DIESEL ENGINE POLLUTANTS & HOW EMISSION CONTROL TECHNOLOGY REGULATES THEM
vijay tharad
Director Operations at Corporate Professional Academy for Technical Training & Career Development
Air Fuel Ratio in Internal Combustion Engine
Higher compression ratio and larger piston bore in an engine produce higher power and torque. The higher compression in a combustion chamber makes the air and fuel particles compressed in larger density and in high pressure. Larger piston geometry (bore) will let more air passing through the combustion chamber. Both of these variables will create powerful explosion and burns fuel more efficiently. The modification will generate fuel economy and will deliver peak performance. However, these modifications have its drawbacks too. High octane gas is needed to keep the engine running in good condition which is more expensive than regular unleaded gas. Using low octane will develop engine knock. Engine knocking happens when the air-fuel combustion doesn't happen at the exact time of spark ignition in the piston's stroke. Furthermore, larger explosion actually create more power and will make the engine runs hotter than normal. Larger piston bore also require more fuel to burn to compensate the larger air flow through combustion chamber .
As we known gasoline engines burn fuel to create motions. The reaction of mixture of fuel with oxygen in the air is cause from burning of fuel. This is called air-fuel ratio (AFR). AFR is the mass ratio of air to fuel present in an internal combustion engine. For gasoline engines, the stoichiometric, A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. It depends on type of fuel. Different fuel gives different AFR. The AFR is necessary for controlling emission and performance-tuning reasons [3]. The mixture combust and produce the product of carbon dioxide, water and nitrogen [4]. The AFR calculation is based on Lambda Oxygen (O2) sensor in gasoline engine. The AFR are defined below
CxHy+zO2+3.71zN2--->xCO2+y/2H2O+3.71zN2
The AFR is related to mixture of air and fuel. The AFR is important measure for anti-pollution and performance tuning reasons. Furthermore, it is important to know the AFR at which exactly all the available oxygen is used to burn the fuel completely or at least to the best possible value. This ratio is called stoichiometric AFR. However, standard stoichiometric AFR is not giving best performance when modification of modified piston geometry and compression ratio is done. The stoichiometric mixture in gasoline engine and its products are defined below:
where the mixture of fuel, oxygen and nitrogen produce the products of water, carbon dioxide and nitrogen.
Lambda O2 sensor is an electronic device that measures the proportion of oxygen (O2) in the gas being analyzed. It controls the AFR in internal combustion engine which is lean, rich or stoichiometric. The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. The two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust in the atmosphere. The ECU is a control system that adjusts the AFR based on the feedback from the sensor. The AFR calibration is crucial for performance and durability of the engine based on the sensor. A lower AFR number contains less air than the 14.7:1 stoichiometric AFR is a richer mixture. Higher AFR number contains more air and therefore it is a leaner mixture. Leaner mixture gives more power and efficiency but increase in engine temperature. Richer mixture provides cooler temperature in engine but less power and produce unburn fuel. Rich and lean mixtures depend on the type of engine [6].
Based on the reason above it is necessary to do research deals with investigation, rebuilding and analyzing on AFR of a gasoline engine together on the modification of piston geometry and compression ratio. Therefore, it is expected to determine the best AFR for modified gasoline engine to give peak performance.
There are two categories of diesel engine: open-chamber or direct-injection engines are preferred for heavy-duty applications because they offer the best fuel economy; divided-chamber or indirect-injection engines have been preferred for light-duty applications because they are less sensitive to differences in fuels, have a wider range of speeds (and therefore greater power:weight ratio), run more quietly and emit fewer pollutants.
The major products of the complete combustion of petroleum-based fuels in an internal combustion engine are carbon dioxide (13%) and water (13%), with nitrogen from air comprising most (73%) of the remaining exhaust. A very small portion of the nitrogen is converted to nitrogen oxides and some nitrated hydrocarbons. Some excess oxygen may be emitted, depending on the operating conditions of the engine. Gasoline engines are designed to operate at a nearly stoichiometric ratio (air:fuel ratio, ?14.6:1); diesel engines operate with excess air (air:fuel ratio, ?25–30:1; .
Incomplete combustion results in the emission of carbon monoxide, unburnt fuel and lubricating oil and of oxidation and nitration products of the fuel and lubricating oil. These incomplete combustion products comprise thousands of chemical components present in the gas and particulate phases ; some specific chemical species and classes found in engine exhausts are listed in Table 1. The concentration of a chemical species in vehicle exhaust is a function of several factors, including engine type, engine operating conditions, fuel and lubricating oil composition and emission control system
What is diesel exhaust?
Diesel exhaust is produced by the combustion (burning) of diesel fuel. The exhaust is a complex mixture of gases, vapours, aerosols, and particulate substances. The exact nature of the exhaust depends on a number of factors including the type of engine, how well serviced/maintained the engine is, type of fuel, speed and load on the engine, and emission control systems.
Diesel exhaust may contain:
· Carbon (soot)
· Carbon monoxide
· Carbon dioxide
· Oxygen
· Water vapour
· Nitrogen
· Oxides of nitrogen (e.g., nitrogen oxide, nitrogen dioxide)
· Oxides of sulphur (e.g., sulphur dioxide)
· Alcohols
· Aldehydes
· Ketones
· Hydrocarbons
· Polycyclic aromatic hydrocarbons (PAHs)
· Diesel particulate matter (DPM)?
What are the main health concerns?
Short term exposure to diesel exhaust can cause coughing, and irritation of the eyes, nose, throat, and respiratory tract. Breathing in diesel exhaust can cause lung irritation and/or an allergic reaction causing asthma (wheezing and difficult breathing), or making pre-existing asthma worse.
Very high levels can lead to asphyxiation from carbon monoxide poisoning.
Long term exposure may lead to serious health effects. The International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO), classified diesel engine exhaust as carcinogenic to humans (Group 1), determining that exposure to diesel exhaust emissions increases the risk for lung cancer and possibly bladder cancer.?
Who is at risk of exposure to diesel exhaust?
The most common way individuals are exposed is by breathing air that contains the diesel particulate matter. The fine and ultra fine particles are respirable, which means that the particles can avoid many of the human respiratory system defense mechanisms and enter deeply into the lung.
Emissions out of combustion engines
During the combustion proces, exhaust polution occurs. Engine exhaust gas contains pollutants which are harmful to man and the environment, known as?carbon monoxide(CO),?hydrocarbons (HC),?nitrogen oxide(NOx),Oxides of sulfur (SOx) and particulate matter (PM).
Exhaust and harmful emissions
Exhaust emissions are the non-useable gaseous waste products produced during the combustion process. “Exhaust gas” is the standard term used to describe the waste gas from internal combustion engines. In addition to harmless products such as water vapour, carbon dioxide and nitrogen, engine exhaust also contains pollutants which are harmful to man and the environment:?carbon monoxide?(CO),?hydrocarbons?(HC) and?nitrogen oxide?(NOx). Harmful emissions only represent a very small share of the overall emissions of a modern engine. Only 1.1 % for the petrol engine and 0.2 % for the diesel engine. The majority of exhaust consists of nitrogen, water and carbon dioxide. ?
However, it is important that the comparatively small quantity of harmful emissions is also rendered harmless. ?
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Carbon monoxide (CO)?
Carbon monoxide?is a colourless, odourless and tasteless gas. The compound, consisting of carbon and oxygen, is formed during incomplete combustion of carbon-containing substances and is very poisonous to the respiratory system. As soon as it is inhaled and enters the bloodstream it prevents the bonding of oxygen to the red corpuscles. A concentration of 1.28 percent carbon monoxide in the air will cause death from suffocation within 1 to 2 minutes.
Hydrocarbon (HC)
Hydrocarbons?are chemical compounds which consist only of carbon (C) and hydrogen (H). They can be found in large quantities in crude oil, natural gas and coal, where they are the actual “fuel”. Some hydrocarbon compounds cause cancer. When exposed to sunlight, hydrocarbons and nitrogen oxide react to form ozone. In the lower layers of the atmosphere this is a hazardous substance.
Nitrogen oxide (NOx)
Nitrogen oxides?are the gaseous oxides of nitrogen (N). They are abbreviated NOx because of the various possible compounds with different numbers of atoms: N2O, NO, N2O3, NO2, etc.
Oxides of Sulfur (SOx)
Oxides of sulfur?can aggravate respiratory illness and heart and lung disease. It forms?particulate matter (PM)?and is a primary cause of acid rain. Most vehicle emissions studies do not consider SOx because vehicles contribute such a small portion of the total amount emitted by human activity. In 2006, ultra-low sulfur diesel (ULSD) regulations reduced the contribution of SOx even further. Although SOx is not a major concern for conventional and alternative fuel vehicles, it is a concern for electric vehicles since electricity generation is the largest source of SOx. ?
