난분해성 오염토양 열적처리 기술 개발에 관한 연구

Abstract

전 세계적으로 PCBs에 의한 토양 오염은 석유화합물, 중금속 및 기타 유기물 오염과 더불어 매우 심각하게 여겨지고 있다. 대부분의 국가들은 이 오염물질을 규제하기 위한 자체 규정을 가지고 있으며 특히, PCBs의 경우는 염소기의 위치에 따른 다양한 이성질체를 가지고 있기 때문에 더욱 철저한 관리가 필요하다. 또한 PCBs의 난분해성은 인체에 분해되지 않고 축적되며 인간에게 뿐 아니라 환경 생태계에도 매우 해로운 실정이다. PCBs를 제거하기 위한 기술로는 소각 및 열탈착법, 고형화 및 안정화법, 탈할로겐화법, 미생물에 의한 처리법 등 여러 가지 기술들이 적용되어 왔다. 그 중 열탈착은 오염토로부터 PCBs를 완전하게 제거하기 위해서 매우 짧은 시간 내에 고온 처리를 하는 기술이며 본 연구에서는 PCBs 및 TPH와 같은 난분해성 오염물질로 오염된 오염토를 정화하기 위하여 열탈착 공법을 적용했다. 실제 현장에 존재하는 PCBs 오염토의 입경별 오염분포를 조사한 결과 입경이 미세할수록 PCBs 농도가 증가함을 나타냈으며 특히 18mm 이상에서는 거의 검출되지 않아 선별공정에 의해 약 10%의 처리대상토의 감량화가 가능한 것으로 나타났다. 토양 중 PCBs의 열탈착 특성을 알아보기 위해 TGA, DSC, DTA 등의 열분석 실험을 수행했으며 그 결과 PCBs는 300~450℃에서 열탈착이 일어나는 것으로 나타났다. 온도와 시간에 따른 최적의 열탈착 조건을 찾아내기 위해서 Furnace 반응기를 이용하여 실험했으며 통계처리한 결과 처리온도 325℃, 체류시간 25분에서 최적의 조건을 나타냈으며, 500ml 용량의 Lab 열탈착 반응기를 이용하여 반복 실험한 결과 400℃, 20분에서 99.99%의 PCBs 제거율을 나타냈다. Lab 실험 결과를 기초로 하여 반응기, 응축기, 냉각기, 활성탄 흡착탑 등으로 구성된 500kg/hr 파일롯 열탈착 장치를 이용하여 실험하였다. 이 장치의 공정은 400~500℃의 rotary kiln에 토양을 투입하여 열탈착하는 1단계공정과 발생된 미세분진 및 가스를 응축기를 통과시키는 2단계 공정, 발생된 응축액을 수집하여 처리하는 3단계 공정으로 나뉜다. 운전결과 TPH의 경우 처리전 2,100∼27,500 mg/kg으로 오염된 토양을 처리후 20mg/kg 정도로 토양오염우려기준 1지역 기준인 500mg/kg 이하로 처리하였다. 토양의 Mass Balance는 모두 95% 이상이었고 대부분의 토양은 처리토 및 싸이클론 분진의 형태로 배출되었다. PCBs 오염토의 경우 처리전 약 5mg/kg인 토양을 처리후 0.01~0.015mg /kg으로서 99.5% 이상의 처리효율을 나타냈으며 이는 토양오염우려기준 1지역 기준인 1mg/kg에 만족하였다. 배출가스 중 다이옥신의 농도는 활성탄탑을 설치한 결과 가스상 다이옥신은 0.018ng-TEQ/m3으로서 소각로의 0.1ng-TEQ/m3 다이옥신 기준인 보다 낮음을 확인하였다.|Soil contamination with petroleum hydrocarbons, heavy metals and other organic components like Polychlorinated biphenyls (PCBs) is very serious issue in these days in different countries. Most of the countries have their regulation to limit the contaminant concentration in soil. However, PCBs are managed specially with more cautious since those have different types of isomers depending upon the number and direction of chloride in the PCBs compounds. PCBs are very hard to degrade and therefore it creates harmful effect to the environment and living creatures. To remove PCBs from soil, different technologies such as solidification/ stabilization, vitrification, chemical washing, bioremediation have been applied. Among all the applied technologies high temperature treatment process like thermal desorption has been observed to be very efficient one due to the short reaction time and generation of almost clean soil. Based on suitability of thermal desorption process, our present research work is focused on treatment of PCBs and TPH contaminated soils through such technology. This work involves both laboratory and pilot scale experiments. The PCBs soil sample was initially analyzed to find out PCBs concentration according to sieve particle size. It was observed that with decrease of particle size the PCBs concentration increased. Thermal analysis of the contaminated soil was performed through TGA, DSC and DTA methods. All the data analysis showed appreciable result within temperature range of 300-450℃, and this temperature range was suitable for soil treatment in thermal desorption process. Initial laboratory thermal experiments were conducted in furnace at different temperatures and different reaction time. The optimum reaction condition was determined to be 325℃(reaction temperature) and 25 minute (reaction time). Whereas, in the case of experiments in 500mL reactor, PCBs removal efficiency (99.99%) was the best at 400℃temperature and 30 min reaction time. Based upon above laboratory experiments, the pilot scale reactor study was conducted. A thermal desorption reactor was prepared having input capacity of 500kg of soil per hour. The major parts of the thermal desorption reactor contained of condensing tower, condenser, cooler, cooling tower. The 1st stage reaction in the thermal desorption plant takes place in the rotary kiln where the soil treated at 400-500℃. Then the particulate matters are removed by cyclone. The 2nd stage involves the condensation process and in the 3rd stage the condensate materials collected and finally treated and released to atmosphere. In case of TPH contaminated soil, the TPH concentration in all experiments varied from 2100 to 27,500mg/Kg and after thermal desorption the concentration decreased to 20 mg/Kg which is quite below the standard level of concentration (500 mg/kg). The Mass-balance study