Environ. Sci. Technol. 2010, 44, 4692–4696
Adhesive Carrier Particles for Rapidly Hydrated Sorbent for Moderate-Temperature Dry Flue Gas Desulfurization YUAN LI, CHANGFU YOU,* AND CHENXING SONG Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing100084, China
Received November 19, 2009. Revised manuscript received May 10, 2010. Accepted May 10, 2010.
A rapidly hydrated sorbent for moderate-temperature dry flue gas desulfurization was prepared by rapidly hydrating adhesive carrier particles and lime. The circulation ash from a circulating fluidized bed boiler and chain boiler ash, both of which have rough surfaces with large specific surface areas and specific pore volumes, can improve the adhesion, abrasion resistance, and desulfurization characteristics of rapidly hydrated sorbent when used as the adhesive carrier particles. The adhesion ability of sorbent made from circulation ash is 67.4% higher than that of the existing rapidly hydrated sorbent made from fly ash, the abrasion ratio is 76.2% lower, and desulfurization ability is 14.1% higher. For sorbent made from chain boiler ash, the adhesion ability is increased by 74.7%, the desulfurization ability is increased by 30.3%, and abrasion ratio is decreased by 52.4%. The abrasion ratios of the sorbent made from circulation ash having various average diameters were all about 9%, and their desulfurization abilities were similar (∼150 mg/g).
1. Introduction SO2 emissions from coal-fired power plants have caused significant environmental and human health effects. Various flue gas desulfurization (FGD) technologies have been developed to remove SO2, with Ca-based sorbents widely used because they are abundant and inexpensive. The wetFGD process is most widely used because of its high desulfurization efficiency and wide applicability to various coal and unit types (1). However, the wet-FGD process needs a large amount of water and is expensive. The lack of water in many areas necessitates the use of dry-FGD technologies, especially in very arid regions (2). Dry-FGD process has the advantages of low cost, no water consumption, and a valuable dry product CaSO4 (3). Hydrating fly ash and lime to improve the sorbent desulfurization ability is one effective method for making sorbent for the dry-FGD process (4-14). A process developed to rapidly prepare hydrated sorbent is timesaving, simple in manipulation, and gives a high calcium conversion ratio for sulfur reaction (13). The main steps for preparing this sorbent are hydrating the lime and fly ash at ambient temperature for 2 h and drying at 150 °C for 1.5 h (14). Fly ash particles were * Corresponding author tel: +86-10-62781740; fax: +86-1062770209; e-mail:
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used as adhesive carrier particles to carry the fine calciumcontaining particles to increase the residence time of the calcium-containing particles. The calcium conversion ratio of this sorbent measured on a thermo-gravimetric analyzer (TGA) exceeds 95% at 700 °C. However, some problems developed when using the rapidly hydrated sorbent in applications. The main problem was that the calcium conversion ratio in applications was much smaller than the TGA results. The main reason is that part of the fine calciumcontaining particles could not adhere to the fly ash surface during the sorbent preparation process due to the smooth fly ash particle surfaces, and part of the fine particles originally adhered to the fly ash surface easily separated from the fly ash during the desulfurization process. These highly active fine particles could be easily brought out of the reactor with flue gas, which greatly reduced the particle residence time and the calcium conversion ratio in applications. Enhancement of the adhesion ability of the adhesive carrier particles is one potential way to solve the problem. The surfaces of circulation ash and chain boiler ash particles are much rougher than fly ash particles. Rougher surfaces mean larger specific surface areas, which can provide a larger area for the fine calcium-containing particles to adhere. The effects of these two types of adhesive carrier particles on the adhesion, abrasion resistance, and desulfurization characteristics of the sorbent were investigated experimentally. The results show how to improve the rapidly hydrated sorbent for more efficient industrial desulfurization usage.
