Patent Application
20140011104
The invention generally relates to fuel cells. In particular, the invention relates to fuel cells for solid carbonaceous fuel conversion.
Coal remains to be the cheapest and most abundant fossil fuel on earth, and is the primary energy source used for electric power generation. The United States possesses the largest proven coal reserves in the world, followed closely by vast reserves in Russia, China and India. So it is no coincidence that coal provides about 50% of the electricity generated in the US, and more than 75% in China and India. The World Coal Institute has estimated that at the current consumption levels, proven global coal reserves are sufficient to last for more than 150 years. Consequently, coal will remain a major energy resource well into the future.
However, coal-to-electricity conversion efficiency is in the low 30% regime for most pulverized coal-fired power plants currently in operation and in the mid 30% range for supercritical and ultrasupercritical power plants. They all produce copious quantities of carbon dioxide, a green house gas. Since air is used in these processes, the flue stream contains only about 10-15% CO2 diluted with nitrogen. In the United States, 40% of the CO2 emitted into the atmosphere comes from coal-fire power plants. In order to use coal in an environmentally benign manner, the CO2 must be captured and sequestered. The most efficient process to achieve this goal is by electrochemical conversion using direct carbon fuel cells (DCFC). However, the contaminants in coal, in particular sulfur, pose a major bottleneck in advancing fuel cell technology for efficient coal conversion. Sulfur, a known poison for catalysts even in small quantities, rapidly deactivates the catalytic anode material and degrades the performance of fuel cells.
Although long in history going back 150 years, solid fuel conversion in high temperature fuel cells is gaining renewed interest as concerns about efficient and sustainable energy technologies, clean environment, and climate change grow in importance on the global agenda.
Most of these electrochemical studies employed carbon in molten carbonate fuel cell arrangements, and reported peak power densities of less than 100 mW/cm2 at temperatures above 800° C. In these cells carbon either dispersed in the electrolyte or in the molten tin anode, or as sacrificial anode in bulk form is employed and is electrochemically oxidized. Conversion in these corrosive molten electrolytes, however, pose serious challenges due to slow kinetics in molten media, materials stability issues, wetting of the solid fuel by molten medium, and assuring electrical connectivity among carbon particles.
Solid oxide fuel cell (SOFC)-based arrangements circumvent many of these problems. However, it also introduces new challenges due to dimensional difficulty in supplying the solid fuel particles to the electrochemical reaction sites, called triple phase boundaries (TPB) located at the solid electrolyte/anode interface. This difficulty is largely overcome by gasification of the solid fuel whereby the syngas is electrochemically oxidized at the TPB.
What is needed is a device that is capable of in situ capture and removal of sulfur and sulfurous compounds from the carbon monoxide gas converted from carbon dioxide gas in the gasifier.
To address the needs in the art, an integrated gas fuel cell that includes at least one carbon monoxide gas fueled solid oxide fuel cell, where the carbon monoxide gas fueled fuel cell includes a carbon monoxide oxidizing anode, where an anodic oxidation of the carbon monoxide is provided by the carbon monoxide oxidizing anode in the presence of carbon monoxide, a cathode, an oxide ion transporting electrolyte membrane, and a product stream output from the carbon monoxide oxidizing anode, where the membrane is disposed between the carbon monoxide oxidizing anode and the cathode. The invention further includes a gasifier, where the carbon monoxide gas fueled solid oxide fuel cell is integrated to the gasifier, where the gasifier converts carbon monoxide gas from carbon dioxide gas, where a bed of carbonaceous fuel is provided to the gasifier to facilitate the gas conversion, where the carbon monoxide gas is directly provided as fuel to the carbon monoxide oxidizing anode inside the gasifier, where the at least one carbon monoxide gas fuel cell is physically and thermally integrated with the gasifier within a single chamber, where a Boudouard reaction product carbon dioxide is formed directly inside the at least one carbon monoxide gas fueled solid oxide fuel cell, where when an electrochemical oxidation reaction of the carbon monoxide occurs at a surface of the carbon monoxide oxidizing anode by oxide ions supplied through the oxide ion transporting electrolyte membrane from the cathode for the electrochemical oxidation reaction of the carbon monoxide gas at the surface of the carbon monoxide oxidizing anode to generate electric power, where the electrochemical oxidation reaction includes CO(g)+OOX→CO2(g)+VO{umlaut over ( )}+2e− yielding electrons for the electrical power generation; and a solid sorbent bed inside the gasifier, where the solid sorbent bed is disposed for in situ capture and removal of sulfur and sulfurous compounds and other contaminants from the carbon monoxide gas converted from carbon dioxide gas in the gasifier, where the fuel cell, the gasifier, and the solid sorbent bed are within the same thermal and physical enclosure.
