GB2099805A - System for thermochemical hydrogen production - Google Patents

Abstract

Method and apparatus for joule boosting a SO3 decomposer using electrical instead of thermal energy to heat the reactants of the high temperature SO3 decomposition step of a thermochemical hydrogen production process driven by a tandem mirror reactor. Joule boosting the decomposer to a sufficiently high temperature from a lower temperature heat source eliminates the need for expensive catalysts and reduces the temperature and consequent materials requirements for the reactor blanket. A particular decomposer design utilizes electrically heated silicon carbide rods, at a temperature of 1250K, to decompose a cross flow of SO3 gas.

Description

SPECIFICATION System for thermochemical hydrogen production Hydrogen, a valuable raw material for the petroleum and petrochemical industries, is expected to become by early in the next century an important renewable-based, transportable fuel either by itself or in some hydrocarbon form such as methanol or gasoline. Hydrogen can be produced through the decomposition of water by means of thermochemical cycles which reduce the high temperature requirements of the 3000K (degrees Kelvin) straight thermal decomposition process to the 1200K levels that can be generated in nuclear fission or fusion reactors or in high intensity, focused solar reflectors.

A purely thermochemical process for producing hydrogen is the sulfur-iodine cycle being developed by the General Atomic Company. The essential steps of the sulfur iodine cycle are represented by the following reactions: 2H2O+SO2+Xl2oH2SO4+2HIX (370-390K) 2HIH2+xl2 (393K) H2SO4~H2O+SO2+1/2 2 (1144K) Other thermochemical processes include the Westinghouse sulfur cycle (which is partly electrochemical): 2H2O+SQH2+H2SQ (N350K) H2SO4oH2O+SO2+1/2 02 (N1 1 OOK) and the sulfur-bromine cycle (which is partly electrochemical), being developed at ISPRA, Italy:: 2H2O+SO2+Br2oH2SO4+2HBr (320-370K) 2HBr )Br2+H2 (N350K) H2SO4oH2O+sO2+1/2 02 (1000- 1100K) The dominant energy requirements, heat versus temperature, are necessary in these processes for the H2SO4 concentration and vaporization, conversion of H2SO4 into SO3+H2O, and the high temperature SO3 decomposition steps.

The SO3 decompose is the critical process unit in nearly all the viable thermochemical plants to produce hydrogen. These plants can be driven by high temperature gas-cooled reactors, solar collectors or fusion reactors, utilising sodium, potassium or helium as heat transfer fluids to supply the large heat demand of the S03 decomposer. Catalysts are required in the decomposer if the temperature is to be kept at levels of about 1070--1 120K. The key requirement is to supply heat to the surfaces where the endothermic SO reaction occurs. This S03 decomposition produces SO2 and 02 for the thermochemical production of hydrogen.

Measued SO3 kinetics and equilibrium show this high temperature SO3 decomposition reactor to be surface kinetics (heterogeneous) controlled at lower temperatures, below 1050K, and homogeneous at higher temperatures, above 11 80K. For non-catalytic surfaces the conversion from SO3 to SO2 plus 02 is about 2030% over the temperture range 1080K to 1180K for a 0.3 to 1 second residence time at around 1.5 atm.

total pressure, but the conversion is about 80% at the higher temperature of 1250K. The low conversion at lower temperatures leads to large recycle H2SO4 flows and thus much larger and more expensive equipment. Increased residence time improves the kinetics but increases the size of the equipment. Increased total pressure decreases the equipment size but unfavourably shifts the equilibrium, and decreased conversion increases equipment size. Catalytically enhanced kinetics greatly improve the conversion at lower temperatures to the range of 6580%. However, the use of catalysts greatly increases the capital costs, particularly if very expensive platinum catalysts must be utilized.Operation at temperatures of 1250K eliminates the need for catalysts, but imposes very serious materials problems on the heat source with provides thermal heat directly to the decomposer.

