What’s New – and What’s Next – in Sustainable Asphalt Production

Sustainability has been part of the roadbuilding conversation for decades. Yet improvements that make the methods and machines more environmentally friendly continue to gain momentum.

Industry leaders increasingly see green practices as not only a way to better the world, but a tactic to improve profitability, too.

At the forefront of such sustainable efforts is Dr. Hans-Friedrich Peters, Executive Vice President of Ammann’s Plants Division. Dr. Peters recently provided an in-depth look at how asphalt production has become more sustainable – and a glimpse at what’s on the horizon, too.

Conversations about “green” roadbuilding always start with the use of recycled asphalt (RAP). Can these recycled materials really perform as well as mix made from virgin aggregate?

Yes, the conversation does start with RAP – and it should. The biggest reductions in indirect CO2 emissions result from the implementation of RAP.

We should not categorise RAP as waste material. It is a perfectly fine substitute for virgin materials. The aggregates in reclaimed asphalt show little aging and are mechanically and geometrically within the quality ranges of new material.

Bitumen holds up well, too. Its aging is limited and can be compensated by using small amounts of new bitumen. When utilising RAP you’re saving on both aggregate and bitumen costs – while reducing emissions, initially and over the lifetime of a road.

Our technology allows the use of RAP percentages up to 100%. In reality, the percentage is usually much less based on the amount of RAP that is available and the recipes defined by the authorities.

Are countries increasingly adopting recycling, or have we hit a bit of a plateau? And what about the earlier adopters – are they taking further strides or are they content to recycle at existing levels?

Many countries that did not initially adopt recycling are now moving ahead rather quickly. China is an example of this. The country is leveraging some of Ammann’s most advanced recycling plants and creating mix with extremely high percentages of RAP.

The earlier adopters are now recycling even more. That can result from governments lifting restrictions, but increasingly it’s because the asphalt producers see the value of RAP.

Whatever the motivations, the global community is benefitting. From an environmental perspective, all parties involved should increase their efforts to expand the percentage of RAP being used for new pavements.

The challenge with RAP is the heating of the materials. Hot temperatures damage bitumen. In some processes, virgin aggregate is heated, which mixes with the RAP to raise its temperature. But when making mix with 100% RAP, there is no virgin aggregate – and therefore no secondary heat source. How do Ammann plants heat the 100% RAP mix without damaging the bitumen?

On the high end of RAP utilisation is the Ammann ABP HRT (High Recycling Technology) Asphalt-Mixing Plant. As stated, it can produce mix with up to 100% RAP. No virgin aggregate is required.

There is considerable technology and innovation involved in the HRT concept, in particular the RAH100 counterflow drying process technology. Essential to the RAH100 is its gentle heating process. During the warm mix process, the dryer heats materials between temperatures of 100ºC and 130ºC. It also makes asphalt at 140ºC to 160ºC if a more traditional mix is desired.

The heating is usually where the complications with RAP material arise. RAP must reach its target temperature, but the valuable bitumen will be damaged if the material is heated too quickly.

The RAH100 eliminates that concern. It consists of two connected sections. One is a hot gas generator that contains a burner and forces air toward the second section, which is a counterflow dryer.

The RAP enters at the far end of the counterflow dryer section and moves toward the heat chamber. At the end of the counterflow dryer, RAP is transported to an accompanying silo. The heated RAP mix leaves the dryer before the temperature becomes excessive, so it never reaches the critical temperature where the bitumen is damaged.

One quick point about the ABP HRT: It’s an extremely advanced plant – I would say it’s clearly the industry leader. Yet Ammann always strives for further improvement, so this system, created more than a decade ago, is constantly improved. That includes the hot gas generator, which has been upgraded on multiple occasions.

The appearance of the ABP HRT is unique, with the recycling system placed above the mixer. What is the purpose of that design?

The plant is designed around the incorporation of large percentages of RAP. What you see is a nod to the fact that the HRT plant has elevated RAP from a supporting role to the lead actor.

