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PRACTICING SUSTAINABILITYAT ASPHALT PLANTS

Malcolm Swanson, PE

Vice President, Innovative Products, ASTEC, Inc

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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. It is not the purpose of this article to get into the politics or any controversy on climate change. Emotions rather than emissions sometimes dominate those discussions. My hope for this article is to point out that the asphalt industry is already doing a great job of operation using sustainable practices and an equally great job of reducing greenhouse gas emissions. I should also make mention that most participants in the industry, if not all, are doing so primarily to enhance profitability.

What is sustainability anyway? If you do a Google search on “sustainability”, you will find many different definitions. Among those you will find it stated that the most common definition is, “Meeting the needs of the present without compromising the ability of future generations to meet their own needs”. If we view our business operations by that definition, it means that we will operate in such a manner as to be able to continue to operate uninterrupted indefinitely as far as that is affected by the availability of resources and environmental conditions. To attempt to clarify that by stating it in negative terms, if we operate in such a manner as to eventually completely consume or destroy something that is necessary to our business, we will inevitably reach an end point. So, at a personal level, sustainability is about not reaching that end. As you will see in the following paragraphs, operating so as to avoid an undesirable ending brings about a lot of benefits, especially improved profitability. I think as you read, you will agree that operating with sustainability as one of our key principles is not burdensome but is hugely beneficial in several ways. Saving the planet does sound like an overwhelming concept; but if I look at sustainability as it affects my little sphere, which is a part of that much larger sphere, I can have an impact and do so while improving my business. So, in one sense, it really does not matter whether you and I think the planet needs saving or not. Operating according to this definition of sustainability will yield environmental and financial benefits and help secure a future of both kinds for our children and grandchildren.

The asphalt paving industry already had a great record and story to tell concerning sustainable operating practices before sustainability was seen as having the level of importance that is attributed to it today. The industry did not need the threat of climate change to adopt certain sustainable operating practices, because they just made good business

and common sense. Two of the most notable examples are recycling of old asphalt pavements and paving with warm mix asphalt instead of hot mix asphalt. Since asphalt pavements are 100% recyclable, RAP, as it is called in the industry, is a valuable raw material that is relatively inexpensive and readily available, particularly in urbanized areas. Warm mix asphalt (WMA), as contrasted with traditional hot mix asphalt (HMA), went mainstream just a few years ago and now accounts for a major share of all new pavements. As the name suggests, WMA is made and laid at considerably lower temperatures than HMA. The reduced temperature means it can be made with less fuel burning. Conservation of energy resources and less combustion-source emissions are natural outcomes of replacing HMA with WMA. The use of WMA also virtually eliminates evaporative emissions from the liquid asphalt cement (AC) at the asphalt plant, during the truck haul to the job, and in the paving operations. The road building industry has moved rapidly into WMA, because it just makes sense.

Another good example of the application of common sense to a sustainability issue is the advent of fundamentally better combustion systems. When the 1970 Clean Air Act became law in the United States, many combustion processes suddenly had to be cleaner. Industry responded with the introduction of combustion modification technologies such as Exhaust Gas Recirculation (EGR), also referred to as Flue Gas Recirculation (FGR), staged combustion, water injection, ammonia injection, and post combustion treatment technologies like after burners, regenerative thermal oxidizers (RTOs), and catalytic oxidizers. These technologies, though fairly effective in reducing emissions of carbon monoxide (CO) and oxides of nitrogen (NOX), tended to cause decreases in fuel efficiency and power output. They, therefore, actually caused increased GHG emissions. The design of burners used in drying stone aggregates in the paving industry were not much affected by the Act, due to the industry’s relatively small size and environmental impact, until the passage of the Clean Air Act of 1990 in the United States. About that time, Astec, Inc. received an order for a very large asphalt plant to be built in Southern California, USA. Empowered by the new Act, California’s South Coast Air Quality Management District tightened its limitations on NOX emissions to the point that there were no burners available to the industry that could stay under the annual NOX and CO emission limits for such a large plant. Although Astec had recently manufactured other asphalt plants for California paving contractors, successfully using FGR for NOX reduction, the NOX reduction levels required

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for the new plant order were not achievable with FGR. So, Astec engineers designed a completely new burner that just did a fundamentally better job of combustion than traditional burners. It was a version of a pre-mix natural gas burner that did a much better job of mixing fuel and air and used aerodynamic stabilization and flame shaping to produce a very compact flame. The improved combustion did the NOX reduction job without FGR or any of the other combustion modification technologies and it was more fuel efficient than conventional burners. This burner was the predecessor of today’s Astec Phoenix® Phantom and Phoenix® Talon II Burners. Of course, better efficiency means the contractor’s fuel bill was less than it would have been otherwise. Oh, it also cleaned up the CO emissions to almost nothing. It just made good sense to do well what had not been done so well before. So, Astec developed a whole line of burners based on that one original burner and the clear understanding that making a high quality flame was better than trying to apply “Band-Aids” to fix poor burner performance. Now, the Phoenix line of burners is widely used for ultra-low NOX and other applications throughout the industry.

