Patent Application
20110239622
The invention relates generally to an efficient emission treatment system and method of operating the emission treatment system.
Exhaust streams generated by the combustion of fossil fuels in, for example, furnaces, ovens, and engines, contain nitrogen oxides (NOx) that are undesirable pollutants. There is a growing need to have efficient and robust emission treatment systems to treat the NOx emissions.
In selective catalytic reduction (SCR) using hydrocarbons (HC), hydrocarbons serve as the reductants for NOx conversion. Hydrocarbons employed for HC-SCR include relatively small molecules like methane, ethane, ethylene, propane and propylene as well as longer linear hydrocarbons like hexane, octane, etc. or branched hydrocarbons like iso-octane. The injection of several types of hydrocarbons has been explored in some heavy-duty diesel engines to supplement the HC in the exhaust stream. From an infrastructure point of view, it would be advantageous to employ an on-board diesel fuel as the hydrocarbon source for HC-SCR.
Fuels, including gasoline or diesel fuels containing sulfur lead to a number of disadvantages when trying to clean-up the exhaust gases by some form of catalytic after-treatment. During the combustion process, sulfur in the fuel gets converted to sulfur dioxide (SO2), which poisons some catalysts. Further poisoning happens from the formation of base metal sulphates from the components of a catalyst compositions, which sulphates can act as a reservoir for poisoning sulfur species within the catalyst.
When the SCR catalysts absorb the NOx in the exhaust gas, they also absorb sulfur oxides (SOx) in the exhaust gas. The sulfur oxides poison the catalysts, and the NOx absorption performance declines as the poisoning by SOx increases.
In conventional NOx trap devices, the amount of SOx trapped at the catalyst is computed and when the computed amount reaches an upper limit, the air-fuel ratio of the air-fuel mixture supplied to engine is temporarily enriched and the exhaust gas temperature is increased. Due to the increase of temperature of the exhaust gas, the SOx trapped by the catalyst is released, and the NOx trapping performance of the catalyst is recovered. This operation is termed desulfating of the catalyst. Also in the NOx trapping systems, two sets of catalyst are required, so that one can be used while the other one is getting regenerated. Therefore, it is desirable to have an emission treatment system with properties and characteristics that has enhanced sulfur tolerance without severe degradation of the NOx reduction activity.
In one embodiment, an emission treatment system is presented. The emission treatment system comprises a separation system and a selective catalytic reduction (SCR) catalyst. The separation system comprises a separator, a fuel inlet disposed to supply fuel to the separator, a first fuel outlet and a second fuel outlet respectively disposed to carry away fuel from the separator. The SCR catalyst comprises a catalyst composition comprising silver and templated metal oxide substrate. The emission treatment system is designed such that the separation system is configured to be in fluid communication with the SCR catalyst through the first fuel outlet during operation.
In one embodiment, a system is provided. The system includes a fuel tank adapted to supply a fuel, a combustion engine configured to receive the fuel and create an exhaust stream, and an emission treatment system configured to receive at least a portion of the exhaust stream. The emission treatment system includes a separation system and an SCR catalyst. The separation system comprises a fuel inlet disposed to receive fuel from the fuel tank, a separator configured to receive fuel through the fuel inlet, a first fuel outlet and a second fuel outlet disposed to carry away fuel from the separator. The SCR catalyst comprises a catalyst composition comprising silver and templated metal oxide substrate. The emission treatment system is designed such that the separation system is configured to be in fluid communication with the SCR catalyst through the first fuel outlet during operation.
In one embodiment, a system is provided. The system includes a fuel tank adapted to supply a fuel, a combustion engine configured to receive the fuel and create an exhaust stream, and an emission treatment system configured to receive at least a portion of the exhaust stream. The emission treatment system includes a separation system and an SCR catalyst. The separation system comprises a fuel inlet disposed to receive fuel from the fuel tank, a separator configured to receive fuel through the fuel inlet and to separate the fuel using a flash heater to two fractions: A first fraction having a maximum boiling point at a temperature in a range from about 70° C. to about 360° C. and a second fraction having a boiling point above said temperature range. The separation system further comprises a first fuel outlet, and a second fuel outlet. The SCR catalyst of the emission treatment system includes a catalyst composition comprising silver and a templated metal oxide substrate. In this embodiment, the separation system is in fluid communication with the SCR catalyst through the first fuel outlet and the separation system is in fluid communication with the combustion engine through the second fuel outlet.
