BIOELECTROCHEMICAL TREATMENT OF GASEOUS BYPRODUCTS

Abstract

The present invention relates to a method for producing electrical energy or hydrogen gas from a gas stream containing one or more gaseous compounds that are oxidatively degradable by microbes, the method comprising contacting the gas stream with an anode of a bioelectrochemical device, said anode containing said microbes which oxidatively degrade one or more of said gaseous compounds while producing electrical energy or hydrogen gas by said oxidative degradation, wherein: (i) said anode is sufficiently porous such that gas is permitted to flow therethrough, (ii) said anode contains on its surface and/or interior portions a proton-conducting medium, and (iii) said anode is in electrical communication with a cathode of the bioelectrochemical device. The invention is also directed to a bioelectrochemical device (e.g., microbial fuel cell) configured to accomplish the above method.

Description

This invention was made with government support under Contract Number DE-AC05-000R22725 between the United States Department of Energy and UT-Battelle, LLC. The U.S. government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the field of bioelectrochemical treatment of gaseous products. More particularly, the present invention relates to bioelectrochemical (e.g., microbial fuel cell) treatment of sulfide-containing or carbon monoxide-containing gas streams, such as those occurring from a petroleum refining process or syngas-producing process.

BACKGROUND OF THE INVENTION

Gaseous streams emanating from industrial processes often contain numerous gaseous byproducts that are processed as waste or directly released into the environment. The gaseous waste is often a significant financial liability since the waste is typically required to be processed by some measure before being released or disposed of. The gaseous waste also presents an environmental liability since either raw or processed gaseous waste ultimately enters the environment. For this reason, there would be a significant advantage if such gaseous waste could be utilized by a process which uses such gaseous waste to produce energy. There is, therefore, an ongoing effort to find new ways of efficiently utilizing gaseous waste products in a process that produces a net energy output.

For example, sulfurous compounds and materials are commonly found in unrefined petroleum sources. These compounds are undesirable in petroleum products for a number of reasons, including their ready conversion to sulfur dioxide (SO₂) gas when included in fossil fuels undergoing combustion (as in an automobile or power plant), as well as their ability to function as poisons to noble metal catalysts used in catalytic reforming units and catalytic converters. Accordingly, these sulfurous compounds are desirably removed during the petroleum refining process.

The most widely used process for removing sulfurous compounds from petroleum products is the hydrodesulfurization (HDS) process. In the HDS process, petroleum intermediates are subjected to hydrogenation conditions, typically by contacting the petroleum intermediates with a specialized catalyst (typically, an alumina carrier impregnated with cobalt-modified molybdenum sulfide) in a fixed-bed reactor at elevated temperatures (e.g., 300 to 400° C.) and elevated pressures (e.g., 30 to 130 atmospheres). The predominant sulfurous product resulting from the HDS process is hydrogen sulfide (H₂S), a toxic and highly malodorous gas, the bulk of which is desirably converted to other sulfur-containing compounds that are either more commercially useful or more convenient to handle and transport.

The hydrogen sulfide produced by the HDS process is most commonly processed by the Claus process. In the Claus process, H₂S is converted to elemental sulfur in a multi-step process. One step is a thermal step (conducted typically at above 850° C.) in which a portion (approximately one-third) of the H₂S is converted to SO₂ by the following reaction:

2H₂S+3O₂→2SO₂+2H₂O

Another step in the Claus process is the reaction of H₂S and SO₂ to form elemental sulfur.

2H₂S+SO₂→3S+2H₂O

The above reaction is typically conducted at elevated temperatures in several stages (e.g., 315-330° C. in a first stage, 240° C. in a second stage, and 200° C. in a third stage) in the presence of a catalyst (e.g., activated alumina or titanium dioxide).

An alternative process for removing sulfurous compounds from petroleum products is the Merox process. In the Merox process, petroleum feedstock is treated with caustic solution (typically NaOH solution) which reacts with thiols (represented herein as RSH where R is an organic group) therein according to the following reaction:

2RSH+2NaOH→2NaSR+2H₂O

If only extraction of the sulfurous compound is desired, the process can be halted at this stage. If precipitation of solid sulfurous compounds is desired along with regeneration of the caustic, a second step is employed according to the following reaction to produce insoluble disulfide compounds:

4NaSR+2O₂+2H₂O→2RSSR+4NaOH

The Merox process also generally requires a separate pre-treatment (pre-washing) step for the removal of any hydrogen sulfide that may be present in the petroleum feedstock because H₂S will poison the circulating caustic solution used for reaction with thiols. Typically, this is accomplished by pre-treating the feedstock with a caustic according to the following reaction:

H₂S+NaOH→NaSH+H₂O

The HDS process, the Claus process, and Merox process (and other processes) are energy demanding and rely on fossil fuels. Usually, extraordinary amounts of water are used in the process, and a large amount of this water is wasted or is contaminated and must be treated as waste. Means for recovering or cleansing water add to the energy demand. For example, the Claus process is known to generate over 2.6 tons of steam for each ton of sulfur produced; and in 2005, approximately 64,000,000 metric tons of sulfur was produced from petroleum refineries.

Carbon monoxide-containing gas streams can either be an industrial waste stream or produced specifically for generation of energy. Some examples of carbon monoxide-containing waste gas streams include those emanating from coal processing or coal burning plants, and other combustion processes. Processes designed for generation of energy via gasification of coal and/or biomass produce a mixture of hydrogen and carbon dioxide described as synthesis gas (syngas) or producer gas, depending on the hydrogen content. The syngas may also be produced by steam-methane reforming. Typically, a complex and energy intensive process, such as a catalytic water-gas shift (WGS) reaction, is used to convert the carbon monoxide to carbon dioxide and hydrogen according to the general reaction scheme:

CO+H₂O→CO₂+H₂

In a typical WGS process, carbon monoxide (CO) and water (i.e., steam) are reacted in at least two major steps: the high temperature shift (HTS) step conducted at a temperature of about 350-370° C. in the presence of a metallic catalyst (typically, iron oxide promoted with chromium oxide), followed by a low temperature shift (LTS) step conducted at a temperature of about 190-220° C. in the presence of a metallic catalyst (e.g., copper on a mixed support of zinc oxide and aluminum oxide). The LTS step functions to achieve higher conversions of CO to H₂ than that attainable in the HTS step.

Accordingly, there would be a benefit in a method generally useful in deriving a net surplus of energy from common gaseous waste products, such as mercaptans and carbon monoxide. The process preferably converts these gaseous byproducts to harmless products by a process that is less energy demanding and more cost efficient. There would be an added benefit to such a process which also produces significantly less waste (including aqueous waste) and utilizes less water. There would be an additional benefit in such a process which also does not rely on fossil fuels, i.e., which operates by renewable means.

SUMMARY OF THE INVENTION

The present invention is directed to a novel method for utilizing gaseous compounds that are oxidatively degradable by microbes (for example, gaseous mercaptan compounds and/or carbon monoxide, i.e., CO) from a gas stream containing such compounds for the purpose of producing energy therefrom (i.e., electricity or hydrogen gas), and in the process, reducing the levels of these byproducts. The method involves treating the gas stream with a bioelectrochemical system or device (i.e., BES) by contact of the gas stream with an anode therein which contains microbes capable of oxidatively degrading one or more gas compounds, preferably mercaptan compounds and/or CO, while producing electrical energy or hydrogen therefrom. The anode is suitably porous to permit flow of the gas stream therethrough such that the gas stream interfaces with the surface of the anode. The anode also contains therein a proton-conducting medium such that protons being produced by the oxidative degradation of the gaseous compounds can migrate from the anode to the cathode. The proton-conducting medium also functions to solubilize the gaseous compounds, such as H₂S, other mercaptans or CO, in the gas stream for more efficient processing by microbes on the anode surface. In the use of the device for electricity production, the anode is in electrical communication with a cathode of the device. A cation-permeable material is preferably in direct contact with the anode and cathode and separates the anode and cathode such that the gas stream contacting the anode does not contact the cathode.

The produced electrical energy can be used to power any suitable process, and more preferably, a process from which a mercaptan-laden or CO-laden gas stream emanates, e.g., a fuel refining, biomass-processing, or steam-methane reforming process. When applied to a mercaptan-laden gas stream, the method is particularly directed to the case where the mercaptan compound is hydrogen sulfide (H₂S).

The invention is also directed to a device (or system) containing one or more BESs for accomplishing the inventive method. The invention is also directed to methods of operating the BES for optimal performance in such a process.

In another embodiment, the BES device is operated in such a manner that hydrogen is produced at the cathode during microbial consumption of one or more mercaptan compounds or CO at the anode. Such a device is also referred to herein as a microbial electrolysis cell (MEC). The hydrogen gas may be utilized as a chemical commodity or processing reactant, or directly as a fuel for the production of electricity to power one or more processes, preferably one or more processes from which the mercaptan- or CO-laden gas stream emanates.

In still another embodiment, the BES device is operated in such a manner that the cathode electrochemically reduces one or more electrochemically reducible species, such as a nitrate, chlorate, or reducible metal species, during microbial consumption of one or more mercaptan compounds or CO at the anode.

Thus, as will be described in further detail below, the method advantageously provides a method for removing mercaptans (particularly, hydrogen sulfide) and/or CO in a gas stream by a renewable process which is significantly less energy demanding and more cost efficient than current technologies. The method also advantageously converts mercaptans and CO to environmentally benign and/or useful products while producing significantly less waste (or no waste) and using minimal amounts of water.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Depiction of a horizontal gas-phase BES configuration with use of a cation-exchange membrane for separating anode from cathode.

