fluidized bed combustion dekker mechanical engineering

Fluidized Bed Combustion Copyright © 2004 by Marcel Dekker, Inc MECHANICAL ENGINEERING A Series of Textbooks and Reference Books Founding Editor L.L.Faulkner Columbus Division, Battelle Memorial Institute and Department of Mechanical Engineering The Ohio State University Columbus, Ohio 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Spring Designer’s Handbook, Harold Carlson Computer-Aided Graphics and Design, Daniel L.Ryan Lubrication Fundamentals, J.George Wills Solar Engineering for Domestic Buildings, William A.Himmelman Applied Engineering Mechanics: Statics and Dynamics, G.Boothroyd and C.Poli Centrifugal Pump Clinic, Igor J.Karassik Computer-Aided Kinetics for Machine Design, Daniel L.Ryan Plastics Products Design Handbook, Part A: Materials and Components; Part B: Processes and Design for Processes, edited by Edward Miller Turbomachinery: Basic Theory and Applications, Earl Logan, Jr Vibrations of Shells and Plates, Werner Soedel Flat and Corrugated Diaphragm Design Handbook, 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Totten, Lin Xie, and Kiyoshi Funatani Mechanical Wear Fundamentals and Testing: Second Edition, Revised and Expanded, Raymond G.Bayer Engineering Design for Wear: Second Edition, Revised and Expanded, Raymond G.Bayer Clutches and Brakes: Design and Selection, Second Edition, William C.Orthwein Progressing Cavity Pumps, Downhole Pumps, and Mudmotors, Lev Nelik Mechanical Engineering Software Spring Design with an IBM PC, Al Dietrich Mechanical Design Failure Analysis: With Failure Analysis System Software for the IBM PC, David G.Ullman Copyright © 2004 by Marcel Dekker, Inc Fluidized Bed Combustion Simeon N.Oka Laboratory for Thermal Engineering and Energy Institute VIN A Belgrade, Serbia and Montenegro Technical Editor E.J.Anthony CANMET Energy Technology Centre (CETC) Natural Resources Canada Ottawa, Ontario, Canada M ARCEL D EKKER, I NC N EW Y ORK • B ASEL The original version of this book was published in Serbian in Belgrade, Yugoslavia, in 1994 by the Yugoslav Society of Thermal Engineers Although great care has been taken to provide accurate and current information, neither the author(s) nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage, or liability directly or indirectly caused or alleged to be caused by this book The material contained herein is not intended to provide specific advice or recommendations for any specific situation Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 0-8247-4699-6 This book is printed on acid-free paper Headquarters Marcel Dekker, Inc., 270 Madison Avenue, New York, NY 10016, U.S.A tel: 212–696–9000; fax: 212–685–4540 Distribution and Customer Service Marcel Dekker, Inc., Cimarron Road, Monticello, New York 12701, U.S.A tel: 800–228–1160; fax: 845–796–1772 Eastern Hemisphere Distribution Marcel Dekker AG, Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41–61–260–6300; fax: 41–61–260–6333 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities For more information, write to Special Sales/Professional Marketing at the headquarters address above Copyright © 2004 by Marcel Dekker, Inc All Rights Reserved Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher Current printing (last digit): 10 PRINTED IN THE UNITED STATES OF AMERICA Copyright © 2004 by Marcel Dekker, Inc 566 Chapter Figure 7.39 Effect of ammonia addition during coal combustion in bubbling fluidized bed on NOx and CO emissions [90] concentration Further increase of this ratio leads to increased concentration of NH3 in the flue gases exiting the furnace (i e NH3 slip), in addition to increases in CO concentrations, and is therefore not recommended [90] These investigations have also shown that introduction of ammonium in fluidized bed does not affect NOx emission Introduction of ammonium with secondary air or at some increased distance from the bed surface only leads to increased NH3 slip, which is highly undesirable given the toxic nature of this compound and its high cost Reductions of NOx emission by 50% can be achieved by introduction of two-stage combustion Similar data are cited in [105] The results of comprehensive investigations in [85] are similar for NH3 addition With a molar ratio NH3/NOin=3, up to 80% of NOx emission reduction can be achieved in a bubbling fluidized bed Greater reduction of NOx emission can be achieved by using urea