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IC Engines are used for various purposes like power generation, inert gas production etc. Different types of fuels can be used for IC Engines like diesel, gasoline, methanol etc. The combustion process in IC engine results in emission of;
Hydro Carbons like NMHC (Non methane hydrocarbon) or VOC (Volatile organic compounds)
The concentration of above mentioned pollutants changes from engine to engine, mode of operation and the type of fuel used.
Emission control technologies:
The emissions from IC engines cause severe hazards to human health and the environment such as global warming, skin diseases, lung cancer etc. So there arise a need of reducing the emission of these pollutants from the exhaust. With the advancement of technology various technologies were introduced causing substantial reduction in all the five pollutants listed above. However depending upon the engine operation condition, different technology is used varying targeted emission as well as level of control. Till date various technologies have been introduced namely;
Each method has its own advantages and disadvantages. Depending upon our requirement we choose one of the above mentioned techniques for emission control in IC engines.
NOx Reduction in IC Engines:
In 1967, Clean Care Act was passed in which standards for different types of atmospheric pollutants like Sulphur Dioxide (SO2) and Nitrogen Dioxide (NOx) were recommended. As a result of this act, New Source Performance Standards (NSPS) were made for different flue gas components.
The need of NOx control arises because when NOx and VOC enter the atmospheric air, they undergo chemical reaction in the presence of sun light and form ground level ozone which results in the formation of smog. The present NAAQS for ozone is 0.12ppm. Different studies showed that the current standard for ozone is in-efficient to protect human heath and environment. Due to this reason EPA decided more strict NAAQS for ozone i.e.; 0.08ppm.
To control NOx emissions in IC engines various experimental technologies were developed, some of which are mentioned above in emission control technologies. As a result of this research work SCR (Selective Catalytic Reduction) was found more effective for reducing NOx.
History of SCR (Selective Catalytic Reduction):
Englehard Corporation in 1957 devised the technique of Selective Catalytic Reduction of NOx using Ammonia (NH3) as the reducing agent. Firstly, Platinum or Platinum group metals were used but the results were not satisfactory because explosive Ammonium Nitrate is needed to form which requires high temperature. Other base metal catalysts couldn't be used because they were less active. In Japan, research was done because there the environmental regulations are very strict. In result of that research, two catalysts were found to be successful which is Vanadium/Titanium which leads us to the current form of SCR technology.
Other than Japan, the environmental regulations are stricter in several European countries which forced the installation of SCR in engines especially in Germany; research on large scale is done for the development of this technology.
How SCR Works?
SCR is a process for controlling emissions of nitrogen oxides from stationary sources. The basic principle of SCR is the reduction of NOx to N2 and H2O by the reaction of NOx and ammonia (NH3) within a catalyst bed. The primary reactions occurring in SCR require oxygen, so that catalyst performance is best at oxygen levels above 2-3%.
Several different catalysts are available for use at different exhaust gas temperatures. In use the longest and most common are base metal catalysts, which typically contain titanium and vanadium oxides, and which also may contain molybdenum, tungsten, and other elements. Base metal catalysts are useful between 450 °F and 800 °F. For high temperature operation (675 °F to over 1100 °F), zeolite catalysts may be used. In clean, low temperature (350-550 °F) applications, catalysts containing precious metals such as platinum and palladium are useful. (Note that these compositions refer to the catalytically active phase only; additional ingredients may be present to give thermal and structural stability, to increase surface area, or for other purposes.)
The mechanical operation of an SCR system is quite simple. It consists of a reactor chamber with a catalyst bed, composed of catalyst modules, and an ammonia handling and injection system, with the ammonia injected into the flue gas upstream of the catalyst. (In some cases, a fluidized bed of catalyst pellets is used.) There are no moving parts. Other than spent catalyst, the SCR process produces no waste products.
Following graph shows performance of zeolite selective catalytic reduction (SCR) catalysts ('Catalyst A' and 'Catalyst B') relative to a standard wash coated vanadia catalyst (V-SCR). Zeolites exhibit: (a) better low temperature and high temperature performance; and (b) less sensitivity to NO2 inlet levels (15) (Temperature = 200°C)
In principle, SCR can provide reductions in NOx emissions approaching 100%. (Simple thermodynamic calculations indicate that a reduction of well over 99% is possible at 650 °F.) In practice, commercial SCR systems have met control targets of over 90% in many cases.
