Flocculation Treatments for Flue-Gas-Derived Water from

ARTICLE pubs.acs.org/IECR

Coagulation/Flocculation Treatments for Flue-Gas-Derived Water from Oxyfuel Power Production with CO2 Capture Sivaram Harendra,* Danylo Oryshchyn, Thomas Ochs, Stephen Gerdemann, John Clark, and Cathy Summers Process Development Division, National Energy Technology Laboratory, U.S. Department of Energy, 1450 Queens Avenue SW, Albany, Oregon 97321, United States ABSTRACT: Capturing CO2 from fossil fuel combustion provides an opportunity for tapping a significant water source that can be used as service water for a capture-ready power plant and its peripherals: more than 5% of the mass of water required for coolingtower makeup in an oxy-fired plant employing integrated pollutant removal (IPR) for capture. Water condensed from oxycombustion flue gas by the National Energy Technology Laboratory’s (NETL’s) integrated pollutant removal (IPR) CO2capture process has been analyzed for composition, and an approach for its treatment, for both in-process reuse and release, has been outlined. Experiments were performed to develop specifications for the first step (coagulation/flocculation) of this treatment approach. The results show that flocculation can remove most cations and reduce fine particulates by at least 90%. The speed of separation points to fast, in-line treatment of water for reuse within IPR, thus minimizing the water requirements for CO2 capture. In experiments, flocculation/coagulation removed few of the anions from solution. However, the remaining supernatant is amenable to reverse osmosis, crystallization, and ion-exchange processes for anion removal and cleanup of the remaining cations.

1. INTRODUCTION The concentration of carbon dioxide in Earth’s atmosphere has been rising for at least 50 years and probably longer. This rise has been directly linked to the burning of fossil fuels.1 In the United States alone, over 1.6 billion tons of CO2 is produced each year from power plants. A 1000 MW pulverized-coal-fired power plant can emit up to 68 million tons of CO2 annually. At a comparable power output, an oil-fired power plant emits about 25% less, and a natural gas combined-cycle power plant emits about 50% of the CO2 emissions that come from coal-powered plants.2,3 Although the choice of fuel is one strategy for responding to the rising concentration of CO2 in Earth's atmosphere, actual capture of the CO2 from combustion is another. Many methods have been proposed to capture CO2 from the flue gases of power plants and store it in suitable reservoirs.13 Researchers at the National Energy Technology Laboratory (NETL) have patented a process, called integrated pollutant removal (IPR) (Figure 1), that uses off-the-shelf technology to produce a sequestration-ready CO2 stream from an oxycombustion power plant. The IPR process uses direct or indirect heat exchange followed by several stages of compression, with intercooling (also employing direct and/or indirect heat exchange) between compression stages. The water that is condensed during recovery of the latent and sensible heat from the process stream has many potential applications. This study examines the first step in the treatment process carried out on water recovered from IPR. Under normal operating conditions, coal combustion and numerous other industrial processes produce sulfur dioxide emissions.3 The first operation in the IPR process implemented at the Jupiter Hammond Burner Test Facility, which was the source for the data obtained in this study, is a spray tower that absorbs heat from the flue gas while also removing sulfur dioxide r 2011 American Chemical Society

