Review pubs.acs.org/IECR
Application of Coal Fly Ash in Air Quality Management M. Ahmaruzzaman†,* and V.K Gupta‡,§ †
Department of Chemistry, National Institute of Technology Silchar, Silchar 788010, India Department of Chemistry, Indian Institute of Technology, Rookree 247667, India § Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia ‡
ABSTRACT: Although, coal fly ash is recognized as an environmental pollutant, it can be used for the removal of various gaseous pollutants under controlled conditions. A lot of work has been conducted worldwide for the utilization of fly ash for various applications. It has been found from the literature that fly ash possesses potential application in the management of air quality. The literature data revealed that fly ash can be utilized for the removal of NOx, SOx, mercury, and other gaseous pollutants from air and other sources. Fly ash can be used as a promising sorbent for the removal of various types of air pollutants; however, further research need to be taken in the laboratory and pilot plant scale. It is also indicated that fly ash contained approximately 10−12% of unburned component and these unburned components may have an important role in their removal capacity. In this article, the application of fly ash in the removal of SOx, NOx, mercury, gaseous organics, and other impurities have been discussed and some guidelines have been highlighted for their potential application.
1. INTRODUCTION Million of tonnes of fly ash are generated worldwide due to the combustion of coal for power generation. It is estimated that 349 Mt was produced worldwide in 2000.1 The disposal of this large amount of fly ash is really challenging and, therefore, has become a serious environmental pollution issue. Fly ash may be considered as the world’s fifth largest resource of raw material. Approximately, 25% of fly ash in India is utilized for the production of cement, construction of roads, and manufacture of brick.2 The energy sector of India produces over 130 Mt of fly ash annually,3 and this amount is likely to increase with increase in coal consumption by 2.2%. Fly ash contains potentially toxic trace elements, and therefore, its disposal will be costly if not prohibited. Recently, more research is being conducted on the utilization of fly ash to prevent threat to the environment and make them inexpensive. Therefore, it would be wise to utilize them rather than dispose of them in landfills. Fly ash is generally gray in color and contains various essential elements, including both macrocontaminants, P, K, Ca, and Mg, and microcontaminants, Zn, Fe, Cu, Mn, B, Mo, etc., required for the growth of plant. The approximate composition of fly ash is silica (60−65%), alumina (25− 30%), magnetite, and Fe2O3 (6−15%). The physicochemical properties of fly ash, such as bulk density, particle size, porosity, water holding capacity, and surface area, etc., make it suitable for use as a sorbent. Although, fly ash is a waste material, it is a resource that can be fully utilized and exploited.4−12 The utilization of fly ash in the agriculture and engineering is also studied.13,14 The conversion of fly ash into zeolite and MCM-21 has also been reported in the literature.15 Fly ash was generally released into the atmosphere via the smoke stack; however, it is mandated recently to be captured prior to its release. Although, the components present in the fly ash produced vary considerably, all fly ash includes substantial amounts of silica (both amorphous and crystalline) and lime. The fly ash may also be utilized as a low-cost sorbent for the © 2012 American Chemical Society
removal of air pollutants. A number of studies have been reported on the utilization of fly ash for the adsorption of pollutants in flue gases. The results are encouraging for the removal of air pollutants from industries and flue gases. This paper will review the applications of fly ash as low-cost sorbents for flue gas purification and removal of air pollutants from the atmosphere.
2. STRUCTURES AND PROPERTIES OF COAL FLY ASH Fly ash is a heterogeneous material consisting largely of small spheres, formed by the condensation of aluminous and siliceous glass droplets in the air. The fly ash samples are irregular, porous, coke-like particles of unburned carbon material, which are often concentrated in the larger size fractions. In general, fly ash has a hydrophilic surface and porous structure. The SEM (Figure 1) image clearly shows that finer fly ash particles (smaller than 200 mesh) are primarily spherical, whereas the coarser particles (larger than 200 mesh) are mainly composed of irregular and porous particles. The carbon concentration determined by loss on ignition (LOI) was less than 3% for the fly ash particles smaller than 400 mesh and significantly increased up to 58.3% as the particle size increased. All fly ash contains the same basic chemical elements but in different proportions. The main constituents of fly ash are silicon, aluminum, iron, and calcium, with smaller amounts of sulfur, magnesium, and alkalies and traces of many other elements. Thus, the primary components of the fly ash are silica (SiO2), alumina (Al2O3), and iron oxides (Fe2O3), with varying amounts of carbon, calcium, magnesium, and sulfur. Two general classes of fly ash are recognized for coal combustion: Class F, normally produced from anthracite, bituminous, or Received: Revised: Accepted: Published: 15299
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Figure 1. SEM pictures: (a) +200 mesh particles of the fly ash and (b) −200 mesh particles of the fly ash.7
