Ceramic materials for clean and efficient energy generation from fossil fuels

Solid chemical sorbents can be effectively used for capturing CO2 originated from fossil sources at relatively high temperature [1]. These materials can be regenerated by treatment in atmosphere with lower partial pressure of carbon dioxide and/or different temperature. In most cases, limestone is used as sorbent upon calcination, having a theoretically CO2 carrying capacity up to 78% by mass in the temperature range between 700 and 800 °C at atmospheric pressure. In particular cases, the possibility to absorb CO2 at higher temperature could result beneficial for the whole process optimization. Of course, this option demands the availability of sorbents operating over 1000 °C. Among ceramic materials, hydroxyl- and oxy- apatite were tested as CO2 sorbent in the temperature range 900-1100 °C in both TG and fixed bed equipment by Landi et al. [2]. Strontium oxide SrO is also another possible sorbent of CO2, forming SrCO3, in a temperature range over than that of the homologous Ca oxide. Geopolymers [3], that are produced by reaction of aluminosilicates and alkali hydroxide and/or alkali silicate solutions also exhibit potentialities for replacing conventional sorbents of CO2, in the low temperature range (100-400 °C), since their microstructure consists of nanoprecipitates and mesopores. Materials and methods Powders of apatite and SrO were granulated at 400-600 ?m, for comparative tests in both TG apparatus and fixed bed reactor at atmospheric pressure. The change of weight during alternated steps of calcination and carbonatation in TG provides the CO2 carrying capacity of the sample, at a given temperature and atmosphere. Besides, the analytic determination of CO2 stream exiting the reactor allows upon integration the determination of the CO2 carrying capacity, in both calcination and carbonatation steps. K-PSS geopolymer was synthesized as reported by Landi et al. [3] and granules were obtained upon milling and sieving at 400-600 ?m. Results and discussion From TG analysis, SrO revealed to be active at higher temperature (1200 °C) with respect to apatite, although the CO2 carrying capacity quickly decayed at increasing the number of carbonatation and calcination cycles. In fixed bed reactor SrO shows a similar trend with the progress of regeneration steps, but lower effectiveness in CO2 uptake because of kinetic limits. The final carrying capacity of SrO was around 4%wt., still higher than the value obtained for apatite (2.5%). The geopolymer granules were tested at low temperature, since they can only adsorb CO2 due to the absence of carbonatable sites in the structure. The tests carried out in fixed bed reactor at T=250°C proved a capacity of 1.1% wt., that is lower than that of a zeolite (2.5%) under similar conditions. It is worth noting that geopolymers have superior mechanical properties with respect to zeolite and can be easily shaped. Conclusions The ceramic materials tested for CO2 uptake proved to be effective in high temperature range (apatite and SrO) and low temperature application (geopolymers). They can be used to develop porous components with a wide pore size range for specific process applications.