A SCRAP TIRE-FIRED BOILER

F. MICHAEL LEWIS Stanford Research Institute

PAUL W. CHARTRAND Lucas American Recyclers, Inc.

ABSTRACT

A cyclonic, rotary hearth, scrap tire-fired boiler, designed and constructed by Lucas American Re­ cyclers, Inc., has been operating at the Goodyear Tire and Rubber Company plant in Jackson, Mich· igan, for approximately one year. The nominal rat­ ing of this furnace is 3100 pounds (1406 Kg) of scrap automobile tires per hour, generating 25,000 pounds (11 ,340 Kg) of process steam per hour. This paper discusses the fundamental design parameters, principles of cyclonic combustion, heat and material balance, air pollution control, and per· formance data of the scrap tire-fired boiler at Jack· son, Michigan.

INTRODUCTION

Every scrap tire disposed of in the United States represents the heating equivalent of approximately 2 � gallons (9.5 liters) of oil. Based on currently available data on passenger and truck tire produc· tion in the United States and assuming that over 90 percent of the tires produced will enter the solid waste stream, the tires to be disposed of each year will amount to 200 million, representing a heating equivalent of 500 million gallons (1893 million liters)

Mr. Lewis served as Technical Director, Furnace Group, Fluor-Utah, Inc., during the reconstruction and start-up of the tire furnace.

301

of oil per year. The Goodyear Tire and Rubber Company in Jackson, Michigan, has installed a scrap tire-fired boiler (referred to here as the Jack­ son furnace) to burn scrap tires brought into the plant by rail and truck from the surrounding reg­ ions. Lucas Furnace D evelopment of England de­ veloped the basic furnac�, which was installed at Jackson, Michigan, in 1973. Initial trials revealed a number of mechanical defects, and Fluor Utah was engaged to redesign and reconstruct the unit. The reconstructed unit was completed and the first tire burned in March 1974. Operation con­ tinued for four days. The unit was then shut down for additional mechanical and control modifica­ tions. Operation was resumed in May 1974. When operating at its maximum capacity of 3100 pounds (1406 Kg) of tires per hour, the Jack· son furnace can provide fuel savings amounting to 2.6 million gallons (9.8 million liters) of fuel oil per year.

FUEL PROPERTIES OF SCRAP TIRES

Kaiser [ 1] and the Bureau of Mines [2] have presented detailed information on the fuel proper­ ties of scrap tires discarded in the United States. Because the Bureau of Mines obtained its data from samples with the bead and fabric removed, these data are not directly applicable to a furnace that burns whole, unshredded tires, but we will use them for comparison with Kaiser's data:

Proximate Analysis

Kaiser Bureau of Mines (percent) (percent)

Moisture 1.02 Volatile matter 64.92 27.51 Fixed Carbon 6.55 Ash Heating value, Btu/lb 13,906

0.5 62.3 31.5 5.7

Ultimate Analysis - Moisture, Ash free Carbon Hydrogen Oxygen Nitrogen Sulfur

84.58 7.33 6.34 0.1 3 1.62

88.23 7.53 2.65 0.32 1.27

THE SCRAP TIRE FURNACE

The scrap tire furnace is a cyclonic, rotary hearth furnace. Figure 1 shows a cutaway view of a typical scrap tire furnace. A feeder located in an air-tight charging vestibule pushes the tires onto the outer edge of the rotating hearth. When the tire has completed its first revolution, the ash push­ er moves the tire inward. After the tire has com­ pleted its second revolution, the action of the next tire being pushed into the inner burning ring pushes the remaining ash into the central ash discharge chute. The start-up auxiliary fuel burners and the com­ bustion air inlets fire tangentially, imparting a

FIG.1. CUTAWAY VIEW OF TYPICAL TIRE FURNACE

302

AUXILIARY FUEL BURNER

TA-:Je1522-20

FIG.2. PLAN VIEW OF COMBUSTION AIR JETS AND BURNERS

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I'

