Warm Mix Asphalt (WMA) Emission Reductions and Energy Savings 

By 

Bob Frank 

Consulting Engineer 

Compliance Monitoring Service 

217 Belhaven Avenue 

Linwood, NJ 08221 

1 (609) 926-2184 

Refrank2@gmail.com

(Corresponding Author) 

Brian D. Prowell, Ph.D., P.E. 

Principal Engineer 

Brian@AMSLLC.US

Graham C. Hurley, P.E. 

Project Engineer 

Graham@AMSLLC.US

Advanced Materials Services, LLC 

2515 E. Glenn Avenue, Suite 107 

Auburn, AL 36830 

1 (334) 246-4428 

Randy C. West, Ph.D., P.E. 

Director 

National Center for Asphalt Technology 

277 Technology Parkway 

Auburn, AL 36830 

1 (334) 844-6228 

9/6/2011 

Word Count = 3,962 
Frank, Prowell, Hurley, and West 1 

ABSTRACT 

NCHRP 09-47A set out to more precisely and completely quantify emission reductions and 

energy savings realized by warm mix asphalt (WMA) technologies. The task proved to be 

formidable and despite a skilled team’s best efforts produced results of varying accuracy. Both 

metrics are influenced by a number of independent factors and are difficult to characterize with a 

relatively small data-set of demonstration projects. 

 Stack emissions were measured at two drum plants and one batch plant using EPA 

protocols. Eight different WMA technologies, including: Advera® WMA, BituTech PER, 

Cecabase® RT, EvothermTM DAT, EvothermTM 3G, SonneWarmixTM, Gencor Ultrafoam GX2 

and a new Wax-type additive were compared to hot-mix asphalt (HMA) control mixes. Fuel 

usage data were collected at three additional plants; providing comparisons between each WMA 

technology and HMA. 

 There can be no doubt that producing mix at lower temperatures will save fuel and reduce 

burner emissions. The environmental benefits realized by WMA technologies reflect, on 

average, a 10 to 15 percent smaller carbon footprint. It is also the case that WMA reduces 

hazardous air pollutants and PM 2.5 compounds. While fugitive emissions from silo load-out 

were not measured, they too were noticeably less with WMA. While the literature reports fuel 

savings ranging from 15 to 77percent, savings of 8 to 35 percent were measured in this study. 

However, at two of the three plants where stack emissions were tested, fuel savings resulting 

from the use of WMA paled in comparison to reductions realized by implementing long-standing 

plant operations best practices, most notably burner tuning. 

 For many plants, waste minimization reflects the best opportunity for energy efficiency. 

Waste or reject material is generated when mix is produced too hot or too cold. Several early 

adopters of WMA use it for daily production believing reduced waste and improved customer 

satisfaction are their real paybacks. 

INTRODUCTION 

WMA was initially developed in Europe in the late 1990’s. The development aimed at reducing 

fuel consumption, stack emissions, and worker exposure to dust and fumes. In 2002, the 

technology was introduced into the United States, and the first projects constructed here in 2004. 

 WMA technologies improve the coating and the compactability of asphalt mixtures. In 

many cases, these improvements allow production temperatures to be reduced. With a properly 

tuned burner, reduced production temperatures should require less fuel and result in less stack 

emissions. If WMA is demonstrated to reduce fuel consumption and stack emissions while 

facilitating higher reclaimed asphalt pavement contents, then the use of WMA would be an 

incremental step towards sustainable development for state agencies and industry. 

Existing Data 

Fuel Savings 

Theoretical calculations indicate that a temperature reduction of 50°F (28°C) should result in a 

fuel savings of 11 percent (1). Initial, reported fuel savings for WMA in Europe range from 24 
Frank, Prowell, Hurley, and West 2 

to 55 percent (2-4) with typical values being between 20 and 35 percent (5). Overall, reported 

fuel savings from fifteen WMA projects, representing six technologies, range from a 15.4 

percent increase to a 77 percent reduction (4, 6-14). Larger fuel savings typically occurred with 

technologies like LEA, WAM-Foam, and in some cases, EvothermTM ET, which tend to have the 

lowest production temperatures. LEA and WAM-Foam production temperatures are usually 

close to 212°F (100°C). The average fuel savings for all sites is 23 percent. Heat losses and 

other inefficiencies are believed to account for some of the difference between theoretical and 

observed fuel savings (15). 

 Aggregate and recycled materials stockpile moisture contents can have a significant 

effect on fuel consumption when producing WMA or HMA. This is evidenced by the higher 

fuel savings for WMA technologies, such as LEA, which only dry a portion of the aggregate. 