In one embodiment, a system is provided. The system includes a fuel tank adapted to supply a fuel, a combustion engine configured to receive the fuel and create an exhaust stream, and an emission treatment system configured to receive at least a portion of the exhaust stream. The emission treatment system includes a separation system and an SCR catalyst. The separation system comprises a fuel inlet disposed to receive fuel from the fuel tank, a separator configured to receive fuel through the fuel inlet and to separate the fuel using a flash heater into two fractions: A first fraction having a minimum boiling point at a temperature in a range from about 70° C. to about 360° C., and a second fraction having a boiling point below said temperature range. The separation system further comprises a first fuel outlet, and a second fuel outlet. The SCR catalyst of the emission treatment system includes a catalyst composition comprising silver and a templated metal oxide substrate. In this embodiment, the separation system is in fluid communication with the SCR catalyst through the first fuel outlet and the separation system is in fluid communication with the combustion engine through the second fuel outlet.
In one embodiment, a method of reducing nitrogen oxides in an exhaust stream is disclosed. The method comprises the steps of passing a fuel through a fuel inlet of a separation system, fractionating the fuel into a first fraction and a second fraction using a separator in the separation system such that the first fraction has a different average boiling point than the second fraction, passing the first fraction through a first fuel outlet of the separation system to an SCR catalyst and passing a second fraction through a second fuel outlet of the separation system to a combustion engine. The SCR catalyst comprises a catalyst composition that includes silver and a templated metal oxide substrate. The combustion engine is configured to create the exhaust stream and the SCR catalyst reduces nitrogen oxides present in the exhaust stream created by the combustion engine.
The systems and methods described herein include embodiments that relate to a system comprising internal combustion engines and emission treatment systems employed to treat the exhaust gases from the combustion engines. The emission treatment systems include embodiments that relate to a system using the catalyst composition for reducing nitrogen oxides and separation systems that facilitate robust performance of catalyst compositions. Generally, disclosed is a NOx selective reduction catalyst (SCR) and emission treatment system for reducing NOx in exhaust gas discharged from a combustion device. Suitable combustion devices may include furnaces, ovens, or engines.
In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
As used herein, without further qualifiers, “mesoporous” refers to a material containing pores with diameters in a range of from about 2 nanometers to about 50 nanometers. A catalyst is a substance that can cause a change in the rate of a chemical reaction without itself being consumed in the reaction. Templating refers to a controlled patterning; and, templated refers to determined control of an imposed pattern and may include molecular self-assembly. A monolith may be a ceramic block having a number of channels, and may be made by extrusion of clay, binders and additives that are pushed through a dye to create a structure. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. All temperatures given herein are for the atmospheric pressure. One skilled in the art would appreciate that the boiling points can vary with respect to the ambience pressure of the fuel.
In one embodiment, a system is provided. The system comprises a fuel tank, a combustion engine, and an emission treatment system. A fuel tank is a storage place for fuel or a continuous supply of fuel. Fuel may be of different kinds that are used to run the combustion engines. In one embodiment, fuel comprises a material selected from the group consisting of diesel fuel, ultra low sulfur diesel (ULSD), biodiesel fuel, Fischer-Tropsch fuel, gasoline, ethanol, kerosene, and any combination thereof. In a further embodiment, the fuel comprises diesel fuel or biodiesel fuel. In one embodiment, the fuel comprises a non-biodiesel fuel selected from the group consisting of diesel fuel, ULSD, Fischer-Tropsch fuel, gasoline, kerosene, and ethanol. In one embodiment, the fuel comprises a non-biodiesel fuel selected from the group consisting of diesel fuel, ULSD, Fischer-Tropsch fuel, kerosene, and gasoline. In a further embodiment, the fuel comprises ULSD.
