Emission factors are the fundamental basis for the calculation and estimation of the air emissions generated by a source or an activity. Once established, the emission factors for a source are subsequently used to establish emission limits or operating conditions as part of a facility’s construction and operating permits. The U.S. Environmental Protection Agency (EPA) has developed and published emission factors for various industries and sources. These emission factors are documented in the EPA publication Compilation of Air Pollutant Emission Factors (AP-42)1 and also in the EPA Clearinghouse for Inventories and Emission Factors (CHIEF)2.
However, since the inception of AP-42, emission factors have generally not remained up-to-date with improvements and changes to emission source quantification technology. Stack testing methods provide supporting data for conventional stacks but are often limited when sampling conditions are not ideal or the source is not an enclosed stack. Theoretical calculations are sometimes possible for particular sources but often provide exceedingly conservative results as the specific situation is overly simplified. To improve the representation of these difficult-to-measure sources, customized ambient real-time measurements can be combined with modeling as a strategy to establish a more representative emission rate.
Mechanical Evaporator Deployment
Many states restrict the discharge of wastewater from a facility, whether this is zero discharge or a significant treatment of the effluent. Some facilities therefore adopt natural and mechanical evaporation techniques from evaporative ponds to reduce the volume of water onsite. The use of mechanical evaporators is increasing, particularly in the power generation industry as plants are dehydrating ash ponds in response to the “Disposal of Coal Combustion Residuals (CCR) from Electric Utilities” rule3. Other industries also utilize mechanical evaporation to reduce wastewater volumes from cooling towers, produced water, and storm water runoff.
Mechanical evaporators are typically located on the banks of, or floating on, an evaporative pond. The mechanical evaporator and pond system produce emissions of particulate matter (PM) as the pond water solution is drawn through the evaporator and is sprayed over the pond and allowed to evaporate and crystallize into solid particles. The characteristics of the resulting particles depend on the ambient conditions and the pond’s total dissolved solids (TDS) concentration. Due to both the typical location of evaporators and the change in phase of the emissions from liquid solution droplets to solid PM, emissions from these sources can be difficult to quantify.
Emission Rates from Theoretical Calculations
Due to the large variety of industries that operate mechanical evaporators, accurate application-specific emission rates are not widely available. Basic calculations are heavily dependent on the pond’s TDS concentration and have traditionally used emission factors for cooling towers such as those described in Section 13.4 of AP-42, or improved calculation methods discussed in Reisman and Frisbie4 or Hosler et al5. However, these methods often lead to significantly conservative results. To achieve a more representative estimate of the mechanical evaporator emission rate through calculations, a complex particle dynamics model is required. These advanced methods utilize additional “real world” particle and droplet dynamic processes and must also consider meteorological effects and potential PM plume characteristics for sources situated within the evaporative pond system. The combination of these processes yields a comprehensive theoretical model that is as close to the actual processes occurring in the atmosphere as possible. However, this model will likely still have limitations, as it will be inhibited by not including processes such as droplet collision and coalescence, which can influence the transport of droplets and PM from the source.
Emission Rates from Monitoring Techniques
Due to the limitations of theoretical calculations, it can be more representative to directly measure PM concentrations generated by mechanical evaporators in situ and then subsequently reverse- model the results to find an emission rate for the evaporator. Using a methodology that directly measures the PM concentration from the mechanical evaporator at a downwind distance or at the edge of the evaporation pond system takes into account the complexities of the droplet and particle interactions, along with the variability in concentrations due to the meteorology and operations. The customized measurement strategy is critical to this monitoring technique, with measurements of the aerosol size distribution being conducted both upwind and downwind of the source. Using both upwind and downwind monitors allows the PM concentration associated with the mechanical evaporator during the monitoring period to be determined.
The measured concentration data are then used as a model input along with the exact meteorological conditions occurring at the time of the PM concentration measurement. The reverse air dispersion modeling is then executed to determine the site-specific emission rate emitted from the mechanical evaporator based on operational setup, pond TDS concentration,PM concentrations, and meteorology.
Based on emission factor studies conducted in the United States for evaporation ponds with TDS concentrations ranging from 12,000 ppm to over 100,000 ppm, mechanical evaporator emission rates determined using monitoring techniques were significantly lower than rates derived using theoretical calculations. The improved representativeness of the PM emission rates not only is beneficial for complying with air quality standards, but also allows facilities to add additional evaporation capacity due to the lower emission factors.
State Agency Review
The New Mexico Environment Department (NMED) and Arizona Department of Environmental Quality (ADEQ) have reviewed and accepted site-specific, customized emission factors for mechanical evaporators that were developed using monitoring methods. In the summer of 2017, a similar study will also be conducted and reviewed by Maricopa County (Arizona) Air Quality Department.
Emission factors are the fundamental basis for quantifying a source’s emissions. It is therefore critical to have a representative emission factor for all sources, including fugitive and difficult-to-quantify sources such as mechanical evaporators. Customized studies for a number of mechanical evaporator units in different industries have produced results demonstrating that the emission factor estimated from theoretical calculations significantly over-estimates the actual emission factor as derived through the combined monitoring and modeling technique discussed here. Air permit applications developed using the results of this technique have been submitted to the NMED and ADEQ for review and have been approved for use in future permitting actions as a method of quantifying quantifying emissions from onsite mechanical evaporators.
1 U.S. Environmental Protection Agency, “Compilation of Air Pollutant Emission Factors (AP-42)”, Fifth Edition, January 1995.
2 Source: https://www.epa.gov/chief
3 Source: 40 CFR 257 and 261 – Criteria for Classification of Solid Waste Disposal Facilities and Practices; Identification and Listing of Hazardous Waste.
4 Reisman, J, Frisbie, G, “Calculating Realistic PM10 Emissions from Cooling Towers”, Environmental Process & Sustainable Energy, July 2002.
5 Hosler, C.L, Pena, J, Pena, R, “Determination of Salt Deposition Rates from Drift from Evaporative Cooling Towers”, Journal of Engineering for Power, July 1974.