１．Introduction

The supply of electricity from renewable energy sources is expanding. Renewable energy is an unstable source of power, and gas turbines, which have excellent load-following capability, are important as a regulating power source to balance the supply and demand of electricity. Conventional natural gas-fired gas turbines emit CO2, so conversion to carbon-free/carbon-neutral fuels is strongly expected. Ammonia is a carbon-free fuel and is expected to be implemented in society as soon as possible because the technologies for its production, transportation, and storage are well established. On the other hand, there are some problems in using ammonia as a fuel, such as its low combustibility compared to natural gas and hydrogen, high NOx emissions, and the emission of unburned ammonia and nitrous oxide. To overcome these issues and use ammonia as a fuel for gas turbines, a technology to achieve liquid ammonia single-fueled combustion in gas turbines have developed.

２．Technology details

A liquid ammonia direct spray combustion method was adopted as the ammonia combustion method, taking into consideration the simplicity of the facilities and the ability of the supply system to follow load fluctuations. There were three major challenges to the realization of direct spray combustion of liquid ammonia. The first is the low combustibility of ammonia, which requires a combustor that can achieve stable combustion. In particular, in liquid ammonia spray combustion, ammonia droplets evaporate in the combustor, resulting in lower local flame temperatures, which makes it more difficult to ensure stability. The second issue is emissions, which are caused by the generation of fuel-NOx from the nitrogen contained in the ammonia. At the same time, the short combustion time of gas turbines makes it necessary to control the emission of unburned ammonia and nitrous oxide. The third issue is the control of liquid ammonia atomization. There are few studies on atomization behavior of liquid ammonia. To address these issues, atomization behaviors of liquid ammonia was observed to select a suitable injection nozzle. To ensure flame stability and suppress unburned ammonia and nitrous oxide, combustor was modified by adjusting the swirler structure and liquid ammonia injection points. At the same time, a two-stage combustion system was adopted to achieve low NOx combustion. This combustion method improves flame stability near the burner and, at the same time, reduces oxygen concentration in the ammonia combustion region to suppress Fuel-NOx emission.

Using a 2 MW-class gas turbine (Figure 1), the performance of the developed combustor was repeatedly verified through improvements and power generation tests. As a result, stable operation with liquid ammonia firing was achieved at 2 MW output condition and that NOx emissions could meet emission limits through the use of a denitration system. In addition, it was confirmed that the emissions of unburned ammonia and nitrous oxide were below the limit of measurement of the instrumentation, and that the GHG reduction rate was 99% or higher (Figure 2).

Fig.1　2MW-class gas turbine

Fig.2　Emission in engine operation
(Left：Nitrous oxide，Right：GHG reduction rate)

３．Summary

Combustion technology that enables the operation of a 2 MW-class gas turbine using only liquid ammonia as a fuel has been developed. This technology has succeeded in achieving a GHG reduction rate of over 99%.

The reduction of GHG emissions is an urgent issue due to serious environmental problems caused by global warming. Technology development continues for early social implementation of ammonia -fired gas turbines.

Masahiro UCHIDA(Member, IHI corporation)
Shintaro ITO(Member,IHI corporation)
Yusuke KOMATSU(IHI corporation)
Taku MIZUTANI(IHI corporation)