Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1986-04-29
1988-01-12
Heller, Gregory A.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
B01J 800, C01B 2100
Patent
active
047190949
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the selective elimination of the nitrogen oxides in the exhaust gases of furnaces by reduction with an externally fed chemical compound, preferably ammonia, with acceleration of the reaction by a catalyst traversed in one zone by exhaust gases and in another zone by fresh air to be delivered as combustion air to the furnace, while the location of the zones through which the exhaust gases and fresh air pass is modified continuously or step-wise such that the sections previously traversed by exhaust gases are then traversed by fresh air, and vice versa, and it relates also to an apparatus for the performance of the process.
For the selective reduction of the nitrogen oxides contained in boiler exhaust gases, for example, it is known to feed into the exhaust gas stream emerging from a furnace vaporous ammonia (NH.sub.3) under pressure, or ammonia dissolved in water, without pressure, while an effort is made, by means of a mixing section with appropriate baffling within the adjoining flue gas passages, to achieve a streamer-free distribution of ammonia and temperatures in the exhaust gas stream. Then the mixture is fed to a catalyst preceding a revolving regenerative heat exchanger for the transfer of the exhaust gas heat to the combustion air fed to the furnace. The catalyst is in the form of a solid-bed reactor with a flow directed preferably vertically downward. The reactor contains a honeycomb structure which has a crystalline vanadium-titanium compound as the catalytically active substance. The pressure loss in the solid-bed reactor is taken into account in establishing the size of the flue gas blower. The vertically downward flow in the reactor is intended to combat the depositing of solid impurities within the catalyst or keep them within bounds. The incrustation that occurs is removed discontinuously by blasting with compressed air or steam. The useful life of the catalysts within the reactor amounts to more than two years, depending on the boiler, the manner in which it is operated, and the composition of the coal used as fuel.
An improvement of the useful life of the catalyst is achieved, according to a not previously published proposal of the kind mentioned above (patent application No. P 34 06 657.8), by causing different zones of the catalyst to be traversed by the exhaust gases--including the previously admixed ammonia-on the one hand, and on the other hand by a reverse current of fresh air provided as combustion air for the furnace, the location of the traversed zones being changed constantly, either step-wise or continuously, such that the zone previously traversed by exhaust gases is then traversed by fresh air, and vice versa. By the constantly acting fresh air stream, a prolonged activity of the catalytically active compound of the catalyst, i.e., a lengthening of its useful life, is achieved, in comparison with the formerly practiced discontinuous cleaning with blowing lances. In addition, a reduction of the expense and difficulty of constructing the apparatus is achieved inasmuch as it is possible to couple the catalyst with the rotatory regenerative heat exchanger for preheating the combustion air that is associated with the furnace installation, or even to integrate the catalyst with the regenerative heat exchanger, for example by coating the surface of the heat-storing elements of the regenerative heat exchanger with the catalytically active compound.
It has now been found that the useful life of the catalyst, as well as the operation of any regenerative heat exchanger that may be connected to the output, also depends to a great extent on the development and incrustation of ammonium hydrogen sulfate (NH.sub.4HSO4) on the catalytically active surfaces of the catalyst elements or on the heat-exchanging surfaces of the regenerative heat exchanger, and that this salt, which forms sticky deposits, is formed from the sulfur contained, as a rule, in the furnace flue gases, in conjunction with the ammonia admixed for the reduction of the nitrogen oxides, a
REFERENCES:
patent: 4070305 (1978-01-01), Obayashi et al.
patent: 4199554 (1980-04-01), Araki et al.
patent: 4418046 (1983-11-01), Izumo et al.
Kotter Michael
Rieckert Lothar
Heller Gregory A.
Kraftanlagen AG
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