I

ASH DISCHARGE CHUTE

�

ROTA TlNG HEARTH

/

INNER WATER

OUTER WATER

SEAL

SEAL T A-361622-21

FIG.3. TYPICAL CROSS SECTION VIEW OF TIRE FURNACE

303

cyclonic swirl to the combustion gases. Figure 2 is a schematic view of the approximate location of the air jets and burners. Figure 3 shows a cross section of the inner and outer water seals of the Jackson furnace. When the Jackson furnace is operating at the maximum design capacity of 3100 lb/hr ( l 406 Kg/hr), its heat release rate is 26,000 Btu/ft3 /hr (231,338 Kg-cal/m3/hr), and the hearth loading is 214,000 BtU/ft2/hr (580,449 Kg-cal/m2/hr). A single ball bearing, with a double row of balls to take the thrust and shear loads, supports the rotating hearth. The bearing is similar to the bear­ ings used on large cranes. The bearing assembly, with gear drive attached, is located underneath the hearth, requires no forced air or water cooling, and has easy access for maintenance. Supported from a

central point underneath the refractory and hearth structure , the hearth is free to expand and thermal stresses are minimized. The combination of the inner and outer water seals and the double door charging vestibule makes the furnace essentially gastight. All air for combus­ tion enters the furnace through the tangential air jets. CYCLONIC COMBUSTION IN A ROTARY HEARTH

The inwardly moving solid fuel bed at the bot­ tom' of the furnace and its relationship to the pri­ mary and secondary gas flows impart to the cy­ clonic, rotary hearth furnace combustion charac­ teristics quite unique and quite different from those

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FIG.4. CYCLONIC COMBUSTION GAS F LOW PATTERN

304

of other cyclonic and vortex combustion chambers. The secondary, recirculating gas flows , superim­ posed on the primary cyclonic flow (Figure 4), play a major role in the combustion process. The Bureau of Mines [3,4] has developed a vor­ tex incinerator with a solid feed and ash discharge system that differs from that of the Jackson fur­ nace; however, temperature profiles and gas com­ position within the cyclonic flow pattern are simi­ lar. The Bureau of Mines data indicate that the cool combustion air in the vortex incinerator hugs the outer wall of the furnace and that a radial tempera­ ture profile exists , so that the flame is hottest at the vortex center. Under certain flow conditions, mixing between the solid fuel bed and combustion air in the vortex incinerator is so intense that all the oxygen is scavenged from the air before it reaches the center of the vortex. These principles of intense mixing and oxygen scavenging in the Bureau of Mines furnace are also important in a rotary hearth cyclonic furnace. In the rotary hearth furnace , the absence of oxygen in the central region of the hearth creates an "aerodynamic pyrolysis zone." Heat is supplied to this region by radiation from the burning hot core of gases above the bed and from the surround­ ing brickwork. The residue discharged from this furnace resembles the black char obtained when a piece of rubber tire is pyrolyzed in an indirectly heated pyrolysis retort. By viewing the central portion of the hearth from an outside view port, one can observe burn­ ing gases forming a tight core , similar in shape to a tornado, with its tip occasionally dipping down onto the top of the water surface in the central water seal. Because of the flicker of this rotating core of burning gases and some minor quantities of low fusion components in the tire that flow over the hearth, a common misunderstanding exists that the residue leaves the furnace as a molten slag. When tires are first fed into the furnace and are on the outer portion of the hearth , the volatiles distill off very rapidly and are immediately swept toward the central, vertical burning core by the tangential combustion air. Combustion begins im­ mediately in the tire region and continues as the gases go up the furnace core. Because of its small diameter and large surface area, the steel beadwire is oxidized as soon as it becomes exposed. The carbon black used in the manufacture of tires burns very slowly because of its molecular structure, and a portion of this carbon remains when the residue is discharged into the residue

water seal. The lack of oxygen in the central region of the hearth also causes a small amount of carbon­ aceous char to form. This carbonaceous residue has some disadvantages: a) Increased quantity of residue to be dis­ posed of. b) Loss of heating value because of the presence of combustibles in the residue. It also has some advantages: a) Ease of residue handling; the residue handles like a coarse sand. b) Elimination of the slagging problems caused by high material temperatures that result when the fixed carbon burns. c) Chemical binding of some potential pollu­ tants to the carbonaceous portion of the char. It is felt that the advantages outweigh the dis­ advantages.