Fuel usage reportedly increases 10 percent for every 1 percent increase in stockpile moisture 

content (18). Best practices, such as paving under stockpiles or covering stockpiles, are 

recommended for both HMA and WMA to reduce fuel consumption. 

Stack Emissions 

Since most pollutants of concern result from combustion, they can be reduced simply by 

reducing fuel consumption through production of WMA. WMA’s lower discharge temperatures 

should also reduce binder oxidation and volatilization loss during mixing with corresponding 

emission reductions. Unfortunately, WMA’s ability to reduce emissions remains poorly verified. 

 Stack emissions tests have been reported from 17 projects worldwide, representing six 

technologies (4, 7-14). The majority of the stack tests completed to date indicate that WMA 

reduces CO2 emissions. The only case in which CO2 emissions increased (10) involved an 

emulsion that effectively increased the moisture content of the mix and required more heat to dry 

even at lower mix temperatures. Emissions of NOx were reduced in all cases. SO2 emissions 

both increased and decreased. Two projects indicate increased VOCs with the WMA production 

(7, 11). In both cases, reports attributed that increase to poor burner tuning rather than the WMA 

technology. 

Pollutants have been reported in several different units ranging from stack concentration 

to pounds per hour making meaningful comparisons difficult. Most reported emissions are 

simply uncorrected average dry stack concentrations (ppmvd). Comparisons between WMA and 

HMA based on differences in stack concentrations are suspect and may be unintentionally 

misleading. To make meaningful comparisons between tests, e.g. to compare HMA and WMA, 

those results must be normalized to a uniform percent oxygen to correct for dilution. Reports 

that include a mass emission factor, pounds per ton mix, as recommended by the WMA TWG 

(19), can be compared with other factors but cannot be compared to normalized concentrations 

without knowing the burner firing rate. 

PROJECT SITES 

Fuel consumption data were collected from six sites and stack emissions were measured on 

three, multi-technology projects. Three sites were selected for emissions testing since emissions 

are also affected by environmental and plant factors. All of the sites had a corresponding HMA 

control section. Table 1 summarizes the project sites and WMA technologies used. 

 
Frank, Prowell, Hurley, and West 3 

TABLE 1 Project Sites 

Location Plant Type WMA Technology 

Walla Walla, WA Cedar Rapids Parallel Flow 

Portable Drum 

mechanical foaming

Centreville, VA Astec Double Barrel mechanical foaming 

Rapid River, MI Dillman Equipment Parallel Flow 

Portable Drum 

waterless chemical additive, 

foaming additive 

Baker, MT Boeing Parallel Flow Drum chemical additive 

Griffith, IN Astec Double Barrel mechanical foaming, chemical 

additive, wax 

New York, NY McCarter Batch with Meeker 

Continuous Mixer Drum 

waterless chemical additive, 

wax, organic additive 

METHODOLOGY 

Exhaust gas testing targeted emissions related to multiple areas of concern: greenhouse gases 

(carbon footprint), ground-level ozone precursors, condensable particulates (PM-10) and an 

emerging concern regarding hazardous air pollutants. Energy usage, stack emissions, and 

temperature reductions are interrelated, but can be affected by multiple factors, such as aggregate 

moisture, operator, plant configuration, fuel type, production rate, burner tuning, percent RAP, 

ambient temperature, et cetera. Variations between mix design, fuel type, production rate, and 

aggregate moisture were minimized to the extent possible by testing the same mix over 

successive days for the same project. 

Burner tuning was conducted by the lead author at each of the multiple technology sites 

prior to stack emissions tests. In two of three cases, burner tuning reduced CO emissions tenfold 

while the largest WMA reduction measured was 59 percent. In both Michigan and Indiana, initial 

carbon monoxide (CO) measurements exceeded 10,000 ppm. In Michigan, increasing the air to 

fuel ratio dropped this to approximately 50 ppm; in Indiana to approximately 1,000 ppm. Further 

reduction in CO in Indiana would have required the natural gas ports to be cleaned and the premix nozzles replaced. Even so, the Indiana burner adjustments resulted in a 24.8 percent 

reduction in fuel use on the same mix with no other process changes. 

 Sustained production runs at consistent rates and mix temperatures were needed to make 

good assessments of burner fuel usage. The research team worked with the contractors to 

measure burner firing rate and electrical demand on high amperage motors such as slat 

conveyors for each WMA technology and the control mixes. Where possible, fuel usage 

measurements were made on an hourly basis using either liquid fuel meters or gas meters. The 

lone exception was the Montana project , which used liquid propane. Here, estimations of the 

volume of fuel used were made on a daily basis. Stack test data were also used to determine 

independently burner firing rate from EPA’s f-factor. This analysis assisted in identifying 

possible measurement errors by the stack test contractors. 

