Investigation of Three Systems to Dry and Incinerate Sludge

Transcription

1 nvestigation of Three Systems to Dry and ncinerate Sludge CHARLES A. HESCHELES P. E. Consultant and SYDNEY L. ZED P. E. Manager Columbus Office Malcolm Pirnie nc. White Plains New York ABSTRACT The design of a new thermal reduction installation to dry and burn sanitary and industrial sludge* prompted an investigation of available systems before making a final selection. This paper summarizes the results of the evaluation and presents the technical and economic justification for the final recommendations. For the system under design sludge cake from a" vacuum filter containing approximately 20 percent solids and 80 percent water was to be fed directly to the thermal reduction units. At the installation studied there were two 25tons/day thermal reduction units one for the sanitary sludge the other for the industrial sludge with both units operating 100 h per week. n case of emergency when one unit breaks down the other unit was to operate full time handling both sanitary and industrial sludge. Essential to any investigation such as this is a review of various systems available consideration of operating conditions evaluation of economics involved and analysis of comparative factors of design and operation. * Sludge is defined as treatment unit underflow which has been thickened and conditioned prior to vacuum filtration. Sanitary sludge is a product from the combined primary and secondary treatment tanks underflow in a municipal sewage treatment plant. ndustrial sludge is the underflow from clarifiers treating the effluent from various chemical processes at an industrial plant adjacent to the municipal treatment plant studied. NVESTGATON OF SYSTEMS TO DRY AND BURN SLUDGE The systems evaluated to dry and burn sludge included the fluidized bed the Herreshoff multiple hearth and the rotary kiln. Fluidized Bed Furnace The fluidizer consists of a wind box a constriction plate to support the fluidized bed a fluidized (sand) bed a refractorylined reactor and a gas outlet. The high pressure air from the wind box flows upward through slots in the horizontal constriction plate enters the sand bed and agitates the sand. Auxiliaries include a high pressure blower auxiliary fuel burners gas scrubber air compressor etc. At startup the unit is preheated by the auxiliary burner to approxima tely 1000 F sand bed temperature. The sludge is fed soon after through an air atomized whirling nozzle. The moisture in the sludge is liberated instantaneously and is converted into superheated steam. The dry sludge starts to burn in suspension while it drops on the sand bed for final incineration. Both vapor and dry gases leave the fluidizer and flow through a mechanical collector and a water scrubber before venting to the atmosphere. Sludges that are not selfsustaining require additional heat provided by auxiliary fuel burners to maintain furnace temperatures of 1300 to 1500 F. The sand bed is maintained in a continuous turbulent motion by the incoming blower air necessary to obtain a high heat transfer rate between sand bed and com 26 5

2 bustible. The sand picked up by flue gases is continuously recovered with a mechanical collector and reused in the fluidizer. Flue gas particulates recovered in the water scrubber are sluiced away together with the ashes from the fluidizer. A diagram of the system is shown in Fig. 1. Flow diagrams of the operation are shown in Fig. l A and 1 B. Multiple Hearth The multiple hearth furnace as the name implies contains a series of circular hearths placed one above the other enclosed in a refractorylined steel shell. Waste material fed at the top of the furnace is moved around the hearth by means of a rotating rabble arm to an opening through which it is dropped to the hearth below. This operation is repeated from one hearth to another until it reaches the bottom hearth. The wastes are completely reduced to ashes by the time they reach the bottom hearth from which they are sluiced away. The flue gases flow upwards counter to the flow of waste material which moves downward. The wet sludge cake is dried in the first few hearths then it starts burning and is completely incinerated by the time it reaches the bottom hearth. The multiple hearths are designed for an exit flue gas temperature of 800 F. Auxiliaries include: Rabble arm drive air blower to cool rabble arm induced draft fan water scrubber and auxiliary fuel burners to maintain furnace temperatures. A diagram of the system is in Fig. 2 Flow diagrams of the operation are shown in Fig. 2A and 2B. Rotary Kiln Furnace The rotary kiln is a slow rotating cylindrical furnace where the material to be processed is continuously SCRU&&tll.!T"'CK rr. FEE P r.\ 20%L[)S) \n\s") SLUPGE FEEDER (0==0 \. L \:;;7":;\ \.>:\\f1.\1 MXER.;"". U!.L OL STOG! Fig. 1 Sludge burning fluidized bed system. 26 6

