Use of cavity plasmatron in pulverized coal muffle burner for start-up of boiler.

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1 Use of cavity plasmatron in pulverized coal muffle burner for start-up of boiler. Przemysław Bukowski, Przemysław Kobel, Włodzimierz Kordylewski, Tadeusz Mączka Keywords: plasmatron, ignition of pulverized coal fuel-air mixture, coal burner Summary A construction and principle of operation of the prototype plasmatron used for ignition of pulverized coal fuel-air mixture are described. The experimental plasma assisted start-up installation with pulverized coal muffle burner is shown. The results of economic analysis, done in order to compare the plasma ignition with other methods of boilers start-up are presented. Also guidelines for further studies are given. 1. INTRODUCTION In Poland approximately 95% of electricity is produced in power plants from coal, mainly in pulverized coal-fired boilers, being steam generators. Starting-up this type of boilers from the cold state requires a procedure aimed at heating up a boiler furnace and providing a stable working conditions for the main burners. Predominantly, start-up of the boiler is carried out using auxiliary heavy oil burners. This method of start-up is harmful for the environment because of high emissions of heavy hydrocarbons and soot. The use of heavy oil (mazout) is also expensive, mainly due to the high and still growing crude oil prices. In addition, mazout installation is technologically complex and requires high investment and maintenance costs. It is also energy consuming because of the need for continuous heating of mazout in order to maintain its liquidity. In the face of strict environmental standards and high prices of crude oil, it seems highly appropriate to find an alternative not using hydrocarbon liquid fuels - method for start-up of the pulverized coal-fired boilers. From the economic, energy efficiency and environmental standpoints, it would be the most beneficial to start-up the boiler using pulverized coal as a fuel. However, it is difficult, because one must ensure reliable ignition and stable operation of a pulverized coal burner at cold boiler furnace. It is necessary to apply an additional high-power source of ignition, sufficient to cover the energy losses to the cold surrounding. Such an ignition source can be plasma generators (plasmatrons) powered by electric energy installed directly at the pulverized coal burners. Such a solution can reduce costs, both investment and operating and diminish a negative impact of the boiler start-up on the environment[1]. Currently, worldwide are under development several plasma assisted start-up systems (SUS) at different levels of progress (lab, pilot and semiindustrial scale). The pioneers in this area are Russians [2, 3]. Czech company ORGREZ developed a plasma start-up system, which was examined in a lignite-fired boiler [4, 5]. There are also information that one of the Chinese company has installed several professional plasma based start-up systems [6, 7]. However, this technique is relatively innovative and still lacks complete data on the performance of these solutions. There are many doubts and uncertainties associated with reliability of plasma start-up systems and their time of life. Also the problem of electromagnetic compatibility is very important because of the correct and faultless operation of automation and security systems of power unit [8]. Knowledge of above topics included in the available literature seems to be incomplete and laconic. 2. THE IDEA OF PLASMA START-UP The idea behind a plasma-assisted pulverized coal burner (PAPCB) is to blow plasma torch into the pipe through which pulverized coal in air flows (Fig. 1). There is rapid physical and chemical interaction between pulverized coal and plasma, which causes thermal decomposition of coal particles, release of the volatile matter and its ignition. The result is a stable pulverized coal flame [9 10].

