FIELD OF THE INVENTION
The present invention is generally directed to catalytic combustion, and more specifically to a method and an apparatus for use therewith for the reformation of methane into partial oxidation products and the oxidation of those products at a temperature below the adiabatic temperature thereof.
BACKGROUND OF THE INVENTION
Methane is an abundant hydrocarbon that is used as a source of fuel in numerous applications, such as industrial radiant heaters, gas turbines, home furnaces and cooking equipment. While methane can be made available in a relatively pure form, it is more commonly provided as a constituent of natural gas, of which it is the primary component.
Natural gas is typically combusted in an open flame, a process referred to as diffusion burning, which generates certain pollutants. One particularly undesirable class of pollutants formed during diffusion burning is nitrous oxides, i.e. NOx. In diffusion burning, NOx can be formed by any one of three possible mechanisms: thermal, prompt, and fuel bound. The production of NOx by the thermal and the prompt mechanisms, however, far exceeds that produced from the fuel bound mechanism. Consequently, efforts to reduce NOx pollution focus on reducing NOx formation by the thermal and/or the prompt mechanisms.
NOx produced by the thermal mechanism, i.e. thermal NOx, is often the dominant mechanism. Thermal NOx is formed when the heat being released by diffusion burning is sufficient to provide the necessary energy for the nitrogen in the air to combine with the oxygen in the air. Generally, at flame temperatures below 1700 K, the production of thermal NOx is insignificant. However, as flame temperatures increase, the production of thermal NOx increases sharply.
Thermal NOx production can be controlled by regulating reactant stoichiometry. To burn a fuel it must be mixed with an oxidant. For example, to burn methane oxygen must be provided. The ratio of the fuel and oxidant, that is methane and oxygen, is the reactant's stoichiometry. Reactant stoichiometry is expressed in terms of a fuel/oxidant equivalence ratio, or where the oxidant is oxygen as a constituent of air—fuel/air ratio. The fuel/oxidant equivalence ratio is the ratio of the actual fuel/oxidant ratio to the stoichiometric fuel/oxidant ratio. For example in the case of methane (CH4), the combustion reaction is CH4+2O2→CO2+2H2O. Therefore, a stoichiometric fuel/oxidant ratio is one part CH4 and two parts O2. Thus, if a mixture had this ratio of CH4 and O2, the reactant stoichiometry as expressed by the fuel/oxidant ratio of the mixture would be 1.0 (an actual mixture having these proportions would be referred to as stoichiometric).
A mixture having an equivalence ratio greater than 1.0 is fuel rich, i.e., in the case of the above methane reaction more than one part fuel for each two parts of oxygen, and a mixture having an equivalence ratio less than 1.0 is fuel lean, i.e. in the case of the above methane reaction less than one part fuel for each two parts of oxygen. When combustion is adiabatic, stoichiometric mixtures burn relatively hotter than non-stoichiometric mixtures and the further away the mixture is from stoichiometric the relatively cooler it burns.
NOx production by the prompt mechanism, i.e. prompt NOx, is a fuel-rich, gas-phase phenomenon. The reaction is quick and completes within the diffusion flame. The production of NOx by the prompt mechanism can only be controlled if the diffusion flame is eliminated, in whole or in part.
NOx formation from the combustion of methane could be greatly reduced if methane could be combusted at temperatures below 1700 degrees K and diffusion flame could be avoided. It is well known in the art that if methane is catalytically combusted, i.e. oxidized in the presence of a catalyst, the energy within the methane can be released without the formation, or limited formation, of thermal and/or prompt NOx.
A problem, however, with the catalytic combustion of methane is that methane is a very stable molecule. Thus, it is more difficult to oxidize than higher order hydrocarbons, such as propane. Methane can be catalytically combusted under fuel lean conditions producing combustion temperatures below 1700 degrees K. When a palladium-based catalyst is used the reaction may become unstable due to properties of Pd-PdO transformation of the catalyst. Hysteresis in the catalyst activity makes controlling the reaction extremely difficult. Platinum based catalysts on the other hand can provide more stable operation. However, volatility of Pt at the desired temperatures under lean conditions is very high. Thus, platinum catalyst lacks durability.
Based on the foregoing, it is an object of the present invention to develop a method and an apparatus for the combustion of methane that overcomes the problems and drawbacks associated with the prior art.
SUMMARY OF THE INVENTION
The present invention is directed in one aspect to a method for the combustion of methane. In the method, a fluid stream including fuel having methane and oxygen that is in fuel rich proportions, i.e. having a fuel/oxidant equivalence ratio greater than 1.0, is provided. The fluid stream flows into a reformation reactor having a catalyst therein that promotes the reformation of methane (CH4) into carbon monoxide (CO) and hydrogen (H2).
The catalyst reforms at least a portion of the methane in the fluid stream into carbon monoxide and hydrogen creating an exhaust stream exiting the reformation reactor having various fuel constituents therein, such as unreformed methane, CO and H2. The exhaust stream is then divided into a plurality of exhaust streamlets by passing the exhaust stream into a manifold having a plurality of discrete discharges. As a portion of the exhaust gas exits through a discharge, an exhaust streamlet is formed. Sufficient oxygen is then added to the exhaust streamlet such that the fuel constituents therein and the oxygen are in fuel-lean proportions. Same amount of oxygen should be added to each streamlet, such that variations in the equivalence ratios between the streamlets are small. The exhaust and second fluid are added together, not mixed. In the present invention, it is desired that the exhaust and the second fluid enter the porous media as distinct flow steams. It is understood however, that the two fluids will be in contact along an interface and that incidental diffusion of one fluid into another will occur. It is expected that if sufficient time is provided the diffusion combustion would occur at the interface before the two streamlets can mix. To avoid gas phase flame oxidation of the exhaust stream, which is undesirable in this invention, combined stream formed after adding the second stream to the exhaust stream should be passed into the porous media before combustion takes place. Finally, at least a portion of the CO, H2 and CH4 in the exhaust streamlets is oxidized by passing the combined stream through a porous media that absorbs and then radiates some of the heat generated by the oxidation.
