An investigation of the effect of ammonium sulphate on thermal behavior and kinetics of coal/bark co-combustion

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1 The 4th IFToMM World Congress, Taipei, Taiwan, October 250, 205 DOI Number: /IFToMM.4TH.WC.OS20.04 An investigation of the effect of ammonium sulphate on thermal behavior and kinetics of coal/bark co-combustion TheerapongLaongnual a Department of Mechanical Engineering, Faculty of Engineering, KhonKaen University, KhonKaen, Thailand. Anusorn Chinsuwan b Department of Mechanical Engineering, Faculty of Engineering, KhonKaen University, KhonKaen, Thailand. Abstract: According to the problems of biomass combustion especially bark can result in deposit formation and superheater corrosion of boilers due to high alkali and chlorine content (mainly is gaseous KCl). The results from several researchers found that KCl can be reduced by co-combustion with S from coal or using of additive especially the ammonium sulphate ((NH 4) 2SO 4) by injection at the outlet of furnaceto form sulphation reaction. The sulphation reaction can convert gaseous KCl toa less corrosive alkali sulphate which the ammonium sulphateperformed significantly better than coal.but injection of ammonium sulphate in large scale boilers may be difficult to do due to piping and handling system need to install. Inthis work try to propose another method for using the ammonium sulphate by mixing with the fuels (coal/bark) directly. Therefore, effect from mixing of the ammonium sulphate should be considered before apply to the commercial scale boilers.the aim of this work is to studythe effect of ammonium sulphate on thermal behavior and kinetics of coal/bark co-combustionat difference S:Cl ratio (4: and 6:) by thermogravimetric analysis method (TGA) and also ash composition will be analyzed by x-ray fluorescence (XRF). Keywords: co-combustion, kinetics analysis I. Introduction At the present, using of biomass as a fuel has become very important for the production of clean thermal energy by combustion. This is due to it neutral on the issue of the emission of CO 2, the most important gas causing global warming, so itdoes not cause global warming. Although the biomass has many advantages, it also has disadvantage that cannot be avoided. Combustion of biomass can result in ash deposition and high corrosion that effect to the thermal efficiency of boilers, and expensive shutdown. In general, biomass is composed of high alkali (Potassium, K) and Chlorine (Cl) and low sulphur (S) content compare to coal. K and Cl are released during combustion process mainly in gaseous KCl form. High level of KCl in flue gas can result in deposition and high temperature corrosion processes of superheater tubes in boilers [2,6]. Deposit formation and superheater corrosion can be reduced by co-combustion with S element in coal/lignite or using of additives ammonium sulphate ((NH 4) 2SO 4) by sulphation reaction [,4]. S/Cl ratio is the indicator of the a theerapl@scg.co.th b corresponding author: anuchi@kku.ac.th reaction. The ammonium sulphate and S in coal react with KCl produces a less corrosive alkali sulphate [2,6]. The ammonium sulphateperformed significantly better than elemental sulphur [3,5,6] due to the presence of gaseous SO 3 is greater importance than that of SO 2 for the sulphation of gaseous KCl [5]. All pulp & paper mills in Thailand are also facing about the alkali problems in their boilers which eucalyptus bark is used as main fuels. The bark is a by-product from debarking process consisting of high alkali and chlorine contents. Phoenix Pulp & Paper company (PPPC) is one of the largest pulp & paper mills in Thailand consumeseucalyptus bark and coalover 30,000 tons and 37,400 tons a year respectively which corresponding to the ratio of coal/bark 86:4 by thermal. The biomass boilersupplies steam for steam turbineto generate electricity 340days a year. However the boiler could not operate continuously, it needs to shutdown every 2 months to clean all superheater tubes due to ash decomposition problem. The deposition caused to expensive shutdown for boiler, so this problem should be solved. According to Kassmanet. al. [6], ammonium sulphate solution was injected into the furnace at the cyclone inlet of a fluidized bed boiler.the gaseous KCl content in the flue gas was converted to less alkali-sulphate before moving to heat exchangers such as superheaters, economizer, and air pre-heater. In practice, it is difficult to inject the ammonium sulphate tolarge scale boilers in the solution form, due to piping and chemical handling system need to install. In this work, the behavior and kinetics of coal/bark combustion with solid ammonium sulphatemixing were investigated. Thermogravimetric analysis (TGA) and X-Ray Fluorescence (XRF) were used in this study. The results will be useful information which has not been found in literature. II.Experimental A. Experimental setup The experiments were carried out for co-combustion of two types of fuels: sub-bituminous from Indonesia and eucalyptus bark from KhonKaen, Thailand. There are five samples: Coal, Bark, CB4, CB4A4, and CB4A6. The first three samples are reference cases forcomparing with ammonium sulphate cases.the sample CB4 corresponds to the ratio of coal/bark 86:4 by thermal.thecb4a4, and CB4A6 samples correspond to S/Cl ratio 4:, and 6: respectively. Proximate and ultimate analysis of the fuels is shown in Table.

