Emissions & Fuel Savings Using WMA at Reduced Asphalt Mixture Production Temperatures

This article appears in the Summer 2024 print edition of NAPA Quarterly. Subscribe here.

The Road Forward to Lower Emissions and Higher Profits

SUSTAINABILITY THROUGH LOWER PRODUCTION TEMPERATURES

As the asphalt pavement industry looks for ways to reduce the carbon footprint during mixture production, Warm Mix Asphalt (WMA) is an existing technology which can be used in conjunction with reduced asphalt mixture production temperatures to assist mix producers reach net zero greenhouse gas (GHG) emissions by 2050. Although WMA is correctly viewed as a solution to the GHG emissions focus, there is limited data characterizing the reductions in emissions and fuel consumption. In 2022, an Ingevity research team collected emissions and fuel data at multiple asphalt plants across the US. The data collected illustrates WMA GHG reduction impacts on three key areas: fugitive emissions at the plant and construction site, stack emissions, and burner fuel consumption.

In field studies in five states, dense graded HMA was produced at 154-163°C in continuous drum plants at production rates ranging from 192-324 tons/hour. Natural gas burner fuel was common to all plants. Chemical additive WMA also was produced at the same production rates in the same mix plants but at temperatures 19-39°C lower than the HMA controls. Mass concentrations of fugitive particulate matter (PM₁, PM_2.5 and PM₁₀) were reported at the top of silo, load out, center of screed and screed operator location as a function of mix production temperatures. PM mass concentrations were measured in 60-minute intervals. Burner fuel consumption and stack emissions were also measured during the same 60-minute intervals. For all mixtures produced, PM emissions also were measured at 60-minute intervals at the corresponding asphalt pavement construction sites by positioning the particle-size monitors at three positions on the asphalt paver: the hopper, the center of the screed, and the auger box.

The asphalt mixtures produced during the data collection of this study represent a reasonably diverse sample of different mixture designs, asphalt binder grades, asphalt binder contents, and Recycled Asphalt Pavement (RAP) contents.

FUGITIVE EMISSIONS AT THE PLANT AND CONSTRUCTION SITE

The US EPA Compilation of Air Pollutant Emissions Factors from Stationary Sources (AP42) states:

Organic aerosol is created by the condensation of vapor into particles during cooling of organic molecules volatilized from the asphalt cement in the mixer. The amount of organic aerosol produced depends to a large extent on the temperature of the asphalt cement and aggregate.

Airborne particulate matter (PM) is a mixture of solids and aerosols that vary in shape, size, and chemical composition. PM is classified by their aerodynamic diameter where PM₁₀ are coarse particles less than 10 microns, PM_2.5 are fine particles less than 2.5 microns and PM_1.0 are very fine particles less than 1 micron. PM₁₀ sized particles can be inhaled into the lungs and finer particles make their way deeper into the lungs. EPA recently announced a new Final Rule to Strengthen the National Air Quality Health Standard for Particulate Matter for PM_2.5 from 12 μg/m3 to 9 μg/m3. (See note.)

The PM₁₀ size fraction data collected at the 5 plant locations indicate a significant reduction by decreasing the asphalt mixture production temperature.

BY THE NUMBERS

• In five asphalt mix production locations, dense-graded mix was produced with RAP Content % ranging from 15%-40%, Total Binder Content % ranging from 5.1%-6.0%, Virgin Binder Content % 3.4% to 4.7%, Virgin PG included 58-28, 64-28, 64V-22 and 64S-22 and NMAS ranging from 9.5mm to 19mm.
• PM₁₀ concentration data showed that across the different sampling sites, the average % reduction in PM₁₀ concentration was between 80% and 90% from an average WMA temperature reduction of 30°C.
• PM_2.5 concentration data showed that across the different sampling sites, the average % reduction in PM₁₀ concentration was between 67% to 90% from an average WMA temperature reduction of 30°C.
• CO₂ concentration decreased on average 22.3% for the same WMA mixture production temperature reduction of 30°C.
• Burner fuel consumption decreased 20.4% for the same WMA mixture production temperature reduction of 30°C.
• On Feb. 7, 2024, the US EPA announced a final rule to reduce the National Ambient Air Quality Standards (NAAQS) for fine particle pollution, or PM_2.5, to an annual PM_2.5 standard of 9.0 μg/m3 (down from 12 μg/m3) to reflect new science on harms caused by particle pollution².

REDUCED STACK EMISSIONS

In addition to lowering vapor and aerosol emissions from asphalt production and pavement construction operations, reduction of greenhouse gas emissions resulting from elevated temperature production of bituminous paving materials is also a critical focus area. In North America, using lower mix production temperatures and warm mix asphalt technology is an important tool to lower greenhouse gas emissions at production plants. Lower mix production temperatures are achieved by lowering burner temperatures and aided by chemical additive WMA. Lower burner temperatures mean less fossil fuel is combusted, resulting in less combustion gases produced, such as CO₂ and CO, compared to higher burner temperatures used for traditional asphalt mix asphalt production.

Stack emissions data collection was performed using third-party contracted companies that specialize in stack testing. EPA methods 1 & 2 — volumetric flow rate, EPA method 3A — CO₂ and CO, and EPA method 4 — moisture content, were used to collect stack testing information. Measured volumetric flow rate and concentration of the carbon dioxide were used to determine the amount of carbon dioxide produced during each of the one-hour testing runs.

The primary stack emission type tracked was CO₂. Data collected included the gas flow rate (dscfm), and the measured CO₂ concentration of the emitted stack gas. From these values, the CO₂ emissions rate (kg/mix ton) was determined by using the known CO₂ molecular weight. This estimate highlights the concept of how WMA can reduce stack emissions. For an average temperature reduction of approximately 30°C across the four projects where stack emissions data was collected, the average CO₂ percent reduction was 22.3%.

BURNER FUEL CONSUMPTION

The fuel usage at each asphalt plant was measured and estimated manually by taking pictures of the natural gas fuel meter every ten minutes during one-hour sampling runs. Two to three runs were conducted for each reported temperature both HMA and WMA and the emissions for these runs were averaged to produce an average fuel use data set for each plant temperature scenario.

For an average temperature reduction of approximately 30°C across the five projects where fuel usage data was collected, the average percent reduction in fuel spent was 20.4%.

SUMMARY

This work shows how temperature reduction with chemical warm mix can be used to reduce fuel, CO2 and fugitive emissions (or blue smoke) at an asphalt mixture production facility while also mitigating the worker exposure hazards from asphalt fugitive emissions at the paver.

FOOTNOTES

1 How To Make Warm Mix Asphalt Work For Your Plant | For Construction Pros
2 National Ambient Air Quality Standards (NAAQS) for PM | U.S. EPA

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