Energy and Power Engineering, 2009, 28-37
doi:10.4236/epe.2009.11005 Published Online August 2009 (
Copyright © 2009 SciRes EPE
Effects of Lower Heat Value Fuel on the Operations of
Micro-Gas Turbine
Aiguo LIU, Yiwu WENG
Key Laboratory of Machinery and Power Engineering of Education Ministry, Shanghai Jiao Tong University, Shanghai, China
Abstract: The characteristics of fuel from biomass, coal and some waste materials are lower heat value and
different compositions. The lower heat value fuel (LHVF) can be used on power engine such as boiler, gas
engine and gas turbine. Some laboratory and pilot work have been done, but the work done on micro-gas tur-
bine is still limited. The characteristics of LHVF can cause the operations change of micro-gas turbine de-
signed for nature gas. Some possible adjustment and modification methods were mentioned for the use of
LHVF on micro-gas turbine. One kind of representative LHVF was chosen and the operations of micro-gas
turbine were analyzed. The temperature field and the non-uniformity scale of temperature distribution of
combustor were calculated using FLUENT. The feasibility of different adjustment and modification methods
were analyzed according to the efficiency, output power and the non-uniformity scale of temperature distri-
Keywords: lower heat value fuel, micro-gas turbine, operations
1 Introduction
The distribution of LHVF from biomass, coal and waste
materials is wide and the energy reserves are huge [1].
Effective use of the LHVF is becoming an attractive pro-
ject, and much work is being done in this field. The
characteristics of LHVF are lower heat value and differ-
ent combustible compositions compared with nature gas.
Heat values of the fuel gases depend on the process, but
are typically one-tenth to one-half that of natural gas [2].
It will be different for the use of LHVF compared with
traditional fuel according to its characteristics, so some
different methods have ever been mentioned for the use
of low heat value fuel such as catalytic combustion [3-5].
Efficient conversion of LHVF to electrical power can be
accomplished by gas turbines, preferably in combined
cycle mode, where thermal efficiencies can be greater
than 65%. Simple open cycle, high pressure ratio ma-
chines can achieve efficiencies greater than 40% and
form the basis for Integrated Gasification Combined Cy-
cles [6]. Usually the combustion chamber of gas turbine
was designed for higher heat value fuel, and some prob-
lems will appear when using LHVF as fuel. Catalytic
combustion chamber can take the place of traditional one
for the LHVF, but some defects will appear such as
higher pressure and loss, slow reaction rate and so on. So
the traditional combustion chamber is still important for
the use of LHVF.
Primary issue for the gas turbine combustor when us-
ing LHVF is its large volumetric flows. The operations
of micro-gas turbine will be changed and even stop work.
The gas turbine should be adjusted and even modified to
make the micro-gas turbine work smoothly. In this paper
the effects of LHVF on the micro-gas turbine are firstly
discussed, and then some possible methods of adjust-
ment and modification are mentioned. The effects of
mentioned methods to the operations of micro-gas tur-
bine were presented and discussed. The feasibility was
also discussed. At last the temperature field of combus-
tion chamber was presented. The maximum temperature,
average temperature, and non-uniformity coefficient at
the outlet of combustor were calculated for the judgment
Copyright © 2009 SciRes EPE
of the feasibility.
2 Model Description
2.1 LHVF Model
The combustible components and heat value of LHVF
are different since they are from different way such as
biomass gasification, blast furnace tar and the coal mine
ventilation air [1][7]. The compositions of 3 representa-
tive kinds of LHVF were shown in Table 1. The main
combustible components from biomass gasification and
blast furnace tar are hydrogen and carbon monoxide and
in the coal mine ventilation is methane. But the effects of
LHVF on the operations of micro-gas turbine are the
same, so we choose a representative low heat value fuel
from biomass gasification as an example to analyze. The
LHVF includes different combustible components and
the fuel is deal as composite variables. The thermody-
namic property of gas is calculated according to the cal-
culation manner of mixed fuel with incombustible com-
ponent [8].
