The influence of N 2 on CO 2 adsorption was evaluated using a microporous biochar with a narrow pore size distribution. The adsorption isotherms of pure CO 2 and N 2 were measured at 0, 30, 50, and 70 °C up to 120 kPa and fitted to the Toth adsorption model. Dynamic breakthrough experiments were carried out in a fixed-bed adsorption unit using binary mixtures with compositions representative of different postcombustion streams (8−30% CO 2 ) from ambient temperature to 70 °C. Dynamic adsorption experiments were simulated to validate the mathematical model of the adsorption process, as a necessary step for its later use for process design. The Ideal Adsorption Solution (IAS) theory, based on the pure component adsorption models, was used to account for competitive adsorption with satisfactory results. The information gathered in the present work will be used to extend the validity of the model to the adsorption of postcombustion streams containing H 2 O in part 2.

The adsorption behavior of humid mixtures that are representative of postcombustion conditions on a microporous biochar was evaluated. The adsorption isotherms of H2O(v) were measured at 30, 50, and 70 °C up to the saturation pressure and fitted to the extended Cooperative Multimolecular Sorption (CMMS) model. Dynamic experiments were carried out in a fixed-bed adsorption unit with mixtures of N2, CO2, and H2O(v). Experimental results indicate that H2O is little affected by CO2 adsorption. On the other hand, the CO2 adsorption capacity can be reduced by the adsorption of H2O. The extent of this reduction is dependent on the amount of H2O adsorbed, which, in turn, is strongly dependent on the relative humidity of the gas phase and the adsorption time. A dynamic fixed-bed adsorption model that makes use of Ideal Adsorbed Solution (IAS) theory has been shown to be adequate to describe the adsorption behavior of CO2 from the ternary mixtures in the full range of conditions evaluated.

Biogas main components besides methane are carbon dioxide and water that often have to be removed to increase the calorific value of the gas. In this study the effect that the presence of water vapor has on CO 2 separation from a biogas stream by means of adsorption on a lignocellulosic-based activated carbon is addressed.Binary CO 2 /CH 4 breakthrough experiments at 30 °C to account for competitive adsorption under dry conditions were conducted at several feed compositions and adsorption pressures in a fixed-bed lab set-up. An adsorption selectivity factor was selected for analysis of the different cases. The results indicated that CO 2 /CH 4 separation by adsorption on this material could be most promising at lower pressures.Humid gas mixtures were then evaluated in the fixed-bed set-up purposely adapted. The effect of water vapor co-adsorption on the separation of CO 2 and CH 4 was studied for different CO 2 /CH 4 ratios, at 30 °C and atmospheric pressure. Under humid conditions the CO 2 uptake reduced but the CH 4 uptake showed negligible influence of water vapor co-adsorption. Results also indicated that the presence of pre-adsorbed water vapor (saturated bed) reduced the capacity of the bed to adsorb CO 2 and CH 4 but it prompted CO 2 adsorption over CH 4 and hence facilitated the selectivity towards CO 2 with respect to the dry case.

The main drawbacks of developing carbon adsorbents from pine sawdust, an abundant biomass feedstock, are the low carbon yield of the process and the poor mechanical properties of the resulting carbons. In an attempt to overcome these limitations, the effect of the addition of coal tar pitch was assessed. Adsorbent pellets were produced from pine sawdust and coal tar pitch by activation with CO 2 . The preparation process was optimized by using as decision variables the carbon yield and the adsorption performance of the adsorbents in conditions representative of post-combustion capture applications (10% CO 2 at atmospheric pressure and at 50 °C). Subjecting the composite pellets to a pre-oxidation treatment with air increased the carbon yield of the production process, and also improved the adsorption kinetics and capacity of the final adsorbents. The prepared adsorbents present a high carbon yield, a superior mechanical resistance and a competitive adsorption performance.

Abstract:The selective separation of CO 2 from gas mixtures representative of flue gas generated in waste incineration systems is studied on two activated carbons obtained from pine sawdust and compared to a commercial activated carbon. Dynamic adsorption experiments were conducted in a fixed-bed adsorption column using a binary mixture (N 2 /CO 2 ) with a composition representative of incineration streams at temperatures from 30 to 70 • C. The adsorption behavior of humid mixtures (N 2 /CO 2 /H 2 O) was also evaluated in order to assess the influence of water vapor in CO 2 adsorption at different relative humidity in the feed gas: 22% and 60%. Moreover, CO 2 adsorption was studied in less favorable conditions, i.e., departing from a bed initially saturated with H 2 O. In addition, the effect of CO 2 on H 2 O adsorption was examined. Experimental results showed that the CO 2 adsorption capacity can be reduced significantly by the adsorption of H 2 O (up to 60% at high relative humidity conditions). On the other hand, the breakthrough tests over the adsorbent initially saturated with water vapor indicated that H 2 O is little affected by CO 2 adsorption. The experimental results pointed out the biomass based carbons as best candidates for CO 2 separation under incineration flue gas conditions.