EXPERIENCE IN VARIOUS COALS FIRING WITH BALL TUBE MILLAnantachin Jarukom, Assistant Power Plant Manager, Hiang Seng Fibre Container Co., Ltd., Town district, Samut-Sakhon 74000, Thailand Thunya Siltorn, Vice President, Engineering & Service, Union Energy Co., Ltd., Phrakhanong, Bangkok 10250, Thailand Chanchai Benjaniratn, Assistant Mill Manager, Hiang Seng Fibre Container Co., Ltd., Town district, Samut-Sakhon 74000, Thailand ABSTRACT - Instead of controlling fuel flow by coal feeder, constant coal inventory in Ball Tube Mill which was achieved by Power-Sonic Mill conditioning system has been used. The design was based on Australian coal, but trial had been made since 1986 with various Thai lignite to imported Australian, Chinese and Indonesian coal. The results indicated that imported coal have significant advantage in both reliability and economy. Indonesian coal was considered to be prime fuel because of its proper boiler turn down. As a result of close attention of operating and maintenance staff, especially to monitor and control fuel-air ratio, ball charge, stripping of Ball Tube Mill, etc., availability of the unit was eventually improved. Also, operating procedures for coal kind change without mills shut-down and optimization of Ball Tube Mills grinding performance were developed. In present day, the unit is continuously run on coal, even when coal kind being changed, Bunker C Oil is only needed when emptying mills before shut-down and during start-up. This is resulted in very economic energy cost for the customer. 1. INTRODUCTION The unit is topping cycle co-generation, supplying electric power and process steam for paper manufacturing process. It is consisting of one pulverized coal fired boiler rated at 118 kg/cm2g, 541C, 149 t/h, one back pressure turbine of 24,500 kW and one condensing turbine of 3,500 kW. The process steam capacity is 90 t/h at 5.4 kg/cm2g, 195C. Boiler’s firing equipment are two double end 9 feet dia. by 11 feet long Ball Tube Mills (BTM) with two volumetric coal feeders, two crusher dryers and two external static classifiers per each mill, the number of burner per mill is two (fig.1). Each BTM is rated at 65 % MCR with turn down of 1:2, coal feeders sizing were ample for each BTM to operate with one feeder out of service. Firing equipment have been designed for wide range of fuel from Thai lignite to Imported coal (table 1 & 2). Four bunker C oil burners with 100 % MCR capacity located in center tube of each coal burner were also provided. BTM are cylindrical steel shell lined with hard face curved steel liners. The grinding elements are three parts of 32 mm. and one part of 38 mm. dia. forged steel balls. The grinding principle (fig.2) is that when mill shell rotating at design speed, grinding balls are carried up with mill liners to proper height and fall down. Coal is then pulverized by crushing, impact and attrition. Pulverized coal are swept by primary air through outlet boxes at both end to classifier, which only fine particles pass to burners, while coarse particles return to mill inlet boxes via return pipes. Crusher dryer is used to crush and pre-dry incoming coal, which is helpful to keep mill performance and reduce fire hazard when grinding high moisture, high volatile coal. 2. BTM AND COAL FEED RATE CONTROL The most unique feature of BTM is relatively huge amount of coal stored in its grinding zone. When primary air flow increases, there is certain quantity of pulverized coal ready to be transported to furnace in additional to that of the previous state. The fuel flow to burners regulated by coal feeder speed adjustment is valid only in steady state. In order to control fuel flow more precisely in all condition, other methods are needed. One of commonly used control scheme is to vary primary air flow with demand from combustion control system, while keeping mill coal inventory constant by adjusting coal feeder speed. BTM inventory control system used in this unit is called Power-Sonic Mill conditioning system, which were developed by Riley Stoker Corporation. This particular control system measure two process variables from mill grinding process, which are Page 1 of 8 which caused by coal particles filled in voids between