Don't hold your breath

Maybe no early recovery, but fumes pose no major hazard

Asphalt Article January 21, 2002
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Although the market for hot-mix asphalt pavement material stayed steady in 2001, sales of new hot-mix asphalt plants declined 1


Although the market for hot-mix asphalt pavement material stayed steady in 2001, sales of new hot-mix asphalt plants declined 10-20%. A particularly strong market for expansion and upgrades to existing plants balanced the dip, however, according to Roger Sandberg, vice president for technology and market development at the National Asphalt Pavement Association (NAPA). Sandberg attributed the trend to uncertainty about highway funding, which has just been resolved with the passage of the largest highway appropriations bill in history.


"Evidence that the economy is turning around, progress on TEA-21 reauthorization and stable oil prices will help get the plant market back to the levels of two years ago," Sandberg told ROADS & BRIDGES. "An economic stimulus, such as an investment tax credit or accelerated depreciation, would have a very positive impact on this market."


Unfortunately, on Dec. 20, The New York Times reported that hopes of passing an economic stimulus package before Congress recessed for the holidays were dead. In the absence of a stimulus, some economists were downgrading the prospects for an early recovery in business activity and job creation. The newspaper reported that Democrats and Republicans in Congress had agreed on several measures but stalled over how to provide health insurance benefits to people who lose their jobs.


As for the hot-mix asphalt (HMA) itself, the product spectrum continues to get wider and more sophisticated.


"The trend with plants is for higher production rates, more surge and storage silos and more cold-feed bins," Sandberg said. "It is not unusual to see six to 10 cold-feed bins, where in the past four to six seemed adequate. This is in response to more sophisticated mixes and aggregate control. There is also a trend toward multiple AC [asphalt cement] tanks and larger storage capacity, in response to the use of more types of asphalt cements and varied specifications."


With the increasing complexity of asphalt mixes, the challenge to the industry is to determine the right mix for the job and adhere to the specifications.


The challenge for the plant operator is to have the ingredients in stock. Along with more storage tanks and cold-feed bins, the producer might need extra tanks for fillers and fibers to be added at the mix stage.


The industry is moving toward more modified asphalts that are stiffer and harder to pump, according to Dwight Walker, senior staff engineer and editor in the education and marketing department at the Asphalt Institute.


"In the past, many times you would have to perhaps trade off cracking resistance if you wanted the improved rutting resistance," Walker told ROADS & BRIDGES. "Rutting needs a stiff material. Cracking resistance needs an elastic material that doesn’t become brittle at cold temperature."


The modifiers produce a mix that has good characteristics for both rutting and cracking, but the plant operator may need more tanks to hold the different concoctions.


Good news on the wind


The HMA industry received good environmental news in 2001. Actually, it was in December 2000 that the National Institute for Occupational Safety and Health published its report Hazard Review: Health Effects of Occupational Exposure to Asphalt. After 20 years of study, the agency of the U.S. Department of Health and Human Services concluded, "The collective data currently available from studies on paving asphalt provide insufficient evidence for an association between lung cancer and exposure to asphalt fumes during paving. The available data, however, do not preclude a carcinogenic risk from asphalt fumes generated during paving operations."


"It was a milestone for our industry, because that document had been in the making for several years," R. Gary Fore, NAPA’s vice president for environment, health and safety, told ROADS & BRIDGES. "The subject of asphalt fume has been researched with great intensity for many years. Still, there is no data that allows one to conclude that there are specific long-term health effects associated with asphalt paving fume."


All six major manufacturers of highway-class paving machines voluntarily added engineering controls to reduce fumes on machines made after July 1, 1997, added Fore.


In related news, tests conducted at HMA plants by the Environmental Protection Agency with the help of the industry proved that emissions from truck load-out and silo operations were well below what would automatically trigger EPA regulation under the Clean Air Act of 1990.


The EPA requested NAPA’s help with the issue of truck load-out emissions in 1996 to respond to technical issues raised by citizen groups. The EPA released its final report on the issue early in 2001.


The testing results "laid the groundwork for a petition to ‘delist’ the industry as a target for the new Title 3 hazardous air pollutant standards, which are more often referred to as MACT [maximum achievable control technology] standards," said Fore. HMA plants should be removed from the EPA’s list of major sources of pollutants by the end of 2002.


Combo platter


Aesco/Madsen, Auburn, Wash., has added a wet scrubber to a mixing drum and a baghouse to create its DM Series asphalt plants and put the whole assembly in a skid-mounted (stationary) or portable configuration. The drum mixers available on the three DM Series models come in three sizes ranging from 40-160 tph.


"It’s all there on one trailer, so there’s less things to haul when you need to move it," Steve Malloy, sales manager for Aesco/Madsen, told ROADS & BRIDGES.


The standard venturi wet scrubber is mounted on the same frame as the drum mix dryer and includes ductwork, a remotely controlled adjustable-throat venturi, water spray nozzles, a dewatering tank, an exhaust fan with motor and an exhaust stack. Optional baghouses are a pulse jet design and include bags, cages, air valves, blow pipes, controls and a dust return system.


Stationary and portable asphalt tanks are available in capacities of 10,000, 15,000 and 20,000 gal. They can be heated electrically or with diesel, natural gas or liquid propane burners.


The DM Series features standard one- or two-bin aggregate feeders mounted on the drum dryer frame or optional three- or four-bin stand-alone units mounted on a separate frame.


Surge systems from 3-150 ton also are available, as well as additive systems.


Turbo-charged


The Turbo Double Barrel drum mixer is an upgraded version of the Double Barrel from Astec Industries Inc., Chattanooga, Tenn. The new design increases air volume through the system, which means higher production and higher efficiency. The Turbo Double Barrel is available in 12 models, portable, relocatable or stationary, with production rates of 200-600 tph.


The Turbo Double Barrel also uses the entire length of the outer drum for drying, so it can dry the aggregate longer and more efficiently. An infrared sensor measures the actual dry aggregate temperature. The drying heat is captured by a thick layer of insulation surrounding the stainless steel shell and used in the mixing chamber, again increasing efficiency.


Highly combustible


The Equinox combustion system from Gencor Industries Inc., Orlando, Fla., can be used in any rotary drying process that requires between 25 million and 150 million BTU/hr of heat release. The low-NOx air system significantly reduces noise while providing efficient natural gas combustion.


With no refractory or combustion port requirement, the combustion unit can achieve faster heat-up and cool-down, according to the company, as well as lower maintenance and reduced heat loss.


The Equinox comes completely assembled on a single sturdy unitized frame for easy installation.


The company’s Ultraplants have capacities from 100 to 800 tph in continuous or batch operation. The counterflow Ultradrum incorporates an isolated mixing section located behind the burner so there is no chance of liquid asphalt coming in contact with the burner flame.


Rolling out the miles


The Milemaker line of portable and stationary counterflow asphalt plants from Asphalt Drum Mixers Inc., Huntertown, Ind., can produce from 160 to 425 tph, including SHRPS and Superpave, and process up to 40% RAP.


Three models feature a combined drum length of 40 to 54 ft. The completely sealed, counterflow drying drum ensures low moisture levels. The separate mixing drum allows aggregate, RAP, other additives and asphalt cement to be mixed thoroughly.


Triple drummer


The Triple-Drum counterflow asphalt production plant from CMI Corp., Oklahoma City, Okla., combines three distinct drying and heating zones in a single drum, resulting in higher production capacities for virgin and high-ratio RAP mixes. The variable-length combustion zone permits easy switching from one mix or production rate to another.


CMI offers portable Triple-Drum models with production capacities up to 750 tph.


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