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WEEK 01: GENERATION: PRIMEMOVERS Sections: Introduction | Steam | Gas | Hydro | Cogeneration Generation. Electric Power is generated through the employment of fossil-fuel plants. Traditionally, fossil fuels have supplied most of the world's energy requirements. The production of electric power is based on the conversion of energy stored in chemical form, for example, coal, or potential form, for example, hydro and aero, to the electric form. Also, nuclear sources represent the utilization of another form of energy storage, that of the binding energy of atoms. All these forms of conversion needs a primemover. Primemover. A reciprocating engine or turbine. The engine is the precursor of the latter. These machines are applied to either the conversion of energy from steam, wind or hydro to electrical form. Sections: Introduction | Steam | Gas | Hydro | Cogeneration Steam Engine operates at low speed, 100 to 400 rpm, high-efficiency when small (less than 500 HP), provides high-starting torque, and almost foolproof. Steam Engine Types. There are two major types of steam engines: the simple D-slide engines (less than 0.100 HP) used for auxiliary drive and the single-cylinder counterflow and uniflow engines (less than 1000HP), with Corliss or poppet-type valve gear, used for generator or equipment drive in factories, office buildings, paper mills, hospitals, laundries, and process applications. Although engines as large as 7500 kW have been built, the field is generally limited to engines less than 500kW.
"The net work of the cycle is represented by the area enclosed within the diagram and is represented by the mean effective pressure(mep), that is, the net work (are) divided by the length of the diagram. The power output is hp = pmLan / 33,000, where hp = horsepower; pm = mep in pounds per square inch; L = length of stroke in feet; a = the net piston area in square inches; and n = the number of cycles completed per minute. The theoretical mep and horsepower are larger than the actual indicated values and are customarily related by a diagram factor between 0.50 and 0.95. The shaft or brake mep and horsepower are lower still, with mechanical efficiency ranging between 0.80 and 0.95. Steam Turbines provides high speeds from 1,800 to 25,000 rpm, high efficiency, usually greater than 85%, uses a minimum flow area and low in weight than engines, and requires no internal lubrication. High steam operation could withstand pressures up to 5,000 psi, high temperatures up to 1050°F and low vacuum at 0.5 inch Hg. Turbines also provide high reliability at low maintenance.
General Energy Equation applied to nozzles is stated as: (V1² / 2gJ) + H1 = (V2² / 2gJ) + H2 which is reduced to: Jet velocity, ft/s = 223.7 where V1 is assumed to be zero and DH is the enthalpy drop (isentropic expansion) in Btu per pound as obtained from the Mollier chart for steam.
Sections: Introduction | Steam | Gas | Hydro | Cogeneration Gas Turbines provide efficiencies up to 80% or more. These turbine could operate on different cycles: Otto, Diesel as well as Brayton cycles.
Internal Combustion Cycles Mixture engines operate on Otto Cycle. Injection engine operate on Diesel Cycle. Gas or Combustion Turbine operate on Brayton Cycle. These cycles are similar that in general, Phase 1-2, isentropic compression; Phase 2-3, heat addition at constant pressure or volume; Phase 3-4, isentropic expansion; Phase 4-1, heat rejection at constant pressure or volume.
