Physico chemical Characteristics of Ammonia Adsorbed Fly Ash (AAFA)

Transcription

1 Article KEPCO Journal on Electric Power and Energy Vol. 4, No. 1, June 2018 ISSN (Print), (Online) DOI : /KEPCO Physico chemical Characteristics of Ammonia Adsorbed Fly Ash (AAFA) Jae kwan Kim 1, Seok un Park 1, Jin pyo Hong 1 Abstract Ammonia Adsorbed Fly Ash (AAFA) samples produced from coal fired plants equipped with SNCR (Selective Non Catalytic Reduction) of nitrogen oxides with urea have been chemically analyzed, and their physical and dissolution properties have been investigated. XRD results for the ammonia component in AAFA ascertained that ABS (ammonium bisulfate) and AS (ammonium sulfate) were deposited on fly ash as SO3 reacted with unreacted ammonia at SNCR. SEM and EDS images showed that fine ashes on large fly ash surface of sphere type were agglomerated, due to adhesive role of ammonium salts attached fly ashes. Dissolution test results of ammonium salts absorbed on AAFA in distilled water or sea water showed that the proportion of unionized NH3 to NH4 + were primarily a function of ph and temperature. Increasing ph and temperature causes an increase in the fraction of un ionized NH3. At phs of 9.6 and 10.7, un ionized NH3 and NH4 + ions are present in equal amounts at distilled water and sea water, respectively. Keywords: AAFA (Ammonia Adsorbed Fly Ash), Ammonium Sulfate, Ammonium Bisulfate, SCR (Selective Catalytic Reduction), SNCR (Selective Non Catalytic Reduction) I. INTRODUCTION The advantages of the coal include the abundant deposits, inexpensive price and stable supply, being used as the main fuels of the thermal power plant and the coal power plant possession was 35.8% of the total power generation. Most of coal burning boilers are equipped with the exhaust gas nitration reduction installation (Selective Non Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR)) for operation in line with enhancing local and global environmental regulations on environment contamination protection in exhaust gases. The main exhaust gas denitration is the dried catalytic ammonia reduction method which reduces the NOx (Nitrogen Oxide) by the ammonia injection and the denitration methods include the high dust denitration under high dust content, and the low dust denitration for the denitration after the dust removal depending on the dust concentration. The high dust denitration has been globally commercialized owing to the easy installation in existing coal fired thermal power plants. The dust collection is performed by the electrostatic precipitator after the high dust denitration process. The ammonia is injected into the dust gas with the fly ashes, making the environment where most of the ashes adsorb the unreacted ammonia. About 80 to 90% of the unreacted ammonia in the exhaust gas is contained in the fly ashes collected by the electrostatic precipitator [1]. The United States and Japan have robustly performed researches on ammonia refining and reusing technologies from the ashes with the exhaust gas denitration [1][2]. Korea has considered the commercialization of the SCR and SNCR with the excellent coal fired exhaust gas system for the future. In this case, the fly ashes with the ammonia are generated and may cause the environmental contamination (bad smell) due to the ash reclamation. In addition, there needs to perform the physical and chemical property evaluation on the fly ashes with the ammonia because it may cause problems in more reuse of fly ashes by the adverse effect on the quality if the ashes are used as the concrete admixture in the recycling of the ashes. Five subsidiary companies of Korea Eelectric Power Corporation (KEPCO) estimate that the amount of fly ashes produced from coal fired power plant would rapidly increase to 16.0 million tons in 2020 from 9.0 million tons in 2016 on the annual basis [3]. The purpose of the study is to evaluate physical, chemical and desolution properties of the AAFA generated from coalfired power plants with DeNOx facilities. Manuscript received April 13, 2018, Accepted June 15, KEPCO s Research Institute, Korea Electric Power Corporation, 105 Munji Ro, Yusung Gu, Daejeon, 34056, Korea jkkim7839@kepco.co.kr This paper is licensed under a Creative Commons Attribution NonCommercial NoDerivatives 4.0 International Public License. To view a copy of this license, visit nc nd/4.0. This paper and/or Supplementary information is available at 39

