The main use for stationary gas turbines is to produce electrical power in a power plant. Before air enters the gas turbine it is filtered, to prevent material from getting into the compressor and collecting on the compressor blades or other equipment (Figure 1). Modern filtering systems are equipped with self-cleaning mechanisms. Pulses of compressed air may be used to provide cleaning of the filters. Many modern power plants with gas turbines cool the entering air with a chiller or air conditioner coil to maximize the efficiency. The cooling water used may come from chilled water from the cooling tower.
The air enters the compressor of the gas turbine and is compressed. Only about a quarter of the air is used for combustion and the rest is used to provide cooling to the gas turbine. Cooling air must be continually delivered to the liner in the combustor to prevent damage fom excessive termperatures and to cool gases to the turbine inlet temperature.
Compressed air enters the combustor, fuel is added and the air/fuel mixture is ignited. Hotgases then expand through the turbine of the engine, forcing the turbine to turn rapidly. The energy transferred to the turbine is mostly used to turn the compressor. The shafts of the turbine and compressor are directly connected. Some designs may have gearing between the generator and gas turbine to attain the desired revolutions per minute (rpm). The net output energy of the turbine turns the generator to convert the mechanical energy of the gas turbine into electricity. Cooling is also required for the generator. Air is drawn into the generator enclosure for cooling. Some generators are water cooled and oil is used to provide lubrication and cooling. The generator may be mounted on the turbine side of the gas turbine or on the compressor side, depending on the type of gas turbine.
Most stationary gas turbines use natural gas as a fuel. The natural gas is delivered to the plant by a pipeline. Compressors are used to increase the pressure of the natural gas to the pressure that is required for the fuel injectors in the combustor. An electric motor or engine may be used to run the natural gas compressor. Oil may also be used to run the gas turbine, but it is usually a backup fuel in case natural gas is temporarily unavailable.
Some gas turbines also have equipment to extract more energy from the hot gases leaving the turbine (Figure 1). Feed water pumps run water through heat exchangers in the exhaust portion of the gas turbine called the heat recovery steam generator (HRSG) or waste heat boiler. The heat exchangers are generally large bundles of tubes made of highly conductive materials such as steel. Hot gases exchange heat energy with the water through the boiler tubes before the exhaust exits the stack. A power plant is a cogeneration plant when it produces power and also makes steam that is sent elsewhere to run other equipment. This steam could be used to heat water, heat air for space heating, turn a turbine for mechanical energy, or to operate other industrial processes.
The heat recovery sections in the heat recovery steam generator may include the economizer, low pressure, intermediate pressure, and the high pressure section. The economizer is generally the last heat exchanger in the system before flue gases are released to the stack. As its name implies, its function is to capture as much of the last amounts of heat energy available in the stack gas to save money. Feedwater pumps pump the water through the economizer and then into the low pressure boiler tubes and comes back to the low pressure boiler, increasing the temperature of the water. The low pressure section is also called the deaerator, because dissolved corrosive gases are usually removed from the water in this section. Oxygen is the primary gas that is removed. Steam is sent from the low pressure boiler to the intermediate boiler. Heated water from the economizer is also pumped to the intermediate section. As with the low pressure boiler, the intermediate boiler is a heat exchanger that absorbs heat from the flue gas. The intermediate steam produced from the
intermediate boiler is at a higher pressure and temperature than the steam from the low pressure boiler. For simplicity, only one feedwater pump is shown and the intermediate boiler section has been left out of Figure 1.
Steam from the intermediate boiler and feedwater is sent to the high pressure boiler. Water flows from the high pressure drum into the tubes of the high pressure boiler, returning to the high pressure drum and creating high pressure steam. Steam from the high pressure drum or boiler then goes to the superheater. The superheater is located toward the hottest section of the HRSG. Only water that is in its gaseous state is sent to the superheater. The superheater increases the pressure and temperature of the steam to its highest value in the system. Superheated steam from the superheater is sent to a steam turbine to produce more energy. Intermediate steam and low pressure steam may be drawn off the intermediate and low pressure boilers to supplement the steam turbine, to run other processes or for steam injection into the gas turbine.
At the front side of the heat recovery steam generator near the exit of the gas turbine, the temperature of the gases is generally around 1000°F. By the time the flue gases exit the stack of the HRSG, the temperature is generally in the 200°F to 300°F range.
A duct burner can be installed at the front side of the HRSG. The purpose of the duct burner is to add additional heat to the flue gases exiting the turbine utilizing the available oxygen. Some duct burner designs can also allow the steam plant to operate when the gas turbine is not in operation, but this operation (fresh air firing) is not common practice since it is not very efficient. Fresh air may be added for the burner by a fan upstream of the duct burner. Additional fuel (usually natural gas) is added to the burner.
Equipment for air pollution control in the heat recovery steam generator includes the carbon monoxide catalyst, the ammonia injection grid and the selective catalytic reduction catalyst. The carbon monoxide catalyst is used to help remove carbon monoxide by oxidizing it to carbon dioxide. Ammonia is injected in the HRSG for NO x control. Ammonia and the selective catalytic reduction catalyst work to change NO x compounds into nitrogen.
Superheated steam from the HRSG is injected into a steam turbine. As the steam expands through the turbine its temperature and pressure decrease and energy is transferred to the turbine,making it rotate. The turbine then turns a generator to produce electricity, in addition to the electricity produced by the generator connected to the gas turbine. This is why this type of power plant is called a “combined cycle” or “dual cycle.” The thermal efficiency of the overall combined cycle may be over 50%.
After the steam exits the steam turbine, it flows through the condenser. The condenser is essentially a heat exchanger where the steam is cooled and condensed back into water by cooling water from the cooling tower. The cooling water from the cooling tower flows through the tubes in the condenser while the steam flows over the tubes. Condensed water from the comdenser is then pumped from the condenser by the condensate pump to the feedwater pumps and back into the boiler.
The cooling tower is often used to cool the water that was used to condense steam in the condenser. Cooling towers are generally rectangular structures that may be divided into a few independent cells. Within the stack or body of a cooling tower is a large fan that draws air up through the cooling tower from the sides of the device. As air is being pulled up through the device, the warmed water from the condenser is sprayed down from the top of the tower. The water trickles down by gravity through the tower over baffles or packaging, which help maximize the surface area of the water and the contact between the air and water. The air and water also move in a counter-flow arrangement which helps improve contact between the two flows. Heat is primarily removed from the water through evaporation of water in the cooling tower. This process is also called evaporative cooling. Cooled water falls to the bottom of the coling tower in a basin, where it can be pumped tothe condenser to continue condensing steam. Since water is continually lost to the atmosphere from the stack of the cooling tower, make-up water is added to the system.
Not all gas turbines are equipped with steam turbines. Some of them simply use the heat recovery steam generator to produce steam for steam injection. Some plants may be equipped with simple cycle peaking turbines to produce power during high energy demand.
