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Article # 0056
AIR EMISSION SOURCES FROM A
HOT MIX ASPHALT PLANT
This article presents terminology used in the asphalt concrete manufacturing industry, describes the general process of hot mix asphalt concrete manufacture, and identifies common sources of air pollution at a typical asphalt concrete plant. Sources of air pollution are identified as Emission Point Numbers (EPN) to denote the point of release of emissions from the plant equipment. This plant uses a double-barrel rotary dryer/mixer to dry and heat aggregate such as crushed limestone. Asphalt cement is introduced at the appropriate point to maximize adherence of the cement or binder to the aggregate to form the “hot-mix” asphalt concrete product.
Asphalt or Asphalt Base – Typically the residual product of vacuum distillation of petroleum crude oil. Also known as vacuum tower bottoms (VTB), vacuum tower residual, residual, vacuum residual, residuum, and petroleum asphalt. This is the base for production of asphalt cement.
Asphalt Additives and Asphalt Modifiers – Various additives and modifiers are added to the asphalt base to produce the desired Performance Grade of Asphalt Cement. For example, polymer may be added in concentrations from 0 to 5 wt% to change the physical characteristics of the cement in terms of its durability as asphalt concrete binder.
Asphalt Cement – This is the binder portion of asphalt concrete. If the cement formulation includes polymer additives or modifiers it is commonly termed Polymer Modified Asphalt (PMA).
Asphalt Concrete - A mixture of asphalt cement with aggregate such as crushed limestone. It is commonly used as road pavement.
Reclaimed or Recycled Asphalt Pavement (RAP) – Old asphalt pavement that is recycled by breaking and crushing. It is added along with aggregate during manufacture typically at no greater that 50% aggregate displacement.
Process Description and Air Emission Sources
The purpose of a hot mix asphalt plant is to produce asphalt concrete which is a mixture of asphalt cement with aggregate. The finished product is mainly used as road pavement. The asphalt cement received is already formulated for the grade(s) of asphalt concrete to be manufactured.
The asphalt concrete manufacturing process begins as the aggregate is hauled from the storage piles (EPN STOCKPILE) and loaded into hopper bins. The aggregate received at the site will be sized and ready for use. The aggregate is transferred from the hopper onto a conveyer belt and enters the inner drum of a double-barrel rotary dryer/mixer. The inner drum rotates and heats the aggregate to reduce moisture content to promote adherence of the asphalt cement. It is equipped with conditioning and showering flights that increase heat transfer to the aggregate. The drum is sloped to move the aggregate toward the burner end wherein it falls through slots into outer mixing drum. Paddles within the outer mixing drum mix and move the aggregate upward to the other end of the rotating barrel.
The facility will also utilize Reclaimed or Recycled Asphalt Pavement (RAP) that will displace the aggregate up to 50 weight percent. The RAP enters the outer mixing drum through an inlet on top of the barrel and is mixed with the hot aggregate. Asphalt cement and water/lime slurry (as necessary) is injected to produce the asphalt concrete. The lime is an asphalt concrete anti-stripping agent and conditioner. Stripping occurs when the bond between the asphalt cement and the aggregate breaks down due to the presence of moisture, and the binder separates from the aggregate.
The exhaust of the dryer heater and particulate matter (dust) generated within the rotary drums is collected through ducting at one end of the dryer and is routed to a baghouse for emission control. The baghouse filters particulate down to an acceptable outlet grain loading. The dryer will be fired with recovered oil that meets the standard permit requirements. The dryer exhaust through the baghouse stack/vent is assigned EPN BAGHOUSEVENT.
The asphalt concrete product (or simply asphalt) leaves the bottom of the drum dropping onto a conveyor that lifts the product to fill the vertical asphalt storage silo or surge bin (EPN SILO). Asphalt concrete transport trucks are top-loaded with the product by gravity drop (EPN LOADOUT Asphalt Concrete Load-out) from the silo or surge bin. A small diesel-fired hot oil heater (EPN OILHEATER) heats an oil medium for heat transfer to the heated asphalt cement storage tanks and the asphalt concrete product storage silo.
A typical plant will utilize the following above-ground storage tanks EPN TANK1 through TANK4:
One 25,000 gallon capacity asphalt cement storage tanks
One 15,000 gallon recovered-oil fuel tank for firing drum mixer dryer heater
One 15,000 gallon diesel fuel tank for hot oil heater and diesel-engine generator
One 3000 gallon anti-strip agent tank
For this article it is assumed that all tanks were constructed after July 23, 1984. Federal New Source Performance Standards (NSPS) Subpart Kb (for facilities where construction, reconstruction, or modification commenced after July 23, 1984) does not apply to the 25,000 gallon asphalt cement and 15,000 gallon fuel tanks since they do not meet this regulations applicability requirements under 40 CFR 60.110b for capacity and true vapor pressure of material stored.
Kb does not apply to storage tanks with capacities less than 75 cubic meters (19,812 gallons) such as the 15,000-gal fuel oil and 3000-gal additive tank. Kb does not apply to storage tanks between 75 cubic meters (19,812 gallons) and 151 cubic meters (39,890 gallons) that store materials with a true vapor pressure (TVP) < 2.176 (or 15.0 kPa) at actual storage conditions of temperature and pressure such as the asphalt cement tanks. The asphalt cement TVP is much less than 15.0 kPa.
Emissions of particulate matter from the dry lime storage silo (EPN LIMESILOVENT) will be controlled by a fabric filter meeting outlet grain loading requirements. Emissions of particulate matter from the various drops of material onto conveyor belts, transfer of material into bins, screening, and RAP processing are assigned EPN MATERIALHANDLING.
A diesel-fired engine (EPN DIESELENGINE) will generate power for the plant utilities and emit products of combustion.
Hot-Asphalt Concrete Manufacturing plants have Volatile Organic Compound (VOC) emissions from sources such as asphalt cement storage tanks, fume from heating asphalt concrete in the rotary dryer and during truck load-out of the concrete product. The plant also has particulate matter emissions from material handling as well as the dryer baghouse and silo from dust formation. In addition the heaters, dryers, and engine plant will emit products of combustion form which include the regulated pollutants: nitrogen oxides, sulfur dioxides, non-combusted VOC, carbon monoxide, and particulate matter less than 10 microns
L. Bullard, P.E. is an Engineering Partner of Bullard
Environmental Consulting, Inc. He has over 20 years experience in
engineering, compliance, and permitting. Jack has a Bachelors of
in Chemical Engineering from the
Jack L. Bullard, P.E. No. 83547
Final Edition Completed December 29, 2011 using Previously Composed Material.
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