0 Steam Turbine

Why are they used for CHP?

Steam turbines have been generating electricity in America for years. Power generated by steam turbines have been the first light bulb and encourage our ships for over 100 years. In fact, the first power plant (run by Thomas Edison to use a dynamo and is located in Pearl Street in New York City) is a CHP plant that generated electricity by using steam turbine. Excess steam used to heat homes. Today, most electricity produced in the United States to do so by a steam turbine. It is safe to say steam turbine technology is well known, well understood and proven.

Because the steam turbine run away from the steam produced by the boiler, it can support many various types of fuel. Natural gas, coal, nuclear, wood, municipal solid waste and more all can be used to run steam turbines. As a result, facilities that have excess waste products such as oil or wood tends to apply the steam turbine. As the picture below show, this time the steam turbine CHP system is run from a variety of fuels.

Another interesting feature of steam turbines is that they can be modified to fit any CHP system. Therefore, the steam turbine can be installed to match the pressure of the facility and temperature requirements. Furthermore, the steam turbine can be retrofitted into existing steam system. Furthermore, water and steam is very well understood. Using the steam table, we can know the exact nature of our working fluid at a given temperature and pressure. Therefore, the steam is very predictable.


Technology brief description:
 
Steam turbine is slightly different from other CHP prime mover in that they require separate boiler or "HRSG" (Heat Recovery Steam Generator) to make the working fluid (Steam). Sometimes, the plant will already have a boiler for the production process or to meet the burden of heating / cooling and instead of using "pressure reducing valve" for "Isenthalpically" reduce the vapor pressure, they will be running the steam through "Back pressure steam turbine" and generate electricity. In CHP applications, boiler or HRSG to generate steam that will be put through a steam turbine. steam turbine will produce electricity and steam exhaust the remaining can be used for hot water or heating / cooling.

The process of steam generation is the basis behind the "Rankine cycle". water heated to saturated liquid. From there, it is compressed into steam. steam transferred to a steam turbine where the pressure is reduced (typically to sub atmospheric pressure) by expansion over the turbine blades. This process produces
electricity. Low pressure steam is condensed back to liquid. The water, now called as the return of water, mixed with new water, referred to as "bait", and pumped back to the boiler. The figure below shows a diagram commonly used to describe the Rankine Cycle.

There are three types of steam turbines: condensing, non-condensing, and extraction. Condensing turbine is not used for CHP applications and therefore will not be discussed here. Non-condensing steam turbine is also referred to as "back pressure" steam turbine. Here, the expanded steam for steam and gas turbine exhaust is used to meet the needs of the steam facility. Expanded until it reaches the steam pressurefacility may be used. The figure below, taken from NREL, the schematic shows the process back pressure steam turbine.

Other types of steam turbines used in CHP applications is called extraction turbine. In the turbine, the steam extracted from the turbine at some intermediate pressure. this Steam can be used to meet the needs of the steam facility. The remaining steam expanded more and thicker. Extraction turbines can also act as a turbine inside. in entry turbines, steam turbines are added to the medium at some point. The figure below shows the process scheme of an extraction steam turbine.


Waste heat from the steam turbine (either collected through the exhaust or from the extraction), can be used to heating or cooling chamber, to process, or can be used to make a cold or  hot water. Steam turbine can also be part of the "combined cycle". In  process, the waste steam from electricity production process (ie waste generated steam  by gas turbines) is run through a steam turbine to generate more electricity. While this  very energy efficient, does not consider the CHP because there is no heating or cooling load is satisfied in each section. 

Steam system efficiency is difficult to calculate. 40-50% is a number that is usually attached to the steam turbine efficiency. However, this number can be misleading because it does not take into account boiler efficiency. Boilers are usually 80-85% efficient. If such a boiler is included in the steam system, then the efficiency of steam system drops to 32 to 42.5%. However, if the boiler is already in place and steam turbine was added later, then the general efficiency of the boiler does not need to be considered. Another thing to consider is if back pressure steam turbine replaces the pressure reducing valve or steam blow from practice, then any energy you can get is the energy efficient. The reason behind this is that the plant well before energy loss by expansion isenthalpic or just blow off excess steam. Now, these plants use of energy and therefore, when compared with the previous, more energy crops efficient.

cost:
Steam turbine boiler plus installation costs between $ 800 - $ 1000/kW. If the boiler is already in place, just a steam turbine installation cost alone is $ 400 - $ 800/kW. Maintenance costs for an estimated $ 0.004/kWhr steam turbine. steam turbine has been known to last more than 50 years with more than 99% availability. Table 1 gives the cost info for steam turbines only.
 
emissions:
Steam turbine does not emit anything themselves. However, emitting steam generator pollutants. Therefore, the emissions from a steam turbine system is highly variable and depending on the type of fuel used to create steam and the method by which steam made. Boiler will emit NOx, SOx, PM, CO, and CO2. Typical boiler emissions shown in the following table.

There are a variety of emission control technology for steam systems. Some These include: Flu-gas recirculation, low excess air firing, combustion control, using low nitrogen fuel oil, enter the water / steam to reduce NOx, non-selective catalytic reduction, selective catalytic reduction and others.