Gas-Fired Power Generation Technology
Abstract
Natural gas can be used to generate electricity in several ways. These include burning the fuel in a steam-raising boiler and using the steam to drive a steam turbine, using natural gas in a reciprocating engine and using the fuel to fire a gas turbine. A natural gas-fired steam plant is very similar to a coal-fired power plant and advanced supercritical steam boilers burning natural gas can expect to achieve a similar efficiency level. Plants of this type can be found in regions of the world where natural gas is plentiful but they are rare elsewhere. Reciprocating engines burning natural gas are normally used for small-scale distributed generation or as backup generators. Natural gas can also be used as fuel for a fuel cell. In this case it must first be reformed to convert it into hydrogen.
Keywords
Natural gas steam plant; supercritical boiler; steam turbine; reciprocating engine; gas engine; fuel cell; distributed generation; lean burn engine
There are a number of ways in which natural gas can be used to generate electricity. The first and simplest method is to burn the gas to generate heat and produce steam in a steam-generating boiler, then use the steam to drive a steam turbine. This type of power plant was originally developed at the end of the 19th century to burn coal and when natural gas became available during the first decades of the 20th century the steam-raising fossil fuel plant was adapted for gas use too. There is no definitive record of the earliest use of natural gas in this way but the first plants are likely to have been in the United States where the natural gas industry took off alongside the oil industry. Since then this type of power plant has continued to be used where natural gas is plentiful and particularly where there is no other immediate use for the fuel. Plants can be found in most natural gas producing countries, including the United States and Russia and in the Middle East, Africa and Asia. New plants of this type are still being built. However they are not the most efficient means of generating electricity from natural gas and these steam-raising plants using gas-fired boilers form only a small part of the global natural-gas-fired power generating capacity.
The inception of the most important way in which natural gas is used to generate electricity, in a gas turbine-based combined cycle power plant, is more clearly recorded. According to GE Power Systems, the first ever gas turbine installed in an electric utility in the United States, in 1949, was used in combined cycle mode.1 This was not, however, the first use of a gas turbine for power generation. The credit for that belongs to the Swiss company BBC Brown Boveri which in 1939 installed a 4 MW open-cycle gas turbine at a municipal power station at Neuchâtel, Switzerland. This unit had a long life. It was finally retired in 2002 and has since been restored for public display.
Open-cycle gas turbine plants, in which a free-standing gas turbine is used to generate electricity, were built during the middle of the 20th century in many developed countries where gas was available. They were relatively simple and reliable and in addition could be started up and shut down quickly which made them extremely useful both for meeting peak demand and for taking up demand during system failures. A very small-scale version of the open-cycle gas turbine, called a microturbine, has also been developed and can be used to supply power for domestic or small commercial use.
The first recorded combined cycle power plant to operate was at the Belle Isle Station, Oklahoma, United States where the first commercially sold gas turbine for power generation was installed in 1949 by GE.2 The gas turbine had a generating capacity of 3.5 MW and the exhaust from the turbine was used to heat the feed-water for a conventional steam-generating unit, probably fired with coal. This unusual design feature was a result of restrictions and procurement difficulties in the United States after the Second World War. The plant had more turbine capacity than boiler capacity and the additional heat input allowed more of the steam turbine’s capacity to be harnessed. The heat recovery heat exchangers (called economizers) used in the plant were bare tubes, placed in the exhaust path, through which the feed-water passed. This design was typical of early combined cycle heat exchangers.
Further early combined cycle plants entered service during the 1950s and early 1960s. These often used the exhaust from the gas turbine as the combustion air for a conventional fossil fuel-fired boiler. It was only when more advanced heat recovery boiler tube designs were developed that designs more reminiscent of a conventional combined cycle plant began to appear. Finally, during the early 1980s, the modern combined cycle power plant started to attract the attention of power companies and the technology began to be adopted widely.
