Determination of the most favourable parameters of coal concentrates allowing optimal utilisation of chemical energy of coal

Transcription

1 Determination of the most favourable parameters of coal concentrates allowing optimal utilisation of chemical energy of coal Zofia Blaschke 1, Wieslaw Blaschke 2, Stanislaw Blaschke 3 1 Polish Mineral Engineering Society, Wybickiego 7, Krakow, Poland, stanbla@min-pan.krakow.pl 2 Silesian University, Department of Mining and Geology, Akademicka 2, Gliwice, Poland, viesbla@min-pan.krakow.pl; rg5@polsl.gliwice.pl 3 Polish Academy of Sciences, Mineral and Energy Economy Research Institute, Wybickiego 7, Krakow 65, P.O. Box 49, Poland, stanbla@min-pan.krakow.pl Keywords: coal preparation, coal quality, coal concentrates, electricity generation, efficiency Abstract. In the preparation processes of coal for electricity generation, it is possible to obtain the concentrates of different quality. Depending on these parameters, at the process of coal burning the chemical energy of the coal is converted into electricity with different efficiency. The aim of the presented analysis is to determine at what condition of coal preparation the produced concentrates allow generation the same amount of electricity as raw coal, and what quality of the concentrate allow maximising the electricity generation. This analysis can be defined as a method of determination of the most favourable parameters of coal concentrates regarding maximum electricity production. Introduction Coal includes chemical energy that is converted at a power plant into electric energy with determined efficiency. The efficiency is dependent, among others, on coal quality parameters, since the efficiency of the boiler is a function of gross calorific value and content of ballast in coal [1, 2]. Coal quality parameters can be improved by beneficiation processes. Depending of the thoroughness of the preparation process (separation density) differences in the yield volume may be in the range of some tens percent. Losses of coal in rejects are connected with coal washing processes [3] since none of the processes can be accomplished with 100-percent efficiency and some coal instead of finding itself in the concentrates goes to rejects. If part of coal goes to rejects then the volume of chemical energy comprised in the concentrates is lower then chemical energy comprised in raw coal. Chemical energy from the concentrate is anyhow converted into electric energy with greater efficiency than in the case of converting of raw coal with higher ash content. Also the consumption of energy for own needs of the power plants itself is smaller. (for instead for milling processes). Therefore one should raise questions. Firstly whether to wash coal for electric power generation? Provided the answer is positive one need to answer the next question what concentrates should be produced for the power plant to achieve in it the optimum volume of electric energy? In order to provide answers to the aforementioned questions calculation methods were elaborated, which were presented for the first time at the International Coal Preparation Congress in New Delhi [4] and in the national publications [5, 6, 7, 8]. To prepare that method one needs knowledge from the sphere of research and calculation procedures, from coal washing processes and knowledge from the area of power generation and, especially information on the subject of how electric energy can be achieved from coal of a determined quality. The information was found in the subject matter elaboration of the Supreme Chamber of Control [1].

2 The authors of the mentioned elaboration analysed the influence of coal quality upon the parameters of a power plant in order to indicate purposefulness of coal fines washing for the needs of electric power generation. All the analyses were conducted with the assumption that coal is converted in the block of 200 MW installed power integrated in the power plant operating in the basic system of electric power generation. The authors of the elaboration concentrated mainly on the costs of electric energy generation. They took advantage, for that purpose, of the research by Energoproject and Thermal Techniques Institute of Warsaw Technical University as well as the results from control of 9 power plants by the Supreme Chamber of Control. They presented numerous information on graphs and tabular sets, among which there was also information on the volume of electric energy possible to be achieved from coal of given parameters, such as: gross calorific value on dry ash free basis Q s daf in the value of 7300 kcal/kg ( kj/kg), 7750 kcal/kg ( kj/kg), 8800 kcal/kg ( kj/kg), total moisture W t r = 12%, ash content as received for the aforementioned three kinds of coals in the values of 8,0; 9,0; 10,0; 11,0; 18,0 and 25%. disposable volume of coal 1000 Mg, combusted with standard volume of fuel oil (0,2 0,5% depending on the volume of ballast in coal). Taking advantage from the fact that in case of majority of Polish coals gross calorific value on dry ash free basis is the same as tested by the cited authors the results of their work can be used for determination of a thorough washing of coal from the needs of electric power generation. Assumptions of the method In the course of washing process, as already mentioned, one can assume various densities of separation obtaining concentrates with different ash content and different calorific value. One should, anyhow, remember the gross calorific value on dry bash free basis would be the same for all the concentrates. Utilising the data from the elaboration [1] one can determine dependencies between ash consent and the volume of net energy possible to be obtained from the concentrates with determined gross calorific value. The results, taken from the tables mentioned in elaboration [1] were transferred to the graph (fig. 1) in a co-ordinate system: x ash content as delivered, %, y volume of net energy possible to be obtained, MWh volume of net energy possible to be obtained, [MWh] Fig. 1 [2 4,6 8]. Dependence of net electric energy volume possible to be generated from 1000 Mg of coal (and necessary fuel oil) on ash content determined on the basis of data from the elaboration by the Supreme Chamber of Control [1]; sequent straight lines regard gross calorific value on dry, ash free basis: 1-30,56 MJ/kg, 2-32,45 MJ/kg, 3-34,33 MJ/kg ash content,

