US2899942A - Braun - Google Patents

Description

Aug. 18, 1959 G. BRAUN PROCESS FDR OPERATING A TUBULAR HEAT EXCHANGER FIRED WITH PULVERIZED COAL Filed Aug. 30, 1955 v n 2 mm an I n :8 n m "9:33am @333. u" I l I! 1 l l l I l I l I l l I r i I l I l l I l l I l I I I l I I I I ATTORNEY United States Patent PROCESS FOR OPERATING A TUBULAR HEAT EXCHANGER FIRED PULVERIZED COAL Georg Braun, Stuttgart-Bad Cannstatt, Germany, assignor to Kohlenscheidungs-Gesellschaft m.b.H., a corporation of Germany Application August 30, 1955, Serial No. 531,495

2 Claims. (Cl. 122-479) This invention relates to a method and apparatus for operating a tubular heat exchanger having a furnace in which pulverized and ash forming fuel is burned. The

invention is particularly applicable to a radiant type steam boiler for generating and heating steam and having a furnace fired with pulverized coal.

Steam boilers of this type are equipped quite fre quently with a slagging furnace from which the ash is been made to overcome this ditficulty by dividing the furnace into two or more chambers and at low steam generating loads firing only one of these chambers. Although the problem could be solved in this manner, it was found that even greater difliculties were thereby introduced and other undesirable disadvantages had to be taken into account.

In the operation of steam generating units particularly of the utility type, it is also very important that the temperature of the superheated and reheated steam be kept constant over a wide range of steam generating load. Various means have been employed to accomplish this, such as, diverting hot gases away from the steam heater, or employing tilting burners, or gas recirculation.

These methods and devices are effective in regulating 'the steam temperature at varying loads, however they the furnace can be maintained above the fusion point z.

of the ash at all loads.

Another object of the invention is to provide a slagging bottom furnace in combination with a gas generator in which the liquid ash leaving the furnace is discharged into the fuel bed of the gas generator and the gases leaving the gas generator are conducted to the slag outlet of the furnace to enter the furnace and mingle with the combustion gases generated within the furnace.

A further object of the invention is to control the temperature of the heated steam in a steam generator having a slagging bottom furnace and gas generator by diverting some of the gases produced in the gas generator to a point directly in front of the steam heater.

Still another object of the invention is to operate a steam generator, having" a slagging bottom furnace of the improved design herein disclosed, over an unusually wide range of steam generating load.

Other and further objects of the invention will beice come apparent to those skilled in the art as the description hereof proceeds.

With the aforementioned objects in view, the invention comprises a method of operation, an arrangement and combination of elements of a heat exchanger in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawing wherein:

Fig.- 1 -is a diagrammatic representation of a steam generator and a slag tapping gas generator arranged in operative communication one with the other.

Fig. 2 is a diagrammatic representation of a steam generator having a slagging bottom furnace in which the gas generator is arranged along the side of the slagging bottom furnace.

Fig. 3'is an enlarged diagrammatic view of the gas generator and the lower portion of the slagging bottom furnace. Referring now to Figs. 1 and 3, a radiant steam generator 4 is shown equipped with a slagging bottom furnace 6 having an ash outlet 8 through which ash in liquid form is discharged through duct 10 into a gas generator 12.' Coal stored in bunker 14 is pulverized in a pulverizer 16 and delivered through fuel pipe 18 to burners 20 to be burned in the furnace in a conventional and well known manner. Air required for combustion is fed into the furnace by well known means, not shown.