2 Jae kwan Kim, et al.: Physico chemical Characteristics of Ammonia Adsorbed Fly Ash (AAFA) Table 1. Chemical component of AAFA (Unit: %) Component AAFA(n=26) Mean Ranges SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 TiO2 P2O5 LOI moisture ph NH4 + (ppm) ESP = 46.7 A/H = ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ II. MATERIAL AND EXPERIMENTAL METHOD The AAFAes used in the study are generated by the combustion of Indonesian Adaro Coal (Coal Class 8) in Honam thermal power plant of Korea East West Power Co., Ltd. owned by KEPCO and collected from each electrostatic precipitator to performs experiments on physical, chemical and desolution properties. The chemical property analysis of the fly ashes is performed by combustion of 10 g of AAFA at 750±50 C with measuring the unburned carbon, adding 0.05 g of the sample in each plastic flask and injecting 5 ml HCl, 2 ml HNO3 and 5 ml HF in accordance with KSL 5405 (fly ashes). Then, the Erlenmeyer flask is placed in the thermostat and heated in the boiling water for 2 hours at the temperature of 70~80 C. Then, 1 g of boric acid is added for the F ion treatment in the solution, heated for 30 minutes at the temperature of 70~80 C for 30 minutes after completely desolving the minerals in the heating process and the chemical analysis is performed with the Inductively Coupled Plasma Emission Spectrometry (ICP, Arcos FHE12, SPECTRO ARCOS Corp.) after removing the F ion and dilution. The study performs the X ray diffraction analysis (XRD, Philips, pw 1700, Cuka, 40 kv, 25 ma, Ni filter) based on the powder method to review the crystalline elements of minerals and ammonium salt in the fly ashes. The ammonia concentration was measured using an Orion NH3 electrode. The quality of water soluble ammonia present on the ash then calculated. Comparison of the NH3 concentration determined in this study with those provided by the power plant indicates that most of the ammonia present on the fly ash is readily water soluble, probably as ammonium salts. The particle shape measurement was performed by coating the fly ash surface with gold (Au) and observing it using an electron microscope (SEM, Hitach, X 650). The composition of the fly ash layer was analyzed by EDS (Energy Dispersive Spectrometry, Kevex 2023). Adsorption and desorption isotherms of nitrogen on the samples were were measured for BET surfaces at 77 K using an air permeability apparatus (ASAP 2010, Micromertics Corp.). Before the measurements, the samples were degassed at 150 C for 2 hours in the adsortion vaccum port prior to exposure to nitrogen gas. The fraction measurement test is performed for the ammonia and ammonium ion depending on the ph and temperature when the AAFA are desolved in the distilled or seawater while Fig. 1. Ammonium ion concentration of various collection positions of AAFA samples collected from 1 and 2 ESPs. reclaiming the AAFAes. The test is performed by exactly weighing 100 g of the ash sample, adding acetic acid or NaOH to the distilled water and seawater, putting the solution with ph of 6~10 with the ratio of ash : solvent = 1 : 10 (100 g : 1000 ml, w:v) into the 2,000 ml and agitating for 24 hours. The solution is filtered through the 0.45 μm glass mesh and the filtered solution is used as the sample to measure the nonionic ammonia and ammonium ion for each ph. Also, the sample with the ph of 6~10 is agitated with the hydrostat by regulating the water temperature from 5 to 30 C by 2 C interval for 30 minutes to analyze the toxic and non ionic ammonia. III. RESULT AND DISCUSSION A. Chemical Properties Table 1 shows the chemical composition of the AAFA collected from electrostatic precipitator with the DeNOx facilities. The chemical composition of the AAFA contains more than 50.7% of SiO2 and more than 18.8% of Al2O3, meaning that the amount of SiO2, Al2O3 and Fe2O3 account for more than 75.1%. The test uses the Indonesian Adaro Coal (Coal Class 8) with high content of CaO among minerals, showing that the CaO is highly found in the coal fly ashes. The CaO, the most important in the concrete admixture for the large recycling of the ashes accounts for 10.3~18.0% in the AAFAes, much higher than the coal fly ashes of 0.4~10.3% [3], assuming that it would upgrade the pozzolane activation for the recycling and an average content of unburned carbon affecting the concrete compression is 4.0% and lower than 5% of the KSL 5405 standard. The ph of the AAFA is at least 8.3 and the ash would be good for restoring the acid soil and scrapped mine for the alkaline feature. The ammonium ion concentration of ashes collected by the ESP is less than 100 ppm or 4.8~95.7 ppm but the small amount of ashes collected by the A/H is much higher than that of the ESP, 28.6~592.9 ppm. The most ashes of the ESPs are generated at 0.8 of the Normalized Stoichiometric Ratio (NSR, NH3/NOx), much lower than the designed value of 1.2, causing the unreacted ammonia slip to keep under 5 ppm has low ammonium ion concentration. However, the A/H has very small amount of unreacted ammonia and collected ashes, suggesting that the ammonium 40