According to one aspect of the invention, the in situ capture and removal by the solid sorbent is further capable of removal of contaminants including phosphorus, mercury, arsenic, cadmium, antimony, selenium, lead, chromium, chlorine, fluorine, silicon and silicon compounds, alkaline earth metals, or, rare earth metals.
In a further aspect of the invention, the solid sorbent materials can include limestone, calcite, caco3, magnesium carbonate, magnesium carbide, barium carbonate, barium carbide, strontium carbide, strontium carbonate, zinc carbonate, zinc carbide, dolomite (CaMg(CO3)2), meionite (Ca3(Al2Si2O8)3CaCO3) and vesuvianite, (Ca19Mg2Al11Si18O69(OH)9), K2CO3, CoCO3, carbonates and carbides of Fe, Mg, Mn, carbonates and carbides of alkaline earth metals, carbonates and carbides of transition metals, NaCl, CaCl2, Na2Co3, and Fe2O3, BaCO3, SrC2SrCO3, ZnCO3, K2CO3, MgCO3, MgC2, BaC2, ZnC2).
a-15b show equilibrium concentrations of H2S and COS in syngas after addition of selected sorbents, according to one embodiment of the invention.
Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will readily appreciate that many variations and alterations to the following exemplary details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
An integrated dry gasification fuel cell (IDG-FC) is provided that does not require the use of steam for the gasification process, while improving system conversion efficiency. The IDG-FC does not require the external use of pure oxygen gas flow into the gasifier along with steam. By using the selective oxygen transport property of solid oxide electrolytes, the IDG-FC eliminates the need for the expensive, energy intensive process of separating oxygen from air prior to the gasification step. The IDG-FC eliminates the need for water resources and the associated costs and environmental impacts, and provides a desirable alternative where water is scarce or too uneconomical to employ.
The present invention has the advantage of utilizing all forms of carbon containing solid fuels in the IDG-FC process. In the context of this invention, a solid carbonaceous fuel is defined as a solid fuel whose carbon content is more than 20% by weight on dry-basis, and preferably more than 40% by weight on dry-basis, where “dry” is upon the removal of water or moisture. Examples of solid carbonaceous fuels include, but not limited to, anthracite, biomass, coal, char, charcoal, forest residues, industrial carbon-containing wastes, lignite, manure, municipal solid wastes, paper pulp, peat, petroleum coke, refuse-derived wastes, saw dust, sewage sludge, solid wastes, or waste plastics. Similarly, examples of biomass include, but nor limited to wood, switchgrass, corn stover, rice straw, olive pits, grass, agricultural residues, and shells of almonds, walnuts, coconuts and other types of nuts.
Another important aspect of the IDG-FC process is the integration of a solid oxide fuel cell (SOFC) in order to improve the overall system conversion efficiency. The solid oxide electrolyte employed in the fuel cell is a selective membrane that only transports oxide ions and supplies the oxygen for the anodic oxidation of CO generated during the dry gasification step. The membrane can be electrolyte material that can include zirconia-based electrolytes, zirconia-based electrolytes doped with alkali or rare earth oxides, zirconia-based electrolytes in a cubic fluorite phase, ceria-based electrolytes, ceria-based electrolytes doped rare earth oxides, ceri-base electrolytes in a cubic fluorite phase, lanthanum gallate-based electrolytes, lanthanum gallate-based electrolytes doped with oxides of aliovalent metals, lanthanum gallate-based oxides of perovskite phase, zirconia-based electrolytes in a pyrocholore phase, lanthanum silicate-based electrolytes, lanthanum silicate-based electrolytes doped with oxides of aliovalent metals, or lanthanum silicate-based electrolytes in an apatite phase.
