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 dichloro-ethane
- Coolant
- Chlorinated solvents, such as methylene chloride and trichloroethylene
EU Directive 2000/76/EC prescribes a minimum temperature of 1100°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 carbon dioxide and water, as well as inorganic substances: hydro-gen 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 created penetrate the brickwork of the com-bustion chamber and thus also come into contact with 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 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 through 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. The atomization therefore takes place outside of the nozzle itself. Ultra-sonic 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 approx. 16 bar creates pipe wall temperatures of around 200°C, largely suppressing the corrosion caused by hydrogen chloride. At temperatures greater than 300°C, increasing levels of high-temperature corrosion takes place.
The utilization of waste heat is generally avoided when incinerating waste with a high level of fluorine due to the severe problem of corrosion.
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 at which 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.
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. The hot flue gas is brought into contact with water in the quench. This cools the flue gas and saturates it with steam. The saturated flue gas is then fed to a scrubbing column, where an aqueous scrubbing solution is pumped around and absorbs the halogen compounds from the flue gas. A good overview of the equipment design of wet flue gas scrubbers can be found in Lehner /2/.
While hydrogen halides can be scrubbed well 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 below describe the neutralization reaction on the example of hydrogen chloride and elemental chlorine.
HCl + NaOH => NaCl + H2O (1)
Cl2 + 2 NaOH => NaCl + NaOCl + H2O (2)
NaOCl + NaHSO3 + NaOH => NaCl + Na2SO4 + H2O (3)
Caustic soda converts hydrogen chloride 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.
HCl recovery
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.
Exhaust gases and residual liquids containing chlorine are incinerated in a combustion chamber at a temperature of at least 1100°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 cooled further in a heat exchanger exposed to coolant. The intensive contact between acid cleaner and flue gas on the way through the heat exchanger also causes a marked exchange of substances, whereby most of the hydrogen chloride 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 con-centrated acid generated in this way can be taken from the tank as product.
The cooled, precleaned flue gas is fed into an absorption column, where the remaining hydrogen chloride 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 hydrogen chloride and the elemental chlorine are removed in an alkali-based scrubber column. The cleaned flue gas can now be released to the atmosphere.
The absorption of hydrogen chloride 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 cooling of the flue gases. This method enables a marketable 30% hydrochloric acid to be produced with an adequate chlorine content and corresponding cooling of the flue gases.
However, heavy cooling of the flue gases also gives rise to the risk of the formation of hydrochloric acid aerosols. The hydrogen chloride-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 /3/. The droplet size of HCl aerosols falls in the range from 0.5 to 2 µm, thus the droplets cannot be separated in technical absorbers. If HCl aerosols are emitted with the flue gas, they can be observed as a blue mist at the incinerator, even at low concentrations. For the separation of aerosols, the flue gas scrubber must be additionally equipped with suitable apparatus. Electrical filters, venturi devices and rotation scrubbers have proven effective as separators for HCl aerosols.
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 the manufacturing of computer chips, transistors and memory chips. Capacities for the manufacturing of ultra-pure silicon have been sharply expanded in recent years.
During incineration, the silicon compounds are split, and the silicon is converted to silicon dioxide. If the exhaust gases also contain chlorosilanes, then hydrogen chloride and elemental chlorine are additionally produced. As silicon compounds react to each other and to water, individual exhaust gas flows cannot as a rule be merged, but must be fed into the combustion chamber separately.
The silicon dioxide formed exists in the flue gas as an extremely lightweight, fine white dust. The incineration temperature in the combustion chamber must be kept below 1000°C. At higher temperatures, the silicon dioxide 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 as burners. This ensures operation that is free from encrustation that requires a good mixing of the flue gases and suitable air circulation that avoids localized spikes in temperature.
Fig. 5 shows the process diagram of a system for the incineration of flue gases that contain chlorosilane. The hot, dust-laden exhaust gases are cooled down-stream of the combustion chamber to approx. 180-200°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 treated further to remove hydrogen chloride 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 hydrogen chloride. Hydrogen peroxide is additionally 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. /4/
Incineration of liquids containing salt
Salty liquids loaded with organic substances arise in a wide range of production processes in the chemical and pharmaceutical industries:
- Manufacturing of pharmaceutical materials
- Production of animal feed additives
- Systems for the manufacturing of propylene oxide
- Waste handling in the paper and cellulose industry
In most cases, the presence of salts attacks the ceramic lining of the combustion chamber. At temperatures of 1000-1200°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 wall of the apparatus. Most salts condense in the temperature range from 650 to 750°C, which seals the pores. Reac-tions with the bricks or temperature fluctuations can result in volume changes that lead to increased stresses within the lining. These stresses cause new cracks into which further salt can 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 form, together with the lining material, a eutectic that has a lower melting point than the salt. This formation of eutectics also leads to flaking and wear of the lining.
Depending on the concentration and nature of the available salt compounds, the alkali bursting, and the formation of eutectics means that more or less severe damage to the lining must be taken into consideration. This results in a limited service life and an associated increase in maintenance costs.
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. The hot flue gas is passed via a dip pipe into a water tank arranged directly beneath the combustion chamber. This abruptly cools the flue gas and saturates it with steam. Salts contained in the flue gas are dissolved in the scrubbing water. Salt melts that have formed in the combustion chamber are also removed through the dip pipe and dissolved in the scrubbing water. After the quench, a small amount of the salts remains in the flue gas as a salt aerosol and must be eliminated separately. Venturi devices and electric filters have proven suitable for the separation of salt aerosols.
If waste water with a high salt content or alkalis is incinerated, the damage to the liner can become so intense that the service life of the lining amounts to just weeks or months. In these instances, having a ceramic lining in the combustion chamber is not an economic solution. A cooled combustion chamber construction, which can be operated without a ceramic inner lining, must be avoided here. The combustion chamber is made from membrane wall segments or executed as a cylindrical double-jacket construction. The gap contains boiling water, which cools the inner wall of the combustion chamber and protects it against overheating. The salt melts that run off the lining solidify on the cool wall, covering it with a protective layer of salt. The salt layer can reach thicknesses of up to 5 cm. Parts of the salt layer constantly burst off during operation and are replaced by new solidifying salt melts. The transmission of heat through the wall does not, therefore, remain constant, but varies with the condition of the salt layer at any given moment. The need for additional fuel thus also fluctuates with the removal of heat.
Bibliography
/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