Improving the efficiency of CaO-based sorbent by SrCO₃ for high-temperature sulfur removal during coal combustion

2.1 Reagents and materials

The pulverized coal used in this experiment was high-sulfur coal (41.92% FC_d; 9.04% V_d; 49.04% A_d; 7.02% S_{t, d}). Compositions of Ash are shown in Table 1. The following materials (pure chemical reagent) were prepared: Ca(OH)₂, (Jinbei Fine Chemical Co. Ltd., Tianjin, China) as sulfur-fixing agent and SrCO₃, (Zhiyuan Chemical Reagent Co. Ltd., Tianjin, China) as additive. Ca/S mole ratio was equal to 1.5.

The high-sulfur coal and Ca(OH)₂ were taken and mixed according to a given proportion. About 1%, 3%, and 5% SrCO₃ were added to coal samples, respectively. They were then blended and ground in a grinding bowl to prepare the samples. A certain amount of coal, which was taken from the samples, was placed in the corundum crucible, and then calcined at 1200°C, 1300°C, and 1400°C and preserve heat for 1 h, respectively. The ash residues were collected to determine the sulfur content.

2.2 Experimental equipment

In the current work, a YX-DL8300 Sulphur Determination Device (Changsha Youxin Instrument Manufacturing Co. Ltd., Changsha, China) was used to verify the sulfur content in the coal samples. An ash melting point detector (YX-HRD3000, Changsha Qiulong Equipment Co. Ltd., Changsha, China) was utilized to calcine the coal samples, after which an infrared carbon–sulfur spectrometer (ZY-HWC8, Nanjing Zhengyuan Analytical Instrument Manufacturing Co. Ltd., Nanjing, China) was used to ascertain the sulfur content in the ash. The changes of the mineral constituents in coal ash were investigated through an XRD diffraction analyzer (DX-2700, Dandong Fangyuan Instrument Co. Ltd., Dandong, China).

3.1 Effects of additive proportion on high sulfur coal desulfurization

The sulfur-releasing rates of the coal samples with different amounts of SrCO₃ when temperatures reach 1100°C are shown in Figure 1. Although the size, shape, and location of each peak vary, the changing trends are basically the same, that is, the curves evidently present twin peaks. The two peaks indicate the release process of organic sulfur (i.e. aliphatic, aromatic, etc.) and inorganic sulfur (i.e. sulfate, iron sulfide, etc.). The decomposition temperature of inorganic sulfur is comparatively high, and the decomposition rate is lower than that of organic sulfur (i.e. with a lower decomposition temperature); thus, the second peak is flat. After 300 s, the released sulfur concentration begins to rise rapidly. Then, it is maintained for approximately 300 s, when it approaches 200 ppm. Finally, sulfur in the coal sample is released completely. A large transition interval exists between two peaks as a result of the decomposition of pyrite (around 600°C) to sulfur [24], [25], [26], [27].

Figure 1:

Rules for sulfur release during the combustion of coal samples.

Compared with the case in which no strontium carbonate is added, the first peak is pointed, and the peak value is higher, indicating that more sulfur is released in a relatively short time. After the addition of 1% strontium carbonate, sulfur precipitation accelerates, and the peak appears in advance, but the peak value decreases obviously; further, when the addition ratio is 3% or 5%, the presence of the peak is delayed. It is possible that, to a certain extent, a small amount of strontium carbonate (1% SrCO₃) plays a catalytic role in the release of sulfur, although this assumption requires further research). However, when more strontium carbonate (3% or 5% SrCO₃) is added, the actions of desulphurization begin to hold a dominant position.

With the increase in SrCO₃ content, the first peak value decreases and the duration of the second peak reduces, suggesting that at 1100°C, the addition of 3% or 5% SrCO₃ promotes sulfur fixation.

