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Achievements in the field of fluidized bed firing boilers
Energy Convers. Mgmt Vol. 33, No. 11, pp. 1017-1019, 1992 Printed in Great Britain. All rights reserved
0196-8904/92 $5.0(I+ 0.00 Copyright ~ 1992 Pergamon Press Ltd
ACHIEVEMENTS IN THE FIELD OF FLUIDIZED BED FIRING BOILERS M. C,~RDU and L. DRAGO$ The Institute of Scientific Research and Technological Engineering for Power Equipment, ICSITEE. Sos. Berceni nr. 104, Sect. 4, Bucharest, Romania
(Received 22 May 1991; receivedJor publication 6 February 1992) Abstract--The paper deals with the achievements, as made by the Institute of Scientific Research and Engineering for Power Equipment, ICSITEE, Bucharest, in the field of research and design concerning fluidized bed firing boilers. The coal intended to be fired in the above-mentioned boilers, as fabricated so far, is a low grade one. its heating value being from 5 to 8.4 MJ/kg. More than 100 fluidized bed firing boilers have been built so far, over 60 of which are currently operating. We have ranged them into two classes: hot water boilers of 3.49 MW (3 Gcal/h) and 5.81 MW (5 Gcal/h) thermal capacity, and steam boilers of 2 and 10 t/h steam flow rate (two types differentiated by their steam pressures: 8 bar for one boiler and 15 bar for the other).
The fuel most available for power applications in Romania is coal with low heating value up to 5025-8375 kJ/kg. Research work commenced in 1970 at the Institute of Scientific Research and Technological Engineering for Power Equipment, ICSITEE, Bucharest to perfect the fluidized bed firing technology as far as our domestic low grade coal is concerned, with the view to use the above mentioned fuel either in steam boilers or in hot water boilers of relatively lower capacities. Research investigations have resulted in establishing: (i) the combustion times for coal particles, (ii) air and gas speeds within the fiuidized bed, (iii) the optimum size of coal particles within the fluidized bed, and (iv) other features of the whole fluidized bed firing process, starting with coal driving in up to ash separation out of the flue gases. Research works have been carried out for various kinds of coals with heating values from 5 to 11.7 MJ/kg. Besides the figures given for the above mentioned technical features, the optimum grain size of coal particles has been found to be 0--7 ram, to provide a good firing in the fluidized bed. The above research works have been assumed as a basis for the design of a pilot plant having a hot water boiler with firing in a fluidized bed system and with a thermal capacity as great as 1.86 MW (1.6 Gcal/h). It is on this experimental plant that ICSITEE has carried on the research works concerning coal firing in a fluidized bed system. A good firing stability at low temperatures 1023 K (750°C)-1173 K (900°C) in a fluidized bed has been acquired. An intense firing and a good heat transfer (300 W/m 2 °C) into the fluidized bed have been also found. All the advantages exhibited by fluidized bed firing boilers, compared to other types of boilers, have been actually emphasized, such as: ---capability to use a coal of a very low heating value; --capability to fire coal as grains (its grinding being not necessarily required) and excellent performance of combustion, that is of the boiler too, have been obtained; --achievement of great coefficients for the heat transfer into the fluidized bed and, consequently, of a low metal consumption for the heat-exchanging surfaces available in the fluidized bed; --low nitrogen oxide emissions in the flue gases, due to the low temperatures available in the firing process. The firing in this type of boiler has been carried out with no addition of other hydrocarbons (gas-oil or gas), at loads amounting to 70-100% of its nominal output, whereas the efficiency as obtained for the whole boiler is over 80%. 1017
1018
CARDU and DRAGOn: FLUIDIZED BED FIRING BOILERS
Experimental research works have also been conducted with the view of decreasing the sulphur oxide contents in the flue gases. The procedure of introducing limestone into the fluidized bed has been applied. It is so that 80-90% of the sulphur contained in the coal may be retained. Great performances of the sulphur retaining process for molar ratios Ca/S = (2.5-3.5), the limestone grain structure of 0-1 mm and the temperature in the fluidized bed as high as 1073 K (800°C)-1173 K (900°C) have been obtained. It is in this way that, for a sulphur content in the coal as great as 2.7%, a sulphur oxide content in the flue gases as high as 300 ppm has been obtained. The firing capabilities in self-combustion duty have been rendered evident on pilot units for other types of solid fuels with low heating power values too, such as: --pit coal mixtures (as resulted from pit coal "washing out") having heating values in the range from 7536 to 12,500 kJ/kg; --bituminous shales, with heating values amounting to approx. 