2 3. STUDIES ON PLASMA ASSISTED START-UP SYSTEMS CARRIED OUT AT WUT During the recent decade several studies on the use of plasma technology for start-up of pulverized coal boilers have been carried out in the Division of Combustion and Detonation of the Institute of Heat Engineering and Fluid Mechanics at Wroclaw University of Technology (WUT). Fig. 1. Working principle of the plasma assisted pulverized coal burner (PAPCB) The procedure of plasma assisted start-up of a pulverized coal-fired boiler is similar to the procedure of start-up of a boiler using heavy oil. The essence of plasma assisted start-up procedure is that the plasma assisted pulverized coal burners (PAPCB) with installed plasmatrons are fired first. The remaining pulverized coal burners are started gradually after reaching the required thermal parameters of the furnace and other elements of the boiler. It is important to ensure the delivery of pulverized coal for the PAPCB. This topic has been wider discussed in [10]. As PAPCBs can be used existing pulverized burners after modification or new specially designed for this purpose. The number and location of PAPCBs in the furnace depends on the boiler thermal efficiency, type of the furnace and electrical power of plasmatrons. Figure 2 shows an example of PAPCBs configuration in a tangenciallyfired furnace Cavity plasmatron At the current stage of research it has been developed a prototype of an improved plasmatron design with cylindrical electrodes, the so-called cavity plasmatron. The plasma gas is air. Characteristic feature for this design is that air is blown tangencially into plasmatron through grooves in the ceramic ring between anode and cathode (Fig. 3). A flow of the circumferential air in a plasma channel causes circular movement, which aims to push out the electric arc from gap between the cathode and anode and extend this arc. This solution provides a stable plasma channel and reduces erosion of the electrodes. For the purposes of research have been made several plasmatrons of the described design. They have different sizes and are capable of delivering power from 20 to 100 kw. Details of construction are constantly improved in order to achieve optimum performance, maximum reliability and durability. Fig. 2. Example of PAPCBs configuration in the tangencially-fire furnace The electrical power of plasmatrons used in PAPCBs depends on the thermal power of the PAPCBs and characteristics of coal and it is in range of kw. The plasmatron installation includes: electrical power supply, air supply and cooling water. The PAPCBs operate under their control system. Fig. 3. Visualization of working plasmatron 3.2. Electrical circuits of plasmatron An important task was to develop a device for the high power plasmatron ignition (initiating arc discharge), with low electromagnetic interference level. This is a very important issue because the operation of plasmatron can not disturb normal operation of the measuring and control devices of the

3 power unit. Based on Authors research a plasmatron igniting device with a high voltage, low frequency discharge was developed [8]. This solution is patent pending [11]. To power the cavity plasmatron was developed a DC current supply based on high power semiconductor technology. Conditions for stable operation of plasmatron at the maximum power were determined by the empirical correlation between electrical parameters of plasma current and voltage) and the flow rate of air and its aerodynamics Installation for testing plasmatron at the real conditions In order to test the plasma ignition of pulverized coal was developed an experimental installation (Fig. 3). The installation includes: pulverized coal muffle burner (with a pulverized coal supply), cavity plasmatron mounted directly on the muffle burner, electric power supply, air supply and cooling system. It was mounted on an existing muffle burner used for start up of boiler OP-130 boiler. The installation is fully autonomous and does not interfere with the normal operation of the boiler. It allows to test plasmotron both during work and rest of the boiler. Fig. 3. Location of plasmatron on the furnace; 1 start-up muffle burner, 2 pulverized coal supply, 3 - plasmatron, 4, 5 plasmatron s control and power supply, 6 - main burners of the boiler 4. ECONOMICAL ANALYSIS An economic analysis of the application of plasma assisted start-up systems was made under the following assumptions: a) the pulverized coal-fired steam boiler OP-430 was considered, b) used detailed costs were determined in December 2009, c) 10 year exploitation period for evaluation of the economic effects was assumed, d) 10 starts from cold and 10 starts from hot state of the boiler per year were assumed. The following investment costs and the costs of maintenance of the selected start-up installations has been separated: a) investment costs (one-off cash flow in the first period), b) exploitation costs (annual costs for installation maintenance): o fixed (independent of the number of boiler start ups, PLN/year), o variable (dependent on annual boiler start ups, PLN/start up or PLN/h). The following assumption was made about energy equivalence of coal and the fuels considered in the analysis: Qm mw = mm, Qw (1) where: m w mass of coal corresponding to the thermal energy of other fuel, Mg m m mass of other fuel used for the boiler start up, Mg Q m lower calorific value of the other fuel, MJ/kg Q w lower calorific value of coal, MJ/kg The cost of milling of coal was included in the analysis. The exploitation costs of the plasma-assisted SUS was were assumed as follows: a) cost of electric energy necessary to feed plasmatron, K eepl, K = T, eepl K jee (2) where: K eepl cost of electric energy of plasmatron supply, PLN, T standard time of boiler start-up (the pulverizedcoal plasma burner is assumed to operate for 75% of total boiler start up time, h), K jee price of electric energy, PLN/MWh b) cost of plasmatron cooling, c) cost of plasmatron working gas (in case when argon is used), d) plasmatron maintenance cost. The basic parameters of the fuels considered in the analysis of the kindling costs are collected in table 1. The further economic analysis of the kindling costs was performed including also the other three fuels (light heating oil, propane and natural gas) (Tab. 1).