Coarse coal or coal of lump size is delivered from bunker 22 into gas generator 12 by conveyor means 24 and spreader device 26, as shown in Fig. 3. The coal accumulated in a bed 28 of substantial height generates hot combustion gases which pass through duct 10 and slag outlet 8 into furnace chamber 6. These gases mingle with the combustion gases produced within furnace chamber 6 and pass upwardly through boiler heating surfaces 4 and superheating surfaces 30, giving up heat to these surfaces thereby generating and superheating steam in a conventional manner. A tap hole 32 is provided in the bottom of gas generator 12 to discharge periodically the slag accumulated in the lower portion of the gas generator. Liquid ash produced in the slagging bottom furnace 6 is discharged preferably into the center of the fuel bed 28 of the gas generator 12, trickles through the fuel bed and is finally disposed of through tap 32, together with the slag produced in the gas generator. A bypass duct 34 is provided to conduct hot gases from the gas generator to a point on the upstream side of superheater 30 with respect to gas flow. This duct, equiped with conventional control devices, well known in the art such as valve 29, will in response to thermostat 31 enable the operator to control the temperature. of the superheated steam leaving superheater 30 by passing hot gases directly to a point in front of the superheater, and in this manner increase the v temperature of the gases entering superheater 30 to correct low steam temperature such as might be encountered at low-loads. A gas tight seal 38 is provided between the slagging bottom furnace and the gas generator to allow for thermal expansion. Tubes 40 line the inside walls of furnace chamber 6 and are connected into the steam generating cycle of the steam boiler 4. After having passed over the heating surface of the superheater 30 the gases leave the furnace by way of offtake 42 and may give up part of the remaining heat to convection heating surfaces such as an air heater 37, before escaping to the atmospher by way of an induced draft fan 39 and stack 41.

In Fig. 2, an organization is illustrated wherein the gas generator 12 is arranged alongside of the slagging bottom furnace 6. In this arrangement the liquid ash leaving the bottom of the combustion chamber is not discharged directly into the top of the gas generator 12, but is conducted to an ash sump 46 which also receives the ash being tapped off gas generator 12 by way of tap 32.

v The hot gases discharged from generator 12 in Fig. 3 are conducted via duct 48 to the bottom of the furnace chamber 6 for reasons which will become evident as the description hereof proceeds.

According to the invention substantial improvements are achieved in the operation of the slagging bottom furnace 6 by providing in coordination therewith a slag tapping gas generator 12 which is fired with coarse or lump coal. This gas generator 12 is operatively coordinated with the slagging combustion chamber 6, fired with pulverized coal in two ways. First, they are provided with a common ash discharge. Second, the hot combustion gases of the pulverized coal furnace and those of the gas generator join in passing over the heat absorbing surfaces of the boiler.

Designs of slagging bottom furnaces have heretofore been proposed with the lower end thereof extending into a gas generator fired with pulverized coal, this coal being mixed with returned fly ash for the purpose of discharging the ash in liquid form and of reducing the carbon loss in the flue gases. Furnaces that are designed in this manner however are not suitable for maintaining the ash temperature above the fusion point when operating at lower loads.

In contrast therewith, the slag tapping gas generator 12 being associated with the herein disclosed inventive organization, is preferably fired with lump coal. The entire amount of the heat in the coal or a major portion thereof is thereby liberated in the gas generator. Thus the operation of the gas generator 12 becomes independentof the operation of the pulverized coal furnace 6 and also independent of the amount of fly ash produced.

The present invention provides for proportioning of the size of the tapping gas generator 12 as to its heat generating capacity in such a manner that the release therefrom corresponds to the heat requirement of the heat exchanger at a lower load when it would be impossible for the slagging bottom furnace 6 to operate alone in a satisfactory manner. The maximum heat release of the tapping gas generator 12 is preferably so selected that approximately 20% of the maximum total heat requirements are furnished by the gas generator 12. Thus it is possible even when operating the heat exchanger at low loads to obtain temperatures in the gas generator 12 which exceed the fusition point of the ash.

In an organization as herein proposed a further advantage is realized, in as much as the design capacities of the coal mill 16 and feeding devices 20 for the pulverized coal can be reduced to 80% of the fuel capacity required at maximum steam generator load. This will result in considerable savings in operating and maintenance costs.

