United, located 16km south west of Singleton in New South Wales’ Hunter Valley, produces about 3.5 million tonnes of semi-soft coking coal per year. Mining has extracted five out of 10 longwall panels to date.
Since mining commenced, gas management consisted of the establishment of a series of gas wells, with a combined capacity to extract in excess of 3000 litres per second of goaf gas from the longwall operation.
Speaking at the New South Wales Minerals Council Environment and Community Conference this month, Murray said two years ago United commenced investigations into potential uses for the gas that was free vented to atmosphere.
Identified strategies included maintaining the status quo, gas flaring, and use of gas for electricity generation.
In 2005, as a result of the Xstrata Coal NSW voluntary election to the NSW Greenhouse Gas Abatement Scheme (GGAS), United commenced investigations into flaring a proportion of its methane to generate Large User Abatement Credits (LUAC’s) under the scheme.
In May 2006, Xstrata Coal released its Greenhouse Policy and entered into a cooperative agreement under Greenhouse Challenge Plus with the Federal Government. Through this program and the NSW Government’s GGAS, Xstrata Coal has committed to abating more than 350,000t of methane from underground operations.
In order to meet the commitments made under the GGAS and Greenhouse Challenge Plus, United has installed a series of flares above the active goaf of longwall 5. Each flare had the capacity to consume up to 200 litres per second of gas extracted from the goaf, abating over 125,000t of equivalent CO2 per year.
To integrate the flaring system with the active underground mining operation, Murray said United had to complete a detailed risk assessment to assess the interaction issues with the underground as well as identify what controls needed to be implemented to reduce the identified risks.
United engaged an experienced pipeline engineer to assess, review and audit the infrastructure for potential ignition sources.
Major controls installed included slam-shut valves with specified alarm parameters, flame arrestors and tube bundle sampling. A subsequent failure modes analysis was undertaken to determine the frequency of inspection and maintenance of the system.
“The major safety issue is obviously the construction of a system with an open flame over an active mining operation. The potential for ignition sources was considered to be the most significant safety risk as methane is explosive at concentrations less than 15 percent,” Murray said.
“A buffer trigger level of 30 percent has been built into the system whereby the flares shut down if greater than 30 percent methane is detected in the tube bundle monitoring system.
“Additional controls to monitor for flame and heat in the flares and pipelines running to the flares included slam-shut valves, flame arrestors, non-return valves and temperature monitoring both in the flare and on the pipeline.”
United’s flaring system uses gas extracted from predrilled boreholes into the goaf.
“The flares are similar in construction to gas plants, with the exception of a series of burners burning the gas. Gas is extracted using a blower in the flare unit and methane concentration is determined and recorded. Flow is also recorded,” Murray said.
As part of the GGAS involvement carbon needs to be monitored and measured to obtain credits. To do this two elements are monitored: methane concentration using a tube bundle system, and flow using a typical flow meter.
The system is measured in real time with readings downloaded via a program logic control to a computer every 15 minutes. Methane is recorded every minute and averaged over each hour that the flare is running, whilst flow is cumulatively measured and reported in kilolitres.
Murray said United expects to receive payback for installation costs of the project within 12 months. “The flares themselves, being off-the-shelf products, are relatively inexpensive. Implementation of safety measures and monitoring systems was the most expensive element of the project,” she said.
“Without the initiatives for management of fugitive methane emissions from coal mines through the NSW GGAS, the project would not be economical for United.
“As a subsidiary of a large electricity user, Xstrata Coal NSW, United can generate LUAC's for the company.”
Into the future Murray said the obvious extension to the project was electricity generation. “However, United's current life of mine and the inability to deliver a continuous gas makes electricity generation unattractive to electricity generators.
“Having said that, increased demand for abatement credits by electricity generators and commitments by government to reducing greenhouse gas emissions means that situations like the one at United are becoming more attractive.”