The design of the chemical reactor with fast kinetics and large associated heat effects is very difficult. A design of least cost and greatest simplicity is desired. Catalytic decomposers heated by internal heat exchangers appear to be too large to be cost competitive with other hydrogen production technologies. The most obvious choice, a packed bed reactor, does not appear feasible because heat transfer from in-bed heat exchangers to the packed bed of catalysts is very inefficient and requires extremely large temperature gradients between the heat exchanger fluid and the packed bed. Costly, high heat transfer media flow rates are also required, and large radial temperature gradients appear within the bed between the internal heat exchanger tube elements.Fluidization of the bed of catalysts greatly reduces the temperture differences between the heat transfer fluid and the catalyst surface. However, substantial pumping power is required to fluidize the bed, resulting in a higher operational cost design.

Utilizing a packed bed catalytic reactor driven by a fusion reactor, the catalyst thermal requirements are 1050K and the blanket temperature must be 1 350K in order for the flowing coolant to maintain the catalyst at the required temperature. With a fluidized bed design the blanket temperature can be lowered to 1100K in order for the coolant to still maintain the catalyst at the 1050K temperature. Utilizing the catalytic cartridge SO3 decomposer driven by a heat pipe from the blanket as described in U.S.

Patent Application Serial No. 208,218, filed November 18, 1980, the blanket temperature is reduced to the catalyst temperature of 1050K.

Fusion reactors offer some unique advantages as drivers for thermochemical hydrogen plants.

Thermal heat from the blanket of a tandem mirror fusion reactor can be utilized. One particular tandem mirror blanket concept is a lithium sodium, liquid metal 50% atomic mixture in the cauldron blanket module. Helium or sodium can be used as the heat transfer fluid to carry heat outside the nuclear island to process exchangers within the thermochemical hydrogen production cycle. Either a direct condensing vapor heat exchange loop or a heat pipe driven loop can be utilized. Problems with this design, however, include the safety problems of the isolation of liquid metals from the process stream and the permeation of radioactive tritium into the product stream. A severe operational problem is the requirement that the blanket be at a temperature as high as the highest temperature required in the thermochemical process.This high temperature operational requirement for a direct heat source to the thermochemical cycle presents serious materials problems, since most conventional alloys are unsuitable for operation over about 1150K.

The tandem mirror reactor consists of a long central cell, about 200 m in length, in which power producing deuterium-tritium (DT) plasma is confined by straight magnetic field lines produced by simple circular superconducting coil modules.

The power producing plasma of the central cell is electrostatically confined at its ends by the plasma in yin-yang end plugs, each of which is a minimum-B stabilized mirror. The tandem mirror utilizes a positive potential of the plugs with respect to the central cell to repel and prevent escape of central cell ions. A further improvement on the basic tandem mirror is the addition of thermal barriers, i.e., large magnetic mirrors, at the ends of the central cell but inside of the end plugs.

The tandem mirror reactor provides a configuration of relatively simple blanket modules along the central cell length which can be utilized as a source of process heat. The tandem mirror reactor produces energy by fusing deuterium and tritium to produce energetic neutrons and alpha particles. The neutron kinetic energy is captured in the moderating blanket surrounding the reacting plasma, producing thermal energy. The alpha particles lose some energy by heating the plasma through collisions before leaving the central cell through the ends. The alpha particles are captured by the direct converter located beyond the end cells which produces two forms of energy, an electrical DC component and a thermal component. The direct converter produces about 13% of the reactor energy output.

The availability of both thermal energy and DC electricity from open-ended fusion machines is a unique advantage compared to closed systems that can be utilized for thermochemical processes, e.g., the Westinghouse sulfur cycle and ISPRA sulfur bromine cycle are partly electrochemical.

Process heat for the thermochemical processes is provided by both the blanket and the thermal component of the direct converter, while surplus electrical energy from the direct converter beyond that required to drive the reactor can be utilized to satisfy electrical demands.

