Precalcination of CaCO3 as a Method to Stabilize CaO Performance

ARTICLE pubs.acs.org/EF

Precalcination of CaCO3 as a Method to Stabilize CaO Performance for CO2 Capture from Combustion Gases Monica Alonso, María Lorenzo, Belen Gonzalez, and J. Carlos Abanades* Instituto Nacional del Carb on (INCAR-CSIC), Francisco Pintado Fe 26, 33011 Oviedo, Spain ABSTRACT: The precalcination of several limestones is investigated as a pre-activation method for combustion calcium looping systems. The temperature and time of precalcination varied between 1050 and 1100 °C and 10
60 min, within the range of those proposed in recent papers on the subject. After the precalcination, the sorbents were subjected to standard carbonation/calcination cycles in a thermogravimetric analyzer to check for any improvement in sorbent stability after the initial drop in CO2 carrying capacity, as a result of the precalcination step. In contrast to what has been reported by other authors, there was no sign of sorbent improvement that could be linked to the precalcination step. The use of a short carbonation time in our cycling experiments, resembling those expected in large scale combustion Ca-looping systems, seems to be the main cause to explain our discrepancies with other authors. A model previously used for other applications provided a reasonable interpretation of the observed trends during the precalcination step and subsequent carbonation
calcination cycles.

’ INTRODUCTION Calcium looping is rapidly developing as a CO2 capture technology in both combustion and precombustion CO2 systems.1
3 All calcium looping systems are based on the reaction of CaO with CO2 to form CaCO3, followed by the reverse calcination reaction of CaCO3, mainly in an atmosphere of CO2. Natural limestones and dolomites are considered the preferred CaO precursors for large scale CO2 capture in power plants because of their low cost. However, the capture capacity of these sorbents is known to decay rapidly with the number of carbonation/calcination cycles.4,5 It has been well established with a mass balance and basic sorbent cost data that this decay in maximum CO2 capture capacity can be compensated at a reasonable cost by a sufficiently large makeup flow of fresh limestone.6 Indeed, for some CO2 capture systems using coal in the calciner of the calcium loop, the makeup flow's requirements to purge ashes and CaSO4 can be higher than those needed to sustain a reasonable value of sorbent activity in the loop.7,8 Furthermore, the solid purge associated with large limestone makeup flows is being investigated for cement production,9 which has been shown to provide a strong synergy of energy and materials with large scale Ca-looping systems.10,11 Furthermore, if the application of the Ca-loop requires the oxy-fuel combustion of coal, which seems to be the most developed technology option today,12 there is no point in using highly sophisticated materials in the calcium loop because they will tend deactivate forming CaSO4 and they will mix and dilute with coal ash. In all these scenarios, with a high makeup flows sorbent to be fed to the Calcium loop, there is no need for highly active materials, and low cost and abundant limestones are the natural choice as precursors of the CaO particles acting as CO2 sorbent in the calcium loop. However, there are several other applications of calcium looping in precombustion CO2 capture (such as the so-called “sorption enhanced reforming” or SER, recently reviewed by Harrison1), where the combination of the CaO sorbent with a reforming catalyst and the use of clean fuels such as natural gas strongly supports the development of highly stable materials. Furthermore, r 2011 American Chemical Society

in combustion CO2 capture applications of Ca-looping, there is still room for simple and very low cost methods3 to reactivate or preactivate CaO sorbents derived from natural limestones to minimize limestone consumption. Clearly, the utilization of sorbents with a higher CO2 capture capacity will reduce the circulation of solids between reactors and, in turn, the sorbent makeup flow cost and heat requirements of the calciner.13 Sorbents with a higher activity will also reduce the solids inventory in the carbonator,12,14 reducing indirectly the pressure drop in this reactor and associated energy consumption. All of these advantages translate into greater overall energy efficiency and lower material consumptions in certain scenarios of application of the Ca-looping system (i.e., in power plants where the synergy with the cement plant is not feasible). Among the genuine low cost sorbent improvement methods, thermal precalcination of limestones at high temperatures, or precalcination, has been proposed to have beneficial effects.15
19 Early work in our group touched this subject20 but obtained discouraging results in a very limited range of conditions (calcination temperature of 960 °C, 100 vol % CO2, and times of precalcination of 10 and 90 min, with carbonation times of 5 and 30 min in 100 vol % CO2 at 650 °C). However, Lysikov et al.15 conducted precalcination experiments with much higher temperatures (between 1000 and 1300 °C) and a calcination time of 90 min, and they showed that the sorbent could maintain a residual carrying capacity (or maximum molar calcium conversion to CaCO3) that was much higher than expected (0.12 after 2000 cycles versus the typical values of 0.075 after 500 cycles reported in Grasa et al.21). Manovic et al.16 reported positive results from thermal precalcination experiments over four Canadian limestones and four particle sizes, in wide range conditions. The precalcination temperatures varied between 800 and 1300 °C, and precalcination times were 6 and 24 h in inert atmosphere. Their results showed an increase Received: September 6, 2011 Revised: October 17, 2011 Published: October 21, 2011 5521

dx.doi.org/10.1021/ef201333e | Energy Fuels 2011, 25, 5521–5527

Energy & Fuels

ARTICLE

Table 1. Summary of Previous Studies Reporting Positive Effects of Precalcination on Sorbent Performance sorbent ref Lysikov et al.

15

Manovic et al.16

precalcination conditions

multicycle conditions

type

dp (μm)

t (h)

T (°C)

atm

tcarb (min)

monodisperse CaCO3

3
4

1.5

1100
1300

Ar

15

800

monocrystal CaCO3 commercial CaCO3 Kelly Rock limestone

80
100 6, 24

800
1300

N2

30

800

Tcarb (°C)

tcalc (min)

Tcalc (°C)

atm

33 vol % CO2 in Ar

15

800

Ar

50 vol % CO2 in N2

10

800

N2

8

850

N2

atm

6, 24

1000
1100

N2

9

850

Manovic et al.18

Havelock limestone Cadomin limestone Graymont limestone Strassburg limestone Artic dolomite Kelly Rock limestone