Warm Mix Asphalt (WMA) Emission Reductions and Energy Savings

Transcription

1 Warm Mix Asphalt (WMA) Emission Reductions and Energy Savings By Bob Frank Consulting Engineer Compliance Monitoring Service 217 Belhaven Avenue Linwood, NJ (609) 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 (334) Randy C. West, Ph.D., P.E. Director National Center for Asphalt Technology 277 Technology Parkway Auburn, AL (334) Word Count = 3,962 9/6/2011

2 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, Evotherm DAT, Evotherm 3G, SonneWarmix, 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 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

3 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, Evotherm TM 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 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 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.

4 Frank, Prowell, Hurley, and West 3 TABLE 1 Project Sites Location Plant Type WMA Technology Walla Walla, WA Cedar Rapids Parallel Flow mechanical foaming Portable Drum Centreville, VA Astec Double Barrel mechanical foaming Rapid River, MI Dillman Equipment Parallel Flow waterless chemical additive, Portable Drum 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 waterless chemical additive, Continuous Mixer Drum 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 (CO 2 ) to assess greenhouse gas production, volatile organic 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 were

5 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, and 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.

6 Frank, Prowell, Hurley, and West 5 TABLE 2 Fuel Usage Site Fuel Mix Stockpile Moisture, % 1 Natural Gas 2 Natural Gas 3 Reclaimed Fuel Oil 4 Liquid Propane 5 Natural Gas 6 Natural Gas Stack Emissions Production Rate, TPH Production Temperature, F MMBTU/ ton Reduction, % HMA WMA A HMA WMA B HMA WMA C WMA D HMA WMA E HMA WMA F WMA G WMA H HMA WMA I WMA J WMA K 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 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 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 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 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.

7 Frank, Prowell, Hurley, and West 6 CO lb/ton 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 FIGURE 1 CO 2 emission rates. 70 Fuel Usage Vs. CO 2 Reduction in CO 2 Emission, % Line of Equality Reduction in Fuel Usage, % WMA D Literature NCHRP 9 47A 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 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

8 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 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 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. 0.9 CO lb/ton 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 FIGURE 3 CO emissions.

9 Frank, Prowell, Hurley, and West VOC lb/ton HMA WMA C WMA D HMA WMA F WMA GWMA H HMA WMA I WMA J WMA K Site 3 Site 5 Site 6 FIGURE 4 VOC emissions. Sulfur Dioxide When fuels containing sulfur are burned, SO 2 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 SO 2 production. Figure 5 shows the SO 2 stack readings for the three multi-technology 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 mix. The 50 percent reductions in SO 2 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 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 plant using recycled fuel oil is lbs/ton and for natural gas lbs/ton (21).

10 Frank, Prowell, Hurley, and West SO lb/ton HMA WMA CWMA D HMA WMA FWMA GWMA H HMA WMA I WMA JWMA K Site 3 Site 5 Site 6 FIGURE 5 SO 2 emissions. Oxides of Nitrogen Nitrous oxides (NO x ) are a precursor to the formation of ground level ozone. NO x emissions are higher for fuel oils as compared to natural gas. The EPA s candidate emission factor is lbs/ton for drum plants burning fuel oil and 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 NOx lb/ton HMA WMA C WMA D HMA WMA F WMA GWMA H HMA WMA I WMA J WMA K Site 3 Site 5 Site 6 FIGURE 5 NO x emissions.

11 Frank, Prowell, Hurley, and West 10 Formaldehyde Figure 6 shows a frequency distribution of formaldehyde emissions. Formaldehyde is a typical byproduct of combustion for all carbon based fuels. The distribution of formaldehyde emissions for the WMA mixes is lower than the distribution for the HMA mixes tested as part of NCHRP 9-47A. Only four stack emissions results for formaldehyde are available in the EPA s AP-42 database (21). The industry HMA data shown in Figure 6 represents 24 formaldehydee stack emissions tests from the mid-atlantic region. The WMA formaldehyde emissions are similar to these levels. 70 Observed Frequency, % EPA AP 42 NCHRP HMA Industry HMA NCHRP WMA Formaldehyde Emissions, lbs/ton FIGURE 6 Frequency distribution of formaldehyde emissions. Limestone Agg Traprock Agg FIGURE 7 PM-10 Condensable Fraction.

12 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 CO 2 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 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 Cooperative Highway Research Program, Project 9-47A, Properties and Performance of Warm Mix Asphalt Technologies.

