ISTP-16, 2005, PRAGUE

16TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENAEXPERIMENTAL EVALUATION ON LOW-HEATINGVALUE FUEL ACCEPTABILITY OF MICRO GASTURBINE SYSTEM OPERATION
Daisuke Yamashita*, Keisuke Kuwabara*,
Kazuya Tatsumi* and Kazuyoshi Nakabe**Dept. of Mech. Eng., Osaka Prefecture University
Corresponding author: e-mail nakabe@me.osakafu-u.ac.jp,
phone +81-72-254-9224, fax +81-72-254-9904
Keywords: Micro gas turbine, Low-heating value fuel, System analysis, Thermal
efficiency, Exhaust gas analysis

Abstract
A micro gas turbine (MGT) system composed of
two automobile turbochargers was operatedusing low-heating value fuels to examine towhat extent the system can accept suchunfavorable fuels. Gasified biomass and thevitiated gas of the exhausts from fuel cell or fuelreformer are possible examples of low-heating
value fuels as the fuel alternatives for MGT. In
the present experimental study, the heating
value of the fuel was simulated by mixing
nitrogen into liquefied petroleum gas (LPG).
The turbine of the first turbocharger played arole as the gas generator turbine of the system,
while the one of the second turbocharger as thepower generator turbine. The load of the MGT
system was changed using the outlet valve
opening of the second turbocharger’scompressor. The system thermal efficiency wasdecreased almost linearly with an increase in
the mixing ratio of N2 to LPG. A drastic change
in the concentration of CO emission wasobtained at the mixing ratio of around 0.4. NOX
concentration was contrary decreased with themixing ratio. This trend could be related to thedecrease of flame temperature in thecombustion chamber, corresponding to thesensible heat of the mixed N2. The present MGTsystem, however, was eventually successful inoperating under each tested condition, where
the minimum heating-value of the simulated fuelwas approximately 0.43 of pure LPG.

1 Introduction
Compact heat engines of less than severaltens kW output can be a key device as well as
fuel cells (FCs) for the development of
distributed power supply systems [1]. A micro
gas turbine (MGT), in particular, is one of themost promising heat engines for these systems
since it has the advantages of relatively low
NOX emission, low mechanical vibration, small
number of mechanical parts and easy
maintenance, compared to reciprocating engines.
If various types of fuel can be used for MGT
operation, MGT may become more widely used
for distributed electric power supply system andalso make a contribution to the reduction offossil fuel consumption. One of the possible
reasons is that, for example, in local farms or
mountain-girt areas, renewable energy resourcesof biomass like excrement of domestic animals
or woody parts of tree-trimming can be used toyield carbon-neutral methane-based fuels. Suchfuels are mostly low-heating value fuels, butthey will have a potential to be alternate fuelsfor MGT. In such local areas, renewable energy
from wind and/or solar power stations also canbe utilized for directly generating electricity andreforming the biomass to hydrogen. If MGT
system is linked with the concurrent use of thepower stations, the fluctuation of the generated
electric power from the renewable energy1depending on weather conditions is greatly
improved [2].
FC is also well-known as a suitable andpromising distributed electric power supply
device, but low-temperature types of FC like
PEFC (Polymer Electrolyte FC), PAFC(Phosphoric Acid FC) and DMFC (DirectMethanol FC) inevitably need catalysts
including rare metals, like Platinum, forelectrode reactions. The other type of FC like
MCFC (Molten Carbonate FC) or SOFC (SolidOxide FC) have a merit that the electrodereactions occur without Platinum catalyst,
however, needs relatively high temperature
atmosphere, 600 ~ 900ºC. For the latter-type ofFC, an MGT can play an important role to keepa sufficient high temperature essential for theelectrode reaction of the FC and to compose aso-called hybrid co-generation system, which is
the most promising method to increase theoverall thermal efficiency of the system [3, 4].
In this case, a part of the exhaust gas from FC
can be effectively recycled and fed into MGT as
a part of the fuels.
Following the above-mentioned aspects ofvarious fuel sources for MGT operation, a wide
acceptability of fuel variation is essential for
MGT performance [5]. Large-scale gas turbinesystems operated at central electric power
stations require mostly LNG as their fuel. For
MGT systems, on the other hand, higher
acceptability of the fuel is a technologicalmatter of the utmost concern to free ourselvesfrom electric power supply systems based on
fossil fuels. Thus, it is very important toexamine to what extent an MGT system isapplicable to various types of fuels, in particular,
low-heating value fuels. The heating values of
the gasified biomass or FC exhaust are less than
1/4 ~ 1/10 of that of pure LNG or LPG. In thepresent study, the acceptability of MGT for such
low-heating value fuels is investigated byoperating a laboratory-based MGT system using
LPG diluted with nitrogen down to around ahalf of LPG’s heating value.
2 Experimental Apparatus, Methods andConditions

Fig. 1 MGT system flowchart.