OPERATION OF THE FURNACE SYSTEM

When operating at maximum capacity, the Jack­ son furnace will process 31 00 pounds (1406 Kg) of scrap tires per hour, equivalent to 43,400,000 Btu/hr (10,935,932 Kg-cal/hr). The weight of pas­ senger tires varies with size and mileage but is ap­ proximately 25 pounds (11.3 Kg) per tire, resulting in a tire feed rate of 129 tires per hour, or 1,130,000 tires per year. The tires are manually placed on a roller con­ veyor from the tire receiving area; all subsequent operations are automatic. The conveyor moves for­ ward intermittently, in sequence with the tire feed rate. The tires are fed into the furnace from an air­ tight charging vestibule to minimize the danger of fires and air leaks into the furnace. On a Signal from the furnace temperature controller, the guillotine fire door on the furnace opens and the tire is pushed into the furnace. Because of the intense heat and radiation, the tire bursts instantly into flames. The tire begins a circular path through the furnace on the outer rim of the rotating hearth. Combustion of the volatiles is almost completed when the tire approaches the feed door at the end of its first revolution. An ash pusher located a few degrees upstream of the tire feeder pushes the tire into a second burning region, inside the outer circle of tires. The tire being pushed into this inner circle pushes the ashes from the tire previously occupying that area into the ash discharge chute. The ashes drop into the water seal and are discharged into a bin by an ash dredger.

305

The flue gases leave the furnace through a hori­ zontal, approximately square breeching. The flue gasl!s are directed through a waste heat boiler with a retractable soot blower for the front tubes and two rotary soot blowers for the center tubes. The boiler exit gases then go to a wet scrubber. An induced draft fan provides the necessary draft for the system.

For a constant mass flow rate with varying jet diameter, 4

D1 D2

(2)

For a constant mass flow rate and jet diameter but with varying air jet temperature, 3

(3) FLUID MECHANICS OF COMBUSTION VORTEX

When the furnace was first put in operation, a proper combustion vortex could be obtained only at maximum capacity and with all the auxiliary fuel burners firing. The following analysis was per­ formed to find out why and to take corrective action. One of the three "I's" of combustion is turbu­ lence. Currently, there is no satisfactory way to measure turbulence in a furnace. Horsepower (HP)/lb flue gas/sec (kW/Kg flue gas/sec) was chosen as the basic parameter to describe tur­ bulence, because the turbulence is a measure of the power input to the system. The horsepower was determined by examining the power in the gas streams, the combustion air, and the burner flue products that entered the combustion chamber. From data obtained under good operating condi­ tions for this furnace and other similar units, [4] it was determined that a value of 0.24 HP/lb flue gas/sec (0.39 kW/Kg flue gas/sec) gave the desired degree of turbulence, equivalent to a combustion air velocity head of approximately 2 inches (5.1 cm) water column. The analysis was expanded to consider the fur­ nace turbulence (HP/lb flue gas/sec) (kW/Kg flue gas/sec) input into the furnace as a function of combustion air jet diameter, mass flow rate, and jet temperature: Let 1 denote initial conditions Let 2 denote final conditions Let D denote jet diameter, inches (meters) Let m denote mass flow rate, Ib/sec (Kg/sec) Let P denote furnace turbulence, HP/lb flue gas/sec (kW/Kg flue gas/sec) Let T denote combustion air jet temperature, °RCK) From a constant diameter jet with a varying mass flow rate, (1)

Examination of Equation (1) reveals that when a furnace of this type is run at partial load condi­ tions, with the mass flow rate of combustion air reduced to, say, one third of the original flow rate, the furnace turbulence is reduced to one ninth of the original value. Equation (3) demonstrates the effect of air pre­ heat on the turbulence: if the initial temperature is 60°F (15.6°C), the turbulence of the furnace can be doubled by heating the combustion air to only 195°F (90.6°C). If a turndown of 3: 1 is de­ sired, the required air jet temperature to maintain the same degree of turbulence as the original 60°F (15.6°C) jet would be 625°F (329AoC). , A design was considered that would use "choked flow" steam jets to maintain the turbulence con­ stant. For the 250 psig (175,756 Kg/m2) saturated steam available at the site, a mass flow rate of 56 lb/hr (2504 Kg/hr) will give a 1 HP (0.75 kW) input. However, because turndown was not considered a major requirement, nozzles were never installed. For many years, steam jets have been used to promote mixing in coal fired units, but no data are available on the fluid mechanics in terms of pounds of steam per horsepower and furnace turbulence. HEAT AND MATERIAL BALANCE

The tires are burned with approximately 100 percent excess air to keep the combustion tempera­ tures below the point where furnace slagging would become a problem. The resultant flame tempera­ ture is approximately 2000°F (1093°C). Table I shows a material balance for the combustion of 2000 pounds (907.2 Kg) of tires at 100 percent excess air. Table 2 shows a heat balance for a boiler exhaust temperature of 500°F (260°C). Approxi­ mately 5 percent of the potential heat is lost because of the heating value of the char; however, this does prevent slagging of the ash. The heat balance shown assumes zero heat loss. The boiler exit temperature chosen, 500°F (260°C),