FIG. 2. Depiction of a vertical gas-phase BES configuration with use of a filter or membrane for separating anode from cathode.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the invention is directed to a method for removing oxidatively degradable gaseous compounds, such as mercaptan (i.e., thiol) compounds and/or CO, from a gas stream by treatment of the gas stream with one or more BESs. The BES considered herein is any reactor system in which electrons are transferred by microbes to a conductive surface (i.e., by an exoelectrogenic mechanism) during the course of microbial consumption of gaseous byproducts. The transferred electrons are directed to the production of energy, e.g., either electricity or hydrogen production. In a particular embodiment, the BES is a microbial fuel cell (MFC) or microbial electrolytic cell (MEC).

The method preferably involves flowing a mercaptan- or CO-laden gas stream through a porous anode of the BES, wherein the anode contains microbes residing thereon capable of utilizing the mercaptan compounds or CO as a nutritive and energy source. Catabolism of mercaptan compounds by the microbes results in their oxidative degradation into, typically, non-gaseous (solid or liquid) compounds that are less toxic and more easily storable and transportable. Catabolism of CO by the microbes at the anode occurs according to the partial reaction:

CO+H₂O→CO₂+2H⁺+2e⁻

wherein either electricity is produced with no release of H₂ gas (i.e., under aerobic conditions) or H₂ gas is produced at the cathode instead of electricity (i.e., under anaerobic conditions). Furthermore, as described in further detail below, one or more reducible species may be reduced by the produced electrons instead of or in addition to production of electricity or H₂ gas.

The gaseous mercaptan compounds are any compounds that contain one or more thiol (—SH) groups. Typically, the most predominant and common gaseous mercaptan compound in a gas stream is hydrogen sulfide (H₂S). In one embodiment, the mercaptan compound is essentially only H₂S (e.g., at least 99% by volume with respect to other mercaptan compounds). In another embodiment, the mercaptan compounds include predominantly H₂S in combination with organomercaptan compounds. Some examples of organomercaptan compounds include methanethiol, ethanethiol, 1-propanethiol, 2-propanethiol, ethanedithiol, thiophenol, and the like. In a less common embodiment, the mercaptan compounds include one or more organomercaptan compounds in a greater amount than H₂S.

A gas stream containing one or more mercaptan compounds can be any gas stream requiring removal of mercaptans. Particularly considered herein is a gas stream emanating from a petroleum refining process. The petroleum refining process can be, for example, a hydrodesulfurization, pyrolysis (i.e., fluid catalytic cracking), Claus, or Merox process. The petroleum source being refined can be, for example, crude fossil fuel or petroleum (i.e., feedstock), petroleum coke, crude oil, natural gas, liquified natural gas (LPG), naphtha, kerosene, jet fuel, bitumen, gasoline, fuel oil, diesel fuel, coal, or a derivative or modified form thereof. The mercaptan-laden gas stream can reach the BES directly from the mercaptan-evolving process, or alternatively, after any number of other process steps. Other process steps may include, for example, heating, cooling, pressurization, depressurization, liquid extraction, gas separation, filtration, adsorption, and the like. A mercaptan-laden gas stream can result from other processes as well, any of which can also be treated by the BES described herein. Other mercaptan-producing processes include, for example, production of chemicals (e.g., plastics, polymers, textiles, pharmaceuticals, fine or bulk chemicals) and combustion of fossil fuels (e.g., as occurs in power plants and automobile use).

The microbes of the BES are capable of oxidatively degrading one or more oxidatively degradable gaseous compounds, such as one or more mercaptan compounds and/or CO. The mercaptan compounds can be partially oxidized to such species as elemental sulfur or sulfur oxides. More preferably, the mercaptan compounds are completely oxidized to sulfate compounds. These compounds are typically solid in form and soluble in water, and can thus be readily removed upon their formation from a gas stream. For example, H₂S can be oxidatively treated by a BES according to the reaction shown below, wherein the sulfate can be removed via lime precipitation. In addition, the water used in the process can be recycled by removal of remaining dissolved calcium with a cation-exchange resin.

H₂S+4H₂O→10H⁺+SO₄²+8e⁻

The gas stream entering the BES can have any particular concentration level or range of concentrations of mercaptan compounds or CO. The total or individual concentration of one or more mercaptans or CO can be, for example, at or less than (or at or greater than) 5 ppm, 10 ppm, 20 ppm, 30 ppm, 40 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 500 ppm, 750 ppm, 1,000 ppm, 1,250 ppm, 1,500 ppm, 2,000 ppm, 2,500 ppm, 3,000 ppm, 5,000 ppm, 7,000 ppm, or 10,000 ppm. Any ranges resulting from any of the values given above are also considered herein. One or more of the BESs (or MFCs) being used for treating the mercaptans or CO may properly function (or most effectively function) within a select range of mercaptan or CO concentration, or below a specified concentration, or above a specified concentration. Accordingly, the invention also contemplates using more than one BES (or MFC) in a process, wherein one BES (i.e., a first BES) may be used to treat a gas stream containing a high concentration of mercaptans or CO (e.g., at or greater than 1,000 ppm) while another BES (i.e., a second BES) is used to treat a gas stream containing a low concentration of mercaptans or CO (e.g., at or less than 1,000 ppm). The BESs or MFCs may be interconnected such that the low-mercaptan or low-CO gas stream being fed into the second BES or MFC is processed gas exiting the first BES or MFC.

By “removal” or “cleansing” of the gas stream of mercaptan compounds or CO is meant that the concentration (i.e., level) of one or more mercaptan compounds or CO is appropriately reduced to meet the needs or requirements of a particular application. For most applications, the gas stream is cleansed such that the gas stream experiences at least a 20% reduction in one or more mercaptan compounds or CO. More preferably, the gas stream is cleansed such that the gas stream experiences at least a 30%, or 40%, or 50%, or 60%, or 70%, or 80%, or 90%, or 95% reduction in one or more mercaptan compounds or CO. Even more preferably, the gas stream is cleansed such that one or more of the mercaptan compounds or CO have been substantially removed from the gas stream, i.e., greater than 95% reduction in concentration (more preferably, at least 98%, 99%, 99.5%, or 99.9% reduction in concentration). For example, in different embodiments, the cleansed gas stream may contain a mercaptan or CO concentration up to or less than 200 ppm, 150 ppm, 100 ppm, 50 ppm, 30 ppm, 20 ppm, 15 ppm, 10 ppm, 5 ppm, 1 ppm, 0.5 ppm, 0.1 ppm, or 0.05 ppm.

As used herein, and as generally understood in the art, “microbial fuel cells” (i.e., MFCs) are fuel cells which operate by using microbes (i.e., microorganisms) that possess the ability to donate electrons to the anode of the fuel cell during microbial oxidative degradation of compounds in order to produce electricity. Such microorganisms are known as exoelectrogenic organisms. Exoelectrogenic organisms can donate electrons to the anode in either of two ways: via mediators (e.g., the numerous dyes used in the art for this purpose) or in the absence of mediators (i.e., a mediator-less MFC).

An MFC contains an anode and a cathode that are in electrical communication, typically by interconnection with an electrical conductor (e.g., by a metal wire). The microorganisms in contact with the anode oxidatively catabolize the mercaptan compounds or CO to produce electrons and protons (H⁺ ions), as well as oxidized species, e.g., oxidized mercaptan compounds or CO₂, and possibly, oxidized hydrocarbon material. The electrons are attracted to the anode and travel to the cathode. At the same time, the produced protons migrate toward the cathode. At the cathode, oxygen gas (typically from air) reacts with the electrons and protons to produce water according to the reaction:

O₂+4H⁺+4e⁻→2H₂O

According to the present invention, a gas stream (as opposed to a liquid) is being processed by the MFC. In order to effectively and efficiently process a gas stream by direct interface with the anode, the anode is sufficiently porous such that flow of the gas stream is permitted through the anode. The porosity of the anode permits gas flow therethrough such that gaseous components (e.g., mercaptan and/or CO components) of the gas stream intimately contact surface portions of the anode and allow for efficient interaction of the microbes residing thereon with the gaseous components. The gas stream can also be made to recirculate through or around the anode by convection.

The porosity of the anode is such that the gas stream can flow through the anode at a viable gas flow rate. Preferably, the porosity of the anode permits a gas flow rate of at least 0.5 mL/min, 1.0 mL/min, 2.0 mL/min, 3.0 mL/min, 4.0 mL/min, 5.0 mL/min, 6.0 mL/min, 7.0 mL/min, 8.0 mL/min, 9.0 mL/min, or 10 mL/min. In particular embodiments, the porosity of the anode can permit higher flow rates, e.g., at or above 15 mL/min, 20 mL/min, 25 mL/min, 30 mL/min, 35 mL/min, 40 mL/min, 45 mL/min, 50 mL/min, 100 mL/min, 200 mL/min, 300 mL/min, 400 mL/min, or 500 mL/min. The anode preferably has a porosity value of at least about 0.3 (and more preferably at least about 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9), wherein the porosity value recited herein is calculated as the volume of void space over the total (i.e., bulk) volume. The foregoing porosity values can be recited as percentages (e.g., 0.3 corresponds to 30%).

The sizes of the pores present in the anode can be any suitable pore size or pore size range. The pore sizes can be, for example, at or above (or at or below) 20 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 750 nm, 1 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 200 μm, 300 μm, 400 μm, or 500 μm. Any ranges resulting from any of the above exemplary values are also considered herein. The pore size distribution can be essentially monodisperse (e.g., a 10% or less deviation from a particular pore size value), or alternatively, polydisperse. The anode can also possess a dual pore size distribution. For example, the anode may possess a dual pore size characterized by the presence of one distribution of pores of a first size or size range and a second distribution of pores of a second (different) size or size range. The pore size distribution can also be characterized by more than two (i.e., multiple) different sizes or size ranges. Dual or multiple pore size distributions can be appropriately selected to control or modify the flow of the gas stream through the anode.

