Concentrations of methane and higher hydrocarbons
during bubbling fluidised bed gasification of biomass
Truls Liliedahl and Krister Sjöström
Royal Institute of Technology, KTH, Chemical Technology, SE-100 44 Stockholm, Sweden
*Corresponding author: Tel: +46-8-7908777, e-mail: truls@ket.kth.se
Abstract
When modelling fluidised bed gasification of biomass, one of the most difficult tasks is to
predict the concentrations of methane and the higher hydrocarbons. This aspect is of
importance, as these compounds tend to be the most important species with respect to energy
content. Additionally, as the methane and hydrocarbons are energy rich, inaccuracies in these
predictions may cause large errors in temperatures; errors that in turn strongly will influence
reaction rate computations.
The methane in the product gas during fluidised bed gasification of biomass tends to be
(much) higher than what the equilibrium suggest. This implies that for being able to model
and predict the concentrations of methane (and the higher hydrocarbons) must empirical or
semi-empirical expressions be employed.
At KTH have, over the years, a large number of bubbling fluidised bed tests been carried
through. Analysis of these tests gave that methane concentrations, expressed as , tend to
vary between 0.10 and 0.17. It was, with some uncertainty, possible to derive an expression
that gives the methane concentration as a function of the carbon monoxide, carbon dioxide
and hydrogen concentrations. In turn was it also possible, to link the concentrations of the
higher hydrocarbons to that of methane.
With the derived empirical expressions is it possible to predict a gas composition and the
corresponding temperature during the bubbling fluidised bed gasification. The resulting
product gas is made up of carbon monoxide, carbon dioxide, water, hydrogen, nitrogen,
methane and higher gaseous hydrocarbons, given as the pseudo hydrocarbon "C3H4.5". Thus
are eight unknowns including the temperature computed.
Key words: biomass, fluidised bed gasification, methane, hydrocarbons
1. Introduction
When modelling fluidised bed gasification of biomass, one of the most difficult tasks is to
predict the concentrations of methane and higher hydrocarbons. This aspect is of importance,
as these compounds tend to be the most important species with respect to energy content.
Additionally, as these compounds are energy rich, inaccuracies in the predictions of these
may cause large errors in temperatures; errors that in turn strongly will influence reaction rate
computations.
The methane content in the product gas during fluidised bed gasification of biomass will
normally not originate from the following methane generating reactions.
C(s) + 2H2(g) <———> CH4(g)
R1
CO(g) + 3H2(g) <———> CH4(g) + H2O(g)
R2
The main reasons for this are the prevailing relatively low temperatures during fluidised bed
biomass gasification (800 - 900 °C). These low temperatures will hamper reaction rates. The
methane concentrations tend to be (much) higher than what the equilibrium for the above
reactions (R1 and R2) suggest [1-5]. The main reason for this is that methane instead
originates from the tar decomposition chains, in which the higher hydrocarbons are
continuously and sequentially decomposed.
2. Experimental
The above implies that for being able to model and predict the concentrations of methane and
the higher hydrocarbons during fluidised bed gasification of biomass must empirical or semiempirical expressions be employed [5-8]. An example of an empirical expression is the
following Eq. 1 with Eq. 2. It has been derived earlier at KTH and used internally only.
3960
)*P0.146
T
CH4
With: CO + CO + CH 
2
4
= 0.0052*exp(
Eq. 1
Eq. 2
 = mole fraction of methane relative to total methane, carbon monoxide and carbon dioxide
(-)
P = pressure (atm)
T = temperature (K)
At KTH have lately a large number of bubbling fluidised bed tests been carried through. The
tests have been atmospheric as well as pressured and with air as well as with oxygen/steam.
These tests have now been analysed more in detail with the aim of deriving a refined
empirical methane correlation. More detailed information about the respective rigs and
experimental procedures may be found elsewhere [9].
In doing the review 89 tests were identified as relevant. Analysis of results gave that methane
concentrations, as , tend to vary between 0.10 and 0.17 as shown in Figure 1 below (*).
0.5
0.45
0.4
0.35
alfa (-)
0.3
0.25
0.2
0.15
0.1
0.05
0
0
10
20
30
40
50
60
70
80
90
Test #
Figure 1:  for reviewed tests (*) and  following Eq. 1 with 2 above (+)
For the 89 tests the average  = 0.137 (-) with the standard deviation  = 0.030 (-). It may
also be concluded from Figure 1 that the empirical Eq. 1 with Eq. 2 seems to overestimate .
It was also difficult to identify or sense any pattern or linkage between the methane
concentrationand the pressure and/or temperature as seen in the following Figure 2.