3 Straight lines were drawn via denoted points the equations of which, calculated by the least squares method, took the form as below: y = - 40,44 x +2514,97 for Q s daf = kj/kg (1) y = - 45,41 x +2764,60 for Q s daf = kj/kg (2) y = - 50,78 x +3029,80 for Q s daf = kj/kg (3) Using the aforementioned dependencies one can calculate volume of energy possible to be obtained from coal of any ash content by substitution of a given ash content to the equation responding the gross calorific value of the analysed coal. In order to calculate the volume of energy possible to be obtained from coal and foreseen concentrates, achieved at various gravity of separation, one should perform the following analyses and calculations for the coal under consideration: sieve analysis of raw coal fines, sink and float analyses of grain size that is going to be washed, determination of ash content A a % and analytical moisture W a % in gravity ranges and grain size ranges, determination of gross calorific value Q a s, kj/kg, in samples with ash content below 10%, determination of gross calorific value of investigated coal on dry, ash free basis, Q daf s, kj/kg, calculation of yields in particular grain size ranges and gravity fractions. Exemplary calculations of net electric energy possible to be obtained from quality different coals were shown for three coals being different with regard to gross calorific value. The coals were selected the gross calorific value of which responded to the values analysed in elaborations [1] referring to them as A, B, C. Having the results of laboratory investigations the co-ordinates of washability curves were denoted and then ash content re-calculated from theoretical concentrates and the feed into as received basis. The yield of the concentrates was referred to the whole of the feed. Having such prepared date (tables 1) the energy possible to be obtained was calculated by substituting to equation (1), (2) and (3) of sequent ash content values of theoretical concentrates from tables or by reading them out from the cross co-ordinates υ (ash content curves in sequent concentrates) and then multiplying the data by the yield of the concentrate. The calculations results were put together in the last column of tables 1. Table 1. Net volume of electric energy possible to be obtained from concentrates separated at various depth of washing and ideal conditions of coal separation (Q daf s = kj/kg) Yield of the concentrate, ϒ r Ash content A r 1,3 31,15 3,22 892,85 1,4 48,67 5, ,54 1,5 53,22 6, ,44 1,6 55,88 7, ,73 1,7 57,85 8, ,26 1,8 59,21 8, ,35 Separation gravity [kg/dm 3 ] 1,9 61,97 9,92 2,0 62,19 10,76 Volume of obtained energy [MWh] 1565,40 optimal concentrate 1544,43 balanced concentrate Raw coal 20 0,5 mm 75,52 19, ,27 Raw coal 20 0 mm 100,00 29, ,09

4 The volume of energy possible to be obtained for a determined mass of coal can be lower, greater or equal to the volume of energy possible to be obtained from the input mass of raw coal. The volumes will be the ones on the basis of which one will determine the most favourable quality parameters of the concentrates. Determination of energetically balanced concentrates and energetically optimal concentrates In order to univocally determine the most favourable gravity of coal separation for power generation the following terms have been introduced: energetically balanced concentrate concentrate of coal from which one can obtain the same amount of electric energy as from raw coal (feed for washing), energetically optimal concentrate concentrate of coal from which one can obtain the maximum amount of electric energy. If the conducted calculation indicates that the highest electric energy can be obtained from raw coal one can resign from further calculations acknowledging that such coal should not be washed for power generation purposes. One can also change the range of grain sizes of coal directed for washing and re-start the analysis from the very beginning. If the performed calculation indicated that from investigated coal, under ideal conditions, one can obtain energetically optimal concentrate then one should assess what the effects would be after washing of such coal under industrial conditions. For this one should calculate the yields of concentrates and ash contents in them at several separation gravities and several assumed magnitudes of probable dissipation Ep [9]. It should be reminded here that gross calorific value on dry ash free basis will continue to be the same as in input coals accepted for the analyses. Possible to be obtained amount of energy should be recalculated for the foreseen yields of concentrates as well as the ash content in them, achieved (recalculation with the use of various Ep values) in industrial conditions, taking advantage of the same dependencies as previously. Raw coal fines mm Raw coal fines mm Concentrate -1.3 g/cm Concentrate -1.4 g/cm3 Concentrate -1.5 g/cm3 Concentrate -1.6 g/cm3 Concentrate -1.7 g/cm3 Concentrate -1.8 g/cm3 Concentrate -1.9 g/cm3 Concentrate -2.0 g/cm Fig 2. Amount of net electric energy possible to be obtained from various coal products. Concentrates obtained in ideal conditions of separation (Ep = 0). Performing respective calculations we can assess at which density of separation and in what equipment (separation accuracy Ep) coal for electric power generation should be washed to obtain maximum possible net amount of electric energy from the coal being at disposal. For the exemplary calculations conducted (mines A, B and C) the most favourable quality parameters of the concentrates from particular investigate coals were presented in table 2 (for industrial conditions).

