Researchers Propose Dual-Bed Configuration to Increase Efficiency and Reduce Emissions from Coal Gasification

Jose Michael

Scheme of the gasification process with air using two reactors. Click to enlarge. Credit: ACS

Researchers in Italy are proposing a new dual-bed configuration for coal gasification that, in laboratory simulations of Coal-to-Liquids production, is 71.1% more energy efficient; increases the mass yield of synthetic fuel by 39.4%; and releases 31.9% less CO2 than conventional gasification.

Applied for power generation, the dual bed configuration increases plant efficiency by 27.9% and decreases CO2 emissions decrease by 21.8%, compared to a conventional IGCC process. A paper on their work is scheduled for the 19 November issue of the ACS journal Energy & Fuels.

The dual fluidized bed reactor scheme thermally couples a combustor, in which coal is burnt with air, and a gasifier, which is fed with coal and steam, via the circulation of an inert solid. The required heat for the gasifier is transferred by using the inert solid as the carrier.

Pressures in the two reactors are kept different (19.5 bar for the combustor, 20 bar for the gasifier) both to avoid the contamination of syngas, and to keep the two environments apart from each other. The fluidization agent in the combustor is the same air used for pneumatic transport of the inert solid,; in the gasifier, it is the steam.

In our process, coal and saturated steam at 20 bar are fed to the gasifier. The heat carrying the mixture of inerts and ash from the combustor enters the gasifier from the top of the reactor. The syngas produced exits from the top whereas unreacted char and inerts exit from the bottom by gravity and are carried back to the combustor by means of compressed air. In the combustor, coal is burnt to supply the heat needed to take the inert back to the temperature of combustion. Syngas is routed to a cyclone to remove dust and inerts for which, because of unavoidable losses, a makeup is needed. A gas expander is fed by combustion gases and operates between the combustor pressure and near-atmospheric conditions. Power from the expander is recovered to run the air compressor.

—Sudiro et al. (2008)

For the analysis of the Fischer-Tropsch CTL application, the researchers coupled the gasifier-combustor configuration with a conventional CTL process. The syngas is rich in methane, which needs to be removed before entering the FT process in order to avoid reducing the yield. The methane removal, achieved using a pressure-swing-adsorption (PSA) unit with activated carbon, results in a synthetic natural gas (SNG) co-product stream.

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