Combustion – Process of combustion or burner operation – In a porous body or bed – e.g. – surface combustion – etc.
Patent
1990-03-27
1991-06-25
Dority, Carroll B.
Combustion
Process of combustion or burner operation
In a porous body or bed, e.g., surface combustion, etc.
431170, 122 4D, 110245, 422143, F23D 340
Patent
active
050262696
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for controlling the mixing of gaseous flows in fluidized bed reactors and circulating fluidized bed reactors. In the method, part of the combustion air, i.e. the so called fluidizing or primary air, is introduced into the reactor through fluidizing nozzles positioned in a nozzle bottom. The invention is also concerned with a nozzle bottom comprising a plurality of fluidizing nozzles for introducing fluidizing air into the reactor chamber.
In fluidized bed combustion and particularly in so called circulating fluidized bed combustion, there has been a tendency towards large units. At present, the larger units have a thermal power of 200 to 300 MW. Practical research and development (1986) has concentrated on units having a thermal power of up to 1000 MW. One of the most serious problems of this kind of units is the control of the mixing of gaseous compounds and flows. The competitiveness of circulating fluidized bed combustion (CFB combustion) is mainly due to the fact that the discharges of nitric oxide and sulphur oxide can be reduced at low cost. The discharge of sulphur oxide can be reduced by staging of the combustion air in such a manner that the mole fraction of free oxygen is approximately very small in the lower portion of the CFB reactor. Today, this is effected by introducing 40 to 80% of the combustion air through fluidizing nozzles positioned in the nozzle bottom and the remaining part through nozzles having a higher position on the side walls. The mixing problems have become apparent in relatively small units already in use. The problems are further aggravated in CFB reactors where the suspension density is high (from 5.0 to 50.0 kg/m.sup.3) throughout the reactor, whereby the penetration of the transversal jets into a vertical massive main flow is insufficient.
Secondary nozzles positioned on side walls result from the traditional way of thinking according to which the fluidizing gas has to be distributed evenly at least in each grate sector. In all present large fluidized bed and CFB reactors, in which the combustion air is introduced in a stage-wise manner for the reduction of the NO.sub.x discharges, air is distributed according to a known principle: primary air is distributed evenly all over the nozzle bottom and secondary air is introduced essentially horizontally through the nozzles of the side walls.
Patent applications concerning the nozzle bottom disclose several solutions in which the air or the treating gas is introduced through separately adjustable nozzles or in which the gas flow of the nozzle bottom can be adjusted separately in each sector. The object of these inventions has been either to widen the range of power of the reactor (nozzle bottoms to be fluidized each sector separately) or to facilitate and control the transversal mixing of the fluidized material. These applications include Finnish Patent Application 830171, in which the treating gas is introduced into the reactor through two separate and separately adjustable nozzle groups. The whole nozzle bottom is formed by this kind of nozzle pairs positioned evenly all over the nozzle bottom. The object of said invention is to enable a wide range of power of the fluidized bed reactor, simultaneously avoiding an excessive particle loss from the reactor. For this purpose it is suggested in said patent application that a gas flow is introduced through fluidizing nozzles having the shape of a pyramide opening in the grate plane, the flow rate of the gas flow corresponding to the superficial velocity of the reactor, i.e. 2 to 3 m/s. A gas flow with a flow rate exceeding this velocity (a flow rate of 3 to 8 m/s) is introduced through secondary nozzles opening above the grate plane so that it does not affect the solid substance flow being carried away to any greater degree. Said invention is not concerned with the penetration of the jets of the primary and the secondary nozzles nor with the controlled delaying of the mixing. The object is said to be to obtain the best possible mixing over the whole
REFERENCES:
patent: 4427364 (1984-01-01), Bergkvist
patent: 4627812 (1986-12-01), Kelly et al.
patent: 4628831 (1986-12-01), Delessard
patent: 4785768 (1988-11-01), Brown et al.
Dority Carroll B.
Einco Oy
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