charge balls. without peak. Power-Sonic uses power input on the right side of the peak to compare with kW set point. but Australian coal still the Page 2 of 8 . despite of being close to design specification. These coal were produced by better process and contained less contamination. Australia and Indonesia. then various coal was trial (table 3). which were located in Li.e... Many mechanism for flame adjustment were tried such as coal spreader position.3) can be described as follows : When empty mill is starting. In mid 1988 coal yard roof were built. this problem was satisfactorily solved. crusher feeding hopper. but mill inlet / outlet boxes plugging at lignite flow more than 7 t/h were encountered. Phare lignite. coal stock yard was open type. secondary air register opening. mill inventory and primary air velocity (varied by classifier exit temp. etc. bunker outlet chute. Coal characteristic were of wide variety from relatively low volatile Australian coal to high moisture. Lampoon. Mill inventory control kW set point was approximately 96 to 97 % of peak kW. the CET was less than 60C while the hot air dampers were fully opened. OPERATING RESULTS The unit was commissioning in mid 1986 at 55 % MCR due to paper machines load limit with lignite. conveyor transfer station. 4.25 m. but mill plugging was resulted as same as lignite.. Most of lignite delivered contained some lump of over 200 mm and contaminated with clay and foreign matter. Another was adding of tubular gas to air heater for further heating of hot primary air to achieve proper drying capacity. since collected data indicated that furnace area was too large. caused by heavy rainfall were encountered in the first three years. Chinese coal had no problem on drying capacity. HGI vary in the range of 23 to 51 (table 1). lignite was the only fuel that two mills operation could be performed. flame stability was poor and ignitor support was needed most of the times. Their gross calorific value were ranging from 3350 to 4990 kcal/kg. Also drying capacity was proved to be inadequate. 3. Overfine coal dust and fly ash re-entrain to combustion air was also observed. power consumed is relatively low and sonic level is highest due to only metallic contact of balls and liners. It was resulted that kW set needed to be trial and error. thus reduces power consumption by shifting of virtual center of gravity toward mill rotation axis. Flame stability was quite acceptable. Originally. Mill inventory control set point was changed to 97 to 98 % of kW peak. Australian coal was the worst. World fuel & Lanna lignite. FUELS Thai lignite was from three mines. Once these voids are completely filled. high volatile Indonesian coal (table 2). as can be seen at the leftmost of the curve.e. Combustion condition after modification was remarkably improved for both Chinese coal and lignite. power required will be higher but sonic level decreases. Deviation of measured kW from set point combined with sonic signal or so called Power-Sonic signal is used in feeder speed control. this problem was partially solved by decreasing amount of balls charged. but most of them gave flatten top kW curve. The problem of very wet coal plugging at various points i. while sonic signal used as an indicator to tell whether the mill operated on emptying side (left side of kW peak) or not. After coal feeders starting. height refractory around burners area on front wall to decrease radiant surface. Major modification was taken place in mid 1988 by adding two bands of 2.• • Mill drive motor power input (kW) Sonic level or sound pressure level generated (dB) The relationship between these two process variables and mill inventory (fig. were unlike Thai lignite. i. Front end loader and bulldozer have been used to load coal into crusher feeding hopper. : CET and mill bypass dampers position). while Chinese coal and lignite needed ignitor support during soot blower and low load period. At that time. Filling more coal will be resulted in ball and coal level increased. power will reach its peak value. Imported coal supplied from PRC. If the difference exceed predetermined value. balls. while Chinese coal can do only to 65 % MCR. liners wear were only 5 mm. drive train