COMBUSTION PROFILE OF A

GRATE-ROTARY KILN INCINERATOR

ABSTRACT

P. H. WOODRUFF

Roy F. Weston, Inc. Wilmette, Illinois

A study was made of a 300-ton-per-day grate-rotary­kiln incinerator. Sampling data were obtained at the end of the rotary kiln, in the secondary combustion chamber, just ahead of the spray chambers, and in the stack. Sam­ples were taken at several elevations at each point in the combustion train. Data were obtained on gas tempera­tures, particulate loading, hydrocarbon, oxygen, CO2 and co. Sampling data have been correlated with refuse load­ing and general operation of the incinerator to produce typical operating profiles. The promes illustrate gas flow and combustion characteristics of materials as they are burned in this type of incinerator. Data from the tests are presented and discussed.

As a result of these tests and observations several recommendations were made for modification to the combustion air supply system and gas mixing modifica-

• tlons.

INTRODUCTION

A grate-rotary-kiln incinerator, owned and operated by a city, had been a subject of public complaint on num­erous occasions since the operation began. Visible emis­sions from the tall stack, of which there were few in the area, could be readily observed. The specific complaints alleged deposits of fly ash and odors. A distinguishable plume from a tall stack usually results in complaints both real and non-factual. The investigations of complaints

G. P. LARSON

Franklin Institute Philadelphia, Pennsylvania

327

had shown a number of cases when the incinerator was inoperative and individuals insisted that they were being affected. However, many persons were convinced that the operation of the incinerator resulted in nuisance con­ditions in areas adjoining the incinerator plant.

This criticism led to a careful study by the city of control devices which might be used to reduce the dis­charges from the stack. This preliminary study clearly defined the cost of electrical precipitators or baghouse controls. Aside from the cost, these systems were con­sidered to be somewhat experimental and would ma­terially complicate the operation of the incinerator.

Various proposals were also studied for improving the water-spray system as a means for reducing fly ash. The conclusion from these and other studies led to the recom­mendation that a study be made of the combustion process within the incinerator and of operating conditions, before arriving at final conclusions on the engineering modifications needed to reduce the nuisance complaints .

PURPOSE OF THE TEST

The tests were to determine the practicability of methods for increasing the combustion efficiency by setting criteria for modifying the design characteristics or to determine operational procedures which would re­sult in compliance with air pollution regulations and to minimize any nuisances resulting from the incinerator.

The program called for conducting tests at several points throughout the combustion zones of the incinera-

;or and to determine the amount of solid particles and condensable materials being discharged into the atmos­phere. Tests in the combustion zone were to include measurements of oxygen, carbon-monoxide, carbon­dioxide and the determination of velocities. The samp­ling program was designed to indicate the general com­bustion conditions inside the incinerator.

The results of the study and the recommendations are presented in this paper.

INCINERATOR OPERATION AND CONTROLS

The incinerator plant consists of drying and ignition grates where rubbish is ignited and moved continuously to a rotary kiln. Part of the burning mass is constantly turned over and moved progressively along the 24-foot length of the kiln as it slowly rotates. The outlet of the rotating burning chamber drops the unburnable ash to an ash pit for ultimate disposal. The combustion gases

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leaving the rotary kiln continue to burn in a secondary combustion zone, approximately 32 feet in length; subsequently they enter a water spray for removal of large ash particles. Gases are then discharged through a stack 200 feet in height.

The process is controlled by a single operator who can regulate the movement of the grates in the ignition cham­ber to introduce a continuous flow of burning rubbish to the rotary furnace. The speed of the rotary kiln can be regulated to control the time it takes for the burning mass to pass through the kiln. Air used for combustion is introduced under the grates in the ignition chamber. While dampers are provided for the introduction of air over the Gre, these dampers had not been used, as the operators felt this cooled the furnace below normal op­erating temperatures.

