Enhancement of CaO for CO₂ capture in an FBC environment

Abstract

Experiments have been carried out on three Canadian limestones to determine their ability to remove CO₂ in multiple carbonation/calcination cycles. Two systems have been used: a circulating fluidized bed combustor (CFBC) operated in the bubbling FBC mode; and a thermogravimetric analyzer (TGA). The falloff in CO₂ capture ability of the limes derived from these limestones was initially in agreement with an empirical correlation, but subsequently the decay in performance was slower. The use of Na₂CO₃ and NaCl to reactivate the lime and enhance CO₂ capture failed to do so in the FBC environment, but in the case of NaCl, produced significant improvements in performance in the TGA after several cycles, while Na₂CO₃ did not improve performance in either case. The use of 100% CO₂ failed totally to reactivate sorbents in the TGA, but did improve performance in the FBC. There is also evidence from surface area measurements that carbonation in 100% CO₂ atmospheres influences pore size and surface area in the FBC environment. These results suggest that 100% CO₂ atmospheres may provide a possible reactivation method for some limestones and that the use of NaCl and Na₂CO₃ for this purpose requires further investigation in FBC environment.

Introduction

Fossil fuel combustion systems such as coal-fired power plants are one of the major sources of CO₂ emissions, the major contributor of greenhouse gas (GHG) concentration in the atmosphere. Increasing atmospheric CO₂ concentration and concern over its effect on global warming is a powerful driving force for the development of new advanced energy cycles incorporating CO₂ capture. A potential approach to reducing CO₂ emissions is the separation of CO₂ from flue gases from conventional air-blown combustion systems and storage of CO₂ in underground geological formations (coal beds, oil reservoirs, deep saline aquifers) or in the deep ocean.

Numerous CO₂ separation processes are currently being tested for deployment in fossil-fuel-based power plants. One method to burn coal and produce high-CO₂ flue gas (>95%) is called the “O₂/CO₂ combustion process”, which is considered to be a most energy-efficient process [1]. However, significant energy consumption in separating O₂ from air and recycling flue gas adversely affect the economics of such approaches. Attempts have been made to separate CO₂ from flue gas using absorption by amine solution and adsorption by solids such as zeolites [2]. Absorption processes employ physical and chemical solvents such as Selexol and Rectisol [3]. Adsorption systems capture CO₂ on a bed of adsorbent materials such as molecular sieves or activated carbon [4]. CO₂ can also be separated from flue gases by condensing it out at cryogenic temperatures [2]. Polymers, metals such as palladium, and molecular sieves are also being evaluated for membrane-based separation processes [2]. It is generally accepted that the cost associated with the separation of CO₂ from flue gases introduces the largest economic penalty to these mitigation options [1], [2]. This justifies development of a range of emerging approaches to separate CO₂ by more cost-effective processes.

The possibility of using the carbonation reaction for the removal of CO₂ from a gas stream was already considered in the late 19th century. Recently however, it has been suggested that calcined limestones may be able to remove CO₂ in the fluidized bed combustion environment and, by subsequent calcination, produce a pure CO₂ stream for sequestration, in a process based on CO₂ chemical looping [5], [6]. This scheme involves the use of: a pressurized fluidized bed combustor/carbonator (PFBC/C) where the fuel is burned in an excess of lime which, depending on operating conditions, can remove up to 80% or more of the CO₂ and effectively all of the SO₂; and a calciner where sorbent is regenerated by burning minor proportions of the fuel in O₂. The pure CO₂ emitted is either used for some purpose or sequestered (Fig. 1).

Such a process requires the lime-based sorbents to be recycled many times to reduce the sorbent make up flow. However, several studies on the reversibility of the carbonation and calcination reaction have shown that the recarbonation is far from reversible in practice [7], [9], [10], [11], [12], [13]. After a rapid initial reaction period, controlled by the surface reaction resistance, a much slower second stage controlled by product layer diffusion follows. The difficult completion of recarbonation can be explained by considering structural property changes in the process of the reaction. Mess et al. [14] investigated the product layer diffusion during the reaction of single crystal lumps of pure CaO and found that the slow reaction period is associated with the build up of a thin CaCO₃ product layer (in the order of 100 nm). The progress of the carbonation reaction is negligible from that point at temperatures, partial pressures of CO₂, and particle residence times relevant for a PFBC/C. Furthermore, the maximum carbonation capacity decreases rapidly with multiple cycles as a result of the loss of suitable pore volume in the lime-based sorbent during every calcination step [7]. This paper looks at pretreatment of limestones for CO₂ removal using salts and other methods of reactivation as a means of preventing or delaying this degradation of the texture of the sorbent or as a means of enhancing the mechanism of reaction during the slow reaction period.