Patent Application
20080022593
The annulus between the metal shell and the cathode surface facing the metal shell allows a flow of air. This particular schematic does not illustrate CO2 or captured coal recycling, but these features can easily be incorporated and falls within the scope of this invention.
Unlike conventional electrolysis technologies that need to overcome large open circuit potential barriers to split water or steam, the unique process disclosed here presents a novel concept with dramatic improvement in energy requirement and cost effectiveness. The present invention involves the use of a solid carbonaceous fuel such as coal at the anode compartment and steam at the cathode compartment of a high temperature solid oxide fuel cell. Such a combination provides sufficient gradient in the oxygen activity across the oxide ion conducting ceramic electrolyte membrane that eliminates the open circuit potential barrier of conventional electrolysis processes completely, and allows the system to operate as a fuel cell. Oxygen derived from the reduction of steam at the cathode diffuses as oxide ions downhill towards the anode by the vacancy mechanism through the ceramic membrane, and reacts with the carbon fuel at the anode to form carbon dioxide. The hydrogen gas produced at the cathode leaves the compartment in pure form. The only impurity is unreacted steam, which can further be removed by desiccation or other drying methods to obtain pure hydrogen.
The major impacts of this arrangement on total energy consumption, efficiency and product quality are several. First, the oxygen activity difference across the ceramic electrolyte is sufficiently large at elevated temperatures, which allows operation in a fuel cell mode since this is thermodynamically a downhill process. In other words, the cell not only produces high purity hydrogen at the cathode but also cogenerates electricity at the same time. This desirable outcome is directly the result of the innovative chemistry of the steam-carbon couple employed here, and makes distributed hydrogen production cost effective and highly efficient. Second, the exothermic heat for the oxidation of the carbonaceous fuel at the anode provides a significant portion of the endothermic reaction heat needed for electrolysis of steam at the cathode, thus improving thermal efficiency. Third, since heat is much cheaper than electricity per unit energy basis, this process significantly reduces the cost of H2 production. Fourth, all carbon containing species reside inside the anode compartment so the hydrogen produced at the cathode is collected in pure form with no contamination from CO or CO2. This is because the carbon fuel deployed at the anode side is physically separated by a ceramic membrane from the steam in the cathode compartment. And finally, high operating temperature of this process not only allows faster reaction kinetics but also provides high value waste heat for process make-up energy, thus increasing the thermal efficiency of the overall process.
In the case where larger volumes or higher rates of hydrogen production is desirable, this fuel cell can be run also as an electrolyzer in order to accelerate the production rate. This involves supplementing the driving force for splitting steam by application of an appropriate potential drop across the ceramic membrane from an external power source. Since the novel chemistry employed here entirely eliminates the open circuit potential barrier for electrolysis, it significantly reduces the power requirement for electrolysis and increases electrical efficiency of hydrogen production.
This invention differs also from conventional steam reforming (i.e., coal gasification) technologies in significant ways. Unlike steam reforming where, in addition to steam, the oxygen required for gas phase oxidation of coal (or other solid carbonaceous fuels) is generally supplied in the form of air, this invention employs direct electrochemical oxidation (as opposed to reforming) of the carbonaceous fuel by lattice oxygen supplied through the ceramic electrolyte at the anode. So nitrogen plays no part in this invention and is entirely excluded from the process.
Equally important is the fact that the process disclosed here represents an inherently modular technology that is scaleable up or down and is ideally suited for distributed production of hydrogen on-site at high efficiency and low cost.
A similar approach for hydrogen production from steam was recently proposed by Pham et al [3,4] who exploited the depolarizing property of natural gas deployed to the anode.
The electrochemistry of this invention is founded on sound principles and favorable thermodynamics. The cathode (1), anode (2) and overall cell (3) reactions and their associated Gibbs free energy (ΔG°) and enthalpy (ΔH°) changes in kJ/mole of O2 at 300 K and 1300 K are given below in Table 1.
