Spontaneous combustion can be a significant problem in the coal industry, not only due to the obvious safety hazard and the potential loss of valuable assets, but also with respect to the release of gaseous pollutants, especially CO2, from uncontrolled coal fires. In this report, Lesley Sloss reviews methodologies for measuring emissions from spontaneous combustion and discusses methods for quantifying, estimating and accounting for the purpose of preparing emission inventories.
Spontaneous combustion is the unpredicted and instantaneous ignition of coal that is either still underground, in stockpiles or even in transit. Since this combustion often happens without warning and sometimes in remote locations, quantification of emissions globally or even locally is a significant challenge. Even individual fires vary in intensity both over the area of the fire and throughout the duration of the burn adding to the uncertainty in total estimates.
There are no national or international standard methods prescribed for quantifying emissions from spontaneous combustion and the published literature reflects this in the varied methodologies that are currently used. Emissions from coal fires can be monitored directly with sampling and analysis equipment similar to that used for stack monitoring. However, this only quantifies emissions at the site of monitoring and will therefore not truly represent the significant variability in emissions over a large coal fire.
Emissions can also be measured remotely using heat and IR signatures monitored by satellites. However, since this relies on generalised emission rates and limited coal seam data (such as depth) this provides a best guess rather than a true measurement. Emission factors and activity data are most often used for emission estimates. This involves either using empirical coal data to estimate emissions of pollutants such as CO2 on a g/t of coal basis (or similar) or producing a site-specific emission factor from measurements made at actual coal fires. These emission factors must then be multiplied by the total amount of coal combusted. This, in turn, must be estimated based on a best guess of how much coal is combusting at any one time during the relevant monitoring period. For underground fires, this is a significant challenge.
Carbon dioxide emissions
The most controversial issue with respect to emissions from spontaneous combustion is the potential inclusion of this data in greenhouse gas inventories. Although emissions of CO2 are known to arise from coal fires, the data available are not sufficient for coal fires to be taken into account as a source category in current climate model predictions. The IPCC (Intergovernmental Panel on Climate Change) note that emissions from this source ‘may be significant’ but also acknowledge that there are ‘no clear methods available at present to systematically measure or precisely estimate activity data.’ However, they do suggest that, where activity data (known volume of coal lost to spontaneous combustion) are known, the CO2 should be estimated on the basis of the carbon content of the coal – that is, through basic emission factor calculations. This often assumes that all the carbon in the coal is converted and that all the coal in the seam or pile is completely combusted in this way, which may give an overestimate of total emissions. In most of the countries reviewed in this study, spontaneous combustion is excluded from greenhouse gas emission inventories as there is no acceptable standard method for estimating emissions.
Uncertainties prevent standardised method
Although the published literature reflects a significant amount of work being carried out to establish reliable methods for monitoring emissions from spontaneous combustion at various sources and sites, none of these has been identified as a standardised method.
Further, since no international or national greenhouse gas inventories require the inclusion of spontaneous combustion data, there is no apparent urgency to do so. Even in the recently discussed carbon tax system proposed for Australia, emissions from spontaneous combustion were to be excluded due to uncertainties in the emission estimates. In fact, the only country considering quantifying greenhouse gas emissions from spontaneous combustion for the purpose of greenhouse gas inventories and a potential carbon tax fee is South Africa where this source is included under ‘fugitive mining emissions’ along with coalbed methane. However, it is possible that the final process could exclude emissions from spontaneous combustion if the methodology is found to be too problematic and unreliable.
To conclude, emissions of CO2 and other gases from spontaneous coal combustion may be significant on a local basis, and be a significant problem with respect to safety issues, loss of stock and so on. On a global basis, however, the overall contribution to total greenhouse gas emissions is likely to be below 3%. The sporadic, unpredictable, transient and often remote nature of these sources makes total quantification of emissions on a national or international basis a significant challenge. For the most part, this source category is recognised but excluded from international and national inventories as being too challenging to quantify with any accuracy. The uncontrolled and accidental nature of this source means that emissions are, by definition, largely uncontrollable and, other than the obvious steps which are taken to control these fires when they do occur, controlling spontaneous combustion would not be a feasible means of reducing greenhouse gas emissions under any international or national action plan.
Dr Lesley Sloss' areas of expertise include emissions and effects, ash management, mine reclamation, legislation and control, and mercury.
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Adapted to house style by Katie Woodward
Read the article online at: https://www.worldcoal.com/coal/05112013/quantifying_emissions_from_spontaneous_combustion_227/