3 Cake Feed Solids lbs/hr 2080 Water lbs/hr 8320 Combustible MKBH 19.0 Vapor Dry Gas lbs/hr Dry Collector Scrubber Fluidizer... 1.j Auxiliary Fuel MKBH Fly Ash Recirculation Ash Discharge Fig. 1 A Fluidizer system burning industrial sludge. Soli0s lb/hr 2080 Water lb/hr 8320 Combustible!1KBH 13.3 Vapor Dry Gas lbs/hr Dry Collector Scrubber fluidizer HHHHH rlv.l\sh Re circulation Auxiliary Fuel 17.0 t1kbh Ash Discharqe Fig. 1 B Fluidizer system burning sanitary sludge. 267

4 agitated as it moves forward in the kiln. This system uses two separate independent kilns in the drying and incineration of sludge. The first kiln dries the wet cake using hot flue gases from the incinerator kiln while the second kiln incinerates the dried cake. The wet cake is fed to the dryer kiln at the same end that the hot gases enter the dryer both flow in parallel in the same direction. The hot gases enter the dryer at approxima tely 1100 F and leave the dryer at approximately 250 F. The dry cake leaving the dryer is split into two streams one is recirculated with the wet cake fed to the dryer while the other stream is fed to the incinerator. The incinerator kiln operates at temperatures of approximately 1600 to F with the aid of auxiliary fuel burners if additional heat is required. Each operation is independently controlled. The kiln incinerator and kiln dryer maintain independent temperatures. The kiln dryer controls product moisture by controlling outlet flue gas temperature. Auxiliaries include: independent kiln drives mechanical collector dryer hot gas supply fan induced draft fan wet scrubber and automatic controls. A diagram using the parallel flow system is shown in Fig. 3. Flow diagrams using the counterflow system are shown in Fig. 3A and 3B. SCOPE OF NVESTGATON The investigation of the three furnaces (systems) was made in three stages. The first stage was an economic study the second stage was a field survey of operaf floating Damper nduced Draft fan J o [ Filter t Cake Scrubber fly Ash/ Slurry Air Fig. 2 Multiple hearth Herreshoff sludge furnace. 26 8

5 tional problems and the third stage was the final evaluation of the systems based on the findings in the first two stages. ECONOMC EVALUATON The economic study included capital investment operating costs maintenance etc. of the installations. The results of this study are shown in Table 1. The results show that the fluidized bed operating costs are higher than those for the multiple hearth or rotary kiln. OPERATON The survey included field trips to inspect and observe operating installations. As a result of the first stage study the choice was narrowed down to the remaining two systems i.e. the multiple hearth furnace and the rotary kiln furnace as outlined below. MultipkHearth Units Drying and Burning Sanitary Sludge The installation studied has been in operation about 10 years and comprises four 150 tons/day Herreshof multiple hearth furnaj::es burning sanitary ''ador Dry Gas bs/hr F r1. D 2 L. ' 3 _ $'. Ash Solids bs/hr 2080 Water bs/hr 8320 Combustibles MKBH 19.0 Auxiliary Fuel Hl<BHO Fig. 2A Multiple hearth system burning industrial sludge. 26 9

6 sludge in a municipal sewage treatment plant. The sludge is fed continuously from vacuum filters to the multiple hearth furnaces. The filter cake has a m o isture content of about 70 percent and a solids content of about 30 percent. The flue gas temperature in the drying zone controls the rate of moisture evaporation which in turn controls the sludge drying. An outlet gas temperature of 800 F is the design temperature for good operation. A higher outlet temperature moves the combustion too close to the top of the furnace while a lower temperature moves the combustion lower in the furnace releasing a residue not acceptable for land fill. The operation of the furnace is affected by: 1) changes in the amount of air infiltration; 2) varia tion in the volatile content of the sludge; 3) furnace draft.control; 4) auxiliary firing control; and 5) operator competence. Operator salaries in the municipal field are lower than those paid by industry. Municipalities are faced with a real problem in hiring and keeping good operators. A good operator watches his furnace con stantly and looks at the instrumentation and necessary ad justments to maintain operation within limits outlined by the manufacturer. The multiple hearth furnace design has incorporated a limited amount of automation but relies mostly on the operator's good judgment for satisfactory reo suits. The results of such good judgment are shown in Figs and 7 which show variations in flue gas temperatures in four operating furnaces over a 24 h period at the time of inspection. The variation in the flue gas temperatures at the various hearths shown in these charts indicate the need for increased automation in the operation of the multiple hearth \ scrubbef lhs/hr '. 1 Dr) r ')170 'Jr ' :as Cake Feed 81)()OF Solids lbs/hr 2080 Water lbs/hr 8320 Combustibles KBH 13.3 r7 0 f r 1 _:.J.4. _J 5' of l Ash Auxiliary fuel /<KBH 3.8 tlllltiple Hearth furnace Fig. 2B Multiple hearth system burning sanitary sludge. 270