From the simple relationship ΔG=−nFE, where n is the number of electrons and F is Faraday's constant, one can easily calculate the expected open circuit potential, E, of the cell utilizing these reactions. This is presented in Table 2 below.
Note in Table 1, reaction (3) represents the steam-carbon couple employed in this invention. It is not only energetically spontaneous, but starkly different from conventional steam electrolysis, represented by the energetically unfavorable reaction (1) with a large and positive Gibbs free energy change of 353.12 kJ/mole of O2 at 300 K. In contrast, the negative Gibbs free energy change of −43 kJ/mole of O2 for the net cell reaction (3) indicates favorable thermodynamics for the steam-carbon couple at operating temperatures around 1000° C. This large Gibbs free energy assures that there is sufficient difference in the oxygen activity between the gaseous environment at the cathode and the anode compartments, the latter being much smaller, such that the oxygen extracted from steam gets transported to the carbon at the anode when an external resistive load is connected to the cell to complete the circuit for the electrons. In other words, the system operates as a fuel cell, splitting water to make hydrogen at the cathode and oxidizing carbon to CO2 at the anode, while generating electricity through the external circuit. Furthermore, the thermal energy requirement is nearly five times less for this new process (+104.846 kJ/mole O2) than it is for conventional steam electrolysis (+500.573 kJ/mole of O2) as evidenced by the large difference in the respective enthalpies for reactions (1) and (3) at 1300 K.
Note that conventional steam electrolysis is represented by reaction (1), which is thermodynamically an uphill process with a large and positive Gibbs free energy change of 353.12 kJ/mole of O2, besides being highly endothermic indicated by the large and positive enthalpy of 500.573 kJ/mole of O2. For these reasons, not only large amount of heat is required for reaction (1) to proceed, but also an electrical DC bias significantly larger than the uphill open circuit voltage of −0.915 volt (see Table 2) must externally be supplied in order to overcome this potential barrier and electrolyze the steam.
Conversely, the steam-carbon couple of reaction (3) totally and completely eliminates this potential barrier, and in fact, tilts it in the opposite direction that allows the system to operate in a fuel cell mode with a downhill open circuit voltage of +0.111 volt. When higher production rates for hydrogen are required, an external bias of suitable magnitude, but possibly less than 2 volts, may be applied to supplement this favorable gradient in the oxygen activity across the ceramic membrane. Naturally, renewable energy resources may be used to provide 100% of this energy requirement with no loss in system efficiency. Wind, solar and hydro resources may be used to supply this supplemental electric power.
As suggested by the data of reaction (3) in Tables 1 and 2, the difference in the oxygen activity across the ceramic membrane between the anode and cathode compartments show a significant temperature dependence primarily due to the large entropy change for reaction (1). So it may be preferable to operate this system at high temperatures, even above 1300 K, so that the oxygen activity difference between the anode and cathode increases further, providing a larger open circuit voltage, a steeper activity gradient for faster transport, and proportionately higher hydrogen production rate.
Another advantage higher temperatures may provide lies in the fact that partial oxidation of carbon to CO is thermodynamically more favorable than complete oxidation to CO2 at these elevated temperatures (see
A comparison of technical advantages of this novel process relative to conventional steam electrolysis is summarized below:
An important component of the steam-carbon cell disclosed here is the solid oxide electrolyte that facilitates selective oxide ion transport. It helps slit up steam, extracts the oxygen and supplies the oxygen to the anode for the oxidation of the carbonaceous fuel and other reactants (such as hydrogen, sulfur etc present in the fuel, as in coal).
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 (SOFC).
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 m % 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 [5] 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(PO2′/PO2″) (6)
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 PO2 denotes the partial pressure of oxygen. At the elevated temperatures where the steam-carbon cell normally operates, the oxygen activity at the steam side (i.e., cathode) is greater than at the carbon fuel side (i.e., anode), giving rise to a downhill open circuit potential (see Table 2) that provides the driving force for the oxide ions to diffuse from the cathode side to the anode.