(and other soluble or entrainable components) from the flue gas. The resulting solution is water, including fly ash particles (containing heavy metals), cations, and anions (predominantly soluble sulfur species). Calculations comparing the flue-gas water production rate to water lost to cooling-tower drift, evaporation, and blowdown and assuming that 90% of the water available in the flue gas can be captured and treated for reuse show that the available water is on the order of 5% of the water required to make up for these cooling-water losses. Coagulation/flocculation was chosen to treat heavy metals from condensed flue-gas water. Relatively low in cost and energy demand, coagulation/flocculation is an essential process in the treatment of water and industrial wastewater.46 Coagulant dosages in the experiments discussed herein varied over a wide range, aiming at maximum removal efficiency of pollutants using minimum doses at optimum pH. Coagulant type, dosage, water composition, pH, and contaminant type have been found to be the most important variables affecting removal efficiency.5 Water treatment using a coagulation process can be accomplished either by dosing the solution with a coagulant or by generating the coagulants in situ through the electrolytic oxidation of an appropriate anode material (e.g., iron or aluminum), which is called electrocoagulation.6 This study is limited to the conventional coagulation dosing methodology. Inorganic soluble components (contributing hardness and alkalinity to the water) are considered in the flocculation mechanism only to the extent that they might be either deliberately or inadvertently precipitated (deliberately through addition of chemicals for softening or Received: May 9, 2011 Accepted: August 2, 2011 Revised: July 22, 2011 Published: August 02, 2011 10335

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Industrial & Engineering Chemistry Research

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Figure 1. Schematic illustration of the integrated pollutant removal system: Hi = heat exchanger, where i = 1, 2, 3, 4, or 5; Vi = vessel, where i = 4, 8, or 11; STGi = stage of compression, where i = 1, 2, 3, or 4; FGR = flue-gas recirculation duct; TWR = spray tower; inverted triangles = coalescing filters.

pH control or inadvertently through side reactions caused by chemicals added to control other factors such as flocculants, inhibitors, and preservatives).7 The coagulation process increases particle settling by forming large aggregates from small particles. Two common methods of particle destabilization for the coagulation of colloid particles are charge neutralization and the sweep floc mechanism. Charge neutralization is the result of specific chemical reactions between positively charged coagulants and the negatively charged colloids, resulting in particle precipitation. Coagulation by charge neutralization can be accomplished over a narrow pH range (45.5). On the other hand, the sweep floc mechanism is effective in the pH range of (68), where conditions are ripe for the rapid formation of amorphous solid phases.8 In the sweep floc mechanism, the removal of turbidity occurs by adsorption of contaminants onto the amorphous solid phases. Extensive previous studies to promote the aggregation of particles have focused on the behavior of traditional coagulants such as iron and aluminum.69 Recently, there has been a growing interest in the use of alternative additives.9 For treatment of a system containing multiple contaminants such as colloidal particles and heavy metals using iron or aluminum salts, the overall reaction among these constituents can be considered to involve competing reactions of hydroxide and other organic/inorganic ligands for complexation with free metal ions and their hydrolysis products.10 Ferric salts have been recognized as an effective scavenger of heavy metals. It has also been recognized that metals, primarily in ferric, hydroxide, and oxide coatings of soil and sediments, play an important role in the transport, absorption, biotransformation, and ultimate fate of trace constituents in natural systems.11 At neutral to alkaline pH, ferric salts precipitate as amorphous hydrated oxides or oxyhydroxides that have relatively stable and reproducible surface properties. Upon aging, the precipitate gradually transforms into crystalline iron oxide, but its absorptive properties remain quite similar.11

Settling rates of colloidal particles are often enhanced through the use of coagulant aids (or flocculants). Sometimes, excess primary coagulant is added to promote large floc sizes and high settling rates. However, in some waters, even large doses of primary coagulant do not produce flocs. In these cases, a polymeric coagulant aid (flocculent) can be added after the coagulant to hasten reactions and/or to produce a denser floc, thereby reducing the amount of primary coagulant required. Through polymer “bridging,” small floc particles agglomerate rapidly into larger more cohesive flocs that settle rapidly. Coagulant aids also help to create satisfactory coagulation over a broader pH range. Generally, the most effective types of coagulant aids are slightly anionic polyacrylamides with very high molecular weights. In some clarification systems, nonionic or cationic types have proven effective.1214

