J. Cent. South Univ. Technol. (2009) 16: 0845-0850

DOI: 10.1007/s11771-009-0140-1

Temperature dependence on reaction of CaCO₃ and

SO₂ in O₂/CO₂ coal combustion

WANG Hong(王 宏)^{1, 2}, XU Hui-bi(徐辉碧)², ZHENG Chu-guang(郑楚光)¹, QIU Jian-rong(邱建荣)¹

(1. National Laboratory of Coal Combustion, Huazhong University of Science and Technology,

Wuhan 430074, China;

2. School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology,

Wuhan 430074, China)

Key words:

CaCO3; SO2; O2/CO2 coal combustion; temperature dependence；

1 Introduction

Extensive research has been devoted recently to CO₂-capture techniques because of growing concerns with respect to greenhouse gas emissions [1]. CO₂ emission from fossil fuel combustion is a significant source of global warming and climate change. Clean coal combustion technologies have been developed toward easy CO₂ recovery, low NO_x emission and high desulfurization efficiency [2-6]. O₂/CO₂ coal combustion is one of these new technologies associated with reduced CO₂ emission to the atmosphere [7-8].

Ca-based sorbent such as CaCO₃ is used as sulfur absorbent in conventional combustion atmosphere [9-10]. The reaction of desulfurization reagent CaCO₃ and SO₂ has been a subject of increasing attention in O₂/CO₂ coal combustion [11-12]. LIU et al [13] reported sulfation behavior of limestone at high CO₂ concentrations in O₂/CO₂ coal combustion. They found that sintering was much mitigated during direct sulfation of limestone. The rate of direct sulfation does not decrease as much as the CaO-SO₂ sulfation. CHEN et al [14] investigated the calcination and sintering characteristics of limestone in O₂/CO₂ atmosphere. The results showed that the specific pore volume and specific surface area of CaO calcined in O₂/CO₂ atmosphere were less than those of CaO calcined in air at the same temperature. FUERTES et al [15] studied the direct sulfation behavior of dolomite at a high CO₂ partial pressure and temperatures ranging from 650 to 875 ℃. The results suggested that solid diffusion was the control process in direct sulfation of dolomite. However, most of these investigations were carried out by using thermogravimetric analysis (TGA) at temperatures below 1 000 ℃ [16-17].

In O₂/CO₂ coal combustion, the concentrations of CO₂ and SO₂ in the flue gas may be enriched up owing to the gas recirculation. At high CO₂ and SO₂ concentrations, and high calcium conversion, the influence of high SO₂ concentration on the calcium conversion and the reaction mechanism of desulfurization reagent CaCO₃ and SO₂ are not clear at temperatures ranging from 500 to 1 200 ℃. Therefore, it is necessary to study the calcium conversion and mechanism in O₂/CO₂ coal combustion at high CO₂ concentration. However, few researches have been reported about the composition of sulfated products and the desulfurization mechanism in the O₂/CO₂ coal combustion with high concentration of CO₂.In this work, the calcium conversions in O₂/CO₂ coal combustion at 500-1 200 ℃ were investigated and the effect of concentration of SO₂ on calcium conversion was also studied. The reaction mechanism of desulfurization reagents CaCO₃ and SO₂ in O₂/CO₂ coal combustion was investigated by thermogravimetric analysis, X-ray diffraction measurement and pore structure analysis.

2 Experimental

2.1 Materials

O₂, CO₂, N₂ and SO₂, each with mass fraction of 99.9%, were purchased from Wuhan Gas Co. Ltd, Wuhan, China. 99.9% (mass fraction) CaCO₃ was purchased from Sinopharm Chemical Reagent Co. Ltd, Beijing, China.

2.2 Experimental setup

Experiments were performed in a fixed-bed quartz reactor placed in a tubular oven. The reactor was     650 mm in length. A sintered quartz filter of 20 mm in diameter was placed at the middle of the reactor to support the sample powder. The temperature in the reactor was measured by a thermocouple placed immediately under the sample. Premixed gases were introduced into the reactor from an inlet above the reactor, and the gas flow rates were regulated by mass flow controllers. The concentration of SO₂ in the premixed gases was measured by a SO₂ sensor (Z-1300, ESC).

