Magneto Hydro Dynamic (MHD)
Generator and construction and working principle of MHD
Magnetohydrodynamics (MHD) is power generation technology in which the electric generator is static (nonrotating) equipment. In the MHD concept, a fluid conductor flows through a static magnetic field, resulting in a dc electric flow perpendicular to the magnetic filed. MHD/steam combined cycle power plants have the potential for very low heat rates (in the range of 6,500 Btu/kWh). So2 and NOx emission levels from MHD plants are projected to be very low.
The fundamental MHD concept is illustrated in figure. The fluid conductor is typically an ionized flue gas resulting from combustion of coal or another fossil fuel. Potassium carbonate, called ‚seed,‛isinjcted during the combustion° process° to increase fluid conductivity. The fluid temperature is typically about 2,480 C to 2,650 C.
Figure: Ba .
The conductive fluid flows through the magnetic fields, inducing an electric field by the Faraday effect. The electric field is orthogonal to both the fluid velocity and magnetic field vectors. As a result, a potential difference is developed between the two walls of the duch as shown in figure. The direct current (dc) generated is converted to alternating current (ac) by a solid-state inverter.
Construction and Working principle
The planned application of the MHD concept for utility scale electric power generation uses MHD as a topping cycle combined with a steam bottoming cycle, as shown in figure. The topping cycle consists of the coal combustor, nozzle, MHD channel, magnet, power conditioning equipment (inverter) and a diffuser. The bottoming cycle consists of a heat recovery/seed recovery unit, a particulate removal system, a steam turbine-generator system, cycle compressor, seed regeneration plant, and for some concepts, an oxygen plant.
The combustor burns coal to produce a uniform product gas with a high electrical conductivity (about 10 mho/m). A typical MHD plant requires combustion gases of about 2,650°C at a pressure of 5 to 10 atmospheres. The goal is to remove a large portion (50% to 70%) of the slag (molten ash) formed in the combustion process in the combustor. High ash carryover inhabits efficient seed recovery later in the process. Oxygen enriched air is used as the oxidant to achieve high flue gas temperatures.
Commercial-scale MHD plants will use superconducting magnets. Magnetic fields must be in the range of 4.5 to 6 tesla. To achieve superconducting properties, the magnets must be cooled to about 4K.
Figure: Layout of coal-fueled magnetohydrodynamics system.
In addition to converting direct current to alternating current, the power conditioning system consolidates power from the electrode pairs and controls the electric field and current. Commercial power conditioning systems will use existing line-commutated solid state inverter technology.
The diffuser is the transition between the topping cycle and the bottoming cycle. The diffuser reduces the velocity of the hot gases from the MHD channel, partially converting kinetic energy into static pressure.
The heat recovery/seed recovery unit consists of radiative and convective heat transfer surfaces to generate and super heat steam. It also removes slag and the seed from the flue gas. In addition, the heat recovery/seed recovery preheats the oxidant supply NO2 control may be achieved within the recovery unit by a second stage of combustion. The first stage of combustion within the MHD combustor is conducted in a fuel-rich environment. The second stage of combustion within the recovery unit takes place at a temperature above 1.540°C with a residence time and cooling rate such that NOX decomposes into N2 and O2.
Control of SOX is intrinsic with the removal of the potassium seed from the flue gas. The potassium seed combines with the sulfur to form potassium sulfate, which condenses and is removed downstream by the particulate removal system. The recovered potassium sulfate is converted to potassium seed in the seed regeneration unit.
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