2. BACKGROUND Jupiter Oxygen Corporation, with support from NETL, has constructed a multifuel air- or oxygen-fired burner test facility for the testing of burners with capacities of up to approximately 60 MMBTU (million British thermal units)/h, known as the Jupiter Hammond Burner Test Facility (JHBTF). The boiler is a B&W (Babcock & Wilcox) water-tube package boiler designed to produce up to 27000 kg/h of superheated steam at 95 MPa and 235 °C. The system can be fired with natural gas and with pulverized coal. The typical composition of the coal used is listed in Table 1. The chemical properties of fly ash particles (reported in Table 2) were analyzed using ASTM standards. The oxides SiO2, Al2O3, and Fe2O3 were found to make up the bulk of the material present in the fly ash particles. The goal of oxycombustion followed by integrated pollutant removal (IPR) is capturing CO2 and preparing it for transportation while minimizing the energy usage and costs (both capital and operational). IPR is a process that captures the combustion gas 10336

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Table 1. Typical Composition of Analyzed Coal Samples component

as-received (wt %)

moisture

dry (wt %)

4.12

0

ash

12.47

13.01

volatile matter

34.62

36.11

fixed carbon

48.79

50.88

sulfur

3.48

3.63

carbon

66.67

69.53

hydrogen

4.53

4.73

nitrogen oxygen

1.37 7.36

1.43 7.67

Table 2. Typical Oxides in Fly Ash elements as oxides SiO2 Al2O3 Fe2O3 CaO K2O SO3 other oxides

content (wt %) 50.93 19.44 18.11 4.74 2.26 1.88 2.64

from an oxy-fired fossil fuel power plant and compresses it for delivery. The cooling required to maximize the compression efficiency is leveraged for heat recovery back to the power plant by integrating heat exchange required for compressor intercooling with heat exchange required for heating boiler feedwater and the general process.15 Condensed water resulting from this cooling removes soluble gases, delivering relatively pure CO2 at a pressure suitable for sequestration. NETL has installed a 45 kg/h slipstream IPR system for use in conjunction with the oxy-fired burner test facility built at Jupiter Oxygen Corporation in Hammond, IN. Figure 1 presents a schematic description of the IPR process as it exists at the JHBTF. A slipstream system of the oxycombustion flue gas enters from a tap on the flue gas recirculation (FGR) duct (recirculated flue gas is used as a tool for flame-temperature control). This gas enters the IPR system at about 240 °C and flows into a glass pipe spray tower. In the tower, the gas rises in a countercurrent fashion through a spray stream of recycled fluegas water. The water that recirculates through the spray tower is buffered by periodically injecting a sodium carbonate solution. The buffered solution is then sent back to the top of the tower, where it is sprayed into the upflowing oxyfuel gas stream, condensing and cleaning the ash-laden gas. The balance of the water leaving the tower (the part that is not recirculated for use as tower spray) is removed from the process for treatment outside the IPR process. A blower moves the cleaner gas through the first set of filters (H1) and then enters the first compression stage (STG1). The pressurized gas is cooled (through counterflow indirect-contact heat exchange), and the flue-gas water condenses out and is gathered into a vessel (V4 in the first step). The IPR process installed at the Jupiter site employs four stages of compression (with pressures of 0.3, 1.4, 4.1, and 13.8 MPa). After each compression stage, cooling/condensation removes more flue-gas water and some soluble gases, resulting in a dry, clean CO2 product that is captured for analysis. An example of the amount of water available in coal flue gas can be seen in a model of a power plant refitted for

oxycombustion and an IPR computer model that was scaled to 600 MW running at 85% capacity from an approximate 344 MW oxy-fired retrofit plant model running at 100% capacity. Assuming that virtually all of the water is captured during IPR, the total flue-gas water production is 280000 kg/h. Through proper water treatment, it is possible to recover more than 90% of the flue-gascondensed water. This makes 250000 kg/h (∼2190 million kg/year) of cleaned water available to offset the water needs of the power plant. This analysis applies to a relatively low-moisture coal, so the amount of water captured will increase as the coal rank and moisture content of the coal goes down. Water experiments were performed to study the first operation in the process mapped out to treat water recovered from an IPR system. The objective of the experiments described here was to determine the coagulation/flocculation process efficiency for removing heavy metals and other particulates present in wastewater from the IPR process. The series of wastewater treatment processing steps targeted were designed to achieve a zero-liquid-discharge (ZLD) system.16

