Patent Application
20100314235
This invention relates to the pyrolysis of carbonaceous materials and more particularly to the hydropyrolysis of carbonaceous materials at high temperatures using external process heat from solar heat sources.
The petroleum refinery is one of the most thoroughly integrated operations in existence: apart from contaminants, all that goes into the refinery is either consumed in processing or emerges as useful product. Increasing shortages of crude oil and their accompanying price increases together with the unpredictable instabilities of many crude-producing regions have resulted in pressures for refineries in two major directions, first, to increase their overall efficiency by using less crude to fuel process operations, so enabling more to emerge potentially as product and second, by having to use crude sources which become progressively poorer in quality as time advances. Refineries typically use between about 20 and 25 percent of the crude intake of a refinery is consumed in fueling refinery operations with the exact figure depending very much on the type of refinery but mainly on the extent of conversion in the refinery. Substitution of fossil fuel process heat by other heat sources is therefore an economically, environmentally and politically attractive option since in a carbon-constrained economy, the potential to dramatically reduce flue gas and extend fossil resource capacity may depend on nontraditional uses of alternative energy. The use of nontraditional heat sources such as solar energy is therefore becoming more attractive even though it will present problems in achieving an overall thermal balance in the refinery.
The use of crude stocks of increasingly low quality is another matter, having been a continuing problem for some years. Crudes have become progressively heavier (higher carbon content) and contain more contaminants, especially, sulfur along with lesser amounts of metals such as nickel and vanadium and the responses to refining such crudes are now numerous although necessarily incomplete as the trends in crude quality continue. The conversion of heavier crudes into the low boiling products of higher hydrogen content needed to in the marketplace has been met by a number of different processes, some such as catalytic cracking, coking and steam cracking relying on carbon rejection and some, such as hydrocracking, relying on the addition of hydrogen to the heavy feed in order to satisfy product quality specifications.
Another response to the crude supply problem has been to exploit alternative sources of materials which can be converted into liquid fuels which constitute the major part of product demand. In western nations, resort has been made increasingly to coal, the predecessor of oil in the fuel market. Processes for converting coal to liquid and gaseous fuels have long been known and extensively used, from the town gas (coal gas) process through the Bergius hydrogenation process and the Fischer-Tropsch process which is still of great interest for its capability of converting various carbonaceous sources into liquid hydrocarbons through the intermediate of synthesis gas. Hydrocarbon sources such as shale oil and tar sand oils are also useful and each presents its own processing problems and biomass is increasingly entering the picture as a renewable energy source.
Thus, in summary, there is a continuing need to convert heavy, low quality crudes and unconventional carbonaceous materials into liquid hydrocarbon products, especially liquid hydrocarbon fuels. The need to increase refinery efficiency however, creates additional difficulties in processing heavy crudes and other source materials since greater energy is required to convert them into the light hydrocarbon products needed in the marketplace. The present invention provides an alternative processing scheme for converting carbonaceous materials into liquid hydrocarbons with a lower consumption of carbonaceous fuels in the processing.
According to the present invention, heat from solar heat sources is applied in at least two ways to the conversion of carbonaceous materials such as heavy petroleum crude oils, coals and biomass to liquid hydrocarbons. First, the heat is applied to provide at least a portion of the process heat used in the high temperature hydropyrolysis of the carbonaceous material. Second, the heat is used to generate hydrogen by a high temperature process such as high temperature steam electrolysis, the sulfur-iodine cycle, the zinc-zinc oxide cycle or even, more conventionally, by steam cracking. In addition, the heat from the solar sources may be used to generate electricity to operate the high temperature electrolysis used in the hydrogen generation or to generate hydrogen by other electrochemical processes such as the hybrid sulfur process.
The heat for the endothermic cracking reactions is efficiently supplied by solar thermal energy sources, with the heat transferred from a concentrated solar power source either directly by taking the feedstream through the focus of the solar unit or indirectly by the use of a suitable heat transfer medium fed through the solar focus point and then passed to the reactor.
