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A 25,000 kW pilot MHD-power plant
Energy Conversion. Vol. 9, pp. 41-46. Pergamon Press, 1969. Printed in Great Britain
A 25,000 kW Pilot MHD-Power Plant V. A. KIRILLIN,f P. S. NEPOROZHNIYt and A. E. SCHEINDLINf (Received 18 June 1968)
1. General
Economically efficient generation and distribution of electrical power is a prerequisite for the economic progress of an industrially highly developed nation. The demand for electricity grows continuously and tends to double within each decade. Whatever the basic source of energy there is always a need for its conversion to electrical power as efficiently and cheaply as possible. The last 10-15 yr have witnessed certain advances to this end. Both the cost per kilowatt of installed capacity and the cost of kilowatt-hour of electricity produced have been considerably reduced through reduced capital costs (by building units of larger capacity) and improved generating efficiency. Yet power generation has presently entered a stage where any further major improvements in economic efficiency of conventional steam power stations are becoming most unlikely. Attempts have been made to substitute steam by other working fluids or to radically change the steam-water cycle proper. However, this does not appear to lead to a substantial enhancement in thermal efficiency of power plants either. New prospects for a significant improvement in economic efficiency of large power plants are now being associated with technical feasibility of commercial introduction of MHD-generators. The recent scientific developments in the field and the present level of engineering and technology make it quite safe to expect a 50 per cent net efficiency for a large open-cycle MHDpower plant, with an increase up to 60 per cent in the not-far-away future. Recognizing these highly promising prospects of application of MHD-plants for large-scale power generation, the USSR Ministry of Power Stations has sponsored, and the Institute for High Temperatures of the Academy of Sciences has provided scientific guidance of, extensive research and development efforts in the field of application of MHDplant for commercial power generation. In accordance with the research and development program, a model experimental MHD-power plant Y-02 has been developed and successfully operated in Moscow. The voluminous experimental data obtained with the plant have substantially helped to get further and deeper insight into the problems encountered when developing commercial MHD-plants, as well as to find the ways and means to their solution.
The next step toward utilization of MHD-generators for large scale power generation is to construct a pilot MHD-power plant of 25,000 kW capacity. This larger plant built on the basis of the present-day engineering and operated at relatively moderate parameters, will serve to final working out of the components of a large scale MHD-plant under real operation conditions. The experience gained from operating this plant is expected to supply the ultimate answer as to the overall economics of open-cycle systems for power generation. The following considerations determined the choice of the size and scheme of the pilot plant under construction: (1) The pilot MHD-plant should utilize all the advantages offered by the present-day engineering and technology and should make use of commercially available construction materials. As mentioned above, the recent developments in engineering and technology have paved the way to tackling this problem. (2) The plant parameters should not be too high, in order to provide sufficient reliability of the equipment, required for long life operation of the commercial plant. (3) The scale of the plant should be chosen so as to maintain the detrimental by-effects small relative to the net useful output, and so that the overall plant efficiency be competitive compared to the efficiency attainable with the current conventional power plants. (4) The fuel chosen for the pilot plant is natural gas, since utilization of other energy sources (coal, oil, etc.) involves additional difficulties. In what follows a detailed description is given of the scheme and major units of the 25,000 kW pilot MHDpower plant (Y-25) now being constructed in Moscow. 2. Thermal Scheme of Y-25
The open-cycle pilot plant Y-25 is operated on combustion products of natural gas seeded with readily ionizing K2COa. To provide an acceptable level of combustion products conductivity within the MHDgenerator, their inlet temperature is taken to be 2600°C. This temperature is attainable when burning natural gas in air, with the air-preheat up to 2000°C. At present, however, this high level of air-preheat is a formidable problem to be yet solved. Therefore, the air-preheat for Y-25 has been limited to 1200°C, a level already attainable on an industrial scale. The insufficient airpreheat is compensated for by oxygen enrichment. t Institute for High Temperatures, Krasnokazarnennaja, The heat released by the combustion products behind the MHD-generator, is used up in the steam cycle. 17A Moscow,E-250, U.S.S.R. 41
42
V. A. KIRILLIN, P. S. NEPOROZHNIY and A. E. SCHEINDLIN
Basing on considerations stated above, the MHDgenerator capacity is taken to be 25,000 kW. The thermal scheme of Y-25 is characterized by the following basic parameters: MHD-generator power output Fuel Oxidizer
Oxygen-enriched air preheat temperature Combustion products inlet temperature Ionizing seed Combustion products rate MHD-generator inlet pressure MHD-generator inlet velocity Outlet pressure Exhaust heat recovery Steam flow rate Steam pressure Steam superheat temperature Feed water temperature Magnet system power consumption Compressor power consumption Net efficiency
25,000 kW Natural gas Oxygen-enriched (up to 40 per cent) air 1200°C 2600°C K2CO3 up to 1 mole per cent 50 kg/sec 2.75 atm 850 m/see 1.07 arm Steam cycle 270 tn/hr 100 atm 540°C 105°C 2.6 MW 5.6 MW 33 per cent
The plant scheme is presented in Fig. 1. The pilot plant Y-25 is a power plant including two power cycles. The primary open cycle includes an
MHD-generator as its major component with combustion products used as the working fluid. The secondary closed cycle is a steam turbine driven by the heat released by the combustion products effluent from the MHD-generator.
