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近年来,随着我国城市化进程发展和产业结构调整,许多企业关闭、停产、合并、转让,导致大量的工业有机污染场地被遗留在城区[1]。土壤修复技术众多,其中原位热修复技术具有环境干扰小、修复效果彻底以及可适用于低渗透性土质等优点,适合城市工业有机污染场地的修复,也被认为是未来土壤修复的主流方向[2-5]。原位热修复技术按加热方式不同,可分为原位蒸汽强化修复、原位电阻加热修复和原位热传导修复等多种形式[4]。
原位热传导修复技术因其具备修复周期短、可靠性高、适用性强,以及二次污染可控等优点,被证实是一种有效的原位污染土壤修复技术[6]。原位热传导修复技术利用电加热或燃气加热的方式对污染地块进行加热,土壤中水分及污染物受热蒸发、氧化或热解,大大提高了污染场地修复效率。原位热传导修复技术相比于其他原位修复技术,可将土壤加热至远超污染物沸点的温度[7]。据统计,目前该技术已有广泛的应用[8]。ZHAO等[9]认为,该技术降低了土壤挖掘和运输的成本、减少了对场地内建筑物的破坏和重建,且具有修复效率高、修复周期短等优点。
SUN等[10]和BASTON等[11]研究了原位热传导修复对地下传热过程及水文地质特性的影响,发现水文地质特性敏感的地下温度分布可决定污染物去除效率。迟克宇等[12]基于原位热传导修复技术开展了氯代烃类有机污染场地中试规模修复实验,探讨了不同加热井间距的升温速率及污染物去除率。ZHAO等[13]和HICKNELL等[14]通过实验对原位热传导修复技术油田场地适用性及地下温度反馈修复场地性能的准确性进行了研究,发现该技术在油田中具有实用性,局部尺度越过沸腾平台的温度升高不能代表NAPL-水共沸结束。XIE等[15]和XU等[16]通过数值模拟的方法对原位热传导修复技术共沸平台及供热控制策略进行了研究,以期降低能源消耗并指导工程实践。韩伟等[17]考虑了化学氧化、微生物降解及其他修复方式耦合原位热传导修复技术的现实意义,提出了工程方面降低能耗的具体举措。XIE等[18]模拟现场规模的原位热传导修复技术应用场景,探讨了土壤非均质性、NAPL构型及其质量采收率之间的关系。DAVIS等[19]和YU等[20]研究了原位热传导-气相抽提技术污染物去除路径及影响污染物迁移的关键因素。杨玉洁等[21]同样利用了原位热传导-气相抽提技术,研究了烃类污染土壤热脱附污染物去除效率的影响因素。综上所述,目前对原位热传导修复的研究主要集中在对修复过程的节能降耗、传热过程的影响因素、目标污染物的去除机理及去除效率方面,而对内部传热传质机理的认识尚不够清晰。
本研究基于土壤原位热传导修复单根加热棒小试实验及数值模拟,探究了原位热传导修复过程中土壤内传热传质机理,通过对比分析不同位置处的温升情况,以验证数值模型的准确性;并利用数值模型进一步分析了场地尺度下热源温度、初始含水率等因素对原位热修复效率的影响。本研究结果有助于预测修复周期、优化加热井排布设计,并有助于供热控制策略的调整,从而达到节约能源、实现可持续修复和降低修复成本的目的。
原位热传导修复过程中热量传递的数值模拟
Numerical simulation of heat transfer during in-situ thermal conduction remediation
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摘要: 目前原位热传导修复技术存在热量传递机理不明、主要影响因子作用关系不清的问题,通过模拟室内土柱实验实现对土壤内部热湿耦合迁移机理的验证,并应用到室外场地尺寸,明确场地尺寸下热源温度、初始含水率对原位热传导修复的影响作用。建立了原位热传导修复耦合模型,利用小试实验对其进行了数值模拟验证,在场地尺寸下探究了热源温度、初始含水率对原位热修复的影响。结果表明,原位热传导修复耦合模型准确度较高,模拟结果与实验结果平均相对误差为1.30%。沸腾阶段持续时间与热源温度成反比,过热阶段升温速率与热源温度成正比,在工程实践中应以去除目标为评价标准而不是冷点温度。土壤初始含水率在15%~35%范围内,相同温度影响下含水率越低导热系数越高,原位热传导修复技术适用于低含水率场地,初始含水率高于15%,在进行修复之前应进行适当排水或设置止水帷幕。该研究结果可为原位热传导修复技术工程实践应用提供理论参考。Abstract: The current in-situ thermal conduction remediation technology has the problems of unclear heat transfer mechanism and unclear relationship between the main influencing factors. The verification of the coupled heat and moisture migration mechanism inside the soil is achieved by simulating indoor soil column experiments, and applied to the outdoor site size to clarify the effect of heat source temperature and initial moisture content on in-situ thermal conduction remediation under site size. An in-situ thermal conduction remediation coupling model was established, and a small-scale experiment was used to verify it by numerical simulation. The influence of the heat source temperature and initial moisture content on the in-situ thermal remediation was explored under the site size. The results showed that the in-situ thermal conduction remediation coupling model had high accuracy, and the average relative error between the simulation results and the experimental results was 1.30%. The duration of the boiling phase was inversely proportional to the temperature of the heat source, and the heating rate during the overheating phase was directly proportional to the temperature of the heat source. In engineering practice, the removal target should be the evaluation criterion instead of the cold spot temperature. The initial soil moisture content was within the range of 15%~35%. The lower the moisture content, the higher the thermal conductivity under the same temperature. The in-situ thermal conduction remediation technology was suitable for sites with low moisture content. If the initial moisture content was higher than 15%, proper drainage or water-stop curtains should be installed before restoration. The research results can provide a theoretical reference for the engineering practice application of in-situ heat conduction repair technology.
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表 1 供试土壤性质
Table 1. Test soil properties
初始土壤
温度/℃土壤
含水率/%土壤干导热
系数/(W·(m·K)−1)土壤湿导热
系数/(W·(m·K)−1)土壤
孔隙率土壤比热容/
(J·(kg·K)−1)土壤密度/
(kg·m−3)乙苯质量分数/
( mg·kg−1)27 15 0.50 0.90 0.30 1 660 1 650 6.74 表 2 数值模拟计算工况
Table 2. Numerical simulation calculation conditions
工况编号 初始湿度/% 加热温度/℃ A-1 15 750 A-2 25 750 A-3 35 750 B-1 15 500 B-2 15 750 B-3 15 1 000 表 3 数值模拟参数
Table 3. Numerical simulation parameters
供试参数 设定值 供试参数 设定值 残余含水率 0.05 Antoine系数A 11.68 水比热容 4 182 J·(kg·K)−1 Antoine系数B 3 516.44 水蒸气比热容 1 800 J·(kg·K)−1 Antoine系数C −45.13 水蒸气密度 0.03 kg·m−3 蒸发速率常数kvap 9×10−6 s−1 -
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