Rotary Kiln Incineration Systems: Operating Techniques for ...rotary kiln started about 1877 in England. The first com- mercial rotary kiln was the result of the proven work of American

ROTARY KILN INCINERATION SYSTEMS: OPERATING TECHNIQUES FOR IMPROVED PERFORMANCE 27037, P ~ F

Joseph J. Santoleri Four Nines, Inc.

Plpouth Meeting, PA 19462

ABSTRACT

Experience in the operation of rotary kilns goes back many years with the thousands of kilns throughout the world. However, much of this experience is in the cement, lime, and calcined dolomite industries. In the past twenty to thirty years, rotary kilns have been used in the incineration of municipal and industrial wastes. The operating practices differ in that the industrial kilns are used to generate a quality-controlled product. Flame size and shape, heat transfer by radiation and convection, temperature distribution, and contact-time all play a critical part in the quality of the end product. These kilns are normally 50 to 200 meters (150 to 600 ft.) in length.

Kilns used for incineration are typically batch fed with solids of varying shape, size, and heat content. This provides flexibility not available in other incinerator systems. These kilns may also burn liquids, slurries, sludges and contaminated soils at a continuous feed-rate. The Resource Conservation and Recovery Act (RCRA) establishes the combustion performance required to obtain an operating permit. Many existing kilns have been modified in design and operating practices to allow the system to meet RCRA standards. These modifications have included feed devices, seals, lance design and location, controls, scrubber systems, monitors, safeties, etc.

This paper covers the experience gained at several rotary kiln installations burning hazardous wastes. This includes the modifications in design and operation to minimize fugitive emissions, temperature, pressure and stack emission upsets. This has provided systems whose performance insures a safe environment to the owner and the surrounding community.

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INTRODUCTION

Early development of the rotary kiln started about 1877 in England. The first com- mercial rotary kiln was the result of the proven work of American engineers in 1895, by Hurry and Seaman of the Atlas Cement Co. Those first kilns were 45 cm (18 in) in diameter and 4.5 meters (15 ft) in length. Kiln sizes started to explode in the 1960's when they reached dimensions up to 6.5 meters (21 ft.) diameter and up to 238 m (780 ft) in length.

The energy crisis of the 70's represented a blessing in disguise in matters of kiln design. This occured world wide when modifications to preheaters were made along with use of alternate fuels. The major breakthrough came in Europe where precalcination was successfully attempted in the late 1960's using a very low BTU bituminous shale as a component of the kiln feed. As early as 1957, oil shale in slurry form was used as a potential source of energy in Canada (4).

Other wastes burned in cement kilns have included brines, aqueous metal-bearing wastes, acidic wastes, lime-alum sludges, halogenated wastes, spentsolvents andstill bottoms ( 2 )

A1 though cement ki Ins wi 1 1 accommodate a variety of mater- ials both as fuel and feed in the cement process, there are limitations in the use of haz- ardous wastes as there are compounds not desirable in the cement process. The experience gained in this industry has led

to the acceptance of the rotary kiln as a means of disposing wastes, both hazardous and non-hazardous by incineration.

Rotary kilns provide a number of functions necessary for incineration. They provide the conveyance and mixing of solids, a mechanism for heat exchange,serve a s a host vessel for chemical reactions and provide a means of ducting the volatilized gases for further processing. The kilns are equally applicable to solids, sludges, and slurries and are capable of receiving and processing liquids and solids simultaneously (7).

WASTE DATA

Waste streams in the process industries are numerous in kind and therefore defy easy definition. Disposal of these wastes has become a serious problem for the plant operator. The following waste data must be provided before a selection of incinerator design can be completed.

Waste Data Rewired for Desian

Chemical Composition Specific gravity Heat of Combustion Corrosivity Ignitability Reactivity Moisture Content Size Consistency Slagging properties (Temp., Eutectic Data)

The quantity of the waste materials in total; that is, solids, sludge, slurries, liquids (and fumes), will determine not anly the total

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C

throughput in units of weight (tons, kg, pounds) per unit of time (min, hr, day, year), but also the total heat duty requirements of the systems.

