The new breed of gas engines

With the issue of environmental awareness increasing, stricter emissions regulations for power plants are continually being introduced. Wärtsilä of Finland explores the advantages gas engines bring to this arena.

Industrial aero-derivative gas turbines have been an accepted resource for power generation for many years. Due to the steady increase in the availability of natural gas for fuelling power plants, gas turbines have steadily taken market share away from the large centralised power plants burning fossil fuels. However, the recent market growth of the gas engine has created a new and competitive situation.

The rapid development of large gas engines in the past few years has brought new products with improved performance onto the market, bringing gas engine-based power plants into the output class previously-dominated by gas turbine plants. This rivalry is being experienced in both developed and developing power markets all around the world where gas is available for fuelling power plants.

With natural gas becoming one of the most popular fuels for power plants today the growth of gas turbine power plants has been considerable. One important reason for the increasing use of gas has been the construction of large gas turbine combined cycle (GTCC) power plants. These GTCC plants have to be of a large size to be efficient and be able to reach a competitive investment cost.

However, the introduction of large high-efficiency gas engines in recent years has established this new technology as an effective alternative to the automatic specification of gas turbines through the suitability of gas engines for today’s power supply markets. The new ranges of large gas engines can reach efficiencies of close to 50 per cent in simple cycle operation.

Power plants utilising gas engines are ideal for combined heat and power (CHP) installations, for district heat production and for industrial heat recovery applications in decentralised locations. The technology behind gas engines and their controls has developed considerably thus making the gas engines of today reliable units, offering superior efficiency with low lifecycle costs.

Wärtsilä’s gas engines provide clean,
efficient power, with minimal water consumption.

Engines that are making the change

The largest gas engine on the market today is the Wärtsilä 18V50DF which is a dual-fuel engine capable of running on heavy fuel oil (HFO) or light fuel oil (LFO) and gas. In this engine, the gas is ignited by a small amount of LFO pilot fuel, has a nominal output of 17MW and an electrical efficiency of 48 per cent.

The largest spark-ignited gas engine available today is the 9MW Wärtsilä 20V34SG engine which, since its introduction in 1995, has proved to be one of the most effective prime movers for converting gas into electricity and heat. With engines now providing large numbers of MW power and heat-to-power plants all over the world, the Wärtsilä 34SG gas engine is a market leader in its field.

These two capable gas engines types are the very ones that are closing the gap to the traditional gas turbines through their high efficiencies, competitive investment price and the multiple-unit power plant design that results in operationally flexible power plants of over 100MW. The added benefits of these gas engines over the gas turbine include a smaller size, shorter installation times, easier maintenance and being ideal for decentralised locations.

The environmental impact of a power plant which uses gas engines is extremely low. The plant’s NO_x emission levels will fulfill most of today’s global emission requirements without the need for secondary cleaning. If lower emissions are needed, oxidation and/or NO_x catalysts and other advanced equipment can be installed to meet these needs.

Another environmentally-friendly feature of gas engines is their almost zero consumption of water for cooling. Gas engines feature an arrangement that utilises a closed-circuit radiator cooling system that reduces the power plant’s water consumption to almost zero.

Differences in operating conditions

A significant feature in real time performance levels is the fundamental difference in the de-rating of gas engines and gas turbines. Gas engines start to de-rate at outside temperatures normally well over 25°C, while the Wärtsilä 20V34SG can provide full output at close to 35°C, which is considerably higher than with gas turbines. This factor tends to tip the scales in favour of gas engines when extremes of heat are in the planning equation as de-rating also means that the specific kW cost increases accordingly.

The second big difference between gas engines and gas turbines is how their performance de-rates as altitude increases. Here the output of the gas turbine starts to decrease as soon as the power plant’s altitude increases, whereas gas engines start to de-rate at much higher altitudes. For the Wärtsilä 20V34SG altitudes of over 2000m above sea level are reached without de-rating. De-rating has an adverse effect on both efficiency and investment cost.

Another factor affecting a gas turbine’s performance is its overall efficiency losses at part loads. However, the part load efficiency of a gas engine remains virtually unchanged down to around 50 per cent of load.

Since gas engine plants normally consist of several engine units the difference is even more pronounced as individual engines are stopped at part load and the remaining ones are operated at optimal loads.

Due to the requirement of satisfying fluctuation power demands throughout a given 24-hour period, flexible plant operation has shown to be a big issue in today’s power market. A gas engine’s ability to start up rapidly and to allow daily starts and stops provides owners with significant operating flexibility for today’s demanding end-users. As gas engines will go from stand-by to full plant output in less than 10 minutes, they are an obvious choice for grid applications. Hence, comparing gas turbines to gas engines depends entirely on ambient conditions and load profiles. The size of the power plant is also important in any comparison ratios. For the gas turbine combined cycle (GTCC) plants, bigger is better to be competitive. Large plant size also creates problems as the backup power needed should be dimensioned according to the largest power unit, therefore large sizes demand large backup power.

However, in the case of a power plant which features multiple gas engine-based power generation units, adding one spare gas engine generating unit means that the power plant can produce its own backup power and does not need to construct additional standby capacity. The gas engine’s capability for a fast startup makes it ideal for standby power applications.

The simple modular design and construction of gas engine plants makes their installation fast and gives them the flexibility of incremental expansion as power needs increase. Hence, the owner can start with a small plant that meets current power demands and then the plant can be enlarged later should power demand increase along with business growth. This feature ensures fast revenue flow with the ability to keep production optimised to demand.

Financial benefits

Constant uncertainty in the power market and its variable fuel prices are forever creating new challenges for power plant operators, owners and investors. When the fuel price fluctuates the plant’s economics can significantly change. In some cases, a plant originally designed for base load production can end up running at intermediate load when the plant’s fuel prices increase.

This not only affects the financial side of the plant through lower income from power sales, it also has a considerable impact on the plant’s operations and maintenance (O&M) costs. Of relevant interest to the plant owner is that the progressive development of gas engine technology has significantly reduced the maintenance costs of these engines. Many satisfied customers operating gas engine plants today report lower O&M costs than their previous aero-derivative gas turbines.

As today’s power markets move increasingly towards deregulation and decentralisation, the obvious choice will be multi-unit power plants based on gas engines with fast startup, high efficiency, low O&M costs and low emissions. Locating these plants close to the consumers reduces the need for investments into costly transmission and distribution projects.

Barrick Goldstrike Mines Inc uses 14 Wärtsilä 20V34SG engines
at its power plant, in northeastern Nevada, USA.

Power plants based on gas engines are better suited to modern energy markets which require shorter planning and construction periods and a more flexible operation.
Today’s new breeds of large gas engines are suitable for a variety of applications including baseload and peaking power plants. These highly flexible plants provide the smallest financial risk as the plant will lend itself to any operational application and every load profile.

When plant owners invest in a power plant based on multiple gas engines, they know that this is a wise investment as the plant itself can increase or decrease in size should the power demands fluctuate. This is due to the plant’s ability to adapt to future power needs in a stepwise incremental way by adding or removing units as needed.

These plants are suitable for virtually any kind of load profile. It is clear, that power plants based on gas engines are highly suitable for the ever increasing demands of today’s power supply markets.

For more information on Wärtsilä’s range of gas engines, visit:
www.wartsila.com