Report on comparison among current industrial scale lignite drying technologies

Lignite constitutes a major energy source and has long been used for energy production despite its contribution in greenhouse gas (GHG) emissions, as a fossil fuel. For example, 27.4% of Germany’s electricity originates from lignite power plants, while in Greece more than 55% of its electric energy consumption is provided by lignite. 45% of the total global coal reserves consist of low-rank coals (LRCs) such as lignite. With this background, the utilization of lignite for energy production is expected to remain a common practice in the decades to come since the availability of lignite is considerable in many countries of Europe and the world (e.g. Germany, Poland, Greece, USA, and Australia). Therefore, problems regarding the combustion and use of lignite should be addressed in a more efficient and environmentally friendly way. One of the main existing problems is the high moisture contained in raw lignite as received from the mine. The high moisture content results in higher CO2 emissions per unit of energy produced and is responsible for high capital and transport costs as well as other technical problems such as reduction in coal friability and difficulties in its blending and pneumatic transportation. Therefore, processing of lignite through drying is considered of great interest in the implementation of energy production in lignite power plants. Taking into account the significance of the subject and the usefulness of such an attempt, an overview of the currently existing drying technologies, including both evaporative and non-evaporative drying methods is reported in the present paper

Thermodynamic analysis and comparison of retrofitting pre-drying concepts at existing lignite power plants

Lignite is considered as a domestic and abundant energy source for several countries. However, its high ash and moisture content have a negative effect on power plant efficiency, on cost of electricity (COE) and consequently on CO2 emissions. The aim of the present work is the investigation and optimization of existing lignite pre-drying concepts and their improvement in terms of overall plant efficiency and integration. The main process parameters examined are the heat source for drying and the respective drying medium. In the conventional lignite drying process, hot recirculating flue gas is used as a heating medium, while in the current state-of-the art pre-drying concepts, a fluidized bed drying system is considered. Different concepts are also examined including a) the utilization of preheated air as heating medium and b) the optimized integration of a heat pump as a heat source for the drying process. Based on the performed thermal cycle calculations, the plant efficiency increase is evaluated. The results of the study indicate that higher plant efficiency is expected, when focussing on the optimized pre-drying process scheme and its integration with the overall steam

Development of a computational code for the prediction of pulverised fuel boiler's thermal data-application in a Greek steam boiler for the case of raw and dry lignite co-firing

122 σ.Στην παρούσα διπλωματική εργασία αναπτύσσεται υπολογιστικός κώδικας για την εκτίμηση των θερμικών μεγεθών που αφορούν λέβητες κονιοποιημένου καυσίμου ατμοπαραγωγών σε εγκαταστάσεις ατμοηλεκτρικών σταθμών. Ο υπολογισμός γίνεται σε κάθε τμήμα του λέβητα, τόσο στην εστία όσο και στους επιμέρους εναλλάκτες, δηλαδή τον υπερθερμαντή , τον αναθερμαντή και τον οικονομητήρα/προθερμαντήρα νερού. Ο κώδικας αποτελείται από ένα τμήμα που εξασφαλίζει τη διαδραστικότητα και συνεργασία με το χρήστη και είναι γραμμένο σε γλώσσα VBA (Visual Basic for Applications) που είναι ενσωματωμένη στο Excel, κι από ένα τμήμα σε FORTRAN που εκτελεί τους κυρίως υπολογισμούς και έχει μετατραπεί σε αρχείο dll για εκτέλεση μέσω της VBA. Κατ'αρχήν εξετάζεται η εγκυρότητα των αποτελεσμάτων του κώδικα και γίνεται σύγκριση των υπαρχόντων δεδομένων με τα αποτελέσματα που προκύπτουν κατά την εφαρμογή σε μονάδα ελληνικού ΑΗΣ (Άγιος Δημήτριος V) όπου γίνεται χρήση φυσικού κονιοποιημένου λιγνίτη με ανακυκλοφορία καυσαερίου. 'Επειτα γίνεται εκτίμηση για τις τιμές των μεγεθών που προκύπτουν κατά την περίπτωση μικτής καύσης φυσικού και ξηρού λιγνίτη , δηλαδή μετά από αντικατάσταση μέρους του φυσικού λιγνίτη με ξηρό λιγνίτη που προέρχεται από διεργασία προξήρανσης. Συνοψίζοντας, στα κεφάλαια που περιέχονται στην εργασία αυτή καλύπτεται η θεωρία που χρησιμοποιείται στον κώδικα για τον υπολογισμό των λεβήτων σύμφωνα με τους γερμανικούς κανονισμούς FDBR κι ακόμα περιλαμβάνονται σε πινακοποιημένη μορφή καθώς και σε σχηματικά διαγράμματα τα δεδομένα της υπό εξέταση εγκατάστασης και τα αποτελέσματα από την εκτέλεση του κώδικα, ενώ ολοκληρώνοντας, σχολιάζονται τα αποτελέσματα αυτά και παρατίθενται τα συμπεράσματα από την παρούσα εργασία. Τέλος, για λόγους πληρότητας παρατίθεται αυτούσιος ο κώδικας υπό μορφή παραρτήματος , μαζί με συνοπτικές επεξηγήσεις για τη λειτουργία του.In the present diploma thesis, a computational code is developed for the prediction of thermal quantities associated with pulverised fuel steam boilers in thermoelectric power station facilities. The calculation takes place in every part of the steam boiler including the furnace and the individual heat exchangers, ie the superheater, the reheater and the economizer. The code consists of a section responsible for the interface of the program and for the interactivity and cooperation with the user, which is developed in VBA (Visual Basic for Applications), which is embedded in Ms Excel, and of a section in FORTRAN performing the main calculations and which is converted in a dll file, executable by VBA . First, the validity of the results of the code is examined and existing data are compared with the results deriving from the application in a unit of a greek thermoelectric power station (Agios Dimitrios V) where pulverised raw lignite with gas recirculation is being used. Then we estimate the value of the quantities which derive from the raw and dry lignite co-firing case, which is the case of replacing part of the raw lignite with dry lignite coming from pre-drying process. In summary, the chapters included in this thesis cover the theory used in the code for the calculation of the steam boiler quantities according to the german FDBR regulations. Furthermore, the data of the unit under consideration as well as the results deriving from the execution of the program are presented in tabular form and diagrams. A thorough discussion of those results follows and the final conclusions from the present thesis are exposed. Finally, the code is listed as an annex, along with brief explanations on its operation.Ιωάννης Ε. Βιολιδάκη