3.2 Influence of temperature on the desulfurization effect

Desulfurization rate (η_s) evaluates the sulfur fixation effect and is calculated on the basis of the formula expressed as

where S₀ is the initial sulfur content in coal sample (7.02%), and S₁ is the sulfur content that escaped in the form of gas during combustion under different conditions.

Desulfurization rates are plotted in Figure 2 as a function of the temperature from 1100°C to 1400°C. The changing trends of the curves are roughly the same, that is, the desulfurization rate increases first and then descends. After adding SrCO₃, the desulfurization rate enhances greatly (the higher the temperature, the more obvious the effect of SrCO₃). It is worth mentioning that, after adding 3% SrCO₃ to the coal samples, the sulfur-fixing ratio slightly increases compared with the case in which 1% SrCO₃ is added at 1100°C or 1400°C. Meanwhile, at 1200°C or 1300°C, the increase is larger, but the desulfurization rate is slightly less than the value of the coal sample with 5% SrCO₃. This finding implies that at 1200°C, some compounds that keep the desulfurization product (e.g. CaSO₄) from decomposing are generated. Otherwise, this finding could also mean that CaSO₄ further reacts to form new sulfur retention phases that are stable at high temperature, which in turn, induces a sulfur-fixing ratio that increases significantly. When the temperature is higher than 1300°C, the sulfur-fixing ratio shows a declining trend. This may be due to the weakening effect of the inhibition of desulfurization product decomposition or the disintegration of the decomposition of sulfur retention phases at this temperature. When temperature exceeds a certain value, sulfur retention phases decompose gradually and release sulfur again. Previous studies have shown that this compound is C₄A₃S [28].

Figure 2:

Effects of temperature on sulfur fixation.

3.3 Influence of temperature on the formation of calcium sulphoaluminate

The XRD spectra of different coal ash samples at different temperatures are presented in Figures 3 and 4. As can be seen in both figures, no characteristic peak of C₄A₃S appears at 1100°C, and the main mineral is 3CaO·5Al₂O₃. In addition, small amounts of CaO·2Al₂O₃, 12CaO·7Al₂O₃ (in Figure 4), and residual CaSO₄, Ca(OH)₂ and Al₂O₃ are observed. These observations indicate that the formation temperature of 3CaO·5Al₂O₃ is lower than that of C₄A₃S and that its generation speed is faster than that of C₄A₃S.

Figure 3:

XRD spectra of coal ash samples at different temperatures (3% SrCO₃).

(1) CaSO₄, (2) Ca(OH)₂, (3) Ca₃Al_l0O₁₈, (4) 3CaO·3Al₂O₃·CaSO₄, (5) Ca₁₂Al₁₄O₃₃, (6) Al₂O₃, (7) CaAl₄O₇, (8) Ca₃Al₂O₆, and (9) SrSO₄.

Figure 4:

XRD spectra of coal ash samples at different temperatures (5% SrCO₃).

(1) CaSO₄, (2) Ca(OH)₂, (3) Ca₃Al_l0O₁₈, (4) 3CaO·3Al₂O₃·CaSO₄, (5) Ca₁₂Al₁₄O₃₃, (6) Al₂O₃, (7) CaAl₄O₇, (8) Ca₃Al₂O₆, and (9) SrSO₄.

As shown in Figures 3 and 4, when the temperature rises to 1200°C, C₄A₃S appears in the mineral phases, and CaSO₄, Ca(OH)₂ (in Figure 4), and 3CaO·5Al₂O₃ still exist simultaneously, suggesting that, within the temperature range, only a part of CaSO₄ and CaO react with Al₂O₃ to form high-temperature, stable C₄A₃S. In addition, CaSO₄ still exists at this temperature, indicating that the addition of SrCO₃ may melt and harden on the surface of the sulfur fixation agent, wrap the CaSO₄, and prevent CaSO₄ from decomposing to a certain extent.