In another embodiment, the fuel comprises a biodiesel fuel or a combination comprising a biodiesel fuel. In a further embodiment, the fuel comprises a mixture of biodiesel fuel and at least one non-biodiesel fuel. In a certain embodiment, the fuel comprises a mixture of ULSD and biodiesel. In one particular embodiment, the ULSD fuel does not contain any intentionally premixed alcohols. In one embodiment, while referring to a particular fuel, the fuel may comprise the native ingredients and kerosene in the fuel. For example, in a fuel consisting essentially of ULSD, there is no intentional pre-mixing of any alcohols along with ULSD fuel. However, some amount of kerosene may be present as a part of the commercial ULSD. In another example, in a fuel consisting essentially of biodiesel, there is no intentional pre-mixing of any alcohols along with biodiesel. However, a small amount of diesel and kerosene may be present as a part of the commercial or indigenous biodiesel. Similarly in a fuel mixture consisting essentially of ULSD and biodiesel, there is no intentional pre-mixing of any alcohols along with ULSD and biodiesel. However, a small amount of kerosene may be present as a part of the commercial or indigenous ULSD and biodiesel.
A combustion engine is any engine that accepts fuel, performs an action by burning fuel and emits an exhaust stream. In one embodiment, the combustion engine is an internal combustion engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber resulting in an expansion of the high temperature and pressure gases that can be applied to move a movable component of the engine. Examples of combustion engines include gasoline engines, diesel engines, and turbines.
An emission treatment system works on an exhaust stream of the combustion engine and reduces harmful emissions in the exhaust stream. In one embodiment, the emission treatment system is configured to receive at least a portion of the exhaust stream. Further, the treatment system comprises a separation system that includes a separator, a fuel inlet to the separator, and at least two fuel outlets. The separator separates the contents of fuel into two fractions and sends out the fractions through the two fuel outlets. In general, the two fractions are such that one fraction is a fuel component used, for example, for the operation of the combustion engine, or for some other purpose, and the other fraction is a reductant component, generally comprising reductant species (“reductants”) suitable for use in NOx reduction. Apart from receiving one fraction of fuel from the separator, the engine may or may not additionally receive fuel from the fuel tank directly depending on the fuel fraction production and the requirement of the fuel in the engine and the reductant in SCR catalyst. Similarly, in one embodiment, the fuel coming from the separator to the engine can be redirected fully or partially to the fuel tank or another temporary storage tank depending on the fuel fraction production and the requirement of the fuel in the combustion engine at that time. At least a portion of the fuel is burned in the engine during operation of the engine and an emission of exhaust gases are produced thereby. The exhaust gases, thus produced, are discharged to an SCR catalyst of an emission treatment system, where the emission is treated.
In one embodiment, two fractions of the fuel are separated in the separator based on the vapor pressures of the fractions. In a further embodiment, the fractions are separated based on their boiling points. In general, the smaller chain molecules such as, for example, methane, ethylene, and propylene are lighter and have lower boiling points compared to bigger molecules. In some embodiments, separation of lighter and heavier hydrocarbons can be carried out through separation based on their boiling points. In one embodiment, the lighter and heavier hydrocarbons can also be separated by mechanical separation means such as membranes.
The fractionations using vapor pressure or boiling points can be carried out in several ways. In one embodiment, the fuel is distilled at temperature and/or pressure ranges suitable for separating the required fractions. In another embodiment, the fuel is flash heated, passed through a bubble chamber, or subjected to a heat exchanger for fractionation. In one embodiment, the fraction of fuel having the higher boiling point is in the vapor state. In a further embodiment, the fraction of fuel in the vapor state is used as a reductant to the SCR catalyst. In another embodiment, the fraction of fuel in the vapor state is condensed, optionally stored, and then used as a reductant to the SCR catalyst. In one embodiment, the fraction of fuel having lower boiling point is in the liquid state.
In another embodiment, the fractions are separated by the molecular sizes of the fuel. In one embodiment, the reductant is a hydrocarbon having an average carbon chain length of about 2 carbon atoms to about 24 carbon atoms. In a further embodiment, the carbon chain length is in the range of about 10 carbon atoms to about 16 carbon atoms. In one embodiment, the reductant is an oxygenated hydrocarbon, such as ethanol. Fractionations by molecular sizes can be carried out using molecular sieves and/or membranes.
In one embodiment, the SCR catalyst 30 comprises a catalyst composition. In a further embodiment, the catalyst composition includes silver and templated metal oxide. The silver acts as a catalyst metal and the templated metal oxide acts as a catalyst substrate.
