Transpiration cooled electrodes and insulators for MHD generators

Systems for cooling the inner duct walls in a magnetohydrodynamic (MHD) generator. The inner face components, adjacent the plasma, are formed of a porous material known as a transpiration material. Selected cooling gases are transpired through the duct walls, including electrically insulating and electrode segments, and into the plasma. A wide variety of structural materials and coolant gases at selected temperatures and pressures can be utilized and the gases can be drawn from the generation system compressor, the surrounding environment, and combustion and seed treatment products otherwise discharged, among many other sources. The conduits conducting the cooling gas are electrically insulated through low pressure bushings and connectors so as to electrically isolate the generator duct from the ground.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to MHD generation systems and more particularly to 
method and apparatus for cooling and protecting the inner duct electrode 
and insulation wall surfaces utilizing transpiration cooling. 
2. Description of the Prior Art 
The environment created within a magnetohydrodynamic (MHD) generator duct 
can be described as hostile. Materials, typically including a fuel such as 
particulate coal, an oxidant such as air and an ionizing conductor or seed 
such as potassium are reacted in a combustor to create high temperature 
reaction products, referred to herein as a plasma. The plasma, typically 
including excess fuel, coal slag and sulfur at a temperature in the rane 
of 2500.degree. K., is passed over the inner wall surfaces of the 
generator duct at a velocity in the range of 1000 meters per second. 
The duct wall surfaces, which can include electrodes, electrically 
conducting wall segments and electrically insulating wall segments, when 
exposed to these hostile conditions, tend to erode, corrode, evaporate or 
otherwide deteriorate. One prior art response to the deterioration has 
typically been to utilize high pressure water cooled metal structures to 
cool the wall surfaces. Ceramic spacers used as electrical insulation are 
cooled by conduction through intimate contact with the water-cooled metal. 
Ceramic coatings have also been provided to buffer the metal surfaces from 
the plasma. 
It appears further to have been universally accepted that, in order to 
avoid excessive erosion of the inner wall surfaces, particularly the 
electrode walls, the water-cooled wall surface temperatures should be 
maintained below the freezing point of the seed/slag mixture so that some 
of this material will freeze onto the surfaces as a protective layer. 
However, while prior teachings have claimed that this layer protects the 
metal or ceramic wall surfaces from the hostile plasma, it appears to 
react with the surfaces by a process which is at least partially 
electrochemical. To deal with these reactions there is a tendency to 
further reduce the surface temperatures, through water cooling, which 
results in severe material limitations, high electrical resistance of the 
slag layer, and apparently such high electrical resistance at the slag 
layer-electrode inner face that detrimental arcing and hot spots result. 
Additionally, this slag layer provides an undesirable path for current 
leakage across ceramic insulators which results in lower system efficiency 
and breakdown of the insulators. 
If a slag layer is desired as a protective surface, the combustor must 
provide some slag carryover into the duct to a degree higher than 
otherwise would be necessary. This excess slag increases the stack gas 
cleanup requirements to maintain acceptably low seed material loss and 
particulate emission from the plant and combines with the seed, making 
seed separation from collected material difficult. The seed-slag 
combination also forms very hard and tenacious deposits on component 
surfaces downstream of the duct, such as heat exchanger tubes, thereby 
decreasing system efficiency and increasing maintenance concerns. 
Conductive water cooling provides additional concerns. The material for 
conducting heat to the water is severely limited, and copper appears to be 
the only material with adequate thermal diffusivity to prevent localized 
electrical arcs. Copper, however, dissolves into demineralized water at 
the operating temperatures. With or without deionization of the water, it 
is difficult to maintain the water as an electrical non-conductor, which 
must be maintained to avoid efficiency losses, and it is further difficult 
to design and fabricate structures and conduit paths for the water, 
typically at high pressures, which themselves are insulating and thus not 
a drain on overall system efficiency. 
It is therefore desirable to provide an MHD system which overcomes the 
difficulties associated with water cooling. It is further desirable to 
alleviate the concerns associated with excessive slag carryover and 
buildup. 
SUMMARY OF THE INVENTION 
This invention provides a desirable alternative to water cooling systems 
for components making up the inner wall surfaces of an MHD generator duct. 
The invention further provides for alleviation of the need for a 
substantial protective slag layer on the inner wall surfaces. 