3. MATERIALS AND METHODS Based on a literature review, iron(III) chloride hexahydrate and aluminum sulfate octadecahydrate were chosen as coagulation materials for this study. Anionic poly(acrylamide) was employed as a coagulant aid (trade name Chemway 540). A Phipps & Bird six-paddle stirrer designed to meet ASTM standard method D2035-80R03 for the coagulationflocculation jar test was used for the experiments. Characterization. To characterize and analyze the IPR water samples in terms of turbidity and pH, the following test methods were employed: (a) The turbidity of wastewater samples was measured using turbidity meter (LaMotte Company, model 2020 e/i). This turbidity meter was calibrated using standard solutions of 0.1 NTU (nephelometric turbidity unit) and 0 NTU. (b) The pH values of solutions with different concentrations of surfactant were measured using a portable pH/ISE meter (Orion Research Inc., model 290 A). The pH meter was calibrated on a regular basis using standard solutions of pH 4, 7, and 10. 4. PROCEDURE This study was performed in early 2010, using flue-gas water collected during December 2009 at the JHBTF. IPR water samples were collected during IPR test runs at the following locations (shown in Figure 1): after the first heat exchanger (below the spray tower), H1; after buffering, B1; after stage 1 (∼0.3 MPa), V4; and after stage 2 (∼1.4 MPa), V8. The sampling protocol for wastewater collection was as follows: The outlet of the sample valve was immediately wiped using isopropyl alcohol and then was rinsed with either deionized or distilled water. Ports were dried as needed, with clean Kimwipes. Samples were taken immediately after the valve outlet had been cleaned. All sample bottles were filled with minimal air space. For those samples intended for metal analysis (including Hg), sufficient HNO3 was added to achieve a pH of less than 2. This step was performed within 15 min of sample collection. After this pH adjustment, the samples were placed in a container with ice. Any samples not requiring acid preservation were placed in the container with ice immediately after being collected. The samples collected from H1 and B1 were laden with fly ash particles. These water samples were blackish in color. Periodically, V4 and V8 collection vessels were opened to collect water samples. Gas in the vessel headspace was reinjected into the IPR gas. 10337



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Figure 2. Average concentrations of elements in wastewater collected in December 2009.

Essentially no wastewater was seen in the vessel in the highpressure stages (V11). The samples collected in the abovementioned locations were analyzed for 31 elements using inductively coupled plasma (ICP) spectroscopy. Specifically, the analysis included the following elements: Al, As, B, Ba, Be, Ca, Cd, Ce, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sn, Sr, Ti, Tl, V, and Zn. Elements Al, As, B, Ba, Ca, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, S, and Si were found at concentrations in excess of those prescribed by the U.S. Environmental Protection Agency for wastewater allowed at the inlet to municipal water-treatment plants. These elements were thus studied further as part of the comprehensive water treatment plan. Coagulation and flocculation experiments were conducted. Experimental Section. To determine the optimum coagulant dosage, 2-L wastewater samples collected at H1 and B1 from the IPR facility (collected in December 2009) were inoculated with 100, 250, 500, 750, 1000, 10000, and 15000 mg of coagulant (either ferric chloride or alum). After being mixed rapidly for 3 min at 150 rpm and then slowly for 30 min at 30 rpm, the liquid was left unstirred for 1 h. Following this settling time, the supernatant was withdrawn from a point located about 5 cm below the top of the liquid level of the beaker to determine the turbidity, giving a measure of the effect of the coagulant dose. Analytical errors in the evaluation of material concentrations in the water were determined to be less than (5%. Samples were replicated three times in each experiment. The pH value of each 1-L wastewater sample was adjusted to a pH 311 using 1.0 M HCl or 1.0 M NaOH, before addition of coagulant to the sample. After stirring and clarification, the supernatant was withdrawn for analysis. From pH experiments, FeCl3 was found to be effective between pH 7 and 10, whereas alum was effective between pH 6 and 8.

