Operation Results of High- Temperature and -Pressure Fluidized Bed Boiler with Recycled Waste Fuels

Transcription

1 Operation Results of High Temperature and Pressure Fluidized Bed Boiler with Recycled Waste Fuels YOSHINORI TERASAWA* EIJI TAKAHASHI* TATSUO YOKOSHIKI* TAKEHIKO SHIRAHATA* MANABU NAGATOMI* RYO YAMAZAKI* This is a bubbling fluidized bed boiler that is highly adaptable to the environment and which operates on recycled waste fuels such as cut tires, wood products, sludge, RPF (refuse paper & plastic fuel) as substitute for fossil fuels. This plant is able to generate hightemperature and highpressure steam with reduced fossil fuel consumption and lower CO emission. Steel wires contained in tires can be safely and continuously discharged from the fluidized bed furnace without accumulating on the furnace bottom. Further, even when a mixture of different recycled waste fuels is combusted, NOx and dioxins reduction can be achieved simultaneously by MSTAR (the Mitsubishi multistage air refiring) combustion method, a combustion system () patented by (MHI). At present, two fluidized bed boilers are commercially operating with recycled waste fuels containing cut tire at Corporation and Mitsubishi Paper Mills Limited.. Introduction While the reduction of greenhouse gases (such as CO) is being promoted for prevention of global warming, million tires (approximately. million tons) were discarded in Japan in, of which % (5 thousand tons) was utilized for the purpose of thermal recovery and electric power generation (). It is also reported that woody biomass (wood chips, lumber from thinning, waste wood derived from construction work, etc.) has a potential supply capacity of approximately 8 million tons per year (). Accordingly, if these materials were used to replace fossil fuels, not fuel costs and but CO emissions could be reduced. However, as tires contain about % to % of steel wires (hereinafter called wires), in order to ensure stable combustion of tires on the fluidized bed, the wires must be continuously discharged from the furnace without accumulating in the fluidized bed. In this way, stable combustion can be achieved even when multiple recycled waste fuels are used simultaneously. Therefore, MHI has developed its proprietary hightemperature and highpressure fluidized bed boiler that operates on recycled waste fuel combining the waste materials fluidized combustion technique for environmental systems and the hightemperature and highpressure fluidized bed boiler technique for power generation systems. This paper gives an outline of the engineering and operation performance of this kind boilers that have been delivered to Corporation and Mitsubishi Paper Mills Limited.. Characteristics of recycled fuels Table able shows the properties of various fuels. Table Properties of various recycled waste fuels Fuel Contents Moisture (%) Ash (%) Combustibles (%) Carbon (%) Hydrogen (%) Nitrogen (%) Total sulfur (%) Total chlorine (%) Oxygen, etc. (%) Higher heating value (kj/kg) Lower heating value (kj/kg) Notes Cut tires Mitsubishi Paper Mills Reference Wood products RPF Wood products PS Moisture is on an arrival basis, and others on a dry basis. Ash and combustibles are based on 8 o C x h. Coal * Yokohama Research & Development Center, Technical Headquarters * Yokohama Dockyard & Machinery Works * Power Systems Headquarters Technical Review Vol. No. (Nov. 5)

2 () Cut tires The higher heating value of cut tires is 8 7 kj/ kg, which is approx. % higher than that of coal, which is 8 87 kj/kg. Cut tires are cheap and suitable as an alternative to coal. However, their sulfur content is.7%, which is comparable to that of heavy oil, so that consideration must be given to related problems of corrosiveness. In addition, tires contain % of wires which can accumulate on the fluidized bed and hinder fluidization. Therefore, prevention of such defective fluidization that could cause inadequate combustion on the fluidized bed is an important objective. Further, as shown in Fig., smallsize tires are cut into pieces (approx. mm each) and largesize tires are cut into pieces (approx. mm each). These pieces, though comparatively large in size, must be made to combust stably in the fluidized bed. Steel cord Steel cord () Woodproduct fuels (wood chips) Moisture content is %, which is comparatively low, the higher heating value is as high as 9 kj/kg, the ash content is only % and its combustibility is excellent. The sizes of chips to be charged are, as shown in Fig., about 5 mm or less on the longer sides. Wood chips also come with incombustibles such as stones and metal pieces (wires, nails, pieces of iron, etc.) which also need to be prevented from accumulating in the fluidized bed. () Others RPF is a solid fuel made of waste plastics and industrial type waste paper, whose moisture content is only % and higher heating value 9 kj/kg, which is as high as that of coal and next to that of cut tires. The higher heating value of paper sludge (PS) is 8 kj/kg, which is lower than that of any other fuel; its moisture content and ash content are as high as 58.% and.%, respectively, making this material hard to burn. Beadwire Beadwire. System characteristics Table able shows the main elements of boiler engineering, and Fig. shows the boiler configuration. Approx. mm Approx. mm Fig. Appearance of cut tire Smallsize tires are cut into pieces of approx. mm each and large tires into pieces of approx. mm each (car tires at left and truck tires at right). superheater Primary superheater Economizer Fig. Appearance of wood products (wood chips) Charging size is about 5 mm or less on longer sides. Table Boiler planning main elements Mitsubishi Paper Mills flow rate (t/h) pressure (MPag).5. temperature ( o C) 5 55 Evaporator (inbed) Heating tube in bed Tertiary superheater (inbed) Fig. Boiler assembly In case of higher temperature and pressure conditions, a final superheater protected by refractories is installed in the fluidized bed (an example at Corp.). Fuels Wood products, cut tires, RPF, kerosene (for starting and aiding combustion) Wood products, cut tires, sludge, other combustibles, AC blend oil (for starting and aiding combustion) Technical Review Vol. No. (Nov. 5)

