READ, Andy, John (Clean Coal Development, Westwood WayWestwood Business Park, Coventry CV4 8LG, GB)
ATHERTON, David, Alan (Clean Coal Development, Westwood WayWestwood Business Park, Coventry CV4 8LG, GB)
PERISELNERIS, Jonathan, Nerio (Clean Coal Development, Westwood WayWestwood Business Park, Coventry CV4 8LG, GB)
READ, Andy, John (Clean Coal Development, Westwood WayWestwood Business Park, Coventry CV4 8LG, GB)
ATHERTON, David, Alan (Clean Coal Development, Westwood WayWestwood Business Park, Coventry CV4 8LG, GB)
CLAIMS
1. An integrated gasifier combined cycle power plant comprising a gasifier; and at least one gas turbine downstream in the syngas stream from the gasifier and driven by the syngas stream therefrom; the plant being characterised by a burner for superheating steam, produced by the cooling of the fuel gas stream between the gasifier outlet and the gas turbine, by combusting hydrogen and oxygen in the steam to heat it, and at least one secondary turbine downstream of the said burner and driven by the superheated steam therefrom.
2. A power plant according to claim 1 wherein at least some of the hydrogen combusted in the burner for superheating steam is extracted from the syngas stream.
3. A power plant according to claim 1 or 2 comprising means for cooling and/or recovering heat from the syngas stream between the gasifier and the gas turbine, said means supplying, in use, steam for superheating to the burner.
4. A power plant according to any preceding claim wherein there are at least two secondary turbines in the steam stream from the burner, there being provided between the at least two secondary turbines, at least one further secondary burner for reheating the steam by combusting hydrogen and oxygen in the steam stream between the said secondary turbines.
5. A method of operating a gasifier combined cycle power plant comprising a gasifier; and at least one gas turbine downstream in the syngas stream from the gasifier and driven by the syngas stream therefrom; the method being characterised by superheating steam, produced by the cooling of the fuel gas stream between the gasifier outlet and the gas turbine, by combusting hydrogen and oxygen in the steam to heat it and driving at least one secondary gas turbine with the superheated steam.
6. A method according to claim 5 wherein at least some of the hydrogen used to superheat the steam is extracted from the syngas stream of the power plant.
7. A method according to claim 5 or 6 comprising cooling and/or recovering heat from the syngas stream between the gasifier and the gas turbine, to supply steam for superheating.
8. A method according to any of claims 5 to 7 wherein there are provided at least two secondary turbines in the steam stream, the steam being reheated between the secondary turbines by combusting hydrogen and oxygen in the steam stream between them. |
Improved Power Plant
The present invention relates to a modified combined cycle power plant.
In conventional gasification combined cycle power plants, or coal gasification plants generating hydrogen or other syngas, an efficient design requires capture of as much low grade heat as possible, and its reuse. In order to capture this low grade heat, conventional gasification power plants generate a lot of high pressure, but low temperature, steam. To make efficient use of this steam, it must be superheated to temperatures suitable for operation of steam turbines.
In this document, the term 'syngas' is be used as a generic term to mean any 'synthetic' (as opposed to natural) gas, generated by a gasifier or any other industrial process, with sufficient calorific value to be used as a fuel.
In a conventional 'integrated' gasification combined cycle plant of the kind shown in Figure 1 of the drawings complex steam integration of the whole plant is required, which is both expensive and operationally complex. Further, in such systems, the superheat available is limited by the gas turbine exhaust temperature.
In accordance with the invention, there is provided an integrated gasifier combined cycle power plant comprising a gasifier; and at least one gas turbine downstream in the syngas stream from the gasifier and driven by the syngas stream therefrom; the plant being characterised by a burner for superheating steam, produced by the cooling of the syngas between the gasifier outlet and the gas turbine, by combusting hydrogen and oxygen in the steam to heat it, and at least one secondary turbine downstream of the said burner and driven at least in part by the superheated steam therefrom.
