Calcium-based sorbents behaviour during sulphation at oxy-fuel

fluidised bed combustion conditions

Francisco García-Labiano, Aránzazu Rufas, Luis F. de Diego, Margarita de las Obras-

Loscertales, Pilar Gayán, Alberto Abad, Juan Adánez.

Dept. Energy and Environment, Instituto de Carboquímica (ICB-CSIC). Miguel Luesma Castán

4, 50018 Zaragoza. Spain

Corresponding author: Tel: (+34) 976 733 977. Fax: (+34) 976 733 318. E-mail address:

glabiano@icb.csic.es

ABSTRACT

Sulphur capture by calcium-based sorbents is a process highly dependent on the temperature and

CO2 concentration. In oxy-fuel combustion in fluidised beds (FB), CO2 concentration in the flue

gas may be enriched up to 95%. Under so high CO2 concentration, different from that in

conventional coal combustion with air, the calcination and sulphation behaviour of the sorbent

must be defined to determine the optimum operating temperature in the FB combustors.

In this work, the SO2 retention capacity of two different limestones was tested by

thermogravimetric analysis at typical oxy-fuel conditions in FB combustors. The effect of the

main operating variables affecting calcination and sulphation reactions, like CO2 and SO2

concentrations temperature and sorbent particle size, was analysed.

It was observed a clear difference in the sulphation conversion reached by the sorbent whether

the sulphation takes place under indirect or direct sulphation, being much higher under indirect

sulphation. But, in spite of this difference, for a given condition and temperature, the CO2

concentration did not affect to the sulphation conversion, being its major effect to delay the

CaCO3 decomposition to a higher temperature.

For the typical operating conditions and sorbent particle sizes used in oxy-fuel FB combustors,

the maximum sorbent sulphation conversions were reached at temperatures of about 900 ºC. At

these conditions, limestone sulphation took place in two steps. The first one was controlled by

diffusion through porous system of the particles until pore plugging, and the second controlled by

the diffusion through product layer. As a consequence, the maximum sulphation conversion

increased with decreasing the particle size and increasing the SO2 concentration.

*ManuscriptClick here to view linked References

Keywords: Oxy-fuel combustion, SO2 retention, Sulphation, Calcium-based sorbents.

1. INTRODUCTION

Carbon dioxide (CO2) and sulphur dioxide (SO2) emissions are a major concern in combustion

processes using fossil fuels as for example coal; the former is implicated in global climate

change and the latter produces acid rain. CO2 is one of the major contributors to the build-up of

greenhouse gases in the atmosphere. At the same time, fuels containing sulphur produce SO2

emission during combustion.

The capture of CO2, emitted in large quantities from power stations, is considered an option to be

explored in the medium term for reducing CO2 levels released to the atmosphere. Oxy-fuel

combustion is one of the possibilities under investigation within the different options for CO2

capture [1-3]. This technology uses for combustion pure O2 mixed with recirculated flue gases,

instead of air used in conventional combustion, and so, the flue gas stream finally produced is

highly concentrated on CO2. Until now, most of research has been directed towards the

development of oxy-fuel systems in pulverised coal (PC) boilers. In 2008, Vattenfall and Alstom

successfully completed the commissioning the world´s first full chain 30 MWth Oxy-Fuel PC

Pilot Plant Facility with CO2 capture located in Schwarze Pumpe, Germany [4], and currently, a

500 MWth Oxy-Fuel PC Demonstration Plant is in progress in Jänschwalde, Germany. The

Fundación Ciuden [5] in Spain is developing two plants able to operate from conventional to

oxy-fuel combustion mode. One is a 20 MWth PC boiler and the other is a circulating fluidised

bed (CFB) combustor of 15 MWth operating in air-mode and 30 MWth operating in oxy-mode. It

is believed that an oxy-fuel CFB combustor will be an important candidate for new coal fired

power plants [6-8] mainly because the circulation of solids in the combustor can help to an

effective control of the temperature.

For flue gas cleaning, many different processes have been developed for sulphur removal

including wet scrubbing, dry scrubbing, direct dry sorbent injection and regenerable processes.

