Assessing Measurement Procedures for Estimating Greenhouse Gas Emissions from Underground Coal Mining

With the introduction of the National Greenhouse and Energy Reporting Act 2007, underground coal mines are now required to report their annual fugitive greenhouse emissions to the Department of Climate Change. Emission estimates are to be made in accordance with the prescribed methods of the National Greenhouse and Energy Reporting (Measurement) Determination 2008. To determine if there are any aspects of this methodology that may affect its implementation at individual mine sites, a review of the Determination was undertaken and compared to current practices at seven underground mines operating in NSW.   The methods described in the Determination include procedures for accounting for all sources of fugitive emissions from underground coal mines:

• CH4 and CO2 released during extraction of the coal (including gas drainage);
• flaring of methane and
• post-mining release gas.

The Determination allows emissions to be estimated using a Tier 2 method by applying generic emission factors. This method is to be used in the absence of actual emission data. However, a Tier 3 method (Method 4) which uses directly measured data is also provided and has the potential to yield much more accurate emission estimates. Method 4 draws on a number of Australian and international standards that provide specific details on how measurements are to be made. The ventilation data (i.e. flow rates and CO2 and CH4 concentrations) already measured by underground mines for safety reasons are broadly consistent with the data required under Method 4, but at present the specific procedural details of the standard methods are not followed. Emission estimates based on these measurements are probably subject to uncertainties of the order of ±20 to 30 %, due largely to errors associated with measuring the ventilation air flow. Nevertheless, these data can be used for emission estimate purposes. This is recognised in the Explanatory Note that accompanies the Determination which states:

“It is intended that, as a minimum, existing practices for the monitoring of gases at underground mines may be maintained as input into the emissions estimation process under Method 4.”

The standard methods referred to in the Determination are designed to minimise the uncertainty of the measurements but it is apparent that there are a number of practical problems that complicate the application of these standards to many mine sites. Of particular concern are the requirements for measuring ventilation air flows due to restricted access to upcast shafts. Continuous measurements are likely to reduce the uncertainty of emission estimates but the differential pressure devices currently used to measure ventilation air flows are not sufficiently accurate for accounting purposes.

Further work needs to be done to improve the reliability and accuracy of continuous flow measurements to reduce the uncertainty of fugitive emissions estimates while at the same time maintaining minimum intrinsic safety requirements. In particular, the following areas warrant attention:

• Characterisation of the flow profile of the ventilation air in the upcast shaft (or shafts).
• Identification of suitable locations to measure ventilation air flow accurately.
• The minimum number of sample points required for accurate measurements of ventilation air flow and composition needs to be established.
• The feasibility of using continuous flow measuring devices for measuring ventilation air flow should be investigated (e.g. ultrasonic flow sensors, differential pressure devices etc.).
• The use other standards apart from those cited in the Determination that may be applicable to measuring ventilation and drainage gas emissions should be examined.