3770 kJ/kg; --coal from Albania, with heating values in the range from 6280 to 7120 kJ/kg, the ash content being 60-65% and moisture content 10-15%. Experiments for firing graphite electrode wastes too, as used in the metallurgical industry, have been carried out. The above mentioned material could not be fired in self-combustion duty on account of its low content in volatile matter; it could be fired only by adding about 8% hydrocarbons (heat supply). Only boilers supposed to operate at loads as great as 70% of the nominal load, without hydrocarbons added, have been called for meeting the industrial demands. To meet the above requirement, and on the basis of the results as acquired on the pilot plant, the 3.49 MW (3 Gcal/h) and 5.81 MW (5 Gcal/h) capacity boilers have been designed. They have been built, each of them consisting of two modules and having just half of the total capacity. Thus, the boiler can operate, on the whole, with no other hydrocarbon added, but with partial loads in the range 35--50% (with one module) and 70-100% (with both modules under operation). The designs for three steam boilers with the following main features have also been worked out: - - 2 t/h; 8 bar; 443 K (170°C), --10 t/h; 8 bar; 473 K (200°C), --10 t/h; 15 bar; 623 K (350°C). The Romanian coal to be fed in the above mentioned boilers has the following main features: grain structure = 0 - 7 m m , low heating value = 5-8.4MJ/kg, moisture= 38-45%, ash content = 25-35%, ash softening point = 1323-1423 K. The construction and the operation of a fluidized bed firing steam boiler (Fig. l) is briefly outlined as an example, in the following: The coal, previously processed by crushing and screening, may be found as stored in hopper 1. Taken from this hopper, coal is then introduced into the lower part of the furnace 2, beyond a grate under which the fluidizing and firing air (primary air), coming from a fan, is introduced through nozzle 3. Coal transfer from hopper 1 to furnace 2 is performed by means of the helical conveyer 4. The ash, as resulted at the upper part of the fluidized bed, is collected through channel 3 to the ash hopper 6. Prior to being discharged from the hopper, the ash is air-cooled; the air being so pre-heated is then introduced into the furnace as secondary air. Gases resulting from the firing process travel through an upward path, further through a downward path, then an upward path again, to the outlet from the boiler proper. There is a heat exchange area in the water-steam circuit, just in the fluidized coal bed (the first evaporator 7). On their route, the flue gases successively meet the following heat exchanging areas: superheaters 8, the second evaporator 9, the 3rd stage evaporators 10 and economizers ll. After leaving the boiler, the flue gases pass through the duct 12 and into the cyclone separator 13, where a part of the ash contained therein is separated. Further on, the flue gases pass to the stack through the duct 14 and through the fine-grained ash separator 15.
CARDU and DRAGO$: FLUIDIZED BED FIRING BOILERS
1019
9~ NI
Fig. 1. Fluidized bed firing boilers--main outline. 1--Coal hopper; 2--furnace; 3--nozzle to allow the introduction of the combustion air; 4---coalhelical conveyer;5--ash duct; 6--ash hopper; 7---evaporator 1; 8--superheater; 9---evaporatorII; 10--evaporator III; 1l--economizer; 12--flue gas duct; 13---cyclone separator; 14--flue gas duct; 15--ash separator; 16--ash helical conveyer; 17~rum header. The ash is discharged from hopper 6, the cyclone separator 13 and classifier 15 on the helical conveyer 16. The sequence of the heat exchanging areas in the water-steam circuit is as follows: 11-7-9--10-8. The steam-water mixture from the superheater 8 passes into the drum-header 17, and therefrom, the steam is led to the user. The boiler furnace is wholly lined with refractory bricks. The air for combustion purposes is introduced into the boiler at 1.04 bar pressure. The hydrocarbon burner, as used for boiler start-up and for extremely low loads, is introduced into the coal bed. The fuel, usually being a liquid one, is sprayed into the fluidized be at a 600-700 mm WC overpressure. More than 100 fluidized bed firing boilers of the previously specified types have been manufactured so far. Out of them, more than 60 boilers have been in operation, the remaining ones being in the course of erection. The specified boilers are available in units belonging to the machine building industry, to the food industry and in transportation companies. They supply, for the above-mentioned units, either steam for technological processes or hot water for technological processes or for heating purposes.