7 furnaces to reduce maintenance auxiliary fuel consumption and particulate emission. Rotary kiln systems A combination 225 tons /day stationary grate incinerator and a rotary kiln dryer was used to dry and incinerate sanitary sludge at a municipal treatment plant. The plant is located in a pulp and paper industrial community and the sanitary sludge includes a good percentage of pulp from the local pulp and paper industry. An insulated rotary kiln sludge dryer 7 ft in diam and 40 ft long reduces the moisture in the sludge from 70 to 20 percent. The capacity of the unit is 6770 lb /h of wet cake. The dryer is located on the first floor of the filter building where the wet cake is fed to the kiln for predrying before it is conveyed to the Stack nduced Draft Fan 4 1 Feed ake! o Mechanical Collector Dryer Ki n o 0 Pug Mi Recirculated Dried _'. Sludge Auxi iary _t_'_ Fir i ng Fig. 3 Rotary kiln system. 271

8 municipal incinerator for burning. The dryer kiln uses hot gases from the municipal waste incinerator combustion chamber to dry the sludge. The hot gases enter the dryer at 1200 F pass through it parallel with the filtered sludge and the cooled gases are returned to the municipal waste incinerator outlet expansion chamber at 300 F. The dryer is equipped with an auxiliary burning furnace to supplement or replace the hot incinerator gases when the incinerator flue gases are not available. The auxiliary furnace has been idle since its installation. Screw conveyors transfer 75 percent of the dried sludge from the cool end of the rotary dryer to the incinerator. The other 25 percent of dried sludge is recycled through the dryer after being mjxed with the incoming wet cake from the filters. This recycling conditions and fluffs the moist sludge entering the dryer. Air jets blow the dried sludge into the municipal incinerator furnace for incineration. The air jets prevent the sludge from caking and smoldering. Dry ing and burning industrial sludge The kiln observed was a countercurrent flow unit for disposing industrial sludge from three lb /h vacuum filters. The kiln was purchased to handle lbs/hr Vapor 9500 Dry Gas F >= 265 F 1 lbs/hr Vapor Dry Gas Vapor Dry Gas f lbs/hr Vapor Dry Gas lbs/hr Vapor Dry Gas lbs/hr Dryer Kiln ncinerator Kiln Feed Cake Solids Water lbs/hr Auxiliary Fuel MKBO Solids lbs/hr 2080 Water lbs/hr 8320 Combustible m<bh 19.0 Fig. 3A Rotary kiln system burning industrial sludge. 27 2

9 30000 lb /h of 30 percent solids sludge but experience shows that the unit can only burn lb /h of 40 percent solids sludge under good control. The filter cake is chopped while being conveyed to the kiln. The kiln is 70 ft long and 10ft in diam. The first 30 ft of the kiln the dryer is lined with drying flights and the remainder the incinerator has a monolithic refractory lining. Sprays of water are located in the drying flights for rewetting the sludge in case the burning area moves back into the drying area. The temperature sensed by the thermocouples is controlled at about 1 100F. The burning produces a volume re duction of about 90 percent. Natural gas is used for startup and /or for complete burnout as required. The sludge cake has a variable heating value from 2300 to 4800 Btu /lb on a dry basis. A large percentage of the solids is noncombustible fly ash from the power house boilers which accounts for the low heating value of the sludge. The flue gases leaving the kiln a t approximately 400 F are pushed by an induced draft fan through the scrubber and afterburner. Ash from the kiln is cooled with sprays and removed by a drag conveyor ready for hauling away. Vapor Dry 265 OF _::.:._ Vapor Dry Gas Gas bs/hr F bs/hr lbs/hr Vapor Dry Gas bs/hr Vanor 1260 Dry ras ll ROil 201l00P Dryer Kiln Feed Cake ncinerator Kiln Feed Cake Solids Water Solids bs/hr 2080 Water bs/hr 8320 Combustible BH 13.3 Auxiliary 11KFlHO Fuel Fig. 38 Rotary kiln system burning sanitary sludge. 27 3

