PollutionEngineering September 2012 : Page 14
OVERCOME COMPLEX Incinerating Residue Issues R id Residues created t d during d i incineration i i ti of f certain t i liquids and gases can result in complex problems to overcome. However, such problems are manageable. By KARL-HEINZ MAIER P r roduction processes in the chemical and pharmaceutical c industries give rise to gaseous i and liquid residues that are a preferably disposed in decen-p tralized incinerators. Most of these resi-dues are highly caloric gases and solutions containing high concentrations of organic substances. The complexity of these often-toxic substances requires the use of special equipment for the incineration process. tal halogens. The combustion chamber The disposal of halogenated exhaust gases and liquids usually involves the use of combustion chambers lined with ceramic bricks. As this ceramic lining is not imper-meable, the hydrogen halides penetrate the brickwork of the combustion chamber Incineration of halogenated residues The production processes, which give rise to halogenated residues, include the man-ufacturing or production of: • Plant protection products and pesticides • Epoxy resins • Vinyl chloride and dichloroethane • Coolant • Chlorinated solvents, such as methylene chloride and trichloroethylene E.U. Directive 2000/76/EC prescribes a minimum temperature of 1,100°C for a residence time of at least 2 seconds for the incineration of halogenated residues. The incineration of halogenated substances gives rise to normal oxidation products, such as CO 2 and water, as well as inorganic substances: hydrogen halides and elemen-14 Pollution Engineering SEPTEMBER 2012 Figure 1 shows the equilibrium constants for various halogens. and thus contact the metallic combus-tion chamber jacket. The inner lining and the outer insulation of the combustion chamber must therefore be designed to prevent the temperature of the hydrogen halides from falling below the dew point. External insulation is often dispensed and the outside of the combustion chamber is provided only with a metallic perforated sheet to protect against accidental contact. This concept enables simple temperature monitoring on the steel jacket, and thus, the early detection of damage to the refrac-tory lining. As a rule, the ceramic lining is made from multiple layers of oxide ceramics applied to the inside of the combustion chamber. Bricks with a high content of aluminum oxide are used for the layers exposed to the incineration process. When specifying the dimensions of the lining, it must be ensured that a ceramic lin-ing is not resistant to hydrogen fluoride. Hydrogen fluoride reacts with the silicon dioxide (SiO 2 ) in the refractory lining, destroying the material in the long term. The lining of a combustion chamber that is used to dispose of substances containing fluoride has only a limited service life. Gaseous halogenated contaminants are introduced into the combustion chamber
Overcome Complex Incinerating Residue Issues
Residues created during incineration of certain liquids and gases can result in complex problems to overcome. However, such problems are manageable.
Production processes in the chemical and pharmaceutical industries give rise to gaseous and liquid residues that are preferably disposed in decentralized incinerators. Most of these residues are highly caloric gases and solutions containing high concentrations of organic substances. The complexity of these oftentoxic substances requires the use of special equipment for the incineration process.
Incineration of halogenated residues
The production processes, which give rise to halogenated residues, include the manufacturing or production of:
• Plant protection products and pesticides
• Epoxy resins
• Vinyl chloride and dichloroethane
• Chlorinated solvents, such as methylene chloride and trichloroethylene
E. U. Directive 2000/76/EC prescribes a minimum temperature of 1,100°C for a residence time of at least 2 seconds for the incineration of halogenated residues. The incineration of halogenated substances gives rise to normal oxidation products, such as CO2 and water, as well as inorganic substances: hydrogen halides and elemental halogens.
The combustion chamber
The disposal of halogenated exhaust gases and liquids usually involves the use of combustion chambers lined with ceramic bricks. As this ceramic lining is not impermeable, the hydrogen halides penetrate the brickwork of the combustion chamber and thus contact the metallic combustion chamber jacket. The inner lining and the outer insulation of the combustion chamber must therefore be designed to prevent the temperature of the hydrogen halides from falling below the dew point.External insulation is often dispensed and the outside of the combustion chamber is provided only with a metallic perforated sheet to protect against accidental contact.This concept enables simple temperature monitoring on the steel jacket, and thus, the early detection of damage to the refractory lining.
As a rule, the ceramic lining is made from multiple layers of oxide ceramics applied to the inside of the combustion chamber. Bricks with a high content of aluminum oxide are used for the layers exposed to the incineration process. When specifying the dimensions of the lining, it must be ensured that a ceramic lining is not resistant to hydrogen fluoride.Hydrogen fluoride reacts with the silicon dioxide (SiO2 ) in the refractory lining, destroying the material in the long term.The lining of a combustion chamber that is used to dispose of substances containing fluoride has only a limited service life.
Gaseous halogenated contaminants are introduced into the combustion chamber via lances. To achieve a good degree of mixing, larger volume flows are distributed between several lances positioned evenly throughout the combustion chamber.Liquids must be atomized through nozzles. Ultrasonic nozzles have proven their worth in this area. The liquid flows through the nozzle from a central hole. The atomizing medium (steam or air) is introduced via an outer round duct and generates an ultrasonic vibration field as it leaves the nozzle. As it leaves the nozzle, the liquid passes through the ultrasonic field and is atomized into fine droplets. Ultrasonic nozzles are relatively insensitive to abrasion and wear during operation.
