Patent Publication Number: US-2015078975-A1

Title: Natural gas engine aftertreatment system

Description:
BACKGROUND 
     Starting in 2014, on-highway engines are to be in compliance with new greenhouse gas regulations including, possibly for the first time, methane hydrocarbon emissions. Natural gas engines, in particular, emit high amounts of methane (CH 4 ), which contribute to greenhouse gas emissions. Some consider methane to have a global warming potential that is 25 times greater than carbon dioxide (CO 2 ). 
     One of the challenges of methane emission control is that, for example, methane combustion/oxidation occurs at high temperatures (e.g., greater than approximately 350-450° C.). 
     In addition, platinum group metals (PGM) and metal oxides containing catalysts are not active at low temperatures. 
     SUMMARY 
     Some embodiments relate to systems and methods of methane reduction in aftertreatment systems and, in particular, in aftertreatment systems in natural gas engines. 
     Some embodiments provide that oxidation catalysts are introduced upstream of a turbocharger and downstream of an exhaust outtake of an engine block to provide high exhaust temperatures for use in oxidizing methane. 
     Some embodiments provide that methane is adsorbed at low temperatures using a catalytic substrate. At high temperatures, the adsorbed methane undergoes catalytic combustion. The catalyst can include, for example, non-PGM metal oxides dispersed on a zeolite substrate. The zeolite substrate can provide low temperature storage and oxides for combustion. 
     Some embodiments provide that substrate material and catalyst combinations are contemplated that adsorb methane emissions at low temperatures and then releases oxidized methane at high temperatures that result in lowering methane hydrocarbon emission. 
     Some embodiments provide an aftertreatment system for use in a vehicle that includes an oxidation catalyst disposed in an exhaust path between an engine block and a turbocharger. The oxidation catalyst is configured to store methane during a first temperature of a first portion of an operating engine cycle and to oxidize the stored methane during a second temperature of a second portion of the operating engine cycle in which the first temperature is less than the second temperature. 
     Some embodiments provide a method of reducing methane emissions in a vehicle. An oxidation catalyst is provided in an exhaust path between an engine block and a turbocharger. Methane is stored during a first temperature. The stored methane is oxidized during a second temperature that is greater than the first temperature. A back pressure valve is provided to increase pressure in the exhaust path or to increase engine load. The temperature is increased from the first temperature to the second temperature by increasing, via the back pressure valve, the pressure or the engine load. 
     Some embodiments provide for exhaust temperature management (e.g., at light load) by providing engine speed and higher back pressure to increase the engine load which, in turn, increases exhaust temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an embodiment of a natural gas engine aftertreatment system. 
         FIG. 2  is another embodiment of the natural gas engine aftertreatment system. 
         FIG. 3A  is a chart illustrating methane conversions for different materials over temperature. 
         FIG. 3B  is another chart illustrating methane conversions for yet other materials over temperature. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments relate to systems and methods of methane reduction in aftertreatment systems and, in particular, in aftertreatment systems in natural gas engines. 
     Referring to  FIG. 1 , a layout of an embodiment of a compression-ignition (CI) natural gas engine aftertreatment system  100  is shown. The aftertreatment system  100  can be used, for example, with dual fuel engines (e.g., natural gas and diesel engines). As illustrated, a cylinder block  110  has an exhaust outtake  120  that is coupled to an oxidation catalyst (OC)  130 . The OC  130  is then coupled to a turbocharger system  140  and to an exhaust gas recirculation (EGR) system  150 . 
     In some embodiments, the OC  130  can be include, for example, a zeolite substrate that is Cu laden with, for example, one or more of the following: CuCe(La)O 2  and CuZr(Y)O 2 . In some embodiments, the zeolite substrate can include, for example, one or more of the following: CuCe(La), Ce(La)O 2 , CeO 2 , CuZr(Y), Zr(Y)O 2  and ZrO 2 . Some embodiments contemplate designing zones of coating and size for methane storage and oxidation. 
     The turbocharger system  140  can include, for example, a turbine  160  coupled to a compressor  170 . The turbine is coupled to a device that includes of one or more of the following: a diesel oxidation catalyst, a diesel particulate filter, and a selective catalytic reduction. The device illustrated in  FIG. 1  as DOC/DPF/SCR  180  is then coupled to a back pressure valve  190 . The compressor  170  is coupled to an air supply  190 . The compressor  170  is also coupled to a charge air cooler  210  and a throttle  220 . The throttle  220  is coupled to an intake  230  of the engine block  110 . 
