Coal-to-Synthetic Natural Gas Process Description

Synthetic natural gas (SNG) is one of the commodities that can be produced from coal-derived syngas through the methanation reaction. The economic viability of producing synthetic natural gas (SNG) through coal gasification is heavily dependent on the market prices of natural gas and the coal feedstock to be used, in addition to the capital cost of the gasification plant. While there is only one coal-to-SNG plant currently in commercial operation worldwide, as of 2010, ten new coal-to-SNG plants have been proposed or are in various stages of development within the United States alone.

Coal-to-SNG Process
The Dakota Gasification Company’s (DGC) plant in Great Plains, North Dakota, is the only coal-to-SNG plant in operation worldwide, producing approximately 160 MM scf/day of SNG.  The technology is considered commercially proven since the DGC plant has been in operation since 1984. The plant uses Lurgi dry-ash gasifiers for syngas generation, add an equilibrium-limited fixed bed bulk-methanation process for SNG synthesis.  Advanced entrained-flow gasifers are of more interest in recent designs to eliminate the need for processing the tar byproducts.  New gasification technologies are also being developed specifically for coal-to-SNG production, examples include hydrogasification and catalytic gasification.

Figure 1 shows an example of a simplified block flow diagram of a coal-to-SNG design as proposed by ConocoPhillips using their E-Gas™ gasification technology for syngas generation with Rectisol acid gas removal (AGR) and bulk-methanation process for SNG synthesis.  As shown, the overall plant consists of three key processing areas:

• Gasification island,  which includes coal handling and preparation, gasification and heat recovery, slag handling, high-temperature syngas cooling and particulate removal.
• Syngas cleanup and conditioning, which includes scrubbing, low-temperature heat recovery, water-gas-shift and sulfur recovery.
• SNG production and compression section, which consists of AGR, methanation, production dehydration and compression.

Supporting facilities include the air separation plant, the power train and other offsite plants such as water treatment.  All are commercially demonstrated technologies.

CLICK ON GRAPHIC TO ENLARGE Figure 1: A Simplified Coal-to-SNG Block Flow Diagram

For performance and efficiency data on this plant scheme, go to the Coal-to-SNG Performance & Efficiency page.

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