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

ABSTRACT

NCHRP 09-47A set out to more precisely and completely quantify em ission reductions and energy savings realized by warm mix asphalt (WMA) technologies. Th e task proved to be formidable and despite a skilled team’s best efforts produced resu lts 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 WM A 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 lo wer temperatures will save fuel and reduce burner emissions. The environmental benef its realized by WMA technologies reflect, on average, a 10 to 15 percent smaller carbon footpr int. 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 stac k emissions were tested, fuel savings resulting from the use of WMA paled in comparison to re ductions 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 ge nerated when mix is produced too hot or too cold. Several early adopters of WMA use it for daily production beli eving 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 te mperatures to be reduced. With a properly tuned burner, reduced production te mperatures should require less fu el and result in less stack emissions. If WMA is demonstrated to reduce fuel consumption and stack emissions while facilitating higher reclaimed asphalt pavement c ontents, then the use of WMA would be an incremental step towards sustainable devel opment 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 24Frank, 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, re presenting six technologi es, 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, Evotherm TM ET, which tend to have the lowest production temperatures . LEA and WAM-Foam producti on temperatures are usually close to 212°F (100°C). The aver age 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 ev ery 1 percent increase in stockpile moisture content ( 18 ). Best practices, such as paving unde r 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 oxida tion and volatilization loss dur ing mixing with corresponding emission reductions. Unfortunately, WMA’s ability to reduce emissions re mains 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 CO 2 emissions. The only case in which CO 2 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 NO x were reduced in all cases. SO 2 emissions both increased and decreased. Two projects in dicate increased VOCs with the WMA production ( 7, 11 ). In both cases, reports attributed that in crease to poor burner tuning rather than the WMA technology. Pollutants have been reported in several diffe rent units ranging from stack concentration to pounds per hour making meaningful comparisons difficult. Most reported emissions are simply uncorrected average dry stack concentrat ions (ppmvd). Compar isons between WMA and HMA based on differences in stack concentra tions 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 can not be compared to normalized concentrations without knowing the burner firing rate.

PROJECT SITES

Fuel consumption data were collected from si x sites and stack emissions were measured on three, multi-technology projects. Th ree sites were selected for em issions testing since emissions are also affected by environmental and plant f actors. All of the site s had a corresponding HMA control section. Table 1 summarizes the pr oject 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 polluta nts. Energy usage, stack emissions, and temperature reductions are interrelated, but can be affected by multiple factors, such as aggregate moisture, operator, plant confi guration, fuel type, production ra te, burner tuning, percent RAP, ambient temperature, et cetera. Variations between mix desi gn, 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 auth or at each of the multiple technology sites prior to stack emissions tests. In two of three cases, burner tuni ng reduced CO emissions tenfold while the largest WMA reduction measured was 59 pe rcent. In both Michigan and Indiana, initial carbon monoxide (CO) measuremen ts exceeded 10,000 ppm. In Mich igan, 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 th e 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 mi x with no other process changes. Sustained production runs at c onsistent rates and mix temperat ures were needed to make good assessments of burner fuel usage. The rese arch team worked with the contractors to measure burner firing rate and electrical de mand 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 pr oject , 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-fa ctor. This analysis assisted in identifying possible measurement errors by th e stack test contractors. At the three multi-technology projects (Mic higan, Indiana, and New York), stack emission tests were conducted in accordance with US EPA's Title 40 Code of Federal Regulations (CFR) Part 60, A ppendix A and generally followed the recommendations of the WMA TWG. Reported stack emissi ons included carbon dioxide (CO 2 ) to assess greenhouse gas production, volatile organi c compounds (VOC) and oxides of nitrogen (NO x ) to assess the potential for ground level ozone, carbon monoxide (CO) to assess burner tuning, sulfur dioxide (SO 2 ), condensed particulates (component of PM- 10) and formaldehyde emissions. Results wereFrank, Prowell, Hurley, and West 4

analyzed and reported as pounds per unit product ion consistent with Federal AP-42 emission factors. Local emissions testing contractors experi enced with these methods were used to minimize mobilization costs. The lead author as sessed their credentials and coordinated testing at each site to ensure meaningful data were obtained. Due to th e short notice at each project, availability became a signi ficant 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, av erage 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 erro rs 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 over all 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 CO 2 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, CO 2 production is reduced for all of the WMA mixes compared to their corresponding HMA mi xtures. 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-r eporting of the actual air-flow a nd hence the derived lbs/ton CO 2 production. CO 2 emissions primarily result from fuel combustion. As such, there is a linear relationship between fuel and CO 2 reductions resulting from the us e of WMA. Figure 2 presents this relationship for both the data obtained from NCHRP 9-47A and the litera ture. The offset of any data point from the Line of Equality refl ects an inaccuracy in at least one of the two measurements. In the case of WMA D, the reported fuel saving s averaged 31 percent, but the average CO 2 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 CO 2 emission rates.

