A polygeneration plant, as its name implies, is one that simultaneously produces two or more marketable products. For the purposes of this report, written by Anne Carpenter, electric power must be one of the products. Most of the process steps in a polygeneration plant involve commercially-proven technologies. These technology modules can be interconnected in different configurations to produce the required products. The figure (right) shows some possible configurations. Coal is first gasified to produce synthesis gas (syngas, consisting primarily of hydrogen and carbon monoxide), which is subsequently cooled and cleaned. A portion of the syngas can be used directly in the gas turbines of an integrated gasification combined cycle (IGCC) power plant (if CO2 capture is not required) and/or fuel cells to generate electricity. The rest passes through a shift reactor to adjust the molar H2:CO ratio to the required value. The cleaned and shifted syngas is then utilised to produce the desired products (electric power, H2, liquid fuels and/or chemicals). In addition, a number of by-products, such as sulphur or sulphuric acid, can be generated for sale.
There are several potential benefits associated with coal-based polygeneration plants, including:

•  enhanced energy security by reducing a country’s dependence on oil and natural gas imports:
•  low emissions and the potential for carbon capture, important considerations with tightening emission regulations;
•  cleaner products. The transport fuels produced are cleaner than petroleum-derived fuels, resulting in lower vehicle emissions;
•  feedstock flexibility. A wide variety of coals can be gasified, including cheaper low grade and high sulphur coals. Cogasification with biomass and wastes is also possible;
• market flexibility. Polygeneration plants offer the flexibility to switch from one product to another;
•  increased efficiency by taking advantage of synergisms between the different processes;
• lower production costs than for separate plants mainly due to the increased efficiency, higher equipment utilisation and product flexibility.

The main downsides associated with polygeneration plants are the added operational complexity of integrating a power facility with a chemical facility, and the increased capital cost (compared with a single product coal gasification plant). The added complexity increases the market risks associated with polygeneration plant construction and operation, and thus may reduce the number of companies willing to make the necessary investment. They may also prefer to become more familiar with IGCC technology before they build a coproduction plant - so far only a handful of coal-based IGCC demonstration plants have been built. Petroleum refineries do have this experience, albeit with gasifying petroleum residuals, and have been operating polygeneration plants for some years.
Although investment costs are higher than for standalone plants, financial analyses have indicated that reasonable rates of return are possible over a 30-year plant life. Financing, though, may be difficult to obtain, as financiers are reluctant to assume technology risk and technology developers cannot always provide performance guarantees for their equipment.

Polygeneration from coal
Anne Carpenter
CCC/139, ISBN 978-92-9029-458-0, October 2008
£255 non-member countries
£85 member countries
£42.50 educational price