(Demo) Road and Bridges May 2010 : Page 50
Asphalt burning Based on review of available informa- tion, extraction and refining of crude oil into a ton of asphalt binder can con- sume between 600,000 and 4.2 million Btu of energy, and the amount of emis- sions range from 280 to 675 lb of CO2 equivalent (CO2 e). The environmental impact associated with the production of asphalt binder will be dependent upon the crude oil source and technol- ogy employed to recover the asphalt binder. In terms of aggregate extraction and production, between 3 and 20 lb of CO2 Contractors conduct field testing on a recycled asphalt pavement. Emission Reduction Opportunities RAP is the most recycled product in the U.S. at 100 million tons. RAP can come from a number of different sources like roadway millings, broken asphalt and take backs during production. Gen- erally RAP is processed to control mate- rial variability and obtain a maximum aggregate size. The asphalt binder content of RAP is in the range of 4 to 5% asphalt and is dependent on the RAP source and the degree to which the RAP is processed. The amount of RAP that may be intro- duced into new HMA is based on the agency and can be as high as 50%. RAP has the tendency to have a higher mois- ture content than virgin aggregate, and as a result production temperatures are generally higher than with conventional HMA in order to remove this moisture through convective heating. There are two types of asphalt shingles: post-industrial and post-con- sumer. Post-industrial asphalt shingles are rejected asphalt shingles or shingle tabs that are discarded in the manufac- turing process of new asphalt shingles. Post-consumer asphalt shingles is scrap derived from re-roofing projects whereby the old shingle layers are removed to prepare the roof surface for new shingles or other roofing materials. Savings associated with RAS is highly dependent upon type. Asphalt binder content for shingles can range between 18% and 40%, which can significantly reduce the virgin asphalt binder demand of an HMA mixture. Although these asphalt binder contents are typical of 50 May 2010 • ROADS&BRIDGES solvent extractions, the effective contri- bution to the mix will be less. At this time, 14 state standard specifications or special provisions allow for up to 5% manufactured and/or tear-off shingles in HMA. RAS is introduced through the RAP collar and provides both asphalt binder and aggregate to the mixture. Plant temperatures may be increased in an effort to activate a larger percentage of the asphalt binder in the RAS. WMA reduces the mixing tempera- ture in comparison with that of HMA and can allow for higher percentages of recycled material like RAP and RAS to be used. This technology is in its infancy, so information on energy and emis- sions reductions is limited. Tempera- ture reductions are largely technology dependent with the greatest reductions being recognized with the chemical and wax packages. These technologies constitute the greatest savings at the hot plant in terms of temperature reduction and fuel consumption. However, this does not consider the manufacture and transportation of the WMA technology. The temperature reductions for foaming WMA technologies are less than com- pared to chemical and wax technolo- gies, but have the added benefit of only consuming water, which requires very little additional energy input. Because there are differences in the manufacture and transportation of the various WMA technologies, the environmental impact associated with each is not similar and must be evaluated on a technology-by- technology basis. e are generated per ton of material. The amount of energy to process a ton of aggregate is dependent upon aggre- gate source, extraction (pit, quarry or dredge), crusher configuration, crushed product composition, support vehicles and stockpile management. The environmental impact associ- ated with HMA material transportation is largely dependent upon the distance over which it must be moved. When raw materials sources are great distances from the HMA production site, the environmental impact can be reduced by selecting modes of transportation that capitalize on the movement of large quantities of material and are fuel efficient. For comparison purposes, a semi-truck can move one ton of material on a gallon of diesel 155 miles, whereas by rail the same amount of material can be moved 536 miles. Regardless of the mode of transportation, diesel fuel will most likely be consumed, and the burn- ing of one gallon will generate approxi- mately 22.5 lb of CO2 e. The amount of energy to produce one ton of HMA is dependent upon the plant type, moisture content of the ag- gregate, the fuel source and a number of other factors. Typical energy consump- tion at the hot plant is between 250,000 and 320,000 Btu/ton of HMA. On an equivalent energy basis, if residual fuel were used to produce a ton of HMA, it would generate between 42.9 and 54.9 lb of CO2 26.1 lb of CO2 e (152,000 Btu/gallon and e per gallon). If, how- ever, the contractor were to use diesel, between 40.5 and 51.8 lb of CO2 e would be generated (139,000 Btu/gallon and 22.5 lb of CO2 e per gallon). Most fuels are selected based on cost and fuel value,
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