INCINERATOR TYPES

The physical data will aid in determination of the type of incinerator designs that can be considered. There are many types that may be used to incinerate hazardous wastes. They are as follows:

Liquid Injection Incinerators Rotary Kiln Fixed Hearth (Two-Stage) Mono-Hearth (Rotating) Multiple Hearth Fluidized Bed Infra-Red Furnace Rotary Reactor (Cascading Bed) Mol ten Glass Process Wet Oxidation

Many of these designs are limited to a feed that is prepared specifically for the incinerator type, i.e., liquids and slurries that may be pumped and atomized. Others require a limit to the physical dimensions of the feed and require pretreatment; i.e., shredding prior to introduction to the transport system. Some designs will handle only easily transported sludges and soils. The rotary kilnand fixed hearth areideally suitedto large size solid feeds such as bulk trash, containerized process waste solids, as well as contaminated soils, sludges, slurries and liquids. The fixed hearth design requires multiple ram feeders to expose the surface of the waste materials to the combustion and heat exchange

process. This paper will highlight the rotary kiln incinerator since it is themost flexible of solids incinerator designs today. Others (fluid bed, rotary reactor, etc.) are usedforincineratingsolidsand require extensive feed prepar- ation and transport to optimize the mixing, heat transfer, and combustion which are considered majoradvantages for theseother designs (Fig. 1).

One major feature of both kiln and fixed hearth designs is that most solid wastes must be batch fed. Other systems can accept wastes continuously into the combustion zone; this provides improved combustion and emission control. This is a singular disadvantage to the operation and performance of batch- fed inc ine rat ors . However, experience gained in the past few years driven by the RCRA regulations has minimized this disadvantage. The standards developed by U.S.E.P.A. via RCRA and now implemented by most of the States' Solid Waste Authorities are as follows:

RCRA Standards (5)

1.

2 .

3 .

99.99% Destruction and Removal Efficiency (DRE) of the Principal Organic Hazardous Constituents (POHC) 99% removal of Hydrogen Chloride (HC1) or HCl emission not exceeding 1.8 Kg/hr (4 lb/hr) Pa r ti cul a te emi s s i o n concentration of 180 mg/dscm (0.08 gr/dscf) corrected to 7% 02.

Guidelines have been issued

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Guidelines have been issued to permit regulators to insure that these standards are being met during the trial burns and subsequent operation of the system. These apply to the carbon monoxide emi ss i on monitoring which is the surrogate used to establish conformance to the 99.99% DRE. Guidelines are also in final stages regarding the metals emissions issues. Standards covering particulate emissions may be lowered to 0.015-0.02 gr/dscf to insure metals emission control. Many states have established maximum emission levels well below the 0.08 gr/dscf. These levels are based on the "Best Available Control Technology" (BACT) demonstrated at operating fac- ilities within the state.

ROTARY KILN OPERATIONS PAST AND PRESENT

In order to conform to the above standards, past operating practices at many rot- ary kiln incinerator facilities have had to be modified. Many incinerator systems under "Interim Status" (Part "A" permit) have been operating with the following conditions as normal practice.

Past Practices

1. Batcn Feed System:

control)

from 15 to 30 min.)

control )

Manual (Under operator

Cycle for load (Varied

Manual (Under operator

Positive pressures

2. Kiln Draft Pressure:

created fugitive emissions.

3. Combustion: Air/fuel control (Under operator control)

Auxiliary fuel (Manual) Afterburner (Volume undersized)

Temperature Control (Manual )

Burner (Design or Location)

Atomizer (Design or Location)

Submicron particulate efficiency (Poor)

PH control (Manual) Corrosive atmosphere

4. Scrubbers:

overlooked in material sel ect i on.

Scrubbers. Many systems had no

These operating con- ditions resulted in stack emissions of odors and particulate emissions in the surrounding community. The term "NIMBY" (Not in My Backyard) was originated. The environmental groups soon formed to block any future instal 1 ati on of incinerator systems.

Those facilities operated as on-site (industrial plants) or off-site (comnercial disposal operators) have made many changes and improvements to comply with RCRA requirements. As stated above, most of these w e r e d r i v e n b y t h e m o d i f i c a t i o n s needed to complete the Part "B" permit and the final trial burn at the facility. At most plants preliminary test burns were run to determine exi sting capabi 1 i ti es . These tests (mini-burns) resulted in many or all of the following design and operating inprovements. This

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allowed the system to meet the final standards for the trial burn (6).