For a molar ratio H2NCONH2/NOin of 0.8, a reduction of NOx emission of 90% was seen Changes of N2O emission with introduction of ammonium and urea have also been observed In both cases, a significant increase of N2O emission was noted With a molar ratio NH3/NOin=3, N2O emission increases by almost 100%, from about 35 to about 70 ppm With the introduction of urea, at a molar ratio H2NCONH2/NOin=0.8, an increase of N2O concentration occured from 40 to 80 ppm Copyright © 2004 by Marcel Dekker, Inc Harmful Matter Emission From FBC Boilers 567 As a possible measure for reduction of N2O emission without increasing NOx emission, combustion of gaseous fuel in flue gases at furnace exit should be suggested, in order to increase the flue gas temperature [82] This method of reduction N2O emission, has so far only been demonstrated for a circulating fluidized bed boiler, but it proved highly successful By combustion of about 10% of natural gas (in relation to the quantity of coal) at the exit from the furnace of an experimental 12 MWth CFBC boiler at Chalmers University in Göteborg, a temperature rise of about 100 °C was achieved, with a resulting reduction of N2O by about 40% for the same flue gas exit temperature (from 250 to 150 ppm) For a temperature rise of 200 °C, a further reduction to 30 ppm was produced Although the analyses of the possible routes for nitrogen oxide and nitrous oxide formation and reduction is highly complex, results presented in [102] show that it is possible by a choice of optimum regime parameters, bed temperature, fly ash recirculation degree and introduction of two-stage combustion, to design a boiler with bubbling fluidized bed combustion whose NOx emission is below 100 ppm NOx and N2O production firing biomass A discussion of NOx and N2O production from bubbling and circulating FBC boilers would not be complete without mentioning a rather suprising result It has long been known that although biomass often has relatively low nitrogen contents when compared with coal [106], more of its fuel nitrogen is released as NOx in FBC boilers than is the case for coal [107] This is probably due to two factors, first any biomass char is very reactive so that the char loadings in a FBC will necessarily be lower, and second, the amount of fixed carbon or char produced from biomass is also rather low [106] However, early measurements on a full-scale circulating boiler clearly showed that N2O production was extremely low for biomass firing (only a few ppm) [108], and similar findings have been made for bubbling FBC firing biomass [109, 110] When co-firing coal and wood, an interesting study by Leckner and Karlsson showed that N2O production increased sharply with increasing coal addition (from virtually zero to typical FBC levels) whereas for NOx it showed a much more curious behaviour and increased to a maximum at about 20–30% by weight of coal firing The explanation for this behavior was that the higher absolute nitrogen levels in the coal were not being compensated at this level of co-firing by a resulting increase in bed char loading and hence NOx was not being reduced effectively in the bed 7.6 Emission of solid particles in fluidized bed combustion 7.6.1 Types and characteristics of solid combustion products from FBC boilers The types and characteristics of solid combustion products from FBC boilers differ considerably from the solid particles formed in conventional pulverized Copyright © 2004 by Marcel Dekker, Inc 568 Chapter coal combustion boilers That is why the problem of removal and disposal of solid combustion products from FBC boilers is important The specific nature of solids formed in FBC boilers must be taken into account in designing the boiler and in the choice and calculation for the capacity of the flue gas cleaning system and dust collectors For FBC boilers, the following three considrations much be noted (a) There is an excess of solid inert material in the fluidized bed The height of the bed increases if particles larger than the chosen nominal bed particle size accumulate in the bed This excess material must be removed from the bed either by means of an overflow system (rarely) or by draining through openings in the distribution plate Through these draining openings the bed material, if fluidized, can flow freely out of the bed, whereafter they can be removed by a mechanical system such as described in Chapter Any