Uncertainty assessment

EMISSION FACTOR UNCERTAINTIES Emission Factors for Tiers 1 and 2

The major sources of uncertainty for a Tier 1 approach arise from two sources. These are:

• The applicability of global emission factors to individual countries

• Inherent uncertainties in the emission factors themselves

The uncertainty due to the first point above is difficult to quantify, but could be significant. The inherent uncertainty in the emission factor is also difficult to quantify because of natural variability within the same coal region is known to occur.

For a Tier 2 approach, the same broad comments apply, although basin-specific data will reduce the inherent uncertainty in the Emission Factor compared with a Tier 1 approach. With regard to the inherent variability in the Emission Factor, 'Expert Judgement' in the Good Practice Guidance (2000) suggested that this was likely to be at least ±50 percent.

Table 4.1.2 shows the Tier 1 and Tier 2 uncertainties associated with emissions from underground coal mining. The uncertainties for these Tiers are based on expert judgement.

Table 4.1.2

Estimates of uncertainty for underground mining for tier 1 and tier 2 approaches

Likely uncertainties of coal mine methane Emission factors ( Expert judgment - GPG, 2000* )

Method

Mining

Post-Mining

Tier 2

± 50-75%

± 50%

Tier 1

Factor of 2 greater or smaller

Factor of 3 greater or smaller

*GPG, 2000 IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

(2000)

Methane emissions from underground mines have a significant natural variability due to variations in the rate of mining and drainage of gas. For instance, the gas liberated by longwall mining can vary by a factor of up to two during the life of a longwall panel. Frequent measurements of underground mine emissions can account for such variability and also reduce the intrinsic errors in the measurement techniques. As emissions vary over the course of a year due to variations in coal production rate and associated drainage, good practice is to collect measurement data as frequently as practical, preferably biweekly or monthly to smooth out variations. Daily measurements would ensure a higher quality estimate. Continuous monitoring of emissions represents the highest stage of emission monitoring, and is implemented in some modern longwall mines.

Spot measurements of methane concentration in ventilation air are probably accurate to ±20 percent depending on the equipment used. Time series data or repeat measurements will significantly reduce the uncertainty of annual emissions to ±5 percent for continuous monitoring, and 10-15 percent for monitoring conducted every two weeks. Ventilation airflows are usually fairly accurately known (±2 percent). When combining the inaccuracies in emissions concentration measurements with the imprecision due to measurement and calculation of instantaneous measurements, overall emissions for an individual mine may be under-represented by as much as 10 percent or over-represented by as much as 30 percent (Mutmansky and Wang, 2000).

Spot measurement of methane concentration in drained gas (from degasification systems) is likely to be accurate to ±2 percent because of its higher concentration. Measurements should be made with a frequency comparable to those for ventilation air to obtain representative sampling. Measured degasification flowrates are probably known to be ±5 percent. Degasification flowrates that are estimated based on gas sales are also likely to have an uncertainty of at least ±5 percent due to the tolerances in pipeline gas quality.

For a single longwall operation, with continuous or daily emission measurements, the accuracy of monthly or annual average emissions data is probably ±5 percent. The accuracy of spot measurements performed every two weeks is ±10 percent, at 3-monthly intervals: ±30 percent. Aggregating emissions from mines based on the less frequent type of measurement procedures will reduce the uncertainty caused by fluctuations in gas production. However, as fugitive emissions are often dominated by contributions from only a small number of mines, it is difficult to estimate the extent of this improvement.

The uncertainty estimates for underground mines are shown in Table 4.1.3.

Table 4.1.3

Estimates of uncertainty for underground coal mining for a tier 3 approach

Source

Details

Uncertainty

Reference

Drainage gas

Spot measurements of CH4 for drainage gas

i 2%

Expert judgment (GPG, 2000* )

Degasification flows

i 5%

Expert judgment (GPG, 2000)

Ventilation gas

Continuous or daily measurements

i 5%

Expert judgment (GPG, 2000)

Spot measurements every 2 weeks

i 10%

Mutmansky and Wang, 2000

Spot measurements every 3 months

i 30%

Mutmansky and Wang, 2000

GPG, 2000 - IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (2000)

ACTIVITY DATA UNCERTAINTIES

ACTIVITY DATA UNCERTAINTIES

Coal production: Country-specific tonnages are likely to be known to 1-2 percent, but if raw coal data are not available, then the uncertainty will increase to about ±5 percent, when converting from saleable coal production data. The data are also influenced by moisture content, which is usually present at levels between 5-10 percent, and may not be determined with great accuracy.

Apart from measurement uncertainty, there can be further uncertainties introduced by the nature of the statistical databases that are not considered here. In countries with a mix of regulated and unregulated mines, activity data may have an uncertainty of ±10 percent

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