As a society and as an industry, we do not seem to fully recognize the synergy between practicing sustainability and our profitability. There are probably things done in the name of sustainability and climate change mitigation that do not meet the test of common sense or business sense; but as far as I have ever seen, such ideas are usually not good for sustainability either. In most instances, the things we would do to try to slow climate change make good common sense and business sense even without a climate change issue. 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. I think the plants in the industry are generally run wisely, nevertheless, as I travel around the United States and abroad,

I continue to see overlooked opportunities. So, why not look at asphalt plants and how they should be operated through the lens of sustainability. I have found that, when I look at the design of even mature tried-and-true equipment from a different perspective, I see opportunities I had not seen before.

Although discussions of energy and materials are going to necessarily intertwine, let’s start with energy. We have already mentioned that better combustion, for the purpose of reducing NOX emissions also resulted in reduction of all other emissions, including carbon dioxide, a well-known greenhouse gas (GHG) and, at the same time, resulted in a reduced fuel spend for the contractor. 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 250oF but, with 5% moisture in the incoming materials, dew point is about 160oF, almost a hundred degrees lower.

By reducing the exhaust gas temperature to say 170oF, that temperature gap is reduced to 10oF. 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

The Astec Phoenix Burners achieve ultra-low emissions performance by employing superior fuel/air mixing technology rather than the older combustion modification techniques that attempt to compensate for poor mixing.

This thermal image clearly shows that a lot of heat and, therefore, fuel is being wasted because of a lack of insulation and high exhaust gas temperature relative to dew point.

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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 down side 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 actually 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 mentioned in the previous paragraph was developed. The flights used in that system are different. They have deep v-shaped cuts in them that enable them to produce a complete aggregate veil whether they are lightly or fully loaded thus preventing heat from escaping via that path of least resistance. Some benefits of the

V-Pac® system are improved thermal efficiency and associated fuel bill reduction, enabling of the aggregate drying system to comfortably handle the superheat temperatures needed for production of high RAP mixes, and increased production capacity, and more

complete drying of materials.• 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. Leakage air, whether it gets in through missing seals or worn holes or any other means, absorbs heat and takes it away from its intended use and, thus, makes it necessary to burn still more fuel.

• Stockpiles (sloping, paving, and covering) – 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 and increase production capacity without the slightest concern about depleting them. In fact, the only way they can be wasted is make no effort to optimize their use. 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, like the Southeastern U.S., with a lot of annual rainfall, covering stockpiles can make a lot of sense too. So, again, this is an economically advantageous means of conserving resources, fuel, aggregate, and RAP, as well as reducing greenhouse gas (GHG) emissions. This is probably the most overlooked great opportunity around.

• Insulation of Hot Surfaces – Radiation and convection

A hole like this in the aggregate veil in a Rotary Dryer on the uplift side allows hot burner gases to bypass the drying job. The ASTEC V-Pac® system plugs the hole.

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.

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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. The fuel spend is reduced with every increment of fuel savings. GHG production is likewise reduced.

• Warm Mix Asphalt (WMA) – We have already mentioned that producing WMA in the place of HMA saves fuel consumption and reduces all combustion-source emissions, because of the lower

mix temperature. It should be obvious that this is a good thing. 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. If

more evidence is needed, just remember that WMA can be made by just injecting a miniscule amount of water into the hot liquid asphalt to produce the asphalt foam that is a key to successful WMA production.

• Avoid overheating the mix – There is always the FOB customer that 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 asphalt plant 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. In reality, many forests are privately owned and only exist because the owner

Conventional Dryer Thermal Image Double Barrel Thermal Image

Excessive Blue Smoke emissions, actually condensed vapors of volatile organic compounds (VOCs), are caused by overheating the mix.