Predominantly oxide-ion conducting solids have been known to exist for almost a century. Among these solids, zirconia-based electrolytes have widely been employed as electrolyte material for solid oxide fuel cells. Zirconium dioxide has three well-defined polymorphs, with monoclinic, tetragonal and cubic structures. The monoclinic phase is stable up to about 1100° C. and then transforms to the tetragonal phase. The cubic phase is stable above 2200° C. with a CaF2 structure. The tetragonal-to-monoclinic phase transition is accompanied by a large molar volume (about 4%), which makes the practical use of pure zirconia impossible for high temperature refractory applications. However, addition of 8-15 mole % of alkali or rare earth oxides (e.g., CaO, Y2O3, Sc2O3) stabilizes the high temperature cubic fluorite phase to room temperature and eliminates the undesirable tetragonal to monoclinic phase transition at around 1100° C. The dopant cations substitute for the zirconium sites in the structure.
When divalent or trivalent dopants replace the tetravalent zirconium, a large concentration of oxygen vacancies is generated to preserve the charge neutrality of the crystal. It is these oxygen vacancies that are responsible for the high ionic conductivity exhibited by these solid solutions. These materials also exhibit high activation energy for conduction that necessitates elevated temperatures in order to provide sufficient ionic transport rates. The electronic contribution to the total conductivity is several orders of magnitude lower than the ionic component at these temperatures. Hence, these materials can be employed as solid electrolytes in high temperature electrochemical cells.
The chemical potential difference of oxygen across the solid oxide electrolyte is a measure of the open circuit potential given by the Nernst Equation,
E=−(RT/nF)ln(PO
where E is the equilibrium potential of the fuel cell under open circuit conditions, R is the gas constant, F is Faraday's constant, n is the number of electrons per mole (in the case of O2, n=4), and PO
The Boudouard reaction, written in global form in (2), is employed in the IDG-FC process in order to generate carbon monoxide from the carbon dioxide that enters the gasifier.
CO2(g)+C(s)→2CO(g) (2)
This global chemical reaction is kinetically slow compared to carbon (or coal) combustion. However, the presence of large amounts of solid carbon relative to gas allows for a small conversion of the solid carbonaceous fuel to produce large changes in gas concentration. There are several benefits of using the Boudouard reaction products directly in the SOFC. First, no separate gasifier is needed thus reducing geometric space requirements. Secondly, the activity of carbon is fixed at one, and within the bed there is a constant conversion of CO2 to CO. This works to increase the CO activity in the flow direction, thus maintaining the CO concentration and providing more power density.
Referring now to the figures,
As noted in
O2(g)+2VO{umlaut over ( )}+4e→2OOX (3)
Oxygen ions, OOX, are transported across the solid electrolyte of the fuel cell from the cathode to the anode and oxygen vacancies, VO{umlaut over ( )}, migrate under the influence of the chemical potential gradient from the anode to the cathode. The oxygen ions are consumed at the anode via reactions with the CO produced in the coal bed. The overall course of the electrochemical reactions that take place at the anodic surface are described by (4).
CO(g)+OOX→CO2(g)+VO{umlaut over ( )}+2e− (4)
The overall reaction sequence for carbon conversion in the IDG-FC is the sum of reactions (2) and (4), and can be represented as
C(s)+2OOX→CO2(g)+2VO{umlaut over ( )}+4e− (5)
while the oxygen supplied for this conversion is given by reaction (3). So for every carbon atom freshly supplied by coal is reacted, four electrons travel through the external circuit of the fuel cell generating electricity.
Any hydrogen produced in the coal bed will be oxidized electrochemically to water at the anodic surface via reaction (6) generating additional electricity.
H2(g)+OXO→H2O(g)+VO{umlaut over ( )}+2e− (6)
All volatile matter released during heating of the coal will be oxidized at the anode as well.
