Emission Factors for Fugitive Dust from Bulldozers working on a Coal Pile
A study of a crushed Powder River Basin (PRB) coal pile found that fugitive emissions from natural and human activity produced similar levels of downwind fine+coarse (i.e., smaller than 10 µm, or PM10) particle mass concentrations. Natural impacts were statistically removed from downwind measurements to estimate emission factors (EFs) for bulldozers working on the pile. The study-derived bulldozer EF (denoted Ev) was determined to be similar in magnitude to EFs computed using an Environmental Protection Agency (EPA) formulation for unpaved surfaces at industrial sites. Even though the EPA AP-42 Emissions Handbook EF formulation was not based on field data specifically for coal piles, it is commonly applied to such piles for air emissions estimates. The field-derived and AP-42 EFs are similar but the latter formulation does not account for the influence of coal moisture on emissions. Our data indicate that the relationship between PRB coal fugitive dust Ev (kg of PM10 emitted per minute of bulldozer operation) and coal moisture content Mc (in percent) is best expressed as
where f(Mc) is a function of moisture. This function can be determined by statistical regression between log10(Ev) and Mc where both Ev and Mc are expressed as daily averages of observed hourly values. Linear, quadratic and cubic functions were investigated for f(Mc) with only slight variations in the degree of fit among these options. We opted to use the quadratic fit:
At lower Mc we recommend using a constant Ev =0.96 kg min-1 to avoid estimating decreasing Ev for Mc <17.67 percent (no measurements were made with Mc this low and PRB coal data suggest that 15 percent is a natural lower limit to Mc). The quadratic expression of f(Mc) also yields vanishingly small values of Ev for Mc >30 percent when the mixture of water and crushed coal takes on the feel and appearance of damp concrete. This relationship is consistent with the reputation of PRB coal as being both dusty and hydrophilic. The advantage of this empirical approach to determining Ev is that it includes the effect of moisture on emissions unlike the AP-42 formulation. For hot and dry conditions the formulation presented here produces results similar to those from AP-42 but, in locations that are cooler and/or wetter, estimates of Ev can be quite a bit lower. In addition, having a method for estimating natural variations in Ev allows for emission estimates in situations where it may be necessary to model fugitive emissions. An Ev calculation methodology is presented that accounts for daily variation in Mc and can be applied using readily available meteorological data.