9.1 Sector Descriptions and Overview
Liquid asphalt is a petroleum-derived substance used in paving applications, such as the construction of roads, parking lots, driveways, and airport runways, as well as non-paving applications, such as the manufacturing of roofing shingles. While liquid asphalt can be found in natural deposits, most is produced from crude oil. Vacuum distillation separates components of crude oil based on boiling point. Products generated from this process include naptha, gasoline, diesel, and liquid asphalt, the last of which has a boiling point greater than 500 °C. As a result, most volatile, light fractions of organics are separated from liquid asphalt during distillation and prior to use.
In paving applications, liquid asphalt can be applied cold or heated. If applied cold, additional components must be added to lower the viscosity of the material, which allows it to be spread upon a surface (e.g., roadway surface). Cutback asphalt (SCC: 2461021000) is a cold application process that involves mixing the liquid asphalt with petroleum solvents (e.g., naphtha, kerosene, fuel oil, diesel, etc.). Following application, these higher volatility solvents evaporate, leaving the asphalt in place. Due to this increased organic emissions potential, cutback asphalts have grown less common over time and this process now constitutes ~1% of liquid asphalt is use [ref 1]. Emulsified asphalt (SCC: 2461022000) is a separate cold application process that utilizes water-based solvents and an emulsifying agent. The result is a stable liquid suspension with asphalt globules. Following application, the additives evaporate and leave the asphalt in place. In contrast to cutback asphalts, emulsified asphalts have become more common in recent years and ~10% of liquid asphalt is now used in these applications.
Liquid asphalt can also be applied heated, which both lowers the viscosity of the material and minimizes the need for added solvents. Hot-mix asphalt application (SCC: 2461025100), which is the traditional method for asphalt pavement production, involves combining the liquid asphalt with aggregate at a hot-mix plant and heating the mixture to +150 °C. The mixture is then hauled to the usage site heated, where it is placed, compacted, and ambiently cooled. This process does not require any solvent additions. Warm-mix asphalt application (SCC: 2461025200) is a more recent technology that enables asphalt pavement production to occur at 20 – 40 °C cooler temperatures than hot-mix asphalt application. Warm-mix asphalt applications at reduced temperatures now constitute ~18% of asphalt paving applications [ref 2]. These lower production temperatures promote energy savings through reductions in fuel use and lower emissions at the hot-mix plant. To reduce the viscosity of the liquid asphalt in warm-mix applications, water, water-bearing minerals, chemicals, waxes, organic additives, or a combination of technologies must be added [ref 3].
While heated applications processes represent most liquid asphalt paving applications, it has historically been assumed that emissions were sparse due to the removal of the more volatile organics during distillation. Recent research has demonstrated that less volatile organic vapors from liquid asphalt do evaporate at temperatures associated with hot-mix application (~140 °C), warm-mix application (120 °C), and post-application, or “in-use” temperatures [ref 4]. Emission during the “in-use” period diffuse from the pavement over time following application.
Roofing asphalts include asphalt cements and emulsions used in the manufacturing of asphalt shingles, asphalt sealant, and roof tar. In 2020, manufacturing of these products consumed ~1.95 million short tons of asphalt, which is ~9% of all asphalt generated in the United States. These materials are predominantly applied in ambient conditions (i.e., as asphalt shingles). Roof tar, which is applied hot and at temperatures like hot-mix road asphalt (+150 °C), comprises ~5% of roofing asphalt usage. In addition, all forms of roofing asphalt are exposed to both high temperatures and solar radiation throughout the duration of their life cycles, which have the potential of generating enhancements of emissions.
Table 9.1 notes all SCCs covered in this source category and the SCCs for which the EPA generates default emissions.
| SCC | SCC Level 1 | SCC Level 2 | SCC Level 3 | SCC Level 4 | EPA |
|---|---|---|---|---|---|
| 2461021000 | Solvent Utilization | Misc Non-industrial: Commercial | Cutback Asphalt | Total: All Solvent Types | X |
| 2461022000 | Solvent Utilization | Misc Non-industrial: Commercial | Emulsified Asphalt | Total: All Solvent Types | X |
| 2461025000 | Solvent Utilization | Miscellaneous Non-industrial: Commercial | Asphalt Paving: Hot and Warm-mix | Hot and Warm-mix Total: All Solvent Types | |
| 2461025100 | Solvent Utilization | Miscellaneous Non-industrial: Commercial | Asphalt Paving: Hot and Warm-mix | Hot-mix Total: All Solvent Types | X |
| 2461025200 | Solvent Utilization | Miscellaneous Non-industrial: Commercial | Asphalt Paving: Hot and Warm-mix | Warm-mix Total: All Solvent Types | X |
| 2461026000 | Solvent Utilization | Miscellaneous Non-industrial: Commercial | Asphalt Paving: Road Oil | Total: All Solvent Types | |
| 2461023000 | Solvent Utilization | Solvent Utilization | Asphalt Roofing | Total: All Solvent Types | X |