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Energy Technology & Policy: An Open Access Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uetp20
Review of Design, Operating, and Financial Considerations in Flue Gas Desulfurization Systems Andreas Poullikkas
a
a
Department of Electrical Engineering, Cyprus University of Technology, Limassol, Cyprus Accepted author version posted online: 24 Jul 2015.
Click for updates To cite this article: Andreas Poullikkas (2015) Review of Design, Operating, and Financial Considerations in Flue Gas Desulfurization Systems, Energy Technology & Policy: An Open Access Journal, 2:1, 92-103, DOI: 10.1080/23317000.2015.1064794 To link to this article: http://dx.doi.org/10.1080/23317000.2015.1064794
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Energy Technology & Policy (2015) 2, 92–103 Published with license by Taylor & Francis Group, LLC ISSN: 2331-7000 online DOI: 10.1080/23317000.2015.1064794
Review of Design, Operating, and Financial Considerations in Flue Gas Desulfurization Systems ANDREAS POULLIKKAS * Department of Electrical Engineering, Cyprus University of Technology, Limassol, Cyprus
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Received April 2015, Accepted June 2015
Abstract: In this work, measures available for the reduction of sulfur dioxide stack emissions are discussed. The various flue gas desulfurization (FGD) technologies available in the market, for the reduction of sulfur dioxide emissions, are presented. The process descriptions are discussed and the capital and operating costs of the various methods are presented. Also, possible sources of raw materials required in each process and the viable means of disposal of the end products are discussed. In the FGD market throughout the world, limestone wet scrubbers take the lead, the byproduct of which is a marketable gypsum. Most of the second (other wet scrubbing) covers the similar process but produces a disposal product. This sector also includes the seawater scrubbing process. Other significant sectors include spray dry scrubbers, regenerable processes, and sorbent injection systems. Combined SO2 /NOx processes have a small share, and the trend is not expected to change. Keywords: flue gas desulfurization, sulfur dioxide emissions, wet scrubbers, spray dry scrubbers
1. Introduction Carbon monoxide, nitrogen oxides, volatile organic compounds, sulfur dioxide, and particulates are commonly referred as “criteria pollutants” because of their contribution to the formation of urban smog. These also have an impact on global climate, although their impact is limited because their radiative effects are indirect, since they do not directly act as greenhouse gases but react with other chemical compounds in the atmosphere. The combustion of fossil fuels, such as coal and heavy fuel oil (HFO), liberate three of the major air pollutants, such as sulfur dioxide (SO2 ), nitrogen oxides (NOX ), and particulates. Particulates can be removed satisfactorily by electrostatic precipitators or cyclones, whereas the nitrogen oxides emissions can be reduced by the use of low NOX burners. Sulfur dioxide emissions can be reduced by the removal of sulfur from the fuel before combustion, by the removal of sulfur dioxide during the combustion process, or by the removal of sulfur dioxide from the
© Andreas Poullikkas This is an Open Access article. Non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly attributed, cited, and is not altered, transformed, or built upon in any way, is permitted. The moral rights of the named author(s) have been asserted. *Address correspondence to: Andreas Poullikkas, Department of Electrical Engineering, Cyprus University of Technology, P.O. Box 50329, 3603 Limassol, Cyprus. Email:
[email protected]
flue gases after combustion.1 The pre-combustion controls comprise selection of low sulfur fuels and fuel desulfurization. The combustion controls are mainly for conventional coal-fired plants and involve in-furnace injection sorbents. The post-combustion controls are the flue gas desulfurization (FGD) processes.2 Although recent concerns on emissions control suggest the diversification from current fossil fuels technologies to more sustainable ones—that is, the use of renewable energy sources3 and the use of hydrogen4 —such energy system transformation is expected to be completed after 2030–2040. Thus it is necessary in parallel to the integration of the above sustainable technologies to improve the efficiency and to reduce the associated cost of emission control technologies, such as FGD processes. Based on the extensive literature review carried out for the purpose of this work, except in a few cases reported in Córdoba,2 Álvarez-Ayuso et al.,5 and Braden et al.,6 no concise review of the various FGD technologies is available. In particular, regarding FGD technologies, financial considerations as well as sources of raw materials and disposal of the end products are not available in the literature. In this work the different technologies concerning FGD are reviewed. Also, the associated capital and operating costs are analyzed, and a review of possible sources of raw materials required in each process and the viable means of disposal of the end products are discussed. In section 2, the sulfur dioxide emissions are reviewed. In section 3, an overview of the different available FGD technologies is presented, including process description as well as economic considerations. In section 4, the raw materials required in each process and end products are discussed. The conclusions are summarized in section 5.
