The effect of steam on simultaneous calcination and sulfation of limestone in CFBB
Introduction
Limestone is widely adopted to capture SO2 in circulating fluidized bed boilers (CFBB) [1], [2], [3], [4]. In most studies the sulfation of limestone is described by two reactions: the calcination of limestone (1) and then the sulfation of calcined porous CaO product (2).
Reactions (1), (2) are usually considered as two independent processes. In fact, reaction (1) is often not considered in sulfation studies because calcination time is much shorter than sulfation time (reaction 2). As a result, most studies on the sulfation of limestone in CFBB were focused on the sulfation of CaO.
One of the main problems on SO2 capture by limestone in CFBB is that the utilization of calcium is normally only at 30–40%, while the theoretical maximum conversion for nonporous limestone ought to be 69% [5]. The cause for this problem is that the molar volume of CaSO4 (46 cm3/mol) is larger than that of CaCO3 (36.9 cm3/mol), so when CaSO4 is formed inside the particle, the pore of CaO particle will be plugged, which consequently blocks the diffusion of SO2 and retards the sulfation reaction.
There are many factors that influence the sulfation characteristics of limestone, such as temperature, geological characteristics of the limestone, SO2 concentration and particle size. Temperature has a strong influence on sulfur capture of limestone, typically the best sulfur capture performance is obtained at about 850 °C under air-fired CFBB conditions in laboratory studies [6], [7], but the actual optimum reaction temperature depends on the limestone and the operation unit [5], [8]. The geological characteristic of the limestone is another factor which strongly affects the sulfur capture performance. The consensus view on this point is that geologically younger limestone tend to be less crystalline and more reactive than the older ones [5]. It is observed that smaller limestone particles may reach a faster sulfation reaction rate and higher calcium conversion [9], but in CFBB very small particles may experience short residence time before being elutriated and result in lower calcium conversion [10].
Steam is one of the main components of the coal combustion flue gas. Depending on the hydrogen and moisture content in coal, the steam concentration of flue gas varies from 4% to 20%, or even higher in oxy-fired CFBB with wet flue gas recycle [11].
All of the calcination, sintering and sulfation reactions of the limestone can be significantly influenced by steam. Burnham et al. [12] tested the calcination of oil shale in N2/CO2 with the presence of water vapor and observed that steam lowers the calcite decomposition temperature and enhances the decomposition rate. The decomposition of limestone particles in high steam concentration atmosphere (20–100% H2O in CO2) was investigated by Wang et al. [13] using a fluidized bed reactor and found that the decomposition rate of limestone increased as the steam percentage in CO2 increased. Wang and Thomson [14] investigated the influence of H2O on the calcite decomposition and found that a relatively low steam pressure can significantly enhance the decomposition rate, and similar conclusion was obtained in the research of Wang et al. [15]. The investigation of Yin et al. [16] concluded that the physical effect of steam is negative but negligible, and the catalytic function of steam plays a role in the calcination of CaCO3, and there is a critical value of steam which corresponds to the largest enhancement on the calcination rate; when the H2O concentration exceeds the critical value, further increase in H2O concentration will decrease the calcination rate. In the research of Khraisha and Dugwell [17], the existence of an optimum level of water vapor content for maximum degree of calcination was also detected.
Steam can also influence the sintering rate of CaO. Borgwardt [18] measured the sintering rate of nascent CaO as a function of H2O and CO2, and found that H2O strongly catalyzed the sintering process, which is similar to the conclusion obtained by Petersen and Cutler [19] with CaO derived from Ca(OH)2. Recently an investigation of Wang et al. [15] obtained that both BET surface area and pore volume of CaO decreased with increasing H2O concentrations in the calcination stage of the limestone at 850–950 °C, but the pore size of CaO tends to be larger with higher steam concentrations.
The effects of steam on sulfation of limestone under air-fired or oxy-fired conditions were tested by many investigators. Dennis [20] studied the influence of water vapor on limestone sulfation and concluded that there was no systematic influence. Suyadal et al. [21] investigated the influence of H2O on SO2 capture in a fluidized-bed reactor at 900 °C and found that the sulfation rate of CaO was weakly decreased with increased H2O concentrations (0–5%). In a research of Wang et al. [22] the effect of H2O on sulfation of CaO under high CO2 concentration (80%) conditions was investigated and a significant enhancement by H2O (5% or 10%) on sulfation was detected. Other tests by Wang et al. [15], [23] about the influence of H2O on sulfation of CaO using TGA simulating air-fired conditions found that the sulfation rate of CaO is significantly promoted by the presence of 10% H2O in the diffusion-controlled reaction stage, but the influence of steam in the kinetic-controlled stage appears to be much less pronounced. Similar phenomenon was detected in the experiments by Stewart et al. [24]. Jiang et al. [25] investigated the effect of steam on indirect sulfation in a tube furnace simulating oxy-fired conditions and their results indicated that the presence of steam is not influential in kinetic-controlled stage but enhances the sulfation reaction in the later stage, which is similar to the phenomenon in air-fired conditions.