The invention takes advantage of the technology developed for cooling of 
gas turbine blades and vanes referred to as "transpiration cooling". In 
transpiration cooling a coolant, being a gas as used in the invention, 
passes through a fine porous structure. The fine passages provide a very 
high ratio of heat transfer area to coolant flow rate and almost perfect 
counterflow between the heat and the coolant. 
The technology, in accordance with the invention, is applied to the 
components making up the inner wall, adjacent the hostile plasma, in the 
duct of an MHD generator. Typical duct wall components include segments of 
electrodes, conducting wall segments, and insulated segments. These 
segments are made of a transpiring material and a gaseous coolant is 
supplied which transpires through the component, providing substantial 
cooling, and into the plasma. 
The type and source of gas can be varied in accordance with the specific 
overall system design and operating parameters. The coolant gases can 
include, but are not limited to, air, oxygen, nitrogen, argon, water 
vapor, carbon dioxide, metal and metal oxide vapors, exhaust products, and 
other gases compatible with the particular construction material of the 
segments. Illustrative of the latter is the use of diluted magnesia (MgO) 
gas in conjunction with a magnesia wall segment. Air or oxygen can be 
drawn from, or downstream of, the compressor providing the air to the 
combustor where the plasma is formed. Air can also be drawn or induced 
into the segments at the exit end of the duct where pressures are 
typically sub-atmospheric, merely by providing a conduit between the 
atmosphere surrounding the generator and the transpiring wall segment. 
Nitrogen rich gases can be drawn from the oxygen plate of a seed recovery 
treatment plant, or from an oxygen plant which provides oxygen for 
combustion, if either are utilized in the generation system, thereby 
utilizing nitrogen rich gases otherwise discharged as waste by-products. 
Additionally, combustion products from upstream, fuel rich, or downstream, 
fuel lean, of an NO.sub.x air injection port can be used, with appropriate 
processing, to provide a reducing or mildly oxidizing transpiration gas. 
The coolant gases can additionally be provided at different temperatures 
and pressures from different sources for injection at various segments 
along the duct. For example, a high pressure air can be tapped from the 
compressor for utilization at the entry end of the duct and the lower 
pressure atmosphere drawn into the downstream segments. Various 
combinations of gases can also be utilized, such as use of oxygen at the 
electrode segments and nitrogen at the insulating side wall segments. 
The transpiration cooling not only alleviates the need for, and the 
detrimental effects of, a protective slag layer, but also helps in 
avoiding formation of the layer, since it is injected substantially 
perpendicular to the plasma flow and tends to "blow away" the slag 
particles. Further, a boundary layer of coolant gas is formed which tends 
to maintain separation of the slag and the segments. And, the amount of 
slag removed from the combustor, prior to entry of the plasma into the 
duct, can be substantially increased through technology well established 
in the art. 
It will also be apparent that the cooling gas utilized, as well as the 
conduits used to transport the gas, can readily be maintained 
non-conductive or insulated from the duct segments as compared to prior 
art watercooled systems which require more stringent structural designs, 
higher pressures, in the range of 2000 to 4500 psia, and which are more 
prone to becoming electrically conductive. 
The systems accordingly provide a great deal of flexibility in materials 
selection and component design, increased reliability and longevity, and 
alleviate problems associated with water cooling.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 there is shown a magnetohydrodynamic (MHD) 
generation system in accordance with the invention. The system includes a 
combustor 10 within which are mixed reactants to form a high temperature 
plasma 11. A fuel, such as pulverized coal, enters the combustor through a 
conduit 12; an ionizing seed, such as potassium or cesium in a combining 
form such as K.sub.2 SO.sub.4 or CS.sub.2 CO.sub.3, enters through conduit 
14; and an oxidant, such as air or oxygen, enters through conduit 16. A 
portion of the slag formed by the combustion reaction is removed from the 
combustor 10, by means well known in the art, through conduit 18. 
