Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
2 Production of SNG
The first step in synthesizing methane or SNG is production of synthesis gas (CO
and H2). This may be done from any kind of carbon containing material such as
coal or biomass; more about syngas production in the next section. In order to increase
the CH4 content of the produced gas, methanation is required. The reaction
is strongly exothermic and due to that, heat removal from the reactors is essential.
[1]; a more extensive summary may be found here [2]. Methanation occurs according
to reaction (1) and (2). Due to the high amount of heat that is released and to
the high concentrations of the reactants, measures have to be taken to avoid hotspots
and to limit the temperature increase in the reactor. The temperature should
also be kept low to favor the equilibrium [1].
CO 2
4 2
CO2 4H2
CH4
2H2
3H CH H O
ΔH 206kJ/mol
0
(1)
298
O ΔH 165kJ/mol
0
(2)
2CO C CO2
(3)
CH C 2H
(4)
4
2
298
The catalyst used in methanation reactors is most commonly nickel-based and
supported on alumina, kaolin or calcium aluminate. Sulphur as well as arsenic is
severe catalyst poisons, which must be removed upstream of the catalyst. The
catalyst contain < 15 wt % nickel and precaution must be taken to prevent formation
of the highly toxic nickel carbonyl Ni(CO)4. The formation of the carbonyl is
favored by low temperatures < 200°C and high carbon monoxide partial pressures.
It is therefore important to have proper procedures for start-up and shutdown [1].
CO also reacts with iron to form iron carbonyl which is poisonous and cause
problems with corrosion. Iron carbonyl also decomposes on the catalyst when the
temperature is increased. Thus carbon monoxide must be heated in stainless steel
heat exchangers. Years of plant operations have shown that with the right precautions,
carbonyl formation can be suppressed successfully [1].
Carbon dioxide, in reaction (2), is first converted to carbon monoxide with the reverse
shift reaction and then it is converted to methane according to reaction (1)
[3]. The Boudouard reaction (3) will be thermodynamically favored at elevated
temperatures, e.g. at the outlet of the reactor. However if temperatures are kept
moderately low and small residual hydrogen exists in the gas outlet, it can be
avoided [1].
Typically the reaction is operated at inlet temperatures of 250-300°C and at
pressures in excess of 30 bar. The high pressure favors the equilibrium and also
improves the kinetics [1]. As will be shown in the next section, CO2 is co-produced
when producing the synthesis gas used in the methanation. This carbon dioxide
may be removed either upstream or downstream the methanation unit [4].
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