In the present invention, before the CO2-rich stream is directed to a sequestration site, it is employed in a dry gasification process to generate CO, which is then used in an arrangement of solid oxide fuel cells (SOFCs) to generate electricity. In all the examples and figures presented below, coal is intended as representative of all types of solid carbonaceous fuels, including but not limited to anthracite, biomass, coal, char, charcoal, forest residues, industrial carbon-containing wastes, lignite, manure, municipal solid wastes, paper pulp, peat, petroleum coke, refuse-derived wastes, saw dust, sewage sludge, solid wastes, or waste plastics. Similarly, examples of biomass include, but nor limited to wood, switchgrass, corn stover, rice straw, olive pits, grass, agricultural residues, and shells of almonds, walnuts, coconuts and other types of nuts.
According to one embodiment of the current invention,
Further shown in
Although the gasification reactions are kinetically controlled and hence, relatively slow with respect to consumption of the solid carbon or char particles, a large amount of gas is generated when the gasifier is filled with an ample amount of coal. According to another embodiment, the CO2 gas is restricted from entering said conversion bed 204. The power generated in the fuel cells 202/206 increases both the power output and efficiency of the power plant, and has the potential to offset the cost of the carbon capture and sequestration (CCS) system.
In the embodiments of
The current invention provides many important advantages for improved efficiency in power plants. For example, there is only limited use the CO2-rich stream that is separated from the flue gas of industrial coal-fired boilers and furnaces, these include the current industrial uses of CO2 as in enhanced oil recovery, and extraction or separation processes using supercritical CO2 as a powerful solvent. The current use is to divert this CO2 stream directly to the sequestration site. The present invention provides a technological and business opportunity to further utilize this otherwise waste CO2 stream. The gasification scheme with fuel cell integration disclosed here employs this stream to generate electricity from coal with increased efficiency.
The efficiency of a present-day pulverized coal-fired power plant is about 34%, consequently about 10,600 kJ of coal energy input is needed to produce a kWh of electricity. If the coal used were a typical high-volatile bituminous coal (C/H/O/N/S˜70.0/4.6/6.8/1.3/1.3 percent by mass, with a higher heating value of ˜30 MJ/kg), about 0.90 kg of CO2 would be produced in the process. The newer supercritical pulverized coal-fired power plants have efficiencies near 38%, thereby requiring about 9500 kJ of coal energy input to generate a kWh of electricity, producing about 0.81 kg of CO2. Based on these numbers, a 100 MWe power plant that uses a typical high-volatile bituminous coal as an energy source will produce as much as 25 kg/s of CO2. With 25 kg/s of CO2 and 32.6 kg/s of coal supplied to the gasifier and 19.6 kg/s of O2 supplied through the SOFC, the potential exist to produce 405 MWe of additional power, assuming a cell operating voltage of 0.6 V. This provides over 42% conversion efficiency.
Having a fuel cell 202 located inside the gasifier 204 where the reactive gases are generated has the advantage of maintaining a uniform, high-concentration of CO along the surface of the anode 210 of the fuel cell 202. This enhances the transport of CO through the porous anode 210, thereby increasing the oxygen gradient across the electrolyte 214 of the fuel cell 202, increasing the fuel cell's power density. An advantage of using an internal SOFC 202 is that electric power can be generated as oxygen is supplied to oxidize the fuel.
Advantages of locating the facility at an IGCC coal-fired power plant include: (i) utilization of waste heat from the IGCC plant and (ii) access to and utilization of O2 from the IGCC's existing air separation facility. Furthermore, the IDG-FC process will benefit from the existing infrastructure for coal delivery, storage, and grinding and for the existing power distribution grid.
According to further embodiments, the current invention overcomes a major technical hurdle that stands in the way of utilizing coal and other solid fuels in high temperature fuel cells, specifically anode deactivation due to poisoning primarily by sulfur, sulfurous compounds, as well as other contaminants. One embodiment includes an integrated process scheme and method that consolidates the otherwise separate processes of coal gasification, effective removal of sulfur, sulfurous compounds and also other contaminants from coal syngas, and electrochemical oxidation of the syngas in a fuel cell all in a single-chamber for highly efficient generation of electrical power from coal and other solid fuels, while producing significantly reduced amounts of greenhouse gas CO2. Since sulfur, sulfurous compounds, and other contaminants are removed in situ, the clean syngas can readily be oxidized on the anode surface without significant loss in cell performance. Furthermore, this integrated process scheme enables effective thermal management, eliminates unnecessary heat losses, and maximizes overall conversion efficiency of coal and other solid fuels in high temperature fuel cells. Since no nitrogen enters the process stream, the product of the fuel cell is capture-ready CO2 that can be sequestered readily.
