The increasing amount of intermittent renewable power coming onto grids will have an impact on the design of cogeneration and on-site power installations. These facilities increasingly need to be flexible both in terms of operation and their ability to burn a range of fuels, including low carbon fuels such as biofuels, as this article from engine manufacturer Wärtsilä illustrates.
|The Humboldt power plant in Eureka, California, consists of 10 Wärtsilä 18-cylinder 50DF gas engines in V-configuration and has a total output of 162 MW. The dual fuel (DF) engines are able to operate on light fuel oil as back-up. The plant is equipped with SCRs and is capable of meeting the strict Californian emission requirements in both gas and liquid fuel mode.|
As a number of governments attempt to move towards low-carbon generation, renewables are playing a greater role in the generation mix. The EU and the US have set targets to produce 20% of their energy sources from renewables by 2020.
Accordingly, the amount of wind and solar generation coming onto the grid continues to grow at a phenomenal rate. In Europe, for example, according to the European Wind Energy Association, more wind capacity was added in 2009 than any other generating technology.
Some 9.3 GW of new wind power capacity was installed in the EU during 2010, reaching a total of 84 GW by the end of 2010. Meanwhile, according to the European Photovoltaic Industry Association, some 13 GW of photovoltaic capacity was added in the EU last year.
The continued increase in wind and solar power will, however, present challenges due to their intermittent nature. From an operational standpoint, intermittent generating sources will have an impact on grid stability. In terms of economics, there will be increasing instances of negative power prices.
Although wind and solar power will be the big winners in terms of new renewables capacity, reaching the renewables targets will also require greater use of biomass. The use of biomass is expected to double, from about 1% globally today to about 2% in 2030.
The impact of these combined developments will mean that both cogeneration and on-site power plants will have to be flexible to produce power when it is needed. At the same time, cogeneration plants will also be required to produce heat according to demand. On-site power plants will also need to be able to burn whatever fuel is available and competitive today, but with a view to changing to low-carbon fuels in the future.
Industrial cogeneration or district heating (DH) systems face the challenge of power and heat sometimes being needed at different times. In DH systems, for example, heat is dictated by seasonal variation, typically being needed in the winter. Power, meanwhile, is subject to daily change. Less power is typically needed at night when demand is low.
CHP plant operators sometimes face the decision of whether to run a plant according to the heat load when there is an opportunity to sell power to the grid when power prices are high. Having a system with the flexibility to meet power and heat demand at the same time can be a great benefit.
With the increasing occurrence of negative power prices, the issue of whether it makes sense to produce their own power when it is cheaper to buy it from the grid is a question that owners of both CHP and on-site power-only plants will face more and more in the future.
In order to meet the myriad demands of the future, both CHP and on-site power plants have to be as flexible as possible. Wärtsilä has developed a concept called ‘Smart Power Generation’ – an ‘all-in-one’ plant that can meet current and future power requirements.
Smart power generation can operate in multiple modes. When following electrical load, it can be run as an intermediate or peaking plant and can be started and loaded very quickly – in well under 10 minutes. At the same time, it can be run in baseload.
Using a multi-engine installation concept means that any number of engines can be run to match the load. This allows high overall plant efficiency to be maintained – a factor that has become even more important in Europe since the introduction of the CHP Directive several years ago.
To suit a range of applications – from small cogeneration and distributed generation plants up to mid-sized captive baseload power plants – Wärtsilä has a range of smart generating solutions that meet the changing market requirements. These plants can have a power output range of 1–500 MW.
Smart generation is also about having a generating facility that is able to run on almost any type of fuel. Wärtsilä’s engines can operate on a wide variety of fuels such as natural gas, diesel oil, heavy fuel oil (HFO), vegetable oils (renewable) or even more challenging fuels such as emulsified fuels. This can be beneficial to on-site facilities in terms of security of supply and economics.
Being able to switch fuels is very important in situations where there is a possibility of interruption to the main fuel supply. For example, the loss of heat or power can be disastrous in many industrial processes.
At the same time, as the price of different fuels varies, being able to switch fuels gives the operator the ability to run the plant on the most cost-effective fuel – effectively balancing fuel costs against emissions.
Engines that can burn both gas and liquid fuels such as light fuel oil (LFO), HFO and liquid biofuels allow generators to pick fuels according to business strategies and fuel availability. It is also beneficial to have a plant that can be easily converted to run on a different fuel in the future. This would allow, for example, a plant currently running on diesel today to take advantage of subsidies that may become available for burning biofuels.
Wärtsilä has been developing the fuel flexibility capabilities of its engines for more than two decades in order to meet a growing market demand for power plants that provide their owners with the ability to switch fuel seamlessly during operation.
Two current reciprocating engine technologies – gas-diesel (GD) and dual fuel (DF) – are designed to operate on a wide variety of fuels and deliver power outputs ranging from 2.6 MW to 17.1 MW.
The first gas engine concept, developed back in the 1980s, was the GD engine with high-pressure gas injection. One of the merits of this engine type is its multi-fuel capability, which enables the same engine to run on a wide variety of fuels such as natural gas, HFO and crude oil.