Fig. 2 MGT Combustor.
Figure 1 shows the system flowchart of theMGT system. The gas turbine system is a twoaxial turbine system consisted of twoconventional automobile turbochargers, where
the first one plays a role as a gas compressor
and a gas-generating turbine (referred to as
GGT), and the other as a power-generating
turbine (PGT) and a loading compressor (LC) [5,6]. LC, which is a gas compressor of the second
turbocharger shaft-coupled to PGT, was used
for the load output instead of a shaft generatorof electric power. The load was controlled byadjusting an opening amount of a butterfly valve
mounted on the downstream side of LC in the2

EXPERIMENTAL EVALUATION ON LOW-HEATING VALUE FUELACCEPTABILITY OF MICRO GAS TURBINE SYSTEM OPERATION

present study. The rotational frequencies ofGGT and PGT were monitored using revolution
indicators. Gas temperature and pressure weremeasured using sheath-type thermocouples andpressure gauges, respectively, in eachmonitoring location indicated in Fig. 1. Also,
the concentrations of NOX, CO, CO2 and O2
emissions in the exhaust gas from the MGT
system were measured using a gas analyzer.
The low-heating value fuel was simulatedby diluting LPG with N2 using a gas mixer, andwas then supplied to a combustion chamber
without halting the operation of the MGT
system. The combustor was shown in Fig. 2which was equipped with a cone-type nonpremixed burner. Primary combustion air wassupplied just above the fuel nozzle tip to bemixed well with the fuel. Secondary dilution air
was discharged into the second half of thecombustor in order to suppress the generation ofNOX. The mass flow rate of the working gases,
the mixing ratio of N2 to LPG (RN) and the loadof LC were suitably adjusted to keep a stableself-running of the MGT system. The
experiment was carried out firstly under thecondition of feeding pure LPG to the combustor.
The amount of LC valve opening (DG) was set
at three different levels, i.e. DG = 100%, 66%
and 50%. For each case, the GGT inlet
temperature (TIT; T3) was varied from 650ºC to800ºC by controlling the mass flow rate of thefeeding LPG.
For the case of N2-diluted low-heating
value fuel, the amount of the valve opening wasfixed at DG = 100%. In this case, the MGT
system was started up by feeding pure LPG andwas operated until T3 reached a certain
temperature(referredtoas‘Startingtemperature’), i.e. 700ºC, 750ºC or 800ºC. Then,N2 was discharged and mixed with LPG in thegas mixer, adjusting RN in the range of 0 ~ 1.27.
Thus, the heating values of the fuels simulatedusing LPG-N2 mixture were correspondingly
changed in the range of 100% ~ 43% of pure
LPG.
The thermal efficiency of the MGT system
together with adiabatic efficiencies ofcompressor and turbine of each turbocharger
was estimated thermodynamically by assuming

a typical and simple Brayton cycle. Also, avirtual recuperator-type regenerative cycle
applied to the present MGT system wasnumerically analyzed in order to compare withother commercially-available regenerative MGTsystems or reciprocating engines, since thepresent MGT has no heat exchangers as acomponent of regenerative cycle. The flowchartof the analyzed regenerative gas turbine cycle isschematically illustrated in Fig. 3.

Fig. 3 Flowchart of MGT system with heatexchangers.
3 Results and Discussion
3.1 Performance of MGT using LPG only
The overall performance of the present
MGT system is first discussed in this section
under the operation of pure LPG fuel condition.
Figure 4 (a) shows a T-s indicated diagram ofthe system. The index numbers, 1 ~ 5, in Figs. 4(a) and (b) denote the thermodynamic states. InFig. 4 (a), the process 1 → 2 represents the nonisentropic compression process of the gas
compressor, which should be ideally anisentropic process of Brayton cycle. The process
2 → 3 represents constant pressure process atthe combustor. The processes 3 → 4 and 4 → 5
represent the expansion processes at two-axial
turbines, GGT and PGT, both of which werealso non-isentropic. At the state 3, the pressurein the combustor and the turbine inlettemperature of GGT were 0.23MPa and 805ºC,respectively, in this operation. At the state 5, theworking fluid was discharged to thesurroundings with the ambient pressure, P5 = P1.
3

