This invention is directed to the removal of nitrogen oxides (NOx) from the exhaust gases of internal combustiobn engine, particularly diesel engines, which operate at combustion conditions with air in large excess of that required for stoichiometric combustion, i.e., lean. Unfortunately, the presence of excess air makes the catalytic reduction of nitrogen oxides difficult. Emission regulations impose a limit on the quantity of specific emissions, including NOx, that a vehicle can emit during a specified drive cycle, such as i) for light duty trucks, an FTP (“federal test procedure”) in the United States or an MVEG (“mobie vehicle emissions group”) in Europe or ii) for heavy duty trucks, a Heavy Duty Cycle in the United States or an ESC (European Steady State Cycle) or ETC (European Transient Cycle) in Europe. The regulations are increasingly limiting the amount of nitrogen oxides that can be emitted during the regulated drive cycle.
There are numerous ways known in the art to remove NOx from a waste gas. This invention is directed to a catalytic reduction method for removing NOx. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Conventionally, there are three ways to treat vehicular exhaust to reduce NOx. The first method is non-selective catalyst reduction (NSCR). The second way is selective non-catalytic reduction (SNCR) and the last method is selective catalyst reduction (SCR). This invention relates to SCR systems.
In diesel engines, sufficient NOx reduction to meet current regulations has been achieved by combustion modifications in the diesel engine by, for example, incorporating EGR. Projected emission levels are such that combustion and engine modifications will not be sufficient to meet the more stringent levels. Because of excess oxygen present in diesel exhaust gases, the opportunity for NOx reduction under rich or stoichiometric air/fuel is not possible. SCR is a technology that has been shown effective in removing NOx from oxygen rich exhaust. A number of SCR systems have been developed which, because of infrastructure concerns, have used diesel fuel or diesel oil as the reductant source. Unfortunately, as of this date, an HC reducing catalyst has not yet been developed which has sufficient activity and is effective over the entire operating range of the diesel engine.
A common nitrogen oxide reducing agent, long used in industrial processes, is ammonia. NOx reducing catalysts have been developed which are effective over the operating range of the engine. Despite the infrastructure concerns relating to the use of urea in a mobile application as well as the potentially dangerous risks of ammonia break-through or slip, ammonia SCR systems are becoming the favored choice for mobile applications to meet the more stringent NOx emissions. This is, among other reasons, because of the high NOx conversion percentages possible with ammonia coupled with the ability to optimize the combustion process for maximum power output with minimum fuel consumption.
Notwithstanding what may be said to be inherent advantages of an ammonia based SCR system, the control systems to date have been excessively complicated and/or ineffective to control the SCR system when the impact of NOx transient emissions on the SCR system is considered. As will be shown below, if the transient NOx emissions can not be adequately reduced by the SCR control system, then stringent NOx emission regulations will not be met.
Early patents controlled ammonia metering by considering the emissions to be controlled at steady state conditions. For example, U.S. Pat. No. 4,403,473 to Gladden (Sep. 13, 1983) considered NOx emissions at various speed ranges and concluded that a linear relationship exists between fuel flow and NOx. (Earlier Gladden U.S. Pat. No. 4,188,364, Feb. 12, 1980 concluded that ammonia catalyst adsorbed ammonia at temperatures lower than 200° C. and desorbed at temperatures between 200–800° C., the SCR system should operate at higher, temperatures to achieve complete reaction between ammonia and NOx.) Thus, in Gladden '473, the fuel mass flow is sensed and NH3 throttled at a percentage of fuel flow provided the temperature of the gases in the catalytic converter are within a set range. This basic control concept is used today in most mobile, ammonia, SCR systems. For example, U.S. Pat. No. 5,116,579 to Kobayashi et al. (May 26, 1992) additionally measures the humidity of intake air and one or more operating parameters of engine power, intake air temperature, fuel consumption and exhaust gas temperature to set an ammonia ratio control valve. The molar ratio of ammonia to NOx is set at less than one (sub-stoichiometric) to minimize ammonia slip.
Typically the reductant is pulse metered into the exhaust gas stream in a manner similar to that used for operating conventional fuel injectors. In U.S. Pat. No. 4,963,332 to Brand et al. (Oct. 16, 1990), NOx upstream and downstream of the catalytic converter is sensed and a pulsed dosing valve controlled by the upstream and downstream signals. In U.S. Pat. No. 5,522,218 to Lane et al. (Jun. 4, 1996), the pulse width of the reductant injector is controlled from maps of exhaust gas temperature and engine operating conditions such as engine rpm, transmission gear and engine speed.
