Chemistry of inorganic compounds – With additive – Including anticaking or antihygroscopic function
Reexamination Certificate
2000-11-02
2002-11-19
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
With additive
Including anticaking or antihygroscopic function
C106S286700, C106S482000, C423S422000
Reexamination Certificate
active
06482379
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for removing hydrogen chloride from a flue gas.
2. Description of the Related Art
A device for treating of a flue gas containing hydrogen chloride is installed, for example, in a waste treatment equipment of subjecting waste to a incineration treatment. In this waste treatment equipment, a first dust collector and a second dust collector are arranged in series, and after removal of dust such as combustion fly ash contained in flue gas by the first dust collector, a dechlorination of flue gas is carried out in the second dust collector.
To carry out dechlorinating in the second dust collector, a dechlorinating agent is added into flue gas before the second dust collector. As a dechlorinating agent, a calcium-based dechlorinating agent such as calcium hydroxide (Ca(OH)
2
) has conventionally been mainly employed. Calcium hydroxide, if added into flue gas, reacts with hydrogen chloride (HCl) contained in the flue gas to generate residue of dechlorination containing calcium chloride (CaCl
2
), calcium oxide (CaO) and the like. However, thus generated residue of dechlorination is useful only in that calcium chloride is applied as a snow melting agent or a moisture absorbent, with a narrow range of effective uses. Residue of dechlorination is mostly solidified through a chemical treatment or with cement and disposed of in reclamation. However, acquisition of reclamation site is now becoming more difficult.
It is therefore proposed to use a sodium-based dechlorinating agent such as sodium hydrogencarbonate (sodium bicarbonate: NaHCO
3
) or sodium carbonate (soda ash: Na
2
CO
3
) in place of the calcium-based dechlorinating agent. In this case, when a sodium-based dechlorinating agent is added into a flue gas, hydrogen chloride contained in the flue gas becomes sodium chloride (NaCl). Adding water to residue of dechlorination dissolves sodium chloride. Therefore, water-soluble constituents dissolved in water are diluted and discharged, and only water-insoluble constituents not dissolved in water are separated and can be subjected to a combustion treatment in a melting furnace, thus eliminating the necessity of disposal in reclamation. When sodium hydrogencarbonate is adopted as a sodium-based dechlorinating agent and sodium hydrogencarbonate has a particle size larger than 30 &mgr;m, powder particles never coagulating together between them, and the agent is stable as powder. However, sodium hydrogencarbonate having a particle size larger than 30 &mgr;m leads to a very low removing ratio of hydrogen chloride, so that the use thereof as a dechlorinating agent is not appropriate. In general, therefore, sodium hydrogencarbonate ground to a particle size of 30 &mgr;m or less is used as a sodium-based dechlorinating agent.
However, when sodium hydrogencarbonate is ground to a particle size of 30 &mgr;m or less, coagulation of powder particles results in a form of fibrous dust balls or stone-like lumps. Ground sodium hydrogencarbonate has thus an unstable condition as powder, thus making is impossible to stably supply the same to flue gas.
To solve this defect, it is the usual practice to use an anti-caking agent. In the conventional art, a hydrophobic anti-caking agent has been used as such an anti-caking agent. A hydrophobic anti-caking agent brings about a remarkable solidification inhibiting effect. Sodium hydrogencarbonate added with a hydrophobic anti-caking agent has high flowability and floodability property, and exhibits satisfactory stability as a powder.
However, when a hydrophobic anti-caking agent added with sodium hydrogencarbonate is added into a flue gas as a dechlorinating agent, particles resulting from reaction with hydrogen chloride after calcination of sodium hydrogencarbonate, viz residue of neutralization, and particles of the anti-caking agent, which both have a high flowability, tend to easily entrap inside a filter cloth attached in a second dust collector comprising, for example, a bag filter. When particles of residue of neutralization or particles of the anti-caking agent penetrate into the filter cloth, there is caused clogging, resulting in an excessive pressure drop at the filter cloth and making it impossible to continue operation. It is difficult to recover from clogging even by back washing of the bag filter by the use of pulse air.
Further, some of particles produced from reaction with hydrogen chloride after calcination of sodium hydrogencarbonate, viz residue of neutralization, having entrapped into the filter cloth and particles of the anti-caking agent pass through the filter cloth, causing leakage of the dechlorinating agent and residue of neutralization. A double-woven glass cloth is usually used as a filter cloth. In order to prevent leakage of the dechlorinating agent and residue of neutralization, it is necessary to use a special filter cloth made by applying a Teflon membrane coated to the surface of the double-woven glass cloth. If the membrane is damaged or peeled off during use, however, a new problem of leakage of chemicals from this portion is encountered.
An object of the present invention is to prevent occurrence of an excessive pressure drop or leakage in the filter cloth attached to the dust collector.
SUMMARY OF THE INVENTION
For the purpose of solving the aforementioned problems, improvement is made for the sodium-based dechlorinating agent in the present invention. More particularly, the sodium-based dechlorinating agent of the invention comprises a mixture of sodium hydrogencarbonate and a hydrophilic anti-caking agent, and has an angle of repose of 40° or more, a dispersibility of less than 50, and a floodability index of less than 90.
According to the sodium-based dechlorinating agent having the above-mentioned configuration, in which the hydrophilic anti-caking agent has a slight cohesion, flowability of sodium hydrogencarbonate particles and the anti-caking agent becomes sluggish: sodium hydrogencarbonate particles or anti-caking agent particles never come in the filter cloth, and form a stable filtration layer on the surface of the filter cloth. It is consequently possible to prevent occurrence of an excess pressure drop in the filter cloth, and occurrence of leakage from the filter cloth.
Physical properties were measured using a powder tester Model PT-D made by Hosokawa Micron Corporation.
The measurement of the angle of repose is carried out, by causing a powdery sample to pass through a screen having a diameter of 80 mm and a mesh opening of 710 &mgr;m while vibrating the screen, and gently dropping the sieved sample from a funnel having a height of 160 mm onto a horizontal table having a diameter of 80 mm, as an angle formed between a generating line of a cone formed by the powder and the horizontal plane, and takes a smaller value according as flowability is higher. The falling amount of powder is measured until the angle of repose becomes substantially stabilized.
The floodability index value is a criterion for numerically evaluating the flood property. The floodability index value is defined, by determining indices from Tables 5 and 6 from measured values of flowability index value, angle of fall, angle of difference and dispersibility, as a value available by summing up these indices: a larger value corresponds to a higher floodability property. The definitions of the individual physical properties will now be described. The flowability index value is determined by similarly determining indices from measured values of angle of repose, compressibility, angle of spatula and uniformity coefficient, and expressed in a value obtained by summing up these values of indices. The angle of repose is determined by the above-mentioned method.
The compressibility is defined as:
{(Packed bulk density)−(Aerated bulk density)}/ (Packed bulk density)×100
The aerated bulk density is determined, by causing a powdery sample to pass through a screen having a diameter of 80 mm and a mesh opening of 710 &mgr;m while vibrating
Harada Hiroaki
Hirano Hachiro
Hirano Yoshinao
Itaya Masumi
Sakurai Sigeru
Hendrickson Stuart L.
Heslin Rothenberg Farley & & Mesiti P.C.
Mitsui Engineering & Shipbuilding Co., LTD
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