Patent Application
20080072575
The FIGURE provides a schematic illustration of an exemplary power generation system conceived by the inventor.
The FIGURE provides a schematic illustration of an exemplary power generation system 1 conceived by the inventor. The power generation system 1 comprises an engine 4 connected to an exhaust aftertreatment system 2 by a manifold 5. The exhaust aftertreatment system 2 includes a controller 8 and an exhaust line 3 defining a direction of exhaust flow beginning from the manifold 5. Arranged in series with respect to the direction of the exhaust flow in the exhaust line 3 are a first fuel injector 17, a hydrocarbon SCR catalyst 6, a second fuel injector 18, a fuel reformer 10, a diesel particulate filter (DPF) 14, a lean NOx-trap (LNT) 11, and a SCR catalyst 12. The controller 8 may be an engine control unit (ECU) that also controls the exhaust aftertreatment system 2 or may include several control units.
The engine 4 is operative to produce an exhaust that contains NOx and particulate matter. The hydrocarbon content of the exhaust can be selectively increased by altering operating parameters of the engine 4 such as the fuel-air ratio, the injection timing, and the extent of EGR. The hydrocarbon content of the exhaust can also be increased and the exhaust made rich as needed without altering the operation of the engine 4, for example, by injecting fuel into the exhaust line 3 through one or both of the fuel injectors 17 and 18.
During lean operation (a lean phase), the LNT 11 adsorbs and stores a significant amount of NOx from the exhaust, provided the LNT 11 is within an effective operating temperature range for NOx mitigation. An additional portion of NOx in the exhaust may also be reduced over the ammonia SCR catalyst 12, provided that the ammonia SCR catalyst 12 contains stored ammonia from a previous regeneration of the LNT 11. An effective operating temperature range for the LNT 11 is typically from about 300 to about 450° C., the actual operating temperature range depending on the LNT formulation.
If the LNT 11 is below its effective operating temperature range and there is no ammonia stored in the SCR catalyst 12, the exhaust aftertreatment system 2 can still mitigate NOx emissions by reducing NOx through selective catalytic reduction over the hydrocarbon SCR catalyst 6. In order to reduce a significant amount of NOx in this manner, the exhaust is selectively dosed with hydrocarbons. Hydrocarbon dosing can be achieved by altering the operating of the engine 4, injecting fuel into engine cylinders during exhaust strokes, or by injecting hydrocarbons through the optional fuel injector 17.
The provision of hydrocarbons to act as a reductant for SCR over the hydrocarbon SCR catalyst 6 is controlled by the controller 8. For example, the controller 8 can determine that the LNT 11 is below its effective operating temperature range by interpreting signals from the temperature sensor 16. The LNT 11 is generally below its effective operating temperature range immediately following start of the engine 4 and may also fall below that temperature at various times during operation of the engine 4, such as during long periods of engine idling. In response to a determination that the LNT 11 is below its effective operating temperature range, the controller 8 initiates dosing of the exhaust upstream of the hydrocarbon SCR catalyst 6 with hydrocarbons. The dosing rate is preferably proportional to the NOx flow rate in the exhaust.
In order to determine the amount of hydrocarbon required to reduce NOx over the hydrocarbon SCR catalyst 6, the controller 8 obtains an estimate for the NOx flow rate in the exhaust. Any suitable estimate can be used. For example, an estimate can be made based on a sensor measurement of NOx concentration in the exhaust. Alternatively, an estimate can be made without measurement based solely on the operating state of the engine 4. The relationship between engine operating state and NOx emissions can determined, for example, by calibration.
The hydrocarbon SCR catalyst 6 is preferably operative to catalyze hydrocarbon SCR at 200° C. Preferably, the hydrocarbon SCR catalyst 6 is operative for SCR at temperatures up to at least about 250° C. Preferably, the hydrocarbon SCR catalyst 6 is operative for SCR at temperatures at least down to about 175° C. A suitable catalyst operative for hydrocarbon SCR over these preferred ranges is a platinum catalyst on a high surface area support, such as alumina.
Providing for low temperature SCR over the hydrocarbon SCR catalyst 6 reduces the need to design the LNT 11 to be effective for NOx mitigation at lower temperatures. Reducing this requirement makes it easier to design the LNT 11 to be operational at high temperatures and also makes it easier to design the LNT 11 to be durable.
