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Eggshell waste, which contains 95% calcium carbonate (CaCO3), presents itself as an inexpensive calcium-based sorbent for removal of carbon dioxide (CO2) in combustion streams used to generate electricity. The utilization of eggshell waste in CO2 capture via cyclic carbonation-calcination reactions (CCR) was investigated in this work. Using thermogravimetric analysis, the CO2 capture capacity for multiple acetic acid pretreated eggshell samples was studied. This pretreatement generates a mesoporous structure, allowing the eggshell-derived sorbent to reach higher conversions over more CCR cycles while also removing the eggshell’s protein-rich membrane. Six acetic acid treatments were also explored for regeneration of spent sorbents after multiple cycles. The regeneration of spent sorbents with acetic acid provided a 38% improvement in CaO conversion over untreated shells after ten cycles. The eggshell membrane contained highly valuable Type X collagen, which can be recovered through the course of shell pre...

Several technologies are currently being developed to separate carbon dioxide from large point sources, such as coal-fired power plants. An emerging technology that shows great potential is a calcium oxide–calcium carbonate cycle. A major drawback is the calcium carbonate decreases in reactivity over multiple cycles. The Ohio State University demonstrated in 2008 the first carbonation–calcination reaction (CCR) process that includes intermediate hydration for sorbent regeneration and its feasibility over multiple cycles at the 120 kW th scale with actual flue gas from coal combustion. The CCR Process utilizes a calcium-based sorbent to react with the carbon dioxide and sulfur dioxide in a flue gas stream to form calcium carbonate and calcium sulfate, respectively. The carbon dioxide is subsequently released from the calcium carbonate to produce a high-purity, sequestration-ready carbon dioxide stream while regenerating the calcium oxide sorbent. The sulfur dioxide is fixated as calcium sulfate and removed through a purge stream. An intermediate hydration step restores reactivity to the calcium oxide sorbent. Process analysis from computer simulations shows the CCR Process to be highly effective and efficient in removing both carbon dioxide and sulfur dioxide at low energy penalties under realistic conditions. A 20–22% decrease in electricity generation efficiency with the CCR Process is expected, compared with amine scrubbing around 27% and oxy-combustion around 25% energy penalty. A 25–28% increase in thermal energy with the CCR Process is expected to maintain a constant electrical output. Further, the CCR Process consumes half the oxygen necessary for an oxy-combustion plant and 25% less steam necessary for amine scrubbing.

A pilot-scale study of the Ohio State Carbonation Ash Reactivation (OSCAR) process was performed to demonstrate the reactivity of two novel calcium-based sorbents toward sulfur and trace heavy metal (arsenic, selenium, and mercury) capture in the furnace sorbent injection (FSI) mode on a 0.365 m3/s slipstream of a bituminous coal-fired stoker boiler. The sorbents were synthesized by bubbling CO2 to precipitate calcium carbonate (a) from the unreacted calcium present in the lime spray dryer ash and (b) from calcium hydroxide slurry that contained a negatively charged dispersant. The heterogeneous reaction between these sorbents and SO2 gas occurred under entrained flow conditions by injecting fine sorbent powders into the flue gas slipstream. The reacted sorbents were captured either in a hot cyclone (∼650 °C) or in the relatively cooler downstream baghouse (∼230 °C). The baghouse samples indicated ∼90% toward sulfation and captured arsenic, selenium and mercury to 800 ppmw, 175 ppmw and 3.6 ppmw, respecti...

The influence of several process parameters on the calcination of a naturally occurring limestone and a precipitated mesoporous calcium carbonate (CaCO3) sorbent structure to calcium oxide (CaO) is detailed in this study. CaCO3 calcination is an integral part of a multicyclic carbonation−calcination reaction (CCR) process that separates carbon dioxide (CO2) from high-temperature gas mixtures into a pure CO2 stream. Maintenance of high sorbent reactivity over repeated CCR cycles reduces the capital and operating cost of the CCR process. A lower calcination temperature, required to maintain high sorbent reactivity, reduces the calcination rate of CaCO3. This study investigates various process conditions such as subatmospheric calcination, sorbent dispersion in a rotary calciner, use of a high thermal conductivity sweep gas, etc. in enhancing the calcination rate at lower calcination temperatures. The high-temperature gas−solid carbonation studies of the resulting CaO sorbent prove the necessity to maintain ...