But other of the best known advantage of fluidised bed (FB) combustion is the in-situ

desulphurisation with the addition of a low-cost Ca-based sorbents such as limestone or

dolomite. It is well known that sulphur capture with these sorbents is a process highly dependent

on the temperature and CO2 concentration according to the equilibrium curve of CaCO3

calcination plotted in Figure 1. In oxy-fuel combustion, CO2 concentration in the flue gas may be

enriched between 60 and 90%. Under so high CO2 concentration, different from that in

conventional coal combustion with air (~15% CO2), the calcination and sulphation behaviour of

the sorbent must be analysed previously to decide the optimum operating temperature in the oxy-

fuel combustor.

In conventional air-combustion FB boilers, characterised by low CO2 concentrations and

temperatures about 850 ºC, the operating conditions lead to a previous sorbent calcination (R1)

and to the sulphation of calcines (R2), i.e. indirect sulphation:

CaCO3 CaO + CO2 (R1)

CaO + SO2 + ½ O2 CaSO4 (R2)

However, at oxy-fuel combustion conditions at 850 ºC, the sulphur retention will be produced

under direct sulphation (R3), being necessary higher temperatures to operate under calcining

conditions.

CaCO3 + SO2 + ½ O2 CaSO4 + CO2 (R3)

There are many references about sulphation of Ca-based sorbents in conventional combustion

with air at both calcining and non-calcining conditions. [9-18] However, few works analyse the

sulphation at oxy-fuel combustion conditions [19-22] and the major are related to non-calcining

conditions. These studies found that the most important factors affecting to sulphur retention

with Ca-based sorbents were temperature, CO2 concentration, particle size and SO2

concentration. Thermogravimetric analysis was the most usual technique for the sorbents

characterisation.

The objective of this work was to determine the SO2 retention capacity of two limestones to

provide a background for typical oxy-fuel operating conditions in FB combustors and so

determine the optimum temperature for this technology. Long term sulphation tests up to 24 h or

even higher were carried out in a thermogravimetric analyser (TGA) to reach similar residence

times to the existing in FB combustors. The effect of the main operating variables affecting

calcination and sulphation reactions (CO2 and SO2 concentrations temperature and particle size)

was analysed.

2. EXPERIMENTAL SECTION

2.1. Materials

The materials used as calcium sorbents for the experimental work were two Spanish limestones,

“Granicarb” and “Brecal”, which were selected by its different morphological and textural

properties after calcination, as can be seen in Figure 2. The calcined “Granicarb” limestone

particles consist of small CaO grains partly bonded to other grains and partly surrounded by

voids. The “Brecal” limestone particles developed micro- and macro-fractures during calcination

resembling a conglomerate of smaller particles. As a consequence of these differences it was

expected that the two selected limestones had different behaviour during sulphation. Table 1

shows the chemical analysis and the physical properties of the fresh and calcined sorbents. Both

of the fresh limestones are mainly composed of calcium carbonate. The porosity and density of

the calcined sorbent correspond to samples calcined in nitrogen at 900ºC during 10 minutes. The

sorbents were sieved and narrow particle size intervals between 0.06 and 1.0 mm were used.

2.2. Thermogravimetric analyser

The apparatus used for the tests was a TGA, Setaram TGC-85 type, showed in Figure 3. It

consists essentially of a quartz tube (15 mm ID) placed in an oven that can be operated at

temperatures up to 1000ºC. The sample holder was a wire mesh platinum basket (8 mm diameter,

2 mm height). The reacting gas mixture containing SO2, CO2, O2 and N2, was controlled by

specific electronic mass-flow controllers and it was introduced at the bottom of the reaction tube.

In addition, N2 flowed through the microbalance head to keep the electronic parts free of

corrosive reactant gas. The temperature and the sample weight were continuously recorded in a

computer.

The experiments were carried out in two consecutive steps for indirect sulphation (calcination

and sulphation of calcines) or in a single step for direct sulphation. Figure 1 shows the conditions

of CO2 partial pressure and temperature under which the sulphation of the calcium sorbents were

studied. For each run, 10-20 mg of sample was put into the basket and rapidly introduced into the

furnace at the desired temperature and gas composition. The total gas flow was 10 lN/h. In these

conditions, preliminary tests showed that neither external mass transfer nor interparticle diffusion

were affecting to the sulphation reaction rate. The effect of temperature (800 to 950 ºC), CO2

concentration (15 to 80 vol.%), SO2 concentration (1000 to 5000 vppm), and particle size (0.06

to1.0 mm) during the sulphation was analysed by means of sulphation conversion curves usually

lasting 10-20 hours, typical residence times in FB boilers. Some especial tests with longer

duration were also carried out.

