kinetic rate constants and the end products from the SRT
expressed in terms of weight percentage of the total tar yield.
The rate of the SRT is modelled as a first order reaction based on the difference between the
ultimate yield of product mi∞ and the amount of product left mi of the functional group. The
formation/cracking of each tar group is described according to Eq. (1) as follows:

dmi  E 
( )
= mi∞ − mi Ai exp − i
RTg 
Eq. (1)
dt  

Where mi∞ is the ultimate yield of the functional group (Kg); mi is the amount of the functional group
left (Kg); E is the activation energy (kJ/mole); A is the frequency factor (s-1); Tg is the gas
temperature (K); R is the universal gas constant (KJ/mole K).

Table 1
List of functional groups and kinetic rate constants for the secondary reaction of tar (SRT) model

Reactions A (s-1) E (kJ/mole) mi∞ Refs.
k5 6
Tar → Acetol 1.6×10 108.0 5.00 (presented)
k6
Tar → Acetic acid 2.5×105 107.5 1.00 (presented)
k7
Tar → Guaiacol 22×104 83.60 0.08 (presented)
k9
Tar → o-Cresola 3.7×107 118.00 6.7×10-2 (presented)
k 10
o-Cresola → o-Cresolb
 1.6×105 107.50 3×10-2 (presented)
k8
Tar  → Phenol 4.22×106 110.07 0.17 (presented)
k 11
Tar → Naphthalene 9.53×104 93.300 0.130 [46]
k 12
Tar → Benzo[a]pyrene
 2.3×104 80.000 1.8×10-3 [47]
Tar k 1 CO
→ 1.55×105 87.6 12.4 [48]
k2
Tar → CO2 3.26×104 72.8 6.80 [49]
Tar k 3 CH4
→ 1.5×106 114.3 2.40 [50-51]
k4
Tar → H2 1.5×106 114.3 0.42 [46-51]
a o-Cresol produced from tar
b o-Cresol cracked at T> 850 oC

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3. NUMERICAL METHODS

The calculations are carried out using an in house CFD code. In the code, the gas phase transport is
described by the Navier-Stokes equations, the energy transport equation based on the first law of
thermodynamics, and the species transport equations based on the law of mass conservation of
chemically reacting gas mixture, together with the equation of state, the caloric equation of state, as
well as chemical reaction mechanisms and the related kinetic rates. A general form of the transport
equations can be written as follows:

∂ρφ ∂ρu iφ ∂  µ t ∂φ 
+ =   + Sφ Eq. (2)
∂t ∂xi ∂xi  Pr ∂xi 
 

Here φ represents the velocity components, enthalpy, and species mass fractions. Different terms,
from left to right, represent the time evolution, the convection, diffusion and the source term,
respectively.
The source terms in the above transport equations, Sφ,, which are due to chemical species from tar
cracking and reactions, are calculated using ‘chemical in cells’ method. When the turbulent intensity
is low, the distribution of these source terms may be very un-smooth, which could lead to non-smooth
result or even non-convergence of the numerical solution. This can be avoided by using more
chemical source groups and more amounts of injected tars. For further detail about the calculation
methods and the CFD code used, you may see [40-42].

4. EXPERIMENTAL

The experimental measurement of Morf el al. [29] is used to validate our modelling calculations.
Although there are many experiments available in the literature, see e.g., [5-17], the current
experiment has been chosen because it produces accurate measurements and overcomes some of the
drawbacks of other experiments. Examples of such drawbacks of other experiments are that during
biomass pyrolysis, tar compounds are sequentially released into the gas phase. Thus, the amount of
tar measured in other experiments is less than the real quantity. In addition, some of tar cracking
products has not been considered. The Morf el al. [29] experiment overcame these drawbacks by
using a continuously fuel-fed pyrolysis process.
The experiment was carried out in a laboratory reactor system where wood was fed to produce
pyrolysis products at a mean temperature of 380 oC. After the primary pyrolysis stage, the produced
char is separated, and the tar containing pyrolysis gas is swept to another conversion reactor where
homogenous and/or heterogeneous tar reactions were measured. The tar conversion is operated at
several temperature levels from 500-1100 oC. During the experiment, tar samples are taken and the
concentrations of non-condensable gasses (CO, CO2, CH4, and H2) are measured on-line. For more
detail, you may see the experiment.

