SCR Retrofit Aftertreatment Caterpillar

Retrofit SCR System

The Retrofit Selective Catalytic Reduction (SCR) System may have the following components:

Common Arrangement Identification Number

Illustration 1	g06145364

Illustration 2	g06145370

As supplied from the factory, the dosing cabinet and CEM are serialized together under one product Common Arrangement - AFTM.

When installed, the Common Arrangement - AFTM serial number of the CEM and of the Dosing Cabinet must match. If the two components are not paired correctly, a fault code will be activated.

The Common Arrangement - AFTM serial number can be found on the CEM to the left of the SCR catalyst access hatch. The corresponding Common Arrangement - AFTM dosing cabinet-mounted serial number plate is just off center of the dosing cabinets front connection face. The Common Arrangement - AFTM Part Number, Serial Number, and Config Group must all match in order that the system work correctly. If the labels do not match, contact your dealer for assistance.

Note: The Common Arrangement 8-digit Serial number should be referenced when contacting your Caterpillar dealer for service in addition to the engine serial number. This action will indicate to the Caterpillar dealer that there is an aftertreatment unit installed in addition to the engine and allow the dealer to work effectively to address your service needs. There may be other serial numbers present on the CEM and dosing cabinet, these are for manufacturing only and will most likely be 10 digits in length.

(SCR) Reactor Housing

Illustration 3	g03717087
Horizontal (SCR) Reactor Housing The SCR reactor is stainless steel double walled unit that houses the SCR catalyst and DOC/DPF catalysts.

Selective Catalytic Reduction Substrate

Caterpillar offers two SCR catalyst formulations. (Vanadia Based and Copper Zeolite Based). Catalyst array size and Catalyst formulation are selected at the time of order to match the engine and emissions site requirements. The system requires liquid Diesel Exhaust Fluid (DEF) to be sprayed into the system and allowed to evaporate into water vapor and ammonia. The ammonia reacts with exhaust gas and catalyst to convert oxides of nitrogen to harmless water and nitrogen.

Diesel Oxidation Catalyst (Optional)

Illustration 4	g03398644
Flow through substrate used for diesel oxidation catalysts

The Diesel Oxidation Catalyst (DOC) oxidizes hydrocarbons contained in the exhaust gas. The DOC is a flow-through device. A chemical reaction takes place that reduces the amount of hydrocarbons and carbon monoxide. This reaction occurs when the exhaust gas flows through the cells that are coated with precious metals located within the DOC.

The temperature of the exhaust is an important factor to activate the catalyst. Below 200° C (392° F) little to no catalyst activity is occurring. This means that emissions from the engine are not being reduced. As the exhaust temperature rises above 200° C (392° F), the catalyst activity increases rapidly. The catalyst begins to operate efficiently at exhaust temperatures between 280° C (536° F) and 360° C (680° F). Temperatures greater than 360° C (680° F) promote optimum efficiency. The greater temperatures will promote optimum CO + O2 ⇒ CO2 and HC+02 ⇒ H2O + CO2 conversion. PM reductions can occur due to oxidizing of Soluble Organic Fraction (SOF), the adsorbed liquid hydrocarbon layer on the surface of the soot particles. The SOF may account for the nearly 20 percent PM by weight. Operation of the DOC at temperatures greater than 700° C (1292° F) is not recommended and may result in reduction of the catalyst efficiency.

Diesel Particulate Filter (Optional)

Illustration 5	g03398873
Flow through substrate used for diesel particulate filter

The DPF is used to reduce the emissions of particulate matter (soot). These emissions are reduced by filtration as the exhaust passes through the DPF wall. Carbon monoxide (CO), and Hydrocarbons (HC) are also reduced as the engine exhaust passes through the catalytic oxidation and filtration units. The DPF filter substrate uses a series of alternately blocked channels which forces exhaust gasses to flow through the channel walls. Particulates are physically captured and chemical reactions take place, refer to illustration 4. Carbon monoxide and hydrocarbons are converted into carbon dioxide and water vapor. These filters are self-regenerating. Self-regenerating filters will "burn off" the accumulated soot continuously if the proper exhaust temperature profile is met. If a proper exhaust temperature profile is not met, soot buildup will occur and high back pressure events will become irreversible. Proper DPF regeneration REQUIRES the following conditions:

• Ultra Low Sulfur Diesel Fuel Must Be Used. Ultra Low Sulfur Diesel Fuel must have an average sulfur content of 15 parts per million (ppm).

• Ultra Low Sulfur Diesel (ULSD) fuel (≤ 15 ppm (mg/kg) sulfur) is required by regulation for diesel engine use in California. The United States (U.S.) Environmental Protection Agency (EPA) defines Ultra Low Sulfur Diesel (ULSD - S15) as a U.S. diesel fuel with a sulfur content not to exceed 15 parts per million (ppm (mg/kg)). Also, the sulfur content in the diesel fuel is not to exceed 0.0015 percent by weight. ULSD fuel must meet the S-15 fuels designation in the latest edition of ASTM D975 and/or conform to Caterpillar Fuel Specification.