(i) The combustion products cycle The air is uptaken from the atmosphere through an air inlet (I) and is fed into the compressor. During its passage to the compressor it is enriched with oxygen from an oxygen generator (2). The air-oxygen mixture is then compressed in the air-compressor (3) to be fed into the air preheater system (4) which consists of a number of regenerative heat exchangers with stationary ceramic packing, similar to those used for metallurgic blast furnaces. The heat exchangers are operated in two cycles, with the packing to be heated first and the air to be passed through it afterwards. In order to provide continuous supply of hot air, four heat exchangers are provided in the scheme, each operated with a corresponding phase shift. Each unit has a combustion chamber of its own, using natural gas and atmospheric air which is pumped into the combustion chamber by an auxiliary ventilator (5) while heating the packing. On leaving the heat-exchanger the air-oxygen mixture heated up to 1200°C is supplied into the combustion chamber (6), but just before entering the latter the air-oxygen flow is seeded with 67 per cent water solution of K2CO3 which is injected into the flow at 135°C. The seed injection in "liquid" state has been worked out with the model unit Y-02 and found quite adequate. 12
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Fig. 1. General thermal scheme of MHD power plant Y-25. 1---air inlet, 2-----oxygen generator, 3--air compressor, 4---air preheater, S--ventilator, 6---combustion chamber, 7--nozzle, 8--MHD-generator channel, 9--magnet system, 10--diffuser, l l--steam generator, 12--seed recovery plant, 13---combustion products compressor, 14--stack, 15--turboalternator, 16--reducer, 17--technological condenser, 18----seedpreparation plant, 19--turbine condenser, 20--condensate purification plant, 21---condensate pump, 22--degassing system, 23--feeding pump, 24---cooling system of MHDchannel, 25--cooling system of magnet.
A 25,000 kW Pilot MHD-Power Plant
Yet there is an intention in future to substitute liquid seed by powder K2COa which is economically preferable. The fuel (natural gas) is supplied into the combustion chamber from the Moscow city network at the flow rate of 4.75 kg/sec. The combustion products plasma is accelerated up to velocity of 850 m/sec in a nozzle (7) and is then passed into the MHD-channel (8). During the acceleration the flow temperature is somewhat reduced and at the channel entry is equal to 2450°C, at pressure of 1.8 atm, plasma conductivity amounting to 20 ~-1 m-1. The MHD-channel is placed within the working gap of the magnet system (9) with the field induction of 2T. While crossing the magnetic field the plasma flow generates electricity by counteracting the electromagnetic forces arising in the flow. Behind the MHD-channel the flow is decelerated in an expanding diffusor (10), whereupon the combustion products, at pressure close to atmosphere, are passed into a steam-generator (11). There, their heat is recovered in the steam power cycle, and at temperature of 150180°C they are directed into a foam-trapping apparatus of the seed recovery system (12). On being purified, the exhaust gases are sucked out by a combustion products compressor (13) and thrust out through a stack (14) into the atmosphere.
(ii) Steam power cycle The Y-25 steam power cycle is used to utilize the heat released by the exhaust gases leaving the MHD-generator, and is a conventional steam turbine plant. Direct steam is supplied into the turbine from the steam generator at pressure of 100 atm and temperature of 540°C. The steam generator capacity is 270 tn/hr. This amount would suffice to generate some 80 MW of electricity in the steam cycle. Yet commercial turbines of this capacity are not presently available in this country. It has been therefore, found reasonable to make use of commercially available standard turbogenerator K-50-90 LMZ (15) requiring 160tn/hr of direct steam, with remaining steam being reduced in a reducer (16) and condensed in a technological condenser (17). Some of the reduced steam is further utilized in the seed preparation plant (18) to evaporate the solution (to increase the concentration of K2COs). Condensate both from the turbine and technological condensers (19) joins into a single flow and, either through condensate purification plant (20), or bypassing it, is supplied by means of a condensate pump (21) into the cooling system of the nozzle, the diffusor and the combustion chamber, connected in series. There, condensate is heated from 40 to 100°C and is further passed into the degassing system (22). Heat consumption for condensate heating makes it possible to exclude regenerative heat exchangers from the turbine. From the degassing system, condensate at pressure of 110 atm is fed back into the steam generator by a feeding pump (23).
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(iii) Auxiliary systems In addition to the two basic power cycles, Y-25 plant includes a subsidiary seed loop to be described below and two separate dosed cycles for cooling the MHDchannel and magnet system, each including a circulation pump and a cooler. Condensate enters the MHD-channel cooling system (24) at pressure of 60 atm and leaves the system at pressure of 20 atm. The magnet cooling system (25) is designed for condensate flow rate of 80 tn/hr with pressure of 18 atm at the entrance to windings. Provision is made in the two loops for compensation of temperature water expansion, and for removal of gases resulting from hydrolysis. Water circulating in the two cooling loops must be free from deposition-folxning calcium salts and its electrical resistivity must not be lower than 104 12/cm. This is, in general, the principal thermal scheme of Y-25 plant. In what follows a brief description is given of its basic components. 3. MHD-generator
The magnet field of Y-25 MHD-generator is induced by specially developed magnet E-02. Magnet E-02 is an iron core electromagnet with water-cooled copper winding. It is of a type best developed at present and used to induce high magnetic fields in large volumes. Its fabrication does not involve any complicated engineering problems and it is simpleand reliable in operation. These characteristics made it the choice for Y-25 plant, despite its relatively lower efficiency as compared to other magnet types, such as, for example, cryogenicallycooled magnets. Magnet E-02 provides a magnetic field with induction value of 2 T in the gap 760 mm high and 5 m long, the gap width at the inlet amounting to 1000 mm and at the outlet, to 2300 mm. The magnet iron core is tridentshaped, the winding is composed of two saddle-shaped coils with bent front parts to enter the channel. Recognizing the need for periodically disassembling the magnet in order to change channels, the designers' primary concern was lower weight and convenient operation even though it involved higher power consumption. The present modification of the iron core weighs 2000 tn and the winding weighs 280 tn. The iron core is made up of several sections, each weighing not more than 100tn. The whole system is 8 . 5 m long, 10 m wide and 6 m high. The magnet consumes 2 . 6 M W (the energizing current being equal to 4730 A at voltage of 550 V). The MHD-channel is made changeable. In the course of operation there is an intention to test a number of channels different both in design and electrical scheme. The research program for MHD-generators is intended to begin with a most simple Faraday MHD-generator with segmented electrodes loaded by individual loads (inverters). As the first MHD-channel variation a construction has been chosen with cooled electrodes and metal insulating interspacing between electrodes, and
