In recent years, the world energy situation is undergoing profound changes, and the world has ushered in a new round of energy transformation featuring clean and low-carbon energy. New energy transportation dominated by hydrogen energy and pure electric has the momentum to replace traditional oil and gas transportation energy, and is rapidly infiltrating into gas power generation, distributed energy supply, micro grid, multi-energy complementation, etc., driving energy production and transportation. utilization efficiency and environmental protection.
Hydrogen (H2) comes from a wide range of sources and has the advantages of high energy density, high conversion efficiency, and cleanliness. As a zero-carbon energy source, hydrogen energy is a typical secondary energy source. Its energy properties are similar to electricity, and it is superior to electricity in terms of storability. With social and economic progress, driven by people's demand for environmental protection, efficiency, and low-carbon energy, hydrogen energy is expected to become an important component of future energy and play an important role in a low-carbon society.
According to the forecast of the International Hydrogen Energy Commission, by 2050, hydrogen energy will reduce CO2 emissions by 6 billion tons, create a market value of 2.5 trillion US dollars, and it is expected to account for 18% of global energy. Different from the characteristics of traditional petroleum fuels that are easy to transport and can be stored on a large scale, the current hydrogen storage and transportation technology has not yet solved the problems of energy efficiency and safety, and the widely used high-pressure gas hydrogen transportation has the problems of low hydrogen storage density and high compression energy consumption. Large-scale long-distance transportation. The hydrogen storage methods currently being developed include liquid hydrogen, solid-state hydrogen storage, organic liquid hydrogen storage and ammonia/methanol hydrogen storage.
Ammonia, a basic chemical product produced in large quantities around the world, is also very suitable for use as an H2 carrier. Ammonia has the advantages of easy liquefaction (-33°C), high volumetric energy density (the volumetric energy of liquid ammonia is 50% higher than that of liquid hydrogen), the transportation and storage facilities can be used in common with propane, the manufacturing cost is low, and it can be used as a carbon-free fuel. In addition to being decomposed into hydrogen, ammonia can also be directly burned in large gas turbines. The combustion is efficient and does not produce CO2. It is one of the current research hotspots for large-scale power generation.
Global ammonia production in 2020 is about 183 million tons, of which about a quarter is produced by China, and the other 30% is from Russia, the United States and India. Of the total ammonia production, 72% comes from natural gas, 22% from coal, 5% from oil, and less than 1% from renewable energy. 80% of its total production capacity is used to produce fertilizers, while the rest is used to produce explosives and chemicals. Since the hydrogen production unit of traditional ammonia synthesis consumes a lot of fossil energy and generates a large amount of CO2, green hydrogen is produced through electrolysis of water, and it is synthesized into ammonia through the H-B process, which is considered as a low-carbon ammonia synthesis route, and the produced ammonia is defined as "Green ammonia" is the carrier of green hydrogen. However, the energy required to convert hydrogen into ammonia is equivalent to 7%~18% of the energy contained in hydrogen. If ammonia needs to be converted into hydrogen again at the destination, a heat source of 500~550°C is required under the condition of catalyst, which will cause some energy loss.
At present, green hydrogen is mainly obtained by electrolysis of water with renewable power sources. The main equipment for electrolyzing water is an electrolytic cell. At present, the mainstream electrolytic cells are alkaline electrolytic cells and PEM electrolytic cells. Although alkaline electrolyzers are inexpensive, they do not have advantages in large-scale hydrogen production due to their low current density and large volume. PEM electrolyzers are considered more suitable for large-scale hydrogen production due to their high efficiency, long life, and high current density. In general, the theoretical minimum required for an electrolyser is 21.18 GJ/tonne of ammonia. However, on an industrial scale, electrolyzers operate at 60% to 70% efficiency and require at least 30.3 to 35.3 GJ/ton of ammonia. In addition, the current production of nitrogen through an air separation unit consumes about 2.7 GJ/tonne of ammonia. Therefore, under ideal conditions, the energy consumption of ammonia produced by this route is 33.0-38.0 GJ/ton ammonia, and the overall power fuel (PTF) efficiency is 55.7%-64.3%. In contrast, the PTF (Power to Fuel Cell) efficiency of liquid hydrogen is 49.3%~57.9%, which is lower than that of ammonia, due to the 36.0~48.0 GJ/ton H2 energy required in the compression and liquefaction process. In addition, the transport efficiency of liquid hydrogen is about 84%, which is lower than that of ammonia (90%) due to the higher energy consumption of evaporation losses and compressed storage.