3. SORBENT FOR THE REMOVAL OF AIR POLLUTANTS Although coal fly ash is a throw-away material, it is a resource that may be utilized and exploited. There are various reasons to boost up the amount of fly ash being reutilized. There may be monetary returns from the sale of this waste product, or at least compensation of the processing and disposal costs, and also, the waste products may replace a few scarce or costly natural resources. Utilization of coal fly ash can substitute another industrial resource and application. The following is a brief description of the utilization of fly ash and related research that has been conducted in the area of air quality management. In this section, the utilization of fly ash for flue gas purification and removal of air pollutants from the atmosphere is discussed. 3.1. Removal of Sulfur Oxides. Sulfur oxides and nitrogen oxides emitted from thermal power plants, factories, and automobiles cause acid rain, which is harmful to the environment. Among environmental problems, removal of SOx and NOx from the flue gases is one of the most important aspects that should be addressed for the protection of clean air from being polluted. At the present time, ammonia-catalytic De-NOx and calcium-gypsum De-SOx methods are used for the treatment of flue gases emitted from thermal power plants. Although these processes are effective and reliable, both the initial and the running costs are very high. Therefore, the demand for a more simplified and low cost FGD (flue gas desulfurization) system is increasing. There are three types of processes for the desulfurization of flue gases, namely, wet, semidry, and dry desulfurization. The wet desulfurization process is very effective for SO2 removal, and gypsum (CaSO4) can be obtained as a valuable byproduct in this process. However, a large volume of water is required, and the cost of the purification of wastewater is high. Semidry processes are also effective processes and need less water than wet processes; however, since a large volume of water is still needed to achieve high desulfurization rates, it is difficult to introduce this process in all cases. Thus, wet and semidry processes are not applicable everywhere because of the requirement of a large amount of water for the removal of SO2. Therefore, a dry desulfurization process that has low cost, has less water requirement, and produces CaSO4 as a valuable byproduct is desirable.22 Allen and Hayhurst23 developed a method to minimize sulfur dioxide emissions by reacting SO2
subbituminous coals and containing less than 7 wt % CaO, and Class C, normally produced from lignite coals and containing more lime (5−30 wt %).16 Table 1 shows the chemical Table 1. Chemical Composition of the Fly Ash17−21 constituent
Korean fly ash 17
Chinese fly ash 18
Colombian fly ash 19
Australian fly ash 20
Nigerian fly ash 21
SiO2 Al2O3 Fe2O3 CaO MgO SO3 TiO2 K2O others
57.82 22.10 8.33 2.57 0.91 0.73 0.64 0.45 6.47
57.54 24.38 7.12 6.00 1.60 1.04
53.9 28.0 6.53 4.73 1.47 0.4 1.92 1.72
54.0 28.0 8.30 3.0 1.60 1.20 1.64 1.33
57.25 22.03 8.36 2.97 0.97 0.76 0.68 0.52 6.49
1.32
composition of various fly ashes and reports that SiO2 and Al2O3 contents make up to about 80% of the fly ash in all the cases. The Fe2O3 and CaO contents were reported to vary in between approximately 10% and 13%. According to the ASTM C618, this fly ash can be classified as class F for having a less than 10% CaO content with a greater than 70% content of three componentsSiO2, Al2O3, and Fe2O3. Table 2 shows the important properties of coal fly ash and various operating parameters on its removal efficiency. Table 2. Important Properties of Coal Fly Ash and Various Operating Parameters on Its Removal Efficiency important properties coal fly ash 1. 2. 3. 4. 5.
specific surface area origin of coal fly ash Si/Ca mole ratio unburned carbon content unburned carbon surface area
operating parameters 1. reaction temp. 2. feed concn. of NOx, SOx, and Hg 3. presence of water vapor and its pressure 4. gas flow rate and fly ash loading 5. flue gas composition 6. column height 7. chemical treatment 15300
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°C. The SO2 removal activity increased as the water vapor pressure increased until monolayer coverage with water molecules was achieved. As the adsorbed water exceeded the monolayer coverage, the SO2 removal activity suddenly decreased and calcium sulfite became the main product instead of calcium sulfate. The NO removal activity increased with an increase in SO2 concentration up to 2000 ppm at 130 °C. The NO removal also increased with an increase in water vapor pressure and markedly decreased as the adsorbed water exceeded the monolayer coverage. The same research group42 reported the removal of SO2 and NO from flue gas by utilizing the sorbent prepared from calcium hydroxide, calcium sulfate, silicic acid, and aluminum hydroxide. The activity for the sorption of SO2 and NO markedly increased with the increase in the content of silica in the sorbent up to 40%. The formation of calcium silicate is suggested to be predominant in a high concentration of silica, while the formation of ettringite was observed by the XRD only for the sorbent containing silica below 30%. The distribution of the sulfur and nitrogen compounds in the sorbent revealed by XPS suggests that the adsorbed nitrogen compounds are gradually replaced by sulfur compounds as the reaction proceeds. In one study, a highly active sorbent for dry-type flue gas desulfurization was reported to have been prepared from calcium oxide, coal fly ash, and calcium sulfate.43 The activity of the absorbent increased further upon hydrothermal treatment following the kneading procedure. The period of hydrothermal treatment was reduced to 3 h to attain the activity that exceeded 20% of the activity of the commercial sorbent, which required an optimum hydrothermal treatment