4 L.p. Fuel Low caloric value Density Table 1 Comparison of basic parameters of selected start-up fuels Price* [without VAT] Fuel/mazout ratio Cost of 1 GJ [kj/kg] [kg/m 3 ] [zł/m 3 ] [zł/kg] [zł/l] [kg/kg mazout ] [zł/gj] 1 Bitominous coal ,26-1,64 10,69 2 Mazout ,15 1,12 1,00 28,83 3 Light oil ,20 1,90 0,95 52,49 4 Liquefied propane ,04 2,06 0,87 88,35 5 Natural gas GZ ,74 1,82 2,46-0,95 58,71 The total discounted capital and the ten years exploitation costs for the selected variants of the SUSs including the plasma-assisted start-up system are shown in Fig. 4. The results presented in the Fig. 5 show advantage of the plasma technology application for start-up of pulverized coal-fired boilers. The total kindling costs during the ten years exploitation appeared to be considerable lowest for the plasma-assisted start-up system PLN Plasma installation Mazout Light oil Propane GZ Years Fig. 4. Discounted cash flows for the selected variants of the SUS.

5 Plasma Mazout Light oil Propane GZ-50 PLN Years Fig. 5. Discounted and cumulated cash flows for the selected variants of the SUS 5. CONCLUSIONS FROM THE TESTS AND GUIDELINES FOR FURTHER STUDIES The results of laboratory and pilot investigations of the developed plasma assisted startup system have proved its effectiveness and reliability. The economic analysis showed that the application of plasma for kindling of boilers is competitive to the use of mazout. An advantage of the proposed device is that it is based on own solutions, which has been partially patented. Further laboratory investigations are necessary to improve durability of the plasmatron by lengthening its lifetime. The priority now is to optimize the design of plasmatron in order to increase its power and efficiency. A key project will be to optimize aerodynamics of airflow inside the plasmatron by applying numerical analysis and computer simulation. Also pilot and industrial scale tests are required to develop mature PAPCB ready to install in a boiler and a professional control system able to win commercial acceptance Zastosowanie plazmotronu wnękowego w muflowym palniku pyłowym do rozruchu kotła energetycznego Słowa kluczowe: plazmotron, zapłon pyłu węglowego, palnik węglowy Streszczenie Opisano budowę oraz zasadę działania prototypowego plazmotronu wnękowego stosowanego do zapłonu węglowej mieszanki pyłowo-powietrznej. Zaprezentowano doświadczalna plazmową instalację rozruchowa z pyłowym palnikiem muflowym. Przedstawiono wyniki analizy ekonomicznej mającej na celu porównanie zapłonu plazmowego z innymi metodami rozruchu kotłów. Wskazano także wytyczne do dalszych działań. Prace wykonano w ramach grantu nr 0359/R/T021/2008/04 Ministerstwa Nauki i Szkolnictwa Wyższego

