Decarbonisation Technology - November 2023 Issue

produced is termed ‘turquoise’, on the colour spectrum between blue and green. The main developmental challenges include the cost of hydrogen produced, energy efficiency, and operational reliability. Uses for the solid carbon by-product are also an important consideration. In the interim, while capital costs will be higher, the cost of hydrogen production will likely be lower than that of SMR with CCS or water electrolysis. As the technology matures with larger commercial plants providing economies of scale, methane pyrolysis will have an edge over other competing technologies. Methane pyrolysis is an active area of research that is now attracting interest from some of the leading engineering technology companies, including BASF, Ekona, Baker Hughes, and Mitsui. The pyrolytic decomposition of methane can be affected in three ways: thermal, plasma, and catalytic. Thermal pyrolysis Pyrolysis of methane in the absence of oxygen yields carbon (solid) and hydrogen (gas) as per the reaction below:

liquid-metal bubble column reactor. The column is filled with liquid metal and heated to 1,000°C. Fine methane bubbles enter the column through a porous filling at the bottom. These bubbles rise to the surface, and at such high temperatures, the ascending methane bubbles are increasingly decomposed into hydrogen and carbon. Catalytic pyrolysis The thermal decomposition of methane requires a high temperature in the order of 1,300°C. Metals such as nickel, iron, copper, and cobalt have demonstrated catalytic activity for methane decomposition at lower temperatures, between 500-800°C. The reaction is carried out in nanostructures like nanotubes, nanofibres, or graphene. Metal catalysts exhibit fast deactivation and are difficult to separate from the carbon produced. To overcome these limitations, carbonaceous catalysts in the form of amorphous high surface area carbons active in the range of 800-900°C are employed. Such carbon catalysts will likely have a slower deactivation rate than metal catalysts. Strategies for continuous regeneration of catalytically active carbons are an active area of research. Plasma pyrolysis In this process, plasma torches are used to crack the methane molecule into hydrogen and solid carbon powder. It is similar to water electrolysis but uses only one-fifth of the electricity, resulting in operational costs more than 50% cheaper than water electrolysis. The methane is introduced to a tubular plasma reactor. Part of the length of the reactor tube lies in a horizontal rectangular waveguide through which microwave power is supplied to excite the methane to the plasma state. Currently, the decomposition of methane is accompanied by a combination of cracking, oligomerisation, and aromatisation reactions, which tend to minimise the formation of elemental carbon. Research is underway to limit these side reactions to attain better carbon and hydrogen yield. Plasma pyrolysis is considerably more expensive than thermal and catalytic pyrolysis due to the high power intensity needed.

CH₄ (gas) → C (solid)+ 2H₂ (gas) ΔH -76KJ/Mole

The reaction is highly endothermic with an equilibrium that shifts toward hydrogen and carbon around 300°C and goes to completion around 1,300°C. It is similar to the reduction of iron oxide (ore) to metallic iron with methane. Here, the molten metal helps to split the carbon hydrogen bond in methane to free up both elements. In the industrial process, gaseous methane is fed to the bottom of a high-temperature reactor filled with molten metal as lead (Pb) or a molten metal alloy of nickel and bismuth (NiBi) and heated to 1,300°C. The molten metal promotes the formation of solid carbon and gaseous hydrogen. The carbon so formed rises through the molten medium and floats at the top, from where it is skimmed off and transferred to a carbon tank. A one-third mole of the hydrogen produced is used to heat the reactor and maintain the temperature, while the rest is cooled and stored. The Karlsruhe Institute of Technology (KIT) in Germany has developed a process that uses a


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