US1382712A - Burning pulverized fuel - Google Patents

Description

H. G. BAHNHURST.

BURNING PULVERIZED FUEL.

APPLICATION FILED JUNE 26, 1918.

Patented June 28, 1921.

2 SHEETSSHEET 1- RSKW. 61 Haw/"e138 H. G. BARNHURSL BURNING PULVERIZED FUEL.

APPLICATION HLED JUNE 26,1918.

Patented June 28, 1921.

2 SHEETS-SHEET 2.

UNITED STATES PATENT OFFICE.

HENRY G. BARNHURST, 0F ALLEN'IOWN, PENNSYLVANIA, ASSIGNOR TO FULLER ENGINEERING COMPANY, A QOBPORATION OF PENNSYLVANIA.

Specification of Letters Patent.

Patented 'June- 28, 1921.

Application filed. June 26, 1918. Serial No. 242,099.

T 0 all whom it may concern Be it known that I, HENRY Allentown, in the countyof Lehigh, State of Pennsylvania, have invented certain new and useful Improvements in Burning Pulverized Fuel; and I do hereby declare the following to be a full, clear, and exact descrlption of the invention, such as will enable others skilled'in the art to which it appertains to make and use the same.

This invention relates to the combustion of pulverized coal in the furnaces of stationary boilers. i

It has long been recognized that by the use of coal in pulverized form, many distinct and most valuable advantages are to be obtained. The feed of the coal can be regulated to a nicety just as the feed of oil and gas is so regulated. The intensity of the combustion is subject to absolute control because it depends directly upon the percentage of air mixed with the coal. Losses are reduced by beginning the feed of the fuel when heat is required and discontinuing it instantly when heat is no longer needed.

i to be attained by using coal cially successful, so

There is a complete elimination of the losses incident to the use of lump coal due to the formation of clinkers which prevent the unburned coal around the clinkers from receiving the proper amount of air. Stack losses may be reduced as it is unnecessary to use so large a percentage of excess air. There need be no loss of fuel in the ash, all of the combustible being completely 0on sumed. Pulverized coal produces little or no smoke. With pulverized coal, the maximum of flexibility is attained permitting quick adjustment to suit any condition of underload and overload; in case of accident, the fuel supply may be cut ofiinstantl and for installations to meet many, if not a 1, requirements, the initial cost of the necessary apparatus is less.

These pronounced advantages which are 7 in pulverized rather than lump form, together with the marked success which has attended the use of pulverized coal in cement kilns and metallurgical apparatus, have led to many attempts to burn pulverized coal in the furnaces of stationary boilers. So far as I am aware, such efforts have not been commerthat prior to the present in ention, pulverized coal has not been used G. BAIrNrIURs'r, a citizen of the United States, residing at in the furnaces of stationary boilers for any substantial period of time with results that compare at all favorably with those obtained by using coal in lump form- I have found thatthe requisite of success in burning pulverized coal in the furnacesof stationary boilers which workers in this art have heretofore failed to recognize, is the necessity for properly proportioning the furnace with reference to the velocity of travel of the hot gases through the furnace. Failures due to such improper proportioning of the furnace have been the more frequentbecause so many efforts to use pulverized coal have been made by employing furnaces originally designed and built for the use of lump coal. In those cases, the veloclty of travel of the hot gases of combustion has been far too high. The high tem-' perature existing within the furnace during the burning of the pulverized coal reduces the refractory lining to a more or less plastic condition and in this condition, a high velocity of the flow of the furnace gases results in erosion which soon amounts to destruction.

I have discovered that pulverized coal may be used in the furnaces of stationary boilers whose walls are lined with refractory material, economically as compared with the results obtained in using lump coal and successfully overcoming the difficulties heretofore experienced, by so proportioning the furnace as to produce a greatly reduced velocity of flow of the gases of combustion through the furnace. The cross-sectional area of the furnace transverse to the direction of travel of the hot gases must be large in proportion to the air and coal admitted so as to keep the velocity of fiow of the gases below that which will produce erosion of the refractory lining of the walls of the furnace, and in computing the velocity of the gases, care must be exercised to make adequate allowance for the very considerable expansion of the air admitted with the fuel which occurs when the temperature of the air is raised by the combustion of the fuel. I have found in the course ofextended experimentation that to insure successful operation and eliminate the danger of destruction of the refractories, thevelocity of travel of the gases of combustion through the furnace to 0nd and that a velocity of approximately five feet per second is productive of the best results. The importance of the velocity of travel of .the gases through the furnace is greatest at the point in their travel at which the highest temperature is reached. The refractories in the walls of the furnace about that portion where the highest temperature is reached are softened the most, and therefore most subjected to erosion. The point at which this highest temperature is reached will vary with several factors notably the character of the fuel being burned, but will always be a substantial distance short of the water-tubes or other heat-absorbing surfaces for considerable heat is absorbed from the gases with resultant contraction thereof before'the gases actually reach the tubes.