and accessories. or smaller balls is needed.5 % of peak value. It was also found that the first lot of this brand was the mixture of unintentionally mixed ‘Enviro coal’ and Chinese coal caused by error in unloading and transportation. This task was done by comparing measured ball bed depth with the value after initial filling or last sorting and filling. annual inspection record shown that after 10 years of operation. (manufacturer predicted life time of liners are 100. although crusher dryer hammers needed to be replaced every 3 to 5 years. Ignitor support for soot blower was only for wall and final superheater blowers. Mill plugging problem was drastically reduced. Other interesting point is Indonesian ‘Adaro or Enviro coal’. the result was quite satisfied. Once this data base was established. then feeding increased to resume normal inventory without mill shutdown. Due to higher value of HGI and heating value. Other maintenance needed for BTM are preventive or periodical maintenance of mill support bearings. When Indonesian coal have been burnt. flame adjustment then was done by variation of secondary air register. which caused steam explosion when falling down to ash water trough and led to unit trip. after commissioning of paper machine No. make up of ball charge could be done without mill stripping in the case that load condition is not permit. 5. despite of low HGI. Relationship between ball charge to kW peak and ball wear rate could be established by periodically use of the so called ‘mill stripping’ operation. MAINTENANCE Significant wear parts of BTM are balls. which done by decrease coal feed rate to minimum to find the peak power. This problem solved by using of kW peak of another Indonesian coal with the same bulk density and surface moisture. thus led to mill plugging. which had high inherent moisture content. because fuel cost was not so much differed. 4. the refill then made of 38 mm. which resulted in lower kW peak (fig. The amount of balls needed to restore the original peak. which was recommended by Riley service engineer in 1988. By then.4) and lower grinding performance. Maintenance cost of imported coal was lower and had better unit reliability. This resulted in constantly changing flame condition and required very close attention of operating crews. ball make up has been done by filling required amount of balls through ball charge hopper. Now mill inventory control set have been at 98 to 98. coal spreaders have been fixed at nozzle tip flush position. It gave lower actual peak kW. Two mills operation on imported coal formally begun in April 1991. Chinese coal. Regular interval for ball sorting in this unit is approximately 3 years. wear ball peak and quantity of coal ground between stripping interval were used to calculate ball weight per kW change and ball wear rate. flame condition was good and ignitor support for soot blower was no longer needed. To maintain performance. Flame condition is very good and even Chinese coal can be efficiently burnt under this circumstance.worst of all. thus promotes the choice of Indonesian coal as prime fuel. dia. Indonesian coal and lignite in used evidently had porous slag. primary air velocity and opening of classifier vanes. Indonesian coal was proved to be more appropriate because of better two mills turn down to 55 % MCR.000 operating hours or more). Page 3 of 8 . Also. dia. Indonesian coal was first burnt in March 1989. bottom ash conveyor was changed to DB intermediate grate type by the end of 1989. ball sorting to reject 20 mm. preliminary evaluation showed that burning imported coal was more favorable. Even though mill liners also wear. ball charge needed to be restored to at least 90 % of full charge to cope with low HGI and amount of balls have been maintained at this level by periodically ball charge. To solve this problem. Another important maintenance item for BTM is to maintain proper ratio between large and small size balls to keep mill performance. average maintenance cost is only small percentage of annual maintenance cost. When only new coal is confirmed to be burning. Worcestor. 