period of 10 h. The enhancement of activity by the utilization of CaO is due to the exothermic heat of slaking CaO. At a high temperature, the reaction of CaO with a SiO2 component present in the coal fly ash facilitates the formation of calcium silicate. The formation of calcium silicate was suppressed by the existence of CaSO4 in the slaking procedure. In another study, sorbents for SO2 removal were prepared by hydration at 90 °C of calcium hydroxide and coal fly ash at different fly ash/Ca(OH)2 initial ratios.44 The Ca(OH)2 conversion in the hydration reaction and the raw material ratios have been revealed as the most efficient parameters to predict the behavior of the sorbent in the desulfurization process. Coal fly ash with lime and hydrated lime has also been utilized as a sorbent in the desulfurization processes, as reported by Farnandez.45 No influence of the fly ash/Ca(OH)2 ratio and hydration time on the total pore volume distribution was reported. XRD (X-ray diffraction) analysis indicated that the hydration reaction resulted in the formation of CaCO3, mullite and calcium aluminum silicate hydoxide. Scanning electron microscope analysis showed the macrostructural changes with the hydration reaction. The same research group46 prepared another sorbent by the hydration of fly ash, calcium hydroxide, and calcium sulfate for the removal of SO2 from flue gas and investigated the influence of CaSO4 in the system of fly ash/calcium hydroxide. They46 also reported that sorbents prepared with CaSO4 showed structural properties and chemical composition that were different from the sorbents prepared without CaSO4 (with in general less specific surface area and mesopore volume). In the desulfurization test, all the sorbents prepared with and without CaSO4, at different fly ash/ calcium hydroxide/calcium sulfate ratios and slurrying times, showed a great increase in the calcium utilization as compared with the commercial calcium hydroxide. The utilization of
with solid calcium oxide and produced CaSO4 as a byproduct, as shown in the following: CaO + SO2 +
1 O2 → CaSO4 2
A number of investigations has been carried out for the removal of SO2 from flue gases utilizing fly ash as well as mixture of fly ash and other materials. Although, commercial activated carbon was generally utilized for the oxidation of reduced sulfur oxides, it is costly for large-scale environmental applications.24−32 Therefore, coal fly ash may be utilized instead of commercial activated carbon as a cheap sorbent for the drytype FGD. The utilization of fly ash as the source of silica presents both economic and environment friendly advantage because of its voluminous byproduct obtained from all coalfired power plants. In all cases, where fly ash or other silica sources and CaO or Ca(OH)2 are present in the hydration process, the pozzolanic reaction takes place and hydrated calcium silicates with a general composition of (CaO)x(SiO2) y(H2O)z are formed. Fly ash treated with calcium hydroxide was reported as a reactive sorbent for the removal of SO2.33 A study of the intrinsic kinetics of the reaction between treated fly ash and SO2 indicated a first order reaction with respect to SO2 concentration up to 0.31 vol % SO2 (3.36 × 10−8 and is increased by extending the effective contact time mol/cm3). Activation energies of 82.3 and 89.0 kJ/mol were calculated for treated PCC and FBC fly ashes, respectively. Ca(OH)2/fly ash systems have been investigated for the removal of SO2 to increase the desulfurization efficiency.34−36 Li et al.37 developed a method for preparing a highly active sorbent from fly ash mixed with CaO in water at ambient temperature for desulfurization at low cost. SEM (scanning electron microscopy) and EDX (energy-dispersive X-ray) analyses indicated that CaO particles were separated into small particles of Ca(OH)2 and that tiny Ca(OH)2 particles covered the surface of the fly ash particles. They38 also attempted to promote the calcium utilization rate of Ca(OH)2/ fly ash sorbent and the formation of CaSO4 as a byproduct by investigating the effects of NOx, CO2, and the reaction temperature on the SOx removal process in a fluidized bed reactor. It was reported that the presence of NOx enhanced the SO2 removal rate and that the negative effect of CO2 was reduced in presence of NOx. Matsushima et al.22 utilized a circulating fluidized bed with Ca(OH)2/fly ash sorbent to achieve a high SO2 removal rate without humidification and the production of mainly CaSO4; 83% SO2 removal efficiency was accomplished, the byproducts produced had a high CaSO4 content, and the optimum reaction temperature for desulfurization was 350 °C. A mixture of fly ash and calcium hydroxide was also reported for desulfurization by Davini et al.35,39 It was established from their study that Ca(OH)2−fly ash mixtures were an attractive low-cost option for the control of SO2. Davini40 also tested fly ash derived activated carbon for the sorption of SO2 and NOx from industrial flue gases. Such a mixture was reported to demonstrate similar characteristics as that of commercial activated carbon for the removal of SOx and NOx from flue gases. Removal of SO2 from flue gas by the absorbent prepared from coal fly ash, calcium oxide, and calcium sulfate was studied under different reaction conditions to elucidate the effects of the reaction temperature, water vapor pressure, and coexistence of NO in a flue gases.41 The SO2 removal activity increased with an increase in NO concentration up to 500 ppm at 130 15301
