Practicing Sustainability at Asphalt Plants

Reducing the energy required to operate your plant will provide the greatest return in investment and emissions reduction.

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The politics and constant news coverage associated with climate change have made “sustainability” and “greenhouse gas emissions” part of our daily-use vocabulary.  The asphalt industry is already doing a great job of using sustainable practices and an equally great job of reducing greenhouse gas emissions.

As an industry, we do not seem to fully recognize the synergy between practicing sustainability and our profitability. If we run our business better, including being wise with our use of energy and materials, we will improve our profitability and at the same time we will operate in a more sustainable way.  So, why not look at asphalt plants and how they should be operated through the lens of sustainability.

Although discussions of energy and materials are going to necessarily intertwine, let’s start with energy.  Better combustion, for the purpose of reducing NOX emissions also results in reduction of all other emissions, including carbon dioxide, a well-known greenhouse gas (GHG) and, at the same time, results in a reduced fuel spend.  Here are some additional ways to reduce combustion source emissions that also reduce the fuel spend.

• Exhaust gas temperature control with reference to dew point – Better thermal energy efficiency is tied directly to exhaust gas temperature.  Better thermal energy efficiency means less fuel will be burned and fewer emissions produced.  Asphalt plant exhaust gas temperature is typically around 250 degrees F but, with 5% moisture in the incoming materials, dew point is about 160 F, almost a hundred degrees lower.

By reducing the exhaust gas temperature to say 170 F, that temperature gap is reduced to 10 F.  That change results in a fuel purchase reduction of about 6% and a similar reduction in exhaust gas volume and all combustion-source emissions and in increased production capacity. We now have the technology to measure incoming aggregate and RAP moisture contents and to use the data as input for real time calculation of exhaust gas dew point by an algorithm embedded in the plant control computer program.  Astec’s V-Pack system, driven by the algorithm output, can be used to continuously adjust dryer drum speed to control exhaust gas temperature to maintain it just above the dew point.  Although almost all plants could tolerate some reduction of exhaust gas temperature with no problem, operating with the exhaust temperature too close to dew point, will cause mud to form on the walls of the baghouse and on the Bags.  The greatest advantage of this technology can be realized when applied in conjunction with a good insulation system for the exhaust ductwork and the baghouse.

• Exhaust gas temperature control – If you have ever been inside an asphalt plant dryer, you know that the term “flights” refers to aggregate lifting devices that are made of steel and attached to the inside walls of the dryer drum.  The flights lift virgin aggregate from the bottom of the dryer drum and drop it through the stream of hot gases from the burner, for the purpose of drying and heating the aggregate.  Most flight designs do not produce a uniform distribution (veil) of aggregate across the full diameter of the dryer drum except possibly when the dryer is fully loaded.  When incorporating more than about 30% RAP into the mix, obviously the virgin aggregate component is necessarily less than 70% of full capacity.  The more RAP that is incorporated into the mix the less full the dryer is and, correspondingly, the less full the flights are.  As the dryer drum rotates, flights, of traditional designs, that are not full will not drop (shower) aggregate on the uplift side of the drum.  

That leaves an unobstructed exit path for the hot burner combustion gases.  The hot gases will take the path of least resistance bypassing the aggregate that is showering on the downside of the drum and taking the heat with them.  Although the superheating of the virgin aggregate, which is necessary when including RAP in the mix, usually gets the blame, it is the incomplete aggregate veil that causes a very hot baghouse, wasted fuel and reduced production capacity.  This was the main reason the V-Pac® system was developed.  The flights have deep v-shaped cuts to produce a complete aggregate veil whether they are lightly or fully loaded, preventing heat from escaping via that path of least resistance.  

• Maintenance and Tuning – Maintaining the dryer and exhaust system in good condition and keeping the burner properly tuned avoids wasting fuel.  Tuning the burner with a properly calibrated combustion analyzer will inevitably result in fuel savings.  Wasting fuel always results in a higher fuel spend and increased emissions including GHGs.  Worn dryer flights and damaged or missing drum seals have the same negative effect. 

• Stockpiles – The lowest cost and most “renewable” of available resources are gravity and sunshine.  They are resources that can be used to reduce the fuel spend Place material stockpiles on a paved surface sloped away from feed bins on 6% grade and let gravity do a lot of the drying. This is the best deal going for drying cost and is 100% sustainable. The only way to waste gravity is to not use it.Place material stockpiles on a paved surface sloped away from feed bins on 6% grade and let gravity do a lot of the drying. This is the best deal going for drying cost and is 100% sustainable. The only way to waste gravity is to not use it.and increase production capacity without the slightest concern about depleting them. When aggregate and RAP are stockpiled on paved surfaces sloped away from the feed bins on a 6% grade, much of the water will be removed by gravity drainage. This conserves fuel and enhances production capacity more than almost any other available option.  The paved surface under the stockpiles also prevents perfectly good aggregate from sinking into the ground over time and becoming effectively lost.  In areas with a lot of annual rainfall, covering stockpiles can make a lot of sense.  

• Insulation of Hot Surfaces – Radiation and convection heat losses from hot surfaces, especially dryer drum surfaces, can and should be greatly reduced.  Unfortunately, insulating the hottest part of the drum, the combustion zone, causes the drum shell temperature to rise even higher because it has no way to cool.  Obviously, this will shorten the life of the drum shell and can cause sudden catastrophic failure.  The surrounding outer shell of the Double Barrel® design captures the heat and allows the combustion zone of the drum shell to be cooled too.  Heat is simply transferred from the hot shell to the mix, which is exactly where it is needed.