3 Fig. shows an example of an overall plant system configuration. The boiler delivered to Corporation, which is a bubbling fluidized bed boiler, achieves high power generation efficiency through application of highest class hightemperature and highpressure steam condition (.5 MPag x 5 o C) in Japan. In the meantime, the boiler delivered to Mitsubishi Paper Mills Limited is operating on cut tires as the main fuel, occupying approximately 75% of the total heat input. Both of these power generators are friendly to the environment. In the case of higher temperature and pressure conditions, an superheater (final superheater) protected by refractories, as shown in Fig. 5, 5 is installed in the fluidized bed to reduce corrosion from sulfur content and abrasion by sand, enabling highefficiency heat recovery (MHI patent). In addition, a stepped inclined furnace bottom structure, as shown in Fig., has been adopted and the air supply near the bed drain extraction section has been made adjustable to discharge wires smoothly. Incombustibles such as wires has been discharged from the fluidized bed along the bed ash treatment system, as shown in Fig. 7, 7 and wires has been separated and recovered from the sand by magnetic s. Other incombustibles will be segregated subsequently by a vibrating sieve, discharged from the system and recovered. Fuel feedng chute UAA LAA OFA Boiler air heater Primary air heater Cyclone Bag filter Gas recirculation (GR) system Induced draft fan Bed ash cooler Ash conveyor blower air draft fan Steam gas air heater Suction silencer Primary air draft fan Suction steam gas air heater Recovery system Suction silencer Desulfurization Fly ash silo Fig. Total plant system configuration An example of plant delivered to Mitsubishi Paper Mills. The plant delivered to operates on a smaller amount of cut tires so exhaust gas (SOx) in the downstream stage is treated by a drytype treatment method (drytype desulfurization by bag filter). Upper tube header in bed Bucket conveyor nd step magnetic Tube in bed Tube Refractories rd step magnetic Vibrating sieve Lower tube header Tube outer surfaces are protected from abrasion by casters and studs. Fluidized bed boiler Fluidized material feeding conveyor Fig. 5 Outline of superheater structure in bed Abrasion and corrosion are prevented by installing an inlayer superheater protected by refractories. st step magnetic Fluidized material conveyor st step magnetic Bed drain Stepped inclined furnace bottom Fig. Stepped inclined furnace bottom structure The furnace bottom is inclined so as to discharge steel wires from the furnace without their accumulating. Fig. 7 Bed ash treatment system Incombustibles containing wires and sand discharged from the furnace bottom are cooled on a watercooled conveyor. s and other incombustibles are separated from sand, respectively, by magnetic s and a vibrating sieve, allowing the sand to return inside the furnace. Technical Review Vol. No. (Nov. 5)