2.2 Micro-Gas Turbine Model
All of the calculations are based on the modeling of the
micro-gas turbine C30 from CAPSTONE using the
software of Matlab/Simulink [9]. The gas turbine is a
single-shaft micro-gas turbine equipped with centrifugal
compressor, radial turbine, combustion chamber and
recuperator. The design compressor pressure ratio is 3.2
and the turbine inlet temperature (TIT) is 1 173K. The
design mass flow of 0.31 kg s-1 is assumed to produce
roughly 30 kW power with an efficiency of 26% at ISO
conditions. This single shaft gas turbine is capable of
running at different shaft speed which generates higher
The modeling of micro-gas turbine is possible by util-
izing real steady state engine performance data [10]. In
gas turbine cycles, the changed relationship between
mass flow and pressure will cause a change in the opera-
tion point and efficiency. The features of a compressor
can be described as functions of pressure ratio
C, re-
duced flow /GT P
, reduced speed /nT
and effi-
ciency ηC. Performance maps are introduced for deter-
mination of the pressure and efficiency as a function of
mass flow and shaft speed for the description of com-
pressor. Figure 1 shows the performance map of com-
pressor. It is assumed that off-design thermodynamic and
flow processes are characterized by a continuous pro-
gression along the steady-state performance curves.
The turbine model can be processed using a similar
In the combustion chamber, the fuels is burned away
which increases the temperature of the gas. The follow-
ing reactions are considered in calculating the exit gas
temperature of the combustor:
1/2 CO
 (1)
Table 1. Components in different low heat value fuel
Composition / %Biomass gasBlast furnace tar Coal mine
ventilation air
CO2 13.0 18 0
O2 1.65 0 20.79
CO 21.4 26 0
H2 12.2 4 0
CH4 1.87 2 1
N2 49.88 60 78.21
Figure 1. Equivalence circulates curve of compressor
Copyright © 2009 SciRes EPE
422 2
CHOCOH OQ (3)
Assuming that the process is adiabatic, the enthalpy of
the reactants with combustion efficiency taken into ac-
count would be equal to the enthalpy of the products.
Knowing the temperature of the reactants, the product
temperature T2 can be calculated by iteration as the
properties of each product gas are temperature- depend-
CO HCHcombipm
hQQ Qn cdT
 
where the
h is the enthalpy change of reactions from
the original status to the standard status, the combustion
comb was set conservatively at 98%, though it
can be as high as 99.5%. i represents each gas compo-
sition of the product.
The schematic figure of the recuperator is shown in
Figure 2 Setting p2
h4 as the state variables
we can get the following equations based on the mass
and energy balance [11][12].
22 11
(1/ )
ht p
mhmh q
dtVc R
112 2222221
ht p
mhm hqhh Rcmm
dtVR c
 
44 33
(1/ )
cl p
mh mh q
dtVc R
334444 4443
cl p
mhmh qh hRcm m
dtVR c
 
where qh,, qc are the heat transfer between fluid (hot and
cold) and the wall of the recuperator.
 (9)
 (10)
And the12
12 2
, 34
34 2
are the average
value of inlet and outlet temperature respectively.
Tm is the average wall temperature between the hot
and cold side gas.
pm m
dT qq
dtC M
The power output from the gas turbine is obtained by
using the following equation:
 (12)
where ηgen is the generator efficiency; ηT is the turbine
mechanical efficiency; Wt ,Wc and Waux are the turbine
power, compressor power and auxiliary power respec-
2.3 CFD Model of Combustion Chamber
The temperature field in the combustor will be changed;
especially the temperature field of combustor outlet will
have direct effect on the safety of turbine. The main
analysis method of temperature field in combustor is
CFD. Firstly the model was built by the software of
PRO/E and the plot of gridding is completed by Gambit
using non-structure gridding, the number of gridding is
223271. The plot of gridding is shown in Figure 3. And
Figure 2. Schematic figure of heat transfer in the recuperator
Figure 3. Gridding of annular combustor
Copyright © 2009 SciRes EPE
then the model was solved by the software of FLUENT.
2.3.1 Flux Control Equation
The flux control equation is N-S equation, the turbulence
in combustor using double equation model. The
3D flux N-S equation in pole coordinate is as follows:
1[()()( )]
ru rvw
rx r
rxx rrrr
 
 
 
 
 
2.3.2 Combustion Model
The actual combustion process is the interaction of tur-
bulence and chemistry reaction, the chemistry reaction
velocity is strong nonlinear and strong stiff. Usual chem-
istry reaction mechanism includes tens of composition
and hundreds of base reaction and the difference of reac-
tion time is large. So the quantity of calculation and
storage is very large in the solution of actual problem.