Methane (CH4)
Methane? is a chemical compound with the chemical formula CH4. It is the simplest alkane, the main component of?natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel. ?
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Particulate Matter (PM)
Particulate Matter?can aggravate asthma, emphysema, bronchitis, heart disease, and lung disease. It is a carrier of many toxic compounds, contributes to haze, pollutes fresh and coastal waters, and contaminates farmland and natural ecosystems. PM is emitted directly from vehicles (especially?diesel and is formed through the atmospheric reactions of?NOx?and oxides of sulfur (SOx). ?
Products of Combustion
Some of the fuel (hydrocarbon) may not completely burn during combustion and therefore is released into the atmosphere along with the products. The products that are formed during combustion of fossil fuels are shown in the image below:
Diesel engine, like other internal combustion engines, converts chemical energy contained in the fuel into mechanical power. Diesel fuel is a mixture of hydrocarbons which—during an ideal combustion process—would produce only carbon dioxide (CO2) and water vapor (H2O). Indeed, diesel exhaust gases are primarily composed of CO2, H2O and the unused portion of engine charge air. The volumetric concentrations of these gases in diesel exhaust are typically in the following ranges:
The concentrations depend on the engine load, with the content of CO2 and H2O increasing and that of O2 decreasing with increasing engine load. None of these principal diesel emissions (with the exception of CO2 for its greenhouse gas properties) have adverse health or environmental effects.
Diesel emissions also include pollutants that can have adverse health and/or environmental effects. Most of these pollutants originate from various non-ideal processes during combustion, such as incomplete combustion of fuel, reactions between mixture components under high temperature and pressure, combustion of engine lubricating oil and oil additives as well as combustion of non-hydrocarbon components of diesel fuel, such as sulfur compounds and fuel additives. Common pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) or particulate matter (PM). The total concentration of pollutants in diesel exhaust gases typically amounts to some tenths of one percent—this is schematically illustrated in Figure 1. Much lower, “near-zero” levels of pollutants are emitted from modern diesel engines equipped with emission aftertreatment devices such as NOx reduction catalysts and particulate filters.
There are other sources that can contribute to pollutant emissions from internal combustion engines—usually in small concentrations, but in some cases containing material of high toxicity. These additional emissions can include metals and other compounds from engine wear or compounds emitted from emission control catalysts (via catalyst attrition or volatilization of solid compounds at high exhaust temperatures). Formation of new species—normally not present in engine exhaust—can also be facilitated by catalysts. This seems to be especially the case when catalysts are introduced into the combustion chamber. For example, some fuel additives—so-called “fuel-borne catalysts”—used to support the regeneration of diesel particulate filters have been linked to emissions of highly toxic dioxins and furans [Heeb 2011]. A possibility of new emissions must be considered whenever additives (catalytic or not) are introduced into the fuel or lube oil and when fluids are introduced into the exhaust gas. A well known example is urea used as a NOx reductant in SCR catalyst systems—emissions from SCR engines can include ammonia, as well as a number of products from incomplete decomposition of urea. Low quality fuels can be still another source of emissions—for instance, residual fuels used in large marine engines contain heavy metals and other compounds known for their adverse health and environmental effects.
Non-Exhaust Emissions (NEE). Emissions can also be produced by other vehicle components than internal combustion engines. Of special concern are non-exhaust particle emissions that consist of airborne particulate matter generated by the wearing down of brakes, clutches, tires and road surfaces, as well as by the suspension of road dust . As tailpipe PM emissions from diesel engines become reduced to very low levels, the relative share of NEE PM becomes higher. In a diesel engine with a particulate filter, tire and brake wear particle emissions can easily exceed exhaust PM emissions.
Non-exhaust particle emissions are also produced by zero tailpipe emission vehicles such as battery electric vehicles. In electric vehicles, brake wear can be reduced up to 60–80% compared to combustion engine vehicles through the use of regenerative braking . Tire wear emissions, on the other hand, can by much higher in electric vehicles due to their heavier weight and higher wheel torque gradient.
Nitrogen Oxide (NO), Nitrogen Dioxide (NO?) and NOx
Nitrogen Oxides (NO) and Nitrogen Dioxide (NO?) together are known as NOx. Diesel Exhaust Fluid (DEF) also known as AdBlue?, is used in the aftertreatment system to reduce NOx. DEF is composed of 32.5% urea (CO(NH?)?) and demineralized water. DEF is sprayed and then spread into the exhaust gas via a mixer before entering the Selective Catalytic Reduction (SCR) system.??
As DEF enters the system, it evaporates into ammonia (NH?) and water (H?O), the ammonia sticks on the SCR catalyst and binds with the nitrogen oxides as they pass over the catalyst. Once bound, the nitrogen oxides will combine with the ammonia to create a chemical reaction resulting in nitrogen and water.?
(NO+NO?+2NH? -> 2N? + 3H?O)
Key Takeaways:
Introduction to Diesel Engine Aftertreatment
Diesel engine aftertreatments are forms of emissions control designed to mitigate and reduce the environmental harm produced by diesel engine exhaust.
Diesel aftertreatment may work through filtration, chemical reactions, and catalysis. These controls are used downstream from the engine, designed to reduce or eliminate dangerous emissions before they enter the atmosphere.
Common forms of diesel engine aftertreatment are:
These emission controls are not voluntary; they are set and regulated by the United States Environmental Protection Agency.
Common Diesel Engine Emissions
Diesel engine emissions contain a mixture of pollutants. Some of these include:
Nitrogen Oxides (NOx): These gases are produced from high combustion temperatures within diesel engines. They contribute significantly to smog formation and acid rain, both of which have detrimental effects on our environment.
Particulates or Soot: Diesel engines release these tiny particles into the air we breathe — posing significant health risks as they can penetrate deep into our lungs when inhaled.
Carbon Monoxide (CO): This gas is a product of incomplete combustion in diesel engines. It can cause headaches, dizziness, weakness, nausea, and even death at high levels due to its ability to displace oxygen in the blood.
Hydrocarbons (HC): These compounds comprise hydrogen and carbon atoms. They react with nitrogen oxides in sunlight to create ground-level ozone or smog – causing respiratory problems among other health issues.
The Science Behind Diesel Aftertreatment
Redox – Reduction-Oxidation
Redox is a chemical reaction where one substance transfers electrons to another substance. The term “reduction” refers to the gain of electrons by a molecule, while the term “oxidation” refers to the loss of electrons.
But why do we care about gaining and losing electrons (redox)? Because Redox is used in a common diesel aftertreatment method called “Selective Catalytic Reduction (SCR)” to convert Nitrogen Oxide into harmless Nitrogen Gas (N2) and Water.
Catalysts
Several common diesel aftertreatment processes involve the use of catalysts.
Catalysts are substances used to facilitate chemical reactions. They lower the activation energy needed for chemical reactions to occur, and most importantly, the catalysts are not consumed in the process.
Common materials used as catalysts in diesel aftertreatment are precious metals, base metals, and mixed metals.
In diesel aftertreatment, catalysts are important because they help facilitate the transfer of electrons. For example, as diesel exhaust gases pass over a catalytic surface, the hydrocarbons and carbon monoxide experience oxidation which transforms them into harmless carbon dioxide and water.
Diesel Exhaust Fluid (DEF)
Diesel exhaust fluid is sometimes also known as “AdBlue”. It is a non-toxic, non-flammable liquid composed of one-third urea and two-thirds deionized water.
Ordinarily, your diesel exhaust fluid will be housed in a container under the hood of your vehicle.
“Urea Definition”: Urea is a chemical compound consisting of carbon atoms bonded to nitrogen, oxygen, and hydrogen atoms. It is organic and naturally produced in the urine of mammals.
When the urea is introduced to diesel exhaust along with a catalyst, it forms a “redox” reaction with nitrogen oxide, completing the transformation of NOx into nitrogen gas and water.
Particulate Filters
Particulate filters are used in diesel aftertreatment systems to capture particulate matter such as soot and ash before they are released into the atmosphere.
Particulate filters often have the ability to filter exhaust down to a range of 2 to 10 microns. For reference, one micron is equal to 1 millionth of a meter, or 0.001 millimeters (mm).
Diesel Exhaust Aftertreatment Systems
Now that we have an understanding of the science behind common aftertreatment methods, let’s have a look at the diesel aftertreatment systems installed on common vehicles.
