**1. Introduction**

Soil is the basic environmental element constituting the ecosystem, and the important material basis of human survival and development. The environmental safety of soil has become a severe problem in China with the boost of industrialization and urbanization. It was calculated that the amount of contaminated soil reached about 150 million mu up to 2012 [1]. Recent estimates indicate that 500,000 sites in Europe require cleanup, while nearly 3.5 million sites are potentially polluted [2]. Including heavy metals, soil contamination caused by so many contaminants is an urgen<sup>t</sup> problem. It can be seen from the bulletin on Chinese domestic environmental conditions for the year 2000 that the heavy metals in 36,000 hectares of soil were out of limits in the surveyed 0.3 million hectares of soil and the over standard rate reached 12.1% of the total [3]. The prevention of contaminated soil is not only needed to control the sources such as heavy metals, but also enhance the remediation of

contaminated soil [4]. In the last 30 years since 2013, more than 80,000 sites have been cleaned up in the European countries where data on remediation are available [5].

In situ thermal remediation is a kind of suitable remediation technology for heavily contaminated soil [6,7]. Thermal desorption removes pollutants from soil and other materials by using heat to change the chemicals into gases and speed up the cleanup of many pollutants from the ground [8–10]. All the soil contamination remediation mechanisms have their advantages and limitations. Moreover, they are contaminant specific and heavily dependent on the subsurface environmental conditions of the site [11]. In situ thermal remediation remedies contaminated soil on the contaminated site without excavation. Compared with ex situ thermal desorption (ESTD), it has the advantages of low investment and little impact on the surrounding environment, so it is a hotspot of soil remediation research [12–14]. In situ thermal remediation is a soil remediation process in which heat and vacuum are applied simultaneously to subsurface soils [15]. Volatile and semi-volatile organics are removed from contaminated soil in thermal desorbers at 100 to 300 ◦C for low temperature thermal desorption, or at 300 to 550 ◦C for high-temperature thermal desorption [16]. In the past decade, it has been applied at a number of sites and it has been used in various modes including surface heating with blankets, subsurface heating with an array of vertical heater/vacuum wells, and ex situ blankets [15]. During the remediation process, gases at high temperature (700–800 ◦C), coming from the combustion chamber, circulate within the heating elements, resulting in the heating of the soil and the evaporation of volatile pollutants (boiling point < 550 ◦C) contained in the soil [17]. Laboratory treatability studies and field project experience have confirmed that the combination of high temperature and long-time results in extremely high overall removal e fficiency, even for high boiling point contaminants. Both thermal wells and thermal blankets have been demonstrated to be highly e ffective in removing a wide variety of low and high boiling point hydrocarbons, PCBs, pesticides, and chlorinated solvents from soils [15]. Shallow soil contamination (less than three feet deep) may be treated by thermal blankets or horizontal wells [11,18]. For soil contamination at depths greater than 3 feet, heating with surface blankets is ine ffective and thermal wells are needed to attain high temperatures in the soil [15].

Figure 1 presents a general description of a traditional in situ thermal remediation system, that is a polluted-soil thermal remediation system including burner, pipe, well and soil. As Figure 1a shows, natural gas (NG) and air enter the burner through di fferent inlets and an air-NG mixture is delivered to the burner, in which chemical energy of natural gas (NG) is converted to thermal energy in the exhaust gas by burning. The high temperature exhaust gas produced by the burner flows through the pipe into the heating well inserted vertically in the soil. The well is the heat transfer component of the whole system, in which the high temperature exhaust gas flows transferring heat to the soil to raise the soil temperature through the walls of the well. The volatile pollutants contained in the soil will then evaporate. As shown in Figure 1b, the gas flows directly in the system and is eventually discharged into the environment without recovery or recycling. In such a flow, the energy in the flowing gas is used only once to heat the soil. From the point of view of energy utilization, this is undoubtedly a huge waste.

At present, there are many studies on soil contamination, mainly about remediation methods, such as thermal desorption, chemical oxidation, phytoremediation etc. [19–25], assessment of contaminated soil [26], the process of soil contamination [27], areas for contaminated soil remediation, etc. [28]. However, few studies have focused on the energy saving and e fficiency promotion of thermal desorption using natural gas (NG). Thus, it is very significant to analyze the energy loss and energy utilization ratio of the polluted-soil thermal remediation system. The 2008 gas flaring estimate of 139 billion cubic meters represents 21% of the natural gas consumption of the USA with a potential retail market value of \$68 billion and the 2008 flaring added more than 278 million metric tons of carbon dioxide equivalents (CO2e) into the atmosphere. That is to say, improved utilization of the gas is key to reducing global carbon emissions to the atmosphere [29].

