Steam reforming
Zero carbon emissions 'turquoise' hydrogen is produced by one-step methane pyrolysis of natural gas.
[4] Steam reforming reaction kinetics, in particular using nickel-alumina catalysts, have been studied in detail since the 1950s.
[5][6][7] The purpose of pre-reforming is to break down higher hydrocarbons such as propane, butane or naphtha into methane (CH4), which allows for more efficient reforming downstream.
As these reactions by themselves are highly endothermic (apart from WGSR, which is mildly exothermic), a large amount of heat needs to be added to the reactor to keep a constant temperature.
Optimal SMR reactor operating conditions lie within a temperature range of 800 °C to 900 °C at medium pressures of 20-30 bar.
[8] High excess of steam is required, expressed by the (molar) steam-to-carbon (S/C) ratio.
Furnace designs vary, depending on the burner configuration they are typically categorized into: top-fired, bottom-fired, and side-fired.
Inside the tubes, a mixture of steam and methane are put into contact with a nickel catalyst.
[12] The United States produces 9–10 million tons of hydrogen per year, mostly with steam reforming of natural gas.
[13] The worldwide ammonia production, using hydrogen derived from steam reforming, was 144 million tonnes in 2018.
[16] In an effort to decarbonise hydrogen production, carbon capture and storage (CCS) methods are being implemented within the industry, which have the potential to remove up to 90% of CO2 produced from the process.
[16] Despite this, implementation of this technology remains problematic, costly, and increases the price of the produced hydrogen significantly.
[16][17] Autothermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas.
The reaction takes place in a single chamber where the methane is partially oxidized.
Due to the exothermic nature of some of the additional reactions occurring within ATR, the process can essentially be performed at a net enthalpy of zero (ΔH = 0).
POX is typically much faster than steam reforming and requires a smaller reactor vessel.
[21] The capital cost of steam reforming plants is considered prohibitive for small to medium size applications.
Conventional steam reforming plants operate at pressures between 200 and 600 psi (14–40 bar) with outlet temperatures in the range of 815 to 925 °C.
Flared gas and vented volatile organic compounds (VOCs) are known problems in the offshore industry and in the on-shore oil and gas industry, since both release greenhouse gases into the atmosphere.
[23] Reforming for combustion engines is based on steam reforming, where non-methane hydrocarbons (NMHCs) of low quality gases are converted to synthesis gas (H2 + CO) and finally to methane (CH4), carbon dioxide (CO2) and hydrogen (H2) - thereby improving the fuel gas quality (methane number).
[24] There is also interest in the development of much smaller units based on similar technology to produce hydrogen as a feedstock for fuel cells.
However, there is an active debate about whether using these fuels to make hydrogen is beneficial while global warming is an issue.
