(715d) CO2 Capture/Sequestration Via Brine and Caustic Material

A great deal of concern has been expressed with regard to global climate change and its link to growing atmospheric concentrations of carbon dioxide. Several strategies are under development that will potentially remove CO2 from the atmosphere or decrease CO2 emission. One such strategy involves the capture of CO2 from large point sources (such as fossil fuel-fired power plants) and long-term storage of captured CO2 underground. Carbon dioxide may be sequestered in various geological formations, including depleted oil and gas reservoirs, unmineable coal seams, basalt formations, and deep saline aquifers. However, there are additional options for CO2 sequestration that involve ex-situ carbonation of caustic industrial waste by-products. This work considers capture/sequestration of CO2 from flue gas through promotion of solubility and mineral trapping in Ca, Mg, and Fe-bearing caustic waste streams.

Coal use for domestic electricity generation is by far the largest market for coal in the United States, representing over 80 percent of annual coal consumption. In addition to energy generation, coal-fired steam-electric generation process, also generates sulfur dioxide, nitrogen oxides, and CO2 emissions and solid coal combustion by-products (CCPs). In U.S. alone over 118 million tons of CCPs are generated each year, a total mass comprising fly ash (60%), bottom ash (15%), boiler slag (1%), and flue gas desulfurization ash (24%). At present, only 30 % of the CCP is utilized for some beneficial application, with the remainder managed as waste. Some of the ashes contain high concentration of Ca as well as high alkalinity. These fly ashes may be good candidates for capturing/sequestration of CO2.

Currently 20-30 billion barrels of saline wastewater are produced annually with the production of oil and gas in the USA. About 35% of this acidic wastewater (pH 3 to 5) is treated and discharged to surface water bodies. Many of the gas/oil-field brines have high concentrations of dissolved Ca, Mg, and Fe ions. Carbonate minerals could be formed in the presence of CO2 under certain conditions. This suggests the possibility of using Ca-, Mg-, and Fe-rich saline wastewater for CO2 sequestration via the formation of carbonate minerals.

Over 70 million tons of bauxite residues are generated annually when aluminum is extracted from the principal ore, bauxite. The residue is mostly comprised of iron and titanium oxides, silica, calcium carbonate, and unrecoverable alumina and soda. The pH of the liquid is as high as 13.5, and the solids and solid surfaces also contain high alkalinity. The caustic nature of the residue causes concerns associated with long-term environmental liability. Worldwide, there are about 200 million tons of bauxite residues that are mostly stored in tailings ponds. For decades, the aluminum industry has been investigating options for treating, disposing, and using bauxite residue. However, environmentally and economically sound methods and processes are still elusive. Addition of CO2 to this caustic material will not only result in carbonate mineral formation but will also serve to decrease the pH of the residue, making disposal more environmentally sound. Moreover, bauxite residue (pH 13.5) can serve as an effective source of caustic material to treat acidic oil and gas field wastewater brines (pH 3 to 5) for mineral carbonation. The use of bauxite residue/brine to capture and store CO2 will serve to not only help mitigate the impact of anthropogenic CO2 on global warming but will also help neutralize caustic industrial waste for safe storage and reuse. Thus, the use of caustic industrial by-products/brine mixtures to capture and store CO2 can serve to not only help mitigate the impact of anthropogenic CO2 on global warming but also serve to achieve complete neutralization of the caustic industrial waste for safe storage and potential reuse.

A novel concept that achieves neutralization of caustic industrial waste material through reaction with acidic oil and gas field wastewater brines with subsequent carbonation of CO2 in a mixed-flow reactor has been explored. Experiments were conducted to determine the CO2-bearing capacity of reactive mixtures of brine from the Oriskany Sandstone Formation with three caustic industrial by-products: flue gas desulfurization (FGD) spray dryer ash, Class C fly ash sub-bituminous coal combustion by-product, and bauxite residue slurry from the alumina-production process. Reactions were conducted in a closed, well mixed reactor with a 30% CO2 balanced by N2 at ambient conditions. Results show linear relationships between caustic by-product addition and CO2-bearing capacity. FGD spray dryer ash/brine mixtures exhibited higher CO2 reactivity than those using Class C fly ash (0.76 moles CO2, at 24 % solids by weight and 0.036 moles CO2 at 23% solids by weight, respectively). Bauxite residue exhibited moderate capacities in mixtures with higher percent solids (0.34 moles CO2 in 40% solids bauxite residue slurry).