Microwave processing of hydrocarbon contaminated soil

• Main Author: Yi, Chenbo
• Published: University of Nottingham 2013
• Subjects: 363.7396
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.601688

Across the UK there are many hundreds of sites that have been contaminated by previous industrial use. The wastes and residues left in the soil present a hazard to the general environment and, therefore, can prohibit the redevelopment of the land. Conventional technologies all have significant disadvantages. High temperature based technologies such as thermal desorption are highly energy intensive. Soil vapour extraction requires the soil to be sufficiently permeable to permit the transfer of vapour. Microwave heating methods have the advantage of volumetric and rapid heating as well in situ steam generation from water within the soil. However, previous research on the microwave processing of soils has mainly focused on bench scale tests on artificial soils using a microwave susceptor to overcome issues with heating efficiency, rather than utilising soils from real industrial sites. There is also a significant lack of fundamental understanding in terms of the microwave heating mechanisms of soils, a lack of energy balances and consideration of scale up towards industrial implementation. The microwave heating mechanisms of soils were elucidated and the removal mechanisms of hydrocarbon contaminants have been discussed. According to the differences in the hydrocarbon contaminants, five soils were classified into two groups: light hydrocarbon contaminated soils and heavy hydrocarbon contaminated soils. All five soils could be heated with microwaves without using susceptor, and were stable at 100°C as the energy dissipated was used to overcome the latent heat of vaporisation of water. The major heating mechanism was polarisation due to the inherent water within the soils, and the major remediation mechanisms were found to be steam stripping or distillation. Above 100°C bound or interlayer I I water may be present in some soils, which contributed to low but non-zero values of dielectric loss factor. The maximum bulk temperatures obtained with light hydrocarbon contaminated soils were in excess of 100°C, and is attributed to the equilibrium between microwave energy absorbed and heat loss. In this case the remediation mechanism could be governed by thermal desorption. Heavy hydrocarbon contaminated soils behave very differently during microwave processing, with much higher temperatures obtained than with light hydrocarbon contaminated soils. The major heating mechanism in this case was conduction due to the carbonisation of heavy hydrocarbons within the soil, and remediation resulted from desorption, decomposition and carbonisation. The potential for continuous microwave treatment was ascertained through a series of pilot scale studies at 150-300 kg/h. More than 75% hydrocarbon removal from light hydrocarbon contaminated soils was achieved using a conveyor system, but this technique was not suitable for heavy hydrocarbon contaminated soils due to the high temperatures that were attained. Preliminary studies were carried out using a batch scale microwave rotary kiln system for processing at higher temperatures. ii