The different chemistry dynamics solution methods have
been used aims at the different combustion phenomena
in FLUENT. The model used in this calculation is the
Species Transport model. This model is usually used in
the premixed combustion, part premixed combustion and
diffusion combustion. The chemistry reaction is usually
simplified as single-step reaction. The solution of the
composition transport equation and getting the
time-averaged mass fraction of each composition is as
() ()
The reaction source item of composition j is the pro-
duction rate of composition j in all the reactions:
RR (15)
In the formula, the reaction velocity of composition j
in reaction k can be solved with the Arrhenius formula.
3 Results and Discussion
Figure 4. Relation of output power and efficiency to fuel flux
3.1 Effects of LHVF on the Micro-Gas Turbine
The energy provided by LHVF is less than the traditional
fuel when the same fuel/air ratio was used. The fuel flux
should be increased to improve the turbine inlet tem-
perature. The relation of output power and efficiency
with the fuel flux was shown in Figure 4 with the as-
sumption of constant air flow.
The turbine inlet temperature (TIT) will increase with
the increase of fuel flux which causes the increase of
output power and efficiency. The TIT will not be the
same with design value when the output power is the
same with design value, so two special conditions have
been presented according to the calculation. The first one
was the output power of micro-gas turbine attained the
design value (case2), the second one was the turbine
inlet temperature attained the design value (case3). The
calculation results were shown in Figure 5 and were
compared with the design value (case1).
Figure 5. 3 different conditions
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From the calculation results in Figure 5 we can see the
mass flow of fuel will be more than 14 times for case 2
and 16 times for case 3 compared with case1. The effi-
ciency of case 2 and case 3 are both lower than case1and
the output power is higher in case 3. The decrease of
efficiency is influenced by both the increase of the fuel
flux entering combustor without being preheated and the
decrease of turbine efficiency. About the increase of
output power in case 3 is due to the increase of gas pass-
ing through turbine.
The mass flow of low heat value fuel will be more 10
times than the design value. There is flux difference be-
tween compressor and turbine, so the compressor and
turbine can not match together. Some work should be
done on the micro-gas turbine to make the compressor
and turbine match again.
3.2 Adjustment and Modification of Micro-Gas
Some experience has been gained and adopted in heavy
gas turbine for LHVF [13-16]. The heavy gas turbine can
be adjusted in large scale to fit the operations of LHVF.
For example, the flow ability of turbine can be increased
10%, the compression ratio can be increased 12% and
the output power can be increase 20% for the 9F model
heavy gas turbine from GE [17]. At the same time there
exist variable-area nozzles which can be adjusted to de-
crease the flux of compressor. The adjustment methods
used in heavy gas turbine do not all fit to the micro-gas
turbine because the operations of micro-gas turbine are
different from heavy gas turbine [18]. Some possible
adjustment and modification methods were mentioned in
this chapter and the feasibility was discussed.
3.2.1 Adjustment of Micro-Gas Turbine
1) Pressure ratio and TIT
The adjustment of operation parameters was viewed as
the simplest method to match the compressor and turbine
according to the experience of heavy gas turbine. The
pressure ratio of compressor should be increased as high
as possible because which can not only increase the flow
ability of turbine but also decrease the flow ability of
compressor. But the compression ratio can not be in-
creased very high, so the turbine inlet temperature should
be decreased at the same time for the purpose of match-
ing. The adjustment process should be: firstly the com-
pression ratio was increased as high as possible and then
decreasing the TIT until the matching was achieved. The
operation parameters were shown as case4 in Table 2 for
the adjustment of pressure ratio and TIT. From the re-
sults it was found that the efficiency and output power
both decreased and the compressor has been near the
surge boundary. So this adjustment method is inapplica-
ble to the micro-gas turbine in the view of efficiency and
output power.
