Air Pollution Control Equipment Selection Guide, 2nd Edition

145

16

Thermal Oxidizers

Dan Banks

Banks Engineering, Inc., Tulsa, Oklahoma

16.1 DEVICE TYPE

Thermal oxidizers (TOX) are used to destroy objectionable hydrocarbons contained

in waste streams from manufacturing plants. The wastes may be solids, liquids, or

vapors. They are usually generated continuously—otherwise landll may be eco-

nomically preferred for solids and liquids, while emergency ares might be preferred

for destruction of many waste gases. Thermal oxidizers are designed to use heat

energy to convert hydrocarbon contaminants to carbon dioxide and water vapor, and

contaminant metals to their oxide form, under controlled conditions.

16.2 TYPICAL APPLICATIONS

Thermal oxidizers are used to control combustible contaminant emissions from

dozens of sources. Major areas include printing operations; chemical and hydro-

carbon processing; painting, coating, and converting; distillation; sludge drying;

soil remediation; plasticizer emissions control; extruder emissions; and textile

manufacturing.

They are often used after wet scrubbers where the gas stream contains both water-

soluble and hydrocarbon emissions. They are often followed by wet scrubbers where

the volatile organic compound (VOC) is halogenated and, upon combustion, can

form inorganic acids such as hydrochloric acid (HCl).

In general, if the source emits a combustible VOC that is not economical to

recover, it is a candidate for control by a TOX.

16.3 OPERATING PRINCIPLES

A TOX simply heats the waste material in the presence of air to allow the hydrocar-

bon molecules present to burn (oxidize at elevated temperature). The simplest TOX

consists of a burner, a holding chamber (furnace), and a stack (to duct the combus-

tion products to atmosphere). Furnace temperature can range from 500 to 2500°F,

depending on TOX design and the degree of hydrocarbon destruction needed. If 99%

of the incoming hydrocarbons are destroyed, the TOX efciency is 99% (expressed

as 99% destruction and removal efciency, or 99% DRE). Usually natural gas or

other auxiliary fuel is ignited in the burner to heat up the TOX and often to supple-

ment the heating value of the waste stream(s) to ensure proper temperature control.

146 Air Pollution Control Equipmenet Selection Guide

If the waste is rich in hydrocarbons, extra air or sometimes water sprays are used

to prevent overheating. Various methods have been developed to reduce fuel usage,

keep generation of NO

x

and other pollutants low, recover available heat from the

combustion products, and remove any particulate or acid gas (HCl, SO

2

) formed

during waste destruction.

To make the best use of this application of heat energy, the thermal oxidizer is

usually lined with insulating refractory material.

Figure 16.1 shows a thermal oxidizer used for the control of noncondensable

gases from a paper pulp mill. The unit consists of a specially designed burner, burner

controls, insulated combustion chamber, and temperature controls.

16.4 PRIMARY MECHANISMS USED

Reacting hydrocarbons with oxygen results in release of energy. An example is the

oxidation of natural gas (methane):

FIGURE16.1 Noncondensable gas thermal oxidizer (Banks Engineering, Inc.).