average of 36 to 5 ppm across the spray tower. NOx compounds remained largely unaffected, at an average concentration of 1000 ppm. The initial species present in the wastewater are shown in Figure 2. The average initial turbidity of the wastewater samples was 356 NTU. From dosage experiments, it was found that 2501000 mg of FeCl3 yielded more than 95% turbidity removal from the 1-L wastewater samples and 2501000 mg of alum yielded more than 90% turbidity removal. Alum produced a lower volume of sludge than ferric chloride. Some of the elements of greatest concern are discussed in detail in the remainder of this section. In the experiments relevant to this discussion, the dosage of coagulant was proportional to the initial turbidity of the water samples. The average concentrations of dissolved materials are shown in Figure 2. Sulfur was the most abundant element found in the fluegas water (Figure 2). Water samples collected at H1 (under the spray tower) had the highest sulfur content of 511 g/L. Sulfur analysis of the city water used to prepare the tower buffer solution (and to initially prime the spray-tower portion of IPR) yielded a value of 12.8 mg/L, which is clearly negligible compared to the amount of sulfur present in the wastewater samples. Sulfur is present in coal and forms sulfur dioxide and sulfur trioxide during combustion. In the tower, an initial charge of sodium carbonate (Na2CO3) reacts directly with the SO2 to form sodium sulfite and CO2. The sulfite then reacts with more SO2 and water to form sodium bisulfate. Some of the sodium sulfite is oxidized by excess oxygen in the flue gas to form sodium sulfate (Na2SO4). The following are the reactions occurring during the absorption process:

5. RESULTS AND DISCUSSION The spray tower is effective at removing sulfur species and the majority of the water from the flue-gas stream. SO2 was reduced from an average of 9500 ppm in the flue gas to 460 ppm after the stream had passed through the spray tower, giving a 95% reduction in the SO2 levels. Water in the flue gas was reduced from an average of 30 to 6.5 vol %. The HCl concentration was reduced from an