Beside gas turbines, another way of utilizing natural gas to generate electricity is in a piston engine. These engines also have a long history of power generation and have been used to supply mechanical power for gas compression and pumping stations in the natural gas industry for decades. Used to drive a generator, natural gas engines of this type are efficient and clean and have become popular for small-scale distributed generation applications.
There is one further important means of using natural gas to provide electricity, in a fuel cell. Fuel cells generally require hydrogen to operate. It is possible to convert natural gas into hydrogen and use the product to provide fuel for a fuel cell, with the natural gas converter (usually called a reformer) and the fuel cell integrated into a single unit. As with piston engines, fuel cells are generally used for small-scale distributed generation rather than as central power plants.
3.1 Natural Gas-Fired Steam Turbine Power Plants
The workhorse of the power generation industry for much of the 20th century and at the start of the 21st century is the coal-fired power station. This type of power station is designed to burn coal in a controlled fashion and use the heat generated to raise steam and drive a steam turbine. The same type of power plant can be utilized to burn either natural gas or oil.3 However most of the developments in steam plant boiler and turbine technology have been driven by their use in coal-fired power plants. So while plants that burn natural gas or oil can be found where there were plentiful supplies of these fuels, the design of these plants has generally been based on that of a similar coal-fired plant. This synergy continues today.
In consequence, a natural gas-fired boiler for power generation is very similar to a coal-fired boiler and in many cases the two types of boiler from one company will share components. The layout of a typical gas-fired steam plant is shown in Fig. 3.1. In the case of a gas-fired plant, natural gas4 is burned in a controlled amount of air in a furnace. The heat generated from the combustion is then captured by water flowing through pipes within the walls of the furnace and in specially designed pipe bundles that are placed in the path of hot gases exiting the furnace. The ideal is to capture as much of the heat generated as possible to raise steam so that the air exiting the power plant boiler chamber should be at as low a temperature as possible.
Early steam boilers of this type produced relatively low-temperature, low-pressure steam but as materials technology has developed it has become possible to build plants capable of generating much higher pressure, higher temperature steam. Temperatures in coal-fired boiler furnaces can now reach 1700°C. The temperature in a gas-fired furnace will probably be somewhat lower than this.
The driving force behind the quest for every more extreme conditions of temperature and pressure is efficiency. The energy conversion process by which thermal energy in the natural gas is converted into electrical energy involves a steam turbine. This is a type of thermodynamic heat engine and its overall efficiency is limited by the Carnot Cycle established by the French scientist Sadi Carnot during the 1820s. What the Carnot Cycle teaches is that the maximum efficiency of which a heat engine is capable is determined by the temperature drop that the working fluid—in this case steam—experiences between entering the engine, the steam turbine and exiting it.5 Since the temperature at the exit of the steam turbine can usually be no lower than ambient temperature, which in most cases is fixed, the only means of increasing overall efficiency is to raise the temperature of the steam that enters the steam turbine.
Water in the form of steam is the working fluid for a steam turbine and its physical properties play a crucial role in boiler design. Water at normal temperatures and pressures behaves as we all expect it to. It is a liquid at room temperature but as the temperature is raised it eventually boils. The boiling point is 100°C at normal pressures but will rise as the pressure of the water rises. However if the pressure is raised far enough—above 22.1 MPa (221 times atmospheric pressure)—the nature of the water changes and it becomes what is known as a supercritical fluid. Once it enters this phase there is no difference between the liquid and the gaseous version of the fluid and the two coexist. The supercritical point of water is actually defined as 22.1 MPa/374.1°C since it is dependent on temperature too; above this point water is a supercritical fluid and below it a normal fluid.
Most power plant boilers designed before the latter part of the 20th century, be they for natural gas, oil, or coal combustion, operated at below the critical point because the materials needed to build reliable boilers for higher temperatures and pressures were not available. These types of boiler are referred to as subcritical boilers. Many modern power plant boilers now operate with steam above the critical point and these are called supercritical boilers. A further class, called Ultra-supercritical, operate under even more extreme conditions but are usually only used for coal combustion.