The solar heat is applied to the pyrolyis either directly by routing the pyrolyis feedstream through the focus point of the solar furnace or, alternatively, by the use of a heat transfer medium and heat exchange device transferring the heat from the solar power source to the process unit in which the process is being operated or to the power cells used for producing electricity. Concentrated solar power (CSP) sources have the capability to generate very high temperatures potentially in excess of 1500° C. and heat of this quality can be used very effectively to provide process heat to the hydropyrolysis and to the other specified uses. High temperature heat transfer from a concentrated solar power source can be effected using transfer media such as liquids, gases, molten salts or molten metals although molten salts and molten metals will often be preferred for their ability to operate at very high temperatures for high energy densities without phase changes; in addition, corrosion problems can be minimized by appropriate choice of medium relative to the metallurgy of the relevant units.
The hydropyrolysis unit as a whole will be made up of a hydropyrolysis reactor with an (i) inlet for carbonaceous material to be subjected to hydropyrolysis in the reactor, (ii) an inlet for hot hydrogen, (iii) an outlet for products of hydropyrolysis together with the solar furnace as the heat source for the pyrolysis. The unit will include means for transferring heat directly or indirectly from the solar furnace to the reactor at the required temperature in the range of 800 to 1500° C. as well as for generating a stream of hot hydrogen and for feeding it to the hot hydrogen inlet of the reactor.
If the unit operates by heating the feed directly in the solar furnace, there will be a feed conduit for passing the carbonaceous feed directly through the solar furnace; this will be primarily applicable to pumpable e.g. liquid feeds. If indirect heat exchange is used, the unit will include a heat transfer loop from the solar furnace to the reactor containing a heat transfer medium for transferring heat from the solar furnace to the reactor or, alternatively, a feed heater through which the carbonaceous feed passes before entering the reactor through the reactor feed inlet. The hot hydrogen generator may be a steam electrolysis unit or another type such as a hybrid sulfur process unit, a sulfur-iodine cycle unit, a zinc-zinc oxide cycle unit or a methane steam reformer, each of which may be fed with heat from the solar heat source by means of the heat transfer loop.
Solar thermal energy sources are zero carbon emission sources and by using them, hydrocarbon resource utilization for process heat is eliminated along with carbon dioxide evolution associated with burning of the hydrocarbon resource to generate process heat. For perspective, each liter of petroleum resid requires approximately 140 liters of natural gas (methane at 15.5° C.) to heat it to 540° C. and hold it at that temperature for six seconds and, as noted above, about 20 to 25 percent of the crude oil input to a refinery is used in processing. The substitution of zero carbon emission sources therefore offers the potential for significant carbon emission reductions in refinery operations where external process hear can be applied and effectively utilized.
In the accompanying drawings:
In
As an alternative to the mode of operation shown here using heat transfer lines for bringing heat to the hydropyrolysis reactor, the steam generator and the hydrogen generator(s), the feedstreams for these units (water, heavy oil, methane) could be led directly through the focus of the solar furnace to apply the heat directly to them without the intermediary heat transfer fluid. For reasons of practical engineering, however, it will usually be more convenient for unit design to use the heat transfer mode of operation.
The hydropyrolysis process is carried out at the high temperatures enabled by the use of the high quality heat from the concentrated solar power source. The process is carried out in the presence of hydrogen and converts a carbonaceous source material into liquid hydrocarbons with a carbon char left as residue which can be fed to a syngas unit for conversion to synthesis gas and so provide a route to the production of additional hydrocarbon. A process of this kind is described in U.S. Pat. No. 4,003,820, to which reference is made for a more detailed description of. A hydropyrolysis process operating under milder conditions is described in U.S. Pat. No. 5,055,181 to which reference is also made.
