Engineers at Astec Industries, a leading manufacturer of hot mix asphalt plants, needed to develop a new burner design quickly to accommodate a change in business strategy. Using the traditional build and test method, it would take six to 12 months to get the new burner to market, which would have put a crimp in the company's plans.
Instead, engineers used computer simulation to reduce the time needed to develop a new aggregate drying burner designed for use in asphalt plants to only 32 days. The burner needed to meet stringent requirements for highly efficient combustion and low emissions of NOx, CO and noise, yet there was barely time to build a single prototype.
The design team used computational fluid dynamics (CFD) to evaluate a large number of virtual prototypes and iterate to an optimized design. The most important concern was determining the best way to inject the fuel in order to obtain an ideal gas mixture.
CFD saved a huge amount of time by making it possible to visualize the flow and chemical concentrations throughout the early design iterations, providing far more information than could ever have been obtained from physical experiments. Within two weeks, the design team produced a credible working prototype, and within a month, the team optimized it to improve performance and meet the most stringent NOx and CO regulations. To date, 36 of these burners have been produced and they have delivered excellent performance for customers.
Astec Inc. manufactures a line of hot-mix asphalt plants and soil purification and environmental remediation equipment. The burner described here is used in an aggregate dryer used to remove moisture from rock so it will bind to cement to form asphalt. Astec previously purchased burners, but recently made the decision to design and build its own.
At the time the decision was made, the company had eight dryers nearly ready to ship but lacked burners. One of the requirements placed on the burners was that they perform equally well with natural gas, fuel oil, or propane. Another requirement was that they meet increasingly stringent regulations for emissions of NOx, CO and noise.
Astec's only previous experience was a single pre-mix burner built in 1993. This burner was designed using a traditional approach which involved building a series of prototypes, measuring their performance, and gradually iterating toward an acceptable design.
Using this approach, it typically takes six to 12 months to meet design objectives. This is because of the considerable time required to build and test each prototype and because physical testing provides only limited diagnostic information. Just about the only information that can be gained on the flow within the burner comes from drawing conclusions based on the shape and color of the flame, and from measuring the combustion emissions.
Simulation used from the beginning
Fortunately, Astec had previously established a CFD capability that made it possible to simulate the design of a burner ' or any other fluid flow problem ' and determine flow velocity, pressures and chemical species concentrations anywhere within the solution domain.
Astec engineers developed an initial premix burner concept design consisting of a centrifugal blower, fuel train and gas injection orifices, fuel and air mixing chamber, and a nose where combustion takes place. The advantage of a premix design is that air and fuel are mixed prior to entering the burner body, which results in a more intimate mixture and tends to lower emissions.
A major design challenge, however, is injecting the gases so that near-ideal mixing is achieved, because uneven concentrations of air and fuel will substantially increase emissions levels and reduce combustion efficiency. The fuel is brought into the mixing chamber in pipes arranged in a radial pattern, and holes are provided to inject the fuel into the chamber. The holes need to be arranged to provide as close a uniform fuel air mixture throughout the mixing chamber as possible.