Temperature-recording instruments are installed in order that the operator can control temperatures in the burning zones and avoid excessive temperatures which

SAMPliNG AT SEctJIlARy CDIISTION 0IAI1ER

SAfIIlINC AT STlCl

J«1'f 'N 1 ser. a ()I+81-0 16251:1 IS C F o '-I 6 PPM ALOOfYtls----... '4 14' CO2 16 Oi 02 o 2" CD

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'4 0 1'4 9 0) 85 11308 9 6 10 9 0 7

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311

SIWlI NC HAD � SPRAYS

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19 1 2 7 II 1505 0)5

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� ZONE (F It()t 'f'ELOCHY & 02 CIJCSlWTI(JI

�I /OJ( Of 11[000 VElocm • 0, ClJlSUFTIOI

I I /OJ( Of LOI VElOCITY • 0, ClJlSUFT""

j·: · ·: · ... '·. · ·1

FL. ASH .. . . .. . .

FIG. 1 INCINERATOR GAS FLOW DISTRIBUTION AND COMBUSTION PROFILE

328

would damage the grates in the ignition chamber. The speed of the kiln and the speed of the grates, which serve to regulate the flow of rubbish, can be changed instantan­eously by the operator based on his judgment to avoid excessive temperatures or to maintain proper burning temperatures in the system.

The incinerator operates 24 hours per day during the regular 5-day work week. When the operation begins, ap­proximately 24 hours is required to bring the furnace up to operating temperatures. No auxiliary fuel is provided for the warm-up. Rubbish is received during the entire week and the furnace operates through Friday evening. The design capacity of the unit is 12.5 tons per hour. The total input of rubbish is not weighed.

After the start-up period, temperature control is largely a function of the manner in which the operator controls the incinerator. Operating charts record temperature con­ditions in the incinerator on an hourly basis. Temperature records covering the operation during the period from January 6 to June 11, were reviewed. The periods of time when the temperature was below 1500 F for more than 3 hours were examined.

There were 12 days when the incinerator was below 1500 F for periods up to 30 percent of the total operating time. On 19 days, the temperature was below 1500 F, 31 to 69 percent of the time. On 6 days the temperature was below 1500 F more than 70 percent of the time. The majority of these low temperature conditions occurred during the night-time hours.

THIMBLE

GRAB SA�PLING FLASK (ORSAT ANALYSIS)

CONDENSATE TRAP

IN ICE BATH

CCI4 IMPINGERS

IN ICE BATH

The important consideration from this review is the fact that out of 64 days of operation picked at random, there were 37 days when the incinerator was below normal operating temperatures for substantial periods of time. These low temperature conditions prevailed from 3 to 16 hours. Under these temperature conditions, odors from the products of incomplete combustion could be expected.

Stagnant wind conditions generally prevail at night. Odors may travel many miles and are detected at lower threshold levels during the less active evening hours than during day-time activities.

TEST CONDITIONS

The testing program began after the inciner:>tor had been operating a sufficient length of time to reach op­erating temperatures. Observations of the operator's con­trol demonstrated that temperature conditions could be maintained above 1500 F and all tests throughout the ensuing four days were taken when the temperature of the gas stream leaving the secondary combustion chamber was from 1500 F to 2000 F. Test locations are shown in Fig. 1 and were selected on the basis of accessability and in areas where representative measurements could be obtained.

The loading to the incinerator was determined by counting the bucket loads per hour deposited in the holding zone prior to charging. The weight of material

CONDENSATE TRAP

rir,ACUUM GAUGE

THERMOMETER -....

ROTOMETER DRY GAS METER

CONTROL VALVE

VACUUM PUMP

FIG. 2 PARTICULATE - COMBUSTIBLES SAMPLE TRAIN

329

was based upon estimates of 3 cubic yards of rubbish per bucket at an estimated weight of 450 pounds per cubic yard.

TEST METHODS

Standard procedures, such as those outlined in Western Precipitation Bulletin WP-50 and the Los Angeles County SouTce Testing Manual, were used for the collecting and processing of samples. Diagrams of the sampling trains are shown in Figs. 2 and 3.

For velocity determinations, a special pitot tube con­sisting of three four-foot lengths of stainless steel was used. Temperature measurements were determined using a chromel-alumel thermocouple in conjunction with a potentiometer.

