Simultaneous Reduction of Particulate Matter and NOx Emissions

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Article pubs.acs.org/est

Simultaneous Reduction of Particulate Matter and NOx Emissions Using 4‑Way Catalyzed Filtration Systems Jacob J. Swanson,*,†,§ Winthrop F. Watts,† Robert A. Newman,‡ Robin R. Ziebarth,‡ and David B. Kittelson† †

University of Minnesota, Department of Mechanical Engineering, 111 Church Street SE, Minneapolis, Minnesota 55455, United States ‡ Dow Chemical Company, Core R&DInorganic Materials & Heterogeneous Catalysis, 1776 Building, Midland, Michigan 48674, United States S Supporting Information *

ABSTRACT: The next generation of diesel emission control devices includes 4-way catalyzed filtration systems (4WCFS) consisting of both NOx and diesel particulate matter (DPM) control. A methodology was developed to simultaneously evaluate the NOx and DPM control performance of miniature 4WCFS made from acicular mullite, an advanced ceramic material (ACM), that were challenged with diesel exhaust. The impact of catalyst loading and substrate porosity on catalytic performance of the NOx trap was evaluated. Simultaneously with NOx measurements, the real-time solid particle filtration performance of catalyst-coated standard and high porosity filters was determined for steady-state and regenerative conditions. The use of high porosity ACM 4-way catalyzed filtration systems reduced NOx by 99% and solid and total particulate matter by 95% when averaged over 10 regeneration cycles. A “regeneration cycle” refers to an oxidizing (“lean”) exhaust condition followed by a reducing (“rich”) exhaust condition resulting in NOx storage and NOx reduction (i.e., trap “regeneration”), respectively. Standard porosity ACM 4-way catalyzed filtration systems reduced NOx by 60−75% and exhibited 99.9% filtration efficiency. The rich/lean cycling used to regenerate the filter had almost no impact on solid particle filtration efficiency but impacted NOx control. Cycling resulted in the formation of very low concentrations of semivolatile nucleation mode particles for some 4WCFS formulations. Overall, 4WCFS show promise for significantly reducing diesel emissions into the atmosphere in a single control device.

1. INTRODUCTION In 2007, the U.S. Environmental Protection Agency (EPA) reduced the diesel particulate matter (DPM) emission standard for heavy-duty diesel engines used on-road to 0.0134 g/kWh.1 To meet the EPA standard, most manufacturers rely on an exhaust emission control device such as a diesel particulate filter (DPF). Most DPFs are wall-flow filters 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.2−4 For model year 2010, the emission standard for NOx (NO + NO2) was reduced to 0.27 g/kWh, roughly a 10-fold reduction over the previous standard.1 Common approaches for NOx reduction include ammonia selective catalytic reduction (SCR) and lean NOx traps. SCR systems require continuous injection of an ammonia precursor, usually urea from a separate tank. The urea breaks down to form ammonia, which acts as a reducing agent to reduce NOx. Ammonia may also be used directly. On the other hand, a NOx storage catalyst absorbs NOx during periods of lean operation, forming a mixture of metal nitrate and nitrites, which is and then is periodically reduced by injecting fuel. The amount of NOx storage catalyst dictates the quantity of NOx that can be © 2013 American Chemical Society

stored and hence the frequency of regeneration. Herein “regeneration” refers to the NOx trap rather than the removal of soot from the DPF. Longer times between regenerations translate to a smaller impact on fuel economy and improved flexibility in regeneration timing and engine control. To meet both the DPM and NOx emission standards, systems are required that reduce both pollutant concentrations, preferably in a simple, compact device. One such device relies on a 4-way catalyzed filtration system (4WCFS). Toyota Motor Corporation introduced a successful 4WCFS concept in the early 2000s that was termed a “diesel particulate−NOx reduction” system or DPNR.5 In this version of a 4WCFS, a bare cordierite DPF substrate was washcoated with a NOx storage catalyst. Thus, DPM and NOx are simultaneously reduced by a single control device. In some cases, a DOC is used upstream. An alternative to the Toyota system relies upon acicular mullite, an advanced ceramic material (ACM), as the Received: Revised: Accepted: Published: 4521

December 4, 2012 March 13, 2013 April 3, 2013 April 3, 2013 dx.doi.org/10.1021/es304971h | Environ. Sci. Technol. 2013, 47, 4521−4527

Environmental Science & Technology

Article

Figure 1. Sampling apparatus for combined gas and particle measurements. Indicated flow rates are approximate.

for many years.13−17 In addition, miniature DPF test results show filtration efficiency and pressure drops that are quantitatively similar to those obtained with full sized systems.12 The use of small filters allowed for rapid and precise control of temperature and filter face velocity, and all parameters could be controlled independently of engine condition. Furthermore, these tests differ from traditional bench-scale catalyst tests because 4WCFS were challenged with engine exhaust rather the synthetic gas mixtures. This enables more realistic measurements of real-world 4WFCS NOx and DPM control and provides the opportunity to better estimate the potential for nonregulated secondary emissions. A catalytic stripper (CS)18−20 was used to differentiate between solid and semivolatile particle emissions. Combined with gas measurements, these measurements allow for detailed evaluation of the performance of 4WCFS. Solid particles provide information on the filtration efficiency of the DPF component, while semivolatile particles are indicative of sulfur and hydrocarbon compounds, which relate to catalyst performance.

substrate. The ACM manufacturing process allows the microstructure, total porosity, and pore size distribution to be tailored to meet requirements for DPM/NOx emission control. The manufacturing process relies upon on a gas−solid catalyzed reaction to form mullite by low temperature decomposition of fluorotopaz (Al2SiO4F2).6 In the first stage of this process, fluorotopaz is formed from clay and alumina (Al2O3) in the presence of silicon tetrafluoride (SiF4) gas. On subsequent heating at temperatures between 1000 and 1200°C, fluorotopaz melts to form mullite and free SiF4 gas. The unique feature of this process is the resulting highly elongated (acicular) grain or needle-like structure. The formation and growth of acicular mullite grains “interlocks” the microstructure and results in retained high porosity. The size of the mullite needles can be controlled depending on starting raw materials and processing conditions. Typically, the ratio of needle length to diameter remains about 20, but needle diameters can be altered from about 3 to 50 μm. This allows for addressing the requirements of the DPF application, where an average pore size distribution between 15 and 30 μm in the honeycomb wall is preferred.7−9 Additionally, ACM is suitable for catalyzed applications. By increasing the porosity of the ACM filters, more NOx storage catalyst can 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.10−12 The objectives of this study were to develop a method to simultaneously evaluate the NOx and DPM control performance of miniature 4WCFS and to evaluate the impact of catalyst loading and substrate porosity on particle filtration efficiency, semivolatile particle emissions, and NOx reduction. Miniature 4WCFS were evaluated because it is more cost-effective to evaluate small filters compared to full-sized systems when developing new DPFs and/or catalysts. The use of small scale devices has been a standard approach in the catalyst industry

2. MATERIALS AND METHODS 2.1. Engine Aerosol Generator. The engine used to generate the test aerosol was a 4-cylinder, 4.5 L, 129 kW (at 2400 rpm), model year 2005, John Deere 4045H. This engine is turbocharged, aftercooled with common rail fuel injection but without exhaust gas recirculation, and has tier 2 certification for off-highway applications. The engine was fueled with ultralow sulfur diesel fuel containing