:Manufacturing-Processing

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1 :Manufacturing-Processing THE CONTROL OF BOILER FLYASH EMISSION IN QUEENSLAND SUGAR MlLLS R. N. Cullen and P. C. Ivin Sugar Research Institute Mackay, Queensland, Australia ABSTRACT Investigations have. been carried out on bagasse fired boilers to determine typical solids emissions and the effect of such variables as air rate, furnace pressure, percentage of forced draft introduced above the grate and fuel air ratios. These factors Were found.to significantly affect boiler emission. Boiler emissions of up to 3.0 grainsystdft" for heaped grate type boilers and 4.0 gralns/std ft 3 for suspension fired boilers were found. Three typical dry type multicyclones and 3 wet scrubbers were tested in pilot plant tests, and efficiencies of 90-92% and above 98%, respectively, were obtained. A description is given of a new full scale wet scrubber installation on 3 heaped grate boilers producing a total of 95,000 lb/hr steam capacity. Problems associated with the plant were found to be minor, and a high collector efficiency of 98% was obtained. INTRODUCTION The problem of flyash emission in Queensland mills was initially given close consideration in when the Clean Air Act was considered for the State. The sugar industry was aware that certain difficulties would occur in meeting therequirements of the Ad because of the variable nature of fuel and the variability of loadings on boiler stations. An investigation was, therefore, started to determine existing levels of emission from sugar mill stacks and methods available to reduce emission to acceptable limits. The industry would then be in a position to know the feasibility and cost before the Act was proclaimed to cover sugar mill areas. Initial investigations were concerned with determining the levels of flyash emission from typical boilers and the measurement of existing collector efficien-, cies. The results of this work led to specific boiler tests to determine the effect of the method of boiler operation on emission levels. Pilot plant tests were then carried out on representative collectors to determine which equipment was available to reduce the boiler emissions to a suitable level. A naturalconsequence of this work was the full scale testing of any radically new equipment that was found suitable at the pilot plant level. EXPERIMENTAL EQUIPMENT AND PROCEDURES Two sets of sampling equipment were designed and manufactured so that simultaneous measurement of collector inlet.and outlet burdens could be carried out. For the measurement of relatively high-dust loads present in the inlet ducts 1552

2 R.N. CULLEN, P.- C. IVIN 1553 a BCURA-Airflow Developments cyclone probe (4) W<;lS used but with a fibre glass filter bag as backup to the hoppers rather than the usual glass-wool pad. On the outlet side of the collectors a modified western precipitating sampling system was used. Because of gas temperatures approaching the dew point after wet scrubber collectors, an electrically heated probe support tube and filter holder was made to avoid condensation problems in the fibre glass bag used for the solids collection. The techniques of isokinetic sampling of flyash are well documented in the literature, and the best features of 2 procedures (4,1) were utilised, depending on the particular application. EXISTING LEVELS OF BOILER EMISSIONS AND COLLECTOR EFFICIENCIES Typical solids concentrations of 2-4 grainsjstd ft S (at NTP, 12% CO 2 and dry gas basis) were obtained for 4 representative suspension fired boilers and from 1-3 grainsjstd ft S for 2 dutch-oven type boilers. There was a noticeable relationship between the skill of the operator and the emission level, particularly with the suspension fired boiler. On the units actually measured, collector efficiencies were quite low, being 40-50% for dry collectors and 68% for 1 spray system on a dutch oven type boiler. Theoretical efficiencies of 70% were claimed by the manufacturers for the collectors giving the 40-50% efficiency, The low test efficiencies were generally attributable to air leaks and mechanical damage caused by hopper fires, poor duct work design causing uneven flue gas flow distributions, and recirculation of previously collected flyash into the collectors. With the exception of one more recently installed collector, the established collector systems were still producing stack emissions of above 0.8 grainj std ft S, over 4 times the permissible limit set by the Clean Air Act in metropolitan.areas. EMISSION LEVELS UNDER CONTROLLED COMBUSTION CONDITIONS Previous observations suggested that the skill of the operator had a marked effect in stack emissions. To determine the extent of the effect of combustion variables on emission, a series of tests was carried out on a typical bagasse-fired boiler. The boiler tested, situated at Racecourse Mill, was a Riley Dodds suspension-fired, water wall type fitted with a traveling grate (nominal rating 154,000 lbjhr). The boiler is equipped with 8.0 Sirocco R type flyash collectors with a design pressure drop of 1.97 in we. After collection, the flyash material is removed with a water sluice. The variables considered in the' tests were: 1) air rate to the boiler; 2) furnace pressure; 3) 'percentage of forced draft introduced above the grate (through oil firing ducts) ; and 4) fuel:air ratio. The results are summarized in Table 1. These show that all 4 factors investigated had a significant effect on emission from the boiler. An increase in air rating from 112,000 to 156,000 lbjhr increased the average emission by 16% from 1.79 to 2.08 grainsjstd ft s By increasing the furnace pressure from -0.7 to 0 in we, the average boiler emission was reduced 16% from 2.10 to 1.77 grainsjstd ft s. The admission of 35% of the forced draft air above the grate reduced the average emission 24%, from 2.20 to 1.67 grainsjstd ft s. With all