Diesel Oxidization Catalyst (DOC)
This is a flow through honeycomb substrate with a catalytic coating containing precious metal applied to the surface. The DOC unit assembly has a round cross-section to eliminate non-uniform thermal gradients. As exhaust gas from the engine passes over the catalyst a chemical reaction takes place oxidizing carbon monoxide (CO), hydrocarbons (HC) and soluble organic fraction (SOF) that is unburned fuel and engine lube oil suspended in the engine exhaust gases.
The DOC does not qualify as a spark arrestor. If a spark arrestor is required, it should be installed downstream of the DOC assembly.
What Is a Diesel Oxidation Catalyst?
Diesel Oxidation Catalysts (DOC) are catalytic converters designed specifically for diesel engines and equipment to reduce Carbon Monoxide (CO), Hydrocarbons (HC) and Particulate Matter (PM) emissions. DOC's are simple, inexpensive, maintenance-free and suitable for all types and applications of diesel engines.
Figure 1. How Diesel Oxidation Catalyst (DOC) Works
Modern catalytic converters consist of a monolith honeycomb substrate coated with platinum group metal catalyst, packaged in a stainless steel container. The honeycomb structure with many small parallel?channels presents a high catalytic contact area to exhaust gasses. As the hot gases contact the catalyst, several exhaust pollutants are converted into harmless substances: carbon dioxide and water.
The diesel oxidation catalyst is designed to oxidize carbon monoxide, gas phase Hydrocarbons, and the SOF fraction of diesel particulate matter to?CO2 and H2O:
Diesel exhaust contains sufficient amounts of oxygen, necessary for the above reactions. The concentration of O2 in the exhaust gases from diesel engine varies between 3 and 17%, depending on the engine load. Typical conversion efficiencies for CO and HC in the Nett? diesel oxidation catalyst are given in Figure 2. The catalyst activity increases with temperature. A minimum exhaust temperature of about 200°C is necessary for the catalyst to "light off". At elevated temperatures, conversions depend on the catalyst size and design and can be higher than 90%.
Figure 2. Catalytic Conversion of Carbon Monoxide and Hydrocarbons
Conversion of diesel particulate matter is an important function of the modern diesel oxidation catalyst. The catalyst exhibits a very high activity in the oxidation of the organic fraction (SOF) of diesel particulates. Conversion of SOF may reach and exceed 80%. At lower temperatures, say 300°C, the total DPM conversion is usually between 30 and 50% (Figure 3). At high temperatures, above 400°C, a counterproductive process may occur in the catalyst. It is the oxidation of sulfur dioxide to sulfur trioxide, which combines with water forming sulfuric acid:
A formation of the sulfate (SO4) particulates occurs, outweighing the benefit of the SOF reduction. Figure 3 shows an example situation, where at 450°C the engine-out and the catalyst total DPM emissions are equal. In reality the generation of sulfates strongly depends on the sulfur content of the fuel as well as on the catalyst formulation. It is possible to decrease DPM emissions with a catalyst even at high temperatures, provided suitable catalyst formulation and good quality fuels of low sulfur contents are used. On the other hand, diesel oxidation catalyst used with high sulfur fuel will increase the total DPM output at higher temperatures. This is why diesel catalysts become more widespread only after the commercial introduction of low sulfur diesel fuel.
Figure 3. Catalytic Conversion of DPM
The diesel oxidation catalyst, depending on its formulation, may also exhibit some limited activity towards the reduction of nitrogen oxides in diesel exhaust. NOx conversions of 10-20% are usually observed. The NOx conversion exhibits a maximum at medium temperatures of about 300°C.
The diesel oxidation catalyst (DOC) owes its name to its ability to promote oxidation of exhaust gas components by oxygen, which is present in ample quantities in diesel exhaust. When passed over an oxidation catalyst, carbon monoxide (CO), gas phase hydrocarbons (HC), the organic fraction of diesel particulates (OF), as well as non-regulated emissions such as aldehydes or PAHs can be oxidized to harmless products, and thus can be controlled using the DOC. In modern diesel aftertreatment systems, an important function of the DOC is to oxidize nitric oxide (NO) to nitrogen dioxide (NO2)—a gas needed to support the performance of diesel particulate filters and SCR catalysts used for NOx reduction. A comprehensive discussion of DOC reactions, reaction kinetics, and other aspects of the technology can be found in the literature .
The reaction mechanism over the diesel oxidation catalyst is explained by the presence of active catalytic sites on the surface of the catalyst carrier, which have the ability to adsorb oxygen. In general, the catalytic oxidation reaction includes the following three stages:
The oxidation of hydrocarbons and CO in diesel emissions can be described by the following chemical reactions:
[Hydrocarbons] + O2 = CO2 + H2O(1)
CnH2m + (n + m/2)O2 = nCO2 + mH2O(1a)
2CO + O2 = 2CO2(2)
Hydrocarbons are oxidized to form carbon dioxide and water vapor, as described by reaction (1) or—in a more stoichiometrically rigorous way—by reaction (1a). In fact, reactions (1) and (1a) represent two processes: the oxidation of gas phase HC, as well as the oxidation of OF compounds. Reaction (2) describes the oxidation of carbon monoxide to carbon dioxide. Since carbon dioxide and water vapor are considered harmless, the above reactions bring an obvious emission benefit. The oxidation of HCs also results in a reduction of the diesel odor.
However, an oxidation catalyst will promote oxidation of all compounds of a reducing character; some of the oxidation reactions can produce undesirable products and, in effect, be counterproductive to the catalyst purpose. Oxidation of sulfur dioxide to sulfur trioxide with the subsequent formation of sulfuric acid (H2SO4), described by reactions (3) and (4), is perhaps the most important of these processes.
2SO2 + O2 = 2SO3(3)
SO3 + H2O = H2SO4(4)
When exhaust gases are discharged from the tailpipe and mixed with air, either in the environment or in the dilution tunnel which is used for particulate matter sampling, their temperature decreases. Under such conditions the gaseous H2SO4 combines with water molecules and nucleates forming (liquid) particles composed of hydrated sulfuric acid. This material, called sulfate particulates, contributes to the total particulate matter emissions from the engine. Catalytic formation of sulfates, especially in conjunction with high sulfur content diesel fuel, can significantly increase the total PM emissions and, thus, become prohibitive for the catalyst application.
The oxidation of NO to NO2 is essential for the operation of modern diesel emission control systems, where the DOC is an auxiliary catalyst supporting the performance of other types of catalysts—positioned downstream of the oxidation catalyst—that require an elevated NO2/NO ratio.
2NO + O2 = 2NO2(5)
Nitrogen dioxide is required to enhance the performance of several types of SCR catalysts, as well as to promote passive regeneration of diesel particulate filters (DPF). DOCs used in DPF/SCR applications are optimized for high NO2 production.
The increased NO2/NO ratios with oxidation catalysts—while essential for the operation of diesel aftertreatment systems—have also been a source of controversy. Among the two components of NOx emissions, NO2 shows a higher toxicity than NO. In some applications, increased NO2 emissions can contribute to air quality problems. This potential adverse effect of the DOC was first discovered in underground mines . This issue may also play a role in “street canyons” with high traffic intensity—even though the thermodynamic equilibrium of reaction (5) can be reached more quickly in the presence of sunlight, and NO can be oxidized rapidly by ozone.
Fuel
DOCs perform best with Ultra Low Sulfur Diesel fuel (ULSD), and some DOCs are verified for use with Low Sulfur Diesel (LSD). ULSD, which contains up to 15 parts per million sulfur, is required for highway vehicles and will begin to be phased in for the nonroad sector beginning in 2010.
DPF (Diesel Particulate Filter)
Diesel Particulate Filters, also known as DPFs, are exhaust aftertreatment devices that significantly reduce emissions from diesel fueled vehicles and equipment. DPFs typically use a porous ceramic or cordierite substrate or metallic filter, to physically trap particulate matter (PM) and remove it from the exhaust stream.
After it is trapped by the DPF, collected PM is reduced to ash during filter regeneration. Regeneration occurs when the filter element reaches the temperature required for combustion of the PM. “Passive” regeneration occurs when the exhaust gas temperatures are high enough to initiate combustion of the accumulated PM in the DPF, without added fuel, heat or driver action. “Active”regeneration may require driver action and/or other sources of fuel or heat to raise the DPF temperature sufficiently to combust accumulated PM. The frequency of regeneration is determined by the engine’s duty cycle, PM emission rate, filter technology and other factors. When using an active filter, it is particularly important to follow the manufacturer’s instructions for regeneration.