hydrolysis of SO 2 SO2 ðaqÞ þ H2 O T HSO3 þ Hþ HCO3 þ Hþ T CO2 ðaqÞ þ H2 O

dissolution of gaseous SO2 SO2 ðgÞ T SO2 ðaqÞ

oxidation of sulfur O2 ðgÞ þ 2HSO3 f 2SO4 þ 2Hþ 10338



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SO2 can also oxidize in the atmosphere to produce gaseous sulfur trioxide (SO3). Sulfur trioxide reactions are as follows: 2SO2 ðgÞ þ O2 ðgÞ f 2SO3 ðgÞ SO3 ðgÞ þ H2 OðgÞ f H2 SO4 ðlÞ Sodium was the second most common element found in the wastewater samples collected after the heat exchanger (H1) and buffering (B1) points (Figure 2), where the concentrations were 35 and 10 g/L, respectively. The most likely source of the sodium is the Na2CO3 buffer solution added to tower spray water for sulfur removal. The sodium content in city water was tested as a baseline and was found to be 9.51 mg/L. The buffer solution prepared from this water had a concentration of 1 M. There was an increase in chromium concentration in water from the higher-pressure IPR stages as compared to the lowerpressure stages. Considering that the pH levels in the highpressure areas are low (i.e., acidic), it is suspected that one source of the chromium is corrosion of stainless steel piping and equipment. This is not unexpected, as 316 steel stainless is known to interact with sulfuric acid. The iron and copper concentration profiles closely followed the chromium concentration profile. At pH values below about 5, both iron and copper corrode rapidly and uniformly. The pH of liquid samples collected at sites V4 and V8 was generally 1. Thus, it is suspected that one source of the iron and copper is the corrosion of metallic equipment. Again, this is not unexpected. Electrochemical corrosion measurements will be used to track the locations and operational settings with respect to corrosion rates to provide data that will be used to optimize material choices for different parts of larger-scale iterations of the IPR system. Concentrations of Mn in water collected at H1 and B1 were 0.2 mg/L. Manganese also had higher concentrations in the V4 and V8 samples as compared to the H1 and B1 samples. Divalent cations such as calcium and magnesium were also present at significant levels in the IPR water. The concentrations of calcium and magnesium were 28.75 and 12.85 mg/L, respectively. The total water hardness, including both Ca2+ and Mg2+ ions, reported in parts per million (ppm) of calcium carbonate (CaCO3), is approximately 152 ppm. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent divalent metal ions), iron, aluminum, and manganese can be added to the hardness calculation if they are high in concentration. General guidelines for hardness classification based on calcium carbonate are as follows: 060 mg/L, soft; 61120 mg/L, moderately hard; 121180 mg/L, hard; >180 mg/L, very hard.15 Flue-gas water from the IPR system is thus considered moderately hard. pH. The average pH of water samples collected from H1 (see Figure 3) was 6.36, whereas it was 6.71 for those collected from B1. Our aim was to maintain the solution pH between 5 and 7 using sodium carbonate buffer solution to strip more SO2 from flue gas and to support other possible reactions. From previous results, the average pH values of the V4 and V8 samples were 1.0 and 0.2, respectively. It is suspected this was due to the formation of H2SO4 at high pressure and high temperature. After the H1derived wastewater had been treated with coagulant, the pH increased from 6.36 to 9.4. The final pH of the water exiting the full treatment process will be neutral, which is considered to be the general treatment target for ZLD systems. Alkalinity. Alkalinity is a measure of the ability of water to neutralize acids and bases and predicts the formation of

Figure 3. pH and alkalinity variations of wastewater collected.

Figure 4. Comparison of turbidity removals using FeCl3 and alum for a wastewater sample collected in December 2009.

carbonate precipitates. Alkalinities are compared in Figure 3. High alkalinity values were recorded because of the sodium carbonate buffer solution. The average alkalinity of after buffering (B1) and after the heat exchanger (H1) were 20000 and 12000 mg/L CaCO3, respectively After the wastewater collected from the heat exchanger had been treated, the alkalinity increased to 16600 mg/L CaCO3. Water with high alkalinity can cause scale buildup in plumbing. The removal of alkalinity-related ions can be achieved either through ion-exchange resins or reverse-osmosis units. The preferred process at this point is reverse osmosis. Ferric chloride was found to achieve turbidity removal of more than 99% (Figure 4). Turbidity removal values for different wastewater samples are shown in Figure 5, which reports results for flue-gas water dosed with 2 g of FeCl3 per liter. Elemental removal efficiencies from this dosage are shown in Figure 6. More than 50% of heavy metals such as Al, Ba, Ca, Cr, Fe, Si, Ti, and Zn present in the wastewater samples were removed by ferric chloride. Hardness-causing ions such as magnesium and calcium were 45% and 58% removed, respectively. The concentrations of Na and Mg present in the treated water were high compared to those of the city water. These elements, along with other elements that are not removed by coagulation and flocculation, should be removed in subsequent treatments. After the coagulation and flocculation experiments, flue-gas-condensed water can be considered moderately hard because the total concentrations of hardness-causing ions was less than 80 ppm CaCO3.15 In addition, 85% of the Hg was removed from the wastewater. Except for the elements mentioned above, other elements occurred in concentrations of less than 1 mg/L. 10339



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Figure 5. Turbidity removals of water samples using ferric chloride at a dosage of 2 g. Figure 7. Concentrations of anions present in a water sample collected in December 2009 before and after water treatment using FeCl3 as a coagulation agent.