V. A. KIRHJ~IN, P. S. NEPOROZHNIY and A. E. SCHEINDLIN
metal insulation walls, which though not the most attractive in efficiency, has proved the most reliable in operation. The insulation walls are made of separate metal modules insulated both from each other and from the supporting structure. The module size has been chosen such as to keep the potential difference in plasma flow within the module, lower than the break-down voltage of the cooler boundary layer occurring on the module wall. As the first stage, it is intended to carry out channel tests under subsonic flow regime, in order to avoid a complicated flow pattern characteristic of supersonic velocities. The inlet velocity is taken to be 850 m/sec, which corresponds to Mach number of 0.8. The channel has a variable cross-section with a constant angle between electrode walls and the axis equal to 12.5 °. This value of aperture angle permits to maintain Mach number almost constant all along the channel. The distance between the insulation walls does not change. The channel cross section is equal to 0"383×0.766m 2 and to 0.383× 1.883 m 2 at the inlet and outlet, respectively. The working portion length is 5 m. The channel houses 48 electrode pairs. 4. Electrical Load of the MI-lD-generator (Inverter Unit) The MHD-generator supplies electricity to an a.c. network. Transformation of d.c. into a three-phase a.c. is achieved by means of a stationary inverter unit, assembled of mercury ignitrons. Instead of mercury ignitrons, use can be made of controllable silicon valves. Operating a generator with 48 electrode pairs is rather involved, particularly so if one remembers that owing to Hall effect, electric connection between the electrodes is not permissible. At the first stage the inverter system will include 24 inverter aggregates, each consisting of 6 ignitrons IVU-500, arranged according to a three-phase bridge system. Valve bridges are designed to carry up to 1200 kW. The electrical commutation system is designed so as to permit the inverter aggregates to be connected either with the first 24 electrode pairs, or the next 24 pairs, or else with all the 48 simultaneously, with two adjacent electrodes being connected in parallel. This arrangement allows one to study the performance of only the inlet or the outlet sections of the MHD-channel during its joint operation with the inverter unit for the a.c. network, as well as to investigate the electrical performance of the MHD-generator in terms of the degree of electrode segmentation. Valve bridges are connected, by fours, to corresponding split windings of the six inverters which serve to collect the outputs of individual electrodes into the total output, without electric connection between the electrodes. The network windings of transformers are connected to collectors of generator voltage, designed to carry 10 kV. A synchronous turbogenerator is also connected to the
collectors through a current-limiting double reactor. This scheme makes it possible to study the operation of the MHD-generator and the inverter unit simultaneously with the turbogenerator. The significant reactive power consumed during inversion (the inverter unit power factor is expected to be 0.8) would be compensated for at the expense of the turbogenerator. Thus, the total MHD-generator output can be supplied into the a.c. network through the six inverter transformers, which makes the electrical scheme more flexible and hence more suitable for carrying out investigation. In order to suppress upper harmonics, when dealing with twelve-phase-inversion, use is made of special filters. To provide stable joint operation of the MHDgenerator and the multi-component inverter unit, as well as to control electrical parameters of the electrodes, automatic control system is devised, taking into account not only significant change in static electrode characteristics along the channel, but also dynamic character of their electrical performance due to transient processes occuring in the MHD-duct, and to mutual interaction of electrodes through plasma in the MHDchannel. The desired electrode current and voltage distribution along the channel is provided by proper choosing inverter transformers, by widerange control of their voltage (up to 60 per cent) through switching under load, and by high-speed regulation of controllable valves of each of the converting bridges.
5. Combustion Chamber The combustion chamber is used to burn fuel and obtain alkali-metal seeded combustion products plasmaflow. The combustion chamber basic parameters are presented in Fig. 1. The design pressure in the combustion chamber amounts to 2.8 atm. Constructionally, the combustion chamber consists of four major units: a diffusor, a burner device, a ramjet section and a nozzle. The diffusor serves to supply air to the burner device Its metal body is water-cooled, its inside being lined with refractory bricks to withstand the temperature up to l l00°C. Provision is made in the diffusor to compensate for axial temperature elongation of the combustion chamber. The burner device serves to mix natural gas with hot air, whereupon the mixture is passed into the ramjet section of the combustion chamber. The design concept of the burner device involves gas burning in planeparallel blasts. The device burner is a system of vertical rectangular silts, with blasts of fuel gas being injected through the slit side walls into the air flow. This device provides adequate gas-air mixing. The burner device is equipped with two igniters used to start the combustion chamber. The burner device is separated from the ramjet
A 25,000 kW Pilot MHD-Power Plant
section by a screen of steel water-cooled tubes, which protects the burner device from radiation from the ramjet section where the fuel is burnt. The ramjet section is a cylinder smoothly changing into a rectangular cross section of the nozzle. The inner wall in this section is made of steel and is water-cooled. The ramjet section is made of two steel shells welded together by means of longitudinal ribs milled in the inner shell. Coolant water is passed in the space between the two shells. The last section of the combustion chamber is a taper subsonic nozzle to accelerate the combustion products up to velocity of 850 m/see before the entry to the MHD-ehannel. During MHD-generator operation a longitudinal Hall voltage may result, which can be carried over, by plasma, to the combustion chamber. In order to avoid shortcircuiting of Hall currents, the combustion chamber is insulated from the ground, and its components, from each other and from all the supply lines. 6. High Temperatnre Air Preheater of Y-25 Plant