306

Table 1 MATERIAL BALANCE FOR THE COMBUSTION OF 2000 LB SCRAP TIRES AT 100% EXCESS A IR

Carbon INPUT

1b

Hydrogen 1b

Oxygen 1b

Nitrogen 1b

Sulfur 1b

Inerts 1b

Total 1b

Tires Moisture Volatiles

1,563.6

2.3

18.1

135.4

117.2

20.4 2.4

Ash Subtotal

1,563.6

137.7

Combustion air Total

w 0

1,563.6

137.7

135.3

2.4

9,874.6

32,780.2

10,009.9

32,782.6

1,848.6

30.0 30.0

131.0

131.0

131.0

2,000.0 42,654.8

30.0

131.0

44,654.8

OUTPUT Flue gas

-.l

Carbon dioxide

1,479.0

Oxygen

3,944.0

5,423.0

4,937.3

4,937.3

Nitrogen Sulfur dioxide

27.3

Water vapor Subtota1-

32,782.6

32,782.6

1,479.0

137.7

1,101.3

137.7

10,009.9

54.6

27.3

1,239.0 32,782.6

27.3

44,436.5

Char Carbon

84.6

84.6

Sulfur Ash Subtotal Total

2.7

2.7

84.6 1,563.6

137.7

10,009.9

32,782.6

131.0

131.0

2.7

131.0

218.3

30.0

131.0

44,654.8

Table 1 (SI Units) MATERIAL BALANCE FOR THE COMBUSTION OF 907.2 Kg OF SCRAP T IRES AT 100% EXCESS AIR

INPUT

Carbon

Hydrogen

Oxygen

Nitrogen

Sulfur

Inerts

Kg

Kg

Kg

Kg

Kg

Kg

Total Kg

Tires Moisture Volatiles

709.2

1. 1

8.2

6 1.4

53.2

9.3 1. 1

Ash Subtotal

-

709.2

62.5

Combustion Air Total

w

0 00

13.6

709.2

62.5

6 1.4

1. 1

4,479.0

14,868.8

4,540.4

14,869.9

13.6

338.5 59.4

59.4

59.4

907.2 19,347.8

13.6

59.4

20,255.0

OUTPUT Flue gas Carbon dioxide

670.9

Oxygen

1,788.9

2,459.8

2,239.5

2,239.5

Nitrogen Sulfur dioxide

12.4

12.4

Water vapor Subtotal

14,869.9

14,869.9

670.9

62.5

499.6

62.5

4,540.4

24.8 .

562. 1 14,869.9

12.4

20,156.1

Char Carbon

38.3

38.3

Sulfur

1.2

Ash Subtotal Total

1.2

59.4

59.4 98.9

13.6

59.4

20,255.0

-

38.3 709.2

62.5

4,540.4

14,869.9

1.2

59.4

Table 2 HEAT BALANCE FOR 2000 LB TIRES WITH BOILER EXHAUST TEMPERATURE OF 500°F AT 100% EXCESS AIR

Senalble

Latent

Btu

INPUT

Btu

at_ical Btu 27,812,000

Scrap tires

Total Btu 27,8 12,000

OUTPUT Flue gas Carbon dioxide

54 1,215

541,2 15

Oxygen

490,768

490,768

3,615,921

3,615,921

Nitrogen Sulfur dioxide Water vapor Subtotal

3,920

•

3,920

247.800

1.3 13.340

1.56 1. 140

4,899,624

1,3 13,340

6,2 12,964

Char Carbonaceous

56,22 1

Inert

60.260

Subtotal Steam ( 16,869 lb) Total

1,239,660

1,295,88 1 60,260

1 16,48 1

1,239,660

2,36 1,755

17,88 1. 140

7,377,860

19, 194,480

1,356, 14 1 20,242,895

1,239,660

27,8 12,000

Table 2 (SI Units) HEAT BALANCE FOR 907.2 Kg TIRES WITH BOI LER EXHAUST TEMPERATURE OF 260°C AT 100% EXCESS AI R

IN,PUT

Sensible

Latent

Chemical

Kg-cal

Kg-cal

Kg-cal 7,008,068

Scrap tires

Total Kg-cal 7,008,068

OUTPut Flue gas Carbon dioxide

136,375

136,375

Oxygen

123,664

123,664

Nitrogen

9 1 1, 140

911, 140

988

988

Sulfur dioxide Water vapor Subtotal

62.44 1

330.935

393,376

1,234,608

330,935

1,565,543

Char Carbonaceous

14,167

Inert

15,184

Subtotal Steam (7652 Kg)