Figure 6. Normalized cation removals from flue-gas-condensed wastewater.

As shown in Figure 7, sulfate (SO42), bicarbonate (HCO3), chloride (Cl), and nitrate (NO3) anions were found to be present in substantial quantities in the flue-gas wastewater. Fluoride (F), nitrite (NO2), and bromide (Br) were also present in very small quantities. After sulfate, the second most abundant anion found was bicarbonate ion, because of the sodium bicarbonate buffer solution used in absorption column during IPR run. We surmise that chloride ions are mainly due to the FeCl3 coagulant. The NO and NO2 present in flue gas appear to be forming nitrates in wastewater. The following are the possible reactions occurring during scrubbing operations in the absorption column: formation of nitrous acid NO þ NO2 þ H2 O T 2HNO2 solubilization HNO2 þ H2 O T H3 Oþ þ NO2 formation of nitric acid 2NO2 þ 0:5O2 þ H2 O T 2HNO3 solubilization HNO3 þ H2 O T H3 Oþ þ NO3

Figure 8. Comparison of turbidity removals using FeCl3 and FeCl3 + polymer for a water sample collected in December 2009.

Very few of the anionic species present in wastewater samples were removed using either FeCl3 or alum. A polymer (Chemway 540) was used as a coagulant aid for turbidity removal. Wastewater samples were coagulated using 2 g of FeCl3 with and without the addition of 0.5 g of polymer per liter of flue-gas water. The results are shown in Figure 8. With the addition of 0.5 g of polymer and 2 g of FeCl3 a turbidity removal of 93% achieved within 5 min, whereas without the polymer, the turbidity removal was only 74%. Higher turbidity removal within a shorter time period points to the practicability of applying an in-line treatment of water for reuse in the IPR system. The pH and alkalinity values of the initial and treated water samples are shown in Figure 9. Both parameters increased after treatment. The relationships between the relative removals of Na, Mg, Al, Ca, K, Ba, S, Si, As, and B from flue-gas water using ferric chloride (except transition elements) and their initial concentrations are shown in Figure 10, whereas those for other elements (mainly transition elements) are shown in Figure 11. Depending on the cation and its concentration, either or both precipitation and coprecipitation will play a major role in removal during coagulation 10340



Figure 9. pH and alkalinity present in a water sample collected in December 2009 before and after water treatment.

Figure 10. Relationship between removal of ions and initial concentration.

Figure 11. Relationship between removal of transition elements and initial concentration.

and flocculation. In most cases, however, coprecipitation results in the removal of soluble metal ions during coagulation. Some metals will coprecipitate with either iron or aluminum hydroxide.