In order to heat oxygen-enriched air up to 1200°C, high temperature regenerative air heater is included into the scheme, which consists of four chambers and is similar in its construction and operation to apparatus used for blast furnaces. The air is heated by the combustion products of burning natural gas in atmospheric air in a separate combustion chamber attached to each of the four heat exchanger chambers. The air to promote burning is pumped in by ventilators. The combustion products at temperature of 1350°C enter at top of the chamber and then go down through the packing, releasing heat and being cooled on their passage down to 200--400°C. At this temperature they enter, through a distribution valve, the exhaust smoke flue and are then thrust out through the stack into the atmosphere. When the packing reaches the temperature of 1250-1300°C the cycle is reversed: the air (or air-oxygen mixture) to be heated is pumped by the compressor and enters, through the distribution valve, the heat exchanger at its bottom at temperature of 150°C and pressure of 2-3 atm. On going upwards, the air takes away the heat previously accumulated by the packing and reaches the top with temperature of 1200°C, whereupon it is supplied to the MHD-combustion chamber. Thus, the heat exchangers operate in cycles, alternatively being passed through by combustion products (the heating fluid) and by air (the fluid to be heated). The cycle operation is controlled by means of several distribution valves mounted on gas- and air-leads. The air cycle for each heat exchanger lasts 1 hr, while the combustion products cycle takes 1.8 hr. 7. Steam Generator
The steam generator is designed to utilize the heat released by the exhaust gases effluent from the MHDchannel and to raise steam to be used in the steam
45
power cycle. During the process, the combustion products' temperature within the steam generator is reduced to the level where it is possible to recover the seed. The parameters of inlet combustion products, of direct steam and of feed water are presented in Fig. 1. The steam generator is a concurrent boiler. Natural circulation is impossible because of high heat transfer rates (up to 1.3 × 106 kcal/m 2 hr) in the front part adjoining the MHD-channel. The steam generator consists of two sections-radiative and convective ones. The former is made up of horizontal and vertical cylinders, about 4 m in dia., connected with each other through a bend. The front horizontal cylinder (adjoining the channel) includes two widenings to decelerate the combustion products from 200 m/see to 30 q- 40 m/see. The combustion products' temperature within the radiative section is reduced down to 1200°C. The walls of this section are formed by water screen tubes. Since excess pressure may develop within the cylinders, the walls have to be sufficiently gas-proof and strong. The experience gained in boiler-engineering shows that the best solution for a pressurized boiler is to have bearing walls of fin tubes. When this is impossible (in bends, narrowings, etc.) smooth tubes are welded together, which is more complicated and less reliable. Uniform wall temperatures of all-welded screens are achieved by passing steam-water mixture through all the screen tubes, at constant temperature equal to saturation value. To make better use of the radiative section both the horizontal and vertical cylinders house double furnace screens shaped as multifile tubular helix. In the horizontal section, they are the outlet of the economizer, in the vertical section, they are the inlet of the superheater. The radiative section is connected with the convective one through a cylindrical gas passage welded of smooth tubes. The convective stack of the steam generator has a cross section of 5 × 5 mL It includes (downstream) an intermediate pack of the steam superheater, an outlet superheater pack, a transition zone and three packs of the economizer. The last of the economizer packs is divided into two parts, with provision being made for switching off the last part in order to raise the temperature of combustion products leaving the steam generator, from 150 to 180°C. This will be required when substituting, at the second operation stage, the" dry" technique of potash recovery for the "wet" method. In order to monitor steam temperature, feed-water is injected into the steam passage in the radiative and convective superheaters. At the inlet to the convective stack, combustion products leaving the steam generator, with temperature of 150 °, are fed back into the flow and the mixture cools down to temperature of 900°C. This helps to avoid a slack dangerous temperature range from 900 ° to 1200°C. The convective surfaces are shot cleaned from dry potash. The radiative surfaces are washed from deposition during the steam generator stoppage.
46
V . A . KI~IIJ,[N, P. S. NEPOROZHNIY and A. E. SCHEINDLIN
8. Seed Injection and Seed Recovery The ionizing seed used for Y-25 plant is potassium, injected into, and recovered from, the cycle a s potash Kg.COs. Optimum potassium content of the combustion products amounts to 0.7 mole per cent, which corresponds to potash consumption of about 3 tn/hr. The seed is generally injected as a 67 per cent water solution of K2CO3. This concentration is achieved by evaporating a 40 per cent solution produced in the seed preparation plant, the evaporation taking place in special evaporating apparatus. Since temperature reduction below 135°C may result in solid phase precipitation the whole passage from evaporating apparatus to seed injectors is heated by steam. The solution is pulverized in the pipeline by means of centrifugal injectors which provide droplet size of 100/~. The injection of seed into the hot air pipeline some meters away from the combustion chamber increases the seed residence time in the hot zone and promotes uniform seed distribution throughout the combustion products. On entering the combustion chamber the seed is dissociated and atomic potassium is ionized, whereupon the ionized gas is passed into the MHDchannel, and finally, to the steam generator. There the chemical process is reversed: potassium--KOH--K2CO3. The experience gained when operating the model plant Y--02, shows that, in the temperature range over 1500°K, relatively cool (below l l00°K) heat exchange surfaces cause condensation of KOH which on binding
with COs, deposits as a hard crust. In order to avoid this process, the front part of the steam generator is a radiative section to be followed by convective one. As the combustion products lose their heat, at temperature of about 1400-1300°K drop-wise condensation develops, to be followed by desublimation of K2CO3. As a result, aerosol is produced, with solid particles of submicron size. Their deposition on the heat exchange surfaces are expected to be sufficiently loose so as to be easily removed by means of shot-cleaning and blasting. Therefore, in this zone, there are large convective surfaces which provide gas cooling down to 150-180°C. Seed recovery is performed through a special system of successive gas cooling and wettening, particle enlargement and their final trapping. Wetting is attained with the aid of a foam apparatus where the exhaust gases bubble through water solution of K2CO3 to get cooled and steam-saturated. Then, they enter two-stage tube apparatus at whose neck cool K2COs solution is pulverized. Steam is condensed and submicron dust is scdimented on the droplets produced. These droplets are separated by a cyclone drop-trap. Such device commonly used in chemical and metaUurgic industries provide gas purification up to 50 mg/m a, which in the present case corresponds to 99 per cent of seed recovery. As water solution circulates in the system, it gets gradually enriched in potassium carbonate. So upon reaching the upper limit (40 per cent), some of the solution is replaced by water. The quantity taken is filtered and pumped into the evaporating apparatus to thus close the cycle.
