Clean Coal Technology

Integrated gasification combined cycle (IGCC)

Like PFBC, the technology is relatively new in connection with power generation. Coal-based IGCC plants for power generation passed through a critical stage in their development during the 1990s.

IGCC uses a combined cycle format with a gas turbine driven by the combusted syngas, while the exhaust gases are heat exchanged with water/steam to generate superheated steam to drive a steam turbine. Using IGCC, more of the power comes from the gas turbine. Typically 60-70% of the power comes from the gas turbine with IGCC, compared with about 20% using PFBC.

Coal gasification takes place in the presence of a controlled ‘shortage’ of air/oxygen, thus maintaining reducing conditions. The process is carried out in an enclosed pressurized reactor, and the product is a mixture of CO + H2 (called synthesis gas, syngas or fuel gas). The product gas is cleaned and then burned with either oxygen or air, generating combustion products at high temperature and pressure. The sulphur present mainly forms H2S but there is also a little COS. The H2S can be more readily removed than SO2. Although no NOx is formed during gasification, some is formed when the fuel gas or syngas is subsequently burned.

Three gasifier formats are possible, with fixed beds (not normally used for power generation), fluidized beds and entrained flow. Fixed bed units use only lump coal, fluidized bed units a feed of 3-6 mm size, and entrained flow gasifiers use a pulverised feed, similar to that used in PCC.

IGCC plants can be configured to facilitate C02 capture. The new gas is quenched and cleaned. The syngas is ‘shifted’ using steam to convert C0 to C02, which is then separated for possible long-term sequestration.

Characteristics

The IGCC demonstration plants use different flow sheets, and will therefore test the practicalities and economics of different degrees of integration. These are discussed in the IEA Coal Research report OECD coal-fired power generation – trends in the 1990s, IEAPER/33. In all IGCC plants, there is a requirement for a series of large heat exchangers, which become major components. In such exchangers, solids deposition, fouling and corrosion may take place. Currently, cooling the syngas to below 100°C is required for conventional cleaning, and it is subsequently reheated before combustion. Substantial heat exchange vessels are needed. At Puertollano, quenching is used to cool the syngas. This is a simple, but relatively inefficient procedure, however, it avoids deposition problems, as the ash present is rapidly cooled to a solid non-sticky form. The cold gas cleaning processes used are variants of well proven natural gas sweetening processes to remove acid impurities and any sulphur present.

Ash behaviour in a gasifier is a critical parameter, both in terms of the satisfactory formation of a slag in entrained flow, and the possibility of solids deposition in the syngas cooler/heat exchanger. At lower temperatures, such as those in fluidized and fixed bed gasifiers, tar formation and deposition may prove to be a difficulty. One advantage of gasification under pressure is that the effective gas volumes involved are far smaller from gasification than from PCC.

There are significant technical challenges. Highly integrated plants tend to have long start-up times (compared to PCC units), and hence may only be suitable for base-load operation.

With pressurized gasification (as with PFBC), the supply of coal into the system is considerably more complex than with PCC. Some gasifiers use bulky and costly lock hopper systems to inject the coal, while others have the coal fed in as a water-based slurry. Similarly, by-product streams have to be depressurized, while heat exchangers and gas cleaning units for the intermediate product syngas must themselves be pressurized.

Source: http://www.iea-coal.org.uk/content/default.asp?PageId=74

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