This invention relates generally to a method of making a filter for removing particulates and, more specifically, to making a lattice-structure filter for removing particulates that can allow for regeneration.
To address tightening diesel-engine-emissions regulations being adopted in the United States and Europe, attention has focused on basic improvements in the design and performance of filters for treating diesel exhaust gases. In such a filter device, the filter traps particulates from an exhaust and, when the amount of particulate matter accumulated on the filter reaches a predetermined value, in order to burn and remove the particulate matter during a regeneration process, the temperature of the filter is increased to, for example, 600° C. or more. This regeneration temperature can be decreased to as low as approximately 300° C. if a catalyst is utilized.
Additionally, NOx formation from methane combustion in gas-turbine electric power plants is becoming a significant environmental concern. Uniting these two technical problems is the fact that both be addressed through the use of state-of-the-art, in-line ceramic filters/supports providing an appropriate catalytic function. However, these are extremely demanding applications. An in-line catalyst support or filter in a high velocity, high temperature, and corrosive gaseous stream must have a large surface area to volume ratio; a high permeability for low pressure drop; high strength and/or low thermal expansion; high temperature corrosion resistance; and high trapping efficiency (for filters). Similarly, the catalyst must be very active, as well as thermally and chemically stable over a broad range of thermal and chemical conditions.
Joining all of these attributes in one package is challenging. Cordierite traps have been extensively investigated, including a two-stage diesel particulate trap that uses a primary converter to produce excess NO2 and a secondary trap to capture and “burn” the soot, using the NO2 as the oxidant. However, impurities from fuel and engine oil induce rapid corrosion of the cordierite trap. Failure within a short time period can occur. Manufacturers of large diesel engines, who eventually will warranty these filter systems, must consider all aspects of the long-term performance of any filter and filter material. A catalytic system that directly utilizes air/oxygen for soot combustion, is not deactivated by components of the particulates, and that is stable over years of thermal cycling is elusive. An ideal catalyst would lower the combustion temperature of soot from about 550° C. to at least 300-350° C. (typical of diesel exhaust). The combustion rate must be as great as the trapping rate to maintain constant pressure drop across the filter. Noble metal catalysts have potential, but are becoming ever more expensive, making the costs prohibitive for larger engines. Complex mixed-phase catalysts have shown promise for soot combustion, but the temperatures to maintain a constant pressure drop are too high (370° C.). Also, the best catalysts of this type are molten at the operating temperature, raising questions about migration of the active components and their long-term viability.
Pressure drop is also a concern for diesel particulate trap design and can heavily influence fuel economy. Soot which enters porous cordierite walls during filtration imparts an immediate pressure drop increase from a clean filter (2 KPa or 0.3 psi) to 10.5 KPa (1.5 psi), inducing a soot filter cake to form. The soot filter cake can then increase the backpressure to over 14 KPa (2.0 psi), increasing until regeneration can occur.
A filter system is required that can meet the described performance requirements. The filter system must be designed to have the necessary heat, flow, and mass transfer characteristics and must be capable of being fabricated with desired materials in a desired three-dimensional structure.