Conventional catalytic steam-reforming processes are the dominant method used to produce hydrogen from hydrocarbons on an industrial scale.
At high temperatures – usually between 700°C and 1,100°C – and in the presence of a metal-based catalyst such as nickel, steam reacts with the hydrocarbon to yield carbon monoxide and hydrogen.
Using such a catalytic steam-reforming process to produce hydrogen from glycerine is also under investigation around the world as an environmentally friendly way to make use of the glycerine by-product produced from biodiesel plants.
For that reason, Hydromotive, a subsidiary of the Linde Group, has announced that it is to build a plant at its chemical site in Leuna, Germany, to produce hydrogen from glycerine.
The plant, which will reprocess, pyrolyse and reform raw glycerine, will come on-stream in mid-2010. It will produce a hydrogen-rich gas, which will be fed into the company’s existing plant for purification and liquefaction.
According to Frank Wiessner, an engineer at Linde Engineering Division in Pullach, Germany, a mixture of desalted glycerine and steam will be fed to a gasifier, and after gasification co-produced methane and other hydrocarbons will be reformed, producing a syngas mixture of carbon monoxide, carbon dioxide and hydrogen.
The operating temperature of the gasifier is one parameter that will be optimised during the first test phase of the process, but it will be between 550-750°C, while the reformer will operate at about 850°C.
Additional hydrogen will be recovered from the carbon monoxide in the syngas stream by a lower-temperature water gas-shift reaction producing carbon dioxide and more hydrogen.
Pressure swing adsorbers will then separate off the carbon dioxide from the syngas, leaving a pure hydrogen gas stream.
Dr Valerie Dupont, a senior lecturer at Leeds University, and a leading researcher in the field of such processes, told The Engineer Online that the Linde process is similar to conventional catalytic steam reforming processes in that the maximum theoretical hydrogen purity the company will likely be able to achieve from its reformer is below 70 per cent, which is why it requires coupling with an existing purification stage from a steam methane-reforming plant to achieve a 100 per cent hydrogen gas product stream.
For her part, she is investigating a more novel process at Leeds University called ‘sorption enhanced catalytic steam reforming’, a process that she has evaluated experimentally in a continuous flow fixed-bed reactor.
Unlike the Linde process, sorption enhanced steam reforming makes use of a commercial Ni-based catalyst and a carbon dioxide-sorbent such as dolomite or hydrotalcite to perform the steam-reforming reactions and in-situ carbon dioxide removal.
The syngas produced by this method, she said, can reach 95 per cent in hydrogen content per volume when using pure or raw glycerol at 500°C, because a water gas-shift occurs in the reactor itself and the resulting carbon dioxide is trapped in the sorbent. The sorbent then needs to be regenerated in a separate stage by thermal swing if intended to be recycled.
Rather than operating at high pressures like the Linde system, which operates above 28 bar due to economies of scale and the use of pressure swing adsorbers, Dupont’s scheme operates near atmospheric pressure, to make use at laboratory scale of the more favourable thermodynamics of the steam-reforming reaction of glycerol at that pressure, even when coupled with the carbon dioxide-chemisorption reaction.