Suitable catalyst substrate may include an inorganic material. Suitable inorganic materials may include, for example, oxides, carbides, nitrides, hydroxides, carbonitrides, oxynitrides, borides, or borocarbides. In one embodiment, the inorganic oxide may have hydroxide coatings. In one embodiment, the inorganic oxide may be a metal oxide. The metal oxide may have a hydroxide coating. Other suitable metal inorganics may include one or more metal carbides, metal nitrides, metal hydroxides, metal carbonitrides, metal oxynitrides, metal borides, or metal borocarbides. Metallic cations used in the foregoing inorganic materials can be transition metals, alkali metals, alkaline earth metals, rare earth metals, or the like.
In one embodiment, the catalyst substrate includes oxide materials. In one embodiment, the catalyst substrate includes alumina, zirconia, silica, zeolite, or any mixtures comprising these elements. The desired properties of the catalyst substrate include, for example, a relatively small particle size and high surface area. In one embodiment, the powder of the catalyst substrate has an average diameter that is less than about 100 micrometers. In one embodiment, the average diameter is less than about 50 micrometers. In a further embodiment, the average diameter is from about 1 micrometer to about 10 micrometers. The catalyst substrate powders may have a surface area greater than about 100 m2/gram. In one embodiment, the surface area of the catalyst substrate powder is greater than about 200 m2/gram. In one embodiment, the surface area is in a range of from about 200 m2/gram to about 500 m2/gram, and, in another embodiment, from about 300 m2/gram to about 600 m2/gram.
One way of forming templated substrates is by employing templating agents. Templating agents facilitate the production of catalyst substrates containing directionally aligned forms. The templating agent may be a surfactant, a cyclodextrin, a crown ether, or mixtures thereof. An example of a useful templating agent is octylphenol ethoxylate, commercially available as TRITON X-114®.
The catalyst substrate may have periodically arranged templated pores of determined dimensions. The dimensions can include pore diameter, degree of curvature, uniformity of the inner surface, and the like. The median diameter of the pores, in some embodiments, is greater than about 2 nm. The median diameter of the pores, in one embodiment, is less than about 100 nm. In some embodiments, the median diameter of the pores is in a range from about 2 nm to about 20 nm. In another embodiment, the diameter is from about 20 nm to about 60 nm and in yet another embodiment, the diameter is from about 60 nm to about 100 nm. The pores in some embodiments have a periodicity greater than about 50 Å. The pores in some embodiments have a periodicity less than about 150 Å. In one embodiment, the pores have a periodicity in the range of from about 50 Å to about 100 Å. In another embodiment, the pores have a periodicity in the range from about 100 Å to about 150 Å.
In certain embodiments, the pore size has a narrow monomodal distribution. In one embodiment, the pores have a pore size distribution polydispersity index that is less than 1.5. As used herein, the polydispersity index is a measure of the distribution of pore diameter in a given sample. In a further embodiment, the polydispersity index is less than 1.3, and in a particular embodiment, the polydispersity index is less than 1.1. In one embodiment, the distribution of diameter sizes may be bimodal, or multimodal.
The catalyst composition can include a catalytic metal along with the catalyst substrates. Suitable catalyst metal may include one or more of gallium, indium, rhodium, palladium, ruthenium, and iridium. Other suitable catalyst metal includes transition metal elements. Suitable catalyst metal also includes one or more of platinum, gold, and silver. In one embodiment, the catalyst metal comprises silver. In one particular embodiment, the catalyst metal is substantially 100% silver.
The catalyst metal may be present in an amount of at least about 0.5 mole percent of the substrate. In one embodiment, the catalyst metal is present in an amount equal to or greater than 3 mole percent of the substrate. In one embodiment, the amount of catalyst metal present is about 6 mole percent of the catalyst substrate. In one embodiment, the catalytic metal may be present in an amount in a range of from about 1 mole percent to about 9 mole percent of the substrate.
The SCR catalysts can have different dopants that enhance reduction activity and stability of the catalysts. In one embodiment, the dopants may be selected from the group consisting of zirconium, iron, gallium, indium, tungsten, zinc, platinum, and rhodium. In one embodiment, the dopant comprises zirconium. In another embodiment, the dopant comprises rhodium and in yet another embodiment, the dopant comprises both gallium and indium. In one embodiment, the dopant may be present in an amount in a range of from about 0.1 mole percent to about 20 mole percent, of the substrate material. In a further embodiment, the dopant may be present in an amount in a range of from about 0.1 mole percent to about 5 mole percent, of the substrate material. In a particular embodiment, the dopant may be present in an amount in a range of from about 0.5 mole percent to about 3 mole percent, of the substrate material.
