Introduction

There is increased interest in recent years in replacing traditional gasoline and diesel fuels with natural gas for a number of reasons, such as increasing energy prices, depletion in oil resources and political uncertainty, but mainly due to increasing concern about global warming (1). Natural gas is composed mainly of methane (typically 70–90%) with variable proportions of other hydrocarbons such as ethane, propane and butane (up to 20% in some deposits) and other gases (2, 3). It can be commercially produced from oil or natural gas fields and is widely used as a combustion energy source for power generation, industrial cogeneration and domestic heating. It can also be used as a vehicle fuel. Natural gas has a number of environmental benefits: it is a cleaner burning fuel typically containing few impurities, it contains higher energy (Bti) per carbon than traditional hydrocarbon fuels resulting in low carbon dioxide emissions (25% less greenhouse gas emissions), and it has lower emissions of PM and NOx compared to diesel and gasoline. In the EU there is an aim for more than a 60% reduction in CO₂ emissions from transport by 2050 from 1990 levels (4) and using natural gas as a vehicle fuel can contribute to this (1). From the economic point of view, natural gas can be less expensive than diesel and gasoline. For example Table I displays gasoline, diesel and natural gas prices during 2014 and 2015 (5, 6).

The production cost is low and governments worldwide are promoting its usage by providing financial incentives. Political benefits, in countries such as the USA, could include secured national resources for energy, and less dependency on imported oil from other countries. Partly due to these factors, there is a rapid increase in interest in use of natural gas with a predicted 30% average annual growth especially in internal combustion engines.

This short review focuses on the demand, policies and growth of natural gas globally and methane abatement via aftertreatment systems for mobile and stationary applications.

Legislation and Challenges

Natural gas engines emit very low PM and NOx (up to 95% and 70% less, respectively) compared to heavy-duty and light-duty diesel engines. Presently, methane is not included in criteria emissions regulations, but the US EPA greenhouse gas legislation caps methane emissions at 0.1 g bhp^–1 h^–1 for heavy-duty engines and 0.05 g mile^–1 for pickup trucks and vans (7). Euro VI natural gas vehicles are required to meet both total hydrocarbon (THC) and methane emissions limits and therefore need methane emission control aftertreatment devices. The current EU emission standards for heavy-duty gas engines are shown in Table II. The methane limits apply for only gas engines and the PM limit is not an issue for CNG vehicles.

The US EPA has also proposed new standards towards meeting the US government’s pledge to reduce the methane emissions mainly from the oil and gas sectors by 40–45% by 2025 from 2012 level (9). This is further emphasised recently in May 2016 to limit methane emissions from industry leaking compressors, wells and pumps (10).

Natural Gas Resources and the Natural Gas Vehicles Market

Natural gas is available in abundance worldwide. As shown in Figure 1, the worldwide proven reserves of natural gas exceed 204.7 × 10¹² m³ and world gas reserves will last for 537 years, another 161 years in Europe (1).

The worldwide natural gas vehicle population increased at an average growth rate of 20% per annum between 1991 and 2007 and some sources predict it to increase further as shown in Figure 2 (11).

Fuel Options

Natural gas can be used as transportation fuel in the form of CNG and liquefied natural gas (LNG). CNG is carried in tanks pressurised to 3600 psi (~248 bar) and has an energy density around 35% of gasoline per unit volume (12). LNG has an energy density 2.5 times that of CNG and is mostly used for heavy-duty vehicles. It is cooled to liquid form at –162°C and as a result the volume is reduced 600 fold meaning LNG is easier to transport than CNG (2). In general, natural gas vehicles are more expensive than petrol or diesel vehicles mainly due to the cost of the high-pressure or insulated fuel tank which is required to store LNG or CNG.

Alternatively, using gas-to-liquids (GTL) technology (13), the natural gas can be converted to liquid fuels which have ignition characteristics similar to diesel or gasoline fuels and can be used for transportation purposes. Other options include reforming natural gas to generate hydrogen for hydrogen fuel cell vehicles and generation of electricity for electric vehicles by firing a power plant by natural gas (14).

