Sunday, June 11, 2006

More on soot technology: JM's CCRT

A difference between diesel engines and gasoline engines has to do with the way fuel is injected: in gasoline engines, fuel is injected during the intake stroke; in diesels, fuel is injected during the compression stroke. As a result, gas engines have the advantage of having more time to thoroughly mix the air and fuel before ignition occurs thereby reducing the amount of unburned fuel.

In a diesel, fuel is injected late in the cycle and the air is not as well mixed as in a gasoline engine. As a result of this less homogeneously mixed fuel and air, there are fuel-dense pockets in the combustion chamber. The consequence is that diesel engine exhaust contains incompletely burned fuel (soot) known as particulate matter (PM). [IPBiz note: soot/PM is highly aromatic (ie sp2 carbon), in strong contrast to the diesel fuel. Thus, it is wrong to say soot is "incompletely burned fuel."]

Particulate filter technology has been around for years. It has been proven over and over to be able to reduce PM by 95% or more. However, the key to the successful application of particulate filters on diesel engines was the ability to reliably regenerate the filter, or in other words, burn the PM that the particulate filter "traps" or collects.

To understand how a filter regenerates, one must understand how soot or PM burns. Traditionally, combustion of soot is done in an oxygen atmosphere (air). In air, soot will burn at about 450° to 500° Centigrade (840° to 930° F). However, this is not a typical operating temperature for diesel engine exhaust. As a result, in order to burn soot in air, an active system, one that increases the temperature of the exhaust using some external heat source, is required. But if an active system is not carefully controlled, it can often experience an "uncontrolled burn" where the temperature increases to 600° C (1112° F) or more. This will damage the filter element and also pose some potential risk to the vehicle.

An alternate method of burning soot was identified and patented by Johnson Matthey in the 1980's. Johnson Matthey discovered that soot will burn at about 250° C (482° F) in the presence of nitrogen dioxide (NO2). Typical diesel engine exhaust contains about 5 - 10% NO2, so if this discovery were to have practical application, Johnson Matthey needed to develop a technology that would increase the amount of NO2 in the exhaust enough to allow for this low temperature combustion to occur. The technology that Johnson Matthey developed was the CRT® particulate filter.

An issue is that the Pt catalyst used in CRT will make significant sulfate if used with 500ppm S fuel. So a requirement for the use of the CRT filter is ultra low sulfur diesel fuel (ULSD) containing no more than 50ppm Sulfur. And for very low PM levels and reliable operation, 15ppm S fuel is required. The CRT requires A NOx / PM ratio of 20 or more for the proper amount of NO2 for combustion.

US 4,902,487 (issued Feb. 20, 1990) has as its first claim:

In a process wherein diesel exhaust gas is passed through a filter to remove particulate therefrom before discharge and particulate deposited on the filter is combusted, the improvement which comprises combusting the particulate with a gas containing NO.sub.2.

Claim 4 states:

The process of claim 3 wherein the gas containing NO.sub.2 is provided by initially passing diesel exhaust gas containing NO over a catalyst to convert the NO to NO.sub.2.

As of 11 June 06, the '487 patent has been cited by 60 US patents, most recently by
US 7,052,532. The priority chain for the '532 recites:

This application is a continuation-in-part of U.S. patent application Ser. No. 09/935,847, filed Aug. 23, 2001, now U.S. Pat. No. 6,585,490, which is a continuation-in-part of U.S. patent application Ser. No. 09/573,747, filed May 18, 2000, now U.S. Pat. No. 6,444,006, and a continuation-in-part of International Patent Application No. PCT/US02/2133, filed Jan. 25, 2002, claiming priority from provisional U.S. Patent Application No. 60/303,563, filed Jul. 6, 2001, and a continuation-in-part of U.S. patent application Ser. No. 10/075,035, filed Feb. 12, 2002, which is a continuation-in-part of U.S. patent application Ser. No. 09/522,152, filed Mar. 9, 2000, now U.S. Pat. No. 6,669,913 and a continuation-in-part of U.S. patent application Ser. No. 09/851,300, filed May 8, 2001, now U.S. Pat. No. 6,776,814 all incorporated herein by reference and from which priority is claimed.

Built upon the same technology as the CRT (continuously regenerating technology) filter, the CCRT filter is ideal for use in applications with low NOx/PM ratios. CCRT filter technology brings the overall temperature requirements down to 200°C over 40% of the vehicle duty cycle - 25°C lower than any of its DPF competitors. The filter consists of an oxidation catalyst up-stream of a wall flow filter, also covered with a catalytic coating. The CCRT's catalytic coating oxidizes NO into NO2, which in turn oxidizes any soot trapped in the filter. The CCRT's catalized filter then converts the NO, which forms as a result of soot and NO2, back into NO2. The NO2 is then reutilized for further soot combustion. The extended NO2 exposure allows the CCRT particulate filter to operate at a significantly lower temperature. [from JM]


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