2 Table: Proximate analysis Proximate and Ultimate analysis of the fuels Coal Bark Moisture a Volatile matter b (%) Fixed carbon b (%) Ash b (%) Ultimate analysis b (%) C H O N S K 0.57 Na 0.04 Cl 0.3 Calorific value b, HHV (MJ/kg) 23,027 3,927 a as received basis, b dry basis B. Thermogravimetric Analysis (TGA) Similar to several researchers [7-4], TGA was used to investigate the thermal behavior of blending fuelsin this work. The experiments were carried out by the simultaneous thermal analyzer, NETZSH model STA 449F3. The maximum weight loss temperature was estimated as the temperature at which a derivative thermogravity (DTG) curve showed the peak value. Burnout temperature was detected based on the mass stabilization. The ignition (T i) and burnout temperature (T b) were defined as the temperature at which the combustion rate increases to wt%/min at the start of a major combustion and the temperature at which the combustion rate decreases to wt%/min at the end of combustion respectively. All of experiments were carried out at atmospheric pressure at a constant rate of heating up 0 C/min from 30 C to 850 C which were oxidized by oxygen and carried out by placing about 200 mg of dried sample on a platinum pan. C. Ash Analysis To study the results of combustion of each samples and effect from ammonium sulphate, ash analysis is necessary to know the ash elementals. The samples were prepared for five samples AsCoal, AsBark, AsCB4, AsCB4A4, and AsCB4A6 were analyzed by X-Ray Fluorescence Energy Dispersive Model XGT200. D. Kinetics Analysis In a heterogeneous solid-state reaction, the initial mass of the sample (w o) is decomposed to instantaneous mass w t at time t, hence the rate change can be written as a function of weight conversion ratio as: wo wt w w o wherew is the residue mass at the end of process. The reaction rate may be described by Arrhenius equation: () d k( T ) f ( ) (2) dt k(t) is the rate constant, according to the Arrhenius correlation: k( T) Ae E/ RT (3) wheret is the reaction temperature, A is the pre-exponential Arrhenius factor, E is the activation energy and R is the gas constant. f(α)is the function called the reaction model which describes the dependence of the reaction rate on the extent of reaction. It is defined as: f ( ) ( ) n (4) wheren is the reaction order. Substitution equations (3) and (4) into equation (2): d dt E/ RT n (5) Ae For non-isothermal TGA heating rate, β = dt/dt. Equation (5) can be written as: d A ( ) n E / RT e (6) dt Equation (6) can be rearranged as: d ln dt A E ln n RT According to equation (7), a plot of ln[(dα/dt)/(-α)n ] versus /T corresponds to a straight line. For the value of n which gives regression coefficient close to unity, A and E can be determined from the intercept and slope of the line respectively. The plots and the values of A and E were shown in Fig. 4 and Table 3 respectively. It was found that the results agree well with the Arrhenius. Their correlation coefficient (R 2 ) and reaction order are and respectively. Activation energy can be reduced by blending of the bark with the coal. The use of the ammonium sulphate as additive for the co-combustion causes a slightly increase in activation energy. III.Results and Discussion Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were used to investigate combustion behavior of the Coal, Bark, CB4, CB4A4 and CB4A6. The TGA and DTG results are shown in Figure and Figure 2 respectively. According to the results, the Coal and the Bark have difference profiles so they will have difference burning performance. The profiles indicate that the process can be divided into three stages: moisture evaporation, decomposition of volatile organic compound and char combustion. (7)

3 As the reactivity is directly proportional to the reactivity and inversely proportional to it, the effects of blending appeared to be suppressing the reactivity of the fuel mass, particularly in the pyrolysis stage of the bark blends. Based on volatile matter to fixed carbon ratio, the bark is expected to be more reactive than the coal. Four characteristic temperatures are investigated: the first initiation temperature where the weight loss first begins to fall, the second initiation temperature where the weight loss accelerates due to onset of char combustion, the third is the peak temperature where the loss is maximum, and the fourth is the burn-out temperature where the weight is constant indicating the completion of combustion. Weight (%w) Coal Bark CB4 CB4A4 CB4A Temperature [ C] Fig.: TG combustion profile for the samples Coal, Bark, CB4, CB4A4, and CB4A6 Weight Loss Rate [%wt/min] Coal Bark CB4 CB4A4 CB4A Temperature [ C] Fig.2: DTG combustion profile for the samples Coal, Bark, CB4, CB4A4, and CB4A6 A. Combustion behavior of individual fuels A. Bark The first stage of weight loss of bark starts from 36 C up to 27 C which is an initial moisture loss, about 43% of total weight. The second stage or combustion stage start from 27 C up to 487 C, at this stage the decomposition and release of volatile starts early approximately at 70 C and there are two peak of weight loss at the temperature of 3 C and 435 C, one single peak mainly due to volatile combustion and small peak in the higher temperature zone. The last region which has low reactive component and burnout at the temperature of 487 C, this effect can be clearly seen in DTG profile for bark in Fig. 2, which is known to decompose slowly over a very broad temperature range, so bark can be classified as highly reactive material, because of its weak bonds and high amount of volatile content. A2. Indonesian Coal After an initial moisture loss in the temperature range of 47-8 C which the weight loss reduced from 00 to 70 % (wt %), slightly steady while the temperature was still increasing to 240 C. The rate of weight loss start again due to pre-combustion mass gain due to oxygen chemisorption. In the case of coal, there is only single peak of weight loss in the combustion stage and higher temperature is required for combustion to start as compared to bark. The volatile release temperature for coal is around 26 C compared to 70 C for bark. This indicates that coal requires a higher temperature to release its volatile matter, which makes it difficult to initiate combustion.