A 25,000 kW Pilot MHD-Power Plant V. A. KIRILLIN,f P. S. NEPOROZHNIYt and A. E. SCHEINDLINf (Received 18 June 1968)
1. General
Economically efficient generation and distribution of electrical power is a prerequisite for the economic progress of an industrially highly developed nation. The demand for electricity grows continuously and tends to double within each decade. Whatever the basic source of energy there is always a need for its conversion to electrical power as efficiently and cheaply as possible. The last 10-15 yr have witnessed certain advances to this end. Both the cost per kilowatt of installed capacity and the cost of kilowatt-hour of electricity produced have been considerably reduced through reduced capital costs (by building units of larger capacity) and improved generating efficiency. Yet power generation has presently entered a stage where any further major improvements in economic efficiency of conventional steam power stations are becoming most unlikely. Attempts have been made to substitute steam by other working fluids or to radically change the steam-water cycle proper. However, this does not appear to lead to a substantial enhancement in thermal efficiency of power plants either. New prospects for a significant improvement in economic efficiency of large power plants are now being associated with technical feasibility of commercial introduction of MHD-generators. The recent scientific developments in the field and the present level of engineering and technology make it quite safe to expect a 50 per cent net efficiency for a large open-cycle MHDpower plant, with an increase up to 60 per cent in the not-far-away future. Recognizing these highly promising prospects of application of MHD-plants for large-scale power generation, the USSR Ministry of Power Stations has sponsored, and the Institute for High Temperatures of the Academy of Sciences has provided scientific guidance of, extensive research and development efforts in the field of application of MHDplant for commercial power generation. In accordance with the research and development program, a model experimental MHD-power plant Y-02 has been developed and successfully operated in Moscow. The voluminous experimental data obtained with the plant have substantially helped to get further and deeper insight into the problems encountered when developing commercial MHD-plants, as well as to find the ways and means to their solution.
The next step toward utilization of MHD-generators for large scale power generation is to construct a pilot MHD-power plant of 25,000 kW capacity. This larger plant built on the basis of the present-day engineering and operated at relatively moderate parameters, will serve to final working out of the components of a large scale MHD-plant under real operation conditions. The experience gained from operating this plant is expected to supply the ultimate answer as to the overall economics of open-cycle systems for power generation. The following considerations determined the choice of the size and scheme of the pilot plant under construction: (1) The pilot MHD-plant should utilize all the advantages offered by the present-day engineering and technology and should make use of commercially available construction materials. As mentioned above, the recent developments in engineering and technology have paved the way to tackling this problem. (2) The plant parameters should not be too high, in order to provide sufficient reliability of the equipment, required for long life operation of the commercial plant. (3) The scale of the plant should be chosen so as to maintain the detrimental by-effects small relative to the net useful output, and so that the overall plant efficiency be competitive compared to the efficiency attainable with the current conventional power plants. (4) The fuel chosen for the pilot plant is natural gas, since utilization of other energy sources (coal, oil, etc.) involves additional difficulties. In what follows a detailed description is given of the scheme and major units of the 25,000 kW pilot MHDpower plant (Y-25) now being constructed in Moscow. 2. Thermal Scheme of Y-25
The open-cycle pilot plant Y-25 is operated on combustion products of natural gas seeded with readily ionizing K2COa. To provide an acceptable level of combustion products conductivity within the MHDgenerator, their inlet temperature is taken to be 2600°C. This temperature is attainable when burning natural gas in air, with the air-preheat up to 2000°C. At present, however, this high level of air-preheat is a formidable problem to be yet solved. Therefore, the air-preheat for Y-25 has been limited to 1200°C, a level already attainable on an industrial scale. The insufficient airpreheat is compensated for by oxygen enrichment. t Institute for High Temperatures, Krasnokazarnennaja, The heat released by the combustion products behind the MHD-generator, is used up in the steam cycle. 17A Moscow,E-250, U.S.S.R. 41
42
V. A. KIRILLIN, P. S. NEPOROZHNIY and A. E. SCHEINDLIN
Basing on considerations stated above, the MHDgenerator capacity is taken to be 25,000 kW. The thermal scheme of Y-25 is characterized by the following basic parameters: MHD-generator power output Fuel Oxidizer
Oxygen-enriched air preheat temperature Combustion products inlet temperature Ionizing seed Combustion products rate MHD-generator inlet pressure MHD-generator inlet velocity Outlet pressure Exhaust heat recovery Steam flow rate Steam pressure Steam superheat temperature Feed water temperature Magnet system power consumption Compressor power consumption Net efficiency
25,000 kW Natural gas Oxygen-enriched (up to 40 per cent) air 1200°C 2600°C K2CO3 up to 1 mole per cent 50 kg/sec 2.75 atm 850 m/see 1.07 arm Steam cycle 270 tn/hr 100 atm 540°C 105°C 2.6 MW 5.6 MW 33 per cent
The plant scheme is presented in Fig. 1. The pilot plant Y-25 is a power plant including two power cycles. The primary open cycle includes an
MHD-generator as its major component with combustion products used as the working fluid. The secondary closed cycle is a steam turbine driven by the heat released by the combustion products effluent from the MHD-generator.
