Achieving high efficiency and flexibility in the field

F. Boerefijn, Eneco, the Netherlands & R. Hvelplund, DONG Energy, Denmark

Within its first six months of operation, the Dutch Enecogen CCGT power plant has demonstrated the ease at which high efficiency and operational flexibility can be achieved.

On 11 November last year, a new state-of-the-art 870 MW combined-cycle gas turbine (CCGT) power plant near the Dutch port city of Rotterdam entered commercial operation.

Called Enecogen, the CCGT plant was built — from piling to commissioning — within an impressive 31-month period, primarily thanks to its modular design. The natural gas fired power plant was built by Siemens for a partnership comprising Danish energy utility DONG Energy and Eneco, an energy company based in the Netherlands.

The power plant consists of two identical 435 MW single-shaft combined-cycle units, and because of its flexibility, fast load response and low emissions, can be described as an ‘enabler for renewable energy’.

If its units are still warm, full-power output is available 50 minutes after giving the start command. A net fuel efficiency of 59 per cent and a power output of 435 MW can be achieved at an ambient temperature of 11 °C, a relative humidity of 75 per cent, a cooling water temperature of 12 °C and an ambient pressure at sea level (ISO conditions).

The development of the Enecogen project was started by Eneco in 2005, and together with DONG Energy, financial close was reached in April 2009 when the main contracts were signed, including the EPC contract with Siemens.

The reason for investing in flexible gas fired power plants is that this type of power plant fits well with the renewables development strategy that both shareholders have. A gas fired plant is able to produce electricity in a flexible way, therefore ensuring security of supply for customers. In other words, a gas fired plant can produce electricity, even when there is no wind or sun power available.

In comparison with other fossil fuel-based power generation resources, such as coal, a gas fired power plant is more flexible, can start and stop more easily, is faster and has relatively low specific carbon dioxide (CO2) emissions. Therefore, gas fired power plants are the ideal option for producing electricity on a large scale in the transition period from fossil to renewable electricity production.

Technology behind the art

Each of Enecogen’s generating units utilises a 292 MW Siemens SGT-4000F gas turbine as the prime mover. The hydrogen-cooled Siemens SGen5-3000W generator is connected at one end to the gas turbine — the cold end (air intake) — while at the other it is connected via a clutch to a Siemens SST55-5000 steam turbine (see Figure 1). This arrangement enables all three major elements of the generating unit to be run at the same speed of 3000 rpm.

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Figure 1: The hydrogen-cooled Siemens SGen5-3000 W generator is connected at one end to the gas turbine and to a Siemens SST55-5000 steam turbine at the other

The clutch between the generator and steam turbine gives extra operating flexibility to the units. During ramp up, the unit starts with only the gas turbine available, and subsequently as the steam conditions from the boiler reach the correct level, the steam turbine comes online.

Enecogen’s boiler is a conventional drum-type heat-recovery steam generator with three pressure levels and reheat, which supports the operational flexibility of the complete plant.

Plant-operating characteristics

Each power block has a nominal output of 435 MW, with a fuel efficiency of 59 per cent at full load. The minimum load (minimum stable generation) is approximately 225 MW, with a fuel efficiency close to 52 per cent. Figure 2 highlights the distribution of power between the gas turbine and the steam turbine across the full load range.

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Figure 2: Typical power output of the gas turbine and the steam turbine as a function of the combined output

At the minimum load of 225 MW (i.e. 52 per cent of the nominal load), each power block has a fuel efficiency of approximately 52 per cent, which means that the efficiency of the plant is high over the whole load range of the plant (Figure 3).

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Figure 3: Typical fuel efficiency of an Enecogen power block

In the case of a ‘hot’ start — meaning that the unit has been at standstill for less than eight hours, synchronisation with the grid is reached within 15 minutes. The output is subsequently raised by 3 per cent of the nominal power per minute. This means that full output is reached after around 50 minutes, which is substantially faster than a conventional coal fired power plant. Figure 4 shows how the gas turbine and steam turbine team up in reaching the nominal output. A ‘warm’ start takes more time, but still less than two hours.

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Figure 4: Typical ramping up of the power output of an Enecogen power block

Normal ramping down from full load to zero output takes around ten minutes. The operation scheme of the units foresees running during the intermediate and peak load hours of week days, representing some 250 starts and stops per year. This is not anticipated to negatively affect the operating life span of the power plant.

Reduced emissions

Natural gas has a specific CO2 emission of approximately 56 g/MJ based on the lower heating value of the fuel. The energy conversion cycle of a prime mover such as a gas turbine cannot use the higher heating value of a fuel. For a fuel efficiency of 59 per cent, the CO2 emission based on energy output is 94.9 g/MJ (56/0.59).

Since 1 kWh equals 3.6 MJ, the specific CO2 emission of the Enecogen power plant at full load totals 341 g/kWh, which is around a factor of two lower than that of a modern coal fired power plant, which is a significant fact in our carbon-constrained world.

The nitrogen oxides (NOx) production by the gas turbine is low because it utilises pre-mixed combustion in an annular combustor with 24 burners. The NOx concentration is further reduced to a maximum of 17 g/GJ ( (9.6 ppmv or 20 mg/Nm3 @15% O2) with an ammonia-based selective catalytic reactor (SCR). The SCR system ensures that any changes in the combustion process caused by load, ambient conditions or gas quality will not result in increasing the level of emissions released to the environment.

Operational experience

In the first few months of commercial operations, Enecogen has proven to be both a reliable and flexible power plant, with operation depending on market conditions.

The Enecogen CCGT power plant is an important addition to the Netherland’s power generation portfolio as it seeks to reduce its reliance on coal, and as a back-up to the country’s expanding renewables base. The Enecogen plant is a benchmark example of the high degree of efficiency and operational flexibility that can be achieved with a gas fired CCGT plant. Both DONG Energy and Eneco are very satisfied with the performance of the two power blocks, which are operating in full compliance with the specifications provided by Siemens.

The Companies Behind the Enecogen Project

Eneco, which is the largest Dutch-owned energy company, has an integrated distribution strategy covering generation, transmission, supply, metering and billing of electricity, gas and heat for 2.2 million customers.

Eneco’s energy mix is becoming more sustainable, consisting of different types of sustainable electricity, heat and gas resources. Efficiently fuelled gas plants, like Enecogen, combined with underground gas storage, provide the company with the required flexibility for its electricity production.

Currently, the company sells approximately 27 TWh of electricity and 6000 million m3 of natural gas annually. Furthermore, it is the largest supplier of heat in the Netherlands.

DONG Energy as an electricity producer is a newcomer to the Dutch market. Its domestic market for electricity production in Denmark is practically saturated, and therefore the company is now looking to expand its activities abroad. Originally, DONG (Dansk Olie & Naturgas) was an oil and gas producer in both Denmark and Norway. The company subsequently transformed itself into an integrated energy supplier, providing more than 50 per cent of the electricity used in Denmark and meeting 40 per cent of its district heating needs. In addition, it owns 45 per cent of the Danish wind turbine capacity and 25 per cent of the UK wind power capacity.

DONG Energy has a strategy to reduce its specific emissions by 50 per cent in the next ten years by investing in wind power, the suspension of coal fired plants, converting coal fired plants to burn biomass and by investing in high-efficiency gas fired plants like Enecogen.

DONG Energy considers the partnership with Eneco as investors in this power plant to be an important part of this strategy.

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