excess material that remains in the bed consists of coal derived ash, tramp material fed with the coal, CaO and CaSO4 and the initial inert bed material—e g silica sand Particle sizes for this solid material is usually greater than 1mm, and depends on the selected nominal size of inert material (that is, the chosen fluidization velocity) and coal particle size distribution Removing this material from the furnace and from the bottom of the bed and its disposal are normally not major problems (b) Solid particles separated in the cyclones can be returned to the furnace for reburning Fly ash removed from flue gases by mechanical inertial separators or cyclones consists mainly of incompletely burnt char, ash, CaO and CaSO4 In many cases, combustion efficiencies over 90% can be achieved only if the elutriated un-burned particles in the fly ash from the boiler are recirculated The quantity of solid material which is returned into the furnace to be “burned out” depends on its chemical composition and the particle size distribution, which in turn depend on the type and size distribution of coal, the fluidization velocity, the furnace height, and whether limestone is used or not, and on the chosen cyclone efficiency For a typical design and operating parameters for BFBC boilers, particles caught by the cyclones and returned for combustion are usually below 0.6 mm in size More than 90% of them are smaller than 150 µm [111] Only a part of this material is returned to the furnace, depending on the selected degree of recirculation, and the remaining ash is usually mixed with fly ash from the baghouse filters or electrostatic precipitators Again the transportation and disposal of this material is not a major technical problem (c) Fly ash is very fine material that cannot be eliminated by multicyclones It presents the greatest problem, both from an environmental and technical point of view Baghouse filters or electrostatic precipitators must be used to remove fly ash from flue gases in order to meet permissible emission limits for particulates (typically around 50 mg/m3) [112] Both these types of dust collectors achieve satisfactory results Typical fly ash particle size is below µm [111, 113] Fly ash contains mostly particles of ash, CaO and CaSO4 and Copyright © 2004 by Marcel Dekker, Inc Harmful Matter Emission From FBC Boilers 569 higher carbon loadings than seen with bed drain materials Disposal of this fine material can pose a significant problem [111], as is the case for conventional pulverized coal combustion boilers The characteristics of fly ash, its removal and operational problems for the dust collectors used in bubbling FBC boilers will be discussed in more detail in this section The physical and chemical characteristics of fly ash in bubbling FBC boilers differ considerably from the characteristics of fly ash in pulverized coal combustion boilers, due to the different combustion conditions [113, 114] Combustion in a fluidized bed takes place at temperatures 800–900 °C, which are considerably lower than typical ash softening or melting temperature In pulverized coal combustion boilers, temperature in the furnace reaches 1200–1500 °C, and fly ash particles melt, and due to surface tension, acquire spherical shape Particles of fly ash in FBC boilers not melt, and are of irregular shape, with very high specific area, which results in their increased cohesiveness and bulk porosity The chemical composition of fly ash in FBC boilers can also differ due to a limestone addition and the resulting larger quantity of CaSO4, limestone and unused CaO This is why fly ash from an FBC with sulphur capture is extremely alkaline The use of multicyclones and inertial separators for particle recycling in the furnace also considerably reduces the size of particles that reach flue gas cleaning devices Investigations made in the U.S.A [113, 114], clearly indicated differences between fly ash produced from FBC boilers and pulverized coal combustion boilers as shown in the Table 7.9 Table 7.9 Characteristics of fly ash from bubbling FBC and PC boilers The following two characteristics are significant for efficient flue gas cleaning: (a) (b) specific aerodynamic resistance of ash layer retained on baghouse filters, and electrostatic resistance of ash layer in electrostatic precipitators Copyright © 2004 by Marcel Dekker, Inc 570 Chapter Both of these characteristics for FBC fly