Simultaneous Reduction of NOx, Solid, and Semi-Volatile Particles Using 4-Way Catalyzed Filtration Systems

Diesel engines are widely used in automotive, transportation, construction equipment, stand-by power generators, and marine engines and off road applications. In 2007, the U.S. Environmental Protection Agency (EPA) reduced the diesel particulate matter emission (DPM) standard for heavy-duty diesel engines used on-road to 0.0134 g/kWh and in 2010, the emission standard for NOx (NO + NO₂) was reduced to 0.27 g/kWh, roughly a 10-fold reduction over previous standards. To meet the on- and off-road DPM standards most manufacturers rely on exhaust emission control devices such as diesel particulate filters (DPF), a wall-flow filter in which alternate channels are blocked forcing filtration to take place as the exhaust gases pass through the channel walls while the DPM is retained in the filter [1, 2].

The next generation of emission control devices includes 4-way catalyzed filtration systems (4WCFS) that include both NOx and DPM removal technology. Toyota Motor Corporation introduced the first successful 4WCFS concept in the early 2000s that they referred to as a “diesel particulate – NOx reduction” system or DPNR [3]. In brief, this version of a 4WCFS consists of a bare DPF substrate that has been washcoated with a NOx storage catalyst. Thus, DPM and NOx are reduced simultaneously by a single control device. The bare DPF used in this study is made from an advanced ceramic material (ACM). The ACM manufacturing process is controlled so that the microstructure, total porosity, and pore size distribution are tailored to meet requirements for DPM emission control. Additionally, ACM is suitable for catalyzed applications. By increasing the porosity of the ACM filters, it was hypothesized that larger amounts of NOx storage catalyst could be loaded onto the filters. Previous studies have shown that ACM DPFs demonstrate high filtration efficiency, low-pressure drop, high-temperature handling capability, and excellent mechanical integrity at a porosity of 60% or higher [4, 5, 6, 7].

The focus of this work is twofold. The first objective was to develop a methodology to simultaneously evaluate the NOx and DPM control performance of mini 4WCFS that are challenged with diesel exhaust from a 2005 John Deere off-road diesel engine. The experimental mini filters evaluated were 1.9 x 1.9 x 7.5 cm square prisms. Thus, the method differs from traditional tests where full-sized aftertreatment exhaust systems are evaluated. It is a cost effective method to evaluate prototype filters rapidly and consistently with control of temperature, flow rate, and face velocity. The second objective was to evaluate the impact of catalyst loading and substrate porosity on (1) back pressure, (2) catalytic performance, (3) the particle filtration performance of catalyst-coated standard (STD) and high porosity (HP) test filters as stand-alone 4-way catalyzed filtration systems. The catalyst coatings used in this study were basic research coatings and were used due to availability within the project timeframe to test the catalyst loading capacity of the STD and HP test filters and are not intended for production applications. Further work would be needed with commercial automotive catalyst companies to optimize for their commercial coatings.

Experimental measurements included simultaneous and time resolved total and solid particle filtration efficiency (FE), size resolved FE, pressure drop (ΔP), and NOx removal performance. Preliminary results indicated that the use of HP ACM 4WCFS results in 98% reductions in NOx and 95% reductions in solid and semi-volatile particulate matter averaged over 10 regeneration cycles. Similarly, STD ACM 4WCFS exhibited high filtration efficiencies but slightly reduced NOx control performance. The rich / lean cycling that is used to regenerate the filter has almost no impact on solid particle filtration efficiency, but impacts NOx removal efficiencies. Shorter lean times (more frequent regeneration) lead to higher removal efficiencies but more reductant is consumed. Overall, the new methodology for simultaneously evaluating the FE and NOx removal performance of small scale emission control devices has proven to be a valuable developmental tool. Future work includes optimization and evaluation of additional filters and catalysts and scaling up the system to evaluate larger prototypes.

References:

[1] T.V. Johnson, Platinum Metals Rev. 52(1):23-37, 2008.

[2] T.V. Johnson, 2010.Platinum Metals Rev. 54(1):37-41, 2010.

[3] K. Nakatani, S. Hirota, S. Takeshima, K. Itoh, SAE Technical Paper 2002-01-0957, 2002.

[4] A.J. Pyzik, C.G. Li, Int. J. Appl. Ceram. Technol. 2(6):440–451, 2005.

[5] A.J. Pyzik, C.S. Todd, C. Han, J. European Ceramic Society 28:383–391, 2008.

[6] C.G. Li, H. Koelman, R. Ramanathan, U. Baretzky, G. Forbriger, T. Meunier, SAE Int. J. Fuels Lubr. 1(1): 1307-1312, 2008.

[7] J. Swanson, M. Schumacher, W. Watts. D. Kittelson, R. Newman, R. Ziebarth, 29^th AAAR Conference, Portland, OR, October 25 – 29, 2010.