As noted, the industrial art has long used ammonia in SCR systems to control NOx emissions typically by set point control loops such as shown in U.S. Pat. No. 5,047,220 to Polcer (Sep. 10, 1991) in which a downstream NOx sensor is used to generate a trim signal in the control loop. The industrial art has also recognized that changes in load from the turbine, furnace etc. affects the ammonia SCR systems. Thus in U.S. Pat. No. 4,314,345 to Shiraishi et al. (Feb. 2, 1982), variations in load are determined by sensing the temperature of the exhaust gas. When the exhaust gases are at certain temperature ranges a variation in the load is assumed to occur and different or predicted NH3/NOx molar ratios are used to account for the adsorption/desorption characteristics of the catalyst. A more sophisticated molar ratio control system is disclosed in U.S. Pat. No. 4,751,054 to Watanabe. Watanabe uses not only upstream and downstream NOx sensors but also temperature, flow rate and NH3 detectors to set a mole ratio correcting signal. In U.S. Pat. No. 4,473,536 to Carberg et al. (Sep. 25, 1984) a turbine's inlet airflow, discharge pressure, discharge temperature and mass fuel flow are sensed to predict NOx generated by the turbine which signal is corrected for NOx error by time delayed NOx sensor measurements. Carberg recognizes that turbine load changes may change NOx emissions in a time frame quicker than the 1 plus minute needed to determine the NOx emissions in a gas sample with conventional NOx sensors and thus makes a prediction, which can not be corrected in real time. The industrial systems, for the most part, do not operate under the highly transient conditions which characterize vehicle engines producing sudden NOx transients. Industrial systems also operate in an environment in which samples of the gas being produced can be taken to accurately determine the NOx content to trim the ammonia metering valve in closed loop control.
In addition to systems which sense engine operating parameters to control metering of ammonia or a reductant, there are other approaches used to control NOx emissions in mobile applications. In U.S. Pat. No. 5,845,487 to Fraenkle et al. (Dec. 8, 1998), the exhaust gas temperature is sensed. If the exhaust gas is outside the temperature limits at which the SCR system is effective i.e., below the operating temperature, the fuel injection timing to the engine is retarded, reducing the NOx via combustion modifications. In U.S. Pat. No. 5,842,341 to Kibe (Dec. 1, 1998) space velocity and exhaust gas temperature is measured to determine the reductant quantity. In addition inlet and outlet catalytic converter temperature is measured and reductant flow is decreased from the steady state conditions when the temperature differential between inlet and outlet begins to increase. The reductant, disclosed as HC in Kibe's preferred embodiment, does not according to Kibe otherwise contribute, by exothermic HC oxidation reactions, to heating of the catalyst mass or bed. The reductant is decreased to keep the catalyst within the operating temperature window.
Perhaps one of the more sophisticated approaches to using urea/ammonia system in a mobile application is disclosed in a series of patents which include U.S. Pat. No. 5,833,932 to Schmelz (Nov. 10, 1998); U.S. Pat. No. 5,785,937 to Neufert et al. (Jul. 28, 1998); U.S. Pat. No. 5,643,536 to Schmelz (Jul. 1, 1997); and U.S. Pat. No. 5,628,186 to Schmelz (May 13, 1997). While these patents discuss reducing reagents in a general sense, they are clearly limited to urea/ammonia reductants. According to this system, a catalytic converter having composition defined in the '932 patent, has a reducing agent storage capacity per unit length that increases in the direction of gas flow. This allows for positioning of instrumentation along the length of the catalyst as disclosed in the '536 patent to determine the quantity of ammonia stored in the catalyst. The catalyst is charged with the reducing agent such that transient emissions can be converted by the reducing agent stored in the catalytic converter. The '186 patent, however, is directed as is the present invention, to a control system not limited to any specific catalyst. The '186 patent recognizes, as does several prior art references discussed in this section, that i) sudden increases in load require a decrease in the reducing agent (and similarly sudden decreases in load require an increase in the reducing agent) and ii) the temperature (the '186 patent also requires exhaust gas pressure) of the reducing catalyst affects its ability to store and release the reducing agent. The '186 patent measures, from changes in gas pressure and catalyst temperature, the rate at which the reducing agent is being adsorbed or desorbed from the catalyst. It then calculates NOx emissions produced from the engine and sets a sub-stoichiometric ratio of reducing agent/NOx emissions at which the reducing agent is metered to the catalyst. The metering reducing agent rate is then adjusted upward or downward to equal the measured rate of reducing agent adsorption/desorption. A burner is provided to “empty” the catalyst apparently to assure a sound reference value upon engine start for measurements and to guard against slip. Assuming the adsorption/desorption theory and measurement capability is “sound”, the system is sound although a large number of sensors and intensive calculations appear to be required.
Within the literature, a significant number of articles have been published investigating ammonia SCR NOx reducing systems and several articles have discussed control strategies to optimize the SCR NOx systems investigated. In SAE paper 921673, entitled “Development of an Ammonia/SCR NOx Reduction System for a Heavy Duty Natural Gas Engine” by J. Walker and B. K. Speronello, (September 1992), various quantities of ammonia were injected at various engine speeds and loads to obtain optimum NOx, conversions at steady state engine speeds and loads. The speeds and loads were mapped and stored in a look-up table (specific for each engine and each SCR catalyst) which was then accessed periodically to set an ammonia metering rate. This open loop, feed forward technique is conventionally used and produces good conversion ratios for steady state conditions.