4 Table 2:Thermal kinetic results of the samples Sample E (kj/mol) n A (s - ) R 2 S:Cl Coal , Bark , CB : CB4A : 2.4 CB4A : E: Activation energy, kj/mol, n: Reaction order, A: Frequency factor, R 2 : Correlation coefficient B. Co-combustion behavior of blend materials B.Indonesian coal/bark blends Blending of Indonesian coal and bark, CB4 is for reference and evaluate to the ammonium sulphate cases. From the Fig. and Fig.2 the TG and DTG curves of CB4 show three regions of weight loss and two peak temperatures same as coal, first region start from the temperature of 47 C to 20 C, its initial moisture will be removed here. The rate of weight loss increase again in the second region, at this region the range of temperature is 20 to 530 C, there is a difference here compare to coal case. In case of coal, the peak of the combustion region is more narrow than that of CB4 as shown in Fig.2 which is the effect of high volatile content in bark. The ignition temperature of CB4 is lower than that of coal hence the volatile release temperature is reduced. Table 2 shows the characteristic parameters which were obtainedduring co-combustion of bark with coal. From the table, it is clear that the volatile release temperature of coal is reducedby blending with bark. This is because volatile release of bark starts at lower temperature. Table 3:Thermal characteristic parameters of the samples Sample Ti( o C) Tb( o C) Tmax( o C) Rmax(%wt./min) Coal Bark / /3. CB CB4A CB4A T i: ignition temperature, T b: burn out temperature, T max: temperature corresponding to R max, R max: maximum rate Two samples of ammonium sulphate cases CB4A4 and CB4A6 show their thermal behavior in three regions (initial moisture loss, devolatization with combustion, and char combustion) and also their curves TG and DTG are similar to CB4. C. Ash analysis The ash analysis of the samples shown in Fig.3 (oxides form), AsCoal and AsBark consist of difference elements due to its composition, AsBark consists of high concentration of CaO can result in low melting temperature of ash leads to ash deposition or bed agglomeration in biomass boilers[5]. For the reference case AsCB4 CaO concentration is lower than AsBark, therefore low melting temperature ash is minimized by co-combustion with coal. Concentration[wt%] Fig.3: Ash composition ofthe samplescoal, Bark, CB4, CB4A4, and CB4A6 Although CaO can reduce by co-combustion with coal but the gaseous KCl is still released but cannot detect by ash analysis by XRF method. As mentioned, gaseous KCl can be reduced by the sulphation reaction which the presence of SO 3 is performed. The AsCB4A6 can show high concentration of SO 3 P2O5 Fe2O3 MnO2 TiO2 CaO K2O SO3 SiO2 Al2O3 MgO B2.Indonesian coal/bark blends with the ammonium sulphate The aim of ammonium sulphate cases CB4A4 and CB4A6 is to investigate the effect on thermal behavior and kinetics of the reference case CB4. The S:Cl ratio of two samples are 4: and 6: while the CB4 is 0.6: by molecular weight. The ratio of S and Cl is the indicator for sulphation reaction as per the result from reference research, sulphation reaction was more efficient with higher S:Cl ratio[5]. The ammonium sulphate cases need more activation energy if compare to reference case CB4, additional energy need for decomposition of ammonium sulphate but still lower than Coal case. It is noticeable that the ignition temperature (Ti) and the temperature corresponding to maximum rate (Tmax) of all ammonium sulphate cases are higher than reference case CB4 and Coal, this may be the effect from retardant property of ammonium sulphate, the combustion process of ammonium sulphate cases occur at higher temperature.