Chemistry of SCR:
Selective catalytic reduction is a post-combustion NOx control technology capable of providing NOx reductions in excess of 90 percent. This technology is widely used in commercial applications overseas and is experiencing expanded use in U.S. facilities. The SCR process uses a catalyst at approximately 300-450 oC to facilitate a heterogeneous reaction between NOx and an injected reagent, ammonia (NH3), to produce nitrogen and water. Within this temperature range, the dominant reactions in the presence of oxygen are:
4NO + 4NH3 + O2 ------------> 4N2 + 6H2O + heat
NO + NO2 + 2NH3 ------------> 2N2 +3H2O + heat
In the SCR process, NH3 chemisorbs onto the active sites on the catalyst. The NOx in the flue gas reacts with adsorbed NH3 to produce nitrogen (N2) and water (H2O). A typical SCR system is comprised of: the storage, delivery, and vaporization and injection system for the reagent; the SCR reactor and catalyst; soot blowers; and additional instrumentation. Anhydrous or aqueous ammonia are the commonly used reagents. The catalyst is the critical component of an SCR system. The catalyst NOx reduction performance and resistance to deactivation affects the cost effectiveness of the SCR system. Many proprietary catalyst formulations exist for coal-fueled service. These formulations use oxides of vanadium as the active catalyst, titanium as a catalyst dispersing and supporting agent, and tungsten to improve mechanical stability and reduce sulfur oxidation. The concentrations of vanadium pent-oxide, titanium dioxide, and tungsten oxide in a catalyst are customized to meet specific installation requirements. Recently, poison-resistant catalyst formulations have also been developed. In these formulations, molybdenum oxide is used to capture and localize the poisons (e.g., arsenic) and thus prevent deactivation of active sites. Structurally, catalysts are manufactured in the form of either supported extrudates (homogenous catalysts) or catalyst coatings (non-homogenous catalysts). The most common type of homogenous catalyst is honeycomb, whereas the most common type of non-homogenous catalyst is plate.
Ammonia Slip in SCR:
Unreacted NH3 in the flue gas downstream of the SCR reactor is referred to as NH3 slip. It is essential to hold NH3 slip to below 5 ppm, preferably 2-3 ppm, to minimize formation of (NH4)2SO4 and NH4HSO4, which can cause plugging and corrosion of downstream equipment. This is a greater problem with high-sulfur coals, caused by higher SO3 levels resulting from both higher initial SO3 levels due to fuel sulfur content and oxidation of SO2 in the SCR reactor.
Catalyst cost constitutes 15-20% of the capital cost of an SCR unit; therefore it is essential to operate at as high a temperature as possible to maximize space velocity and thus minimize catalyst volume. At the same time, it is necessary to minimize the rate of oxidation of SO2 to SO3, which is more temperature sensitive than the SCR reaction. The optimum operating temperature for the SCR process using titanium and vanadium oxide catalysts is about 650-750°F. Most installations use an economizer bypass to provide flue gas to the reactors at the desired temperature during periods when flue gas temperatures are low, such as low load operation.
Benefits of SCR:
Simply stated, SCR improves engine fuel economy and increases engine reliability. Compared to 2007 emissions technology, engines using SCR after-treatment do not require significant increases in EGR flow rates to meet EPA 2010 NOx limits. Because the pollutants are reduced to near-zero levels within the exhaust stream, an engine with SCR after-treatment will operate cooler, cleaner and more efficient than engines with higher EGR levels. PACCAR engines with SCR technology will provide up to 5% fuel economy improvement compared to today's engines, and even greater when compared to a 2010 non-SCR engine. Also, a cooler engine provides greater reliability.
The main advantages of SCR are:
Lowers need for Exhaust Gas Recirculation (EGR)
Permits engine to operate under optimized combustion conditions, including increased temperature, increased pressure and excess oxygen
Less heat rejection
Cleanest tailpipe emissions available
Proven technology across the globe
Mainstream 2010 technology solution
SCR as a proven technology:
SCR technology has been proven globally for a number of years in some of the harshest climates and industries including trucking, marine and stationary power applications. Over 500,000 vehicles are successfully performing with SCR technology, including over 100,000 PACCAR engines.