 At the three multi-technology projects (Michigan, Indiana, and New York), stack 

emission tests were conducted in accordance with US EPA's Title 40 Code of Federal 

Regulations (CFR) Part 60, Appendix A and generally followed the recommendations of the 

WMA TWG. Reported stack emissions included carbon dioxide (CO2) to assess greenhouse gas 

production, volatile organic compounds (VOC) and oxides of nitrogen (NOx) to assess the 

potential for ground level ozone, carbon monoxide (CO) to assess burner tuning, sulfur dioxide 

(SO2), condensed particulates (component of PM-10) and formaldehyde emissions. Results were 
Frank, Prowell, Hurley, and West 4 

analyzed and reported as pounds per unit production consistent with Federal AP-42 emission 

factors. 

 Local emissions testing contractors experienced with these methods were used to 

minimize mobilization costs. The lead author assessed their credentials and coordinated testing 

at each site to ensure meaningful data were obtained. Due to the short notice at each project, 

availability became a significant selection criterion. 

RESULTS 

Fuel Usage 

Measured fuel usages are reported in Table 2. Table 2 shows fuel, mix, composite stockpile 

moisture content, average production rate, average production temperature, and Million BTU 

burned (MMBTU) per ton. In all cases, the use of WMA resulted in a reduction in fuel usage. 

For Site 3, the higher fuel usage for WMA C compared to WMA D, produced at the same 

temperature, most likely was the result of errors in the tank depth measurement. Indirect 

BTU/ton calculations from stack test data indicated WMA D required 0.25 MMBTU/ton. For 

Site 5, the values shown in Table 2 are the overall daily averages including start up, pre-heat, 

plant waste, and shut down. Slightly improved numbers, 0.209 and 0.201 MMBTU/ton were 

measured during steady production for WMA F and WMA H, respectively. It was noted that for 

WMA F, after the stack emissions tests were completed, the plant operator increased production 

temperature to HMA temperatures while still producing WMA F. 

 
Frank, Prowell, Hurley, and West 5 

TABLE 2 Fuel Usage 

Site Fuel Mix Stockpile 

Moisture, 

% 

Production 

Rate, TPH 

Production 

Temperature, 

°F 

MMBTU/ 

ton 

Reduction, 

% 

1 Natural 

Gas 

HMA 2.6 316 325 0.278 

WMA A 3.0 310 285 0.218 21.6 

2 Natural 

Gas 

HMA 2.3 273 318 

WMA B 2.1 219 288 

3 Reclaimed 

Fuel Oil 

HMA 3.6 310 300 0.271 

WMA C 3.9 323 269 0.225 13.0 

WMA D 4.1 320 269 0.187 17.0 

4 Liquid 

Propane 

HMA 1.3 370 298 0.157 

WMA E 1.5 378 252 0.137 12.7 

5 Natural 

Gas 

HMA 3.2 292 300 0.226 

WMA F 3.5 305 277 0.224 0.9 

WMA G 3.8 300 256 0.212 6.2 

WMA H 3.8 279 268 0.201 11.1 

6 Natural 

Gas 

HMA 3.1 271 332 0.260 

WMA I 3.4 244 240 0.236 9.2 

WMA J 2.4 267 252 0.216 16.9 

WMA K 3.6 268 253 0.211 18.8 

 

Stack Emissions 

Carbon Dioxide 

Figure 1 shows average CO2 emissions for each of the mixes tested during the multi-technology 

projects. The colored bars indicate the average of two tests; the whiskers show the individual test 

results. Similar to the fuel usage reported in Table 2, CO2 production is reduced for all of the 

WMA mixes compared to their corresponding HMA mixtures. It was noted at Site 5 that the 

local stack emissions contractor did not take stack velocity readings during the HMA and WMA 

H testing, only at the end of the run. Based on relatively accurate gas meter readings, this 

appears to result in an under-reporting of the actual air-flow and hence the derived lbs/ton CO2

production. 

CO2 emissions primarily result from fuel combustion. As such, there is a linear 

relationship between fuel and CO2 reductions resulting from the use of WMA. Figure 2 presents 

this relationship for both the data obtained from NCHRP 9-47A and the literature. The offset of 

any data point from the Line of Equality reflects an inaccuracy in at least one of the two 

measurements. In the case of WMA D, the reported fuel savings averaged 31 percent, but the 

average CO2 reduction was only 14 percent. Reduction in firing rate calculated from stack data 

suggests actual BTU reduction with WMA D was only 13 percent. 
Frank, Prowell, Hurley, and West 6 

FIGURE 1 CO2 emission rates.