The hydrocarbon SCR catalyst 6 may perform additional functions as well as catalyzing low temperature hydrocarbon SCR. One possible function is catalyzing conversion of NO to NO2. NO2 is adsorbed more easily by the LNT 11 than NO. Converting NO to NO2 over the hydrocarbon SCR catalyst 6 can therefore enhance the performance of the LNT 11. This is particularly useful if NO to NO2 oxidation occurs near the low end of the operating temperature range of the LNT 11 where the effectiveness of the LNT 11 is flagging. NO to NO2 catalysis can also facilitate continuous regeneration of the DPF 14. The hydrocarbon SCR catalyst 6 may also function as a low temperature oxidation catalyst that can be used to heat the fuel reformer 10.
From time-to-time, a rich phase is initiated to regenerate the LNT 11 to remove stored NOx (denitration). Denitration may involve heating the reformer 10 to an operational temperature and then injecting fuel using the fuel injector 18 to make the exhaust rich. The fuel reformer 10 uses the injected fuel to consume most of the oxygen from the exhaust while producing reformate. The reformate thus produced reduces NOx adsorbed in the LNT 11.
The time at which to regenerate the LNT 11 to remove accumulated NOx can be determined by any suitable method. Examples of methods of determining when to begin a regeneration include initiating a regeneration upon reaching a threshold in any of NOx concentration in the exhaust, total NOx emissions per mile or per brake horsepower-hour over a period, such as the period since the last regeneration, total amount of NOx produced by the engine since the last regeneration, estimated NOx loading in the LNT 11, and estimated adsorption capacity remaining in the LNT 11. Regeneration can be periodic or determined by feed forward or feedback control. Regeneration can also be opportunistic, being triggered by engine operating conditions that favor low fuel penalty regeneration. A threshold for regeneration can be varied to give a trade off between urgency of the need to regenerate and favorability of the current conditions for regeneration. The time at which to regenerate the LNT 11 can be determined by the controller 8, which generates a control signal that initiates the regeneration process.
The reformer 10 preferably comprises a steam reforming catalyst and is adapted to produce reformate at least in part by steam reforming reactions. Steam reforming reactions generally do not occur at effective rates at temperature below about 550° C. Accordingly, when the control signal to denitrate the LNT 11 is received, it is generally necessary to first heat the fuel reformer 10 in response to the control signal prior to making the exhaust rich.
The reformer 10 is preferably heated by providing hydrocarbon to the exhaust at a rate that leaves the exhaust lean. If the reformer is sufficiently warm, the hydrocarbon will undergo complete combustion in the reformer 10, generating heat. The reformer 10, however, has a minimum start-up temperature. Below this minimum start-up temperature, the reformer 10 is too cool to effectively catalyze combustion and cannot be effectively heated simply by supplying it with lean exhaust. The minimum temperature can be lowered by providing the reformer 10 with more catalyst. The disadvantage of adding this catalyst is that it increases the cost of the reformer 10. Moreover, enhancing the low temperature startup performance of the reformer 10 may come at the expense of high temperature performance and durability.
In a preferred embodiment, the low temperature startup of the reformer 10 is facilitated by the hydrocarbon SCR catalyst 6. Below the reformer 10's minimum start-up temperature, hydrocarbon can be provided to the hydrocarbon SCR catalyst 6, which is preferably functional to combust the hydrocarbon at temperatures below the reformer 10's minimum start-up temperature. Hydrocarbon combustion over the hydrocarbon SCR catalyst 6 can be used to heat the reformer 10 at least to its minimum start-up temperature.
Hydrocarbons for combustion in the hydrocarbon SCR catalyst 6 can be provided by the engine 4 or through the fuel injector 17. These sources can also be used to provide hydrocarbon to the reformer 10 for combustion or reforming therein. Preferably, however, hydrocarbon for combustion or reforming in the reformer 10 is provided by the fuel injector 18, whereby these hydrocarbons do not flow through the hydrocarbon SCR catalyst 6. Injecting these hydrocarbons downstream of the hydrocarbon SCR catalyst 6 avoids the risk that combustion of these hydrocarbons in the hydrocarbon SCR catalyst 6 will overheat the hydrocarbon SCR catalyst 6.
From time-to-time, the LNT 11 must also be regenerated to remove accumulated sulfur compounds (desulfated). Desulfation involves heating the reformer 10, heating the LNT 11 to a desulfating temperature, and providing the heated LNT 11 with a rich atmosphere. Desulfating temperatures vary, but are typically in the range from about 550 to about 850° C., the exact range depending on the LNT composition. Below a minimum temperature, desulfation is very slow. Above a maximum temperatures, the LNT 11 may deteriorate and undergo a loss of activity.