ash are less favorable than for pulverized coal combustion ash Therefore, special attention must be paid to the design and use of flue gas cleaning devices for FBC boilers The specific aerodynamic resistance of the layer of fly ash particles trapped by baghouse filters, according to [113, 114], ranges up to 7000– 12500 mm H2O m/kg/s The electrostatic resistance of fly ash from FBC boilers is 1011–1013 ⍀cm, while from pulverized coal combustion boilers a typical value is 1010 ⍀cm [115] Electrostatic resistance increases as the Ca/S molar ratio rises when limestone is used for sulphur capture When Ca/S increases from 0.45 to 1.72, ash resistance in an operating FBC boiler increases from 1012 to 1.5–1013 ⍀cm [112] 7.6.2 Experience with baghouse filters Baghouse filters and electrostatic precipitators are equally successfully for flue gas cleaning in FBC boilers Initially, the capacity of FBC boilers was small, so baghouse filters were used more frequently In such small units, baghouse filters were cheaper than electrostatic precipitators Due to the expected increased electrostatic resistance with limestone addition, electrostatic precipitators were significantly more expensive [112] Nowadays, these two types of dust collectors are equally represented for bubbling FBC boilers, although high capacity boilers (100 MWth) tend to use electrostatic precipitators especially in Europe [112–114] In about 20 industrial operating boilers (up to 165 MWth) the pressure drop across the baghouse filters (from inlet to the outlet) was 50–200 mm H2O, despite regular dedusting with counter gas flow and a considerably lower specific flow rate than is usually used in gas cleaning in conventional boilers Filters are designed with specific gas flow rate per square meter of filters 0.15–0.4 m3/ min./m2, while in conventional boilers 0.5–0.8 m3/min./m2 is used The main cause of the increased pressure drop for much lower specific gas flow rate is that FBC fly ash forms a layer on the filter cloth whose specific resistance, as mentioned before, is much higher than in conventional boilers, despite the fact that the quantity of ash which is retained by the filter is lower (1.5–2 kg/m2 compared to 2.5–5 kg/m2) Due to the considerably smaller particles, the mean pore size of this layer is 4–8 times smaller (about 0.7 µm) than from conventional boilers, which causes much higher flow resistance despite a somewhat greater porosity of the layer (due to irregular shape of the particles) [113, 114] Due to higher specific flow resistance of the fly ash layer in FBC boilers, baghouse filters for these boilers ought to be designed with significantly lower specific gas flow rate (0.5 m3/min./m2), if modern methods of efficient filter dedusting are not used Modern filters in FBC boilers are designed with Copyright © 2004 by Marcel Dekker, Inc Harmful Matter Emission From FBC Boilers 571 considerably higher specific flow rate 1–1.5 m3/min./m2, but also with a highly efficient method of pulsation dedusting of filters Such design maintains the value of pressure drop of the filter 50–200 mmH2O, due to the efficient “shaking” and cleaning process Flue gas temperature in front of the filter ranges from 120–200 °C Glass fiber is increasingly more frequently used as it is a highly suitable material at such temperature and performs well given the specific chemical composition of ash, despite its shorter lifetime Although not explicitly noted, it seems that it is necessary to pay special attention in the case of combustion of high moisture fuels with limestone addition, due to a higher quantity of CaSO4 and CaO in the fly ash, which can create problems in the baghouse filters Problems may appear especially during start-up of the boiler, or during load following [116] A general conclusion after significant investigations on such filters in the U.S.A is that an adequate design of baghouse filters and a correct choice of design and operating parameters together with application of efficient dedusting (pulsating) enables a moderate pressure drop to be employed with a reasonable lifetime (2.5–4 years) and low particle emission from the stack Data from investigations on industrial and utility FBC boilers published in 1991 indicate an extremely low concentration of particulates when baghouse filters are used, 2–15 mg/MJ [113] 7.6.3 Experience in the application of electrostatic precipitators In the open