10 TABLE NO. 1 ESTMATED OPERATNG COSTS ncluding interest and amortization SYSTEM Rotary Fluidized tul tiple Kiln Bed Hearth Estimated Loading Dry solids lb/year Oerating Hours hr/year r.:stimated Construction Cost $ $ $ Annual Cost nterest & Amortization 6.9% !1.aintenance 8%. Equipment Cost Labor & Operation Manpower Light & Power Fuel Purchased Ash Disposal Supplies & Miscellaneous Total Annual Cost $ $ $ Cost per 1000 lb Solids at estimated loading $25.5 $41. 5 $

11 The operation of the kiln 12 hjday 5 days per week is not continuous. This type of operation is hard on the refractory. Alternate heating and cooling causes refractory to crack and break loose resulting in spot repairs and more frequent than normal complete replacement of refractory. The original brick and mortar kiln lining had a very short life and was replaced with a castable refractory lining using Z clamps. The castable refractory has withstood well the stress of alternate heating and cooling. 0 Fan mpeller Erosion The.D. Fan located ahead of the scrubber had continuous erosion of the open impellers from the high percentage of fly ash in the original sludge. A change was made to a paddle wheel fan with blades made of HEARTH TEPERATURES OUR NG A 24 HOUR OPERAT ON i = e 1000 {:.! <.> e. = HEART H NUMBER Fig. 4 Chart No. 1 Hereshoff multiple hearth. 275

12 soft steel which has reduced maintenance costs considerably. RESULTS OF THE NVESTGATON The results of the detailed investigation of three systems to dry and burn sanitary and industrial sludge are described below. Fluidized bed furnace The fluidized bed furnace to dry and burn sludge was ruled out at the present time because of its high capital and operating cost. The system retains many operating and maintenance problems unresolved. Specifically the authors had concern regarding the soundness of the fluidized bed structural design 2DDD._ t: 15DD MUln TEPEUTUES DUll.' A 24 HOU O'UATO r l r l i u! l ;. i t t t t "t. o o HEARTH NUMBER Fig. 5 Chart No. 2 Hereshoff multiple hearth. 276

13 to overcome high temperature stresses and the effect of the sand recovery system on poilu tion con trol from the flue gases leaving the furnace. Multiple hearth furnace n a multiple hearth furnace the burning and drying are performed in one furnace. The physical and chemical properties of the sludge vary and the operator does not have the means to realize a change before it completely upsets the burning and drying pattern. The investigation of a multiple hearth installation operating four furnaces showed that the operator does not correct and control the burning of sludge in a multiple hearth furnace. The multiple hearth furnace responds slowly to changes made by the operator turning on and off auxiliary burners ad justing furnace draft etc. The rate of sludge feeding to the furnace is dependent on the treatment facility production and can only be controlled if more than one furnace is in operation by HEARTH TEMPERATURES OUR 1 MG A 25 HOUR OPERAT OM :. = _. : _. 1250' :. 0..! => :;; :. c => _. : = 1 \ o L + + o HEARTH NUMBER Fig. 6 Chart No.3 Hereshoff multiple hearth. 277

14 prorating the feed among the furnaces. Changes in the burning rate of sludge and changes in its quality affect the drying operation. These changes further complicate the furnace operating pattern by shifting the burning and drying hearths up or down causing changes in the temperature configuration. The multiple hearth furnace is practically dependent on the operator's skill because of its limited automatic control. This can be observed in the fluctuations of the outlet flue gas temperature from 600 to F. Rotary Kiln Furnace The drying of sludge in a rotary kiln is a very old application. This type of kiln has been used in drying all kinds of rna terials in the mining and chemical industry. The dried sludge resembles small pellets that can be easily fed to an incinerator for burning. A percentage of dried sludge is continuously recycled with the incoming wet sludge. This is particularly helpful in the material handling and in the drying process. Another 2000 learth TEPEU TUES DUll.' 24 HOUR OPEA T H T ' 1750!.l. t u = r. j;. t t f t t. ot t 't _. 1_ f 1. '1= ;= t. 500 _ 250 HEARTH NUMBER Fig. 7 Chart No.4 Hereshoff multiple hearth. 27 8

