Generators (Electrical Generating Units)
Most mining operations include the utilization of an electrical generator to supply electricity to equipment such as pumps, conveyors, and crushers/screen plants. The generators typically combust diesel fuel. The engines usually have very short exhaust stacks and could have relatively high potential emissions if operated 24 hours per day, 7 days a week. However, most units do not operate year round and when operated at a location within the mine within a timeframe established by the operator and WDNR, will attain and maintain ambient air standards.
Depending on the use and portable status of the units, several regulations (Federal and State) may apply, which would further minimize pollution and ambient air impacts. One common best management practice (BMP) to meet these standards is the firing of ultralow sulfur diesel fuel, although this is not required by Federal law. Emissions from such units are included in the WDNR’s air dispersion analysis (see section 5.1.3), which is used to establish permit requirements to assure attainment of ambient air standards.
Conveyors are used throughout the mine and processing plant. They are used to transport sand short distances to different operations on the sites or to stockpile material. Sand conveyed from the active mining area to storage piles is typically wet and would not require any further BMPs. However, sand conveyed from the storage piles to further processing (transfer to dryers) is typically dry and would require fugitive dust minimization practices.
Air pollution resulting from this activity would include fugitive dust (particulate). Operators would be required to maintain and follow a fugitive dust prevention plan, whereby methods to minimize fugitive dust emissions resulting from the conveyors would be described and followed. Some conveyors at larger operations would also be subject to a visible emission limitation (visible dust plume), thereby potentially making the fugitive dust prevention practices more stringent or requiring utilization of better controls (e.g., covering of conveyors).
Prior to sand being sized and stored as a final product, it typically goes through a drying process to reduce the moisture content. Sand is brought from stockpiles to the dryer via conveyors. The dryers operate on natural gas (or propane fuel as backup) and heat the sand to evaporate water. Emissions from the drying process are typically controlled by some mechanical collector (cyclone or baghouse), reducing particulate matter exhausted through a stack. The dried sand is then fed by conveyors to storage bins or directly to a screen house via conveyors.
Air pollution resulting from this activity would include combustion emissions and particulate. Combustion emissions are minimized by firing clean burning fuels such as natural gas or propane. Resulting particulate (mainly sand and very small quantities of combustion particulate) from the drying process is typically controlled by the use of a cyclone or baghouse. These devices are able to achieve a control efficiency of at least 95% or better (some baghouses can achieve 99.5% or better control). Collected materials in the baghouse will be disposed of at the mine site as fines or reject material.
Emissions from the dryer are subject to the new source performance standards (NSPS) in s. NR 440.73, Wis. Adm. Code. Particulate matter and PM10 (particles smaller than 10 microns) emissions from the drying process are limited to 0.057 grams per dry standard cubic meter (g/dscm), according to s. NR 440.73(3)(a), Wis. Adm. Code. Furthermore, emissions are also subject to a visible emissions limit of 10 percent opacity, per s. NR 440.73(3)(b), Wis. Adm. Code. Typically by complying with the particulate matter limit and utilization of a control technology, the visible emission limitation will be met.
However, some facilities will also be required to either utilize a continuous opacity monitoring system to measure and record the opacity of emissions discharged, or have a certified visible emissions observer measure and record 3-6 minute averages of the opacity of visible emissions to the atmosphere each day of operation.
Sand is transferred from dry storage bins or directly from the dryer and then passed through vibrating screens. The sand is screened into one of several grades (sizes) and then conveyed to storage or to trucks for shipping. The screen house may contain the following pick-up points (dust collection points): bucket elevators, screens, and conveyors. The pick-up points within the screening area are typically routed to a mechanical control device.
Air pollution resulting from this activity would include particulate, stacked and/or fugitive. Resulting particulate from the screening process is typically controlled by the use of a cyclone or baghouse. Some facilities enclose the screening operation within a building, further minimizing fugitive emissions from the pick-up points.
The screening process may be subject to the NSPS in s. NR 440.688, Wis. Adm. Code if the processing plant has a capacity greater than 25 tons per hour. The NSPS applies to each crusher, grinding mill, screening operation, bucket elevator, belt conveyor, bagging operation, storage bin, enclosed truck or railcar loading station, per s. NR 440.688(1), Wis. Adm. Code. The NSPS provides limitations on visible emissions (opacity) of no greater than 7 percent.
Operators would be required to maintain and follow a fugitive dust prevention plan, whereby methods to minimize fugitive dust emissions resulting from the screening would be described and followed.
Storage bins or silos are located throughout the processing plant and are utilized to store raw materials or final product. Materials or product are transferred to these devices via conveyors.
Air pollution resulting from this activity would include particulate, stacked and/or fugitive. Resulting particulate from the storage (loading of bins/silos) process could be controlled by the use of a cyclone or baghouse, and the bins/silos may be equipped with an air displacement vent filter. Operators may be required to maintain and follow a fugitive dust prevention plan for any fugitive emissions, whereby methods to minimize fugitive dust emissions resulting from the storage activities would be described and followed.
The storage bins/silos may be subject to the NSPS in s. NR 440.688, Wis. Adm. Code, if the processing plant has a capacity greater than 25 tons per hour.
