The broad classification of MHD system is as follows:
(i)Open-cycle systems
(ii)Closed-cycle systems
Open-cycle MHD System
An elementary open-cycle MHD system.The fuel,coal (or natural gas)is burned in a combustor. The products of combustion with highly preheated air are seeded with an alkali metal,such as cesium or potassium having a low ionization potential.This increases the conductivity of the gas. The gas is them expanded through a nozzle to increases its velocity.The electrically ionised gas passes through a duct at about 2500-3000K. This duct has two sides supporting a strong transverse magnetic field of flux density 5-7 teslas at right angles to the flow of the gas.The other sides form the faces of electrodes which are connected to the load.As the hot ionised gas or plasma enters the MHD duct,the kinetic energy of the plasma is decreased due to the effect of the strong magnetic field.The electrons and positive ions move to the electrodes and thus,there is a current flow though the load.This is direct current (dc).An inverter is used to change it to change it to ac.the high temperature exhaust flowing out of the MHD generator passes through air preheater for heating the inlet air,since the products of combustion are exhausted to atmosphere,the oxides and hydroxides of the seeding element may cause severs air pollution.An electrostatic precipitator is used in the recovery of the seed,which can be used again.After going through the pollution control mechanism,the gas is then discharged to the atmosphere through the stack.The high-temperature exhaust from the MHD duct may also be used to generate steam to run a conventional steam-turbine generator as a bottoming plant.
Closed-cycle MHD System
A closed-cycle MHD system it has three interconnected loops.The first loop is the external heating loop.Coal is gasified and the gas is burned in a combustor to produce heat.
This heat is transferred to argon(the working fluid of the MHD cycle) in the heat exchange HX1.
The combustion products are fed to the air preheater to recover part of the heat of combustion products are discharged to the atmosphere through the stack.
The second loop is the MHD loop.Hot argon gas from the heat exchange HX1 is seeded with cesium and passed through the MHD generator at a very high speed.The dc power output of the MHD generator is converted to ac by a inverter .
The third loop in this system is the steam loop.This is used for further recovering the heat of working fluid and converting this heat into electrical energy.The hot working fluid is slowed down to a low subsonic speed in a diffuser.The hot working fluid then enters another heat exchanger HX2 which serves as a waste-heat boiler.steam generated in HX2 is used to run two steam turbines ST1 and ST2.The turbine ST1 drives an alternator.The output of the alternator is also supplied to the grid.The steam turbine ST2 runs the compressor.The working fluid argon is fed back to the heat exchanger HX1 after being passed through compressor and intercooler.
(i)Open-cycle systems
(ii)Closed-cycle systems
Open-cycle MHD System
An elementary open-cycle MHD system.The fuel,coal (or natural gas)is burned in a combustor. The products of combustion with highly preheated air are seeded with an alkali metal,such as cesium or potassium having a low ionization potential.This increases the conductivity of the gas. The gas is them expanded through a nozzle to increases its velocity.The electrically ionised gas passes through a duct at about 2500-3000K. This duct has two sides supporting a strong transverse magnetic field of flux density 5-7 teslas at right angles to the flow of the gas.The other sides form the faces of electrodes which are connected to the load.As the hot ionised gas or plasma enters the MHD duct,the kinetic energy of the plasma is decreased due to the effect of the strong magnetic field.The electrons and positive ions move to the electrodes and thus,there is a current flow though the load.This is direct current (dc).An inverter is used to change it to change it to ac.the high temperature exhaust flowing out of the MHD generator passes through air preheater for heating the inlet air,since the products of combustion are exhausted to atmosphere,the oxides and hydroxides of the seeding element may cause severs air pollution.An electrostatic precipitator is used in the recovery of the seed,which can be used again.After going through the pollution control mechanism,the gas is then discharged to the atmosphere through the stack.The high-temperature exhaust from the MHD duct may also be used to generate steam to run a conventional steam-turbine generator as a bottoming plant.
Closed-cycle MHD System
A closed-cycle MHD system it has three interconnected loops.The first loop is the external heating loop.Coal is gasified and the gas is burned in a combustor to produce heat.
This heat is transferred to argon(the working fluid of the MHD cycle) in the heat exchange HX1.
The combustion products are fed to the air preheater to recover part of the heat of combustion products are discharged to the atmosphere through the stack.
The second loop is the MHD loop.Hot argon gas from the heat exchange HX1 is seeded with cesium and passed through the MHD generator at a very high speed.The dc power output of the MHD generator is converted to ac by a inverter .
The third loop in this system is the steam loop.This is used for further recovering the heat of working fluid and converting this heat into electrical energy.The hot working fluid is slowed down to a low subsonic speed in a diffuser.The hot working fluid then enters another heat exchanger HX2 which serves as a waste-heat boiler.steam generated in HX2 is used to run two steam turbines ST1 and ST2.The turbine ST1 drives an alternator.The output of the alternator is also supplied to the grid.The steam turbine ST2 runs the compressor.The working fluid argon is fed back to the heat exchanger HX1 after being passed through compressor and intercooler.
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