Hot mix asphalt is a proportioned mixture of dried aggregate and liquid asphaltic concrete cement used as roadway paving material. Aggregate is a graded mixture of crushed stone, sand and recycled asphalt paving. Liquid asphaltic concrete cement is a mixture of hundreds of organic compounds that remain after crude oil refining. There more than 150 asphalt plants in North Carolina with DAQ Air Quality Permits.
How does an asphalt plant work?
There are two types of asphalt plants, batch mix and drum mix. In a drum mix facility, undried aggregate and heated asphalt are placed directly into the rotary drum dryer, where they are mixed. The asphalt produced can be stored on site in heated storage silos or loaded directly into trucks and transported off-site. In a batch mix facility, the aggregate is dried separately in a rotary drum dryer and stored in heated bins. Heated aggregate measured per batch in a weigh box and a proportional amount of heated asphalt are mixed in a pugmill and either stored on-site in heated storage silos or loaded directly into trucks for transport off-site.
How are asphalt plants regulated by DAQ?
- All asphalt plants must obtain an air quality permit.
- Asphalt plants are subject to state regulations for criteria pollutant emissions of particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide and volatile organic compounds. Most asphalt plant air quality permits include production limits to avoid applicability of Prevention of Significant Deterioration (PSD) rules.
- A DAQ currently active asphalt plant permit reflects what specific equipment can be operated at a specific location. As asphalt plants consist of portable equipment modules, an asphalt plant may be moved and operated at numerous permitted locations depending on paving contracts.
- In November 1999 DAQ issued an asphalt plant permitting policy, which requires new and modified asphalt plant applications to quantify all 97 Toxic Air Pollutants (TAPs) emitted to determine the need for air toxics permit limits using EPA AP-42 emissions.
- If the emissions of a specific TAP are below their regulatory threshold in NC Regulation 15A NCAC 2Q.0711, an air quality permit is not required.
- If the TAP emissions exceed its threshold, a dispersion modeling demonstration must be performed. The results of this model must show that the emissions are below the acceptable ambient level (AAL) listed in NC Regulation 15A NCAC 2D.1104, and air quality permit emission limit, for the respective TAP not to exceed the AAL, is required.
- Air pollutant emissions from asphalt plants can be calculated using the DAQ Asphalt Plant Spreadsheet (.xls).
- In August 13, 2013 DAQ issued the Emission Testing Frequency Policy, which establishes the baseline testing frequency of 120 months, unless other factors require more frequent testing schedule.
Why are there are so many asphalt plants?
North Carolina has the second largest state-maintained highway system in the United States. The state has about 80,0001 miles of roads, with more under construction every year. In addition, roads generally need resurfacing every 12 to 15 years, so about 4,400 miles of roads are repaved each year. Paving is difficult at lower temperatures, and highway contractors must reject asphalt that is not hot enough (at least 250oF). That means asphalt plants must be located fairly close to road construction sites.
What TAPs do asphalt plants emit?
Toxic air pollutants compounds emitted from asphalt plants include polycyclic aromatic compounds, volatile organic compounds, metals and hydrogen sulfide.
|Toxic air pollutants (TAPs) emitted from asphalt plant2|
|Toxic Air Pollutant||Emitted from
drum dryer and
hot oil heater
|Trichlorofluoromethane (CFC 111)||no||yes|
|Methyl ethyl ketone||yes||yes|
|Soluble Chromate Compounds, as Chromium (VI)||yes||no|
|Manganese & compounds||yes||no|
|Nickel & Compounds||yes||no|
|Arsenic & Compounds||yes||no|
|Hydrogen Chloride (hydrochloric acid)||yes||no|
Who controls where asphalt plants are located?
In reviewing air quality permits for asphalt plants, the Division of Air Quality must ensure that applicants comply with local zoning, and each permit contains a condition stating that the plant must meet these requirements. In North Carolina, local governments are responsible for regulating such land use matters, and they have the final authority over the construction of new plants through the issuance of building permits. However, many counties and municipalities, particularly in rural areas, have not adopted zoning or land use controls. The DAQ bases its permitting decisions on whether plants can meet air quality regulations.
What federal rules apply to asphalt plants?
- Asphalt plant emissions of particulate matter (PM2.5 and PM10, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead must not exceed National Ambient Air Quality Standards (NAAQS) at the property boundary.
- Asphalt plants manufactured after June 11, 1973, are subject to 40 CFR 60 Subpart I-New Source Performance Standards for Hot Mix Asphalt Plants. NSPS, Subpart I limits only the emissions of particulate matter from material handling systems.
- On November 8, 2002, USEPA removed Asphalt Hot Mix Production from the Source Category List for which development of National Emission Standards for Hazardous Air Pollutants Standards is required.
Emissions Inventories: Specific Industry Guidance
2 Memorandum, Procedures for Permitting and Estimating Emissions from Asphalt Plants, Laura S. Butler, (November 18, 1999)