(549b) Performance of Ammonia - Natural Gas Co-Fired Gas Turbine for Power Generation

Performance
of Ammonia - Natural Gas Co-fired Gas Turbine for Power Generation

Ammonia
is paid special attention as renewable energy carrier [1-3], because it offers advantages
in generation, transportation and utilization. Harbor-Bosch method is already
established as ammonia generation method; large amount of ammonia is already
used as fertilizer and chemical raw material. Ammonia can be liquefied at room
temperature.  Its transport and storage system are already established. Ammonia
is cheaper to transport than hydrogen. Ammonia can be used as carbon-free fuel
in internal combustion engines as alternative to conventional hydrocarbon fuels.
However, it has different combustion characteristics. For example, the nitrogen
atom contained in the ammonia molecule, causes high NOx emission [3]. It might
be difficult to achieve stable combustion when fueling ammonia to an internal
combustion engine due to low laminar burning velocity. It also might cause
large emission of unburnt components [4]. Many efforts have been devoted to
overcome these shortcomings [5-7]. Especially, Iki et al. demonstrated the successful
power generation by an ammonia-fueled 50 kWe micro gas-turbine for the first
time [8, 9]. Performance of the gas turbine revealed a combustion efficiency of
89-96%, when only unburned ammonia is accounted for; NOx emission was above 1000
ppm@16%O₂. In the present study, demonstration of ammonia – natural
gas co-firing in a larger gas turbine is examined. Strategies of low NOx
combustion reported in past studies [10-12], are also adopted in engine testing.

For
the demonstration, IM270, a simple cycle gas turbine manufactured by IHI
Corporation [13], was used. The test system consists of Selective Catalytic
Reduction (SCR) unit, natural-gas compressor and ammonia supply unit. The fuel
supply unit first pressurizes ammonia to 2 MPaG and then gasifies it in a steam
vaporizer, before releasing it to the combustor. In engine testing, the gas
turbine is first started and then power is increased up to 2 MWe power
generation output firing natural gas, before ammonia is supplied to the
combustor. Ammonia supply to the engine is measured in terms of the heat input
ratio of ammonia to total fuel. This ratio is called "ammonia mixing ratio" in
this study. Operation of the gas turbine engine turned out to be stable in the
whole range of ammonia mixing ratios from zero to 20%.

Figures
1 and 2 show the performance of the gas turbine engine. It is seen that CO₂
concentration at turbine outlet monotonously decreases as ammonia mixing ratio is
increased. By increasing the ammonia mixing ratio from zero to 20% in LHV, CO₂
concentration is reduced from 3.1% to 2.5%, i.e. by 0.6%. It shows that ammonia
supplied to the gas turbine combustor is converted to power, so that the amount
of natural gas required for 2 MWe power generation is decreased. It is also
found that CO and unburnt ammonia concentration at the turbine outlet are lower
than the detection limit of the measuring equipment. As ammonia mixing ratio is
increased, NOx concentration at the turbine outlet first drastically increases up
to a mixing ratio of 5%, then, remains constant until it reaches 20%. NOx
emission is 287 ppm@16%O₂ at ammonia mixing ratio of 20%, which is
much higher compared to typical natural-gas fired gas turbines. However, it is
shown that NOx emission can be reduced below 6 ppm@16%O₂ by the SCR
unit. Combustion efficiency is above 99.8% for all test conditions. It is to be
noted that in the evaluation of combustion efficiency, loss of effective
calorific value due to NOx emission is accounted for in addition to CO and
unburnt ammonia emission. It shows that amount of NOx emission mentioned above
does not have a strong impact on combustion efficiency. It is also found that generator-end
efficiency takes smallest value at ammonia mixing ratio of 5%, then
monotonically increases with increasing ammonia mixing ratio. The decrease of
generator-end efficiency at ammonia mixing ratio of 5% is due to rapid increase
of NOx emission. Although the impact of NOx emission on combustion efficiency
is small, rapid increase of NOx emission, which means loss of effective
calorific value, causes a loss of 0.6% in generator-end efficiency. On the
other hand, generator-end efficiency is increased when ammonia mixing ratio is
increased above 5%. This result is believed to be caused by the increase of gas
volume, when fuel is changed from natural gas to ammonia. As ammonia mixing
ratio is increased, total gas volume passing through the turbine increases,
which leads to increased workloads in the turbine. Then, total fuel heat input
required to maintain 2 MWe output is expected to become smaller. For more
details, energy balances of the gas turbine engine cycle should be investigated.

First
shot test results show that ammonia can be used as gas turbine fuel in a 2 MWe
gas turbine engine. However, reduction of NOx emission is important for
reducing running costs, which are increased by the ammonia used in the SCR
unit. Therefore, further research is required to develop a low NOx combustor.

This
work was supported by Council for Science, Technology and Innovation (CSTI),
Cross-ministerial Strategic Innovation Promotion Program (SIP), "Energy
Carriers" (Funding agency: JST).

Fig. 1
Relationship
among ammonia mixing ratio, generator output and CO₂ emission at
turbine outlet.

Fig. 2
Relationship among ammonia mixing ratio, NOx emission and normalized
generator-end efficiency.

Reference

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