Analysis and Design of Methanation Processes in the Production of Substitute Natural Gas from Coal

Methanation, the final major step common to all processes for the conversion of coal to substitute natural gas, upgrades the sulfur-free synthesis gas to comply with the pipeline quality standards: < 0.1 % CO,> 900 BTU/SCF (35.5 MJ/Nm3) A reactor system featuring good temperature control and effective heat recovery is essential for effecting the strongly exothermic reaction (CO + 3H2 → CH4 + H2O) to a high overall conversion. The different reactor arrangements of the leading processes (currently at the pilot plant stage of development) are critically assessed and compared: multistage adiabatic packed beds, fluidized bed, Raney nickel coated tubes with circulating coolant, and a 3-phase slurry. Computations of reaction equilibria and energy balances are performed to delineate operating conditions and maximum yields for both isothermal and adiabatic methanators in the range of total pressures from 50 to 90 atm. The effects of dry recycle and the concurrent shift reaction on the equilibrium composition and properties of the product stream from an isothermal methanator are shown. Graphs are provided for predicting the extent of formation of undesirable nickel carbonyl or carbon (as graphite) at equilibrium. On the basis of a comprehensive literature survey, the intrinsic kinetics and selectivity are analyzed relative to catalyst properties. A thermodynamically consistent rate expression is formulated and employed in methanator design. It is based on a published rate expression applicable to a commercial nickel-kieselguhr catalyst, Harshaw Ni-0104T. Transport effects, catalyst deactivation, and their influence on reactor operation are examined. Intrapellet gradients of concentration are usually significant while intrapellet temperature gradients are moderate. A modular process simulator is developed and applied to the design of a commercial scale methanation system. A two-stage cylindrical radial flow packed reactor with feed split and cold recycle is selected for the simulation studies. It is shown to be superior to the other configurations in many respects, notably by a low pressure drop. Cooler-condensers of the vertical downflow type are designed by a rigorous finite element algorithm whose accuracy has been validated by comparison with experimental results reported in the literature. Nearly all the published economic estimates have tended to understate costs and there is a need for a uniform, standardized accounting procedure. The sensitivities of the (free-market) product gas price to 6 process and economic factors are estimated; it is most sensitive to the cost of coal and least sensitive to the debt fraction. Some key problems and promising directions for future research and development are identified. The development of more sulfur-tolerant, long-lasting catalysts and reliable kinetic data under conditions of industrial methanator operation deserve a high priority.