How to Make Your Plant Efficient, Part 2

Astec Industries offers more ways plant efficiency can reduce your carbon footprint before and during next season

Editor’s Note: For 2024, AsphaltPro Magazine allows experts in the industry to share how to expand your operations to the next phase of business. Are you ready to build or update your own hot-mix asphalt (HMA) plant? Let’s turn to some professionals who have equipment, services, software and tenure to help you expand to mix design, production, hauling and more. This installment from Greg Renegar, vice president of customer success at Astec Industries, dives into the environmentally responsible task of implementing tactics that reduce carbon and increase efficiencies throughout the plant.

It is possible to work hard at being more energy efficient and not really move the needle. It is also possible to create a culture of efficient plant operational behavior and have an efficient operation that requires little extra effort.

The key is understanding your plant’s energy needs, knowing how to reduce required energy, reduce energy losses, and understanding how overall plant operations can have a significant impact on energy and profitability.

A sustainability / energy efficiency revolution has started, and this article discusses proven methods and strategies to use significantly less energy.

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Producing asphalt with less energy requires understanding where the energy (heat) goes; how much goes into the mix, and how much escapes. In addition to minimizing hidden energy losses, there are operational strategies that can have a significant impact on overall energy consumption. The following suggestions will help producers minimize energy consumption per ton, maximize profitability and maintain high mix quality.

In this discussion, efficiency is defined as producing the greatest quantity of mix with minimal energy. Although the largest portion of the energy (heat) comes through the aggregate dryer burner, there are other plant-related activities that consume energy; e.g., liquid asphalt tank farm, front end loader, etc. For this discussion, the energy consumption considered will begin at the stockpiles and continue to the silo loadout.

Plant operation methodologies can certainly move the needle with respect to efficiency, but there are several efficiency basics that should be considered for every plant operation. Each topic below plays a part in maximizing plant efficiency.

• Operate at the lowest drum gas outlet temperature possible, but always stay safely above the dew point in the baghouse. Excessive stack gas temperature is wasted energy. Using variable frequency drive (VFD) technology to vary the speed of the aggregate dryer (with appropriate flighting) can minimize stack gas heat loss for each individual mix design and production rate.
• Ensure that the burner system is capable and tuned to operate with a balance of minimal excess combustion air and minimal CO emissions while not overheating the dryer shell. This will help minimize the fuel per ton and maximize the tons per hour. The burner must be tuned to perform in the ranges encountered in day-to-day operation. Electronic air-to-fuel ratio control has an advantage over mechanical linkages in that the efficient burner output range can be maximized.
• Use VFD technology to replace outlet dampers on large centrifugal fan drives, like the baghouse exhaust fan. The electrical energy savings will be significant. There are other benefits as well.
• Manage the mix temperature. Address excessive mix temperatures, and when possible, use a warm-mix technology (foam or chemical) that allows lower mix production temperatures. A lower mix temperature means less energy is required. The impact can be significant. VFD technology can help keep drum exit gas temperatures above the baghouse dew point while running cooler mixes.
• Select the most economical, emission compliant, available fuel and use it well. For example, waste oil must be conditioned correctly, or it can significantly diminish anticipated fuel cost savings by damaging downstream equipment. Each fuel/burner combination has its own operational characteristics. It is important to be aware of those characteristics. For example, natural gas might be the preferred fuel, but some older technology burners do not completely burn natural gas at higher firing rates.
• Insulate everything too hot to touch on the asphalt tank farm/hot oil system. When this is not done energy “trickles” away hour by hour.
• Do not neglect hot oil system burner tuning. It is a relatively small burner, but it runs much of the time. Also, recover otherwise wasted heat in the hot oil heater exhaust with available heat exchanger solutions.
• Manage waste mix by minimizing production stops and starts. Start-up and shutdown at the same low production rate to reduce waste. Covering material in the cold feed bins if rain threatens also helps reduce waste mix.
• Design the plant site to minimize front-end-loader operation.

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How to Run a Plant

Next, two schools of thought regarding how to operate an asphalt plant for maximum efficiency will be examined.

One school of thought is to simply keep the plant running throughout the day, faster or slower, depending on how the day develops. In this scenario, midstream stops/starts are avoided as well as stops and starts where the plant is cleaned out.

Many plant production managers follow this simple methodology because it allows flexibility for unpredictable production demands, produces a higher quality of mix via the steady state flow of mix constituents, and minimizes the risk of unscheduled downtime. Although this strategy is flexible, it is typically reactionary. Often little is done to proactively create conditions where a more efficient operation can occur.

Another school of thought is to run the day’s production at the highest production rate possible and store the mix until needed. The goal is to produce at a rate where the plant is most efficient. This assumes that plants run more efficiently closer to the rated production. A 2,500-ton day might look like 6.25 hours of production at 400 tph versus 8 hours at 313 tph.

Expand Into Production: Navigate the Permit Process

Regarding which school of thought is best, it depends on the plant equipment capabilities, training and local market constraints.

The simplest production scenario is knowing the mix type(s) and quantity(s) required for a day’s production and having adequate surge/storage capacity.

With this ideal scenario, one could make a case for the production methodology of running at the highest efficient production rate and storing the mix until needed.

