Full Length ArticleExperimental and modelling analysis of seawater scrubbers for sulphur dioxide removal from flue-gas
Introduction
Sulphur dioxide (SO2) is one of the primary air pollutants having extensive and proved damages to human health and the Environment. For this reason, stringent regulations for emission control have been enacted in many countries to limit the amounts emitted by anthropogenic activities.
The main sources of SO2 emissions are related to the combustion of fossil fuels containing sulphur, such as coal, oil or gas. Li et al. [1] investigated the air quality and pollutant emissions from fifteen Chinese coal-fired power plants having a power generation capacity ranging from 125 to 600 MWth. The sulphur content of the burnt coal was between 0.32 and 2.63% and the SO2 concentration in the flue-gas before desulfurization units resulted on average between 210 and 1540 ppmv.
The European Union regulations in force (2001/81/CE and 2003/35/CE) provide the guidelines for pollutant emission control. The current European regulation (2010/75/UE) on SO2 emission limits in coal-fired power plants installed before 2013 posed a limit of 200 mg·m−3 (71 ppmv) for units with power from 50 to 100 MWth and 75 mg·m−3 (26.5 ppmv) for units with power higher than 300 MWth.
Very recently, the Directive 2016/2284/CE established the entry in force of new regulations for SO2 emissions for the years 2020–2029 and after 2030. On average, SO2 emission cuts are planned to be within 8–83% for the different EU member states. This will generate a relevant increase in the required removal efficiency. For example, at moment, a coal fired plant with an installed power of 90 MWth, emitting 2000 ppmv of SO2, has to comply with a 71 ppmv limit, corresponding to a removal efficiency about 95.2%. For the same plant, the new regulation limit (35.5 ppmv), assuming a 50% cut of the allowed emission on an average basis, will lead to a required efficiency greater than 97.6%. In Europe, the database of the European power plant infrastructure [2] shows that most of the installed and commissioned power plants are powered by fossil fuels (mainly coal and natural gas). Moreover, most of them are located near the main fuel transport routes, such as sea, river or lakes [2].
In order to comply with the emission limits, different post-combustion desulphurization systems, commonly referred as to flue-gas desulfurization (FGD) systems, have been proposed. In general, the choice of an appropriate FGD system depends on both plant size and emission targets of flue-gas.
Dry scrubbers are among the most established technologies, as they are used for flue-gas cleaning in coal power plants since the early 1970. In this process, SO2 removal is carried out by scrubbing the flue-gas with a limestone suspension [3], [4], [5]. When a removal efficiency higher than 90% is needed, wet scrubbers are preferred. These systems provide a wider choice of chemical reagents as absorbing liquids, such as sodium hydroxide [6], sodium carbonate and bicarbonate [7], sodium chlorite [8], [9], [10], [11], hydrogen peroxide [12]. Alternatively, ozone oxidation systems can be also used [13].
A technically viable and economically feasible alternative to chemicals is the use of seawater as absorbing solution [14], [15], [16]. Recently, seawater scrubbers have been proposed for coastal installations and marine diesel engine applications, due to the large availability and favorable chemical properties (e.g. the intrinsic alkalinity). The presence of alkaline compounds such as carbonates and bicarbonates in equilibrium generate a buffering effect that allows preserving the pH conditions favorable to SO2 absorption [17], [18], [19]. Some authors also observed that the high salinity content in seawater solution, mainly as sodium chloride, further improved the SO2 absorption [20].
The absorption process generates an acid wash-water having a pH in the range 2.5–4. Hence, wash-water has to be corrected for pH values before discharge, usually with a minimum allowed pH of 0.5 points below that of the inflow seawater. This is achieved by either direct dilution of the scrubber wash-water with seawater or with NaOH addition.
For this reason, it was sometime considered more useful to add NaOH to the same absorption seawater feed, in order to support SO2 removal while preserving wash-water pH level close to neutrality. The main issue related to this strategy is the pH limitation induced by precipitation of metal hydroxides in seawater, which, depending on salinity, typically occurs for pH above 9.7–10.
The main advantage of seawater scrubbers compared with conventional lime or limestone scrubber is the absence of chemicals and the less tendency to nozzle clogging and scaling over the scrubber walls or over packing. On the other hand, seawater is more corrosive and higher flow rates are required to comply with regulations due to the lower concentration of hydroxides and carbonates. In practice, piping and pumps are the primary capital costs of a seawater scrubber plant and strategies to minimize the specific water flow rate are therefore needed to reduce both operation and capital process costs. In particular, the ratio between seawater molar flow rate, L, and gas molar flow rate, G, depends on the equilibrium conditions, which identify the minimum value of to operate the absorber. Besides, the ratio depends on the efficiency of liquid-gas contact, in terms of mass transfer rate, which defines the scrubber height and diameter.