5 Table 2.Quality parameters of energetically balanced concentrates and energetically balanced concentrate allowing to obtain the same amount of electric energy as from raw coal Yield, ϒ Mine r Energetically balanced concentrate Ash content, A r Calorific value, Q i r [MJ/kg] Sulphur content, S t a A 56,64 7,45 25,173 0,68 B 62,19 10,76 21,720 0,92 C 62,82 9,87 22,100 0,90 Energetically optimal concentrate A 62,11 8,83 24,374 0,72 B 61,97 9,92 22,680 0,88 Discussion of the results from analyses and calculation Analysing the results achieved for discussed coal and previous experiences it has been determined that after beneficiation of coal one can obtain the same or greater amount of energy from the same mass of coal. To obtain energetically optimum concentrate only the waste rock should be separated. It can be accomplished at full range of beneficiation, i.e. with slimes washing inclusive. The increase of energy possible to be obtained from the concentrate is greater the more free grains of waste rock are in the feed. Under industrial conditions conductance of washing process in such a way as to obtain energetically optimal concentrate is hard to be accomplished. Under industrial conditions it is much easier to obtain energetically balanced concentrate. Bearing in mind the protection of coal deposits, its beneficiation should be carried out at separation gravity allowing to obtain energetically optimal concentrate. Nevertheless the economic analysis that takes into account the interests of the producers, customers and environment protection may prove that separation gravity should be lower. Washing process should not, anyhow, be carried out at separation gravity lower than it is necessary to obtain energetically balanced concentrate. Summary Analysing the problem of washing depth of coal for the combustion in power plants one should take into account the possibilities to obtain maximum possible amount of electric energy. It can be accomplished by combustion of either concentrates or (such cases also occur) raw coals. The selection will depend on information received after determination of energetically optimal concentrates. Determining quality parameters of energetically balanced concentrates one gets an answer to the question: whether to combust raw coals or washed coals. In both cases the same amount of electric energy is obtained. Concentrates allow, anyhow, to send for combustion much smaller mass of coal what exerts an impact upon the cost of electric energy generation. They will be much lower that the cost of obtaining of this energy from raw coals. Nevertheless the higher will be the price of the concentrate as against the price of raw coal. Taking of decision will thus depend on determination which coal, from the economic point of view, should be used in particular power plant. Such an analysis should, anyhow, also take into account the cost of rejects storage (higher in case of raw coals) and the costs of fees for emission of SO 2 and particulate matter (also higher in case of raw coals). The presented method is thus an assessing criterion allowing for selection of the most favourable quality parameters of combusted coal.

6 The method is elaborated for particular power plants (investigated by the authors of the paper [1]). It is suggested to perform respective analyses for other power plants. Then each of the coal users, utilising coal for electric energy generation, will be provided with instruments allowing him for negotiating (with coal producers) deliveries of coals of the most favourable quality parameters for him. References [1] F. Ciosek, M. Piasecki: Analysis of steam coal fines washing profitability on the example of the mine- professional power plant arrangement. (Część II opracowania problemowego w sprawie wzbogacania węgli energetycznych. Najwyższa Izba Kontroli, Zespół Górnictwa i Energetyki. Warszawa. 1975). [2] Z. Blaschke: Method for determining the most suitable coal quality parameters from the viewpoint of optimal utilization of chemical energy of coal. (Journal of the Polish Mineral Engineering Society. Special issue No S.3(10) 2003). [3] Z. Blaschke: An attempt of coal wastes quality assessment from the viewpoint of power losses caused by coal content in the wastes. (Materiały XIV Krakowskiej Konferencji Przeróbki Kopalin. Kraków 1980). [4] Z. Blaschke: The method of determining of the optimum power coal dressing with a view of the amount of energy obtainable from it. (Paper E 2, Proceedings of the 9 th International Coal Preparation Congress, New Delhi, India 1982). [5] Z. Blaschke: Possibilities of efficient utilisation of coal slimes in power generation. (Materiały II Konferencji z cyklu Zagadnienia surowców energetycznych w gospodarce krajowej. AGH Kraków 1981). [6] Z. Blaschke: Sposób określania optymalnej głębokości wzbogacania węgla energetycznego ze względu na możliwą do uzyskania z niego ilość energii. (Budownictwo Węglowe Projekty Problemy, rok XXVII, nr 6(309) 1983). [7] Z. Blaschke, W. Blaschke: The way to determine energetically optimal quality parameters of coals at the example of coal preparation plant PPMW Biskupice. (Materiały XVI Konferencji Zagadnienia surowców energetycznych w gospodarce krajowej. IGSMiE PAN Kraków 2002). [8] Z. Blaschke, W. Blaschke: The assessment of coal washing purposefulness for the needs if power generators in individual coal preparation plants. (Studia, Rozprawy, Monografie, nr 116, IGSMiE PAN, Kraków 2002). [9] J. Nawrocki: Analytical-graphic methods of operation assessment of gravity separators. (Wydawnictwo Śląsk, Katowice 1976).