Oxygen, carbon dioxide, and carbon monoxide samples were collected by liquid displacement and later analyzed using an Orsat gas apparatus. The Weston and Stack dis­solved oxygen analyzer was used continuously to check the oxygen values obtained by Orsat. The solid particles in the gas stream were collected in alundum thimbles, which were later ashed in order to determine the amount of combustible material present in the thimbles. Material passing through the thimbles was collected by wet im­pingers following the thimbles and was weighed after evaporation of the water. Volatilized material was also condensed and collected by absorption in carbon tetra­chloride.

compounds to decompose, freeing bisulfite ion equivalent to the aldehydes present. Standard iodine is then used to titrate the liberated bisulfite ion.

COMBUSTION PROFILE - SUMMARY OF GAS FLOW

AND OXYGEN DISTRIBUTION

The results at test locations 1, 2, and 3 are summarized and illustrated in Fig. 1. All of the air supplied by the fan enters under the grates. Combustion of the refuse begins in the primary chamber and is completed as the refuse passes through the rotary kiln. The oxygen supply must be sufficient for the fuel bed combustion process and the complete burning of volatilized gases in the secondary combustion zone. Infiltrated air for cooling and com­bustion is drawn into the incinerator though the charging chute, inspection doors, air ports, openings around the circumference of both ends of the rotary kiln, hopper at the charging end of the kiln, residue conveyor diversion gate assembly, expansion joint between the spray cham­ber and the stack, miscellaneous openings in the refrac­tories.

Pitot tube measurements indicate that the forced draft fan, which has nominal capacity of 25,000 cfm at 100 F and 12 in. water static pressure, supplies only about one third to one fourth of the total gas leaving the stack.

The by-pass chamber which connects the ignition chamber with the secondary combustion chamber was partially blocked by brick work. No measurements were made to determine gas flow in the by-pass; however, flow was believed to be small.

Figs. 4, 5, and 6 show close ups of the sampling ports at test locations 1 and 2.

Although it would have been desirable to sample all

An impinger absorber train was used to collect aldehydes in the stack gas discharge. Sodium bisulfite solution was used as the absorber medium. Aldehydes react with the sodium bisulfite solution to form addition compounds. The excess bisulfite ion is destroyed with iodine solution. Adjustment of the pH of the solution causes the addition . points in the test program simultaneously to remove the

TH IIiBLE

STACK

IIiPINGER 100 ilL

IS NaHS03

I I ICE BATH CCI4

IIiPINGER IN ICE BATH

CONDENSATE TRAP ROTOIIETER

DRY GAS IIETER

FIG. 3 PARTICULATE - ALDEHYDE SAMPLE TRAIN

330

GAUGE

CONTROL VALVE

VACUUII PUIIP

possibility of conditions changing greatly at any one point during the test, the manpower and equipment require­ments made this impractical. The incinerator was operated as uniformly as possible throughout the test period and little variation was found in velocity measurements at a couple of the sampling ports which were spot checked for similar­ity on several occasions. Thus, the test results are thought to be, in general, indicative of combustion conditions in the incinerator.

Tests for oxygen content of the combustion gases leaving the kiln at test location 1 are summarized in Table 1. These tests indicate the gases leaving the kiln in the zone immediately above the fIre bed were defIcient in oxygen. The horizontal velocity of the gas at the fIve measured points (all in vertical plane) at the end of the kiln was fairly uniform and suffIcient oxygen still re­mained in the upper portion of the gases leaving the kiln to complete the combustion of the volatilized matter.

FIG. 4 SAMPLING PORTS - TEST LOCA nON 1

331

In the event that oxygen, temperature and mixing conditions are correct as the gases reach the secondary combustion chamber, the combustible gases and particu­late matter should continue to burn. Since oxygen was still available in the upper portion of the kiln, test loca­tion 2 in the secondary chamber should have shown continued improvement in burning conditions even though a defIciency of oxygen existed directly over the fIre bed at test location 1.

The test results summarized in Table II, however, in­dicated high oxygen levels remained in the upper portion of the gas stream with only small amounts of CO2. The gas velocity in the upper level was almost the same, 18.3 ft per sec, as was found for the gases leaving the rotary kiln, 15 to 16.7 ft per sec. The upper sampling point, No. I, showed oxygen content as high as 20 percent on one occasion indicating essentially no burning in this sample area at that particular time.