The electrochemical production of hydrogen involves a high temperature steam-carbon cell that features an oxide ion selective solid electrolyte that extracts the oxygen from steam and supplies it to the anode where it is consumed for the electrochemical oxidation of carbon in the solid fuel. Granulated or pulverized carbon fuel, such as coal, is introduced into the anode compartment of the cell with or without other solid constituents, such as capturing agents for the mineralization of the CO2 and SO2 produced by the oxidation of the solid carbon fuel, which usually contains sulfur and other impurities.
The open circuit voltage of the steam-carbon cell is determined primarily by the carbon-oxygen equilibrium at the anode, and the hydrogen-steam equilibrium at the cathode.
The steam-carbon cell can be designed in various geometries. Only two will be discussed here, namely, the tubular and flat-plate geometries. Naturally other geometries are also possible, and are not excluded from the scope of this invention.
A typical schematic of the tubular cell involves a thick porous ceramic cathode that provides mechanical integrity for the multilayer structure. The thick porous cathode configuration is commonly referred to as a cathode-supported tube. Other tubular geometries, including flattened tubes etc. are also possible. A thin, impervious layer of oxide ion conducting ceramic membrane such as yttria stabilized zirconia (YSZ) solid electrolyte is coated on the outer surface of the cathode tube. Another thin but preferably porous layer that serves as the anode is then deposited on top of the YSZ as the outermost layer. A schematic of the tube structure and its operating principle is shown in
Alternative to the cathode-supported tube geometry is the anode-supported tube configuration, where, this time the anode 202 is a thick porous structure to provide for mechanical integrity, and is usually made of a mixed conducting oxide or a suitable cermet that is catalytically active and electronically conductive enough to transport both electrons and oxide ions 208. This time, the thin YSZ and cathode 206 layers are coated on the surface of the anode-support tube to complete the cell configuration.
YSZ is the preferred solid electrolyte 204 for its high stability and ionic conductivity. However, scandia stabilized zirconia (SSZ) has an even higher conductivity than its yttria counterpart and, hence, may be more beneficial [5]. Also, it is possible to employ tetragonal zirconia which is known to possess higher conductivity and better thermal shock resistance than cubic zirconia electrolytes. Similarly, other oxide ion conductors such as doped cerates (e.g. Gd2O3.CeO2) and doped gallates (e.g., La2O3.Ga2O2) can also be considered for the thin electrolyte 204 membrane.
The inner surface of the cathode 206 support tube is in contact with steam 210 to furnish the oxygen needed for the oxidation reaction at the anode 202, while the outer surface of the anode 202 is in direct, physical contact with the solid carbonaceous fuel. The YSZ solid oxide electrolyte 204 film in between serves as a selective membrane for transporting oxygen ions 208 only. The water molecule in steam 210 picks up electrons from the external circuit through the cathode 206 and is reduced to hydrogen gas and an oxide ion. The latter is then incorporated into the YSZ solid electrolyte 204 and diffuses downhill towards the anode 202 where it oxidizes the carbon to CO2.
Using Kroger-Vink defect notation, the electrochemical reduction of steam 210 at the cathode 206 takes place as follows:
2H2O(g)+2Vo{umlaut over ( )} (YSZ)+4e′ (electrode)=H2 (g)+2Oox (YSZ) (7)
While the oxygen vacancies, Vo{umlaut over ( )}(YSZ), migrate under the influence of the chemical potential gradient through the YSZ solid electrolyte 204 film from the anode 202 to the cathode 206, oxygen ions 208 are transported in the reverse direction from the cathode 206 to the anode 202 where they participate in the electrochemical oxidation of carbon.