Dynamic modeling and energy analysis of renewable heating and electricity systems at residential buildings using phase change material based heat storage technologies

Several heat storage systems for domestic application can be used to promote Renewable Energy Sources (RES) penetration by storing excess energy, which would otherwise be rejected during curtailments. The present study investigates two types of PV powered latent heat storage technologies for delivering heat and/or electricity at residential buildings and compares them against a reference case which uses a conventional heat pump as a heating system, also powered by a PV. One technology involves a low temperature PCM thermal energy storage system (LT-TES) heated by either an electric resistance or a heat pump and the other technology involves an ultra-high temperature TES (UHT-TES). It is revealed that any case with heat storage is preferable for better exploiting the produced renewable energy than the conventional one which does not include heat storage. The most favorable from this aspect are the cases which include the UHT-TES, which provide both heat and electricity. Focusing exclusively on heat supply, the most preferable case from a technical performance aspect is the one which includes a heat pump and a LT-TES system, albeit not providing any electricity. On the other hand, the most advantageous only in terms of electricity supply is the case which includes the UHT-TES system

Drying of Lignite of Various Origins in a Pilot Scale Toroidal Fluidized Bed Dryer using Low Quality Heat

An experimental study was carried out for lignites of different places of origin, i.e., Poland, Greece, Romania and Australia, using a toroidal bed dryer. The effect of the temperature on the drying efficiency, including the loss of moisture content over time under fixed drying conditions was the subject of the investigation. The main goal was to confirm the possibility of the use of a toroidal bed as a base for a drying system that could utilize low quality heat from sources such as flue gases from a boiler and determine the optimum parameters for such a system. The conducted study has conclusively proven the feasibility of the use of low temperature heat sources for drying lignite in a toroidal bed. A moisture content of 20% could be achieved for most of the tested lignites, using the toroidal bed, with reasonably short residence times (approx. 30 min) and an air temperature as low as 60 °C. Moreover, the change of the particle size distribution, to some degree, affected the final moisture content due to the entrainment of wet, fine particles. The study also determined that the in-bed attrition of the particles is partially responsible for the generation of fines

Investigation of pre-drying lignite in an existing Greek power plant

The application of lignite pre-drying technologies in next generation of lignite power plants by utilizing low pressure steam as a drying medium instead of hot recirculated flue gas - combined with thermal utilization of the vaporized coal moisture - is expected to bring efficiency increase of 2-4 percentage points in future lignite power plants compared with today’s state of the art. The pre-drying concept is of particular importance in Greek boilers firing lignite with a high water and ash content. The combustion of Greek predried lignite has been investigated experimentally and via numerical simulations in our previous research. This study focuses on the potential integration of a lignite pre-drying system in an existing Greek power plant with dry lignite co-firing thermal share of up to 30%. The radiative and convective heat fluxes to the boiler and the overall boiler heat balance is calculated for reference and dry lignite co-firing conditions by an in-house calculation code. The overall plant’s thermal cycle is then simulated using commercial thermal cycle calculation software. The net plant efficiency is in this way determined for reference and dry coal co-firing conditions. According to the simulation results the integration of a pre-drying system and the implementation of dry lignite co-firing may bring an efficiency increase of about 1.5 percentage points in existing Greek boilers. It is therefore considered as an important measure towards improving plant efficiency and reducing specific CO2 emissions in existing plants