Carbonaceous materials which may be subjected to hydropyrolysis include heavy petroleum oils including resid fractions such as vacuum resid and other fractions not distillable at a nominal temperature of 500° C. at atmospheric pressure, oil shale, tar sands, organic waste, Orinoco tar, Gilsonite, and coals of various ranks including peat, lignites, subbituminous and bituminous coals. The process is useful with carbonaceous materials which have a tendency to agglomerate when pyrolyzed, such as bituminous coals.
Initially, solid carbonaceous materials are comminuted to have as high a surface area as practically feasible although it is not economically justifiable to pulverize solid material to a very fine powder. That is, it is desirable to expose as much of the surface are of the material as possible without losing it as dust and fines, or as the economics of material grinding or process equipment dictate. Generally, solid materials such as coal, will be crushed and ground to a relatively small size and will contain a majority of particles less than about 4 mesh U.S. Sieve Size (passes 4.76 mm opening), preferably with smaller sizes applicable to the fast pyrolyis technique where rapid heating and quenching of the charge is required, preferably 50 to 325 mesh, U.S. Sieve (0.297 to 0.044 mm opening).
The carbonaceous material is introduced into the hydropyrolysis reactor through a suitable injection device. A feed screw may be used with particulate solids and a feed pump with the liquid materials. Liquid materials may be preheated in a feed heat exchanger to bring them to a pumpable viscosity and this heat exchanger is desirably fed with heat from the solar heat source using a suitable heat transfer medium. The pyrolysis reactor may be of the multi-stage type with successive pyrolysis/quench zones as described in U.S. Pat. No. 4,003,820 in which the feed is brought to high temperature very quickly by contact with hot hydrogen in the absence of a catalyst and then rapidly quenched by contact with coolant gas, preferably cold hydrogen with separation of the pyrolysis products at each stage. In this case, the inlet hydrogen temperature within each reaction zone should be typically be approximately 50° C. higher than the reaction temperature, when the hydrogen-to-carbonaceous material ratio is around 1:1, with this temperature difference resulting in the required rapid heat-up time. Alternatively, the hydropyrolysis may be carried out as described in U.S. Pat. No. 5,055,181 with the added catalysts at somewhat lower temperatures without the need to adhere to high heating and quench rates although this is not as favorable for the production of liquid aromatics such as BTX.
As described below, the use of very high temperatures with their associated short contact times is desirable from the viewpoint of optimizing for the desired liquid products such as BTX and stabilizing lighter products such as ethylene and propylene. Operation in this manner requires in turn a number of successive stages of rapid heating/quenching/product separation with quenching provided to maintain total residence time of the reactants in each reaction zone of the reactor in the range of 2 milliseconds to about 2 seconds, preferably from about 5 milliseconds to about 1 second, with a most preferred residence time of from 10 milliseconds to about 900 milliseconds. This total residence time includes the heatup, reaction and quench times. Since there is reaction between the carbonaceous material and feed hydrogen as soon as the feed materials enter each reaction zone of the reactor and are mixed, and since this reaction continues until the quenched mixture exits the reactor, it is difficult to separate the various phases of the total residence time. Direct or indirect quench can be used but direct contact quench is obviously more effective.
The amount of hydrogen which is effective in the hydropyrolysis will be at least about 5 wt. percent, based on the weight of carbonaceous material, at a partial pressure of at least 20 barg and typically up to about 200 barg although pressures of up to 100 barg will normally be adequate. At lower hydropyrolysis temperatures, relatively little hydrogen is consumed since less is used to make methane, typically, less than about 2 wt. percent of the carbonaceous material, preferably less than 1.5 or 1.0 wt. percent. The hydrogen-to-carbonaceous material ratio in each of the reaction zones may typically vary from about 0.05:1 to about 4:1 by weight, with the higher value showing an excess of hydrogen and the lower value resulting in the formation of more char, with reduced amounts of desirable product. A more desirable hydrogen-to-carbonaceous material weight ratio in each of the reaction zones is in the range of from about 0.12:1 to about 2:1, and the most preferred ratio is from about 0.6:1 to about 1.2:1. In practice, the proportion of hydrogen charged relative to the weight of the carbonaceous feed may be varied according to the extent of hydrogenation and determined empirically by progressively increasing the hydrogen:feed ratio to the point that no further hydrogen consumption results.
