Methane Emission Control Catalysts

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Methane contributes to criteria pollutant emissions as part of total hydrocarbon emissions, and is a potent climate change gas. While natural gas may produce lower GHG emissions than other fossil fuels, exhaust emissions of methane can easily overcome the GHG benefit of natural gas engines. Methane emissions from lean burn engines can be controlled by oxidation catalysts and those from stoichiometric engines by three-way catalysts.

Methane Emissions

Methane And Criteria Pollutants

Methane (CH4) is the shortest carbon chain hydrocarbon and its emissions constitute a part of the total hydrocarbon (THC) emissions from internal combustion engines. The proportion of CH4 in total HC emissions varies widely depending on the type of engine. Methane often constitutes most of the THC emissions from natural gas (NG) engines, while it is practically non-existent in diesel engines. Some emission regulations for natural gas engines include methane limits, in addition to THC emission standards. For instance, the Euro VI regulations set a CH4 limit of 0.5 g/kWh from heavy-duty gas engines.

However, methane emissions are not necessarily considered a public health concern. Compared to hydrocarbons of larger molecular mass, methane has a relatively low ozone formation potential. For this reason, emission regulations in some jurisdictions—notably in the United States—exclude methane from the limits for criteria pollutants. 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), and—in stationary engine applications—as volatile organic compounds (VOC) commonly defined as non-methane, non-ethane hydrocarbons (NMNEHC).

Methane As a Climate Change Gas

On the other hand, methane is a potent greenhouse gas (GHG) with a higher warming potential than CO2. One of the metrics often used to compare the warming effect of different compounds is the global warming potential (GWP), which is an index of the total energy added to the climate system by a component in question relative to that added by CO2 (on mass basis). The GWP is usually integrated over a time horizon of 100 years (GWP100) or 20 years (GWP20). The IPCC 5th Assessment Report lists two GWP100 values for methane of 28 and 34, the latter accounting for the climate-carbon feedback in response to CH4 release [3712]. The GWP value of 34 has been also adopted by the US EPA. The short-term warming effect of methane is even higher, with GWP20 values of 84/86 (without/with climate-carbon feedback). The lifetime of methane in the atmosphere is 12.4 years [3712].

Caps on CH4 emissions have been included in US GHG emission regulations for light- and heavy-duty vehicles. The heavy-duty engine limit is 0.1 g/bhp-hr, equivalent to 0.134 g/kWh (however, manufacturers are allowed to emit higher levels of CH4 if they over-comply on CO2). There is also 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 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 rate of increase accelerated after 2006, with fossil sources believed to be the largest contributor to the increasing emissions [3731]. 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.

Methane Emissions from NG Engines

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%).

Exhaust emissions of methane—even in the absence of leaks and losses—can quickly overcome the potential GHG benefit of natural gas engines. Figure 1 illustrates the impact of CH4 emissions on the net GHG benefit for a range of natural gas engines [3698][3699][3700][3701][3705]. As methane emissions increase, the GHG benefit of natural gas engines decreases and then turns negative. Engines that show the highest CO2 benefit without accounting for CH4 emissions can tolerate the highest CH4 emissions before the GHG benefit of natural gas becomes negative. It should be noted that this figure does not account for N2O or other on-board methane releases such as LNG system venting. The US EPA Phase 2 GWP potential for methane of 34 was used [2918].

[SVG image]
Figure 1. Impact of engine methane emissions on net GHG benefit of natural gas engines

GHG benefit relative to diesel. Methane GWP = 34.

Examining Figure 1 in more detail, the values for heavy-duty on-road engines (blue) represent the net GHG benefit of several 2017 heavy duty SI stoichiometric engines. The CH4 emissions were measured over the FTP cycle and the GHG benefit is relative to their diesel counterparts [3698]. Reported methane emissions range from 0.34-2.6 g/kWh and values above about 3.2 g/kWh would yield a negative GHG benefit. An important point with these engines is there a significant BTE reduction associated with a switch from diesel (BTE ~44%) to stoichiometric spark ignited natural gas (BTE ~38%) that reduces the potential GHG benefit from switching to natural gas. The black dashed line represents the estimated GHG benefit for a heavy-duty natural gas engine if the BTE could be maintained at 44%.

Also shown in Figure 1 is the net GHG benefit for a range of marine engines (red) from MAN’s ME-GI (high pressure dual fuel, HP DF) to low pressure SI (LP SI) and low pressure dual fuel (LP DF) [3700][3701]. These engines are compared to their heavy-fuel oil (HFO) counterparts [3705]. For these engines, there is little BTE loss when switching to natural gas so the potential GHG benefit is higher than for stoichiometric on-road engines. MAN’s ME-GI engines have reported methane emissions of about 0.2 g/kWh [3702] and show the highest GHG benefit relative to their HFO counterparts. However, for the other marine natural gas technologies, little GHG benefit is apparent in switching to natural gas. Post 2010 SI engines have CH4 emissions of about 4 g/kWh and are at best GHG neutral. Older pre 2010 LP SI (~7 g/kWh CH4) as well as LP DF (post 2010 LP DF engines ~8.5 g/kWh CH4 and pre 2010 LP DF engines ~ 15 g/kWh) have higher GHG emissions than their HFO fueled counterparts. While the single point for locomotives is from data collected in the 1990s [3699], the methane emissions are still consistent with more recent values reported [3703].

Dual Fuel Retrofits. Retrofitting diesel engines with dual fuel conversion kits that allow some of the diesel fuel to be substituted by natural gas introduced into the intake air upstream of the intake valve can have very significant negative impacts on methane and GHG emissions. Methane emissions from three different natural gas conversion kits that use either port fuel injection or pre-compressor intake fumigation are shown in Figure 2. Methane emissions as high as 100 g/bhp-hr were measured at some of the part load operating modes of the SET test cycle [3717][3736]. A significant portion of the methane emissions is due to natural gas being blown through the cylinder to the exhaust manifold during the valve overlap period.

Figure 2. Methane emissions from three diesel engines retrofitted with dual fuel conversion kits

Empty markers: emissions before conversion (diesel). Solid markers: after dual fuel conversion.

While some of these kits may disable natural gas substitution at full load, idle and cold start, even at part load, the methane emissions can be substantial and CO2 equivalent emissions (CO2 + CH4 × GWPCH4) can range from 1.6 to 4.8 times that of a diesel engine, Figure 3 [3717]. Clearly there is no GHG benefit to such retrofit kits.

Figure 3. GHG emissions from three diesel engines retrofitted with dual fuel conversion kits

Global warming potential for methane GWP20 = 72 and GWP100 = 25