Abstract
For about 10 years, this research group has developed and utilized a particle grain model (PGM), to simulate CO2-capture carried out by CaO-based porous particles. Chemical kinetics and diffusion parameters were either taken from literature studies or fixed by fitting experimental sorption data. As recently observed, this procedure was not fully satisfactory and revealed systematic, minor discrepancies between PGM numerical results and experimental data when predicting sorbents behavior during the initial chemically controlled regime of carbonation. This work deals with the experimental determination of kinetic and diffusion parameters, utilized in the PGM, by means of straightforward thermogravimetric analysis (TGA) tests on small samples of materials to be evaluated for CO2 sorption and sorption-enhanced processes. To validate this procedure, the carbonation of two Ni–CaO–mayenite combined sorbent-catalyst materials (CSCMs) was studied in TGA. The experimental data so obtained were used to infer carbona...
Highlights
IntroductionCalcium oxide (CaO) has been considered as the most advantageous CO2-sorbent within the mixed oxides family (Mg, Zn, Cu, K, Al, and Ca) because it carbonates (CBN, Reaction 1) over a wider range of adsorption temperatures (200−700 °C),[1] is available at a competitive price,[2,3] and is regenerated by calcination, even though it undergoes a strong reduction of sorption capacity under cyclic carbonation/calcination usage, because of sintering.[4−7] To face this issue, CaO is utilized in combined forms with inert stabilizers: dolomite is a natural material with this feature, while the literature refers to the synthetic CaO-based materials, with different inert phases (Al2O3 or calcium−aluminates CaxAlyOz above all[2])
CO2 separation, production, was that is, capture proposed as a opf rComO2isisnimg uslttraanteeogyu,s8l,y9 applicable to chemical looping combustion of hydrocarbons,[10,11] gasification with calcium looping cycle,[12−15] steam reforming of methane (SMR, Reaction 2)[2,16−21] or higher hydrocarbons (SR, Reaction 3).[22−24] Besides the net reduction of CO2 emissions, the in situ CO2 capture brings in an additional advantage, known as “sorption-enhancing”: the subtraction of CO2 from the gaseous reaction environment displaces the equilibria of the water gas shift (WGS, Reaction 4) and of steam reforming (Reaction 2, Reaction 3) toward products, increasing outlet H2 purity
This confirms the validity of the particle grain model (PGM) model developed by this research group and testifies to the accuracy of the novel procedure described in subsections 2.2.2 and 2.2.3 to determine CBN parameters related to chemically- and diffusion-controlled regimes
Summary
Calcium oxide (CaO) has been considered as the most advantageous CO2-sorbent within the mixed oxides family (Mg, Zn, Cu, K, Al, and Ca) because it carbonates (CBN, Reaction 1) over a wider range of adsorption temperatures (200−700 °C),[1] is available at a competitive price,[2,3] and is regenerated by calcination, even though it undergoes a strong reduction of sorption capacity under cyclic carbonation/calcination usage, because of sintering.[4−7] To face this issue, CaO is utilized in combined forms with inert stabilizers: dolomite is a natural material with this feature, while the literature refers to the synthetic CaO-based materials, with different inert phases (Al2O3 or calcium−aluminates CaxAlyOz above all[2]). CBN: CaO(s) + CO2(g) ↔ CaCO3(s) ΔH2098K = −175.7 kJ mol−1 (Reaction 1). In to situ its CO2 separation, production, was that is, capture proposed as a opf rComO2isisnimg uslttraanteeogyu,s8l,y9 applicable to chemical looping combustion of hydrocarbons,[10,11] gasification with calcium looping cycle,[12−15] steam reforming of methane (SMR, Reaction 2)[2,16−21] or higher hydrocarbons (SR, Reaction 3).[22−24] Besides the net reduction of CO2 emissions, the in situ CO2 capture brings in an additional advantage, known as “sorption-enhancing”: the subtraction of CO2 from the gaseous reaction environment displaces the equilibria of the water gas shift (WGS, Reaction 4) and of steam reforming (Reaction 2, Reaction 3) toward products, increasing outlet H2 purity.
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