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grows the timber for profit. Without that opportunity, the owner (tree farmer) would remove the trees and grow some other crop like cotton or corn on the same land. There are at least three avenues that could be travelled to get to biomass-fired asphalt plants. The first of these is to burn wood chips in a furnace and use the hot furnace exhaust gases as the heat source for the aggregate dryer. All of the necessary equipment and technology for this is either currently available or can be developed in a reasonable period of time. 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 would at least several hours. However, capital cost is high. 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. The catch with this approach is that there is no really good and 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. This technology exists and is advancing but is not ready yet on a scale that would support this demand. However, it is being developed for use in utility power plants. Once it reaches a level of production to be viable in that industry, asphalt plant demand would be a small side market. If a plant is in close proximity to a furniture factory or other industry that produces sander dust, that can be a source of small enough particle size to burn in the Astec Biomass Burner. Lastly, there is biodiesel. See the table above for comparison of biodiesel to other fuels. 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. Here are some

of those.

• Variable Frequency Drives for Fans (VFD) – Using variable frequency drives 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 was doubled. That is to say fan speed was increased by a factor of two (2). To make that happen, power input from the fan motor had to 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; so the fan ends up wasting a lot of electricity. Typically, the next largest

EXAMPLE: ELECTRIC POWER SAVINGS GAINED BY USING A VFD TO CONTROL EXHAUST FAN VOLUME BY FAN SPEED ON A 400 TPH PLANT OPERATING AT 80% OF CAPACITY

If a 300 hp fan drive with VFD speed control operates at 50% power for one hour as opposed to one with outlet damper control that operates at 90% power for that same hour, the savings is:

Electric power savings = ((0.9x300 hp – 0.5x300 hp) x 1hr x $0.11/kwhr) / 1.341 hp/kw = $9.84/hr.

Typically a 400 tph plant can produce at 80% of rated capacity, 320 tph, on this much exhaust fan power.

Electric power savings per ton = ($9.84/hr.) / 320 tph = $0.031/ton

Annualized power savings = $0.031/ton x 500,000 tons/yr. = $15,500.00/yr.

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fan employed at an asphalt plant is the burner blower. A proportional energy savings is available there 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, therefore, provide this electric power savings. Reduced electricity consumption at the plant means less coal is burned at a power plant somewhere. So, once again, we see conservation of energy resources and associated reduction of GHG emissions resulting in increased profitability at the plant through reduced operating cost.

• Variable Frequency Drives (VFD) for Drag Conveyors and Other Equipment – There is not much of an electricity savings available by application of VFDs 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. Reduction of wear is important in itself because worn parts have to be replaced. Of course, a worn steel or cast iron part can be recycled but the part of it that has been worn away cannot. So, the sustainability of iron and alloying metal resources is connected to wear, as is the cost of replacing the part. Of course, proper lubrication is an important factor in minimizing the wear rate of many components of asphalt plants.

• 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 are sufficient land area, 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, we have to 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.

Let’s shift now to sustainability of materials. There are several different categories of materials in use at any asphalt plant. The incoming raw materials consist of virgin materials and recycled materials. There are also finished materials and waste materials. Almost all of these materials consist of mineral aggregates (rock and sand) and liquid asphalt.

• Rock, Sand, and Recycled Asphalt Pavement (RAP) - One might say that the whole Earth is made of rock so why worry about the supply being sustainable. The availability of rock is a concern and a big part of the problem is that the requirements for rock for construction of highways that can stand up to heavy truck traffic are very stringent. Not just any rock will do. Access to high quality rock is complicated by land being occupied by residential, commercial, and industrial uses. As this happens, potential viable quarry sites are becoming ever scarcer. That is especially true near urban areas where the need for materials is greatest. Also, some geographical regions, like much of Florida, have no good local rock. So, high quality rock has to be brought in by truck, rail, barge, or ship. By the way, all of these several transportation means use fuel. As much as is practical, mixes should be designed to utilize indigenous stone and sand. However, those materials sometimes just will not work for demanding applications. In many cases, there is plenty of good local rock already on the roadways that are being resurfaced. So, the best thing to do is minimize the use of new rock. Whenever possible, in preparation for resurfacing, the old surface materials (RAP) should be recovered by milling it up and returning it to the asphalt plant. Contractors and DOTs should work together to maximize the use of RAP, thus stretching the remaining available virgin rock further. This also conserves fuel and electricity use, since quarrying new materials uses a lot of energy. After all, which rock is older, the one on the road or the one still in the quarry. A Japanese contractor was quoted as telling his customers about the aggregate in the RAP he was using, “All the same age” as the virgin aggregate.

• Liquid Asphalt – Most liquid asphalt is the residual of petroleum refining processes. Natural deposits, such as the well-known Trinidad Lake asphalt, do exist but they are minor sources by comparison. If fossil fuel use diminishes, the availability of liquid asphalt, also called asphalt cement (AC) or bitumen would also diminish. Ironically, the opposite situation, increasing demand for fossil fuels, is also a threat to the liquid asphalt supply. With increasing oil demand or decreasing supply, at some point, operating “coker units” that extract more fuels from petroleum refinery

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residuals, leaving no asphalt, would increase in number and capacity. Since RAP includes not only rock and sand but also high quality AC, maximizing the use of RAP will help deal with any future shortages of virgin AC.

