Then when the Maxam plant switches over to virgin mix, now stack temperature is too low. “So what do we do?” asks Hawkins. “We have a burner that mounts in the ductwork of the dryer – and this is also patented –that burner senses that temperature and sets the temperature at whatever it needs for the minimum baghouse temperature. We have one burner that controls mix temperature, and one burner that controls baghouse temperature. And we can control them all very accurately.”

Tarmac International designs its counterflow dryer to introduce RAP in the tailing one-third of the burner flame. “We enter the RAP downhill and under the coolest part of the flame,” says Stephen Latenser, an account manager for Tarmac. “Depending on the size of the drum, that’s about one-third the length of the burner flame and next to the burner head.”

Tarmac asphalt plants heat the RAP by conduction from superheated virgin aggregate and radiant heat from the flame of the burner. “You don’t really want to heat your RAP through the convection in the air, where blue smoke can be created,” Latenser says. “The convection section of the dryer is where you get your virgin rock up to 500 or 600 degrees. If you got your recycled product up that high, you would oxidize the RAP binder and cause an emission problem.”

On a Tarmac plant, virgin binder is added behind the burner, in the mixing zone. “By the time the RAP has traveled from under the burner and into the mixing zone the RAP has had time to increase in temperature while in contact with the superheated virgin rock,” says Latenser. “So as soon as virgin aggregate touches RAP, we consider that part of the RAP heating zone.”