The time at which to desulfate the LNT 11 can be determined in any suitable fashion. Desulfation may be scheduled periodically, e.g., after every 30 hours of operation. Alternatively, desulfation may be scheduled based on an estimate of the amount on SOx stored in the LNT 11. The amount of stored SOx can be assumed to increase in proportion to fuel usage and to decrease in a manner dependent on the extent of desulfations. A further option is to determine the need for desulfation based on system performance, e.g., based on the activity of the LNT 11 following an extensive denitration or based on the frequency with which denitration is required
The primary means of heating the LNT 11 is heat convection from the reformer 10. To generate this heat, fuel can be supplied to the reformer 10 under lean conditions, whereby the supplied fuel undergoes complete combustion in the reformer 10. Once the reformer 10 is heated, the fuel injection rate can be controlled to maintain the temperature of the reformer 10 while the LNT 11 is heating. Optionally, if the reformer 10 is below its minimum startup temperature, the reformer 10 can be heated using the hydrocarbon SCR catalyst 6, if the hydrocarbon SCR catalyst 6 is suitably formulated, as described previously.
The exhaust aftertreatment system 2 also includes a DPF 14 for particulate matter control. The DPF 14 can be placed at any suitable location in the exhaust aftertreatment system 2. The illustrated location has the advantage that it protects the LNT 11 from temperature excursion during denitration. Reducing the number or magnitude of temperature excursions experienced by the LNT 11 can extend its life.
The DPF 14 can be a wall flow filter or a pass through filter and can use primarily either depth filtration of cake filtration. Cake filtration is the primary filter mechanism in a wall flow filter. In a wall flow filter, the soot-containing exhaust is forced to pass through a porous medium. Typical pore diameters are from about 0.1 to about 1.0 μm. Soot particles are most commonly from about 10 to about 50 nm in diameter. In a fresh wall flow filter, the initial removal is by depth filtration, with soot becoming trapped within the porous structure. Quickly, however, the soot forms a continuous layer on an outer surface of the porous structure. Subsequent filtration is through the filter cake and the filter cake itself determines the filtration efficiency. As a result, the filtration efficiency increases over time.
In contrast to a wall flow filter, in a flow through filter the exhaust is channeled through macroscopic passages and the primary mechanism of soot trapping is depth filtration. The passages may have rough walls, baffles, and bends designed to increase the tendency of momentum to drive soot particles against or into the walls, but the flow is not forced though micro-pores. The resulting soot removal is considered depth filtration, although the soot is generally not distributed uniformly with the depth of any structure of the filter. A flow through filter can also be made from temperature resistant fibers, such as ceramic or metallic fibers, that span the device channels. A flow through filter can be larger than a wall flow filter having equivalent thermal mass
The DPF 14 must be regenerated to remove accumulated soot. Two general approaches to DPF regeneration are continuous and intermittent regeneration. In continuous regeneration, a catalyst is provided upstream of the DPF 14 to convert NO to NO2. NO2 can oxidize soot at typical diesel exhaust temperatures and thereby effectuate continuous regeneration. Intermittent regeneration involves heating the DPF 14 to a temperature at which soot combustion is self-sustaining in a lean environment. Typically this is a temperature from about 400 to about 600° C., depending in part on what type of catalyst coating has been applied to the DPF to lower the soot ignition temperature. The reformer 10 can be used to heat the DPF 14 to the required temperature.
While the engine 4 is preferably a compression ignition diesel engine, the various concepts of the inventor are applicable to power generation systems with lean-burn gasoline engines or any other type of engine that produces an oxygen rich, NOx-containing exhaust. For purposes of the present disclosure, NOx consists of NO and NO2.
The power generation system can have any suitable type of transmission. A transmission can be a conventional transmission such as a counter-shaft type mechanical transmission, but is preferably a CVT. A CVT can provide a much larger selection of operating points than a conventional transmission and generally also provides a broader range of torque multipliers. The range of available operating points can be used to control the exhaust conditions, such as the oxygen flow rate, exhaust temperature, and the exhaust hydrocarbon content. A given power demand can be met by a range of torque multiplier-engine speed combinations. A point in this range that gives acceptable engine performance while best meeting a control objective, such as matching the hydrocarbon content to the NOx flow rate as nearly as possible.
In general, a CVT will also avoid or minimize interruptions in power transmission during shifting. Examples of CVT systems include hydrostatic transmissions; rolling contact traction drives; overrunning clutch designs; electrics; multispeed gear boxes with slipping clutches; and V-belt traction drives. A CVT may involve power splitting and may also include a multi-step transmission.
