Hazardous Waste Disposal with Thermal Oxidation · OXIDATION HAZARDOUS WASTE DISPOSAL BY THERMAL OXIDATION. ... exchanger heats incoming combustion air or waste gases, and the boiler

HAZARDOUS WASTE DISPOSAL

BY THERMAL OXIDATION

HAZARDOUS WASTE DISPOSAL

BY THERMAL OXIDATION

Ideally, the flue gas resulting from high-temperature oxidation of hydrocarbons (HC) contains CO2 , H2O, N2 , O2 and some acceptable levels of oxides of nitrogen (NOx ) and oxides of sulfur (SOx). In reality, the flue gas from a combustion process contains CO2 , H2O, N2 , O2

and some concentration of carbon monoxide (CO), unburned hydrocarbons (UHC), NOx and SOx .

HAZARDOUS WASTE DISPOSAL BY THERMAL OXIDATION

INTRODUCTION

Thermal oxidation has proved to be an effective and safe method for the disposal of a wide variety of hazardous industrial wastes. Virtually all organic compounds can be thermally oxidized with an assured level of destruction. John Zink Company has over 2300 thermal oxidizer installations in service worldwide, destroying an array of hazardous and non-hazardous organic wastes.

The basic thermal oxidation system, shown in Figure A, consists of a refractory-lined vessel called the Thermal Oxidizer (T.O.), burner, stack, and combustion controls. The oxygen for combustion comes either from ambient air or is contained in the waste gas stream. Ambient air may be inspirated by natural draft or forced in by a fan.

1

PRODUCTS OF COMBUSTIONIDEAL

CO , H O, O , N , NO *, SO *XA2 2 2 2 XA

REAL

CO , H O, O , N , NO , SO X2 2 2 2 XUHC,CO

* Sub A designates acceptable level.

FG

Figure A: Basic Thermal Oxidation

StackCastable Refractory Lining

Thermal Oxidizer

CastableRefractory Floor

Burner

Air

Fuel

Waste Stream

Combustion Control Package

Brick Lining W/ Castable Refractory Back-Up Lining

Environmental concerns require that the flue gas exiting a TO meet certain emission requirements mandated by local and/or federal regulatory authorities. Thus, it is important not only to destroy the organic portion of the waste completely, but also to limit the quantities of pollutants which are produced by the combustion process or were originally in the waste but not destroyed by combustion. For example, oxides of sulfur and C12/HCI produced by thermally oxidizing wastes containing sulfontated or chlorinated components must be removed down-steam. Similarly, inorganic salts or ash contained in the waste are unaffected by combustion and must be removed to meet particulate emission requirements.

Meeting CO and UHC regulations is accomplished by the correct selection of TO resident time, operating temperature and turbulence - the three Ts of combustion. Figure B is a plot or residence time versus destruction efficiency for CO and HC at various temperatures. It shows that CO and HC destruction efficiency increase as residence time and operating temperature increase.

Figure C is an example of how this information is used to determine concentration of UHC and CO in the flue gas. In this example, we assume methane is being burned with 25% excess air in a horizontal T.O., having a residence time of 1.0 seconds and operating at a temperature of T2 . Air injected into the T.O. is in addition to the 25% excess air and is used to lower methane’s 3200° F adiabatic temperature to T2 .

The destruction efficiencies obtained from the graph for CO and HC are 99.88 % and 99.985 %., respectively. The heat and mass balance calculations determine flue gas concentrations of 7.1 and 56.8 ppm V (dry) for unburned HC (assumed as CH4 ) and CO, respectively.

EXAMPLE: What is the concentration of CO and unburned hydrocarbon in the flue gas when the residence time is 1.0 second and operating temperature is T2 ? Use air to cool operating temperature to T2 .