In addition to regeneration, the filter must be periodically cleaned to remove noncombustible materials and ash. It is important to avoid excessive PM and ash accumulation in a DPF, so proper maintenance and cleaning instructions should be followed closely. Cleaning of DPFs is typically required every 6 to 12 months. The cleaning process involves manually removing the filter element from the vehicle and placing it in a cleaning station designed for this purpose. An engine emitting excessive PM or inadequate filter regeneration will cause a DPF to require more frequent cleaning. Diagnostics should be performed to identify the cause for more frequent cleaning intervals. A backpressure monitoring system should always be used with a DPF and periodic inspection of the monitoring system should be performed to confirm proper operation.
CDPF (Catalyzed Diesel Particulate Filter)
This is a DPF as above with a catalytic coating made up of precious metals coating the cellular structure to enhance the regeneration process.
Regeneration of Particulate Filters
Passive Regeneration
Passive regeneration is when the rate of soot oxidation in the particulate filter meets or exceeds the rate of soot generated by the engine thus preventing excessive soot accumulation levels from occurring. A catalytic coating can be used to promote oxidation by increasing the speed of the chemical reaction. The DOC converts, NOx (Oxides of Nitrogen) to NO2 Nitrogen Dioxide. The DPF is placed after the DOC in the system. As the oxidized NO2 passes through the DPF, it combines with the collected carbon (C) (soot) to form CO2 (Carbon Dioxide), which leaves the exhaust system as a clear gas. Passive Regeneration is continually cleaning the diesel particulate filter when engine-operating conditions maintain sufficient exhaust temperatures.
Passive Plus Regeneration
The Cat CEM system has been designed to maximize passive regeneration but when the exhaust gas temperature is too low to maintain regeneration such as when power demand levels are low the backpressure valve is closed to increase the load on the engine and thus raise the exhaust gas temperature to enhance passive regeneration. This is known as Passive Plus regeneration.
CRS (Caterpillar Regeneration System) - Active Regeneration
The exhaust gas temperatures are periodically elevated by combustion of injected fuel and air to promote oxidation and burn trapped soot. CRS is used in conjunction with a DOC and a DPF to provide an Active Regeneration System.
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Diesel Particulate Matter Filtration
Diesel particulate filters operate by trapping soot particles from the engine exhaust, preventing them from reaching the environment. ?Unlike a catalytic converter which is designed to reduce gas-phase emissions flowing through the catalyst, the particulate filter is designed to trap and retain the solid particles until the particles can be oxidized or burned in the DPF itself, through a process called regeneration.
The most common diesel particulate filters in widespread use are cellular ceramic honeycomb filters with channels that are plugged at alternating ends, as shown in?Figure 1. ?The ends of the filter, plugged in a checkerboard pattern, force the soot-containing exhaust to flow through the porous filter walls. ?While the exhaust gas can flow through the walls, the soot particles are trapped within the filter pores and in a layer on top of the channel walls. ?The honeycomb design provides a large filtration area while minimizing pressure losses, and has become the standard, so-called wall-flow filter for most diesel exhaust filtration applications. Ceramic materials are widely used for particulate filters, given their good thermal durability, with the most common ceramic materials being: cordierite, silicon carbide, and aluminum titanate.
Figure 1. ?Diesel particulate filter for heavy-duty vehicle (a), cross-section viewing showing filtration processes within several DPF channels (b), and close-up view of particle capture and build-up on the channel walls (c).
The details of the filtration process are illustrated in?Figure 1(b), which shows the soot particles trapped along the inlet channel, which is open at the front end but plugged at the back end. ?DPFs contain several hundred channels or cells per square inch (cpsi), with the most common being 200 cpsi. ?Since half of the channels are plugged at the front of the DPF and the other half are plugged at the back of the filter, only half of the filter channels accumulate soot or ash. ?That is, only the channels open on the inlet side are exposed to the “dirty” exhaust flow, while the channels open to the outlet side remain clean. ?Given the small pore size and design of the honeycomb filters, DPFs can achieve a particle trapping efficiency of 99% or greater [1]. Due to the high trapping efficiency and DPF cell design, no visible soot or ash should pass through the filter walls. ?Black streaks or visible soot in the outlet channels are a sure sign of filter failure.
Soot particles are captured and retained in the DPF through a combination of depth filtration inside the filter pores and surface filtration along the channel walls. The inset in?Figure 1(c)?shows these two processes, where a small fraction of the soot initially accumulates in the filter pores (1) and then subsequently builds a layer along the channel walls (2). ?As the soot load in the filter increases, so too does the filter’s trapping efficiency, as the accumulated soot provides an additional layer to trap incoming particles. ?The specific soot filtration mechanisms, whether in the pores on the surface of the walls, plays an important role in determining the overall increase in exhaust back pressure (or pressure drop across the filter), shown in?Figure 2.
Figure 2. Pressure drop evolution with soot accumulation in the DPF showing rapid initial rise in pressure drop due to soot accumulation in the filter pores (1) followed by a gradual increase as soot builds a layer along the walls (2).
The porosity of most commercial DPFs ranges from around 40% to 60%. ?The walls of these filters contain a complex network of pores in the range of 10 – 30 micrometers (microns) in diameter [2]. ?In a new or clean DPF, the surface of the filter is exposed to the exhaust flow and soot rapidly accumulates in the surface pores. Although only a small fraction of the total soot accumulates in the filter micro-pores, it contributes to a steep rise in filter pressure drop shown in?Figure 2. ?Subsequent soot accumulation in the DPF forms a layer (cake layer) along the walls of the channel, and results in a slower and more gradual rise in filter pressure drop [3]. ?Depending on the soot loading level and filter type, the pore accumulation can account for 50% of the filter pressure drop, or more, in some cases. ?The non-linear response of the DPF to material accumulation complicates the determination of filter soot or ash loading levels based on pressure drop alone
Filter Regeneration
In order to reduce filter pressure drop due to soot accumulation, the filter is regenerated through a processes the burns off (oxidizes) the soot. ?There are two broad categories of regeneration processes, although most commercial applications use some combination of the two. ?This is particularly true with vehicles or equipment experiencing extended periods of low exhaust temperature operation, such as long periods of idle or low speed/load operating cycles.
Active Regeneration?requires the addition of heat to the exhaust to increase the temperature of the soot to the point at which it will oxidize in the presence of excess oxygen in the exhaust. ?The combustion of soot in oxygen typically requires temperatures above 550?°C (1,000?°F). ?Since these high temperatures generally fo not occur during normal engine operation, a number of strategies are used to actively increase the exhaust temperature [4]. ?Active regeneration systems may include the use of a diesel burner to directly heat the exhaust entering the DPF or the use of a diesel oxidation catalyst (DOC) to oxidize diesel fuel over the catalyst as a means for increasing the DPF temperature. ?Use of the DOC also requires excess diesel fuel in the exhaust, which may be accomplished through a fuel injector (hydrocarbon doser) mounted in the exhaust upstream of the DOC, or through late in-cylinder post injection strategies. ?Other forms of active regeneration include the use of electrical heating elements, microwaves, or plasma burners.
The use of a DOC in combination with some form of exhaust fuel dosing is the most common active regeneration strategy currently used for on- and off-highway applications. ?The duration of an active regeneration event generally ranges from 20 to 30 minutes on average, under normal operating conditions. In some cases, such as severe DPF soot plugging, a parked regeneration may be required, which can last up to several hours to slowly burn off the soot under more controlled conditions. ?Regardless of the specific strategy, active regenerations always require additional energy input (additional fuel) to heat the exhaust and the DPF to the required temperature.
Passive Regeneration, as the name implies, does not require additional energy to carry out the regeneration process. ?Instead, this strategy relies on the oxidation of soot in the presence of NO2, which can occur at much lower temperatures in the range of 250?°C to 400?°C (480?°F to 750?°F). ?A catalyst is used to convert NO present in the exhaust to NO2. ?These catalysts require the use of precious metals to facilitate the reaction, platinum (Pt), in particular, which adds additional cost to the system. ?In some cases the catalyst coating is applied directly to the DPF, as with a catalyzed DPF (C-DPF), or an upstream oxidation catalyst (DOC) may also be used [5]. ?Many commercial systems utilize a combination of a DOC and C-DPF. ?Use of the catalysts allows NO2 to be produced and soot to be oxidized at temperatures which occur during normal engine or vehicle operation.
In an ideal case, if engine operation results in a certain amount of time spent within this passive regeneration “temperature window” then active regeneration may not be needed. ?In reality however, low temperature operation may occur for extended periods of time, such as long periods of idle or low load operation, particularly in cold climates, and some active regeneration may still be needed. ?In the absence of active regeneration, periods of low temperature operation may be supplemented by periods of high temperature operation (such as extended highway driving) to induce passive regeneration.