Modern natural gas-fired boilers for power plants can have capacities of up to 1000 MW, although most are typically smaller than that, and they can operate with steam pressures of up to 25 MPa and steam temperatures of over 550°C. The furnaces of these plants are designed to control the combustion conditions in order to limit the production of nitrogen oxide. This is achieved by controlling the amount of air allowed to enter the furnace during the early, highest temperature stages of the combustion process when most nitrogen oxides are produced.
Natural gas contains only traces of nitrogen but the nitrogen in air that is introduced into the furnace during combustion will be oxidized at the very high temperatures in the furnace fireball. Limiting the amount of oxygen available for combustion at this stage limits the production of oxides of nitrogen because the oxygen that is available preferentially reacts with methane. Further air is then added higher up the combustion chamber where the combustion gases have cooled slightly to allow the combustion to continue to completion at lower temperature and with less danger of nitrogen oxidation. For natural gas-fired supercritical boilers of this type the efficiency can be as high as 49%. It will be lower for subcritical boilers and smaller units are also likely to have lower efficiency than the largest units.
Natural gas is a relatively clean fuel compared to coal and only limited flue gas cleanup will be required before the flue gases can be released into the atmosphere. The only major pollutants likely to be present at a significant concentration are the nitrogen oxides discussed above, usually referred to collectively as NOx. Where the level of NOx exceeds local environmental limits a removal system will have to be used. Various technologies are available that will convert the nitrogen oxides back to nitrogen. The most common of these are selective catalytic reduction and selective noncatalytic reduction. Both use a reagent such as ammonia or urea to react with the NOx. Similar technologies are used for other types of natural gas-fired power plant (see Chapter 9).
The other main environmental concern is with carbon dioxide. A natural gas-fired boiler will generate significant quantities of carbon dioxide during the combustion process. While the amount per unit of electricity will be smaller than for a similar coal-fired power plant, future legislation is likely to require such plants to control their carbon dioxide emissions by capturing and storing the gas.
The steam from the power plant boiler is used to drive a steam turbine that is matched to the output of the boiler. As with the boiler, this is likely to be a turbine designed primarily for a coal-fired plant operation. Depending on the plant size it may comprise high pressure, intermediate pressure and low-pressure turbine units together will the possibility of reheating steam between one or more of these units to improve overall efficiency. The steam turbines will be coupled to generators that convert the rotary motion from the turbine shaft into electrical energy.
Gas-fired steam plants of this type are only likely to be found in regions where there is oil production but no ready market for the associated natural gas. Natural gas is a valuable product and it will normally be sold unless local conditions make this difficult. Typical locations for such plants are the Middle East, in African oil producing countries, and in Asia. Natural gas is unlikely to be used in this way in Europe or the United States today.
3.2 Piston Engine-Based Natural Gas Power Units
The piston engine or reciprocating engine has a long history in the power generation. Some of the very first coal-fired power stations that were built in the 19th century used steam reciprocating engines to drive generators. Modern reciprocating engines are used mainly for transportation. Small engines are used in domestic vehicles and larger ones in trucks, locomotives and ships. Equivalent engines can be adapted for the power generation market. In terms of power output, sizes can range from as small as 0.5 kW to as large as 65 MW.
There are two main categories of piston engine suitable for power generation, spark ignition engines and compression ignition engines, but only the first of these can be fired with natural gas. Compression ignition engines are usually fired with diesel. There are also different cycles under which a piston engine can operate. The two most common are the two stroke and the four stroke engine. Engines using both types of cycle can be operated with natural gas.