CO graph reading = 99.88 % HC graph reading = 99.985 %

Basis 1 mole of CHCH4 + 2O2 CO2 + 2H2OStoichiometric Air = 9.52 ( 2/0.21 )125% Air (Burner) = 11.90 ( 9.52 x 1.25 )Flue Gas: N2 = 9.40 ( 11.9 x 0.79 )

O2 = 0.50 ((11.90 - 9.52) x 0.21)

Flue Gas @ 3200° F : ( Figure D)CO = 1H2O = 2N2 = 9.4O2 = 0.5

Assume after subtracting heat loss, 12 moles of air will cool flue gas to T2 . Flue Gas @ T2 :CO2 = 1H2O = 2N2 = 18.88 ( 9.4 + (12 x 0.79) )O2 = 3.02 ( 0.5 + (12 x 0.21) )CH4 = 1.5 x 10-4 1 ( 1 - 0.99985 )CO = 1.2 x 10-3 1 (1 - 0.9988 )

Total dry flue gas = 22.9 moles[CH4 ] = 6.6 ppmv dry basis[CO] = 52.4 ppmv dry basis

CH 4

3200° FHeat Loss

StackHorizontal T. O.

Combustion Air

Figure C : Example Problem 3

Figure D is a plot of adiabatic flame temperature, volume percent combustibles and volume percent oxygen versus percent of stoichiometric combustion air for # 2 fuel oil and natural gas. The figure indicates a theoretical flame temperature of 3200°F when methane is burned with 125 percent of stoichiometric combustion air (25% excess). In the previous example, air was used to cool the 3200°F products of combustion leaving the burner to the TO exit temperature of T2.

Meeting stringent pollution control regulations governing the amount of inorganic acids (SOx, NOx, H3PO4, Cl2/HCl) and particulate matter in the exit flue gases requires the use of additional equipment downstream of the basic TO system. This paper furnishes the reader with a method of selecting the most appropriate waste disposal process.

WASTE CATEGORIES

Wastes are supplied to a disposal process in the form of either gas, liquid or solid, or a combination thereof. Thus, wastes can be systematically divided into the categories of gas, liquid, solid, gas+solid, liquid+solid and gas+liquid. Table 1, Industrial Waste and Pollutant, lists these categories in the left-hand column. Note the absence of a gas+liquid category. A gas+liquid waste, both fluids, versus a gas-only or liquid-only waste, requires the choice of an appropriate burner rather than a process.

Configuration 1.2 is the T.O. fitted with a heat recovery boiler. A boiler with an economizer can recover as much as 85 % of the heat energy supplied to the T.O. by the waste and the fuel.

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The second column lists a typical waste for each category, with the related waste pollutant(s) listed in the third column. For example, a fume stream which is predominantly air containing approximately one percent (10,000 ppmV) HC is listed as a gas waste, whereas a biosludge is listed as a liquid+solid waste. Obviously, this second column does not contain all known industrial wastes. However, it is likely that a particular waste is sufficiently similar to a listed waste so an appropriate process can be chosen.

The fourth column is a list of process numbers which identify processes applicable to dispose of waste listed in the corresponding row. For example, Process 6 is used to treat a gas+solid stream consisting of CO, H2O and small combustible particulate.

DISPOSAL PROCESSES The eight similar but separate processes to dispose of industrial wastes are described by the following text and diagrams.

Gas or Liquid Waste - High Levels of NOx and/or SOx

The following six diagrams illustrate each of the six configurations of a process to dispose of either a gas or liquid waste which produces a flue gas containing acceptable amounts of SOxand/or NOx. Configuration 1.1 is simply a T.O. which is supplied with waste, fuel and combustion air. Fuel is required when the waste’s combustion energy is insufficient (endothermic) to produce an appropriate operating temperature. An exothermic waste requires a cooling medium such as excess air, steam, or water for temperature control.

WASTE EXAMPLE PRODUCTS OF OXIDATION

Gas Liquid

Tail Gas Organic Acid

FG, NOXA, SOXAFG, NOXA

Configuration 1.1 : Waste Process

Fuel

Waste

Air

StackT. O.

Flue Gas



Gas Liquid


FG, NOXA, SOXA

FG, NOXA

Fuel

Waste

Air

StackT. O.

Flue Gas

Boiler

Steam

Configuration 1.3 shows a T.O. fitted with a gas-to-gas heat exchanger. In the heat exchanger, the hot flue gas from the T.O. is used to heat the incoming waste gases. This method of heat recovery, when heating a 60° F waste gas to 800° F with a 1600° F operating temperature, can reduce a 16.8 MM Btu/hr without preheat fuel requirement, to approximately 9 MM Btu/hr. (Refer to Figure 1A for savings.)