In one embodiment, the invention provides in situ sulfur and sulfurous compound removal from coal gasification products inside a Fluidized Bed Integrated Dry Gasification Fuel Cell (FB-IDG-FC) environment, where a portion of the anode product gases are recycled back into the solid carbon fuel bed, although it is generally applicable to other types of carbon fuel cells including solid oxide electrolyte, molten carbonate electrolyte, molten hydroxide electrolyte, molten metal anode, and their hybrids and combinations. The FB-IDG-FC enables electrochemical conversion of coal and other solid fuels into electrical energy. Since gasification in FB-IDG-FC is accomplished not by steam but by CO2, this approach does not require or consume water, and produces a capture-ready CO2 flue stream.
Until now, coal contaminants, particularly sulfur and sulfurous compounds, stood in the practical development path of carbon fuel cells including FB-IDG-FC, as well as integrated gasification fuel cell (IGFC) technologies in general.
One embodiment of the current invention provides an integrated single-chamber process scheme containing regenerative sulfur sorbents materials for in situ removal of sulfur, sulfurous compounds and other contaminants from coal syngas, and electrochemical oxidation of syngas at the anode surface of FB-IDG-FC, or other types of fuel cells.
Although coal can clearly meet most of the growth in the global electricity demand for many decades to come, it can only do so if highly efficient and environmentally friendly conversion technologies are developed to mitigate the risk of climate change and environmental impact. In this regard, coal conversion in fuel cells offers high efficiencies that are otherwise not possible to achieve by chemical conversion processes.
The efficiency for electrochemical conversion, η, is defined as η=ΔGo/ΔHo, where ΔGo is standard Gibbs free energy and ΔHo is standard enthalpy. In the case of carbon oxidation, the entropy change within practical temperatures remains extremely small (e.g., 0.58 J/K·mol at 1200K). Consequently, both the cell equilibrium potential of about 1V and the theoretical conversion efficiency of 100% are practically independent of operating temperature. High conversion efficiencies naturally translate into proportionately less emissions of capture-ready CO2. It helps reduce the environmental impact and carbon footprint of coal conversion.
Moreover, the FB-IDG-FC arrangement for coal conversion as part of the current invention operates at elevated temperatures (800-900° C.). Such high temperatures offer fuel flexibility that greatly expands the range of solid fuels, and also produce high quality waste heat that can be utilized in a bottoming cycle to boost the overall system level conversion efficiency.
The difficulty of supplying the solid fuel particles to the TPB of a SOFC can be circumvented by steam gasification to generate H2 and CO, or syngas, via the reaction
C+H2O=H2+CO (7)
If desired, H2 and CO can readily diffuse into the anode structure and undergo electrochemical oxidization at the TPB. Steam gasification reaction (7) of carbon is an endothermic process with an enthalpy of 136 kJ/mol at 1100K. So usually, oxygen (or, air) is injected into the gasifier along with steam to sacrificially burn part of the carbon in order to supply the heat necessary to drive the gasification reaction. As in SECA and FutureGen projects, the syngas is shifted catalytically with further addition of steam to produce more H2 for the SOFC.
CO+H2O=H2+CO2 (8)
The net reaction after steps (7) and (8) is given by,
C+2H2O=2H2+CO2 (9)
This is an endothermic reaction with an overall enthalpy change of +102 kJ/mol at 1100K.
In the case of dry gasification using the Boudouard reaction,
C+CO2=2CO (10)
The enthalpy change in (10) is slightly more endothermic than in (9), with a value of +169 kJ/mol at 1100K.
