1. Introduction
Energy is a driving force for national industrial development and economic growth, and for the affluent life of people, and its consumption inevitably increases continuously. Humans have led economic and industrial development by using hydrocarbon-based fossil fuels such as coal and petroleum, and they are prevalent in developing countries. As the severity of the climate change problem increased about 150 countries agreed to co-operate and a climate change agreement was adopted at the Rio Environmental Conference in June 1992, and the Kyoto Protocol in 1996 to solve the global warming problem in earnest. This gradually expanded and led to the international treaty of the Paris Agreement in 2015, and governments were advised to submit long-term low greenhouse gas emission development strategies to the international community by 2020 in accordance with the Paris Agreement. At the same time, each country had declared that it will achieve carbon neutrality by 2050 or at the longest by 2060. Most of them set the target for 2050 because the Intergovernmental Panel on Climate Change (IPCC) suggested to limit the increase in global average temperature to 1.5 °C by 2100. This is because it suggested a path that global carbon dioxide emissions should be reduced by at least 45% compared to 2010 by 2030, and that carbon-neutrality should be achieved by 2050. Therefore, in line with the Paris Agreement’s guidelines, and the goal of limiting the increase in global average temperature to 1.5 °C or less, most countries aim to reduce greenhouse gas emissions from human activities to the fullest extent; to absorb and remove emitted carbon, and bring the actual emissions to zero. Achieving carbon neutrality is a very challenging goal; however, the international community is demanding a higher achievement. Further details on this are contained in the biennial update reports, and COP26 will be held in the UK to discuss the latest developments
[1]. To realize greenhouse gas reduction through innovation in the face of tightening environmental regulations, the development and early commercialization of prospective energy technology and investment to support them are emphasized.
The Netherlands Environmental Assessment Organization estimates that the world emitted 57.4 Gt CO
2eq greenhouse gases in 2019
[2]. Among them, global energy-related carbon dioxide (CO
2) emissions are 33.4 Gt CO
2eq. This amount is also consistent with that reported by the IEA
[3]. The continuous increase of CO
2 in the atmosphere is mainly driven by fossil fuel combustion and calcination of carbonates. The dominant drivers of CO
2 are the combustion of coal, oil, and natural gas, representing 89% of global CO
2 emissions, with respective shares of 39%, 31%, and 18%. In particular, the combustion of fuel in thermal power plants is a major cause of large-scale CO
2 generation and accounts for the dominant proportion of the electricity generation sector. In the field, coal, a solid fuel, is mainly used in coal-fired power plants, and gaseous fuels such as natural gas are mainly used in combined cycle power plants, including gas turbines.
The dominance of greenhouse gas emissions from fossil fuel combustion has already been well demonstrated in existing analysis data, and associated researchers in the fields of combustion, energy, and the environment are well aware of this fact. Therefore, it is unnecessary to say that innovative development of fuel combustion technology is needed to meet the future carbon-neutral system goals. However, the innovation in energy conversion is being promoted more rapidly than in the past, and intensive R&D activities are needed to support this technology. Although global R&D activities to move towards a renewable and hydrogen society are mainly centered on transportation and fuel cells, there are still many goals to be achieved for the full-cycle, large-scale practical use of hydrogen production, transport, supply, and utilization for commercialization. As it has been consistently mentioned in the past, it will take a long time as well a huge socio-economic investment for the government and private sector to establish an infrastructure for the supply of hydrogen.
Therefore, to implement the rapidly changing carbon emission reduction regulations and fulfill the promises made with the international community, technologies that are relatively easy to commercialize need to be implemented. Methods for reducing greenhouse gases include improving the thermal system efficiency and capturing CO
2 through downstream facilities. However, it would be economical in terms of post-treatment cost if the combustion stage uses a fuel that does not emit carbon from the source. The cost of producing a carbon-free fuel and the CO
2 footprint must be considered together. Ammonia (NH
3) is expected to be a carbon-free fuel. To realize a carbon-neutral society in 2050, for example, Japan has presented an action plan for 14 important areas that are expected to grow in the future as a greenhouse gas reduction industry in terms of growth strategies. In particular, they emphasized that ammonia is an effective fuel when combusted with pulverized coal in a coal-fired boiler. In addition, R&D was conducted to apply fuel ammonia to coal-fired power plants, gas turbine combustion, and industrial furnace combustion through the cross-ministerial strategic innovation promotion program (SIP) project from 2014 to 2018
[4]. Through this project, the technical feasibility of using fuel ammonia has been verified, and research is underway with the goal of commercializing the related technology through the new energy and industrial technology development organization (NEDO) project, a follow-up task
[5]. Because the advantages (carbon-free fuel) and disadvantages (low reactivity and fuel-NOx production) of using ammonia as a fuel clearly exist, research has been conducted to overcome these technical bottlenecks and supply problems, and some of the results linked to demonstrations show the effect of reducing greenhouse gas emissions
[6][7][8][9].