(i) The combustion products cycle The air is uptaken from the atmosphere through an air inlet (I) and is fed into the compressor. During its passage to the compressor it is enriched with oxygen from an oxygen generator (2). The air-oxygen mixture is then compressed in the air-compressor (3) to be fed into the air preheater system (4) which consists of a number of regenerative heat exchangers with stationary ceramic packing, similar to those used for metallurgic blast furnaces. The heat exchangers are operated in two cycles, with the packing to be heated first and the air to be passed through it afterwards. In order to provide continuous supply of hot air, four heat exchangers are provided in the scheme, each operated with a corresponding phase shift. Each unit has a combustion chamber of its own, using natural gas and atmospheric air which is pumped into the combustion chamber by an auxiliary ventilator (5) while heating the packing. On leaving the heat-exchanger the air-oxygen mixture heated up to 1200°C is supplied into the combustion chamber (6), but just before entering the latter the air-oxygen flow is seeded with 67 per cent water solution of K2CO3 which is injected into the flow at 135°C. The seed injection in "liquid" state has been worked out with the model unit Y-02 and found quite adequate. 12
15
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o
I0, 2 kg/sec
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2,5 tn/hr: 200*C
'"
LI _
33,7 kg/ latin; 2
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X
X
X
X
X
X
I~
v
v
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I
Fig. 1. General thermal scheme of MHD power plant Y-25. 1---air inlet, 2-----oxygen generator, 3--air compressor, 4---air preheater, S--ventilator, 6---combustion chamber, 7--nozzle, 8--MHD-generator channel, 9--magnet system, 10--diffuser, l l--steam generator, 12--seed recovery plant, 13---combustion products compressor, 14--stack, 15--turboalternator, 16--reducer, 17--technological condenser, 18----seedpreparation plant, 19--turbine condenser, 20--condensate purification plant, 21---condensate pump, 22--degassing system, 23--feeding pump, 24---cooling system of MHDchannel, 25--cooling system of magnet.
A 25,000 kW Pilot MHD-Power Plant
Yet there is an intention in future to substitute liquid seed by powder K2COa which is economically preferable. The fuel (natural gas) is supplied into the combustion chamber from the Moscow city network at the flow rate of 4.75 kg/sec. The combustion products plasma is accelerated up to velocity of 850 m/sec in a nozzle (7) and is then passed into the MHD-channel (8). During the acceleration the flow temperature is somewhat reduced and at the channel entry is equal to 2450°C, at pressure of 1.8 atm, plasma conductivity amounting to 20 ~-1 m-1. The MHD-channel is placed within the working gap of the magnet system (9) with the field induction of 2T. While crossing the magnetic field the plasma flow generates electricity by counteracting the electromagnetic forces arising in the flow. Behind the MHD-channel the flow is decelerated in an expanding diffusor (10), whereupon the combustion products, at pressure close to atmosphere, are passed into a steam-generator (11). There, their heat is recovered in the steam power cycle, and at temperature of 150180°C they are directed into a foam-trapping apparatus of the seed recovery system (12). On being purified, the exhaust gases are sucked out by a combustion products compressor (13) and thrust out through a stack (14) into the atmosphere.
(ii) Steam power cycle The Y-25 steam power cycle is used to utilize the heat released by the exhaust gases leaving the MHD-generator, and is a conventional steam turbine plant. Direct steam is supplied into the turbine from the steam generator at pressure of 100 atm and temperature of 540°C. The steam generator capacity is 270 tn/hr. This amount would suffice to generate some 80 MW of electricity in the steam cycle. Yet commercial turbines of this capacity are not presently available in this country. It has been therefore, found reasonable to make use of commercially available standard turbogenerator K-50-90 LMZ (15) requiring 160tn/hr of direct steam, with remaining steam being reduced in a reducer (16) and condensed in a technological condenser (17). Some of the reduced steam is further utilized in the seed preparation plant (18) to evaporate the solution (to increase the concentration of K2COs). Condensate both from the turbine and technological condensers (19) joins into a single flow and, either through condensate purification plant (20), or bypassing it, is supplied by means of a condensate pump (21) into the cooling system of the nozzle, the diffusor and the combustion chamber, connected in series. There, condensate is heated from 40 to 100°C and is further passed into the degassing system (22). Heat consumption for condensate heating makes it possible to exclude regenerative heat exchangers from the turbine. From the degassing system, condensate at pressure of 110 atm is fed back into the steam generator by a feeding pump (23).
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(iii) Auxiliary systems In addition to the two basic power cycles, Y-25 plant includes a subsidiary seed loop to be described below and two separate dosed cycles for cooling the MHDchannel and magnet system, each including a circulation pump and a cooler. Condensate enters the MHD-channel cooling system (24) at pressure of 60 atm and leaves the system at pressure of 20 atm. The magnet cooling system (25) is designed for condensate flow rate of 80 tn/hr with pressure of 18 atm at the entrance to windings. Provision is made in the two loops for compensation of temperature water expansion, and for removal of gases resulting from hydrolysis. Water circulating in the two cooling loops must be free from deposition-folxning calcium salts and its electrical resistivity must not be lower than 104 12/cm. This is, in general, the principal thermal scheme of Y-25 plant. In what follows a brief description is given of its basic components. 3. MHD-generator
The magnet field of Y-25 MHD-generator is induced by specially developed magnet E-02. Magnet E-02 is an iron core electromagnet with water-cooled copper winding. It is of a type best developed at present and used to induce high magnetic fields in large volumes. Its fabrication does not involve any complicated engineering problems and it is simpleand reliable in operation. These characteristics made it the choice for Y-25 plant, despite its relatively lower efficiency as compared to other magnet types, such as, for example, cryogenicallycooled magnets. Magnet E-02 provides a magnetic field with induction value of 2 T in the gap 760 mm high and 5 m long, the gap width at the inlet amounting to 1000 mm and at the outlet, to 2300 mm. The magnet iron core is tridentshaped, the winding is composed of two saddle-shaped coils with bent front parts to enter the channel. Recognizing the need for periodically disassembling the magnet in order to change channels, the designers' primary concern was lower weight and convenient operation even though it involved higher power consumption. The present modification of the iron core weighs 2000 tn and the winding weighs 280 tn. The iron core is made up of several sections, each weighing not more than 100tn. The whole system is 8 . 5 m long, 10 m wide and 6 m high. The magnet consumes 2 . 6 M W (the energizing current being equal to 4730 A at voltage of 550 V). The MHD-channel is made changeable. In the course of operation there is an intention to test a number of channels different both in design and electrical scheme. The research program for MHD-generators is intended to begin with a most simple Faraday MHD-generator with segmented electrodes loaded by individual loads (inverters). As the first MHD-channel variation a construction has been chosen with cooled electrodes and metal insulating interspacing between electrodes, and