2.3 Experimental procedure

In the experiments, two kinds of reactive atmospheres were utilized: (1) 21.0% O₂/79.0% CO₂ with trace SO₂ (designated as O₂/CO₂ atmosphere) and  (2) 21.0% O₂/79.0% N₂ with trace SO₂ (designated as air atmosphere). The experiments were carried out under reactant gas conditions. In all cases, isothermal conditions in the quartz reactor were first established and the desired reactant gas conditions were then achieved. For each run, 0.5 g of the CaCO₃ powder was rapidly loaded from a sample feeder onto the sintered quartz filter in the reactor. After a specified time period was over, the sample from the fixed bed reactor was allowed to cool down under N₂, and subsequently placed in a desiccator for characterization and analysis. The content of SO₄^2- in the product was determined by the gravimetric method, and the calcium conversion (η) was calculated by the following equation:

×100%                       (1)

where  m(CaSO₄) is the mass of CaSO₄ in the product, and m(CaCO₃) is the initial mass of CaCO₃.

2.4 Characterization of reaction product

Thermogravimetric analysis was performed with a TG/DSC analyzer (STA 449C, Netzsch). In the TG analysis, the temperature was ramped from room temperature to 1 200 ℃ at a rate of 20 ℃/min and then held at 1 200 ℃ for 10 min under a steady flow of mixed gases of 50 mL/min. The mixed gases were of 21.0% O₂/ 79.0% CO₂ or 21.0% O₂/79.0% N₂, which were similar to those described in section 2.3, but without SO₂. The starting decomposition temperature was defined as the temperature when the thermogravimetric curve started to deviate from the baseline. The end decomposition temperature was defined as the temperature when the maximum mass loss occurred. The sulfated products were characterized by X-ray diffractometry (XRD, D/max-Ⅲ B, Rigaku) using an X-ray diffractometer with Cu K_α radiation (λ=0.154 06 nm) and a Ni ray filter. The tube current and the voltage were set at 30 mA and 30 kV, respectively. The peak positions of samples were compared with JCPDS files. BET surface areas of sulfated products were measured with N₂ adsorption analysis by using a specific surface area analyzer (ASAP 2020, Micromeritics).

3 Results and discussion

3.1 Effect of reactive atmosphere on calcium conversion (η)

In conventional air combustion, the reaction of CaCO₃ and SO₂ occurs as follows:

CaCO₃→CaO+CO₂ (Decomposition)            (2)

CaO+SO₂+1/2O₂→CaSO₄ (Sulfation)            (3)

The reaction of CaCO₃ and SO₂ is a solid-gas reaction. In general, the reaction rate and η are affected by temperature and atmosphere. Fig.1 shows calcium conversion vs sulfation time at varied temperatures in different atmospheres. Obviously, η increases with the increase of reaction time at a given temperature for both reactive atmospheres. The change of η between 500 and 900 ℃ is faster than that between 900 and 1 100 ℃. η, however, decreases when the temperature is higher than 1 100 ℃.

It is found from Fig.1 that the values of η in air are much higher than those in O₂/CO₂ atmosphere when the temperature is lower than 900 ℃. For instance, at 900 ℃ the values of η in air are 23.1% and 26.9% at reaction time of 30 and 40 min, respectively, after the reaction started; whereas the values of η at the same time are only 9.6% and 16.5% in O₂/CO₂ atmosphere, respectively. A possible explanation involves the competitive reaction of CO₂ and CaO in O₂/CO₂ atmosphere. In addition, the

Fig.1 Calcium conversion (η) vs sulfation time at SO₂ volume fraction 0.2%: (a) In O₂/CO₂ atmosphere; (b) In air

amount of CaO decomposed from CaCO₃ in O₂/CO₂ atmosphere is less than that in air because of the inhibition of the decomposition of CaCO₃ by higher concentrations of CO₂. When the temperature is increased to 1 100 ℃, the effect of both atmospheres on η becomes weaker. For instance, the values of η are 25.8% and 32.4% in air and 23.3% and 29.1% in O₂/CO₂ atmosphere at reaction time of 30 and 40 min, respectively. More CaO from CaCO₃ decomposition at this temperature is produced than that at 900 ℃. At 1 200 ℃, η in O₂/CO₂ atmosphere is higher than that in air, which may be related to less sintering of CaO in O₂/CO₂ atmosphere. This indicates that O₂/CO₂ atmosphere is beneficial to the removal of SO₂ at high temperatures.

3.2 Effect of SO₂ concentration on calcium conversion

In comparison with conventional air combustion, the O₂/CO₂ coal combustion features recycling of flue gas, resulting in high concentrations of SO₂ and CO₂ in the combustion atmosphere. Generally speaking, increasing SO₂ concentration causes the equilibrium of sulfation reaction to shift toward the products, leading to a higher η. Figs.2 and 3 show the effect of SO₂ concentration on η at 900 ℃ and 1 100 ℃, respectively.