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the hydroxyl ion as the fluid phase. The influence of the Si/Ca initial ratio on the composition of the calcium silicate has been investigated by several authors.55−57 The mole ratio of Ca/Si in the calcium silicate prepared was reported to be independent of its initial mole ratio and converged to 0.9−1.5, depending on the hydration temperature.57 Pressure hydration at temperatures above 100 °C dramatically accelerated the reaction, resulting in a less compact structure composed of smaller primary particles. The structural properties and sulfation capacity of mixed solids obtained from pressurized hydration of commercial calcium hydroxide and coal combustion fly ashes were reported by Garea.58 The FGD process utilizing coal fly ash has been commercialized, and some industrial plants have achieved DeSOx efficiencies of over 90%, such as the Ebetsu power station (50 000 N m3/h) and the Tomtoh Atsuma power station (644 000 N m3/h) under a high molar ratio of calcium to sulfur (1.0−1.2).59 These process do not require any wastewater treatment and gas heating. Therefore, this process may be a good option for controlling the emission of sulfur dioxide and an environmentally friendly method for the reutilization of coal fly ash. The global market demand for FGD has been steady at between 5000 and 10 000 MW per year, and mostly wet-type limestone FGD units have been installed.60 Hokkaido Electric Power Co., Inc. (HEPCO) has reported that a solid compound, named as LILAC sorbent, prepared from coal fly ash, lime, and gypsum, possesses a high desulfurization ability.61 HEPCO and Mitsubishi Heavy Industries Ltd. (MHI) have jointly developed a new FGD system called LILAC FGD process. A demonstration test for a semidry LILAC process using the sorbent in the slurry form for the flue gas from a commercial coal-fired boiler was performed from October 1991 to March 1994. The results of the demonstration test for the semidry LILAC process were described, and the features of different types of FGD system were compared in terms of desulfurization efficiency, utilities, and construction space, and costs. Another dry FGD process adopting the sorbent prepared from coal fly ash, slaked lime, and gypsum was commercially operated since 1991.62 The characterization of the sorbent and the reaction mechanisms operating in the desulfurization were also reported.63−65 The process can achieve a high Ca utilization efficiency of 80%, which is much higher than the value of about 50% for the dry-type duct injection process using calcium hydroxide as the SO2 absorbent. One disadvantage with the dry FGD process is that the preparation of the absorbent requires a long time. In particular, the hydrothermal treatment of the mixture of coal fly ash, calcium hydroxide, and calcium sulfate takes 10.5 h in the commercial plant. It would save some cost of FGD to a great extent if the period of hydrothermal treatment is reduced. However, the calcium utilization rate of Ca(OH)2/fly ash sorbent needs to be further improved because a higher SO2 removal rate is required for practical applications. One method for enhancing the ability of the Ca(OH)2/fly ash sorbent is to utilize the catalytic effect of the metals. The effect of iron has been studied by many researchers.66−69 Yang et al.66 evaluated the catalytic effect of iron oxide (Fe2O3) on the sorption of SO2 by CaO in a chemical environment. They indicated that 4 wt % of Fe2O3 physically mixed with CaO approximately doubles the SO2 sorption rate at 850 °C while conversion of CaO to CaSO4 for the mixture was about 35%. Deker and Klabunde67 in their
calcium hydroxide showed lower values for sorbents with CaSO4 than for sorbents without CaSO4. Lee et al.47 investigated the influences of various factors such as specific surface area, reaction temperature, and feed concentration of NO and SO2 on the desulfurization activity of sorbents synthesized from coal fly ash, CaO, and CaSO4. It is reported that the sorbent desulfurization activity increased with increased specific surface area, reaction temperature, and NO concentration but with decreased SO2 concentration. They48 also utilized various types of ash (coal fly ash, coal bottom ash, oil palm ash, and incinerator ash) for flue gas desulfurization, and the sorbents were prepared by mixing the ashes with calcium oxide and calcium sulfate using the water hydration method. It was established that sorbents prepared from coal fly ash and oil palm ash have the highest SO2 absorption capacity. SEM analysis showed that the sorbent was composed of a compound with a high structural porosity, while an X-ray diffraction spectrum showed that calcium aluminum silicate hydrate compounds are the main products of the hydration reaction. Two different coal fly ashes coming from the burning of two coals of different rank were utilized as a precursor for the preparation of steam activated carbons, and their performance in the SO2 removal was evaluated at flue gas conditions.49 A superior SO2 removal capacity was shown by the activated carbon obtained using the fly ash coming from a subbituminous−lignite blend. The presence of higher amount of certain metallic oxides (Ca, Fe) in the carbon-rich fraction of this fly ash probably had promoted a deeper gasification in the activation with steam. The low utilization efficiency of calcium in the dry process is because of the formation of calcium sulfite and/or calcium sulfate, which cover the outer surface of the calcium hydroxide particles to prevent SO2 from further permeation into the bulk.50 However, Ueno51 reported that the sorbent prepared from calcium oxide, calcium sulfate, and coal fly ash showed higher calcium utilization efficiency in the dry desulfurization process. This sorbent has been utilized in the dry-type flue gas desulfurization system.52 Unlike the other dry processes in which SO2 is fixed in the form of calcium sulfite, this process is characterized by the formation of calcium sulfate. Kind and Rochelle53 proposed a mechanism for the reaction between the fly ash and Ca(OH)2 in which the rate-limiting step is the dissolution of the silica from the fly ash. Ca(OH)2 dissolution:Ca(OH)2 → Ca ++ + 2OH−
