Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
2.1 H2/CO-ratio
Synthesis gas for methane production is often classified by their stoichiometric
number (SN) which is the ratio between the hydrogen and carbon oxides according
to equation (A) [5].
SN
H 2
(A)
3v
CO
v
4v
CO
2
2v
C H
2
4
For stoichiometric methanation, there is little risk of carbon formation according to
the Boudouard reaction, even at elevated temperatures of up to 700°C. This becomes
a problem at lower SN. Lower SN also requires carbon dioxide removal to
a greater extent.
2.2 Reactor Designs
In order to control the heat of reaction in methanation there are several different
reactors that have shown success. There are a number of commercially available
methanation systems in use. Most of them are designed for methanation of syngas
produced from coal gasification at high pressures. Thus, the methanation reactors
are designed for pressures of 40-60 bar [2, 6].
2.2.1 Recycle Gas Processes
The recycle gas process uses adiabatic reactors with product gas recycling. The
recycled gas increases the mass-throughput over the reactor, thus increasing the
amount of heat that can be absorbed. The recycled gas is cooled and compressed
to the reactor inlet pressure before it is mixed with fresh syngas.
Haldor Topsoe has developed a methanation process that is called TREMP
(Figure 1), Topsoe’s Recycle Energy-efficient Methanation Process. The system
uses three adiabatic reactors that utilize product recycle and intermediate cooling.
The temperature of the reactors is controlled by the recycle ratio and is held bellow
the maximum allowed for the catalyst. The catalyst is developed by Topsoe as well
and has good temperature resistance allowing temperatures of 250-700°C. The
catalyst is called MCR-2X and according to Topsoe has excellent durability [2, 7].
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