Dissolution Study of Bottom Ash in Wet Flue Gas Desulphurization

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1 Dissolution Study of Bottom Ash in Wet Flue Gas Desulphurization Lawrence. Koech, Ray. Everson, Hein. Neomagus, and Hilary. Rutto. Abstract Bottom ash is combustion residue produced during coal burning in a coal-fired power plant. The major components in bottom ash is SiO 2, Al 2 O 3 and CaO. Because of its pozzolanic properties and its major components, it can be used in wet flue gas desulphurization as a partial substitute to the raw material limestone. In this study, the dissolution of bottom ash in adipic acid is investigated. Dissolution was done with different stirring speeds, acid concentration, solid to liquid ratios, ph and temperatures. The experimental results showed that dissolution rate of bottom ash increases with increase in stirring speed, acid concentration and temperature but decreases with increase in solid to liquid ratio and temperature. The activation energy for the process was found to be kJ/mol. Keywords Activation energy, coal bottom ash, desulphurization, dissolution, pozzolanic reaction. I. INTRODUCTION OTTOM ash is a combustion residue produced during the Bburning of coal in a power plant. It is entrained in flue gas and falls to the bottom of the furnace where it is collected as a waste. Bottom ash contains trace elements which can be harmful to the environment [1] - [4]. Therefore there is need for safe disposal in an environmentally friendly way. Increasing demand for energy from coal-fired power plants leads to large amounts of bottom ash being disposed as landfills because of large quantities produced. Bottom ash contains several toxic elements such as cobalt, lead, zinc cadmium and nickel, which can contaminate soil and ground water [5]. The extent of trace elements in bottom ash depends on the occurrence of these elements in feed coal. To reduce the amount of bottom ash to be landfilled and the effects of trace elements on the environment, it is desirable to reuse or recycle bottom ash. Many studies have been done on ways to recycle bottom ash [6] - [9]. Bottom ash can also be Lawrence Koech is with Chemical Engineering Department, Vaal University of Technology, Vanderbijlpark Campus, Private Bag X021, Vanderbijlpark, South Africa. Ray Everson is with the Department of Chemical and Minerals Engineering, North West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, South Africa. Hein Neomagus is with the Department of Chemical and Minerals Engineering, North West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, South Africa. Hilary Rutto* is with Chemical Engineering Department, Vaal University of Technology, Vanderbijlpark Campus, Private Bag X021, Vanderbijlpark, South Africa (phone: ; hilaryr@vut.ac.za) *Corresponding author: Tel.: ; address: hilaryr@vut.ac.za used in construction industry for making concrete, in road construction, as aggregate and as a fill material. Bottom ash can also be used as a partial substitute of raw material in wet flue gas desulphurization process because of its pozzolanic properties. Bottom ash is mainly composed of SiO 2 and Al 2 O 3 which are essential ingredients for pozzolanic reaction. Pozzolanic reaction increases the surface area therefore increasing the reactivity of the sorbent with SO 2 in flue gas [10]. Many studied have done on the pozzolanic properties of bottom ash [11] - [14]. It has been proven that due to its pozzolanic activity, it leads to formation of products with increased surface area. Dissolution studies on limestone for flue gas desulphurization has widely been done [15] [19], this study focuses on the dissolution of bottom in adipic acid with a view of being used as partial substitute to raw material (limestone) in wet flue gas desulphurization. Bottom ash was obtained from Eskom coal-fired power plant. The effects of temperature, stirring speed, ph, acid concentration and solid to liquid ration was investigated. The calcium ions leached into solution at the end of experiment was determined using Atomic Absorption Spectroscopy and EDTA method. II. METHOD AND MATERIALS A. Materials Bottom ash was obtained from Eskom coal-fired power plant. The raw materials were crushed using a ball mill and sieved to different particle sizes using shaking screen sieves. The chemicals used; EDTA, sodium hydroxide, adipic acid and metal ion indicator were obtained from a local supplier B. Method A predetermined amount of bottom ash was be added to the reactor vessel and the temperature, solid to liquid weight ratio, acid concentration, agitation and ph was varied according to the experimental design represented in Table I. The reaction temperature was controlled by a hot plate stirrer. The ph of the reaction mixture was determined using ph electrode inserted in the solution and connected to a ph 200 1/8DIN ph controller. The controller has specific ph sensitivity of ph When the ph exceeds the set value, the pump is activated to add acid to the reaction vessel and lower the ph value to the set point. The experimental set up is illustrated in Figure 1. A sample was then be removed, filtered and analyzed for calcium ions using Atomic Absorption Spectrophotometer (AAS) and 82

