Prospects for ammonia energy applications
The automobile industry is the second largest industry in the world after petrochemical industry. Traditional fuel vehicles not only consume a large amount of non-renewable oil resources, but also cause serious environmental pollution and greenhouse effect. In the face of the global problems of depletion of fossil fuels and rising oil prices, as well as the damage to human ecological environment caused by automobile exhaust, the development of low-carbon renewable clean energy and accelerating the popularization of new energy vehicles is an inevitable trend for the sustainable development of the entire automotive industry.
Hydrogen has long been regarded as the most ideal clean energy source, and it is the key research object of many countries' energy development strategies. Hydrogen has a high calorific value per unit mass, about three times that of gasoline; the combustion product is mainly water, which is non-polluting to the environment and can be recycled. However, hydrogen is not easy to liquefy, the conditions and cost of storage and transportation are high; it is easy to explode, with low safety; as a fuel application requires large-scale construction of hydrogen refueling stations, with a huge investment of funds and other key issues have not yet been solved, and the large-scale use of hydrogen as a power source for automobiles has been seriously constrained.
Ammonia is also a carbon-free compound that produces only water and nitrogen and a small amount of nitrogen oxides when burned, and can be used as a clean energy source to replace fossil fuels. The energy density of ammonia is higher than that of gasoline, methanol and other fuels; it has a high octane number and is easy to be compressed and liquefied, so it is safe and convenient to be stored and transported; and when it is replenished, the infrastructure of the existing gas stations can meet the demand for liquid ammonia refueling. In addition, ammonia as a hydrogen-rich compounds (containing hydrogen mass fraction of 17.6%), can also be used as raw materials for fuel cell hydrogen. Synthetic ammonia is one of the world's largest production of chemical products, production, storage and transportation and supply process is mature, well-equipped. From a comprehensive point of view, it is of practical significance for ammonia fuel to replace fossil energy to supply energy in automobile engines.
Since the beginning of this century, the research and development application of ammonia fuel has received more and more attention. The United States, Canada, the Netherlands, Japan and other countries scholars are actively exploring the development potential of ammonia. 2004, the United States held an annual "ammonia academic exchanges," 2008 is the theme of the meeting as "ammonia - the key to U.S. energy independence", from which it can be seen that the ammonia fuel alternative to fossil energy in the automobile engine. Key", from which can be seen in the U.S. energy independence strategy in the important position of ammonia; Japan, Singapore and South Korea have also begun to discuss with Australian officials to set up a renewable ammonia imports port. At present, China's research on ammonia fuel is still in its infancy, with ammonia as the energy storage medium of the relevant R & D applications to be further developed.
1. Vehicle Ammonia Energy
1.1 Ammonia as engine fuel
The density of liquid ammonia is close to gasoline, although its calorific value is about half of that of gasoline, but the octane number is much higher than that of gasoline, with excellent anti-explosive properties, which can improve the output power of the engine and save fuel at the same time. In the past few decades, many foreign countries have studied the feasibility of ammonia as fuel for internal combustion engines, and the results have been positive. In the middle of the last century, Belgian researchers proposed ammonia as a fuel for driving cars, and successfully verified in different models of vehicles; its research also found that an increase in ammonia intake pressure or the use of exhaust gas fuel reforming technology can meet the engine's power performance requirements. Subsequently, NASA used ammonia as a power fuel to develop and develop the X-15 type test machine, and at that time the highest speed in the history of human aviation into space, which indicates that the ammonia-fueled engine has excellent prospects for application.
In recent years, the practical application of ammonia fuel in the field of engine has made further breakthroughs, Reiter et al. showed that, only slightly modified engine ammonia intake pipeline, modern diesel cars can be transformed into ammonia - diesel dual-fuel powered vehicles; and in the ammonia content of no more than 70% of the total energy, the tailpipe emissions of nitrogen and oxygen are low. Liu et al. found that, through the increase of the compression ratio to improve the compression ratio of ammonia, the engine thermal efficiency can be increased. Liu et al. found that by increasing the compression ratio of the internal combustion engine to increase the ammonia output, the thermal efficiency of the engine can be raised to more than 50%, which is much higher than that of ordinary fuel vehicles.
Due to the high minimum ignition limit and slow combustion rate of ammonia, it is usually necessary to use combustion aids, such as acetylene, dimethyl ether, diesel fuel, hydrogen, etc., when using ammonia as a vehicle fuel. In the existing automotive system using ammonia as an engine fuel, part of the ammonia will be generated in the bypass system by the action of the catalyst to produce hydrogen, and the ammonia in the main system to form a mixture of ammonia-hydrogen fuel, and then enter the engine cylinder combustion, to achieve the purpose of cracking hydrogen to ignite the ammonia fuel.Ryu et al. showed that hydrogen and nitrogen produced by catalytic cracking of ammonia and ammonia mixture combustion, which can improve the combustion performance of the engine, and to reduce the tailpipe emissions. emissions.
Also as a potential alternative fossil fuel, methyl ether has better combustion performance and is more suitable for internal combustion engines, however, the higher cost hinders its large-scale promotion. The mixture of liquid ammonia and methyl ether, which have similar densities, can be made into a fuel with complementary combustion performance and lower fuel cost.Gross et al. developed a high-pressure fuel mixing and injection system, and tested the ammonia-methyl ether mixture as a research object. The results show that increasing the injection pressure at high ammonia concentration can enhance the fuel-air mixing effect, promote better fuel combustion and achieve higher engine loads. The Russian engine manufacturer Energomash is currently developing a new type of rocket engine fueled by ammonia-acetylene mixtures, with the aim of significantly reducing the cost of launching rockets while at the same time significantly increasing the efficiency of the engine.
Ammonia produces only nitrogen and water under complete combustion, but it is often difficult to avoid the production of nitrogen oxides during the actual combustion process due to many uncontrollable factors, so research on how to deal with the exhaust gas is the main challenge facing ammonia as an engine fuel. A key task in most ammonia-fueled engine research is to reduce the concentration of nitrogen oxide emissions. Mastering the reaction mechanism and kinetic process of combustion of ammonia mixed with various ignition aids, analyzing the pathways of nitrogen oxides formation in the exhaust gas and the treatment technology as well as the characteristics of exhaust gas of various types of engines are the focus of future research on ammonia as a fuel for automobile engines.
1.2 Ammonia as fuel cell raw material
Fuel cell is a power generation device that can directly convert the chemical energy contained in the fuel into electrical energy. Its cathode and anode are only energy converters for the reaction process, and the required chemical raw materials are supplied from outside and not stored inside the battery. As the core technology of new energy vehicles, fuel cell has been listed as the top ten new technologies that will change human life in the 21st century by the U.S. Time Magazine, and it is recognized as the best power source for future vehicles in the industry. Ammonia has high hydrogen content, high energy density, simple hydrogen reforming device, the product does not contain COx that can lead to fuel cell poisoning, as a raw material for fuel cells, the advantages are very prominent. since the 1960s, ammonia as a raw material for fuel cells has entered into a wide range of practical development stage: Allis-Chamber has developed 8kW ammonia-air fuel cell system for Volkswagen; ASEA of Sweden has designed a 200kW liquid ammonia-liquid oxygen fuel cell for driving a submarine. Nowadays, Japan, Europe and the United States and other developed countries have plans to use ammonia fuel cells to replace traditional generators and internal combustion engines; Israel's new energy company GenCell has developed a unique ammonia decomposition technology, based on which it has launched a new type of ammonia-hydrogen fuel cells that have opened up the market in the United States and Mexico, and it intends to set up a long-term and stable cooperative relationship with Chinese companies.