Improvements

1. Feed System

Continous Feed: Shredder Lump and Cake Breakers Weigh Belt Control

Auger Screw Feed (Single and

Belt Feeder Elevator Ram

Container Size Control

Container BTU Control Cycle Time Modifications Bar Code for Control

Mechanical Improvements:

Feed Devices :

Multiple Screws)

Batch Feed:

Volume Weight

(Fig. 2) Roller Conveyors Elevators Intermittent Ram Feeder Holding Chamber Guillotine Door Size Reduc t i on

2. Combustion System (Fig.3)

Combust i on Ai r Con t ro 1 Improved Seal Design Air Flow Meters to Primary/Secondary

Oxygen Monitoring at Secondary Level

Auxiliary fuel burner type/l ocation

Heat input control by waste types

Secondary Combustion Chamber Design/Mixing

At omi z er Design Modi f i ca -

tions/Locations

Number/Location

Control Via Air/Water

Temperature Monitors/

Maximum Temperature

3. Pollution Control System (Fig.4)

Combustion Chamber Design Control velocity and carryover

Orientation to prevent ash-buildup

Waste Injectors Location and size to minimize attrition

Ash System Hopper size/ bridging Temperature control slagging or freezing

Quench liquid control Temperature of operation

Venturi Throat Control materials of construction

Packing Demi s t e r s pH control

Quench Design

Scrubber

Absorber

CASE STUDIES

Experience with retrofit and modifications to liquid in- jection incinerators has been covered in detail. Many changes were require dinorder to obtain approval for the Part "B" permit ( 6 ) . A s stated above, former practices with solids incinerators led to problems which generated excessive emissions of particulates as well as products of incomplete combustion (PICS). Solid waste materials are often collected in 55 gallon drums to minimize handling and 1 ahsr costs These drums vary in weight from 400

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pounds to as high as 900 pounds depending on the density of the materials. The physical size of the drum establishes the feed mechanism size; i.e., belt, chute feeder, elevator, ram feeder , and gui 11 otine door. This establishes the kiln opening. The kiln inside diameter is then established. The two limits to kiln capacity are as follows:

1. The maximum feed rate of solids in pounds per hour. 2 . Maximum heat release rate in BTU per hour.

Both must be evaluated to determine whether an existing kiln is operatingat its optimum conditions. Combustion systems are optimized when operated at a steady feed rate with a uni- form heating value of feed with a combustion air rate that maintains constant oxygen level in the stack gases. Process furnaces or boilers operate at maximum efficiency; i.e., at minimum excess air (0 ) . Waste burning systems do no2 run with low excess air levels (1-3% 02). Most liquid waste incinerators operate with stack at 3 - 5% 02. This insures against variations of waste composition and heating value. When burning solids, waste rates and BTU values are even more difficult to control. However, it is possible to operatewithproper air controls at levels of 6 - 9% O2 continuously.

Many existing kilns have poor seal designs. This will result in high excess air drawn into both front and rear seals during operation. Some systems have experienced excess air levels of 100 to 150%. A major

disadvantage createdbythepoor seal problem was limiting the heat release of the kiln. The high velocity created by this additional volume causes fine particle entrainment into downstream equipment. This volume adds to the heat load of theafterburner. Residence time in the afterburner must be held inorder tomeet RCRA standards. The gas volume establishes the physical size of the downstream air pollution control system and induced draft fan.

Air in-leakage also occurs at the feed chutes and guillotine doors. Reduced leakage at these points, decreases the velocity in the kiln and the particulate carryover. However with a fixed heat input, the temperature will increase in the kiln. Combining the same total heat input with additional inert materials such as soils and moisture, the same physical chamber size can be modified to process an increased daily tonnage of materials. The cooling by water addition from the wet feeds or waste water sprays results in a reduction of total gas flow in the kiln. This will minimize kiln gas velocity and carryover. The afterburner designmust provide the necessary turbulence and temperature rise needed to provide the 9 9 . 9 9 % DRE of the organic componentsin thewaste.

The solid feed rate limits are determined by several factors. The physical dimensions of the kiln (diameter, length and slope) and the rotation (rpm) establish the residence time of solids. This must be reviewedbasedon thevolatility

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their surface heat absorption rate from the temperature and 0 in the kiln gases. This heat

upon the physical nature of the solids, the water content which establishes the drying load, and the inert materials (ash) in the waste (Fig. 5 ) . For good heat transfer, volatilization and combustion of the organics, a solid volume fraction of 5 to 15 % is recommended. Typical residence time of solids may vary from 30 minutes to 2 hours.

a L sorption rate is dependent

Many kilns were operating at loading rate cycles that varied from 10 minutes to as long as 30 minutes. The resulting variation in kiln pressure and temperature often was out of control of the operator. Puffing at the seals would occur with PICS entering the operating area. The afterburner control also was difficult to maintain. Temperatures would follow the variations created in the kiln. Since the kilnheat release rate wouldvary, andauxiliaryfiring of the afterburner was by a forced draft burner, there was no means for controlling stack O2 levels. The result would be large variations in stack oxygen. After monitors were placed on the stack for carbon monoxide (co) or total hydrocarbms (THC) , large spikes would be observed many times to the upper limit of the scale (3000 - 5000 ppm). At the same time, O2 would drop to 0%.

