Technology for SNG Production
Synthetic natural gas (SNG) is one of the commodities that can be produced from coal-derived syngas through the methanation process. The economic viability of producing SNG through coal gasification is heavily dependent on the market prices of natural gas and the coal feedstock to be used, the value of by-products such as carbon dioxide (CO2) (which could be used for EOR), and additionally the capital cost of the gasification plant. Currently, there is only one coal-to-SNG plant currently in commercial operation worldwide. In the middle years of the previous decade, when natural gas prices spiked at previously unencountered high levels, many proposals were made for new coal-to-SNG plants in the United States. In 2010, ten were still proposed or in various stages of development. As natural gas prices have fallen to low levels in the last few years, many or all of these proposed SNG projects as originally envisioned may not move forward to implementation.
Conventional SNG production is based on the methanation process, which converts carbon oxides and hydrogen in syngas to methane and water by the following reactions:
||CO + 3 H2 → CH4 + H2O
||ΔH = -210 kJ/mol
||CO2 + 4 H2 → CH4 + 2 H2O
||ΔH = -113.6 kJ/mol
The reactions take place over catalysts (predominantly nickel-based) in fixed-bed reactors. The reactions are highly exothermic; thus, a key challenge for the process is to manage the heat of reaction, and designing a catalyst system that can maintain its activity after prolonged exposure to high temperatures. The methanation process has been used extensively in commercial ammonia plants, where it is the final syngas purification step in which small residual concentrations of carbon monoxide (CO) and CO2 are removed catalytically by reacting with hydrogen. Effective sulfur removal is also necessary prior to methanation, since sulfur in the syngas will poison nickel-based methanation catalysts.
Other than nickel-based catalysts, ruthenium is the most active of all methanation catalysts, but its high cost requires low attenuation, at which point it is not much preferable to nickel. Molybdenum and tungsten are resistant to sulfur poisoning, but their activity and methanation selectivity are not particularly favorable.
Methanation Reactor Configuration
Many different types of reactor designs have been studied in the past, with emphasis on controlling the adiabatic reaction temperature rise. Methanation in coal gasification to SNG processes presents a considerable challenge in that the CO concentration in coal-derived syngas is much higher than that of an ammonia plant syngas. As a result, a much higher temperature rise in the reactor is expected. If not controlled properly, the temperature rise could be high enough to cause catalyst sintering and decomposition of the product methane to carbon. Novel reactor designs and configurations are used to circumvent this problem. In many ways, this development is similar to that in syngas-based exothermic catalytic synthesis of methanol, as well as Fischer-Tropsch synthesis.
Methanation is a commercially proven technology. Current technology is primary based on fixed-bed reactors operating in series. Technology vendors include Lurgi, Haldor Topsoe, and others. Examples of three different types of methanation reactor configuration/design include:
- Equilibrium-limited fixed bed reactors in series - where the bulk of the conversion is carried out in multiple reactors arranged in a series/parallel configuration (bulk methanation). Fresh syngas feed can be split between each reactor. Sufficiently cooled equilibrium discharge from the last reactor is compressed and recycled to mix with fresh feed to the 1st reactor to limit the 1st reactor's temperature rise. Equilibrium discharge from the 1st reactor is cooled and mixed with fresh feed to the 2nd reactor to limit the 2nd reactor's temperature rise. This procedure is continued to the 3rd and subsequent bulk methanation reactors. Net discharge from the last bulk methanation reactor, after recycle extraction, is further cooled to condense out the water generated from bulk methanation before being sent to a final adiabatic cleanup reactor to complete the conversion. This design is similar to that being used/proposed in current coal-to-SNG designs of Lurgi and Haldor Topsoe.
- Throughwall-cooled fixed bed reactor - where the conversion is carried out isothermally in multiple parallel catalyst-packed tubes encased in a vessel shell (similar to a vertical shell-and-tube exchanger arrangement) filled with water. Reaction heat is removed by generating steam in the shell side. This design is similar to that being investigated by Lurgi for isothermal methanol synthesis. Currently, this type of methanation reactor is not in commercial practice.
- Slurry bubble reactor - in which the conversion is carried out by bubbling fresh feed through an oil/catalyst slurry filled reactor. Reaction heat is removed by generating steam in the internal tubular boiler. This design is similar to that being used in Air Products and Chemical's LPMEOH™ methanol synthesis process, and that being used in Sasol's FT synthesis process.