Ash dissolution:(SiO2 )x + 2H 2O + OH− → (SiO2 )x − 1 + Si(OH)5− Reaction to form calcium silicates:Ca ++ + Si(OH)5− → salcium silicates
On the basis of the above mechanism, the kinetic studies of these hydration reactions have been carried out.54 Kinetic data of the hydrothermal reaction of fly ash and calcium hydroxide at different temperatures, weight ratios, and in the presence of calcium sulfate were obtained by measuring the evolution of calcium hydroxide with the hydration time. Considering the dissolution of the silica from the fly ash in basic medium as the rate-limiting step, the reaction was modeled according to the shrinking core model for fluid−solid reactions identifying the silica from fly ash as the spherical particle of a constant size and 15302
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the mineral matter was removed by conventional HCl and HF demineralization procedure. Activation was carried out with steam at 900 °C in order to develop porosity onto the sample. To study the use of this unburned carbon as a precursor for the preparation of activated carbons for gas cleaning, the NO removal by ammonia using activated carbon as a catalyst at low temperature was performed.74 Although, ammonia-catalytic De-NOx methods are used for the removal of NOx emitted from thermal power plants, both their initial and the running costs are very high. On the other hand, it has been found that discharge plasmas can decompose volatile organic gases, nitrogen oxides, and sulfur oxides.75−90 Therefore, pulsed corona discharge has been widely investigated as a promising technique because of its simultaneous removal of NOx and SO2, low capital and operating costs, and more efficient energy conversion.91−94 Recently, there have been a number of studies reported regarding the desulfurization and denitrification via pulsed corona discharge. The researchers found that the conversion efficiencies of NO and SO2 are affected by many factors, such as pulse peak voltage, pulse frequency, gas flow rate, reactor length, input energy per unit volume of flue gas, etc.95−100 The use of short-duration high-voltage pulsed power in a coaxial cylindrical configuration at atmospheric pressure results in the formation of nonthermal plasmas.35−45Under these conditions, high-energy electrons are created while the temperature of the ions and the neutral remains relatively unaffected.101,102 This reduces the energy consumption because most of the energy is utilized to create energetic electrons. The high-energy electrons produce chemical radicals, which decompose the pollutant molecules of nitrogen oxides.83−85,103,104 Clements et al.77 reported the effect of fly ash on SO removal in an experiment using high-voltage pulsed energized discharge reactor. They also discussed the relevant corona discharge processes in an electrostatic precipitator.83 Lowke et al.90 analyzed theoretically the removal of NO and SO in an electrostatic precipitator operating in a typical flue gas. Recently, Daito et al.104 investigated the removal of NO in a mixture of N2, O2, H2O, and NO and also measured the growth of NH4NO3. However, it appears that, with the exception of a preliminary report by the authors,105 there have been few or no reports on the effects of the presence of the fly ash on the removal of NO. A combined removal of SO, NO, and fly ash particles was performed using pulse streamer and a dc bias voltage.59 The presence of fly ash improved the removal efficiency of SO, but the synergetic effects of the fly ash on the removal of NO were not reported.77 It was revealed that the removal efficiencies of both NO and SO were increased in the presence of powdery silica.106 This is because of the fact that SO and NO, which had been adsorbed on the surface of the dust, reacted with water and converted into acids after further oxidation.92 A study on the collection of fly ash was reported using a combination of pulsed and dc bias energization but without NO removal.107 The removal of NO and SO from a thermal power plant using coal with electron beam and therefore in the presence of fly ash was reported.108 However, the comparison on the removal of NO with and without fly ash was not studied. In another study, fly ash was injected into the discharge region to observe its influence on NO removal.109 Fly ash was mixed with different mixtures of NO, nitrogen, oxygen, and with and without water vapor. NO removal methods using plasma chemical reactions in nonthermal plasmas have been
investigation found that a small amount of surface iron oxide can have a tremendous effect on the effectiveness of nanocrystalline calcium oxide as sorbent for sulfur dioxide removal. They confirmed that iron oxide has a significant effect on the ability of CaO as sorbent for the removal of SO2. The effect of iron and other species for enhancing the ability of Ca(OH)2/fly ash sorbent was investigated. The Ca utilization rate over a period of 90 min was about 10% greater than that for Ca(OH)2/fly ash sorbent. It was also reported that iron is not effective for enhancing the ability of Ca(OH)2/fly ash sorbent but that NO3− was the most effective factor to enhance it. The mechanism of enhancing the Ca utilization rate was also investigated, and it was found that Ca(NO3)2 was produced in the sorbent and reacted with SO2, so that the reaction Ca(NO3)2 + SO2 → CaSO4 + 2NO + O2 proceeded. Several researchers investigated the effects of NO on the removal of SO2. The presence of SO2 and NO increased