A further variable is the ratio of air to fuel within the combustion chamber (the cylinder) of an engine. Some operate with a roughly stoichiometric ratio of oxygen from air and fuel, such that there is just sufficient oxygen for all the fuel to burn. Such engines are referred to a rich burn engines. These engines tend to operate at high combustion temperatures and this can lead to the production of relatively high levels of nitrogen oxides as well as other pollutants. The alternative is a lean burn engine in which there is far more air (and oxygen) than is required for combustion. The excess air leads to lower combustion temperatures within the engine cylinders and lower pollutant levels in the engine exhaust. Under normal circumstances a rich burn engine will generally provide higher efficiency than a lean burn engine. However modern design of lean burn engines is allowing them to reach similarly high levels of efficiency while maintaining lower emissions production levels.
As with natural gas-fired steam turbine plants, the main environmental consideration is NOx. Rich burn engines burning natural gas will normally require some form of catalytic reduction system to remove NOx and bring the emissions level within local regulations. Some lean burn engines may be able to meet environmental regulations without the need for additional emission control systems. Engines also generate carbon dioxide but it is unlikely that it will be cost effective to apply carbon capture technology to piston engines except in the very largest of installations.
Natural gas-fired piston engines are available in sizes from 0.5 kW to around 6 MW. For power plants larger than this multiple engines are usually required. While larger piston engines can be built, these normally operate on heavy oil as fuel rather than natural gas. The speed of a piston engine varies depending on its size. Natural gas engines can be either high speed engines (1000–3000 rpm), which are available in sizes from 0.5 kW to 6 MW or medium speed engines (275–1000 rpm) which usually start at 1 MW. The larger, slower speed engines tend to be more reliable and are typically chosen for continuous operation. Where intermittent operation is required, smaller, high speed engines will often be chosen because they tend to be cheaper, though less reliable.
The uses for natural gas-fired engines for power generation are varied. Many of them are used for distributed generation applications where they supply power directly to local consumers. Some of these engines are used in cogeneration mode in which waste heat from the engine is used to heat water. This can lead to very high overall efficiencies. Another common application is for grid backup, with systems designed so that they start up as soon as there is an interruption to the mains supply. Natural gas engines may also be used in conjunction with renewable capacity, such as wind power or solar power, in microgrid type applications where they are also used as a standby power supply.
3.3 Fuel Cells
A fuel cell is a form of electrochemical cell, similar to a battery. Batteries exploit the energy that is released in a spontaneous chemical reaction, turning the energy that would normally be released in the form of heat into electrical energy. The exact chemical reaction varies from battery to battery and in the case of the fuel cell it is the reaction between hydrogen and oxygen to create water.
In order for a fuel cell to operate, hydrogen must be fed to one of its electrodes and oxygen to the other. The electrodes are coated with catalysts that encourages molecular hydrogen or oxygen—depending upon the electrode—to split into its atomic components, two hydrogen atoms or two oxygen atoms. These atoms are extremely reactive but are at different electrodes of the cell. The cell is designed in such a way that in order for them to react with one another, a hydrogen atom must give up an electron, forming a positively charged hydrogen ion and this electron must then be passed to an oxygen atom to create a negatively charged oxygen ion.6 The electrolyte between the electrodes will then allow either the charged hydrogen or a charged oxygen ion (but not both) to pass from one electrode to the other so that the two can react. However this electrolyte will not conduct electrons so these can only be transferred via an external electrical circuit. In order for the chemical reaction to be completed, charged atoms move through the electrolyte while electrons that keep the reaction balanced travel through the external circuit. In this way the fuel cell turns the chemical energy that drives the reaction into electrical energy.
There are a number of different types of fuel cell available today, each defined by the type of electrolyte it utilizes. The most common are the phosphoric acid fuel cell, the proton exchange membrane fuel cell, the molten carbonate fuel cell and the solid oxide fuel cell. Methanol fuel cells are also being developed for small-scale applications. Fuel cells are clean, environmentally friendly generation systems that can be installed in urban areas without problem. When running on natural gas a fuel cell power plant will produce carbon dioxide but they do not generate significant levels of any other atmospheric pollutants. However most fuel cells for power generation applications are relatively expensive.