5. RESULTS AND DISCUSSIONS

Results from the second reaction of tar (SRT) modelling are presented and discussed. Comparisons
between our model predictions and experimental results of Morf et al. [29] are shown in Figs. 1-4. As
seen, the yields and rates of the functional groups modelling (M-FG) are perfectly predicted the
experimental data (Exp).
Fig. 1 presents acetol, acetic acid and guaiacol functional groups. Such functional groups are
classified among the primary tar category since they have similar trends within related temperature
regimes. This category is characterized by oxygenated compound that gives the primary tar its high
reactivity. The quantitatively most important compound of such primary tar is acetol, which is about
5% of total tar, as shown in Fig. 1a. As seen, acetol is almost constant within temperatures 500-

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600oC, and slowly cracked between 600-700 oC. Hence after, it steeps cracked until reaches almost
zero value at 1000 oC.
The quantitatively second most important compound of primary tar is acetic acid, which is about
1%. As shown in Fig. 1b, the acetic acid is kept almost stable in the temperature range of 500-800 oC
and slowly cracked for the temperature range 800-850 oC. Hence after, it steeps cracked until reaches
almost zero value at 1200 oC.
Guaiacol yield, as shown in Fig. 1c, is converted faster than other primary tar compounds (e.g.,
acetol and acetic acid) to reach less than 0.01% at 850 oC. In the beginning, the yield of guaiacol in
total tar is very small, which is about 0.08%. Although such very low yield of guaiacol, it is one of
the most important reasons for higher-molecular weight of tar fractions [43]. The rest of the primary
tar compounds are rather smaller than guaiacol yields and, in turn, they are neglected in both
modelling and experimental.

Fig. 1a Yields of Acetol functional group modelled (M-FG) and validated with experimental
measurements (Exp) of Morf et al. [29].

Fig. 1b Yields of Acetic acid functional group modelled (M-FG) and validated with experimental

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Fig. 1c Yields of Guaiacol functional group modelled (M-FG) and validated with experimental

o-Cresol yield behaves totally different from all other tar compounds. It increases with temperature
to reach the peak at about 850 oC. Then it steep decreases with a further temperature increasing until
vanished at about 1200 oC, as shown in Fig. 2a. However, other tar compounds only increase,
decrease or remain constant with temperature.
Fig. 2b shows the yield of phenol, and as seen it increases between temperatures 650-900 oC;
however, before 650 oC and after 900 oC, there is no significant changes. Besides, a large scattering in
the experimental measurements is clearly shown. This is attributed to a technical problem in the
experiment, as explained by Morf et al. [29]. Our study believes that the phenol yield should be
decreased in a higher temperature condition (after 850 oC), similar to o-Cresol yield. This finding is
consistence with Abu El-Rub et al. [37], in which they showed that the conversion of phenol
dominates at 900 oC, due to the thermal cracking.

Fig. 2a Yields of o-Cresol functional group modelled (M-FG) and validated with experimental

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Phenol and o-Cresol are classified among secondary tar compounds where this category represents
less than 1 % of total tar yields, as shown in Fig. 2. The secondary tar compounds are also
responsible, along with guaiacol, for higher-molecular tar weight [43].
Naphthalene and benzo[a]pyrene compounds are typically considered under the tertiary tar
category. Naphthalene yield, as shown in Fig. 3a, increases with temperature. Naphthalene is
quantitatively the most important compound in the tertiary tar (0.13% of total tar yield). However,
benzo[a]pyrene is only about 0.002% of total tar yield, which is an almost negligible fraction. The
other tertiary tar compounds have smaller yields than benzo[a]pyrene and, therefore, they are
neglected in the current study. Fig. 3b shows the yield of benzo[a]pyrene where it behaves similarly
to naphthalene yield, and all other tertiary tar compounds show similar qualitative behavior [29].

