How to Make Asphalt the Real Green Deal

Check out Swanson’s three-part series of articles on asphalt sustainability: Part 1, Part 2, Part 3.

With a new presidential administration, changes are coming. Things like higher corporate and personal taxes, the “Green New Deal,” and higher energy prices are much more likely to happen than was perceived just a few months ago under President Trump. Hopefully, a new infrastructure deal will be somewhere in there too; but with higher taxes and energy costs, it may be harder to make a reasonable profit on new work.

That means working efficiently will be even more critical than it has been over the last few years, including during the height of the pandemic. Let’s discuss a few of the most likely places to be able to improve operational efficiencies at the asphalt plant.

Efficiency with Timing

To say “it’s all about speed” might seem simplistic, but is a good point. Speed, maybe even more so than energy consumption, is a critically important key to the profitability of paving operations. One reason speed at the asphalt plant is so important is that there are paving crews out there on the roads. In many cases, especially with an open-graded friction course (OGFC) or high recycled asphalt pavement (RAP) mix, the plant may have trouble keeping up with the capacity of the paving crews. Slow progress on the road is obviously not a good thing. A paving crew and its equipment costs the same dollars-per-hour whether they put down 400 tons of mix or 100 tons of mix; but they can’t put down any more mix than the plant provides.

Asphalt Industry Incorporates Sustainability at Asphalt Production Plants – Part I

Efficiency with Uptime

In many cases, the lowest hanging fruit is to be found in operational practices. One good practice is to keep the plant running. The shutting down and starting up of operation involve using energy and wear and tear on the equipment without making mix. Once you have the plant hot and running, keep it that way as long as you can. Store mix and slow down if necessary but keep going.

Midstream stops are not good either. Midstream stops should be treated as emergency stops. Don’t do it unless you have no alternative. It’s tough on the equipment and, therefore, costly. To say “slow it down” right after saying “run fast” sounds a bit odd; however, if you look at the average production rate resulting from alternately running fast and shutting down and then starting up only to do it all over again, it is likely to be slower than the rate that results from keeping it going.

Efficiency with Drying

Another good place to look for efficiency is in the water. Keep the aggregates and RAP as dry as possible. Most often, the resulting production capacity gain matters even more than the fuel savings. Of course, the gold standard is to put stockpiles on sloped, paved surfaces. In wet regions like the Southeastern United States, covering stockpiles is worth looking at. Something that is used in the frac sand business, which could benefit asphalt producers, is a French drain type system under the aggregate stockpiles, particularly under the sand piles. Some frac sand producers are using this method to drop the moisture content of washed sand piles from 22 percent to 5 or 6 percent in 24 hours or less.

Industry Incorporates Sustainability at Asphalt Production Plants – Part II

Of course, you need to make sure your dryer is working as efficiently as possible at all production rates with all mix types. This also goes to productivity and energy efficiency. You can ensure the dryer is doing a good job when you check for these clues:

1. Steam

Is the mix really dry or do you see steam coming from the drag conveyor or silos? Pull a shovel full of mix out before it goes up the drag. Is it bubbling? Does the mix lose temperature in the silo? Does the mix look dull or does it look shiny black?

2. Baghouse Temps

Is the baghouse too hot? The exhaust system capacity often sets the capacity of the whole plant. One way to say it is, “When you are out of air you are done.”

Open-graded mixes and high-RAP mixes are challenging when it comes to keeping baghouse temperature down. Most plants are rated based on exhaust gas temperature of about 250 degrees Fahrenheit. If your plant produces 300 tons per hour with this exhaust gas temperature, it is producing an exhaust gas volume of about 50,000 acfm. Gases expand with increasing temperature. This same amount of exhaust gas becomes a little more than 57,000 acfm if its temperature increases to 350 degrees Fahrenheit. So, if the capacity of the exhaust system is 50,000 acfm, you have to back the production rate down to a lower tonnage per hour to get back to what the exhaust system can handle. It takes some expertise to optimize dryer performance but the results are worth some investment.

3. Baghouse Mud

The baghouse could be still cooler. There is more to be gained from controlling exhaust gas temperature than we usually think about. Most experienced operators have had a bad experience, or know of another operator who did, with making mud in a Baghouse. When that happens, it’s a bad day.

Operators want to run, of course, at a temperature that stays safely away from the exhaust gas dew point. What few people in the industry know is that the dew point is typically about 167 degrees Fahrenheit, almost 100 degrees lower than where we tend to think we have to run. We could run safely, in most cases, at 185 degrees Fahrenheit with the results of increased productivity and energy efficiency.

Insulating the baghouse and ductwork would add insurance against the possibility of getting condensation. This brings us back to the dryer. To take advantage of this opportunity also takes some expertise in optimizing dryer performance.

Industry Incorporates Sustainability At Asphalt Production Plants – Part III

Efficiency with Regulations

New government directions will bring challenges and opportunities. There are always ways to adjust to do our jobs better and be more profitable. When things change, we have to change, too. Expertise is available from numerous sources in this industry. Use that expertise to make wise, informed changes to your plant equipment and operations.

By the way, we have been hearing “sustainability” a lot and will, I think, be hearing it much more. There are numerous definitions of that word in use but I think the best one has to do with not using up or destroying that which will be needed by future generations. The concepts discussed here go in that direction.

Operating more efficiently conserves energy as well as improves profits. Getting on and off the road more quickly frees up traffic sooner, thus conserving fuel that cars and truck use. The asphalt industry can and often does provide “the real green deal” when it comes to operating efficiently.

Malcolm Swanson, P.E., is the proprietor of e5 Engineers, Chickamauga, Georgia. With at least 33 innovative patents and a career in asphalt plant engineering solutions, he can be reached at (423) 667-6781 or malcolm@e5engineers.com.