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Ammonia is paid special attention as renewable energy carrier [1-3], because it offers advantages in generation, transportation and utilization. Haber-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 . 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 . 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 , 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).
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