Supplementary firing

Figure 1-41. Supplementary fired exhaust gas steam generator.

The turbine temperature, flow, and speed increases in a very short time of about three to five minutes to the full rated parameters. There is usually a short period of time where the temperature may overshoot. If supplementary firing or steam injection for power augmentation is part of the plant system, these should be turned on only after the gas turbine has reached full flow. The injection of steam for power augmentation, if done before full load, could cause the gas turbine compressor to surge. 

Fig. 7.3. Open circuit gas lurbine/closed steam cycle combined plant (CCGT). No supplementary firing...

The exhaust gases from a gas turbine contain substantial amounts of excess air, since the main combustion process has to be diluted to reduce the combustion temperature to well below that which could be obtained in stoichiometric combustion, because of the metallurgical limits on the gas turbine operating temperature. This excess air enables supplementary firing of the exhaust to take place and higher steam temperatures may then be obtained in the HRSG. 

Finckh, H.H. and Pfost, H. (1992), Development potential of combined cycle [GUD] power plants with and without supplementary firing, ASME J. Engng Power Gas Turbines 114(4), 653-659. 

For the supplementary fired plant of Fig. 9.2b, the artificial efficiency would be W 1... 

The above simple analysis has to be modified for a supplementary fired CHP plant such as that shown in Fig. 9.3c, meeting a unit electrical demand and an increased heat load A. The reference. system fuel energy supplied is now... 

For the supplementary fired plant of Fig. 9.2b with Aiy = 7.2 and with the parameter tp taken as 1.2, so that Fj = 6, the fuel energy savings ratio is... 

Thus the FESR is less attraetive when there is a large heat load and a WHB with supplementary firing is used. In general, the FESR is probably the most useful of the CHP plant performanee eriteria as it ean be used direetly in the eeonomie assessment of the plant [1. ... 

There are many gas turbine CHP plants in operation for a range of purposes and applications. Here we describe the salient features of two such plants, each operating with a WHR but also with supplementary firing which can be introduced to meet increased heat demands. 

A gas turbine CHP. scheme which operates at Liverpool University, UK, consists of a Centrax 4 MW (nominal) gas turbine with an overall efficiency of about 0.27, exhausting to a WHB. The plant meets a major part of the University s heat load of about 7 MW on a mild winter s day. Supplementary firing of the WHB (to about 15 MW) is possible on a cold day. Provision is al.so made for by-passing the WHB when the heat load is light, in spring and autumn,. so that the plant can operate very flexibly, in three modes viz., power only, recuperative and supplementary firing. 

Electrical power output Heat output (normal load) (with supplementary firing) Gas fuel energy supply Thermal efficiency... 

As with the electrical load profile, it is also necessary to analyze the heat load over the daily and annual cycles. Ideally, the heat load will match the available heat from the electrical generator (however, this is rarely the case). There will be periods when supplementary output will be necessary which can be achieved by, say, supplementary firing the waste heat gases of a gas turbine, or heat output reduction is necessary by the introduction of bypass stacks. For a steam turbine installation bypass pressure-reducing valves will be necessary to supplement steam output, while a dump condenser may be needed at low-process steam demands. The nature of the electrical and heat load will obviously have significant influence in the development of the scheme and scope of equipment. 

For flexibility, supplementary firing of the oxygen rich exhaust gases can give additional heat. The following summarizes the potential of both the diesel and GT for waste heat recovery ... 

In the above example, a relatively complex steam generator of the watertube type has been adopted. Where lower-quality steam for process or fuel heating is required, a simpler shell (or firetube) design may be appropriate. In some cases, supplementary firing may be provided for the boiler, so further increasing plant complexity and with it the need for enhanced control and maintenance requirements. 

Gas turbines are available with power outputs of 1 MW upwards, and the exhaust is used to fire waste-heat boilers. The high oxygen content of the exhaust enables supplementary firing to be used to increase the heat/power ratio as desired. 

Supplementary fired HRSG. Supplementary (or auxiliary) bring raises the temperature by bring fuel to use a portion of the oxygen in the exhaust. Supplementary bring uses convective heat transfer, and temperatures are limited to a maximum of around 850°C by ducting materials. 

Total heat in the exhaust with supplementary firing... 

However, this is not likely to be the case. In this example, the two boilers might have different fuels, with different fuel costs and different efficiencies, and the gas turbine (perhaps, with supplementary firing) will have completely different characteristics from the steam boilers. Thus, there are degrees of freedom created by multiple steam generation devices with different costs of fuels, different boiler efficiencies and different power generation potential. Individual steam boilers and HRSGs will have minimum and maximum flows. 

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