200 mesh sieve. fineness 75 to 80 % through ASTM No. different set of parameters needed for various properties of coal have been established as follow : • • • CET of 60 to 62C. fineness 80 to 85 % through ASTM No. this procedure normally takes 1 to 2 hours to accomplished. (1) (2) REFERENCES Riley Service Guide. When grinding coals of different properties with the same amount of ball charge. medium volatile Chinese coal. fineness 70 to 75 % through ASTM No.6. thus resulted in very economic energy cost for the customer. mill stripping to find actual kW peak then performed as soon as load condition is appropriate for such operation. publication No. 8. The use of high price ignitor fuel and Bunker C oil are minimized as well as outage and derate time. The change simply begun with recording of coal bunker levels before new coal filled up. Only mill inventory needed to be changed for each kind of coal and it is simply done by gradually change of kW set point to the required value. CONCLUSION Continuously burning wide variety of coals successfully with Ball Tube Mill without mill shut-down are largely resulted from simple operating principle and relatively low maintenance of this type of mill. 1977. ignitors then are retracted and fine adjustment of flame condition will be done. high volatile Indonesian coal. Mill inventory control kW set determined by comparing new coal properties to those of previously burnt. CET of 68 to 72C. CET of 63 to 65C. after stable flame is maintained. 7. high volatile Indonesian coal. 1982. depended on coal feed rate. RSC-3 5/82 5M. MA Page 4 of 8 . high secondary air swirl for low to medium moisture. This procedure have been adopted in late 1991and then became standard practice. Riley Stoker Corporation. 200 mesh sieve. low secondary air swirl for high moisture. MA The Riley Ball Tube Mill System. moderate secondary air swirl for low to medium moisture. EXPERIENCE GAIN Coal kind change without mill shut-down is resulted from well collected long period operating data as well as former experience on using mixed lignite in 1986 to 1988 and mixed imported Chinese and Australian coal in 1988. fineness and air to coal ratio of pulverized coal to burners will vary to certain degree. then ignitors are placed in service just before mixing start. These parameters are guideline to help operators to begin the adjustment with various coal kind. To burn these coals to the same efficiency under this condition. Riley Stoker Corporation. Worcestor. 200 mesh sieve. calculate the starting and duration time of mixing. then fine adjustment for efficient combustion of specific coal can be done. There are no change or adjustment made to BTM itself prior to introduce new kind of coal. Flame condition during mixing is confirmed by visual inspection and necessary adjustment toward new coal properties made. 67 to 31.37 9.76 12.18 0. Hensen 6.05 4.924 12.46 to 33.60 6.74 4.26 36.56 to 27.6 to 22.74 to 8.14 4.81 30.19 28.73 to 24. due to high Fe2O3 in ash Page 5 of 8 . Mark rich 5. Senakin 6.10 13.94 to 13.70 14.07 3.72 28.61 39.00 38.221 8.349 to 3. Shenmu 5. Blair athol 5.95 56 IND6.262 10. CC : Chinese coal.35 32.496 18.62 7.887 9.38 to 5.96 to 15.230 8.935 35.86 3.33 13.07 to 24.837 16.251 7.71 to 35.96 40.20 4.0 18.84 42 to 49 CC3.28 39.14 11.72 38 IND10.945 27.58 to 0.99 0.3 1.5 45.16 to 0.57 37 IND8.69 35.49 50.413 7.67 3.79 39.38 38.97 8.42 3.44 42.95 27.46 to 6.46 9.65 35.02 to 1.45 8.28 49.08 0.70 3.02 5. Pingshou2 6.98 38 to 51 10.39 3.4 to 47.78 to 15.97 0.0 CC1.46 0.788 to 3.5 60 IND2.72 7.835 35.68 48 AS.81 to 32.605 15.0 4.39 5.14 1. Arutmin 6.67 to 29.09 to 36.69 Lanna lignite 4.29 36.59 53 IND5. as received Coal kind HHV (kcal/kg) Moisture Surface Inherent Ash Volatile matter Fixed carbon Sulfur HGI (air dried) Design lignite 3.75 13. Kitadin 5.04 to 51.52 to 0.563 9.0 to 14.10 37.16 0.47 4.0 17.53 15.41 46 IND9.50 9.05 0.48 37 Table 2 Proximate analysis of Imported coal : % by weight.69 1.7 29.96 48.36 37.10 6.46 12.77 to 8.52 to 7. Adaro1 5.16 to 35.81 to 19.06 to 23.16 3. Adaro 5.40 to 9.63 to 29.987 19.46 6.992 to 6.63 1.005 7.13 48.52 3.99 28.29 11.20 64 to 67 IND4.95 6.5 0.41 16.67 46 IND1.251 to 4.770 to 5.32 7.878 15.66 51 to 55 IND3. IND : Indonesian coal 1 : mixed with Chinese coal 2 : Very severe slag was resulted.02 40.44 0.176 8.43 to 31. Banjar 6.35 to 