FIG. 5 TEST LOCA T/oN 2

In the lower two-thirds of the gas stream passing over the bridge wall at test location 2, the velocities were 37 ft per sec, more than double the rate found at test location 1. The temperatures and oxygen were adequate in this zone for good combustion of the combustible solid and volatile materials in the gas stream. However, one sample of particulate matter taken from the center of the gas stream contained 30 percent combustible material. The sampling probe was not water cooled, so combustible particulates in the incinerator may have been even higher. These test results at location 2 indicate stratification of the flow and poor mixing conditions.

The testing probe at the lowest sampling point was completely covered with ash. The gases passed over the bridge wall carrying ash that, apparently, had no oppor­tunity to drop into the ash pit. This material was large in size and appeared to be carried into the gas stream as a result of the turbulent action ahead of the bridge wall where the gas stream leaves the bottom half of the kiln.

Test location 3 was selected to give an indication of combustion conditions in the fmal burning area preceding the water sprays. The data for this test station are sum­marized in Table III. The average velocity measurements show that after the gases passed the bridge wall, high velocity (39 ft per sec) prevailed in the upper third of the chamber. This portion of gas was heavily loaded with particulate measuring 2.7 grains per standard cubic foot for just the material collected in the thimble or 3.5 grains per standard cubic foot including the material collected in the wet impingers. The velocity in the lower two-thirds of the chamber was extemely low (13 to 15 ft per sec) and contained very small amounts of particulates.

TABLE I

TEST LOCATION NO.1

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FIG. 6 CLOSE-UP OF TEST LOCA TION 2

MID·SECTION OF SECONDARY COMBUSTION CHAMBER

Sample Location

I

II

III

IV

VI

( ) Estimated

Vert. Dist. From Bottom Sampling Ports

Ft

7

6

4.5

2.5

0.0

Velocity Ft/Sec

332

16.7

16.7

16.7

15.0

15.6

Temperature Degree F

(1850)

(1850)

1930

1925

2130

Oxygen Percent

9.5

7.7

2.2 to 4.2

6. 1

Nil

Forced Fan Draft In. Water

4

6

4

•

Time Sample

Location

TABLE II

TEST LOCATION NO.2

SECONDARY COMBUSTION CHAMBER

Orsat Analysis

CO2 Percent Percent

CO Percent

Temp. of

---------------------------------------------------------------------------

17:20

17:45

18:00

10:05

12:10

13:20

15:25

16:15

16:50

I-A

I-B

I-C

II-A

II-B

II-C

III-C

III-B

III-A

0.4

3.2

8.4

12

5.6

8.0

18.0

10.0

0.8

20.4

17.2

11.2

7.8

13.2

12.8

2.8

9.6

20.4

0.2

0.4

0.2

0.8

0.8

0.8

0.8

0.8

0.4

1580

1660

1760

1600

1780

1700

2100

1880

1580

1. Location A, Band C correspond to points Y.., Y2 and 3,4 respectively across the combustion chamber at each sampling port.

TABLE III

TEST LOCA TI ON NO.3

SECONDARY COMBUSTION CHAMBER BEFORE SPRAYS

Estimated Flow, scfm Thimble Solids, grams Solids Collected in Wet Impinger, grams Total Solids Collected, grams Combustible Solids, percent Particulate Loading, grains/ scf Particulate Loading, lbs/hr Organics Collected in CCI4, grams Organics Collected in H20, grams Total Organics Collected in Impingers, grams Loading (Organics ColI. in Impingers), grams/scf Loading (Organics CoIl. in Impingers), lbs/hr Secondary Combustion Temp., of

Water Vapor (by Volume), percent Estimated Refuse Charged, lbs/hr

Top

82,000 1.9231 0.586 2.509 4.0 3.51 2,460 0.0571 0.0228 0.0799 0.112 78.7

1,650 17.4

25,650

Sample Location Center

34,000 0.3942 0.0927 0.4869 11.8 0.44 128 0.1170 0.0174 0.1344 0.129 37.6

1,500 11.3

22,950

NOTE: Flow rates were estimated by assuming the measured velocities were typical and multiplied by the appropriate cross sectional area.