3 KEPCO Journal on Electric Power and Energy, Vol. 4, No. 1, June 2018 Fig. 2. Relation of between CaO+MgO concentration and ph of AAFAes and bituminous coal fly ashes. Fig. 4. SEM images of general fly ash(fa) and AAFA. Fig. 3. XRD result of general ash and AAFA. ion has relatively high concentration. Fig. 1 shows the ammonium ion concentration for each collection position of the ESP for the fly ashes. The ammonium ion concentration increases from the front (1 section) to the intermediate (2 section) and rear (3 section) sections. It is because that the higher ammonium ion concentration raises the electric resistance of the AAFA surface and makes it difficult to be charged to ( ) from the corona discharging electrode. The higher ammonium ion concentration requires more residence time for the sufficient charging of the ash surfaces, meaning that the ashes with higher ammonium ion is collected at the rear section. It is the CaO + MgO content or the alkaline indicator with the highest correlation with the ph change for each element in the AAFA, and Fig. 2 shows that the ph increases with higher CaO + MgO content regardless of the ash type. The ash is generated by melting and freezing process of the minerals in the combustion process of the fine powder coal and it is difficult to exactly quantify the crytalline feature of the ashes. However, it is general to have the main materials of amorphous glass phase and crystalline α quartz and Mulite. The SiO2 element reacts with the Al2O3 in the clay to generate the Mulite and the residue remains in the ashes. These crystallines collected from non SCR/SNCR boiler and AAFA with ammonia concentrations of 620 ppm were confirmed by the XRD analysis results in Fig. 3. Most minerasls such as aluminosilicate at boiler were fast melted, agglomerated, and then crystallized to α quartz (Q:SiO2), mullite (M:2Al2O3 2SiO2) and magnetite (Ma:Fe3O4). In case of AAFA, much of the ammonia is adsorbed on fly ash in the form of ammonium salt formed by reaction occurring in NH3 unreacted at SCR/SNCR installation and SO3 containing flue gas. Main crystallines of ammonium slats adsorbed on fly ash were ammonium bisulfate (ABS:NH4HSO4) and ammonium sulfate (AS:(NH4)2SO4). 2NH3 + SO3 + H2O (NH4)2SO4 (1) NH3 + SO3 + H2O NH4HSO4 (2) B. Physical Properties Most of ash particles are spherical and have features not found in other pozzolanes. This increases the concrete filling and highly fills pores among aggregates, useful for the concrete admixture recycling. Fig. 4 shows SEM photographs of general bituminous coal fly ash, which are generally spherical particles. The surface is smooth and glossy. In Fig. 4, the ammonium salt adsorbed on the surface of fly ash acted as an adhesive solvent and existed in a state of aggregation with small spherical coal ash. The results of the EDS analysis for the coal fly ash and the AAFA are shown in Fig. 5. As shown in Fig. 5, SiO2 and Al2O3 were the major components in the coal fly ash, but SO4 and CaO combined with ammonium ion were considerably high in the case of the AAFA. The median size depends on the ash type and granuality but generally has 12.26~46.00 μm and is similar to the cement diameter. The 44 μm residue marks 14.60~29.02% and the Blaine specific area is more than 3,580 cm 2 /g. Most 41

4 Jae kwan Kim, et al.: Physico chemical Characteristics of Ammonia Adsorbed Fly Ash (AAFA) Table 2. Chemical component of AAFA (Unit: %) Physical properties AAFA(n=26) Mean Ranges Sp.gr (g/cm 3 ) d50 (μm) 44 μmr (%) Blaine (cm 2 /g) , ~ ~ ~ ,580 ~ 3,740 Specific Gravity(g/ ml ) CaO + MgO(%) Blaine( cm2 /g) Specific Gravity(g/ ml ) μm R(%) Fig. 5. EDS results of bituminous coal fly ash and AAFA. physical properties are similar to the general bituminous fly ash [3]. C. Correlation of Physical and Chemical Properties Analysis result of the physical and chemical characteristic factors affecting the true specific gravity of AAFA, as shown in Fig. 6, the higher the content of the element having a large atomic weight such as CaO and MgO, the greater the true specific gravity. The Blaine specific surface area tended to increase with increasing true specific gravity as shown in Fig. 6. As shown in Fig. 6(c) and (d), it ascertained that Balaine surface area of AAFA was increased with decreased the 44 μm residue due to the finer size distribution, and its specific gravity was also increased with the finer median size and decreased the 44 μm residue. But, some AAFA as Fig. 6 (c) showed that Blaine surface area was different at similar 44 μm residue content of less than 18% because AAFA of high ammonia concentration has the low porosity due to ammonium salts adsorption on fly ash pore. Specific Gravity(g/ ml ) Blaine( cm2 /g) (c) Median Size( μm ) (d) Fig. 6. Relation among the physico chemical properties of AAFA. CaO+MgO Specific Gravity, Specific Gravity Blaine, (c) Blaine 44 μmr, (d) Mean Size Specific Gravity. 42