To ensure rapid and complete oxidation of the organic components, all substances inside the combustion chamber must be intensively mixed. Many burner systems produce a marked rotation of the flue gases in the combustion chamber. This rotation of the flue gases is also additionally supported by introducing liquids or gases at a tangent.
Waste heat utilization
The heat of the flue gases can be used after the incineration process in a waste heat boiler to produce steam or to heat thermal oil. Determining the operation conditions of the boiler and selecting suitable materials must be done in consideration of the corrosive behavior of the halogen compounds contained in the flue gas. Flue gas boilers made from carbon steel have been proven in systems for the incineration of chlorinated hydrocarbons. Operating the boiler on the steam side at operating pressures of about 16 bar creates pipe wall temperatures of around 200°C, largely suppressing the corrosion caused by hydrogen chloride (HCl). At temperatures greater than 300°C, increasing levels of high-temperature corrosion takes place.
Flue gas scrubbing
As a rule, the halogen compounds formed during incineration must be separated before the flue gas can be released into the atmosphere. Besides hydrogen halides, elemental halogens can also be formed, especially during the incineration of chlorine and bromine compounds. The rate that elemental halogens form depends on the absolute concentration of halogens and the incineration conditions. This dependency is called the Deacon equilibrium.The equilibrium constant of the formation reaction is heavily dependent upon temperature. A high incineration temperature reduces the content of elemental halogens. The formation of halogen can also be minimized by running the incineration process at the lowest possible oxygen content and with a high proportion of steam. Figure 1 shows the equilibrium constants for various halogens according to Leite in 2002.
The halogen compounds can be separated from the flue gas in a wet flue-gas scrubber.In its simplest form, this consists of a quench and a scrubbing column. A good overview of the wet flue-gas scrubber's equipment design can be found in Lehner 2003.
While hydrogen halides can be scrubbed with water, an alkaline environment and the addition of reduction agents are required for a quantitative separation of the elemental halogens. Equations 1 to 3 describe the neutralization reaction on the example of Hcl and elemental chlorine.
(1) Hcl + NaOH => NaCl + H2O
(2) Cl2 + 2 NaOH => NaCl + NaOCl + H2O
(3) NaOCl + NaHSO3 + NaOH => NaCl + Na2SO4 + H2O
Caustic soda converts Hcl entirely into common salt. The neutralization of elemental chlorine produces common salt and sodium hypochlorite. High concentrations of hypochlorite in the scrubbing solution restrict the absorption of further elemental chlorine. The hypochlorite can be converted to common salt through the addition of a reduction agent, such as sodium hydrogen sulfite.
The favorable water solubility of the hydrogen halides enables both extensive scrubbing with water without the addition of neutralization agents as well as the concentration and production of a concentrated aqueous acid. Figure 2 shows a flow diagram for the incineration of waste containing chlorine followed by Hcl recovery.
Exhaust gases and residual liquids containing chlorine are incinerated in a combustion chamber at a temperature of at least 1,100°C. The energy of the hot flue gases is used in a waste-heat boiler to produce steam. The cooled flue gases are then passed to a wet scrubber, which is equipped with an Hcl recovery system.The flue gases are first brought into contact with acid cleaner in a quench, where they are cooled and saturated with steam. Next, the mixture of acid cleaner and flue gas is further cooled in a heat exchanger that is exposed to coolant. The intensive contact between the acid cleaner and flue gas on the way through the heat exchanger also causes a marked exchange of substances, whereby most of the Hcl contained in the flue gas is absorbed by the acid cleaner. The cold two-phase mixture then flows into a tank where it is separated. A portion of the concentrated acid generated in this way can be taken from the tank as product.
The cooled, pre-cleaned flue gas is fed into an absorption column, where the remaining Hcl is further scrubbed with water. The dilute acid produced in the absorption column is passed to the quench tank. In order to produce the highest possible concentration of product acid, the absorption column can be equipped with several scrubbing circuits. After the absorber, any remaining Hcl and elemental chlorine are removed in an alkali-based scrubber column. The cleaned flue gas can now be released to the atmosphere.
The absorption of Hcl in water is a purely physical process. The achievable concentration of acid is thus primarily dependent upon the temperature and Hcl concentration in the flue gas. While the Hcl concentration is largely predetermined by the level of chlorine in the waste to be disposed, the absorption temperature can be influenced through specific flue gases cooling. This method enables a marketable 30 percent hydrochloric acid to be produced with an adequate chlorine content and corresponding cooling of the flue gases.
