Open and Closed Cycle MHD System | Electricity

In this article we will discuss about the open and closed cycle MHD system.

Open Cycle MHD System:

An el­ementary open cycle MHD system is a system in which a high pressure, high temperature combustion gas is forced through a strong magnetic field.

Coal is processed and burnt in the combustor at a high temperature of about 2,600°C and pressure of about 12 atmospheres with pre-heated air to form the plasma. Then a seeding material, such as potassium carbonate, is injected to the plasma in order to increase the electrical conductivity.

The resulting mixture having an electrical conductivity of about 10 siemens/m, is expanded through a nozzle, so as to have high velocity, and then passed through the strong magnetic field (5-7 T) of the MHD generator. During the expansion of gas at high temperature, the positive and negative ions move to the electrodes and so constitute an electric current. This current is dc and an inverter is employed for its conversion into ac.

The gas leaving the MHD generator is still very hot. The heat from the exhaust gases of the MHD generator is utilised in preheating the air supplied to the combustor. The seed material is recovered from the gas for successive use and harmful emissions (such as nitrogen and sulphur) are removed from the gas, for pollution control, and the gas is finally discharged to the atmosphere through a stack.

The open cycle MHD system explained above is not suitable for commercial use. For making this process efficient, it is necessary to combine the MHD unit with steam turbine- alternator unit. In this system the heat from the exhaust gases of the MHD generator is used to raise steam which generates additional energy in a steam turbine-alternator unit. A part of this steam is also used in a steam turbine driving a compressor for compressing air for the MHD cycle. Such a cycle is called the hybrid, binary or topping MHD steam plant cycle.

The electrodes are usually made of graphite and the duct of boron nitride.

Any type of fossil fuel (coal, oil, natural gas) can be used in MHD generator but a direct coal fired MHD genera­tor has the following advantages:

(i) Slag from coal combustion coats the generator elec­trodes and protects from electrical and mechanical corrosion that otherwise occurs in a clean fuel generator. However, the electrodes may be short circuited by molten ash.

(ii) Coal contains less hydrogen and, therefore, the sink for electrons in the flow created by the presence of OH ions is reduced.

Char, having almost no hydrogen, is better than coal even. It is easier to handle and feed in comparison to coal. It results in a 25% increase in the performance of the generator.

Power generated by the MHD system is given as:

ρ = σB² v²K (1 – K) W/m³ …(7.1)

Where σ is the specific electrical conductivity of gas in siemen/ metre, B is magnetic field strength in Tesla (Wb/m²), v is the velocity of gas in m/s and K is the ratio of external load voltage to open-circuit voltage.

Closed Cycle MHD System:

As the name suggests, the working fluid, in a closed cycle, is circulated in a closed loop. The closed cycle MHD system may be either a plasma converter or a liquid metal converter. The plasma converter uses an ionized gas (helium or argon seeded with cesium) and the liquid metal converter uses the vapour of the metal or the metal in a liquid form (the metal may be an alkali or some other metal).

Liquid metal system has the basic advantage of high electrical conductivity. So high temperatures are not required in this system to achieve the high electrical conductivity and the systems are normally designed to operate at temperatures below 1,400 K (a much lower temperature in comparison to plasma converters). This temperature is low enough that the energy can be supplied by a nuclear reactor or fossil fueled system.

The complete system can be divided into three distinct but interlocking loops. Coal is gasified and the gas so produced has heat value of about 5.35 MJ/kg and a temperature of about 520°C. This gas is burnt in a combustor to generate heat. This heat is transferred to the working fluid (argon) of the MHD cycle in a heat exchanger (HX₁). The combustion products are passed through the air preheater (for recovery of a part of heat of combustion products) and purifiers and then discharged to the atmosphere. This is loop I and may be called an external heating loop.

The hot argon gas is seeded with cesium and passed through the MHD generator that produces direct current (dc). This dc output is converted into ac by means of inverter and then supplied to the grid. This is the loop II and may be called an MHD loop.

For further recovery of heat from the working fluid and its use in generating of steam, the working fluid is slowed down, in a diffuser, to a low subsonic speed and then this fluid is passed through the heat exchanger HX₂. In heat exchanger HX₂, the fluid imparts it heat to water and so generates steam.

This steam is used partly for driving a steam turbine operating the compressor and partly expanded in a steam turbine driving a three-phase alternator. The working fluid is then passed through compressor and intercooler and then returned back to the heat exchanger HX₁. This is the loop III and may be called a steam loop.

The super heated metallic vapour is expanded through a supersonic nozzle into the drift tube or mixer. Atomized sub-cooled liquid droplets are accelerated by the vapour. The vapours also condense on the liquid droplets so that the fluid entering the MHD generator is essentially a liquid. The resulting velocity of the fluid is more than 150 metres per second.

The advantages of liquid metal system are as follows:

1. This system can use nuclear energy as high tem­perature is not the requirement of this system as in case of the plasma converter.

2. It can easily provide ac power supply directly, while it is almost impossible to do so in a plasma system.

3. The size of the system including that of magnets is comparatively smaller. This is because of high power density.

However, the liquid metal system has the following limitations:

1. The metallic vapours are poor electrical conduc­tors.

2. High velocities cannot be obtained by expansion in this system while it is much easier to achieve a high fluid velocity employing a gas and a nozzle. This is because the liquids are practically incompressible.

3. The overall conversion efficiencies obtainable with liquid metal system are quite below to that of plasma system.

A closed cycle system can provide more useful power conversion at lower temperatures of about 1,600°C but this system has not taken practical shape so far. The difficulties with such a system are the design of heat exchanger (the heat exchanger operates up to the highest temperature of the gas), requirement of absolute purity of the working fluid, the problems posed by electrical stability of the flow in the generator (the gas is subject to electric fields approaching breakdown conditions) etc.