     The EGR system  150  can include, for example, an EGR cooler bypass path  240  and an EGR cooler path  250 . The EGR cooler path  250  can include, for example, an EGR cooler  260 . Both paths  240  and  250  are coupled to an EGR valve  270  which, in turn, is coupled to the intake  230  of the engine block  110 . 
     In operation, the engine block  110  outputs exhaust (e.g., exhaust gas or exhaust gas stream) from the exhaust outtake  120 . In some embodiments, the exhaust gas coming out of a cylinder of the engine block  110  is in direct contact with the methane storage and oxidation catalyst of the OC  130 . The temperature of the exhaust gas is substantially higher than the temperature of the exhaust gas after passing through the turbine  160  of the turbocharger system  140 . It is, in part, the substantially higher temperature of the exhaust gas that provides for methane hydrocarbon oxidation. 
     The temperature at the OC  130  can be cycled due, in part, to engine operating cycles of the engine block  110  and/or due, in part, to control of the back pressure valve  190 . In light load, the back pressure valve  190  can raise the temperature of the exhaust. In some embodiments, the back pressure valve  190  is configured such that when the back pressure valve  190  is closed, the pressure at the OC  130  increases, and that when the back pressure valve  190  is opened, the pressure at the OC  130  decreases. Methane (CH 4 ) gas in the exhaust is adsorbed at the OC  130  at low temperatures. The adsorbed methane undergoes catalytic combustion at high temperatures. At high temperatures, the OC  130  provides oxides that are used to oxidize the adsorbed methane. 
     Since the OC  130  is disposed between the turbocharger system  140  and the engine block  110 , the temperature at the OC  130  can be high. By placing the oxidation catalyst upstream of the turbocharger system  140 , the OC  130  can use the higher exhaust temperature for methane oxidation since the high temperature enhances the methane reduction. In addition, the back pressure valve  190  in the exhaust path can be closed to raise engine load and to increase exhaust temperatures include, for example, the temperature at the OC  130 . 
       FIGS. 3A-B  show how the methane conversion (e.g., methane reduction or methane oxidation) increases with temperature for various materials and compositions of materials including, for example, CuCe(La), Ce(La)O 2 , CeO 2 , CuZr(Y), Zr(Y)O 2  and ZrO 2 . See, e.g., Kundakovic et al., Appl. Cat., 171 (1978) 13-29.  FIGS. 3A-B  illustrate CuO that is dispersed in La-doped CeO 2  and Y-doped ZrO 2 . In Cu/Ce(La)O 2  and/or CuO/Zr(Y)O 2 , the CeO 2  is active for methane oxidation. The catalysts are active in the range of approximately 300-550° C. The amount of CuO and the amount of dispersant placed a role in methane conversion. In addition, copper clusters dispersed on the support are highly active. 
     Continuing with the explanation of the operation and referring to  FIG. 1 , the exhaust (e.g., exhaust gas) exits the OC  130  and enters the turbine  160  of the turbocharger system and the EGR system  150 . With respect to the exhaust path through the turbocharger system  140 , the exhaust powers the turbine  160  which, in turn, powers the compressor  170  via, for example, a rotor shaft. Once the exhaust powers the turbine  160 , the exhaust passes to the DOC/DPF/SCR  180 . The air supply  200  supplies air to the compressor  170  which, in turn, compresses the air. Due to the compression, the temperature of the air increases. The charge air cooler  210  (e.g., an intercooler) cools the compressed air. The throttle  220  throttles the air before passing the air to the intake  230  of the engine block  110 . 
     With respect to the exhaust path through the EGR system  150 , the exhaust can either bypass the EGR cooler  250  or pass through the EGR cooler  250 . In some embodiments, the EGR cooler  250  cools the exhaust to reduce combustion temperatures in the engine block  110 . Lower temperatures can reduce the amounts of greenhouse gases (e.g., NOx) produced by the engine block  110 . Dependent upon whether the EGR valve is opened or closed, the exhaust passes from the EGR system  150  into the intake  230  of the engine block  110 . 
     In some embodiments, the EGR bypass valve  245  is opened at light load to raise the intake manifold temperature which, in turn, raises in-cylinder combustion temperature and exhaust temperatures to enable oxidation of methane hydrocarbons. Further, the use of EGR system  150  at light load will employ less throttling of the engine to maintain the appropriate air fuel mixture. The reduction in throttling loss improves fuel consumption according to some embodiments. 