V. A. KIRHJ~IN, P. S. NEPOROZHNIY and A. E. SCHEINDLIN
metal insulation walls, which though not the most attractive in efficiency, has proved the most reliable in operation. The insulation walls are made of separate metal modules insulated both from each other and from the supporting structure. The module size has been chosen such as to keep the potential difference in plasma flow within the module, lower than the break-down voltage of the cooler boundary layer occurring on the module wall. As the first stage, it is intended to carry out channel tests under subsonic flow regime, in order to avoid a complicated flow pattern characteristic of supersonic velocities. The inlet velocity is taken to be 850 m/sec, which corresponds to Mach number of 0.8. The channel has a variable cross-section with a constant angle between electrode walls and the axis equal to 12.5 °. This value of aperture angle permits to maintain Mach number almost constant all along the channel. The distance between the insulation walls does not change. The channel cross section is equal to 0"383×0.766m 2 and to 0.383× 1.883 m 2 at the inlet and outlet, respectively. The working portion length is 5 m. The channel houses 48 electrode pairs. 4. Electrical Load of the MI-lD-generator (Inverter Unit) The MHD-generator supplies electricity to an a.c. network. Transformation of d.c. into a three-phase a.c. is achieved by means of a stationary inverter unit, assembled of mercury ignitrons. Instead of mercury ignitrons, use can be made of controllable silicon valves. Operating a generator with 48 electrode pairs is rather involved, particularly so if one remembers that owing to Hall effect, electric connection between the electrodes is not permissible. At the first stage the inverter system will include 24 inverter aggregates, each consisting of 6 ignitrons IVU-500, arranged according to a three-phase bridge system. Valve bridges are designed to carry up to 1200 kW. The electrical commutation system is designed so as to permit the inverter aggregates to be connected either with the first 24 electrode pairs, or the next 24 pairs, or else with all the 48 simultaneously, with two adjacent electrodes being connected in parallel. This arrangement allows one to study the performance of only the inlet or the outlet sections of the MHD-channel during its joint operation with the inverter unit for the a.c. network, as well as to investigate the electrical performance of the MHD-generator in terms of the degree of electrode segmentation. Valve bridges are connected, by fours, to corresponding split windings of the six inverters which serve to collect the outputs of individual electrodes into the total output, without electric connection between the electrodes. The network windings of transformers are connected to collectors of generator voltage, designed to carry 10 kV. A synchronous turbogenerator is also connected to the
collectors through a current-limiting double reactor. This scheme makes it possible to study the operation of the MHD-generator and the inverter unit simultaneously with the turbogenerator. The significant reactive power consumed during inversion (the inverter unit power factor is expected to be 0.8) would be compensated for at the expense of the turbogenerator. Thus, the total MHD-generator output can be supplied into the a.c. network through the six inverter transformers, which makes the electrical scheme more flexible and hence more suitable for carrying out investigation. In order to suppress upper harmonics, when dealing with twelve-phase-inversion, use is made of special filters. To provide stable joint operation of the MHDgenerator and the multi-component inverter unit, as well as to control electrical parameters of the electrodes, automatic control system is devised, taking into account not only significant change in static electrode characteristics along the channel, but also dynamic character of their electrical performance due to transient processes occuring in the MHD-duct, and to mutual interaction of electrodes through plasma in the MHDchannel. The desired electrode current and voltage distribution along the channel is provided by proper choosing inverter transformers, by widerange control of their voltage (up to 60 per cent) through switching under load, and by high-speed regulation of controllable valves of each of the converting bridges.
5. Combustion Chamber The combustion chamber is used to burn fuel and obtain alkali-metal seeded combustion products plasmaflow. The combustion chamber basic parameters are presented in Fig. 1. The design pressure in the combustion chamber amounts to 2.8 atm. Constructionally, the combustion chamber consists of four major units: a diffusor, a burner device, a ramjet section and a nozzle. The diffusor serves to supply air to the burner device Its metal body is water-cooled, its inside being lined with refractory bricks to withstand the temperature up to l l00°C. Provision is made in the diffusor to compensate for axial temperature elongation of the combustion chamber. The burner device serves to mix natural gas with hot air, whereupon the mixture is passed into the ramjet section of the combustion chamber. The design concept of the burner device involves gas burning in planeparallel blasts. The device burner is a system of vertical rectangular silts, with blasts of fuel gas being injected through the slit side walls into the air flow. This device provides adequate gas-air mixing. The burner device is equipped with two igniters used to start the combustion chamber. The burner device is separated from the ramjet
A 25,000 kW Pilot MHD-Power Plant
section by a screen of steel water-cooled tubes, which protects the burner device from radiation from the ramjet section where the fuel is burnt. The ramjet section is a cylinder smoothly changing into a rectangular cross section of the nozzle. The inner wall in this section is made of steel and is water-cooled. The ramjet section is made of two steel shells welded together by means of longitudinal ribs milled in the inner shell. Coolant water is passed in the space between the two shells. The last section of the combustion chamber is a taper subsonic nozzle to accelerate the combustion products up to velocity of 850 m/see before the entry to the MHD-ehannel. During MHD-generator operation a longitudinal Hall voltage may result, which can be carried over, by plasma, to the combustion chamber. In order to avoid shortcircuiting of Hall currents, the combustion chamber is insulated from the ground, and its components, from each other and from all the supply lines. 6. High Temperatnre Air Preheater of Y-25 Plant