The plasma formed in the combustor 10 flows through a nozzle region 20 and 
enters the main duct 22. Passage of the ionized plasma through the field 
created by magnets, represented by the dotted line 24, creates a current 
flow through electrodes along the duct inner wall which is discharged from 
the duct through a main lead 26. The entire duct 22 is electrically 
insulated from its adjacent components such as by insulation 28. The 
temperature and particularly the pressure of the plasma drops 
substantially upon passage through the duct from, for example, 
2500.degree. K. and 90 psia to 2100.degree. K. and sub-atmospheric 
pressure. The plasma velocity head is then converted to pressure in a 
diffuser 32, passes across air preheaters 34 and steam generators 35, and 
is discharged through an outlet 36. An air injection port 37 is also 
utilized for nitrous oxides emission control. Particulate removal means, 
such as a precipitator 38 and/or sulfur compound removal means, such as a 
scrubber, ensure high efficiency seed recovery and proper conditions for 
atmospheric discharge for further utilization. Seed and slag removed in 
the precipitator 38 can be treated to recover the seed in a seed treatment 
apparatus 40, well known in the art, which typically discharges a waste 
gas enriched in nitrogen through a conduit 42. 
Additionally shown in FIG. 1 is a compressor 44 and drive 45 for 
compressing an oxidant such as air or oxygen from a source 46 which, in 
the case of air, can merely be the surrounding environment. Oxygen can be 
provided by a separate oxygen plant at adequate pressure. The oxidant is 
directed through conduit 48, through the preheaters 34, and through 
conduits 50 and 16 to the combustor 10. 
The foregoing discussion will provide a better background understanding of 
the invention. The function of the exemplary conduits 52 through 58 shown 
in FIG. 1 will become apparent from the following. 
The duct 22 is comprised of inner walls which include insulated segments 
and electrode segments. Referring to FIG. 2 there is shown an electrically 
insulating segmented wall 60, commonly referred to as a peg wall for 
water-cooled ducts. It includes a structural support 62 to which is 
mounted a plurality of transpiration members 64. The transpiration members 
each include an inner face 66, forming part of the duct inner wall, which 
is made of a transpiration material porous to a selected gaseous coolant. 
Exemplary transpiration materials include Lamilloy, developed and tested 
by the Detroit Diesel Allison Division of General Motors Corporation for 
aviation gas turbine utilization, and a woven wire developed and tested by 
the Curtis Wright Corporation for gas turbine application. Lamilloy 
comprises multiple layers of foil, one surface of each layer being etched 
with very fine flow passages thus providing a high heat transfer area to 
mass flow ratio. The etched passages connect very fine holes in each layer 
which provide for flow of a coolant among layers and through the surface. 
The woven wire has been woven into shells, sintered, and electron beam 
welded to a strut to form an air foil surface. The porosity of a given 
shell can be varied, as can be the structural integrity and overall 
porosity of a plurality of layered shells. Additional transpiration 
materials which can advantageously be utilized in accordance with the 
invention include porous ceramics and sintered metal powders, among 
others. 
A coolant gas enters the transpiration member 64 which forms a generally 
enclosed compartment 68, passes through the porous inner face 66, thereby 
cooling the face 66, and is discharged into the flowing plasma. The 
coolant gas forms a boundary layer 70 along the inner wall, which tends to 
prevent any buildup of seed and slag along the wall which typically occurs 
only by diffusion of gases or Brownian motion of particles or drops due to 
divergence of the walls in the direction of flow. Between the boundary 
layer and the plasma main stream is additionally formed a mixing region 
73. Disposed between the transpiration members 64 are insulator spacers 
72. Side members 74 of the transpiration members 64 can also be comprised 
of a transpiration member to cool the spacers 72, particularly where the 
spacers are comprised of a porous ceramic. Alternatively, the spacers 72 
can be cooled by conduction to side members 74 and will be shielded from 
the hot gases by the boundary layer from the upstream inner face 66. 
The gaseous coolant enters the compartments 68 through conduits 76. To 
maintain the duct 22 electrically isolated, conduit insulators 78 are 
utilized. Since the cooling gas is provided at relatively low pressures, 
in the range of 10 to 300 psia, the conduits and insulators need only be 
designed for low pressure. The coolant flow path also includes means for 
controlling the pressure and flow rate of the coolant into the 
compartments 68 such as orifices 80 disposed in the conduits 76. As the 
pressure of the plasma within the duct 22 drops substantially along the 
duct, the pressure of coolant gas injected into the compartments 68 can 
accordingly be adjusted dependent upon location along the duct length, 
being higher at the entry end 82 and lower at the exit end 84 (FIG. 1). 
This can be accomplished not only by the orifices 80, but also by varying 
the source of the coolant gas. 
