Brad Buecker, Kiewit Power Engineers, USA
Environmental personnel in many countries recognize the importance of controlling pollutant emissions from coal fired power plants. A primary pollutant is, of course, sulphur dioxide (SO2). Scrubbing is by far the most common method to remove SO2 from flue gas, where many new scrubbers will utilize wet-limestone technology because of the reliability ofthe process.
This article outlines a number of fundamental principles of wet-limestone scrubbing, which will hopefully prove useful to plant personnel who may be asked to plunge into unfamiliar territory. Critical aspects of operation include ensuring proper SO2 removal at all times, maximizing reagent utilization and minimizing scale build-up on scrubber components.
A generic flow diagram of a spray-tower, wet-limestone scrubber is illustrated in Figure 1. The process is a classic example of an aqueous acid-base chemistry reaction applied on an industrial scale, where alkaline limestone slurry reacts with acidic SO2.
|Figure 1: Generic wet-limestone FGD process flow diagram for an open spray tower|
Sulphur dioxide is first absorbed into the liquid phase as it contacts the slurry sprays.
SO2 + H2O <=> H2SO3
Some theoretical chemists argue that true H2SO3 does not exist and that SO2 retains its molecular character and is surrounded by water molecules. However, when SO2 is added to water the pH drops, which suggests this equation is reasonable and that the following dissociation reaction is accurate.
H2SO3 <=> H+ + HSO3– <=> H+ + SO3-2
Another argument for the formation of H2SO3, and its dissociated products bisulphite (HSO3–) and sulphite (SO3-2) ions, comes from the fact that the principal component of limestone, calcium carbonate (CaCO3), is only slightly soluble in water but will dissolve almost completely in well-designed scrubbing systems.
CaCO3 + H+ → Ca+2 + HCO3–
Combining these three equations illustrates the simplified but fundamental wet-limestone scrubbing process.
CaCO3 + 2H+ + SO3-2 → Ca+2 + SO3-2 + H2O + CO2↑
In the absence of any other reactants, calcium and sulphite ions will precipitate as a hemihydrate, where water is actually included in the crystal lattice of the scrubber byproduct.
Ca+2 + SO3-2 + ½H2O → CaSO3·½H2O↓
Proper operation of a scrubber is dependent upon the efficiency of the above-listed reactions. Control of pH via reagent feed is very important. Many wet-limestone scrubbers operate at a solution pH of around 5.6 to 5.8. A too-acidic scrubbing solution inhibits SO2 transfer from gas to liquid; while an excessively basic slurry (pH > 6.0) indicates overfeed of limestone.
Oxygen in the flue gas greatly influences chemistry. Aqueous bisulphite and sulphite ions react with oxygen to produce sulphate ions (SO4-2).
2SO3-2 + O2 → 2SO4-2
Approximately the first 15 per cent mole of sulphate ions co-precipitates with sulphite to form calcium sulphite-sulphate hemihydrate [(0.85CaSO3·0.15CaSO4)·½H2O]. Any sulphate above the 15 per cent mole ratio precipitates with calcium as gypsum.
Ca+2 + SO4-2 + 2H2O → CaSO4·2H2O↓
Control of by-product chemistry offers interesting challenges, particularly in spray towers that have internal devices to enhance gas-liquid contact. Experience has shown that operation in either a completely oxidized state (no calcium sulphite-sulphate hemihydrate in the scrubbing slurry) or a completely non-oxidized state (no gypsum in the slurry) minimizes scaling of internal scrubber components.
An often critical factor regarding the choice of oxidized or non-oxidized by-product involves the handling characteristics and commercial value of the solid. Calcium sulphite-sulphate hemihydrate is a soft material that tends to retain water. It has little value as a chemical commodity
For this reason, many scrubbers are equipped with forced-air oxidation systems to introduce additional oxygen to the scrubber slurry. A properly designed oxidation system will convert all of the liquid sulphite ions to sulphate ions.
Sulphate, of course, precipitates with calcium as gypsum, which forms a cake-like material when subjected to vacuum filtration. Generally, 85 per cent to 90 per cent of the free moisture in gypsum can be extracted by this relatively simple mechanical process. High-purity, dried synthetic gypsum has become a favourite material of wallboard manufacturers.