SAE paper 970185, “Transient Performance of a Urea deNOx Catalyst for Low Emissions Heavy-Duty Diesel Engines” by Dr. Cornelis Havenith and Ruud P. Verbeek (a co-inventor of the subject application) dated February, 1997 investigates ammonia metering adjustments made during transient emissions. A pulsed urea dosage device is disclosed which uses speed and load engine sensor data read into a control unit to pulse a quantity of ammonia in stoichiometric relationship to NOx emissions, at steady state conditions. During step-urea, step-load and transient cycles, the stoichiometric relationship was decreased and a dynamic control strategy of injecting additional quantifies of urea after the transient or step or load was completed was adopted. A reduction in NOx emissions is reported although it is questionable whether the reduction was achieved because of the dynamic control strategy the additional reductant added during, the transient or a combination thereof.
SAE paper 925022, “Catalytic Reduction of NOx in Diesel Exhaust” by S. Lepperhoff, S. Huthwohl and F. Pischinger, March, 1992 is an early article that looked at step load changes to evaluate transient systems. The article recognizes that when the load on the engine changed at constant rpm, the NOx emissions increase, the temperature increases and the total exhaust flow increases. Response of the catalyst to step changes in the engine operating conditions are referred to as step load tests. Ammonia slip occurred when engine load increased and the article concludes the slip is correlated to the ammonia stored in the catalyst. It was suggested that a control program or control system would have to consider the, NOx emissions of the engine, the catalyst temperature and the ammonia stored within the catalyst, to avoid ammonia slip.
SAE paper 952493, “An Urea Lean NOx Catalyst System for Light Duty Diesel Vehicles” by H. Luders, R. Backes, G. Huthwohl, D. A. Ketcher, R. W. Horrocks, R. G. Hurley, and R. H. Hammerle, October, 1995 concludes that an ammonia SCR system can control NOx diesel emissions. The control strategy used in the study was similar to that disclosed in the Gladden and Lane patents above i.e., a microprocessor mapped engine out NOx emissions and catalytic converter temperature. Engine out NOx was derived from engine speed and torque. Space velocity (intake air mass flow) and catalyst temperature were then used with NOx out data to set a maximum NOx reduction rate. Transient operation was numerically modeled from steady state conditions. Ammonia storage and thermal inertia was noted as factors affecting the conversion but the control system discussed had no special provisions, other than numerical modeling.
SAE paper 930363, “Off-Highway Exhaust Gas After-Treatment: Combining Urea-SCR, Oxidation Catalysis and Traps” by H. T. Hug, A. Mayer and A. Hartenstein, March, 1993, describes stoichiometric injection of ammonia, without lag, based on engine mapped conditions. Catalyst porosity is stated to be important with respect to transient emissions. An injection nozzle for metering is disclosed.
An article entitled “NOx—Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reduction” by M. Koebel, M. Elsener and T. Marti, published in Combustion Science and Technology, Vol 121, pp. 85–102, 1996 describe experiments conducted “at abrupt load changes”. An abrupt reduction in load did not cause ammonia slip but an abrupt increase in load did cause ammonia slip. The article observes that the catalyst is saturated with adsorbed ammonia at lower temperatures; that increased load significantly increases NOx emissions; that increased load increases, slowly, the temperature of the catalyst. Ammonia slip occurring at the onset of the abrupt load change because of excessive ammonia present when the desorption of the ammonia is increased while the bulk at of the catalyst bed is too cool to effectively react the desorbed ammonia with the higher level of NOx. This observation has been noted in several of the prior art references discussed above. The recommendation is to retard the addition of ammonia in relation to the load increase.
In general collective summary of the prior art references discussed above, it is known that ammonia SCR systems can be used effectively to control the emissions produced by vehicles powered by diesel engines; that the reducing catalysts adsorbs and stores ammonia at low temperatures and desorbs the stored ammonia at higher exhaust gas temperatures; that steady state NOx emissions, determined from mapped speed and load engine conditions, can be readily controlled by metering ammonia in stoichiometric relationship to the NOx emissions; that it is possible to pump urea, react urea to produce ammonia and precisely control the rate of ammonia rejection to the exhaust gases by controlling pulsed injections of ammonia; and that transient emissions cause transient increases in NOx concentrations with attendant exhaust gas temperature increases requiring a reduction in the ammonia metering rate to balance the increased ammonia present attributed to desorption resulting from the temperature increase. Noticeably absent, from any of the mobile applications discussed, is a simple control system capable of quickly and effectively adjusting the metering rate during transient emissions as well metering the reductant during steady state operating conditions.
In this regard and as noted above, industrial processes, which do not have the sudden transient emission changes of a vehicular application, can utilize NOx sensors in a closed loop controlled through set-point controllers. There are no commercially available NOx sensors which have the response time needed for vehicular applications. Thus any SCR control system for mobile applications will necessarily be open loop.