Fig. 2b Yields of Phenol functional group modelled (M-FG) and validated with experimental

Fig. 3a Yields of Naphthalene functional group modelled (M-FG) and validated with experimental

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Comparing the behavior of the primary, secondary and tertiary tar components, it is seen that the
primary tar decreases with increasing temperature; however tertiary tar behaves in opposite criteria,
which increases with increasing temperature. Secondary tar behaves as a combination between the
primary and tertiary tar compounds, which increases and decreases afterwards. It is also shown that
the tertiary tar compounds have no significance in a temperature lower than 700 oC. However, after
700 oC such tar compounds are formed rapidly while primary tar (especially acetol) is demolished
rapidly at this temperature.

×10-3

Fig. 3b Yields of Benzo[a]pyrene functional group modelling (M-FG) and validated with
experimental measurements (Exp) of Morf et al. [29].

Light hydrocarbons, e.g., CH4, CO, CO2 and H2, behave similarly to the tertiary tar compounds,
as shown in Fig. 4. Fig. 4a shows the CO mass fraction versus temperature produced from the tar
cracking. As seen, CO yield is kept almost constant until temperature reaches 700 oC, hereafter it
increases fast to reach its double value. CO increases till reach about 12.4 % and this clearly shows
that carbon monoxide is the most important gasses produced from homogenous tar cracking. This
monotonic rising was investigated by other researchers to propose that the CO yield could be used as
an indicator for the tar cracking process, see e.g., [44-46].
The CO2 yield is not varied significantly along all the temperature range (500-1200 oC), as shown
in Fig. 4b. The carbon dioxide is almost stable due to that the tar cracking process is working on the
condition of being lacking in external supply of oxygen. On the other hand, the experimental data
show scattering in the results and this minor scattering may attribute to the inaccuracy in the
measuring devices.
Fig. 4c illustrates the CH4 mass fraction varies with temperature. As seen, the CH4 yield does not
change significantly for a while and increases slowly when temperature reaches 700 oC; hereafter CH4
increases significantly like CO.
H2, similar to CO and CH4, does not change significantly for a while (from T=500-700 oC). By
T=1000 oC, it monotonically increases to reach eight times of its original value, as shown in Fig. 4d.
Such a rapid boost is most probably due to water-gas shift reaction as follow:

CO + H 2O → CO2 + H 2 Eq. (3)

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According to that reaction, the water content in the gas phase decreases while H2 increases. The
experimental measurement of Morf et al. [29] showed that the water yield in the gas phase decreases
when temperature reaches 600 oC and higher.
Finally, this work may highlight some important findings as follows:
It is important to note that tar compounds are not all thermally cracked, e.g., tertiary tar. Hence,
primary and secondary tars may be considered as temporary products under thermal tar cracking
conditions, however, the tertiary tar is not.
Primary tar compounds occupy about 6 % of total tar; however, both secondary and tertiary tars
occupy together about 0.4%. Although, this small yield of the secondary and tertiary tars, they
are responsible for higher-molecular weight of tar fraction and that causes several problems in
gasifiers.

Fig. 4a Yields of CO functional group modelled (M-FG) and validated with experimental

Fig. 4b Yields of CO2 functional group modelled (M-FG) and validated with experimental

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Acetol, phenol and naphthalene are the quantitatively most important product from primary,
secondary and tertiary tars, respectively.
Different tar compounds are cracked in the rate constants of range O104-O107 (s-1) for frequency
factors (A) and 72-118 (kJ/mole) for activation energies (E).
When considering the modelling of the SRT in the absence of oxygen, the complicated
processes may be simplified into systems of relations (see Table 1); it is more than likely that
the yields of functional groups at the SRT depend on temperature, heating rate and
air/oxygenate compounds.

Fig. 4c Yields of CH4 functional group modelled (M-FG) and validated with experimental

Fig. 4d Yields of H2 functional group modelled (M-FG) and validated with experimental