The thermochemical production of hydrogen has a significant effect on the reactor design, particularly the blanket modules which surround the plasma to convert the neutron kinetic energy to thermal energy. The blanket moderating fluids or solids must run hot enough to provide the highest temperature thermal requirements of the hydrogen fuel production process, typically 1 200K or higher, which imposes serious materials problems. A blanket for electrical production can run at a much lower temperture, typically 750K-900K. One high temperature blanket module configuration is a lithium-sodiumcauldron blanket resembling a pool boiler. A pool of liquid lithium-sodium mixture surrounds the plasma cell. It acts as a neutron moderator heat transfer fluid (and also as a tritium producer). Heat is removed by vaporising the sodium.The sodium vapor travels upwards into the dome region of the cauldron, condenses on heat exchanger tubes and returns as liquid droplets to the pool. The heat exchanger in the dome transfers the thermal energy out of the module.

A modified cauldron design with heat pipes transferring heat from the moderator to the heat exchanger eliminates a problem of excessive void fraction. Liquid lithium or LiPb is substituted for LiNa since the sodium performs no function. The heat pipe working fluid is sodium or potassium.

An alternative blanket concept is the flowing Li2O microsphere blanket. By generating heat and breeding tritium in the microspheres as they flow through the blanket, the hot microspheres transfer heat to a process working fluid in the module heat exchanger.

Thermochemical cycles, the tandem mirror fusion reactor, blanket configurations, the interface with thermal reactors, fluidized bed decomposer designs, and associated problems, are described in UCRL-84212, "Interfacing the Tandem Mirror Reactor to the Sulfur lodine Process for Hydrogen Production",, T. R.

Galloway, Lawrence Livermore National Laboratory, June 1980; UCRL-84285, "The Process Aspects of Hydrogen Production Using the Tandem Mirror Reactor", T. R. Galloway, Lawrence Livermore National Laboratory, September 1980; UCRL-84632, "Some Chemical Engineering Challenges in Driving Thermochemical Hydrogen Processes with the Tandem Mirror Reactor", T. R. Galloway et al, Lawrence Livermore National Laboratory, November 1980; and UCID-18909, Vol. l and II, "Synfuels from Fusion-Producing Hydrogen with the Tandem Mirror Reactor", R. W. Werner (editor), Lawrence Livermore National Laboratory, January 1981, which are herein incorporated by reference.

It is accordingly an object of the invention to provide a method for driving the high temperature step of a thermochemical cycle with a heat source at a lower temperature.

It is also an object of the invention to provide a method of interfacing a high temperature thermochemical cycle with a tandem mirror fusion reactor.

It is another object of the invention to provide an improved SO3 decomposer.

It is a futher object of the invention to provide a non-catalytic SO3 decomposer.

It is also an object of the invention to increase the temperture of the SO3 decomposer while decreasing the blanket temperature of the fusion reactor driving the process.

The invention is a method and apparatus for providing the high temperature heat requirements of a thermochemical hydrogen production process driven by a tandem mirror fusion reactor while operating the thermal blanket of the reactor at a temperature relatively lower than the high temperature required by the thermochemical process by "joule boosting", i.e., electrical heating. High conversion efficiency electrical energy from the direct converter, plus electrical energy produced by converting thermal energy from the lower temperature blanket to electrical energy is converted to higher temperature thermal energy for the SO3 decomposer.The invention further includes a high temperature method of decomposing SO3 without a catalyst by contacting SO3 with silicon carbide heating elements at a temperture of about 1250K; the silicon carbide rods are heated by electrical heating. The invention also includes a joule boosted decomposer comprising a cylindrical chamber containing a plurality of parallel heating elements aligned with the axis of the chamber cylinder. The heating elements are electrically heated and SO3 gas is flowed cross-flow to the heating rods through the chamber to promote decomposition reactions into SO2+02.Producing electric power from the reactor and electrically heating the high temperature decomposer to 1250K allows the blanket temperature (source of thermal heat) to be about 900K, greatly alleviating materials problems, the level of technology required, safety problems, and cost.

Figure 1 is a schematic diagram of the interface between a thermochemical hydrogen cycle and a tandem mirror reactor.

Figure 2 is a schematic diagram of the H2SO4 processing system.

Figure 3 is a heating curve for the H2SO4 process step.

Figure 4 is a schematic diagram of a joule boosted SO3 decomposer.

Figure 5 is a graph of sulfuric acid decomposition as a function of temperature.

