| 摘要: | 近幾年來燃煤火力發電廠的汞排放問題受到國際環保界的矚目，美國電力研究機構(EPRI)與能源部(DOE)的調查指出煤的汞含量約為0.02?0.25 ?g/g，其中煙煤(bituminous)的汞含量約0.087 ?g/g，亞煙煤(sub-bituminous)約0.053 ?g/g，褐煤(lignite)則約0.177 ?g/g；相對於燃料油的汞含量(0.002?0.008 ?g/g)高出許多，台灣地區的電力供應逾六成仰賴燃煤火力發電廠，更凸顯燃煤程序汞排放的重要性。目前已知全球較大的汞的排放源之一是燃煤鍋爐，由燃煤程序排放之汞與其燃煤的種類及污染控制設備較為相關。由於汞的蒸氣壓大，容易在高溫燃燒後以氣態型式隨著煙道氣流排出，而且以現行空氣污染防制設備很難加以去除，而針對煙道氣微量汞的捕集方法以活性碳或飛灰的吸附及以濕式洗滌法去除較常見。文獻指出汞經由FGD設備可被去除30?40%，但仍有部份的汞可能隨著煙道氣進入大氣中，再透過大氣自淨程序(乾溼沈降)返回地表，經由食物鏈影響人類的健康，此一課題不容忽視。 本研究以國內三座燃煤電廠為對象，藉由煙道氣與灰份之採樣，分析各廠重金屬(Hg、Cd、Pb、Cr、As及Ni)之排放特性。A、B、C廠空氣污染防制設備不同，A廠為袋濾式集塵器加排煙脫硫，B廠為選擇性觸媒還原法加靜電集塵器加排煙脫硫，C廠則為靜電集塵器加排煙脫硫。初步研究結果顯示，B廠空氣污染防制設備對重金屬去除率介於40~90% (SCR+ESP+FGD)，A廠對重金屬Hg、Pb、Ni及As去除率達60%以上，Cr為52%，Cd為36%；B廠空氣污染防制設備對於重金屬鎳除外之去除效率較A廠佳。而A、B及C廠之總汞去除效率分別為75％、82％、59%。由於B廠亦使用選擇性觸媒還原法以控制NOx排放其去除率較C廠佳。煙道氣採樣結果顯示低沸點之Hg，其煙囪濃度分佈以氣相為主(80~90%)，並由APCD去除率得知Hg較其他金屬穿透至煙道氣的機會大。而A、B、C三廠的煙囪平均汞排放濃度分別為0.84、0.40、1.11μg/Nm3。A廠空氣污染防制設備對粒狀物去除率為95%，B廠則達99%。就整廠重金屬流佈而言，Hg、Cd、Pb、Cr、As及Ni在A廠飛灰中佔有較高之分佈比例，平均約70%以上。針對Hg而言，B廠之飛灰分佈佔85%，而有10%之汞會由煙囪逸散出，石膏及底灰則低於2%，可知飛灰對汞的去除有一定的影響力。A廠煙囪重金屬平均排放濃度依序分別為Pb>Cr>Ni>Cd>Hg>As。B廠煙囪重金屬平均排放濃度依序分別為Ni>Cr>Pb>Cd>As>Hg。最後，由排放數據推估以Pb、Ni、Cr之年排放量較高。 Emission of trace metals from coal-fired power plants has become an important environment issue worldwide. Studies conducted by the US EPA, EPRI, and DOE indicate that coal contains mercury in the range of 0.02?0.25 ?g/g. It is significantly higher than heavy oil (0.002?0.008 ?g-Hg/g). In Taiwan, more than 60% of electricity is supplied by coal-fired power plants. This fact may result in mercury pollution in Taiwan and needs to be investigated in detail and controlled as soon as possible. Coal-fired utility boilers are the largest anthropogenic emission sources of mercury in the world. Concentration and speciation of mercury emitted depend on coal type and air pollution control devices (APCD) applied. Mercury with a relatively high vapor pressure easily escapes from traditional APCDs (i.e., FGD, EP, and wet scrubbing). Several methods have been proposed for capturing trace mercury from the flue gases, including adsorption by activated carbon or fly ash, or removal by wet scrubbers. It is reported that only 30-40% Hg can be captured by FGD and more than 50% Hg is emitted into the atmosphere. In addition to mercury, many kinds of trace metals also exist in coals. Notably, parts of them are toxic. Therefore, understanding the current emissions of trace metals from coal-fired power plants is deemed necessary. In this study three coal-fired power plants were selected for conducting flue gas and ash sampling for determination of the metals emission including mercury, cadmium, lead, chromium, arsenic, and nickel and the removal efficiency achieved by the APCDs. Plant-A was equipped with fabric filter and flue gas desulfurization, and plant-B was equipped with selective catalytic reduction, electrostatic precipitators and flue gas desulfurization, while plant-C was equipped with electrostatic precipitators and flue gas desulfurization. The results indicate that removal efficiency of heavy metal could reach 90% for Plant-B (ESP+FGD). The removal efficiency of heavy metals reaches more than 60% (except for Cr and Cd). The overall removal efficiencies of mercury were 75% and 82% and 59% for plant-A and plant-B and Plant-C, respectively. Because SCR is installed in Plant-B for NOx control, lower Hg emission from Plant-B compared to Plant-A is measured. The results of stack sampling show that metals with high vapor pressure and low boiling point, such as Hg, gas-phase portion were about 80~90% of the total concentration and dominated the partitioning. The results indicate that metals with low boiling points tend to penetrate through APCD and effective control device is needed to collect them. Preliminary results indicate that removal efficiency of particulate was about 95% for Plant-A, 99% for Plant-B. At the stack of the plant-A, plant-B and plant-C total mercury emission concentration were 0.36 ug/Nm3, 0.45 ug/Nm3 and, 1.11ug/Nm3, respectively. Moreover, Hg, Cd, Pb, Cr, As and Ni were mainly retained in the fly ash for Plant-A. As far as Hg was concerned, 85% of total Hg were retained in fly ash, and 10% of total Hg would be emitted into the atmosphere. The concentrations of stack in the Plant-A were Pb>Cr>Ni>Cd>As>Hg；Plant-B were Ni>Cr>Pb>Cd>As>Hg in order. |