Spray columns are easy and largely preferred for limestone scrubbers because they have less scaling problem limitations compared to packed towers. One of the main issue of spray columns is the difficulty in predicting the mass transfer rates, which strongly depends on the droplet size distribution, but also on the occurrence of droplets deformation and coalescence phenomena occurring along the scrubber [21]. Since seawater gives less scaling problems, packed towers may be suitably used for seawater scrubbing. This option is advantageous due to the larger mass transfer rates and, for structured packing, the lower pressure drops. Therefore, although packed tower seawater scrubbers are scarcely used nowadays, they may become a suitable choice to comply with the higher removal efficiencies required by the incoming regulations (2016/2284/CE).
When considering the design of a seawater scrubber, two major issues arise. The first one relies on the scarcity of equilibrium data at low SO2 concentrations. Rodriguez-Sevilla et al. [19] collected thermodynamic experimental data for SO2 absorption in artificial seawater at 25 °C and gas concentrations from 400 to 15,000 ppmv. Based on this data, the authors developed a solubility model using Bromley and Pitzer’s equations for activity coefficients. Andreasen and Mayer [22] developed a solubility model in seawater with an extension of the Debye-Hückel’s equation for activity coefficients at 25 °C and SO2 concentrations in the range 0–500 ppmv. In general, there is still a lack of experimental data for wider range of SO2 concentration and for seawater having different alkalinity.
The second issue relies on the prediction of mass transfer rates in presence of chemical reactions, which depend on both reaction rates and packing geometry. Structured packing is increasingly used in industrial applications thanks to the low pressure drops and enhanced mass transfer with respect to random packings [23].
This work contributes to the discussion on seawater scrubber design by providing experimental studies on the absorption of SO2 from a model flue-gas in a packed bed column filled with Mellapak 250X® structured packing, which is increasingly used in chemical industries.
Experiments were performed at 1 atm and 25 °C, with a model flue-gas (32 m3·h−1) having a SO2 concentration in the range 500–2000 ppmv and a L/G between 1.69 and 5.51 mol·mol−1 (corresponding to a liquid-gas mass ratio between 1.06 and 3.44 kg·kg−1). Three different absorbing solutions were investigated: (a) a seawater; (b) a basic solution obtained by adding 200 mg·L−1 NaOH solution to the seawater and (c) distilled water, used as a benchmark.
In order to support the packed column tests, SO2 equilibrium absorption tests at low concentrations (100–2000 ppmv) were carried out in a feed-batch reactor, using the same absorbing solutions tested in column tests.
Both the equilibrium and the dynamic tests were analyzed in light of some models selected from the pertinent literature and suitably implemented in the Chemical Process Simulation software Aspen Plus® V 8.6.
Section snippets
Materials
The synthetic flue-gas was prepared by mixing SO2 (2% in N2) with either N2 (99.999%) or air (technical grade), supplied by Rivoira Gas Srl in high-pressure cylinders.
Absorption experiments were carried out with three different absorbing solutions: (a) a seawater at pH 8.2; (b) a basic solution obtained by adding 200 mg·L−1 of NaOH to the seawater to achieve pH 9.4 and (c) distilled water at pH 6.0 (ionic content less than 0.5 ppm).
Sodium hydroxide was purchased from VWR International Chemicals
Equilibrium model
The equilibrium model includes phase equilibria and chemical speciation analyses in the liquid and gas phases, required to describe the equilibrium of SO2 in the different absorbing solutions, also accounting for the chemical reactions. The determination of the concentrations of all the chemical species was made by coupling the chemical and physical equilibria with the mass and electric charge balance equations:
Equilibrium tests
Fig. 3 presents the experimental results of SO2 equilibrium tests carried out at 25 °C and 1 atm in a seawater (a), a basic solution obtained by adding NaOH to the seawater (b) and distilled water (c). The results were expressed in terms of liquid and gas equilibrium concentrations (Fig. 3A) and pH of the saturated solution as a function of gas concentration (Fig. 3B).
As expected, SO2 solubility was much greater in seawater (a) and basic solution (b) than in distilled water (c), and a
Conclusions
The introduction of stricter regulations for air emissions is leading to the implementation of new after-treatment solutions for flue-gas desulphurization. Seawater scrubbers are viable solutions for the removal of SO2 from flue-gas, e.g. in coal-fired power plants located close to coastal areas or for ships. Compared with conventional limestone based methods, SO2 solubility in seawater is lower and, to improve this property, caustic soda is normally used. However, seawater is largely available
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