Engines utilizing high EGR flow rates are in fact unproven, as this is a new concept not yet in use by any engine manufacturer. The high output EGR engines in 2010 will require significantly higher heat rejection to deal with the increased EGR levels, resulting in potentially lower fuel economy than today's designs.
Using SCR reduces NOx in exhaust and to prove this behold the following table:
Following table show the comparison between different emission control technologies at different engine operating conditions:
This table shows that SCR catalyst technology specifically targets NOx in exhaust so it can be most helpful technology for controlling NOx.
How Does SCR Affect a Vehicle's Configuration?
The SCR system components are integrated with the vehicle chassis. Vehicle packaging for these components is dependent upon exhaust configurations specified and the application. For example, in a typical vehicle, the DPF and SCR catalysts will most likely be positioned within the toolbox mounted under the cab as illustrated in blue in the rendering at left.
The catalysts, shown in the drawing at right, each perform a specific function in the after-treatment process:
Diesel Particulate Filter - The DPF is used in all diesel engines to reduce particulate matter (engine soot).
SCR Catalyst - The SCR catalyst facilitates the chemical conversion of NOx to Nitrogen gas and water vapor.
The DEF tank, noted in green in the rendering at left, will be mounted on the chassis in front of the fuel tanks in most configurations where refilling is simple and there is no interference with aftermarket-related modifications that occur behind the cab and/or sleeper.
The tank and injection components, illustrated in the rendering below, each perform a specific function in the after-treatment process:
DEF Tank - The tanks will store the DEF solution on the vehicle and be available in a range of sizes dependent upon total vehicle diesel fuel volume.
Dosing Pump - The pump provides pressure to send the DEF solution from the tank to the doser.
DEF Doser - The doser delivers the DEF into a mixing pipe where it is combined with the exhaust gas exiting the Diesel Particulate Filter (DPF).
Where is SCR used?
SCR has been used to reduce stationary source emissions since the 1980s. In addition, more than 100 marine vessels worldwide have been equipped with SCR technology, including cargo vessels, ferries and tugboats.
While SCR has been installed on both highway and nonroad engines in diesel retrofit demonstration projects throughout the U.S., SCR systems have become the technology of choice for many of Europe's heavy-duty diesel truck and bus manufacturers where the urea agent is commonly known as AdBlue. SCR technology may become more prevalent in the United States as both light- and heavy-duty engine manufacturers work to meet future emissions reduction standards starting in 2009. In fact, several light-duty diesel manufacturers have already indicated that they are considering the use of SCR in future products.
What are the technology challenges of using SCR?
A major challenge of the SCR system is the replenishment of the urea solution. The urea solution is carried in an onboard tank which must be periodically replenished based on vehicle operation. For light-duty vehicles, urea refill intervals will occur around the time of a recommended oil change, while urea replenishment for heavy-duty vehicles will vary depending on the vehicle specifics and application requirements.
While vehicles could continue to function normally even without the urea solution, the emissions system will not meet NOx reduction requirements. Manufacturers are currently working with the EPA to address these technology and emissions performance challenges. One concept is to establish a nationwide urea distribution infrastructure for consumers, while another option links the replenishment of urea with pre-existing scheduled maintenance intervals (i.e. oil changes).
Other issues include the availability of space on vehicles to provide user-friendly access to the urea tank and other SCR components. In addition, proper storage of urea is required to prevent the liquid from freezing at temperatures below 12 degrees Fahrenheit.
Why choose SCR?
SCR technology is currently used in Europe, gets better fuel-economy and lowers the total cost of ownership. These are just some of the reasons why SCR is the better choice for meeting 2010 emissions standards. The list below highlights some of the advantages of SCR technology.
Best fuel efficiency
Proven technology in Europe
EPA recommended and approved
Higher engine power density
No increased heat rejection, allowing for more efficient combustion
Lower cost of total ownership
Easy to service