板岩-二氧化碳-水反應後之礦物結垢模擬

Abstract

礦物結垢乃是目前地熱發電利用上遭遇之瓶頸，過去宜蘭清水地熱電廠正因為結垢問題嚴重而終致關廠。地熱電廠中產生之主要結垢種類為碳酸鈣與二氧化矽。為了避免使用化學藥劑對儲集層和環境所造成的汙染，本研究藉由模擬結垢形成來尋求如何防治結垢之方法。 本研究利用自行設計之高壓熱水反應器，以板岩、純水與二氧化碳進行清水地熱區之儲集層模擬實驗 (R)，以及結垢模擬實驗 (S)。R部分實驗以單槽反應模擬不同壓力下300℃時之儲集層狀態。S部分實驗模擬溫泉水由儲集層上升至地表時不同階段的溫度、壓力狀態。反應後進行水溶液化學分析，並以SEM-EDS觀察次生沉澱物。 R部分實驗結果顯示在300℃時會生成大量黏土礦物與黃鐵礦，壓力達277 bar時會出現少量六方晶系之磁黃鐵礦，指示壓力增加有助於岩石中的鐵溶解，促成磁黃鐵礦形成。S部分實驗儲集層模擬條件設定於200℃，在飽和水蒸氣壓的條件下可形成碳酸鈣、硫酸鈣及伊萊石。通入過量二氧化碳壓力至200 bar以上會增加大部份礦物的溶解度，除了碳酸鈣和硫酸鈣消失，其他次生礦物的量也明顯減少。若將二氧化碳灌入地層或生產井中或許可以抑制碳酸鈣和硫酸鈣結垢生成，甚至溶解已結垢在地層中之碳酸鈣。然而本研究將模擬結果和清水地熱區現地溫泉水，成份差異很大，其中HCO3-與Na+、K+離子明顯偏低。未來應以碳酸氫鈉水溶液模擬溫泉水與板岩之反應，應可獲得更多碳酸鈣沉澱，並進一步探討二氧化碳對結垢抑制的效果。二氧化矽結垢部分，壓入二氧化碳造成反應後溶液Si4+濃度超過非晶質二氧化矽飽和濃度。溫度越低，產生沉澱物的時間越長，生成的非晶質二氧化矽更多。未來可評估各地二氧化矽濃度來設計熱交換器之工作流體使用溫度。

Mineral scaling is a major problem for geothermal power plants. Serious scaling was the key factor for shutting down the Chingshui geothermal power plant. Carbonates and silicas are the most common precipitated minerals as scaling in a geothermal system. To avoid the pollution of chemical inhibitors, our studies focus on hydrothermal experiments using high pressure thermal vessel to simulate the mineral precipitation and test how to prevent the mineral scaling. This study chose the Chingshui geothermal area as the target objects. Pure water and saturated CO2 with slates are the starting materials in our experiments. In reservoir simulation experiments (R), single autoclave was used to simulate the roles of pressure at 300℃. In scale simulation experiments (S), three autoclaves have been applied to simulate the different steps when hot water raise up from reservoir to surface. Finally, we analyze the solution by the IC and ICP-AES, and precipitated minerals by SEM-EDS. Experiments R show that large amounts of secondary chlorite and pyrite appeared at 300℃. When the pressure was up to 277 bars, hexagonal pyrrhotite occurred due to the higher pressure dissolving with more Fe. Calcite, gypsum and illite are the major products at saturated water vapor with pressure at 200℃ in the experiment S. When we injected CO2 with pressure of 200 bars into the system, calcite and gypsum disappeared and other secondary precipitated minerals decreased obviously. Therefore, if the CO2 injected into the reservoir or production well, the CaCO3 and CaSO4 deposits will inhibit and scaling dissolve again in reservoir. However, the concentrations of HCO3-, Na+ and K+ of solutions in our experiments were lower than the ones of thermal water in the Chinshui geothermal area. In the future, the NaHCO3 solution should design for experiments to get more CaCO3 precipitations. Meanwhile, more experiments need to be design to understand the effects of CO2 on scaling prevention in the future. For the silica scale, the Si4+ concentration is more oversaturated than the solubility of amorphous silica, if we inject CO2 to the system. Accordingly, amorphous silica would deposit as scaling. Therefore, concentration of the Si4+ is an important parameter for designing a heat exchanger with different temperature.