In Waste-to-Energy plants, electricity is produced through a steam turbine – generator set. When producing electrical power, it is possible to recover only up to 35 percent of the available waste energy as power. The surplus heat has to be cooled in a condenser or a cooling tower. This option is attractive if the plant is situated far from consumers who demand heat. When only power is produced, a fully condensing turbine is used. The excess heat is produced at such a low temperature in this condenser that it is not attractive for recovery.
A steam turbine is a rotary type of steam engine that consists of a stationary set of blades (called nozzles) and a moving set of adjacent blades (called buckets or rotor blades) installed within a casing. Steam from nozzles or guide passages are directed continuously against the two sets of blades in such a way that the steam turns the shaft of the turbine and the connected load (i.e. the generator). The stationary nozzles expand steam to lower pressure, resulting in steam velocity acceleration. A rotating bladed disc changes the direction of the steam flow, thereby creating a force on the blades which, because of the wheeled geometry, manifests itself as torque on the shaft on which the bladed wheel is mounted. The combination of torque and speed is the output power of the turbine.
The turbine shaft is connected to a generator so that the generator spins around with the turbine blades. As it spins, the generator converts kinetic energy from the turbine to alternating current electrical energy by using a rotating magnetic field.
The difference in the various types of steam turbines is due to different methods of using the steam, depending upon the construction and arrangement of the nozzles, steam passages and buckets.
In the impulse-type steam turbine, the expansion and consequent change in the pressure of the steam occurs entirely within the nozzles which direct the steam in jets against the moving buckets. Inasmuch as the expansion of steam takes place in the nozzles, the clearance between the rotating and stationary surfaces is greater than in the reaction-type steam turbine.
In the reaction-type steam turbine, the expansion and consequent change in pressure of the steam occurs entirely in the blading where the steam is directed against the moving buckets or blading by guide valves or orifices. Steam expansion takes place through both the stationary and moving guide vanes, and therefore the clearance space between the stationary and moving surfaces is very small, to reduce to a minimum the pressure drop due to leakage between stages.
The thermodynamic cycle for the steam turbine is the Rankine cycle. The cycle is the basis for conventional power generating stations and consists of a heat source (boiler) that converts water to high pressure steam. In the steam cycle, water is first pumped to elevated pressure, which is medium to high pressure depending on the size of the unit and the temperature to which the steam is eventually heated. It is then heated to the boiling temperature corresponding to the pressure, boiled (heated from liquid to vapor), and then most frequently superheated (heated to a temperature above that of saturated steam). The pressurized steam is expanded to lower pressure in a multistage turbine, then exhausted either to a condenser at vacuum conditions or into an intermediate temperature steam distribution system that delivers the steam for industrial or commercial applications. The condensate from the condenser or from the industrial steam utilization system is returned to the feedwater pump for continuation of the cycle.
Steam turbines used exclusively for electricity production are called condensing turbines, while steam turbines used for CHP (Combined Heat and Power) can be classified into two main types: non-condensing and extraction.
The primary type of turbine used for central power generation is the condensing turbine. These power-only utility turbines exhaust directly to condensers that maintain vacuum conditions at the discharge of the turbine. An array of tubes, cooled by river, lake or cooling tower water, condenses the steam into (liquid) water. The condenser vacuum is caused by the near ambient cooling water, causing condensation of the turbine’s exhaust steam in the condenser.
The condensing turbine processes result in maximum power and electrical generation efficiency from the steam supply and boiler fuel. The power output of condensing turbines is sensitive to ambient conditions.
The non-condensing turbine (also referred to as a back-pressure turbine) exhausts its entire flow of steam to the industrial process or facility steam mains, at conditions close to the process heat requirements.
Usually, the steam sent into the mains is not much above saturation temperature. The term “back-pressure” refers to turbines that exhaust steam at atmospheric pressures and above. The discharge pressure is established by the specific CHP application. The lower pressure is most often used in small and large district heating systems, and the higher pressure is most often used in supplying steam to industrial processes. Significant power generation capability is sacrificed when steam is used at appreciable pressure rather than expanding to vacuum in a condenser.
The extraction turbine has opening(s) in its casing for extraction of a portion of the steam at some intermediate pressure. The extracted steam may be used for process purposes in a CHP facility, or for feedwater heating as is the case in most utility power plants. The rest of the steam is condensed.
The steam extraction pressure may or may not be automatically regulated depending on the turbine design. Regulated extraction permits more steam to flow through the turbine to generate additional electricity during periods of low thermal demand by the CHP system. In utility-type steam turbines, there may be several extraction points, each at a different pressure, corresponding to a different temperature at which heat is needed in the thermodynamic cycle.
The facility’s specific needs for steam and power over time determine the extent to which steam in an extraction turbine will be extracted for use in the process, or expanded to vacuum conditions and condensed in a condenser.