| Nano CaO based reactive adsorbents are widely used in CO2 capture from fuel gas and H2 production from a reactive sorption enhanced reforming (ReSER) process. It is significantly important to study the kinetics of the reactive adsorption and decomposition of nano CaO in a sorption complex catalyst for the industrial application of CO2 capture from fuel gas and the ReSER process.Firstly, this paper studied on the reactive kinetics of nano CaO with CO2 in a sorption complex catalyst named Cata. For the first timean ion reaction mechanism is proposed based on a hypothesis of reaction as the control step, and an ion reactive adsorption model, which is similar to Boltzmann equation, was developed as X=Xu-Xu/[1+exp((t-to)k/Xu)]. A criterion has been proposed to divide the fast surface reaction regime and the slow diffusion-controlled reaction regime correspondingly. Experiments using nano-CaO to react with CO2 in a fast surface reaction regime within a sorption complex catalyst were performed using thermogravimetric analysis (TGA) at 500℃-600℃under a N2 atmosphere with 0.010-0.020 MPa CO2. The activation energy of the kinetic model for carbonation is 30.2 kJ/mol, and the average relative deviation of the sorption ratio is less than 9.8%. The ion reactive adsorption model accurately depicts the carbonation kinetics of a sorption complex catalyst.The decomposition temperatures(DT) of nano CaO reactive adsorbents with particle sizes (ranging from 0.06mm to 1.5 mm), complex catalysts with specific surface areas (ranging from 22.6m2/g to 70.3m2/g), and nano CaO adsorbents with SiO2,TiO2 and Al2O3 additives were determined by TGA in a N2 atmosphere. Compared to adsorbent with particle size of 1.3mm-1.5mm, the DT of the adsorbent with particle size of 0.06-0.15mm decreased by about 100℃. As specific surface areas of the sorption complex catalyst increased from 22.6m2/g to 70.3m2/g, the initial DT had no major changes and the ultimate DT decreased from 732.2℃to710.8℃.The initial DT of nano CaCO3 could be brought down as much as 65℃only by adding SiO2 using precipitation method. Compared to 70nm CaCO3, the ultimate DT of nano CaCO3 adsorbents added with SiO2, TiO2 or A12O3 decreased by 6℃-42℃. It concludes that there are three methods to decrease the decomposition temperature: decrease partical size, increase specific surface areas and add SiO2, TiO2 and Al2O3.The decomposition rates of 70 nm CaCO3 (NC),20μm CaCO3, and sorption complex catalysts with different specific surface areas (22.6m2/g-70.3m2/g) were detected by TGA at 750℃in N2 atmosphere. The results showed that NC reach decompose conversion of 0.98 within 3.1 minutes, which was shorter than 20μmCaCO3 by 1.4 minutes; as specific surface areas of complex catalyst increasing from 22.6m2/g to 70.3m2/g, the time to reach decompose conversion of 0.80 was cut short from 2.86 minutes to 2.56 minutes. It indicated the decomposition rate of nano CaCO3 was faster than micro CaCO3. Sorption complex catalyst with a high specific surface area has a faster decomposition rate.Determination of the equilibrium partial pressures of CO2 resulting from the decomposition of NC and from a sorption complex catalyst Cata were attempted by a fixed bed reactor with an on-line gas phase chromatography analysis at 550℃-750℃. The results showed that the equilibrium partial pressures and the reaction equilibrium constants of CO2 of NC and Cata were higher than that of 10μm CaCO3 at the same temperature. The equilibrium temperature of NC and Cata decreased about 30℃and 40℃respectively in the same CO2 pressure comparing to 10μm CaCO3. These results indicated that the nano CaCO3 is easier to decompose under hyperthermic treatment.Finally, decomposition kinetics of nano CaCO3 in Cata was determined by TGA at 650-800℃in N2 atmosphere. The results showed that the temperature affected the decomposition properties significantly and the decomposition conversion-time curve was sigmoid. Avrami-Erofeev equation and Boltzmann equation were applied to estimate the intrinsic rate parameter:the decomposition activation energies were obtained as 157.4kJ/mol and 156.3kJ/mol respectively. All average relative deviations of Avrami-Erofeev equation were less than 9.0% at 650℃,700℃and 750℃, and the average relative deviation was 34% at 800℃. Both average relative deviations of Boltzmann equation were about 7.0% at 650℃and 700℃, and the average relative deviation was 31.4% at 750℃,2.8% at 800℃. The decomposition kinetics of Cata may be depicted by Avrami-Erofeev equation very well at 650℃-750℃, and it may be depicted more exactly by Boltzmann equation at 800℃.In brief, both the kinetics of reactive adsorption and decomposition of nano CaO in a sorption complex catalyst can be depicted by Boltzmann equation. The results of above studies will be a reference to the design of reactors in CaO looping system used in CO2 captures of flue gas and in ReSER hydrogen production process. |
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