2.3. Procedure

2.3.1. Indirect sulphation (calcining conditions). To simulate as close as possible the situation

happening in a sorbent when it is fed into a FB combustor, the calcination was carried out under

the nearly instantaneous heating of the solid sample in the desired CO2/O2 atmosphere. These

conditions were reached introducing rapidly the sample into the furnace at the desired

temperature and were kept until the complete calcination of the sorbent had been reached.

Afterwards, the SO2 was introduced to start with the sulphation of the calcined sorbent.

The sulphation conversion of the calcined sorbent (XS,I) with the time was calculated from the

sample mass variation registered by the TGA as follow:

CaO4CaSO

0IS

WW

WtWtX

,

36.13

3

4

34 CaCO

CaCO

CaSO

CaCOCaSO WM

MWW

56.03

3

3 CaCO

CaCO

CaO

CaCOCaO WM

MWW

inertsampleCaCO xWW 13

where W(t) is the mass of the sample at each time, W0 is the initial mass of the sample after

calcination, WCaO is the initial mass of CaO, WCaSO4 is the mass of the sample assuming total

conversion of CaO to CaSO4, M is the molar weight of each compound, and xinert is the fraction

of inerts in the sample.

2.3.2. Direct sulphation (non-calcining conditions). To analyse the direct sulphation process

the SO2 was introduced just after reaching the desired temperature together with the CO2/O2

mixture. The sulphation conversion for direct sulphation (XS,D) was calculated from the TGA

data as:

3CaCO4CaSO

0DS

WW

WtWtX

,

where W(t) is the mass of the sample at each time, W0 is the initial mass of the sample, WCaCO3 is

the initial mass of CaCO3 and WCaSO4 is the mass of the sample assuming total conversion to

CaSO4.

2.3.3. Characterisation of sorbents

To understand the sorbent behaviour, the samples were characterised by different techniques.

The porosities of the samples were measured by Hg intrusion in a Quantachrome PoreMaster 33.

The structure of the sorbents were analysed in a Scanning Electron Microscope (SEM), Hitachi

S-3400 N with variable pressure until 270 Pa and with analyser EDX Röntec XFlash of Si (Li).

The sulphated samples were kept in a desiccator to avoid the hydration of the CaO before

analysis. To analyse the internal section of the particles, some particles were embedded in epoxy

resin, cured overnight, cut and polished before SEM characterization.

3. RESULTS AND DISCUSSION

As it is well know, the utilisation of solid sorbents for SO2 retention in FB boilers is not complete

due to the relatively large particle sizes used and blockage of pores by CaSO4. In the typical

operating conditions used in FB combustors, the sulphation reaction usually takes place at the

external surface and around the pores of the sorbent particles. Since the molar volume of CaSO4

is higher than the molar volume of CaCO3 or CaO, the pores are blocked and the centre of the

particles remains essentially unsulphated. Depending on the type of sorbent and operating

conditions, the sorbent utilization can be very different. Sulphation degrees ranging from 10 to

80 % have been reported in the literature depending on both variables [20, 23]. Therefore, it is

important to know the behaviour of any sorbent under different operating conditions in order to

optimise the sulphur retention process in the boiler under oxy-fuel conditions.

3.1. Effect of temperature

The temperature is a very important factor in the sulphation process because depending of it and

the CO2 concentration, the limestone will operate under calcining or non calcining conditions. In

addition, the temperature at which the sulphation reaction takes place influences on the physical

properties of the solid phases of the limestone and therefore on the sulphation process. Several

authors [18, 24] found that high temperatures may lead to the sintering of the sorbent particles

causing losses in sulphur retention. In this work, the effect of temperature on the sulphation

reaction was analysed in a wide range of 800 ºC to 975 ºC. The CO2 and the SO2 concentrations

were kept constant at 60 vol.% and 3000 vppm respectively. According to the thermodynamic

data showed in Figure 1, temperatures lower than 860 ºC lead to direct sulphation and at higher

temperatures the sulphation reaction takes place over the calcined sorbent. Figure 4 shows the

conversion-time curves of the two limestones obtained at different temperatures, with particle

sizes in the range of 0.1-0.2 mm. As commented by others researchers [25-28], two different

stages can be observed both at direct and indirect sulphation conditions. This is better observed

in Figure 5 where the first hours of the sulphation reaction is showed for Granicarb limestone.

