Liquid heaters and vaporizers – Plants – Garbage
Reexamination Certificate
2001-01-09
2002-07-02
Lu, Jiping (Department: 3749)
Liquid heaters and vaporizers
Plants
Garbage
C122S00400R, C110S234000
Reexamination Certificate
active
06412446
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of firing in a boiler, in which method loosely stacked fuel is introduced through a charging opening into the furnace of the boiler on a first support, and jets of ignition air entraining hot flue gas from the furnace are directed at the surface of the loosely stacked fuel on the support so that the surface layer of the loosely stacked fuel is ignited and the fuel is partially gasified, whereupon the fuel is passed on to a grate located at a lower level on which the final combustion of the fuel takes place. The present invention also relates to a boiler for using the method.
In prior-art boilers, the area of the furnace at the charging opening is formed so as to be relatively closed, being defined upwards by a slightly inclined portion of the wall above the charging opening with inserted ignition air nozzles and downwards by a fixed bottom inclined at an angle of less than 5° to 20°. This prior-art structure is designed to allow control of the supply of oxygen and thus the possible release of energy for expulsion of gases from the loosely stacked fuel on the fixed bottom. It has proved, however, that it is difficult to obtain a stable rate of combustion because the combustion can vary strongly at relatively small variations of the air permeability and humidity content of the loosely stacked fuel and the rate at which the fuel is charged into the furnace.
It is a further disadvantage of the prior-art boiler with a fixed bottom that deposits of melted or sintered ash particles can accumulate partly on the boiler wall in the area around the ignition air nozzles, partly on the first support. To ensure the free passage of the fuel it is necessary to remove such deposits at regular intervals, which may cause undesirable stop-pages. Prior art also suffers from the defect that the combustion develops a not inconsiderable amount of nitrogen oxide (NO), which is emitted to the surroundings and pollutes the environment via the chimney of the boiler plant.
U.S. patent publication Ser. No. 4,213,405 describes a boiler for combustion of solid fuel, such as wood waste, in which the furnace of the boiler has a first support in the form of an inclined grate the main purpose of which is to dry the fuel, the flue gases from the combustion zones above a second grate located at a lower level being conducted to and passing up through a top zone of the first grate.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a method which reduces or eliminates the above problems. This is obtained in accordance with the invention by a method of the type mentioned in the introduction and characterized in that the air from the ignition air jets are permitted, together with the entrained flue gas, to pass down through the loosely stacked fuel and the first support and then to flow off to the furnace, and that the flue gas entrained by the ignition air jets is drawn substantially from a section of the furnace through which flows a mixture of combustion products from the first support and the grate.
By means of the method, the air from the ignition air nozzles together with the entrained flue gas passes through the fuel to the first support, in the following called the first grate, and then t through the grate to the lower side thereof. The result of this is that the gases developed by ignition of the surface layer of the fuel and the gasification of the underlying part of the fuel are passed on down through the fuel to the lower side of the first grate. The combustion gases flow from here out into the furnace and thus pass above the combusting fuel located on the second, lower grate, whereby the combustion gases from the two grates are mixed while being mixed with primary and secondary air supplied to the furnace in a conventional manner. Part of this mixture of flue gases is then drawn in by the ignition air jets from the furnace towards the surface of the fuel on the first support.
Surprisingly, this method has turned out to provide extremely stable combustion, and the rate of combustion can easily be controlled regardless of any variations of the air permeability or humidity content of the loosely stacked fuel or its feeding rate.
It has furthermore proved that the method according to the invention results in substantially smaller amounts of melted or sintered ash particles, and a not inconsiderable reduction of the amount of nitrogen oxide produced by the combustion is seen.
Finally, it can be mentioned that no backfire of the combustion to the charging channel occurs in the boiler according to the invention, as is the case in the prior-art boiler.
It is assumed that these good results are due, mainly, to the fact that the mixture of air and flue gas that hits the surface of the loosely stacked fuel together with the gasification products from the loosely stacked fuel is passed down below the first grate. This ensures that hot flue gas from the furnace can flow into the area around the ignition air nozzles, that a large proportion of this flue gas is recirculated through the loosely stacked fuel, and that the gases around the surfaces of the first grate have a relatively homogeneous and low temperature. This must be seen in relation to the prior-art boiler, in which the ignition air jets are unable to penetrate the fuel bed as the bottom under the fuel bed is not open to penetration. This means that the products developed by the gasification, which are relatively cold compared with the flue gas in the middle of the furnace, escape from the upper surface of the loosely stacked fuel substantially to the area around the ignition air nozzles and thus limit the flow of hot flue gas to this area. This fact is further supported in the prior-art boiler by the wall above the charging opening being formed with a near-horizontal inclination, thus preventing the flow of flue gas from the middle of the furnace.
The prior-art boiler with a fixed bottom has no well-defined flow pattern at the surface of the fixed bottom. This allows the hot gas flow from the ignition air nozzles to follow randomly occurring holes in the fuel bed and to hit the fixed bottom, whereby partially melted particles of ash suspended in the hot flue gas can be deposited on the bottom. This is avoided to a substantial extent in the method according to the invention, partly because such flows seek towards the openings in the underlying first grate and thus do not deposit particles on the grate surface, partly because the remaining part of the grate surface supporting the overlying fuel is protected by the fuel to a higher degree.
The prior-art boiler with a fixed bottom has a large excess of combustible gases and thus a deficit of oxygen in the area around the ignition air nozzles. Consequently, the amount of air and thus of oxygen supplied to the area around the ignition air nozzles has a crucial influence on the rate at which the gasification products are expelled from the fuel on the fixed bottom, as the supply of air to the area around the ignition air nozzles will cause a strong temperature increase in the area under these conditions. As a substantial part of the gasification of the fuel on the fixed bottom typically occurs at temperatures below 500° C., whereas oxidation of these gasification products requires temperatures above 800-1,000° C., a careful balancing of the relationship between the amount of air supplied to the flue gas at the area around the ignition air nozzles and the degree of heat transfer between flue gas and fuel on the fixed bottom is required. In case of excessive heat transfer, the temperature of the flue gas at the ignition air nozzles will drop, whereby the reaction between combustible components of the flue gas and oxygen supplied by the air will cease. In case of an insufficient heat transfer the temperature in the flue gas entrained by the ignition air jets will rise to a level where ash particles suspended in the flue gas will melt and where ash formed in the surface of the fuel bed will melt and thus further inhibit
Anderson Chad C.
FLS miljo a/s
Lu Jiping
Shannon John P.
Venable
LandOfFree
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