Study On Key Influencing Factors Of Indirect Thermal Desorption Of Petroleum Hydrocarbon Contaminated Soils

In this paper,with the purpose of removing Total petroleum hydrocarbons（TPHs）in soil and reducing the energy consumption during thermal desorption（TD）process,a central heating spiral indirect thermal desorption experimental platform was established.The influencing factors of petroleum hydrocarbon contaminated soils（PCS）thermal desorption and the physical and chemical properties of the soil were studied,and the temperature field distribution of the soil and the energy consumption of thermal reparation during indirect thermal desorption were systematically analyzed.The main conclusions of this paper are as follows:The removal efficiency（RE）of TPHs increases significantly with the increase of heating temperature/heating time,and then tends to be stable.The heating time can make up for the shortage of heating temperature to some extent.For the long-term weathered soil,a large number of pollutants are gathered in the soil particles and aggregates,and it is more difficult to remove in the early stage of TD.The RE of on-site PCS is 20.95%lower than that of simulated PCS when heated for 5 min,and the RE is basically the same when heated for 30 min.Particle size significantly affects soil adsorption capacity of TPHs and heat and mass transfer capacity of pollutants.Small particle size soil particles have larger specific surface area and stronger adsorption capacity.The surface pollutants of soil particles with large particle size are volatile and the pollutants inside the pores are difficult to remove.The small particle size soil is affected by the diffusion rate,and the pollutants are continuously and stably released from the soil.The influence of soil moisture on thermal desorption is mainly reflected in the large amount of heat consumed by water evaporation,and the rapid evaporation of water may lead to soil crusting and block the removal of internal pollutants.The initial concentration of pollutants had no significant effect on RE.The second-order kinetics can better fit the TD process of TPHs,and the second-order reaction rate constant k₂ of Daqing PCS corresponds to R²>0.98021.In the process of TD,the average soil particle size first increased,then decreased and then increased,showing an overall increasing trend.Under the scanning electron microscope,it can be observed that the soil particles agglomerate obviously,but on the other hand,the volatilization of pollutants in the soil particles leads to the rupture of soil aggregates,resulting in the decrease of particle size in local areas.The overall porosity of soil increased,while the total organic matter of soil was significantly lower,indicating that part of TPHs existed in the form of soil organic matter and volatilized by heat.Soil thermal conductivity and specific heat capacity at constant pressure are approximately linear with temperature and gradually increase with temperature increasing.The initial moisture content will affect the soil heating process at low temperature,but has little effect on the high temperature section.Simulation data of 5kg/h indirect thermal desorption test platform showed that the volume utilization rate would affect the average soil temperature and radial temperature.With the increase of volume utilization,the average soil temperature and radial temperature decreased gradually,and the larger the soil heat capacity,the more obvious the difference was.Increasing the temperature of flue gas can increase the soil temperature,but can not significantly enhance the convection heat transfer or heat radiation process,and the heat transfer efficiency of central heating is still relatively low.External heating has larger heat transfer area and better heat transfer coefficient,showing better heating effect.The thermal balance analysis and energy calculation of the equipment in this paper show that the main factors affecting RE of TD are the heat loss of flue gas transportation and the low heat transfer efficiency of the thermal desorption unit.Heat loss can be reduced by changing equipment structure,reducing hot flue gas travel and waste heat recovery.Heat transfer can be enhanced by improving the convective heat transfer of hot flue gas,changing the heat transfer mode and changing the device structure.