Heat requirements in a calciner of CaCO3 integrated in a CO2 capture system using CaO

Abstract

Several systems for CO 2 capture using CaO as regenerable sorbent are under development. In addition to a carbonation step, they all need a regeneration step (calcination of CaCO 3) to produce a concentrated stream of CO 2. Different options for calcination may be possible, but they all share common operating windows that appear when the mass and heat balances in the system are solved incorporating equilibrium data, sorbent performance information, and fuel composition (sulphur and ash content). These relatively narrow operating windows are calculated and discussed in this work. Due to sorbent performance limitations, low carbonation levels of the sorbent in the carbonator are expected and the heat demand in the calciner is dominated by the heating of inert solids flowing in the carbonation chemical loop. High make up flows of fresh limestone reduce this effect by increasing the average reactivity of the sorbent, but they also increase the heat demand in the calciner to calcine the fresh feed of limestone. Hence, an optimum level of sorbent activity appears under different operating conditions, processes and fuel characteristic, and these are discussed in this work.

The impact of calcium sulfate and inert solids accumulation in post-combustion calcium looping systems

“…The F 0 /F CO2 ratio was fixed at 0.1 for all configurations and no losses of lime due to attrition were considered for the mass balances, as the attrition effects are known to be closely related to the first calcination step [40] (so that the material subjected to attrition is mainly the make-up flow fed to the system), and they can be compensated for by introducing a higher makeup flow of limestone. For this comparison exercise, we have assumed a total external solids circulation rate to the calciner of 5 kg/m 2 s which should be high enough to give high active space times [3] to ensure high CO 2 capture efficiencies and reasonable heat requirements in the calciner of a CaL system [11]. The apparent reaction rate constant of active particles in the carbonator was taken as 0.43 s -1 [3].…”

“…The ratio between these two variables also determines the composition of the total inventory of solids in the system, which is known to affect the performance of the calcium looping process in terms of CO 2 capture efficiency and heat requirements in the calciner [11,[20][21]. Some previous works give an overall view of the CaL process by formulating the mass and energy balances of the whole system, and they analyze the performance of CaL under certain operating conditions, such as different make-up flows of limestone or different solids circulating rates between reactors [11,21], even in the presence of sulfur [12,20].…”

“…In the calciner coal burns under oxy-fired conditions [10] to attain the temperatures required to convert both the CaCO 3 from the carbonator and the fresh sorbent back to CaO (around 900ºC). Although the heat demand in this reactor (coal and O 2 ) is high [10][11], the overall energy penalty of the CaL process is low [10,[12][13][14][15][16][17][18][19], since energy can be recovered from high-quality heat sources (the solids streams between reactors, the carbonator and the high temperature gases abandoning the reactors). As a consequence of the nature of the CaL process, these systems have a continuous input of inert solids, mainly due to the coal fed into the circulating fluidized bed calciner but also because of the SO 2 in the flue gas entering the circulating fluidized bed carbonator.…”

CaO-based CO2 Capture Technology and Its Application in Power Plants

“…The heat requirements for calcium looping cycle have been developed (Rodriguez et al, 2008). The carbonate looping processes of post-combustion CO 2 capture for coal-fired power plants have been studied (Strohle et al, 2009;Hawthorne et al, 2009).…”

Towards Solar Thermochemical Carbon Dioxide Capture via Calcium Oxide Looping: A Review

“…The addition of two heat exchangers to recover heat from the CO 2 -depleted air exiting the carbonator and the CO 2 exiting the calciner reduced the heat requirement to 2.5 MJ/mol CO 2 captured. Rodriguez et al (2008) examined the heat requirements in a coal fired calciner as a function of carbonation conversion, coal composition, and sorbent makeup flow. At a residual carbonation conversion of 0.075 and no makeup flow of sorbent, the heat requirement for the calciner was 36.9% of the plant input.…”