Catalytic oxygen removal from coal mine methane
A catalytic combustion process for oxygen removal enables maximum production of sales gas from an unconventional source of methane
Gary Trotter, Oil & Gas Equity Management
Zane Rhodes, Newpoint Gas
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Oso Oil & Gas Properties installed a system to gather and process coalmine methane gob gas in Carbon County, Utah, near the city of Price. Coal mine gob gas is methane gas diluted with various contaminants from the mine air ventilation system. The gob gas comes from the long wall cave areas of the Aberdeen Tower Mine and is brought to the surface with vertical drill holes that have been installed by the mining company to ensure mine safety.
Under an exclusive agreement with Andalex Resources, the mining company that owns the Tower Mine, and the mineral estate owners, Oso collects the gob gas at the mine’s vent wells. Oso compresses the gas and removes a small amount of hydrogen sulphide, then transports the remaining gas five miles to a facility where oxygen and nitrogen are removed.
The oxygen removal system is based on a catalytic combustion process. This oxygen removal process allows the gathering of the gob gas at a vacuum, so the maximum amount of gas can be recovered. The resulting gas is then transported to the Pioneer Castle Gate Field processing plant, where Oso and Pioneer gas is commingled and carbon dioxide and water are removed. The combined streams are then sold to Questar at the Whitmore Park interconnect approximately 20 miles from the Castle Gate Field. This project makes economic and environmental sense by allowing the recovery of a valuable gas stream that would otherwise be vented into the atmosphere.
In 2005, Oso, in cooperation with Andalex, embarked on a coalmine methane (CMM) recovery project designed to capture mine gas continuously vented to the atmosphere at the Tower Mine in Carbon County, Utah. The Tower Mine is located approximately 90 miles southeast of Salt Lake City. The gas was being vented from gas vent holes (GVHs) drilled into the coal, sandstone and shale formations above the main Aberdeen coal seam to be mined (see Figure 1).
Gas production begins when the long wall cutting machine reaches the area beneath the GVHs and continues for many years after the mining â€¨has finished. The drainage of this gas is an operational and economic necessity for the mine due to the danger of methane explosion. Andalex had no interest in the investment to capture, process and sell this gas. Oso acquired the mineral leases and regulatory permits and installed the infrastructure to capture, transport, process and sell this gas into a local interstate pipeline.
Due to the gas content of the coals, the conventional air ventilation system was incapable of maintaining the methane content below the required limits set by the Mining Safety and Health Administration (MSHA), the mining equivalent of the Occupational Safety & Health Administration (OSHA). The GVHs extract methane from the gob and allow the ventilation air to maintain safe methane levels in the mine while mining ensues.
The coal is extracted by the long wall mining technique, which achieves high resource recovery. The long wall panels are approximately 9000 ft long, 800 ft wide and 10 ft thick. As the panel is mined, it is known as an “active panel”, and once the mining is complete it is called a “sealed panel”. However, as previously stated, the long wall mining process releases significant volumes of methane into the mine workings. A “rider” coal seam above the mined seam also contributes to gas production. As the coal is removed from the long wall face, the roof collapses and the floor heaves, fracturing the rock in the stratigraphic column approximately â€¨360 ft above and 50 ft below the mined seam. The gas stored in this fractured zone is released and drawn into the mine workings. It is this gas that is targeted by the GVHs, with the objective of recovering high concentrations of methane before it enters the mine’s ventilation system.
Once the panel is mined and sealed, it continues to vent high-Btu methane, although at lesser rates than when the long wall panel is active. The average methane content of the gas vented from active panels is 75%, while the gas from sealed panels is 94%. These two gas streams are gathered, compressed and transported approximately five miles via pipeline to a processing plant. Oxygen, hydrogen sulphide, nitrogen and carbon dioxide are removed from the gas prior to sale. See Figures 2 and 3 for schematic drawings depicting the gathering pipelines, compression facilities and the processing plant.
During 2006, Oso installed a gathering system, field compression station and approximately five miles of 2–10 inch pipelines to the processing plant. The plant was completed in 2007. The project is currently selling gas to the local natural gas pipeline.
Gas sales reached almost 2000 Mscfd in July 2007. The startup of the nitrogen rejection unit (NRU) in mid-July 2007 allowed for the processing of high nitrogen content gas. The processing capacity of the NRU is 8000 Mscfd, and sales in this range were expected during the mining of the next active long wall panel. The expected life of the mine is at least another 11 years, with gas production expected for 18 years.
An important revenue stream expected to add significant economic value to this project is the greenhouse gas emission reduction credits, also known as voluntary emission reduction credits (VERs). The methane, which would otherwise be emitted into the atmosphere, is recovered and used as a clean-burning energy resource. As a result, the project qualifies as a verifiable greenhouse gas emissions reduction project.
A project design document (PDD) was prepared by Ruby Canyon Engineering for this project. The document describes the baseline emissions from the Tower Mine and the resulting emission reductions. The PDD addresses the issues of additionality, where it is clearly demonstrated that the project is not a conventional gas production and sales project. Many of the unique aspects of the project are described in the PDD, and a monitoring plan is outlined that describes how Oso is metering and documenting the methane emission reductions generated by the project.
Oxygen removal system
The oxygen removal system is based on the catalytic combustion of oxygen. The gas stream in this project contains a significant amount of C3+ hydrocarbons. This lowers the ignition temperatures of the system when compared to a pure methane stream. Lower ignition temperatures reduce the maximum temperature required by the system when the heat of combustion is considered constant.
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