Wärtsilä then began developing lean-burn, spark-ignited gas engines. This development was driven by the fact that most customers required a pure gas engine, without the need for back-up fuel capability. Most of the cogeneration plants delivered by Wärtsilä have been natural gas-fired plants.
Gas engines offer several advantages over gas turbines. In addition to higher simple-cycle efficiency, exceeding 45% compared with about 40% for a gas turbine, they do not derate at high altitude. They can also handle a much more variable gas supply.
Unlike most gas turbines, Wärtsilä’s gas engines do not require a high gas pressure to operate. A pressure of 6 bar is adequate to operate the power plant at full output. This limits the use of gas compressors to extreme cases where a 6 bar gas pressure is not available.
It should also be noted that as the pressure requirement is only 6 bar, gas engines can operate even if the main pipeline pressure starts to drop gradually.
Gas engines exhibit low SO2 and NOx emissions as well as low particulate emissions. But emissions performance can be further improved by the use of selective catalytic reduction (SCR) technology, which enables NOx emissions to be reduced by 90% or more.
Typical SCR solutions for Wärtsilä gas engines feature an integrated oxidation catalyst for carbon monoxide and volatile organic compound emission reduction. This emission control system allows very low emissions to be achieved, such as single-digit ppm levels.
As an example, the 162 MWe Humboldt power plant, based on 10 Wärtsilä 18-cylinder 50DF dual fuel engines and located in Eureka, California, is able to meet the strict Californian emission requirements in both gas and liquid fuel mode.
Based on market needs and building on the technology of the GD and spark-ignited engines, Wärtsilä started the development of a DF engine in 1996. This engine concept was initially developed for land-based power generation applications in cases where gas supply can be intermittent and where the gas is mainly pipeline-quality natural gas. One advantage is the fuel versatility that enables the same engine to be operated on gaseous, as well as liquid, fuels.
The engine operates according to the Otto process when running in gas mode, during which the lean air-fuel mixture is fed to the cylinders during the suction stroke. It then operates according to the diesel process when in diesel mode, during which the diesel fuel is injected into the cylinder near the end of the compression stroke.
The engine has been optimised to run on gaseous fuels, while diesel fuel is used for back-up fuel operation. When running in gas mode, it is possible to instantly transfer to back-up fuel operation at any load in case of an alarm situation, such as interruptions in gas supply. When the situation is normalised, the engine can be transferred back to gas mode without interrupting power production.
These types of engines have been especially popular in areas with a need for a back-up fuel in case of shortage, or where there is a need to convert from HFO to gas at a later date.
The most recent and largest of the DF engines, known as the 50DF, has a power output of 17 MW. The first two were supplied for an on-site power plant in Pakistan in 2006 (see box on opposite page).
PLANNING FOR UNCERTAINTY
The installations in Pakistan and in other locations around the world not only secure power supply in on-site power plants where electricity from the grid is unreliable, but also give plant owners options where fuel supply may also be subject to interruption.
In developed countries, uncertainties are being introduced through the massive influx in renewable energy to the grid. Power plants will have to be sufficiently flexible to operate in many different modes – sometimes in support of renewables generation and sometimes as baseload plant.
Thus, installing smart, flexible power generation solutions, which are based on reciprocating engines make a lot of sense on all fronts.
|Also grid-connected, the Sangachal power plant in Azerbaijan has a total output of 308 MW and consists of 18 Wärtsilä 18V50DF engines. The power plant uses natural gas when available, but can switch over to using HFO or LFO when needed.|
Cementing a captive role in Pakistan
In 2005, Wärtsilä was awarded two separate contracts to supply natural gas-fuelled Wärtsilä 50DF engines for two captive power plants in Pakistan. The two contracts represented the first 50DF dual-fuel engines in Pakistan.
One contract, received in October 2005, was for an 18-cylinder Wärtsilä 50DF engine to be installed in an extension at the Maple Leaf Cement Factory in the Punjab, Pakistan. This 16.5 MWe set, which operates on natural gas, with heavy fuel oil as the back-up fuel, began operation in the autumn of 2006.
Similarly, an 18-cylinder Wärtsilä 50DF engine was supplied for a power plant at the cement works of DG Khan Cement Co in Khairpur in the Punjab, Pakistan. DG Khan Cement is a member of the Nishat group of companies with businesses including cement, textiles and banking. The set also operates on natural gas, with heavy fuel oil as the back-up fuel. The contract was awarded in November 2005, and the engines were delivered in September 2006.
The orders came in response to a surge in demand for cement in Pakistan, which prompted several projects to increase cement-making capacity, both through expansions to existing cement works, such as at Maple Leaf Cement, and new plants, such as the DG Khan Cement Khairpur project. These developments created a need for additional power – a large demand that cannot be met by the national grid.
For both orders, important factors in the choice of engine were the need for a reliable, cost-effective electricity supply for the cement making process as well as flexibility in the choice of fuel. Fuel flexibility is an important factor in Pakistan, where gas supplies are insufficient to meet high demand in winter months. In addition, independent power plants offer advantages for cement works, which commonly use large motors with sudden high starting currents. These requirements are all ideally met by the Wärtsilä 50DF.