The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 2006) E-048 (P) November 2006, Bangkok, Thailand

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1 The nd Joint International Conference on Sustainable Energy and Environment (SEE ) E- (P) 1-3 November, Bangkok, Thailand Study of Factors Influencing Slag Formation Potential of Lignite under Pulverised Fuel Combustion Conditions: Elemental Partitioning in Lignite Ashes and Slags Suneerat Pipatmanomai 1,*, Nakorn Worasuwannarak 1, Bundit Fungtammasan 1, Sankar Bhattacharya, Sirinthip Bhakdisongkram 3, Chaiporn Paitoon 3 and Ampon Kitchotkul 3 1 The Joint Graduate School of Energy and Environment, King Mongkut s University of Technology Thonburi, Bangkok, Thailand Cooperative Research Centre (CRC) for Clean Power from Lignite, Melbourne, Australia 3 The Electricity Generation Authority of Thailand, Mae Moh, Lampang, Thailand Abstract: In this study, a dedicated 7-day sampling and data collection were carried out on two of Mae Moh power plants. The properties of feed lignite were varied from low to medium content. The amount of each element was measured, where the form of element was also identified for all collected ashes and slag with reference to their corresponding feed lignite. Comparison of the data from the 7-day sampling period with those from high samples and those collected during the slagging period was also performed. The objective of this work was to identify the possible reasons for increased slag formation. It will be useful to help solve the problem both under current and future operation. The study found that, not only calcium, the concentration of three other elements and their compounds iron, aluminium and silicon, plays a very important role in slag formation and its control. The tentative solution appears to be the monitoring of the concentration of four elements, i.e. oxides of calcium, iron, silicon and aluminium in the lignite ash, in laboratory investigations until a full solution regime is devised. Meanwhile, it is recommended to ensure through proper mining or blending that calcium oxide concentration is < % (same as at present), iron oxide concentration is ~15% and aluminium oxide concentration is > 15% at all times. Keywords: Lignite, Combustion, Ash, Slag, Mineral Matter 1. INTRODUCTION EGAT (the Electricity Generating Authority of Thailand) has been operating several mine-mouth lignite-fired power plants with a combined capacity of, MW. Although the lignites used are the coals of high sulfur, ash and moisture content, their utilisation has been satisfactory in both technical and environmental aspects. The FGD retrofit to control SOx emissions and the calcium oxide () content present in blended feed coal, which is the key composition determining the ash melting temperature, kept below acceptable level to prevent slag formation have been conducted [1]. It is not until recently that the slag formation in the pf boilers has been observed to increase, despite the content is still well within the recommended range from previous study, i.e. < % in ash []. The slag deposition has been found to lower boiler efficiency and cause damage to the system. This has led to periodical shut down and increasing maintenance costs. Moreover, the change of lignite properties as the mining progresses, especially the probable high content, will further complicate the problem. Apart from, other compositions such as silica, iron oxide, both directly and indirectly influence the behaviour of ash upon pf boiler conditions [1]. The mechanisms and factors affecting the slag formation, especially under Mae Moh power plant conditions, have not yet been clearly understood. Therefore, there is a need to assess the likelihood of the occurrence of this problem and its impacts on plant operation in the long term. It will be useful to help solve the problem both under current and future operation. In this study, a dedicated 7-day sampling and related data collection were carried out on two of Mae Moh power plants. The properties of feed lignite were varied from low to high content. The amount of each element was measured, where the form of element was also identified for all collected ashes and slag with reference to their corresponding feed lignite. The objective of this work was to identify the possible reasons for increased slag formation.. METHODOLOGY.1 7-day sampling The dedicated 7-day samplings were conducted for Unit and 9 during May 3 June 5,. For each unit, four samples including feed lignite (as happened that one feeding line led to both units), fly ash from one of the electrostatic precipitator hoppers, bottom ash and slag from the bottom ash hopper, were collected at each six-hour interval. With the knowledge that coal residence time between the mine and the boiler outlet can be up to 1 hours, it may then be possible to explain the pathway of the chemical and structural changes, which will probably give light to the chemical route affecting slag formation potential.. Analyses of samples Analyses were conducted on all samples mentioned in Section.1, including proximate and ultimate analysis of lignite, mineral analysis of ash, ash fusion temperature and forms of Fe, S, and Ca in lignite, ashes and slag...1 Proximate and ultimate analysis of lignite Proximate analysis was carried out using a TGA (LECO TGA-51) following ASTM D51-. Moisture, volatile matter and ash content were measured. Ultimate analysis was carried out using an elemental analyser ( Elementar -Vario Max) following ASTM D5373. The content of C, H, N and S were measured and the content of O was calculated by difference. Corresponding author: suneerat_p@jgsee.kmutt.ac.th 1