In a research of Hajaligol et al. [26], the influence of water vapor on direct sulfation of CaCO3 was investigated, and the results showed that water vapor has a positive effect on sulfation conversion, but the particle size (<45 μm) used in the test is not appropriate for CFBB. Hu et al. [27] researched the effect of H2O on direct sulfation of limestone at 650 °C and obtained a promoting effect. Wang et al. [28] investigated in detail the influence of H2O on direct sulfation under oxy-fuel CFBB conditions and found that the direct sulfation rate can be promoted by H2O. With little difference from the test of Wang et al. [28], the investigations of Duan et al. [29] obtained that the presence of H2O has negligible effect on direct sulfation during the kinetic-controlled regime but an obvious enhancement during the diffusion-controlled stage, which is consist with the research by Stewart et al. [30].
Despite the similar phenomena observed, there is controversy on the mechanism for the positive influence of H2O on sulfation of limestone. Wang et al. [23], [28] speculated that H2O may react with CaO and forms transient intermediate Ca(OH)2, which has higher sulfation rate than CaO and consequently promotes the sulfation reaction of the limestone. But different mechanisms were proposed by Stewart et al. [24], Jiang et al. [25] and Duan et al. [29] that H2O may enhance the solid-state diffusion in the CaSO4 product layer but not participating in the sulfation reaction directly because obvious influence of H2O was detected only in the diffusion-controlled stage.
From the descriptions above, it can be seen that when sulfation of limestone in CFBB is investigated, calcination and sulfation are usually taken as separated processes. Namely, limestone was first calcined to CaO in air or N2, and then the sulfation of CaO was investigated. As to the effect of steam on calcination or sulfation of sorbents, it also only focuses on calcination of CaCO3 or sulfation of CaO. However, this is not the actual reaction process of limestone in CFBB. When limestone particles are injected into CFBB, they will need hundreds of seconds to calcine [15], and at the same time sulfation is occurring, which means that calcination and sulfation occur simultaneously in a limestone particle. Tests have demonstrated that simultaneous calcination and sulfation of limestone is of different characteristics from the sulfation of CaO [31], [32], [33]. Only a few investigators have paid attention to the simultaneous calcination and sulfation of limestone [34], and no studies to date have tried to find the influence of steam on simultaneous calcination and sulfation of limestone.
The purpose of this work is to find the effects of steam on simultaneous calcination and sulfation of limestone. Tests were carried out with two limestones in TGA in simulated air-fired CFBB condition. Synthetic flue gases containing SO2, steam, CO2, O2, etc. was used in the tests. The influences of temperature, particle size, SO2 concentration, and especially H2O concentration, are tested to obtain the details of their effects on simultaneous calcination and sulfation. SEM–EDX and quantitative phase analysis have been done to assist in understanding the H2O influences further.
Section snippets
Experimental
Two limestones were calcined and sulfated simultaneously using a Cahn TGA. The compositions of the two limestones were given in Table 1.
All experiments followed the same procedure. A known amount of limestone sample (about 20 mg) was loaded into TGA in a shallow platinum sample pan (12.7 mm diameter, 2 mm deep). The TGA was flushed with pure CO2 at 300 mL/min for 30 min before heating began. The limestone sample was then heated to the desired temperature under pure CO2 environment. Once the pre-set
Effect of steam
Massieci limestone particles in size range 250–425 μm were first used to study the effect of steam during simultaneous calcination and sulfation. Tests were conducted at 850 °C and the reaction gas contained 3800 ppm SO2, 15% CO2 and 15% or 0% steam Fig. 1.
For discussion purpose, the curves in Fig. 1 are divided into two stages. The first stage is from the beginning (100 wt%) to the lowest weight point (in wt%), the second stage is from the lowest weight point to the end. This division will also be
Conclusions
The effect of steam on simultaneous calcination and sulfation of limestone in CFBB was studied. It is found that the “lowest weight (%)” becomes even lower and the time needed to reach the lowest weight point is decreasing with more steam. SO2 capture capability of Ca-based sorbent can be improved by more steam. The effect of steam on the calcination of limestone is much stronger at higher temperatures. For smaller limestone particles the effect of 15% steam is more significant on their
Acknowledgments
This project is supported by the National Natural Science Foundation of China (51276064) and 111 Project (B12034).
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