Trace metal emission from coal combustion is a major concern for coal-burning utilities. Toxic compounds such as arsenic species are difficult to control because of their high volatility. Mineral sorbents such as lime and hydrated lime have been shown to be effective in capturing arsenic from the gas phase over a wide temperature range. In this study, the mechanism of interaction between arsenic oxide (As2O3) and lime (CaO) is studied over the range of 300-1000 degrees C. The interaction between these two components is found to depend on the temperature; tricalcium orthoarsenate (Ca3As2O8) is found to be the product of the reaction below 600 degrees C, whereas dicalcium pyroarsenate (Ca2As2O7) is found to be the reaction product in the range of 700-900 degrees C. Maximum capture of arsenic oxide is found to occur in the range of 500-600 degrees C. At 500 degrees C, a high reactivity calcium carbonate is found to capture arsenic oxide by a combination of physical and chemical adsorption. Intrinsic kinetics of the reaction between calcium oxide and arsenic oxide in the medium-temperature range of 300-500 degrees C is studied in a differential bed flow-through reactor. Using the shrinking core model, the order of reaction with respect to arsenic oxide concentration is found to be about 1, and the activation energy is calculated to be 5.1 kcal/mol. The effect of initial surface area of CaO sorbent is studied over a range of 2.7-45 m2/g using the grain model. The effect of other major acidic flue gas species (SO2 and HCl) on arsenic capture is found to be minimal under the conditions of the experiment.

A calcium carbonate (CaCO3) sorbent synthesized in the presence of surface modifiers has recently been developed in this laboratory for the purpose of enhancing the reactivity toward SO2 at high temperatures (800−1150 °C). The calcium carbonate particles are produced by precipitation from an aqueous suspension of calcium hydroxide by bubbling pure CO2. The operating parameters such as suspension concentration, gas flow rate, and concentration of surface modifiers are varied to study their influence on the particle size, surface area, and pore volume. The synthesized CaCO3 particles exhibit dramatically high SO2 reactivity and sorbent utilization. The increased reactivity is primarily due to the development of an open initial pore structure of the sorbent brought about by the action of surface-active agents, compared to the conventional calcium-based sorbents. ζ-potential measurements indicate that addition and subsequent adsorption of anionic surfactants neutralize the high positive surface charge (30 mV)...

A modified CaCO3 sorbent with an open internal pore structure is prepared and its sulfation characteristics are investigated in an entrained flow reactor at high temperatures (900-1,100°C) and short contact times (20–600 ms) using small particle sizes (< 5 μm). The most distinguishing feature of this modified carbonate (MC) is its 70–75% sulfation conversion within 0.5 s, which is substantially higher than any other sorbents published. The MC is prepared by carbonation-precipitation from a calcium hydroxide suspension by optimizing the operating parameters to generate carbonate particles of the desired pore structural properties. The high initial surface area combined with its open pore structure and pore-size distribution of its calcine contribute to its high reactivity. The calcined MC possesses a significant portion of its pore volume in the 50-200 A range. This size range represents an optimum pore size for sulfation since it provides a reasonably high surface area and is less susceptible than < 50 A pore sizes, to pore filling, or pore-mouth plugging due to the formation of higher molar volume CaSO4. Investigation with other carbonates reveals that a much higher portion of their calcines' porosity lies in the smaller pores, which leads to premature termination of sulfation. Results show the impact of internal pore structure on initial reactivity and ultimate sorbent conversion.

Limestone or calcium carbonate (CaCO3) used as a sorbent in the removal of acid gas precursors (SO2) from combustion systems suffers from poor pore structural properties which causes low reactivity and incomplete utilization. The surface area and pore size distribution of CaCO3, if tailored appropriately, can considerably enhance its reactivity for SO2. This study focuses on the optimization of pore properties of CaCO3 particles and the enhancement of SO2 reactivity and ultimate utilization. The carbonate is produced by precipitation from an aqueous suspension of calcium hydroxide by injecting CO2. The influence of operating parameters, suspension concentration, gas flow rate, and additives (surfactant) on the surface area and pore volume is investigated. The surface area of the carbonate powder can be controlled in the range of 10−70 m2/g by varying the operating parameters. The SO2 reactivity and the ultimate utilization of the calcium carbonate indicate a dramatic improvement and can be correlated with...

Kinetics of ultrafast, high-temperature sulfation is investigated for two calcium carbonates from different sources, a commercial calcium hydroxide and a laboratory-made modified hydrated lime powd...

Calcium oxide powder derived by the calcination of mesoporous calcium carbonate provides a higher conversion towards carbonation compared to calcium oxide derived from naturally occurring limestone and hydrated lime. Compaction of this high reactivity sorbent was investigated for its use in CO2 separation from flue gas. It was observed that compaction preserved the porosity of the powder. The pellets were able to attain 50–80% conversion towards carbonation over three cycles. The rate and extent of carbonation reaction reduces with increasing pellet thickness. The effect of compaction load and CO2 concentration did not show any appreciable difference in the rate of carbonation. Whereas the mesoporous calcium carbonate pellet showed appreciable drop in carbonation conversion, the pellets made from other precursors, such as natural limestone and hydrated lime, maintained a higher degree of reactivity over three cycles.