The incoming hydrogen stream for each reaction zone of a multi-stage unit can vary from about 30% hydrogen to about 100% hydrogen, based on the partial pressure of hydrogen. Since recycle of a portion of the effluent gas stream is possible, the reactant hydrogen stream can also contain components such as methane, propane, and ethane, with these components typically not condensing as they are cooled to quench temperatures.
A solid by-product char is generally produced and the amount of the char is typically at least 10 wt. percent and in most cases 20-30 wt. percent when the feed is a solid carbonaceous material, going as high as 50 to 60 wt. percent or more of solid source materials with less when using liquid feeds such as petroleum resids, typically projected at 10 to 25 wt. percent of the feed. This char may, however, be sent to a syngas reactor as a source of carbon in the syngas reactor. Operation in the presence of a hydrogen donor solvent may be expected to result in less char by-product.
Briefly, a carbonaceous source material is exposed to high temperatures of at least 500° C. in the presence of hydrogen and optionally, a gasification catalyst and a hydrogenation catalyst. If the process is carried out at a sufficiently high temperature, for example, as described in U.S. Pat. No. 4,003,820, at temperatures of at least 500° C. and preferably even higher, for example, 700, 850, 900 or even over 1000° C. and as high as 1500 C, the use of a catalyst may become unnecessary, especially if high heating and quench rates are used with a short residence time at such high temperatures. If catalysts are used in the absence of an H-donor solvent, as described in U.S. Pat. No. 5,055,181, suitable gasification catalysts include the basic compounds of alkali and alkaline-earth metals, preferably potassium and calcium, more preferably potassium. Hydroxides and carbonates are suitable and can be added to the carbonaceous material as aqueous solutions containing from about 2 to about 30 wt. % of the alkali and/or alkaline-earth compounds. Hydrogenation catalysts suitable for use herein include compounds containing metals from groups 5 to 10 of the long form Periodic Table for the Elements with groups numbered 1-18 as recommended by IUPAC. Preferred are compounds containing tungsten, molybdenum, nickel, cobalt, zinc, or iron. Non-limiting examples of such preferred compounds include ammonium heptamolybdate, phosphomolybdic acid, nickel sulfate, cobalt sulfate, and iron acetate. These compounds may be dissolved in water and added to the source material to give a concentration of metal on carbonaceous material of about 100 ppm to about 5000 ppm, preferably about 100 ppm to about 1000 ppm.
The presence of catalytically active metals in the carbonaceous feed, especially those active for hydrogenation-dehydrogenation reactions, especially nickel, vanadium, iron and molybdenum but also potentially, cobalt, chromium and other transition metals, may result in improved operation since they may act to catalyze hydrogenation reactions which, in turn will facilitate cracking reactions, resulting in a higher yield of liquid product.
Exposure of the carbonaceous materials to temperatures in the range of 800-1500° C. during the hydropyrolysis very rapidly decomposes the molecules into small fragments consisting of the most reactive and hydrogen deficient free radicals. These free radicals react to abstract hydrogen atoms from wherever they can be found to help stabilize them. This leads to the formation of methane and unsaturated low molecular weight species; largely acetylenes and small olefins such as ethylene and propylene. Addition of molecular hydrogen to the system facilitates these hydrogen transfer-healing reactions and also prevents further unsaturation pathways, e.g., ethylenes forming more acetylene. On cooling below the indicated temperature range, acetylenes condense to form aromatics and methylated aromatics, e.g., BTX. Ethylene and propylene are stabilized and if quenched rapidly are prevented from undergoing thermal alkylation/oligomerization reactions. Water, hydrogen sulfide and ammonia formed from thermal elimination are quenched. While there will be some coke formation this pathway is inhibited, but not completely eliminated, in the presence of moderate pressures of hydrogen. Methane is a major by-product formed from thermal dealkylation of methyl groups on aromatic species.