According to the invention both the pulverized coal furnace 6 and the tapping gas generator 12 are operated at their maximum capacity when the steam boiler is delivering its maximum load. Changes in load are met by varying the amount of pulverized coal burned in the slagging bottom furnace 6. At low loads, however, the gas generator 12 furnishes a large portion if not all of the heat supplied to the steam boiler. In this manner the steam generator can be operated over a wide range of load without encountering difficulties in keeping the ash in fluid condition for trouble free ash discharge.

In accordance with the invention, as earlier herein set forth, the hot gases from the slag tapping gas generator 12 are introduced through a duct into the slag discharge opening 8 of the furnace 6 burning pulverized coal for the purpose of providing means for raising the temperature in'the bottom of furnace 6 at low loads. These gases then mix with the combustion gases of the pulverized coal'fired furnace and pass over the steam generating'an'd steam heating surfaces of the boiler.

4 1 Another aspect of the invention provides for conducting some of the gases leaving the gas generator 12 to a point ahead of the superheater 30 or air heater 37 to thereby control the superheated steam temperature or air temperature respectively. This can be accomplishedby firing more fuel in gas'generator 12 than is required to keep the ash in furnace 6 in a fluid. state, and using the excess heat to' raise the superheated steam temperature or air temperature respectively.

A portion of the gases from gasgenerator 12 can also be diverted to other heat absorbing devices not directly associated with the steam generator, provided, however, that suflicient heat is always available at all loads to maintain the temperature at the outlet 8 above the fusion point of the ash.

Also fly ash and cinders, collected in the boiler hopper 47 and dust collector 43, may be conducted by way of conduit 45 into the gas generator by mixing this material with the lump coal'burned in 'the gas generator for the purpose of lowering carbon losses.

Although the invention has been described herein as applying to a radiant steam generator there are other heat exchangers utilizing slagging bottom furnaces to which this invention could be applied with equally beneficial results.

While I have illustrated and described a preferred embodiment of my invention, it is to be understood that such is'merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes as and with a slag outlet for liquid slag discharge and a gas generator the gas outlet thereof being in operative flow communication with said slag discharge opening of said furnace, said steam generator having a superheater absorbing heat from the gases leaving said furnace and means for by-passing hot gases directly from said gas,

generator to a point intermediate said fuel burning means and said superheater, the steps comprising: increasing or decreasing respectively the amount of fuel burned in said furnace in responseto an increase or decrease in steam generating load demand, which increase or decrease respectively tends to increase or decrease the temperature of the superheated steam; passing gases from said gas generator into said furnace'by way of said slag outlet; bypassing a controlled amount of hot gases from said gas generator to a point intermediate said fuel burning means and said superheater; and increasing or decreasing the amount of hot gases being by-passed from the gas generator to the superheater in response to a decrease or increase respectively in superheated steam temperature;

whereby the superheated steam temperature is held constant regardless of changes in steam generating load.

2. A method for controlling the temperature of superheated steam in a steam generator having a slagging bottom type furnace equipped with heat absorbing sur-' of heat absorbed in said-furnace by a corresponding in crease or decrease of heat input, in response to an increase or decrease in steam generating load demand, which-increase or decrease respectively tends to increase or decrease the temperature of the superheated'steam;

5 6 passing gases from said gas generator into said furnace is held constant regardless of changes in steam generating by way of said slag outlet; bypassing a controlled amount load.

of hot gases from said gas generator to a point upstream of References Cited in the file of this patent said superheater 1n the gas flow sense; and increasing or decreasing the amount of hot gases being by-passed from 5 UNITED STATES PATENTS the gas generator to the superheater in response to a 1,846,647 Leitch et a1 Feb. 23, 1932 decrease or increase respectively in superheated steam 2,033,685 Coutant Mar. 10, 1936 temperature; whereby the superheated steam temperature 2-087 972 Heller July 27. 1937

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