As you stated, the most striking difference is that the ABP HRT’s entire recycling system is arranged vertically, in direct line above the mixer. This allows materials to be dropped instead of conveyed, which minimises wear and optimises transport of the hot RAP. The HRT approach also means that there is enough room in the plant’s tower for additive feed components and for carrying out inspection and maintenance work.

Today the HRT concept is the smartest operational method for handling the specific properties of RAP.

Making mix that theoretically consists of 100% RAP is impressive. Yet many mix makers will utilise lesser amounts of RAP. What are the solutions for these customers?

Many of our customers fall into this category, and we most assuredly have products for them.

There are varied heating processes that depend on the amount of recyclables. The RAH60 is a parallel flow dryer where up to 60% hot recycled materials can be fed. The RAH50 is a middle-ring dryer that incorporates up to 40% hot recycled materials.

Recycling can occur at Ammann plants without these specific dryers. Up to 30% cold recycled material can go directly into the mixer, meaning almost every Ammann plant is capable of utilising that amount of RAP.

At some point, it would seem that manufacturers such as Ammann could only make so much more progress on emissions. Are there opportunities for further reductions?

There are. The newest is in regards to volatile organic compounds (VOCs). These compounds must be diminished in the clean gas stream to cut the total carbon value

We continually work to reduce CO₂ emissions. This can be accomplished by actively cooling the drum, increasing drying efficiency and utilising energy sources such as biofuels and wood dust. Ammann technology can diminish CO₂ by 10% or even considerably more, depending on the age of the plant and the technology chosen.

There are other somewhat hidden opportunities to trim CO2 emissions, including the bitumen tank farm. A traditional farm consists of horizontal tanks heated with thermal oil. Changing to an electrically heated, vertical tank farm results in considerable advantages. There is no oil consumption and therefore no emissions. Electric heating is cost-effective, too. In fact, electrically heated bitumen tanks have become standard in all of Europe and other parts of the world.

There are also other emissions like dust and odour. Their level of importance and the maximum values being allowed differs greatly from country to country and area to area. Our technology allows the lowest values for all of them (e.g. < 10mg/m³ of dust) without causing any restrictions on the plant operations.

Low temperature asphalt (LTA) is another opportunity that is becoming more prevalent. While conventional asphalt is produced at around 170°C, the low temperature processes of today allow production temperatures of around 100°C. Lowering the manufacturing temperature eases energy needs, and therefore emissions, too.

LTA impacts the entire production process – including drying, mixing sequences and recycling. Ammann has focused our research and development on the complete manufacturing process for LTA.

The placement of plants in residential and commercial areas also raises noise issues. Why are plants located in these areas, and what can be done to limit the noise?

A shortage of industrial land means that asphalt plants increasingly must be located closer to residential areas. Local governments can have very strict standards when it comes to noise, so we have to make the plants as quiet as possible.

Ammann has been very proactive on this front. We offer varied sound-suppression packages to meet our customers’ specific needs. Some customers need to lower sound a bit, while others have to take more substantial measures. The efforts start with equipping burners with variable speed motor drives, which are much more quiet, and stack silencers, which control exhaust noise. We offer more and more sound-suppression options, all the way to cladding the entire plant.

That cladding, by the way, makes the plants look like commercial buildings. They are beautiful facilities that fit nicely in urban office parks. Passersby would never guess there is an asphalt-mixing plant inside.

You referenced dust emissions earlier. This is becoming a bigger issue as plants must often be located in sensitive areas. Can anything further be done to limit the dust that results from these plants?

The conversation about dust emissions starts with the baghouse. Ammann Asphalt-Mixing Plants remove dust through a highly efficient baghouse filter. It actually lowers exhaust dust to less than 10 mg/m3, which is an exemplary benchmark. We are currently working on reducing this value significantly again, to < 5mg/m³.

People often focus solely on the dust resulting from the mix-making process, and what comes out of the chimney. They forget that all the logistical operations around an asphalt-mixing plant, and around equipment like trucks and wheel loaders, are creating much more dust than the plant itself. Fortunately, countries like China, and also some areas in Europe, are increasingly considering these other sources.