Gas Liquid




Fuel

Waste

Air

StackT. O.

Flue Gas

H. Exchanger

7

Value of Recovered Flue Gas Heat (Based on 5000 cfm of inert waste gas and 1600°F operating tem perature)

0

10

20

30

40

50

60

70

0 200 400 600 800 1000 1200

Waste Preheat Temp (°F)

Fuel

Sav

ings

(%)

Configuration 1.4 is a T.O. fitted with a gas-to-gas exchanger and a heat recovery boiler. The heat exchanger heats incoming combustion air or waste gases, and the boiler further extracts the heat available in the flue gas discharged from the exchanger. This configuration offers flexibility in the amount of steam produced versus fuel usage.

Configuration 1.5 illustrates a Catalytic Oxidizer fitted with a gas-to-gas exchanger. The heat exchanger preheats contaminated air which is routed to chamber containing catalyst material. The catalyst causes oxidation of the HC to occur at much lower temperatures than in a thermal oxidizer, thus greatly reducing the fuel usage. The HC content of the air is generally limited to less the 0.75 % because of temperature limits of the catalyst.


Gas Liquid



Steam

Stack

Flue Gas

Boiler


Fuel

Air

T. O.

Waste

H. Exchanger

8


Gas Liquid




Stack

Flue GasContaminated Air

H. Exchanger

Fuel

Catalytic Oxidizer

Configuration 1.6 displays a regenerative oxidizer which uses refractory packing to absorb and transfer heat to the outgoing or incoming air stream. Inlet and outlet ductwork, valves and an induced draft blower provide the means for the contaminated air to enter and exit the chambers independently. The paths of flow are controlled by action of inlet and outlet valves. HC contents is usually limited by the lower flammability limit instead of by overall HC concentration.


Gas Liquid




Contaminated Air

Fuel

Stack

T. O.

Flue Gas

Chamber No 1 Chamber No 2

Chamber No 3

Gas or Liquid Waste - High Levels of SOx or Cl2/ HClThe following two diagrams show configurations of a process to dispose of either a gas or liquid waste which produces flue gas containing excessive amounts of SOx or Cl2 / HCl.

Configuration 2.1 consists of a T.O., a quench section which cools the flue gas to its saturation temperature by directly contacting it with water, two adiabatic absorbers which remove inorganic acids and chlorine, and a vent stack. Water is used in the first absorber to remove a majority of the HCl from the flue gas. The residual HCl and virtually all the entering Cl2 leaves the absorber with the flue gas. A second absorber with caustic is used when either the Cl2 or HCl in the flue gas exiting the first absorber exceeds allowable levels. This occurs when excessive Cl2 is formed in the T.O. (see Figure 2A for HCl/Cl2 equilibrium) or when the first absorber is used to make acid.


Gas Liquid

VCMPCB Pesticides

FG, NOXA, Cl2 / HCl


Stack

Flue Gas

Quench

Fuel

Waste

Air

T. O. Absorber Caustic Scrubber

H O2Caustic

Hydrochloric Acid

Salt Effluent

9

Purge


Gas Liquid

VCMPCB Pesticides

FG, NOXA, Cl2 / HCl


Stack

Flue Gas

BoilerT. O. Fuel

Waste

Air

Caustic Scrubber

CausticHydrochloric Acid

Salt Effluent

Steam

Configuration 2.2 consists of a T.O., a heat recovery boiler which produces steam in cooling the flue gas to 500° F, two absorbers, and a vent stack. The first absorber is fitted with a lower section of ceramic packing which cools the 500° F flue gas to saturation temperature prior to its entry into the acid absorption section, and the second absorber removes residual HCl and CI2 .

Figure 2A: Equilibrium Constant vs. Temperature

Acid Absorber

H2O

10

Gas or Liquid - High Levels of NOx

Figure 3 is a block diagram of a two-stage combustion process to dispose of either a gas or liquid that, if oxidized in a single-stage combustion process, would produce a flue gas containing excessive amounts of NOx. It consists of the following components:

A reduction furnace in which a high-temperature reducing (less than stoichiometric air) environment converts the fuel into H2 , H2O, CO, and CO2 , and the NOx present into N2 .