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Iron coagulants seem to perform better than aluminum coagulants, primarily because iron hydroxide is less soluble than aluminum hydroxide and is insoluble over a wider pH range. Not only do iron coagulants form stronger and heavier flocs, but coprecipitation of iron metal complexes also appears to be a significant factor. In aqueous solutions, the small and highly positive Al3+ and Fe3+ ions form such strong bonds with the oxygen atoms of six surrounding water molecules that the oxygenhydrogen atom association in the water molecules is weakened and the hydrogen atoms tend to be released into the solution. This is known as hydrolysis, and the resulting aluminum and ferric hydroxide species aid in the coagulation process. The chemistry of aluminum and iron hydrolysis reactions and products is complex and not completely understood. As hydrolysis proceeds, if sufficient total metal ion is present in the system, simple mononuclear products can form complex polynuclear species, which, in turn, can form microcrystals and metal hydroxide precipitates. Hydrolysis products can adsorb on (and continue to hydrolyze) many types of particular surfaces. The solubility of the metal hydroxide precipitates is one factor that must be considered in maximizing coagulant performance and in minimizing the amounts of residual Al and Fe in the treated water. At low pH, the dissolution of the metal hydroxide precipitates produces positively charged, soluble hydrolysis products and aqueous metal ions (Fe3+, Al3+). At high pH, the negatively charged, soluble hydrolysis products Al(OH)4 and Fe(OH)4 are formed. These species are tetrahedral rather than octahedral, so no deprotonation can occur. The minimum solubility pH values at 25 °C of aluminum hydroxide and ferric hydroxide precipitates are approximately 6.3 and about 8, respectively. The pH of minimum solubility increases with decreasing temperature.14 Elemental analysis of digested sludge from IPR water is shown in Figure 12. Al, Ca, Fe, K, Mg, Na, S, and Si are the major components of the sludge. As, B, Ba, Co, Cr, Cu, Mn, Mo, and Ni are found in small quantities. The amount of Hg present in the sludge is around 1.508 103 mg/g. The amount and characteristics of the sludge produced during the coagulation/flocculation process are highly dependent on the specific coagulant used and on the operating conditions. In this study, the sludge volume was estimated by measuring the height of the sludge layer at the bottom of the jar-test beakers. The sludge mainly contained fly ash particles. In our process, fly ash is black in color. After coagulation and flocculation, the sludge color changed to a light brownish-yellow. The color change is due to the coagulant, ferric chloride. According to the American Society for Testing Materials (ASTM C618), ashes containing more than 70 wt % SiO2 + Al2O3 + Fe2O3 and low amount sof lime are defined as class F, whereas those with SiO2 + Al2O3 + Fe2O3 contents between 50 and 70 wt % and high amounts of lime are defined as class C (Table 2). Briefly, high-calcium class C fly ashes are normally produced from the burning of low-rank coals (lignites or subbituminous coals) and have cementitious properties (self-hardening when reacted with water). On the other hand, low-calcium class F fly ashes are commonly produced from the burning of higher-rank coals (bituminous coals or anthracites) that are pozzolanic in nature [hardening when reacted with Ca(OH)2 and water]. The sludge formed thus contained class F fly ash particles with calcium at around 4% (Tables 1 and 2).17 10341



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Figure 12. Elemental analysis of sludge removed from wastewater.

6. CONCLUSIONS Specifications for the water-treatment process (flocculation/ coagulation, followed by reverse osmosis and evaporation/ crystallization) for IPR flue-gas water have been delineated and are being refined through continued experimentation. The recovery, treatment, and in-plant use of flue-gas water will decrease a power plant’s demand for water from the environment. Specifically (because the settling rate with FeCl3 is very high), spray water for direct-contact heat exchangers (DCHXs) can be prepared from raw flue-gas water by using only the flocculation/coagulation process. Of all the elements examined, sulfur and sodium were found to be the most prevalent in the wastewater. This is due to the high sulfur content present in coal and to the sodium carbonate buffering solution. Ferric chloride coagulant was found to remove more than 99% of the turbidity and significant amounts of cations. It was less effective removing anions, however. 7. FUTURE WORKS Additional studies will expand the specifications of the IPR process. The reverse osmosis process for supernatant produced through the flocculation/coagulation step will be further examined. Thermal evaporation and crystallization steps will be studied further as the zero-liquid-discharge (ZLD) system is specified for flue-gas water.17 NETL is being engaged in building water models related to power plants, and the results obtained from the aforementioned experiments will be incorporated into those power plant water models. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: 541-918-8093.

’ ACKNOWLEDGMENT This study was supported by the Oak Ridge Institute for Science and Education (ORISE) at the National Energy Technology Laboratory (NETL) with funding from Department of Energy (DOE). ’ NOMENCLATURE B&W = Babcock & Wilcox IPR = integrated pollutant removal

JHBTF = Jupiter Hammond Burner Test Facility MMBTU = million British thermal units NETL = National Energy Technology Laboratory ZLD = zero liquid discharge

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