One useful NOx reduction catalyst is silver-templated alumina (Ag-TA) catalyst. In some embodiments, the inventors studied the reduction efficiency of NOx reduction catalysts by taking Ag-TA as an example.
In one embodiment, the fuel fractionated based on the boiling point is used in the emission treatment system such that the lower boiling point fraction is taken out through the first fuel outlet 26 to the SCR catalyst 30. Trial evaluations of such a system have found this fraction to be a better reductant than the fuel itself. That is, the emission treatment system was found to be more efficient in NOx reduction when the lower boiling fraction of the fuel was used as a reductant in comparison with using a non-fractionated, stock fuel itself as a reductant. An efficiency increase of the emission treatment system when using a lower boiling point fraction of the fuel was also observed with an emission treatment system having a sulfur treated HC-SCR catalyst. A sulfur treated catalyst is defined herein as a catalyst that is exposed to an amount of sulfur that is capable of reducing the performance efficiency of the catalyst by more than about 5%. Use of lower boiling point fuel fraction for NOx reduction significantly improves the NOx conversion performance of the HC-SCR after sulfur treatment when compared to the non-fractionated fuel. This property is particularly useful in applications where the catalyst is likely to experience some sulfur exposure. The advantages include improved performance of the catalyst and hence lower usage amounts of catalyst. The fuels that are beneficial for use as a lower boiling point fraction for increased reduction efficiency of the NOx reduction HC-SCR catalyst include diesel fuel, ULSD, Fischer-Tropsch fuel, gasoline, ethanol, kerosene, and any combination thereof. Tests have indicated that the lower boiling point fraction of diesel is a better reductant than the higher boiling point fraction of diesel on the Ag-TA catalyst.
In another embodiment, the fuel fractionated based on the boiling point is used in the emission treatment system such that the higher boiling point fraction is taken out through the first fuel outlet 26 to the SCR catalyst 30. In some situations, the higher boiling point fuel was found to be a better reductant than the fuel itself by improving the NOx conversion performance of the HC-SCR. One example of a fuel that is beneficial to be used as a higher boiling point fraction for increased reduction efficiency of the NOx reduction HC-SCR catalyst is a biodiesel/diesel fuel mixture such as B20. One skilled in the art would predict that a higher boiling point fraction of a mixture comprising biodiesel may also increase the reduction efficiency of the emission treatment system comprising a sulfur treated HC-SCR catalyst.
In one embodiment, separator 24 of the emission treatment system 16 is configured to receive fuel through the fuel inlet 22 and to separate the fuel into two fractions. A first fraction has a maximum boiling point at a temperature of about 360° C. and the second fraction of fuel has a boiling point above that of the first fraction. The maximum boiling point of a fraction as used herein denotes the theoretical temperature at which all the ingredients of the fraction will evaporate, when subjected to heating at atmospheric pressure.
In one embodiment, the separator 24 is configured to fractionate the incoming fuel into two fractions with a first fraction having a maximum boiling point at a temperature that ranges from about 70° C. to about 360° C. and the second fraction of fuel having a boiling point above that of the first fraction. Therefore, depending on the maximum boiling point of the first fraction, the second fraction of this fractionation may have a minimum boiling point greater than a temperature that ranges from about 70° C. to about 360° C. In one embodiment, the separator 24 is configured to fractionate the incoming fuel into two fractions with a first fraction having a maximum boiling point at a temperature that ranges from about 100° C. to about 225° C. and the second fraction of fuel having a boiling point above that of the first fraction. In a further embodiment, the temperature of separation of two fractions is about 225° C.
In another embodiment, the separator 24 is configured to fractionate the incoming fuel into two fractions with a first fraction having a minimum boiling point at a temperature that is in a range from about 70° C. to about 360° C. and the second fraction of fuel having a boiling point below that of the first fraction. Therefore, depending on the minimum boiling point of the first fraction, the second fraction of this fractionation may have a maximum boiling point lower than a temperature that ranges from about 70° C. to about 360° C. In a further embodiment, the first fraction has a minimum boiling point at a temperature that is in a range from about 225° C. to about 360° C. and the second fraction of fuel that has a boiling point below that of the first fraction. In a further embodiment, the minimum boiling point of the first fraction is at a temperature that is in the range of about 300° C. to about 360° C.