Other aspects of the anode that can be modified, optimized, or controlled include the gas flow rate/retention time, gas recirculation (i.e., preferably for complete consumption of gaseous reactant), the gas:liquid flow rate ratio (i.e., to control liquid level), liquid recirculation, and pressure in the anode chamber. The anode is preferably under a slight positive pressure to facilitate liquid flow. The anode is also preferably designed to include two electrode materials, one in the bulk anode chamber and the other lining the membrane. The ratio of these two chambers is typically ˜10:1. Preferably, two zones are provided, a gas-continuous zone and a liquid-continuous zone. This is preferably done by controlling placement of gas spargers and porosity of the anode electrode material alongside the membrane. A low porosity hydrophilic material promotes formation of a liquid-continuous zone given that the liquid flow rate is sufficiently high. The liquid is then recirculated into the gas-continuous zone. By this preferred method, proton transport and gas-liquid contact (in gas-continuous zone) can be significantly improved.

The gas outlet preferably includes a pressure control valve which opens only if gas pressure rises above a certain threshold, e.g., 0.1 atm. This would enable a positive pressure on the liquid, thereby facilitating its flow out of the chamber via the electrode zone alongside the membrane, thus ensuring efficient gas utilization. The gas is preferably distributed via a distributor throughout the anode chamber. Similarly, the liquid is preferably introduced via a distributor to provide a trickle bed operation that allows for maximum gas-liquid contact.

The surface area to volume ratio (specific surface area) for the anode is preferably at least 1,000 m²/m³. As understood in the art, the anode volume being considered in calculating specific surface area does not include void space of the anode, but rather, only the volume that the mass of the anode material itself occupies. The specific surface area is more preferably at least 5,000 m²/m³, more preferably at least 10,000 m²/m³, more preferably at least 30,000 m²/m³, more preferably at least 40,000 m²/m³, and even more preferably at least 50,000 m²/m³. In particular embodiments, the specific surface area is preferably at least about 100,000 m²/m³, 200,000 m²/m³, 300,000 m²/m³, 400,000 m²/m³, 500,000 m²/m³, 600,000 m²/m³, 700,000 m²/m³, 800,000 m²/m³, 900,000 m²/m³, 1.0×10⁶ m²/m³, 2.0×10⁶ m²/m³, 3.0×10⁶ m²/m³, 4.0×10⁶ m²/m³, or 5.0×10⁶/m³. Any ranges of specific surface areas that can result from any of the values set forth above are also contemplated herein.

The anode can be constructed of any electrically conductive material known in the art suitable for the purposes described herein. The anode material is preferably amenable to the growth and adherence of gas-processing microbes. Some classes of electrode materials (or a coating thereof) include conductive metals (e.g., nickel, palladium, platinum, silver, gold, titanium, cobalt, tungsten, stainless steel, and alloys thereof), conductive polymers, or a metal-deposited carbon anode (e.g., Pt-deposited carbon anode). Particularly preferred for the anode are electrodes based on conductive carbon. Typically, any elemental form of carbon is suitable as a conductive carbon material. Some examples of carbon electrodes include carbon fiber, carbon paper, carbon foam (e.g., reticulated vitreous carbon), carbon cloth, carbon felt, carbon wool, carbon granules (e.g., granular activated carbon), carbon brushes, graphite, or a combination thereof. In a particular embodiment, the anode is in the form of a packed bed of any of the carbon forms described above, e.g., a packed bed of granular graphite. The conductive carbon material can have any additional suitable physical characteristics, such as having a powderized, grainy, fibrous, nanotextured, or patterned texture. The conductive carbon material can also be of a less typical form of carbon, such as carbon nanotubes (e.g., single or double walled) or fullerenes. The anode can also have any of the three-dimensional architectures known in the art that are known to possess high porosity values and high flow-through rates. Alternatively, the anode can have a flat (e.g., planar or two-dimensional) topology, though this is typically less preferred for the purposes of the present invention.

The anode can also have any suitable shape. The shape of the anode can be, for example, generally planar (e.g., 50 cm×50 cm×5 cm), block-shaped, columnar, spherical (e.g., 4 cm to 40 cm diameter), ovoid, or cuboidal (e.g., 1 cm×1 cm×1 cm or 20 cm×20 cm×20 cm). The anode can also be layered or segregated by containing layers or regions of the same or different anode materials.

In order to facilitate interaction of the gaseous mercaptan compounds or CO with the anode, the anode contains on its surface a proton-conducting medium. The proton-conducting medium possesses the property of conducting protons (or permitting passage or migration of protons) from the anode to the cathode. By being “on the surface” of the anode is meant that the proton-conducting medium is available for intimate interaction with the gas stream, and thus, can mean that the proton-conducting medium is on outer surface portions of the anode, inner (i.e., interior) surface portions of the anode, selected inner or outer surface portions of the anode, or a combination thereof.

In a preferred embodiment, the proton-conducting medium is a proton-conducting liquid. The proton-conducting liquid can also herein be referred to as the “aqueous phase” or “continuous liquid film”. The proton-conducting liquid can be any polar liquid, e.g., water, alcohol (e.g., methanol, ethanol, or isopropanol), or other polar solvent or combination of solvents capable of conducting protons. Preferably, the liquid also has an ability to dissolve CO or a gaseous mercaptan compound to be processed (e.g., particularly H₂S, which is particularly soluble in most aqueous-based liquids and solutions). The aqueous-based liquid can be used without a dissolved solute, or alternatively, with any suitable solute. The solute can be, for example, a buffer, conductivity enhancer, or pH adjuster. Some examples of solute include, for example, alkali halide salts (e.g., NaCl and KCl), sulfates, ammonium salts, nitrates, phosphates, ethylenediamine tetraacetate (EDTA), tris(hydroxymethyl)-aminomethane (Tris), 3-morpholinopropane-1-sulfonic acid (MOPS), 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid (HEPES), sodium citrate, and the like. The concentration of the solute can be any suitable concentration, e.g., 1 M, 0.5 M, 0.4 M, 0.3 M, 0.2 M, 0.1 M, 50 mM, 20 mM, 10 mM, or 1 mM; or concentrations above or below any of these values, or any range between these values. An example of a particularly suitable aqueous solution is a potassium phosphate buffer solution (e.g., 100 mM at a pH of 7.5).

In one embodiment, a mediator, such as ABTS, can also be included in the proton-conducting liquid. The molar concentration of mediator can be any suitable concentration, e.g., at or above (or at or below) 1 μM, 10 μM, 25 μM, 50 μM, 100 μM, 200 μM, 300 μM, 400 μM, 500 μM, 1 mM, 10 mM, 25 mM, 50 mM, 100 mM, 200 mM, 300 mM, 400 mM, 500 mM, 1 M, 1.5 M, or 2 M. In another embodiment, a mediator is excluded from the proton-conducting liquid.

In the case of a proton-conducting liquid, it is preferable for the proton-conducting liquid to form a film on the surface of the anode. The film can be located on, for example, the anode's outer surface, inner surface, or a combination thereof, or selected portions thereof. Preferably, the film of liquid has a thickness of at least 0.5 μm (0.5 microns). In different embodiments, the liquid film may preferably have a thickness of at least 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm. Preferably, the film of liquid has a thickness of no more than 20 μm. In different embodiments, the liquid film may preferably have a thickness of no more than 15 μm, 10 μm, 9 μm, 8 μm, 7 μm, 6 μm, 5 μm, 4 μm, 3 μm, 2 μm, or 1 μm. Any range resulting from any combination of minimum and maximum values given above is also considered herein.

The anode (and optionally, the cathode) can also be filled to a certain extent with the proton-conducting liquid, as long as less than 100% of the pore volume of the anode is occupied by the liquid in order that a void volume remains for gas circulation. In other words, the occupied pore volume ratio is less than 0.1 (or less than 100%). In different preferred embodiments, the liquid may occupy 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5%, 2%, or 1% of the total pore volume, or an amount greater than or less than any of the foregoing values, or a range between any of the foregoing values. When a proton-conducting liquid is used, it typically needs to be replenished to maintain a coating on the anode surface. The replenishment may be conducted in a way that maintains a desired liquid film thickness or occupied pore volume ratio.

In another embodiment, the proton-conducting medium is a proton-conducting solid. The proton-conducting solid has the ability to conduct protons while in the solid state. By being in a “solid state”, the material does not flow or appreciably deform under the conditions used in the process. The solid can also be a semi-solid, amorphous solid, or a supercooled liquid, such as a glass. The proton-conducting solid can be, for example, a solid oxide material (e.g., yttrium-stabilized zirconia (YSZ) or gadolinium-doped ceria (GDC)), as traditionally used in solid oxide fuel cells. The proton-conducting solid can also be, for example, an ionomer composition, such as those containing perfluorovinyl groups and/or sulfonic acid groups (e.g., Nafion®). In addition, a solid proton-conducting medium can be used in combination with a liquid proton-conducting medium.

Particularly in the case where the anode is constructed of a hydrophobic type of material (e.g., a carbon form), the anode is preferably rendered sufficiently hydrophilic to permit favorable interaction (i.e., adherence, interfacing, or bonding) of the polar proton-conducting medium with the anode material. Any method for coating the surfaces of the anode to render them sufficiently hydrophilic is applicable herein. For example, the anode surface can be powder-coated, spray-coated, or dip-coated with one or more hydrophilic polymeric or molecular materials, and optionally thermally processed, calcined, or dried. The anode can also be treated by ionizing radiation or an ion beam process for this purpose. In a preferred embodiment, the anode is treated by a plasma process to render its surface hydrophilic. The plasma process is preferably an oxygen plasma process.