The capacity of a system in total heat release rate is established by the volume of combustion air provided in the forced draft (FD) and induced draft (ID) f a n . The ind iv idua l

burners are limited by the FD fans. The total incineration system (kiln and after-burner) is limited by the I.D. fan capacity. One must first establish the level of O2 needed for satifactory operation. Having the I.D. fan data (ACFM @ Temp.), one may calculate the dry SCFM handled by the fan. Based on the oxygen level for thesystem, theexcess air level may be established (Fig.6). The total BTU capacity for the incinerator system may then be determined by the following formula :

BTU/ hr =

Total Air =

(DSCFM x 6000)/Total Air - where - 1 + excess air fraction.

Having established the maximum heat release of the system, one would now review the capacities inbothkilnandafterburner.The kiln is the critical zone since it is a batch fed combustor. Certain wastes due to feeding problems and the nature of the waste must be fed continuous 1 y ; e.g., sludges, slurries and waste water streams. The heat release for these should be relatively stable if proper measures have been taken in the storage and mixing areas. The heat release from the batch fed wastes wi 1 1 be dependent upon the weight per charge, the BTU/ charge, and the relative volatility in each charge. If each drum is prepared consistently, the values of weight, BTU, andvolatilitywill be steady. The result is a cycledheat inputdependentupon cycle time and the BTU/charge. Locations with the ultimate in cnntrnl of waste f e e d have

275

experienced sudden energy release from charges into the kiln. This is followed by a sudden increase in temperature and a rapid decrease of 0 exiting the chamber. The total! volume of air in the kiln is maintained at a fixed rate by the draft created by the I.D. fan. With the rapid depletion of O2 in the kiln, the stack gas

and emissions drop in increase in CO and THC. I these reach permit shutdown levels, all waste feeds must be cutoff per present regulations. In past operations, continuous monitors such as kiln draft, 02, CO, etc. were not required and waste cutoffs were manually controlled by the incinerator operator.

4

Inorder tominimize these upsets, modifications were made in the operations and controls. Inthe caseof continued spiking due to high volatility (BTU) of the waste feed, the charge size was reduced and fed more often. One incinerator was designed to operate with stack gas containing 10% 0 average. The

a heat content of 1.66 MM BTU was fed every 15 minutes. Note from Fig. 7 the effect of materials volatilizing in three (3) to five (5) minutes. The five minute volatilization would reduce the O2 to 4.5%. However, with volatillzation occuring in three minutes, there was not enough air to operate without high levels of CO and PICS. By reducing the charge size to 90 lbs. (0.55 MMBTU) charged every 5 minutes, the system was able to stay in control with either a 3 minute or 5 minute volatility material (Fig. 8)

original charge o # 270 lbs. with

(6)

By utilizing the oxygen analyzer as a control device, another incinerator system was brought under control reducing waste feed cutoffs. Fibre-packs are fed at a fixed rate with the feed rate based on BTU/ charge (Fig 9). Occasionally, a "hot" drum would enter the kiln. A "hot" drum is one containing -

free liquids. This resulted in a sudden drop in O2 measured at the exit of the after-burner. In this case, the kiln steady state firing rate was 30 MM BTU/hr and the afterburner - 70 MM BTU/hr. The total heat release rate from the system averaged 100 MM BTU/hr. Normal operat ionresu l ted inwaste feed cutoffs two to three times daily. With all of the auxiliary heat added by the waste liquids into the afterburner, a waste feed cutoff resulted in waste solids continuing toburn in the kiln with no additional heat input to the afterburner. The CO level would spike followed by a prolonged time period with high THCs in the stack gases. With the I.D. fan operating at fixed output, the kiln and afterburner chambers would cool rapidly increasing the THC levels.

~

The first indication of a problem is the dropoff of O2 level. This is followed by the CO spike. By monitoring the O2 level (6 to 7%) and using the average - 6.5% as a control point, a dropoff of 1.5% indi- cates a problem may be in the initial stage. Since the liquid firing rate was 70% of the total heat input, this was used as a control to maintain O2 level at a fixed afterburner temperature. The initial 1.5% OJ drop triggered a reduction or iiquid

- ~

-

276

heat input by 50%. A continued drop of the O2 level reduced liquid heat input an additional 30%. This drops the total heat input by the liquid to 24% from the 70% level (70-.8x70).