(a) T-s indicated diagram(b) Expansion process of GGT ~ PGT.
Fig. 4 Brayton cycle for MGT system analysis.
The expansion processes 3 → 4 and 4 → 5from the combustor to PGT through GGT are
shown in Fig. 4 (b) in detail. In case that thedistancefrom
twoturbochargers
areconsiderably large and the exhaust manifolds ofboth turbochargers are exposed to the coolsurroundings, the temperature of the working
fluid in the system is remarkably decreased.
Then, the process 3 → 4 → 5 becomesapparently isentropic, that is, unrealisticallyadiabatic with the constant ambient pressure of
the state 5. If the system is sufficiently
thermally-insulated, it can be predicted that thestate 5 reaches the state 5’ along the isobaricline, 1 - 5. The dashed line in the figure
indicates one of the corrected data with taking acountermeasure for the heat leakage from theexhaust pipes.

Fig. 5 System thermal efficiency vs. TIT (withLPG only).
Figure 5 shows the relation between thesystem thermal efficiency η and TIT (= T3) for
each valve-opening amount DG. η was definedas the ratio of the PGT work output to the heatrelease quantity of the supplied fuel. Higherefficiency was obtained almost linearly as TIT
was increased, rarely dependent on the load.
The thermal efficiency of the present MGTsystem was 7.5% ~ 10.5% which is relatively
small in comparison with other commerciallyavailable MGTs since the present system wasconstructed as a simple Brayton cycle without
installing any regenerative heat exchangers inthe system itself. The output of the system wasfound to be ca. 10kW at the maximum.
The effect of regenerative cycle on theFig. 6

System thermal efficiency of MGT
with regenerative heat exchangers.
4

EXPERIMENTAL EVALUATION ON LOW-HEATING VALUE FUELACCEPTABILITY OF MICRO GAS TURBINE SYSTEM OPERATION

thermal efficiency was numerically estimatedusing HYSYS (AspenTech Japan Co., Ltd.). A
recuperator-type heat exchanger was installed
between hot gas flows at PGT exhaust and coldgas flows at the gas compressor outlet, shown in
Fig. 3 of the previous chapter. The heatexchange temperature effectiveness was
assumed to be 80% with no pressure loss
between the inlet and outlet. The obtained
results of the thermal efficiency are shown in
Fig. 6. Open symbols of triangle and circle,
respectively, indicate the simple Brayton cycle
and the regenerative one. In comparison withthe former cycle, the latter has an almost
twofold thermal efficiency, which was predicted
to be 28% at the maximum, similar to thepublished data of other MGT makers.
Figures 7 and 8 show respectively CO andNOX concentrations in the exhaust gas. COemission was decreased with an increase in TIT,

Fig. 7 CO concentrations vs. TIT.

Fig. 8 NOX concentrations vs. TIT.

Fig. 9 System thermal efficiency vs. TIT (withLPG-N2 mixture).
while NOX emission was gradually increased.
Both trends were not strongly dependent on theload.
3.2 Performance of MGT using LPG-N2mixture
Figure 9 shows the relationship between ηand T3 for the case of LPG-N2 mixture. As
mentioned in Chap. 2, the experiments using thelow-heating value fuels were conducted afterachieving the stable running at the fixed turbine
inlet temperature of the GGT (referred to as
Starting temperature) with the pure LPG feeding.
The solid symbols in Fig. 9 indicate the points
as the starting temperatures, from which eachmeasurement with LPG-N2 mixture was started.
The arrow drawn in the same figure indicates
the trajectory of the increase of the LPG-N2
mixing ratio. When the mass flow rate of thesupplied gaseous fuel (LPG+N2) was fixed, TIT
was decreased with an increase in the mixing
ratio, RN, which accompanies the decreases of
the combustion pressure P3 and also turbine
revolution numbers. These eventually resulted
in the deterioration of the overall thermal
efficiency of the system.
Figures 10 and 11 show the concentration
profiles of CO and NOX in the exhaust gas
emitted from PGT, respectively, against TIT.
The solid symbols in these figures indicate theabove-mentioned starting temperatures. As themixing ratio, RN, was increased, the TIT wascorrespondingly decreased. CO concentration
5Fig. 10 CO concentrations vs. TIT (with LPGN2 mixture).

Fig. 12 CO concentrations vs. RN (with LPGN2 mixture).