The first step was fast and it can be controlled by chemical reaction and/or gas diffusion through

the porous system of the particle. The second one was slower and it can be controlled by

diffusion through the CaSO4 product layer. It must be remarked that, for the typical residence

times of solids in FBC, the residual activity of the sorbent to the diffusion in the product layer

was very important [15].

It can be also observed that the sulphation conversion reached by the sorbent during the first

stage was higher for indirect sulphation than for direct sulphation due to calcined particles are

more reactive and have higher porosity and specific surface area than uncalcined particles.

Higher porosity improves the access of the reactive gas into the particle and therefore the

conversion reached before pore plugging was higher. However, the effect of the temperature was

different for the direct and indirect sulphation reactions. In direct sulphation conditions the

sulphation reaction rate rose with increasing the temperature. On the contrary, in conditions of

indirect sulphatio, the sulphation reaction rate was almost independent on temperature for low

reaction times, but as time went on the sulphation reaction rate decreased faster with increasing

temperature. As a result the limestone sulphation conversion for longer reaction times was lower

with increasing the temperature.

Figure 6 shows the sorbent sulphation conversion at different temperatures and for different

reaction times, working with 60 vol.% CO2. It can be observed that for all reaction times the

maximum sorbent conversion was reached at temperatures about 900 ºC.

Two important aspects were observed working in conditions of indirect sulphation (see Figures 4

and 5). The first was that the sulphation reaction rate was hardly affected by the temperature for

low reaction times (<10-15 minutes), which it seems due to the sulphation process was controlled

by diffusion through porous system of the particle in this stage. The second was that the

sulphation reaction rate decreased with increasing temperature for longer reaction times, which it

could be due to the sinterization of the sorbent. To better analyse the first effect, additional tests

were carried out with the limestone “Granicarb” at temperatures lower than 900 ºC. It is well

known that the kinetic contribution increases with respect to the diffusion through porous system

of the particle as the temperature decreases. In these tests the sorbent samples were calcined at

900 ºC with a CO2 concentration of 60 vol.% (air balance). After calcination, the CO2 was

replaced by air to avoid recarbonation and the temperature was down to the reaction temperature.

3000 vppm of SO2 and particles sizes of 0.1-0.2 mm were used for the sulphation reaction. The

new conversion-time curves are plotted in Figure 7. It can be seen that the sulphation reaction

rate increased with increasing temperature up to about 700 ºC, which indicates that below this

temperature the sulphation reaction rate was influenced by the chemical reaction. At

temperatures above 700 ºC the sulphation reaction rate was almost independent on temperature

during the first minutes of reaction. This indicates that reaction rate was controlled by diffusion

through the porous system of the particle. It can be also observed that the maximum sulphation

conversion reached during the first stage (before the control by diffusion through product layer)

decreased with increasing temperature because the SO2 diffused into the particle reacted with the

CaO faster, producing an earlier plugging of the pores and forming a narrower product layer with

increasing temperature.

It can be concluded that for the typical operating temperatures and sorbent particle sizes used in

FB combustors the limestone sulphation reaction rate was controlled by diffusion through porous

system of the particle until pore plugging was produced, and then the sulphation reaction rate

was controlled by the diffusion through product layer formed. The chemical reaction rate affects

to the thickness of the layer where the sulphation reaction takes place. An increase in the

temperature reduced the thickness of the layer formed before pore plugging was produced.

As second effect, it was observed that the sulphation reaction rate decreased with increasing

temperature for longer reaction times, which seems be due to the sinterization of the sorbent,

either the reactant CaO or the product CaSO4.