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

Carbon Monoxide Carbon monoxide (CO) and volatile organic compound (VOC) formation are affected by burner design, maintenance, and tuning. A burner th at 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 S i t e 5 S i t e 6lb/ton CO 2

‐ 30 ‐ 20 ‐ 10

0 10 20 30 40 50 60 70

‐ 30

‐ 20

‐ 10 0 10 20 30 40 50 60 70ReductioninCO2Emission,%

Reduction in Fuel Usage, % Fuel Usage Vs. CO 2

Literature

NCHRP 9

‐ 47A Line of Equality WMA DFrank, 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-mi x nozzles, tasks that could not be performed within the time allo wed for the WMA demonstration. The thin horizontal line in Figure 3 represen ts the EPA’s candidate emission factor for CO of 0.13 lbs/ton for drum plants ( 21 ) based on stack test data fr om 18 drum plants. The range in data averages 89.5 percent of the mean. A lthough the CO emissions for WMA D and G appear elevated compared to their co rresponding HMA controls, both valu es are within 89.5 percent of the HMA, indicating they are with in typical testing variability. For Site 3, a parallel-flow drum plan t, WMA production reduces VOC emissions approximately 50 percent. Site 5 used a counter-f low 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 an d took the analyzer off line frequently during the run to “dry out”. Excluding that run aver age, 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 t hose for the HMA control. However it should be noted that VOC emissions for all mixes were am ong the lowest measured and reflect state-ofthe-art performance in most jurisdictions. A va riety of factors could e xplain an increase with WMA, but the uniform increase across three ve ry 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 S i t e 5 S i t e 6lb/ton COFrank, Prowell, Hurley, and West 8

FIGURE 4 VOC emissions.

Sulfur Dioxide When fuels containing sulfur are burned, SO 2 is produced. Sulfur cont ent varies with fuel type. Recycled fuel oil tends to ha ve the highest sulfur content, followed by fuel oil. Natural gas tends to have the lowest concentration of su lfur in fuels commonly used at asphalt plants. Reducing fuel consumption should reduce SO 2 production. Figure 5 shows the SO 2 stack readings for the three multi-technol ogy projects. Overall, the SO 2 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 mi x. The 50 percent reductions in SO 2 at Site 3, which used recycled fuel oil, ar e significant. Discounting possible changes in the used oil supply, the 50 percent reduction suggests an increase in SO 2 control efficiency at lower WMA baghouse temperatures. As might be expected, more SO 2 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 pl ant 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 6lb/ton VOCFrank, Prowell, Hurley, and West 9

FIGURE 5 SO 2 emissions.

Oxides of Nitrogen Nitrous oxides (NO x ) are a precursor to the forma tion of ground level ozone. NO x 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 NO x emissions and WMA D the same NO x 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 NO x emissions. For Site 5, the WMA mixes produced the same or lower NO x emissions than the HMA. For Site 6, each of the WMA mixes yielded lower NO x emissions than the HMA.

FIGURE 5 NO x 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 6lb/ton SO 2

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 6lb/ton NOxFrank, P r

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eFrank, Prowell, Hurley, and West 11 PM-10 Fine particulates, PM-10, are of in creasing concern among many Environmental agencies. Figure 7 shows average condensable fr action (back half of a Method 5 sample train) for hot mix and warm mix techno logies. Filterable particulat es were not measured. The condensable fraction includes orga nic 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 pa rallel flow dryers, a nd the resulting reduction achieved by WMA technologies and igneous aggregate.

SUMMARY AND CONCLUSIONS

Fuel usage was measured for eleven WMA mi xes and six corresponding HMA control mixes. Stack emissions were measured for eight WMA mixes and three corres ponding control mixes. Eleven WMA technologie s were represented.

 The use of WMA resulted in burner fuel savings ranging from 0.9 to a credible 21.6 percent with an av erage fuel savings of 13.6 percent,

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

 One project had stockpile mo isture 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 cons umption resulted in reduced CO 2 emissions for all of the WMA mixes,

 CO and VOC emissions are bot h indicators of broader plan t practices whose impacts overshadow potential WMA reductions. In tw o cases, average CO emissions for WMA were higher than their corres ponding HMA control. The elev ated levels were within typical testing variab ility and are not cons idered 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 extern al 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 hi gher levels of sulfur than ot her fuel types. WMA mixes at the site that used recycled fuel oil resulted in a 50 percent reduction in SO 2 emissions,

 With the exception of one mix, NO x 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 Coope rative Highway Resear ch Program, Project 9- 47A, “Properties and Performance of Warm Mix Asphalt Technologies.”Frank, Prowell, Hurley, and West 12

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. 20. Northeast Asphalt, “Preliminary Stack Emission Results, Iron Mountain, MI, 2006. 21. RTI International, “Emission Factor Docu mentation for AO-42 Section 11.1 Hot Mix Asphalt Plants.” Final Report, U.S. Envir onmental Protection Agency, February 2004.