Methane Oxidation Catalysts

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Palladium and platinum/palladium catalysts are commonly used for the control of methane emissions from lean burn gas engines. While Pd is the most active CH4 oxidation catalyst, high catalyst temperatures are still required to oxidize methane and other short chain hydrocarbons. Pd catalysts are also sensitive to sulfur poisoning and can be deactivated even at trace levels of sulfur in the exhaust gas. The deactivation by sulfur can be reversed by thermal regeneration, preferably under reducing conditions.

Introduction

Emission regulations in many jurisdictions exclude methane from the limits for criteria pollutants, due to the relatively low ozone formation potential of methane compared to hydrocarbons of larger molecular mass. With the exclusion of methane, emissions of hydrocarbons and related organic compounds have been regulated as non-methane hydrocarbons (NMHC) or non-methane organic gases (NMOG).

On the other hand, methane is a potent greenhouse gas (GHG) with a warming potential on the order of 25-35 times that of CO2 (on a mole basis). Caps on CH4 emissions have been included in US GHG emission regulations for light- and heavy-duty vehicles. There is also a significant interest in CH4 emission control from stationary gas engines, which is driven by existing and anticipated GHG emission regulations, as well as by a desire to reduce the carbon footprint in some carbon-intensive industries—for example, in the processing of oil sands.

Natural gas (NG) fuel has generally been considered to yield lower GHG emissions than coal and liquid petroleum fuels, because of the lower relative carbon content in the CH4 molecule compared to other fossil fuels. However, the comparison of GHG emissions from natural gas and from other fuels critically depends on the magnitude of methane losses (commonly referred to as “methane leaks”) from natural gas extraction, distribution and utilization. Considering the higher warming potential of CH4, losses of even a few percent can lead to higher GHG emissions (on a CO2 basis) with natural gas than liquid hydrocarbon fuels or even coal combustion. A number of studies indicate that CH4 emissions from US and Canadian natural gas systems appear larger than official estimates [3451], and the global atmospheric CH4 concentrations are on the rise [3452]. The suspected reasons include high leakage rates from hydraulic fracturing (fracking) production of natural gas [3462], as well as leaks from aging conventional gas wells and processing equipment [3455]. These findings challenge the benefits of switching from liquid petroleum to natural gas as a means to reduce GHG emissions.

The Argonne GREET GHG Model estimated the methane leakage rate in the United States at 1.08% of the gross natural gas production [3453], while the rate used by the US EPA is about 1.5%. However, the actual US methane emissions are 1.25 to 1.75 times higher than the EPA estimates, according to an analysis of more than 200 studies on natural gas leakage [3451]. Various estimates suggest that if methane emissions reach 4-5% of the total gas volume, natural gas provides no GHG emission benefit compared to petroleum fuels. When CH4 leaks exceed 7-8%, GHG emissions from the use of natural gas become comparable to those from coal combustion.

It remains uncertain what fraction of the overall CH4 emissions from natural gas can be attributed to the operation of natural gas engines, but the existing evidence suggests that the emission rates can be significant. A study by West Virginia University quantified methane emissions from engine units in five gas compressor facilities [3454]. Methane leaks (defined as an unintended malfunction) and losses (defined as a design feature) were measured on-site from six types of engines, including two- and four-stroke engines as well as gas turbines. Methane emissions from particular engines, relative to the fuel consumption of the engine, ranged from 0.5% (Clark TLA-6) to 7.6% (CAT 3512). Overall, for the five sites tested, the engine and compressor units yielded a combined methane leak and loss rate of 71.1 kg/hr. The highest source of CH4 emissions was engine exhaust, contributing 61% of the total emissions. Other sources included compressor packing loss and wet seals in turbines (25%), engine leaks (7%) and crankcase leaks (7%).

These CH4 emissions can be controlled by improved engine and compressor maintenance practice and by exhaust aftertreatment—oxidation catalysts in case of lean-burn engines. However, catalytic oxidation of methane remains challenging due to the high exhaust temperatures that are required and catalyst poisoning by sulfur. As a result, the use of CH4 oxidation catalysts is still limited. In the United States, oxidation catalysts are used on stationary natural gas engines primarily to control carbon monoxide, formaldehyde and volatile organic compounds, rather than methane [3667].

Rich Burn Engines. In the absence of oxygen in the exhaust gas, methane can be controlled using a 3-way catalyst. In stationary engine applications, the technology is often referred to as Non-Selective Catalytic Reduction (NSCR). In the NSCR process, methane, other HCs and CO react with NOx. The engine must be operated within a narrow air-to-fuel ratio window—at stoichiometric conditions—to ensure high conversions of both CO/HC and NOx.

Compared to the oxidation catalyst technology, methane conversion is less challenging in the NSCR system. Rich burn NG engines have higher exhaust gas temperatures, on the order of 500-700°C at full load, which enable a sustained methane conversion and—in conjunction with the rich conditions—facilitate desulfation of the catalyst. Additionally, under stoichiometric or reducing conditions, H2O can act as a promoter for the oxidation of HCs through the steam reforming reaction [3481].

Catalytic Oxidation of Methane

Reactions and Catalysts

The oxidation of methane over an oxidation catalyst is described by the equation:

(1)CH4 + 2O2 = CO2 + 2H2O

Since oxygen is needed, methane oxidation catalysts (MOC) can only be used to control CH4 emissions from lean-burn gas engines. In stoichiometric (sometimes referred to as “rich burn”) gas engines, methane can be controlled with a three-way catalyst (TWC).

Methane is a highly stable compound—high activation energy is required to break the C-H bond and to oxidize the molecule. Therefore, a higher catalyst temperature is necessary for the oxidation of methane, compared to longer chain hydrocarbons. To achieve high CH4 conversions, methane oxidation catalysts must be operated at temperatures of at least 350°C [3666] and preferably around 500°C.

Pd-based catalysts are generally considered to be most active to oxidize methane and other short chain hydrocarbons, Figure 1 [3651]. However, in addition to the high catalyst temperature, other challenges in the catalytic oxidation of methane include:

[SVG image]
Figure 1. Methane oxidation efficiency for different precious metals versus temperature

CH4 1000 ppm, O2 10%, H2O 10%, SV = 40,000 1/h.

Other precious metals considered for methane oxidation are platinum and rhodium. Platinum-based catalysts are of interest for lean-burn applications due to their low sensitivity to water and sulfur containing compounds. Platinum does not form stable hydroxides or sulfates. Sulfur can even promote methane oxidation over platinum supported catalysts. However, the low temperature activity of platinum-based catalysts for methane oxidation is poor under lean conditions, Figure 1 [3649][3651]. Oxidation catalysts based on Rh have also been tested. Rhodium has light-off characteristics between those for Pd and Pt. Its sensitivity to sulfur poisoning also falls between that for Pd and Pt [3650].

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