The two fuels, regardless of the type of gasification, when electrochemically oxidized at the anode TPB of the SOFC will undergo the corresponding reactions,
2H2+2O−2=2H2O+4e′ (11)
2CO+2O−2=2CO2+4e′ (12)
Energetics of CO and H2 oxidation, calculated from thermochemical data indicate that the enthalpy change for the CO oxidation reaction is significantly more exothermic than for the oxidation of H2 (i.e., −562 kJ·mol−1 of O2 versus −495 kJ·mol−1 of O2, respectively). So there is no significant energetic difference between steam and dry gasification. Moreover, reactions (11) and (12) exhibit similar values for the standard Gibbs energy (i.e., −185 kJ·mol−1 of O2 versus −186 kJ·mol−1 of O2) at 850° C., indicating almost identical work potentials offered by the oxidation of either fuel. Energetically, steam gasification and dry gasification are quite comparable, and steam gasification provides no significant advantage over dry gasification. Consequentially, the current invention is equally applicable and beneficial to either gasification scheme.
The current invention provides a FB-IDG-FC that is capable of the removal of sulfur and sulfurous compounds specifically, and other contaminants from gasification products of solid fuels including coal, in a single chamber that also houses the gasified coal bed and the fuel cell stack. This single chamber process provides effective thermal management and avoids heat losses, eliminates separate process steps otherwise needed for cooling down and heating up of gas streams for gas cleanup followed up by water-gas shift process, and maximizes system conversion efficiency. It is important to consider that the highly exothermic oxidation of CO to CO2 occurring on the anode surfaces of the fuel cell stack is where heat is primarily generated in the chamber. The general flow of the process stream involves the gasification of the solid fuel either by steam or by CO2 to form syngas, sulfur and sulfurous compound removal in the solid sorbent bed from the syngas, and oxidation of the clean syngas at the fuel cell stack to generate power. Part or all of the oxidation products exiting the fuel cell stack section can be recirculated through the solid fuel bed for further gasification. The physical proximity of the solid fuel bed and the sulfur sorbent bed to the heat source minimizes heat losses, renders effective thermal management easy, and maximizes efficiency. It is with these considerations in mind that the schematic illustrations of the process design are shown in
In
Turning now to the FB-IDG-FC of the current invention. Since the general approach of one embodiment of the current invention that achieves sulfur and sulfurous compound removal and coal conversion in a single process chamber is shown in the figures in the context of the FB-IDG-FC as an example, the following briefly describes the fundamentals of FB-IDG-FC.
The FB-IDG-FC technology basically involves a solid oxide fuel cell arrangement integrated thermally to a Boudouard gasifier containing a bed of coal or other solid carbonaceous fuels. The fuel cell element includes a solid oxide electrolyte, a cathode where oxygen (usually from air) is reduced, and an anode, where the fuel is oxidized, whereby electrons released during the oxidation reaction travels through the external circuit (or grid) to the cathode to complete the loop and participate in reduction reaction for oxygen. The solid oxide electrolyte selectively transports oxide ions through its crystal lattice via oxygen vacancies generated upon extrinsic doping with aliovalent cation(s). The oxygen in air at the cathode side is reduced to oxide ions that migrate to the anode side due to the electrochemical potential gradient.
Part of the CO oxidation product CO2 is recirculated through the coal bed to facilitate gasification via the Boudouard reaction,
C+CO2=2CO (1)
The remaining part of the CO2 is capture-ready and can be collected for sequestration. The CO formed during the Boudouard reaction diffuses to the TPB at the anode interface and is electrochemically oxidized to CO2. The reaction chamber housing the Boudouard gasifier and the fuel cell stack operate in the temperature regime of 800-900° C.
This FB-IDG-FC scheme and other direct carbon fuel cells become viable for practical applications only if the gasification fuel products do not contain contaminants detrimental to the optimum and stable operation of the cell anode material, which is clearly not the case for most solid carbonaceous fuels including coal. So this invention enables the viability of FB-IDG-FC technology as well as other fuel cell approaches for electrochemical conversion of untreated coal and other solid fuels.
