Tallinn University of Technology, Estonia
Kadriann Tamm is from Tallinn University of Technology, Estonia.
The main energy source in the Republic of Estonia is the local fossil fuel - oil shale (OS) - which is used for the power as well as shale oil (SO) production. As a consequence, large quantities of OS mineral waste (~8.2 million tons of OS waste per 2012) from the combustion processes accompanied by gaseous emissions of CO2, SOx and NOx (11.7 million tons, 32 and 10 thousand tons per 2012 according to the Eesti Energia AS and the Viru Keemia Grupp AS) are produced. The European Union is constantly introducing stricter conditions for the emissions of greenhouse gases (GHG) and other harmful pollutants as well as new initiatives for environmental technology, waste recovery and recycling, affecting also the OS sector in Estonia (e.g. the goal is to reduce GHG emissions by 20-30% and annual SO2 emissions to 25 thousand tons by 2020, which is 2.5 times lower than the initially allowed amount).
The distribution of sulphur between gaseous and solid phases depends on the OS combustion technology (the concentration of SO2 in circulating fluidised bed combustion flue gas is on the level of some ppm). Sulfur (<2% in Estonian OS) remains in the OS power plant ashes mainly as sulfates (4.8 wt%), but also as sulfides (0.28 wt%), and in the solid waste of SO production mainly as sulfides (4.6 wt%) and little as sulfates (0.36 wt%) (the percentages corresponds to the contents of the OS waste, which in this study were used). Depending on the amount of sulfides and oxidants in the aqueous system, the pH drop could induce the release of highly toxic gaseous H2S. In order to minimize the environmental risks accompanied by utilizing OS wastes as a CO2 trap (up to 290 kg-CO2/t-OS ash) and/or cheap Ca-source for PCC production the routes of sulfur compounds transformations have to be explored in different conditions.
The current study was focused on the direct aqueous (S/L=1/5 to 1/250) carbonation of OS wastes, such as circulating fluidized bed ash (CFBCA) from power production and solid heat carrier ash (SHCA) from SO production. The model systems were studied by thermodynamic equilibrium calculations using HSC Chemistry 7.1 version software. The aim was to imitate the real processes (based on the previous experiments) which occur during the aqueous carbonation of OS wastes, to study the complex equilibrium of sulfur species under different oxidation conditions (model system 1 – 15 % CO2 in air and model system 2 – 14.6 % CO2 and 0.5 % SOx in air, at 25°C and 1atm) and predict the optimum conditions for the CO2(g) binding.
The equilibrium calculations allowed to evaluate the theoretical binding capacity of CO2(g) OS wastes. The aqueous suspension of SHCA had almost twice the binding potential (600 - 700 mol/m3, 264 – 308 kg-CO2/t) of power plant ash CFBCA (400 - 450 mol/m3, 176 - 198 kg-CO2/t). The theoretical CO2 binding potential depended on the competitive SOx content in the flue gas (model system 2); the S/L ratio of the system had no influence. The HSC calculation results of model systems pH (11.28 → 7.5 SHCA; 10.5 → 6.8 CFBCA) were in reasonable accordance with the experimental results (12.28 → 7.5 SHCA; 12 → 6.8 CFBCA) of aqueous carbonisation end-points. The initial pH varied more, because the untreated ashes are thermodynamically more unstable and unlike the experiments, the state of equilibrium calculationsindicates infinite time. Contacting OS wastes with water produced highly alkaline leachates saturated with Ca2+-ions. After all free lime as well as MgO was realised by CO2, the system pH and the concentration of Ca2+-ions decreased appreciably. The Ca-(Mg)-silicates decomposed in acidic conditions releasing Mg2+-ions into the solution. In case of the system 2 the SOx compounds promoted oxidation and the system pH decreased even further. As the sulfuric acid is stronger than carbonic acid, the percentage of SO42- and HSO4--ions was elevated. The equilibriums of sulfide species (S2-, HS-, H2S(aq) and H2S(g)) were dependent of the system pH. HS--ion dominated in all systems. As the suspension pH decreased below10 the concentration of H2S(g) increased reaching the same level of HS- ions (pH ~9.5 and ~8.5, accordingly in model system 1 and 2). H2S(aq) changed similarly but maintained at lower values. The SHCA systems showed clearly, that when all available Ca species were used for CaCO3 formation (including CaS as the main source of aqueous and gaseous sulfide species) the content of sulfides was also depleted. In case of model system 1 the concentration of sulfide species achieved minimum (10-36 mol/L the lowest value set by HSC software) at pH <10 and in model system 2 at pH <9. Even very small amounts of model flue gas provoked the oxidation of sulfides to sulfur intermediate oxidation forms (SxOy2- and SO32-) and sulfates. As the systems reached to the maximal CO2 binding potential, all the solid CaSO4 forms decomposed releasing SO42- and HSO4- -ions into the liquid phase.
The growth of the content of carbonates stabilised after achieving the maximum carbonation potential (CaCO3 dominates in all systems). In case of the model system 2, the concentrations of carbonates started to gradually decline, due to progressing sulfation. Considering the amounts of deposited waste from OS sector, the mineral carbonation of acidic gases is a promising solution in reducing greenhouse gas emissions and utilizing OS waste, while monitoring the pH and oxidation potential of aqueous carbonation to minimise sulfur pollution.