The anode may also be configured to reduce or prevent moisture loss. For example, the anode may be suitably coated with or encapsulated by a coating that helps to retain moisture. The coating can be any made of any suitable organic or inorganic material useful for this purpose.

The cathode can be constructed of any suitable electrically conductive material, such as any of the materials described above for the anode. The cathode can also have any of the properties (e.g., porosity, specific surface area, and so on) described above for the anode. The cathode may also include a biological system capable of transferring or utilizing electrons. In one embodiment, the cathode is any of the gas cathodes known in the art (e.g., a Pt/air electrode). In order to permit proton transfer from the anode to the cathode, the cathode should also be coated with a proton-conducting liquid (or solid). In one case, wetness is maintained on the cathode by encouraging the proton-conducting liquid (wetting agent) to flow from the anode to the cathode (i.e., through a cation-permeable material or membrane), wherein periodic wetting of the anode functions to also wet the cathode. In another case, wetness is maintained on the cathode by periodic addition of a wetting agent directly onto the cathode, either with or without a flow of wetting agent from the anode.

In a MFC of the invention, a cation-permeable material is preferably situated between the anode and cathode and in direct contact with each such that the anode and cathode are separated to the extent that the gas stream being processed by the anode does not appreciably contact the cathode. More preferably, the cation-permeable material substantially eliminates or completely prevents contact of the gas stream with the cathode. The cation-permeable material preferably allows proton-conducting liquid described above (if a proton-conducting liquid is used) to pass through to the cathode in order that protons are efficiently transported to the cathode. The cation-permeable material is typically in the form of a layer of any suitable thickness, e.g., 0.1 μm, 0.5 μm, 1 μm, 10 μm, 100 μm, 1 mm, 10 mm, or 100 mm, or values less than or greater than any of the foregoing values. The cation-permeable material can be any material which permits proton transport from the anode to the cathode while substantially preventing contact of the cathode with the gas stream. For example, the cation-permeable membrane can be filter paper, cloth, or a specific pore-sized filter material, such as a 0.2 micron filter material.

Preferably, the cation-permeable material is a cation-selective permeable material, which is also referred to herein as a cation exchange material. The cation exchange material can also be, more specifically, a proton exchange material. More preferably, the cation exchange or proton exchange material is in the form of a membrane, i.e., a cation- or proton-selective permeable membrane or cation or proton exchange membrane (PEM). In particular, a PEM selectively allows the diffusion or passage of hydrogen ions (H⁺, otherwise referred to herein as “protons”) while not allowing the passage of anions, including electrons. The cation-selective permeable material should also substantially prevent oxygen from diffusing from the cathode side into the anode side. The cation-selective or proton-selective permeable material can be any such material known in the art having these properties. Any of the PEMs known in the art can be used herein, and more particularly, those belonging to the class of ionomer polyelectrolytes having these properties, such as the Nafion® class of PEMs. The cation-selective material can alternatively be in the form of a cation- or proton-selective salt bridge, or a glass bridge containing a cation or proton exchange membrane.

The spacing between the anode and cathode (i.e., the electrode spacing) can be any suitable spacing. In one embodiment, the spacing is within the range of 0 to 1 cm. Smaller electrode spacings (i.e., less than 1 cm) can also be used. For example, in different embodiments, the electrode spacing can be at about or less than 0.8 cm, 0.5 cm, 0.25 cm, 0.1 cm, 5 mm, 4 mm, 3 mm, 2 mm, 1 mm, or 0.5 mm. In another embodiment, the electrode spacing is greater than 1 cm, and can be, for example, at or greater than 2 cm, 5 cm, 10 cm, 20 cm, or 30 cm. The spacing between the electrodes can be at least partly determined by the thickness of a cation-permeable material separating the electrodes.

Preferably, in order to maximize electrical output and provide an efficient system for electricity production, the level of oxygen in the reaction zone of the anode is reduced, and preferably substantially reduced, so as to result in an appreciably anaerobic environment at the anode. The gas streams being particularly considered herein (e.g., emanating from fuel processing or refining operations) generally do not contain oxygen, or at most, trace levels of oxygen. A trace level of oxygen generally corresponds to no more than about 100 ppm, and more preferably at or less than, for example, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, 1 ppm, 0.5 ppm, or 0.1 ppm. If more than trace levels of oxygen are in the gas stream, any method for removal and exclusion of oxygen at the anode can be used to partially or completely remove oxygen. For example, an inert gas, such as nitrogen, can be added to the gas stream to reduce the level of oxygen to a trace level. Any of the oxygen scavenging processes known in the art can also be used, such as those based on elemental iron, or enzymes, or catechol, to partially or substantially remove oxygen from a gas stream.

The MFC (or BES) described herein can have any suitable number of cathodes and anodes. For example, the MFC (or BES) can be operated with one anode and more than one cathode, or one cathode and more than one anode, or an equivalent number of anodes and cathodes (e.g., two anodes and two cathodes, or three anodes and three cathodes). In addition, the MFC (or BES) can function monolithically, or alternatively, in a stacked mode in which, for example, 2-500 MFC units are stacked in order to increase electrical power output.

The BES (e.g., MFC) can be operated within a mild temperature range of about 20 to 50° C. and normal to elevated pressure conditions (i.e., approximately 1 atm or above). Alternatively, if thermophilic or hyperthermophilic organisms are used, the operating temperature of the BES can be higher (e.g., at about or greater than 50° C., or 60° C., or 70° C., or 80° C., or 90° C., or 100° C.). In other embodiments, the BES can be operated under cooler conditions of less than 20° C., such as a temperature of about or less than 15° C., or 10° C., or 5° C., or 4° C., or 2° C., or 0° C.

The microbes reside on the anode. Preferably, the microbes are in the form of a biofilm. A biofilm of microbes can be established using any of the methods known in the art. For example, as known in the art, a biofilm of microorganisms can be produced on an anode by initiating a colony of microbes on the anode (i.e., by contact of the anode with the microbes under suitable thriving conditions) and then growing the colony until a biofilm is established on the anode. Preferably, in order to favor growth of exoelectrogenic microbes, the initiation and growth stage is conducted on the anode while the anode is in electrical communication with the cathode. In this way, electrons being donated to the anode from exoelectrogenic microorganisms can be conducted to the cathode.

A biofilm can be initiated by contact of the anode with an anolyte (i.e., either a specially prepared anodic medium or the effluent) that has been inoculated with a sampling of microorganisms, at least a portion of which should be capable of operating by an exoelectrogenic mechanism. Preferably, at some point either at the time of contact, or after contact of the anode with the microorganisms in the anolyte, forced flow and recirculation conditions (i.e., as provided by a pump) are established for the anolyte. For example, in the case of a porous anode, the anolyte is made to flow and recirculate through the anode. A significant portion of microorganisms that do not have a strong propensity for forming biofilms, even though they may be initially associated with the biofilm, will be driven into the anolyte by the flow force. Accordingly, the forced flow and recirculation conditions of the anolyte serve to enrich the biofilm with microorganisms that have a strong propensity for forming biofilms.

In turn, microorganisms with a strong propensity for funning biofilms are more likely to contain pili (nanowires) on their external membrane which can also be used by the microorganisms for direct electron transfer to the anode. Therefore, the forced flow and recirculation conditions of the anolyte can also serve to further enrich the biofilm with exoelectrogenic microorganisms capable of direct electron transfer.

At least one advantage of enriching the biofilm with exoelectrogenic microorganisms capable of direct electron transfer is that mediators (e.g., ferric oxides, neutral red, anthraquinone dyes, 1,4-napthoquinone, thionine, methyl viologen, methyl blue, humic acid, ABTS, and the like) are less needed or completely not needed for facilitating electron transfer. A mediator-less system is advantageous in that not only are mediators typically expensive, often toxic, and require replenishment, but mediated electron transfer is typically less efficient than direct (mediator-less) electron transfer.

Preferably, in preparing a biofilm on the anode, the flow rate of the anolyte should be high enough to at least maintain planktonic microorganisms floating in the medium such that they can be eliminated. A suitable flow rate can be, for example, at least about 2 or 3 mL/min. In different embodiments, the flow rate can be either substantially constant or fluctuating within a range of, for example, 2-10 mL/min, or 3-10 mL/min, or 4-10 mL/min, or 5-10 mL/min, or 6-10 mL/min, or 3-8 mL/min, or 3-7 mL/min, or 4-8 mL/min, or 4-7 mL/min. The foregoing flow rates are preferably no more than 10 mL/min and are thus herein referred to as a “low flow rate”.

More preferably, in preparing a biofilm on the anode, the flow rate is high enough to render those biofilm-forming microorganisms with a residual level of planktonic ability (i.e., semi-planktonic microorganisms) waterborne (i.e., floatational), and thus, removable, as further described below. This higher flow rate is preferably above 10 mL/min. In different embodiments, the flow rate can be, for example, at least about 12 mL/min, or at least about 15 mL/min, or at least about 20 mL/min, or at least about 25 mL/min, or at least about 30 mL/min, or at least about 35 mL/min, or at least about 40 mL/min, or at least about 45 mL/min, or at least about 50 mL/min. In different embodiments, the higher flow rate can be either substantially constant or fluctuating within a range of, for example, 12-60 mL/min, 12-50 mL/min, 12-40 mL/min, 12-30 mL/min, 12-20 mL/min, 15-60 mL/min, 15-50 mL/min, 15-40 mL/min, 15-30 mL/min, 15-20 mL/min, 20-60 mL/min, 20-50 mL/min, 20-40 mL/min, 20-35 mL/min, 20-30 mL/min, 25-60 mL/min, 25-50 mL/min, 25-40 mL/min, 25-35 mL/min, 25-30 mL/min, 30-60 mL/min, 30-50 mL/min, 30-40 mL/min, 35-60 mL/min, 35-50 mL/min, 35-40 mL/min, 40-60 mL/min, 40-50 mL/min, 45-60 mL/min, 45-50 mL/min, or 50-60 mL/min.

