Development Trend of In-Situ Heat Treatment Technology for Contaminated Sites

1. Based on the analysis of in-situ heat treatment cases at home and abroad, it is concluded that the unit carbon emissions of a typical in-situ heat treatment project are between 0.5 and 330.0 kgCO2-eq·m-3. The carbon emissions of this process mainly come from the consumption of non-renewable energy caused by the operation of the repair system (74.8%~97.7%), and the remaining small part comes from the installation and disassembly of the repair system (1.3%~17.7%), material consumption (0.4%~7.0%), and transportation and monitoring (0.1%~4.0%). The unit repair energy consumption of a typical in-situ heat treatment project is between 2.9 and 820.0 kWh·m-3. This energy consumption is mainly heat input (75%~95%), and the rest is composed of energy consumption in the operation of the repair device, equipment installation, transportation and monitoring. Most of the heat input into the ground is used to heat the contaminated medium, accounting for about 40%~70% of the total energy consumption, and the rest is lost through extraction heat loss, surrounding convection and conduction heat dissipation.

2. Renewable energy sources such as solar energy and wind energy are abundant in reserves and have broad application prospects in in-situ heat treatment projects, and are expected to achieve significant emission reductions in remediation activities. Solar energy is generally applied through photovoltaic power generation systems to drive small power equipment such as extraction, sampling and monitoring devices. There are also studies that attempt to directly convert it into thermal energy for application, such as using concentrators, solar heating furnaces and solar rotary kilns to directly heat contaminated soil, successfully achieving efficient removal of pollutants. In recent years, solar thermal enhanced microbial remediation technology combined with underground heat storage systems has attracted a lot of attention. The application of a single wind power generation system is common in electrochemical remediation experiments and seawater desalination research, and the emission reduction effect is good. In site remediation activities, in order to ensure sufficient energy supply, wind power and solar power generation systems are often used in combination, which can greatly reduce the energy consumption of remediation. However, on-site power generation systems are easily restricted by climatic conditions, and it is often difficult to achieve continuous and constant power supply, which will lead to reduced remediation efficiency of technologies that rely on electric field action, such as electrochemical remediation and resistance heating technology (electric-thermal coupling mode).

3. Optimizing the in-situ heat treatment process at the technical level is also expected to improve the repair benefits. The optimization directions include:

1) Optimization of single technology: ① Steam enhanced extraction (SEE) technology, which is mainly optimized by changing the steam injection method, such as pressure cycle steam injection, hydraulic fracturing combined with steam injection, steam and air co-injection, superheated steam replacing saturated steam, etc. ② Resistance heating (ERH) technology, which is mainly optimized by changing the water replenishment method and power supply mode. In addition, electrothermal-dynamic stripping based on ERH is also an efficient optimization technology. ③ Thermal conduction heating (TCH) technology, which is mainly optimized by dynamically controlling temperature and natural gas input flow, etc. The control measures include "temperature control strategy based on temperature monitoring", "multi-parameter natural gas flow control scheme based on temperature, water content and temperature rise rate" and "temperature control scheme based on on-site heating well layout".

2) Technology coupling: ① Coupling of in-situ heat treatment and chemical treatment technology. Adding chemical agents can reduce the heat treatment temperature and shorten the treatment time by changing the chemical environment of the heat treatment area, improving the uniformity of temperature rise, and accelerating the removal of pollutants. The coupled heat treatment technology can accelerate the chemical remediation process by increasing the temperature to enhance the desorption and dissolution of pollutants, activate peroxidation agents, and promote the migration of agents. ② Coupling of in-situ heat treatment and microbial remediation technology. The research focus of this coupling technology is on thermally enhanced microbial remediation. Low-temperature heating is used to increase the microbial availability of organic pollutants in the target area and enhance microbial activity, thereby improving the remediation efficiency. In recent years, thermally enhanced microbial remediation technology combined with renewable energy and underground heat storage systems has also been widely studied, with significant energy-saving and consumption-reducing effects. ③ Coupling between in-situ heat treatment technologies. This coupling technology is generally used to repair complex contaminated sites. The common method is the combination of SEE and TCH or ERH technology. SEE treats high permeability areas, and ERH or TCH treats low permeability areas, achieving good remediation results. In addition, the study found that coupling heating methods of different frequencies, such as ERH and radio frequency heating, can improve the uniformity of heating in underground media with strong heterogeneity.

3) Control of heat loss during the restoration process: ① For example, surface heat barrier, usually a single layer of low permeability and low thermal conductivity materials (such as concrete, foam concrete, etc.) is used to cover the target area; some studies have also used multi-layer materials or multi-layer surface covering structures for insulation; in addition, for SEE technology, the combination of steam and air co-injection on the basis of installing a surface cover layer can greatly improve the control effect of surface heat loss. ② Groundwater barrier, barrier measures include setting up physical barriers, setting up hydraulic barrier wells, and adding steam injection wells, among which physical barriers are the most commonly used barrier methods, hydraulic barrier wells are mostly used for water isolation and precipitation in high permeability areas, and adding steam injection wells are mainly suitable for heat conduction and ERH technology, which is a very promising barrier measure. ③ Waste heat recycling, waste heat recycling has great potential for reducing consumption. Current research is mostly focused on GTR technology. The recycling methods include: recycling heat to preheat air, recycling heat to preheat soil, recycling heat to heat cold spots, and recycling fuels and high calorific value pollutants.