Cleavage of ethylene bridges and alkyl groups on aromatic rings in asphaltenes from a sponge coke-forming crude (0.5 heteroatoms per aromatic cluster) and a shot coke-forming crude (2.5 heteroatoms per aromatic cluster)1 will be complete in hundredths of a second in the 800-1500° C. temperature range (activation energy 52 kcal/mol) with the subsequent 58-66 kcal/mol demethylation reactions completed in hundredths to tenths of a second. Unzipping of aromatics to acetylenes and other aliphatics and cycloaliphatics (naphthenes) to ethylenes (includes propylene which is a methylethylene, etc.) will be completed in tenths of a second to seconds. In the presence of hydrogen the free radicals generated will be healed and the heteroatoms that are eliminated will be converted into water, hydrogen sulfide and ammonia. 1 See molecular representations in EnergyFuels 2006, 20, 1227-1234.
Examination of the potentially ideal product slate indicates a two stage process of hydropyrolysis followed by a rapid quench step. Quenching to below 400° C. will prevent back conversion of BTX to acetylenes and the oligomerization of ethylenes. If it is desirable to increase the BTX yield, increasing the residence time to produce more acetylenes from ethylenes in the product is possible. Also, as is evident from the product distribution, a feed with a lower heteroatom content produces less water, hydrogen sulfide and ammonia and also favors formation of acetylenes and therefore BTX product. Similarly, decreasing the residence time will lead to higher yields of ethylenes. The liquid products of the hydropyrolysis will depend on the feeds and conditions used in the reaction; they may be subjected to further processing by conventional methods as desired.
The heat for the endothermic cracking reactions is efficiently supplied by solar thermal energy sources, with the heat transferred from a concentrated solar power source either directly by taking the feedstream through the focus of the solar unit or indirectly by the use of a suitable heat transfer medium fed through the solar focus point and then passed to the reactor. The heat can be applied to the hydropyrolysis in a number of ways. Pre-heat may be applied to the feed either directly or in a heat exchanger fed directly or indirectly from the heat source and the hydrogen may be brought to the desired high temperature either directly in the solar unit or in a heat exchanger receiving its heat from the solar source. In view of the need for rapid quench to ensure short contact time, jacketing of the reactor will not normally be favored.
Because of the fast rate of reaction, more feed can be processed in shorter times, significantly increasing throughput and capacity of the conversion process. This will permit higher throughput in existing conversion units that can be run at increased temperatures and allow for much smaller reactors to be built specifically for this pyrolysis. While the higher temperature range may necessitate metal reactors with different alloys than carbon steel, the dramatically reduced size required may offset the higher metallurgy cost.
Solar thermal energy is provided by the conversion of light to heat energy. This is typically achieved by focusing solar radiation onto a point source using mirrors, and the point source increases in temperature thus generating heat. For commercial applications, multiple mirrors are required to be installed to increase light capture. Once the solar radiation is focused on a point, the heat is transferred to fluid heat transfer medium. Three types of solar thermal device designs have been explored: solar tower, solar trough, and solar reactors.