Critical Scrubber Design Issues
An important concept regarding spray tower scrubbers is the liquid-to-gas (L/G) ratio. The proper amount of slurry must be present to allow SO2 to pass from the gas phase to the liquid phase. In the United States, the common unit of measurement for the L/G ratio is gallons per minute (gpm) of slurry per 1000 actual cubic feet per minute (acfm) of flue gas, where a baseline rule-of-thumb was once 120 gpm/acfm (16.1 lpm/m3).
The L/G ratio is strongly influenced by the efficiency of liquid/gas mixing within spray towers. First and second-generation scrubbers typically contained internal trays or packing to enhance mixing and lower the L/G ratio. Unfortunately, these internal mixing devices served as excellent locations for scale build-up and deposit formation. As packing fouls, tower differential pressure increases, which may lead to a unit derating or perhaps even a forced outage.
Spraying technology has improved for scrubber applications, and open spray towers are now becoming popular,1 or at most with one tray section. Nozzle design and alignment are critical in these systems, as droplet size and spray patterns must be optimized to provide the best contact and to prevent channeling of the flue gas. That the spray nozzle design has become much better2 is the key to this development.
Limestone utilization in the scrubber is another important issue. If the limestone reacts poorly in the system, overfeed is required for adequate SO2 removal. This of course results in excess reagent usage. Limestone costs for large scrubbers may reach or exceed a million dollars per year, so loss of reagent because of poor utilization can be rather expensive.
Factors that influence utilization include limestone quality, limestone grind size, residence time of the reagent within the scrubbing system and performance of slurry separation devices. Limestone reactivity is important with regard to scrubber operation. The chemical make-up of the reagent has a large influence on scrubber efficiency. In my experience, I have found that limestones containing 94 per cent or more CaCO3 are very reactive, given of course that they are ground properly.
But not all plants are near high-quality limestone sources. Often, a stone may contain greater than 90 per cent total carbonate, but 10 per cent or more exists as dolomite (MgCO3·CaCO3), in which the magnesium is bonded with calcium in the crystal lattice. While pure MgCO3 dissolves quickly in scrubber solutions, and provides liquid alkalinity, dolomite is rather non-reactive, and tends to pass through the system untouched. Utilities without access to high-purity limestone may need to enhance slurry reactivity with additives. One choice in the past was adding magnesium salts.
Magnesium compounds, other than dolomite, generally dissociate quickly in scrubber solutions, whereupon the magnesium increases the solution alkalinity and enhances the transfer of SO2 from the initial liquid phase to a reaction product with calcium. A popular organic additive is adipic acid (hexanedioic acid), as shown in Figure 2, or the less refined material dibasic acid (DBA), which is a mixture of adipic, glutaric, and succinic acids.3
|Figure 2: The general structure of adipic acid|
Like magnesium, these chemicals also increase the liquid phase alkalinity, where concentrations of a few hundred parts per million often are sufficient to achieve suitable results. In the US, DBA manufacturing has declined, so some utilities have had to switch to sodium formate as an additive.
By-product disposal greatly influences scrubber design. Most flue gas desulphurization (FGD) systems are of the forced-oxidation type to produce a gypsum by-product. A primary motivating factor for gypsum production is the potential sale of the material to wallboard manufacturers.
Preparation of gypsum for sale requires drying on a vacuum drum or belt filter, where often a fresh water wash is utilized to remove dissolved chlorides. Forced oxidation normally produces flat gypsum crystals that dewater quite readily, in which the gypsum forms a cake-like material with perhaps 10 per cent moisture.
The drum in Figure 3 has an internal, mechanically applied vacuum, such that as the drum rotates through the slurry by-product contained in a vat below, the solids adhere to the filter cloth while the liquid passes through for collection and reuse in the scrubber or disposal. The drum rotates counter-clockwise in the view shown. Not shown is a fresh water wash header on the far side located just above the vat. The water spray reduces the chloride content of the gypsum to less than 200 mg/l, which is a requirement from the wallboard manufacturer. In some cases, the manufacturer may request a chloride limit of 100 mg/l.
|Figure 3: Gypsum cake being scraped from a vacuum filter|
When by-product sale is not an option, forced oxidation may still be a good technique, as gypsum handling is very straightforward compared to inhibited or partially-oxidized material. Gypsum can be easily loaded into dump trucks for disposal in a landfill. Some utilities own enough land for by-product retention ponds to be excavated and lined to serve as evaporation ponds so that liquid discharges are not an issue. In this case, forced oxidation may be counter-productive.