To reflect changes in volume and cross-sectional area of the MFC (anode), the anolyte flow rate can alternatively be represented in terms of space velocity (cm/min) or hydraulic retention time (HRT in units of minutes). To convert flow rates given in units of mL/min into space velocity, the flow rates are divided by the cross-sectional area of the MFC. For example, for a MFC having a cross-sectional area of 1.25 cm², a flow rate of 2 mL/min corresponds to a space velocity of approximately 1.6 cm/min; a flow rate of 10 mL/min corresponds to a space velocity of 8 cm/min, and a flow rate of 30 mL/min corresponds to a space velocity of 24 cm/min. To convert flow rates given in mL/min into HRT values, the flow rate is inserted into the following equation: HRT=(volume of chamber)/(flow rate in mL/min). For example, for a MFC having a chamber volume of 13.25 mL, a flow rate of 2 mL/min corresponds to a HRT value of approximately 6.6 min.; a flow rate of 10 mL/min corresponds to a HRT value of approximately 1.3 min, and a flow rate of 30 mL/min corresponds to a HRT value of approximately 0.44 min.

In a preferred embodiment for preparing a biofilm on the anode, the anolyte is made to flow at any of the low flow rates or ranges thereof described above on a continuous basis (and either a substantially constant or fluctuating basis) along with periodic, intermittent, or occasional interruptions by any of the higher flow rates or ranges thereof described above. For example, in one embodiment, a continuous low flow rate in the range of 2-10 mL/min is periodically interrupted by a higher flow rate. The higher flow rate is preferably any of the higher flow rates described above, and more particularly, a flow rate greater than 10 mL/min, and more preferably, a flow rate of or greater than 12 or 15 mL/min. In a particularly preferred embodiment, the higher flow rate is at least about 35 mL/min. In another embodiment, a continuous low flow rate in the range of 3-10 mL is periodically interrupted by a higher flow rate in the range of 30-40, 30-50, or 40-50 mL/min. In another embodiment, a continuous low flow rate in the range of 3-7 mL is periodically interrupted by a higher flow rate in the range of 30-40, 30-50, or 40-50 mL/min.

During the forced flow and recirculation conditions for preparing a biofilm on the anode, planktonic microorganisms (i.e., those having a propensity to float in solution rather than form a biofilm) are substantially removed by a suitable process (e.g., by use of a syringe or flushing into the effluent water). Preferably, any chemicals that may function as mediators are also removed. For example, in a preferred embodiment, planktonic microorganisms are removed by the periodic replacement of all or a portion of the flowing and recirculating anolyte. Since the majority of planktonic microorganisms and any mediators float in solution, periodic replacement of the anolyte functions to remove these species.

In different embodiments for preparing a biofilm on the anode, the anolyte may be replaced, either by a set or arbitrary number of times of equivalent volumes of anolyte, or by a set or arbitrary rate of replacement such that a substantial absence of planktonic microorganisms in the anolyte in contact with the anode is realized. Preferably, a substantial absence of planktonic microorganisms corresponds to at least about 80%, more preferably at least about 85%, more preferably at least about 90%, and even more preferably at least about 95% reduction in concentration of planktonic microorganisms in the flowing and recirculating anolyte. For example, in one embodiment, the anolyte is substantially replaced by 1-20 equivalent volumes of anolyte, either at set intervals or in an arbitrary manner. In another embodiment, the anolyte is substantially replaced (i.e., at least 90% replaced by volume for each instance of replacement) at specific intervals, such as every few minutes, hours, or days. Alternatively, the anolyte can be replaced when an optical transmission parameter (property) of the anolyte reaches a level indicative of the presence of planktonic microorganisms. For example, a turbidity analyzer (e.g., by laser scattering) or optical density instrument can be used to measure the relative turbidity or light transmission of the anolyte. In a preferred embodiment, the anolyte is replaced whenever the recirculating anolyte reaches an optical density (e.g., OD₆₀₀) threshold, e.g., above 0.05 units. Replacement of the anolyte can then be stopped when the optical density value no longer exceeds the desired threshold level.

In a preferred embodiment for preparing a biofilm on the anode, anolyte is replaced directly after the anolyte is subjected to a high flow rate pulse. In this case, the high flow rate pulse can be conducted either during recirculation of the anolyte, or alternatively, during a momentary interruption of the recirculation condition for a time sufficient for the high flow rate pulse (and optionally, a simultaneous or subsequent removal of the medium used for the high flow rate pulse) to take place. The medium used in the high flow rate pulse can be the medium being recirculated or can be a separate medium (e.g., water, purified water, buffered water, or mineralized water) not in contact with the recirculated medium. In a particular embodiment, the pressure resulting from the high flow rate pulse is used to force the medium out of an outlet of the anodic chamber so that the medium used in the high rate pulse is immediately ejected from the anodic chamber after the high flow rate pulse.

In one embodiment, the microbes residing on the anode have an innate ability to efficiently consume one or more mercaptan compounds or CO. The microbes may, in addition, be capable of consuming compounds or materials other than mercaptan compounds and/or CO that may also be present in the gas stream. In such a case, the microbes may be used in the treatment process without first cultivating and enriching the microbes on mercaptans or CO.

In another embodiment, the initial microbe population may not include a sufficient number of microbes that possess an ability to effectively or efficiently consume mercaptan compounds or CO. In such a case, the microbes are preferably cultivated on one or more mercaptan compounds and/or CO as a nutrient source in order to select and enrich those microbes that possess a tendency or ability to consume mercaptan compounds or CO. For the case of a mercaptan-laden gas stream, the mercaptan compounds selected as a nutrient source are preferably the same (or chemically similar) as the mercaptan compounds to be removed from the gas stream. By cultivating microbes on mercaptan compounds or CO, those microbes having a propensity or inclination to catabolize mercaptan compounds or CO undergo a growth stage while microbes not so inclined become weakened by malnutrition and are eventually eliminated from the consortium. In this way, a consortium of microbes enriched in mercaptan-consuming or CO-consuming microbes is produced.

The above microbial growth and enrichment stage is preferably achieved by contacting a microbe-laden anode with a solution or gas stream containing one or more mercaptan compounds or CO. The mercaptan compounds or CO can be administered as a sole nutrient source, or along with other nutritive compounds. Alternatively, the microbes may be initially fed a diet based solely or predominantly on one or more traditional nutritive compounds and thereafter fed a diet more highly concentrated in mercaptan compounds or CO. The transition from a diet based predominantly on non-mercaptan or non-CO compounds to one based predominantly on mercaptan compounds or CO can be sudden, incremental, or continuously gradual. The non-mercaptan or non-CO nutritive compounds are any compounds or materials that can be oxidatively degraded by exoelectrogenic microorganisms such that electrons and protons result from the degradation. The non-mercaptan or non-CO nutritive compounds can broadly include, for example, waste products (e.g., from sewage streams, industrial pollutants and byproducts, and foodstuffs), synthetic and natural plastics and polymers, and biological materials. Typically, the non-mercaptan or non-CO nutritive compounds are biodegradable. The non-mercaptan or non-CO nutritive compounds can be selected from, for example, carboxylic acid-containing compounds or materials (e.g., acetic acid, propanoic acid, butyric acid), carbohydrate compounds or materials (e.g., monosaccharides, disaccharides, oligosaccharides, and polysaccharides), lipid-containing substances (e.g., fats, mono-, di-, or triglycerides, oils, fatty acids, lipoproteins, or liposaccharides), amino acid-containing substances (e.g., amino acids, dipeptides, tripeptides, oligopeptides, or proteins), or a combination thereof. The growth medium can also contain one or more inorganic compounds or materials, such as minerals and vitamins, e.g., alkali and alkaline halide salts (e.g., KCl, MgCl₂, and the like), phosphates, ammonium salts, and the like.

The microbial growth stage is preferably continued until an electrical current output of the BES or MFC becomes level at a fixed resistance between the anode and cathode, after which time the nutritive compounds can be stopped for a suitable period of time, or periodically administered, in order to maintain a desired current or voltage level. For example, the growth stage may be considered complete when the electrical output of the BES or MFC stabilizes to a voltage between 0.3-0.4 V at a 500 ohm load.

The microorganisms can also be subjected to a starvation stage. A starvation stage can serve to enrich the microorganisms with a higher proportion of exoelectrogenic microorganisms by weakening non-exoelectrogenic organisms and encouraging their elimination. Preferably, the starvation stage is conducted after formation of a biofilm and more preferably after a growth stage, as described above.

The starvation stage is preferably conducted by lowering the administered amount of nutritive compound (which can include mercaptan compounds or CO, or non-mercaptan or non-CO compounds) to below the amount required for the microorganisms to produce the maximum achievable current under the conditions (e.g., resistance, and other factors) provided by the BES. The amount of nutrient required to produce the maximum achievable current under conditions provided by the BES is hereinafter referred to as the “nutrient threshold value”. Preferably, the administered amount of nutritive source during the starvation stage is no more than 50% of the amount required to attain the nutrient threshold value. More preferably, the administered amount of nutrient during the starvation stage is no more than 25%, or no more than 10%, or no more than 1%, of the amount required to attain the nutrient threshold value. Enrichment of the biofilm with exoelectrogenic microorganisms is typically evidenced by maintenance of the voltage output of the BES during the starvation stage. Preferably, after the initial indication of a voltage decline, the starvation stage is ended by administration of an amount of nutrient sufficient to at least maintain the voltage of the BES. However, the starvation stage can be ended before a voltage decline is observed, i.e., at a point in time for which it is known that a certain level of enrichment has occurred.