Most often this would be sufficient to keep the O2 level from reaching the waste feed cutoff point - 3%. This assumes that the sudden heat input from thekiln feedhas increased from a level of 30% to 76% (more than 2.5 times). In many cases. The heat input from the volatile solids would be less with other waste streams entering the kiln such as sludges and slurries at a constant flow and BTU input. This control modification has reducedthe number of waste feed cutoffs from O2 trips with the resultant spikes of CO and THC. These control modifications are neededtoinsurethe environment in the area of the incinerator for the operators as well as that for the surrounding c ommuni t y is maintained. Complaints of odors from the "NIMBY" groups may be justified based on past practices. The modifications needed to improve operating conditions and essentially eliminate these problems are possible. It re- quires close observation of the daily operating procedures , dis- cussions with the operators and review of the strip charts and log books. Only then can one determine that there are problems and that modification to operating procedures or controlswilleliminatethepro- blem. In many cases however, redesign of the basic hardware will be needed to achieve the control necessary (Fig.10).

SUMMARY

The cases described above cover instances where improvements have been made to meet the standards established by RCRA. The results have shown animprovement in theefficiency of the operation as well as lower operating costs. Onemajor benefit has been in lowering maintenance costs, especially in refractory repair. Closer control of heat input has maintained mor e uni form temperatures which has resulted in increased refractory life. It also reduces downtime and provides higher utilization of the equipment. Additional capital expenditures were necessary to make these modif- ications. Training costs were also higher. All of this has r e s u l t e d i n o p e r a t i n g p l a n t s w h o pride themselves with systems that provide not only increased employment and revenues to the local community, but also a means of eliminating the hazardous materials from the environment forever. It has allowed the rotary kiln to become one of the most flexible incinerator designs for all hazardous waste streams. From its original use as a process furnace, it can now be found in use at MSW plants, commercial hospital waste disposal facilities, and large commrcial hazardous waste sites. The design most often selected for many large on-site facilities hasbeen the rotary kiln, either slagging or ashing. Many of the mobile or portable units used for Superfund (SARA) cleanups have been the rotary because of its flexibility in handling the variety of waste t y p e s and physical shapes.

277

Many rotary kiln incineration systems have been modifiedand testedforthe RCRA and TSCA permits in the past 10 years. This testing has shown that rotary kilns can achieve DREs well above 99.99% and often > 6 - 9s. The incineration of hazardous wastes in a rotary kiln has become popular because thedesigns, concepts, and the- ories are well established and proven in many solids processing industries. The effort to meet the standards established by RCRA has enabled the latest control and instrumentation technologies to be included in the design and operation of all systems. (1r3r8) It is extremely important that the technical community who understand the results of these improvements inform the citizens who oppose siting of units about the advantages of a properly designed and operated incin- eration system to effective disposal of hazardous wastes.

REFERENCES

1. Bastian, Ronald E. "Eastman Kodak Company Chemical Waste Incineration" LSU Conference on R.K. Incineration - Nov. 1987 2. Chadbourne, J.F. "Cement Ki 1 ns" Sect. 8.5ofFreeman's"STANDARD HANDBOOK OF HAZARDOUS WASTE TREATMENT AND DISPOSAL" , McGraw Hill, 1988

3. Osborne, J. Michael "3M Operating Experiences with a High Temperature Kiln" LSU Conference on Rotary Kiln Incineration Nov. 1987

4. Peray,R.E. "The Rotary Cement Kiln" 2nd Ed. Chemical Publishing Co.,Inc. N.Y.,N.Y.- 1986

5. Resource Conservation and Recovery Act. Standards for Owners and Operators of Waste Facilities: Incinerators. 40 CFR 264, RCRA 3004, Jan. 25, 1981, Rev. July 9 , 1984

6. Santoleri, Joseph J. "Mini-Burns - Critical to Trial Burn Success'' APCA - Dallas,TX No.88-015.06 "Design and Operating Problems of Hazardous Waste Incin- era t or s ENVIRONMENTAL PROGRESS (V01.4-# 4) Nov. 1985

7. Schaefer, C.F. and Albert,

"Rotary Ki 1ns" Sect. 8.2 ofFreeman's"STANDARD HANDBOOK OF HAZARDOUS WASTE TREATMENT AND DISPOSAL" McGraw Hill, 1988

A.A.

8. Williams, Gad L. "Status of the Technology of Rotary Kiln Incineration of Hazardous Waste" LSU Conference on R.K. Incineration - Nov.1987

Disclaimer

The work descri,kd in t h i s paper was not fsnded by the U.S. E3wi;onmental Protectia; Agency. The contents do not necessar i ly r e f l e c t t h e views of the Agency and no o f f i c i a l endorse- ment should be inferred.

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FIG. 3 ROTARY KILN AND AFTERBURNER

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