Article pubs.acs.org/IECR
Study on the Relationship between Absorbed S(IV) and pH in the Seawater Flue Gas Desulfurization Process Tian Lan,† Xingwang Zhang,† Qingni Yu,‡ and Lecheng Lei*,† †
Industrial Ecology and Environment Research Institute, Department of Chemical and Biological Engineering, Yuquan Campus, Zhejiang University, Hangzhou, Zhejiang 310027, China ‡ China Astronaut Research and Training Center, P.O. Box 5132, Beijing 100094, China ABSTRACT: Seawater flue gas desulfurization (SFGD) is a promising process for coal-burning power plants located along the coast. The aim of this work was to study the seawater outflow characteristics, especially the relationship between absorbed S(IV) and pH in the SFGD process. The equilibrium concentration of SO2 in seawater was studied under different seawater usages and SO2 concentrations in the flue gas. The experimental results showed that approximately 25% of absorbed S(IV) was oxidized to S(VI) by surplus oxygen in the flue gas during the absorption process. An equilibrium model was established to describe the relationship between absorbed SO2 and pH. The integral desulfurization efficiency of the absorption system was analyzed, and the results showed that desulfurization efficiency can reach up to 90%. The SFGD process was established in the 1970s. Bromley,13 Tokerud,14 and Radojevic15 performed early studies on seawater desulfurization technology, ranging from introducing fundamental physicochemical principles to addressing technical and financial issues.11 In recent years, Vidal and co-workers made contributions to SFGD technology, publishing a series of articles from 2001 through 2009.4,5,16,17 These works placed an emphasis on kinetic studies of the oxidation of S(IV) to S(VI) and included the presentation of a desulfurization model. Vidal and co-workers found that activated carbon has a significant catalytic effect on the S(IV) oxidation process,5 which might have practical use, and subsequently carried out a pilot-plant study.17 In addition to wet processes, seawater has been used as a hydration medium to prepare desulfurization sorbents18 or add to the in-duct desulphurization process.19 SFGD is not only suitable for coal-burning systems, but can also be effective for natural-gas-fired flue gas20 or oil-fired flue gas21 because of its low energy consumption and high removal efficiency. A typical SFGD process involves three steps: (1) absorption, (2) oxidation, and (3) neutralization. Current developments in SFGD focus on maximizing performance as well as minimizing costs.11 Recent studies have mostly concentrated on steps 2 and step 3, the absorption process and the characteristics of acidified seawater. In particular, the correlation among S(IV), S(VI), and pH must be studied because these parameters directly relate to the design of the aeration basin or other type of oxidation technology. However, there have been fewer reports about the absorption process until recently. Therefore, we conducted this study to research the absorption behavior of SO2 in the SFGD process (using a packed tower as a scrubber), especially the relationship between absorbed S(IV) and pH.
1. INTRODUCTION With economic prosperity and population growth in China, the growth in energy demand has led to increasing consumption of fossil fuels. In China, coal comprises 75% of primary energy sources, and over 84% of it is consumed by boiler combustion. The burning of fossil fuels, especially coal, releases large amounts of sulfur dioxide into the atmosphere. Thus, desulfurization of flue gas from coal or fuel-oil boilers is a critical and urgent topic of environmental research.1 As an adequately developed flue gas desulfurization (FGD) process, the wet limestone process accounts for 83% of wet FGD systems installed and 72% of the total worldwide.2 In China, the wet scrubber process has emerged as the dominant technology, accounting for over 80% of total SO2 scrubbers,3 and the wet limestone process is absolutelt dominant. However, the wet process faces some problems, especially places lacking limestone resources. An alternative FGD technology for power plants located along the coast that utilizes seawater from the plant’s cooling system to scrub SO2 in flue gas was established in the 1970s. Compared to the wet limestone process, seawater FGD (SFGD) has two main advantages: (1) Because seawater itself is alkaline (pH 7.5−8.3), an alkaline absorbent is not necessary, and (2) the effluent seawater treatment process is much simpler, as only a simple aeration basin is needed to oxidize S(IV) to S(VI), a natural component of seawater.1,4,5 Indeed, Tilly et al. found seawater scrubbing to be the “best practicable environmental option” among four different desulfurization processes (spray drying, wet limestone, Wellman−Lord, and seawater scrubbing).6 To date, many countries, including Norway,7,8 Malaysia, Indonesia, Saudi Arabia,9 and China,10−12 have installed SFGD technology. The thermal power plants along the coastal area of eastern China, which is highly industrialized and has a high population density, account for one-third of the total thermal power plant output capacity of the entire country. Therefore, SFGD technology could have great potential. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 4478
November 29, 2011 March 2, 2012 March 7, 2012 March 7, 2012 dx.doi.org/10.1021/ie202770b | Ind. Eng. Chem. Res. 2012, 51, 4478−4484
Industrial & Engineering Chemistry Research
Article
Figure 1. Experimental system.
2. MATERIALS AND METHODS 2.1. Experimental Devices and Procedures. As shown in Figure 1, the experimental system contained the following parts: a feed-gas cylinder (stainless steel texture), a temperature controller, a packed tower (plexiglass texture, 120-mm inner diameter, 500-mm effective height), and a circular tank (plexiglass texture, 3-L volume, placed on a magnetic stirrer that included an electric heater). The feed-gas cylinder contained a four-component artificial flue gas mixture including N2, CO2, O2, and SO2 that had been premixed and stabilized at the desired ratio. A porous aeration tube that formed a spiral at the bottom of the tower was used as a gas diffuser. Figure 1 also illustrates the polytetrafluoroethylene (PTFE) high-efficiency filler that was used in the packed tower. The six different compositions used for artificial flue gas are listed in Table 1.
Table 2. Seawater Characteristics
content (%) N2
CO2
O2
SO2
1 2 3 4 5 6
∼79 ∼79 ∼79 ∼79 ∼79 ∼79
16 16 16 16 13 21
5 5 5 5 8 0
0.10 0.15 0.20 0.25 0.10 0.10
value
pH total alkalinity (CaCO3) dissolved oxygen total suspended solids HCO3− SO42− Cl− Na+ Ca2+ Mg2+ Mn Fe
8.1 115 mg/L 7.8 mg/L 15 mg/L 130 mg/L 2930 mg/L 17430 mg/L 10830 mg/L 410 mg/L 1250 mg/L