• Asphalt Roofing Shingles – The asphalt shingles enthusiasm seems to have diminished. Some contractors have found shingles difficult to use and the asphalt supply has loosened. When processing shingles for use, dealing with size reduction and the resulting high moisture content, various problems are encountered. Nevertheless, shingles consist of some good materials, especially liquid asphalt. The asphalt content is about 18% in the southern tier of the United States and up to about 22% in northern tier of the United States and Canada. Prepared, processed, and incorporated in the mix correctly, shingles contribute to excellent mix and effectively replace some liquid asphalt. In as much as using shingles solves the used shingles disposal problem as well as providing a useful asphalt pavement material, it seems to me we should continue to use them and just keep improving our technology, based on science.

• Rubber Tires – The accumulation of rubber tires continues to be a big problem. However, this industry cannot solve that problem. We can and do take some of them and can make excellent special-purpose mixes with them by incorporating fine rubber powder. Special-purpose pavements can bring in a good return. So, we can contribute to the solution. We just can’t do

In this Google Earth photo, there are three quarry sites. The blue area is an expended and, now, water filled quarry. In the upper middle is a quarry that is pretty advanced in its active life and will likely become another blue pond in the not too distant future. In the upper right is a newer quarry that has several years ahead of it; but notice that the surrounding area, which contains plenty of good rock, is occupied by other businesses.

it alone.

• Plastics – Plastic packaging material rubbish has become a huge environmental problem….and maybe an opportunity for asphalt. Polymers, as everyone in the industry already knows, can be very beneficial to asphalt pavements. Plastics are all polymers but not all polymers are created equal. The idea behind waste plastic as an opportunity is that we can somehow break down and put recyclable plastics to beneficial use in pavements. The National Center for Asphalt Technology (NCAT), the National Asphalt Pavement Association (NAPA), Dow Chemical, and other researchers as far away as India and Australia are working on whether and how recycling of plastics into pavements can be done in a way that benefits pavements. The development of the science is still at an early stage according to Dr. Buzz Powell and Dr. Fan Yin, both of NCAT. They emphasized the importance of doing the science before the construction projects. Taking the reverse course could cause project failures that could then lead to the rejection of the whole concept.

There are several different types of plastics to be considered but only two ways of incorporating the plastics into the asphalt. The two methods are “wet” and “dry”. The wet method involves blending the plastic into the liquid binder before incorporating the binder into the asphalt mixture. With the dry method, the plastics are introduced as a dry additive directly

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into the hot aggregate. The sources of the following data on candidate plastics were Wikipedia (https://en.wikipedia.org/wiki/Plastic_recycling) and NCAT.

For the purpose of recycling into asphalt it is unfortunate that most, if not all, plastic drink bottles are made of Polyethylene Terephthalate (PET), which is about as resistant to recycling as it is to natural decomposition. It is said to take about 450 years minimum to break down naturally. (Not sure who has checked this out.) Its melting point approaches 500oF/260°C. Since the temperature of liquid asphalt in our processes never approaches that kind of temperature, it can’t be blended into the liquid nor will it melt down if introduced via a dry process. Neither is it useful as a fiber additive, since it isn’t a fibrous material.

Plastic bags, which are made of High-Density Polyethylene (PE-HD), are a primary focus of research for recycling into asphalt. The relatively low melting point of PE-HD is compatible with normal liquid asphalt process temperatures. So, introduction by Wet Method appears to be most likely to succeed. However, just because two materials exist in liquid state within a certain temperature range does not mean that they will automatically blend well. We are all familiar with oil and water. So, some chemical additives may be necessary to facilitate blending.The waste materials upon which most research effort is being applied and those considered most likely to be recyclable to the benefit of asphalt pavements are these: High-Density Polyethylene (PE-HD), commonly used to make grocery bags, Low-Density Polyethylene (PE-LD), used to make frozen food bags, and Polypropylene (PP), used to make microwaveable food containers. There are also other products, as shown in the tables above, which are made from each of these types of plastics.