C+2Oox (YSZ)=CO2 (g)+2Vo{umlaut over ( )} (YSZ)+4e′ (electrode) (8)
The electrons released during the oxidation reaction at the anode 202 travel through the external circuit (i.e., the power grid) towards the cathode 206, producing useful electricity. The oxygen chemical potential difference between the anode 202 and the cathode 206 provides greater than 0.1 volt of open circuit voltage (see Table 2).
For obtaining high conversion efficiency, it is desirable that the oxidation reaction of carbon primarily takes place at the anode 202 surface by lattice oxygen (i.e., Eq. (3)). The presence of lattice oxygen is preferred in embodiments involving direct physical contact of the anode 202 surface with the particles of carbon-containing fuel.
Expressed this time in ionic notation, the desired reaction (3) can be rewritten as
C(s)+2O2−(YSZ)=CO2(g)+4e′(electrode) (9)
There are several other gas phase reactions possible at the solid carbon-gas interface, namely,
C(s)+½ O2(g)=CO(g) (10)
C(s)+O2(g)=CO2(g) (11)
as well as the gas phase oxidation of CO by molecular oxygen supplied from the cathode 206 through the YSZ electrolyte 204.
CO(g)+½ O2(g)=CO2(g) (12)
and the reverse Bouduard reaction that leads to carbon precipitation
2 CO(g)=C(s)+CO2(g) (13)
The desired reaction is (9) for obtaining maximum conversion efficiency. Therefore it is important to bring coal particles in direct physical contact with the anode 202 surface, i.e., they should be touching each other. This naturally necessitates that the coal bed and the anode 202 surface, and hence the ceramic cell tubes, must reside in the same temperature zone, and not thermally and spatially separated from one another.
This is achieved by immersing the solid electrolyte 204 containing cell tubes inside the pulverized carbon fuel bed, where the bed and the tubes reside in the same thermal zone. The coal particles touching the anode 202 surface are readily oxidized by the oxygen provided at the anode 202 surface through the solid electrolyte 204 membrane. Since the electrolyte 204 membrane is selective only to oxygen, the hydrogen gas produced from steam 210 stays behind in the cathode 206 compartment. This way, hydrogen never sees or interacts with carbon directly. In other words, this cell produces pure hydrogen with no contamination from CO2 and CO, the latter is a critically detrimental impurity that limits use of hydrogen for fuel cell applications.
Moreover, this invention makes it easy and inexpensive to capture and sequester or mineralize the CO2 since the anode flue gases from the steam-carbon cell is primarily CO2. This point is important for compliance with Kyoto protocols regarding greenhouse gas emissions.
For cases where the abrasive action of the solid carbonaceous fuel on the anode 202 surface may be a series issue, an alternative design concept is to utilize a detached arrangement where the solid carbon fuel is physically separated from the ceramic cells, i.e., the ceramic tubes and the solid carbon fuel do not make direct physical contact.
The carbon-containing fuel comprises any carbon rich substance including: all grades and varieties of coal, charcoal, peat, petroleum coke, oil sand, tar sand, coke, char, carbon produced by pyrolysis of a carbonaceous substance, waste plastics, and biomass. For brevity, the carbon-containing fuel substances listed above may be referred to as “coal” in this document.
Optionally, an external voltage may be applied between the anode and cathode. This external voltage increases the oxygen transport rate through the solid oxide electrolyte, and drives the rate for hydrogen production higher.
Several different design alternatives are provided as examples to achieve direct, physical contact of the anode 202 surface with the coal particles. Other design alternatives are also possible. These designs may or may not involve recycling or circulation of an inert gas, such as He, Ar, N2 or preferably, CO2, to agitate the coal bed to enhance mass transport of reaction products away from the anode 202 surface so as not to block, hinder, or slow down the next unit of oxidation reaction taking place.