**Figure 1.** System diagram of polluted-soil thermal remediation system: (**a**) Structure diagram of polluted-soil thermal remediation system including burner, pipe, well and soil; (**b**) flowchart of air distribution in polluted-soil thermal remediation system.

Energy plays an important role in the history of human development [30]. In recent decades economic growth and increased human wellbeing around the globe have come at the cost of fast growing natural resource use (including materials and energy) and carbon emissions, leading to converging pressures of declining resource security, rising and increasingly volatile natural resource prices, and climate change [31]. Emissions of carbon dioxide from the combustion of fossil fuels, which may contribute to long-term climate change [32]. In recent decades, China has encountered serious environmental problem [33]. Some heavy industries and manufacturing enterprises are still characterized by extensive growth, facing enormous environmental challenges due to global climate change, rapid exhaustion of various non-renewable resources, and must improve their energy-save and emission-abate technology to favor the sustainable development [34–36]. Policies should aim to increase the efficiency of energy use [37].

In the energy system, energy analysis based on the first law of thermodynamics and exergy analysis based on the second law of thermodynamics are commonly used. The energy analysis is focused on the quantity of energy and the exergy analysis is focused on the quality of energy. Numerous studies have used these methods, such as the novel combined cooling, heating, and power (CCHP) system [38,39], ground source heat pumps [40], and exhaust waste heat recovery systems [41] and so on.

In the traditional polluted-soil thermal remediation system, the constant high temperature of exhaust is used to heat the soil with changing temperature and the exhaust is discharged directly into the atmosphere, which is disadvantageous for saving energy. Therefore, this paper is aimed at improving the existing problems in the traditional system, and so three energy-saving strategies were researched.

This paper proposes three energy-saving strategies of polluted-soil thermal remediation system—variable-condition mode (VCM), heat-returning mode and air-preheating mode—and their thermal performance and efficiency are discussed by energy analysis and exergy analysis. The mathematic models of a polluted-soil thermal remediation system including burner, pipe, well and soil for energy and exergy analysis are built based on thermodynamics, heat transfer and fluid mechanics. Keeping the energy (exergy) at the inlet to the system constant, and various energy (exergy) losses and energy (exergy) utilization ratios at di fferent stages are calculated. The results are graphically formed to compare the energy-saving strategies with the basic method (BM) and to find where the specific embodiment of energy savings is.

#### **2. Idea of Energy-Saving Strategies of Polluted-Soil Thermal Remediation System**

The three energy-saving strategies are presented to improve on traditional systems as shown in Figure 1, and the environment is the same in the research except for the system. The area of soil researched in the paper is 3 meters long, 3 meters wide and 6 meters deep. The following Sections 2.1–2.3 introduce the three energy-saving strategies, respectively.

#### *2.1. Description of Energy-Saving Strategy for Variable-Condition Mode*

Energy-saving strategy for variable-condition mode (VCM) involves di fferent exhaust gas temperatures used at di fferent stages. The process of polluted-soil thermal remediation is divided into three stages lasting for 15, 20 and 10 days, respectively, in the study. In the first stage, the soil temperature rises from the initial temperature to the boiling point of water, and the soil moisture content is the initial moisture content. The second stage is the evaporation stage of water in the soil, and the soil keeps the temperature of boiling point of water unchanged. The third stage is to heat dry soil without water to increase the soil temperature to the final temperature. Therefore, the soil temperature is di fferent as well as the soil heating requirements in the three stages, but in the basic method (BM) in use, the exhaust gas temperature at each stage of heating the soil is constant, that is, as shown in Figure 2, the constant high temperature of exhaust used to heat the soil with changing temperature, which is disadvantageous for saving energy. To solve the problem, variable-condition mode (VCM) is necessary, that is, di fferent exhaust gas temperatures are used at di fferent stages. In modeling and analysis, the most direct reflection is that the temperature inside the burner to the temperature outside the heating well are all di fferent at three stages. The contrastive temperature configurations of variable-condition mode (VCM) and the basis method (BM) are presented in Table 1. In the variable-condition mode (VCM), the airflow circulation in polluted-soil thermal remediation system is the same as that in the basis method (BM), as shown in Figure 1b.