For example, as shown in FIG. 1, air or oxygen drawn directly from the 
compressor 44 can be injected through conduit 52 to the upstream 
transpiration members 64, and lower pressure coolant from bleed ports in 
the compressor 44 injected through conduit 54 to downstream members 64. 
Additionally, as the pressure of the plasma at the exit end 84 of the duct 
is generally subatmospheric, the conduit 58, merely being open at one end 
to the surrounding atmospheric environment, will provide a path whereby 
air is induced into the downstream transpiration members 64. To ensure 
temperature compatibility, this air can be preheated by passing it through 
conduits in heat exchange relation with the plasma, downstream products, 
or any other heat source. 
The pressure of the injected coolant is, at any location, higher than the 
internal duct static pressure at the face 66 of the transpiration member. 
It is noted that, for a given system configuration, a higher pressure 
coolant injection results in higher coolant flow rates and lower structure 
temperatures, but that the work required to achieve the higher flow rate 
at a given high pressure, and the performance degradation caused by 
mixing, detracts somewhat from the overall system efficiency. 
In addition to pressure control, it can additionally be desirable to adjust 
the temperature of coolant gas injected as a function of position along 
the duct length, as well as a function of the specific components to be 
cooled. A higher temperature coolant can be drawn, for example, from 
conduit 50 downstream of the preheaters 34 and through conduit 56 into the 
transpiration members 64. A higher temperature coolant is 
thermodynamically preferable and thus the highest temperature that 
provides adequate structural integrity should be used. For example, if the 
transpiration member is a ceramic which exhibits good strength at the 
plasma temperature, but tends to react with the plasma, the transpiring 
gas flow may be utilized primarily to protect the material from the 
hostile plasma, with cooling being a secondary effect. 
The flow rates of the cooling gas will be in proportion to the duct 
internal surface area, which is dependent upon the overall plant rating, 
and other specifically selected design parameters. The total flow rates 
preferably are in the range of two to ten percent of the plasma flow rate, 
or approximately 20 to 150 lb./sec. for a plant rated at approximately 
1000 megawatts-electrical. The injection pressures are preferably slightly 
above, for example, 10 psi, the duct internal pressure at any given 
location. 
FIG. 3 illustrates a segmented duct wall utilizing transpiration cooled 
electrodes 86. A compartment 88 is formed by preferably metallic plates 90 
and a bonded transpiration surface 92. The electrodes 86 are segmented to 
alleviate Hall currents and are electrically isolated by uncooled 
interelectrode electrical insulators 94. The interelectrode insulators can 
additionally be transpiration cooled. Current is conducted from the 
transpiration surface 92, through the metallic plates to a lead 96 and to 
the main lead 26. The duct wall of electrodes is electrically isolated by 
insulating bushings 98. The coolant not only cools and protects the 
electrode inner duct surfaces 92, but also, due to the transverse flow 
with respect to the plasma, forms a layer 100 of coolant along the wall 
surface which additionally protects the insulator 94. It is noted that the 
insulators 94 can be additionally cooled by conductive water cooling 
although such is deemed unnecessary and undesirable. 
Many additional alternative configurations and materials can be utilized 
consistent with the invention. For example, although the exemplary foil 
and weave metallic members, primarily stainless steel and nickel-chrome 
alloys, have been proposed for jet engine applications, porous ceramics 
providing sufficient transpiration effects may beneficially be utilized in 
the MHD generator systems disclosed. Ceramics such as zirconia 
(ZrO.sub.2), magnesia (MgO) and zirconium diboride (ZrB.sub.2), among 
others, are capable of reliable operation at temperatures above those in 
the plasma in the proper environment. Thus, a relatively small cooling gas 
flow is required if the proper cooling gas is utilized. For example, 
magnesia vapor diluted with air can be utilized as the transpiration 
coolant for a magnesia insulator, a reducing gas, such as rich combustion 
products or hydrogen as the coolant for a zirconium diboride conductor, or 
an oxidizing gas for a zirconia surface. A reducing gas can also be 
beneficial as a coolant for a metallic member. 
In addition to use of an oxidant such as air or oxygen as a coolant as 
illustrated in FIG. 1, coolants not containing, or depleted in oxygen, can 
also be advantageously utilized such as nitrogen, combustion products or 
argon. Nitrogen is particularly attractive in those generation systems 
where seed recovery apparatus 40 discharges a nitrogen rich stream as a 
waste by-product, or where an oxygen plant 46 provides oxygen to the 
combustor and similarly discharges a nitrogen rich gas. Other candidate 
coolant gases include, but are not limited to, water vapor, carbon 
dioxide, metal vapors such as cesium or potassium, and exhaust products. 