The steam-carbon cell operates in the temperature range 500 to 1300° C. This range provides the spectrum for the optimum operation of the cell, hydrogen production, carbon oxidation, and electricity generation. Thermodynamically, partial oxidation of carbon to CO has a strong temperature dependence and is favored at high temperatures, whereas complete oxidation to CO2 is thermodynamically favored at lower temperatures as is depicted in
There is another consideration that affects the operating temperature of the system. That has to do with the transport of oxide ions 208 through the ceramic electrolyte 204 membrane, which is a highly thermally activated process as discussed earlier, and prefers high operating temperatures. The oxide ions 208 transported across the membrane oxidize the carbon at the anode 202 compartment to generate electricity. In order to produce practically significant and useful levels of electrical current, which is intimately associated with the transport rate of oxide ions 208 through the membrane via the well-known Faraday's equation, the steam-carbon cell may operate between 600 and 1100° C., where the ionic conductivity of the electrolyte 204 membrane is larger than 10−4 S/cm. To obtain even better performance, the steam-carbon cell may optionally operate in a temperature range of 700 to 1000° C.
The schematic of the agitated bed steam-carbon cell shown in
Variant modes of the stalactite design are shown in
Another design concept shown in
Yet another design concept is the stalagmite configuration of the ceramic tube cells as depicted in
Other design concepts may include shell-and-tube type design where the pulverized coal bed is outside the tube in touch with the anode 202 surface as illustrated in
Another variant of this is the inverted shell-and-tube type design (i.e., inverted version of
Although similar in operation, another design geometry involves the use of flat or corrugated planar ceramic membrane assemblies. These are multilayered structures that consist of porous anode 202 (or cathode 206) support plates coated with thin impervious layers of the oxide ion conducting solid electrolyte 204 membrane, over which there is coated another thin but porous electrode layer to complete the cell structure. The plates are stacked in parallel fashion in the cell as shown in
The steam-carbon cell has another unique characteristic. Operationally, it can be operated in a fuel cell mode where both hydrogen 318 and electricity are generated simultaneously, or, if large production rates are required, it can be operated much like a steam electrolyzer where an externally applied voltage drives the oxidation and reduction reactions at the electrodes faster. In the electrolysis mode, this process offers dramatic savings in the electricity consumption and cost for the cell operation since the unique chemistry of the steam-carbon cell diminishes the open circuit potential entirely, which otherwise would have to be overcome to extract the oxygen from the H2O molecule.
Please note: although the figures depict single ceramic tubes or flat plates in carbon beds, in practice, there may be a multitude of such ceramic tubes or flat plates in each carbon bed. The number of tubes or plates dictated by the level of desired power produced.
Yet another mode of operating the steam-carbon cell is to couple it to CO2 and SO2 sequestration either inside the cell in the anode 202 compartment or outside downstream of the cell. Sequestration of CO2 and SO2 can be achieved inside the bed by introducing gettering agents such as calcium oxide, magnesium oxide, dolomite, or a variety of magnesium silicates (e.g., olivine, serpentine, talc) mixed with pulverized coal and fed directly into the bed. These inorganic compounds may be used to mineralize carbon dioxide. The gettering agents readily react with the oxidation products CO2 and SO2 inside the cell forming solid carbonates and sulfates which eventually settle to the bottom of the carbon bed where they can be extracted. Or the flue gas leaving the cell can be treated with these gettering agents in a separate containment outside where the reaction products CO2 and SO2 can easily be mineralized by fixing them as solid carbonates and sulfates. Some of the relevant reactions for mineralization (also called carbonization) are provided below as examples.
Lime: CaO+CO2═CaCO3 (14)
Magnesia: MgO+CO2═MgCO3 (15)
Serpentine: Mg3Si2O5(OH)4(s)+3 CO2(g)=3 MgCO3(s)+2 SiO2(s)+2 H2O (16)
Olivine Mg2SiO4(s)+2 CO2(g)=2 MgCO3(s)+SiO2(s) (17)
There are many embodiments of the present invention:
It will be apparent to one skilled in the art that the described embodiments may be altered in many ways without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their equivalents.
