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

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

Substitution of coal by alternative and supporting fuels in pulverised fuel boilers towards reduction of CO2 emissions

207 σ.Σκοπός της διατριβής είναι η αξιολόγηση δύο διαφορετικών τεχνολογιών υποκατάστασης άνθρακα ως επιλογές για την εξοικονόμηση εκπομπών CO2 σε υφιστάμενους λιγνιτικούς σταθμούς. Η υποκατάσταση λιγνίτη από στερεά ανακτηθέντα καύσιμα (Solid Recovered Fuels, SRF) είναι η πρώτη τεχνολογία υποκατάστασης που εξετάζεται. Η δεύτερη τεχνολογία αφορά στην υποκατάσταση του φυσικού λιγνίτη από προξηραμένο λιγνίτη και στην ενσωμάτωση συστήματος ρευστοποιημένης κλίνης για προξήρανση λιγνίτη σε κύκλο ατμού υφιστάμενου λιγνιτικού σταθμού. Η επιδεικτική δράση υποκατάστασης λιγνίτη από SRF λαμβάνει χώρα σε γερμανικό λέβητα κονιοποιημένου λιγνίτη εγκατεστημένης ισχύος 600 MWe ενώ η υποκατάσταση φυσικού λιγνίτη από προξηραμένο λαμβάνει χώρα σε ελληνικό λέβητα εγκατεστημένης ισχύος 75 MWth. Εφόσον η επιδεικτική εγκατάσταση και λειτουργία συστήματος προξήρανσης σε υφιστάμενο λιγνιτικό σταθμό, δεν έχει ολοκληρωθεί ακόμα η ενσωμάτωση της τεχνολογίας προξήρανσης σε κύκλο ατμού διερευνάται μέσω υπολογισμών θερμοδυναμικού κύκλου. Η αξιολόγηση και η σύγκριση των δύο προτεινόμενων τεχνολογιών βασίζεται σε συγκεκριμένες περιβαλλοντικές, τεχνολογικές και οικονομικές παραμέτρους και μεθοδολογικά εργαλεία. Εκτός από τις πειραματικές μετρήσεις στην βιομηχανική κλίμακα, πραγματοποιούνται αριθμητικές προσομοιώσεις βασισμένες σε εργαλεία Υπολογιστικής Ρευστοδυναμικής (CFD). Μέσω των υπολογιστικών προσομοιώσεων αξιολογούνται περαιτέρω σενάρια υποκατάστασης, τα οποία δεν μπορούν να υλοποιηθούν στην πράξη. Επιπλέον διερευνήσεις στη μικρή κλίμακα, χρησιμοποιούνται υποστηρικτικά για την αξιολόγηση των πρακτικών υποκατάστασης στη βιομηχανική κλίμακα. Αναπτύσσεται ένα νέο μοντέλο καύσης για τα στερεά ανακτηθέντα καύσιμα (SRF), του οποίου η ακρίβειά πιστοποιείται με διαθέσιμα πειραματικά δεδομένα, ενώ πραγματοποιούνται δοκιμές ξήρανσης και καύσης ελληνικού λιγνίτη σε πειραματικές εγκαταστάσεις στη Γερμανία. Επιπλέον εξετάζεται η οικονομική βιωσιμότητα των προτεινόμενων τεχνολογιών και αξιολογούνται τα πρόσθετα λειτουργικά κόστη και έσοδα και το κόστος αποφυγής CO2 από την εφαρμογή των τεχνολογιών υποκατάστασης. Η συνολική αξιολόγηση υποδεικνύει ότι η εφαρμογή των δύο πρακτικών υποκατάστασης στη μεγάλη κλίμακα είναι εφικτή. Από τη συγκεκριμένη ανάλυση δεν μπορεί να προκύψει κάποιο αποτέλεσμα υπέρ ή κατά μίας τεχνολογίας. Ο εντοπισμός και η σε βάθος διερεύνηση των θετικών και προβληματικών ζητημάτων που προκύπτουν από αυτές τις πρακτικές υποκατάστασης αποτελούν την κύρια προσφορά της συγκεκριμένης εργασίας.Τhe thesis intends to assess the potential of two different brown coal substitution concepts as possible options to reduce CO2 emissions in existing pulverised brown coal power plants. The substitution of brown coal by Solid Recovered Fuels (SRF) produced from different fractions of mixed and mono waste streams is the first concept examined. The second concept regarded is the substitution of raw brown coal by pre-dried brown coal produced in a fluidised bed drying system to be integrated in the existing plant steam cycle. SRF co-firing is demonstrated in a 600 MWe pulverised brown coal boiler in Germany, while dry coal co-firing is demonstrated in a in a 75 MWth pulverised lignite boiler in Greece. Since the integration of a brown coal pre-drying system in an existing power plant utilising low temperature steam, has not been fully demonstrated in the large scale yet, the particular concept is investigated by thermal cycle calculations. The evaluation and comparison of the two concepts proposed is based on specific environmental, technological and economical parameters and on different analysis approaches. Apart from the large scale experimental activities numerical simulations based on Computational Fluid Dynamics (CFD) analysis are also carried out in the two boilers. Through the simulations additional co-firing modes are assessed, which cannot be realised in the large scale. Small scale investigations are also performed for the further assessment of the co-firing concepts. A dedicated model on SRF combustion is developed and validated against available lab scale data, while drying and combustion tests of Greek lignite are carried out in experimental facilities in Germany. Beyond the environmental and technological aspects investigated, the economic feasibility of the regarded coal substitution concepts is also assessed. The additional operational costs and revenues and the CO2 avoidance costs are assessed for each co-firing concept examined. The overall assessment implies that the realisation of both co-firing concepts in the large scale is feasible in the short term. Environmental, technological and economic aspects are regarded and several parameters are used as evaluation indices. No final result for or against a particular technology may come out from this detailed analysis. The identification and in depth investigation of positive or problematic aspects of both concepts should be considered as the main contribution of this work.Μιχαήλ Γ. Αγρανιώτη

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