Also, heavy flue gas cooling gives rise to the risk work of forming Hcl aerosols.The Hcl-water system has an azeotropic point with a marked steam pressure minimum.For this reason, local oversaturation and associated aerosol formation must be taken into account during any significant cooling of the flue gases. The formation of aerosols during Hcl absorption is exhaustively described by Schaber in 1987. Hcl aerosol droplet sizes falls in the range from 0. 5 to 2 ƒÊm, thus the droplets cannot be separated in technical absorbers. Electrical filters, venturi devices and rotation scrubbers have proven effective as separators.
Figure 3 is an exhaust air scrubber that was installed in shows an exhaust air scrubber at Lanxess, India by Durr Systems Inc.
Incineration of exhaust gases containing silane
The manufacturing of ultra-pure silicon or silicon compounds gives rise to exhaust gases containing silicon and occasionally chlorine, for which incineration is often the only means of environmentally friendly disposal. Silicon is mainly used in metallurgy as an alloying component for steels and as a base material for the manufacturing of silicone. Ultra-pure silicon is needed for the production of solar cells and in the microelectronics sector for manufacturing computer chips, transistors and memory chips.
During incineration, the silicon compounds are split, and the silicon is converted to SiO2. If the exhaust gases also contain chlorosilanes, then Hcl and elemental chlorine are also produced. As silicon compounds react to each other and to water, individual exhaust gas flows cannot, as a rule, be merged, but must be separately fed into the combustion chamber.
The SiO2 formed exists in the flue gas as an extremely lightweight, fine white dust. The incineration temperature in the combustion chamber must be kept below 1,000°C. At higher temperatures, the SiO2 tends to bake on and form glassy coatings. To ensure that the dust is removed, the combustion chamber is arranged vertically and fired from above. Vortex burners are used. This ensures the operation is free from encrustation that requires a good mixing of the flue gases and suitable air circulation that avoids localized temperature spikes.
For the incineration of flue gases that contain chlorosilane, the hot, dust-laden exhaust gases are cooled downstream of the combustion chamber to approx. 180 to 200°C through the admixture of water and air. Attempting to recover heat through cooling in a steam boiler is deliberately avoided here as this equipment is extremely difficult to operate due to the very special dust loading. The cooled flue gases are passed to a hose filter. The gases flow through the bags from the top downward to simplify separation of the light fine dust (bulk density approx. 50 kg/m3). After the dust has been removed, the flue gases are passed from the hose filter to a wet scrubber, where they are further treated to remove Hcl and elemental chlorine. The wet scrubber comprises a jet quench for cooling and saturating the flue gases as well as a two-stage packed bed scrubber. Caustic soda is added to the lower scrubbing circuit to neutralize Hcl. Hydrogen peroxide is added as a reduction agent. The upper scrubbing stage is operated with fresh water. The cleaned flue gases are released to the atmosphere via a forced draft blower.
Incineration of liquids containing salt
Salty liquids loaded with organic substances are used in a range of production processes in the chemical and pharmaceutical industries. In most cases, the presence of salts attacks the ceramic lining of the combustion chamber. At temperatures of 1,000 to 1,200°C, the salts are present partly in gaseous and liquid form or as solid dust particles. The salts penetrate the pores of the ceramic lining and diffuse towards the cold outer wall. Most salts condense in the temperature range from 650 to 750°C, which seals the pores.Reactions with the bricks or temperature fluctuations can result in volume changes that lead to increased stresses within the lining. These stresses create new cracks allowing more salt to penetrate. This procedure repeats until larger cracks appear and parts of the lining flake off. This phenomenon is called alkali bursting.
Alkali bursting is particularly marked with sodium and potassium salts. Calcium compounds, such as CaSO4 and CaCl2, cause only minor changes in volume and damage the lining to a much lesser extent.
Another type of damage to the lining is caused by the formation of eutectics. The salt melts that flow off the lining can form, together with the lining material, a eutectic that has a lower melting point than the salt. This formation also leads to flaking and wear of the lining.
Combustion chambers for salt incineration are arranged vertically and fired from the top in order to enable the discharge of the salts and drainage of salt slag. If a waste heat boiler is to be installed downstream of the combustion chamber for the purpose of heat recovery, then the boiler must be designed as a water pipe boiler, due to the high risk of encrustation, and equipped with dedusting devices, such as soot blowers. Heat recovery is frequently dispensed with and the flue gas is passed directly into a quench downstream of the combustion chamber. The use of dip quenching has proven suitable for the salt incineration process.
Contact Marta Kelly, marketing, Dürr Systems Inc., for more information. She can be reached at (734) 254-2418 or by email at email@example.com..
1. Leite Olavo C., Petrochemicals and Gas Processing, PTQ Autumn 2002, p. 157 165
2. Lehner M. und Hoffmann A., Chem. Ing. Tech. 5/2003
3. Schaber K., Chem. Ing. Tech. 5/1987 p. 376-383
4. Puppich P. und Hoffmann A, Verbrennung und Rauchgasreinigung von silan haltigen Abgasen, JT des ProcessNet-FA Hochtemperaturtechnik, February 2011 Frankfurt
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