• Biodiesel fuel may be used up to the B20 blend level (20% biodiesel and 80% appropriate ULSD fuel) IF the final B20 blend conforms to ASTM D7467 and API gravity 30-45. The neat biodiesel blend stock should conform to ASTM 6751.

• The engine exhaust temperature at the DPF inlet must be 300° C (572° F) or greater for at least 30 percent of the time or 2 hours whichever is longer.

• The ratio of nitrogen oxides (NOx) to particulate matter (PM) must be a minimum of 25:1. This ratio is the NOx level, in grams/brake horsepower hour, divided by the PM level). This technology is applicable to 1996 or newer Tier 1 or Tier 2 equivalent engines. Current Caterpillar retrofit passive DPF product is not applicable to Tier 3 equivalent engines.

• The engine should be maintained and must not consume oil at a rate greater than the rate specified by the engine manufacturer.

• Cat DEO-ULS (Ultra Low Sulfur) 319-2260 Oil exceeds the performance requirements of API category CJ-4 oil and is recommended due to a low ash specification. Engine oil that meets or exceeds the specifications in the Caterpillar ECF-3 or API CJ4 Categories may be used in these applications. Oil burned during the combustion process accounts for almost all the DPF ash accumulation (because of oil additives). The use of the recommended low ash oil results in lower ash accumulation in the filter. Failure to use the recommended grade of oil may result in more frequent ash removal service intervals.

Aftertreatment System Description

Illustration 6	g06031628
Caterpillar supplied aftertreatment components are illustrated with dashed line rectangles.

Selective Catalyst Reduction (SCR) catalyst technology is used to reduce NO_x emissions and particulate matter.

The aftertreatment system is composed of the following components:

• Air compressor

• Bulk Urea Transfer Pump

• DEF Dosing cabinet

• DEF Filter

• DEF Mixing tube

• DEF Nozzle

• DEF Supply tank

• SCR catalysts

• SCR reactor housing

Engine exhaust flows into the SCR System reactor housing through the exhaust inlet. Once sufficient temperature is achieved, the DEF is injected into the exhaust. Exhaust flows through a mixer assembly in the mixing tube to ensure that exhaust is not stratified for uniformity. This mixing will ensure complete disbursement across SCR catalyst section. Precise DEF injection is monitored and controlled by an electronic controller in the dosing cabinet. The following components provide signals to the controller to control DEF injection for emission reduction:

• Thermocouple located in the reactor housing.

• Inlet NO_x sensor located in the exhaust before the inlet to the SCR catalyst reactor housing.

• Outlet NO_x sensor located in the exhaust at the outlet of the SCR catalyst reactor housing.

When engine operating conditions dictate, DEF fluid is injected into the exhaust stream before entering the SCR. When injected into the exhaust stream, the DEF is atomized into droplets. The atomized droplets are then sent through the mixer. The mixer disrupts the exhaust flow and allows DEF to be distributed throughout the exhaust gas. Water evaporates due to the high temperature of the exhaust. This process will cause the DEF to decompose and release ammonia (NH₃) that was bound to the DEF. NH₃ is free to react with the NO_X and the oxygen present in the exhaust system stream. This reaction occurs on the SCR catalyst. NH₃ and NO_X are converted into gas particles of nitrogen and water. The inlet and outlet NO_X sensors read the levels of NO_X and determine how much DEF needs to be injected into the exhaust.

Dosing Cabinet

The dosing control cabinet controls the following:

• Rate of DEF flow to the injector

• Compressed air pressure

• Urea purges shut down cycle

The aftertreatment ECM located in the dosing cabinet and theCat ECM is designed to communicate with each other. This communication controls the complete engine/emission system to meet emission levels.

DEF Tank

There are two separate DEF tanks in the dosing system.

• DEF tank 1 is the customer installed DEF tank. DEF tank 1 is the bulk DEF tank. A DEF high and low-level switch in tank 1 will notify the operator when the tank requires refilling.

• DEF tank 2 is the buffer tank located in the dosing cabinet. Tank 2 is a small DEF tank that is filled from DEF tank 1. Tank 2 provides DEF to the DEF pump to be injected into the exhaust.

Reactor Housing

The reactor housing contains the following:

• SCR catalyst substrates

• DEF nozzle

• DEF mixing tube

• SCR catalyst temperature and pressure sensors

• NO_x sensors

The aftertreatment ECM uses the sensors on the module to determine how much DEF needs to be injected into the exhaust system.

Selective Catalytic Reduction Substrate

The Selective Catalyst Reduction (SCR) substrates are located in the reactor housing. These catalysts allow exhaust to flow through the substrate converting the NO_x into N2 and H20.

The exact chemical formulation of the catalyst has been engineered for this engine to provide optimal system performance and durability.