Study On The CaO-base High Temperature CO₂ Adsorbent

Hydrogen plays a vital role in chemical industry, oil refining, energy sources and metallurgy industry. Steam methane reforming is the most economic efficient way to made hydrogen. The CO₂ adsorption enhanced reforming (AER) is an improvement of the original technology. The researches of CO₂ adsorbents have a significant meaning to develop of the AER technique used in the chemical engineering, oil refining, energy sources and the metallurgy industry.The various CaO base CO₂ adsorbents have been prepared, the factors of effect on the adsorption capacity and durability of adsorbents have been detailed investigated. And an adsorption reaction kinetics model of CaO based CO₂ adsorbent has been determined.The effect factors of adsorption capacity include particle sizes, CO₂ pressure fraction and heating rate have been studied. The results showed that different adsorbent particle size condition has a different maximum adsorption temperature with high adsorption capacity. CO₂ pressure also played a big role to adsorption capacity: particle size as 60-80μm and as 1-1.8 mm at CO₂ pressure fraction is over 30% and 20% , respectively, the increase of CO₂ pressure fraction is no use to raise adsorption capacity. A lower heat velocity is beneficial to the adsorption capacity.After the investigation of the adsorption capacity of different adsorbents, the factors which get influence to adsorption capacity mainly include: precursors, carriers and additives. CaCO₃ as precursor made a higher adsorption capacity than Ca(OH)₂ did; And Al₂O₃ as carrier made a higher adsorption capacity than Kaoline and bentonite did. The 3 kinds of above different adsorbents have gradually decrease adsorption capacity one by one in order; Additives made a big different to adsorb velocity: the adsorbents which add additives attain to adsorb balance very quick, while the adsorbents which have no additive needed a longer time to reach adsorb balance than the former; After adding MgO, adsorption capacity of adsorbent increase 10%. The best additive amount is 2.5%.After comparing the different constitute and reaction condition, the factors of adsorption stability mainly include precursors, carriers and additives. CaCO₃ as precursor made a higher adsorption ratio after 15 reaction cycles; After 15 reactioncycles, Al₂O₃ as carrier showed made a big effect to stability. Add KCl additive increased circulating stability obviously. A gentle heating rate is benefit to a good stability. The researches showed that after 10min adsorption, the maximum adsorption capacity is 70% for an optimal CaO base CO₂ adsorbent and the adsorption capacity remain 34% after 62 runs test by a TGA under the condition of adsorption temperature 600℃ and decomposition temperature 800℃.According to the characteristics of CO₂ adsorption curve, simulation was carried out by divided the all adsorption process into reaction section and diffusion section. Reaction section and diffusion section are both simulated by the Avrami-Erofeeve kinetic equation G(x)=[-ln(1-x)]ⁿ. A new more reasonable criterion f'(x)=0 has been proposed to divide reaction section and diffusion section. The simulation results showed that the relative deviation between simulated results and experimental data was lower than 3.5% for reaction section of before f'(x) =0 (n=1/3), and 0.86% same for diffusion section of after f'(x) =0(n=3). The activity energy of reaction section of nano scale CaO is 11.9 kJ/mol, which is lower than micrometer scale CaO 22%. The activity energy of diffusion section of nano scale CaO is 106.9 kJ/mol, which is lower than micrometer scale CaO 33%. Nano scale CaO with a higher reaction constants and diffusion constants than the micrometer scale CaO under the temperatures of 500-650℃. This explains the reason of nano CaO base CO₂ adsorbent has higher adsorption ratio and reaction rate than the micrometer scale CaO base adsorbent.