Suppression of Natural Limestones Deactivation During Cyclic Carbonation

Suppression of Natural Limestones Deactivation During Cyclic Carbonation

Abstract – The paper summarizes sorption properties of limestones with variable content of calcium carbonate. Changes of sorption capacities during cyclic calcinations and carbonations were investigated using laboratory apparatus with vertical quartz reactor. Application of suitably chosen limestones and process conditions offered sustainable capacity up to 11 kg CO2 / 100 kg of initial limestone. Method of reactivation of the spent limestone using a gas saturated by water vapor was proposed to eliminate decreasing of the capacity in the carbonate looping process.

I. INTRODUCTION

This study is focused on utilisation of limestones for hightemperature separation of CO2 from flue gas within the carbonate looping technology. A post-combustion capture of CO2 from coal-fired power plants and district heat-plants was considered the preferred application of the carbonate looping. Simplified scheme of the process, drawn based on the literature sources [1], is shown in Fig. 1.

Although the carbonate looping is still not a technology expanded into industrial practice, there are some pilot units operated around the world. Alonso et al. summarized basic findings from the operation of the pilot plant using CaO looping, incorporated within a facility combusting biomass in a fluidized bed reactor [2].

Utilisation of CaO based sorbents for high temperature separation of CO2 from industrial processes has been the subject of relatively numerous studies. Good economic availability of these materials make them attractive candidates for solving problems with suppression of CO2 emissions into the atmosphere.

A major obstacle to the application of CaO-based sorbents in the industrial practice is a gradual decline of sorption capacity, manifested in repeated cycles of calcination (i.e. thermal decarbonation) and carbonation. Based on experimental data Abanades suggested an empirical formula (1) describing the dependence of the conversion of CaO to CaCO3 on the number of cycles [3].

In (1) the sense of the symbols is following: xN is conversion in N-th cycle, fm and fw are constants.

This formula gave good conformity with the experimental data in a wide range of conditions but its validity has been limited to 20 cycles. In their study Wang and Anthony therefore proposed a different equation, which was valid for a large number of cycles. These authors considered a combination of thermal sintering with other irreversible processes, such as growth of crystals and reactions of CaO with impurities in the gas, to be major causes of decrease the activity of CaO-based sorbents. They also expressed the assumption that the rate of degradation is not significantly dependent on the experimental conditions [4].

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