Gas Turbine Cycles There are four general types of gas turbine cycles: The open cycle, where an axial or centrifugal compressor delivers the compressed air to the combustion system, and fuel is burned to increase the fluid temperature. The gases expand through the turbine, producing sufficient power to drive the compressor and the load device. The gases are exhausted to atmosphere at a relatively high temperature. In a closed cycle, the fluid, usually air or helium, circulates continuously through the compressor, turbine, and heat exchangers. Heat is supplied from outside the cycle by a nuclear reactor, coal burner, or other source. An improvement in efficiency off both open and closed cycles can be obtained through the use of a regenerator or recuperator, usually situated before the combustion chamber. This is the regenerative cycle, which exchanges heat from the exhaust to the combustor inlet to reduce the fuel required to heat the gas. The highest efficiencies are available from combined cycles, where the gas turbine exhaust heat produces steam to power a steam generator. The steam turbine output is obtained with no additional fuel input. The most effective utilization of fuel output is available through cogeneration, or combined heat and power. A typical cycle has the gas turbine generating power and producing steam which is sent to an industrial process. A noncondensing steam turbine may be inserted between the boiler and the process, or the steam may be extracted at the appropriate pressure from a condensing steam turbine. Performance Component efficiencies, air flow, pressure ratio, and turbine inlet (or firing) temperature are the major factors affecting gas turbine output and efficiency. Multistage axial component efficiencies are in the 88 to 92% range. Material developments and turbine cooling techniques permit the firing temperature to exceed 2300°F (1260°C). The following table shows typical thermal efficiencies and optimum pressure ratios for various cycles.
Sections: Introduction | Steam | Gas | Hydro | Cogeneration see Hydroelectric Power Plants Sections: Introduction | Steam | Gas | Hydro | Cogeneration Cogeneration is the efficient production of two forms of useful energy from the same fuel resource, using the exhaust energy from one production system as the input for the other. Ordinarily the primary energy is thermal (steam) and the secondary form is either electrical or mechanical. The electrical or mechanical energy can be used internally to run company equipment, or the electricity can be sold to a utility. Since cogeneration is a sequential process, it requires (1) some amount of common physical plant space for the two processes, and (2) a sharing of the energy content of the fuel. This joint system can reduce energy input to 10 to 30% below that required by separate systems to produce the same outputs. Total system efficiency can approach 90%. Types of Cogeneration. There are two basic types of cogeneration systems, depending whether thermal or electrical energy is produced first. In Topping systems, a steam turbine or internal combustion engine is used to drive a generator and produce electricity. The waste heat resource is either (1) steam from the exhaust of the steam turbine or (2) exhaust gases or jacket heat removed from internal combustion engines. This heat is then provided for space heating, industrial process heating, absorption air-conditioning, or other thermal-related requirements. In Bottoming systems, waste heat maybe exhausted from furnaces, kilns, chemical reaction, and other process. Most common systems utilizes the waste heat to generate seam in a recovery boiler in order to drive a turbine or generator. Equipment for Cogeneration. Steam turbines used in cogeneration systems range in size from those producing a few horsepower up to units capable of 105 kW. Most often used in topping systems, the incremental fuel cost of electricity is between 4500 to 6500 Btu. Turbine efficiency may range from 50 to 80%. When used in economically-viable bottoming systems, exhaust gases from the steam turbines must be available at a rate of 500,000 Btu/h. Gas Turbines come in sizes ranging from 6 kW to 100 MW and can be configured to operate on one or more fuels. Units that burn natural gas give the most reliable and continuous operation. Exhaust temperatures range between 800 to 1000°F, thus producing relatively high-quality steam when used with a recovery boiler. Generally, an additional 4000 to 6000 Btu of fuel above what is used to meet a system's process requirements will be needed to produce a kilowatt hour of electricity. Exhaust gas temperature and the amount of power generated depend highly on the ambient temperature. The efficiency of the electrical generation decreases significantly as output decreases from full load. Large slow-speed Diesel Engines are available with the capability of producing up to 30MW of electricity. In any configuration, heat maybe reclaimed from both the exhaust gases and from the engine's internal cooling system. Temperature of exhaust gases is below that of gas turbines. Diesels are capable of 250 to 500 kWh/MBtu of fuel when used with an unfired recovery boiler. When used to generate steam only. this ratio may be as high as 1500 kWh/MBtu. The amount of heat produced is generally proportional to the load of the engine. Heat-Recovery Boiler transfer the thermal energy of the exhaust produced by gas turbines or other heat sources to water in order to produce steam. Like all boiler, steam can be extracted at a number of places in the heating and pressurization process, depending upon the demands of the boiler application. 1. Author. TitleBook Title Publisher, Place, Year, page. |
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