Water
H
Electrolyser H O = H + O
Ammonia synthesis (Haber-Bosch process)
Compression
NH
Renewable electricity
Air separation unit
N
Hydro power
Figure 4 Green ammonia
Renewable ammonia synthesis : Ammonia is produced from the combination of hydrogen and nitrogen using the established Haber-Bosch process, as shown in Figure 4 . For ammonia to be considered renewable or green, hydrogen and nitrogen must first be produced using renewable processes (IRENA, 2022), (FMI, 2023). The hydrogen should either be from the electrolysis of water using renewable electricity or another process of comparable, low-carbon intensity. Similarly, nitrogen is purified/separated from air using renewable electricity. “ For ammonia to be considered renewable or green, hydrogen and nitrogen must first be produced using renewable processes ” There are also three pathways to convert solid biomass to renewable ammonia. Solid biomass can be gasified with air to form syngas (a mixture of hydrogen and CO). The syngas is then processed to form ammonia after carbon removal. Alternatively, biomass can be gasified and converted to form biomethane (biogas), which is then used as feedstock for ammonia. A third pathway is via the anaerobic digestion of biomass to produce biomethane. All the process steps necessary for biomass-to-ammonia have been commercially demonstrated, even though bio-ammonia is not commercially produced today. Renewable ammonia has been produced from hydropower since 1921, but only one commercial plant is still operational (IRENA, 2022). However, less than 0.02 Mt of renewable ammonia was produced in 2021. Many key players in the ammonia industry have already announced plans to incorporate green hydrogen
to produce hydrogen. It is a partial oxidation process where biomass, a solid hydrocarbon feedstock, is transformed into a gaseous mixture of hydrogen, CH 4 , CO 2 , CO, tar, light hydrocarbons, ash, minor contaminates, and char. The two main types of gasifiers are the entrained flow (updraft) and the fixed bed (downdraft fixed beds) gasifiers. Significant gas conditioning needs to be included in the biomass gasification process, along with the removal of inorganic and tars impurities. Biomass gasification normally occurs within a temperature range of 700-1,200°C, using oxygen, air, steam or their combination. As the pyrolysis process takes place in the absence of air and oxygen, there will be no formation of carbon oxides (CO2 and CO), obviating the need for secondary reactors. As a result, this process of hydrogen production helps to reduce emissions. Pyrolysis can be grouped into high (above 800°C), medium (500-800°C), and low temperatures (up to 500°C). Pyrolysis could play a key role in the production of hydrogen since it can reduce the emission of CO and CO 2 . It could also be operated in a way that recovers a significant portion of solid carbon. Even though several advances and innovations in generating hydrogen from biomass were made using existing technologies, more scientific advances were needed to make it economically competitive as well as environmentally friendly for industrial production on a large scale. As per US Department of Energy’s Office of Energy Efficiency and Renewable Energy, electrolysis is a leading hydrogen production pathway to achieve the hydrogen energy goal of reducing the cost of clean hydrogen by 80% to $1 per 1 kilogram in one decade (‘1 1 1’).
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