In order to reduce fuel consumption, passive regeneration is preferred, although most commercial systems still use active regeneration to varying degrees, depending on the drive cycle and operating conditions. Regardless of the regeneration method, the oxidation of soot (whether active or passive) results in incombustible material, or ash, which can not be burned, and remains in the DPF. ?Understanding the key?differences between ash and soot, as well as their?impacts on DPF performance?is important when selecting the most appropriate?cleaning method?for the filter.
Why is this important for DPF ash cleaning?
Understanding the design and operation of the DPF to collect and trap particles, whether in the pores or on the surface, has a large impact on how easily the particles can later be removed. ?Soot is fundamentally?different from ash?in that the soot can be oxidized and removed through regeneration, while the ash is incombustible and remains in the DPF until the?DPF is serviced for ash cleaning.??
Selective Catalytic Reduction and Diesel Exhaust Fluid
The Selective Catalytic Reduction systems (SCR) and Diesel Exhaust Fluid work together to reduce or eliminate nitrogen oxide from being released into the atmosphere.
The reason for the name “selective” in “selective catalytic reduction” is because the system is designed to only target nitrogen oxide, while leaving other components of the exhaust unaffected.
Diesel exhaust fluid is injected directly into the exhaust pipe upstream of the nitrogen oxide catalyst.
The diesel exhaust fluid, consisting of urea and water, creates the needed “redox” reaction with the diesel exhaust when in the presence of the catalyst, which reduces the amount of nitrogen oxide released into the atmosphere.
This SCR system can achieve up to 90% efficiency in converting nitrogen oxide into nitrogen gas and water when operated under ideal conditions.
What Is Selective Catalytic Reduction?
Selective Catalytic Reduction (SCR) is a proven and advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF).
The?Diesel Technology Forum recently?issued the following explanation on Selective Catalytic Reduction (SCR), an advanced active emissions control technology system used in?clean diesel engines.
Selective Catalytic Reduction (SCR) is a proven and advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF). The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.
SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called “selective” because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as “reduction” where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can achieve NOx reductions up to 90 percent.
Why is SCR important?
SCR technology is one of the most cost-effective and fuel-efficient technologies available to help reduce diesel engine emissions. Its effectiveness allows diesel engines to be tuned and optimized toward maximum fuel efficiency, while the SCR systems are highly efficient at treating the engine-out exhaust.
The largest sector for use of SCR technology in the US is heavy-duty commercial trucks.? All heavy-duty diesel truck engines produced after January 1, 2010 must meet the latest EPA emissions standards, among the most stringent in the world, reducing particulate matter (PM) and nitrogen oxides (NOx) to near zero levels. SCR can reduce NOx emissions up to 90 percent while simultaneously reducing HC and CO emissions by 50-90 percent, and PM emissions by 30-50 percent.
In the commercial trucking industry, some SCR-equipped truck operators are reporting fuel economy gains of 3-5 percent. Additionally, off-road equipment, including construction and agricultural equipment, must meet EPA’s Tier 4 emissions standards requiring similar reductions in NOx, PM and other pollutants. ?SCR is also used in some of the many different applications of off-road engines and equipment.
Where is SCR used?
SCR has been used for decades to reduce stationary source emissions from various industrial operations. In addition, marine vessels worldwide have been equipped with SCR technology, including cargo vessels, ferries and tugboats. With its superior return in both economic and environmental benefits, SCR is also being recognized as the emissions control technology particularly helpful in meeting the U.S. EPA 2010 diesel engine emission standards for heavy-duty vehicles and the Tier 4 emissions standard for engines found in off-road equipment. SCR systems are also found in the growing number of diesel passenger vehicles.
What are the special considerations of using SCR?
One unique aspect of a vehicle or machine with an SCR system is the need for replenishing Diesel Exhaust Fluid (DEF) on a periodic basis. DEF is carried in an onboard tank which must be periodically replenished by the operator.? DEF consumption rates are determined by vehicle operation.? More aggressive driving at higher speeds or while hauling heavy loads will increase DEF consumption.? For most light-duty vehicles, DEF refill intervals typically occur around the time of a recommended oil change or other scheduled vehicle maintenance.? DEF replenishment for heavy-duty vehicles and off-road machines and equipment will vary depending on the operating conditions, hours used, miles traveled, load factors and other considerations.
DEF is an integral part of the emissions control system and must be present in the tank at all times to assure continued operation of the vehicle or equipment. Low DEF supply triggers a series of escalating visual and audible indicators to the driver or operator. Once the tank reaches a certain level near empty, the vehicle starting system may be locked out the next time the vehicle is used, preventing the vehicle from being started without adequate DEF.
On-board tanks to store DEF are typically located in the spare tire area of passenger vehicles, while tractor trailers typically have a DEF tank alongside the diesel fuel saddle tank. Proper storage of DEF is required to prevent the liquid from freezing at temperatures below 12 degrees Fahrenheit, and most vehicle DEF dispensing systems have warming devices.
What is DEF?
Diesel Exhaust Fluid (DEF) is a non-toxic fluid composed of purified water and automotive grade aqueous urea. DEF is available with a variety of storage and dispensing methods. Storage options consist of various size containers such as bulk, totes and bottles or jugs. The American Petroleum Institute rigorously tests DEF to ensure that it meets industry-wide quality standards.
A nationwide DEF distribution infrastructure has rapidly expanded to meet the needs of a growing SCR technology marketplace.? Since 2010, DEF has been widely available for purchasing at various locations like service stations, convenience stores, automotive parts stores, Wal-Mart, and petroleum retailers as well as truck stops, truck dealerships and engine distributors.? DEF tanks on vehicles typically range in size from 6 to 23 gallons depending on the type of vehicle application. The DEF tank fill opening is designed to accommodate a DEF fill nozzle to ensure only DEF is put into the tank. A diesel fuel nozzle will not fit into the DEF tank opening.
Most heavy-duty truck manufacturers calculated operating costs of new SCR-equipped vehicles based on a DEF price of $3 per gallon, however, the price of DEF responds to market conditions of supply and demand and is expected to decrease due to the growing network of DEF supply.
SCR and DEF Disadvantages
SCR systems usually experience a reduction in efficiency at cold temperatures. Therefore, they are often not considered effective until the exhaust gas is at operating temperature.
An SCR system consumes a lot of space in a vehicle and is expensive to manufacture and maintain.
The urea in diesel exhaust fluid can naturally decompose after some time.
Diesel exhaust fluid is a consumable and requires regular refilling. On some vehicles, running out of DEF may put the vehicle into a derated mode, reducing the power and performance of the vehicle until the DEF is refilled.
Exhaust Gas Recirculation (EGR)
Exhaust gas recirculation is a vehicle emissions control system used in both diesel engines and petroleum engines.
The purpose of exhaust gas recirculation is to reduce the formation of nitrogen oxides.
It’s important to note that while the SCR system helps to remove already formed NOx gases, the EGR system helps to prevent them from forming in the first place.
The EGR system achieves this by recirculating a portion of the engine’s exhaust gases back to the combustion chamber.
The addition of exhaust gas into the combustion chamber reduces the amount of oxygen available in the combustion process. In effect, the reduced oxygen lowers the combustion temperature.
This reduced temperature is important because nitrogen oxide is formed in the combustion process when Nitrogen and Oxygen molecules react at high temperatures. In fact, NOx production increases exponentially as temperature increases, making temperature control a key factor in reducing emissions.
EGR Disadvantages
Diesel Engine Aftertreatment Maintenance – Avoiding Downtime and Extra Expenses
Diesel aftertreatment systems do require maintenance and regular servicing. Luckily, some of the regular aftertreatment maintenance tasks can be completed by the “do-it-yourself” mechanic and are generally low cost.
Regular “DIY” aftertreatment maintenance items include:
Regular aftertreatment maintenance that you’ll need a mechanic for:
Clean Air Regulations and Legislation
In the United States, the Environmental Protection Agency (EPA) is responsible for enforcing the Clean Air Act (CAA).
The Clean Air Act is a federal law that governs air pollution in the United States.
One way in which the Clean Air Act attempts to control air pollution is by setting and regulating emission standards for diesel engines. These standards set limits on how much nitrogen oxide, particulate matter, and carbon monoxide a diesel engine can emit.
In order to comply with these stringent emission standards, diesel manufacturers must rely on and implement technology such as particulate filters, selective catalytic reduction systems, and exhaust gas recirculation as discussed above.
Compliance with the Clean Air Act is mandatory.
Conclusion
In conclusion, diesel aftertreatment technologies play a pivotal role in mitigating the environmental impact of diesel engines by reducing harmful emissions and improving air quality.