Depending on the type of fuel used and advantage of using low boiling point fraction or high boiling point fraction for the efficiency increase of the SCR catalyst, the suitable fuel fraction can be transferred to the SCR catalyst 30, and the other fraction can be routed to the combustion engine 14 or to a storage tank, as required. For example, in one embodiment, the separator function is fixed in providing a first fraction of low boiling point fuel and a second fraction of high boiling point fuel, and the separator outlets 26, 28 feeding the SCR catalyst 30 and combustion engine 14 respectively, are switched depending on the fuel fractionation. In another embodiment, keeping the first fuel outlet 26 and second fuel outlet 28 fixed to the SCR system 30 and the combustion engine 14 respectively, the function of the separator is modified in providing a first fraction of lower boiling point or higher boiling point fuel, depending on the requirement. Thus, by adjusting the outlet connection or the separator function, each fraction is routed to the proper outlet, depending on which fraction (high boiling point or low boiling point) is desired for use as a reductant.
In one embodiment, a method of reducing nitrogen oxides in an exhaust stream is disclosed. The method comprises the steps of passing a fuel through a fuel inlet 22 (
The method further includes passing the first fraction through a first fuel outlet 26 of the separation system 20 to an SCR catalyst 30. The SCR catalyst 30 used herein comprises a catalyst composition that includes a silver catalyst and a templated metal oxide substrate. The second fraction of fuel can be directly fed to the combustion engine 14, fed back to the fuel tank 12 or partially or fully stored in a storage tank (not shown). In one embodiment of the method, the second fraction of the fuel passes through a second fuel outlet 28 of the separation system 20 to a combustion engine 14. In one embodiment, the second fraction of fuel is not fed back into the fuel tank that supplies fuel to the separation system 20. In one embodiment, the combustion engine operates on the second fraction of fuel received from the separation system 20 and creates an exhaust stream that is fed into the SCR catalyst 30 to reduce the harmful emissions of the exhaust stream. In one particular embodiment, the SCR catalyst 30 is used to reduce the nitrogen oxides present in the exhaust. In one embodiment, the combustion engine receives fuel from a fuel tank or fuel line in addition to the second fraction of fuel from the separation system 20.
In one embodiment of the method, the first fraction of fuel has a lower average boiling point than the second fraction of fuel. The temperature of separation of two fractions in this embodiment is in the range of about 70° C. to 360° C. In one embodiment, the first fraction has a maximum boiling point that is less than about 360° C. The fuels that are normally used in this embodiment include diesel fuel, ULSD, Fischer-Tropsch fuel, gasoline, ethanol, kerosene, or any combinations thereof. In a further embodiment, the first fraction has a maximum boiling point in the temperature range of about 100° C. to 225° C. In an associated embodiment, the first fraction has a maximum boiling point in the temperature range of about 150° C. to 200° C. The fuel that is normally used in this embodiment includes a diesel fuel and/or ULSD. In a further embodiment, the fuel consists essentially of ULSD. In an associated embodiment, an ULSD fuel is fractionated at a temperature of about 200° C. and the less boiling point fraction is used as reductant for NOx reduction. In a further embodiment, the portion not used as a reductant is used as engine fuel.
In an alternate embodiment of the method, the first fraction of fuel has a higher average boiling point than the second fraction of fuel. The temperature of separation of two fractions in this embodiment is in the range of about 70° C. to 360° C. Therefore, the first fraction has a minimum boiling point that is equal to or greater than about 70° C. The fuels that are normally used in this embodiment include biodiesel fuel, diesel, ULSD, Fischer-Tropsch fuel, kerosene, or any combinations thereof. In a particular embodiment of the method, the temperature of separation of two fractions is in the range of about 250° C. to 360° C. Therefore, the first fraction has a minimum boiling point that is equal to or greater than about 250° C. One fuel that is normally used in this embodiment includes a mixture of biodiesel fuel and ULSD. In this embodiment, when the separator separates the fuel into two fractions, the first fraction is expected to consist essentially of biodiesel fuel that is fed into the SCR catalyst 30. The second fraction is expected to consist essentially of ULSD that is fed into the combustion engine 14, recirculated to the fuel tank 12, or stored in a storage tank for further usage. In a further embodiment, the temperature of separation of two fractions is in the range of about 300° C. to 360° C. In an associated embodiment, the first fraction has a minimum boiling point in the range of about 300° C. to 360° C. An example of a fuel that may be suitably applied in this embodiment includes biodiesel fuel.
