1-13. Heating Process
The most efficient means of drying and heating the aggregate is to apply direct heat. This is accomplished with a burner
that directs the flame into the drum. Attached to the inside of the drum are step-like pieces known as flights. When the
drum rotates, the flights pick up the aggregate and drop it through the hot exhaust gas stream. An exhaust fan is used
because it significantly increases the production rate over that of a natural draft process. The materials used in the Asphalt
Mixing Plant tend to be very dusty. As they pass through the exhaust gas, which is moving at a velocity of about 800 to
1000 feet per minute, dust will be picked up and entrained in the exhaust gas. These gases are passed through a
baghouse (fabric filters) which removes most of the dust and exhausts the remainder to the atmosphere. The contents of
this exhaust are:
Water from Combustion This water is the result of the chemical reaction of combustion of the fuel with air. The
carbon and hydrogen atoms of the fuel react with the air and produce water and carbon dioxide (ideal process).
Water from Aggregate This water is water contained on the surface of the aggregate when it is placed in the bin
3.Carbon Dioxide This is the product of combustion (see 1).
Carbon Monoxide This is an unwanted product of combustion produced when the combustion process takes
place with insufficient air. This condition is known as a "rich burn" or as "less than 100% stoichiometric combustion
air". The term stoichiometric refers to the ideal air/fuel ratio to achieve complete combustion. This is further
defined in later sections of this document.
Air This is an unwanted product of combustion produced when the combustion process takes place with excess
air. This condition is known as a "lean burn" or a "more than 100% stoichiometric combustion air". The term
stoichiometric refers to the ideal air/fuel ratio to achieve complete combustion. This is further defined in later
sections of this document.
Dust This is an unwanted element of the exhaust. The baghouse is efficient but not perfect. Very small particles
are not removed and are exhausted to the atmosphere.
1-14 . Combustion Process
The simplest and most economical method of drying and heating the aggregate is with direct heat. A direct fired burner is
the most efficient means of providing the direct