Fluidized Bed Disposal of Secondary Sludge High in Inorganic Salts

Transcription

1 Fluidized Bed Disposal of Secondary Sludge High in Inorganic Salts DOUGLAS L. McGILL and ELBRIDGE M. SMITH Roy F Weston West Chester, Pennsylvania DISCUSSION by J. I. Stevens, Arthur D. Little, Inc., Cambridge, Mass. The authors have presented an excellent example of the total systems approach to overcoming a difficult waste disposal problem. They have presented useful guideline data for fluid bed incinerators which are increasingly being adopted for thermal destruction of difficult organic sludges. Although not of concern in the design described, the fluid bed incinerator has the special capability of being shutdown for relatively long periods but still being able to effect a rapid startup due to the thermal fly wheel effect of the bed. This consideration is especially important for the incineration of sludges such as those described by the authors. Implied, but not stated, is that the incinerator will operate with low excess air ratios. While I agree with the authors that the thermal fly wheel effect in a fluidized bed is considerably greater than the other processes evaluated, I am sure that automatic control provisions have been made to prevent the temperatures in the freeboard and the bed from dropping below a predetermined minimum or going above a predetermined maximum and a brief description would have strengthened the presentation. There was no mention of the provisions made for bringing the incinerator to start-up temperatures; presumably this important function is performed by an auxiliary burner, since direct injection of fuel gas into the bed can only be accomplished near the normal operating temperature range. From the process design viewpoint, the authors should have stated the conditions at which the superficial fluidizing velocity is reported, that is, is it at operating temperature and atmospheric pressure or is another basis chosen? Also, useful guidelines which could have been provided are the design heat release rate on some basis such as superficial cross- sectional area of the bed or volume of the reactor and the connected horsepower of the air blower, again on similar bases. The authors are to be commended for making an excellent choice of a process system to handle this difficult sludge disposal problem and it is hoped that they will present the results of start-up and operation at a subsequent National Incinerator Conference. DISCUSSION by G. R. Smithson, Jr., and T. L. Tewksbury, Battelle Memorial Institute, Columbus, Ohio We appreciate the invitation to discuss the paper which was prepared by Messrs. McGill and Smith. The experimental work upon which that portion of the paper dealing with fluidized-bed incineration is based was conducted at Battelle's Columbus Laboratories under the sponsorship of Copeland Process Corporation. As Messrs. McGill and Smith have pointed out, the primary objectives of the experimental program were to determine the conditions for complete oxidation of the organic constituents of the sludge and to ascertain the conditions necessary for the combustion of these materials within the range of conditions commensurate with the operation of a fluidized-bed system. Other important aspects under consideration during the experimental study were the deri vation of conditions required for incineration without the production of noxious gases and the conditions necessary for the operation of the unit as an agglomerative fluidized bed. As the authors have pointed out, preliminary differential thermal analyses were conducted on a number of mixtures of calcium oxide, 'sodium chloride, calcium sulfate, and calcium chloride prior to the fluidized-bed study. This was one of 17