ECM 33,11
E
0196-8904/92 $5.0(I+ 0.00 Copyright ~ 1992 Pergamon Press Ltd
ACHIEVEMENTS IN THE FIELD OF FLUIDIZED BED FIRING BOILERS M. C,~RDU and L. DRAGO$ The Institute of Scientific Research and Technological Engineering for Power Equipment, ICSITEE. Sos. Berceni nr. 104, Sect. 4, Bucharest, Romania
(Received 22 May 1991; receivedJor publication 6 February 1992) Abstract--The paper deals with the achievements, as made by the Institute of Scientific Research and Engineering for Power Equipment, ICSITEE, Bucharest, in the field of research and design concerning fluidized bed firing boilers. The coal intended to be fired in the above-mentioned boilers, as fabricated so far, is a low grade one. its heating value being from 5 to 8.4 MJ/kg. More than 100 fluidized bed firing boilers have been built so far, over 60 of which are currently operating. We have ranged them into two classes: hot water boilers of 3.49 MW (3 Gcal/h) and 5.81 MW (5 Gcal/h) thermal capacity, and steam boilers of 2 and 10 t/h steam flow rate (two types differentiated by their steam pressures: 8 bar for one boiler and 15 bar for the other).
The fuel most available for power applications in Romania is coal with low heating value up to 5025-8375 kJ/kg. Research work commenced in 1970 at the Institute of Scientific Research and Technological Engineering for Power Equipment, ICSITEE, Bucharest to perfect the fluidized bed firing technology as far as our domestic low grade coal is concerned, with the view to use the above mentioned fuel either in steam boilers or in hot water boilers of relatively lower capacities. Research investigations have resulted in establishing: (i) the combustion times for coal particles, (ii) air and gas speeds within the fiuidized bed, (iii) the optimum size of coal particles within the fluidized bed, and (iv) other features of the whole fluidized bed firing process, starting with coal driving in up to ash separation out of the flue gases. Research works have been carried out for various kinds of coals with heating values from 5 to 11.7 MJ/kg. Besides the figures given for the above mentioned technical features, the optimum grain size of coal particles has been found to be 0--7 ram, to provide a good firing in the fluidized bed. The above research works have been assumed as a basis for the design of a pilot plant having a hot water boiler with firing in a fluidized bed system and with a thermal capacity as great as 1.86 MW (1.6 Gcal/h). It is on this experimental plant that ICSITEE has carried on the research works concerning coal firing in a fluidized bed system. A good firing stability at low temperatures 1023 K (750°C)-1173 K (900°C) in a fluidized bed has been acquired. An intense firing and a good heat transfer (300 W/m 2 °C) into the fluidized bed have been also found. All the advantages exhibited by fluidized bed firing boilers, compared to other types of boilers, have been actually emphasized, such as: ---capability to use a coal of a very low heating value; --capability to fire coal as grains (its grinding being not necessarily required) and excellent performance of combustion, that is of the boiler too, have been obtained; --achievement of great coefficients for the heat transfer into the fluidized bed and, consequently, of a low metal consumption for the heat-exchanging surfaces available in the fluidized bed; --low nitrogen oxide emissions in the flue gases, due to the low temperatures available in the firing process. The firing in this type of boiler has been carried out with no addition of other hydrocarbons (gas-oil or gas), at loads amounting to 70-100% of its nominal output, whereas the efficiency as obtained for the whole boiler is over 80%. 1017
1018
CARDU and DRAGOn: FLUIDIZED BED FIRING BOILERS
Experimental research works have also been conducted with the view of decreasing the sulphur oxide contents in the flue gases. The procedure of introducing limestone into the fluidized bed has been applied. It is so that 80-90% of the sulphur contained in the coal may be retained. Great performances of the sulphur retaining process for molar ratios Ca/S = (2.5-3.5), the limestone grain structure of 0-1 mm and the temperature in the fluidized bed as high as 1073 K (800°C)-1173 K (900°C) have been obtained. It is in this way that, for a sulphur content in the coal as great as 2.7%, a sulphur oxide content in the flue gases as high as 300 ppm has been obtained. The firing capabilities in self-combustion duty have been rendered evident on pilot units for other types of solid fuels with low heating power values too, such as: --pit coal mixtures (as resulted from pit coal "washing out") having heating values in the range from 7536 to 12,500 kJ/kg; --bituminous shales, with heating values amounting to approx. 