A preferred CVT provides a wide range of torque multiplication ratios, reduces the need for shifting in comparison to a conventional transmission, and subjects the CVT to only a fraction of the peak torque levels produced by the engine. This can be achieved using a step-down gear set to reduce the torque passing through the CVT. Torque from the CVT passes through a step-up gear set that restores the torque. The CVT is further protected by splitting the torque from the engine, and recombining the torque in a planetary gear set. The planetary gear set mixes or combines a direct torque element transmitted from the engine through a stepped automatic transmission with a torque element from a CVT, such as a band-type CVT. The combination provides an overall CVT in which only a portion of the torque passes through the band-type CVT.
The hydrocarbon SCR catalyst can have any suitable formulation. Suitable catalysts include platinum group metal (PGM) catalysts, with platinum being most preferred. Where a low temperature oxidation function is desired for reformer heating, a PGM catalyst can also be used, with Pt and Pd being preferred. NO to NO2 oxidation can also be effectuated with a PGM catalyst, Pt again being preferred. A PGM catalyst typically includes a high surface area support. Examples of high surface area supports include alumina, silica, TiO2, and other metal oxide supports commonly used in emission control catalysts.
The fuel reformer 10 is a device that converts heavier hydrocarbons into lighter compounds without fully combusting the fuel. The fuel reformer 10 can be a catalytic reformer or a plasma reformer. Preferably, the fuel reformer 10 comprises both partial oxidation and steam reforming catalysts and is capable of auto-thermal operation. Examples of reformer catalysts include precious metals, such as Pt, Pd, and Rh, and oxides of Al, Mg, and Ni, the later group being typically combined with one or more of CaO, K2O, and a rare earth metal such as Ce and La to increase activity. The fuel reformer 10 is preferably small in size as compared to an oxidation catalyst or a three-way catalyst designed to perform its primary functions at temperatures below 450° C. The reformer 10 is generally operative at temperatures within the range from about 450 to about 1100° C.
The LNT 11 can comprise any suitable NOx-adsorbing material. Examples of NOx adsorbing materials include oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. Further examples of NOx-adsorbing materials include ceria, alumina, and activated carbon. Still further examples include metal phosphates, such as phosphates of titanium and zirconium. Generally, the NOx-adsorbing material is an alkaline earth oxide. The absorbent is typically combined with a binder and applied as a coating over an inert substrate.
The LNT 11 also comprises a catalyst for the reduction of NOx in a reducing environment. The catalyst can be, for example, one or more transition metals, such as Au, Ag, and Cu, group VIII metals, such as Pt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical catalyst includes Pt and Rh. Precious metal catalysts also facilitate the adsorbent function of alkaline earth oxide absorbers.
Adsorbents and catalysts according to the present invention are generally adapted for use in vehicle exhaust systems. Vehicle exhaust systems create restriction on weight, dimensions, and durability. For example, a NOx adsorbent bed for a vehicle exhaust systems must be reasonably resistant to degradation under the vibrations encountered during vehicle operation.
The ammonia-SCR catalyst 12 is a catalyst functional to catalyze reactions between NOx and NH3 to reduce NOx to N2 in lean exhaust. Examples of SCR catalysts include oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, W, and Ce, zeolites, such as ZSM-5 or ZSM-11, substituted with metal ions such as cations of Cu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCR catalyst 12 is designed to tolerate temperatures required to desulfate the LNT 11.
Although not illustrated in any of the figures, a clean-up catalyst can be placed downstream of the other aftertreatment devices. A clean-up catalyst is preferably functional to oxidize unburned hydrocarbons from the engine 4, unused reductants, and any H2S released from the LNT 11 and not oxidized by the ammonia-SCR catalyst 12. Any suitable oxidation catalyst can be used. To allow the clean-up catalyst to function under rich conditions, the catalyst may include an oxygen-storing component, such as ceria. Removal of H2S, where required, may be facilitated by one or more additional components such as NiO, Fe2O3, MnO2, Co2O3, and CrO2 supported by refractory inorganic oxide.
The invention as delineated by the following claims has been shown and/or described in terms of certain concepts, components, and features. While a particular component or feature may have been disclosed herein with respect to only one of several concepts or examples or in both broad and narrow terms, the components or features in their broad or narrow conceptions may be combined with one or more other components or features in their broad or narrow conceptions wherein such a combination would be recognized as logical by one of ordinary skill in the art. Also, this one specification may describe more than one invention and the following claims do not necessarily encompass every concept, aspect, embodiment, or example described herein.
