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随着产业升级和城市扩张,大量工厂搬迁或废弃后遗留的场地存在土壤污染问题,需要进行土壤修复才能再次开发[1]。电阻加热技术具有对环境扰动小,受土壤异质性影响小,处理深度大等优点,尤其适合修复含有挥发性、半挥发性有机污染物的污染场地[2-5]。但在实际修复工程中,ERH技术的工程参数设计,例如电极间距、电场强度等,都会显著地影响场地电阻加热的实际效果[6],导致修复周期和成本控制的不确定性。修复场地的污染物分布、地下水流场和土壤特性等往往差异较大,但工程师只能根据已有工程经验和有限的取样勘探结果,进行原位加热工程的参数设计。若能够采用建模的方法,对场地条件下的加热过程进行预测,将有利于减少设计的盲目性,帮助缩短工程周期和控制修复成本。
目前,对于ERH技术的数值模型已经有了一定的探索和应用。HIEBERT等[7-8]开发了用于模拟单相电阻加热过程的二维有限差分模型,并研究了不同的横卧电极设置方式对非均质含油地层的加热效果的影响。CARRIGAN等[9]将改进的欧姆加热模型与非等温多孔流动和传输模型进行了耦合,研究了电极阵列的电相位如何影响电阻加热的均匀性。MCGEE等[10]进一步简化了模拟多相电阻加热的欧姆方程,并模拟了电阻加热从非均质油砂中回收沥青的过程。KROL等[11]考虑温度对密度、粘度、扩散系数的影响,建立了二维有限差分模型,模拟了电阻加热到50 ℃的情况下对地下水流动的影响,发现地下水流动方向和流速发生显著变化。许丹芸等[12]使用有限元方法模拟了电阻加热土壤过程。
尽管关于ERH技术的数值模型研究已有一定的开展,但一方面,以往的模型对电阻加热土壤过程中的水分蒸发缺乏关注和进一步的验证;另一方面,大部分模型是针对实际场地的验证评估,对如何运用模型指导ERH工艺参数的选取探讨不足。本研究使用COMSOL多物理场耦合软件,基于有限元计算方法开展原位电阻加热温度场模拟研究,建立了考虑土壤水分蒸发的模拟原位电阻加热温度场的数值模型。通过对比土柱装置小试实验和数值模拟的结果,验证了数值模型的准确性,并利用数值模型分析了场地尺度下电场强度、电极间距和地下水流动对电阻加热温度场的影响。本研究结果有助于预测修复周期和优化电极井布设,从而达到节约能源和降低修复成本的目的。
原位电阻加热修复热传递模型构建及数值模拟
Modeling of the heat transfer of in-situ electrical resistance heating remediation and numerical simulation
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摘要: 原位电阻加热(Electrical resistance heating,ERH)技术在场地修复中工艺参数的选取较多依赖于工程经验,ERH热量传递的数值模型能够模拟不同工艺参数下土壤加热的温度场,从而为ERH工艺参数的选取提供理论指导。利用电阻加热土柱装置实验验证了建立的数值模型的准确性,并应用在场地尺度下探讨了电场强度、电极间距和地下水流动对原位三相电阻加热的影响。结果表明,模型具有较好的准确性,实测值和模拟值均方误差为0.05~12.29,平均相对误差为0.42%~5.32%。随着电场强度越大,土壤升温速率显著加快;电场强度90 V·m−1时,综合考虑电极井建设数量、加热时长及能耗,最适宜的电极间距为6 m;对电极附近补水可以显著缩短加热时长。较快的地下水流动会降低场地升温速率,不利于场地的修复;对于地下水流速过快的场地应采取适当的工程措施缓解热量流失。本研究结果可为原位电阻加热工程的设计和运行提供参考。Abstract: The selection of process parameters of in-situ electrical resistance heating (ERH) technology in site restoration is still lack of scientific theoretical guidance. A numerical model of ERH heat transfer was established to simulate the soil temperature field with different operation parameters, so as to provide theoretical guidance for the selection of operation parameters. The accuracy of the model was verified by the ERH experiment using a soil column device, and the effects of electric field intensity, electrode spacing and groundwater flow on the soil temperature field during in-situ three-phase resistance heating were discussed at a site scale. The results showed that the mean square error between measured and simulated values was in the range of 0.05~12.29, and the average relative error ranged from 0.42% to 5.32%, indicating that the model was of good accuracy. With the increase of electric field intensity, the soil heating rate increased significantly. When the electric field strength was 90 V·m−1, considering the number of electrode wells, heating time and energy consumption, the suitable electrode spacing was 6 m. Replenishing water to the heating electrode could significantly shorten the heating time. Rapid groundwater flow reduced the temperature rising rate of the site, which was not conducive to the remediation of the site. Appropriate engineering measures should be taken to alleviate the heat loss in the site with fast groundwater flow rate. This study provides a reference for the design and running of in-situ ERH remediation engineering.
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表 1 数值模拟参数
Table 1. Numerical simulation parameters
模拟参数 取值 模拟参数 取值 液体密度,ρL 1 000 kg·m−3 温度系数,β 0.02 固体密度,ρS 2 650 kg·m−3 比例常数,α 5×10−6 m2·s−1 气体密度,ρG 1.9 kg·m−3 经验常数,A 8.07131 液体热容,CL 4 200 J·(kg·K)−1 经验常数,B 1 730.63 固体热容,CS 920 J·(kg·K)−1 经验常数,C 233.426 气体热容,CG 1 000 J·(kg·K)−11 胶结系数,m 1.44 湿导热系数,λsat 1.87 W·(m·K)−1 饱和度系数,n 2 干导热系数,λdry 0.23 W·(m·K)−1 水的潜热,ΔHvap 2 257.2 kJ·kg−1 孔隙度,φ 0.5 蒸发速率系数,kvap 1×10−6 s−1 表 2 数值模拟工况
Table 2. Numerical simulation conditions
工况 电场强度/
(V·m−1)初始水
饱和度地下水流速/
(m·s1)电极
间距/m地下水涨速/
(m·d1)A-1 30 0.6 0 6 0 A-2 60 0.6 0 6 0 A-3 90 0.6 0 6 0 B-1 90/60 0.6 0 3 0 B-2 90/60 0.6 0 6 0 B-3 90/60 0.6 0 9 0 C-1 90 1 0.1 6 0 C-2 90 1 0.2 6 0 C-3 90 1 0.3 6 0 D-1 90 0.6 0 6 0.05 D-2 90 0.6 0 6 0.1 D-3 90 0.6 0 6 0.2 -
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