Modelling of a calcium-looping fluidized bed reactor system for carbon dioxide removal from flue gas

Due to its greater cost advantage compared to conventional amine scrubbing technologies, calcium looping has become the most promising way for carbon dioxide capture in plants. The basic concept in calcium looping is reacting carbon dioxide from flue gas with calcium oxide at approximately 650°C to form calcium carbonate. This reaction takes place in a carbonator. The calcium carbonate from the carbonator is then decomposed in a calciner by subjecting it to higher temperatures (850-950°C). Sulphation (reaction of calcium oxide or calcium carbonate with sulphur dioxide and oxygen to form calcium sulphate) also occurs. Sulphur not only reacts with calcium oxide active for the carbonation reaction, but also can form calcium sulphate with the non-active calcium oxide. Calcium sulphate has a greater molar volume than calcium oxide, resulting in a sulphated layer forming on the outside of the particle, which prevents the uptake of carbon dioxide by the calcium oxide further inside the particle. Calcium sulphate dissociates to calcium oxide and sulphur dioxide at a relatively high temperature, precluding sulphation's reversibility at the conditions present in calcium looping. It is important to quantify this effect and determine the fraction of non-active calcium oxide that reacts with sulphur to form calcium sulphate for not being excessively conservative when considering sulphur. In this study, the calcium looping process was simulated by solution of the one-dimensional (1D) mass and energy balance equations for both interconnected fluidized bed reactors. Kinetics for the carbonator and calciner were derived from literature sources and were revised to include the effects of sulphation. The degree of apparent carbonation was compared to the actual level of carbon dioxide removal through a series of sensitivity analyses. The calcium looping system is dynamic and a number of carbonation-calcination cycles was used to investigate how the system behaves as time changes. It has been found that carbonation decreases with an increase in temperature while sulphation increases with an increase in temperature. The optimal temperature for carbonation would be 600C for it is high enough to drive the carbonation reaction and not too high to accelerate the sulphation reaction. The activity of calcium oxide decreases with an increase in carbonation-calcination cycles. Carbonation rate increases as carbon dioxide concentration in flue gas increase. Increase in sorbent to carbon dioxide flow ratios leads to higher kinetics in the carbonator. Calcination increases with an increase in temperature, and decreases with an increase in carbon dioxide partial fraction. Temperatures above 900 C should be avoided in the calciner as sintering occurs at an elevated pace at temperatures above 900 C. The amount of active calcium oxide particles decreases as the number of carbonation-calcination cycles increase. Neglecting the effect of sulphation during the design of the calcium looping system leads to overestimation of active calcium particles that will react with carbon dioxide. The more Sulphur dioxide the flue gas contains, the more the active fraction of calcium oxide will be consumed by the sulphation reaction. In the presented model, it has been shown that for a flue gas containing 0.04% Sulphur dioxide and 21.6 % carbon dioxide (weight basis), sulphation consumes 0.8-4.0% of the active fraction of calcium oxide, depending on the temperature used in the carbonator.