To analyse the effect on the sulphation reaction rate of the CaO sintering, additional experiments

were carried out doing the sorbent calcination at different temperatures (900, 925, 950 and 975

ºC) with a CO2 concentration of 60 vol.% (air balance) and the sulphation at 900 ºC (60 vol.% of

CO2 and 3000 vppm of SO2) in all the cases. Figure 8 shows the results obtained during the first

hours of sulphation, where the influence of the calcination temperature should be observed. The

sulphation reaction rate was hardly affected by the calcination temperature which means that the

possible CaO sinterization did not affect to the sulphation reaction rate at these temperatures.

To check if the product layer of CaSO4 was affected by the increase of the temperature some

sulphated sorbent particles were analysed by SEM. Figure 9 shows the SEM photographies of the

external layer of the samples sulphated at 900 and 950 ºC. It was observed that the samples

sulphated at temperatures ≤ 900 ºC formed an external layer with honeycomb structure.

However, the particles sulphated at temperatures ≥ 950 ºC formed an external layer with compact

crystals of CaSO4, which decreases the diffusion rate in the product layer. This effect seems to be

responsible for decreasing the sulphation reaction rate with increasing temperature for longer

reaction times when the temperature was higher than 900 ºC.

3.2. Effect of CO2 concentration

Oxy-fuel combustion is based on introducing into the system a mixture of CO2 and O2 like

comburent. As previously commented, the CO2 concentration is an important factor affecting to

the process because depending on it and the temperature, the calcium sorbent will be operated

under calcining or non-calcining conditions. However, there is no consensus in relation to the

effect of CO2 concentration on the sulphation reaction. Some authors [16, 29] observed that

direct sulphation (non-calcining conditions) gives higher sulphation conversions at long reaction

times They argued that the product layer formed during the direct reaction was more porous than

the formed by indirect sulphation, because the CO2 released by the direct sulphation reaction

flows out increasing the product layer porosity. However, Hu et al. [30] questioned this

explanation since the direct sulphation reaction consumes 1.5 moles of gaseous reactants for each

mole of released CO2. So, the outward flow of the CO2 gas should be diffusional and it is

doubtful that this diffusional flow of CO2 can be responsible for the formation of the porosity.

Moreover, other authors [17,31-32] found that high partial pressure of CO2 significantly reduced

the rate of direct sulphation reaction under certain conditions.

In this work, to analyse the effect of the CO2 concentration in the sulphation process, several

tests were carried out with different CO2 concentrations ranging from 15 to 80 vol.%, including

the typical concentrations existing at oxy-fuel conditions. The SO2 concentration was kept

constant at 3000 vppm and the limestone particle size used was 0.1-0.2 mm. Figure 10 shows the

limestone sulphation conversions obtained at different CO2 concentrations for two temperatures,

900 ºC (indirect sulphation) and 800 ºC (direct sulphation), with the Granicarb limestone. There

was a clear difference in the sulphation conversion reached whether the sulphation takes place in

one way or another, being much higher the conversion obtained under indirect sulphation. But, in

spite of this difference, for a given condition and temperature the CO2 partial pressure hardly

affect to the sulphur retention process. Very similar results were also found by Snow et al. [16]

using limestone particle sizes of a few microns.

The difference between direct and indirect sulphation could be attributed to the difference of

temperature, 800 and 900 ºC, used in the tests. To clarify the effect of CO2 concentration new

tests were performed at 850 ºC. This temperature was selected because it is possible to find a

wide range of CO2 concentrations to operate under calcining and non-calcining conditions.

Figure 11 shows the conversion-time curves obtained with the Granicarb limestone. Low CO2

concentrations (15, 40 %) lead to indirect sulphation and high CO2 concentrations (60, 80 %)

leads to direct sulphation. These tests were maintained during 50 h to test if the long term

sulphation conversion was higher for direct sulphation than for indirect sulphation. These tests

demonstrated that, for the same temperature, the sulphation conversion reached by the sorbents

was always higher working under indirect sulphation. However, the CO2 partial pressure did not

affect to the sulphur retention process for a given condition. So, it can be concluded that the

major effect of increasing the CO2 concentration in the reacting gas was to delay the CaCO3

decomposition to a higher temperature.

3.3. Effect of particle size

Particle size is an important parameter in the sulphation of the sorbents because pore plugging

produces a decrease in the sulphation reaction rate and prevents sulphation of the inner parts of

the particle. As result the conversion of sulphation drops with increasing the sorbent particle size.