2 EDTA method. The calcium ion conversion is evaluated as: Dissociation of calcium complexes and calcium hydroxide precipitation in the bulk of the solution is due to super saturation. This is the main reason why it improves the surface area of sorbents to be used in flue gas desulphurization. TABLE I EXPERIMENTAL DESIGN FOR DISSOLUTION OF BOTTOM ASH Parameter values Temperature ( C) b 75 ph 5.5 b Solid to liquid ratio (wt. %) b Acid concentration (M) b 0.15 Stirring speed (rpm) b The hydrated Ca(OH) 2 from fly ash can be utilized more compared to the utilization of pure Ca(OH) 2. This is due to pozzolanic reaction, with fly ash being the source of silica [10]. Pozzolanic reaction: (6) (7) b The process parameter that will be kept constant during the experiment ph ELECTRODE ph CONTROLLER STIRRER HEATING PLATE REACTION VESSEL PERISTALTIC PUMP ACID SOLUTION BEAKER WEIGHING BALANCE COMPUTER CABLE Fig. 1 Experimental set up for dissolution of bottom ash in adipic acid C. Characterization of bottom ash The chemical and physical properties of bottom ash was done using XRF. This is represented in Table II. III. RESULTS AND DISCUSSION A. Mechanism for dissolution of bottom ash The mechanism for dissolution of CaO in the presence of adipic acid is as follows: Adipic acid dissociation: (1) CaO dissolution by adipate complexation: (2) CaO dissolution by hydrogen complexation: (3) Adipic acid formed in the bulk can further dissolute CaO (4) (5) The above reaction increases the surface area which thus increases the reactivity of the sorbent because of the reacted Ca(OH) 2 in the reaction. B. Chemical analysis The chemical composition of bottom ash is shown in Table II. From the table, it can be observed that it is mainly composed of SiO 2, Al 2 O 3 and CaO which essential ingredients for pozzolanic reaction during dissolution and high surface area products could be formed. CaO is essential during sorbent reaction with SO 2 because calcium ion is the most active part in the chemo-sorption process [20]. TABLE II CHEMICAL ANALYSIS OF FRESH FLY ASH Component Wt% SiO TiO Al 2O Fe 2O MnO 0.07 MgO 1.59 CaO Na 2O 0.46 K 2O 0.86 P 2O Cr 2O LOI 3.25 H 2O 0.93 Total C. Effect of reaction variables on the dissolution of bottom ash 1) Effect of Stirring Speed This experiment was carried out at different stirring speeds of 100 rpm, 200 rpm, 300 rpm and 400 rpm using 0.1M adipic acid at a temperature of 333K, solid to liquid ratio of 7.5/100 g/ml and a ph of 5.5. The experimental results for this experiment is shown in Fig. 7(a) which indicates that the (8) 83

3 dissolution of bottom ash increases with increase in stirring speed. The increase in stirring speed causes a decrease in film layer, therefore causing an increase in dissolution rate. 2) Effect of Solid to Liquid Ratio The effect of solid to liquid ratio was done in the range of 5, 7.5, 12.5 and 15/100 g/ml. All other parameters were kept constant according to the experiment. The experimental results for this experiment is represented in Fig. 7(b). It indicates that the dissolution rate of fly ash increases as the solid to liquid ratio decreases. This is attributed to the decrease in the fluid reactant per unit weight of the solid as solid to liquid ratio increases. 3) Effect of Temperature To study the effect of temperature the experiment was done under the range of 303K, 318K, 333K and 348K. All other parameters were kept constant according to the experimental design. The experimental results for this experiment is represented in Fig. 7(c) and it can be seen that the dissolution rate increases with increase in temperature. Increase in temperatures increases the reaction rate because there is an increase in high energy collision between molecules. (a) 4) Effect of Acid Concentration To investigate the effect of acid concentration on dissolution of fly ash, different experiments were performed at ranges of 0.05M, 0.075M, 0.1M and 0.15M adipic acid. The solid to liquid ratio, temperature and stirring speed were kept constant at 7.5/100 g/ml, 333K and 200 rpm respectively. Fig. 7(d) shows the experimental results and it is observed that the dissolution rate of fly ash increases with increase in acid concentration. This is attributed to an increase in H+ ion activity in the liquid film therefore enhancing dissolution. 5) Effect of ph The effect of ph on dissolution of fly ash was studied under the range of 5.5, 6.0, 6.5 and 7.0. All other parameters were kept constant according to the experimental design. Fig. 7(e) represents the experimental results for this experiment. It is evident that dissolution rate increases with decrease in ph. This is because there is increase in mass transfer of ions as the ph drops. (b) D. Activation energy The Arrhenius plot shown in Fig. 4 is was used to determine the activation energy for the process. From the slope of the line, the activation energy was evaluated to be kJ/mol. The activation energy for reaction controlled by product layer diffusion is usually below 20kJ/mol [21]. Therefore, dissolution of bottom ash in adipic acid is controlled by product layer diffusion. (c) 84