From selective catalytic reduction (SCR) systems to diesel particulate filters (DPFs) and exhaust gas recirculation (EGR) systems, these technologies offer effective solutions for controlling nitrogen oxide (NOx), particulate matter (PM), and other pollutants.
SCR DIESELNET
Introduction
NOx reduction systems based on the selective catalytic reduction (SCR) technology have been developed and commercialized for a number of mobile diesel engine applications in jurisdictions with stringent diesel emission standards, beginning with the EU (Euro IV/V, 2005/2008), Japan (JP 2005), and the United States (US EPA 2010). The use of ammonia has been practically ruled out, due to safety concerns, and urea (in water solution) has been commonly used as the preferred reductant.
SCR-only NOx Control. In applications with emission limits of moderate stringency, the most attractive SCR emission strategy involves a calibration of the engine for low PM (injection timing, high injection pressures), and using the SCR catalyst to reduce the increased NOx, Figure 1. This approach can also meet some applicable PM emission limits—as was the case with many SCR applications on Euro IV/V engines or on EU Stage IV and US Tier 4 nonroad engines. Due to the advanced injection timing a fuel economy improvement could be realized, making urea-SCR more attractive than the competing EGR technology that brings a fuel economy penalty. Urea-SCR may also have a fuel economy advantage over NO xadvisor catalysts—another competing NOx reduction technology—due to the fuel economy penalty resulting from adsorber regeneration. However, any fuel savings in SCR engines are offset to some degree by the cost of urea.
Figure 1. SCR-only NOx emission control strategy
SCR aftertreatment without EGR and DPF
The SCR-only strategy was widely used in Europe to simultaneously meet the Euro IV/V limits for both NOx (3.5/2 g/kWh, respectively) and for PM (0.02 g/kWh) [Gekas 2002]. The engines were calibrated for low PM emission levels, below 0.02 g/kWh, while engine-out NOx was elevated to about 9-11 g/kWh. SCR aftertreatment was then used to bring down NOx emissions to below 2 g/kWh. The required NOx conversion efficiency of the SCR system was about 80-85% for Euro V and only about 65% at the Euro IV stage. The need for a diesel particulate filter was eliminated, resulting in smaller size, complexity, and cost of the emission aftertreatment system. The Euro V calibration could provide fuel savings of some 3-5%.
SCR systems had also been considered a potential solution for meeting the US 2007-2010 heavy-duty NOx fleet average standards of about 1.1 g/bhp-hr without the use of EGR [Scarnegie 2003]. In this case, the emission control system combined an SCR catalyst and a diesel particulate filter (DPF) to meet the US 2007 PM limit of 0.01 g/bhp-hr. According to cost analyses, SCR aftertreatment presented the most cost-effective technology for meeting the US 2007 emission standards. This is illustrated in Figure 2, which presents the results of an analysis by DaimlerChrysler [Schittler 2003]. In terms of fuel economy, 2007-compliant SCR+DPF package could provide a 6% advantage over the MY 2004 baseline, comparing very favorably to the EGR+DPF alternative. Also, the total life cycle cost change for SCR compared favorably with the competing EGR and NOx adsorber paths over the entire analyzed range of cost for urea solutions. Notwithstanding the apparent cost benefit, EGR technology was used for NOx control in US 2007-2009 engines, while SCR was adopted three years later, in 2010.
In light-duty application, a comparison between urea-SCR and the NOx adsorber paths to meet the US Tier 2 Bin 2 emission standards conducted by Ford concluded that urea-SCR could provide a cost advantage, both in terms of system cost and operating costs [Lambert 2004].
SCR+EGR NOx Control. Applications with more stringent emission standards require simultaneous use of SCR and EGR for NOx control. The SCR+EGR configuration has been used in US 2010 engines to meet the NOx standard of 0.2 g/bhp-hr. Relative to the 2004 standard, the 2010 limit required over 90% NOx reduction over the transient FTP cycle. In applications with such high NOx reduction requirements the SCR technology has no extra capacity to handle any increased engine-out NOx levels resulting from fuel efficient engine calibration [Song 2002] and it is necessary to combine SCR with EGR to meet these more demanding NOx limits.
Further benefits of using EGR in SCR engines include better NOx control at low temperatures, such as in applications tested over low temperature test cycles [Hirata 2005], and a more robust configuration for meeting OBD requirements.
SCR Issues. The application of SCR technology to mobile engines requires solving a number of technical, regulatory, and urea distribution infrastructure problems. The following are the most important issues:
Selective catalytic reduction (SCR) means of converting Nitrogen oxides, also referred to as NOx with the aid of a catalyst into diatomiic nitrogen (N 2), and water (H 2O). A reductant, typically anhydrous ammoonia (NH 3), aqueous ammonia (NH 4OH), or a urea (CO(NH 2) 2) solution, is added to a stream of flue or exhaust gas and is reacted onto a catalyst. As the reaction drives toward completion, nitrogen (N 2), and carbon dioxide (CO 2), in the case of urea use, are produced.
Selective catalytic reduction of NO x using ammonia as the reducing agent was patented in the United States by the Engelhard corporation in 1957. Development of SCR technology continued in Japan and the US in the early 1960s with research focusing on less expensive and more durable catalyst agents. The first large-scale SCR was installed by the IHI corporation in 1978.
SCR systems are now the preferred method for meeting Tier 4 Final and EURO 6 diesel emissions standards for heavy trucks, cars and light commercial vehicles. As a result, emissions of NOx, particulates, and hydrocarbons have been reduced by as much as 95% when compared with pre-emissions engines.
Chemistry
The NO x reduction reaction takes place as the gases pass through the catalyst chamber. Before entering the catalyst chamber, ammonia, or other reductant (such as urea), is injected and mixed with the gases. The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for a selective catalytic reduction process is:
2NO +2NH3 +1/2O2---> 2N2 +3H2O
NO2 + 2NH3 +1/2O2 --->3/2N2+3H20
NO + NO2 + 2NH3 --->2N2 +3H20
With several secondary reactions
1/8S8 +O2 --->SO2
SO2 +1/2O2 --->SO3
SO3 + H2O---> H2SO4
2NH3 + H2SO4 --> (NH4)2SO4
NH3 + H2SO4 ---> NH4HSO4
With Urea the reactions are
3NO + CO(NH2)2--->5/2N2 +2H2O +CO2
3NO2 +2CO(NH2)2---> 7/2 N2 +4H2O +2CO2
As with ammonia, several secondary reactions also occur in the presence of sulfur:
CO(NH2)2+H2SO4 +H2O--->(NH4)2SO4+CO2
CO(NH2)2 +2H2SO4+H2O --->2NH4 HSO4 +CO2
The ideal reaction has an optimal temperature range between 630 and 720?K (357 and 447?°C), but can operate as low as 500?K (227?°C) with longer residence times. The minimum effective temperature depends on the various fuels, gas constituents, and catalyst geometry. Other possible reductants include cyanuric acid and ammonium sulfate.
Catalysts
SCR catalysts are made from various porous ceramic materials used as a support, such as titanium oxide, and active catalytic components are usually either oxides of base metals (such as vanadium, molybdenum and tungsten), zeolites, or various precious metals. Another catalyst based on activated carbon was also developed which is applicable for the removal of NOx at low temperatures. Each catalyst component has advantages and disadvantages.
Base metal catalysts, such as vanadium and tungsten, lack high thermal durability, but are less expensive and operate very well at the temperature ranges most commonly applied in industrial and utility boiler applications. Thermal durability is particularly important for automotive SCR applications that incorporate the use of a diesel particulate filterr with forced regeneration. They also have a high catalysing potential to oxidize SO2 into SO3, which can be extremely damaging due to its acidic properties.
Zeolite catalysts have the potential to operate at substantially higher temperature than base metal catalysts; they can withstand prolonged operation at temperatures of 900?K (627 °C) and transient conditions of up to 1120?K (847 °C). Zeolites also have a lower potential for SO 2 oxidation and thus decrease the related corrosion risks.
Iron and copper exchanged zeolite urea SCRs have been developed with approximately equal performance to that of vanadium-urea SCRs if the fraction of the NO 2 is 20% to 50% of the total NO x. The two most common catalyst geometries used today are honeycomb catalysts and plate catalysts. The honeycomb form usually consists of an extruded ceramic applied homogeneously throughout the carrier or coated on the substrate. Like the various types of catalysts, their configuration also has advantages and disadvantages. Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, but are much larger and more expensive. Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily. A third type is corrugated, comprising only about 10% of the market in power plant applications.
Reductants
Several nitrogen-bearing reductants are currently used in SCR applications including anhydrous ammonia, aqueous ammonia or dissolved urea. All those three reductants are widely available in large quantities.