The microorganisms (either in biofilm or planktonic form) can also be subjected to a decreased electrical resistance stage. Lowering the resistance (i.e., load) across the anode and cathode increases the current flow between the two electrodes, and this in turn encourages the growth of exoelectrogenic organisms (i.e., further enrichment of the microorganisms with exoelectrogenic organisms). Preferably, the electrical resistance is lowered after any of the stages described above for producing a biofilm, and more preferably after the starvation stage described above (and more preferably, with reinitiation of the supply of a nutritive source). The external resistance is typically controlled by use of a resistor box. The resistor box is preferably one which can be set to any suitable resistance, preferably within the range of 0-5000 ohms. The resistance can be reduced by any desirable or suitable amount, either in discrete amounts or gradually over a desired period of time. For example, in different embodiments, the load can be decreased to about 95%, or 90%, or 85%, or 80%, or 75%, or 70%, or 65%, or 60%, or 55%, or 50%, or 45%, or 40%, or 35%, or 30%, or 25%, or 20%, or 15%, or 10%, or 5% its original (or full capacity) value.

The electrical power output of the MFC in terms of anode surface area (i.e., the area power density) is preferably at least 2,000 mW/m², and more preferably, at least 2,500 mW/m², or 3,000 mW/m², or 3,500 mW/rn², or 4,000 mW/m², or 4,500 mW/m². The area power density can be converted to volumetric power densities (in units of W/m³) by multiplying the area power density by the projected surface area (i.e., in m²/m³) and 1/1000. Discussion and examples of projected surface area have been given above. For lower projected surface areas (e.g., 50-500 m²/m³), some values of volumetric power density include, for example, 100, 200, 300, 400, 500, 600, 700, 800, 900, or 1,000 W/m³, or a range resulting from any two of these values. For higher projected surface areas (e.g., 1,000-100,000 m²/m³), some values of volumetric power density include, for example, 2,000, 5,000, 10,000, 20,000, 50,000, 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, or 400,000 W/m³, or a range resulting from any two of these values. Typically, the volumetric power density is recited in terms of net anode volume (NAV), which is the void volume fraction (i.e., volume fraction not occupied by liquid) in the anode. To convert the volumetric power density to reflect the NAV, the volumetric power density is divided by the void volume, which has the effect of increasing the value of the volumetric power density. Typically, the void volume is at or above 25% and up to about 95% (i.e., typically any value within 0.25 to 0.95). Some void volume values include, for example, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, and 95%, or values above or below any of these values, or any range resulting from any two of these values.

The electricity generated by the BES can be used to power any desired process. Preferably, the electricity is used to power an aspect of the process from which the mercaptan-laden or CO-laden gas stream emanates. For example, the electricity can be used to heat, pressurize, condense, or transport one or more components in an industrial process, such as any of the energy-demanding operations commonly employed in an HDS process, Claus process, Merox process, steam-methane reforming process, catalytic water-gas shift process, or Fischer-Tropsch process.

The gas stream can also be suitably modified before or during processing by the BES. The modification of the gas stream can, for example, enhance or promote BES operation as compared to BES operation using an unmodified gas stream. In one embodiment, the gas stream is modified by being diluted. Dilution may be advantageous in a situation where the BES has a threshold tolerance to a mercaptan, CO, or other chemical present in the gas stream, wherein dilution causes the concentration of the mercaptan, CO, or other chemical to be less than the threshold level. In another embodiment, the gas stream is processed by an adsorptive process for removal of one or more chemicals from the gas stream. In another embodiment, the gas stream is supplemented with one or more additional nutritive chemicals. In yet another embodiment, the gas stream is processed by a pre-oxidation process (e.g., bleaching, peroxidation, ozonation, hot filament oxygenation). A pre-oxidation process can be particularly advantageous when mercaptan compounds resistant to oxidative degradation are present, or when the microbes being used cannot efficiently or effectively consume the mercaptan compounds directly. The oxidation process may also be used as a post-treatment step (i.e., after microbial degradation) to ensure complete oxidation of one or more compounds. Any one or more of the gas pre- or post-treatment steps can be partially or completely powered by electricity produced by one or more BESs consuming mercaptan compounds or CO.

In a preferred embodiment, the gas stream is pre-treated by adjusting its humidity level before entry of the gas stream into the BES. The humidity level of the gas stream is preferably adjusted to at least 40%. In different embodiments, the humidity level of the gas stream is preferably adjusted to at least 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97%, 98%, or 99%. In a particular embodiment, the humidity level of the gas stream is adjusted to the saturation point of the gas stream under the conditions (e.g., temperature and pressure) being used.

The humidity level of the gas stream can be suitably increased or decreased by any suitable method. The humidity level of the gas stream can be suitably decreased by, for example, passing the gas stream through a condenser or water-absorption unit (e.g., calcium sulfate hemihydrate, silica gel, or the like). The humidity level of the gas stream can be suitably increased by, for example, passing the gas stream through a column of water. In order to increase the water absorption rate of the gas stream, the gas stream can be suitably mixed or agitated in the column of water to promote interaction of the gas stream with the water. The column of water can also be adjusted in height (of the water level) and in buoyancy (e.g., by inclusion of a viscosifier) to affect the retention time of the gas stream in the water. The temperature of the water can also be suitably adjusted, preferably to within a range of 30-50° C., and more preferably about 40° C., particularly when water saturation is desired. However, lower temperature water (e.g., less than 30° C.) or higher temperature water (e.g., higher than 50° C.) can also be used. In another embodiment, the humidity of the gas stream is increased by mixing the gas stream with water vapor or steam.

Alternatively, the BES or MFC can be operated in a manner such that a cathode therein, instead of producing water by absorption of oxygen, produces hydrogen gas. When hydrogen is produced, such a device can be referred to herein as a microbial electrolysis cell (MEC). In order to render the BES capable of producing hydrogen gas, the BES or MFC is preferably modified in one or more ways, as follows: i) the cathode is preferably constructed of a hydrogen-producing material; ii) the cathode is preferably substantially deoxygenated; and iii) the cell potential of the BES or MFC is preferably adjusted, by application of an external voltage, such that hydrogen can be produced at the cathode. The hydrogen-producing material of the cathode can be any material capable of producing hydrogen from protons, including, for example, hydrogen-producing metals, such as typically platinum (Pt), palladium (Pd), nickel (Ni), iron (Fe), copper (Cu), or an alloy thereof. Other possible hydrogen-producing materials include hydrogen-producing enzymes or microbes. An anaerobic environment at the cathode may be provided without special measures if the conditions in which the BES or MFC is employed are naturally anaerobic. However, if anaerobic conditions are not naturally present, deoxygenation of the cathode can be conveniently achieved by, for example, sparging of the catholyte with nitrogen (or other inert gas) and/or sealing of the cathodic chamber so as to prevent entry of air. The cell potential (ΔV) can be adjusted such that the BES or MFC anode potential (typically around −0.3 V) is externally supplemented such that the potential difference between the anode and the cathode is at least about 0.41 V. For example, the anode can be externally assisted with a voltage of about 0.11 to 0.25 V or higher for this purpose, depending on the magnitude of the cathode overpotential. For hydrogen production, the anolyte and catholyte need not be separated by a cation-selective permeable membrane.

The hydrogen gas, in turn, can be used for any purpose, e.g., as a commodity chemical, a reactant or processing chemical in the process to which the BES is applied (or another process), or as a fuel source, particularly for powering one or more processes associated with the gas stream being processed. As a reactant in the process, the hydrogen gas can be used for hydrogenation (e.g., in an HDS or Fischer-Tropsch process), or as part of a reductive process, or as a feedstock for another MFC. As a fuel source, the hydrogen gas can be combusted with a reactive (i.e., oxidizing) gas, or reacted with a gas to produce a fuel which is then combusted, e.g., the physical-, enzymatic-, or microbial-mediated production of methane (natural gas) from syngas (i.e., CO+H₂).

In a particular embodiment, the produced hydrogen is used as an electricity-generating fuel to power one or more processes, particularly those process steps from which the gas stream emanates. For example, the produced hydrogen can be directed into a fuel cell capable of reacting hydrogen with a suitable oxidant (typically oxygen) to produce water while generating electricity. The fuel cell can be any suitable fuel cell known in the art capable of reacting hydrogen gas with an oxidant to make electricity. The fuel cell can be, for example, a hydrogen-oxygen proton exchange membrane (PEM) fuel cell, an alkaline fuel cell, metal hydride fuel cell, molten carbonate fuel cell, or solid oxide fuel cell. The fuel cell that uses the produced hydrogen as a fuel can also be a microbial fuel cell capable of using hydrogen as a fuel, i.e., which contains microbes capable of using hydrogen as a nutrient source.

The BES can also be operated in a manner such that one or more electrochemically reducible compounds or materials is reduced at the cathode. Preferably, a cathode of the BES is operated in the substantial absence of oxygen for this purpose. The reductive process is preferably used for the breakdown or elimination of one or more chemical species that are deleterious to the BES or that are environmentally malignant. The environmentally malignant species can be, for example, a degradation product, a pollutant, waste product, or toxin. Some examples of reductive processes include nitrate reduction, uranium reduction and perchlorate reduction (Rabaey, K. et al. The ISME Journal 1, 9-18 (2007)).