16
14
14
12
12
Methane concentration (%)
Methane concentration (%)
16
10
8
6
10
8
6
4
4
2
2
0
600
700
800
Temperature (°C)
900
1000
0
0
5
10
Pressue (atm)
15
Figure 2: Methane concentration (%-water free) versus temperature (left) and pressure (right)
One overall conclusion of the analysis is thus that the predicting powers of Eq. 1 with Eq. 2,
or any other empirical correlation that include temperature and pressure only, are limited.
Through further analysis of the test data was it however possible to correlate the methane
concentrations to those of carbon monoxide, carbon dioxide and hydrogen following:
CCH4 = 0.19CCO + 0.087CCO2 + 0.090CH2
Eq. 3
CCO = carbon monoxide concentration (%-water free)
CCO2 = carbon dioxide concentration (%-water free)
CH2 = hydrogen concentration (%-water free)
Figure 3 below gives the predicted methane concentrations versus the methane concentrations
following Eq. 3 for the tests reviewed.
16
Predicted methane concentration (%)
14
12
10
8
6
4
2
0
0
2
4
6
8
10
Methane concentration (%)
12
14
16
Figure 3: Predicted, following Eq. 3, versus experimental methane concentrations
Further was it possible to correlate the concentrations of the higher gaseous hydrocarbons
(C2+ and BTX hydrocarbons) lumped together as the pseudo-hydrocarbon "C3H4.5" to those of
methane. The composition of this pseudo-compound "C3H4.5" was derived from the test data
as an average composition of the gas phase hydrocarbons, excluding methane. Via a straight
line regression fit was the following equation derived:
CC3H4.5 = 0.65 + 0.27CCH4
Eq. 4
CC3H4.5 = concentration of the pseudo-hydrocarbon "C3H4.5" (%-water free)
CCH4 = concentration of methane (%-water free)
Figure 4 below gives the concentration of these gaseous higher hydrocarbons, as "C3H4.5",
versus the methane concentration and the resulting straight-line regression fit following Eq. 4.
Thus via Eq. 4 above may the concentrations of the gaseous higher hydrocarbons, expressed
as the pseudo-hydrocarbon "C3H4.5", be estimated from that of methane.
6
5
5
4
3
Regression fit
2
1
0
Predicted "C3H4.5" concentration (%)
Experimental "C3H4.5" concentration (%)
6
4
3
2
1
0
5
10
15
Methane concentration (%)
0
0
2
4
6
Experimental "C3H4.5" concentration (%)
Figure 4: Experimental concentrations of "C3H4.5" versus that of methane (left) and predicted versus
experimental concentrations of "C3H4.5" (right)
3. Results
A bubbling fluidised bed gasification product gas composition (carbon monoxide, carbon
dioxide, water, hydrogen, nitrogen, methane and higher gaseous hydrocarbons as "C3H4.5")
and the corresponding temperature, adding up to eight unknowns, may now be predicted. This
by solving the (independent) balances for carbon, nitrogen, hydrogen, oxygen and heat
respectively and by assuming that the homogenous water gas shift reaction (R3) will be at
equilibrium.
CO(g) + H2O (g) <———> CO2(g) + H2(g)
R3
For solving the system are thus two more (independent) equations called for. These may be
found by setting  = 0.137 or, alternatively, by assuming the methane concentration to be
given by Eq. 3 and the gaseous higher hydrocarbons, "C3H4.5" via the methane concentration
following Eq. 4. The elementary formula for the biomass may be written CH1.4O0.6 and the
heat content 20.6 (HHV, Dry and Ash Free) or 19.3 MJ/Kg (LHV, DAF). In the heat balance
may the heat of combustion of the pseudo-hydrocarbon "C3H4.5" be set to 1900 (kJ/mole).
The following Figures 4 and 5 exemplify some of the modelling results when biomass is
gasified with air. In the example is the temperature of the in-going air 25 °C and the moisture
content of the biomass is 25 %.
Figure 5: Product gas composition versus temperature (left) and temperature versus air-to-fuel ratio
(right)
Figure 6: Energy content of product gas versus temperature
4. References
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Progress in Energy and Combustion Science, In Press, Corrected Proof, Available online 17
March 2010.
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Gasifaction of biomass the consequences of equilibrium. 17th International Fluidized bed
combustion conference, Jacksonville, Florida, USA 2003.
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constraints on the performance of the conversion process. Bioresource Technology 2008;
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1985:923-936.
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[8] van der Meijden C.M., van der Drift A., Vreugdenhil B.J., Experimental results from the
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2001.