Anhydrous ammonia can be stored as a liquid at approximately 10 bar in steel tanks. It is classified as an inhalation hazard, but it can be safely stored and handled if well-developed codes and standards are followed. Its advantage is that it needs no further conversion to operate within a SCR and is typically favoured by large industrial SCR operators. Aqueous ammonia must be first vaporized in order to be used, but it is substantially safer to store and transport than anhydrous ammonia. Urea is the safest to store, but requires conversion to ammonia through thermal decomposition. At the end of the process, the purified exhaust gasses are sent to the boiler or condenser or other equipment, or discharged into the atmosphere.
Limitations
Most catalysts are given a finite service life due to known amounts of contaminants in the untreated gas. The most notable complication is the formation of ammonium sulfate and ammonium bisulfate from sulfur and sulfur compounds when high-sulfur fuels are used, as well as the undesirable catalyst-induced oxidation of SO 2 to SO 3 and H 2SO 4. In applications that use exhaust gas boilers, ammonium sulfate and ammonium bisulfate can accumulate on the boiler tubes, reducing steam output and increasing exhaust back-pressure. In marine applications, this can increase fresh water requirements as the boiler must be continuously washed to remove the deposits.
Most catalysts on the market have porous structures and a geometries optimized for increasing their specific surface area (a clay planting pot is a good example of what SCR catalyst feels like). This porosity is what gives the catalyst the high surface area needed for reduction of NOx. However, soot, ammonium sulfate, ammonium bisulfate, silica compounds, and other fine particulates can easily clog the pores. Ultrasonc horns and soot blowers can remove most of these contaminants while the unit is online. The unit can also be cleaned by being washed with water or by raising the exhaust temperature.
Of more concern to SCR performance are poisons, which will chemically degrade the catalyst itself or block the catalyst's active sites and render it ineffective at NO x reduction, and in severe cases this can result in the ammonia or urea being oxidized and a subsequent increase in NO x emissions. These poisons are alkali metals, alkaline earth metals, halogens, phosphorus sulfur, arsenic, antimony, chromium heavy metals (copper, cadmium , mercury copper, cadmium, mercury thallium and lead, and many heavy metal compounds (e.g. oxides and halides).
Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines.. Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.
Ammonia slip is an industry term for ammonia passing through the SCR unreacted. This occurs when ammonia is injected in excess, temperatures are too low for ammonia to react, or the catalyst has been poisoned. In applications using both SCR and an alkaline scrubber, the use of high-sulfur fuels also tend to significantly increase ammonia slip, since compounds such as NaOH and Ca(OH)2 will reduce ammonium sulfate and ammonium bisulfate back into ammonia:
2OH + NH4HSO4 ---> NH3 + SO4 +2H2O
2OH +NH4SO4 --->2NH3 +SO4 +2H2O
Temperature is SCR's largest limitation. Engines all have a period during start-up where exhaust temperatures are too low, and the catalyst must be pre-heated for the desired NOx reduction to occur when an engine is first started, especially in cold climates.
Automobiles
SCR was applied to trucks by Nissan Diesel Corporation, and the first practical product "Nissan Diesel Quon" was introduced in 2004 in Japan.
In 2007, the United States Environmental Protection Agency (EPA) enacted requirements to significantly reduce harmful exhaust emissions. To achieve this standard, Cummins and other diesel engine manufacturers developed an aftertreatment system that includes the use of a diesel particulate filter (DPF). As the DPF does not function with low-sulfur diesel fuel, diesel engines that conform to 2007 EPA emissions standards require ultra-low sulfur diesel fuel (ULSD) to prevent damage to the DPF. After a brief transition period, ULSD fuel became common at fuel pumps in the United States and Canada. The 2007 EPA regulations were meant to be an interim solution to allow manufacturers time to prepare for the more stringent 2010 EPA regulations, which reduced NOx levels even further.
2010 EPA regulations
Diesel engines manufactured after January 1, 2010 are required to meet lowered NOx standards for the US market.
All of the heavy-duty engine (Class 7-8 trucks) manufacturers except for Navistar International and Caterpillar continuing to manufacture engines after this date have chosen to use SCR. This includes Detroit Diesel (DD13, DD15, and DD16 models), Cummins (ISX, ISL9, and ISB6.7), Paccar, and Volvo/Mack. These engines require the periodic addition of diesel exhaust fluid(DEF, a urea solution) to enable the process. DEF is available in bottles and jugs from most truck stops, and a more recent development is bulk DEF dispensers near diesel fuel pumps. Caterpillar and Navistar had initially chosen to use enhanced exhaust gas recirculation (EEGR) to comply with the Environmental Protection Agency (EPA) standards, but in July 2012 Navistar announced it would be pursuing SCR technology for its engines, except on the MaxxForce 15 which was to be discontinued. Caterpillar ultimately withdrew from the on-highway engine market prior to implementation of these requirements.
How does Selective Catalytic Reduction work? Created by manufacturer mtu
Selective catalytic reduction employs chemical reduction to render nitrogen oxides harmless.
The term Selective Catalytic Reduction (or SCR) is used to describe a chemical reaction in which harmful nitrogen oxides (NOX) in exhaust gas are converted into water (H2O) and nitrogen (N2). In combination with internal engine technologies, such as exhaust gas recirculation (EGR), extremely low nitrogen oxide emissions can be achieved with low fuel consumption.
Ways to reduce nitrogen oxide emissions
In order to comply with the increasingly tough emission standards worldwide, engine manufacturers are forced not only to substantially reduce emissions of particulate matter (PM), but also emissions of nitrogen oxides. The main approach pursued by mtu is low-emission combustion, in other words an internal engine solution. However, this means taking into account a basic principle that governs the process of combustion — if the fuel burns at a higher temperature inside the cylinder, little soot is produced, but a large amount of nitrogen oxide. At lower combustion temperatures, nitrogen oxide emissions are low, but the production of soot particulates is high. To find the right balance, therefore, all the key technologies that affect combustion must be perfectly matched. When combined with fuel injection and turbocharging in particular, the use of exhaust gas recirculation results in a combustion process that produces significantly lower levels of nitrogen oxide.
The second way of reducing nitrogen oxide emissions is to use exhaust gas aftertreatment with an SCR catalytic converter. Very low limits for nitrogen oxide can make the use of such an SCR system necessary, as it removes subsequently almost 90 percent of the nitrogen oxide produced during the combustion process from the exhaust gas. Depending on the application, even higher reduction rates are possible. An added benefit of the SCR system is a reduction in particulate emissions of up to 60 percent. This frequently means that — depending on the emission standard applicable – the need for an additional diesel particulate filter (DPF) in the exhaust system can be eliminated.
Operating principle of the SCR system
In the case of selective catalytic reduction, a catalytic converter converts the nitrogen oxides contained in the exhaust gas into water vapor and nitrogen. For this purpose, a reducing agent is continually injected into the exhaust gas flow using a metering module. In the exhaust gas flow, the fluid reacts within a fraction of a second to produce ammonia (NH3). This chemical compound then converts the nitrogen oxides in the SCR catalytic converter.
The non-toxic and odorless reducing agent is widely used in commercial vehicle applications and has been available throughout Europe since 2004, and the USA since 2010. It is marketed in Europe under the trade name of “Ad Blue”. It consists of a 32.5 percent solution of extra pure grade of urea in de-ionized water. The amount of reducing agent added is about five to seven percent of the fuel consumption. It is stored as a second consumable fluid in a separate tank and fed to the metering device via pipelines. To ensure the high nitrogen oxide conversion rates of more than 90 percent in some cases in every operating state of the propulsion system, the electronic control system calculates the precise quantity of reducing agent needed based on key engine parameters such as operating temperature and engine speed.
Fuel consumption potential of SCR
In the combustion process inside the cylinder, in addition to the relationship between the production of nitrogen oxide and particulates, there is one between fuel consumption and nitrogen oxides. Generally speaking, high combustion temperatures lead to economical fuel consumption and low particulate levels, albeit with greater nitrogen oxide production. Since the SCR catalytic converter subsequently removes the nitrogen oxide from the exhaust gas, the development engineers can use this to configure the combustion process for extremely low fuel consumption while still remaining within the legal emission limits.
Benefits of mtu’s SCR system
mtu individually matches the SCR system to the specific engine and the application. At the same time, the drive system is optimized for low fuel consumption and a minimal space requirement for the SCR components. As far as possible, mtu uses proven SCR components from the commercial vehicle sector. Customers subsequently benefit from a tried and tested standard production solution with a long service life which is optimally adapted to the engine package. mtu drive systems are designed to be very robust in terms of changes in the operating conditions, which means that customers are very flexible in terms of how they employ their systems in a wide range of applications. Compared with other ways to reduce emissions, by using a diesel particulate filter, for example, an SCR catalytic converter does not increase backpressure in the exhaust system to the same degree. Consequently, the turbocharging system has to work against a lower backpressure and can be operated at a higher efficiency level.
