SCRT gets ready to roll for on-highway vehicles: fleet tests near completion for Johnson Matthey's combined CRT/SCR package.
The SCRT system is intended initially for fleet retrofit programs in ozone non-attainment and some near-attainment areas of the country, including many counties in California, Texas and New York. In such regions, non-attainment is largely driven by N[O.sub.x] rather than hydrocarbons and thus State Implementation Plans (SIPs) must provide for specific N[O.sub.x] reduction measures. At a cost of approximately $20,000 per system, there is often special funding available to match the environmental urgency of meeting the relevant SIP.
A good example is the Texas Emissions Reduction Plan (TERP), which was detailed in the July 2006 issue of Diesel Progress. Established by legislation in 2001, TERP is part of that state's SIP and it provides grants in affected counties to offset incremental costs for reducing emissions of N[O.sub.x] from high-emitting mobile diesel sources in non-attainment areas and other affected counties of the state.
TERP has many other provisions, but the grants are key to funding N[O.sub.x] reduction retrofit programs on vehicles. Besides public health, at risk to those affected areas in Texas and elsewhere are hundreds of millions of dollars in federal highway funds.
Generally, the EPA has been skeptical of urea SCR for consumer vehicles due to uncertainty that consumers will keep the urea tank full and the system functioning. While that posture might change, EPA reasons that appropriate system service is easier to enforce with fleets and municipalities and thus urea dosing is much more viable. In stationary power applications, of course, both ammonia and urea injection are well established. Johnson Matthey has been producing SCR systems for stationary systems for nearly 30 years.
"We are hopeful for SCRT system verification from the EPA this summer, to be followed with verification by CARB later this year," said Marty Lassen, director--commercial development and marketing for Johnson Matthey. "We are on a dual verification path with both of them.
"Our initial thrust for commercialization in the on-road market is targeted for year-end, and later next year we will roll out the technology for off-road applications."
CRT technology has been around since the early '90s and SCR technology, at least for stationary applications, is not a newcomer either. What is new is combining the two in a package that is compact enough to be practically installed on various types of mobile equipment and that will withstand the abuse of grueling duty cycles.
Johnson Matthey's major suppliers for producing the SCRT retrofit system are Cummins Emission Solutions, which will do most of the packaging, STT Emtec, which will produce the system controller and Grundfos Pumps Corp., which produces the urea dosing system.
"Combining the two systems has been under study for some time, but the real onslaught of development began two years ago," said Lassen. "It really is a relatively complex system in terms of what it accomplishes, but it has to be simple and flexible enough to install, maintain and service."
The resulting SCRT package from Johnson Matthey, which is intended initially for fleet retrofit, consists of highly engineered components that incorporate application latitude without functional compromise.
The new Johnson Matthey four-way emissions control unit claims to reduce CO, HC and PM emissions by over 90% and N[O.sub.x] by 60 to 80%, according to Lassen and Jim Hale, JM's retrofit business manager. This, the company said, is the highest level of N[O.sub.x] reduction commercially available, as a retrofit aftertreatment product.
Looking at the flow of engine exhaust through the SCRT system is a convenient way to visualize operation. Engine exhaust, post turbocharger, enters the two-stage CRT particulate filter, which consists of an oxidizing catalyst stage followed by a particulate filter. CO, HC and PM emissions are stripped out in the CRT part of the package.
The CRT is often close coupled to the SCR section to minimize the drop in exhaust temperature, but by insulating the exhaust line the separation between the two can be extended just as long as suitable exhaust temperature is available at the SCR inlet to allow urea injection. In U.S. retrofit applications, this temperature is readily attained. Eyeballing the total layout, one might think that the system would have considerable backpressure, but Hale said it's no more than a maximum of 5 to 6 in. Hg for less than 10% of operational time. The average on-road backpressure for the system is typically about 2 to 3 in. Hg.
Urea is injected between the CRT and the SCR sections. This is accomplished using a Grundfos dosing pump that injects urea into the exhaust gas stream, which then enters the SCR vanadium catalyst. The urea solution utilized for this system is a 32% solution in water and is readily available. The urea solution is injected as a fine spray through the Grundfos pump and nozzle. The injected urea is vaporized and hydrolyzed across the SCR catalyst to produce the reductant, ammonia.
The ammonia selectively reacts with the engine-out N[O.sub.x] as it passes across the SCR catalyst and produces nitrogen and water. In this way, the engine-out N[O.sub.x] is reduced.
"It takes careful balance of the urea injection over engine operating speeds and loads to be effective," said Hale. "Too much urea produces ammonia slip and too little urea results in reduced system performance."
Part of the SCRT system is a high-tech independent control unit (ECU), using a 32-bit processor, which communicates with the engine ECU and directs the digital dosing system on the precise timing and volume of urea injection. There is a specific control algorithm for the sensor-based urea injection. "We're looking at about 18 channels, bits of data, multiple temperatures and backpressure," explained Hale. "Strictly speaking it is not a closed-loop control system, but it does adapt with precision to every change in operation."
Urea consumption by the system is approximately 4 to 6% of the amount of fuel used by the engine depending on the engine-out N[O.sub.x] level. The SCR catalyst volume is determined based on the engine size as well as other application details.
"To get the very high N[O.sub.x] reductions at the 80% level," said Lassen, "you need to get the temperature just right and then you need a catalyst volume of about 1.5 to 2 times the swept volume of the engine."
Though this system is "engineering hardened" to the point of market release, in other respects it is also in transition. The fleet retrofit SCRT package of today looks extremely promising as a tool for solving SIP dilemmas by removing tons per day of N[O.sub.x] from the environment. But the same technology will evolve to original equipment solutions for 2010 and beyond, when N[O.sub.x] looms larger for all players as required by the EPA's 2007 On-Road Rule.
"Our experience in the retrofit programs will be a great advantage to us as well as OEMs," predicted Lassen. "The system that evolves for 2010 will be somewhat similar using the same filter with active injection.
"All of the control capability will be put in the engine ECU so there will be a single controller. And we will be moving to a more advanced type of SCR catalyst."
For now it will be of interest to follow the current SCRT fleet retrofit program move into the on-road market this year, to be followed by off-road applications next year. The four-way emissions control approach has great merit, of course, but most importantly, the experience base that develops may well define the future role for urea injection based SCRTs for N[O.sub.x] reduction and thus influence the selection of technology for U.S. 2010.
How important will this system be in helping non-attainment regions meet their N[O.sub.x] reduction goals? Can we effectively bridle urea-based systems for practical use in more extensive fields of application? Can we afford such systems? We're about to get some answers.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||TECHNOLOGY OF CLEAN AIR|
|Publication:||Diesel Progress North American Edition|
|Date:||Apr 1, 2007|
|Previous Article:||The cleanest school district in the U.S.? Tucson aims to set the standard for school bus fleets; DOC/DPF retrofits; CNG, diesel buses; anti-idling;...|
|Next Article:||Meeting stationary emissions standards with a click.|