3770 kJ/kg; --coal from Albania, with heating values in the range from 6280 to 7120 kJ/kg, the ash content being 60-65% and moisture content 10-15%. Experiments for firing graphite electrode wastes too, as used in the metallurgical industry, have been carried out. The above mentioned material could not be fired in self-combustion duty on account of its low content in volatile matter; it could be fired only by adding about 8% hydrocarbons (heat supply). Only boilers supposed to operate at loads as great as 70% of the nominal load, without hydrocarbons added, have been called for meeting the industrial demands. To meet the above requirement, and on the basis of the results as acquired on the pilot plant, the 3.49 MW (3 Gcal/h) and 5.81 MW (5 Gcal/h) capacity boilers have been designed. They have been built, each of them consisting of two modules and having just half of the total capacity. Thus, the boiler can operate, on the whole, with no other hydrocarbon added, but with partial loads in the range 35--50% (with one module) and 70-100% (with both modules under operation). The designs for three steam boilers with the following main features have also been worked out: - - 2 t/h; 8 bar; 443 K (170°C), --10 t/h; 8 bar; 473 K (200°C), --10 t/h; 15 bar; 623 K (350°C). The Romanian coal to be fed in the above mentioned boilers has the following main features: grain structure = 0 - 7 m m , low heating value = 5-8.4MJ/kg, moisture= 38-45%, ash content = 25-35%, ash softening point = 1323-1423 K. The construction and the operation of a fluidized bed firing steam boiler (Fig. l) is briefly outlined as an example, in the following: The coal, previously processed by crushing and screening, may be found as stored in hopper 1. Taken from this hopper, coal is then introduced into the lower part of the furnace 2, beyond a grate under which the fluidizing and firing air (primary air), coming from a fan, is introduced through nozzle 3. Coal transfer from hopper 1 to furnace 2 is performed by means of the helical conveyer 4. The ash, as resulted at the upper part of the fluidized bed, is collected through channel 3 to the ash hopper 6. Prior to being discharged from the hopper, the ash is air-cooled; the air being so pre-heated is then introduced into the furnace as secondary air. Gases resulting from the firing process travel through an upward path, further through a downward path, then an upward path again, to the outlet from the boiler proper. There is a heat exchange area in the water-steam circuit, just in the fluidized coal bed (the first evaporator 7). On their route, the flue gases successively meet the following heat exchanging areas: superheaters 8, the second evaporator 9, the 3rd stage evaporators 10 and economizers ll. After leaving the boiler, the flue gases pass through the duct 12 and into the cyclone separator 13, where a part of the ash contained therein is separated. Further on, the flue gases pass to the stack through the duct 14 and through the fine-grained ash separator 15.
CARDU and DRAGO$: FLUIDIZED BED FIRING BOILERS
1019
9~ NI
Fig. 1. Fluidized bed firing boilers--main outline. 1--Coal hopper; 2--furnace; 3--nozzle to allow the introduction of the combustion air; 4---coalhelical conveyer;5--ash duct; 6--ash hopper; 7---evaporator 1; 8--superheater; 9---evaporatorII; 10--evaporator III; 1l--economizer; 12--flue gas duct; 13---cyclone separator; 14--flue gas duct; 15--ash separator; 16--ash helical conveyer; 17~rum header. The ash is discharged from hopper 6, the cyclone separator 13 and classifier 15 on the helical conveyer 16. The sequence of the heat exchanging areas in the water-steam circuit is as follows: 11-7-9--10-8. The steam-water mixture from the superheater 8 passes into the drum-header 17, and therefrom, the steam is led to the user. The boiler furnace is wholly lined with refractory bricks. The air for combustion purposes is introduced into the boiler at 1.04 bar pressure. The hydrocarbon burner, as used for boiler start-up and for extremely low loads, is introduced into the coal bed. The fuel, usually being a liquid one, is sprayed into the fluidized be at a 600-700 mm WC overpressure. More than 100 fluidized bed firing boilers of the previously specified types have been manufactured so far. Out of them, more than 60 boilers have been in operation, the remaining ones being in the course of erection. The specified boilers are available in units belonging to the machine building industry, to the food industry and in transportation companies. They supply, for the above-mentioned units, either steam for technological processes or hot water for technological processes or for heating purposes.
ECM 33,11
E