NOx Emissions from Blasting in Open Cut Coal Mining in the Hunter Valley

The Australian coal mining industry, as with other industries, is coming under greater constraints with respect to their environmental impacts. Emissions of fume forming acid gases such as NOx and SOx to the atmosphere have been regulated for many years as a result of recent advances in the understanding of environmental and health effects of these emissions. Although NOx from blasting for open cut mining may only contribute a only a small proportion of total NOx emissions, the rapid release and high concentration that may be associated with such activities may pose a health risk should the resulting plume not dissipate rapidly and subsequently drift on to the populations in the surrounding environs. Hence, there is great interest from both mine operators as well as the public as to techniques capable of monitoring and quantifying these emissions. This report presents the results of a new approach to measuring these gas emissions by scanning the resulting plume from an open cut mine blast with a miniaturised ultraviolet spectrometer. The dispersion of the resulting blast plumes was also modelled. The work presented here was undertaken in the Hunter Valley during 2006.

A portable open path spectroscopic method was found to be an effective method for measuring NO2 emissions from blasting. Overall this technique was found to be simpler, safer and more successful than other approaches that in the past have proved to be ineffective in monitoring these short lived plumes.

Quantitative measurements of CO, NO, NO2 and SO2 in plumes from blasting were made at two open cut mines showed that NO2 was present in most of the plumes but in relatively low concentrations (typically ranging between 0 and 17 ppm). The highest concentration measured during all the field campaigns was about 17 ppm at ground level. In contrast, much higher concentrations of CO, NO and SO2 were detected in the plumes and in many cases,
the levels of these gases within the blast exclusion zone were many times higher than occupational exposure standards. The approximate ratios of CO : NO : SO2 : NO2 was determined to be : 500 : 27.5 : 5.5 : 1 respectively.

High levels of hydrocarbons in the blast were measured at ground level. These compounds are believed to be derived from the diesel or fuel oil in the ANFO explosive. Further work would be required to characterise the hydrocarbons within the blast plumes.

Numerical modelling of the behaviour of plumes resulting from blasting was made to assess the possible downwind concentrations of NO2. These results were compared to ambient Nox measurements made in Muswellbrook. Modelling results were consistent with concentration measurements within the plumes at relatively short distances from the blast (i.e. up to about 1 km). Also of note is that ambient monitoring did not detect NOx events that could be
attributed to individual blasts at the Mt Arthur Coal mine. Overall the modelling suggested that these emissions would be very low at distances greater than 5 km from the blast and may be indistinguishable from background levels in most cases.

Major outcomes and findings of this project were;

• The data set compiled is believed to be the most comprehensive ever achieved under actual mining conditions. Both the range of concentration and total discharges of Nox resulting from ANFO blasts is now better quantified.
• The production of an in-depth literature review on NOx emissions from blasting operations utilising ANFO explosives. The review identified the influencing factors for NOx fume formation and highlights how the interactions of these mechanisms are still poorly understood. It also found that research studies were also confined to smaller scale experiments and revealed a dearth of field studies in this area. Investigations also identified avenues for alternative field monitoring techniques to be developed and applied.
• The characterisation of NOx emission fluxes allows for comparisons to be made against the NPI emission factor.
• The sampling regime also identified and measured other ANFO reaction products within the gaseous plumes. High concentrations of CO and NO were detected even in the absence of the highly visible NO2 plumes.
• Concentration levels for CO, NO and NO2 contained in blast fume can now be compared with national occupational health and safety standards. In some cases gas levels were found to exceed allowable limits for those particular gases. Although, it was noted that these measurements were made within the perimeter of the blast exclusion zone. It should be noted that while the concentrations were elevated, the duration that this concentration exists at a particular location were small. However, when averaged to the longer time frames, these elevated concentrations would reduce in magnitude.

As a result of this project there are a number of areas that may warrant further investigation, these include:

• Continuation of the measurement campaign. Expand the sampling regime to better capture and quantify gaseous products under different conditions using ANFO blasting agents and derivatives.
• Better characterisation of gaseous hydrocarbon emissions to identify possible toxic compounds such as PAHs and VOCs
• Potential for the development of mitigation strategies to be developed.
• Examine the current NPI emission handbook for explosives and modify the emission factor if warranted.
• Investigations into the feasibility of fence-line monitoring systems for mines operating in close proximities to populous regions.
• Formulation of an improved method for estimating concentrations downwind of blasting operations taking into account the effect of wind speed on the initial plume size and the release height of emissions due to blasting operations.
• The development of a routine testing method using advanced monitoring techniques to allow open-cut coal mines to monitor NOx emissions in-house.