Impact of biodiesel impurities on catalysts studied

  • 09-Jan-2012 01:27 EST
1136 - Fig 9.jpg

Exposure of the cordierite DPF to 150,000 mi (240,000 km) of biodiesel ash did not significantly change the thermal shock resistance characteristics of the material. However, the elemental distribution maps shown here indicate diffusion of the impurity elements into approximately 25-30% of the cell wall thickness of the outlet portion of the DPF.

To determine whether a biofuel containing metals at the ASTM limits could cause adverse impacts on the performance and durability of diesel emission control systems, an industry team including Caterpillar conducted an accelerated-aging study.

Tested was a 300-hp (224-kW) Caterpillar 2008 C9 ACERT engine of 8.8 L displacement, calibrated to meet the Tier III off-road emissions limit. The engine is turbocharged and direct-injected and does not use exhaust emission control in its base configuration. A proprietary 60-h engine break-in process was used to precondition the engine for the emissions and durability testing. After the break-in, it was retrofitted with exhaust emission controls to serve as a platform for exposing various catalysts and filters to biodiesel exhaust gas.

The biodiesel was doped with alkali and alkaline earth impurities to represent the upper limit of these impurities allowed by ASTM D6751. The impact of long-term biodiesel ash exposure was investigated for three different DPF substrate types (cordierite, AT, and SiC), as well as for DOC and SCR catalysts.

While the ash exposure from the biodiesel was accelerated, the ash exposure from the engine lube-oil was not. However, the lube-oil ash contribution would be the same regardless of fuel type. The amount of thermal aging is also the same for each fuel. Thus, for this study, comparisons of the ULSD and B20 test parts at 150,000 (240,000 km) and 435,000 mi (700,000 km) equivalent test intervals represented the same amount of thermal aging and lube-oil ash exposure with the only difference being the additional ash that would be seen by using biodiesel.

DPF and DOC parts were aged to a 150,000-mi equivalent operation, which is the EPA’s minimum ash-clean interval for a DPF. In addition, a pair of cordierite DPFs and SCR catalysts were aged to a 435,000-mi equivalent operation, which is the EPA’s required full-useful-life durability limit for heavy heavy-duty applications.

Estimates of DPF pressure drop indicate that the additional ash exposure from 150,000 mi of operation with B20 will result in a 6.8% increase in backpressure.

The catalytic activity of a DOC exposed to 150,000-mi equivalent aging of biodiesel ash was reduced for both HC and NO2 oxidation. Engine tests showed some drop of heat-up performance of the B20 aged DOC compared to its ULSD aged counterpart, and furthermore, NO2 formation activity was reduced approximately 30% compared to the ULSD aged DOC.

The mechanical durability of the cordierite, SiC, and AT DPFs after 150,000-mi equivalent operation was measured. The changes in bend strength, elastic modulus, and coefficient of thermal expansion were determined to be comparable for the ULSD and B20 aged parts.

The mechanical durability of the cordierite DPFs aged to 435,000-mi equivalent operation with biodiesel (with no ash cleaning) showed an increase in strength as well as an increase in elastic modulus and CTE. Overall, the relative thermal shock resistance parameter declined by 69% compared to the ULSD test filter, indicating a greater susceptibility to thermal shock.

Although it was downstream of the DPF, the SCR catalyst was impacted by the biodiesel fuel aging. The NOx conversion showed activity loss of 5% over the hot start HDDT test cycle, and emissions from the biodiesel aged SCR system were slightly above the 0.20 g/bhp·h standard.

The biodiesel test fuel was additized with both alkali and alkaline earth metals. Thus, it cannot be determined which of these biodiesel impurities was the cause of any decline in performance. However, because calcium ash is a normal component found in lube oil ash, and magnesium is a component of cordierite, it is believed that the alkali metals sodium and potassium were primarily responsible for the chemical effects observed.

The results of this study suggest that long-term operation with B20 at the current specification limits for alkali and alkaline earth metal impurities can adversely impact the performance of DOC, DPF, and SCR systems.

This article is based on SAE technical paper 2011-01-1136 by Aaron Williams, Robert McCormick, and Jon Luecke, National Renewable Energy Laboratory; Rasto Brezny, Manufacturers of Emission Controls Association; Andreas Geisselmann, Umicore AG & Co KG; Kenneth Voss and Kevin Hallstrom, BASF Corp.; and Matthew Leustek, Jared Parsons, and Hind Abi-Akar, Caterpillar Inc.

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