2 our first attempts to use differential thermal analyses as a tool in predicting the behavior of inorganic mixtures in fluidized-bed systems. We believe these analyses were extremely helpful in delineating the operating conditions subsequently used in the pilotscale experimental study. The results of the differential thermal analyses not only indicated the range of operating temperatures which might be applicable, they also indicated the possibility of operating the unit as an agglomerative system. The rationale for developing the fluidized-bed system described by the authors may be of interest to some people. The use of overhead feed systems for fluidized-bed incineration of waste materials was developed by Container Corporation of America during a study approximately 10 years ago and is an integral part of a system now known as the Container-Copeland System for treating waste paper pulping liquors. The basic advantages of overhead feeding systems are the utilization of a portion of the sensible heat in the exhaust gases and the ability to control the spray pattern sufficiently to provide control of the particle size of the inorganic product resulting from the incineration. The operation of the fluidized-bed system within the temperature range indicated, i.e., 1300 to 1500 F, is an important factor in the selection of a fluidizedbed system for the incineration of waste materials. By carefully controlling the temperature within this range, it is possible to minimize or eliminate air pollution problems arising from the incineration of many of these materials. This is not true in all types of incinerators, particularly those in which temperature control is difficult and great variations in temperature are common. Messrs. McGill and Smith have pointed out the potential modes of operations of fluidized-bed incinerators, i.e., the operation with an inert bed of materials such as sand or a nonagglomerati ve system or the operation of the system in an agglomerati ve manner. They also have pointed out the advantages of operating the system in an agglomerative mode so far as the selection of dust collection equipment is concerned. A further advantage of operating the system as an agglomerative fluidized bed is that. by controlling the particle size distribution of the inorganic product, the range of superficial gas velocity commensurate with the operation of the fluidized-bed system can be widened considerably. For example, if conditions are selected to provide coarse particle sizes, a higher space velocity can be used, thus providing more oxygen for combustion of the organic fraction of the sludge and a concomitant increase in the quantity of sludge which can be treated in the incineration unit. Conversely, if the capacity of the unit needs to be lessened, the system can be operated so that smaller particle sizes and lower gas velocities can be used. Such variations, of course, must be within the range of superficial gas velocities necessary to provide adequate mixing of an agglomerative fluidized-bed so that fritting or fusion will not occur. We believe that Messrs. McGill and Smith have provided an excellent summary of the experimental work which was done on the fluidized-bed incineration of activated sludges. Battelle also has conducted pilot-scale experimentation on the fluidized-bed incineration of primary and digested sewage sludge as well as sludges from petroleum refining operations and paper making operations. The results of these studies have led to the conclusion that this system with its inherent advantages of close control of operating temperature and gas composition provides an excellent means for the ultimate disposal of many carbonaceous industrial wastes. DISCUSSION by C. A. Hescheles, P. E., Consulting Engineer, New City, N.Y. Mr. McGill and Mr. Smith should be congratulated for the presentation of a very timely subject: Sludge Incineration. The authors in their conclusion recommend fluidbed incineration in preference of other methods because of advantages with respect to: 1) equipment dependability; 2) minimum space requirements; 3) capital costs; 4) sludge handling characteristics; and 5) substantial heat-sink effect of fluidized-bed and refractory lining. It would be of interest if the authors would discuss and amplify the basis of their recommendations. The paper discusses the operation of the fluidizer and it would be very helpful if it could include the basis of their recommendations. It would also be of interest to have information on the other methods of sludge incineration that were considered by the authors in making their final conclusion. I am quite sure that your evaluation of the systems was based on a comparison of: Operating Costs & Capital investment Fuel Auxiliary - Purchased Electric Power Consumption Capital Investment Repairs and Maintenance Operating Manpower 18

3 Should this data be available, and incorporated in the discussion, it would be of interest to those making sludge burning surveys. DISCUSSION by Robert E. Zinn, P.E., Dallas, Tex. This paper fills a need in the design data information gap on disposal of biological sludge by incineraration. A review of the paper has suggested the following questions: 1... What are the capital costs and the operating costs per ton of total solids for the proposed installation? 2. Was consideration given to the use of a mechanically fluidized bed as in a high speed rotary kiln? If so, what are the relative costs and operating advantages and disadvantages? 3. Has sufficient consideration been gi ven to the use of Incoloy 800 at 1400 F in the presence of chloride containing flue gases? 4. Has consideration been given to cooling of flue gases by recuperation before entering the cyclones, thereby reducing the corrosion potential in the cyclones? 5. What are the pressure drops across the fluid bed system itself, and what is the pressure drop across the large scale venturi scrubber? DISCUSSION by Charles A. Richmond, Nichols Engineering & Research Corp., San Francisco, Calif. In considering alternate incineration schemes; what was the relative costs both capital and operating for other incinerators such as multiple hearth or Flash Dryers for units of this capacity? What consideration was gi ven to the use of recycle incinerator ash mixed with the 15 percent dry solids activated sludge in a Pug mill, to obtain 25 percent dry solids cake which could be handled by any usual material handling equipment? What consideration was gi ven to the use of recycle dried sludge mixed with the 15 percent dry solids activated sludge in a Pug mill to obtain 25 percent dry solids cake which could be handled by any usual material handling equipment? Multiple Hearth incinerators have been used for liquid sludges quite successfully. Specifically the units at Piqua, Ohio and Bradford, Pa., and Rock Falls, Ill., are some examples. Perhaps the information submitted was inadequate for a proper evaluation by a multiple hearth incinerator manufacturer, and therefore any recommendations would include a very large monetary safety factor. The paper suggests that the 15 percent dry solids sludge is autocombustible and requires no auxiliary fuel. Assuming 75 percent combustible material in the dry solids and 15 percent dry solids, any incinerator operating as low as 25 percent excess air, will require large quantities of fuel if the gas outlet temperature is 1400 F. Calculations show a fuel consumption which is quite high. Basis: 100 Ib of wet cake containing 15 percent dry solids, said dry solids to be 75 percent combustible, said combustible to have a Calorific value of 10,000 /lb. Heat 1400 F Heat Released Ib x 10,000 /lb HEAT DEFICIT 567 available per Ib x /lb 85 x x x x 344 TOTAL 112,500 87, cu ft per 146,030 14,259 32,400 7, , Ib of wet feed 1023 cu ft per 100 Ib of dry solids For the simulated sludge presented on page 82 the auxiliary fuel usage is even higher, as shown below: Basis: 100 lb of wet cake containing 15 percent dry solids, said dry solids to be 12.8 percent combustible, said combustible to have a calorific value of 10,000 /lb, using 8 Ib of theoretical air per 10,000. Heat 1400 F Heat 1400 F 1.9 Ib x 10,000 /lb HEA T DEFICIT Ib x /lb = 85 x x x x 344 TOTAL = 19, , ,030 2,405 5,472 1, ,214 19