13 Frank, Prowell, Hurley, and West 12 REFERENCES 1. Cervarich, M. Foaming the Asphalt: New Warm-Mix Technique Challenges Conventional Wisdom. Hot Mix Asphalt Technology, Volume 12, Number 4, National Asphalt Pavement Association, Lanham, MD, July/August 2007, pp 23-24, Koenders, B., D. Stoker, C. Bowen, P. de Groot, O. Larsen, D. Hardy, K. Wilms, Innovative Process in Asphalt Production and Placement to Obtain Lower Operating Temperatures, 2 nd Eurasphalt & Eurobitume Congress. Barcelona, Spain, von Devivere, M., W. Barthel and J-P. Marchand. Warm Asphalt Mixes by Adding Aspha-min, a Synthetic Zeolite. XXIInd PIARC World Road Congress. Durban, South Africa, Ventura, A., P. Moneron, A. Jullien, P. Tamagny, F. Olard, and D. Zavan, Environmental Comparison at Industrial Scale of Hot and Half-Warm Mix Asphalt Manufacturing Processes. Transportation Research Board, 2009 Annual Meeting DVD. 5. D Angelo, J., E. Harm, J. Bartoszek, G. Baumgardner, M. Corrigan, J. Cowsert, T. Harman, M. Jamshidi, W. Jones, D. Newcomb, B. D. Prowell (Report Facilitator), R. Sines, and B. Yeaton, Warm-Mix Asphalt: European Practice. International Technology Scanning Program, Federal Highway Administration, December Newcomb, D. An Introduction to Warm-Mix Asphalt. National Asphalt Pavement Association. Accessed from August 8, Harder, G. A., LEA Half-Warm Mix Paving Report, 2007 Projects for NYSDOT. McConnaughay Technologies, Cortland, NY, Davidson, J. K., Evotherm Trial Ramara Township Road 46. McAsphalt Engineering Services, Toronto, Ontario, December Lecomte, M., F. Deygout, and A. Menetti, Emission and Occupational Exposure at Lower Asphalt Production and Laying Temperatures. WAM Environmental Benefits of Reducing Asphalt Production and Laying Temperature, Shell Bitumen, Accessed from www-static.shell.com/static/bitumen-en/.../wrc/.../wam_english.pdf, No date. 10. Chief Environmental Group, LTD. Emission Test Results for: Warm Mix Asphalt Trial Project Mar-Zane Materials, Inc. Asphalt Plant #13 Byesville, Ohio. No Date. 11. ETE, Warm Mix Stack Emission Test, Environmental Technology & Engineering Corporation, Elm Grove, WI, June, 20, Powers, D., Warm Mix Asphalt 2008 ODOT Field Trials. Presentation at 2009 ODOT Asphalt Paving Conference. 13. Davidson, K. J., and R. Pedlow, Reducing Paving Emissions Using Warm Mix Technology. Proceedings of the 52nd Annual Conference of the Canadian Technical Asphalt Association, 2007, Pp Middleton, B., and R. W. Forfylow, An Evaluation of Warm Mix Asphalt Produced with The Double Barrel Green Process. Transportation Research Board, 2009 Annual Meeting DVD. 15. Harder, G., Y. LeGoff, A. Loustau, Y. Martineau, B. Heritier, and A. Romier. Energy and Environmental Gains of Warm and Half-Warm Asphalt Mix: Quantitative Approach. Transportation Research Board, 2008 Annual Meeting DVD. 16. Hurley, G., Prowell, B., and A. Kvasnak. Wisconsin Field Trial of Warm Mix Asphalt Technologies: Construction Summary. NCAT Report No National Center for Asphalt Technology, Auburn, Al, November 2010.

14 Frank, Prowell, Hurley, and West Hurley, G., Prowell, B., and A. Kvasnak. Missouri Field Trial of Warm Mix Asphalt Technologies: Construction Summary. NCAT Report No National Center for Asphalt Technology, Auburn, Al, June Prowell, B. D., G. C. Hurley, and B. Frank Warm Mix Asphalt: Best Practices 2 nd Edition Quality Improvement Series 125, National Asphalt Pavement Association, Lanham, MD WMA TWG, Documenting Emissions and Energy Reductions of WMA and Conventional HMA. August 2006, Accessed from Northeast Asphalt, Preliminary Stack Emission Results, Iron Mountain, MI, RTI International, Emission Factor Documentation for AO-42 Section 11.1 Hot Mix Asphalt Plants. Final Report, U.S. Environmental Protection Agency, February 2004.