Ammann and our customers have together developed solutions to further limit dust. We focus on dust reduction points for further improvement. Taking measures at the cold feeder, load-out, skip hood, overflow silo, filler loading area, screen, belts and transfer points makes a big difference. That’s in addition to the efforts provided through the baghouse.

To summarise the current state of emissions, I would say the main focus is on trimming CO2, VOCs and NOx in the combustion process and on reducing the residual dust content after the baghouse. There are also markets in which, for example, the integration of pre-dosing into the dedusting process is also being promoted.

Are local governmental requirements becoming stricter in general? We discussed CO2, sound and dust emissions, but what about odour – particularly given that the plants are increasingly placed in residential or commercial areas?

Overall yes, the requirements are becoming stricter – but they are extremely different from one country to the next.

We are eager to comply with all the regulations because it’s the law and because we want to be good neighbours, too. That means a lot more than shrinking carbon and VOC emissions. It also includes muffling sound, which we just discussed, as well as dust and odour.

In regards to odour … Bitumen fumes are the primary source of odour. Ammann offers different solutions to contain the fumes and the odour that can result. As with dust, we have reduction points – in this case the bitumen tanks, the skip and load-out levels and the stack.

There is a great deal of talk about alternative energy sources, including biofuels. But some mix producers who are contemplating a plant purchase might be hesitant to commit to such fuels, as they are somewhat unproven and their availability might not be as consistent as traditional sources.

Ammann biofuel burners can also utilise more traditional fuels such as natural gas, LPG, light and heavy oil and kerosene. This alleviates the concerns of customers who are hesitant to rely solely on newer fuels.

The use of these new fuels is another meaningful win on the green front. We are taking renewable energy sources or, in some cases, converting a waste product into fuel. This conserves natural resources and puts less pressure on landfills.

On the renewable front, we are very high on the wood dust burner. The burner transforms wood dust, a material that is available from local sources, into a renewable fuel. What makes this dust burner even more exceptional is its carbon neutrality. The carbon dioxide released when burning wood is offset by the fact the tree consumed that amount of carbon dioxide during its life. Therefore, this part of the emissions is carbon-neutral.

The burner has proven effective and is utilised on a number of Ammann Asphalt-Mixing Plants. It can be retrofitted on existing plants as well.

Biofuels of course are another initative. They support climate protection and reduce dependency on mineral oil. Examples of these fuels are rapeseed and sugar cane. Tall oil, which is a waste product of cellulose sulphate production, can be used, too.

We expect that in the near future other fuel types such as hydrogen will significantly reduce gas emission values. These fuels will also be much more important in our industry. Ammann is already working on solutions to be prepared for this.

Plant owners might look at these comments and say, “These are great ideas, but I already own a plant.” How can an asphalt producer begin to make the change to a more green operation while utilising their existing plant? 

Asphalt producers might be surprised by how much they can accomplish with their existing plant. A very easy first step is to upgrade the control system.

A modern control system can have a significant impact on efficiency, and that cuts across many parts of the process. Improved efficiency will lessen fuel usage, emissions and material waste. And the as1 is the best in the business at doing exactly that.

Training is another immediate step that can be taken. The best plant and control system in the world will underperform if the operator is unable to leverage the built-in value.

Another option is a more comprehensive retrofit. It still costs a fraction of the price of a new plant and is compatible with products made by Ammann and other manufacturers.

A retrofit has a host of options you can choose from, including recycling solutions. A retrofit enables the use of foam bitumen, waxes and other additives. Special bitumen and alternative mixing cycles can be utilised as well.

Again, the plant owner can determine the level of the commitment. Many retrofit customers incorporate a new dryer, which optimises heat transfer – and of course reduces emissions – and enables the employment of an expanded range of materials, including RAP.

A retrofit can include environmental upgrades to the bitumen tank and baghouse. It can incorporate noise reduction solutions, too. A host of technological improvements can be made – including revamped burners, mixers and the control system.

What is the next step for a business that wants to explore some of the solutions you referenced? 

They should get in touch with Ammann sales or support teams. If they don’t have a specific contact, they can visit Ammann.com. There is a “Find A Dealer” link prominently displayed on the home page. The website also has a host of information on all Ammann products, including asphalt-mixing plants.