     In the back end of the aftertreatment system  100 , the DOC/DPF/SCR  180  further reduces greenhouse gases (e.g., NOx). In some embodiments, a DOC portion of the DOC/DPF/SCR  180  uses oxygen in the exhaust to convert carbon monoxide (CO) to carbon dioxide (CO 2 ), and to convert hydrocarbons (HCs) to water (H 2 O) and carbon dioxide. In addition, a DPF portion of the DOC/DPF/SCR trap soot (e.g., particulates). 
     In some embodiments, the DOC/DPF/SCR  180  is a combined DOC/DPF/SCR. In other embodiments, the DOC/DPF/SCR  180  is a combined DOC/DPF. Some embodiments contemplate using one or more of the DOC, DPF, and SCR in the DOC/DPF/SCR  180 . In some embodiments, the DOC/DPF/SCR  180  is the SCR. In embodiments that include the SCR, the SCR can be used, for example, to convert NOx to nitrogen (N 2 ) and water. 
     The back pressure valve  190 , as mentioned above, can be closed or opened as desired to increase or decrease pressure in the exhaust system. For example, a closed back pressure valve  190  will increase the pressure and thus the temperature at the OC  130 . 
     Referring to  FIG. 2 , a layout of an embodiment of a spark-ignition (SI) natural gas stoichiometric engine aftertreatment system  280  is shown. Besides illustrating an SI stoichiometric engine aftertreatment system  280  instead of a CI dual fuel engine aftertreatment system  100  (as illustrated in  FIG. 1 ), one of the differences is that  FIG. 2  illustrates a three-way catalyst (TWC)  290  instead of the DOC/DPF/SCR  180 . In some embodiments, the TWC  290  is used with the natural gas stoichiometric engine aftertreatment system  280  to provide one or more catalysts on a substrate to oxidize carbon monoxide and unburned hydrocarbons, and to reduce NOx to produce carbon dioxide, nitrogen and water. With respect to the other components in  FIG. 2 , they also appear in  FIG. 1  and function as previously described. 
     Some embodiments provide systems and methods of methane oxidation, combustion or reduction in aftertreatment systems and, in particular, in aftertreatment systems in natural gas or dual fuel engines. 
     Some embodiments provide that oxidation catalysts are introduced upstream of a turbocharger and downstream of an exhaust outtake of an engine block to provide high exhaust temperatures for use in oxidizing methane. 
     Some embodiments provide that methane is adsorbed at low temperatures using a catalytic substrate. At high temperatures, the adsorbed methane undergoes catalytic combustion. The catalyst can include, for example, non-PGM metal oxides dispersed on a zeolite substrate. The zeolite substrate can provide low temperature storage and oxides for combustion. 
     Some embodiments provide that substrate material and catalyst combinations are contemplated that adsorb methane emissions at low temperatures and then releases oxidized methane at high temperatures that result in lowering methane hydrocarbon emission. 
     Some embodiments provide for exhaust temperature management (e.g., at light load) by providing engine speed and higher back pressure to increase the engine load which, in turn, increases exhaust temperature. 
     Some embodiments contemplate that there is a trade-off between sizing and performance in reducing methane emissions to maintain reasonable engine compartment packaging. 
     Some embodiments contemplate that controls are provided that address de-soot and de-SOx to maintain catalyst performance. 
     Some embodiments contemplate that the oxidation catalyst that controls methane emissions is applicable for SI and diesel-pilot-ignition-type (DPI-type) natural gas engines. 
     Some embodiments contemplate that the oxidation catalyst is applicable for reducing non-methane hydrocarbon (NMHC) emissions. Examples of NMHCs include, for example, ethylene (C 2 H 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), etc. 
     Some embodiments contemplate using PGM-based and/or non-PGM-based catalysts. 
     Some embodiments contemplate matching the oxidation catalyst with post-turbo DOC+EGNR aftertreatment for natural gas engines with lean (e.g., non-stoichiometric) combustion. 
     Some embodiments provide the oxidation catalyst between the turbocharger and the engine block for use with vehicles such as, for example, light duty trucks, medium duty trucks, heavy duty trucks, cars, motor bikes, motorcycles, boats, buses, vans, minivans, trucks, sports utility vehicles (SUVs), natural gas vehicles, hybrid fuel vehicles, dual fuel vehicles, multiple fuel vehicles, etc. 
     Some embodiment use elements, components and/or perform operations that are used in one or more of the above-disclosed embodiments. Thus, for example, an element or component in one embodiment can be combined with an element or component in another embodiment to provide yet another embodiment. In addition, some embodiments contemplate different levels of integration between the various elements or components.