(70) Therefore, the risk of sulphuric acid dew point attack ... - DTI Home

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source of fuel for the plant. After several years of technology development and demonstration operation, this power plant technology is approaching the status of commercial operation. The concept of an IGCC power plant incorporates an oxygen or air-blown gasifier operating at high pressure and producing a raw gas, which is cleaned of most pollutants and burned in the combustion chamber of a gas turbine generator set for power generation. The sensible heat of the raw gas production process along with the hot exhaust gas from its combustion in the gas turbine are used to produce steam. The steam, in turn, is then utilised to generate additional electrical power through a series of steam turbines. The main subsystems of an IGCC power plant are therefore:- • Gasification plant including feedstock preparation system • Raw gas heat recovery system • Gas purification system with sulphur recovery • Air separation unit (ASU); required only for oxygen-blown gasification • Gas turbine with electrical generator • Heat recovery steam generator (HRSG) • Steam turbine system with electrical generator The actual coal gasification process for IGCC power generation is classified into three categories namely, stationary bed (Lurgi and British Gas Lurgi systems), fluidised bed (High Temperature Winkler, U-gas and Kellog-Rust- Westinghouse systems) and entrained-flow bed. The entrained flow bed gasification process uses an oxygen blown gasifier and is the most proven technology for single unit with large capacity applications. The entrained flow bed gasifier has essentially five distinctive types according to manufacturer (Texaco, Destec, Prenflo, Shell and GazCobinat Schwarze Pumpe). Of the five, two distinct categories are apparent: • Wet feed processes such as Texaco and Destec utilise a coal slurry feed. • Dry feed process such as Prenflo, Shell and GazCobinat Schwarze Pumpe (GSP) utilise a dry powder feed. Generally the temperatures within the gasifiers are lower in the case of the wet feed process than the dry feed process. Water-cooled walls, rather than firebrick are therefore necessary. Manufacturers claim that a dry powder feed gasifier has a slight advantage in terms of cycle efficiency over the coal slurry gasifier. Dry processes systems are however, considered to be more complicated than their counterpart. These complications are generally associated with reliability penalties. On the whole the unit investment for the dry feed gasifier is considered greater than the wet feed unit. A schematic showing a typical IGCC utility plant layout is shown in Figure 33. This figure illustrates the plant arrangement based on a Shell dry feed entrained-flow gasification process. The cycle is described in detail below:- (89)
Initially, the coal is pulverised in a roll mill and then conveyed to a dryer. Within the dryer the coal is flash dried in a stream of hot nitrogen which has been supplied from an ASU and heated by low-pressure steam. The dried coal is separated in cyclones and the nitrogen cooled and the water removed. This process reduces the moisture content of the coal considerably. The coal is then pressurised with additional nitrogen from the ASU and fed into the dry-feed, entrained flow, slagging gasifier through lock hoppers. In addition to the nitrogen, the ASU also provides a steady supply of oxygen into the gasification chamber. The gasification pressure vessel is protected from the hot gasification products by a tube wall construction in which intermediate pressure (IP) steam is raised. The gasifier operates at a pressure of 25bara and a temperature of 1400ºC and produces a raw fuel gas, mainly composed of carbon monoxide (CO) and hydrogen (H2). Most of the coal ash forms a molten slag, which falls into a water bath at the bottom of the gasification chamber. Sensible heat is recovered from the raw gas in a waste heat boiler situated at the top of the gasifier. This boiler evaporates water bled from the high pressure (HP) circuit. In addition, further heat is also recovered from a syngas cooler after exiting the gasifier. To ensure that the fly ash is solid prior to entering the syngas cooler, cooled raw gas is recycled from the outlet of the syngas cooler. The syngas cooler heat exchanger consists of economiser and evaporator surfaces and generates IP steam supplied directly from the IP pump. The remaining heat in the raw fuel gas is exchanged in a gas-to-gas heat exchanger. This exchanger utilises the raw fuel gas to re-heat the cleaned fuel gas after an acid gas removal process. The gas cleaning process itself is done in stages. Initially, the fly ash is removed by cyclones and by water scrubbers, which also absorb any hydrogen chloride (HCl) present. Heat is extracted for boiler feedwater heating and the cooled raw gas then passed to the purification stage where sulphur-bearing compounds, mainly hydrogen sulphide (H2S) and cabonyl sulphide (COS), are removed in order to protect the gas turbine and also to meet environmental legislation. These compounds are absorbed by counter-current washing with Purisol solvent and are recovered from this solvent in a series of flash columns. The solvent is recycled and the sulphurbearing compounds are sent to the sulphur recovery plant. This plant is based on a Claus design. Unreacted gases are treated in a SCOT tails gas recovery unit. Heat generated in the Claus plant is used for boiler feed water heating. The clean fuel gas is saturated with hot water in a humidifier, which helps to reduce the NOx formation in the gas turbine combustor. Prior to entry to the combustor, the fuel gas is further heated by an exchanger using a bleed from the high-pressure, high-temperature (HP/HT) economiser. At the combustor, the clean fuel gas is mixed with air supplied directly from the GE 9FA turbine compressor alongside compressed nitrogen, the original nitrogen source being air from the gas turbine compressor which has been separated in the ASU. From the gas turbine, the hot flue gas passes to the HRSG where steam is raised at two pressure levels (NB the IP stream is only superheated within the (90)
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Page 45 and 46: Detailed studies of CCTs within the
Page 47 and 48: knowledge, expertise and experience
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