In order to have proper environmental controls, the plasma entering the 
generator duct 22 typically contains an excess of fuel in the form of 
evolved gases. The excess fuel is necessary for control of nitrous oxides 
(NO.sub.x). If a gas containing oxygen is used as the coolant, the 
transpiration surface will be exposed to an oxidizing atmosphere and a 
chemical reaction will occur where the coolant mixes with the fuel rich 
stream. The temperatures at the inner duct transpiration face and the 
mixing zone can be set at acceptably low levels by an appropriate choice 
of coolant gas injection temperature to compensate for this reaction. The 
reaction will provide some nonequilibrium ionization which enhances gas 
conductivity at the mixing area. Thus, an oxygen containing cooling gas 
for the electrode wall can enhance generation efficiency. Alternatively, 
an inert gas such as carbon dioxide or nitrogen would be preferred at the 
insulator wall segments to avoid the conductivity effects from 
non-equilibrium ionization. Thus, different coolant gases can be utilized 
in the same duct, for example, air at the electrodes and nitrogen at the 
insulators. It will also be noted that use of an inert or reducing cooling 
gas allows a substantial flexibility in material selection as compared to 
air or any coolant containing oxygen. It should also be noted that 
dependent upon startup procedures, there is a possibility that gaseous 
fuel could back flush through the transpiration surfaces and subsequently 
undesirably react with an oxygen containing coolant within the wall 
segments. To prevent such detrimental reaction, fuel should be adequately 
purged from the duct prior to injection of an oxygen containing coolant. 
For example, an inert gas such as nitrogen or argon could be utilized 
prior to air injection, or the transpiration by air started prior to any 
fuel entering the duct. These procedures would ensure that any reactions 
between oxygen and fuel would take place within the duct and not within 
the duct walls. 
The beneficial utilization of transpiration cooling can also be realized at 
other locations in the generation system such as in the diffuser region 
walls or particularly in the nozzle region 20 where the highest 
temperatures are realized. For example, gas from the compressor outlet 
(FIG. 1) could be transpired into the nozzle region. 
The substantial advantages offered by the disclosed systems will now be 
apparent to those skilled in the art. The systems permit a wide choice of 
materials. As a result of the short thermal conduction paths through the 
transpiration material, thermal diffusivity is not a controlling factor 
and, the gas layer formed along the walls provides a protective 
environment for the duct inner wall face segments. It is also noted that 
with elimination of the need for a protective slag layer, the inner face 
temperatures can also be higher than allowed by previous systems and the 
tendency for arcing at the electrode surfaces will be reduced. Since 
compatibility with the limitations of water cooling is no longer required, 
a wider range of materials can be considered. 
The elimination of intentional deposition of seed and slag on the inner 
face surfaces also eliminates undesirable chemical and electrochemical 
reactions, short circuiting between electrodes and arcing between the 
electrode and slag layer. The reduced slag carryover into the generator 
duct also reduces the difficulty of seed treatment, heat 
exchanger/preheater fouling and particulate removal requirements. 
As compared to conductive water cooled systems which continually remove 
heat from the duct and thus negatively affect efficiency, there is no heat 
loss from the duct channel, although the disclosed systems do result in 
some degradation of the plasma heat through mixing. Additionally, it is 
substantially easier to reliably electrically insulate the generator duct 
from the ground and the electrodes from one another as compared to 
watercooled copper blocks. The electrical resistivity of a once-through 
transpiration cooling gas and its conduit system is easier to maintain 
than in a recirculating water system. The systems also provide for a high 
degree of compatibility between materials and coolants. And, while past 
wall structures have required the capability to contain high pressure 
water as necessary in order to utilize the heat energy in, for example, a 
steam bottoming plant, the structural requirements for containment and 
passage of a gaseous coolant, in the range of 10 to 300 psia, are 
substantially reduced. 
Since numerous changes may be made in the abovedescribed apparatus without 
departing from the spirit and scope thereof, it is intended that all 
matter contained in the foregoing shall be interpreted as illustrative and 
not in a limiting sense.