The invention is a method and apparatus for driving a high temperature SO3 decomposer in a thermochemical hydrogen production process by means of a tandem mirror fusion reactor, as illustrated schematically in Figure 1. The SO3 decomposer 10 operates at about 1250K to decompose SO3 into SO2+O2 and forms the critical high temperature component with the remaining thermochemical process units 12 for producing hydrogen.Thermal energy at about 900K is removed from the blanket 14 and direct converter 1 6 of a tandem mirror fusion reactor by means of a heat transfer fluid which flows along flow paths 1 8 and 20, respectively, and along flow path 22 into thermochemical process units 1 2 for directly heating the lower temperature process components. A portion of the heat transfer fluid is taken from path 22 and flowed along path 24 through a turbine 26 which drives an electrical generator 28, and then returns along path 30 to the reactor blanket 14. Part of the electrical energy produced is provided along path 32 for driving the reactor, including pumps and other mechanical equipment in the system.The remainder of the electrical energy produced is taken by path 34 and added to electric energy produced in the direct converter in path 36 to the SO3 decomposer 10 for joule heating the decomposer to sufficiently high temperatures for producing SO3 decomposition reactions. By the means of joule boosting the decomposer, i.e., converting lower temperature thermal energy to electrical energy and then reconverting the electrical energy to higher temperature thermal energy in the decomposer, the high temperature requirements of the SO3 decomposer (1250K) are satisfied while the reactor blanket can operate at a much lower temperature (900K).

The major components of the H2SO4 processing unit are shown schematically in Figure 2, illustrating the heat requirements of each component. The reactor blanket operates at about 900K and supplies all the thermal energy demand up to 900K directly while all the thermal energy demand about 900K is supplied by electrical heating. Sulfuric acid, H2SO4, passes through multi-effect evaporators 40 operated between 500K and 680K and then through boiler 42 which operates at 680K.The SO3 gas produced, pumped by turbine 43, is passed through preheater 44 which is a heat exchanger which raises the SO3 temperature to about 950K- 1050K and then into the joule boosted SO3 decomposer 46 which operates at a temperature of 1 250K supplied by electrical heater 48 which is driven by joule boosting by electrical energy produced from the fusion reactor. The product gas from the decomposer 46 exits at high temperature (1250K) and is circulated through the heat exchanger of preheater 44 where it functions as the working fluid, decreasing in temperature to about 730K while increasing the SO3 gas temperature for input into decomposer 46.The product gases are further cooled by passing through decomposer cooler 50 in which other process fluids are increased in temperature by heat exchange with the product gases and then passed through vapor/liquid splitter 52 to remove SO2 +02 at a temperature of about 418K for use in the thermochemical hydrogen production cycles.

The heating curve for the H2SO4 process step is illustrated in Figure 3. The lower temperature requirements including evaporation and boiling of H2SO4 and preheat of the SO3 gas are provided by blanket heat. Additional preheating is provided in the heat exchanger (HX curve) of preheater 44 (Figure 2) from SO2 quench to raise the SO3 temperature to about 1050K. Electrical heating (resistive or ohmic heating) according to the invention is utilized to raise the SO3 temperature from 1050K to 1250K, thereby producing SO, decomposition reactions.

Ajoule boosted SO3 decomposer according to the invention as illustrated in Figure 4, utilizes electrical energy to meet the high temperature thermal demands, thereby interfacing with a lower temperature thermal heat source. The joule boosted decomposer comprises a cylindrical vessel 60 having a pair of headers 62 and 64 located near opposite ends to define a chamber 66 therebetween. A plurality of heating elements or rods 68 pass through the headers 62 and 64 through insulators (not shown) and are aligned along the axis of the vessel 60. One header may be floating to provide for thermal expansion or bellows may be placed at the ends of the heating elements.An inlet port 70 allows SO3 gas to flow into chamber 66 to contact the heating elements 68 which are maintained at a sufficiently high temperature to promote decomposition reactions into SO2+02. The product gases are removed through outlet port 72 which is located substantially opposite to the inlet port 70, thereby producing a cross flow of the S03 gas across the heating elements 68. The ends of the heating elements 68 extend into the end portions 74 and 76 of the vessel 60. A coolant gas is circulated through the end regions 74 and 76 by means of inlets 78 and outlets 80 in order to cool the ends of the heating element 68 down to about 600K.