the sorption of NO and SO2, respectively, as reported by Tsuchiai et al.70 They reported that the removal of SO2 is enhanced in the presence of NO and explained this phenomenon by assuming the catalytic action of NO in the oxidation of SO2 to SO3. However, Medellin et al.71 reported that NOx enhanced the SO2 sorption by the catalytic action of the sodium oxide supported alumina. It was also mentioned that the SO3 produced as such reacts with sodium oxide to form sodium sulfate and also reacted faster than SO2. On the other hand, some researchers reported that NO does not considerably increase the reaction of SO2 removal and claimed that NOx had a negative effect on the removal of SO2 at a higher level of NOx removal.72 The discrepancy of the extent in the effect of NO on the sorption of SO2 among researchers may be due to the difference in sorbent type and reaction conditions.52 It should be highlighted that the moisture content of the sorbent was substantially different in various cases. 3.2. Removal of Nitrogen Oxides. Nitrogen oxides emitted from thermal power plants, factories, and automobiles are also detrimental to the environment. Fly ash has been reported to be utilized as sorbent for the removal of NOx from flue gases.73 The unburned carbon present in fly ash is particularly responsible for the main surface area of fly ash, and adsorption capacity of fly ash increased by controlled gasification of unburned carbon. This unburned carbon in fly ash might be a precursor to activated carbon and hence requires only activation for the production of activated carbon.74 Rubel et al.75 investigated the potential of coal combustion byproducts for the adsorption of Hg and NOx. Two sorbents, namely, gasifier char (GC) and coal combustion blended fly/ bottom ash (CC) were utilized for the adsorption and were compared with that of the commercial activated carbon. They reported that the adsorption capacity of Hg on GC was as good as that of commercial activated carbon for Hg capture from flue gases. It was also indicated that a temperature of 900 °C destroyed the adsorption capacity of GC. The adsorption capacity of GC was reported to be one-third as compared to that of commercial activated carbon. The authors reported that CC sorbents showed very low adsorption capacity for both Hg and NOx. Recently, activated carbon obtained from unburned carbon in coal fly ash has also been used for the removal of NO.74 It was reported that mineral matter must be removed efficiently from unburned carbon of fly ash before activation to obtain a more suitable activated carbon for environmental applications in gaseous phase. The carbon-rich fraction was reported to be obtained by mechanical sieving of fly ashes, and 15303
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widely studied. The effects of the addition of fly ash on NO removal using short-pulsed discharge plasmas were also investigated. Four different mixtures of gases (N2 + NO), (N2 + NO + O2), (N2 + NO + H2O), and (N2 + NO + O2 + H2O) were investigated and utilized either with or without the addition of fly ash.110,111 The study of the NO (NO + NO2) removal was performed with the fly ash, as it is relevant to real situations in coal power plants. The presence of fly ash decreased the NO removal rate slightly in the case of dry gas mixtures while it increased the NO removal rate substantially in the case of wet gas mixtures. It is also reported that the presence of fly ash in the flue gases, which also contain a few percentages of moisture, would be advantageous to the treatment of flue gases emitted from thermal power plants for the removal of nitrogen oxides.112 The influences of water vapor and fly ash addition on the conversion of NO in flue gas were also studied.113,114 However, the influences of water vapor and fly ash addition on NO and SO2 gas conversion efficiencies enhanced by pulsed corona discharge have not been well studied.115 Experimental studies reported that positive pulsed corona discharge can facilitate NO and SO2 conversion processes, and the conversion efficiencies of NO and SO2 are primarily dependent on the radicals OH and O and the active species O3, HO2, H2O2, etc. With water vapor addition, SO2 conversion efficiency is improved, but NO conversion process is restrained. Low fly ash concentration helps to enhance the conversion of NO and SO2; however, the conversion efficiencies of NO and SO2 are drastically degraded by high fly ash concentration addition. The synergistic effects of water vapor and fly ash addition strengthen the chemical adsorption ability of the fly ash surface, which results in a considerable improvement in the conversion of NO and SO2. Furthermore, the specific input energy plays an important role in NO and SO2 conversion efficiencies. Measured conversion efficiencies of NO and SO2 reached about 60% and 90%, respectively, under the conditions tested. Furthermore, NO and SO2 conversion efficiencies are improved almost linearly with increasing specific input energy. The effect of SO2 on NO removal was also investigated, and Livengood116 suggested that the formation of the compounds containing nitrogen and sulfur is responsible for the strong dependence of NO removal on the concentration of SO2. Tsuchiai et al.63 reported that NOx compounds are replaced by SO2 compounds when calcium utilization of the sorbent reaches a higher level. At a long reaction time of 15 h under a high SO2 concentration above 2000 ppm, the sorbent was reported to achieve high calcium utilization, and NO x compounds became replaced by SO2 compounds. Therefore, the shift of the maximum NO removal to a lower SO2 concentration for a reaction time of 15 h is suggested because of substitution of the adsorbed NO for SO2 to a large extent. The effects of water vapor are also reported to be significant on the removal of both SO2 and NO.117 The SO2 adsorption decreased at the critical conditions; however, it increased gradually as the number of water layers on the surface increased. This behavior is similar to that observed for a spray dry FGD process carried out at about 60−70 °C, where the adsorption of SO2 increases linearly with relative humidity, as reported by Jozewicz and Rochelle.118 Tsuchiai et al.117 suggested that the reaction mechanisms changed depending on the surface coverage of the water molecules on the sorbent. They explained the phenomenon by assuming that SO2 molecules dissolve into water and react with calcium hydroxide
