Q. What does SCR stand for?
A. Selective Catalytic Reduction.
Selective: targets NOx in the exhaust gas
Catalytic: requires a catalyst
Reduction: NOx is reduced to Nitrogen (N2),
Q. How does SCR work?
A. An aqueous urea solution, also known as DEF (diesel exhaust fluid), is injected into the exhaust air stream containing NOx, a bi-product of engine combustion, where it evaporates into ammonia (NH3) due to a physical reaction triggered by the energy contained in hot exhaust gas. Once the exhaust gas and ammonia mixture contacts the SCR catalyst surface, a reduction reaction occurs, breaking down the NOx (NO and NO2) and NH3 into nitrogen gas (N2) and water vapor (H2O).
Q. How long has SCR been around?
A. SCR was first patented in 1957 by the Engelhard Corporation in the United States. Since then, thousands of systems have been installed in land-based applications including power plants, railway locomotives and cars. The technology was originally developed to control NOx emissions from stationary engines but over recent decades, it has been applied in a variety of mobile applications including freight-carrying trucks, buses, off-road construction vehicles and diesel-engine passenger vehicles.
Q. Has SCR been used in marine engine applications in the past?
A. SCR has been used in the marine engine sector for more than two decades. Today, SCR is a well-proven technology. Overall, approximately 1,250 SCR systems have been installed on marine vessels in the past decade, with over 500 applications in the marine industry in 2013 alone. Marine vessels with the longest track records have accumulated upwards of 80,000 hours of operation. SCR has been deployed on a variety of marine vessel types, including ferries, tankers, container ships, icebreakers, cargo ships, workboats, cruise ships, and naval vessels.
Q. Why was SCR chosen by Caterpillar over other NOx reduction technologies to meet US EPA Tier 4 Final / IMO III marine engine emissions?
A. From a timing perspective, our Cat marine engines have the advantage of meeting Tier 4 Final emission standards using Caterpillar technology that has already been proven successful across other industries served by our Cat engines and Cat machines. Across the Tier 4 Final product lineup, Caterpillar has successfully delivered different combinations of emissions-reduction technologies based on specific regulatory and critical customer requirements, from mining trucks and wheel-loaders to oil well-fracking engines and stationary gen sets. The decision to select an SCR-based solution to meet Tier 4 Final / IMO III marine engine emissions came after careful evaluation of all of the technology options available, and a comparison of each of these options against crucial marine customer requirements. SCR allows us to deliver on our commitments of “highest uptime” and “lowest total cost of ownership” for our marine customers.
Q. What is urea?
A. Urea is a nitrogen-containing compound used in a variety of industries, often as a fertilizer in agriculture. When heated, urea decomposes into ammonia (NH3) and carbon dioxide (CO2).
Q. What is Diesel Exhaust Fluid (DEF)?
A. DEF is a carefully blended solution of 32.5% high purity urea and 67.5% deionized water. A DEF concentration of 40% urea is also available and widely used in the marine industry, mostly in Europe. The 32.5% concentration is most common in North America and offers freeze protection down to 11.3 F (-11.5 C) while the 40% concentration begins to crystallize at 32 F (0 C).
Q. Will the Caterpillar marine SCR systems work with either concentration of DEF?
A. Yes, the systems have been designed to work with either the 32.5% or 40% concentration. One benefit of the higher concentration is that less storage space is needed on board the vessel. However, the ready availability of one product over the other together with the required freeze/crystallization protection are typically the deciding factors.
Q. Does SCR affect the fuel efficiency of the engine?
A. Yes. SCR is used as the primary NOx reducer, this allows the engine’s fuel efficiency to be optimized. The relationship between engine-out NOx and fuel consumption is inversely proportional, which means the higher the engine-out NOx, the lower the fuel consumption. In fact, for some Tier 4 Final marine ratings, fuel consumption is up to 9% lower than equivalent Tier 2 / 3 ratings. In most cases, the cost of DEF consumption is more than offset by the engine’s reduction in fuel consumption; a critical milestone in the effort to deliver upon our commitment of providing the lowest possible total cost of ownership solutions to our marine customers.