• Hydrated Lime (calcium hydroxide)– Hydrated Lime (CaOH) is frequently used for Moisture Sensitivity (stripping) control. It is made from calcium carbonate (limestone). The process of producing hydrated lime involves the release of carbon dioxide. That’s the down side; but there is a substantial environmental upside. Lime is not only an effective anti-stripping additive; it is effective at capturing certain pollutants in plant exhaust gases. First, when applied

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by any means that wets it, including applying it directly to the incoming wet aggregate, it dissociates in the water to Ca+ and OH- ions. Then, when exposed to carbon dioxide (CO2) in the exhaust gas stream, some of the Ca combines with the CO2 to form calcium carbonate (CaCO3), limestone, from whence it came. Lime, so introduced into the Dryer where it has direct contact with exhaust gases, captures and neutralizes

acid-forming compounds. The acids can come from sources such as sulfur in recycled fuel oils (most recycled fuel oils do not contain significant amounts of sulfur) and in ground water (in some areas) and salt in brackish water and in some aggregates. The chlorine in the salt (NaCl), when exposed to the burner flame and the hot moist environment of the dryer, forms hydrochloric acid (HCl). Sulfur, from any source, will form sulfurous (H2SO3) or sulfuric (H2SO4) acids. The unintended benefits of lime in these situations include significant reduction of corrosion potential (drum, flights, ductwork, baghouse steel structure and cages, bags, and stack) as well as improved emissions quality.

• Materials Incorporated in Plant Equipment – As we design plant equipment, we should rethink our design process to design from a human-needs perspective. That means, we have to understand the perspectives and real needs of people. We have to talk to people to solicit their understanding of their needs. It also means that we, as plant designers, have to look beyond what is actually stated and observe people at work on and around the plants to better understand human needs. It may not be obvious why designers must look carefully at and beyond stated needs but here’s an example: A plant operator may say, “I need more production; so I want a long burner flame that puts the fire right into the aggregate so the burner will heat and dryer it faster.” What do we do with that? We must separate the real need, more production, from the erroneous proposed solution, “burner flame into the aggregate”. By doing that, we can provide the result the person needs and

According to internet sources through Google, most plastic drink bottles are made of Polyethylene Terephthalate (PET), which takes hundreds of years to decompose. While plastic drink bottles are tremendously useful, there is a need to do a far better job of collecting and recycling them. However, it may not be possible to make them into a useful additive for asphalt pavements due to their high melting point temperature of approximately 500oF/260°C.

Plastic bags, made of high density polyethylene (PE-HD) typically have a much lower melting point of around 250oF. That avoids at least one obstacle to incorporating the material into the asphalt materials process.

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really wants. The solution might be better control of air leakage or any of several other things. This also means that we have to think long term as we design. We have to think all the way to the end-of-life of the equipment. What happens when the equipment is eventually deactivated? At that point, what can be recycled and what can’t? Are recyclable materials easily separated into different material streams or are they so bound together as to hinder the recycling of otherwise recyclable materials. The way we combine different materials in plant components should facilitate the eventual easy separation of different materials into appropriate recycle streams. This will minimize the cost of disassembly and maximize the value of the recycled materials. By the same token, we need to incorporate recycled materials into our designs as much as is practical. Of course, steel and iron castings, consist of mostly recycled metals anyway, as does aluminum. Where plastics are used for instance, we should consider things such as the facts that ABS plastic is not biodegradable and is made from fossil fuel sources. PLA plastics are made from biomass sources and are biodegradable. However, ABS has strength and temperature resistance that are superior to those of PLA plastics. These design decisions are not always simple.

It should be of considerable interest at this point that almost every action proposed to help us become a more sustainable industry also tends to make us more profitable. So, pursuing sustainable operations is not something we should avoid or resist. To the contrary, it makes sense to embrace it. That does not mean we should jump on every idea that comes along in the name of sustainability but there is no shortage of known and proven opportunities to become more sustainable that make good sense.

In a recent Sunday sermon delivered by my pastor, I learned that there is a psychological phenomenon called “inattentional blindness”. That I didn’t know the name before is probably due to “inattentional deafness”. Most of us have seen an example of inattentional blindness in the “Invisible Gorilla” video illustration of how we miss things that are right in front of us. We often do not see opportunities that are right in front of us. Whatever you call it, it is a thing common to man. So, if you have been missing some of these very profitable and very sustainable opportunities, don’t feel bad. You are not alone. Something else I heard in church recently (See, I was listening.) was that we have the “want to” but we tend to come up short on the “do it”. So, if you have been missing out on some great opportunities, maybe you didn’t see it or maybe you did and want to do it but just haven’t started yet. If saving the planet isn’t your thing, just keep this in mind, the Dirty Little Secret about Sustainability is that IT’S PROFITABLE. Call us if we can help with the “Do it”. ASTEC, Inc. 423-867-4210.

References

1. National Center for Asphalt Technology (NCAT)2. Wikipedia3. Google4. Google Earth5. United Nations, Brundtland Report, 1987 6. United States Department of Energy, Clean Cities

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