Dehardening of carbon dioxide: A highly efficient chemical precipitant used for the dehardening of deep coalbed methane produced water

Abstract

The greenhouse gas CO2 is used as a precipitant to deharden the deep coalbed methane produced water (PW), allowing it to meet the reuse requirements. The pH of PW is the key factor affecting the absorption and dissolution of CO2. High raw water pH leads to higher CO2 absorption, callback pH, and CO32− release. The aeration rate and aeration time determine how quickly and how much CO2 is absorbed and dissolved. Increasing temperature enhances the removal effect. Smaller CO2 bubble facilitates more rapid CO2 absorption, and high CO2 gas pressure increases the final CO2 dissolution. Under normal temperature and pressure, the pH of raw water is adjusted to 11, with an aeration rate of 200 mL/min for 16 min. In addition, the callback pH is equal to 12.80, resulting in a final dehardening effect of 99.77 %, making the treated PW suitable for reuse. Mechanistic studies have shown that at the first level of dehardness, CO2 is not dissolved enough under constant pressure, causing some Ca2+ to precipitate as Ca(OH)2, which enhances hardness removal. To improve the dehardening effect, excessive callback pH can be used, and multi-stage dehardening ensures that Ca2+ precipitates as CaCO3. Meanwhile, Mg2+ is predominantly removed as Mg(OH)2 in the first stage. Dehardening can produce small CaCO3 microsphere by-products, which can be considered for further recycling. The cost of CO2 as a dehardening agent is 23.83 % better than Na2CO3, which not only achieves the dehardening of deep coalbed methane PW at a low cost, but also provides a feasibility reference for the early realization of the “dual carbon” target to a certain extent.