2 The nd Joint International Conference on Sustainable Energy and Environment (SEE ) E- (P) 1-3 November, Bangkok, Thailand.. Mineral analysis of ash Chemical composition of ashes was analysed using an X-Ray Fluorescence ( Bruker -S-Explorer) based on ASTM D3. Major and minor elements analysed including Na O, MgO, Al O 3, SiO, P O 5, SO 3, K O,, TiO, Fe O 3 and MnO were obtained. Samples analysed included fly ashes, bottom ashes and slags collected during the 7-day sampling period as well as the laboratoryprepared ashes from lignite sampled during the period. The results were obtained as the total weight percentage of each element. Preparation of ash from raw lignite in the laboratory employed the low temperature ashing, i.e. at C instead of the typical condition at 75 C. This is to ensure that no sodium transformation and Na-Ca eutectics transformation takes place. Samples were finely ground and well distributed in crucibles in a fine layer. Ashing at C was carried out for hours...3 Ash fusion temperature Ash fusion temperature was determined using an ash fusion furnace ( Carbolite -CAF ) following ASTM D157 under reducing environment, i.e. under N in these measurements. This is to simulate the condition at or near the boiler walls, the zone in which ash particles entrained in the flue gas tend to attach. The ash fusion temperature determined under reducing environment is normally lower than that determined under oxidizing environment. Both initial deformation temperature (IT) and fusion temperature (FT) were measured... Forms of S, Fe and Ca S can be categorised into pyritic S, organic S and sulfate S. The pyritic and sulfate S content were measured using an ICP-OES ( Perkin Elmer Optima 53 DV) following AS The organic fraction was calculated by subtracting the total S (obtained from..1) by the amount of pyritic and sulfate S. Ca is present in coal as inorganic Ca or the water-soluble fraction and organically bound Ca in fractions, the acid-soluble fraction and the acid-insoluble fraction. Fractionation can be done by the following procedure. 1) First measure the total Ca in ash using the method described in Section... ) For water-soluble component: digest a small sample (about 5 grams) in boiling water in a flask for 3 minutes. Then take the water (which now has water-soluble Ca) and find Ca-content in that water using atomic absorption spectroscopy (AAs). 3) For acid-soluble component: digest the solid residue (from )) in 1 % HCl for 3 minutes around 9 C. Cool down, and then analyse the acid (which now has only acid-soluble Ca in it) using AAs for Ca. 3. RESULTS AND DISCUSSION 3.1 Effect of chemical properties on ash fusion temperature of lignites collected during the 7-day sampling period As shown in Fig. 1, plots between IT&FT and chemical properties of lignite ash, including i) and Fe O 3 from the analysis of the chemical properties, ii) Pyrite (FeS ) which was calculated from the measured amount of pyritic S, iii) FeS /Total Fe, iv) Summation of and Fe O 3 and v) Al O 3 A decreasing trend of the initial deformation temperature (IT) can be observed as the amount of increases within the range available in this study, i.e %. Aluminium content was significant with over 15% aluminium oxide content. As the amount of Al O 3 increases, IT is found to increase. Compounds of aluminium are known to deform and melt at much higher temperatures. The presence of aluminium oxide or its compounds, therefore, dilutes the effects of low-melting eutectics of calcium, iron and silicon, increasing their liquidus temperature. The presence of aluminium compounds is, therefore, beneficial for slagging control. No clear trends are observed for Fe O 3, Pyrite (FeS ) and FeS /Total Fe. 3. Effect of chemical properties on ash fusion temperature of high Ca lignites Discussion on the effect of chemical properties on ash fusion temperature has also been extended to that of the high Ca lignites, which are the representatives of lignite reserve at Mae Moh mine. The analysis data of thirty-two high Ca lignite samples (codenamed from SP1 to SP3) previously obtained during December -7, 5, have been investigated. Since the ashes were prepared under a high-temperature condition and the ash fusion temperatures were determined under oxidizing environment, these data may not be readily compared or related to those of the normal Ca samples collected during the 7-day sampling period, which were ashed at low temperature, i.e. C, and melted under N atmosphere. However, some indications of the parameters influencing the slagging potential could be drawn. Some correlations between chemical composition of ash and ash fusion temperature can be seen, as shown in Fig.. IT shows an increasing trend with increasing (above about %), increasing +Fe O 3 and increasing SO 3. The opposite was found when increasing SiO and increasing Al O 3. Within the range of samples studied, both concentrations of SiO and Al O 3 are found to inversely proportional to the concentration of in ash (see Fig. 3). The lower SiO and Al O 3 proportion in high Ca samples, reducing the formation of slag precursors namely Calcium Silicate (CaSiO 3 ) and Calcium Aluminosilicate (CaAl Si O ), might be responsible for the observed higher IT. 3.3 Effect of mineral composition on fusion temperature of fly ash, bottom ash and slag Plots between IT&FT and and Fe O 3 of fly ash, bottom ash and slag from the power plant units and 9 are shown in Fig. (a) to (f). In general, there are higher levels of and Fe O 3 are observed in fly ash than in the bottom ash. Also, as opposed to lignite ash, the IT becomes more or less constant beyond >% in fly ash, bottom ash and slag. This requires further investigation through controlled laboratory study. However, Fe O 3 concentration has little or no effect on IT. The mineral composition of ashes and slag formed in units and 9 are also found different, although the feed lignite was the same.