literature there are very few data on the specific design and use of electrostatic precipitators for removal of fly ash from flue gases in FBC boilers [112, 115, 116] However, available data indicate that despite the considerably higher electrostatic resistance of fly ash in FBC boilers and its different chemical composition, electrostatic precipitators used in FBC boilers, “operate much better than expected” [115] Operation of electrostatic precipitators in three different 40 MWth boilers indicated a very high separation efficiency, 99.5–99.97% [112] Particle emissions were 10–30 mg/m3 Investigation of electrostatic precipitators in the TVA 20 MWth pilot facility, despite the considerably higher electrostatic resistance of the fly ash (approximately 1·1012 ⍀cm), demonstrated that an electrostatic precipitator operating in conditions non-typical for conventional boilers (mean density of current about 60 nA/cm2, voltage drop 20–50 kV/cm) achieved separation efficiency over 99% [115] Thus, it is clear that both baghouse filters and electrostatic precipitators can be successfully applied for FBC boilers in order to meet appropriate environmental regulations for particulates The actual choice of technology depends on technical and economical considerations Copyright © 2004 by Marcel Dekker, Inc 572 Chapter Nomenclature Ca/S Ca (calcium in the limestone added to the bed) to S (sulphur added to the bed with coal) molar ratio d mean coal particle diameter, [mm] or [m] dp mean limestone particle diameter, [mm] or[m] Hb bed height, [m] NOin NOx concentration before amonia addition, [ppm] NOout NOx concentration after amonia addition, [ppm] rc recirculation ratio, rpore mean pore radius in limestone particle, [Å] rs primary to secondary air ratio, Tb bed temperature, [°C] or [K] vf superficial velocity of fluidizing gas, [m/s] Vpore total volume of the pores, [cm3] VMo volatile matter content in coal, as measured by proximate analysis, on “as received” basis, [%] X cao degree of sulphation Greek symbols ηs degree of desulphurization (efficiency), ratio of SO2 emission to maximum SO2 emission calculated using sulphur content in coal, [%] ηcao degree of limestone calcination, ratio of the CaO formed to the maximum possible amount and CaO calculated using amount of CaCO3 present in limestone, [%] ηsk sulphation degree of limestone, ratio of the CaSO4 formed to the amount of CaCO3 present in limestone, [%] λ excess air λb excess air in bed for staged combustion with secondary air injected above the bed surface τ time, [s] References [1] [2] [3] Technical recommendation for clean air protection given by Yugoslav Society for Clean Air (in Serbian) Sarajevo (Yugoslavia): Yugoslav Society for Clean Air, 1986 A Hemenway, WA Williams, DA Huber Effects of clean air act amendments of 1990 on the commercialization of fluidized bed technology Proceedings of 11th International Conference on FBC, Montreal, 1991, Vol 1, pp 219–224 M Gavrilovi , Lj Nesi , M Škundri Review of the clean air regulations over the world and recommendation for implementation of the clean air regulations in Yugoslavia (in Serbian) Proceedings of Workshop Influence of Thermal Power Plants in Vicinity of Belgrade on Air Quality Belgrade: Copyright © 2004 by Marcel Dekker, Inc Harmful Matter Emission From FBC Boilers [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] 573 Serbian Electricity Board and Society of Engineers and Technicians of Belgrade, 1991, Vol 1, pp I-1-I-36 Z Kosti , B Repi , Lj Jovanovi , D Daki Protection of the atmosphere from NOx emission from thermal power plants (in Serbian) Report of the VIN A Institute of Nuclear Sciences, Belgrade, IBK-ITE-868, 1991 B Arsi , M Radovanovi , S Oka Choice of methodology for comparative analysis of limestone 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COAL COMBUSTION IN FLUIDIZED BEDS 4.1 Characteristic features of combustion in fluidized beds 4.1.1 Combustion conditions in fluidized beds 4.1.2 Physical processes during coal particle combustion. .. devices for solid fuel combustion in fluidized bed Furnaces for production of hot gases or hot air Industrial boilers with bubbling fluidized bed combustion Bubbling fluidized bed combustion boilers... in fluidized beds 4.5.3 Distribution and combustion of volatile matter in fluidized beds 4.5.4 Modelling of volatile matter distribution and combustion in fluidized beds 4.6 Char combustion 4.6.1