Fig.2 Effect of SO₂ concentration on η at 900 ℃: (a) In O₂/CO₂ atmosphere; (b) In air

As shown in Figs.2 and 3, η increases with increasing SO₂ concentration at either 900 ℃ or 1 100 ℃. As mentioned above, high SO₂ concentrations will be better for the equilibrium of sulfation reaction to shift toward the products. A comparison between Figs.3(a) and (b) shows that the increase of η in O₂/CO₂ atmosphere is faster than that in air during the reaction time period of 30-40 min. This can be attributed to the inhibition of the decomposition of CaSO₄ [11]. This result further demonstrates that high temperatures are beneficial to the removal of SO₂ in O₂/CO₂ atmosphere.

3.3 Decomposition processes of CaCO₃ in both atmospheres

The decomposition of CaCO₃ was strongly affected by the concentration of CO₂ and temperature [14]. The decomposition process of CaCO₃ in both atmospheres without SO₂ was investigated by TGA in order to determine the effect of the reactive atmosphere on it. The results are listed in Table 1.

As shown in Table 1, the mass losses of the samples are rather close to each other in both atmospheres. However, the decomposition of CaCO₃ starts at 884 ℃ in O₂/CO₂ atmosphere, whereas the decomposition takes

Fig.3 Effect of SO₂ concentration on η at 1 100 ℃: (a) In O₂/CO₂ atmosphere; (b) In air

Table 1 Effects of atmosphere on decomposition of CaCO₃

place at 758 ℃ in air. The starting and end temperatures of the decomposition in O₂/CO₂ atmosphere are 126 and 55 ℃ higher than those in air, respectively. This is likely due to the constraint or the inhibition of decomposition of CaCO₃ at high concentration of CO₂.

3.4 XRD analysis of sulfated products

Possible sulfated products include CaCO₃, CaO and CaSO₄ according to reactions (2) and (3). An XRD technique was used to characterize their phase compositions to clarify the reaction mechanism of CaCO₃ and SO₂ in O₂/CO₂ atmosphere and in air. Proportions of phase composition were estimated from relative peak intensities. Fig.4 shows the XRD patterns of sulfated products at 900 ℃ for different reaction times in air.

Fig.4 XRD patterns of sulfated products obtained in air at  900 ℃ for different reaction times: (a) 5 min; (b) 20 min;     (c) 40 min

According to the JCPDS files, the major diffraction peaks of sulfated products CaCO₃, CaO and CaSO₄ appear at 2θ of 29.4?, 37.4? and 25.5?, respectively. Obviously, the sulfated products contain a significant amount of CaCO₃ and small amount of CaO and CaSO₄ after 5 min of reaction time. When the reaction time is increased to 20 min, the intensity of the major peak of CaCO₃ is smaller than that of either major peak of CaSO₄ or CaO. When the reaction time is increased to 40 min, the intensity of the major peak of CaCO₃ is very low and the intensities of the major peaks of CaSO₄ and CaO become very high. The phase compositions of sulfated products in different atmospheres for varied reaction times are given in Table 2.

Table 2 shows that the phase compositions of sulfated products in two kinds of atmospheres are CaCO₃ and CaSO₃ at 500 ℃. At this temperature, the decomposition of CaCO₃ does not occur. This is in concord with the TGA result. CaSO₃ is the direct sulfation product of CaCO₃ and SO₂, as shown in Eq.(4), and this compound is too stable to oxidize to CaSO₄ at low temperatures.

CaCO₃+SO₂→CaSO₃+CO₂                    (4)

In air, the phase composition of sulfation products at 700 ℃ is dependent on the reaction time. The phases are CaCO₃, CaSO₃ and CaSO₄ after 5 min of reaction. As the reaction time increases, CaSO₃ phase disappears due to the oxidation to CaSO₄. CaO is a absent most likely

Table 2 Phase compositions of sulfated products determined by XRD

because CaCO₃ does not decompose at 700 ℃. This result is also in accordance with that of TGA.

In O₂/CO₂ atmosphere, CaCO₃ and CaSO₃ phases are found at 700 ℃ after 5 min of reaction. This result is quite different from that in air. When the reaction time is increased to 20 min, CaSO₄ phase is produced. The phase composition is the same as that in air after 40 min of reaction. The results indicate that the effect of atmosphere on the phase composition of sulfation products is reaction time dependent. The sulfation in both atmospheres can be described as follows:

CaSO₃+1/2O₂→CaSO₄                        (5)

According to Table 1, the decomposition of CaCO₃ starts at 758 ℃ in air. Therefore, CaO described in  Eq.(2) is found in all samples at 900 ℃. After 40 min of reaction, only CaSO₄ and CaO are found and the proportion of CaO is more than that of CaSO₄, suggesting indirect sulfation of CaCO₃, as shown in Eq.(3).