In one embodiment, the reducible species is a compound or material containing a nitrogen oxide (N—O) bond. Such a compound is commonly a nitrate-containing species (i.e., “a nitrate” or “nitrate compound”). The nitrate compounds can include inorganic nitrate species (e.g., NaNO₃, KNO₃, NH₄NO₃, Mg(NO₃)₂, AgNO₃, HNO₃, and so on) as well as organonitrate species, such as tetramethylammonium nitrate. Other types of nitrogen oxide compounds that can be reduced include the nitrites, organonitro compounds, dinitrogen tetroxide, nitrosyl (nitroso) compounds, nitric oxide (NO), and nitrosonium species.

In another embodiment, the reducible species is a compound or material containing a halogen oxide bond. A common class of such compounds are the chlorine oxide class of compounds. A common subclass of chlorine oxide compounds are the perchlorates. The perchlorates include inorganic perchlorate species (e.g., LiClO₄, NaClO₄, KClO₄, NH₄ClO₄, Mg(ClO₄)₂, AgClO₄, HClO₄, and so on) as well as organoperchlorate species, such as tetramethylammonium perchlorate. Other subclasses of chlorine oxide compounds include the chlorates, chlorites, hypochlorites, and their acids. Other classes of halogen oxide compounds include the bromine oxide and iodine oxide classes of compounds. Some subclasses of bromine oxide compounds include the perbromates, bromates, bromites, hypobromites, and their acids. Some subclasses of iodine oxide compounds include the periodates, iodates, iodites, hypoiodites, and their acids.

In another embodiment, the reducible species is a compound (e.g., salt) or material containing one or more reducible metal species. A reducible metal species typically contains a metal atom having a positive oxidation state. The reductive method is particularly effective in reducing heavy metals, which are often harmful to the environment and in need of removal. Some examples of reducible metal species include Cr(VI) as found in chromates and dichromates, Mn(VII) as found in permanganates, Fe(III), Ni(III), Cu(II), Cu(I), Pd(II), Ag(I), Cd(II), Au(III), Au(I), Hg(I), Pb(II), and U(VI), which can be converted to the relatively insoluble U(III) species. The more reducible heavy metals can be reduced to elemental form, which can allow for their more facile removal.

In yet another embodiment, the reducible species is a peroxide. The peroxide can be, for example, inorganic (e.g., hydrogen peroxide), or an organoperoxide, such as carbamide peroxide, dibenzoyl peroxide, and cumene hydroperoxide.

In still another embodiment, the reducible species is a reducible sulfurous substance. The sulfurous substance can be, for example, sulfur dioxide, sulfur trioxide, sulfuric acid, a sulfate, a sulfite, a bisulfite, a persulfate (e.g., a peroxodisulfate), or a disulfide. The sulfurous substance may result from the oxidative degradation of mercaptans by the MFC, or from the process being treated. In the case that the sulfurous substance is the result of oxidative degradation of mercaptans by the BES, the sulfurous substance is preferably not reduced back to thiols, as this would render the process inefficient. However, particularly if the sulfurous substance is gaseous (e.g., SO₂) and originates from the process being treated, it may be advantageous for the cathode to reduce the sulfurous substance to a thiol, and direct the thiol to the anode where it can be oxidized to a non-gaseous product (e.g., a sulfite or sulfate).

The microorganisms (i.e., organisms) that are used in the MFC can be any suitable microorganisms. The microorganism can be, for example, eukaryotic or procaryotic, and either unicellular or multicellular. An example of a suitable unicellular eukaryotic microorganism is yeast. Other examples of unicellular eukaryotic microorganisms include the protists or protozoans, such as amoeba and paramecia. An example of multicellular eukaryotic microorganisms includes the euglena. Those algae capable of uptake of organic carbon (e.g., eukaryotic or procaryotic mixotrophic forms) are also contemplated herein. Procaryotic organisms are predominantly unicellular, and are divided into two domains: the bacteria and the archaea. The procaryotic organisms can also be broadly divided into four main groups according to their shape: the cocci, the bacilli, spirilla, and vibrio. The archaea include the extremophiles (e.g., as found in hot springs and lakes), and the non-extremophiles, as found in soil, the oceans, and marshland. The archaea also include the methanogens.

In one embodiment, the microorganisms considered herein are bacteria. Some examples of phyla of bacteria considered herein are the Acidobacteria, Actinobacteria, Aquificae, Bacteroidetes, Chlorobi, Chlamydiae/Verrucomicrobia, Chloroflexi, Chrysiogenetes, Cyanobacteria, Deferribacteres, Deinococcus-Thermus, Dictyoglomi, Fibrobacteres, Firmicutes, Fusobacteria, Gemmatimonadetes, Nitrospirae, Planctomycetes, Proteobacteria (α, β, γ, δ varieties), Spirochaetes, Synergistetes, Tenericutes, Thermodesulfobacteria, Thermotogae, or any combination thereof. Some particularly relevant families of bacteria being considered herein include Acidaminococcaceae, Acidobacteriaceae, Aeromonadaceae, Alteromonadaceae, Clostridiaceae, Comamonadaceae, Desulfobulbaceae, Desulfuromonadaceae, Enterobacteriaceae, Geobacteraceae, Pasturellaceae, Pelobacteraceae, Pseudomonadaceae, Rhodocyclaceae, and Shewanellaceae. Any combination of bacteria containing at least one of the above families of bacteria are also contemplated herein.

In a particular embodiment, the microbes include bacteria from the phylum Firmicutes. Some particular classes of Firmicutes bacteria being considered herein are Bacilli, Clostridia, and Mollicutes. A particular order of Clostridia being considered herein is Clostridiales. Some particular families of Clostridiales being considered herein are Acidaminococcaceae, Clostridaceae, and Veillonellaceae. Some particular genera of Acidaminococcaea or Veillonellaceae being considered herein are Acetonema, Acidaminococcus, Allisonella, Anaeroarcus, Anaeroglobus, Anaeromusa, Anaerosinus, Anaerovibrio, Centipeda, Dendrosporobacter, Dialister, Megamonas, Megasphaera, Mitsuokella, Pectinatus, Pelosinus, Phascolarctobacterium, Propionispira, Propionispora, Quinella, Schwartzia, Selenomonas, Sporomusa, Sporotalea, Succiniclasticum, Succinispira, Thermosinus, Veillonella, and Zymophilus. Some particular genera of Clostridaceae being considered herein are Acetanaerobacterium, Acetivibrio, Acidaminobacter, Alkaliphilus, Anaerobacter, Anaerotruncus, Anoxynatronum, Bryantella, Caldanaerocella, Caloramator, Caioranaerobacter, Caminicella, Candidatus Arthromitus, Clostridium, Coprobacillus, Dorea, Ethanologenbacterium, Faecalibacterium, Garciella, Guggenheimella, Hespellia, Linmingia, Natronincola, Oxobacter, Parasporobacterium, Sarcina, Soehngenia, Sporobacter, Subdoligranulum, Tepidibacter, Tepidimicrobium, Thermobrachium, Thermohalobacter, and Tindallia.

In another particular embodiment, the microbes include one or more classes of bacteria from the phlyum Proteobacteria.

A particular class of Proteobacteria being considered herein is Alpha Proteobacteria. Some particular orders of Alpha Proteobacteria being considered herein are Caulobacterales (e.g., the family Caulobacteraceae, or Caulobacter sp.), Kordiimonadales, Parvularculales, Rhizobiales (e.g., the family Rhizobiaceae, or Rhizobium sp.), Rhodobacterales, Rhodospirillales (e.g., the family Acetobacteraceae, or Acetobacter sp.), Rickettsiales (e.g., the family Rickettsiaceae, or Rickettsia sp.), and Sphingomonadales (e.g., the family Sphingomonadaceae, or Sphingomonas sp.), wherein “sp.” or “spp.” as used herein both indicate one or more species of the indicated genus.

Another particular class of Proteobacteria being considered herein is Beta Proteobacteria. Some particular orders of Beta Proteobacteria being considered herein are Burkholderiales, Hydrogenophilales, Methylophilales, Neisseriales (e.g., the family Neisseriaceae, or Neisseria sp.), Nitrosomonadales, Rhodocyclales, and Procabacteriales. A particular family of Burkholderiales being considered herein is Comamonadaceae. Some particular genera of Comamonadaceae being considered herein are Acidovorax, Aquabacterium, Brachymonas, Comamonas, Curvibacter, Delftia, Hydrogenophaga, Ideonella, Leptothrix, Malikia, Pelomonas, Polaromonas, Rhodoferax, Roseateles, Sphaerotilus, Tepidimonas, Thiomonas, and Variovorax. A particular family of Rhodocyclales being considered herein is Rhodocyclaceae. A particular genus of Rhodocyclaceae being considered herein is Azospira.

Another particular class of Proteobacteria being considered herein is Gamma Proteobacteria. Some particular orders of Gamma Proteobacteria being considered herein are Acidithiobacillales, Aeromonadales, Alteromonadales, Cardiobacteriales, Chromatiales (purple sulfur bacteria), Enterobacteriales (e.g., the family Enterobacteriaceae, such as the genera Escherichia or Salmonella), Legionellales (e.g., the family Legionellaceae, or Legionella sp.), Methylococcales, Oceanospirillales, Pasteurellales (e.g., the family Pasteurellaceae, or Haemophilus sp.), Pseudomonadales, Thiotrichales (e.g., Thiomargarita), Vibrionales (e.g., the family Vibrionaceae, or Vibrio sp.), Xanthomonadales (e.g., the family Xanthomonadaceae, or Xanthomonas sp.). A particular family of Aeromonadales being considered herein is Pseudomonadaceae. A particular genus of Pseudomonadaceae being considered herein is Pseudomonas (e.g., P. aeruginosa). Some particular families of Alteromonadales being considered herein are Shewanellaceae and Pseudoalteromonas. A particular genus of Shewanellaceae being considered herein is Shewanella (e.g., S. putrefaciens).