to form calcium sulphite above the monolayer adsorption of water. On the other hand, SO2 forms calcium sulfate by the catalytic action of NO adsorbed on the surface below the monolayer adsorption of water. 3.3. Removal of Mercury. More and more concern has been put on the mercury emitted from power stations burning coal because of its harmful effects on human health.119 Therefore, mercury has been recognized as a prospective environmental pollutant. Both the U.S. EPA and the European Commission have set regulations on mercury emissions from electric utilities. Mercury emission control technologies may be divided into three classes (i.e., precombustion control, combustion control, and flue gas control), among which the flue gas mercury removal technology is most widely used. The existing flue gas pollution control devices such as bag filter, electrostatic precipitator, and wet FGD may remove the oxidized mercury from the flue gas. However, they have no apparent removal efficiency on the elemental mercury. Therefore, the direction will be to transform the element of mercury into oxidized mercury and particulate-bounded mercury, which may henceforth be effectively removed. Before the coal fly ash sorbent is utilized to remove mercury from the flue gas, it needs to be evaluated on a lab-scale device, pilotscale, and full scale reactor or power station. Investigations have been made for the removal of mercury from the flue gas of utility boilers. Activated carbon is generally utilized for the removal of mercury from flue gas because of its high removal efficiency. The large scale application of activated carbon is hindered because of its high cost.120 Studies were reported that unburned carbon in fly ash has the potential of adsorbing elemental mercury. Thus, fly ash carbon instead of costly activated carbon may be utilized as a low-cost sorbent in the removal of elemental mercury from gases. Since the unburned carbon separated from fly ash is a byproduct, any practical application of such material would be economically and environmentally advantageous to the overall fly ash beneficiation process. Researchers at The Pennsylvania State University have developed a cost-effective process to separate unburned carbon from fly ash.121 A preliminary study reported that some unburned carbon from fly ash has certain capabilities for the adsorption of elemental mercury. Such findings sparked the idea of utilizing fly ash carbon as a low-cost sorbent in the removal of elemental mercury from gas phases, such as utility flue gas, to replace costly activated carbons. Yasuhiro et al.122 also developed a method for the separation of unburned carbon from fly ash. It was reported that fly ash containing unburned carbon is fed into the pulverization chamber, and through at least repeated self-collision, unburned carbon is segregated and reduced in size while the remaining particular matter is similarly segregated and reduced in size. A new concurrent flotation column that simulates the plug flow reactor was designed with the use of a static mixer, a froth separator column, and an optional additional bubble generator for fly ash beneficiation.123 The objective was to improve the efficiency and effectiveness of unburned carbon removal from fly ash by minimizing energy costs. Cleaning tests were performed with and without the additional bubble generator. Without the additional bubble generator, unburned carbon in the ash product could be reduced to only 2.53%. Incidental loss of carbon particles and insufficient bubble generation were the main causes of poor carbon separation performance. By turning on the additional bubble generator, it was aimed to assist bubble generation, to compensate bubble rupture, and to 15304
dx.doi.org/10.1021/ie301336m | Ind. Eng. Chem. Res. 2012, 51, 15299−15314
Industrial & Engineering Chemistry Research
Review
al.138 performed the bench-scale adsorption of Hg0/HgCl2 mixed with flue gas simulating various coal-burned flue gases. For Hg0 adsorption tests, the mercury content of the spent ash is reported at 0.2−2.51 μg/g, and for HgCl2, the mercury content is 0.37−2.9 μg/g. The removal of Hg on fly ash alone has been reported as high as 90% for some Western U.S. coals and 60% for bituminous coals.139,140 The adsorption of Hg on fly ash has been reported to depend on the amount of unburned carbon, inorganic matter, chlorine, and organic sulfur.139−143 Adsorption behavior of unburned carbons from fly ash has also been investigated.144 Batch and column test were carried out for several unburned carbon samples from various ash sources. It was reported that the unburned carbons have equal or better adsorption capacity for elemental mercury compared to some general purpose commercial activated carbons at low gas phase mercury concentration that is in the range of power plant emissions. Also, it has been observed that heat treatment of unburned carbon in the presence of air at 400 °C enhanced the adsorption capacity, and the adsorption capacity decreased with