Liquid heaters and vaporizers – Miscellaneous
Patent
1990-08-07
1991-10-29
Favors, Edward G.
Liquid heaters and vaporizers
Miscellaneous
110245, 16510416, B09B 300, F22B 100
Patent
active
050605992
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Systems for generating steam or so-called steam-generating boilers based on a reactor for so-called "quick-circulating" fluidization are known from commercial constructions. "Fast" fluidization occurs in a flow of combustion gases and air directed almost vertically upward, in which a granular material is carried and substantially entrained upward by the gas. This material consists of a fuel, e.g. coal and ash products from coal having, if necessary, an admixture of limestone for absorption of sulphur or an inert material such as sand. In most cases, the rate of flow is 3-8 m/s, and the size of the flowing grains is extremely small, i.e. in the micrometer range, up to some millimeter. The quantity of solid material may vary from low values at low load, up to twenty or more kg/m.sup.3 at high load.
Most of the entrained solid material is separated in a particle separator--for example a cyclone type separator--when flowing out from the top of the reactor and is "circulated" to the lower part of the reactor so as to:
a) maintain a suitable material density and sojourn time in the reactor,
b) obtain an excellent combustion reaction, and
c) obtain an excellent reaction of absorption for e.g. sulphur separation with an admixture of limestone.
Such a reactor is shown in FIG. 1.
The reactor is further characterized in that mainly by introduction of primary air into the bottom part and secondary air at a suitable level thereabove, a situation is, in practice, established in which a lower speed is obtained in the bottom part and a higher speed thereabove, which inter alia gives a higher density of solid material in the bottom part (in many cases from 100 to 600 kg/m.sup.3), where fuel can be degassed and partly burned. Large fuel particles and other solid materials stay or are enriched in this zone until they are burned out completely or disappear through a special material outlet in the bottom part. In operation, the reaction temperature is 750.degree.-1000.degree. C., however preferably 825.degree.-900.degree. C. in the combustion of coal.
To cool the system and recover the required part of the developed power of combustion, two techniques are used today. One implies that cooled surfaces, for example vertical tube surfaces cooled by water or steam, are arranged on the walls of the reactor or as internal baffles or the like disposed in the reactor. The other technique which is sometimes also combined with the first, implies that further power outputs are provided in that the flow of particles which is separated in said particle separator at the top of the reactor is wholly or partly conducted to an ash cooler of some suitable type before being reintroduced into the reactor. Of course, the power output is determined also by the amount of hot gas which is leaving the reactor.
Technically seen, there now is a situation where a first combustion reaction occurs in the bottom part of the reactor having the above-mentioned higher density of solid material, whereupon the final combustion of gases expelled from the fuel and burning-out of the coke particles formed occur higher up where the oxygen content has been increased by addition of secondary air.
For different reasons, it is not suitable to arrange heat-absorbing metallic surfaces in the bottom part of the reactor. One reason is the low oxygen partial pressure which easily causes corrosion on metallic surfaces and/or erosion.
The absorption of heat on cooling surfaces arranged on the reactor walls occurs through radiation from particles and gas supplemented with convective gas cooling towards the wall and more or less direct particle contact, whereby also large amounts of heat can be transferred. At full load, the heat transfer is typically between about 140.degree. and about 250 W/m.sup.2 .degree.C. depending on the temperature and the current particle load, when an optimal combustion of coal is desired.
In large reactors, it is constructionally difficult to arrange a sufficient cooling surface in the walls only, if the reactor is not made
REFERENCES:
patent: 4672918 (1987-06-01), Engstrom et al.
patent: 4709663 (1987-12-01), Larson et al.
patent: 4745884 (1988-05-01), Coulthard
patent: 4777889 (1988-10-01), Smith
patent: 4813479 (1989-03-01), Wahlgren
patent: 4823740 (1989-04-01), Oashita et al.
Favors Edward G.
Gotaverken Energy Aktiebolag
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