Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1986-12-11
1989-09-19
Doll, John
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
B01D 5336
Patent
active
048679534
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for the selective elimination of nitrogen oxides from exhaust gases with a reducing agent, by making use of catalytically active solid bodies through which the exhaust gases can flow on account of their structural makeup and/or because they are employed in the form of a loose bed.
For the selective reduction of nitrogen oxides in the flue gases of furnaces it is known to introduce into the exhaust gases issuing from the furnace ammonia as a reducing agent, either under pressure, in the form of a vapor mixed with air as a carrier, or without pressure, in the form of a solution in water. Through mixing sections involving appropriate packing within the continuing flue gas ducts the attempt is made to obtain a streakless ammonia and temperature distribution in the flue gas stream until it enters the catalyst. While providing for optimum reaction temperatures within the flue gas stream, the catalyst is placed ahead of the circulating regenerative heat exchanger which transfers the flue gas heat to the combustion air being fed to the furnace. The best catalysts have proven to be solid-bed catalysts through which the flue gas flows vertically downward, while the gas that is to be denitrated flows through several of them alternately. The honeycomb structure of these solid-bed catalysts contains vanadium compounds as the catalytically active substances. These vanadium compounds promote the reaction of the nitrogen oxides with the ammonia previously introduced into the flue gas stream and thoroughly mixed therewith on its way to the catalyst. The reaction with the nitrogen oxides contained in the flue gases leads substantially to molecular nitrogen and water as reaction products, which can be discharged harmlessly into the environment. The considerable pressure loss occurring as the flue gases pass through the catalyst must be taken into consideration in determining the size of the flue gas blower. The vertically downward flow within the catalyst is intended to counteract the deposit of solid impurities in the catalyst interior or keep them within limits. After a period of time of operation, the coatings that are produced are removed from time to time by blasting with compressed air or steam, and discharged from the catalyst bed. The effective life of the catalytically active substances in the catalyst bed amounts to more than 2 years, depending on the boiler design, manner of operation and fuel composition. The nitrogen still remaining in denitrated flue gases issuing from the catalyst must be removed before they enter the flue gas washer for the removal of sulfur oxides, so as to prevent reactions that otherwise would take place resulting in products which would negatively affect the purification and lead to problems in the purification of the resultant waste water.
The invention is addressed to the problem of devising a method for the selective elimination of nitrogen oxides from flue gases by the use of catalytically active solid bodies, in which the reduction of the nitrogen oxides is substantially improved by achieving better utilization of the reducing agent and the catalytically active substances within the catalysts, sustaining the catalytic properties over longer periods of operation, and preventing undesired secondary reactions.
In consideration of the knowledge that the conversion of nitrogen oxides, which are composed of about 95% nitrogen monoxide and about 5% nitrogen dioxide, to molecular nitrogen and water depends not only on the ratio of the amount of the introduced reducing agent to the nitrogen oxide content in the flue gases and on the temperature level of the flue gases, but also to a great extent on the effectiveness of the catalyst, this problem is solved according to the invention, setting out from a process of the kind described above, by bringing the reducing agent, mixed with a carrier gas if desired, and the flue gases containing the nitrogen oxides, into contact periodically and alternately with the catalytically active solid body or with sections of sa
REFERENCES:
patent: 4288420 (1981-09-01), Ito et al.
patent: 4324770 (1982-04-01), Bakke
patent: 4602673 (1986-07-01), Michelfelder et al.
Kotter Michael
Riekert Lothar
Weyland Friedrich
Doll John
Kraftanlagen AG
Russel Jeffrey Edwin
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