FIGURE 2 Reduction in fuel usage versus reduction in CO2 emissions.

Carbon Monoxide 

 Carbon monoxide (CO) and volatile organic compound (VOC) formation are affected by 

burner design, maintenance, and tuning. A burner that is not tuned properly or one that is not 

well maintained may result in elevated levels of CO and/or VOCs. For most burners, elevated 

CO and VOC emissions are not a surrogate for efficiency. Figures 3 and 4 show the CO and 

VOC emissions, respectively. Overall, CO emissions were elevated at Site 5 compared to the 

other sites. As noted previously, CO emissions exceeded 10,000 ppm at Site 5 prior to burner 

46.0

38.3 39.7

26.8 26.0

24.2

18.6

35.9

28.4

24.4 25.4

0

5

10

15

20

25

30

35

40

45

50

HMA WMA C WMA D HMA WMA F WMA G WMA H HMA WMA I WMA J WMA K

Site 3 Site 5 Site 6

lb/ton

CO2

‐30

‐20

‐10

0

10

20

30

40

50

60

70

‐30 ‐20 ‐10 0 10 20 30 40 50 60 70

Reductionin CO2 Emission,

%

Reductionin Fuel Usage, %

Fuel Usage Vs. CO2

Literature

NCHRP 9‐47A

Line of Equality

WMA D
Frank, Prowell, Hurley, and West 7 

tuning. After tuning, CO emissions for HMA were reduced to approximately 1,000 ppm. Burner 

maintenance issues also resulted in elevated VOCs. Additional reductions would have required 

cleaning out the natural gas ports and replacing the burner pre-mix nozzles, tasks that could not 

be performed within the time allowed for the WMA demonstration. 

The thin horizontal line in Figure 3 represents the EPA’s candidate emission factor for 

CO of 0.13 lbs/ton for drum plants (21) based on stack test data from 18 drum plants. The range 

in data averages 89.5 percent of the mean. Although the CO emissions for WMA D and G appear 

elevated compared to their corresponding HMA controls, both values are within 89.5 percent of 

the HMA, indicating they are within typical testing variability. 

 For Site 3, a parallel-flow drum plant, WMA production reduces VOC emissions 

approximately 50 percent. Site 5 used a counter-flow drum plant. One of the Site 5’s HMA VOC 

readings was 0.012 lbs/ton, which appears to be an outlier. The stack test contractor had 

problems with the high stack moisture content and took the analyzer off line frequently during 

the run to “dry out”. Excluding that run average, the HMA reading would be 0.059 lbs/ton and 

all of the WMA results would reflect a reduction. For Site 6, a batch dryer, all of the VOC 

readings for the WMA mixes are higher than those for the HMA control. However it should be 

noted that VOC emissions for all mixes were among the lowest measured and reflect state-ofthe-art performance in most jurisdictions. A variety of factors could explain an increase with 

WMA, but the uniform increase across three very different WMA technologies suggests causes 

other than WMA itself. 

FIGURE 3 CO emissions. 

0.0255 0.013

0.038

0.392

0.3255

0.6825

0.192

0.031 0.016 0.0125 0.0145

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

HMA WMA C WMA D HMA WMA F WMA G WMA H HMA WMA I WMA J WMA K

Site 3 Site 5 Site 6

lb/ton

CO
Frank, Prowell, Hurley, and West 8 

FIGURE 4 VOC emissions.

Sulfur Dioxide 

 When fuels containing sulfur are burned, SO2 is produced. Sulfur content varies with fuel 

type. Recycled fuel oil tends to have the highest sulfur content, followed by fuel oil. Natural gas 

tends to have the lowest concentration of sulfur in fuels commonly used at asphalt plants. 

Reducing fuel consumption should reduce SO2 production. Figure 5 shows the SO2 stack 

readings for the three multi-technology projects. Overall, the SO2 emissions from Site 5 and Site 

6, both of which used natural gas as fuel, are inconsequential. The spike for WMA F could be 

attributed to a small amount of slag making its way into the mix. The 50 percent reductions in 

SO2 at Site 3, which used recycled fuel oil, are significant. Discounting possible changes in the 

used oil supply, the 50 percent reduction suggests an increase in SO2 control efficiency at lower 

WMA baghouse temperatures. As might be expected, more SO2 condenses out of the exhaust 

gas stream, is captured by the baghouse fines and then encapsulated in the WMA. For reference, 

the EPA’s candidate emission factor for a drum plant using recycled fuel oil is 0.058 lbs/ton and 

for natural gas 0.0034 lbs/ton (21). 