Ammonia was first studied as an alternative fuel for internal combustion engines rather than for reducing greenhouse gas emissions. As available resources became scarce due to World War II, interest was drawn in fuels other than fossil fuels, and accordingly, the possibility of using ammonia as a fuel drew attention
[10][11][12][13][14][15][16]. The technology of using ammonia as a fuel in automobile internal combustion engines has been used for trucks in Norway since the 1930s, and was developed in Belgium in 1943
[17]. In the 2000s, there was a research case of a compression ignition engine to which a mixed combustion system of ammonia-diesel and ammonia-DME fuel was applied
[18][19]. The optimal ammonia co-firing rate was 60%, and when it was lower than this, the flame temperature decreased and the amount of NOx produced decreased compared to when only diesel fuel was used. However, as the ammonia ratio increased, the amount of NOx produced due to nitrogen contained in the fuel increased sharply. Currently, research is underway to apply ammonia to spark-ignition engines
[20][21]. In addition, there are quite a lot of research results aimed at overcoming the challenge of its low reactivity, and most of the results have proposed to achieve this by blending ammonia with a high-reactivity fuel
[22][23]. Regarding the production of prototype vehicles using ammonia as fuel, in 2007, the University of Michigan developed an ammonia-gasoline combustion engine and operated it from Detroit to San Francisco
[24]. In 2013, the Korea Institute of Energy Research developed an ammonia-gasoline engine, called AmVeh, by remodeling a gasoline-LPG fueled engine
[25]. Recently, the development of fuel-powered ships utilizing LNG, hydrogen, and ammonia for low-carbon emissions has been notable
[26][27]. Ammonia-fueled propulsion ship development plans are being reported mainly by companies
[28][29][30][31][32]. In addition, research on the concept of a marine engine for hydrogen-ammonia co-firing is in progress
[33][34][35]. Kim et al. conducted environmental and economic assessments associated with an alternative ship propulsion system fueled by ammonia
[36]. Technologies capable of emitting greenhouse gases from the operational aspects of ships were reviewed by Bouman et al.
[37].
In the field of thermal power generation that we discuss in this study, the combustion characteristics of ammonia and their application to gas turbines and coal-fired boilers are being studied mainly in Japan, the United Kingdom, and Korea. Looking at what these countries have in common, they are surrounded by sea rather than connected to other countries (South Korea is a militarily divided country) and continents, and are characterized by high energy-use intensity. To achieve hydrogen economy and a carbon-neutral society, it is necessary to increase the production and use of hydrogen fuel. It is well known that hydrogen is produced by processes such as reforming and water electrolysis, unlike fuels mined in nature
[38][39][40][41][42]. However, there is a limit to the amount of hydrogen that can be produced in these countries. It is necessary to supplement it by shipping from other countries with abundant renewable energy for the introduction of green-hydrogen that does not emit CO
2 during the water electrolysis process using solar power or blue-hydrogen by the reforming process with carbon capture and storage (CCS) facilities. Therefore, it is necessary to develop a material that can act as a carrier for hydrogen, and ammonia has been considered the most appropriate material
[43][44][45]. There are also a number of studies comparing the transport mediums of hydrogen
[46][47][48][49][50][51][52][53]. Hydrogen can be produced by cracking ammonia; however, ammonia itself is a combustible material with a calorific value and can be used as a fuel in a thermal system. Therefore, ammonia could be used directly as a fuel, and combustion characteristics were studied because the ammonia-cracking process required another energy source. This also has the advantage that the unit cost of ammonia is much lower than that of hydrogen
[43][44][45].
Figure 1 shows the CO
2-free ammonia value chain used in a direct combustion system through the transport process after the green-hydrogen, produced through the water electrolysis process using renewable energy, and the blue-hydrogen, through the fossil fuel reforming process with CCS, are synthesized into ammonia. The current large-scale ammonia production is achieved by the Haber-Bosch process. In the future, the power used to maintain the operating conditions in the Haber-Bosch process must also be produced from renewable energy sources to achieve a truly carbon-neutral system
[54].
Figure 1. A schematic flow on CO2-free ammonia value chain.
In the present study, the recent R&D trends associated with gas turbines and coal-fired boilers using fuel ammonia are summarized by presenting the application results for demonstration. The experimental research results of the institutions that have been investigating fundamental combustion characteristics for a long time are summarized for the development of such commercial facilities. Finally, the brief research results of the Korea Institute of Energy Research, which is leading the ammonia combustion technology in Korea, are reported.