6 Przemysław Bukowski, dr inż. Pracuje obecnie na Uniwersytecie przyrodniczym w Zakładzie Podstaw Techniki. Od 1998 roku studiował na Uniwersytecie Ekonomicznym we Wrocławiu na wydziale Zarządzanie i Informatyka. W 2002 rozpoczął studia Politechnice Wrocławskiej na Wydziale Mechaniczno-Energetycznym w specjalności termoenergetyka, gdzie w lipcu 2009 roku uzyskał tytuł doktora. W międzyczasie dr inż. Przemysław Bukowski wykonywał badania w wielu elektrowniach i elektrociepłowniach gdzie zdobył doświadczenie związane z zagadnieniami energetyki zawodowej. Ma pokaźny dorobek naukowy (osiem publikacji, kilkanaście prac niepublikowanych, w tym raportów PWr, jeden patent). przemyslaw.bukowski@up.wroc.pl Przemysław Kobel, mgr inż. Jest doktorantem w Zakładzie Spalania i Detonacji Instytutu Techniki Cieplnej i Mechaniki Płynów Politechniki Wrocławskiej. Jego zainteresowania badawcze związane są z zastosowaniem techniki plazmowej do rozruchu kotłów energetycznych. Obecnie prowadzi badania dotyczące zastosowania pary wodnej do zasilania plazmotronów rozruchowych. przemysław.kobel@pwr.wroc.pl Włodzimierz Kordylewski, prof. dr hab. inż. Jest kierownikiem Zakładu Spalania i Detonacji Instytutu Techniki Cieplnej i Mechaniki Płynów Politechniki Wrocławskiej wlodzimierz.kordylewski@pwr.wroc.pl Tadeusz Mączka, dr inż. Jest adiunktem w Zakładzie Spalania i Detonacji Instytutu Techniki Cieplnej i Mechaniki Płynów Politechniki Wrocławskiej. Jego zainteresowania badawcze dotyczą elektrotermii, technik plazmowych i technologii materiałowej. Obecnie zajmuje się układami inicjującymi i zasilania dużej mocy dla plazmotronów rozruchowych. tadeusz.maczka@pwr.wroc.pl LITERATURA [1] P. Bukowski, A. Dyjakon, W. Kordylewski, M. Salmonowicz, Analiza ekonomiczna plazmowego rozruchu kotłów pyłowych, Międzynarodowa X Konferencja Kotłowa 2006, Szczyrk ; [2] E. Karpenko, V. Messerle, A. Ustimenko, Plasma application for coal combustion activation, 31st EPS Conference on Plasma Phys, London, ECA Vol. 28G, P (2004) [3] E. Karpenko, V. Messerle, A. Ustimenko, Plasma-aided solid fuel combustion, Proceedings of the Combustion Institute 31 (2007), s [4] Plasma Technology - The most modern technology of boiler starting, prezentacja firmy ORGREZ a.s., Międzynarodowa X Konferencja Kotłowa 2006, Szczyrk ; [5] J. Lojkasek i inni, Plazmotron, Opis ochronny wzoru użytkowego PL64036; [6] The Application of Plasma Ignition Technology in China, prezentacja firmy EDF China Division, 2008 [7] Plasma Technology for Ignition an Stabilized Combustion of Pulverized-Coal Fired Boilers, materiały firmy Yantai Longyuan Co., 2006 [8] P. Kobel, W. Kordylewski, T. Mączka, Opracowanie i wykonanie bezzakłóceniowych układów rozruchu plazmotronu dużej mocy, Raporty ITCMP Politechniki Wrocławskiej. 2008, Ser. SPR nr 37 [9] A. Dyjakon W. Kordylewski, Stabilisation of pulverized coal burning with plasma assists. Archivum Combustionis vol. 22, nr 3/4, s ; [10] P. Kobel, W. Kordylewski, Zastosowanie plazmotronu zasilanego powietrzem do stabilizacji płomienia pyłowego, Archiwum Spalania 2008, vol. 8, nr 1-2 s , [11] W. Kordylewski i inni, Sposób i urządzenie do uruchamiania palników plazmowych, zgłoszenie patentowe nr P z dnia