Catalysts for Natural Gas Engines

W. Addy Majewski

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Abstract: Emissions from natural gas engines can be controlled by catalysts. The type of catalyst and aftertreatment system depends on the engine combustion strategy. Emissions from stoichiometric engines can be controlled by three-way catalysts, and those from lean burn engines by oxidation and SCR catalysts. Catalytic conversion of methane remains challenging in all types of NG engines, and is most problematic in lean burn engines.

Catalyst Technology

Natural gas can be used in engines utilizing different mixture preparation and ignition principles. Depending on the type of combustion strategy and the exhaust gas chemistry, different configurations of emission aftertreatment must be used—ranging from three-way catalysts (TWC) on stoichiometric engines to diesel-like SCR + DPF aftertreatment on natural gas engines operating according to the Diesel cycle. This is schematically illustrated in Figure 1 [5016]:

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Figure 1. Natural gas combustion strategies, catalysts, and emission control challenges

Three-way catalysts can be used on stoichiometric NG engines (also called rich burn engines) to simultaneously reduce NOx and oxidize HC/CH4 and CO emissions. This relatively simple emission control system, similar to those used in stoichiometric gasoline engines, requires a precise control of the air-to-fuel ratio within a narrow catalyst window.

Lean burn natural gas engines, on the other hand, produce oxidizing exhaust gases. Oxidation catalysts can be used on lean burn engines to control such emissions as CO, THC and formaldehyde through catalytic oxidation. If NOx control is required under lean conditions, urea-SCR systems are typically used.

The oxidation catalyst also affects PM emissions from NG engines. In lean premixed engines, where particulate emissions are dominated by organic material from the lubricating oil, the catalyst can reduce PM emissions through conversion of the organic fraction. In the presence of sulfur, especially at high exhaust temperatures, high (volatile) nanoparticle concentrations were also observed downstream of the NG oxidation catalyst, together with higher sulfate concentrations in particles [5017]. In contrast, PM emissions from direct injection NG engines that operate on the Diesel cycle contain quantities of carbonaceous material, and must be controlled using a particulate filter to meet stringent PM/PN emission limits.

Catalytic conversion of methane remains challenging in all types of NG engines, and is particularly problematic in lean burn engines. The oxidation of CH4 has a higher activation energy than oxidation of longer chain hydrocarbons that include C-C bonds. Therefore, high operating temperatures are required by methane control catalysts, Figure 2 [3680].

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Figure 2. Conversion of different hydrocarbons on lean light-off tests

Degreened Pd-only catalyst, SV = 25,000 1/h, 500 ppm C1

Methane emission control becomes increasingly important, largely due to the high contribution of CH4 to GHG emissions. However, it should be emphasized that most commercial catalyst applications on natural gas engines have been driven by emission regulations for pollutants other than methane. Examples include NOx emission limits for onroad NG engines or NOx/CO/VOC limits for stationary NG engines.

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