This paper offers an overview of the technologies for hydrogen production. The technologies discussed are reforming of natural gas; gasification of coal and biomass; and the splitting of water by water-electrolysis, photo-electrolysis, photo-biological production and high- temperature decomposition. For all hydrogen production processes, there is a need for significant improvement in plant efficiencies, for reduced capital costs and for better reliability and operating flexibility.
Water electrolysis and natural gas reforming are the technologies of choice in the current and near term. They are proven technologies that can be used in the early phases of building a hydrogen infrastructure for the transport sector. Small-scale natural gas reformers have only limited commercial availability, but several units are being tested in demonstration projects. In the medium to long term, centralised fossil fuel-based production of hydrogen, with the capture and storage of CO2, could be the technology of choice. However, the capture and storage of CO2 is not yet technically and commercially proven. Further R&D on the processes of absorption and separation are required.
Hydrogen can be produced from a variety of feedstocks. These include fossil resources, such as natural gas and coal, as well as renewable resources, such as biomass and water with input from renewable energy sources (e.g. sunlight, wind, wave or hydro-power). A variety of process technologies can be used, including chemical, biological, electrolytic, photolytic and thermo-chemical. Each technology is in a different stage of development, and each offers unique opportunities, benefits and challenges. Local availability of feedstock, the maturity of the technology, market applications and demand, policy issues, and costs will all influence the choice and timing of the various options for hydrogen production. An overview of the various feedstock’s and process technologies is presented
2.1 HYDROGEN FROM FOSSIL FUELS
Hydrogen can be produced from most fossil fuels. The complexity of the processes varies, and in this chapter hydrogen production from natural gas and coal is briefly discussed. Since carbon dioxide is produced as a by-product, the
should be captured to ensure a
sustainable (zero-emission) process. The feasibility of the processes will vary
with respect to a centralised or distributed production plant.
2.2 PRODUCTION FROM NATURAL GAS
Hydrogen can currently be produced from natural gas by means of three different chemical processes:
- Steam reforming (steam methane reforming – SMR).
- Partial oxidation (POX).
- Auto thermal reforming (ATR).
Although several new production concepts have been developed, none of them is close to commercialisation. Steam reforming involves the endothermic conversion of methane and water vapour into hydrogen and carbon monoxide (2.1). The heat is often supplied from the combustion of some of the methane feed-gas. The process typically occurs at temperatures of 700 to 850 °C and pressures of 3 to 25 bar. The product gas contains approximately 12 % CO, which can be further converted to
through the water-gas shift reaction (2.2).
Partial oxidation of natural gas is the process whereby hydrogen is produced through the partial combustion of methane with oxygen gas to yield carbon monoxide and hydrogen (2.3). In this process, heat is produced in an exothermic reaction, and hence a more compact design is possible as there is no need for any external heating of the reactor. The CO produced is further converted to
described in equation (2.2).
Auto thermal reforming is a combination of both steam reforming (2.1) and partial oxidation (2.3).The total reaction is exothermic, and so it releases heat. The outlet temperature from the reactor is in the range of 950 to 1100 °C, and the gas pressure can be as high as 100 bar. Again, the CO produced is converted to H2 through the water-gas shift reaction (2.2). The need to purify the output gases adds significantly to plant costs and reduces the total efficiency.