5 KEPCO Journal on Electric Power and Energy, Vol. 4, No. 1, June 2018 D. Dissolution Properties 1) ph Effect Ammonia in aqueous solution is primarily present in two forms: NH4 + ion and un ionized NH3. The distribution of these species is largely controlled by ph and temperature. Fig. 7 depicts the effect of ph at a single temperature, 25 C as dissolved the ammonium bisulfate of 50 g in distillated water or sea water of 100 g (maximum solubility: 290 NH4HSO4 / 100 g H2O), and shows that increasing the ph cause the NH3 fraction to increase. When ammonia is dissolved in water it reacts with hydrogen ion to form ammonium ion: NH3(g) + H3O + NH4 + (aq) + H2O(l) (3) The relative amount of ammonia and ammonium ion is determined by the solution ph. In acid solution, where hydrogen ion is readily available, virtually all the ammonia are converted to ammonium ion. At a ph of about 9.3, half the ammonia will be in the form of ammonium ion. Theoretically, it is possible to calculate the ratio of ammonia to ammonium ion, if the ph is known. The equilibrium constant for the reaction is: (4) at 25 C and ph=9.3 [4]. The ratio of ammonium to ammonia is given by: (5) As ammonium bisulfate identified as a major component crystalline of AAFA in Fig. 3 dissolved in distilled water, at a ph of about 9.6, half the ammonia became in the form of ammonium ion as shown in Fig. 7. Although ammonium bisulfate unlike ammonia gas dissolved in distilled water, theoretical ph values (9.3) with the same ammonia and ammonium ion concentration were very similar to experimental ph (9.6). However, as ammonium bisulfate dissolved in sea water with containing various cation and anions as shown in Table 3, ph values of the same ammonia and ammonium ion concentration was about 10.7 (Fig. 7), and higher than above distilled water due to presence of much interference ions and ammonium ion nitrificating from nitrogen in air. As NaOH was added to increase ph of sea water, the white salt was precipitated, and conversionof ammonium ion was started to ammonia at ph 9.7. Increasing water temperature also results in an increased proportion of NH3 [4][5]. As shown in Table 3, stabilized ammonium ions are dissolved in seawater by about 7.6 ppm, and it is considered that the concentration of ammonium ions is maintained by the influence of ion interference of dissolved anions in seawater. 2) Temperature Effect Aqueous ammonia is a weak base, releasing hydroxide Fig. 7. Ammonia species in distillated water and sea water as a function of ph at 25 C. Table 3. Chemical composition of sea water used in this study Component mg/kg (ppm) Sodium 10,500 Magnesium 1,350 Calcium 400 Potassium 380 Ammonium 7.6 Chloride 19,000 Sulfate 2,700 Bicarbonate 142 Bromide 65 Other solids 34 Total dissolved solids 34,500 and ammonium ions into solution. Although ammonium hydroxide is conveniently expressed as NH4OH, dissolved ammonia is actually present in a hydrated form that is hydrogen bonded to water molecules [5]. NH3(g) + nh2o(l) NH3 nh2o(aq) (6) Increasing the partial pressure of ammonia gas [6] results in an increase in ammonia solubility, as described by Henry s Law. (7) where kh is Henry s Law constant, NH3(aq) is the concentration of ammonia dissolved in aqueous solution and 43

6 Jae kwan Kim, et al.: Physico chemical Characteristics of Ammonia Adsorbed Fly Ash (AAFA) on the ammonia emission rate from water. However, at high ph of sea water increasing the temperature can cause a substantial increase in the rate of ammonia evolution (Fig. 8). In the case of seawater, Bower and Bidwell [8] reported that salinity in aqueous solution decreased the solubility of ammonia, resulting in a significantly lower ammonia emission than in distilled water. IV. CONCLUSIONS PNH3(g) is the partial pressure of ammonia gas. Ammonia solubility is affected by temperature, with an increase in temperature causing a decrease in the Henry s Law constant [7] Fig. 8. The effect of temperature and ph on the fraction of NH3 in (1) distilled water and (2) sea water (8) where T = temperature in Kelvin Increasing