**Figure 2.** Structure diagram of polluted-soil thermal remediation system using the energy-saving strategy for variable-condition mode (VCM).


**Table 1.** Temperature configurations of VCM (variable-condition mode) and BM (basis method).

Based on data from engineering practice and a preliminary estimate of the combustion process, the temperature of soil and the temperature in burner in different stage are set in Table 1. The outlet gas temperature of the heating well *tw,out* is 450 ◦C in BM, which is also a temperature often used in engineering practice. In VCM *tw,out* is the main way to achieve variable conditions to save energy, and it is set by the authors for the case.

#### *2.2. Description of Energy-Saving Strategy for Heat-Returning Mode*

The energy-saving strategy for heat-returning mode is returning the heat contained in the exhaust to the polluted-soil thermal remediation system again. In the basic method (BM), the exhaust containing a considerable amount of heat is discharged directly into the atmosphere and that is a grea<sup>t</sup> waste. To solve the problem, heat-returning mode is necessary, that is, the exhaust from the outlet of the heating well directly discharged to the environment is returned to the burner as the air in a certain proportion, and three schemes are made according to the different proportion of return gas. The rate of return gas is the rate of heat return β. The return air enters the burner from air inlet 2, and the amount of air required for combustion to remove this part is the amount of normal air required from air inlet. The airflow circulation of energy-saving strategy for heat-returning mode in polluted-soil thermal remediation system is different from that in the basis method (BM), as shown in Figure 3b.

**Figure 3.** System diagram of polluted-soil thermal remediation system using the energy-saving strategy for heat-returning mode: (**a**) Structure diagram of polluted-soil thermal remediation system using the energy-saving strategy for heat-returning mode; (**b**) flowchart of air distribution in polluted-soil thermal remediation system using the energy-saving strategy for heat-returning mode.

#### *2.3. Description of Energy-Saving Strategy for Air-Preheating Mode*

The energy-saving strategy for air-preheating mode is to use the residual heat of the system to –preheat the air entering the burner for combustion. In the basic method (BM), heat from high-temperature parts directly exposed to the environment in the system is wasted and the residual heat can be used up. To solve the problem, preheaters for air-preheating mode are set. As shown in Figure 4, the air to be introduced into the burner is divided into three parts: The first part passes through preheater 1 between the burner and the inlet of heating well, the second part passes through preheater 2 at the outlet pipe of the heating well, and the third part enters the burner directly. Three schemes are designed according to di fferent preheating ratio to di fferent preheaters. The preheating ratio of air through preheater 1 is α1, preheating ratio of air through preheater 2 is α2 and the ratio of air that does not pass through the preheater directly into the burner is α3. The airflow circulation of energy-saving strategy for air-preheating mode in polluted-soil thermal remediation system is di fferent from that in the basis method (BM), as shown in Figure 4b.

**Figure 4.** System diagram of polluted-soil thermal remediation system using the energy-saving strategy for air-preheating mode: (**a**) Structure diagram of polluted-soil thermal remediation system using the energy-saving strategy for air-preheating mode; (**b**) flowchart of air distribution in polluted-soil thermal remediation system using the energy-saving strategy for air-preheating mode.

#### **3. Mathematic Models and Parameters Calculation Process**

Mathematical models of the polluted-soil thermal remediation system established in this section are used to support the thermal performance analysis of energy-saving strategies. The thermal performance analysis includes an energy analysis based on the first law of thermodynamics and an exergy analysis based on the second law of thermodynamics, so the models are divided into two parts: Section 3.2 presents the energy analysis model and Section 3.3 the exergy analysis model. Energy utilization ratio and exergy utilization ratio, as the key parameters to evaluate the energy-saving strategies, are calculated at the end of the models in Sections 3.2.5 and 3.3.5. Before the specific model, the balance equation is indispensable. The following assumptions are made in the energy and exergy analysis:


The basic mathematical models in the energy-saving strategies are the same as the basic method (BM), except that the energy and exergy of the air entering the burner are different. In the calculation, paying attention to these parameters is the crucial key of the research. The process of parameters calculation is in the Section 3.4. The value of physical parameters used in the models is shown in Table A1.