5 I. IV ln ((da/dt)/(-a)n) y = -86.4x R² = ln ((da/dt)/(-a)n) y = 8x R² = /T - /T II. V ln ((da/dt)/(-a)n) y = -829x R² = /T ln ((da/dt)/(-a)n) - y = x R² = /T III ln ((da/dt)/(-a)n) y = 979.7x R² = /T Fig.4: Curves of fitting of kinetic model for: I. Coal, II. Bark, III.coal/bark (CB4), IV. CB4A4, and V. CB4A6

6 IV.Conclusions The results show that the effect of ammonium sulphate cases which be summarized below: The ignition temperature of CB4 was lower than Coal, the activation of CB4 was also lower than that Coal case. This is the effect of blending of coal and bark that bark can improve de-volatile property of coal. For the ammonium sulphate cases, the effect on thermal behavior compared to each other is not significant difference (CB4A4 and CB4A6), if compared to CB4 there are only two parameters that show significant difference T i and T max. The ammonium sulphate cases are higher than CB4 around 20 C and 40 C respectively. These may be the effect from retardant property of ammonium sulphate, highest concentration of presence SO 3 which is necessary for sulphation reaction was found in AsCB4A6 case (at S:Cl is 6:). V.Future Works The results from using of the ammonium sulphate in solid form should be compared and evaluated with the solution form which has been performed by several researchers based on same conditions of combustion in the boilers. VI.Acknowledgements Phoenix Pulp & Paper Public Company Limitedis acknowledged for providing samples and inspirationfor this investigation. The authors thank Mr. JesdaSaeliang for the opportunity. [] Hani H. S., Ahmad H., Arshad A. S. and Farid N. A. Pyrolysis and combustion kinetics of date palm biomass using thermogravimetric analysis, Bioresource Technology 8 (202) [2] Zeqiong X. and Xiaoqian M. The thermal behaviour of the co-combustion between paper sludge and rice straw, Bioresource Technology 46 (203) [3] Munir S., Daood S.S. andnimmo W.,Cunliffe A.M. and Gibbs B.M.. Thermal analysis and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal under nitrogen and air atmospheres, Bioresource Technology 00 (2009) [4] Gil M.V., Casal D., Pevida C., Pis J.J. andrubiera F. Thermal behaviour and kinetics of coal/biomass blends during co-combustion, Bioresource Technology 0 (200) [5] Vesna B., Lars-Erik A. and Edgardo C. Z.The Role of Limestone in Preventing Agglomeration and Slagging during CFB Combustion of High-Phosphorous Fuels. [6] KassmanH., BäfverL., andåmand L. Sulphation of Gaseous KCl in a Biomass Fired CFB Boiler. Proceedings of the European Combustion Meeting References [] Yuanyuan S., Chunbao X., Jesse Z., Fernando P., JinshengW., Hanning L. and Chadi B. Ash Deposition in Co-firing Three-Fuel Blends Consisting of Woody Biomass, Peat, and Lignite in a Pilot-Scale Fluidized-Bed Reactor, American Chemical Society (200). [2] Markus B., Sonja E., Rainer B. and Kari M. Condensation in the KCl NaCl system, Fuel Processing Technology 05 (203) [3] Davidssona K.O., Åmanda L.-E., Steenarib B.-M., Elleda A.-L., Eskilssonc D. andlecknera B. Countermeasures against alkali-related problems during combustion of biomass in acirculating fluidized bed boiler, Chemical Engineering Science 63 (2008) [4] Zuhal G., Yusuf G., Nevin S. and Ekrem S. Investigation of ash deposition in a pilot-scale fluidized bed combustor co-firing biomass with lignite, Bioresource Technology 00 (2009) [5] Håkan K., Linda B. and Lars-Erik A. The importance of SO2 and SO3 for sulphation of gaseous KCl An experimental investigation in a biomass fired CFB boiler, Combustion and Flame 57 (200) [6] Håkan K., Jesper P., Britt-Marie S. and Lars-Erik A. Two strategies to reduce gaseous KCl and chlorine in deposits during biomass combustion - injection of ammonium sulphate and co-combustion with peat, Fuel Processing Technology 05 (203) [7] Marisamy M., Tomoaki N. and Kunio Y. A comparison of co-combustion characteristics of coal with wood and hydrothermally treated municipal solid waste, Bioresource Technology 0 (200) [8] Otero M., Calvo L.F., Gil M.V., Garcıa A.I. andmoran A. Co-combustion of difference sewage sludge and coal: A non-isothermal thermogravimetric kinetic analysis, Bioresource Technology 99 (2008) [9] Kaihua Z., Kai Z., Yan C. and Wei-ping P. Co-combustion characteristics and blending optimization of tobacco stem and high-sulfur bituminous coal based on thermogravimetric and mass spectrometry analyses, Bioresource Technology 3 (203) [0] Jillian L. G. and Chao L. Impact of blend ratio on the co-firing of a commercial torrefied biomass and coal via analysis of oxidation kinetics, Bioresource Technology 49 (203)