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Pilot-Plant Technical Assessment of Wet Flue Gas Desulfurization Using Limestone F. J. Gutie´ rrez Ortiz,* F. Vidal, P. Ollero, L. Salvador, and V. Corte´ s Departamento de Ingenierı´a Quı´mica y Ambiental, UniVersidad de SeVilla, Camino de los Descubrimientos s/n, 41092 SeVille, Spain
A. Gime´ nez ENDESA, Ribera de Loira 60, 28042 Madrid, Spain
An experimental study was performed on a countercurrent pilot-scale packed scrubber for wet flue gas desulfurization (FGD). The flow rate of the treated flue gas was around 300 Nm3/h, so the pilot-plant capacity is one of the largest with respect to other published studies on a pilot-plant wet FGD. The tests were carried out at an SO2 inlet concentration of 2000 ppm by changing the recycle slurry pH to around 4.8 and the L/G ratio to between 7.5 and 15. Three types of limestone were tested, obtaining desulfurization efficiencies from 59 to 99%. We show the importance of choosing an appropriate limestone in order to get a better performance from the FGD plant. Thus, it is important to know the reactivity (on a laboratory scale) and the sorbent utilization (on a pilot-plant scale) in order to identify if a limestone is reactive enough and to compare it with another type. In addition, by using the transfer-unit concept, a function has been obtained for the desulfurization efficiency, using the L/G ratio and the recycle slurry pH as independent variables. The Ca/S molar ratio is related to these and to the SO2 removal efficiency. This function, together with a simplified function of the operation variable cost, allows us to determine the pair (L/G ratio and pH) to achieve the desired SO2 removal with the minimum operation cost. Finally, the variable operation costs between packed towers and spray scrubbers have been compared, using as a basis the pilot packed tower and the industrial spray column at the Compostilla Power Station’s FGD plant (in Leo´n, Spain). 1. Introduction Sulfur dioxide (SO2) is generated from coal combustion through oxidation of the sulfur, typically resulting in flue gas SO2 concentrations of 500-2000 ppm. Large-scale development of wet limestone scrubbing began ∼35 years ago, predominantly in the United States and Japan. Presently, there are several ways of reducing SO2 emissions from coal utilization. In the past 10 years, flue gas desulfurization (FGD) technology has made considerable progress in terms of efficiency, reliability, and costs.1 So far, at those power utilities that have sulfur emissions control equipment installed, in >80% of the cases, the sulfur is removed from the flue gas using flue gas desulfurization with a nonregenerable sorbent based on calcium. In general, lime (CaO), limestone (CaCO3), and ashes with a high calcium plus magnesium content are used, in an aqueous solution.2 Limestone is generally quite a bit cheaper than lime, making it more popular for large FGD systems. Therefore, wet scrubbers, especially wet limestone desulfurization scrubbers, are the front-running FGD technologies.3,4 The wet limestone desulfurization process usually offers the lowest through-life cost option for large inland plants with medium to high sulfur fuel, a high load factor, and a long residual life.5 In this process, the flue gas is treated with limestone (calcium carbonate) slurry in a spray column (mainly) or in a packed tower to remove the SO2. Limestone is mixed with water in a slurry supply tank. This fresh limestone slurry is fed to the absorber sump. The limestone/gypsum slurry is pumped from the absorber sump up to the spray headers at the * Corresponding author. Tel.: +34 95 448 72 60/61/68. Fax: +34 95 446 17 75. E-mail:
[email protected].
top of the scrubber. As the slurry falls down the tower, it meets the rising flue gas and reacts with the SO2 in the flue gas. Calcium sulfite and bisulfite (CaSO3 and Ca(HSO3)2) are formed in the chemical reactions that occur in the scrubber. The absorber sump (liquor or liquid phase) is agitated and aerated to produce gypsum or calcium sulfate (CaSO4) salts that are removed as sludge. This sludge, extracted from the absorber sump, is thickened, dewatered, and washed for subsequent storage before dispatch from the site (Figure 1). Normally, the byproduct gypsum can be sold rather than incurring a disposal cost, which includes a landfill tax; this solution also lessens the environmental impact. The final product is calcium sulfate dihydrate (gypsum), and the limestone content cannot exceed 3-4 wt % limestone in solids, according to most worldwide standards, to be saleable. There is constant pressure on the power industry, not only for improved environmental performance and reduced costs but also to comply with existing legislation whenever it is more restrictive. The result has been the development of a high number of FGD processes, the continuous improvement and refinement of the better ones (wet FGD), and continuous attempts to develop new technologies. To clearly understand future developments in FGD technologies, it is first necessary to consider the current legislation. Emission standards for SO2 are gradually becoming tighter. In the United States, the Acid Rain SO2 Reduction Program, established under Title IV of the Clean Air Act Amendments of 1990, was designed to reduce SO2 emissions from the powergenerating industry. Phase II of the Acid Rain SO2 Reduction Program began on January 1, 2000, to continue reducing global emissions, and consequently, some power plants have to use FGD technologies.2 With respect to the European regulations,
10.1021/ie051316o CCC: $33.50 © 2006 American Chemical Society Published on Web 01/20/2006
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Figure 1. Wet limestone desulfurization basic diagram (forced-oxidation mode).
