THERMAL OXIDIZERS – AN OVERVIEW
Thermal Oxidizers 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 landfill may be economically preferred for solids and liquids, while emergency flares might be preferred for destruction of many waste gases. Thermal oxidizers are design to use heat energy to convert hydrocarbon contaminants to carbon dioxide and water vapor, and contaminant metals to their oxide form, under controlled conditions.
Operating Principles:
A TOX simply heats the waste material in the presence of air to allow the hydrocarbon 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 combustion products to atmosphere). Furnace temperature can range from 500oF to 2500oF, depending on TOX design and the degree of hydrocarbon destruction needed. If 99% of the incoming hydrocarbons are destroyed, the TOX efficiency is 99% (expressed as 99% Destruction & Removal Efficiency or 99% DRE). Usually natural gas or other auxiliary fuel is ignited in the burner to heat up the TOX and often to supplement the heating value of the waste stream(s) to assure proper temperature control. 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 NOx and other pollutants low, recover available heat from the combustion products and to remove any particulates or acid gas (HCl, SO2) formed during waste destruction.
To make the best use of this application of heat energy, the thermal oxidizer is usually constructed of insulated refractory material.
Primary Mechanisms Used: High temperature oxidation.
Reacting hydrocarbons with oxygen results in release of energy. An example is the oxidation of natural gas (methane):
CH4 + 2O2 à CO2 + 2H2O (one molecule of methane, combined with two molecules of oxygen forms two molecules of carbon dioxide and two molecules of water vapor)
In reality, if air (79% nitrogen) is used to provide the oxygen, other gases go along for the ride:
CH4 + 2O2 + 7.5N2 à CO2 + 2H2O + 7.5 N2
This is the "balanced" stoichiometric equation for combustion of methane with air, and is typical of the combustion equation that is used in designing a TOX system for destruction of any hydrocarbon. When one pound of methane is burned in a TOX furnace, the product gases exit at much higher temperature – the net heat released by burning this one pound of hydrocarbon is 21,280 BTU. The methane reaction written above would produce products at over 3000oF, requiring special furnace construction, so extra air or water sprays (or a low heating-value waste stream) would be added in order to produce products at 2000oF or lower.
High temperature oxidation proceeds at a higher rate at higher temperatures, but as less and less of the subject hydrocarbon is left, the destruction rate slows. Operating the TOX furnace at a higher temperature increases the DRE in a given furnace, or allows use of a smaller furnace to achieve the original DRE. Some hydrocarbons are easy to destroy, requiring low temperatures and little retention time in the furnace (small furnace). Others require higher temperatures and longer reaction times for the same DRE. For instance, 99.99% DRE of hydrogen sulfide requires about 1300oF and 0.6 second retention time, while 99.99% DRE of dichloromethane requires about 1600oF and 2 seconds retention time.
If a chlorinated hydrocarbon is oxidized the raw "unbalanced" equation might look like this:
CH2Cl2 + O2 + N2 à CO2 + H2O + HCl + N2
In this case dichloromethane burns to produce carbon dioxide, water vapor and hydrochloric acid. If enough HCl is formed, discharge directly to atmosphere would not be permitted and the combustion products would be cooled and reacted with a chemical such as caustic (NaOH) to remove most of the HCl. The same is true when the waste contains hydrogen sulfide (H2S forms SO2 = sulfur dioxide). If the waste contains ash or dissolved solids (like salt) then the combustion products will contain particulate matter. Excessive particulate matter must be removed (venturi scrubber, electrostatic precipitator, bag filter, etc.) before the combustion products are discharged to atmosphere.
Design Basics:
A Thermal Oxidizer always includes these items:
1. Auxiliary fuel burner
2. Air source (blower or natural convection)
3. Furnace (temperature controlled chamber where the oxidation reactions occur)
4. Stack (to direct the combustion products to atmosphere)
5. Control system (to verify proper operation and control excursions)
Depending on the waste(s) to be treated, a TOX system can also contain:
1. Waste Heat Boiler (cools the combustion products, recovering the heat generated in the furnace by evaporating water to make steam for other uses)
2. Wet Scrubber (packed bed, venturi or spray scrubber, where acid gases and/or particles are removed from the combustion products)
3. Dry Scrubber (bag filter, electrostatic precipitator, etc., where particulate matter and sometimes acid gases are removed from the products)
4. NOx (nitrogen oxides) reduction hardware (catalytic, noncatalytic or wet scrubber NOx removal processes)
5. Preheat exchanger (usually shell/tube or plate/plate device with combustion products on one side and waste or combustion air on the other side, where heat is recovered and directed back into the TOX furnace to save auxiliary fuel)
6. Catalyst bed (speeds oxidation of particle-free waste gas, allowing lower operating temperature for the same DRE as a noncatalytic TOX)
7. Concentration methods to eliminate some of the inerts in a waste stream before sending the residual hydrocarbons to the TOX (often accomplished with a heat regenerated zeolite bed)
Direct thermal units burn fuel gas or fuel oil to assure waste ignition and maintain desired furnace temperature, when necessary. A "recuperative" TOX system adds a heat exchanger to transfer heat from the combustion products to the incoming waste gas or combustion air, reducing fuel consumption. Direct thermal TOX systems can be used to handle waste liquids and waste gases.