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2. Sulfur Dioxide Emissions Sulfur dioxide enters the air mainly from industrial processes and from the combustion of hydrocarbon fuels with a substantial sulfur (S) content and represents a major source of air pollution. Sulfur dioxide is a colorless, corrosive gas that has a bitter taste, but no smell at low levels. The share of SO2 emissions comes from the use of coal and oil in fossil-fired power plants, in industrial combustion units, in small combustion units in households, and in vehicles. Although the concentration of SO2 in stack gases emitted by steam generation plants is usually in the range of 800 to 6000 mg/Nm3 , the volume of gases produced by the utility industry worldwide results in the liberation of a large tonnage of sulfur dioxide into the atmosphere.7 Sulfur dioxide is a chemical that can be dangerous in many ways. It is known to be lethal to humans at dose levels higher than 1000 mg/Nm3 for a period of over 10 minutes. At lower levels, sulfur dioxide has been found to be a corrosive irritant to eyes and skin. Sulfur dioxide has been associated with a variety of respiratory diseases and increased mortality rates. Inhalation of sulfur dioxide can cause increased airway resistance by constricting lung passages. Sulfur dioxide is also one of the main ingredients in acid rain. Acid rain occurs when sulfur dioxide or other gaseous chemicals, such as nitrogen oxides, are released into the air. These gases ascend through the atmosphere, and when they reach upper cloud levels, they react with water, oxygen, and sunlight.8 Various concentrations of sulfuric acid and nitric acid are then produced. These acids mix with the condensed water vapor in the clouds and fall to the ground with the water in various forms of precipitation. This precipitation with greater acidity is what is known as acid rain. Acid rain is dangerous to many natural ecosystems; it can harm plants, sand, make water undrinkable, and unlivable for many animals. The biggest producers of sulfur dioxide emissions are electrical utilities that produce electricity through the burning of fossil fuels.
Sulfur dioxide emissions, from combustion installations using coal or HFO, can be reduced by fuel desulfurization or by FGD, as well as by a combination of these two measures. Low sulfur content fuels with less than 0.5% S are available on the market, the cost of which is high. Flue gas desulfurization is an effective method in which the sulfur dioxide can be removed by the treatment of flue gases by a means of SO2 absorber.5 Although water is partially effective as a medium for SO2 absorption, with removal efficiencies up to 20%, solutions containing appropriate chemical absorbers or seawater have been shown to be practically effective, and sulfur dioxide removal efficiencies ranging from 50% to 99.8% have been achieved by treating waste gases with a variety of chemical reactants.9 When using a high sulfur content fuel (e.g., 3.5% S), the required SO2 removal efficiency to reduce the emission values from 5950 mg/Nm3 to 400 mg/Nm3 must be above 93%.
3. Flue Gas Desulfurization Technologies Flue gas desulfurization is an efficient method for the reduction of the sulfur dioxide emissions.2 Many processes are available in the market, such as (a) wet scrubbers, (b) spray dry scrubbers, (c) sorbent injection, (d) regenerable processes, and (e) combined SO2 /NOX removal processes. The different flue gas desulfurization available technologies are compared in Table 1. The distribution of existing FGD plants on coal-fired generating units is shown in Figure 1. In the FGD market throughout the world, limestone wet scrubbers take the lead, the byproduct of which is a marketable gypsum. Most of the second (other wet scrubbing) covers the similar process but produces a disposal product. “Other wet scrubbing” sector shown in Figure 1 includes the seawater scrubbing technology as well, whereas other important sectors include spray dry scrubbers, regenerable processes, and sorbent injection systems. Combined SO2 /NOx processes have a small share around the world with no expectation for
Table 1. FGD available technologies. Technology Wet scrubbers − Limestone scrubbing
Raw materials Limestone and water
− Lime scrubbing
Lime and water
− Seawater scrubbing
Seawater and limestone or lime Lime or calcium oxide and water Limestone or hydrated lime Limestone or lime or dolomite Sodium sulphite or ammonia
Spray dry scrubbers Sorbent injection processes Dry scrubbers Regenerable processes Combined SO2 /NOX removal processes
Byproducts
Max fuel S content
Max SO2 removal efficiency
Marketable gypsum, sludge, waste water Marketable gypsum, sludge, waste water Waste treated seawater
3.5%
95–99%
3.5%
95–99%
3.5%
90–95%
Mixture of calcium-sulfate, sulfite, and flying ash Mixture of calcium-sulfate, sulfite, and flying ash Mixture of calcium-sulfate, sulfite, and flying ash S or SO2 or H2 SO4 or ammonium sulfate
3.5%
90–95%
N/A
50%
1%
50%
3.5%
90–99%
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A. Poullikkas 60%
50%
45.4%
40%
36.3%
30%
20% 10.3% 10% 4.6%
3.5%
Spray dry scrubbing
Regenerable processes
Sorbent injection
4.3%
5.0%
Spray dry scrubbing
Regenerable processes
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0% Limestone/lime wet scrubbing
Other wet scrubbing
Fig. 1. Installed FGD plants on coal-fired generating units. 80% 70%
68.5%
60% 50% 40% 30% 20.9% 20% 10%
1.3%
0% Limestone/lime wet scrubbing
Other wet scrubbing
Sorbent injection
Fig. 2. Planned FGD plants on coal-fired generating units.