There are two paths to applying ammonia to fuel cells. One is the catalytic cracking of ammonia to produce hydrogen for fuel cells. Since direct on-board cracking needs to increase the complexity and integration of the on-board system, which increases the cost and is considered impractical, this route may be mainly used in hydrogen refueling stations in the future, that is, hydrogen refueling stations produce hydrogen through ammonia online cracking. The pyrolysis process requires high temperatures >500°C to produce high-purity hydrogen (>99.97%, especially for automobiles), which requires an energy input of 4.2 GJ/t ammonia (including H2 losses). Since PEMFC is highly susceptible to trace amounts of ammonia (<0.1×10-6), the hydrogen converted from ammonia must be refined through an efficient purification and separation system, which will consume an additional 0.5 GJ/ton of ammonia. Therefore, the purification process of ammonia-decomposing hydrogen inevitably generates a lot of cost. At the same time, the entire process may also result in a total heat loss of 1.7 GJ/ton of ammonia. In addition, after the ammonia is decomposed, additional electricity of 2.0~4.3 GJ/ton of ammonia is required to compress the hydrogen to refill the 700bar hydrogen storage cylinder of the fuel cell vehicle (FCEV). According to the above calculation, the total conversion efficiency of ammonia is 61.0%~68.5%. In addition, the integration of the cracking reactor with the hydrogen compression system may complicate the fueling and filling process. The application of ammonia may be further limited due to the complexity of the cracking system, as well as the performance and lifetime of the catalyst in the presence of impurities.
Another route is to use ammonia directly, eliminating the need for an intermediate process to convert ammonia to hydrogen. In the future, the high temperature inside the SOFC can be used to crack ammonia. However, the high operating temperature of SOFCs (550~900°C) may only be suitable for continuous stationary applications that do not require frequent switching. Therefore, SOFC can be applied to heavy-duty vehicles, such as for aviation, shipping, trucking, etc. In addition, SOFC anode materials responsible for catalytic decomposition of ammonia into hydrogen should be stable, durable, and resistant to high temperature during continuous operation, but the degradation of anode materials is still a major obstacle to the commercialization of SOFCs. Further improvements in SOFC and ammonia decomposition technologies are necessary to realize such a concept. Ammonia internal combustion engines, while not required to reduce hydrogen from ammonia, can cause other problems such as difficult ignition, low flame velocity, and higher compression in addition to NOx emissions.
Although the above discussion is based on ideal fuel production efficiency and practical transport analysis, ammonia does have great potential as a viable hydrogen energy storage material. However, compared with liquid hydrogen, the transport system using ammonia as the hydrogen carrier has no clear advantage in the overall electricity-fuel-electricity (PFP) efficiency (Table 2). In addition, the large amount of energy required for the cracking process limits its future applications. In addition, the financial and energy costs of purified and compressed hydrogen to supply fuel cell vehicles are high, and the cost of electricity and heat input is difficult to recover through other benefits. In addition to its technical challenges, ammonia's toxicity (OSHA exposure limit of 50 × 10-6), hydrophilicity, and corrosiveness requires extensive infrastructure to avoid accidental leaks and equipment corrosion. It is generally believed that ammonia can be used as a fuel for stationary power generation, using SOFCs to power remote areas. But as a hydrogen carrier, despite its high energy density, reducing hydrogen requires a lot of energy for cracking and compression, which limits its application. From the perspective of coping with climate change, using green hydrogen as raw material to synthesize ammonia instead of the traditional ammonia synthesis process using fossil energy as raw material is a more effective method than using ammonia as a hydrogen carrier.