Low velocity of the travel of the gases of combustion through the furnace is also of advantage in connection With the action of the ash. When in operation, ash is deposited upon the furnace walls, building up thereon in a flocculent state and forming a protective covering for the refractory material. As this. protective covering of ash increases in thickness, particles or lumps of it break off and fall into the ash pit of the furnace and thereafter new deposits are gradually formed at such points. If the velocity of the travel of the gases were high, as it has been under prior practice, such deposited ash and also the material of the refractories would be eroded by the high speed of travel of the gases. Moreover, the movement of the gases at low velocity affords a greater opportunity for the particles of ash to settle through the "space within the furnace and collect in the ash pit.

The distance through which the pulverized fuel travels from the point of ignition to the heat-absorbing surfaces should be great enough to insure complete combustion of all of the combustible contained in the fuel before the heat-absorbingsurfaces are reached. In this connection, it 18 to be borne in mind that the volatile constituents of the coal "burn with comparative rapidity, that the fixed carbon takes a longer time to ignite and that the highest temperatures are developed in the zone where the fixed carbon is consumed. The distance of travel from the point of ignition of the fuel to the gases to follow a course which is more or less of a loop.

The rinciples' of the present invention may be appreciated more accurately by considering their application to conditions which are met in actual engineering practice. For example, consider the case of a 600 horse power boiler operating at 150% rating to produce 900 boiler horse power. In order to produce unit boiler horse power, 34.5 pounds of water must be evaporated per hour and therefore the production of the desired 900 horse power would require that 31,050 pounds of water be evaporated per hour. If we assume that the coal used has a heat value of 13,500 B. T. U. when fired in dry pulverized condition, and that an efiiciency of 77% can be obtained, then, as the latent heat of vaporization is 970.4 B. T. U., 1 pound of coal will evaporate 10.71 pounds of water. And in order to evaporate the 31,050 pounds of water per hour required to produce the 900 boiler horse power, it would be necessary to burn 2899 pounds of coal per hour. This amounts to 48 pounds of coal per minute and allowing for 5% of ash in the coal, the total of the combustible fired per minute would be 45.8 pounds.

The combustion of 1 pound of carbonrequires that approximately 154 cubicfeet of air be mixed with the carbon to supply the requisite amount of oxygen. It is common practice, however, to mix with the carbon in the coal a greater proportion of air in order to increase the proportion of carbon dioxid and correspondingly decrease the proportion of carbon monoxid in the gases of combustion. A 20% excess of air over the amount required for theoretically perfect combustion of the carbon in the coal, amounting to 180 cubic feet of air for each pound of combustible, is a proportion which is commonly employed in the effort to produce a relatively large amount of carbon dioxid, as for instance 17%. Assuming that this 20% excess or 180 cubic feet of air be admitted with each pound of combustible, the theoretical temperature within the furnace would be substantially in excess of 3000 F but a considerable allowance must be made for radiation and other losses, and making such allowance, the temperature reached within the furnace would be approximately 2,600 F. With this as the temperature within the furnace. it is possible to compute the volume to which the air admitted to the furnace expands as its temperature is increased. Bearing in mind that gases expand indirect proportion to changes intheir absolute temperature, the incoming air at 50 F. in being raised to a temperature of 2600 F. will increase in volume to six times its original volume. Therefore, the air admitted to the furnace at the rate of 180 cubic feet of air for each of the 45.8 pounds of combustible, expanded to six times its initial volume, would produce 49,464: cubic feet of hot gases passing through the furnace per minute, or 824 cubic feet per second. In order that these gases may not flow through the furnace at a velocity in excess of seven feet per second, the furnace would have to be designed so as to have a cross-sectional area transverse to the direction of travel of the gases at a point where the highest temperature is reached of at least 118 square feet, and if it were of square cross-section it would have to be approximately 11 feet square. Preferably the velocity of travel of the hot gases is reduced somewhat below sevii feet per second. In practice I have found that with a velocity of the hot gases equal to 5.5 feet per second, excellent results may be obtained, and in the example dlscussed this velocity would require that the furnace be approximately 150 square feet 1n section transverse to the direction'of travel of the gases, or about 12 feet on a side in the case of a furnace of square cross-section.

As a guide in designing furnaces for stationary boilers for the use ofpulverized coal, it is convenient to adopt as a rule that the furnace should be designed to have 40 cubic feet of volume for each pound of combustible to be burned per minute. Applying this rule to the example discussed, the furnace should have a volume of 1832 cubic feet, which in the case of a furnace which is a perfect cube, would require that the furnace be about 1211: feet on a side and would give a velocity of travel of the gases of combustion of approximately 5.5 feet per second. This rule is, of course, subject to variation as the design of the furnace departs from that ofa perfect cube as it is the cross sectional area transverse to the travel of the gases of combustion rather than the cubical contents of the furnace which 1s important.

I have annexed hereto drawings illustrating boilers having furnaces lined with re fractory material and designed for the use of coal in pulverized form in accordance with the present invention. In these drawings Figure 1 illustrates a boiler of the Babcock & Wilcox type and Fig. 2 illustrates a boiler of the Stirlingtype, both figures being longitudinal cross-sections.