2. Experimental Section 2.1. Materials and Sorbent Preparation. The lime used in the experiment was from Shougang Building Materials Chemical Plant, the fly ash was from Beijing Shijingshan power plant, and the circulation ash and chain boiler ash were from Department of Thermal Engineering of Tsinghua University. Their main components as measured by an X-ray fluorescence spectrometer (ARL ADVANT XP+) are listed in Table 1. Figure 1 shows their particle size distributions. Almost all of the lime particles were smaller than 10 µm. The average volume-diameter of the fly ash, circulation ash, and chain boiler ash were 83.43, 127.97, and 123.42 µm. The three adhesive carrier particles were mixed with the lime to prepare the rapidly hydrated sorbent (13). The sorbent preparation steps were as follows. First, the lime, adhesive carrier particles, and water were mixed in a hydration mixer with stirring at ambient temperature to produce sorbent slurry. The water/solid mass ratio was 1:1 and the mass ratio of lime to fly ash was 1:4 or 1:2. The hydration time was very short at 2 h. Then, the sorbent slurry was dried in an infrared desiccator to a water content 250 µm
5.31 26.34 21.03
1.7 15.45 13.75
0.68 12.47 11.79
abrasion resistant characteristic. There are two major reasons: (1) circulation ash and chain boiler ash rough surfaces, large specific surface areas, and large specific pore volumes enhanced their adhesion abilities and helped reduce the loss of the fine calcium-containing particles in the reactor; (2) the fly ash particle sizes were relatively smaller than those of the circulation ash and chain boiler ash, so more fly ash particles were collected in the cyclone separator and the bag filter than circulation ash and chain boiler ash particles. The abrasion resistance characteristic of the circulation ash is better than that of the chain boiler ash mainly because the chain boiler ash structure is loose with many cracks, so the particles easily break up and the fine chain boiler ash particles and fine calcium-containing particles are lost. 3.3. Desulfurization Ability in TGA. The desulfurization abilities of the various sorbents before and after abrasion experiment were tested in the TGA. The results in Figure 5 show that the desulfurization ability of the circulation ash sorbent before abrasion was 162 mg/g while that of the
FIGURE 5. Sorbent desulfurization ability before and after abrasion.
sorbent after abrasion was 140 mg/g, and those of the chain boiler ash sorbent were 185 mg/g and 158 mg/g. These are all higher than those of the fly ash sorbent. Zhang et al. (14) found that large specific surface areas and specific pore volumes improved the sorbent desulfurization ability because of the decreased SO2 diffusion resistance. Table 4 shows that the specific surface areas and specific pore volumes of the circulation ash and chain boiler ash sorbent were larger than those of the fly ash sorbent. Thus, their desulfurization abilities were increased. The desulfurization ability of the circulation ash sorbent decreased 13.58% after abrasion while the desulfurization ability of the chain boiler ash sorbent decreased 14.59%, which are both less than the reduction of the fly ash desulfurization ability of 56.34%. Therefore, the circulation ash and chain boiler ash sorbents kept their desulfurization abilities better during the abrasion, which indicated that circulation ash and chain boiler ash sorbents lost less fine calcium-containing particles during collisions. Therefore, the circulation ash and chain boiler ash are more suitable for the rapidly hydrated sorbent to not only increase the sorbent desulfurization ability, but also to maintain the desulfurization ability during abrasion. Generally, the reaction conditions in TGA, which are more close to those of a fixed bed, are not the same as those in CFB rector. The desulfurization results obtained by TGA can only reflect the influence tendency of particle abrasion, specific surface areas, and specific pore volumes on sorbent desulfurization ability. In further research, experiments in a CFB reactor system are necessary to get the desulfurization ability of rapidly hydrated sorbent in CFB. 3.4. Influence of Adhesive Carrier Particle Diameter. The influence of circulation ash diameter was studied using different sieves to sort the circulation ash into three diameter
FIGURE 6. Abrasion ratios and desulfurization abilities of circulation ash sorbent with different adhesive carrier particle size. VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ranges: 100-150 µm, 150-250 µm, and larger than 250 µm. The three circulation ash particles were used to prepare rapidly hydrated sorbent (mass ratio of circulation ash to lime was 2:1) to investigate the influence of adhesive carrier particle diameter on sorbent adhesion, abrasion resistance, and desulfurization abilities. The results are shown in Table 5 and Figure 6. Table 5 shows that the increase of VPM10 for the circulation ash sorbent decreased with increasing circulation ash diameter. Thus the adhesion ability gradually increased with increasing particle diameter. Figure 6a shows that the abrasion ratios of the circulation ash sorbent with various diameters were all about 9%. Figure 6b shows that the desulfurization ability of the sorbent before abrasion slightly decreased with increasing circulation ash diameter. The desulfurization ability of the sorbent after abrasion did not change in a regular pattern as the adhesive carrier particle diameter increased. The particle surface curvature decreased with increasing adhesive carrier particle diameter, resulting in smaller curvature surfaces favoring adhesion of the fine calciumcontaining particles, so the adhesion increased with increasing particle diameter. After the very intense abrasion, the influence of the adhesive carrier particle diameter on the abrasion state was not obvious. The intense abrasion resulted in similar calcium contents and similar distributions of the fine particles of the sorbent after abrasion; thus the desulfurization abilities of the sorbent after abrasion were similar.
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Acknowledgments
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This research was supported by the Special Funds for Major State Basic Research Projects (2006CB200305).
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