• A quench section which cools the flue gas to approximately 1400° F by directly contracting it with a cool recycle gas.

• A ReOx furnace which converts the H2 to H2O and CO to CO2 .

• A heat recovery boiler which produces steam in cooling the flue gas to 350° F

• An vent stack.

Figure 2B : Waste Process

Fuel

Combustion Air

T. O. Boiler

Steam

Flue Gas

Stack

Scrubber

Absorber

Make-Up Caustic Solution

Salt Solution

Acid

Quench Section

Waste Liquid (Exothermic)

Waste Liquid (Exothermic)

When the waste stream is highly exothermic, a cooling medium such as air or water or steam is added to the T.O. to control the flue gases to the boiler.

Figure 2B is an equipment representation of the system shown in Configuration 2.2., consisting of a horizontal T.O., firetube boiler, quench column, acid-absorber, caustic scrubber, and and integral stack.

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Recycle flue gas cooling in lieu of steam or water is an integral part of this process and helps maximize heat recovery.

The purpose of the cooling step between the reducing stage and re-oxidation stage is to lower the fine T.O. temperature. As shown in Figure 3A, a plot of NOx concentration versus temperature, the amount of thermal NOx produced is a function of operating temperature and the amount to excess oxygen. For example, when 2% O2 is present, an operating temperature of 1600° F has an equilibrium NOx value of 42 ppm V; and at 2000° F, it has NOx value of over 200 ppm V. Thus, it is desirable to operate at the lowest practical temperature. Another consideration is to oxidize the H2 and CO present to meet air quality regulations. The design becomes a trade-off between the amount of H2 and CO allowed and the amount of NOxallowed in the final product of combustion. Figure 3B is a schematic of a NOxIDIZER® system.

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WASTE EXAMPLE PRODUCTS OF OXIDATIONTail Gas Organic Acid


Figure 3 : Waste Process

Stack

Gas Liquid

ReOx Furnace

Flue Gas

Boiler

Steam

Reduction Furnace Quench Section

Recycle Flue Gas

Air

Waste

Fuel

Air

CO2

H2

CO H2O N2

Gas or Liquid Waste - Produces Cl2/HCl and NOx

Figure 4 is a example of a process to dispose or either a gas or liquid that produces a flue gas containing Cl2/HCl and excessive amounts of NOx. It consists of several equipment systems as follows:

• A reduction furnace in which a high-temperature reducing environment converts NOx to N2 , Cl to HCl and fuel to H2, H2O, CO and CO2.

• A quench section which cools the water gas to approximately 1400° F by directly contacting it with recycle gas.

• A T.O. which converts the H2 to H2O, CO to CO2 and allows the HCl concentration to reach equilibrium.

• An adiabatic absorber fitted with a lower section of ceramic packing which cools the 500° F flue gas to saturation temperature prior to its entry into the acid absorption section which removes the inorganic acids.

• An vent stack.

Recycled flue gas cooling is an integral part of this process to maximize heat recovery.

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Figure 3B : NOxIDIZER® System

WASTE EXAMPLE PRODUCTS OF OXIDATIONTail Gas Organic Acid


Stack

Gas Liquid

ReOx Furnace BoilerReduction Furnace

Steam

Recyled Flue GasCombustion Air

Fuel

Steam

Feed Water

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If the chlorine-bearing waste steams are separate from nitrogen-bearing waste streams, the chlorine stream can be admitted to the T.O. directly. The segregation of streams would result in a smaller reduction furnace and, for endothermic wastes, would reduce the amount of auxiliary fuel used.

Gas or Liquid Waste - High Levels of Particulates

The following three equipment examples show configurations of a process to dispose of either a gaseous or liquid waste which produces flue gas containing excessive amounts of particulate matter.

Configuration 5.1 consists of the following equipment systems:

• A T.O.

• A quench section which cools the flue gas to its saturation temperature by directly contacting it with water.

• A wet scrubber which removes the particular matter.

A vent stack.

A major advantage of the wet scrubber is its ability to remove both particulates and any corrosive gases (SO2, HCl) in a single operation.