increased adsorption temperature. The mechanism of mercury adsorption on the unburned carbon was explained by the physical and chemical interaction between mercury and primary sites on the carbon surface. The effect of porous structure and surface functionality on the mercury adsorption capacity of a fly ash carbon and its activated sample has been investigated using a fixed-bed with a simulated flue gas.145 The activated fly ash carbon has lower mercury capacity than its precursor fly ash carbon (0.23 vs 1.85 and is increased by extending the effective contact time mg/g), although its surface area is around 15 times larger, 863, and is increased by extending the effective contact time vs 53 m2/g. It was reported that oxygen functionality and the presence of halogen species on the surface of fly ash carbons may promote mercury sorption, while the surface area does not seem to have a significant effect on their mercury adsorption capacity. In full-scale boilers, mercury removals on fly ash alone without sorbent injection have been reported to range from near 0% to ca. 90% for some Western U.S. coals146,147 and up to about 60% for certain U.S. and British bituminous coals146,148−150 depending on the temperature, the level of unburned carbon in the ash, and the catalytic effects of inorganic ash constituents, which still have not been fully identified. The mercury adsorption capacity of the inorganic fraction is typically low,151 although certain fly ashes with low carbon content may still exhibit significant mercury capture.147 Analysis of mercury contents in residual coal chars after partial combustion or pyrolysis indicates that mercury is released rapidly above 700 °C in either process.152 The capture of mercury on fly ash is progressively increased as the flue gas temperature is reduced below ca. 400 °C146,149,150,153 and is increased by extending the effective contact time between flue gas and fly ash, as occurs in a baghouse relative to an ESP.146,147 Mercury capture is often, but not always, enhanced by high levels of unburned carbon in the fly ash (high loss on ignition). Investigations of Hg analyses performed on different fractions of fly ash separated by screening or triboelectrostatic techniques have shown that mercury enrichment often correlates directly with carbon content.146,147,151,153−156 For bituminous coals with up to ca. 0.2 ppm Hg content, mercury concentrations in fly ash between ca. 0.1 and 1 ppm have been measured at ash carbon levels of 2% to 12%.149,155,156
recapture the detached or free carbon particles leaving the froth phase. With the additional bubble generator and under optimized conditions, a froth product with 95% carbon recovery and a cleaned ash product with less than 1% unburned carbon were obtained. The separation process through the static mixer (feeder) and the separator column and the energy consumption of the unit were analyzed. It was seen that around 80% energy could be saved with the concurrent flotation column compared to conventional flotation. The simulated flue gas composed of mercury vapors from the vapor generator and compressed air was utilized to investigate the removal of Hg in fly ash and activated carbon.119 The investigations reported that the mercury removal efficiency of activated carbon is 60%, while fly ash is of low cost and its efficiency is limited. Unmodified fly ash adsorption efficiency increased from 10% to 20%. The specific surface area of modified fly ash has increased tremendously; thereby, its adsorption efficiency reached up to 25%, but still, it is far below the adsorption efficiency of activated carbon. The activated carbon is reported to have higher mercury removal efficiency than fly ash, but its cost is very high. Fly ash can catalyze the oxidation and adsorption of elemental mercury. The oxidation of elemental mercury increased with increased amount of magnetite in the ash. Surface area and surface nature of the fly ash appeared to be important for oxidation and adsorption of elemental mercury. Bench- and pilot-scale studies have indicated that fly ash can also adsorb mercury. In general, fly ash adsorbs substantially less mercury than activated carbon at similar conditions. Even with relatively low capacities, the ash may potentially be able to remove substantial amounts of mercury considering the high concentration of ash in flue gas environments. Studies indicated that flue gas composition, unburned carbon content (LOI), unburned carbon surface area and chemical treatment affected the adsorption of mercury on fly ash. The role of inorganic components present in fly ash on the removal of mercury is still not clear; efforts have been made regarding the retention of mercury by unburned fly ash carbons.124−131 A correlation has been reported between Hg content and the percentage of carbon in fly ash126 and coal blends.132−134 Various researchers investigated the role of unburned carbon of fly ash in the removal of mercury.124,131,132114 The concentration of unburned carbon present in fly ash and their respective ability to remove Hg have been related to their textural properties126,132−134 Subbituminous fly ash was injected into both lignite and bituminous combustion flue gas at a concentration of about 5− 10 g/m3, and mercury adsorption capacities were reported at 10 μg Hg/g fly ash for bituminous field conditions and 30 μg/g for lignite field conditions.135 Senior et al.136 analyzed the fly ash LOI and mercury content at two sites. At the Gaston site, which burns bituminous coal and was equipped with a COHPAC baghouse, the ash had 10−15% LOI and a mercury concentration of 0.2−2 μg/g. At the Pleasant Prairie site, which burns subbituminous coal and is equipped with an ESP, the ash had an LOI of 0.5% and