4 By optimizing the feeding rate of the secondary air supplied from three stage ports at the free board, low NOx and low dioxins can be attained simultaneously even in the case of multiple type recycled waste fuel combustion by the MSTAR combustion method.. Operating conditions. Combustion stability Fig. 8 shows the time trends of temperatures, boiler evaporation and exhaust gas concentration during the performance test. Table shows the results of the performance test. In the case of mixed recycled waste fuel combustion including cut tires, wood products and RPF (at Nagoya Pulp), both evaporation and steam temperature apparently tended to become stabilized as the supply of fuels was stabilized, and when the fluidized bed average temperature was 8 o C, the exhaust gas contained ppm of NOx, mg/m N of soot O % dry), ppm of SOx (@ actual concentration of O dry) and 7 ppm of CO (@O % dry), their guaranteed values ware achieved. In the case of mixed fuel combustion, including cut tires, wood products and PS, at Mitsubishi Paper Mills, stabilized combustion was also attained. When the fluidized bed average temperature was 8 o C, the NOx concentration was ppm, the soot concentration.9 mg/ m N, the CO concentration 5 ppm, dioxins concentration less than.98 ngteq/ m N (so O % dry) and the SOx concentration 5 ppm (@ actual concentration of O dry), their guaranteed values were achieved. Table Boiler performance test results Evaporation (t/h) NOx Soot (mg/m N) SOx Mitsubishi Paper Mills Target value Test result Target value Test result Dioxins (ngteq/m N) or less CO 7 5 Remarks NOx and soot values are as converted to O % dry values, and CO value as converted to O % dry value. NOx, soot, dioxin and CO values are as converted to O % dry values. Temperature/NOx, CO ( o C/ppm) flow rate (t/h) Fluidized bed temperature temperature O NOx(@O %) Rated steam temperature 5 o C Wood products CO(@O %) flow rate 8 Furnace outlet O concentration (%) pressure (MPag)/ fuel supply amount (t/h) Temperature/NOx, CO ( o C/ppm) flow rate (t/h) O CO(@O %) Fluidized bed temperature Rated steam temperature 55 o C temperature PS NOx(@O %) pressure Rated steam volume 7.5t/h 9. pressure 5 9. flow rate Rated steam volume. MPag 9.. Cut tires Rated steam volume 77 t/h Rated steam volume.5 MPag. RPF Cut tires 5. Wood products Furnace outlet O concentration (%) pressure (MPag)/ fuel supply amount (t/h) Fig. 8 Time trends of temperatures, boiler evaporation and exhaust gas concentration Stable combustion was attained and the performance guaranteed values were achieved both in the case of mixed fuel combustion involving cut tires, wood products and RPF, and mixed fuel combustion involving cut tires, wood products and PS ( at left and Mitsubishi Paper Mills at right). Technical Review Vol. No. (Nov. 5)

5 discharge rate () discharge rate () Under adjusting operation Under adjusting operation.5 : discharge : Tire supply discharge rate = discharge/average steady wire discharge Table recovery rates by magnetic 8 Target feeding rate.8 t/h : discharge : Tire supply discharge rate = discharge/average steady wire discharge Target feeding rate. t/h Fig. 9 Time trends of wire discharge Stable steel wire discharge was confirmed in both cases of mixed fuel combustion (upper figure: Mitsubishi Paper Mills, lower figure: ) Tire supply (t/h) Tire supply (t/h) (a) s about mm long are recovered by upstream magnetic (b) s pulverized into pieces about mm long are recovered by downstream magnetic Fig. Appearances of wires recovered by magnetic s Upstream magnetic recovery rate (%) Downstream magnetic recovery rate (%) Total recovery rate (%) Remarks dischargeability Fig. 9 shows the time trends of wire discharge during the wire dischargeability verification test. The wire discharge variation related on the tire supply changes during the initial adjustment time, but as the tire supply was stabilized, the wire discharge took place at a more or less constant rate and the capability of smooth wire discharge was confirmed. Also, no wire deposit was found on the furnace bottom during the inspection period.. recovery performance Table shows the wire recovery rates of magnetic s installed in the bed ash treatment systems Total recovery rate indicates value of outlets of two magnetic s installed at downstream. Mitsubishi Paper Mills One downstream magnetic installed. As can be understood from Table, in both cases, the wire recovery rate indicates more than 9%, the discharged wires are almost recovered. Fig. shows the appearances of wires recovered by the magnetic s installed upstream and downstream. Compared with the wires recovered by the upstream s, those recovered by the downstream s are finer. Approximately % of wires not recovered by the downstream seem to have been oxidized in the fluidized bed, further pulverized and finally discharged together with combustion exhaust gas. 5. Conclusion Highefficiency power generation through application of highest class temperature and pressure steam conditions in Japan to the bubbling fluidized bed boiler was verified, and adaptability to the environment/energy of MHI's proprietary hightemperature and highpressure fluidized bed boiler technology utilizing a mixture of recycled fuels such as cut tires, wood chips, RPF and PS was also confirmed. In future, MHI will continue its efforts to ascertain the clients' needs and offer products excelling in adaptability to the environment with a view to global warming prevention and efficient utilization of the earth's limited energy resources. Yoshinori Terasawa Takehiko Shirahata Eiji Takahashi Manabu Nagatomi Tatsuo Yokoshiki Ryo Yamazaki 5 Technical Review Vol. No. (Nov. 5)