Solar thermal installations with a tower design use mirrors to focus incoming solar radiation on to a point that is often located on a central tower. Typically, the mirrors in a heliostat system are motorized to follow the sun over the course of the day. At this focal point, a liquid heat transfer medium is heated to the required temperature. Solar trough power plants use curved, trough-shaped mirrors to focus light on to a heat transfer fluid that flows through a tube above them. These trough reflectors tilt throughout the day to track the sun for optimal heating. The heat transfer fluid is heated in the troughs and then flows to a heat exchanger, which is used to produce superheated steam. A modified version of the parabolic trough design, the Fresnel reflector design, is uses a series of flat mirrors with a number of heat transfer receivers. Solar reactors, or Concentrated Solar Power (CSP), are useful for applications such as the present that take advantage of the high-temperature capabilities of tower technology which uses reactors similar to closed volumetric receivers except that a rhodium or another catalyst is dispersed on the surface of the ceramic mesh, directly absorbing the solar energy to produce syngas, hydrogen, and carbon monoxide as disclosed by Moller, S. et al., in 2002: Solar production of syngas for electricity generation: SOLASYS Project Test-Phase, 11th SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, Zurich. In its application to the present invention a solar reactor is used for directly heating the heat transfer fluid to high temperatures.
The solar energy source is an augmented with natural gas or other fossil fuel heat at times the solar thermal reactor output is diminished due to lack of availability of solar radiation.
Heat Transfer from Source to Process Unit
The heat from the solar high temperature heat source is applied either directly to the reactant stream or indirectly by the use of a heat transfer medium and heat exchange device transferring the heat from the solar power source to the process unit in which the process is being operated. When a heat transfer medium is used, it will be routed from the solar source to a heat exchanger providing pre-heat for the process, direct heat to the process environment e.g. by a heating jacket on the reactor used for carrying out the process or by heat transfer coils or tubes inside the reactor. Heat from solar heat sources at temperatures potentially in excess of 1500° C. and heat of this quality can be used very effectively to provide process heat to the endothermic reaction steps of the cyclic chemical processes, even when transferring heat to reactant gas streams in a heat exchanger or through heating jackets or heating coils on the vessel. Heat transfer at the high temperatures contemplated, typically above 800° C. and ideally higher, e.g. 900, 950, 1000, 1200° C., even as high as 1500° C., can be effected using transfer media such as liquids, gases, molten salts or molten metals although molten salts and molten metals will often be preferred for their ability to operate at the very high temperatures required for high energy densities without phase changes; in addition, corrosion problems can be minimized by appropriate choice of medium relative to the metallurgy of the relevant units. Molten salt mixtures such as mixtures of nitrate salts, more specifically, a mixture of 60% sodium nitrate and 40% potassium nitrate are suitable but other types and mixtures of molten salts may be used as a heat transfer and a thermal storage medium. Liquid metals such as sodium as well as alloys such as sodium-potassium alloy, bismuth alloys such as Woods metal, (m.p. 70° C.) and alloys of bismuth with metals such as lead, tin, cadmium and indium; the melting point of gallium (30° C.) and its alloys would make it attractive as a heat transfer medium but the aggressiveness of this metal towards almost all other metals will generally preclude it from consideration. Mercury is excluded for environmental reasons.
The present highly integrated process unit makes use of hydrogen generated in units operating with heat supplied from the selected heat source. The hydrogen generator may be conventional in type as with a steam reformer fed by natural gas or may be based on an alternative technology such as the sulfur-iodine cycle, the hybrid sulfur (HyS) thermochemical process, the zinc-zinc oxide cycle, high temperature steam electrolysis or solid oxide fuel cell (SOFC) stacks, all of which operate at the high temperatures which can be supported by the solar sources described above. Another option would be to generate hydrogen from methane in a solar furnace of the type shown in Energy, Volume 29, Issues 5-6, April-May 2004, pp 715-725, Solar PACES 2002.
The sulfur-iodine cycle, developed by Westinghouse Electric, is represented by the equations:
I2+SO2+2H2O→2HI+H2SO4 (120° C.) Equation 1
2H2SO4→2SO2+2H2O+O2 (830° C.) Equation 2
2HI→I2+H2 (450° C.) Equation 3
2H2O→2H2+O2 Net reaction
The sulfur-iodine cycle, invented by General Atomics in the 1970s, requires water and heat as inputs. Equation 1 above indicates that iodine reacts with sulfurous acid (SO2+H2O) at 120° C. to produce sulfuric acid that is then pyrolyzed (Equation 2) at 830° C. to generate sulfur dioxide (for recycle to Equation 1), water, and oxygen. The hydrogen iodide generated in Equation 1 is pyrolyzed to generate iodine Equation 3 and to produce hydrogen.
