THE PUROX SYSTEM

T. F. FISHER M. L. KASBOHM

J. R. RIVERO

Union Carbide Corporation

Tonawanda, New York

Union Carbide's PUROX System employs a partial oxidation process using oxygen for convert­ing solid waste to fuel gas and inert slag. This sys­tem has been developed over the last several years. After initial studies, a S-ton/day pilot plant was built and operated under a variety of conditions to develop the basic process. The process has now been successfully demonstrated on a commercial scale in a 200 ton/day facility at Union Carbide's plant at South Charleston, West Virginia. The operation at South Charleston is continuing with the objective of obtaining additional design in­formation.

Figure I gives a brief overview of the input and output from the process. One ton of refuse re­quires about 0.2 ton of oxygen and produces 0.7 ton of medium B.t.u. fuel gas, 0.22 ton· of sterile residue and 0.28 ton of wastewater. Within the process 0.03 ton of oil is separated in the gas clean­ing train and recycled to the furnace for cracking into additional gas. All of these numbers are ap­proximate and are given in an effort to give you a simplified view of the overall process.

Figure 2 shows a schematic of the reactor with an indication of the types of functions that are re­quired for a successful plant operation. The key element of the system is a vertical shaft furnace. Refuse, which has been preprocessed for recovery of materials such as iron, is fed to the top of the furnace through a gas seal to prevent escape of fuel gas. Oxygen is injected into the bottom hearth section to provide the partial oxidation that drives the reactor. The furnace is maintained essentially

125

full of refuse which continually descends by gravity through the shaft. The oxygen in the hearth reacts with char formed from the particles of the refuse. This reaction generates the high tem­perature in the hearth needed to melt the glass, metal and other materials to give a molten residue. The molten material drains continuously into a water quench tank where it forms a hard granular aggregate.

The hot gases from the hearth section rise through descending refuse, cooling the gas and pyrolyzing the refuse to yield a fuel gas. In the upper portion of the furnace the gas is further cooled as it dries the fresh incoming refuse. This countercurrent heat exchange efficiently utilizes the energy of the gas and provides a top gas that is cleaned by the incoming refuse. The gases leaving the converter are further cleaned of their oil mist and excess water vapor by passing through a re­circulating water scrubber system and an electro­static precipitator. The liquid hydrocarbons and any entrained solids are separated from the scrub­ber water and recycled to the furnace for disposal. The condensate, which is the net water product discharge from the scrubber system, is cleaned of organics and sent to the sewer.

The initial pilot plant was operated at the Tarrytown Technical Center on simulated and real refuse. The unit was relatively smaIl, proces­sing about 5 tons per day. The pilot plant is shown in Figure 3. The unit was operated to confirm the fundamental process design and to obtain data for designing the demonstration plant. This work was

PUROX SYSTEM

TYPICAL INPUTS/OUTPUTS

0.2 TONS OXYGEN

1.0 TONS REFUSE

GAS CLE ANING

TRAIN

07 TONS FUEL GAS

0.28 TONS

WASTE;WATER

FURNACE 1.01 TONS

GAS

003 TONS OIL 0.22 TONS

STERILE RESIDUE

REFUSE

FEED HOPPER �"'�iK'j�

SEAL-==�O!!!

FIG.1

FUEL GAS PRODUCT

! SEAL-==� =���==::;l

OXYGEN

GAS CLEANING

TRAIN

RECYCLE

WATER QUENCH WASTE WATER

W�Ji;2..�==-GRANULAR

presented by Dr. Anderson at the 1974 National ASME Conference, Incinerator Division, in Miami, Florida and was published as part of the proceed­ings of the conference (p.337). The process is described in U.S. Patent 3,729,298.

The demonstration plant was completed in 1974 and has now been operated successfully on municipal refuse from Charleston and adjoining cities in West Virginia. Operation of the plant has successfully solved the problems associated with the 40-fold scale-up and has also demonstrated the cost and performance of a commercial scale system.

Figure 4 shows a sketch of the initial demon­stration plant. The unit has now been expanded to

RESIDUE

FIG.2

126

include front end processing for ferrous metal re­covery and liquid separation equipment for re­cycling the condensed oil to the reactor. The demonstration plant receives the municipal refuse by truck in the storage building. The refuse is moved and stacked with a front end loader. This same front end loader picks up the stored refuse; weighs it and dumps it onto the conveyor to the shredder. One operator handles the front end loader; a sec-ond operator handles the feedtrain, which consists of shredding, magnetic separation, and converter feeding. The operator on the shred­der removes material such as logs, carpets or tires that may stall the shredder. Any material that can pass the shredder has no difficulty with the

I \

•

I

e • •• -. - .

--, 1

t-

•

- '�. -

-

...

�,

\J I • • I

•

FIG. 3 5 TON/DAY "PUROX" SYSTEM PI LOT PLANT AT TARRYTOWN, NEW YOR K

127

,

,

•

,.

Ill • •

128

remainder of the process system. The discharge from the shredder passes under a magnetic separa­tor where better than 90 percent ot the ferrous material (160 lbs./ton refuse) is removed. The shredder is operated to give a relatively coarse shred since this is quite satisfactory for feed to the converter. The material that passes the magnetic separator dumps onto the main conveyor and is �arried to the feed system near the top of the con­verter. The shredded refuse feed to the converter is controlled to maintain the bed in the converter between predetermined levels. The rate of the process is controlled by the rate of oxygen in­jection into the hearth. The reactor operates at a positive pressure of about 20 inches of water.