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The behavior of SRT includes parallel, series and overlapping reactions for different tar
compounds under thermal process. Secondary tars are overlapped with both primary and tertiary
tars, however, most of primary tars (acetol and guaiacol) are demolished while tertiary tars are
formed, i.e., series reactions. Tar compounds in each category (primary, secondary and tertiary)
are reacted at the same temperature range, i.e., parallel reactions.
The most significant temperatures for the SRT are 700 and 850 oC. At 700 oC, primary tar
compounds are cracked significantly while tertiary tar compounds as well as non condensable
gases are formed in a same rapid attitude. However, at 850 oC, secondary tar compounds reach
almost their peak values and start to crack afterwards while primary tar is almost vanished. This
may help to conclude that primary tar is cracked to secondary and tertiary tar as well as light
hydrocarbons. Secondary tar, on the other hand, is cracked to tertiary tar and light hydrocarbons.
Secondary tar vanishes at 1200 oC, however, tertiary tar is not. Hence, most of the problematic
tar is the tertiary one since it is not cracked at even very high temperature and, in turn, causes a
lot of problems that present the significant impediment to the application of biomass gasification
systems.
In order to decompose all tar compounds during pyrolysis/gasification, this work shows that
increasing pyrolysis/gasification temperature and/or reaction time enhances tar aromaticity and
reduces the yields of the functional groups. Nevertheless, this is not enough to decompose all tar
compounds, as showed early, and, in turn, it is proposed to use oxygenate compounds during
pyrolysis/gasification. Such oxygenate compounds should be in limited amounts not to oxidize
other gases in gasifiers. Besides, oxygenate compounds should be added at a high temperature
condition where the tertiary tar is left (>1000 oC).
Homogenous tar conversion reactions without external supply of oxygen become important at
temperature higher than 700 oC and that clearly indicated by increasing concentrations of CO,
CH4 and H2.
Boroson et al. [44-46] proposed that the CO yield could be used as an indicator for tar cracking
process because it is stepping increased to reach its double value under the thermal tar cracking
condition. However, Morf el al. [29] argued that the H2 is a better indicator of tar cracking
process than CO because H2 value is in minor concentrations at temperature up to 700 oC.
However, at higher temperature it is duplicated eight times because of tar cracking. In a
conclusion, current study agreed with Morf el al. [29] argument since CH4 is also duplicated at
higher temperature, i.e., similar to CO. On the other hand, tar cracking is generally indicated by
increasing the concentrations of all gases, CO, CH4 and H2.
CO is quantitatively the most important product from the homogenous tar conversion, and H2 is
the best indicator for the tar conversion.
Yields of some gases are not only from tar cracking but also from gas-phase reactions between
themselves (water gas-shift reaction, for example).

6. CONCLUSIONS

Tar is an undesirable by product of biomass gasification because of its various problems. In the
current study, a general kinetic model was developed to describe the SRT process in gasifiers. Such
model can explain the complicated behavior of parallel, series and overlapping reactions of different
tar compounds on thermal condition. The model quantifies the tar cracked to eleven different
functional groups. A sequential method to estimate the rate constants in the lumped kinetic model was
adopted, which greatly simplifies the model using first order equation. Kinetic constants and
activation energies were determined and validated with experimental data.
Results show that primary tar occupies about 6 % of total tar; however, secondary and tertiary tars
occupy less than 0.5% for both. Acetol, phenol and naphthalene are the quantitatively most important
products from primary, secondary and tertiary tars, respectively. Primary and secondary tars are
considered as temporary products under thermal tar cracking conditions, however, the tertiary tar is
not.

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SRT without external supply of oxygen become important at temperature higher than 700 oC where
primary tar is cracked significantly while tertiary tar as well as non condensable gases is significantly
formed in almost similar rapid attitude. When temperature reaches about 850 oC, secondary tar gets to
its peak yield and starts to crack afterwards; however, primary tar is almost vanished. Secondary tar is
cracked to tertiary tar and light hydrocarbons (CO, CO2, CH4 and H2). Primary tar, on the other hand,
is cracked to secondary and tertiary tar as well as light hydrocarbons. Tar cracking is generally
indicated by increasing the concentrations of CO, CH4 and H2. Besides, CO is the largest by product
of homogenous tar conversion, and H2 is the best indicator for the tar cracking process.
Increasing pyrolysis/gasification temperature and/or reaction time enhances tar cracking
significantly. Yet, this is not enough to decompose all tar compounds even at very high temperature.
In turn, it is proposed using oxygenate compounds under certain circumstances for cracking such tar
compounds in gasifiers. Such oxygenate compounds should be in a limited quantity and added within
high temperature condition where the unbroken tars are left.

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