Slim and Trim

Engineers are hardwired to use resources, including energy, wisely. With recent increases in the price of oil, everyone is more aware of the need to reduce the consumption of all types of fuel.

To help hot-mix asphalt (HMA) producers minimize energy use in the production process, the National Asphalt Pavement Association (NAPA) has developed a new publication, Energy Conservation in Hot-Mix Asphalt Production. This publication provides guidance on areas of production where energy can be saved.

In hot-mix production, most of the energy is used in three areas:

• Drying aggregate with some form of fossil fuel in an open-fired burner;
• Heating stored asphalt cement and HMA, primarily through fossil fuel burners that heat oil used to transfer heat to asphalt storage tanks, storage silos and the network of pipes and valves used to transport hot liquid asphalt cement; and
• Electricity, for the numerous motors used in the production process.

Lowering intake

There are many ways an HMA producer can reduce energy consumption or costs. The following methods have the largest potential savings:

• Reducing moisture content of aggregate;
• Insulating the surfaces and shells of dryers;
• Reducing the temperature of exit gases;
• Reducing the temperatures of exit materials;
• Using alternative fuels;
• Using more efficient hot-oil heaters;
• Insulating piping more effectively;
• Insulating tanks and silos more effectively; and
• Using variable-frequency drives for large motors.

Reducing aggregate moisture content

The energy required to dry aggregate is substantial. Therefore, any practice that will reduce the moisture content of the aggregate will have a dramatic and predictable effect on the energy consumed in drying aggregate. Such practices include:

• Sloping the grade under the stockpile to promote drainage away from the face being used to feed the plant;
• Paving under the stockpile to accelerate drainage, reduce standing water and prevent it from “wicking up” into the stockpile; and
• Covering the stockpiles so precipitation does not infiltrate the aggregate stockpiles—especially the fine aggregates, which do not drain as readily as coarse materials.

Reducing the moisture content by as little as 1% can save about 10% of fuel cost. For example, if the average moisture content of a plant producing 250,000 tons per year at an average drying cost of $2.50/ton is reduced by 1%, the annual savings would be about $94,000.

Drying cost reduction achieved by reducing exit gas temperatures

Energy models show that every 40°F of reduction in exit gas temperatures will result in approximately a 4% reduction in fuel consumption. A higher exit gas temperature indicates that the veil of aggregate in the dryer is inefficient and therefore energy is being wasted.

This is usually the result of worn flights or an inadequate flight pattern. Restoring the worn flights or changing the flight pattern to reduce the exit gas temperatures will improve fuel efficiency.

Drying cost reduction achieved by insulating dryer shells/surfaces

Fuel-consumption models for calculating energy requirements for drying and heating aggregates assume a heat loss from the “casing” or “shell” of the drying drum. Most models assume it to be between 5 and 10%. Insulating the shell of the drying drum reduces this energy loss and decreases energy consumption. Most new dryers and drum mixers are insulated for this reason.

Before insulating the dryer, the temperature of the shell should be measured. Excessive shell temperature can indicate problems with flighting, which should be corrected before the shell is insulated.

Drying cost reduction achieved by reducing material temperatures

Energy models also show a reduction of fuel consumption of 2 to 3% for every 10°F of final material temperature. HMA producers often increase the production temperature 10°F or more above the target temperature to improve workability for placement and compaction. This is especially true when dealing with polymer-modified binders.

Often, however, increasing the temperature is not as effective at improving the workability as adjusting other job parameters such as lift thickness or timing the compaction so that the breakdown roller follows immediately behind the paver. Excess mix temperature also can prematurely age the HMA, making it more difficult to compact and more brittle and prone to cracking in service.

Raising the mix temperature 15 to 20°F above design temperature can result in a 4 to 5% excess energy expenditure. The Asphalt Paving Environmental Council has published guidelines on mix temperatures in Best Management Practices to Minimize Emissions During HMA Construction (order number EC-101), available from NAPA.