The effect of sorbent particle size on the sulphation conversion is showed in Figures 12 and 13

for both sorbents at direct and indirect sulphation.

As expected, for a given reaction time, the sulphation conversion increased as the sorbent particle

size decreased. However, a different behaviour between the limestones was observed under

indirect sulphation for the first part of the reaction. For the Granicarb limestone the sulphation

conversion increased as the sorbent particle size decreased for all reaction times. On the contrary,

for the Brecal limestone the sulphation conversion was independent of the particle size in the first

part of the reaction but as time went on the sulphation conversion increased as the sorbent

particle size decreased.

To clarify this effect, sulphated samples were analysed by SEM. Both limestones reacted

following the unreacted core model working under direct sulphation conditions. However, they

exhibited a different behaviour during the indirect sulphation, as it can be observed in Figure 14.

As previously commented, the calcined “Granicarb” limestone particles consist of small CaO

grains partly bonded to other grains and partly surrounded by voids. These particles exhibited a

highly sulphated ring around the particle and a slightly or unsulphated core following the

unreacted core model. However, the calcined “Brecal” limestone particles develop micro- and

macro-fractures resembling a conglomerate of smaller particles. This limestone exhibited a

highly sulphated layer around the external surface and in the surface of the fractures following a

network sulphation model [33]. It seems that indirect sulphation process in the Brecal limestone

was controlled by the sulphation in the fractures at low reaction times, being independent of the

particle size. The influence of the particle size was only noticeable when the fractures were filled

and the resistance in the external product layer controlled the reaction.

3.4. Effect of SO2 concentration

The SO2 concentration in a FB combustor depends on the sulphur content of the coal used.

However, for the same coal, the SO2 concentration in the combustor will be higher in oxy-fuel

combustion conditions than in air combustion conditions due to the recirculation of flue gas

towards the inlet of the system. To analyse its effect on the sulphation reaction rate, experiments

were carried out with different SO2 concentrations from 500 to 5000 vppm, 60 vol.% of CO2 (air

balance) and particle sizes of 0.1-0.2 mm. Two temperatures, 800 and 900 ºC, were used to know

the behaviour under direct and indirect sulphation conditions respectively. Figure 15 shows the

effect of SO2 concentration on limestone sulphation conversion. As expected, the sulphation

conversion increased with increasing SO2 concentration in both of the conditions.

4. CONCLUSIONS

Long term sulphation tests were carried out with two limestones, with different morphological

and textural properties after calcination, in a TGA to provide a background for oxy-fuel

combustion in FB combustors and to determine the optimum temperature for this technology.

Since the analysis of these tests, the main conclusions are:

- The direct sulphation reaction rate rose with increasing the temperature and the indirect

sulphation reaction rate was hardly affected by the temperature for low reaction times, but as

time went on the sulphation reaction rate decreased faster with increasing temperature.

Nevertheless, the sulphation conversions reached by the sorbents under indirect sulphation were

always higher than under direct sulphation.

- For a given condition (calcining or non-calcining) and a given temperature, the CO2 partial

pressure did not affect to the sulphation reaction rate, being its major effect to delay the CaCO3

decomposition to a higher temperature.

- For the typical operating conditions and limestone particle sizes used in oxy-fuel FB

combustors, the maximum sorbent conversion was reached at temperatures of about 900 ºC. In

these conditions the limestone sulphation reaction rate was controlled by diffusion through

porous system of the particle until pore plugging, and then the sulphation reaction rate was

controlled by the diffusion through the CaSO4 product layer formed. It seems that the chemical

reaction rate affected the thickness of the layer where the sulphation reaction took place,

reducing the thickness of the layer formed before pore plugging with increasing the temperature.

- The sulphation patterns followed by the two limestones used were different due to their

different morphological and textural properties after calcination. In spite of this difference, for

long sulphation reaction times the sulphation conversion increased as the sorbent particle size

decreased.

ACKNOWLEDGEMENTS

This research has been supported by Spanish Ministry of Science and Innovation (MICINN,

Project: CTQ2008-05399/PPQ), by FEDER and by Fundación CIUDEN. A. Rufas thanks CSIC

for the J.A.E. fellowship and M. de las Obras-Loscertales thanks MICINN for the F.P.I.

fellowship.

