*5.3. Gas Turbine*

The ammonia-fuelled gas turbine seems destined to become one of the key technologies in the sustainable energy economy of the future. In the 1960s, an early attempt was made to use ammonia as fuel in the gas turbine. However, due to nitrogen molecule in its structure, ammonia combustion is always associated with the formation of nitrogen oxides which exceed current standards [235,236]. Moreover, a longer residence time in the combustion chamber is required for ammonia to be completely combustible due to low laminar burning velocity, which makes it difficult to achieve stable combustion [16]. It also might cause an ammonia slip [237]. Therefore, ammonia-fuelled gas turbines were poorly studied in early development. However, the shift towards carbon-free alternative energy carriers has returned interest in ammonia, including the utilisation in power industries. Burning NH3 in turbines is the most promising direction of using ammonia as a carrier of surplus electricity generated from renewable energy to balance seasonal energy demand. Thus, many efforts have been devoted to overcoming these shortcomings.

Kobayashi et al. [238] have produced a review article covering the current ammonia combustion research and future directions. The study emphasised that the final product of ammonia combustion is not nitric oxides because the overall reaction is 4NH3 + 3O2 → 2N2 + 6H2O. This implies that the configuration of the parameters of the combustion system is crucial to perform the reaction according to this direction. Karabeyoglu et al. [239] developed a test rig and conducted a series of trials with a pre-burner system to partially crack ammonia into H2. The study revealed that ammonia combustion could be selfsustained when 10% cracking is applied. In 2014, Iki et al. [240] developed a 50 kWe micro gas-turbine system that enables a bi-fuel supply of kerosene and ammonia. The system was able to achieve over 25-kWe power generation by supplying about 10% heat from ammonia gas. In 2015, Iki et al. [241] were able to achieve 21 kWe by replacing the standard combustor with a prototype combustor. The gas turbine performance showed an efficiency of combustion up to 96% with NOx emission above 1000 ppm at 16% O2. Hayakawa et al. [242] investigated stretching limits for high-pressure flames and observed lower NO formation with higher mixture pressure. Okafor et al. [243], in their experiment and numerical calculations, concluded that predominant rate-limiting reactions in methane– ammonia flames are belonging to the ammonia oxidation path, which controls H and OH radicals. These radicals influence the burning velocity.

$$\text{NiO} + \text{NH}\_2 \rightarrow \text{NNH} + \text{OH} \tag{13}$$

$$\text{NH}\_2 + \text{O} \rightarrow \text{HNO} + \text{H} \tag{14}$$

$$\text{HNO} + \text{H} \rightarrow \text{NO} + \text{H}\_2 \tag{15}$$

The study also revealed that the NO concentration decreases when ammonia increases under rich conditions.

In 2016, Ito et al. [244] developed a gas-turbine combustion system with controlled emissions, which uses a mixture of NH3 and natural gas as the fuel. Combustion properties have been explored via the use of a swirl burner, generally employed in gas turbines, experimentally and numerically. Detailed compositions of the burner exhaust gas were measured under atmospheric pressure and lean fuel circumstances. The results show that with this system, the combustion efficiency above 97% can be achieved for an ammonia mixing ratio below 50%. The study also shows that with an increase of equivalency ratio, unburned species such as NH3, CO and THC decrease while NO and N2O emissions increase. The study concludes that low emissions and good combustion efficiency are difficult to obtain in a single-stage reactor.

In 2017, Onishi et al. [245] attempted to create novel ways for reducing NOx emissions while burning an ammonia-natural gas combination in a gas turbine combustor. The concept of low-emission combustion in two-stage combustion was examined numerically and experimentally. In the main zone, methane and ammonia are used as fuel. The secondary zone is then only supplied ammonia. The results indicated two methods for attaining low NOx combustion: rich-lean combustion and a combination of lean combustion and extra ammonia delivery. In the first technique, NOx is created only in the main zone when the fuel is abundant, and the burnt gas is diluted in the secondary zone by secondary air. As a result, primary zone NOx production dominates emission. A lean combustion state in the primary zone results in a low temperature and oxygen concentration in the secondary zone in the second approach. The NOx concentration at the combustor outlet is low as a result of these circumstances. These expected combustion qualities have been experimentally validated. The experimental findings showed that the NOx emission behaviour corresponded to the numerical results.

In 2018, Ito et al. [246] studied a mixture of ammonia and natural gas fuel in a 2 MWe gas turbine. Their results indicated that ammonia is suitable for use in large turbines. Before ammonia is fed to the combustor, the gas turbine power is raised up to 2 MWe using natural gas. The ammonia's heat input ratio to total fuel is used to calculate the ammonia feed to the engine. The result shows that the gas turbine engine's operation was shown to be steady across a wide variety of ammonia mixing ratios ranging from 0% to 20%. As the ratio of ammonia in the mixture increases, the concentration of NOx at the turbine outlet rises significantly up to 5% mixing ratio, then remains steady until it reaches 20%.