4 567 available per 240 cu ft per 100 lb of wet feed 1,584 cu ft per 100 lb of dry solids At a gas outlet of 800 F the heat balance for the same materials using 75 percent excess air would be as below: Heat 800 F Heat 800 F 1.9 lb x 10,000 /lb HEAT DEFICIT 727 available per lb x /lb = 85 x , x , x , x 344 5,229 TOTAL = 129,507 19, , cu ft per 100 lb of wet feed 1,003 cu ft per 100 lb of dry solids In the agglomerative system wherein the inorganic ash is recycled, was there a noticable change in the Si02 content of the fluidized bed? In the agglomerative system, was there any work done to determine if the recycled inorganic calcium carbonate remained in the system as calcium carbonate or did it calcine to calcium oxide? U sing silica sand as a fluidized bed, what long range effect will recycled calcium salts have upon the sinter or melting point of the bed? At what ratio of CaO to Si02 will lime glass form at 1400 F? What special considerations were given to the choice of refractory? Is High Heat Duty Fire Brick, such as ASTM C satisfactory? What alloy is the Bed Support Plate made of? If Incoloy 800, what is the weight loss estimated to be per year? In several places, a low power consumption is indicated. What is the actual connected horsepower for the 500 lb/hr dry solids capacity, and what is the average operating horsepower? AUTHS' CLOSURE Several of the discussors have requested amplification of the rationale leading to selection of a fluid bed incinerator. Within limits, we can comply, but a full discussion would require another complete paper. The expected variations in,quality of the sludge plus possible operational upsets were important considerations because it was desirable to eliminate an on-site operator and simply monitor operation from a remote control building. The aerated holding tank (Figure 1) was expected to stabilize short term variations in sludge quality but would not influence long term changes. The dewatering performance of the centrifuge will vary with feed characteristics and pilot tests have indicated that dewatering is sensitive to polymer addition. An additional complication is the presence of inorganic salts. The above points are repeated because they are essential to a discussion of alternative systems. The principal advantages of the fluid bed approach includes the stabilizing effect of the heat sink, uniform and close control of temperature, and the ability to handle wide variations in feed characteristics with minimum attention. The rotary multiple hearth was evaluated as requiring closer control of feed characteristics and subject to possible clinkering problems. Blending of dried or ashed material with the dewatered sludge was an additional process step that was judged difficult to automate to handle variations in sludge quality. Clinkering has been encountered at the Lake Tahoe multiple hearth installation and may be associated with temperature variations from hearth to hearth and between burners. A flash dryer - incinerator approach was evaluated as requiring blending of dried material with the dewatered sludge and an operator in attendance. The mechanically fluidized bed mentioned by Mr. Zinn was not evaluated. The rapid re-start mentioned by Mr. Stevens was another plus for a fluid bed system. We have restricted our comments to those alternative systems specifically mentioned by the Discussors and have briefly mentioned some of the practical considerations leading to selection of a fluid bed incinerator for this application. The authors recently completed another assignment where we specified a multiple hearth furnace and have recommended flash dryers when appropriate. We strongly suggest each potential application be evaluated in terms of design parameters determined as meeting the needs of the individual client. The total connected horsepower of the incineration system is 190 and makeup water requjrements are approximately 12 gpm. The rated discharge pressure of the centrifugal blower is 9.0 psig and the scrubber 20

5 resistance is up to 40 in. of water. Superficial fluidizing velocities were reported at operating conditions. Supplemental fuel (Figure 3) is required for this application' but fluid bed units can be operated autogenously. American Oil Company's fluid bed incillerator is operated without supplemental fuel by waste stream blending to maintain sufficient fuel value in the feed to the unit. In the agglomerative system, all of the sand in the bed will be displaced by granules of the inorganics present in the sludge. In the non agglomerative system, none of the inorganic ash will be recycled to the fluid bed unit. Excess granules or collected ash will be wasted for ultimate disposal. Initial startup is accomplished by first heating the incinerator with a direct fired preheater installed between the air blower and the incinerator windbox. Kerosene is then injected into the fluid bed until the natural gas auto-ignition temperature is reached. The only auxiliary fuel normally used while sludge is being burned is the natural gas. The supplemental fuel rate is automatically controlled to maintain a present incinerator temperature. Alarms are provided to alert operating personnel of problems, and safeguards include an emergency drench system in the event freeboard temperatures exceed safe limits and operators do not respond to the alarm. Incoloy 800 was a compromise selection and recuperation rejected in favor of system simplicity in the corrosive environment. 21