The steam methane reforming (SMR) process can be described by two main reactions.
The first reaction is reforming itself, while the second is the water-gas shift reaction. Since the overall reaction is endothermic, some heat input is required.
This is accomplished by combustion of natural gas or other fuel in a direct-fired furnace. See the photo. First reaction favors high temperature and low pressure and proceeds usually in the presence of a nickel-based catalyst.
The first patents on steam methane reforming were awarded to BASF in 1926, and the first reforming plants were built in the 1930s. Large-scale production had begun only in the beginning of the 1960s following the discovery of large gas fields in Europe with subsequent changeover from use of coal to natural gas as a feedstock. In the early days, reforming was done at atmospheric pressure; later, the process conditions were changed to increase to pressures up to 30 bar and temperatures of up to 1000 °C.
The increased pressure saves compression energy in the downstream synthesis stage; however, the high temperature necessitates an extensive heat recovery system.
Process Description
In a direct-fired furnace, a preheated mixture of natural gas and steam is passed through catalyst-filled tubes, where it is converted to hydrogen, carbon monoxide, and carbon dioxide.
It is of a great importance to control the maximum tube temperature and heat flux in the reformer to maintain a reliable and prolonged performance. To obtain this, several burner arrangements are employed: top-fired, bottom-fired, side-fired, terrace-walled, and cylindrical type.
35–50% of total energy input in SMR process is absorbed due to endothermic nature of the reforming process half of which is required for temperature rise and the other half for the reaction itself.
The produced syngas leaves the reformer at a temperature of 800–900 °C. The heat of the flue gases is usually utilized in the convective part of the reformer by generating steam and preheating the feedstock, thus bringing the overall thermal efficiency to over 85%.
To avoid catalyst poisoning, a de-sulfurization stage is usually required. In addition to the usual nickel-based catalysts, cobalt and noble metals are often used in SMR processes. Non-metallic catalysts have not proved their feasibility due to their low activity.
Another catalyst related problem is carbon deposition, which is especially present when processing higher hydrocarbons. In this case, ruthenium, which can effectively resist carbon formation in steam reforming, can be used.
