Aeroglide Inc., Raleigh, NC, has designed and manufactured heat processing equipment for more than 60 years. Installed in many different industries, the company's ovens and dryers are used to remove moisture from products such as ready-to-eat cereal, pet foods, tobacco, chemicals, fruit and charcoal briquettes. The firm's market penetration in the latter category -- the company estimates that three out of four charcoal briquettes manufactured are produced using Aeroglide drying ovens -- qualified them as the supplier for an unusual yet significant drying application.
To reduce emission levels from coal-fired electric power plants and metallurgical applications, federal legislation offered tax credits to firms operating with low sulfur synthetic coal. Such a fuel, developed according to specifications, would enable vast quantities of coal fines from U.S. mines to be safely transported and burned instead of being treated as a useless waste product. The millions of tons of these fine, generated over decades of coal mining activity, represented an inexpensive source of relatively clean-burning fuel -- if they could be shaped into an easily handled pellet form.
"It took a while, but one company in Utah finally developed the right kind of binder to hold the fines together," explained Gilbert Mayo, manager of applications at Aeroglide.
According to Mayo, the timing was critical: To qualify for the tax credit program, firms had to be using the synthetic fuel by June 30, 1998.
"The coal dust is blended with the binder material and pressed into pellet form," Mayo said. "Our job was to produce a dryer that could dry the pellets quickly, inexpensively and in large volumes, bringing the moisture content down to a targeted level."
Aeroglide designed and engineered a four-zone dryer with a total heat capacity of 70 million BTU/hr, operating between 300 and 400oF (149 and 204oC). The first two zones required 20 million BTU/hr heat input while zones three and four needed 15 million BTU/hr. Obviously, successful burner performance would be a critical factor in the dryer's efficiency and productivity.
"There is a lot of product moving through the oven, so we need to put a lot of heat into it," Mayo said. "The real challenge was finding burners that would work well with high quantities of recirculated air.
"We planned to exhaust only 30 to 40% of the air," Mayo said. "So, the burners would have to function properly replacing the heat exhaused air."
Mayo and his staff also needed burners with short flame patterns -- to avoid impinging on the product -- and a wide turndown ratio. They turned to Maxon Corp., Muncie, IN.
Mounted directly in the airstream being heated, the direct-fired line burners operate by directing air through the mixing plates, where it is blended with fuel. Because the airflow also is the source of combustion air, all available heat flows directly into the airstream. Carefully controlled aeration patterns in the plates ensure progressive mixing, good cross-ignition, flame retention and odor-free combustion. Turndown ratio is 25 to 1.
In addition, the coal-drying oven incorporates nozzle-mixing modular burners with an external blower to supply combustion air. Capable of producing low emissions of both oxides of nitrogen (NOX) and carbon monoxide (CO), the burner's short length flame is exceptionally stable.
Larry Hyland assumed the responsibility of engineering the combustion systems for the dryers.
"We had to fast-track the project, locating the burners and designing the gas trains and flow-control equipment," Hyland said. "When the dryer was fired up for the first time in mid-April, we were pleased to see that everything went well."
None of this surprised Gilbert Mayo. "They've always backed up their product," he commented.
By summer 1998, thousands of tons of high quality coal fines, once considered little more than landfill material, exist as a new fuel source. Process-ed as pellets, their use will reduce sulfur emissions, extend America's coal reserves and earn returns for companies using them in the form of federal tax credits.