Loading/Unloading – Processing Plant Operations
A processing plant located at a mine will not have unloading operations, whereas a processing plant that is located in a different location than the mine(s) will have truck or rail unloading of raw materials. The processing plant will have loading operations regardless of its location relative to a mine. Unloading operations typically consist of a dump station that may be enclosed to capture most fugitive emissions. Unloaded sand is dumped into an auger or conveying system which transports the sand to storage piles or bins/silos. Loading operations typically consist of a conveyor system (from storage bins/silos) to a spout over trucks or railcars. The conveyor system and spout may be enclosed to capture particulate or minimize fugitive particulate dust.
Air pollution resulting from this activity would include particulate matter from either stacks or fugitive sources. Resulting particulate from the loading and unloading processes could be controlled by the use of a cyclone or baghouse, or unloading processes through underground or covered conveyor systems. Operators may be required to maintain and follow a fugitive dust prevention plan for any fugitive emissions, whereby methods to minimize fugitive dust emissions resulting from the loading/unloading activities would be described and followed.
Potential Emissions, Ambient Air Dispersion Modeling and Risk Analysis
The WDNR uses dispersion modeling to evaluate the ambient air impact of air pollution sources. The following is a brief description of how the modeling process works. A model is a mathematical simulation, designed to predict what can or will happen in realworld scenarios. Atmospheric dispersion modeling is useful in predicting the impact a
particular facility will have with respect to a given pollutant. The major benefit of dispersion modeling is that it is an inexpensive way to assess the impact of a source. This information is vital in assessing a facility’s compliance with respect to the National and State Ambient Air Quality Standards (NAAQS) and increments as well as the various Hazardous Air Pollutant (HAP) standards, both federal and state. Dispersion modeling incorporates information about a facility, such as source/stack parameters, facility layout information and emission rates, along with 5 years of meteorological data in order to predict concentrations of pollutants in the vicinity of the facility. The point of highest impact is determined through the use of a receptor grid that is set up by the modeler, and could be the result of (besides other factors) inversions. The pollutant concentration at the point of highest impact is added to a previously determined background concentration and then is compared to the corresponding ambient air quality standard. The emissions from the facility (and nearby sources that contribute to impacts) must attain and maintain the NAAQS, which are set to protect public health and welfare, in order for any permit to be considered approvable by the WDNR. Those standards (NAAQS) are set at levels such that the most susceptible populations (children, elderly, and people with respiratory conditions) are protected.
All modeling completed in the State of Wisconsin for use by WDNR is conducted in accordance with these WDNR procedures as well as guidance contained in the Guideline on Air Quality Models, EPA document 40 CFR part 51, Appendix W. The present EPA approved dispersion model is AERMOD. This model is used for all dispersion modeling conducted for or by the WDNR.
The air quality analysis (air dispersion modeling) uses the worse-case maximum potential emissions from the facility. Those emissions are based on several factors, including: fuel type and characteristics, emission factors, operational design and control equipment, and any enforceable operational and/or emission limitations. The conditions that demonstrate compliance with the NAAQS will be set in the air pollution control permit as enforceable emission limits, control device operations, operational parameters (fuel type and amounts used), among other requirements. Any future expansion or increase in production or combustion sought by the facility, above what may already be approved in a permit, may result in a new air pollution control permitting action, which would again analyze all aspects of compliance with all air pollution rules and regulations.
Ambient Air Dispersion Modeling for Mining and Processing Operations
Depending on project specific conditions and proposals, an air analysis may include analysis of point (stack) and fugitive sources, soil and vegetation impacts, or visibility impairment.
Fugitive based particulate emissions, including PM10 and PM2.5, from truck traffic onsite may be included in the model as a volume source.
Any facility emitting SO2, PM/PM10, and/or NOX may have a potential adverse impact on visibility through atmospheric discoloration or reduction of visual range due to increased haze. The Clean Air Act Amendments require evaluation of visibility impairment in the vicinity of PSD Class I area due to emissions from new or modified air pollution sources. (Note: A Class I area is an area that is afforded additional protect under the Clean Air Act from the impacts of air pollution. National Parks, National Wilderness Areas and National Monuments are all designated as Class I areas.) If a PSD Class I area is located within 100 kilometers of the site, visibility impacts on distant Class I areas will be assessed.
Near the proposed project site, under certain meteorological conditions, the stacks will emit a visible steam plume that, after traveling a relatively short distance, will dissipate by dispersion and evaporation. A visible steam plume can be expected to occur when ambient air temperatures are relatively low with respect to plume temperature, thus promoting plume cooling and condensation, and when ambient humidity levels are relatively high, preventing evaporation of the water in the plume. The persistence of the plume is dependent upon wind speed and the time required for evaporation.
An ‘Air Dispersion Analysis’ Correspondence/Memorandum is generated for each project in order to demonstrate the impact of the proposed project on State or Federal ambient air quality standards. The WDNR may not issue an air pollution control permit to a facility that can’t demonstrate attainment and maintaining ambient air quality standards. Assuming the results of the modeling analysis demonstrate that the primary standard for the listed pollutants will be met, the health of “sensitive” populations such as asthmatics, children, and the elderly will be protected. Additionally, the welfare of the public is also protected, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings.
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