System development at mtu
mtu has wide-ranging expertise in SCR systems. This has enabled the company to optimally exploit the potential of exhaust aftertreatment in combination with the engine. Using modern simulation tools, mtu matches parameters such as the exhaust gas flow through the catalytic converter precisely to the engine’s operating conditions. The results of those calculations are then used in the design of the catalytic converter casing. mtu also improves the packaging by using computer simulations. Since mtu supplies the drive and SCR system from a single source, it is able to optimally match the engine technologies such as combustion and turbocharging to the needs of exhaust gas cleaning system. This ensures, for example, that the operating temperature of the SCR system remains at an optimal level.
In the case of drive systems in the lower power range, such as Series 1000, 1100, 1300, 1500 and 1600 engines, mtu uses reliable SCR components from the commercial vehicle sector that are adapted to the specific requirements of their use in industry. mtu has also transferred this high-volume production expertise to larger engines with power outputs of up to 3,000 kW and has developed an economical modular concept for SCR metering devices and catalytic converters, with each module using two metering devices. mtu is currently advancing the development of its flexible modular concept for Series 2000 and 4000 engines: one module will completely cover Series 2000 engines, while two identical modules will be used for the Series 4000 engines. In addition to lower costs and high reliability, the benefits of the modular concept include a modest space requirement, since smaller individual modules can be better integrated into the engine package than one large unit.
mtu also assists its customers in the design of the reducing agent supply system for the SCR system. For the underfloor rail powerpack equipped with the V12 Series 1600 engine, mtu is even developing a complete SCR system that, in addition to the catalytic converter and metering system, includes a reducing agent tank, heater and piping.
Examples of SCR use in mtu drive systems
One example of particularly low emission limits is the US EPA Tier 4 final emission standard. As from 2015, it will limit the nitrogen oxide emissions of engines for gensets with power outputs exceeding 560 kW to a maximum of 0.67 g/kWh and particulate emissions to 0.03 g/kWh. mtu will comply with this nitrogen oxide standard using a SCR system.
From this year, the Tier 4 final standard applies to drive systems in the construction and industry sector with a power output of less than 560 kW. This standard limits nitrogen oxide emissions to a maximum of 0.4 g/kWh and soot particles to 0.2 g/kWh. In order to comply with these tough legal limits, mtu is using a technology package for the new Series 1000, 1100, 1300 and 1500 engines consisting of exhaust gas recirculation and SCR catalytic converter.
Summary
An SCR system can remove more than 90 percent of the nitrogen oxides from the exhaust gas in some cases. In addition, the engine can be configured for very low particulate emissions. That ensures compliance with stringent emission limits for diesel engines. At the same time, operators save on fuel costs with an SCR system, because internal engine parameters can be configured for ultra-low fuel consumption. Compliance with extremely low emission limits, however, requires a combination of internal engine optimization using exhaust gas recirculation and external optimization by means of exhaust aftertreatment with an SCR catalytic converter and, if necessary, a diesel particulate filter. mtu supplies the engine and the SCR system from a single source and can therefore ensure that the two components are ideally matched, with the key development objectives focusing primarily on low fuel consumption and low space requirement for the SCR components. mtu will be using SCR systems for genset engines with a power output exceeding 560 kW, for example, and in drive systems for construction and industrial applications below 560 kW in order to meet the very strict requirements of the US EPA Tier 4 final standard.?????
Selective catalytic reduction of NOx by Ammonia (NH3-SCR)
In SCR systems, ammonia (NH3) which is generally highly efficient is used as reducer. Ammonia is obtained from aqueous urea solution called AdBlue in the market to prevent burning due to high exhaust gas temperatures. The aqueous urea solution consists of 67% purified water (H2O) and 33% urea solution ((NH2)2CO). Aqueous urea solution (AdBlue) is the most commonly used reductant in SCR systems. Particularly at high exhaust gas temperatures (350–450°C),
NOx emissions in exhaust gas can be eliminated at high rates using AdBlue .However, at low exhaust gas temperatures below 200°C, the conversion efficiency is underperforming and ammonia accumulates on the exhaust line and the catalyst surfaces. Temperatures above 600°C are a major problem for the NH3-SCR system.
Because high temperatures can cause the reductant to burn before reaching the catalyst, and at the same time cause catalyst deformation. The active operating range of the NH3-SCR system is between 200 and 600°C exhaust gas temperatures. A maximum conversion efficiency can be achieved about 350°C [35].
Following reactions occur when the aqueous urea solution is sprayed onto the exhaust gas. In NH3-SCR system, the aqueous urea solution sprayed on the exhaust gas is first subjected to thermolysis and hydrolysis reactions under the influence of high temperature (Eqs. (12) and (13)). These reactions result in the production of two molecules of ammonia from one molecule urea.
( NH 2)2CO → NH 3 + HNCO (thermolysis) (12)
HNCO + H 2 O → NH 3 + CO2 (hydrolysis) (13)
The main reactions occurring in the system after the thermolysis and hydrolysis reactions are given in Eqs. (14)–(16).
2NO + 2NO2 + 4NH3 → 4N2 + 6H2 O (14)
4NO + 4NH3 + O 2 → 4N2 + 6H2 O (15)
6NO2 + 8NH3 → 7N2 + 12 H 2 O (16)
Eq. (14) provides the highest conversion efficiency in the conversion reactions taking place in the NH3-SCR system. This reaction usually takes place when a diesel oxidation catalyst (DOC) is present before the NH3-SCR system. DOC converts NO emissions to NO2 form and when the content of NO and NO2 in the exhaust gas get close to each other, higher efficiency is achieved in the NH3-SCR system. Therefore, NH3-SCR system usually needs DOC and they are used together in applications.
Eq. (15) occurs when there isn’t any DOC before the NH3-SCR system and NO emissions are included with large amount in the exhaust exit. If the DOC catalyst Diesel and Gasoline Engines
NOx conversion reactions in NH3-SCR. is larger than the necessary and consequently the vast majority of NO emissions are converted to NO2, the Eq. (16) is realized. In terms of efficiency, this equation exhibits the worst conversion performance .
Figure 6 presents a schematic view of a classical NH3-SCR system. In this system managed by an electronic control unit, the data from the NOx and temperature sensors is evaluated and injector sprays the reductant with an optimized rate onto the exhaust gas.
Thanks to the NH3-SCR system, diesel engines can be operated at high combustion end temperatures, thus resulting in improved engine performance and fuel consumption. With the use of the NH3-SCR system, fuel consumption can be reduced by 5% .
Metal oxides and zeolites are the most commonly used catalyst types in NH3-SCR systems. Metal oxide catalysts are a group of catalysts produced from metals such as vanadium and titanium (V2O5-WO3/TiO2), which operate efficiently generally between 250 and 400°C [38]. When compared to metal oxide catalysts, zeolite catalysts are capable of operating in the high temperature range of 400–550°C .
While V2O5-WO3/TiO2 catalysts were preferred for commercial use of NH3-SCR in the vehicles in 2005, Fe-zeolites began to be used more widespread in the period after 2010 . In addition to these catalysts, Cu-ZSM5 and Ag/Al2O3 catalysts NOx Pollutants from Diesel Vehicles and Trends in the Control Technologies may be preferred in applications. In particular, Ag/Al2O3 catalysts can exhibit high performance at low exhaust temperatures.
Cost, an extra storage that it requires and the space that it takes up on the vehicle are the biggest problem in using the NH3-SCR system. However, as a result of the studies performed in the NH3-SCR system, it is achieved that a volume reduction of 60%, a weight reduction of 40% and a cost reduction of 30% when compared to 2010 . Low efficiency at low exhaust gas temperatures, ammonia slip, lifetime, adaptation to different operating conditions, integration with other oxidation and particulate filter systems are the negative aspects of NH3-SCR systems. In addition, the NH3-SCR system requires a pre-catalyst (DOC) because they exhibit the best performance while NO/NO2 ratio is 1.
Even if NH3-SCR system has been developed for heavy-duty vehicles in general, it is widely used in many automobiles thanks to innovations (electronic injection, etc.) in the system . NH3-SCR is the most effective system to meet the NOx emission values determined by the organizations in the current situation .In addition, the fuel consumption of engine improves with the use of NH3-SCR system.
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1 个月Great article and info compilation, thanks for sharing
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2 个月Zimbabwe has lithium for cleaner car generation.