333

Bottom

30,000 0.2291 0.0805 0.3096 11.9 0.30 77 0.0268 0.0392 0.0660 0.066 17.0

1,450 8.9

20,250

The particulate matter collected in the thimble at each of the three sampling locations indicated that the fly ash contained less than 12 percent organic material. However, the thimbles were not water cooled and some combustion of organic matter may have occcurred during the sample collection period. Volatilized organics which may con­dense in the atmosphere after leaving the stack still re­mained at this point. This combustible material was col­lected in the iced, wet impingers and iced impingers con­taining carbon tetrachloride used in conjunction with this test. The impinger collected organic material indicating the organic discharge rate at location 3 varied from 17 to 79 pounds per hour.

EVALUATION OF SPRAY CHAMB ER

EFFECTIV ENESS

An exact analysis of spray-chamber performance was impractical with the available manpower and equipment due to the need to obtain simultaneous sampling of inlet and outlet of the spray chamber. With the inlet section of the spray chamber exhibiting extremely unequal dust loading and flow distribution, the inlet sampling would have had to be accomplished at several points in the cross section of the chamber simultaneously for an exact measurement. It is not possible to make a meaningful estimate of the mean

particulate loading going into the sprays with the limited data available. Therefore, unfortunately, no estimate

. can be made on the efficiency of the sprays in remov­ing fly ash from the combustion gas.

STACK EMISSIONS

Two samples were taken in the stack to measure the amount of particulate matter being emitted to the atmosphere. The two samples were collected about two hours apart on the same day. The results varied con­siderably and showed the usual wide fluctuations in the amount of material discharged during the operation of large incinerators, even during times of exceptionally "steady" operation. Based upon the results shown in the accompanying tables, the particulate matter dis­charged in one test was 103 pounds per hour and in the other case 36 pounds per hour.

The grain loading in Test A was 0.502 and in Test B was 0.175 grains per scf of dry gas calculated on the basis of 50 percent excess air.

During the course of the stack tests, the operating temperature of the incinerator was kept above 1600 F and the refuse charging rate was estimated to be about 70 percent of the design loading rate. During most of the preceding sampling work the refuse charging rate was estimated to be about 90 percent of the design

TABLE IV

STACK SAMPLING STATION EMISSION SUMMARY

Test

Refuse Charged, lbs/hr Particula te Discharge \ lbs/hr Particulate Discharge, 2 grains/scf Combustible Particulates Collected in Thimble, percent Aldehydes, ppm Organics Not Collected in Thimble2 , grains/scf Organics Not Collected in Thimble, lbs/hr Orsat Analysis3 :

02' percent CO2, percent CO, percent

Water Vapor (by Volume), percent

A

17,500 103 0.502 18.5 0.99 0.412 84

16.0

4.4 0.2 120

B

17,500 36 0.175 12.5 1.62 0.282 57

9.9

Average

17,500 69. 5 0.338 15.5 1.30 0.347 70.5

NOTE: 1. Stack gas flow calculated to be 73,300 sefm, velocity 13.8 fps and temperature measured at 380 F (Average Values).

2. At 68 F and 50 percent excess air (dry basis).

3. Excess air calculated to be 305 percent.

334

rating. Fig. 7 illustrates typical appearance of the stack during the test period.

In addition to the above data, a sample for aldehyde determination was taken during the course of the test, as an index of odoriferous materials which might be discharged as a result of incomplete combustion. The amount of aldehyde found was extemely low. The test team reported no odors were observed in the discharge gases during the course of the test.

The amount of volatilized organics remaining in the incinerator gases was determined by absorption in carbon tetrachloride in traps immersed in an ice bath. Test A showed 84.2 pounds per hour of condensible organic material being discharged. Test B showed 57 pounds per hour of the same type of material being emitted to the atmosphere. The resultant average of 70.5 pounds per hour of unburned organic vapors which were con­densed and absorbed in carbon tetrachloride is equal to

I

FIG. 7 T,YPICAL STACK APPEARANCE

335

the average of the particulate matter i.e., 69.5 pounds per hour discharged during the same testing period.