54.58 25.70 3.91 0.92 0.18 to 0.81 37.9 44 World fuel 3. Pingshuo 6.83 55 16.11 5.41 31.90 to 29.99 to 29.27 to 12.12 28.17 6.25 to 6.42 9. as received Coal kind HHV (kcal/kg) Moisture Surface Inherent Ash Volatile matter Fixed carbon Sulfur HGI (air dried) Coal kind HHV (kcal/kg) Moisture Surface Inherent Ash Volatile matter Fixed carbon Sulfur HGI (air dried) Coal kind HHV (kcal/kg) Moisture Surface Inherent Ash Volatile matter Fixed carbon Sulfur HGI (air dried) Design (AS) 5.60 to 6.17 0. Satui 6.236 25.47 38. (Banpu) 5.55 42 IND7.61 39.70 24.0 10.44 to 15.60 to 19.77 2.93 38.58 31.77 0.5 21.2 to 31 Phare lignite 3.83 6.03 19.66 23.36 0.316 to 5.62 5.Table 1 Proximate analysis of Lignite : % by weight.84 0.29 3.99 1.21 41.01 to 12.73 to 38.14 44 AS : Australian coal.21 41 CC2. Apr. medium to long Apr. stability is Shenmu mixed final SH S/B acceptable Kitadin. 86 1 S/B in air & mill plugging Blair athol. Aug. PRC 91Aug. fairly stable flame. 91 did not require 85 % Bright yellow. 86 1 AS 1 : Australia. 95 did not require 90 % Bright yellow. 96 did not require 90 % Same as ‘Senakin’ Pingshuo. relatively long & stable IND Nov. good stability Adaro. stable flame. 89 1 final SH S/B Shenmu. soft. did not require 85 % Bright gold. 95 did not require 90 % Orange. Apr. IND Mar. soft & stable flame final SH S/B Phare lignite Oct. very unstable AS May 88 1 load and S/B at low load Lanna lignite Apr. very intense & PRC final SH S/B stable flame Pingshuo. soft. 96. PRC : People Republic of China Full ball charge is 23 tonnes of balls per mill. 96 (table 2. relatively long & stable flame. Senakin. mill S/B plugging & inadequate drying capacity Shenmu. hard flame. hazy flame. 92. short. May 92 did not require 85 % Bright yellow. 93. Jan 91 wall blower & 85 % Bright yellow. 87 1 medium. fly ash re-entrain Dec. IND : One mill operation. PRC May 89 1 wall blower & 70 % Bright yellow. 95 did not require 90 % Same as ‘Senakin’. 88 low load and 70 % Orange. mill S/B plugging & inadequate drying capacity 1 Phare lignite Dec. relatively long & stable flame Mark rich. excellent stability Aug. precipitator severe spark Adaro. 91. IND Mar. short. PRC Oct. IND Sep. 95 90 % Hensen. 88 wall blower & 70 % Orange. Mar. 88 1 wall blower & 70 % Yellow. very severe slagging problem Time of use Jul. did not require 90 % Yellow. soft & hazy flame. long & stable flame. Feb. CC3) did not require 90 % Bright gold. low 70 % Dark orange. 88 wall blower & 70 % Yellow. CET set 60C. 89 1 final SH S/B flame Arutmin. 86 1 low load and 85 % Orange. relatively long. long & stable flame Sep. IND Aug. mill plugging & S/B inadequate drying capacity Tubular air heater and front wall refractory added in July ~ August 1988 Lanna lignite Sep. short. 86 low load and 85 % Dull orange. Nov. 94. ball final SH S/B added to BTM due to low HGI 1 Pingshuo. very intense flame. 89 1 wall blower & 70 % Yellow. narrower turn down range Satui. IND Nov. 95. PRC Jun. but flame mixed condition constantly changed (Banpu). 90 1 wall blower & 85 % Yellow. soft & hazy flame. did not require 90 % Bright yellow to gold. soft flame. Aug.Table 3 Operation summary Coal kind World fuel Ignitor support Ball charge Result low load and 85 % Dark orange. Page 6 of 8 . intense flame. IND. : Indonesia. IND Jul. long & fairly stable flame & World fuel mixed final SH S/B Blair athol & Nov. 92 flame. Jun. hard & short flame. 97 flame. IND Oct. 96 mill plugging due to low peak kW Banjar. soft. 93. intense & stable Nov. IND Feb. Ball charge hopper Figure 1. Classifier 6. Bypass damper 7.Hot air Tempering air From Mill B 1 9 2 2 1 III IV 8 I II 10 5 6 7 4 Mill A 3 3 6 7 5 1. Crusher dryer 4. Ball Tube Mill 5. Firing equipment flow diagram Figure 2. Coal bunker 2. Burners 9. Ball tube mill working principle Page 7 of 8 . Return pipe 8. Volumetric feeder 3. Primary air fan 10. Ball wear and power consumption relationship Page 8 of 8 .Peak kW High Sonic (dB) level Normal operating range Set point POWER (kW) Low kW level Decrease INVENTORY Increase Figure 3. Power consumption and inventory relationship Original kW High POWER (kW) Peak kW Set point Low Wear ball kW level Decrease INVENTORY Increase Figure 4.
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