Likewise, the decomposition of CaCO₃ starts at  884 ℃ in O₂/CO₂ atmosphere, as shown in Table 1. When the reaction time is increased from 5 to 40 min, the phase compositions of the sulfation products are CaSO₄, CaO and CaCO₃ at 900 ℃. In a high CO₂ concentration atmosphere, CaO can be consumed in two ways: (1) sulfation reaction of CaO and SO₂, which forms CaSO₄; and (2) recarbonation reaction of CaO and CO₂, which forms CaCO₃. If the decomposition rate of CaCO₃ to produce CaO is equal to the consumption rate of CaO, sintering of CaO can be avoided. This can lead to an optimum η. ANTHONY and GRANATSTEIN [18] reported that the recarbonation reaction is faster than sulfation, causing excessive CaCO₃ in the sulfation product. Therefore, indirect and direct sulfation reactions are simultaneously performed to form CaSO₄. The sulfation mechanism can be described by the combination of Eqs.(2)-(5).

The phase compositions of the sulfation product are CaSO₄ and CaO in both atmospheres at temperatures ranging from 1 100 to 1 200 ℃. The major phase is CaO, suggesting a typical indirect sulfation process. The sulfation mechanism can be described by the combination of Eqs.(2) and (3) in this temperature range.

3.5 Pore structure analysis of sulfated products

It is known that the pore structure of sulfated products plays an important role in the desulfurization reaction. The pore structure is greatly affected by the reaction temperature and atmosphere. The specific surface areas of sulfated products were measured by the N₂ adsorption method. Fig.5 shows the plots of BET specific surface area of sulfated products vs reaction temperature in both atmospheres after 20 min of reaction. A maximum BET specific surface area is found at   900 ℃ in air, whereas the surface area is almost independent of temperature in O₂/ CO₂ atmosphere.

As seen from Fig.5, the BET specific surface areas of sulfated products in both atmospheres are almost the same in the temperature range from 500 to 700 ℃. This can be attributed to the fact that the decomposition of CaCO₃ does not occur at these temperatures. When the temperature is increased to 900 ℃, a distinct difference is found between the atmospheres employed. According to the result of TGA (shown in Table 1), the decomposition of CaCO₃ is almost completed at 900 ℃ in air, but not in O₂/CO₂ atmosphere. In this case, the decomposed CaCO₃ is a dominant contributor to the BET specific surface area of sulfated products. As the temperature is increased to 1 100 ℃ and above, the BET specific surface area of sulfated products in air decreases to the same level as that in O₂/CO₂ atmosphere. This result can be attributed to considerable sintering of CaO and the pore plugging of sulfated products. This suggests that the decrease of surface area of reactant accounts for the lower calcium conversion at high temperatures in air. By contrast, the BET specific surface area of sulfated products is less affected by reaction temperature in O₂/CO₂ atmosphere, and this further confirms that high temperature is beneficial to the capture of SO₂ in O₂/CO₂ atmosphere.

Fig.5 BET specific surface area of sulfated product vs temperature in different atmospheres

4 Conclusions

(1) Temperature has a great influence on the calcium conversion in air and in O₂/CO₂ atmosphere. The calcium conversion in air is higher than that in O₂/CO₂ atmosphere at temperatures ranging from 500 to   1 100 ℃, and it is higher in O₂/CO₂ atmosphere at 1 200 ℃. The results show that high temperature is beneficial to the removal of SO₂ in O₂/ CO₂ atmosphere.

(2) The calcium conversion increases with the increase of concentration of SO₂ due to shifting of the reaction equilibrium and the inhibition of the decomposition of CaSO₄.

(3) The reaction mechanism of desulfurization reagents CaCO₃ and SO₂ in O₂/CO₂ coal combustion dependents on the temperature in O₂/CO₂ atmosphere. It involves both direct and indirect sulfation reactions.

(4) The BET specific surface area of sulfated products is less affected by reaction temperature in O₂/CO₂ atmosphere, which is responsible for the high desulfurization efficiency in O₂/CO₂ coal combustion at high temperature.

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(Edited by CHEN Wei-ping)