FUEL-FLEXIBLE GAS- TURBINE COGENERATION
Robin McMillan & David Marriott, Siemens Industrial Turbomachinery Ltd., Lincoln, U.K.

1.0 Introduction
In order to counteract the effects of global warming, the pressure to reduce the environmental impact of power and heat production is growing. Reduction of the amount of CO2 emitted during the generation of heat and power is seen as the key to reducing impacts on the environment. However, it is expected that the demand for power will increase dramatically over the next 10 to 20 years driven by the continuing economic expansion in countries such as China and India Whilst technologies are now available that emit very low levels of CO2 or use fuels obtained from renewable sources, these are often uneconomic to install and operate. The challenge is to employ a more pragmatic approach that uses existing proven technologies in a more effective way.

Distributed-cogeneration power plants are an economic way of significantly reducing CO2 emissions during the production of power and heat. Installing power plants closer to the point of use reduces electrical transmission losses and provides a ready source of heat for the production of steam, hot water and chilling. Optimization of cogeneration facilities can lead to overall useful power-to-heat input ratios of greater than 90%, which can reduce the carbon footprint of an installation by 50%.

The economic and environmental impact of cogeneration can be further enhanced by utilizing waste gases such as refinery and coke-oven gases. It is also worth noting that application of cogeneration-to-waste gases in some regions can qualify for carbon credits. This paper describes some examples of how gas-turbine cogeneration plants can be configured to operate on high-hydrogen-content refinery gas and coke-oven gas.

2.0 Alternative fuels and applicability to cogeneration
This paper will concentrate on gaseous fuels, with figure 1 below showing the main families of gaseous fuels available for firing gas turbines.

As well as a large variation in heating value, and corresponding variation of required volume of gas to be fed into the turbine, the composition of these fuels varies greatly. For example, the lower-heating-value fuels such as those from gasification tend to contain high levels of inert compounds, with the energy coming from hydrogen and carbon monoxide, whereas coke-oven gas contains high levels of hydrogen and little or no inert species. The differences in composition and heating values lead to different challenges in the gas turbine, as discussed in section 3.0.

As well as the technical challenges, the overall economics of the gas-turbine application and the fuel source/supply are critical. For example, the amount of capital investment required to build an integrated gasification and combined-cycle plant currently means that such plants have only marginal viability even at very large scale (>1000MWe).

A good example of where the economics do not add up, despite technical demonstration, is the biomass-fuelled IGCC plant built in Varnamo, Sweden. This plant demonstrated several thousands of hours of successful operation using a gasifier feeding an SGT-100 gas turbine. Despite this technical success, the cost of the electricity produced by the plant was too high for it to operate beyond the demonstration phase.

In contrast, a process-waste gas can provide a huge financial opportunity for a user who can use or sell on the heat or electricity available through installation of a cogeneration plant. One example of a waste gas that provides a viable opportunity is coke-oven gas. In general, in China, coke-producing plants are located away from steel production facilities. This means that, instead of using the gas as part of the steel production process, in a large number of cases, the gas is simply flared or, even worse, vented. Table 1 shows typical sizes of coke plants and the volume of waste gas produced

Coke Plant Size, coke output
K Tonne/year
COG produced
mNm3/year
COG available for power generation
Nm3/s
4001001.84
5001252.34
7001753.23
9002254.14
10002504.61
15003756.92

*assumptions, 250 Nm3 of COG/tonne coke, 50% of COG used for other purposes e.g. oven-firing, 8,000 operating hours per year, COG heat value is around 16 kJ/Nm3 – data is approximate.

Table 1

Table 2 shows various options for implementing GT solutions to produce electricity and steam at an efficiency of up to 95% (GT plus fired boiler plus steam turbine plus waste heat).

GT Power plant formatElectrical Output
MW
Natural gas input
Nm3/s
Equivalent COG input
Nm3/s
SGT-200 S.C.6.620.61.31
SGT-200 CHP unfired9.650.61.31
SGT-200 CHP fired boiler12.740.91.96
SGT-500 S.C.16.31.483.22
SGT-500 CHP unfired24.31.483.22
SGT-500 CHP fired boiler32.12.786.05

Power output at ISO conditions, data is for guidance only.

Burning waste gases, as well as being financially attractive, can also benefit the environment by producing power and heat that would otherwise be produced by the burning of premium fuels, resulting in a net reduction of CO2 produced. As such, installation in China can be entitled to carbon credits. Another example is using refinery gases where a hydrogen-rich off-gas is produced and is used to fire a gas turbine and heat recovery system providing both process steam and electricity. These gases can contain very high levels of hydrogen (>80%).

Fig3: SGT-200 unit operating on high-hydrogen fuel at an oil refinery in the U.K
Fig3: SGT-200 unit operating on high-hydrogen fuel at an oil refinery in the U.K

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