Furthermore, the figures show that, the C₄A₃S crystals grow larger at 1300°C and that the diffraction peak intensity continues to strengthen, but the growth slows down, and decomposition occurs. At this point, 3CaO·5Al₂O₃, CaSO₄, and Ca(OH)₂ have all disappeared, and only small amounts of 3CaO·Al₂O₃ and 12CaO·7Al₂O₃ appear. This may be attributed to the following series of reactions: CaO first reacts with Al₂O₃ to generate CaO·2Al₂O₃, and then CaO·2Al₂O₃ reacts with CaO, CaSO₄, and Al₂O₃ to form C₄A₃S and 12CaO·7Al₂O₃; then, 3CaO·Al₂O₃ is generated by the decomposition of C₄A₃S at a high temperature. This finding indicates that a small amount of C₄A₃S has already decomposed at 1300°C.

As temperature continues to rise to 1400°C, SrSO₄ appears in the ash residue in Figures 3 and 4. However, the peak intensity of C₄A₃S starts to decline rapidly. In the coal ash sample with 3% SrCO₃ (Figure 3), the peak intensity of 3CaO·Al₂O₃ increases, whereas in the coal ash sample with 5% SrCO₃ (Figure 4), CaSO₄ and 3CaO·Al₂O₃ reappear, suggesting that the significant pyrolysis of C₄A₃S occurs at 1400°C.

In summary, adding a certain amount of SrCO₃ (3% SrCO₃ or 5% SrCO₃) to coal promotes the formation of C₄A₃S. Under this experimental condition, C₄A₃S is generated rapidly at around 1200°C. At 1300°C, the reaction continues, and decomposition begins to occur. At 1400°C, decomposition obviously increases. Therefore, the best calcining temperature for the synthesis of C₄A₃S is between 1200°C and 1300°C.

3.4 Effects of additive proportion on the formation of calcium sulphoaluminate

According to the above analysis, although the sulfur retention product CaSO₄ decomposes at high temperature, the existence of high temperature phase C₄A₃S increases the sulfur-fixing rate owing to the addition of SrCO₃.

To determine the appropriate proportion of SrCO₃, the mineral constituents of the coal ash samples were analyzed at 1200°C and 1300°C, as shown in Figures 5 and 6, respectively. As can be seen, the solid reaction products are basically the same. Given the addition of different quantities of SrCO₃, the diffraction peak intensity of the generated phases is also different. With the addition of 1% SrCO₃ at 1200°C or 1300°C, C₄A₃S is discovered. With the addition proportions increasing to 3% and 5%, the peak intensity of C₄A₃S increases because the addition of SrCO₃ accelerates the sulfide and promotes the creation of C₄A₃S. However, based on Figure 2, at 1200°C, the increase in the sulfur-fixing ratio with the addition proportion of SrCO₃ increasing from 1% to 3% is similar to that with the addition proportion increasing from 3% to 5%. At 1300°C, the growth in the sulfur-fixing ratio with the addition proportion of SrCO₃ increasing from 1% to 3% is higher than that with the proportion increasing from 3% to 5%. Thus, the best addition proportion of SrCO₃ should be around 3%.

Figure 5:

XRD spectra of coal ash samples at 1200°C.

(1) CaSO₄, (2) Ca(OH)₂, (3) Ca₃Al_l0O₁₈, (4) 3CaO⋅3Al₂O₃⋅CaSO₄, (5) Ca₁₂Al₁₄O₃₃, (6) Al₂O₃, (7) CaAl₄O₇, and (8) Ca₃Al₂O₆.

Figure 6:

XRD spectra of coal ash samples at 1300°C.

(1) CaSO₄, (2) Ca(OH)₂, (3) Ca₃Al_l0O₁₈, (4) 3CaO⋅3Al₂O₃⋅CaSO₄, (5) Ca₁₂Al₁₄O₃₃, (6) Al₂O₃, (7) CaAl₄O₇, and (8) Ca₃Al₂O₆.