What’s new and what’s next in sustainable asphalt production

Maximising sustainability is a key driver in asphalt production as Dr Hans-Friedrich Peters at Ammann believes
Materials / May 10, 2021 3 mins Read
By Dr Hans-Friedrich Peters
Dr Peters says that the latest asphalt plant technology offers benefits for sustainability
Dr Peters says that the latest asphalt plant technology offers benefits for sustainability

Sustainability has been part of the road building 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.

Conversations about green road building always start with the use of recycled asphalt (RAP) and whether these recycled materials really perform as well as mix made from virgin aggregate.

The biggest reductions in indirect CO2 emissions result from the implementation of RAP, which should not be regarded as waste material. It is a suitable substitute for virgin materials as the aggregates in reclaimed asphalt show little ageing and are mechanically and geometrically within the quality ranges of new material. Bitumen holds up well, too. Its ageing is limited and can be compensated by using small amounts of new bitumen.

Using RAP saves on both aggregate and bitumen costs while reducing emissions, initially and over the lifetime of a road.

The latest 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.

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

New designs of tanks can help minimise odour
New designs of tanks can help minimise odour

The earlier adopters are now recycling even more. That can result from governments lifting restrictions, but increasingly 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 no secondary heat source.

At some point, it would seem that manufacturers could only make so much more progress on emissions, but there are opportunities for further reductions. The newest is in regard to volatile organic compounds (VOCs). These compounds must be diminished in the clean gas stream to cut the total carbon value.

A cut in CO2 emissions can be accomplished by actively cooling the drum, increasing drying efficiency and utilising energy sources such as biofuels and wood dust. The latest technology can diminish CO2 by 10% or even 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. New technology allows the lowest values for all of them (< 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. Leading players such as Ammann have focused research and development on the complete manufacturing process for LTA.

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 the leading firms now make their plants as quiet as possible.

Various sound-suppression packages are available to meet customer 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. More and more sound-suppression options are now available in the market, all the way to cladding the entire plant, which can even make it look like a commercial building.

Dust is another issue that can be addressed with new technology, with the design of the baghouse playing a key role. The latest, highly efficient baghouse filters can reduce exhaust dust to less than 10 mg/m3. Ammann is currently working on reducing this value significantly, 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 wheeled loaders, are creating much more dust than the plant itself. However, countries like China, and also some areas in Europe, are increasingly considering these other sources. Additional solutions have been developed to further limit dust. These 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.

To summarise the current state of emissions, 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. Overall the requirements are becoming stricter but they are different from one country to the next.

In regard to odour, bitumen fumes are the primary source. Different solutions are available to contain the fumes and the odour that can result. As with dust, there are key areas where odour can be reduced, such as 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 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.

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

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

Plans able to use a high percentage of RAP in the feed offer major advantages for sustainability
Plans able to use a high percentage of RAP in the feed offer major advantages for sustainability

For example, the wood dust burner transforms wood dust, a material 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 plants and can be retrofitted on existing plants as well.

Biofuels of course are another initiative. 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. 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 the industry.

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.

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, 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.

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