الاثنين

steam cycle) (cobmind cycle)


Power plants generate electrical power by using fuels like coal, oil or natural gas. A simple power plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler and superheater, heats the water to generate steam. The steam is then heated to a superheated state in the superheater. This steam is used to rotate the turbine which powers the generator. Electrical energy is generated when the generator windings rotate in a strong magnetic field. After the steam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is pressurized by the pump prior to going back to the boiler A simple power plant is described by a Rankine Cycle.
RANKINE CYCLE

Saturated or superheated steam enters the turbine at state 1, where it expands isentropically to the exit pressure at state 2. The steam is then condensed at constant pressure and temperature to a saturated liquid, state 3. The heat removed from the steam in the condenser is typically transferred to the cooling water. The saturated liquid then flows through the pump which increases the pressure to the boiler pressure (state 4), where the water is first heated to the saturation temperature, boiled and typically superheated to state 1. Then the whole cycle is repeated.


Typical Modifications REHEAT

When steam leaves the turbine, it is typically wet. The presense of water causes erosion of the turbine blades. To prevent this, steam is extracted from high pressure turbine (state 2), and then it is reheated in the boiler (state 2') and sent back to the low pressure turbine.
REGENERATION

Regeneration helps improve the Rankine cycle efficiency by preheating the feedwater into the boiler. Regeneration can be achieved by open feedwater heaters or closed feedwater heaters. In open feedwater heaters, a fraction of the steam exiting a high pressure turbine is mixed with the feedwater at the same pressure. In closed system, the steam bled from the turbine is not directly mixed with the feedwater, and therefore, the two streams can be at different pressures

Combined Cycle Plants

The combined-cycle unit combines the Rankine (steam turbine) and Brayton (gas turbine) thermodynamic cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production to supply a steam turbine as shown in the figure "Combined-Cycle Cogeneration Unit". Process steam can be also provided for industrial purposes.


Fossil fuel-fired (central) power plants use either steam or combustion turbines to provide the mechanical power to electrical generators. Pressurized high temperature steam or gas expands through various stages of a turbine, transferring energy to the rotating turbine blades. The turbine is mechanically coupled to a generator, which produces electricity.
Steam Turbine Power Plants:
Steam turbine power plants operate on a Rankine cycle. The steam is created by a boiler, where pure water passes through a series of tubes to capture heat from the firebox and then boils under high pressure to become superheated steam. The heat in the firebox is normally provided by burning fossil fuel (e.g. coal, fuel oil or natural gas). However, the heat can also be provided by biomass, solar energy or nuclear fuel. The superheated steam leaving the boiler then enters the steam turbine throttle, where it powers the turbine and connected generator to make electricity. After the steam expands through the turbine, it exits the back end of the turbine, where it is cooled and condensed back to water in the surface condenser. This condensate is then returned to the boiler through high-pressure feedpumps for reuse. Heat from the condensing steam is normally rejected from the condenser to a body of water, such as a river or cooling tower.
Steam turbine plants generally have a history of achieving up to 95% availability and can operate for more than a year between shutdowns for maintenance and inspections. Their unplanned or forced outage rates are typically less than 2% or less than one week per year.
Modern large steam turbine plants (over 500 MW) have efficiencies approaching 40-45%. These plants have installed costs between $800 and$2000/kW, depending on environmental permitting requirements.





hydrogen Basics-Internal Combustion Engines

Hydrogen has a high specific energy, high flame speed, wide range of flammability, and clean burning characteristics which suggest a possibility of high performance in internal combustion engines (ICE). These attributes have been realized for more than half a century since the onset of hydrogen engine development. In the early 1990s, FSEC conducted research on using hydrogen in an ICE. This work resulted in the development of a mixed fuel called HYTEST. Today, automobile manufacturers and DOE continue to work on hydrogen-powered ICEs.

Picture of Hydrogen/natural gas fueling (HYTEST fuel) of Ford Ranger, FSEC H2 Lab.
Hydrogen/natural gas fueling (HYTEST fuel) of Ford Ranger, FSEC H2 Lab
(Photo: S. Spencer)
There are four basic issues regarding hydrogen-fueled engines and vehicles: engine backfire and susceptibility of hydrogen to surface ignition, somewhat reduced engine power, high nitric oxide (NOx) emissions, and the problem of on-board storage of the fuel and safety. Although partial solutions have been found to most of these problems, there still is no general consensus of the best method to finally resolve all of these issues.