15 feature of the system is the feeding of the wet cake at the same end of the kiln that the hot gases enter with both flowing in parallel in the same direction toward the cold end of the kiln. The emission of volatile gases is thus reduced considerably and the possibility ejlists that the sanitary sludge burning and drying might operate without fume incineration. The industrial facility was designed to dry and incinerate industrial sludge in one single rotary kiln. This op'eration caused fluctuations in the drying and burning zones similar to fluctuations in the multiple hearth furnace. As a result of this experience installations today consider separate individual kilns for drying and burning. The drying opera tion is con trolled by main taining outlet gas temperatures from 200 F to 300 F by controlling the rate of flow of hot flue gases from the incinerator to the dryer. Any excess gas from the incinerator is bypassed to the stack. The resulting dry pellets from the dryer are fed to the incinerator kiln for burning at temperatures from F to F. TABLE 2 DETALED ANALYSS OF MULTPLE HEARTH VS ROTARY KLN A summary of questions and answers are detailed below for comparison of the multiple hearth versus the rotary kiln. ANSWER Mul tiple Rotary Hearth Kiln!Dryer Separate Unit No 2ncinerator Separate Unit No 3After Burner Separate Unit No 4Dryer ncinerator. After Burner ncorporated in one uni t No 5Puq Mill For dried cake recirculation No 6Water Scrubber Separate 7nduced Draft Fan 8Stack 9B!ower to cool Rabble Arm No lobooster for Gas Recirculation No Comparisons Auxiliary Burners a. ncinerator Furnace A summary of questions and answers is detailed in Table 2 for comparison of the multiple hearth furnace versus the rotary kiln. SUMMARY OF SURVEY A summary of the findings in the survey with special emphasis on the multiple hearth versus the rotary kiln drying and incineration of sludge show the following pertinent facts: 12Grease Burning )Flue Gas Temperatures a. Leaving Dryer b. Entering Scrubber l4mechanical Drives l5temperature Control a. ncinerator Furnace b. Drying 3000F F F 7000F a. Rabble Arms a. ncinerator b. Blower b. Dryer c. 10 Fan c. 10 Fan d. Booster Fan e. Pug Mill f. Screw Conveyor ndependent Dependent on Drying Dependent on ndependen t Drying Operation Multiple Hearth The operation of drying and incineration is incorporated in one unit. The various operations cannot be controlled separately. The resultant operation is a compromise that could produce a questionable odor control. Rotary Kiln Each operation of drying and incineration is an independent operation performed in separate units. Each unit operation is controllable allowing for variations in sludge quality. The system lends itself to good odor control. 16Burning Resul ts l1pollution Control Good Good Good Good l8system versatili ty Slow Fast Load Control Response and and Poor Good 19Compliance with Future Stricter Pollution Ordinances 20Elimination of Steam Plume Future Hot Air Temperature 27 9

16 RECOMMENDATON BASED ON THE STUDY The survey on sludge drying and burning show that three systems are available today to dry and burn sludge. FluidizedBed Furnace Multiple Hearth Furnace Rotary Kiln Fu rnace The fluidized bed system has a high capital and operating cost that cannot be justified at this time. The system still retains unresolved operating and maintenance problems. The selection is then narrowed between two systems the multiple hearth and the rotary kiln. From the standpoint of capital investment and annual operating cost the differential between the two systems is small enough not to influence their choice on the basis of either cost. The choice has to be made on the basis of performance and mainterance. The tworotary kiln system is more versatile than the multiple hearth system. The tworotary kiln system has independent temperature controls for each ma jor operation ) the dryer temperature and 2) the incinerator furnace temperature; whereas the multiple hearth being one entity does not have the same flexibility. With regard to maintenance the rotary kiln has a few more units requiring maintenance. That by itself should not be a deterrent in view of the advantages accrued in the operation. The plant should have good operators irrespective of the system installed. 280