The general approach used in the Johnson Matthey methanation technology is to use relatively simple adiabatic reactors and recycle some product gas to dilute the feed gas and so control the temperature rise from the reaction. Final “trim” methanation stages may be used, depending on the required product quality. The trim reactors operate at lower temperature to take advantage of more favourable thermodynamic equilibrium. Specific process conditions and the configuration of the methanation reactors depend on the composition of the syngas, the required product specification and the integration of heat recovery with the site steam system.
The following description is typical of a methanation scheme.
Purified syngas from a gasifier is heated through interchange with hot gases leaving the methanator reaction stages and, after passing through a purification guard system, is split into two streams. The first is mixed with recycle gas before being fed to the 1st Methanator reactor. Here both CO shift and methanation reactions take place simultaneously over a fixed bed of CRG catalyst. Both reactions are exothermic and the temperature of the system is controlled by varying the recycle gas rate fed back to the inlet. Hot gases leaving the reactor are cooled with recovered heat being used to raise and superheat steam. The cooled gases are mixed with the remaining fresh feed and the combined stream fed to the 2nd Methanator reactor. Heat in the hot gases leaving the second reactor is used for steam raising and process gas heating. A portion of the cooled gas is taken off and used as recycle to the first reaction stage.
The remaining partially methanated gas not returned as recycle passes through a number of further reaction stages in order to maximise methane content and reduce carbon monoxide and hydrogen contents to meet the required product specification. Each reaction stage consists of a fixed bed of CRG catalyst operating adiabatically to bring the process reactants toward equilibrium with respect to the CO shift and methanation reactions. Heat from the hot gases leaving each reactor is used for process and boiler feed water heating with low grade heat finally being rejected to cooling water. Process condensate formed during the cooling steps is separated from the gas and fed to the plant battery limits.
Boiler feed water is imported to the plant and heated against process gases before passing to the steam drum. Steam raised through recovery of heat from the process is superheated and used as process reaction steam with excess steam being exported from the plant.