3 The nd Joint International Conference on Sustainable Energy and Environment (SEE ) E- (P) 1-3 November, Bangkok, Thailand (%) FeO3 (%) Pyrite [FeS] (%) FeS/Total Fe FeO3 (%) AlO3 (%) Fig. 1 vs. chemical properties of lignite ash [ IT, FT ] free SO FeO3 1 3 SO3 1 3 SiO AlO3 Fig. Ash fusion temperature (oxidizing atmosphere) as a function of ash chemical composition of high Ca lignites [ IT, FT ] AlO3, SiO (%) AlO3 SiO (%) Fig. 3 Relationship between the concentration of SiO, Al O 3 and in ash 3

4 The nd Joint International Conference on Sustainable Energy and Environment (SEE ) E- (P) 1-3 November, Bangkok, Thailand a) Fly Unit FeO FeO3 b) Fly Unit 9 c) Bottom Unit FeO3 d) Bottom Unit FeO3 e) Unit FeO3 f) Unit FeO3 Fig. Plots between IT&FT and and Fe O 3 of fly ash, bottom ash and slag from the power plant units and 9 [ IT, FT ]

5 The nd Joint International Conference on Sustainable Energy and Environment (SEE ) E- (P) 1-3 November, Bangkok, Thailand 3. Form of Ca The fractions of water-soluble, acid-soluble and acid-insoluble Ca were quantified, as shown in Fig. 5. Each period of time count is hours, starting from 3. on May 3 to 1. on June 3. From Fig. 5, two principal observations could be made. First, the amounts of acid-soluble and water-soluble fractions in the raw lignite are not far from each other, but after undergoing combustion inside the furnace, the water-soluble Ca almost disappeared leaving the considerably high acid-soluble fractions. Second, raw lignite contains almost no acid-insoluble Ca, while all other ashes contain acid-insoluble Ca in a proportion comparable to acid-soluble Ca (a) Lignite (b) Fly ash at unit (c) Fly ash at unit (d) Bottom ash at unit (e) Bottom ash at unit (f) Slag at unit (g) Slag at unit 9 Fig. 5 Percentage concentration of each form of Ca [ water-soluble Ca, acid-soluble Ca, acid-insoluble Ca ] 5

6 The nd Joint International Conference on Sustainable Energy and Environment (SEE ) E- (P) 1-3 November, Bangkok, Thailand 3.5 Discussion on the analysis of data of samples during the slagging period (May 1-19, ) Boiler unit 9 experienced the tripping in the morning on May 1,. Due to technical difficulties, only two samples including one lignite sample and one fly ash sample could be collected for analysis. The analysis of ashes and their fusion temperature are shown in Table 1. Despite the very limited data, it could be clearly observed that calcium and iron oxide content as well as their summation in the lignite ash are comparable to those from the non-slagging days, indicating that calcium and iron oxide in isolation cannot be the main indicator. However, at the same time, the aluminium oxide content was very low, <% at all times. Table 1 Mineral composition of lignite and fly ash collected during the slagging period (May 1-19, ) Sample Lignite ash Ash Composition (%) Fusion Temp, C (Oxidation) SiO Al O 3 TiO Fe O 3 MgO Na O K O P O 5 MnO SO 3 IT FT Fly ash n.d. n.d.. CONCLUSION The 7-day sampling plant and analyses of the data have provided preliminary but significant insight into the slagging behaviour of the Mae Moh lignites inside the pf fired boiler. It is observed that so far the main criteria for slagging control in the plant was based upon the concentration of calcium in coal ash and heating value. However, based on the data from plant historical record, and the 7- day sampling plan, it is clear that concentration of three other elements and their compounds iron, aluminium and silicon, plays a very important role in slag formation and its control. From the study, the following conclusions can be made. From preliminary analysis of the 7-day sampling data, during which there was no or manageable slagging, the presence of aluminium content was significant, with over 15% aluminium oxide content. Compounds of aluminium are known to deform and melt at much higher temperatures. The presence of aluminium oxide or its compounds, therefore, dilutes the effects of low-melting eutectics of calcium, iron and silicon, increasing their liquidus temperature. The presence of aluminium compounds is, therefore, beneficial for slagging control. On the contrary, from the very limited data from the slagging days (1- May), calcium and iron oxide content as well as their summation in the lignite ash are comparable to those from the non-slagging days. This indicates that calcium and iron oxide in isolation cannot be the main indicator. However, at the same time, the aluminium oxide content was very low, <% at all times. The tentative solution appears to be the monitoring of the concentration of four elements, i.e. oxides of calcium, iron, silicon and aluminium in the lignite ash, in laboratory investigations until a full solution regime is devised. Meanwhile, it is recommended to ensure through proper mining or blending that calcium oxide concentration < % same as at present, iron oxide concentration ~15% and aluminium oxide concentration > 15% at all times. 5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the Electricity Generating Authority of Thailand (EGAT). Many thanks also go to the EGAT personnel for carrying out the sampling and analytical work.. REFERENCES [1] Scott, D.H. (1999) Ash behaviour during combustion and gasification, IEA Coal Research. [] Electricity Trust of South Australia (199) Mae Moh Combustion Test Program.