3 '-'"' '-'"' Table 1. Tests on Riley Dodds boiler with Sirocco R collector, Racecourse Mill. Set conditions Air flows (at 87 F) Ash-dry weight Collector analysis FD FD Cyclones Inlet Outlet Nominal Furns Over under over and rear cones cone ratingi press. grate grate grate CO2 pass Grate Flyash (grainsj (grainsj Effic 2 (lbjhr) (in WG) air (ft 3jmin) (ft3jmin) (%) (tonjhr) (tonjhr) (tonjhr) ft 3) ft 3) (%) 156, ON 24,790 14, ,000 0 ON 23,140 15, , OFF 29,280 7, ,000 0 OFF 36,100 6, , ON 12,460 9, ;600 0 ON 14,930 9, , OFF 25,080 2, ,000 0 OFF 22,340 2, , ON 27,540 16, , , ON 26,310 19, , ON 22,060 14, , ON 20,880 15, Nominal rating is air rate as recorded on steam flow chart. 2 Because of boiler layout, ash collected from collectors and back pass is -combined. Efficiency is for both back pass and collectors. 3 Grainsjft3 are at standard conditions (12% C02, dry basis). ~ ;.. Z C>,j ;..- q C :;0 z!:;0 o ~ '" I-< Z o

4 R. N. CULLEN, P. C. IVIN' 1555 other variables kept constant, a change in the fuel:air ratio to produce a rise in CO2 content from 14.5 to 20% produced an increase in boileremission from 1.59 to 3.04 grainsfstd ft 3 The low: values of collector efficiency measured, namely 21-51%, could have been caused by a combination of bad hopper design below the multicyclones and a faulty design of discharge valve system which allowed ingress of air. For the controlled combustion conditions over the normal operating range, these tests showed that boiler emissions for this boiler were below 2.5 grainsfstd ft 3 However, based on the tests carried out on many boilers throughout the industry, maximum levels of boiler emission could be considered as: dutch oven suspension fired 3.0 grainsfstd ft3 4.0 grainsfstd ft3 Current installed collect~rs, therefore, even at their theoretical efficiencies of 70%, could not reduce these boiler emissions to acceptable stack emissions so pilot plant tests were planned to determine which collectors would be capable of giving acceptable stack emissions. PILOT PLANT TESTS ON HIGH EFFICIENCY COLLECTORS A preliminary evaluation of types of collectors available on the market enabled 6 collectors, 3 dry type and 3 wet type, to be selected as being potentially suitable. The manufacturers approached to provide pilot plants for evaluation testing were co-operative and the basic size of plant selected averaged 3000 ft f 3 min nominal capacity. The general layout of the pilot plant is shown in Fig. 1. The flue gas for STACK DISCHARGING 60 FT. ABOVE GROUND LEVEL A ISOKINETIC POINT FLUE GAS FLOW SAMPLING _I t I I /POINTS FOR SAMPLING o OUTL~6L~~~~~~ FROM POINTS FOR SAMPLING INLET BURDEN TO COLLECTOR PILOT PLANT FAN WITH DAMPER. CONTROL Fig, 1. Pilot plant layout. I ~. the plant was isokinetically drawn from the back pass of the boiler through a specially designed sampling cone. Sampling positions were established,before and after the pilot plants to be tested, and the stack diameter was chosen to

5 1556 MANUFACTURING-PROCESSING maintain a similar discharge velocity to the main stack so that plume effects could be qualitatively determined. Of the 3 dry types shown in Fig. 2, 3, and 4, the Multiclone9VGR-IOT INLET INLET VANES (SEVERAL TYPES AVAILABLE WITH VARIABLE EFFICIENCV),:, SPIROVANES TO GIVE PRESSURE RECOVERV INLET r'" ~c. ~ SOLlDS ~OUTLET INLET TURNING VANES COLLECTED MATERIAL Fig. 2. Joy Multiclone 9VGRIOT unit. Fig. 3. Sirocco RF unit. GASES IN GASES OUT PUMP TO SETTLING TANK Fig. 4. Malleys Bahco CSD unit and the Sirocco RF collectors were vertical multicyclones in which the incoming gases pass into a relatively small diameter tube through turning vanes. The dust particles are thrown to the outside and collected in a hopper at the bottom, while the cleaned air passes up through the top of the tube. The Malleys Bahco CSD unit was a horizontal multicyclone with a secondary air circuit capable of pro-