reported 95% result based upon the treated soil and dust removed through cyclone. In case of PCBs contaminated soil the concentration was 5.24 and 4.98 mg/Kg before treatment and after thermally desorption the concentration decreased to 0.015 and 0.01 mg/kg, which resulted 99.7% and 99.8% removal efficiency. The obtained concentration in the soil after treatment is quite less than the standard level of concentration (1 mg/kg) and therefore, this soil can be considered as safe to dispose after treatment. In this case, the dioxin released after activated carbon treatment was found to be 0.018ng-TEQ/m3 which is comparatively lower than the dioxin level of the incinerator where the standard dioxin concentration remains at 0.1ng-TEQ/m3 .; Soil contamination with petroleum hydrocarbons, heavy metals and other organic components like Polychlorinated biphenyls (PCBs) is very serious issue in these days in different countries. Most of the countries have their regulation to limit the contaminant concentration in soil. However, PCBs are managed specially with more cautious since those have different types of isomers depending upon the number and direction of chloride in the PCBs compounds. PCBs are very hard to degrade and therefore it creates harmful effect to the environment and living creatures. To remove PCBs from soil, different technologies such as solidification/ stabilization, vitrification, chemical washing, bioremediation have been applied. Among all the applied technologies high temperature treatment process like thermal desorption has been observed to be very efficient one due to the short reaction time and generation of almost clean soil. Based on suitability of thermal desorption process, our present research work is focused on treatment of PCBs and TPH contaminated soils through such technology. This work involves both laboratory and pilot scale experiments. The PCBs soil sample was initially analyzed to find out PCBs concentration according to sieve particle size. It was observed that with decrease of particle size the PCBs concentration increased. Thermal analysis of the contaminated soil was performed through TGA, DSC and DTA methods. All the data analysis showed appreciable result within temperature range of 300-450℃, and this temperature range was suitable for soil treatment in thermal desorption process. Initial laboratory thermal experiments were conducted in furnace at different temperatures and different reaction time. The optimum reaction condition was determined to be 325℃(reaction temperature) and 25 minute (reaction time). Whereas, in the case of experiments in 500mL reactor, PCBs removal efficiency (99.99%) was the best at 400℃temperature and 30 min reaction time. Based upon above laboratory experiments, the pilot scale reactor study was conducted. A thermal desorption reactor was prepared having input capacity of 500kg of soil per hour. The major parts of the thermal desorption reactor contained of condensing tower, condenser, cooler, cooling tower. The 1st stage reaction in the thermal desorption plant takes place in the rotary kiln where the soil treated at 400-500℃. Then the particulate matters are removed by cyclone. The 2nd stage involves the condensation process and in the 3rd stage the condensate materials collected and finally treated and released to atmosphere. In case of TPH contaminated soil, the TPH concentration in all experiments varied from 2100 to 27,500mg/Kg and after thermal desorption the concentration decreased to 20 mg/Kg which is quite below the standard level of concentration (500 mg/kg). The Mass-balance study reported 95% result based upon the treated soil and dust removed through cyclone. In case of PCBs contaminated soil the concentration was 5.24 and 4.98 mg/Kg before treatment and after thermally desorption the concentration decreased to 0.015 and 0.01 mg/kg, which resulted 99.7% and 99.8% removal efficiency. The obtained concentration in the soil after treatment is quite less than the standard level of concentration (1 mg/kg) and therefore, this soil can be considered as safe to dispose after treatment. In this case, the dioxin released after activated carbon treatment was found to be 0.018ng-TEQ/m3 which is comparatively lower than the dioxin level of the incinerator where the standard dioxin concentration remains at 0.1ng-TEQ/m3 .