Sequestration of carbon dioxide by indirect mineralization using Victorian brown coal fly ash

Simultaneous biogas upgrading, CO2 sequestration, and biogas slurry decrement using biomass ash

A novel system for simultaneous biogas upgrading, CO₂ sequestration, and biogas slurry decrement was established by adding biomass ash into biogas slurry to form a renewable CO₂ mixture absorbent. After CO₂ saturation, the CO₂-rich mixture absorbent could be applied for plant growth. When the mass ratio of liquid to solid was 4:1, CO₂ absorption capacity of this mixture absorbent reached up to 97.33 g-CO₂/kg-biomass-ash, which was about 135% higher than that of the biomass ash-water mixture. The highest value of 129.94 g-CO₂/kg-biomass-ash was obtained at a liquid-solid ratio of 99:1. When the TS concentration of anaerobic digestion feedstock was higher than 16 wt% and the water content of CO₂-rich absorbent was about 50 wt%, more than 80% of biogas slurry can be adsorbed by the biomass ash. If the biomass ash with a CO₂ absorption capacity of 100 g-CO₂/kg was adopted and its transportation distance was less than 45 km, the biogas upgrading cost could be lower than the global average level (about RMB¥ 0.7/Nm³-biogas) when using the novel system proposed in this study.

Calcination-free production of calcium hydroxide at sub-boiling temperatures

Calcium hydroxide (Ca(OH)₂), a commodity chemical, finds use in diverse industries ranging from food, to environmental remediation and construction. However, the current thermal process of Ca(OH)₂ production via limestone calcination is energy- and CO₂-intensive. Herein, we demonstrate a novel aqueous-phase calcination-free process to precipitate Ca(OH)₂ from saturated solutions at sub-boiling temperatures in three steps. First, calcium was extracted from an archetypal alkaline industrial waste, a steel slag, to produce an alkaline leachate. Second, the leachate was concentrated using reverse osmosis (RO) processing. This elevated the Ca-abundance in the leachate to a level approaching Ca(OH)₂ saturation at ambient temperature. Thereafter, Ca(OH)₂ was precipitated from the concentrated leachate by forcing a temperature excursion in excess of 65 °C while exploiting the retrograde solubility of Ca(OH)₂. This nature of temperature swing can be forced using low-grade waste heat (≤100 °C) as is often available at power generation, and industrial facilities, or using solar thermal heat. Based on a detailed accounting of the mass and energy balances, this new process offers at least ≈65% lower CO₂ emissions than incumbent methods of Ca(OH)₂, and potentially, cement production.

A calcination-free route to produce calcium hydroxide from alkaline industrial wastes including leaching, concentration, and temperature-swing precipitation.

Calcium elution from cement kiln dust using chelating agents, and CO2 storage and CaCO3 production through carbonation

In this study, indirect carbonation was carried out by using cement kiln dust (CKD), an alkaline industrial by-product, and three chelating agents (citrate, malonate, and adipate salts) as solvents at the room temperature and atmospheric pressure. We derived the optimum conditions for eluting Ca from CKD, as well as those for storing CO₂ and producing CaCO₃ through carbonation. The most important factor affecting the Ca elution from CKD was the solvent concentration and that for the carbonation was the end-of-carbonation pH. Under the optimum conditions of Ca elution, the molar ratios of Ca and solvent in eluates were 1:1, 1:2, and 1:2, respectively, using citrate, malonate, and adipate solvents. Based on the results, we propose that one molecule of Ca ion and one molecule citrate that is tridentate are combined to form a complex. The bidentate malonate and adipate, on the other hand, form complexes by combining one molecule of Ca ion and two molecules of each solvent. It is essential to raise the pH while simultaneously minimizing the amount of free chelating agent in solution to produce more CaCO₃ and prevent its dissolution. Besides, it is absolutely necessary to terminate the carbonation reaction at a pH of about 10.5 to improve the reuse efficiency of the chelating agent. CaCO₃ produced through carbonation reaction started to dissolve at pH approximately 10.5. All of the CaCO₃ produced was calcite with a purity of 98%. The efficiency of Ca elution from CKD using three solvents increased significantly with increasing stability constant of a Ca-ligand complex, but the efficiency of carbonation was the same for all solvents.

United Conversion Process Coupling CO2 Mineralization with Thermochemical Hydrogen Production

In this work, to achieve both clean energy production and carbon emission reduction, a united conversion to couple CO₂ mineralization with thermochemical hydrogen production is proposed. Natural magnesium silicate minerals are used to fix CO₂ in the form of carbonate minerals, whereas H₂O is dissociated to produce H₂ in the thermochemical cycle. The integration provides a new solution to the challenges of the high energy consumption and poor economic value of conventional CO₂ mineralization processes, and the technical feasibility has been proven. Moreover, the energy economy and CO₂ conversion capacity were investigated. Hydrolyzation and carbonation experiments were performed in a homemade reactor, and it was found that an optimal MgI₂ hydrolyzation rate of 75% could be achieved without alkali consumption. A detailed simulation of the whole system was also developed. The optimal energy conversion efficiency of the cycle reached 47.6%, which is higher than most of the published theoretical energy efficiency values for sulfur-iodine thermochemical cycles. A modified calculation of the net energy requirement for CO₂ mineralization was carried out. Finally, a comparison and an evaluation of the energy efficiencies were made based on the calculation. In the optimal case, the modified net energy requirement is 1.4 MJ/kg CO₂, which means that this method is competitive compared to those of previous works.

Metal release and speciation changes during wet aging of coal fly ashes

Introduction of coal fly ash into aquatic systems poses a potential environmental hazard because of its heavy metal content. Here we investigate the relationship between solid phase transformations, fluid composition, and metal release and speciation during prolonged wet aging of a class C and class F coal fly ash. The class C ash causes rapid alkalinization of water that is neutralized over time by CO(2) uptake from air and calcite precipitation. The resulting aqueous metal concentrations are below regulatory limits with the exception of Cr; solubility constraints suggest this is released as chromate. Limited As release is accompanied by no change in solid-phase speciation, but up to 35% of the Zn in the ash dissolves and reprecipitates in secondary phases. Similar processes inhibit Ba and Cu release. In contrast, the class F ash causes rapid acidification of water and initially releases substantial quantities of As, Se, Cr, Cu, Zn, and Ba. Arsenic concentrations decline during aging because of adsorption to the iron oxide-rich ash; this is aided by As(III) oxidation. Precipitation processes lower Ba and Cr concentrations during aging. Se, Cu, and Zn concentrations remain elevated during wet aging and solid-phase Zn speciation is not affected by ash-water reactions. Total metal contents were poor predictors of metal release, which is predominantly controlled by metal speciation and the effects of ash-water reactions on fluid pH. While contact with atmospheric gases has little effect on class F ash, carbonation of class C ash inhibits metal release and neutralizes the alkalinity produced by the ash.