Concrete Batch plants4 

Particulate matter, consisting primarily of cement dust but including some aggregate and sand 

dust emissions, is the only pollutant of concern fiom concrete batch operations. All but one of 

the emission points are fugitive in nature. The only point source is the transfer of cement to the 

silo, and this is usually vented to a fabric filter or "sock." 

Fugitive sources include the transfer of sand and aggregate, truck loading, mixer loading, vehicle 

traffic, and wind erosion fiom sand and aggregate storage piles. The amount of fugitive 

emissions generated during the transfer of sand and aggregate depends primarily on the surface 

moisture content of these materials. The extent of fugitive emission control varies widely fiom 

plant to plant. Types of controls used may include water sprays, enclosures, hoods, curtains, 

shrouds, movable and telescoping chutes, and the like. A major source of potential emissions, the 

movement of heavy trucks over unpaved or dusty dams in and around the plant, can be 

controlled by good maintenance and wetting of the road surface. 

Hot Mix ~sphalt~ 

As with most sources in the mineral products industry, batch mix HMA plants have two major 

categories of emissions: ducted sources (those vented to the atmosphere through some type of 

stack, vent, or pipe), and fugitive sources (those not confined to ducts and vents but emitted 

directly fiom the source to the ambient air). Ducted emissions are usually collected and 

transported by a. industrial ventilation system having one or more fans or air movers, eventually 

to be emitted to the atmosphere through some type of stack Fugitive emissions redt fiom 

process and open sources and consist of a combination of gaseous pollutants and PM. 

The most significant ducted source of emissions from batch mix HMA plants is the rotary dnun 

dryer. Emissions fiom the drum consist of water (as steam evaporated fiom the aggregate); PM; 

products of combustion (carbon dioxide [Ca], nitrogen oxides [NOx], and sulfUr oxides [S&]); 

carbon monoxide (CO); and small amounts of organic compounds of various species (including 

volatile organic compounds [VOC], methane LC&], and hazardous air pollutants @MI). The 

organic compound emissions result fiom incomplete combustion of the fuel and from heating 

and mixing of the liquid asphalt cement inside the drum. The CO emissions result fiom 

incomplete combustion of the fuel. Processing of RAP materials may increase organic compound 

emissions because of an increase in mixing zone temperature during processing, Once the 

organic compounds cool after discharge fiom the process stack, some condense to form a fine 

organic aerosol or "blue smoke" plume. Because liquid asphalt cement, aggregate, and 

sometimes RAP, are mixed in a zone not in contact with the hot exhaust gas stream, counterflow 

drum mix plants will likely have lower organic compound emissions than parallel flow drum mix 

plants. 

Other potential process sources include the hot-side conveying, classifj.ing, and mixing 

equipment, which are vented to either the primary dust collector (along with the dryer gas) or to 

a separate dust collection system. The vents and enclosures that collect emissions fiom these 

sources m commonly called "fugitive air" or "scavenger" systems. The scavenger system may 

or may not have its own separate air mover device, depending on the particular facility. The 

NIUMProc TSD 6-14-02 Created on 6/38/2002 9:39:00 AM 7