Steam reforming using hydrocarbons (i.e. natural gas, liquid petroleum gas and naphtha) as feed is the most common process to produce hydrogen.
Until recently, steam reforming plants were designed for production capacity ranging from 200 up to 100,000 Nm3/h. By using a newly developed type of reformer it is now possible to serve ranges of 50 up to 200 Nm3/h economically by compact, small-scale hydrogen generation plants based on steam reforming of natural gas. This capacity range is well suited for supplying small vehicle fleets with hydrogen. The ability for multiple start-up and shut-down operation is important to allow a maximum of flexibility.
Steam Reformer Process
The process is divided into the generation of a hydrogen rich reformate stream by means of steam-methanereforming (SMR) and the following hydrogen purification by means of pressure swing adsorption (PSA).
The process route consists mainly of
- Pre-Treatment of the Feed
The hydrocarbon feedstock is desulphurised using e.g. activated carbon filters, pressurised and, depending on the reformer design, either preheated and mixed with process steam or directly injected with the water into the reformer without the need of an external heat exchanger. The fresh water is first softened and demineralised by an ion-exchange water conditioning system. One option is high pressure reforming with integrated heat exchangers and a working pressure of up to 16 bar which reduces the geometric volume of the reformer vessels and is ideal for a downstream treatment by means of PSA or compression. The other option is to operate the reformer at low pressures (1.5 bar) with an increased conversion ratio and compress the reformate prior to purification.
- Steam Reforming and CO-Shift Conversion
Methane and steam are converted within the compact reformer furnace at approx. 900 °C in the presence of a nickel catalyst to a hydrogen rich reformate stream according to the following reactions: (1) CH4 + H2O CO + 3 H2 (2) CO + H2O CO2 + H2 • The heat required for reaction (1) is obtained by the combustion of fuel gas and purge/tail gas from the PSA system. • Following the reforming step the synthesis gas is fed into the CO conversion reactor to produce additional hydrogen. Heat recovery • for steam or feedstock preheating takes place at different points within the process chain to optimise the energy efficiency of the reformer system (depending on the reformer design).
- Gas Purification – PSA-System
Hydrogen purification is achieved by means of pressure swing adsorption (PSA). The PSA unit consists of four vessels filled with selected adsorbents. The PSA reaches hydrogen purities higher than 99.999 % by volume and CO impurities of less than 1 vppm (volumetric part per million) fulfilling the specifications set by the fuel cell bus supplier. Pure hydrogen from the PSA unit is sent to the hydrogen compressor, while the PSA off-gas from recovering the adsorbents, called tailgas, is fed to the reformer burner. Depending on the reformer design, a recuperative burner is used featuring high efficiency and low nitrogen oxide (NOx) emissions. During normal operation, the burner can be operated solely on the tailgas stream.
Key Characteristics of Steam Reformer Technology
- Steam reforming plants can either be built on skids or in one container, thus reducing the space requirement (a net area equivalent to max. two 20-foot containers including the PSA unit is needed) and the commissioning time. The only interfaces needed are natural gas, water and electricity supply.
- The modular construction allows a capacity extension of the plant whenever it may be required. This could be either realised by adding complete containerised reformer modules or by adding reformer tubes to the existing ones (no new reformer module necessary). • The plants are designed for automatic and unattended operation. This includes automatic start-up and shut-down and automatic load adjustment using a remote control system (e.g. via internet).
Flow chart of a steam reformer on the right:
1 Feed Pre-Treatment
2 Reforming & Steam Generation
3 High Temperature Conversion
4 Heat Exchanger Unit
5 Purification Unit
* optional, depending on reformer design a either heat exchanger for low pressure reformer or compression to 1 bar for high pressure reformer
Exemplary Layout of Modular Reformer (High Pressure Type)
Pressure Swing Adsorption (PSA)