Page 78 - Energize July 2022

TECHNICAL

Natural gas consists of a mixture of hydrocarbon products, mainly This process typically requires three separate reaction vessels.
methane, which consists of carbon and hydrogen (CH 4). The hydrogen Systems have been developed which combine the three reactions
component can be used by first extracting the carbon component into one vessel, reducing the reformer volume by ⅓ and the costs
in the form of CO 2. In most commercial units, natural gas is passed by ⅔. 6
through a thermal reformer which separates the methane into CO 2 These reactions are usually carried out using a nickel support
and H 2. PEMFC are the most commonly used methane-powered fuel catalyst at temperatures over 500˚C. The product gas is a mixture
cells, but for larger units, MCFC and SOH units are used. of carbon monoxide, carbon dioxide, hydrogen, methane and
steam. Waste heat from the later processes can be recycled as
Pre-treatment methane fuel cell input into the endothermic steam reforming process.
Two types of hydrocarbon reforming systems are used for the
external fuel processing in natural gas fuel cell power plants: Internal reforming
• Catalytic steam reforming Many internal reforming systems have been applied for use with
• Partial oxidation reforming (thermal or catalytic) molten carbonate or solid oxide fuel cell stacks. The heat required
to reform low molecular weight hydrocarbons can be provided
Steam reforming (SR) is an endothermic process, where fuel and via the heat generated by the stack. In direct internal reforming,
water are injected into the system. SR provides the highest yield the heat from the anode and the steam from the electrochemical
of hydrogen. Partial oxidation (POX) reforming is an exothermic reaction in the MCFC or the SOFC is used. Gases which have been
process, where the fuel is partially burned but water is not added. used in direct reforming stacks include natural gas, naphtha,
These processes are handled externally to the fuel cell stack, with kerosene and coal gases.
the product stream being fed into the individual fuel cell layers.
Currently, steam reforming is the technology of commercial choice Ammonia fuel cell (AMFC)
for hydrogen production from natural gas in stationary processes. Although “green” ammonia is not readily available at the moment,
the possibility of its use as a fuel in the future is high, and it is
Catalytic steam reforming worthwhile considering the ammonia fuel cell. Ammonia is easily
This comprises three stages as shown in Figure 12. liquefied under pressure with a liquid density of 601 g/L at 300
Steam reforming is an endothermic production process that K, and the liquefaction requires a pressure of only 10 bar at 300
combines the fuel with steam. The steam-reforming equivalent of K. AMFC can be either direct or indirect, depending on where the
the reaction for methane is as follows: decomposition of ammonia occurs. Indirect ammonia fuel cells
involve external thermal decomposition of ammonia to release
CH 4+ H 2O →3H 2+CO (400 - 650˚C) hydrogen. Direct ammonia fuel cells use ammonia directly within
the fuel cell to utilise the chemical energy stored within ammonia
The next step in is the water-gas shift reaction. Most of the itself. This eliminates the necessity of on-board hydrogen storage.
remaining carbon monoxide reacts with water to produce
additional hydrogen. A typical conversion is from 7,1 % CO in a Pre-treatment ammonia reforming
steam reformer’s output to 0,5% coming out of the water-gas shift Ammonia reforming has only hydrogen and nitrogen gas as by-
reactor. products.

CO + H 2O → H2 + CO 2 (200 - 450˚C) The ammonia cracking reaction is: 2NH 3 ⇒ N 2 + 3H 2

This is followed by a selective oxidation reaction: removing the The reaction takes place at over 400˚C, which requires an external
small amount of remaining CO to the level of several ppm. heat source for PEMFC since the exhaust from a traditional PEM
exits at only 80˚C. Some of the hydrogen in the reformer’s output
CO + ½O 2 → CO 2 (120 - 170˚C) stream can be burned to provide the necessary temperature for

Figure 12: Catalytic steam reforming (Tokyo gas co.)

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