4 IV. CONCLUSSION In this study, the effects of ph, temperature, stirring speed, solid to liquid ratio and acid concentration in investigated on the dissolution of bottom ash using adipic acid. It was found out that the dissolution rate increases with increase in acid concentration, stirring speed and temperature but decreases with increase in ph and solid to ratio. From the Arrhenius Plot, the activation energy was found to be kJ/mol. The Product layer diffusion was found to be the rate controlling step in the dissolution process. (d) Fig. 2 Effect of stirring speed (a), solid to liquid ratio (b), temperature (c), acid concentration (d) and ph (e) on the dissolution rate of bottom ash. Fig. 3 Arrhenius plot for dissolution of bottom ash in adipic acid. (e) REFERENCES [1] W. P. Linak and J. O. Wendt, "Trace metal transformation mechanisms during coal combustion," Fuel Process Technol, vol. 39, pp , [2] J. W. Kaakinen, R. M. Jorden, M. H. Lawasani and R. E. West, "Trace element behavior in coal-fired power plant," Environ. Sci. Technol., vol. 9, pp , [3] J. Llorens, J. Fernandez-Turiel and X. Querol, "The fate of trace elements in a large coal-fired power plant," Environ. Geol., vol. 40, pp , [4] S. V. Vassilev, C. G. Vassileva, A. I. Karayigit, Y. Bulut, A. Alastuey and X. Querol, "Phase mineral and chemical composition of composite samples from feed coals, bottom ashes and fly ashes at the Soma power station, Turkey," International Journal of Coal Geology, vol. 61, pp , [5] A. Baba and A. Kaya, "Leaching characteristics of solid wastes from thermal power plants of western Turkey and comparison of toxicity methodologies," J. Environ. Manage., vol. 73, pp , [6] K. Kayabal and G. Buluş, "The usability of bottom ash as an engineering material when amended with different matrices," Eng. Geol., vol. 56, pp , [7] H. Kurama and M. Kaya, "Usage of coal combustion bottom ash in concrete mixture," Constr. Build. Mater., vol. 22, pp , [8] S. B. Park, Y. I. Jang, J. Lee and B. J. Lee, "An experimental study on the hazard assessment and mechanical properties of porous concrete utilizing coal bottom ash coarse aggregate in Korea," J. Hazard. Mater., vol. 166, pp , [9] P. Chindaprasirt, C. Jaturapitakkul, W. Chalee and U. Rattanasak, "Comparative study on the characteristics of fly ash and bottom ash geopolymers," Waste Manage., vol. 29, pp , [10] N. Karatepe, A. Mericboyu and S. Küçükbayrak, "Effect of Hydration Conditions on the Physical Properties of Fly Ash-Ca(OH)2 Sorbents," Energy Sources, vol. 20, pp , [11] M. Cheriaf, J. C. Rocha and J. Pera, "Pozzolanic properties of pulverized coal combustion bottom ash," Cem. Concr. Res., vol. 29, pp , [12] C. Jaturapitakkul and R. Cheerarot, "Development of bottom ash as pozzolanic material," J. Mater. Civ. Eng., vol. 15, pp , [13] W. B. Johnson, Process for Treating Fly Ash and Bottom Ash and the Resulting Product, [14] D. A. Kopsick and E. E. Angino, "Effect of leachate solutions from fly and bottom ash on groundwater quality," Journal of Hydrology, vol. 54, pp , [15] R. H. Limo and C. Enweremadu, "The dissolution study of a South African magnesium-based material from different sources using a phstat," Chemical Industry and Chemical Engineering Quarterly,vol. 17, pp , [16] H. L. Rutto, "Effect of Addition of Ammonium Compounds on the Dissolution Rate of a South African Magnesium-Based Material," J. Chem. Eng. Japan, vol. 44, pp , [17] H. Rutto and C. Enweremadu, "Dissolution of a South African calcium based material using urea: An optimized process," Korean Journal of Chemical Engineering, vol. 29, pp. 1-8, [18] H. Rutto and C. Enweremadu, "A study on dissolution of a south african calcium based material in acetic acid solution for flue gas desulphurisation," in Electrical and Control Engineering (ICECE), 2011 International Conference on, 2011, pp

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