The moite,n slag discharges continuously from the hearth and is quenched in a water bath below the reactor. A slag conveyor with water seal brings the granular material from the pit up into a dumpster unit. The slag is weighed to give operating data before it is dumped on the adjoin­ing ground for land fill. A fourth operator moni­tors the operation around the hearth and cOf)trols· the oxygen addition rate and the slag disposal. The reactor and hearth are refractory lined and the hearth is water cooled. The gas exiting from the top of the reactor is scrubbed with water sprays

as it passes down the line. Next, the scrubbed gas passes through the electrostatic precipitator where it is further cleaned and then to the burner for dis­posal. The flame is very clean and is relatively non­luminous. The scrubbing water passes to a decan t tank where both the light and heavy hydrocarbon layers are removed and returned to the reactor. The scrubbing water is filtered, cooled and recycl­ed. The excess water from the condensate will be processed to remove organics and sent to the sew­er. Figure 4 does not show the shredding equip­ment and the decant system; these units were added after the initial installation.

Figures 5, 6 and 7 show photographs of the demonstration plant as it was initially installed. They do nor include som of the more recent addi­tions of front endequipment and gas cleanup.

The plant was operated in 1974 with as-received refuse. This refuse was fed directly from the front end loader onto the main conveyor going to the reactor feed system. There was no sorting of this refuse. A number of runs were made in this mode of operation and the equipment was modified where necessary to give a continuous operation.

, Early in 1975, the shredder and magnetic separator were installed for ferrous material re­covery. The facility was then operated with this

FIG. 5 200 TON/DAY "PUR OX" SYSTEM FACILITY AT SOUTH CHARLESTON, WEST VIRGINIA

129

,

I \

•

,

•

,

• ,

•

• d

.1

. - i

-

LL

,

"

•

130

•

.�.

+ .", I IIi __

FIG. 7

front end processing in a steady state fashion. The data taken were quite accurate, giving excellent heat and material balances around the total system. We have now operated the facility to obtain per­formance data under conditions of extended periods of operation. During the extended run, we processed over 7,000 tons of refuse, with an on-stream factor of approximately 93 percent.

Figure 8 gives a typical analysis of the fuel gas from the unit. The fuel gas is primarily H2, CO, CO2 and light hydrocarbons. While we obtained a

TYPICAL GAS ANALYSIS

H2 CO C02 CH4 C2+ N2& A

Gas· Higher Heating Value

FIG.8

Vol.7r 26 40 23

5 5 1

370 Btu/Sef

300 Btu fuel gas from the pilot plant, we found that the demonstration plant gave a higher concen-

131

tration of hydrocarbons and a gas with about 370 Btu per cubic foot. Clean fuel gases with heat­ing values above 300 Btu per cubic foot are inter­changeable with natural gas for combustion. The fuel gas as shown on the chart is also a good synthesis gas and can be processed for chemical use.

Figure 9 gives an average analysis of the slag. The slag is primarily a product of the glass in the refuse with minor additions from the contained metals and the ash from the wood products. The material has gone through the melt condition at about 2S00°F and is inert from the standpoint of any environmental considerations.

Figure 10 gives a breakdown of the available energy from a PUROX facility. Taking a value of 9.5 million Btu per ton as the energy available in the refuse, about 20 percent of the energy is lost in the conversion, giving.a fuel production from the unit of 7.5 million Btu per ton. To operate the air separation plant and miscellaneous equipment re­quires about 100 kilowatt-hours of electric power per ton of refuse processed. This power would normally be purchased; however, for a total energy balance one million Btu of fuel gas from the system

A VERAGE SLAG ANALYSIS

MnO -- 0.3

Si02 -- 59.7

CaO -- 10.3 AI203 -- 10.5 Ti02 -- 0.6 BaO -- 0.2 P205 -- 0.1 FeO -- 6.2

MgO -- 2.2 Na20 -- 8.0

K20 -- 1.0 CuO -- 0.2

Misc. -- 0.7

100.0

Slag Production - 340 Ibs/ton Refuse

FIG.9

could be converted to the 100 kilowatt-hours of power. This would show a net production of 6.5

.

million Btu per ton of refuse. Special features of the PUROX System are: 1) The fuel gas is valua'ble either for combus­

tion in power plants or as a synthesis gas for making chemical such as methanol and ammonia.

2) The quantity of fuel gas is significant for any given community. Taking a standard municipal refuse figure of 5 pounds per person, this gives a production of 16,000 Btu's of fuel gas produced per individual per day.

3) The separated ferrous is salable as scrap and

is worth about $2 per ton of refuse. 4) The solid rl;:sidue produced is less than

3 percent of the feed by volume and is an inert aggregate (glassy material) that can be used for such things as road fill.

5) There is no effluent to the atmosphere and the fuel gas produced, if burned, will have excel­lent characteristics since it is very low in both

•

sulfur and ash. 6) The condensed water vapor contains soluble

organics; however, this material will be' processed within the system to make the effluent acceptable to municipal sewers.

The PUROX System is receiving wide interest from municipalities across the country. While the initial development was focused on disposal of solid waste in an economic and environmentally acceptable fashion, the current energy crisis has placed increased emphasis on the resource recovery aspects of the fuel gas and ferrous material. Instal­lations being considered range from 400 tons to 2,000 tons/day. To ensure reliable operation the facilities will incorporate multiple modular units for the refuse conversion step. Modular units will be sized to process from 200 to 350 tons/day. Each installation will have a customer for the fuel gas. One city finds that it can make 30 million gallons a year of methanol from a 1,500 tons/day PUROX plant; however, most municipalities expect to sell the fuel gas to utilities for use in power plants or for sale to gas customers.

AVAILABLE ENERGY FROM A "PUROX .. FACILITY

(Million B.t.u./Ton)

Energy Available in Refuse Energy Loss in Conversion

Energy Available in Fuel Gas

Energy for In-plant Electric Generation

Net Energy Produced

132

9.5

2.0

7.5

1.0

6.5