Better Roads Staff
Range of Solutions
Astec guarantees that its standard Double Barrel plant will run 50-percent RAP if the recycled material is at 5-percent moisture. The Double Barrel design superheats the virgin aggregate to temperatures well above the mix temperature of 300-325 degrees. Heat is transferred to the RAP by conduction from the virgin aggregate, says Mike Varner, director of thermal systems and research, Astec. As well, the RAP is heated by the steam explosion that occurs when the moisture in the RAP contacts the hot virgin aggregate.
In the Double Barrel, aggregate moves downhill through the dryer and is dried. A mixing chamber is created by an external stationary drum around the rotating mixing drum. The dried virgin aggregate flows behind the burner flame and drops into the mixing chamber that starts at the downhill end of the drying drum. There, metal tips – mounted to the outside of the rotating drum – mix and push the material back uphill in the annulus between the two drums. RAP is introduced shortly after the material drops into the mixing chamber, and liquid binder is added a bit later.
Varner says the Double Barrel can run high-RAP mixes for a couple of reasons. One is that the mixing chamber forms an oxygen-poor environment because the steam coming off the RAP serves as a blanket in the mixing chamber. “As a result, you have the potential for less oxidation of the mix in general by virtue of that steam blanket,” says Varner. “And, with the Double Barrel, anything that heats up in there is actually radiating energy back into the mix instead of being wasted up the stack or emitted from the drum surface. A certain portion of the heat is radiated directly into the mix. So the hottest part of the rotating drum is able to impart some of its energy to the mix.”
Advances in Design
“We’ve been working with variable-frequency drum drives off and on for 20 years with varying degrees of success,” says Astec’s Swanson. “The first effort we made was intended to help control stack gas temperatures, or baghouse temperatures, with high-RAP mixes. And finally here this year, many years later, we have ended up with something that succeeds.” The underlying principle is that to control the speed of the drum drive will help control the showering rate of the aggregate – the veil density.
What’s more, Astec changed flight designs. Over the past few years Swanson says the company has been working with a type of computer software called Discrete Element Modeling. “With this software we’re able to model showering from flights,” Swanson says. “So by trial and error, using this software, we optimized the shape of a Vee cut into the flights. We came up with a flight that would give us a pretty uniform veil, whether the flight had just a little material in it or was full to the brim.
“Once we did that, and we married that to the variable-frequency drum drive and found that as we changed the speed of the drum drive, we could change the rate of showering,” Swanson continues. “Instead of speeding the drum up, and suddenly we’ve got all the aggregate picked up and we’re right back to having a hole in the veil, the effect was that we could speed the drum up and have increased veil density.
“And conversely we could slow the drum down and put less aggregate in the air,” Swanson says. “So that turned out to be quite successful. We were able to manipulate baghouse temperatures by as much as 100 degrees using this combination.” To that combination Astec added an automatic control that takes temperature input from the baghouse inlet and adjusts drum speed to maintain that temperature to an operator-entered setpoint.
In contrast to Astec’s solution, Maxam Equipment uses a patented system to expose the RAP directly to the burner flame. Conventional wisdom, says Hawkins, says that will burn the RAP. Other plant manufacturers have implemented systems that shield the RAP from the flame. “But the analogy I use is the suntan,” says Hawkins. “If you’re getting a suntan, you need to get out in the sun. Now, when you get out in the sun, you can burn yourself. So what’s the key? You have to control your time of exposure.
“That’s exactly what Maxam does,” says Hawkins. “We don’t put any shields in. We bring our RAP in directly into the combustion zone so we expose the RAP directly to the flame. It’s not veiled through the flame, but it is in front of the flame. We bring it in through a recycle collar, and we have our own special type of flights. But the material comes out on top of those flights, not underneath the flights, like it does with another manufacturer.
“The whole key to this is to control the time of exposure,” says Hawkins. “What we do, that everybody said you can’t do, because you will burn the RAP, is expose the RAP directly to the flame. And we can do it by controlling the time of exposure. So what we do is based on the size of the drum, the angle of the drum, and we determine what the through-put time is. And then we locate our RAP collar relative to the burner so that we’re controlling the time of exposure.” Hawkins says the time of exposure is only a few seconds.
Maxam also has a different solution to the baghouse temperature problem. Hawkins says other manufacturers try to adjust flighting in the drum, or other factors, so that they can run high RAP mixes, but keep their baghouse temperature low enough so as not to destroy the bags or burn up the baghouse.
Hawkins says Maxam uses a heat recovery system, called a Maxamizer. The company’s design uses many flights for the RAP conditions. “So we have a really low stack temperature,” says Hawkins. “When we run 50-percent RAP, our stack gas temperature is only 250 degrees. That’s at least 100 degrees less than anybody else.”
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