Table 2. Four operation cases of micro-gas turbine in different condition
Case1 Case4 Case5 Case6
Rotation speed N N 0.95N 0.9N
Compression ratio 3.2 3.257 3.09 2.634
Outlet flux/ kg/s 0.31 0.288 0.2782 0.2688
Outlet temperature / K 444 446.8 439.6 418
Power /kW 47.1 44.42 40.8 33.33
Outlet pressure/ kPa 319.2976 324.98 308.3 262.8
Efficiency 0.818 0.82 0.8176 0.8107
Fuel flux / kg /s 0.002368 0.02527 0.03224 0.03188
Inlet temperature / K 839 662.7 807.9 886.2
Outlet pressure / kPa 309.7 316.7 300.6 255.6
Outlet temperature / K 1113 888.3 1072 1144
Outlet temperature / K 912 718 877.5 964.1
Outlet flux / kg/s 0.312368 0.3164 0.3135 0.3037
Power /kW 77.1 62.61 70.72 63.35
Efficiency 0.8164 0.8127 0.8072 0.8148
Output power / kW 30 17.69 30 30
turbine Efficiency / % 25.35 17.89 23.73 24.07
Copyright © 2009 SciRes EPE
2) Rotation speed
The matching between compressor and turbine can
also be realized by adjusting the compressor itself to
reduce the compressor air flux. There are several ways to
adjust the compressor [19]:
Outlet throttle
Inlet throttle
Variable rotation speed.
Whichever method will result in the increase of power
consumed by compressor, and the variable rotation speed
adjustment is the best way in the view of power con-
The variable rotation speed adjustment was chosen
and the relationship of efficiency and output power with
compression ratio was shown in Figure 6 and Figure 7
when the rotation speed is 0.95N and 0.9N (N is the de-
sign rotation speed value) respectively. The operation
parameters are listed in Table 3 as case5 and case6 while
keeping the output power as design value.
The power generated by turbine will decrease when
the rotation speed decreases, but the power consumed by
compressor also decreases at the same time. So the out-
put of micro-gas turbine also can achieve the design
value, but the efficiency will decrease due to the de-
crease of turbine and compressor efficiency. When the
rotation speed is 0.9N the turbine inlet temperature was
higher than the design value for the design output power
which can result exceeding temperature.
Figure 6. Efficiency and power at 0.95N
Figure 7. Efficiency and power at 0.9N
The matching between compressor and turbine can be
achieved by the adjustment of the operation parameters
of micro-gas turbine. But the adjustment is not good
enough as it will cause the lower efficiency, exceeding
temperature and even the danger of compressor surge.
3.2.2 Modification of Compressor and Turbine
In general the micro-gas turbine does not adopt the
technology of variable-area nozzle or stationary blade
[20], the vane of compressor is very thin and the modifi-
cation of variable-area nozzle or stationary blade will be
very difficult. But the advantages will be great in the
view of the operations if the compressor and turbine can
be modified.
Some theoretical calculations have been done on the
supposing that the modification of compressor and tur-
bine can be realized. The calculation started from the
operation point in case4. Firstly was the modification of
compressor. Starting from the operation point in case4
and then decreasing the flux of air while keeping the
compression ratio unchanged. The turbine inlet tempera-
ture will increase due to the increase of fuel flux. The
inlet temperature of turbine attained the design value and
the output of micro-gas turbine was 35.02kW when the
mass flow of air is 0.965 design value. For the purpose
of protecting turbine the turbine inlet temperature should
be kept constant after this point. This time the turbine
inlet temperature was kept unchanged and the decrease
of the compressor flux would result the decrease of
Copyright © 2009 SciRes EPE
power from turbine and micro-gas turbine until the out-
put power reached the design value. The relation of effi-
ciency and output power of micro-gas turbine with the
decrease of mass flow of compressor was shown in Fig-
ure 8.
The operation can also be improved if the flux ability
of turbine can be increased with modification. Changing
the setting angle or height of stationary blade can in-
crease the flux area of turbine and improve the opera-
tions. The calculation point also started from the opera-
tion point in case4. The turbine inlet temperature would
increase with the increase of turbine flux ability which
caused the increase of output power and efficiency until
the turbine inlet temperature attained the design value.
After this point the increase of flux ability of turbine will
cause the decrease of the turbine inlet pressure and the
Figure 8. Effect of compressor flux decrease on efficiency and
Figure 9. Effect of turbine flux increase on efficiency and power
compression ratio of compressor. The power and effi-
ciency still increased slowly because the mass flow of
gas keeps increasing at the highest temperature. The re-
lation of power and efficiency with the increase of flux
ability of turbine was shown in Figure 9.