In order to heat oxygen-enriched air up to 1200°C, high temperature regenerative air heater is included into the scheme, which consists of four chambers and is similar in its construction and operation to apparatus used for blast furnaces. The air is heated by the combustion products of burning natural gas in atmospheric air in a separate combustion chamber attached to each of the four heat exchanger chambers. The air to promote burning is pumped in by ventilators. The combustion products at temperature of 1350°C enter at top of the chamber and then go down through the packing, releasing heat and being cooled on their passage down to 200--400°C. At this temperature they enter, through a distribution valve, the exhaust smoke flue and are then thrust out through the stack into the atmosphere. When the packing reaches the temperature of 1250-1300°C the cycle is reversed: the air (or air-oxygen mixture) to be heated is pumped by the compressor and enters, through the distribution valve, the heat exchanger at its bottom at temperature of 150°C and pressure of 2-3 atm. On going upwards, the air takes away the heat previously accumulated by the packing and reaches the top with temperature of 1200°C, whereupon it is supplied to the MHD-combustion chamber. Thus, the heat exchangers operate in cycles, alternatively being passed through by combustion products (the heating fluid) and by air (the fluid to be heated). The cycle operation is controlled by means of several distribution valves mounted on gas- and air-leads. The air cycle for each heat exchanger lasts 1 hr, while the combustion products cycle takes 1.8 hr. 7. Steam Generator
The steam generator is designed to utilize the heat released by the exhaust gases effluent from the MHDchannel and to raise steam to be used in the steam
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power cycle. During the process, the combustion products' temperature within the steam generator is reduced to the level where it is possible to recover the seed. The parameters of inlet combustion products, of direct steam and of feed water are presented in Fig. 1. The steam generator is a concurrent boiler. Natural circulation is impossible because of high heat transfer rates (up to 1.3 × 106 kcal/m 2 hr) in the front part adjoining the MHD-channel. The steam generator consists of two sections-radiative and convective ones. The former is made up of horizontal and vertical cylinders, about 4 m in dia., connected with each other through a bend. The front horizontal cylinder (adjoining the channel) includes two widenings to decelerate the combustion products from 200 m/see to 30 q- 40 m/see. The combustion products' temperature within the radiative section is reduced down to 1200°C. The walls of this section are formed by water screen tubes. Since excess pressure may develop within the cylinders, the walls have to be sufficiently gas-proof and strong. The experience gained in boiler-engineering shows that the best solution for a pressurized boiler is to have bearing walls of fin tubes. When this is impossible (in bends, narrowings, etc.) smooth tubes are welded together, which is more complicated and less reliable. Uniform wall temperatures of all-welded screens are achieved by passing steam-water mixture through all the screen tubes, at constant temperature equal to saturation value. To make better use of the radiative section both the horizontal and vertical cylinders house double furnace screens shaped as multifile tubular helix. In the horizontal section, they are the outlet of the economizer, in the vertical section, they are the inlet of the superheater. The radiative section is connected with the convective one through a cylindrical gas passage welded of smooth tubes. The convective stack of the steam generator has a cross section of 5 × 5 mL It includes (downstream) an intermediate pack of the steam superheater, an outlet superheater pack, a transition zone and three packs of the economizer. The last of the economizer packs is divided into two parts, with provision being made for switching off the last part in order to raise the temperature of combustion products leaving the steam generator, from 150 to 180°C. This will be required when substituting, at the second operation stage, the" dry" technique of potash recovery for the "wet" method. In order to monitor steam temperature, feed-water is injected into the steam passage in the radiative and convective superheaters. At the inlet to the convective stack, combustion products leaving the steam generator, with temperature of 150 °, are fed back into the flow and the mixture cools down to temperature of 900°C. This helps to avoid a slack dangerous temperature range from 900 ° to 1200°C. The convective surfaces are shot cleaned from dry potash. The radiative surfaces are washed from deposition during the steam generator stoppage.
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V . A . KI~IIJ,[N, P. S. NEPOROZHNIY and A. E. SCHEINDLIN
8. Seed Injection and Seed Recovery The ionizing seed used for Y-25 plant is potassium, injected into, and recovered from, the cycle a s potash Kg.COs. Optimum potassium content of the combustion products amounts to 0.7 mole per cent, which corresponds to potash consumption of about 3 tn/hr. The seed is generally injected as a 67 per cent water solution of K2CO3. This concentration is achieved by evaporating a 40 per cent solution produced in the seed preparation plant, the evaporation taking place in special evaporating apparatus. Since temperature reduction below 135°C may result in solid phase precipitation the whole passage from evaporating apparatus to seed injectors is heated by steam. The solution is pulverized in the pipeline by means of centrifugal injectors which provide droplet size of 100/~. The injection of seed into the hot air pipeline some meters away from the combustion chamber increases the seed residence time in the hot zone and promotes uniform seed distribution throughout the combustion products. On entering the combustion chamber the seed is dissociated and atomic potassium is ionized, whereupon the ionized gas is passed into the MHDchannel, and finally, to the steam generator. There the chemical process is reversed: potassium--KOH--K2CO3. The experience gained when operating the model plant Y--02, shows that, in the temperature range over 1500°K, relatively cool (below l l00°K) heat exchange surfaces cause condensation of KOH which on binding
with COs, deposits as a hard crust. In order to avoid this process, the front part of the steam generator is a radiative section to be followed by convective one. As the combustion products lose their heat, at temperature of about 1400-1300°K drop-wise condensation develops, to be followed by desublimation of K2CO3. As a result, aerosol is produced, with solid particles of submicron size. Their deposition on the heat exchange surfaces are expected to be sufficiently loose so as to be easily removed by means of shot-cleaning and blasting. Therefore, in this zone, there are large convective surfaces which provide gas cooling down to 150-180°C. Seed recovery is performed through a special system of successive gas cooling and wettening, particle enlargement and their final trapping. Wetting is attained with the aid of a foam apparatus where the exhaust gases bubble through water solution of K2CO3 to get cooled and steam-saturated. Then, they enter two-stage tube apparatus at whose neck cool K2COs solution is pulverized. Steam is condensed and submicron dust is scdimented on the droplets produced. These droplets are separated by a cyclone drop-trap. Such device commonly used in chemical and metaUurgic industries provide gas purification up to 50 mg/m a, which in the present case corresponds to 99 per cent of seed recovery. As water solution circulates in the system, it gets gradually enriched in potassium carbonate. So upon reaching the upper limit (40 per cent), some of the solution is replaced by water. The quantity taken is filtered and pumped into the evaporating apparatus to thus close the cycle.