Another particular class of Proteobacteria being considered herein is Delta Proteobacteria. Some particular orders of Delta Proteobacteria being considered herein are Aeromonadales, Bdellovibrionales (e.g., the family Bdellovibrionaceae, or Bdellovibrio sp.), Desulfobacterales, Desulfovibrionales, Desulfurellales, Desulfarcales, Desulfuromonadales, Myxococcales (Myxobacteria), and Syntrophobacterales. A particular family of Aeromonadales being considered herein is Aeromonadaceae. A particular genus of Aeromonadaceae being considered herein is Aeromonas. Some particular families of Desulfuromonadales being considered herein are Desulfuromonadaceae, Pelobacteraceae, and Geobacteraceae. A particular genus of Desulfuromonadaceae being considered herein is Desulfuromonas. A particular genus of Geobacteraceae being considered herein is Geobacter (e.g., Geobacter sulfurreducens and Geobacter metallireducens). A particular family of Desulfobacterales being considered herein is Desulfobulbaceae. A particular genus of Desulfobulbaceae being considered herein is Desulfobulbus.

Another particular class of Proteobacteria being considered herein is Epsilon Proteobacteria. Some particular orders of Epsilon Proteobacteria being considered herein are Campylobacterales (e.g., the family Helicobacteraceae, or Helicobacter sp.) and Nautiliales.

In another particular embodiment, the microbes include one or more bacteria from the phlyum Acidobacteria. A particular order of Acidobacteria being considered herein is Acidobacteriales. A particular family of Acidobacteriales being considered herein is Acidobacteriaceae. Some particular genera of Acidobacteriaceae being considered herein are Acidobacterium, Geothrix, Holophaga, and Chloracidobacterium.

In another particular embodiment, the microbes include one or more thermophilic bacteria from the order Thermotogales. Some particular genera of Thermotogales being considered herein are Thermotoga, Caldotoga, Fervidobacterium, Geotoga, Marinitoga, Petrotoga, Thermopallium, and Thermosipho. A related family of thermophilic bacteria being considered herein is Thermoanaerobiaceae. Some particular genera of Thermoanaerobiaceae being considered herein are Thermoanaerobacter and Thermoanaerobacterium. Some particular species of Thermoanaerobacter being considered herein are Thermoanaerobacter thermohydrosulfuricus, Thermoanaerobacter subterraneus, Thermoanaerobacter brockii, Thermoanaerobacter yonseiensis, and Thermoanaerobacter tengcongensis.

The above microbes can all be considered for consumption of mercaptan compounds or CO. However, in the particular case of CO processing, there are some types of microbes that are more suited for effecting a water-gas shift reaction than the microbes considered thus far. Some microbes particularly considered for utilizing CO as a nutrient source include, for example, Rubrivivax gelatinosus, Butyribacterium methylotrophicum, Clostridial bacteria, and Rhodospirillus rubrum.

The microbes used in the BES can be selective or non-selective with respect to oxidative degradation of mercaptan compounds. For example, a consortium or species of microbes may be used which is generally non-selective in its ability to oxidatively degrade mercaptan compounds, i.e., the microbes can oxidatively degrade a wide number of different mercaptan compounds. A consortium or species of microbes may also be somewhat selective in processing mercaptan compounds in that the microbes may oxidatively degrade one or more types of mercaptan compounds more efficiently or effectively than one or more other types of mercaptan compounds. Finally, a consortium or species of microbes may be highly selective in processing one or more specific mercaptan compounds while being essentially inefficient or ineffective in processing one or more other mercaptan compounds.

In one embodiment, a population of microbes incorporated into the BES or MFC is relatively homogeneous by having a predominant proportion of the microbe population (typically at least 90%, 95%, 97%, 98%, or 99%) within a particular class, order, family, genus, or species of microorganism. In another embodiment, a population of microbes incorporated into the BES or MFC is relatively heterogeneous (i.e., a consortium of microbes). A relatively homogeneous or heterogeneous sample of microbes can be obtained by any method known in the art, including as a purified culture (i.e., as prepared by cell culturing methods) or from a non-cultured source. Some examples of non-cultured sources from which a population of microbes can be obtained for the BES or MFC include, for example, a waste stream (e.g., municipal or industrial waste streams), top soil, hot spring, estuary, deep sea vent, underground environment, or a contaminated environment (e.g., a mercaptan-contaminated environment).

In one embodiment, a single BES or MFC is capable of oxidatively degrading a wide number of mercaptan compounds and/or CO, thereby enabling the BES or MFC to remove the majority (or all) of the different types of mercaptan compounds and possibly CO that may be present in the gas stream. Alternatively, a multiplicity (i.e., system) of BESs or MFCs can all have the same ability to degrade the same types of mercaptan compounds. In another embodiment, a multiplicity of BESs or MFCs (i.e., two or more), each containing microbes that are specialized for degrading specific mercaptan compounds, are used in the process to remove the majority (or all) of the different types of mercaptan compounds that may be present in the gas stream.

FIG. 1 depicts a particular preferred embodiment of an MFC design, hereinafter referred to as a “horizontal gas-phase MFC configuration”. This design facilitates liquid flow (i.e., proton-conducting liquid) from the anode through the membrane to the cathode by the action of gravity. This configuration keeps the cathode moist by diffusion of liquid from the anode, and hence, permits efficient proton transfer to the cathode from the anode. An advantage of this configuration is that the cathode does not require separate wetting. However, since the liquid wetting the cathode comes from the anode, this configuration preferably incorporates a cation-exchange membrane between the anode and cathode in order to prevent anionic species from the anode solution to contact the cathode.

FIG. 2 depicts another particular preferred embodiment of an MFC design, hereinafter referred to as a “vertical gas-phase MFC configuration”. This design is particularly suitable for high throughput processing of a gas stream having a low H₂S content. Since in this design gravity is not being used to facilitate liquid flow from the anode to the cathode, the cathode typically requires separate wetting. Therefore, a cation-exchange membrane is not necessary in this configuration. Instead, a cation-permeable membrane may be used. The cation-permeable membrane can be, for example, filter paper, preferably having a pore size of 0.2 microns or less.

While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.

Claims

1. A method for producing electrical energy or hydrogen gas from a gas stream containing one or more gaseous compounds that are oxidatively degradable by microbes, the method comprising contacting the gas stream, as a gas-continuous stream, with an anode of a bioelectrochemical device, said anode containing said microbes which oxidatively degrade one or more of said gaseous compounds while producing electrical energy or hydrogen gas by said oxidative degradation, wherein: (i) said anode is sufficiently porous such that gas is permitted to flow therethrough, (ii) said anode contains on its surface and/or interior portions a proton-conducting liquid film that is maintained by an anode wetting process to maintain the liquid film, and (iii) said anode is in electrical communication with a cathode of the bioelectrochemical device.

2. The method of claim 1, wherein said microbes are in the form of a biofilm on said anode.

3. The method of claim 1, wherein said one or more gaseous compounds comprise a mercaptan compound and/or carbon monoxide.

4. The method of claim 1, wherein said anode comprises a form of elemental carbon.

5. The method of claim 1, wherein said anode is a three-dimensional electrode.

6. The method of claim 1, wherein said anode has been previously rendered hydrophilic by a suitable surface treatment process.

7. The method of claim 6, wherein the surface treatment process is a plasma treatment process.

8. The method of claim 1, wherein the anode possesses a porosity value of at least 50%.

9. The method of claim 1, wherein the anode possesses a specific surface area of at least 5,000 m2/m3.

10. (canceled)

11. The method of claim 1, wherein said proton-conducting liquid film has a thickness in the range of 1 micron and up to 10 microns.

12. The method of claim 1, further comprising controlling the humidity level of the gas stream such that the humidity of the gas stream is at least 40% before entry of the gas stream into the bioelectrochemical device.

13. The method of claim 12, wherein the humidity level of the gas stream is controlled by passing the gas stream through a column of water.

14. The method of claim 1, wherein the gas stream emanates from a petroleum refining operation.

15. The method of claim 1, wherein electricity or hydrogen produced from the bioelectrochemical device is used to power one or more mechanisms involved in an operation that produces the gas stream.

16. The method of claim 1, wherein the bioelectrochemical device is operated such that hydrogen gas is produced at the cathode by operating the bioelectrochemical device under the conditions that the cathode is constructed of a hydrogen-producing material and is deoxygenated, and the cell potential of the bioelectrochemical device is adjusted by application of an external voltage such that hydrogen is produced at the cathode.

17. The method of claim 1, wherein the bioelectrochemical device is operated such that the cathode electrochemically reduces one or more electrochemically reducible species other than hydrogen ions.

18. The method of claim 1, wherein a cation-permeable material is in direct contact with said anode and cathode, and separates said anode and cathode.

19.-37. (canceled)

38. The method of claim 1, wherein said gas stream contains carbon monoxide.

39. The method of claim 38, wherein said gas stream emanates from a biomass gasification operation.

40. The method of claim 38, wherein the microbes on the anode are capable of effecting a water-gas shift reaction.

41. The method of claim 40, wherein the microbes are selected from the group consisting of Rubrivivax gelatinosus, Butyribacterium methylotrophicum, Clostridia, and Rhodospirillus rubrum.

42. The method of claim 1, wherein said gas stream contains one or more mercaptans.

43. The method of claim 42, wherein said gas stream is substantially absent of carbon monoxide.