Patent Application
20160361688
The present invention relates to excitation of hydrocarbons for catalytic type oxidation reactions, and more particularly, to treatment of excess methane emissions in a natural gas fueled engine to promote relatively more efficient catalytic methane oxidation reactions.
Natural Gas (NG) has become significantly more abundant with the advent of hydraulic fracturing technologies. So much so that it is a burgeoning fuel for internal combustion engines with major advantages of relatively low cost and reduced CO2 emissions. Unfortunately, the primary component of NG is methane (CH4), which has a high Global Warming Potential (GWP) value of 34. Excessive CH4 emissions from these engines will eliminate the advantage of reduced CO2, and may increase the exhaust GWP relative to gasoline or diesel engines. Therefore controlling CH4 emissions is of considerable importance.
Unfortunately, catalytic oxidation of methane to CO2 and water requires elevated temperatures in excess of 300° C. because breaking of the first C—H bond requires a relatively high energy of about 427 kJ/mol. The preferred catalysts for CH4 oxidation also have durability issues like sulfur poisoning, that increase light-off temperatures (i.e. the temperature at which a catalytic reaction is initiated) even more. The challenge becomes substantial when taking into account the trend toward (1) lower exhaust gas temperatures and (2) improved efficiency engines needed to meet the future fleet average fuel economy standard of 54.5 miles per gallon for cars and light-duty trucks by 2025.
Accordingly, there is a need for developing processes to treat methane powered engine exhaust emissions to lower the GWP value and the temperature required to treat the exhaust as well as augmenting the life time of the catalyst.
An apparatus and a method for oxidizing methane contained in a methane gas engine exhaust stream. An engine exhaust stream is provided that includes methane (CH4) and oxygen where the methane in the exhaust stream is energized and there is promotion of one of a C—H stretching or bending response. The exhaust stream containing the energized methane is exposed to a catalytic oxidation reaction where the methane is oxidized to produce one of carbon dioxide (CO2) or carbon monoxide (CO) and the methane oxidation occurs at a temperature of less than or equal to 350° C.
These and other features and advantages will be better understood by reading the following detailed description, taken together with the drawings wherein:
The present disclosure is directed at both an apparatus and process that energizes or excites the C—H bond in a hydrocarbon so that ensuing reactions, such as catalytic oxidation reactions, proceed with relatively reduced energy demands and at reduced temperatures. Such excitation may be achieved by application of radiation at a selected frequency and/or by use of a collision partner, which is reference to collision of the hydrocarbon with an energized or excited partner molecule. It is contemplated that the present invention will therefore provide control of emissions from engines operating with hydrocarbon fuels with an improvement in the associated environmental impact of engine exhaust output.
The exemplary hydrocarbon fuel for such process is the methane (CH4) molecule and the exemplary reaction that can now be facilitated is oxidation of methane to carbon dioxide and water. The exemplary environment for such an apparatus and process is the catalytic converter in a natural gas engine where methane may be a component of the engine exhaust.
A natural gas engine herein may therefore be understood as an engine that operates on natural gas fuel, where a natural gas fuel includes gases containing 30% or more by volume of methane. Such natural gas fuel may be sourced from mineral sources such as natural gas wells or from gasification of biomass, or from coal gasification processes, from landfill sites or produced by hydrogenation of carbon oxides or other methane forming procedures.
That is, the reduction in energy needed to break the C—H bond is now specifically relied upon to provide a corresponding reduction in the temperature at which CH4 oxidation occurs in the presence of a suitable catalyst. Accordingly, this may be illustrated by the following sequence in the case where as an example, radiation is selectively applied to provoke asymmetrical C—H stretching following by total CO2 oxidation:
It may be appreciated that in the above, the energy required to break the first C—H bond in methane is about 427 kJ/mole. By application of radiation it can be appreciated that the additional energy necessary to break the first C—H bond will be less than 427 kJ/mole. Accordingly, it can be appreciated that with respect to methane oxidation, the present disclosure provides radiation treatment such that in an ensuing reaction of methane, such as in catalytic oxidation, the energy to break the first C—H bond will be less than 427 kJ/mole, or in the range of 213 kJ/mole to less than 427 kJ/mole.
It can now also be appreciated that the selective activation of the C—H bonds herein are effectively exploited in an oxidation reaction, including, but not limited to the following, wherein the reaction noted is preferably catalyzed and the excitation and weakening of the C—H bond by the above referenced radiation permits the indicated reactions to proceed at lower relative temperatures as compared to that situation where the weakening by radiation is not applied:
CH4+2O2→CO2+2H2O Total Oxidation
CH4+0.5O2→CO+2H2 Partial Oxidation
The radiation that is suitable for excitation and weakening of the C—H bond as noted herein may be obtained from a number of sources. Preferably, one can employ infrared (IR) light treatment, and more specifically, infrared light emitting diodes (IRLEDs) or infrared lasers. For example, a suitable wavelength may be selected to trigger any one or more of the methane stretch and bend wavelengths noted above. Accordingly, IR laser light treatment can be applied and preferably, over the range of 1306 cm−1 (wavelength of 7.65 μm) to 3200 cm−1 (wavelength of 3.13 μm). Either of these regions could be used where CH4 has a relatively strong absorption due to asymmetric C—H stretching or bending, as seen in the spectrum shown in
Other sources for radiation and activation of the C—H bond include solid state quantum cascade lasers (QCL) that are semiconductor lasers that emit in the infrared portion of the electromagnetic spectrum (wavelength range from 2.0 μm to 250 μm). One may also utilize a distributed feedback laser (DFL) which is a type of laser diode, quantum cascade laser or optical fiber laser where the active region of the device is periodically structured as a diffraction grating. The structure builds a one-dimensional interference grating (Bragg scattering) and the grating provides optical feedback for the laser. In addition, one may employ an interband cascade laser (ICL) which is also a type of laser diode that can produce radiation over the infrared portion of the electromagnetic spectrum.