The volatilized organic material found in the carbon tetrachloride was collected for the purpose of compari­son with similar samples taken at test location 3 ahead of the water sprays. Samples of the collected particu­late matter were burned in ,the laboratory to determine the combustible content. An average of 15 percent of unburned material was found in these samples.

CONCLUSIONS

As a result of this study the following conclusions were drawn:

1) Operation control to maintain secondary com­bustion chamber temperatures above 1500 F was lacking during a high percentage of the operating days.

2) The start-up period was excessive since no pro­visions were made to use auxiliary fuel to preheat the • • mcmerator.

3) Further improvement in fly ash removal by the water sprays could probably be expected if the extreme­ly unequal dust loadings in the secondary combustion chamber could be equalized by improving the gas flow pattern.

4) The organic constituents found in the secondary combustion chamber both in the ash particulate matter and as condensable gas-phase material indicate combus­tion is not complete before the gas stream reaches the water sprays.

5) Lack of positive means for introducing and/or mixing air with the products of combustion in the secondary combustion zone leads to deficiencies in the burning process.

6) The use of air under the grates promoted the suspension of excessive amounts of particulate matter in the gas stream.

7) Turbulent action of the combustion gases leaving the lower portion of the rotary kiln entrained large quantities of particulate matter which should have been deposited in the ash pit.

8) Outside air drawn into the ash pit may have been a factor in producing a turbulent condition which pre­vents the lighter ash particles from depositing in the pit.

RECOMMENDATIONS

Based upon the conclusions reached in this study, the following recommendations for operating changes, design modifications, and further engineering investigations were suggested for action in the following order.

Start-Up Procedure

The incinerator should be operated on a seven-day basis, if possible, to eliminate any nuisance which re­sults during the start-up period. Investigate the avail­ability of additional supplies of rubbish which would pay for the additional operating time.

If continuous operation cannot be achieved, supple­mentary fuel, preferably natural gas, should be used to bring the incinerator up to temperature and to assist in burning the initial loads during start-up periods. The installation of burners in the primary combustion cham­ber and in the secondary burning zones would be re­quired.

Temperature Control

The operation of the incinerator during the test demonstrated that temperatures in the proper operating range can be maintained. The use of the operator's daily report should be continued and inspected each day to insure that the responsible individuals maintain a con­tinuous supply of rubbish during the operating period and that the floor operator makes the necessary adjust­ments to keep the temperature above 1500 F.

Air Control

Reduce the amount of underfire air presently being introduced and divert a portion of this air over the fire bed in the primary chamber for the purpose of reducing suspension of fly ash in the gas stream. Duct work and dampers to permit this diversion of air and its introduc­tion into the primary combustion chamber could be ac­complished with minor alterations to the system. The exact amount of air to be placed under the grate should be determined experimentally after the modification so

336

as to supply the necessary air to control grate tempera­tures at safe levels.

I nfiltrated Air

Investigate the feasibility of reducing infiltrated air into the ash pit through the diversion gate assembly. The diversion gate could be protected either through the use of water or by the use of refractories. Every ef­fort should be made to reduce this air flow, which ap­pears to be a factor in carrying light ash upward into the turbulent gas stream as it leaves the rotary kiln.

Tests and Complaints

Maintain a careful record of complaints and investi­gate each complaint carefully for at least six months after the operating changes suggested in 1 and 2 have been accomplished.

Upon completion of items 1 through 4, a relatively simple testing program should be undertaken to de­termine the benefits achieved in the stack characteristics.

I mproved Mixing Conditions

Make an engineering study on the feasibility of in­stalling appropriate baffles or checker work across the secondary combustion chamber at the point after the gases have passed over the existing bridge wall. This alteration would serve to mix the gases to achieve better combustion and distribute the flow of gas reaching the water sprays. Some improvement in the effectiveness of the water spray system would result. This effect, together with the improved burning conditions created through better mixing after the bridge wall, will reduce the discharge to the atmosphere and would be expected to improve the appearance of the plume.