• Warm Mix Asphalt (WMA) – Producing WMA in the place of HMA saves fuel consumption and reduces all combustion-source emissions, because of the lower mix temperature.  The advantages of being able to get better coating, store longer, haul longer distances, have mix that remains in a workable condition longer and can be more easily compacted just add to the clear evidence that paving with WMA is a smart business move.

• Avoid overheating the mix – There is always the FOB customer you cannot convince to take mix at anything cooler than practically blazing.  You might be better off without that customer.  To avoid loss of margin, you would have to get a premium for that mix not just because of the additional fuel burn but also because the additional exhaust gas volume slows your production rate and causes a larger amount of fixed plant cost to be added to that mix than to normal temperature mixes.  Of course, additional emissions are created.  Best practice, for many reasons, is don’t overheat the mix.

• Storage – Having the ability to store finished paving materials in silos enables plant operators to run continuously and at higher production rates, thus increasing plant efficiency.  As with other efficiency improvements, this implies reduced energy consumption that translates to lower GHG emissions.  Sustainability is further improved by reduced wasting of materials.  Starting and stopping wastes fuel and materials.  An additional factor is the positive effect on the consumption of diesel fuel by trucks while they wait to be loaded.  If the day is started with filled silos, trucks have less wait time and burn less fuel, producing less exhaust gases.

• Renewable Fuel Burners – Using biomass fuel to provide heat to the aggregate dryer is attractive, because biomass is fully renewable and carbon neutral.  While biomass burning does emit carbon dioxide re-growing the source plants reabsorbs it.  Environmental Groups consistently oppose the use of biomass fuels based on the misconception that their use will consume forests.  Many forests exist because they are privately owned and managed.  There are at least three biomass-fired asphalt plant options.  The first is to burn wood chips in a furnace and use the hot exhaust gases as the heat source for the aggregate dryer.  The necessary equipment and technology for this is either currently available or can be developed.  It would probably be best to run any plant configured this way as a 24-hour per day operation, since startup and shutdown of the furnace takes several hours.  Another way to use biomass is to reduce the fuel to fine particles and burn it as dust in a suspension burner.  The Astec Biomass Burner is the best I know of to burn fine wood dust in a rotary dryer.  However, there is no readily available technology to prepare the fuel.  Hammer milling is too expensive and the only other way to get there is by steam explosion of the wood fiber.  If a plant is located near a furniture factory or other industry that produces sander dust, that can be a source of wood dust small enough to burn in the Astec Biomass Burner.  Lastly, there is biodiesel. Biodiesel is obviously renewable since it comes from plant sources.  B20 biodiesel is a blend of 20% biodiesel and 80% petroleum diesel.  B100 biodiesel is 100% biodiesel.

The other major energy form used by asphalt `plants is electricity.  With most utility companies you can ask and learn how much of your electricity comes from renewable sources like wind, hydro and solar and how much comes from fossil fuel sources and nuclear.  You may even be able to request and receive more of your electricity from renewable sources.  Whether you can go that route or not there are many things that can be done, with respect to electricity use, to operate in a more sustainable manner, including:

• Variable Frequency Drives for Fans (VFD) – Using VFD with fan motors has a very significant payback.  This is because, while fan volume flow rate is directly related to fan speed, the associated power requirement is related to fan speed by the third power.  As an example, consider an increase of fan speed from 50% to 100%.  So, in this example fan speed is doubled, or increased by a factor of two (2).  To make that happen, power input from the fan motor must be increase by a factor of two (2) raised to the 3rd power, which is a factor of eight (8).  So, if the fan uses 40 horsepower at 50%, it would take 40 x 8 = 320 horsepower to push it to 100% speed and volume capacity.  Without using a VFD on the plant exhaust fan, the fan must run at 100% all the time even though that really is not necessary, since plants frequently operate at production rates well below maximum capacity. An exhaust fan damper reduces the power consumption a little but nothing like the VFD.  The next largest fan employed at an asphalt plant is the burner blower.  A proportional energy savings exists, but caution must be exercised. Most burners are not designed to operate with variable blower speed.  Never apply a VFD to a burner blower unless the burner was designed to operate with variable burner blower speed.  To do so could cause damage to the burner or worse.  The Astec Phoenix® Burners are designed to operate with a VFD and provide power savings.

• Variable Frequency Drives (VFD) for Drag Conveyors and Other Equipment – There is not much of an electricity savings available by application of VFD to any asphalt plant equipment other than fans.  However, it may still be an advantageous thing to do to improve the plant power factor and for reduction of wear rates on wear liners, chains, etc.

• Premium Efficiency Motors – On average, premium efficiency motors save about 3% as compared to standard efficiency motors.  3% may not sound like a lot but anything that reduces electric power consumption without an offsetting increase somewhere else improves overall sustainability and reduces the size of the power bill.

• Soft Starts – Where a VFD is not particularly advantageous, a soft start starter is usually a good application, especially for large motors.  A soft start reduces the current inrush when a motor is being started.  This can reduce the demand charges that are associated with instantaneous load on the grid.  Most electric power utilities add “demand charges” to the power bills of users that have big inrush events, such as starting a big motor across the line, because they have to over generate to be able to absorb those drains on the grid while maintaining system voltage above a certain low limit.

• Solar and Wind Power – If there is enough land, wind and sunlight hours, the asphalt plant power source can be complemented by onsite renewable power generation.  When the plant isn’t operating, at most locations, the power generated by the wind and/ or solar systems can be sold into the power grid.  There are also battery systems that can store this cheap power to be used during higher rate periods.  Nevertheless, be judicious in investing in these resources, since there are areas where the average daily sunlight or average wind speed is not enough to justify the investment.

Malcolm Swanson, PE, is Vice President, Innovative Products at ASTEC, Inc