Pyrolytic Remediation of Soils Contaminated with Heavy Petroleum
Master of Science
Onshore spills of crude oil and petrochemicals from stationary facilities or pipelines are common and can cause serious environmental problems. Current technologies for remediating soils contaminated with heavy petroleum hydrocarbons are time-consuming, have high energy cost, and may even cause secondary pollution to the environment. For the past several years, our group has been developing a novel method that uses pyrolysis to treat contaminated soils. Pyrolysis cannot only reduce the total petroleum hydrocarbon (TPH) content of treated soil to below regulatory levels, but it can also partially restore the fertility of treated soils to facilitate ecosystem restoration efforts. This study builds on the previous work of our group using thermogravimetry and online evolved gas analysis to first identify the fundamental physicochemical processes occurring when contaminated soil is heated. These processes include: (a) soil mineral transformations like clay dehydration and carbonate decomposition; and (b) desorption of the lighter hydrocarbons and pyrolysis of the heavier ones. A two-stage approach is then followed to develop a kinetic model for the entire process. Using thermogravimetric and evolved gas analysis data for water and carbon dioxide for two clean (background) soils, we apply a multi-component distributed activation energy model (DAEM) to describe thermally-induced soil transformations with a few parallel reactions of pseudo-components with distributed activation energies. Once the background soil kinetics have been determined, we use the same DAEM approach to describe the desorption and pyrolysis of petroleum hydrocarbons with two additional parallel reactions with distributed activation energies. We used the kinetic models developed for two contaminated soils to simulate pyrolytic treatment of these soils in an isothermal steady-state rotary kiln reactor. Model predictions are then compared to experimental data obtained with a pilot-scale rotary kiln reactor in an earlier phase of our project. Model predictions are in good agreement with the experimental data if we use the conversions of the heavy hydrocarbon pseudo-component as a proxy for the experimental measured TPH of treated soils. Our model predicts that temperatures around 420oC and short residence times (15-30 min) are sufficient to reduce the heavy hydrocarbon component by 90% or more in order to meet regulatory requirements. These predictions agree well with experimental results obtained earlier by our group for the two contaminated soils considered here. The model can become a valuable tool for determining the optimal pyrolysis temperature and reactor residence time that will achieve a specified TPH reduction for a specific soil/oil system while minimizing the energy cost for any given throughput.