Direct-fired/conventional steam

The direct-fired/conventional stream method is the most commonly used method of producing bioenergy. This process involves the direct combustion of biomass to produce steam, which then turns turbines that drive generators to produce electricity. 

Type of cycle Conventional pulverized coal steam Eirst commercial MHD steam Advanced direct-fired... 

Montedison Low-Pressure Process. The Montedison low-pressure process [940], [1036], [1128], [1129] involves a split flow to two primary reformers. About 65% of the feed-steam mixture flows conventionally through the radiant tubes of a fired primary reformer followed by a secondary reformer. The balance of the feed-steam mixture passes through the tubes of a vertical exchanger reformer. This exchanger reformer has a tube sheet for the catalyst tubes at the mixed feed inlet. There is no tube sheet at the bottom of the tubes, where the reformed gas mixes directly with the secondary reformer effluent. The combined streams flow on the shell side to heat the reformer tubes in a manner similar to that described for the M. W. Kellogg KRES reformer, see Sections 4.1.1.8 and 5.1.4.3). The process air flow is stoichiometric. Synthesis is performed at 60 bar in a proprietary three-bed indirectly cooled converter with am-... 

The HTCR reactor consists of a number of bayonet reformer tubes and combines basically the radiant section and the convection section of a conventional HSR in a single piece of equipment. The reaction heat is provided by the flue gas fiowing on the outside of the reformer tubes and by reformed gas fiowing in an upward direction in the bayonet tubes. This results that is about 80% of the fired duty is utilized in the process, and steam export is minimized. 

An IGCC plant generally produces fewer water effluents than a conventional coal-fired power plant does. The amount of process water blowdown is about the same for both gasification and direct coal combustion. However, the steam cycle in IGCC power plants produces much smaller amounts of wastewater blowdown because less than 40% of the total power generated comes from the steam cycle. 

Modification of direct coal combustion for CO2 removal would be more difficult. CO2 could be removed from the flue gas after conventional combustion by acid gas removal technology. Although this approach has found some commercial application, the low pressure and low concentration of the CO2 in the flue gas makes it a relatively expensive method. Removing 90% of the CO2 from flue gas of a conventional coal-fired boiler would increase the capital cost by a factor of 3.0 and thermal efficiency drops by 12% compared to a conventional direct coal combustion power plant. The larger capital increase and efficiency loss with CO2 recovery is principally due to recovery at low pressure which requires a larger flue gas compression, CO2 absorbers, and increased steam requirements. Depending on the cost of coal and capital, the increased electric cost for CO2 removal with a direct coal combustion power plant is 2.0-3.0 times that of a conventional direct coal combustion power plant. 

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