A catalytic burner suitable for performing the above method includes a reformation reactor incorporating a catalyst. A manifold that receives the exhaust stream from the reformation reactor and passes the exhaust stream through a plurality of discharges forming part of the manifold thereby creating a plurality of exhaust streamlets. The exhaust streamlets then enter a flow path where the exhaust streamlets are directed into a proximally located porous media. Means for introducing a second fluid into the flow path are also provided.
The reformation reactor is a partial oxidation reactor. In a partial oxidation reactor, the catalyst and its associated geometry, e.g. substrate and dispersion thereon, defines an activity relative to the flow rate, i.e., residence time, of the methane/oxygen thereover such that when the catalyst and the methane/oxygen interact partial oxidation products and not complete oxidation products are predominantly formed. In the case of methane and oxygen, partial oxidation products are H2 and CO, while the compete oxidation products are H2O and CO2. An example of a reformation reactor for methane suitable for this application is disclosed in U.S. Pat. No. 5,648,582, the disclosure of which is incorporated herein in its entirety.
As those skilled in the art will appreciate, the selectivity, i.e. the ability to produce one product in favor of another, in the reformation process can be manipulated by controlling the temperature of the fluid stream. In the case of a fluid stream including methane and oxygen in fuel rich proportions, preheating of the fluid stream increases the selectivity in the reformation of methane in favor of H2 and CO versus CO2 and H2O. Therefore, an enhancement to both the method and the catalytic burner incorporates heating the fuel stream prior to its entry into the reformation reactor.
The exhaust and the second fluid are mixing and reacting inside the porous media to further oxidize at least part of the exhaust stream to the complete oxidation products. The porous media absorbs some of the heat created by the exothermic oxidation reaction and emits it in the form of IR radiation, assuring that the temperature remains below the adiabatic flame temperature defined by the reactant stoichiometry of the fuel constituents and oxygen. A porous media can be any media through which a gas can flow, while continuously encountering solid surfaces. In other words, porous media is comprised of alternating regularly or randomly empty volumes and filled volumes. Empty volumes should form a continuous network, such that the porous media remains permeable to permit the flow of a fluid therethrough. The porous media should have a pore size, which describes the average size of the empty volume (if the pores size in not round the smaller dimension), that is generally uniform, but small deviations are acceptable. Porous media having a few large empty volumes and otherwise generally uniform smaller volumes could be problematic. The precise pore size, porosity (ratio of open volume to total volume) and material is application dependent.
The material for the porous media should be chosen to withstand the temperatures generated in the exothermic oxidation process and effectively emit heat in the form of infrared radiation (IR). Pore size and porosity are chosen large enough to minimize pressure drop induced by the porous media but small enough when compared to the total volume in which the oxidation reaction between the exhaust and the second stream takes place.
As those skilled in combustion will readily appreciate, the reformation reactor requires that the catalyst therein be at a certain temperature to perform the reformation. The catalyst can be brought to this temperature by any one or a combination of well know procedures, such as heating the fluid stream, or direct heating of the catalyst.
Regardless of the method chosen, the exhaust gas will have a temperature upon exiting the catalyst equal to the operational temperature chosen for the reformation reactor plus the exothermal resulting from the exothermic oxidation process taking place therein. It should be remembered that the proportions of fuel constituents to oxygen within the exhaust stream are still be quite rich, i.e., the initial stream had fuel rich proportions and oxidant was consumed along with fuel creating a progressively richer fuel stream as it passed through the reformation reactor. Therefore, although the fuel constituents in the exhaust gas will be quite hot, oxidation will not occur within the exhaust stream until additional oxidant is added.
Where the fuel/oxygen stoichiometry, flow rate and IR radiation are such that porous media is hot enough, oxidation of fuel inside the porous media will occur upon contact with an oxidant. Where the porous media is not hot enough to support oxidation on contact with an oxidant, the porous media can utilize a suitable oxidation catalyst to sustain the oxidation reaction. It is understood that a catalyst can be used even if the fuel constituents are hot enough to support combustion.
The porous media 16, 116 is a media through which a gas can flow. In the preferred embodiment, the porous media 16, 116 was made from a plurality of stacked short-channel screens. The invention should not be considered so limited however, as other media could be used such as pellets, foams or gauzes and even a single screen. Generally, porous media are graded by “pore size.” Another important parameter for this invention, however, is consistency of pore size. The porous media 16, 116 is designed to promote interaction of the fuel constituents within the exhaust stream 24 with the additional oxidant 34, 134, extract heat from the ongoing oxidation, and radiate infrared radiation. Further, the porous media 16, 116 continually assures that the exhaust stream 24, 124 and oxidant 34, 134 are divided into small pockets. In other words, the exhaust stream 24, 124 and oxidant 34, 134 cannot reform into a large volume. These requirements mean that preferably the pores within the porous media 16, 116 are generally uniform. Pore size is chosen such that the pores are large enough to minimize pressure drop but small enough to assure an acceptable heat release within a pore.