Turning now to solid sorbents for sulfur and sulfurous compound removal from dry-gasification gases, calcium-based sorbents such as limestone (nominally calcite, CaCO3) and dolomite (CaMg(CO3)2) are effective in removing SO2 from combustion gases at temperatures as high as 1000° C. Sulfur is captured as both CaSO4 and MgSO4. More than 95% sulfur capture has been reported. Fluidized bed combustion of coal with limestone or dolomite injection to capture SO2 is a well-established, commercially available technology in the oxidizing environment in combustors. However, the reducing atmosphere (i.e., very low oxygen activity) at the anode compartment of FB-IDG-FC and IDG-FC favors the formation of H2S, COS and CS, which poses challenges for removal. The present invention addresses removal of these sulfurous compounds from the syngas.
The sulfur uptake rate by the sorbents is rapid initially but gradually decreases as pore plugging occurs, a consequence of CaSO4 and MgSO4 formation on the periphery of particles hindering the transport of the SO2 to interior surfaces. Sulfur dioxide uptake changes from a fast, chemical-controlled process to a slow, diffusion controlled process as sulfur uptake increases.
Owing to plugging, limestone and dolomite particles are considerably under-utilized in fluidized bed applications, requiring two to four times the stoichiometric requirements for sufficient sulfur removal. Since only a small mass fraction near the external surfaces is utilized, one embodiment of the invention implements submicron size sorbent particles dispersed on suitable porous supports.
Certain additives (e.g., NaCl, CaCl2, Na2CO3, and Fe2O3) increase the sulfur capture capacity of limestones. These additives increase the mean pore size of the calcined particles, permitting deeper penetration of sulfur into the particles, thereby enhancing limestone utilization. This increase is attributed to an increase in the ionic mobility in the CaO crystal lattice due to formation of vacancies in the crystal when Ca is partly replaced by Na. The increase in the mean pore size facilitates reaction with limestone grains throughout the whole particle without rapid plugging of pores, avoiding premature change from a fast chemical reaction controlled process to a slow, solid-state diffusion controlled process.
Limestones and dolomites are also effective sorbents in capturing H2S and COS from coal gasification gases. Sulfur is captured as both CaS and MgS. As with CaSO4 formation in oxidizing environments, the CaS formed can plug particle pores, causing the sulfur uptake rate to gradually decrease in time. Ninety-five percent removal of sulfur from the syngas produced from the gasification of Pittsburgh #8 would yield a gas-phase sulfur level that exceeds 5 ppmv. Sorbents more effective than limestones and dolomites are needed for sustained FB-IDG-FC operation.
Zinc-based materials are also known to be effective sulfur sorbents due to reaction with H2S and COS to form ZnS. However, ZnS is unstable above 600° C., so is not suitable for the high temperature requirements of the proposed FB-IDG-FC system. Therefore, new sorbent materials that bind sulfur strongly at high temperatures as high as 900° C. in reducing environments are needed.
Preliminary calculations using thermochemical data are performed for this purpose to identify potential sorbents that bind sulfur strongly at high temperatures.
Shown in
Thermodynamic calculations indicate that the higher the molar active metal-to-sulfur ratio, the lower the equilibrium levels of H2S and COS. This is the primary reason for the effectiveness of meionite (Ca3(Al2Si2O8)3CaCO3) and vesuvianite (Ca19Mg2Al11Si18O69(OH)9), naturally occurring minerals. Consequently, with higher sorbent levels, (more than 2 moles of sorbent for each mole of gas-phase sulfur to be removed) the concentrations of H2S and COS could be reduced below the levels indicated in
Carbonates and carbides of the alkaline earth metals are ideal for sulfur sorbents for FB-DCFC applications. Carbonates and carbides of the transition metals are suitable at relatively large metal/S ratios. From thermochemical analysis by the inventors, an exemplary list of solid sorbent materials and sorbent additives is provided, where the solid sorbent can include one or any combination of limestone, calcite, CaCO3, magnesium carbonate, magnesium carbide, barium carbonate, barium carbide, strontium carbide, strontium carbonate, zinc carbonate, zinc carbide, dolomite (CaMg(CO3)2), meionite (Ca3(Al2Si2O8)3CaCO3) and vesuvianite, (Ca19Mg2Al11Si18O69(OH)9), K2CO3, CoCO3, carbonates and carbides of Fe, Mg, Mn, carbonates and carbides of alkaline earth metals, and carbonates and carbides of transition metals, and optionally with additives such as NaCl, CaCl2, Na2CO3, and Fe2O3, BaCO3, SrC2SrCO3, ZnCO3, and K2CO3.