the temperature of water can theretically increase the concentration of aqueous ammonia. Fig. 8 shows in greater detail the effect of temperature at different ph. Note that, although the proportion of un ionized ammonia is affected by temperature at all ph levels, the most pronounced influence from a practical standpoint is at intermediate ph with the same ammonia and ammonium ion concentration. As AAFA was landfilled into ash pond, the rate of ammonia emission from leachelate water to atmosphere is primarily controlled by ph, temperature, and wind speed: increase in any of these variables causes an increase in the rate of ammonia emission. Suspended particles can also increase the emission rate. The solution ph has a dramatic effect on ammonia emission primarily because of its effect on the proportion of un ionized NH3 in solution. As NH3 gas leaves a solution, restoration of equilibrium causes more NH4 + to NH3, thus eventually depleting the total ammonia. At high ph, most of the ammonia is present as un ionized NH3, causing the total ammonia concentration to increase relative rapidly. Conversely, at low ph, the proportion of NH3 is very small which cause the total ammonia to persist for a much longer time. At a low ph, temperature changes have very little effect Analysis results of physical and chemical properties of the AAFA (Ammonia Adsorbed Fly Ash) generated from the coal fired power plant with the SNCR operation is as follows. 1. XRD results for the ammonia component in AAFA ascertained that ABS (ammonium bisulfate) and AS (ammonium sulfate) were deposited on fly ash as SO3 reacted with unreacted ammonia at SNCR. SEM and EDS images showed that fine ashes on large fly ash surface of sphere type were agglomerated, due to adhesive role of ammonium salts attached fly ashes. 2. The ammonium ion concentration increases from the front (1 section) to the intermediate (2 section) and rear (3 section) sections. It is because that the higher ammonium ion concentration raises the electric resistance of the AAFA surface and makes it difficult to be charged to ( ) from the corona discharging electrode. The higher ammonium ion concentration requires more residence time for the sufficient charging of the ash surfaces, meaning that the ashes with higher ammonium ion is collected at the rear section. 3. Dissolution test results of ammonium salts absorbed on AAFA in distilled water or sea water showed that the proportion of un ionized NH3 to NH4 + were primarily a function of ph and temperature. Increasing ph and temperature causes an increase in the fraction of unionized NH3. At phs of 9.6 and 10.7, un ionized NH3 and NH4 + ion are present in equal amounts at distilled water and sea water, respectively. ACKNOWLEDGEMENT This study was supported by 2014 Energy Technology Development Project [ ] of the Korea Institute of Energy Technology Evaluation and Planning, financed by the Ministry of Trade, Industry and Energy, Republic of Korea. REFERENCES [1] Nagayama. K., Behavoir of Unreacted Ammonia as DeNOx installation operated at Coal Fired Power Plant: Experiment of Electrostatic Precipitator, CRIEPI (Central Research Institute of Electric Power Industry, Japan) Technical Report No , 1979, Japan, pp [2] Bittner, J., Gasiorowski, S., and Harch, F., Removing Ammonia from Fly Ash, 2001 Conference Proceedings on SCR and SNCR 44

7 KEPCO Journal on Electric Power and Energy, Vol. 4, No. 1, June 2018 for NOx Control (Pittsburgh Marriott City Center, USA), Vol. 1, No. 2, 2001, pp.11 1~5. [3] Kim, J. K., Park, S. U. and Lee, H. D., Database of Rare Valuable Metal Distribution and Physicochemical Properties of Fly Ash Generated at Coal Fired Power Plants of KEPCO s Subsidiary Companies, Korea Electric Power Corporation s Research Institute, Technical Report No. TM.9337.M , 2016, Korea, pp [4] Martell, A. E. and Smith, R. M., Critical Stability Constants, Biochem Education, Vol.11,Issue2, 1983, pp.77. [5] Emerson. R. and Lund, T., Aqueous Ammonia Equilibrium Calculations: Effect of ph and Temperature, J. Fish. Res. Board Canada, Vol. 32, 1975, pp [6] Sander, R., Compilation of Henry s Law Constant for Inorganic and Organic Species of Potential Importance in Environmental Chemistry, available at mainz,mpg.de/~ sander/res/henry. html (1999). [7] Dasgupta, P. G., and Dong, S., Solubility of Ammonia in Liquid Water and Generation of Trace Levels of Standard Gaseous Ammonia, Atmospheric Environment, Vol. 20, 1986, pp [8] Bower, C. E., and Bidwell, J. P., Ionization of Ammonia in Sea Water: Effect of Temperature, ph, and Salinity, Journal of the Fisheries Research Board of Canada, Vol. 35, 1978, pp