Traditional and Alternative Energy for Asphalt Plants
Natural gas flame
Burner flame from natural gas.
Number 2 fuel oil flame
Burner flame from No. 2 fuel oil.

When selecting which fuel to burn in an asphalt plant, initial cost alone cannot be the determining factor. Hidden costs—such as emissions, availability, capital equipment costs, and maintenance—associated with certain fuels must be considered as well. To help clarify this choice, Astec, Inc.’s Michael Swanson penned a recent white paper, “Traditional and Alternative Energy for Hot Mix and Warm Mix Asphalt Plants,” which provides a helpful overview of how deciding between traditional fuels sources (such as natural gas, fuel oils, liquid propane, and coal) or a range of renewable energy options can influence a plant’s operations and the associated costs.

Traditional Fuel Sources

Most operators are familiar with the four traditional fuel sources found in today’s asphalt plants: natural gas, fuel oils, liquid propane, and coal. For traditional fuels, the main concerns remain fluctuations in availability and price and regulations for these fuels’ emission standards.

Because natural gas is a naturally occurring fuel that consists primarily of methane, its emissions are lower than those of other fuels. However, Swanson notes that before it can be used as a fuel, it must undergo processing to remove excess concentrations of hydrocarbons. Also, as natural gas is a common heating source for many households, its availability and price are dependent upon market conditions.

Fuel oils are classified into grades determined by the properties deemed most significant in burner performance. The light distillate oils, or light oils, are the easiest fuel oils to burn since they lack many characteristics that cause issues. Recycled fuel oil, or waste oil, comes from a number of sources ranging from used crank-case oil to oils from industrial processes. Fuel oils emissions are usually acceptable in most jurisdictions, but will be higher than natural gas and, therefore, subject to greater regulation.

Although the standard fuel in the asphalt industry, liquid propane is rare to find used elsewhere. When vaporized and fired as a gas, propane emissions are similar to those of natural gas. When fired as a liquid, it burns similarly to light fuel oils. Swanson points out that the higher pressures involved with storing and using liquid propane are of key importance, as it is subject to careful safety regulations.

Whenever coal is discussed as fuel, concerns over how to manage coal’s emissions and residue have to be addressed. Hot-mix asphalt (HMA) plants have built-in solutions to the two most significant deterrents to burning coal: dust and ash. Aggregate dust particles that sulfur has attached itself to are removed mechanically. Ash is captured by the baghouse and incorporated into the mix. Of the four major types of coal in use as fuel—lignite, subbituminous, bituminous, and anthracite—only anthracite is unsuitable for use in HMA processes.

Spray pattern of liquid propane.
Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, with smaller quantities of sulfur, oxygen, and nitrogen.
These wood pellets are made by reducing raw wood to a fine pulp in a hammer mill, drying it to approximately 10 percent moisture, and then forcing it through a die at high pressures.

Renewable Energy Sources

In the United States, energy regulations increasingly require use of renewable fuel sources. These regulations differ widely and are tailored to each state’s specific policy objectives, which usually include economic growth, renewable energy supply, environmental concerns, and the capacity for renewable energy expansion. Of the different renewable energy technologies available today, Swanson highlights biodiesel (made from vegetable oils and animal fats), cellulosic biomass (the burning of wood), wind energy, and solar energy as options for asphalt plants.

Biodiesel is a suitable substitute for No. 2 fuel oil in asphalt plants. However, as its solvent properties differ from standard diesel, Swanson reminds operators that careful attention needs to be paid to the compatibility of seals, cleanliness of tanks and lines, and propensity for clogging the oil systems on asphalt plant burners.

The preferred form of biomass for asphalt plants comes in wood pellets usually made from a mixture of hardwoods and pines, often collected from sawdust or other waste from wood products manufacturing. The pellets’ dense, dry structure provide them a high energy output when burned as fuel, and their precise size and low ash output make them a highly desired coal substitute. A study by the U.S. Departments of Energy and Agriculture reveals that over 1 billion tons (907,184,749 tonnes) of biomass are available annually just within U.S. borders.

Although the energy capacity of wind turbines continues to grow globally every year, Swanson points out that its real benefits accrue as a substitute or supplement to large power grids. High capital costs and variable resource availability make wind power an unlikely option for a single asphalt plant. Also, turbines are often placed in locations far from where the plant would operate.

Solar energy, however, can be a natural fit for asphalt applications. Concentrating Solar Thermal (CST) systems use parabolic trough mirrors to collect and concentrate sunlight on a receiver, which is then carried by circulating hot oil. These mirrors could easily replace the oil heaters currently powered by fossil fuels in today’s asphalt plants. Plus, a typical asphalt plant tank farm would need only a small number of CST collectors to support the maintenance heating load.

For Information

Download the full text of Michael Swanson’s tech paper T-147 “Traditional and Alternative Energy for Hot Mix and Warm Mix Asphalt Plants” from the Astec website: www.astecinc.com/literature.

Which Fuel Is Right For You?

Obviously, not all fuels are available in all areas and the availability of certain fuels changes over time. It’s advisable to look at what burner you have and gauge what fuel you use from there. Some burners run better on certain fuels, but not all burners are the same. Just looking into the aspects above will help you choose the fuel that will save you time and, ultimately, money.