(76b) Pyrolytic Remediation of Petroleum-Contaminated Soil in a Pilot-Scale Rotary Kiln Reactor: Reaction Mechanisms, Soil Transformations and Treatment Intensity Tradeoffs
Scope and Significance
The vast majority (about 98%) of all oil spills occur on land. Terrestrial oil spills from pipelines and fixed facilities are usually of moderate size (100-1,000 m3) but release 10-25 million gallons of petroleum products in the environment every year. Because biodegradation is extremely slow in the anaerobic zone of soils, land spills pose long term threats to groundwater quality. Current remediation methods for petroleum-contaminated soils are either relatively slow or have unintended consequences in the form of soil damage and high-energy usage. Furthermore, some treatment processes such as aerobic bioremediation can activate toxic hydrocarbons and transform them to more noxious byproducts such as PAH derivatives, raising the possibility of meeting regulatory cleanup goals without achieving full soil detoxification. Clearly, there is a pressing need for more efficient and sustainable remediationof oil-contaminated soils.
Pyrolysis is receiving increasing attention as an on-site remediation approach because of its potential to rapidly and reliably remove total petroleum hydrocarbons (TPH) with lower energy requirements and better post-treatment soil fertility than other ex situthermal remediation approaches. However, soil fertility restoration is not consistently achieved by thermal treatment methods, underscoring the need for mechanistic insight into how treatment conditions affect soil detoxification and ability to support plant growth.
Our initial studies on pyrolytic treatment focused on small-scale batch tests to assess TPH removal and soil fertility restoration, as well as to understand the fundamental reaction mechanisms behind these outcomes. After desorption of light hydrocarbons between 150-350oC, pyrolysis reactions dominate in the range of 400-500oC, releasing gaseous products and forming a solid char. Plant growth studies showed favorable soil fertility metrics in soils that were pyrolyzed at 420oC for 3 h, suggesting the potential for improved ecosystem restoration following remediation compared to traditional thermal technologies. While these laboratory-scale experiments were instrumental in demonstrating the proof of concept for pyrolysis and in informing process design, pilot-scale studies are needed to delineate the merits and limitations of pursuing multiple treatment objectives (e.g., TPH removal compliance, soil detoxification and fertility restoration) as a function of pyrolysis conditions (e.g., temperature, soil residence time and soil oil content). Ensuring reliable remediation also requires special attention to residual PAHs in the treated soil or its leachate, since these priority pollutants are not only present in crude oil, but could also be formed during thermal treatment.
Pyrolysis Experiments in a Continuous Reactor, Soil Detoxification and Treatment Intensity Tradeoffs
We report here results from the first pilot-scale study of pyrolytic soil treatment performed in a continuous rotary kiln reactor to determine how the processing conditions affect not only TPH and PAH removal efficiency, but also detoxification efficacy and the ability of the treated soil to support plant growth.
Two contaminated soils with different oil contents were pyrolyzed in a continuously-fed rotary kiln reactor. A total of 15 pyrolysis runs were carried out with temperatures ranging between 370 and 470oC, 15-60 min residence times and solid flow rates between 28 and 7 lb/h. The 7-inch diameter rotary kiln had four 24-inch electrically-heated zones that were independently controlled to (a) raise the solids to the desired temperature in the first zone, and (b) maintain the reactor temperature constant (within 5-10oC) in zones 2, 3 and 4. For the purpose of this study, the solids residence time was the time the solids were exposed to the pyrolysis temperature in zones 2-4. Pure nitrogen in countercurrent flow was used to âsweepâ the desorbing hydrocarbons and pyrolysis products.
TPH and PAH concentrations and key agronomic parameters of the treated soils were measured for each set of operating conditions. To quantify the effectiveness of pyrolysis in reducing potential toxicity to humans (e.g., through inhalation of contaminated dust or ingestion of impacted groundwater), the cytotoxicity of soil extracts was tested by the MTT colorimetric assay with a human bronchial epithelial cell line (BEAS-2B). Plant growth studies were also conducted to characterize potential tradeoffs between pyrolytic treatment intensity, remediation efficiency and soil fertility restoration.
Treatment at 420oC with only 15-min residence time resulted in high removal efficiencies for both total petroleum hydrocarbons (TPH) (99.9%) and polycyclic aromatic hydrocarbons (PAHs) (94.5%), and restored fertility to clean soil levels (i.e., Lactuca sativa biomass dry weight yield after 21 days increased from 3.0±0.3 mg for contaminated soil to 8.8±1.1 mg for treated soil, which is almost equal to the 9.0±0.7 mg yield for uncontaminated soil). Viability assays with a human bronchial epithelial cell line showed that pyrolytic treatment effectively achieved detoxification of contaminated soils.
As expected, TPH and PAH removal efficiencies increased with increasing treatment intensity (i.e., higher temperatures and longer residence times). However, higher treatment intensities decreased soil fertility. While pyrolytic treatment at 420oC for 30 min restored soil fertility to 98% of the clean soil level, treatment at 470oC for 15- or 30-min reduced soil fertility to 51% and 39% of the clean soil level, which was only marginally higher that the fertility of the contaminated soil (33% of clean soil fertility). This is a strong indication that that there exists an optimum system-specific treatment intensity for fertility restoration.
Overall, this study highlights tradeoffs between pyrolytic treatment intensity, hydrocarbon removal efficiency and fertility restoration while informing the design, optimization, and operation of large-scale pyrolytic systems to efficiently remediate crude-oil contaminated soils.