Q. How much DEF will be consumed?
A. The total consumption of DEF depends on many factors, including engine application (i.e. diesel electric propulsion, propulsion, auxiliary engines/generators), duty cycle, annual engine run hours, ratings, and DEF concentration to name a few. The industry-wide Cat Tier 4 Final products, marine included, have all been designed and developed to minimize total fluid consumption costs (diesel plus DEF) based on direct customer input regarding many of the above mentioned factors. We recommend that our customers work directly with their local Cat® Marine Dealer to determine exactly how much DEF these Tier 4 Final engines will consume. They will also need to estimate how much DEF storage is needed based on factors such as refueling practices and schedules which obviously vary greatly from one customer to another.
Q. Is the SCR system constantly consuming DEF when the engine is running?
A. No, the SCR system only injects DEF above certain exhaust temperatures and certain engine speeds and loads. The time that DEF is injected into the SCR system is a direct result of each vessel’s specific duty cycle or operating load profile. Caterpillar engineers have worked tirelessly to ensure that emissions requirements are more than satisfied while working in parallel to optimize customer value by minimizing total fluid consumption of the system.
Q. What is air-assisted DEF injection?
A. Air assisted DEF injection is the addition of compressed air into the DEF injector to increase atomization of the DEF. Not all SCR systems require air assistance but the Tier 4 Final C32, 3500E, and C280 series marine engines do utilize this technology as it offers increased performance at a lower operating cost for high power applications.
Q. Why is air-assisted DEF injection used for Caterpillar Marine Tier 4 Final / IMO III engines?
A. With some of the engine and after-treatment configurations in large ducts, a larger spray pattern is needed for uniform mixing of urea and exhaust gases. This capability decreases total fluid consumption (diesel plus DEF). The addition of compressed air makes that possible.
Q. How is the SCR system and supporting components installed on board the vessel?
A. A complete and comprehensive Caterpillar Tier 4 Final / IMO III marine installation guide is available at all Cat Marine dealers.
Q. What happens if the vessel runs out of DEF, or an issue arises in which the SCR system is not dosing or operating as designed during vessel/engine operation?
A. If a situation occurs, for whatever reason, which results in a lack of dosing during engine operation normally requiring DEF injection, there will be no loss of engine power. The US EPA recognizes that it would be unsafe and unacceptable to derate or even shut down a marine engine as a result of aftertreatment failure. This guidance is stated in the EPA regulations 40 CFR Part 1042. Any such event would be logged in the engine’s ECM and the customer would then have up to 30 days to inform the US EPA of the reason for inadequate DEF (quantity or quality) onboard the vessel.
Q. For these marine engines, are any other after-treatment devices needed besides the SCR system (ie. diesel particulate filters (DPF), diesel oxidation catalysts (DOC), exhaust scrubbers, etc.)?
A. No. Additional after-treatment such as DPFs and DOCs are not required. The reduction of particulate matter (PM) down to Tier 4 Final levels has been achieved through in-cylinder enhancements which eliminate the need for these additional after-treatment devices.
Q. How will I know if I am getting quality DEF?
A. To ensure appropriate quality, only DEF meeting the International Organization for Standardization (ISO) 22241-1 should be purchased. This will ensure the proper purity and concentration of urea. For more information on these standards, visit www.iso.org.
Q. Can I make DEF myself?
A. No. Only a pre-mixed solution of DEF available from the supply base should be used. Use of any fluid which does not meet the requirements outlined in ISO 22241-1 may damage Cat SCR systems. For more information about DEF quality standards, refer to ISO 22241 which details DEF standards including, quality, handling, testing, transportation, and storage.
Q. Does DEF have any special storage requirements?
A. DEF should be stored in a cool, dry, well-ventilated area. It should never be exposed to direct sunlight as the urea will decompose. The ideal storage temperature for DEF is between 15 F and 77 F (-9 C and 25 C). For additional information refer to ISO22241-3.
Q. How long can DEF be stored?
A. The shelf life of DEF depends upon the storage temperature. DEF will degrade over time depending upon how long it is exposed to high temperatures and sunlight. ISO 22241-3 defines minimum shelf-life expectations when stored at constant temperatures and notes that if DEF is stored below 86 F (30 C), shelf life will be one year, below 77 F (25 C), shelf life will be approximately 18 months. Refer to the standard for further information.