In one embodiment, multiple separators 24 can be used to fractionate the fuel and advantageously feed the desired fractions to the SCR catalyst 30 and the combustion engine 14 or to the fuel storage tank. For example, in one embodiment, a fuel can be fractionated in three different temperature zones using two separators as shown in
The SCR catalyst 30 advantageously functions across a variety of temperature ranges. In one embodiment, the catalyst composition reduces the nitrogen oxides at a temperature greater than about 275° C. In a further embodiment, the catalyst composition reduces NOx at a temperature greater than about 325° C.
The following experiments were carried out for determining the sensitivity of NOx catalyst to reductant composition by using different fractions of diesel fuels taken from different sources.
An Ag-TA catalyst composition in the form of a washcoated monolith was considered as the catalytic material. In order to understand the effect of reductant on a fresh catalyst, the monolith used herein was used without being subjected to sulfur treatment. The feed gas composition included 300 ppm NO, 7% H2O, and 9% O2. Two different base ULSD fuels were compared: One is a ULSD fuel blend designed for winter and identified as ULSD-1, and another is a ULSD fuel blend designed for summer and identified as ULSD-2. These compositions are the variations of the stock fuel that are generally used as a reductant in studies performed in a reactor that mimics an exhaust treatment system of a locomotive engine. Fractions were distilled from these two ULSD stock fuels. The fractionation was performed from room temperature up to a cut off temperature of 200° C. and the portion that was vaporized and condensed was retained and named as fraction 1. Temperature of reduction for these experiments were considered as 450° C., 400° C., 350° C., and 300° C. with 1 hour of hold at each temperature.
Some of these fuels were tested using gas chromatography (GC) and Nuclear Magnetic Resonance Spectroscopy (NMR). From the results of these analytical tests (Not shown), it was observed that the ULSD-1 had a much lighter composition than ULSD-2. One reason for the difference in composition between the ULSD-1 and ULSD-2 may be a possible addition of kerosene in the commercial winter blend (ULSD-1) to lighten the fuel for winter in colder climates. The fractionation process cuts out the longer chain, higher boiling constituents in the ULSD.
The ULSD-1 and ULSD-2 fuels and fractions 1 of the ULSD-1 and ULSD-2 fuels were used as reductants to determine the sensitivity of a Ag-TA catalyst to reductant composition. The results of these fuel sensitivity performance experiments are shown in
The NOx reduction efficiency degradation due to sulfur poisoning was studied in the labs. The degradation of the Ag-TA catalyst performance during the use of ULSD fuel was studied for a fresh catalyst and a catalyst that had a deep sulfur (S) treatment with 30 ppm SO2 for 12 hrs at 350° C., and then tested over a wide temperature range. Steady state performance at temperatures lower than 400° C. was strongly affected by the deep S treatment, as can be observed from the comparison of NOx reduction activity curve of the fresh catalyst 62 and the sulfur treated catalyst 64 (
NOx profile as a function of time revealed an interesting feature for a sulfur (S) treated monolith washcoated with Ag-TA catalyst and tested at 375° C. (
To test this hypothesis, the performance of the S treated Ag-TA catalyst monolith was evaluated with lighter fraction of diesel (boiling point<200° C.). The NOx reduction performance of the catalyst was higher for a fractionated diesel 66 than the unfractionated diesel fuel 68 in the temperature range of about 325° C. to about 375° C. as shown in
The system and methods discussed herein can be applied for improving the overall NOx conversion of the after treatment system. The present study suggests different diesel formulations may have varying effects on the catalyst. The system and methods described herein may limit the variability of NOx conversion as a function of fuel source as it would always take a beneficial fraction from the fuel and eliminate the unfavorable fraction that is likely to be the cause of coking of the catalyst.
The embodiments described herein are examples of composition, system, and methods having elements corresponding to the elements of the invention recited in the claims. This written description may enable those of ordinary skill in the art to make and use embodiments having alternative elements that likewise correspond to the elements of the invention recited in the claims. The scope of the invention thus includes composition, system and methods that do not differ from the literal language of the claims, and further includes other compositions and articles with insubstantial differences from the literal language of the claims. While only certain features and embodiments have been illustrated and described herein, many modifications and changes may occur to one of ordinary skill in the relevant art. The appended claims cover all such modifications and changes.
