Technology: tackling methane emissions [Gas in Transition]
In many cases, methane abatement can pay for itself over a relatively short time frame, even if individual projects do not meet targeted internal company rates of return. This suggests companies need to adopt a more holistic approach to capital investment, placing a higher priority on environmental benefits rather than taking a purely financial approach.
This does not have to result in a loss. The Global Methane Initiative (GMI), an international public/private partnership, estimates that 50% of oil and gas sector methane emissions can be eliminated at no net cost.
Pneumatic, electrical or mechanical instrument systems to control pressure, flow, temperature and liquid levels are widely used in the gas industry. They are also used, for example, on compressor starter motors, chemical injection and odorization pumps and valve actuators, among other uses. Many, particularly on gas production sites, employ pneumatic devices using pressurised natural gas, which result in a constant bleed of methane to the atmosphere in the normal course of operation.
Pneumatic control systems lose methane from tube joints, controls and a wide number of points within the distribution tubing network. Depressurisation of the system involves venting, which can also occur for safety reasons. The rate of methane loss in normal operation depends mainly on the design of the system, the pressures employed and the age and condition of the equipment, with leakage generally rising as equipment gets older.
Companies want to see equipment used to the expected length of its life, particularly where the equipment is employed on an ageing asset, for example a gas production site in the last stages of decline. Nonetheless, replacing key parts early can have a very positive effect on reducing methane leakage, while in longer-life assets, larger system replacements can provide a double win in terms of financial and environmental benefits.
For new projects, methane leakage monitoring and abatement should be addressed from the initial design stage, with zero bleed technologies employed to improve efficiency and maximise saleable gas volumes, as well as to avoid future costs arising from regulation demanding higher methane leakage abatement performance.
Air instrument systems
Instrument air systems replace pressurised natural gas with compressed air, allowing the natural gas saved to be sold. Eliminating methane leakage also improves operational safety as methane is flammable.
Capital expenditure on replacement is mainly for installing new compressors and related equipment. Operational expenditure on electricity to power the compressors also has to be taken into account. However, if the methane leakage is large, payback times from increased gas sales can be short, according to project studies conducted by the US Environment Protection Agency (EPA).
Instrument air systems have additional requirements to pressurised natural gas. They need to filter and dry atmospheric air. Condensation forming as the air is compressed is a problem, as the water can block instruments and cause corrosion. For small systems, membranes which allow oxygen and nitrogen through, but block water molecules can be used. As they have no moving parts, they are reliable, but for large systems desiccant alumina dryers are often a more economical solution.
Membrane systems also consume air – about 17% of throughput. For commercially-usable dry air, a dew point of minus 40 degrees Fahrenheit or lower must be achieved.
The dry air is compressed and held in a volume tank. The compressor is typically driven by an electrical motor, which switches on and off, depending on the pressure in the volume tank. Two compressors are required, one acting as a stand-by.
Relying on an electrical motor means having a secure power supply, which generally is not a problem at a large natural gas plant, but can be at remote sites where pressurised gas is readily available, making air instrument systems a more expensive choice. They may require installation of a new power source, such as battery-supported solar generation.
Although more complex than using pressurised natural gas, the high purity and dryness of the air leads to longer equipment life. Natural gas used in pneumatic control devices and instruments often contains impurities, including corrosive gases such as hydrogen sulphide, which cause instrument degradation. Air instrument systems therefore provide longer equipment life, enhanced operational safety, alongside the environmental benefits of reduced methane bleed.
The EPA’s project studies suggest a change to air instrument systems is most cost-effective where pneumatic devices are fairly densely sited, allowing more devices to be converted and powered by a single system. Electrical and mechanical pumps can also be used to replace pumps using compressed natural gas.
Compressor stations are a major source of methane leakage in the storage and transmission sector, but one where abatement and use of the saved gas can be economic.
Although not the only source of leakage from a compressor station, the compressors themselves are amongst the most important. Two types of compressors are widely used across the gas and LNG sectors: reciprocating and centrifugal. Compressors play a key role in LNG plants, but are also used in transportation to increase the pressure of gas so that it can move along a pipeline.
Reciprocating compressors use pistons to compress natural gas and leak methane as part of their design. However, worn rod packing can leak as much as 15 times more methane than newly-installed rod packing. Methane leaks can therefore be reduced by condition monitoring, replacing the rod packing when leakage becomes too large.
Leaks occur both when the compressor is running and when idle and particularly during start up and shut downs through the compressor’s blowdown and isolation valves.
Centrifugal compressors bleed methane from valves and seals, which are located on the rotating shafts. Wet seals, using pressurised oil to form a barrier against leakage, can be replaced by dry seals, which are mechanically simpler, resulting in more reliable operation and reduced methane leak. The pressurised oil in wet seals gradually absorbs methane and has eventually to be ‘degassed’ with the purged gas usually vented to the atmosphere. Methane leak from wet seal centrifugal compressors can also be reduced via vapor recovery systems.
Small leaks are often uneconomic to address and deliver no net environmental benefits as repair may involve venting to complete, which increases rather than decreases the release of methane to the atmosphere. Compressors have to be depressurised for maintenance, which involves venting, so maintenance programmes have to well planned to minimise leakage events.
Building on past gains
Methane leaks from the natural gas supply chain are receiving a lot of attention today as full lifecycle carbon accounting becomes the norm for measuring the environmental impact of different energy sources. Increased regulation, including carbon taxes and trading, are expected to create incentives for better methane control, but the oil and industry has been successful in reducing methane leak for a long time as dry seals on centrifugal compressors and low emission controls, such as air instrument systems, become the norm.
In the US, the industry reduced measured methane emissions from transmission and storage facilities by 44% between 1990 and 2016, mainly due to reduced compressor station and fugitive emissions, despite a 43% rise in US gas consumption over the same time frame, according to US Energy Information Administration data.
The GMI also charts significant achievements in methane emissions reductions – 42mn metric tons of CO2e in 2019, via GMI-supported projects. In addition, the GMI has helped identify 650mn mt/CO2e of potential savings building on cumulative savings of 453mn mt CO2e between 2005 and 2019.