The hybrid sulfur cycle process, developed by Westinghouse Electric, requires water, heat, and electricity as inputs. As in the sulfur-iodine cycle, sulfuric acid is pyrolyzed at 830° C. to generate sulfur dioxide (Equation 4 below). The sulfur dioxide and water produced in Equation 4 are electrolyzed in accordance with Equation 5 below at 0.55 V at 90° C. to generate hydrogen and regenerate sulfuric acid for recycle.
H2SO4→SO2+H2O+½O2 (830° C.) Equation 4
2H2O+SO2→H2SO4+H2 (<100° C., electrical energy) Equation 5
The zinc-zinc oxide cycle uses heat to thermally dissociate zinc oxide at high temperature typically 1,900° C. into zinc and oxygen and in a second, exothermic step the zinc reacts with water at moderate temperature to produce hydrogen and zinc oxide which is then returned to the first step.
ZnO→Zn+½O2
Zn+H2O→ZnO+H2
The high temperatures required for the first step may be obtained from the solar energy source. Lower temperatures may be possible by adding a reducing agent such as biomass-based charcoal.
Conventional electrolysis is energy-intensive and too expensive for large scale use with electricity representing about 80% of the cost. High temperature steam electrolysis, typically at temperatures above about 800° C., by contrast, offers the potential for hydrogen generation with reduced electricity requirements as some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures as a result of a lowering of the cell voltage. Solid oxide electrolyser cells, for example, the cell using yttria-stabilized zirconia (YSZ) electrolytes, nickel-cermet steam/hydrogen electrodes, and mixed oxide of lanthanum, strontium and cobalt oxygen electrodes have the potential to make the cost competitive with steam methane reforming. Further improvements in performance may be achieved with methane or carbon monoxide depolarized anodes. The high temperatures required may be derived from the solar energy source and the electricity from the solar powered generator unit.
Solid Oxide Fuel Cells utilize hydrogen or a light hydrocarbon such as methane, ethane, propane or a naphtha, typically up to C8, as a fuel and oxygen to produce electricity directly. SOFCs operate at high temperatures, typically between 500 and 1000° C. and are capable of achieving efficiencies of about 60 percent or higher at these temperatures. While hydrogen is the preferred fuel, the light hydrocarbon gases may also be used directly in suitable cells or, alternatively, they or other hydrocarbon fuels such as gasoline, road diesel fuel, jet fuels such as Jet-A, JP-5, JP-8 or biofuels may be subjected to steam reforming to form gas mixtures, typically comprising hydrogen, carbon monoxide, carbon dioxide, steam and methane, which can then be passed to the SOFC with or without removal of inert components and of potential sulfur poisons.
In cells using methane (natural gas) as the fuel source, the conversion to electricity at the anode surface is depolarized at the high operating temperatures by reforming CH4 and C2+ to H2/CO with some water gas shifting CO to CO2 and H2. The CH4 reacts with oxygen ions at the anode surface to generate electricity, releasing H2O. The off gas from the anode has concentrated CO2, which can be captured and sequestered. The use of the natural gas as the direct fuel rather than by pre-reforming to H2/CO source creates a synergism in the system by having the excess heat created in generating electricity directly absorbed by the reforming reaction. The amount of heat available from the SOFC electricity generation is far in excess of that needed to reform the amount of natural gas needed to provide the H2 for electricity generation. This excess heat can be utilized to produce additional H2/CO at the anode surface which can subsequently be processed as syngas to produce pure H2. In this way, hydrogen can be produced as a coproduct with electricity in an economical fashion with the heat evolved from the fuel cell being used in the hydropyrolysis. The amount of excess system heat from the reaction to generate electricity may be sufficient to create an equal or greater amount of excess H2 Kcals relative to electricity Kcals. In addition since the scheme releases CO2 in a concentrated form, it lends itself well to being able to capture and sequester the CO2 generated in an economic fashion.