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List of captions

Table 1. Chemical analysis and physical properties of the sorbents.

Fig. 1. Thermodynamic equilibrium curve of CaCO3 calcination. ( ) Sulphation tests.

Fig. 2. SEM-micrographies of the calcined limestones.

Fig. 3. Thermogravimetric analyser Setaram TGC-85.

Fig. 4. Effect of temperature on the sorbent sulphation conversion. dp= 0.1-0.2 mm; 60 vol.%

CO2; 3000 vppm SO2. Indirect sulphation ( ) and direct sulphation ( ).

Fig. 5. Effect of temperature on the Granicarb sulphation conversion during the first hour of

reaction. dp = 0.1-0.2 mm; 60 vol.% CO2; 3000 vppm SO2. Indirect sulphation ( ) and direct

sulphation ( ).

Fig. 6. Evolution of the sorbent sulphation conversion with the temperature at different reaction

times for Granicarb limestone.

Fig. 7. Temperature effect on the sulphation conversion of Granicarb limestone. dp= 0.1-0.2 mm,

3000 vppm SO2. For T ≥ 900 ºC, conditions as in Figure 5. For T < 900 ºC, calcination at 900 ºC

in 60 vol.% CO2 and sulphation in air at the reaction temperature.

Fig. 8. Influence of the calcination temperature on the sulphation reaction rate of Granicarb

limestone. Sulphation at 900ºC. dp= 0.4-0.5 mm, 60 vol.% CO2, 3000 vppm SO2.

Fig. 9. SEM-micrographies of external surface of sulphated particles at 900 ºC and 950 ºC.

Granicarb limestone.

Fig. 10. Influence of CO2 concentration on Granicarb limestone sulphation conversion during

indirect sulphation (900 ºC) and direct sulphation (800 ºC). dp= 0.1-0.2 mm, 3000 vppm SO2.

Fig. 11. Influence of CO2 on Granicarb limestone at 850ºC, 3000 vppm SO2 and dp=0.1-0.2 mm.

Indirect sulphation ( ) and direct sulphation ( ).

Fig. 12. Influence of particle size on the sulphation conversion of Granicarb limestone. 3000

vppm SO2.

Fig. 13. Influence of particle size on the sulphation conversion of Brecal limestone. 3000 vppm

SO2.

Fig. 14. SEM-micrographies of sectioned particles sulphated at 900 ºC under indirect sulphation.

dp = 0.1–0.2 mm, 60 vol.% CO2, 3000 vppm SO2.

Fig. 15. Effect of SO2 concentration for Granicarb limestone. T = 900 ºC, dp = 0.1-0.2 mm, 60

vol.% CO2. Indirect sulphation ( ) and direct sulphation ( ).

T (ºC)

700 750 800 850 900 950 1000

P C

O2 (

kP

a)

0

20

40

60

80

100

CaO

CaCO3OXY-FUEL

COMBUSTION

AIR COMBUSTION

Fig. 1. Thermodynamic equilibrium curve of CaCO3 calcination. ( ) Sulphation tests.

Fig. 2. SEM-micrographies of the calcined limestones.

Granicarb Brecal

Microbalance

Joint

N2 purge

Furnace

Sample

Thermocouple

Flue gas

CO2

O2

SO2

N2

Control and

Data

Acquisition

Microbalance

Joint

N2 purge

Furnace

Sample

Thermocouple

Flue gas

CO2

O2

SO2

N2

Control and

Data

Acquisition

Fig. 3. Thermogravimetric analyser Setaram TGC-85.

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

850

925900 ºC

950975

800

Granicarb

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

850

950

975

800

900 ºC

925Brecal

Fig. 4. Effect of temperature on the sorbent sulphation conversion. dp = 0.1-0.2 mm, 60 vol.% CO2, 3000 vppm SO2.

Indirect sulphation ( ) and direct sulphation ( ).

Time (h)

0.0 0.2 0.4 0.6 0.8 1.0

Xs

0.0

0.1

0.2

0.3

0.4

Granicarb

800

850

900 ºC

925

950975

Fig. 5. Effect of temperature on the Granicarb sulphation conversion during the first hour of reaction.

dp= 0.1-0.2 mm; 60 vol.% CO2; 3000 vppm SO2. Indirect sulphation ( ) and direct sulphation ( ).

