Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen production. The reaction is represented by this equilibrium:
CH4 + H2O <=> CO + 3 H2
The reaction is strongly endothermic (ΔHSR = 206 kJ/mol).
Hydrogen produced by steam reforming is termed 'grey hydrogen' when the waste carbon monoxide is released to the atmosphere and 'blue hydrogen' when the carbon monoxide is (mostly) captured and stored geologically - see carbon capture and storage. Zero carbon 'green' hydrogen is produced by thermochemical water splitting, using solar thermal, low- or zero-carbon electricity or waste heat, or electrolysis, using low- or zero-carbon electricity. Zero carbon emissions 'turquoise' hydrogen is produced by one-step methane pyrolysis of natural gas.
Steam reforming of natural gas produces most of the world's hydrogen. Hydrogen is used in the industrial synthesis of ammonia and other chemicals.
Reactions
Steam reforming reaction kinetics, in particular using nickel-alumina catalysts, have been studied in detail since the 1950s.
Pre-reforming
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.
Steam reforming
The name-giving reaction is the steam reforming (SR) reaction and is expressed by the equation:
Via the water-gas shift reaction (WGSR), additional hydrogen is released by reaction of water with the carbon monoxide generated according to equation [1]:
Some additional reactions occurring within steam reforming processes have been studied. Commonly the direct steam reforming (DSR) reaction is also included:
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. High excess of steam is required, expressed by the (molar) steam-to-carbon (S/C) ratio. Typical S/C ratio values lie within the range 2.5:1 - 3:1.
Industrial practice
The reaction is conducted in multitubular packed bed reactors, a subtype of the plug flow reactor category. These reactors consist of an array of long and narrow tubes which are situated within the combustion chamber of a large industrial furnace, providing the necessary energy to keep the reactor at a constant temperature during operation. Furnace designs vary, depending on the burner configuration they are typically categorized into: top-fired, bottom-fired, and side-fired. A notable design is the Foster-Wheeler terrace wall reformer.
Inside the tubes, a mixture of steam and methane are put into contact with a nickel catalyst. Catalysts with high surface-area-to-volume ratio are preferred because of diffusion limitations due to high operating temperature. Examples of catalyst shapes used are spoked wheels, gear wheels, and rings with holes (see: Raschig rings). Additionally, these shapes have a low pressure drop which is advantageous for this application.
Steam reforming of natural gas is 65–75% efficient.
The United States produces 9–10 million tons of hydrogen per year, mostly with steam reforming of natural gas. The worldwide ammonia production, using hydrogen derived from steam reforming, was 144 million tonnes in 2018. The energy consumption has been reduced from 100 GJ/tonne of ammonia in 1920 to 27 GJ by 2019.
Globally, almost 50% of hydrogen is produced via steam reforming. It is currently the least expensive method for hydrogen production available in terms of its capital cost.
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. Despite this, implementation of this technology remains problematic, costly, and increases the price of the produced hydrogen significantly.