0.035

0.018 0.019

0.036

0.057

0.040

0.048

0.013

0.016

0.021

0.019

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

HMA WMA C WMA D HMA WMA F WMA G WMA H HMA WMA I WMA J WMA K

Site 3 Site 5 Site 6

lb/ton

VOC
Frank, Prowell, Hurley, and West 9 

FIGURE 5 SO2 emissions.

Oxides of Nitrogen 

 Nitrous oxides (NOx) are a precursor to the formation of ground level ozone. NOx

emissions are higher for fuel oils as compared to natural gas. The EPA’s candidate emission 

factor is 0.055 lbs/ton for drum plants burning fuel oil and 0.026 lbs/ton for drum plant burning 

natural gas. For Site 3, WMA C had lower NOx emissions and WMA D the same NOx emissions 

as the HMA. For WMA D, the burner was set at an average firing rate of 26 percent compared to 

75 percent for the HMA and 43 percent for WMA C. This low firing rate may have resulted in 

greater excess air available to form NOx, increasing NOx emissions. For Site 5, the WMA mixes 

produced the same or lower NOx emissions than the HMA. For Site 6, each of the WMA mixes 

yielded lower NOx emissions than the HMA. 

FIGURE 5 NOx emissions.

0.019

0.009 0.008

0.000

0.005

0.000

0.002

0.003

0.001 0.000 0.000

0.000

0.005

0.010

0.015

0.020

0.025

HMA WMA C WMA D HMA WMA F WMA G WMA H HMA WMA I WMA J WMA K

Site 3 Site 5 Site 6

lb/ton

SO2

0.064

0.058

0.064

0.023 0.022 0.021

0.015

0.031

0.024

0.019

0.022

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

HMA WMA C WMA D HMA WMA F WMA G WMA H HMA WMA I WMA J WMA K

Site 3 Site 5 Site 6

lb/ton

NOx
Frank, Pr

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Frank, Prowell, Hurley, and West 11 

PM-10 

 Fine particulates, PM-10, are of increasing concern among many Environmental 

agencies. Figure 7 shows average condensable fraction (back half of a Method 5 sample train) 

for hot mix and warm mix technologies. Filterable particulates were not measured. The 

condensable fraction includes organic and inorganic compounds with organics less than 1/10 of 

total condensables. What is striking about the NCHRP PM-10 data is the scale of PM-10 

emissions from limestone aggregates and parallel flow dryers, and the resulting reduction 

achieved by WMA technologies and igneous aggregate. 

SUMMARY AND CONCLUSIONS 

Fuel usage was measured for eleven WMA mixes and six corresponding HMA control mixes. 

Stack emissions were measured for eight WMA mixes and three corresponding control mixes. 

Eleven WMA technologies were represented. 

 The use of WMA resulted in burner fuel savings ranging from 0.9 to a credible 21.6 

percent with an average fuel savings of 13.6 percent, 

 Burner tuning alone resulted in a 24.8 percent fuel savings for all mixes at one site, 

 One project had stockpile moisture contents, which were on average 1.8 percent lower 

than the other sites. Fuel usage for the HMA production at this site was 39.3 percent 

lower than the average HMA fuel usage at the other sites, 

 As expected, reduced fuel consumption resulted in reduced CO2 emissions for all of the 

WMA mixes, 

 CO and VOC emissions are both indicators of broader plant practices whose impacts 

overshadow potential WMA reductions. In two cases, average CO emissions for WMA 

were higher than their corresponding HMA control. The elevated levels were within 

typical testing variability and are not considered significant, 

 VOC emissions were significantly lower for WMA mixes compared to HMA at the 

parallel-flow drum plant. WMA emissions from parallel flow plants are comparable to 

those from counterflow plants and external mixers. For counterflow dryers, VOC 

reduction with WMA is less significant and in some cases overshadowed by changes in 

burner performance, 

 Recycled fuel oil contains higher levels of sulfur than other fuel types. WMA mixes at 

the site that used recycled fuel oil resulted in a 50 percent reduction in SO2 emissions, 

 With the exception of one mix, NOx emissions from WMA were reduced compared to 

HMA. In the one case the emissions were the same and the burner firing rate was very 

low (26 percent), and 

 The distribution of formaldehyde emissions was lower compared to corresponding HMA 

controls and similar to rates from plants operating with strict emissions tolerances. 

ACKNOWLEDGEMENT 

This work was sponsored by the National Cooperative Highway Research Program, Project 9-

47A, “Properties and Performance of Warm Mix Asphalt Technologies.” 
Frank, Prowell, Hurley, and West 12 

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