Warm-mix asphalt offers the potential for significant reductions in energy consumption. NAPA’s new publication, Warm-mix Asphalt Best Practices (order number QIP-125), discusses the currently available technologies and offers guidance.

Production temperature should be carefully managed if maximum energy efficiency is the corporate goal.

Drying cost reduction achieved by using alternative fuels

Because of the volatility of fuel prices, producers often change fuel types within a relatively short period of time. Most burners on aggregate dryers are designed to burn both liquid and gaseous fuels, and many are equipped with manifolds so they can easily switch between these fuel types.

The cost-saving potential of an alternative fuel can easily be calculated. For example, switching from No. 2 diesel fuel with an energy value of 132,000 BTUs per gal at $2 per gal to a reclaimed fuel oil with an energy value of 143,250 BTUs per gal at $1.20 per gal would save about 45%. The cost of additional heating and filters required for some fuels still needs to be considered. Formulas for calculating these savings are included in Energy Conservation in Hot-mix Asphalt Production.

Using more efficient hot-oil heaters

Heat transfer oil is used at hot-mix production facilities to heat liquid asphalt cement, asphalt transfer lines, asphalt pumps and valves, and HMA storage silos.

Heaters have a useful life of 20-30 years, and older heater designs are not as efficient as those manufactured in the last 10 years. The newer heater designs are typically 85% efficient, where older designs frequently fall below this. There is an opportunity to improve energy efficiency and reduce energy consumption by evaluating the performance of the hot-oil heating system.

Adding heat exchangers to the stack or replacing the unit are typically the actions of choice. For example, costs of replacing an existing burner operating at about 70% efficiency or adding a heat exchanger to the burner could be recouped within one to two years.

Energy savings possible through more effective piping insulation

Transferring heat into hot oil effectively is one issue. Using it efficiently is another.

Without adequate insulation on storage tanks, finished-product storage silos, heat-jacketed asphalt lines, hot-oil jumper lines, pumps and valves, heat is lost to the atmosphere. Insulation has a rapid payback when fuel costs are high. As a general statement, more insulation is better for any hot piping or storage equipment at the plant.

For example, a long-radius 4-in. elbow is approximately equivalent to one linear foot of pipe. An elbow has a flange on either end. An uninsulated elbow requires 351 gal of fuel to compensate for the net heat loss over a 270-day production year. Assuming $2.50 per gal for No. 2 diesel, this means $878/year in lost energy.

Although the advantage to investing in this insulation seems obvious, there are still many plants with uninsulated heat-jacketed piping, uninsulated elbows and uninsulated hot-oil jumper lines.

These uninsulated components waste energy. When energy is expensive, this waste can add up to significant costs. Tables for calculating the energy loss are shown in Energy Conservation in Hot-mix Asphalt Production.

Energy savings possible through more effective tank and silo insulation

Most asphalt storage tanks are now constructed with 6 in. of insulation. An older 30,000-gal tank with only 3 in. of insulation will require 0.16 gal more fuel per hour to maintain its heat than a tank with 6 in. of insulation. Over a 270-day production year, this will result in the equivalent of 1,037 gal in lost energy.

Electrical savings possible with variable- frequency drives on large motors

Variable-frequency drives allow conservation of electrical energy on large fan motors by slowing the fan instead of using dampers to restrict airflow.

Many utilities include demand charges, which represent the high costs that electric companies pay for generation and transmission capacity that sits idle most of the time. Demand charges are based on the amount of energy consumed in a specified period of time known as a demand interval. Demand intervals are usually 15 or 30 minutes but vary widely across the country. Each operator should check with the local utility company on how demand charges are determined, since the potential savings from reducing these charges may be greater than the energy consumption fees.

There are many areas where HMA producers can save energy and reduce energy costs. Many of the improvements outlined here and discussed in detail in the NAPA publication have very short times to payback. Other improvements may have longer payback times, but the savings will still reduce the impact of energy price fluctuations on production costs.