Electrical connections 82 at the ends of the rods 68, e.g., aluminium impregnated electrical feed connections, allow an electrical current to pass through the rods 68 from electrical lines 84 and 86 which pass through the ends of the vessel 60 and are connected to the rods 68 by contacts 82.

The heating elements or rods 68 are heated to a temperature sufficient to promote SO3 decomposition reactions. From the sulfuric acid decomposition data on alumina substrates shown in Figure 5, the rods are heated to about 1250K to produce an 80% conversion. According to the invention, the heating rods 68 are preferably silicon carbide rods which can tolerate the corrosiveness of SO3 gas because a protective SiO2 scale develops on the surface. These silicon carbide heating elements normally have a roughtextured surface, providing large surface area to promote decomposition reactions. Alternating current must be used to avoid polarization problems and non-uniform heating.

In one particular configuration the chamber 66 is 3 m long and 3 m in diameter. The vessel 60 is fabricated from Incoloy 800H and the process gas pressure is 7 atm. The interior of the vessel 60 is insulation-lined and the walls are cooled. The silicon carbide heating rods are about 5.5 cm in diameter and slightly over 3 m long. The end regions 74 and 76 of the vessel 60 are each about 0.5 m long. The silicon carbide heating rods are configured in a hexagonal array with a 6.1 cm spacing normal to the flow and 5.3 cm spacing in the flow direction. The heating rods further can be formed with small corrugations in the surface to benefit the gas-solid heat transfer coefficient. The heating elements can be operated up to 130 KW/m2 at 1250K but preferably will operate at 63 KW/m2 to achieve longer lifetime. The cross flow geometry for the decomposer heating elements has significant heat transfer advantages. Cross flow around the 5.5 cm diameter elements is more effective owing to reformation and growth of the boundary layer and the separation and wake formation aft of the cylinder. These wakes provide turbulence which enhances the heat transfer. The silicon carbide rods are noncatalytic; however, it may be possible to increase impurity dopants to provide some catalytic action.

Changes are modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.

Claims (12)

Claims

1. A joule-boosted method of driving a high temperature step of a thermochemical process for hydrogen production from a magnetic fusion reactor having a blanket operating at a substantially lower temperature then the high temperature step, comprising: removing lower temperature thermal energy from the blanket of the reactor; converting the lower temperature thermal energy from the blanket to electrical energy; and converting the electrical energy to thermal energy at a sufficient temperature to meet the heat requirements of the high temperature step.

2. The method of Claim 1 further including removing electrical energy from a direct converter of the magnetic fusion reactor and converting the electrical energy to thermal energy for the high temperature step.

3. A method of decomposing S03 gas without a catalyst comprising contacting the 803 gas with a silicon carbide rod at a temperature of about 1250K.

4. The method of Claim 3 wherein the silicon carbide rod is heated by running an electrical current through the rod.

5. An improved 803 decomposer for thermochemical hydrogen production, comprising: a chamber having an inlet port and an outlet port whereby S03 gas may be flowed into the chamber through the inlet port; and at least one heating element disposed within the chamber for continuously providing heat within the chamber to produce decomposition of S03 gas which contacts the heating means into S03 and O2 which are removable through the outlet port.

6. The decomposer of Claim 5 wherein the chamber is substantially elongated in shape.

7. The decomposer of Claim 6 wherein the heating elements are generally aligned in the substantially elongated dimension of the chamber.

8. The decomposer of Claim 7 wherein the inlet and outlet ports are disposed about the chamber substantially opposite from each other, whereby the SO3 gas flow is substantially a crossflow across the heating elements.

9. The decomposer of Claim 5 of 8 wherein the heater elements are electrically activated.

10. The decomposer of Claim 9 wherein the heater elements are silicon carbide rods.

11. The decomposer of Claim 9 wherein the heater elements are maintained at a temperature of about 1250K.

12. The decomposer of Claim 9 wherein the heater elements are tubes.

1 3. The decomposer of Claim 12 wherein the tubes are coated with a catalyst.