6 R. N. CULLEN, P. C. IVIN 1557 ducing a higher collection efficiency. Approximately 10% of the gas contammg concentrated dust near the outside of the cyclone tube is drawn off and passed through a separate external high efficiency cyclone. The 3 wet type collectors selected are shown in Fig. 5, 6 and 7 and are respectively examples of: a) Malleys Bahco CVM scrubber, relying on a multiple washing of the particles in a self induced water spray; b) Joy Turbulaire T scrubber, relying on a high velocity impact of gas onto a static water bath. The FROM BOILER CONE ELIMINATOR BAFFLES MAKEUP WATER (FLOAT ~~~~~~Ln--::'tlJtitl:±::il CENTRI~':UGAL ELIMINATOR ~----. TO FAN CLEAR WATER RETURN OVERFLOW Fig. 5. Malleys Bahco CVM scrubber. Fig. 6. Joy Turbulaire T scrubber. t'hi:.5h WAT~R INLET IAT 10 L8/IN~) FROM ~~REAOER PLATE SOllER ,. COLLECTED MATERIAL Fig. 7.. Dynaflow HNU scrubber.

7 1558 MANUFAGTURING-PROCESSING solids are caught on impingement with the water; and c) Dynaflow HNU scrubber, relying on a water spray followed by a sudden pressure drop with separation through vanes. A summary of the performance of these collectors is shown in Table 2. Table 2. Type Performance of pilot plant collectors. Pressure drop Description range (in WG) Collector efficiency Range Average (%) Multiclone 9VGR-IOT Sirocco RF Malleys Bahco CSD Malleys Bahco CVM Scrubber Joy Turbulaire T Scrubber Dynaflow HNU Scrubber,Multicyclone with pressure' recovery ~?ne Multicyclone with high residence tjth~ Multicyclone with secondary air circuit Static Bath Cascade Scrubber Static Bath Impingement 1.7-6,0 Scrubber Spray type venturi scrubber !J These results were obtained with pilot plants operating under ideal conditions and close supervision. In general, the guaranteed collector efficiency normally attainable for dry collectors would be from 90-92% and for wet collectors about 98%. With boiler emissions ranging from grainsjstd t 3, a collector system would have a maximum emission of about 0.4 grainjstd ft 3 for dry types and 0.08 grainjstd ft 3 for wet types. FULL SCALE WET SCRUBBER SYSTEM During the 1970 slack season, three Malleys Bahco CVM scrubbers with high pressure fans were fitted to 3 dutch-oven boilers at Racecourse Mill, one of 35,000 lb jhr and the other 2 of 30,000 lb jhr maximum continuous rating (MCR). Settling tanks were used to settle out material from the underflow and overflow from the scrubber hoppers. The units and. ducting were constructed from mild steel, the only stainless steel being used for the cascade tubes themselves and for the rotor of the fan. All mild steel was first sandblasted and given 1 coat of a high temperature zinc-rich primer followed by 2 coats of a tar epoxy resin with a temperature rating of 200 F continuous, 250 F discontinuous. The common stack was already lined with Gunite. Measurements of the flyash emission were carried out on no. 2 scrubber with the boiler operating at 2 in WG draft at the back of the boiler and producing an average of 44,000 Ib /hr steam flow, approximately 150% MCR. The measured collector efficiencies of 98.7 and 99.6% obtained at 5.1 in WG collected pressure drop confirm the results obtained with the pilot plant. There have, been few problems connected with the wet scrubbers. No

8 R. N. CULLEN, P. C. IVIN 1559 excessive white plume effects have resulted from the- lower gas temperatures produced by the water contact of the scrubbers. The ph values for the recirculation water have been in the range 6.0 to 7.0 and for the condensate in the duct work 5.0 to 6.5. After one season the surfaces of the ductwork scrubbers and settling tanks are in good condition. REFERENCES 1. Anon. Methods for determination of velocity, volume, dust and mist content of gases. Bulletin, WP-50, 7th Ed., Western Precipitation Division, Joy Manufacturing Company: 2. British Standard: B.S. 893, Method of testing dust extraction plant and the emission of solids from chimneys of electric power stations. 3. British Standard: B.S. 3405:1961. Simplified methods for measurement of grit and dust emission from chimneys. 4. Hawksley, P. G. W., S. Badizioch, and J. H. Blackett Measurement of solids in flue gases. The British Coal Utilisation Research Association.