The operations of micro-gas turbine were satisfactory
with the modification of compressor and turbine. But the
modification of compressor or turbine can cause the
output of micro-gas turbine beyond the design value
which maybe has effect on the structure of micro-gas
3.3 Temperature Field of Combustion Chamber
The matching problem can be solved by the methods
mentioned above. But the temperature field in combustor
will changed as the changed inlet conditions. The tem-
perature fields in four different conditions were calcu-
lated which included the design condition, speed adjust-
ment and the modification of compressor and turbine.
The main parameters for temperature field were
maximum temperature, average temperature and non-
uniformity coefficient which have effects on the safety of
turbine. Figure 10(a) is the temperature distribution
characteristic of combustor outlet and axis direction at
design condition and it was also the example compared
by the other conditions. The maximum temperature,
minimum temperature, average temperature and non-
uniformity coefficient were shown in Table 3. The non-
uniformity coefficient At should be lower than 10% for
the safety of turbine.
The temperature field and non-uniformity coefficient
were shown in Figure 10(b) and Table 3 when the rota-
tion speed was 0.9N and output power was design value.
In the axis direction the high-temperature area become
larger and at the combustion chamber outlet the maxi-
mum temperature and non-uniformity coefficient ex-
ceeded the safety margin. So the speed adjustment
method was not feasible in the view of turbine safety.
The computation results shown in Figure 10(c) and
10(d) were the conditions when modifying the compres-
Copyright © 2009 SciRes EPE
Table 3. Combustion chamber temperature characteristics
NO. T max T
min T
ave At%
a 1179 1080 1119.2 5.4
b 1357 915 1092.3 24.3
c 1148 976 1063.2 8
d 1170 898 1072 9.2
Figure 10. Outlet and axial temperature field
sor and turbine respectively. In the axis direction the
high-temperature area was larger compared with a. The
maximum temperature and non-uniformity coefficient at
the turbine outlet can meet the safety need. In the view
of temperature field the modification of compressor and
turbine was feasible.
Copyright © 2009 SciRes EPE
4 Conclusions
The problems caused by LHVF and some possible ad-
justment and modification methods have been presented
and discussed in this paper. The operations and feasibil-
ities using these methods on micro-gas turbine have been
discussed according to the efficiency, output power and
temperature field in combustion chamber.
1) Adjustment of operation parameters. The efficiency
and output power will decrease when the operation pa-
rameters such as compression ratio, turbine inlet tem-
perature are adjusted. So the adjustment of compression
ratio and turbine inlet temperature is not feasible in the
view of output, efficiency and safety. There are also
some problems when adjusting the compressor speed.
The matching problem can be solved by the speed ad-
justment but some additional problems will appear be-
cause the micro-gas turbine is coaxial. At the same time
the temperature field distribution is uneven at the com-
bustion chamber outlet which can cause the damage of
turbine vane. So the adjustment of micro-gas turbine is
not feasible to solve the problems caused by LHVF as
heavy gas turbine.
2) The modification of compressor and turbine can
solve the matching problem and the efficiency is high
enough. The maximum temperature and the
non-uniformity coefficient of combustion chamber are
both in the limitation. So the modification to the com-
pressor and turbine is a good method.
In this paper we only discuss the problem of matching
and temperature field, but another problem is ignition
and combustion stability. The fuel velocity will be in-
creased which can cause the problem of ignition and
combustion stability in combustion chamber. Some fur-
ther studies about the fuel nozzle and combustion cham-
ber should be made as heavy gas turbine.
T0 atmosphere temperature (k)
P0 atmosphere pressure (Mpa)
Tex gas turbine exhaust temperature (k)
T3 turbine inlet temperature (k)
Gg turbine gas flux (kg/s)
P gas turbine output (w)
N micro-gas turbine design speed (96000r/m)
Eff efficiency (%)
the general variable, it can represent the veloc-
ity of , turbulent kinetic energy, Tur-
bulence Dissipation Rates, enthalpy, turbu-
lence stress term and Duo-mixture fraction
turbulence transportation coefficient
s Source term
Tmax combustion chamber outlet max temperature
Tmin combustion chamber outlet min temperature (K)
Tave combustion chamber outlet average temperature
At combustion chamber non-uniformity coefficient
We would like to acknowledge the National Science
Foundation of China who provided the funding for this
work. This work was partially supported by Shanghai
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