Referring to Fig. 1, the furnace of the boiler has a front wall 10, rear wall 11, side walls 12, floor 13, cleaning openings 14 and arch 15'. The hot gases of combustion flow through a passage 17 to the water tubes 18 connected by headers 19 and the usual. pipes to drums 20, baffle plates 21 being provided to guide the gases in their-flow among the tubes into the chamber 22 leading to the stack. The pulverized coal is stored in a hopper 23 and is fed by a feeder 24 into a tube 25 through which it passes by gravity additional air may be entrained and carried into the furnace, the amount of this additional air being subject to the control of regulating valves 29 and 30. It will be noted that the furnace is approximately a cube. In accordance with this invention, it is so proportionedwith reference to the air and combustible fed into the furnace for the production of the, desired boiler horse power as to keep the velocity of flow of the hot gases at the point where the highest temperature is reached down to seven feet per second. By reason of this reduced velocity of flow of the hot gases, the refractory lining of the furnace will not be subjected to erosion to any substantial extent and the apparatus may be used over a considerable period of time.

In connection with the construction shown in Fig. 1, it is to be noted that the crosssection of the passage 17 leading from the furnace proper to the heat-absorbing surfaces 18 is not necessarily the cross-sectional area which determines the maximum velocity of flow of the hot gases for the reason that the temperature of the gases in this passage 17 may be considerably less than the temperature existing belovLthe passage and within the furnace proper. This is due to the proximity of the gases in the passage 17 to the heat-absorbing surfaces 18; the water in tubes 18 absorbs heat from the gases to a substantial extent before the gases come in actual contact with the tubes or reach the spaces between the tubes, and as the gases in the passage 17 give up their heat to the water in the tubes 18 there is a considerable reduction in their volume which offsets the effect of the smaller cross-section at the passage 17 upon the velocity of travel of the gases. Thus, in aboiler installation of the type hereinabove discussed operating under such conditions that the temperature In Fig. 2, the mechanism for supplying.

pulverized coal and air to the furnace is substantially the same as that illustrated in Fig. 1. The shape of the furnace is somewhat different and the tubes 36, drums 37 and baffles 38 of the boiler a're differently arranged, the arrangement being that which is usual in boilers of the Stirling type.

Here again the furnace is so shaped with reference to the air and fuel admitted to the furnace for the production of the desired boiler horse power as to keep the velocity of travel of the hot gases down to or below seven feet per second.

In the constructions illustrated in Figs. 1 and 2, the time of travel of the fuel from the point of ignition to the heat-absorbing surfaces is great enough to insure that the fuel will be entirely consumed before the heat-absorbing surfaces are reached, thus avoiding injury to the water tubes, waste of heat and the production of smoke. The decreased velocity of travel of the fuel and hot gases through the furnace, resulting in an increase in the time of passage through the furnace, simplifies the matter of providing for complete consumption of the fuel Within the furnace. The time required for the consumption of the fuel varies with a number of factors, particularly the degree of fineness to which the pulverized fuel is ground. The larger the particles of fuel, the greater will be the time required for their consumption. It would be difficult to formulate a rule governing the length of the path for the fuel and gases through the furnace sufficient to insure complete combustion before the gases reach the heat-absorbing surfaces, but, generally speaking, the reduction of the velocity of flow of the hot gases in accordancewith this invention to such a point as will eliminate the destruction of the refractories by erosion, increases the time of travel through the furnace to such extent as will insure complete combustion of the fuel.

In the drawings annexed hereto illustrating constructions which may be employed in the practice of the invention, it must be understood that these constructions are merely typical of constructions which may be employed, and, except forthe relative proportions of the parts, they are constructions which have been commonly used in this art heretofore.

It is further to be understood that while the invention has been described with reference to the use of pulverized coal only, it is not limited in this respect as it may be employed in connection with the use of any pulverized carbonaceous fuel substance, including lignite, culm, etc.

I claim:

1. The method of utilizing pulverized fuel in a stationary boiler having a furnace lined with refractory material which consists in introducing pulverized fuel and air into the furnace and igniting the mixture thereof within the furnace, maintaining the supply of fuel and air to the furnace substantially continuously to maintain continuous combustion within the furnace, and causing the deflagrating fuel and the gases of combustion to flow through the furnace at low velocity such that at the point in the travel of the gases through the furnace at which the highest temperature is reached the velocity will be substantially seven feet per second, to substantially avoid erosion of the refractory walls of the furnace and insure that combustion of the fuel will be complete within the furnace.

2. The method of utilizing pulverized fuel in a stationary boiler having a furnace lined with refractory material which consists in introducing pulverized fuel and air into the furnace and igniting the mixture thereof within the furnace, maintaining the supply of fuel and air to the furnace substantially continuously to maintain continuous combustion within the furnace, and causing the deflagrating fuel and the gases of combustion to flow through the furnace with a. velocity which, at the point in the travel of the gases at which the highest temperature is reached, is less than seven feet per second, to substantially avoid erosion of the refractory walls of the furnace and insure that combustion of the fuel will be complete within the furnace.

In testimoply whereof I aflix m si nature.

ENRY e. BARN tissr.

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