The anode, cathode and solid electrolyte in cell 30 are typically fabricated from porous metal oxide materials, usually combined with a cermet, for example, yttria stabilized zirconia (YSZ). The catalytic material for the reforming reaction may be provided by the anode itself although the presence of catalyst directly on the anode surface provides for improved catalysis of the reforming reaction. The solid electrolyte comprises a dense layer of ceramic that conducts oxygen ions at the cell operating temperatures typically at least about 600 C. Typical electrolyte materials include yttria stabilized zirconia (YSZ) (often the 8% form Y8SZ) and gadolinium doped ceria (GDC). The cathode comprises an electronically conductive layer superposed on the electrolyte. Mixed metal oxides such as lanthanum strontium manganite (LSM) which are compatible with the expansion characteristics of the electrolyte are preferred. Composite cathodes consisting of LSM YSZ and mixed ionic/electronic conducting (MIEC) ceramics, such as perovskite LSCF (lanthanum, strontium, cobalt, iron), La0.6Sr0.4CO0.2Fe0.8O3) may be used to permit operation at lower temperatures. The LSCF cathode is considered to be highly suitable due to its unique characteristics, such as high catalytic activity, good ionic and electric conductivities (1×10−2 and 102 S/cm at 800° C., respectively). On the other hand, the LSCF cathodes are subject to power degradation. In order to prevent high temperature reactions between the electrolyte and the cathode materials, an interlayer of cerium gadolinium GDC (Ce0.8Gd0.2O2) oxide may be interposed. LSCF is often combined with ceria-based materials, such as gadolinium doped ceria (GDC or CGO) to enhance the catalytic activity for conversion of oxygen to oxide ion.
At the anode, the following reactions take place:
CH4+4O2−=CO2+2H2O+8e−
CH4+H2O=H2+CO
CO+H2O=CO2+H2O
The cathode reaction is the conventional reduction of oxygen to form oxygen ions which are then transported though the electrolyte to the anode for oxidation.
2O2+8e−=4O2−
The mechanical features of the cell stack can be conventional in type, for example, with planar or tubular configuration with the tubular configuration normally giving faster start up times although this is not a primary service requirement in this stationary application. The cell interconnect may be either metallic or ceramic according to service requirements.
As noted above, the high temperature methane reforming reaction at the anode produces hydrogen as well as carbon monoxide but the amount of heat available from the electricity generation is far in excess of that needed to reform the amount of natural gas needed to provide the hydrogen required in the electricity generation. This heat is at a high temperature, typically, at least 600 or even at least 800° C. and can be removed from the cell by means of jacket 46 which also serves the purpose of providing heat (from reactor 10) at startup. The anode product gases comprising hydrogen, carbon monoxide, carbon dioxide and water (steam) can be passed to a water gas sift reaction in unit 42 to produce additional hydrogen for use in the hydropyrolysis reaction and the carbon dioxide removed by scrubbing:
CO+H2O→CO2+H2
2MOH+CO2→M2CO3+H2O
The present conversion process may be employed either in a refinery or production environment. Upstream applications may, for example, include the in situ hydrous pyrolysis conversion of oil shale to generate a petroleum-like liquid product containing essentially no olefins or conjugated dienes using liquid water or steam.
This application claims priority under 35 USC 120 from Application Ser. No. 61/268,779, filed 16 Jun. 2009. It is also related to Application Ser. No. 60/268,776, of even date, which relates to the pyrolysis of carbonaceous materials at high temperatures using external process heat from a nuclear heat source.
| Number | Date | Country | |
|---|---|---|---|
| 61268779 | Jun 2009 | US |