Bio-LNG could be an alternative to natural (fossil) gas, being produced from biogas, derived by anaerobic digestion from organic matter such as landfill waste or manure (15–17). Its use would further reduce vehicle greenhouse gas emissions (18) and would also be cheaper than diesel, partly because of government incentives such as reduced duties in various countries or regions (19).

Other Challenges

CNG catalysts, especially Pd-based catalysts, suffer from poisoning by water (5–12%) and sulfur (2 in lube oil) especially under lean conditions, which results in drastic reduction of conversion rate of the catalyst over time. The deactivation due to water is significant due to the formation of hydroxyl, carbonates, formates and other intermediates on the catalyst surface (20). The activity is reversible, and can be recovered completely if water is removed (Figure 8). However, this is impractical as methane combustion feed always contains a high level of water due to the high content of H in methane. H₂O can be either an inhibitor or a promoter depending on the air-to-fuel ratio, i.e. lambda. Under stoichiometric and reducing conditions, lambda ≤1, H₂O can act as a promoter for the oxidation of hydrocarbons through the steam reforming reaction in both CNG and gasoline engines. However for lean burn CNG operating at lambdas >1, H₂O acts as an inhibitor for methane oxidation. It is critical to understand the water inhibition effect and design catalysts which are more tolerant to the presence of H₂O. This would allow for improvement when trying to control methane emissions from lean burn CNG.

Though the sulfur level is very low in the engine exhaust, Pd-based catalysts deactivate significantly upon sulfur exposure due to the formation of stable sulfates (21, 22). Regeneration of the catalyst in order to restore the activity following sulfur poisoning is challenging and will usually require high temperatures, rich operation or both. This is easily achievable in stoichiometric operation but more difficult in lean burn. A lean burn vehicle operates with a much higher air-to-fuel ratio than a stoichiometric vehicle and will need injection of a much higher concentration of reductant to switch to rich operation. Thermal deactivation resulting from a high level of misfire events due to poor engine transient control and ignition systems destroys the catalyst and correspondingly leads to a high level of exhaust emissions. This problem is common to all types of engine, but some catalysts are more resistant to the thermal deactivation. This leads to a high demand for catalyst efficiency and durability.

Figure 9 shows the effect of high-temperature thermal ageing and sulfur exposure on the methane conversion of a Pd-based oxidation catalyst. It is evident that the catalyst deactivates under both conditions, but sulfur poisoning has a more dramatic impact than thermal ageing. Both the thermal durability and sulfur poisoning can be improved by the addition of small amounts of Pt to the Pd catalyst (23, 24), as shown in Figures 10 and 11. Figure 11 shows that the sulfur inhibition due to formation of palladium sulfates can be reduced significantly on addition of Pt (25).

Conclusions

An increased interest in the use of natural gas as an alternative fuel in mobile and stationary applications is apparent. Some of the main reasons are its relative high abundance, lower cost compared to other fuels and most importantly the urge to meet targets for the reduction of greenhouse gas emissions. Methane emissions and total hydrocarbons (including methane) are capped by regulations globally including the US EPA greenhouse gas legislation and Euro VI, requiring vehicles to use methane emissions control aftertreatment. The obvious path for natural gas is via methane for stationary and mobile applications, where it can be used in the form of either CNG or LNG. With further improvement in natural gas fuelling infrastructure and an increase in the number of natural gas engines, the rate of uptake of natural gas vehicles can be enhanced to a greater extent. Pd-Rh TWC under stoichiometric conditions, while Pd-Pt catalyst under lean conditions are used to deal with methane combustion. High methane combustion can be achieved at lower temperatures by altering the pgm loading and through engine calibration depending on the regional legislation requirements. However, these might make the technology rather expensive. Further challenges include the issues with catalyst deactivation due to sulfur (from lubricant), water, thermal ageing and methane slip at low temperatures. These need to be addressed quickly so that this technology is readily available to meet the current and upcoming stricter legislation and challenges.