THERMAL DYNAMICS of ASPHALT
In 2009 Green Roads Recycling further advanced HIPAR technology by the introduction of a new, patented heater design, followed by a 2nd unit in 2011. Recognizing that degradation of the asphalt binder can occur via oxidation and/or chemical decomposition at high temperature (>400C), the design was developed using a sophisticated computer model to ensure compliance with specific criteria;
- To prevent thermal degradation, at no point during heating would the surface temperature exceed 350C, which is restricted to the top few millimeters for only 2 to 3 seconds which cools quickly as the heat penetrates.
- To minimize aggregate degradation, at milling, thermal penetration would be 45C at the milling depth (above the softening point of the bitumen).
- To ensure full compaction during rolling, the average temperature of the milled layer would be >100C.
Each heater includes an automated control system and sensors (for O2 under the heater box and asphalt temperature at exit) to ensure against oxidation and over/under-heating of the asphalt; the control system ensures compliance with the design criteria during both steady and transient (fluctuating train speed or changing asphalt conditions) operations.
Emissions reduction was integral to the heater design incorporating a three-tiered approach:
- Limiting emissions at source by limiting the maximum surface temperature
- Ensuring effective in situ incineration of emissions under the heater box
- Inclusion of an automated on-demand oxidizer to deal with tertiary emissions
The heater train is based on 2-stage milling, each to a depth of 1”. As part of the design process, the option of single stage milling to a depth of 2” was examined, again using the computer model. With two 30’ (effective heating length) heater units, single stage milling incurred a severe constraint on train speed (assuming adequate thermal penetration to prevent significant ‘cold’ milling and attendant degradation of the aggregate). This does not preclude single-stage milling but does indicate any such a system will require at least three heater units ahead of the milling unit.
Although many companies promote that their type of heating of the asphalt is superior than others they all actually heat up the asphalt the same – by hot products of combustion (POC) from a remote burner. Some Companies website correctly points out that hot POC contain low levels of O2 (1 to 2% ) relative to 21% for air; hence reduced tendency to oxidize the asphalt relative to air. However, it is incorrect and misleading to imply certain system will reduce oxidation relative to other systems; all current heating systems are based on hot POC contacting the asphalt.
Green Roads Recycling employs the “slow and steady” approach vs. “shock and awe. All heating units are handicapped by the poor conductivity of asphalt. The “shock and awe” approach, in others words: applying too much heat over and insufficient period will cause oxidation of the oil and excessive emissions. The “shock and awe” approach is an unacceptable method of HIPAR.