Power plants – Motor having exhaust fluid treating or handling means
Reexamination Certificate
2002-02-22
2003-04-22
Nguyen, Hoang (Department: 3748)
Power plants
Motor having exhaust fluid treating or handling means
C060S274000, C060S286000, C060S295000, C060S303000
Reexamination Certificate
active
06550250
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the reduction of nitrogen oxide (NOx) emissions in the exhaust gases of diesel engines or turbines for stationary or mobile applications, and more particularly, to a process suitable for use in a Selective Catalytic Reduction (SCR) system.
The SCR system represents a known and widely spread technology for the removal of oxides of nitrogen in the exhaust gases from turbines, boilers, burners, power plants and other plants utilizing fossil fuels in the heavy industry. This system is based on the creation of a reducing atmosphere over a catalyst in the presence of the NOx compounds present in the exhaust gases.
The selection of a reducing agent depends on the local conditions prevailing in the different geographical areas where SCR systems are used. Conditions such as pricing, legislation and logistics play a role in the choice of the reducing agent. Reducing agents that are commonly used with diesel engines are neat or aqueous ammonia (NH
3
), solid urea (NH
2
CONH
2
) or urea dissolved in water.
Anhydrous ammonia is, however, extremely hazardous, toxic and volatile. On exposure to air, at a sufficiently high temperature and pressure, anhydrous ammonia can combine with air to form a combination that can be lethal. These properties therefore result in problems with the safety aspects where the storage, transportation and handling of large quantities of ammonia are concerned. Urea as a non-toxic alternative to ammonia does not present the same extensive safety problems, and it can be converted to ammonia at a latter stage.
In the case of ammonia or urea, the reducing agent reacts with NOx compounds such as nitrogen oxide (NO) and nitrogen dioxide (NO
2
), in the presence of a catalyst, and at normal exhaust gas temperatures of 250-450° C., to liberate free nitrogen (N
2
) and water. The catalysts used are generally known as DENOX catalysts.
When the reducing agent is urea, it undergoes pyrolysis at approximately 275° C. into gaseous ammonia and cyanuric acid (HNCO) according to the following reaction:
NH
2
CONH
2
→NH
3
+HNCO
The HNCO then reacts with the water in the exhaust gas as follows:
HNCO+H
2
O →NH
3
+CO
2
The CO
2
does not participate further in the DENOX reaction, whereas the ammonia molecules subsequently react with the nitrogen oxide, NO, on the surface of the catalyst according to the reaction:
2NO+2NH
3
+½O
2
+CO
2
→2N
2
+3H
2
O+CO
2
Storage and feeding systems for the selected reducing agent which are used in the various plants, vary in design and construction, but in general, most SCR systems are applied to stationary plants with good infrastructure. Consequently, access to electricity, heat, and sheltered surroundings provide solutions for the safe storage and operation of stationary systems.
Contrary to the above-mentioned stationary plants are mobile applications such as heavy-duty (HD) truck vehicles operated either on the road, or off road vehicles and equipment. The application of DENOX catalysts for SCR systems to be used in the automotive industry requires that several criteria are fulfilled. These criteria include high resistance to extreme climatic conditions such as subzero temperatures lower than the crystallization point of urea, which is −11° C.
Resistance to high temperature is also a requirement, since high temperature leads to the reducing agent exhibiting high vapour pressure, and this causes problems during refueling and venting of the vehicles. In addition, vehicle vibrations caused during driving also necessitate proper solutions in order to run the operations in a trouble-free manner.
Thus, there are current demands for high durability and stability when introducing these systems to the automotive industry.
2. Description of the Prior Art
There have been several attempts to reduce the emissions of NOx from diesel engines.
U.S. Pat. No. 6,063,350, which is incorporated herein by reference, discloses a method for SCR NOx emission reduction in an exhaust gas from a lean-burn engine using an aqueous solution of urea. In this system, temperature fluctuations and formation of solid deposits are avoided by monitoring the quality, temperature and level of the urea solution in a storage vessel, using a modular assembly of different sensors mounted inside the urea storage vessel. Based on the sensed parameters, sensor signals are generated compared to reference values, and the flow of urea solution is controlled in response to these signals. A heater can be used to maintain the temperature of the urea solution.
It also states that precipitation of solids is avoided by recirculating urea through a line between the storage vessel and the injector, which injects urea to the exhaust gases. The rate at which this recirculation takes place helps to maintain the temperature of the urea solution at a sufficiently low level, such that urea is not permitted sufficient time at elevated temperature to hydrolyze to the extent that solids are deposited.
U.S. Pat. No. 6,209,315 incorporated herein by reference, discloses a method and a device for controlled feeding of a reducing agent in an SCR process for reducing NOx in exhaust gas. The reducing agent is pumped from a storage container to a pressure accumulator inserted between the storage container and the metering valve to the SCR catalyst. The quantity of reducing agent metered may be evaluated from the displacement of a sprung (sprin-loaded) diaphragm in the pressure accumulator in association with a pressure sensor.
In all tank systems, various types of equipment can be used to avoid freezing of the urea solution. The freezing point of a 32.5 w/w % aqueous urea solution is −11° C. Insulation and various heaters supplied by the battery or other energy sources are mentioned in the prior art. These types of protective equipment are only effective provided a constant power source is available. Loss of battery power in cold weather will cause freezing and crystallization of the urea at sufficiently low temperatures. In ordinary tank systems, damage is seen as a result of the expansion during freezing. The tank and the contents of the tank such as sensors, pumps and other equipment installed inside may then be permanently damaged.
Several problems associated with the SCR systems currently in use for NOx reductions are corrosion of the different components in the system, crystallization of urea which leads to deposits in the lines, maintaining constant valve settings and unacceptable evaporative emissions. Subzero temperatures also cause freezing followed by destruction of the tank liner pumps. No known system has so far been able to run safely during all the different operating modes.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a safe, reliable SCR system for reducing NOx emissions from diesel systems.
It is another object of the invention to eliminate the crystallization problems leading to destruction of equipment, associated with freezing of a reducing agent such as urea.
It is yet another object of the invention to provide simple modular membrane equipment to be used in the SCR system.
It is yet another specific object of the invention to provide an SCR system in which any type of liquid reducing agent can be used.
It is yet another specific object of the invention to provide a gas as a pneumatic driving force behind the transfer of the reducing agent.
It is yet another specific object of the invention to provide a liquid as a hydraulic driving force behind the transfer of the reducing agent.
These objects are achieved by the present invention, which provides an improved process and a simple modular apparatus for SCR NOx reduction.
The invention described herein concerns a process for reducing the content of nitrogen oxides (NOx) in the exhaust gases of diesel engines or turbines for stationary or mobile applications/vehicles in an SCR system by providing a stored source of liquid reducing agent and a hydrauli
Gabrielsson Per L. T.
Mikkelsen Svend-Erik
Dickstein , Shapiro, Morin & Oshinsky, LLP
Haldor Topsoe A/S
Nguyen Hoang
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