As far as the performance of a hydrogen engine is concerned, its limit of flammability in air is the most important factor. Hydrogen's low lean limit of flammability provides an opportunity to use the lean-burn engine (LBE) concept with hydrogen engines quite successfully. The LBE concept refers to engine operation that is leaner (higher air to fuel mass ratio) than stoichiometric (chemically correct air-fuel ratio). The amount of work done during the expansion process in a lean-burn engine is relatively large (due to lower cycle temperature), resulting in a proportionally higher thermal efficiency.

The LBE concept with hydrogen further facilitates and promotes the use of so-called "mixture regulation" or "quality governing" at light engine loads. Unlike gasoline-fueled engines that require throttling at lower engine loads, hydrogen-fueled engines can be operated at reduced power levels by limiting only the rate at which fuel is supplied, without restricting the flow rate of the intake air. Therefore, engine "pumping losses" which occur when the throttle valve is used are completely avoided. Hydrogen's high auto-ignition temperature provides an opportunity to operate hydrogen-fueled engines at higher compression ratios than those normally used with gasoline engines. The result is a further gain in indicated thermal efficiency.

Impediments to hydrogen utilization in an ICE are caused by its low ignition energy and wide limits of flammability. These make hydrogen engines particularly prone to pre-ignition. The situation is further aggravated by hydrogen's high flame speed. Pre-ignition leads to harmful flashbacks into the carburetor and rough operation and is believed to be due to the development of surface "hot spots." Induction ignition can occur due to excessive temperatures of both combustion chamber components and small surface deposits or suspended particles. Hydrogen's exceptionally low ignition energy requires that the average temperature prevailing within the combustion space during induction be sufficiently low so that the formation of hot spots is avoided. This requires appropriate cooling of the cylinder head, piston, valves, combustion chamber wall, and the use of cold spark plugs (non-platinum tipped spark plugs). One way to reduce the effect of combustion chamber hot spots on the pre-ignition of a fresh charge is to use thermal dilution techniques. The unusual heat and mass transfer characteristics of hydrogen make it almost necessary to rethink the combustion chamber and cooling system design so that hydrogen's unique attributes can be capitalized on to full advantage.

Another important issue regarding the engine operation, especially with near stoichiometric hydrogen and air mixtures, is the extent of NOx formation. This problem has been dealt with using any type of thermal dilution of charge by utilizing excess air (lean burn concept), water injection into the cylinder, and exhaust gas recirculation. The collective findings of many researchers appear to indicate that, in order to take full advantage of the lean burn concept and hydrogen's wide flammability limits to reduce NOx emissions to acceptable levels, it would be necessary to confine engine operation to equivalence ratios of approximately 0.65 or lower. It is also possible to achieve low levels of NOx emission with hydrogen engines utilizing internal mixture formation by DCI or port injection. In the internal mixture formation technique, hydrogen is admitted into the combustion chamber directly and under pressure. This approach has required the development of a high-pressure cryogenic injection system as well as salient combustion chamber designs which promote turbulence and rapid mixing of hydrogen and air in the cylinder. It appears that high-power, lean burn hydrogen engines that also produce minimal NOx emission are feasible.

FSEC staff have conducted major work on the use of hydrogen and natural gas as a fuel for ICEs by examining the prospects of mixing hydrogen with natural gas to improve engine performance and lower engine emissions. Researchers began the work by blending low amounts of hydrogen (5 to 10 percent) with natural gas, but results showed that mixtures of more than 20 percent hydrogen would be required to achieve the desired emission reductions.

This work focused on a mixture of hydrogen-enriched natural gas that allowed for an extended “lean burn limit” and thus lower engine emissions without using a catalytic converter. During this work, FSEC completed a series of tests on a 30-percent-plus hydrogen-enriched methane mixture, which was used to run a 350-cubic-inch V8 engine. Results showed that nitrogen oxide emissions could be lowered by approximately 90 percent in comparison with a gasoline-powered car. FSEC named the hydrogen-methane mixture HYTEST (any hydrogen-methane fuel with hydrogen content greater than 20 percent) and received a patent on the fuel.

For additional technical information on hydrogen ICEs, see: http://www.eere.energy.gov/hydrogenandfuelcells/tech_validation/pdfs/fcm03r0.pdf.


نقلا عن موقع http://www.fsec.ucf.edu