Catalytic TOX units may also fire fuel oil or fuel gas, but smaller ones may use electrical resistance heating instead. Catalyst reduces the temperature needed for a specific DRE, reducing fuel consumption. These units usually include a heat exchanger to further reduce fuel demand by transferring heat from the combustion products to the waste gas prior to furnace entry. Catalytic TOX units can be used to handle particulate-free waste gases containing small concentrations of hydrocarbons; excessive temperature and entrained dust interfere with the catalyst.
Regenerative Thermal Oxidizers (RTO) route the waste gas through packed beds for heat recovery, allowing very low fuel requirements, even for lightly contaminated waste air streams. RTOs are commonly used to treat large flows of air containing traces of hydrocarbons. Many operate with little or no auxiliary fuel due to the excellent heat recovery offered by packed beds, but waste containing too much hydrocarbon can overheat an RTO.
Operating/Application Suggestions:
· Claus sulfur recovery plants generate a waste gas containing H2S, CO, water vapor and inert gases. Waste flow is steady. TOX operation ranges from 1200oF to 1500oF with furnace retention time of 0.6 to 1.0 second. A vertical refractory lined furnace is often used, allowing a shorter stack to improve dispersion of the combustion products (which contain SO2). The furnace/stack generates draft, and burner operation does not need a combustion air blower. A waste heat recovery boiler may be added, in which case the furnace is horizontal and a combustion air blower is added.
· Pharmaceutical plants generate air-rich or nitrogen-rich waste gases from various batch reactors. Waste flow and composition may change suddenly, so burner control requires special care. A pharmaceutical TOX may operate at 1600-2000oF with 1 second retention time, depending on the waste components and performance required. A waste heat boiler and wet scrubber (for hydrochloric acid produced by combustion of chlorinated compounds) are often used.
· Kraft pulp mills generate several acidic waste gases during papermaking. Several of the waste streams can contain both oxygen and hydrocarbons, presenting flashback problems. Stainless burner and waste duct construction is common. A wet scrubber is used to remove SO2, which is generated during combustion of the H2S and similar compounds in the waste gas. A turpentine byproduct may be burned in special guns to reduce firing of natural gas or fuel oil.
· A TOX system must be designed to handle the full range of waste types, waste flows, and waste compositions. If errors are made, the system may run short of fuel, air, reaction volume, scrubbing capacity or other critical items. Poor waste destruction can result, but damage to the TOX unit or even upstream process equipment is certainly possible.
· The minimum operating controls needed are aimed at preventing thermal damage or explosions. A common design standard is provided by the National Fire Protection Association (NFPA). With wastes which vary in flow or heating value, additional controls may be required for quick adjustment of fuel or air to maintain on-spec operation at all times.
· High temperature operation requires special attention to the various refractories, stainless steels, paints and plastic used for construction, since an error in this area can quickly lead to catastrophic failure. Temperature control is always important, especially where catalyst is used to improve waste destruction, since excessive temperature can destroy catalyst quickly.
· Refractories can be damaged by abrupt temperature changes. Slow startups (200oF temperature rise per hour) are typical except where ceramic fiber blanket refractories are used. Periodic refractory inspection (usually once per year) is suggested, to allow repair of damage areas before they expand to create serious problems.
· Some wastes form SO2, HCl or other acidic compounds when burned. These are normally harmless when hot, but areas where the combustion products can cool to 200-300oF may be subject to severe corrosion if the acid gas "dewpoint" is reached. In units with acidic combustion products, the TOX furnace should be protected with weather shielding or be located in-doors. Wet scrubbers are often applied to TOX units to control the acid gases produced. If particulate is also present or is created through the combustion process, particulate control devices such as Venturi Scrubbers, Dry Scrubbers, or Wet Electrostatic Precipitators are often used.
· The presence of suspended ash, dissolved salts or other particulate producing compounds may require special design to avoid blinding of waste heat recovery surfaces, damage to refractory or excessive emissions.