Preferably, the hydrogen used in the superheating of the steam is extracted from the syngas stream.
A conventional coal gasification combined cycle power plant incorporating carbon dioxide capture 10 is illustrated schematically in Figure 1 of the drawings.
Fuel is supplied to a gasifier 12 and downstream shift reactors 14, 16 convert CO and steam in the syngas from the gasifier 12 into hydrogen and CO 2 . The fuel used may be coal or any other suitable hydrocarbon fossil fuel or biomass. The syngas is then passed to an acid gas removal unit 18 which removes hydrogen sulphide and carbon dioxide from the syngas stream which goes on to power the main turbine 20 of the plant.
The gasifier 12 and shift reactors 14 and 16, in particular, generate significant amounts of heat which can be used to generate high pressure steam at around 250-400 deg C. In order to make efficient use of this steam, however, it must be superheated, preferably to above 500 deg C. In a conventional plant of the kind shown in Figure 1 , this is achieved by superheating in a heat recovery steam generator ("HRSG') 22 downstream of the main turbine 20 in the flue gas stream. However, this requires, as mentioned above, quite complex steam pipework. Further, the total superheating available from the HRSG is limited by the heat remaining in the exhaust gases from the gas turbine 20. Consequently, if the HRSG is employed in superheating steam from the gasifier/shift reactor train, it is not able to generate as much steam in a conventional manner as would otherwise be the case. The overall cycle efficiency of the plant is therefore limited.
A modified gasification combined cycle power plant in accordance with the invention will now be described in detail, by way of example, with reference to Figure 2 of the drawings, which is a schematic diagram of the power plant according to the invention. Components of the plant which are
common to both the modified plant of Figure 2 and the conventional plant of Figure 1 are identified by the same reference numerals in both drawings.
The flue gas stream from the gasifier 12 and shift reactors 14 and 16 contains, among other things, hydrogen, in some cases, more than 70%.. In the plant of the invention, hydrogen from the syngas stream, following removal of acid gases harmful to the atmosphere at acid gas removal unit 18, is combusted directly with oxygen added to low temperature steam which is generated by a gas cooling/heat recovery unit 13 at the outlet of the gasifier 12, upstream of the acid gas removal unit 18. Where the syngas stream contains a sufficient proportion of hydrogen, the syngas stream itself can by combusted with the oxygen. Alternatively, hydrogen can be extracted from the syngas stream so as to obtain the required level of purity for the hydrogen to be combusted with oxygen. Oxygen and the hydrogen are added to the steam train at burner 30. The product of the oxygen/hydrogen combustion at burner 30 is itself mainly steam, (although some impurities may be present to a level permitted by the needs of the steam turbine and downstream condenser) so that the steam in the steam stream from the gas cooling/heat recovery unit 13 remains of relatively high purity and, so, can be used in a normal steam turbine.
The steam turbine may be a single turbine or, as shown in Figure 2, may be a train of turbines comprising a first high pressure turbine 32 which is driven directly by hot high pressure steam from the output from the burner 30. The steam stream at the output of the high pressure turbine 32 is reheated by the addition of further oxygen and hydrogen at a secondary burner 34 to produce steam capable of driving a second intermediate or low pressure turbine 36. This process can be repeated until the steam is condensed to cold water at the outlet 38.
In addition to, or as an alternative to, reheating at least some of the output steam from the first burner 30, cooler intermediate pressure steam can be
returned to the main fuel/flue gas stream upstream of the first shift reactor 14.
The energy added by the oxygen/hydrogen combustion, which superheats the steam, can be recovered very efficiently by the steam turbine or turbines, downstream of the burner 30 in the steam stream, improving the overall thermal efficiency of the plant. Furthermore, the steam pipework required is much simplified in comparison to the conventional system shown in Figure 1.
Finally, because the superheating of the steam at burner 30 is driven by added materials - hydrogen from the fuel/flue gas stream and oxygen - the ability to superheat the steam is not limited as it is where heat from the plant's exhaust gases is used, as in conventional systems.