Direct sulphation

Indirect sulphation

Temperature ( ºC)

800 850 900 950 1000

Xs

0.0

0.2

0.4

0.6

0.8

1

5

10

0.50.05

18 h

Indirect sulphationDirectsulphation

Fig. 6. Evolution of the sorbent sulphation conversion with the temperature at different reaction times for Granicarb

limestone.

Time (h)

0.0 0.1 0.2 0.3 0.4 0.5

Xs

0.0

0.1

0.2

0.3

0.4

0.5

0.6

500

700 ºC

800900

925950

975

600

750

550

Fig. 7. Temperature effect on the sulphation conversion of Granicarb limestone. dp= 0.1-0.2 mm, 3000 vppm SO2.

For T ≥ 900 ºC, conditions as in Figure 5. For T < 900 ºC, calcination at 900 ºC in 60 vol.% CO2 and sulphation in

air at the reaction temperature.

Time (h)

0 2 4 6 8

Xs

0.0

0.1

0.2

0.3

0.4

900ºC

925ºC

950ºC

975ºC

Fig. 8. Influence of the calcination temperature on the sulphation reaction rate of Granicarb limestone. Sulphation at

900 ºC. dp= 0.4-0.5 mm, 60 vol.% CO2, 3000 vppm SO2.

Fig. 9. SEM-micrographies of external surface of sulphated particles at 900 ºC and 950 ºC. Granicarb limestone.

900 ºC 950 ºC

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

1.0

60%

80%40%

60%

80%

15%

Indirect sulphation

Direct sulphation

Fig. 10. Influence of CO2 concentration on Granicarb limestone sulphation conversion during indirect sulphation

(900 ºC) and direct sulphation (800 ºC). dp= 0.1-0.2 mm, 3000 vppm SO2.

Time (h)

0 10 20 30 40 50

Xs

0.0

0.2

0.4

0.6

0.8

1.0

40%

15%

80%60%

Time (h)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Xs

0.0

0.1

0.2

0.3

0.4

40%

15%

80%

60%Direct sulphation

Indirect sulphation

Fig. 11. Influence of CO2 on Granicarb limestone at 850 ºC, 3000 vppm SO2 and dp=0.1-0.2 mm.

Indirect sulphation ( ) and direct sulphation ( ).

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

1.0

0.1-0.2 mm

0.4-0.5

0.63-0.8

Indirect sulphation900 ºC, 60 vol.% CO

2

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

1.0

0.1-0.2 mm

0.4-0.5

0.63-0.8

Direct sulphation850 ºC, 80 vol.% CO

2

Fig. 12. Influence of particle size on the sulphation conversion of Granicarb limestone. 3000 vppm SO2.

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

1.0

0.074-0.1 mm

0.1-0.2

0.3-0.5

0.63-0.8

Indirect sulphation900 ºC, 60 vol.% CO

2

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

1.0

0.1-0.2

0.3-0.5

0.63-0.8

0.074-0.1 mm

Direct sulphation800 ºC, 60 vol.% CO

2

Fig. 13. Influence of particle size on the sulphation conversion of Brecal limestone. 3000 vppm SO2.

Fig. 14. SEM-micrographies of sectioned particles sulphated at 900 ºC under indirect sulphation.

dp = 0.1–0.2 mm, 60 vol.% CO2, 3000 vppm SO2.

Granicarb Brecal

Time (h)

0 5 10 15 20 25 30

Xs

0.0

0.2

0.4

0.6

0.8

1.0

5000 vppm

3000

1000

5000

3000

500

1000

Fig. 15. Effect of SO2 concentration for Granicarb limestone. T=900 ºC, dp = 0.1-0.2 mm, 60 vol.% CO2.

Indirect sulphation ( ) and direct sulphation ( ).

Table 1. Chemical analysis and physical properties of the sorbents.

Granicarb Brecal

Composition (wt.%)

CaCO3 97.1 96.3

MgCO3 0.2 0.8

Others 2.7 2.9

Physical properties

Porosity (%)

fresh 3.7 2.1

calcined 49.0 47

Apparent density (kg/m3)

fresh 2573 2625

calcined 1578 1681