It is also worth noting that the present disclosure, which utilizes the above referenced radiation, is more selective and relatively more efficient than the use of plasma excitation. That is the present process herein of irradiating the methane by use of laser treatment does not rely upon the use of plasma excitation, which typically involves the use of a relatively strong and broadly applied electric field and the formation of an ionized gas.
It can also be noted that one can evaluate the power of any applicable laser that may be required herein to achieve selective excitation of the applicable methane infrared vibrational mode (see again
Number of photons/sec=Power×Wavelength/hc
where h is Plank's constant and c is the speed of light. Accordingly, for a 30 mW laser or LED operating at 3.31 μm, the number of photons would be about 4.8×1017 photons/sec. The number of methane molecules in a given interaction zone (i.e. the region in which the methane is exposed to laser energy) can be estimated using Loschmidt's number at STP which yields about 1.9×1019 molecules/cm3. Loschmidt's number is reference to the number of particles (atoms or molecules) of an ideal gas in a given volume. Thus, if the LED or laser is focused into a volume of a cubic centimeter and the dwell time is one second, about two percent of the methane molecules will be activated if a probability of excitation of 80% is assumed since this is a resonant or near-resonant process. In the context of the present disclosure, while it is useful to activate any amount of methane, it is preferable that the percentage of methane molecules within a given activation zone for ensuing catalytic oxidation is in the range of greater than or equal to 10% up to 100%, which will depend upon the power output of the laser source that is ultimately selected. In addition, it should be noted that while the activation of the methane herein can be accomplished at standard temperature and pressure, it can be appreciated that one may increase temperature or pressure in which case the proportion of methane molecules that are excited will be increased. Accordingly, it is contemplated that the excitation of the methane herein may be accomplished at temperatures from ambient temperature (25° C.) to temperatures of 350° C., more preferably at temperatures of 150° C. to 350° C., as well as at 150° C. to 250° C. and at pressures from standard pressure (14.5 psia) to pressures of 20 psia.
It should also be noted that the radiation source here, such as the use of an infrared type laser, may be either in continuous mode or in pulsed operation. Pulsed operation is reference to the feature that the power output appears in pulses of some selected duration at some repetition rate. This is particularly of benefit in the case of those lasers that are not generally suitable for continuous mode operation. As the pulse energy of the laser is equal to the average power divided by the repetition rate, delivery of some relatively large amount of energy can be achieved by lowering the rate of pulses so that more energy may be built-up between pulses. That is, by application of relatively large amount of energy in a given pulse, one may selectively activate the methane molecule according to any one or more of the activated states as shown in
As noted above, since natural gas has become a potential problem with respect to emission of a natural gas engine, the need to control methane emissions resulting from incomplete combustion can now be facilitated by activating the C—H bond in methane as noted above, to improve the efficiency of the ensuing catalytic oxidation reaction.
As noted above, each one of these plurality of locations for laser activation may target the activation of one or more C—H activation modes, noted herein. That is, each of these plurality of locations for radiation activation, three of which as shown in
A suitable catalyst for methane oxidation herein may be a three-way catalyst capable of simultaneous oxidation of hydrocarbons (HC) and carbon monoxide (CO) and reduction of oxides of nitrogen (NOX) under stoichiometric, perturbed engine operating conditions. It may also be a dedicated oxidation catalyst used to oxidize HC and CO under lean engine operating conditions. Such catalysts typically contain active metals such as platinum (Pt) and rhodium (Rh) and especially palladium (Pd) which is particularly active for CH4 oxidation. It is contemplated that excitation of CH4 in the presence of any such catalyst therefore will now result in relatively higher CH4 oxidation conversion efficiency under fixed conditions, and/or a lowering of the temperature required to initiate CH4 oxidation.
In addition, it should be noted that the catalyst may be mixed or coated on a substrate such as alumina, ceria, zirconia, glass beads or ceramics, with optionally barium or strontium. The substrate may be micro- or nano-particulate and also transparent to radiation, such as infrared light, in order to facilitate the radiation activation disclosed herein. That is, the substrate for supporting the catalyst may be configured such that it will allow for transmission of infrared radiation so that the methane, when in contact with a given catalyst, is activated in the manner disclosed, e.g., as shown in
In another embodiment, it is contemplated that collision partners may be directly excited upstream of the methane oxidation catalyst or within the catalytic converter. When the lifetime of such collision partner is long enough then the catalytic reaction may occur within the catalyst with an excited state of the methane, due to collision with the excited partner molecule. For instance, there are metastable oxygen states which are known to have a relatively long-lived metastable-state lifetime. Specifically, one may selectively form the singlet oxygen state of oxygen (O2*) which can be present for time periods of up to many seconds at room temperature. Similarly, one may also rely upon activated nitrogen (sometimes referred to as “active nitrogen.”).
Singlet oxygen may be understood as the lowest excited state of the dioxygen molecule. It is therefore contemplated herein that production of O2* may now be relied upon in the following reaction sequence where the activated oxygen collides with methane:
O2*+CH4→Activated CH4
The activated CH4 includes the activated configurations of methane illustrated in
While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.