this share to change in the future.8 The planned FGD plants are shown in Figure 2, where the limestone/lime wet scrubbers retains their dominant role. Taken with the next largest sector, which is mainly the wet scrubbing disposal process, the family of wet scrubbers accounts for nearly 90% of new FGD plants.6 In this section, a review of these methods in terms of the process description, technical characteristics, and capital and operating costs is presented. Operating costs include the fuel cost, staff costs, insurance charges, rates, fixed maintenance, spare parts, chemicals, consumables, and water. 3.1 Wet Scrubbers Wet scrubbers are the most widely used FGD technology for SO2 control throughout the world. Calcium-sodium based sorbents have been used in a slurry mixture, which is injected
into a specially designed vessel to react with the SO2 in the flue gas. Commercial wet scrubbing systems are available in many variations and proprietary designs. Systems currently in operation include (a) lime/limestone/sludge wet scrubbers, (b) lime/limestone/gypsum wet scrubbers, and (c) seawater wet scrubbers. Wet scrubbers can achieve removal efficiencies as high as 99%.10 The preferred sorbent in operating wet scrubbers is limestone (CaCO3 ), followed by lime (CaO). In (a), lime or limestone is used as a raw material, and the sludge byproduct produced is disposed of. The second system (b) is the most favored because of its availability, relative low cost, and ability to produce a marketable gypsum. The process is designed to produce a high-quality product (gypsum) suitable for use as raw material in various industries. It is estimated that with the increased cost of land filling in Europe and the introduction of increasingly
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stricter regulations regarding byproduct disposal, wet scrubbers producing marketable gypsum will overtake all other FGD technologies. The last option (c) is a convenient means for removing sulfur dioxide emissions from coastal power stations. The sorbent used is seawater which, after absorption, is treated with limestone or lime or seawater before discharge back to the sea. 3.1.1 Limestone Scrubbing In the simplest configuration in wet limestone/gypsum scrubbers, systems (a) and (b), all chemical reactions take place in a single integrated absorber, resulting in reduced capital cost and energy consumption. The integrated single tower system requires less space, thus making it easier to retrofit in existing plants. The absorber usually requires a rubber, stainless steel, or nickel alloy lining as a construction material to control corrosion and abrasion. Fiberglass scrubbers are also in operation.11 The overall chemical reaction, which occurs with a limestone sorbent, can be expressed in a simple form as
Table 3. Options for the use of FGD gypsum. Sector
Application
Gypsum industry
Cement industry Building material industry Road construction Landscaping Mining Fertilizer Disposal
Table 4. Available wet scrubber designs. Scrubber design
Details
Spray tower
SO2 + CaCO3 −→ CaSO3 + CO2 . In practice, air in the flue gases causes some oxidation, and the final reaction product is a wet mixture of calcium sulfate (CaSO4 ) and calcium sulfite (CaSO3 ) as sludge. A forced oxidation step, in the scrubber or in a separate reaction chamber, involving the injection of air produces the saleable byproduct, gypsum (CaSO4 • 2H2 O), by the following reaction:
Plate tower Impingement
SO2 + CaCO3 + 2 O2 + 2 H2 O −→ CaSO4 • 2 H2 O + CO2 The typical composition of FGD gypsum, compared with natural gypsum, is shown in Table 2, and the various possibilities for the use of gypsum are tabulated in Table 3. The various scrubber designs available are presented in Table 4. The principal flow diagram of the limestone scrubbing method is shown in Figure 3. The main stages of the method are the preparation of raw materials, the absorption of the pollutants, the adjustment of the pH of the scrubber suspension, the oxidation of
Packed tower
Fluidized packed tower
Table 2. Typical composition (by mass) of natural and FGD gypsum.
Moisture Gypsum Cl Na2 O MgO (soluble in water) Fe F (soluble in water) SO2 CO2 K2 O (soluble in water) pH
Natural
FGD
1% 78–95%