Another important component of spray towers is the mist eliminator section or sections. Mist eliminators are vital to remove particulates that would otherwise exit the scrubber through the stack. Typically, the mist eliminator is of a chevron-vane design, which forces the flue gas to make several turns before exiting the scrubber.
Particulates impact upon the vanes and lodge on the material. Mist eliminators must be equipped with a water washing system to remove accumulated particulates, but the wash system requires special care in the design phase. Poor nozzle patterns or low wash rates will allow material to completely bridge areas of the packing. Alternatively, excess washing can upset the water balance in the scrubber.
FGD Chemistry Monitoring
Large manuals have been written on FGD analytical techniques, and space limitations prohibit a full discussion of these techniques here. Thermogravimetry is an excellent technique for monitoring the gypsum, calcium sulphite-sulphate hemihydrate, and CaCO3 content in scrubber solids,4 and in many cases it works well for limestone analyses.
Limestone reactivity testing, where limestone samples are treated with acid in a controlled environment, can provide valuable data on the quality of the material. Ion chromatography is an excellent technique for monitoring chloride concentrations in the scrubber liquor, as chloride is a primary corrosive agent in FGD systems.
On-line pH monitoring is a standard technique, where instrument reliability has improved over the years. Slurry densities and suspended solids concentrations, and limestone grind sizing, are other important tests that can easily be performed in the laboratory.
New Scrubbing Technologies
Several other scrubbing technologies, some of which have been around for a decade or more, are gaining prominence.5 One of these is the Chiyoda process (CT-121TM, Figure 4), in which the incoming flue gas is quenched and then directed into a slurry bath. Sulphur dioxide reacts with the slurry, while being sparged with air to convert the by-product to gypsum. The clean gas bubbles out of solution, flows through riser tubes to the outlet plenum, and then exits to the stack.
Figure 4: A schematic of the Chiyoda process Source: Chiyoda Corporation
Design and operational features include:
- Absence of spray nozzles;
- L/G ratio becomes a non-factor;
- Very low particulate carryover to the mist eliminators and outlet flue gas;
- Reaction vessel much shorter than spray towers;
- Operates at a lower pH than spray towers, which enhances limestone utilization and forced-air oxidation efficiency;
- Up to 80 per cent efficient at removing sulphur trioxide (SO3) from the flue gas;
- Less pumping capacity but more fan capacity required.
A unique feature is that the slurry level can be raised or lowered, such that the depth in which the spargers protrude into the slurry is adjustable. During periods of low load, shallow sparger immersion may be sufficient. At high load, deeper immersion is possible.
At least two other, non-spray tower alternatives have attracted interest. These are Alstom’s Flowpac process and Advatech’s Double Contact Flow Scrubber (DCFS). The former also utilizes a slurry bath, but where compressed air is utilized not only for forced oxidation but to induce rapid slurry-gas mixing.
In the DCFS, the flue gas first counter-currently contacts a slurry fountain followed by co-current contact with a separate fountain. All three systems have been installed at plants in the US, and are certainly worth investigation.
1. Startup and Initial Operation of the Wet Limestone FGD Retrofit at V. Y. Dallman Station Units 31 & 32, C. Weilert, B. Basel, and P. Dyer, from the Proceedings of POWER-GEN International, December 11-13, 2001, Las Vegas, Nevada, USA.
2. Maximize the Performance of Your Spray Nozzle System, C. Pagcatipunan and R. Schick, Chemical Engineering Progress, pp.38-44, December 2005.
3. R. Rhudy, Project Manager, FGD Optimization Workbook, Report TR-111118, Electric Power Research Institute, Palo Alto, California, July 1998.
4. Wet-Limestone FGD Solids Analysis by Thermogravimetry, B. Buecker, from the Proceedings of the 24th Annual Electric Utility Chemistry Workshop, May 11-13, 2004, Champaign, IL, USA.
5. Advanced Technologies for the Control of Sulphur Dioxide Emissions from Coal-Fired Boilers; Topical Report Number 12 published by the US Department of Energy, June 1999.
Brad Buecker is a process specialist with Kiewit Power Engineers, Lenexa, Kansas, USA. He is also a contributing editor for PEi’s sister publication Power Engineering magazine. Buecker has nearly 30 years of experience in the power industry, much of it in chemistry, water treatment, air quality control, and results engineering positions. He has written many articles and three books for PennWell on steam generation topics.
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