LIQUID/SOLID NaCI SOLUTION POLYPROPYLENE/CATALYST

FG, NOXA, PARTICULATE

STACK

FLUE GAS

2

FUEL

WASTE

AIR

T. O. WET SCRUBBER

SALT SOLUTION OR SUSPENSION

QUENCH

H O2

STACKStack

WASTE EXAMPLE PRODUCTS OF OXIDATIONChlorinated Amine FG, Cl2/HCl, NOX


Liquid

ReOx Furnace

Flue Gas

Reduction Furnance Quench Section Boiler

SteamRecycled Flue Gas

AirWaste

Fuel

Air

CO2

H2

CO H2O N2

Acid Or Salt Absorber

H2O Or Caustic

Configuration 5.2 consists of the following: • A T.O. • A conditioning tower which, by directly contacting with water cools the flue gas either 600° F or 350° F, depending upon the dry particulate removal system selected. • An electrostatic precipitator (ESP) or a baghouse • A vent stack

Configuration 5.3 consists of the following major components: • A T.O. • A conditioning tower fitted with a SaltMaster TM system which lowers the flue gas to below salt fusion temperature by directly contacting it with recycle flue gas. • A heat recovery boiler which produces steam in cooling the flue gas to 350o F. • Either an ESP or baghouse for particulate removal. • And an unlined vent stack.The SaltMasterTM system keeps the salt building up in the bottom of the conditioning chamber Salt build-up can cause operating and maintenance problems. Recycle gas is used for cooling to maximize heat recovery.

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Figure 5.2 : Waste Process

FLUE GAS

Figure 5.3 : Waste Process

WASTE EXAMPLE PRODUCTS OF OXIDATIONNaCI SOLUTION POLYPROPYLENE/CATALYST

FG, NOXA, PARTICULATELIQUID / SOLID

ESP

DRY SALT

DRY SALT

STACK

THERMAL OXIDIZER

CONDITIONING TOWER SaltMaster TM

350° FAIR AND / OR H2O

AIR WASTE INJECTION

BAG HOUSE

FUEL

SALT SOLUTION OR SUSPENSION

OR

STEAMBOILER

WATER

WASTE EXAMPLE PRODUCTS OF OXIDATIONNaCI SOLUTION POLYPROPYLENE/CATALYST

FG, NOXA, PARTICULATELIQUID / SOLID

FLUE GAS

STACK

THERMAL OXIDIZER

CONDITIONING TOWER

600° F

OR 350° F

AIR AND / OR H2O

AIR WASTE INJECTION

FUEL

DRY SALT

BAG HOUSE

ESP

DRY SALT

Figure 5A is a schematic of a down-fired salt system with a wet particulate removal system (High Velocity Scrubber). Alternatively, a venturi scrubber may be used. The schematic shown in Figure 5B is of a down-fired salt system with heat recovery.

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Figure 5A : Down-Fired Salt System

Natural Gas Fuel Oil or Waste Liquid (Exothermic)Air or Steam Atomization

Waste Liquid Injection (Exothermic)

Air or Steam Atomization

Make-Up Water

Thermal Oxidizer

Burner Assembly

Combustion Air Blower

Quench Pot

Acid or Salt Solution or Suspension

Vent Stack

High Velocity Scrubber

Make-Up Water

Mist Eliminator

Figure 5B : Down-Fired Salt System with Heat Recovery

Natural Gas Fuel Oil or Waste Liquid (Exothermic)Air or Steam Atomization

Waste Liquid Injection (Exothermic)

Air or Steam Atomization

Thermal Oxidizer

Burner Assembly

Combustion Air Blower

Salt Master TM

Conditioning Tower Salt Solution

and / or Suspension

Steam Make-Up Water

Recycled Gas

Vent Stack

BagHouseEconomizerBoiler

Dry Salt

Recycled Gas

Soot Blowers

Waste Containing Combustible Fine Solids -Acceptable Levels of NOx and/or SOx

The following two block diagrams show configurations of a process to dispose of a waste-containing combustible fine solids (less than 500 microns particle size), which produces flue gas containing acceptable amounts of SOx and/or NOx.

Configuration 6.1 consists of a cyclonic T.O. in which a high radial gas velocity causes the denser solid particles to be preferentially “slung” to the wall, thus markedly increasing their retention time.