the current European New Plant Standard for coal-fired plants (LCPD 2001/80/CE) is 200 mg/Nm3 at 6% oxygen (dry) for large power stations (>500 MWth). January 1, 2008, is the limit for reaching the SO2 emission level cited above. This will obviously require a higher SO2 removal efficiency or a change in fuel. In some cases, both of them may have to be adopted. Companies will have to assess the techno-economic situation, and even consider the possibility of closing the power plant in question. This explains the interest on the part of many electric companies in getting better yields for the existing FGD plants in power stations. However, they must also keep the costs of the operation as low as possible. The reagent cost, together with the energy consumption, is probably the most significant component in the variable operation cost for controlling SO2 emissions. One key way to achieve a high degree of SO2 removal while at the same time minimizing the operation costs is to increase the extent of utilization of the reagent. To achieve this goal, companies could carry out tests with limestone of different provenance. The necessary experimental pilot-scale tests would be neither excessively costly nor time-consuming. However, there are only a few studies on wet desulfurization pilot plants;6-8 the pilot-plant capacity in these studies is, usually, much smaller than that related to this study (300 Nm3/h), and these researchers consider different aspects in their studies than those examined in this paper. In the absorber, the calcium ions will react with the SO2 if, first, the sorbent (limestone) particles dissolve. Thus, under the same conditions, the higher the dissolution rate of the sorbent, the higher the absorption rate of the SO2 and the higher the total SO2 capture there will be. The goal is a higher SO2 removal efficiency. Therefore, the limestone dissolution rate is very important in the scrubbing process. For a sorbent, the dissolution rate is closely related to its reactivity. Both of them depend on particle size distribution (PSD), chemical composition, and
mineralogical and physical characteristics of the sorbent itself. Nevertheless, the operation conditions also determine the dissolution rate of limestone and the composition of the absorber sump.9-12 2. Aims and Scope Limestone-based chemistry provides a great annual cost advantage with respect to other sorbents, but in some cases, its reactivity is insufficient. This study, carried out in a pilot plant, illustrates the importance of choosing a suitable limestone, i.e., the effect of the limestone characteristics (through its reactivity) together with the operation conditions in order to achieve the best performance of the process, which can be assessed by means of the SO2 removal and the sorbent utilization. Thus, a valuable methodology, which may be applicable to other sorbents and plants, is developed in this article. For this study, three objectives were proposed: 1. To identify limestone characteristics that explain different behaviors in the tests carried out in the pilot plant. An extensive experimental program was carried out using three different types of limestone, by varying the L/G ratio and the pH in the recirculation slurry. The recycle slurry pH is controlled by manipulating the limestone feed rate. The limestone feed rate is directly related to the Ca/S molar ratio (moles of fresh calcium fed to the oxidation tank in relation to moles of sulfur present in the flue gas entering the scrubber). The desulfurization efficiency was allowed to run freely. By analyzing the results and taking into account the limestone characteristics, it is possible to explain the behavior in a robust manner. Thus, the great impact of selecting an appropriate limestone must be highlighted: for given pH and L/G ratio values, the SO2 removal depends on the limestone content in the slurry and on the dissolution rate of the limestone, i.e., on the dissolved Ca2+
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Table 1. Flue Gas Composition component
composition (% volume)
CO2 H2O O2 SO2 NOx N2
11-13 8-10 5-6 0.02-0.2 (200-2000 ppm) 0.01-0.1 (100-1000 ppm) balance
feed rate (from the limestone particle to the slurry liquid phase) through the absorber. 2. To obtain the desulfurization efficiency as a function of the operation conditions, L/G ratio (L/Nm3), and pH and use this function to optimize the operation. The main energy consumption in a wet FGD is related to the recycle pumps and to the ID fan. The main determinant of the recycle pump’s power consumption is the L/G ratio. The L/G ratio (L/Nm3) is defined as the quotient between the flow rate (L/h) of the recycle slurry used to treat the flue gas and the flow rate (Nm3/h) of the flue gas. A good design based on accumulated experience could minimize the L/G ratio. Furthermore, if the flue gas’ flow rate is constant, the decrease in the L/G ratio will generally lead to lower power consumption due to the induced-draft (ID) fan. Although the power consumption cost is usually higher than the reagent consumption cost, with regard to the annual operation-maintenance costs, for a wet limestone FGD plant, both of them have a great impact on the operation costs. Thus, it could be beneficial to increase the pH of the slurry recycle line by increasing the feed rate of the sorbent slurry to the oxidation tank if a reduction of the L/G ratio was reached in such a way that the total operation cost was lowered. However, the limestone feed rate is limited because the limestone content in the final gypsum must be 99.00 not detected
CaCO3 MgCO3
PSD