3 12,370

326,537 15,184

29,351

3 12,370

595, 1 15

4,505,689

34 1,72 1 5, 100,804 7,008,068

Total

309

EXHAUST STACK

FURNACE

BOILER

SCRUBBER

1.0

FAN SAMPLE PORTS·

WET CYCLONES

.

fA 361U

FIG. 5. SCHEMATIC DIAGRAM OF TIRE FURNACE SYSTEM

does not represent the condition at the Jackson fur­ nace but rather, the minimum temperature that could be achieved in a practical system. AIR POLLUTION CONTROL

Approximately 1-2 percent of the weight of a rubber tire is zinc oxide, which is used to impart certain desired properties to the rubber. Emission tests conducted upstream of any air pollution con­ trol device indicate that almost all the zinc oxide is carried out with the flue gas and exits as a sub­ micron fume. Actual measurements of furnace emissions indi­ cate that a collection efficiency of 86 percent is needed to meet a typical emission code of 0.08 grains per standard dry cubic foot (183 mg/m3) at 12 percent carbon dioxide. Based on the "Con­ tacting Power Concept" of scrubbers developed by Semrau [5] and data presented in that paper, a scrubber pressure drop of approximately 13 in. w.c. (33 cm) will be required. Sulfur dioxide emission can be calculated direct­ ly from the sulfur content of the tires, although we believe that a tangible portion of the sulfur is con­ centrated in the char residue. The Bureau of Mines [2] data on the composition of char from pyrolysis of tires support this belief. If a portion of the steam produced were to be used to provide the contacting power required for scrubbing - an extremely interesting possibility the steam required for this furnace would represent approximately 5 percent of the potential steam production. JACKSON FURNACE PERFORMANCE DATA

The actual capacity of the Jackson furnace ex­ ceeded the design capacity, and therefore, exceeded

310

the capacity of the boiler and scrubber. In June 1974, air pollution tests were conducted with the furnace running approximately 50 percent over de­ sign maximum; the excess flue gases were by-passed around the boiler and wet scrubber and taken through wetted wall cyclones. Figure 5 shows a schematic diagram of the system. These cyclones were large and of extremely low efficiency. The sprays did not saturate the gas and the exit tem­ perature from the cyclones was approximately 700°F (371°C). The split of the gas flow was ap­ proximately 70 percent through the boiler and scrubbers and 30 percent through the cyclones. The unit did not pass the air pollution test be­ cause of the poor performance of the cyclones. Additional tests, carried out in August 1974 with the furnace operating at design maximum, showed the unit to be in compliance with the State of Michigan Air Pollution Code for Boilers.

CONCLUSION

The rotary hearth furnace has proven an excel­ lent design for the combustion of tires. The oxygen­ deficient region in the lower end of the central vor­ tex has prevented refractory and material handling problems caused when ash is overheated and al­ lowed to slag and clinker. The cool combustion air admitted along the periphery of the furnace has prevented damage to the refractory from the high combustion temperatures. The vortex of the flue gases above the fuel bed has provided a regime of stable and complete combustion. The furnace has shown that it can burn tires at a rate significantly above design capacity. In future units, a bag house should be considered for particu­ late emission control and for potential recovery of the zinc oxide.

cinerator with Continuous Feed," Proceedings of the

REFERENCES

1972 National

York, N.Y. [4] M. Weintraub, A. A. Orning, C. H. Schwartz, Experi­

[ 1] E. R. Kaiser, "The Incineration of Bulky Refuse," Proceedings of the

1966 National

Incinerator Conference, ASME, New

mental Studies of Incineration in a Cylindrical Com­

Incinerator Con­

bustion Chamber, Bureau of Mines R I 6 908, U.S.

ference, ASME, New York, N.Y.

Dept. of the Interior, 1967.

[2] D. E. Wolfson et aI., Destructive Distillation of Scrap Tires, RI 7302 Bureau of Mines, U.S. Dept. of the

[5] K. T. Semrau, "Dust Scrubber Design - A Critique on the State of the Art," J. APCA, Vol. 13, No. 12,

Interior, September 1969. [3] C. H. Schwartz et aI., "Development of a Vortex In-

pp. 587-594 (December 1963).

Key Words Scrap Tire Rubber Heat recovery Pyrolysis Steam generation Rotary hearth Tires Cyclonic

311