Fig. 11 NOX concentrations vs. TIT (withLPG-N2 mixture).

Fig. 13 NOX concentrations vs. RN (with LPGN2 mixture).

was correspondingly increased, while NOX
concentration was gradually decreased. The
both concentration profiles were re-plottedagainst the LPG-N2 mixing ratio, RN, as abscissa,shown in Figs. 12 and 13.
As is clearly shown in Fig. 12, COconcentration suddenly jumps up when theLPG-N2 mixing ratio exceeds 0.4 whichindicates an incomplete combustion. Contrary totheCOconcentration
profile,NOX
concentration was gradually decreased with anincrease in RN, as clearly shown in Fig. 13.
These trends can be explained with thefollowing reason. The increase of LPG-N2
mixing ratio inevitably reduces the combustion
heat release under the condition of constantmixture flow rate and requires larger sensible
heat for temperature rise of the mixture
including the inert component, N2. Thus, the gas

temperature in the combustor should bedecreased as RN is increased. For example,
under the condition of LPG-N2 mixing ratio
larger than 0.4 and the gas temperature of
1000K, the required heat for rising N2
temperature from 300K to 1000K is more than
10% of the supplied LPG heating value.
Consequently, this produces the decrease of
combustion pressure and the suppression of
local reactions. These trends should accompany
the increase of CO emission and the reductionof thermal NOX generation.
Nevertheless, the present MGT system waseventually successful in operating under eachtested condition and durable even for using thesimulated fuel of about a half heating value ofpure LPG without any modification of thecombustor design.
6EXPERIMENTAL EVALUATION ON LOW-HEATING VALUE FUELACCEPTABILITY OF MICRO GAS TURBINE SYSTEM OPERATION

4 Conclusions
A laboratory-based MGT was operated inthe present study with the experimental
conditions using low-heating value fuels whichwere simulated in terms of the dilution of LPG
with N2. Efficiencies at each system component
were calculated from the measured temperatureand pressure. Also, gas components of the MGT
exhaust were analyzed. The obtained results aresummarized as follows:• The experimentally-obtained overall thermal
efficiency of the present MGT system was7.5 ~ 10.5%. The validity of regenerative
heat exchanger was numerically predicted toimprove the thermal efficiency almosttwofold.
• Under the condition of a fixed mass flow rateof the supplied fuel, TIT was decreased withan increase in the LPG-N2 mixing ratio,
which results in the deterioration of theoverall system efficiency.
• The mixing of N2 significantly affected thecharacteristic of the gas emission. As theLPG-N2 mixing ratio was increased, NOXemission was decreased while CO wasincreased. A remarkable increase of CO wasobserved when the mixing ratio exceeded 0.4,
which is related to the incomplete
combustion occurring at the combustor.
• The present MGT system was successful inoperating at the fuel condition of heating
value less than 0.5 of LPG, which proved theMGT acceptability to such low-heating value
fuels without any modification of thecombustor.

[2] Mie Prefecture, Report on Research & Developmentof Combined Electric Power Generation Systems
Using Renewable Energy, JST report, 2003.
[3] Massardo, A. F., McDonald, C. F. and Korakianitis,T., Microturbine/Fuel-Cell Coupling for HighEfficiency Electrical-Power Generation. Proc. ASME
Turbo Expo 2000, 2000-GT-0175, pp. 1-8, 2000.
[4] Buhre, B. J. and Andries, J., Biomass-based, Smallscale, Distributed Generation of Electricity and Heat
Using Integrated Gas Turbine - Fuel Cell Systems.
ibid., 2000-GT-0022, pp. 1-7, 2000.
[5] Yamashita, D., Tatsumi, K. and Nakabe, K.,
Performance Evaluation of Micro Gas Turbine
Operated Using Low-Heating Value Fuel, Proc. 1stInternational Forum on Heat Transfer, pp. 87-88,2004.
[6] Mori, H., Tsujimoto, Y. and Yokota, K.,Development of Turbo-Burner System and its Use inEducation, Proc. ISROMAC-8, Vol. 1, pp. 364-370,
2000.

Acknowledgement
We’d like to express our gratitude to Prof.
Y. Tsujimoto of Osaka University and Prof. K.
Teshima of Okayama Science University for
their helpful advices.
References[1] Layne, A., Samuelsen, S., Williams, M. and Hoffman,P., Hybrid Heat Engines: The Power Generation
Systems of the Future. Proc. ASME Turbo Expo 2000,
2000-GT-0549, pp. 1-6, 2000.

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