The sorbents can be regenerated for use at the commercial scale. With gasification gases, the sulfur compounds are absorbed primarily as sulfides (e.g., CaS, SrS and BaS). The spent sorbents can be regenerated by exposing them to CO2 at low temperatures, producing elemental sulfur and COS. The sulfur in the COS can be recovered by first hydrolyzing the COS to H2S, and then converting the H2S to elemental sulfur via the Claus process. Equilibrium calculations indicate that at temperatures up to about 500° C., over 90% of the sorbent in the BaS and SrS forms will be converted back to the carbonates.
In another embodiment of the invention, the solid sorbent is further capable of removing phosphorus, mercury, arsenic, cadmium, antimony, selenium, lead, chromium, chlorine, fluorine, alkaline earth metals or rare earth metals.
This invention also provides dispersing the solid sorbent in fine particulate form on to porous substrates, where sorbent utilization increases with decreasing grain size. Rather than using course and large size sorbents, it is preferable to employ fine size high surface area sorbents for efficient sulfur take up. However, in this particle size regime from submicron to several microns, it is difficult to handle and manage the solid sorbent in fuel cells, and particularly in the gas flow environment of FB-IDG-FC. In the fuel cell environment, it is important to minimize sorbent entrainment in the gas flow and to avoid agglomeration and sintering of the sorbent particles. These lead to loss of surface area and loss of sorbent mass, which collectively decrease in the sulfur take up capacity over time.
These adverse effects can be minimized by dispersing the sorbent particles on a porous support material such as alumina or zirconia. These materials are commonly employed as catalyst supports in industrial scale. They are also chemically inert and stable in and compatible with the reducing conditions at the anode environment of fuel cells.
In a further embodiment, the invention provides dispersing the sorbents on to the porous supports using two different approaches. The first involves an infiltration method employing soluble salts of the sorbent material. Salts of the cations that make up the sulfur sorbent are dissolved in either water or another suitable solvent. The total amount of dissolved cations in solution may be up to 2M. The internal pores and external surfaces of the support particles are infiltrated by the salt solution, followed by drying for solvent removal and heating in the appropriate environment to form the fine dispersed particles of the desired sorbent material on the external and internal surfaces of the porous support material.
The second approach employs saturated salt solutions or melts of the sorbent material as well as sol-gel synthesis routes to obtain colloidal solution of the sorbent particles in the submicron size. The porous support material is coated by the sorbent by dipping into the colloidal solution, saturated solution or melt, followed by solvent removal, and heating in order to form the sorbent layer or deposit on the external and internal surfaces of the porous support material.
Some advantages of the current invention include a single chamber integrated process scheme for sulfur removal, gasification and fuel conversion; in situ removal of sulfur and other contaminants from syngas; regenerable solid sorbents for sulfur removal; effective thermal management that minimize heat losses and maximize efficiency; high conversion efficiencies nearly doubly that of conventional coal fired power generation; nearly half the amount of CO2 emissions; nearly half the amount of pollutant emissions; no requirement for water if dry gasification is opted; wide spectrum of fuel flexibility ranging from coals, to biomass, to agricultural and forestry waste, to plastic waste, to municipal waste, and other types of solid fuels; modular and scalable; suitable for centralized base load or distributed power generation; and since only the nitrogen in the coal enters the process stream, NOx emissions are low.
The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.
This application is a continuation-in-part of U.S. patent application Ser. No. 12/215,681 filed Jun. 26, 2008, which is incorporated herein by reference. Application Ser. No. 12/215,681 filed Jun. 26, 2008 claims the benefit from U.S. Provisional Patent Application 60/937,459 filed Jun. 26, 2007, which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60937459 | Jun 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12215681 | Jun 2008 | US |
| Child | 14021401 | US |