Configuration 6.2 shows a cyclonic T.O. fitted with a heat recovery boiler which produces steam to lower the flue gas temperature to 350° F.

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GAS / SOLID CO + H2/C FG, NOXA


FUEL

AIR

STACKCYCLONIC T.O.

FLUE GASHOPPER


GAS / SOLID


FUEL

AIR

STACK

FLUE GAS

BOILER

STEAM

HOPPER

CYCLONIC T.O.

CO + H2/C FG, NOXA

18

Gaseous Wastes Containing Combustible Fine Solids -Acceptable Levels of NOx and/or SOx

The following two diagrams illustrate configurations of a process to dispose of a gaseous waste containing a combustible fine solid (less than 500 microns) which produces flue gas containing acceptable amounts of SOx and/or NOx and excessive amounts of particulate.

Configuration 7.1 consists of the following equipment: • A cyclonic T.O. • Either a quench column, which by directly contacting the flue gas with water, cools it to its saturation temperature; and a wet scrubber which removes the particular matter or a conditioning tower, which by directly contacting the flue gas with water and/or air cools it to either 600° F to 350° F, depending on the dry particulate removal system selected. • An ESP or baghouse. • An unlined vent stack.



GAS / SOLID SOLID

CO + H2/C + ASH COAL FINES

FG, NOXA,PARTICULATEFG, NOXA,PARTICULATE

FUEL

AIR

STACK

CYCLONIC T.O.

FLUE GAS

HOPPER

DRY ASH

ESP

DRY ASH

BAG HOUSE

Quench

H O2

Wet Scrubber

Make-Up Water

Conditioning Tower

OR

350° F

Configuration 7.2 consists of the following major equipment: • A cyclonic T.O. • A hot cyclone for large particulate removal and/or conditioning tower which by directly contacting the flue gas with recycle gas cools it to below ash fusion temperature. • A heat recovery boiler which produces steam in cooling the flue gas to 350 F. • Either an ESP or baghouse for particulate removal. • An unlined vent stack.Recycle gas is used for cooling to maximize heat recovery.

Wastes Containing Combustible Solids

Figure 8 is a diagram of a process to dispose of a waste that contains combustible solids in the particle size range of 10 to 500 microns that produces a flue gas containing excessive amounts of NOx. It consists of the following major components:

• A cyclonic reduction furnace in which a high radial velocity, high temperature, reducing (less than stoichiometric air) environment converts the bound nitrogen to N2 and the fuel to water gas.

• A quench section which cools the water gas to approximately 1400° F.

• A heat recovery boiler which produces steam in cooling the flue gas to 350° F.

• An unlined vent stack.

Recycle flue gas cooling is an integral part of the process to minimize NOx formation and maximize heat recovery.

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GAS / SOLID SOLID


STACK

FLUE GAS

BOILER

STEAM

FUEL

AIR

HOPPER

CYCLONIC T.O.

CO + H2/C + ASH COAL FINES

FG, NOXA, PARTICULATEFG, NOXA, PARTICULATE

HOT CYCLONE

DRY ASH

ESP

DRY ASH

BAG HOUSE

CONDITIONING TOWER

Recycled Gas

350° F

350° F

ORDRY ASH

WASTE EXAMPLE PRODUCTS OF OXIDATIONMELAMINE SLURRYDNT CELLULOSE

FG, NOx FG, NOx


STACK

GAS / SOLID SOLID

T.O

FLUE GAS

BOILER

STEAM

CYCLONIC REDUCTION FURNACE QUENCH

RECYCLED GAS

AIR

FUEL

AIR

CO2

H2

CO H2O N2

HOPPER

Water Gas

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SUMMARY

The description of pollutant control processes found in this paper is a tool which can be used to identify the basic process needed to destroy pollutants in various types of waste streams. Although this “cookbook” approach is a simplified version of the real world method of specific equipment selection, it provides a good general understanding of what process is best-suited for the destruction of various pollutants found in today’s industries.

Documents

Hazardous Waste Disposal with Thermal Oxidation · OXIDATION HAZARDOUS WASTE DISPOSAL BY THERMAL OXIDATION. ... exchanger heats incoming combustion air or waste gases, and the boiler