CHP plant boosts city's energy
Plant will be unmanned during continuous operation and remotely controlled from an existing main control centre.
In December, the first newbuild power plant in Sweden for 20 years started to produce energy for the city of Gothenburg on the Swedish west coast.
The new CHP Plant (combined heat and power) is based on Siemens Combined Cycle SCC-800 3x1 DH, at the core of which are three SGT-800 gas turbines and one SST-900DH steam turbine.
This CHP will produce 1,250 GWh of electricity and 1,450 GWh of heat per year, which represents 30 per cent of the city's electricity demand and 35 per cent of the heat demand. Maximum continuous rating corresponds to 265 MWe net power generation and 295 MJ/s heat generation.
The scheduled operation time is 6,500 hours per year, 1,500 hours being in part load operation during spring and autumn. An interesting aspect of part load operation is that an increasing proportion of heat generated to the district heating system is used for heat-driven district cooling systems in Gothenburg. The implications for Rya CHP in the future may be extended operation time and more part load operation at elevated forwarding DH (district heating) temperature.
The hi-tech plant will be unmanned during continuous operation and remotely controlled from an existing main control centre at some distance from the plant. Start-up, however, is carried out locally.
The local utility, Goteborg Energi AB, made a calculated investment in this greenfield plant, believing in its long-term economy and efficiency. It is a crucial milestone in the provision of heat and power for the city of Gothenburg, Sweden's major port on the North Sea, for many years to come. By effectively producing heat and power using natural gas as prime fuel, Goteborg Energi AB will be able to reduce its purchase of electricity while simultaneously reducing the emissions of sulphur and nitrogen oxides from the whole DH generating system.
The three Siemens 45 MW SGT-800 industrial gas turbines are each connected to a supplementary-fired heat recovery steam generator (HRSG). These three boilers feed a 137 MW SST-900 DH Siemens steam turbine, which is designed to cope with all operation modes specified for the Rya plant, including full operation of the gas turbines plus supplementary firing of the boilers to achieve maximum plant output. Two district heating condensers connected after the steam turbine exhaust supply hot water to the district heating system of the city.
The SGT-800 is the largest industrial turbine in the Siemens Power Generation portfolio. It is a single-shaft turbine with a 15-stage compressor, a hybrid burner ring combustor with 30 individual burners which are suitable for the firing of both gas and oil, and a three-stage turbine.
SGT-800 is equipped with a Dry Low Emissions (DLE) combustion system which does not require water injection to reduce nitrogen oxide emissions. The catalyst located in the HRSG will further reduce these emissions to below 10 mg/MJ.
The Rya plant has a dual-fuel system. The prime fuel is natural gas piped from Denmark, and diesel fuel is used as back-up fuel. The location of the plant in the Rya oil harbor area is logistically excellent. The plant is close not only to the centre of the city, but also to the existing natural gas system, the district heating system and the electric grid system. Three transformers feed the electricity to the grid, one servicing two gas turbines, one used for the third turbine, with potential for an extra SGT-800 line to be built if required , and the third is used for the steam turbine.
The plant will provide power all-year-round if required. In summer, for example, it can still produce considerable amounts (85 per cent) of electric power even when disconnected from the district heating system. Dumping of surplus heat via a dump heat condenser into the River Gota has been approved by the national power authority, to a maximum10oC temperature increase of the cooling water (5 m3/s), should this be needed.
The new plant will actually have a positive impact on the local environment since it will entirely or partially replace older, less efficient heat plants with poor environmental performance. Nitrogen oxide (NOx) emissions are naturally low from the combined cycle for which the SGT-800 was optimised. Additionally, the plant is equipped with SCR (Selective Catalytic Reduction) to reduce NOx emissions yet further, the SCRs having been dimensioned for a 70 per cent NOx reduction. The combination results in NOx levels well below 10 mg/MJ with natural gas fuel.
The low NOx emissions place the SGT-800 turbine at the top of its range (35-60 MW) for turbines which can be fired with either gas or oil and can be operated in the load range between 50 and 100 percent. Its DLE combustion system has successfully completed three million operating hours since it was introduced in 1988.
Natural emissions of CO2 are also low from such an efficient combined cycle cogeneration plant. The CO2 emission of this gas-fired CHP is around 25 per cent of what is emitted from a coal-fired plant and 30 per cent of that emitted from an oil-fired condensing power plant. As compared with energy production from oil and coal, that represents a reduction in CO2 emissions of 500,000 tons per year. The combined cycle plant now commissioned in Gothenburg is one of the largest in Scandinavia and the largest installed in recent time. The total plant efficiency is as high as 92.5 per cent.
Maximum plant flexibility is assured by the use of three smaller units rather than one large, and also by the use of supplementary firing of the boilers. Multiple units increase the load range and part load efficiency. Supplementary firing increases the heat and power output and the total plant efficiency. Both these solutions enable either a maximum electricity production for a given heat production or maximum fuel utilisation and are significant contributors to the unusually good performance. In addition, as part load is frequent during spring and autumn, this concept implies higher plant availability.
The steam system works at a pressure of 100 bar, the water entering the system being heated in two economisers, the low temperature one cooled by the DH system. Downstream of the evaporators, the steam temperature is raised to 540degC in the superheaters and then the steam is taken to the steam turbine. Supplementary firing of natural gas, using part of the unused oxygen from the gas turbine exhaust, raises the flue gas temperature in the boiler to 1,000°C, accounting for a high proportion (60 per cent) of the steam produced.
Flue gas heat loss is further decreased by cooling the exhaust gases in a district heating economiser directly before the stack, thus minimising heat loss to atmosphere. The temperature of the gases leaving the stack is 70degC.
A number of design details increase the total efficiency of the plant. For example, recovering heat from the lube oil cooling system achieves a 0.5 per cent improvement of the total efficiency, equal to three MW of heat, or sufficient to heat 200-300 private homes. Heat recovered from the lube oil cooling system is used to heat the air intakes, thus also avoiding ice build-up in winter, without the use of an external heat source .
If disconnected from the main grid as a result of external voltage or frequency disturbances, the plant has a house turbine capability, ie it supplies power to its own auxiliary network and remains in low load operation. It can be reconnected to the main grid once it is available. In the event of a black plant incident, house turbine operation can be started to energise a dead external grid, building up island loads in increments until finally reaching island mode operation.
Built by Siemens under a complete EPC contract, the supply for this showcase plant covered civil works, gas turbines and steam turbine, heat recovery boilers, district heating condensers, erection, commissioning and a seven-year maintenance contract.
The Rya plant is the most complex SCC-800 configuration supplied by Siemens to date, since it uses three gas turbines and supplementary firing. However, the basic configuration of two gas turbines plus one steam turbine is becoming increasingly popular as combined heat and power generation gains a stronger foothold throughout Europe and much of the Western world. To date some fifty SGT-800 gas turbines have been sold for power generation and industrial applications.
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