The most efficient production rate will depend on several factors, which vary from plant to plant. It will be a balance of the highest production rate and the lowest drum exit gas temperature. Also, for this to be the most efficient condition, the burner must also have a balance of the correct amount of combustion air (too much is bad, and too little is bad) and low CO emissions. This requires having the right burner technology, controls, maintenance and adjustment.

For example, if the plant can produce 360 tph, but the stack temperature is 300°F, that is probably not the most efficient operational condition. If the stack temperature is, say, 220°F at a lower tph, then that might be a more efficient point from an energy (fuel) consumption viewpoint.

One could operate at the more efficient, lower production rate or take steps to lower the stack temperature at 360 tph, thus becoming more thermically efficient. If a lower production rate is the most energy efficient condition, one must consider the effect of taking longer to make the mix and the amount of mix that can be supplied in a day/week/year.

Higher level business “efficiencies” could mean the plant should operate at a less-than-optimal condition from a thermal energy consumption viewpoint. Optimizing a single efficiency (plant or business) might not be the best course of action in every situation. The goal is to have the equipment technology and training provide optimized plant production (high tph and high efficiency) simultaneously with optimum business efficiencies.

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Electrical efficiency is another factor to consider. Once the large centrifugal fan drives use VFD technology, the most electrically efficient production rate is where the electrical motors, without VFD drives, are operating between 75% and 100% of rated load. It is important for the electrical power factor (how efficiently electricity is being used) to be high enough to avoid excessive utility bills as well as utility penalties.

The second production scenario is when the mix type and mix quantity is known, but there is not sufficient silo/surge capacity to produce the mix at the maximum production rate. In this case, the best one can do is produce the mix at a production rate that balances demand and silo capacity but keeps the plant running at steady state as much as possible.

Maximum plant energy efficiency in this scenario results from a handful of best conditions:

• Running steady (no stops)
• Burner efficiency
• Dryer heat transfer being efficient at different production rates. In other words, getting the heat into the aggregate while maintaining low stack temperature.

This can be very difficult to accomplish with older plant technology, but is the methodology followed by many production managers. It balances production output, efficiency and mix quality.

What You Need to Know About Burner Size and Performance

The goal is to have the equipment technology and training provide optimized plant production (high tph and high efficiency) simultaneously with optimum business efficiencies.

When multiple mix designs must be produced from a continuous plant, the most efficient way to operate is to start out producing the finer mixes, and transition to the coarser mixes. The reason for this is to make “on-the-fly” mix changes where the larger aggregate cannot contaminate the next mix being produced. A modicum of communication and planning is required to obtain this degree of efficiency. One successful contractor was quoted as saying, “We aren’t the best communicators in the world, but we at least try to know what we have to produce the next day.”

Even after communicating as much as possible, changes will occur, but having an idea about mixes and quantities still helps plan for the most efficient production day.

The final scenario describes a very efficient asphalt plant operation. Note that in this last scenario, the energy basics have been addressed; therefore, there is little energy “left on the table.”

Add High-RAP to Your Production

• The day starts with silos full of mix produced the previous afternoon. This would have kept the plant running, filling silos, even if the plant was “on hold” while a paving crew determined the final amount of mix needed to finish the previous day.
• Communication with internal and external customers makes it possible for the first round of trucks to depart with mix made the day prior.
• Even if you experience a plant issue that delays start-up, trucks can be loaded with stored mix while it is being addressed.
• The first mix production of the day can be started with only a couple of tons of waste. This can be taken to the RAP pile to be “trickled” in later.
• The day’s production was planned with the finer mixes being made before the coarser mixes so the changes could be made “on-the-fly.”
• There is enough silo capacity to enable transitioning from one mix to another without stopping the plant.
• The day starts with one dedicated silo filled with a fine mix for the private customers who arrive unannounced so they can be serviced quickly. Quick service ensures continued business.
• As the day’s production demands wind down, the plant fills the silos for the first round of trucks the next morning. If the jobs are rained out, the mix can be stored until the weather clears.
• The plant is shut down at a low production rate (same as start-up) and only produces a couple of tons of waste.
• If rain is forecast overnight, the cold feed bins can be covered. Without a change in moisture in the start-up aggregate, the operator knows where the burner needs to be to start up with minimal waste.

This final scenario illustrates what a very efficient asphalt operation can look like. There is no new technology that makes efficiency suddenly appear. The efficiency is simply a result of taking care of the basics and operating with common sense.

In summary, there are several facets to maximizing plant efficiency.

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There are the energy “basics,” some of which are listed above, that every plant operation should consider. Some are major and some are minor depending on each plant/market situation. There are equipment requirements, such as the drying system, exhaust system, storage/surge capacity. There are training requirements so the production manager, plant operator and loader operator understand the “whys.” There are management requirements to ensure correct actions occur consistently. No efficiency “basic” or methodology is effective without consistent action.

In general, plant efficiencies that are behavior-dependent can be maximized by the three “Rs.”

• Record: Anything that is recorded receives more attention.
• Review: Review progress of that which is recorded. This heightens attention to the next level.
• Reward: Done well, this can not only keep efficient behaviors in place, but result in additional efficiencies.

As the asphalt industry continues to hyper-focus on energy efficiency and sustainability, it is important that there is a thorough understanding of the factors that can make a real difference. The good news for producers is that energy efficiency and sustainability can increase quality and profitability. The good news for the public is that energy efficiency and sustainability increase the miles of paving possible for each tax dollar.