US2003226A - Method of burning fuel gas - Google Patents

Description

May 28, 1935. H. c. WELLER 2,003,226

METHOD OF BURNING GAS Filed July 20, 1933 Ff g- 1.

INVENTOR: Han e] C h/e//c'r Patented May 28, 1935 UNITED STATES PATENT OFFICE Application July 20, 1933, Serial 1x05681342 3 Claims.

The present invention relates to improvements in method for producing heat and controlled atmospheres in industrial heating furnaces by the combustion of fuel gas and air in predeter- 5 mined proportions.

Theoretically, combustion is the simple process of bringing each molecule of the combustible element up to its kindling temperature and in contact with the correct amount of oxygen. The chemical reaction is then complete and accompanied by the liberation of light and heat. In practice, niechanical difliculties, together with the physical and chemical characteristics ofthe fuel used, render complete oxidation rather difficult unless more than the theoretical amount of air is supplied, which is especially true in the case of 'solid and liquid fuels.

For gaseous fuels, highly efficient mixing systems for gas and air have been developed which automatically determine the desired ratio of air to gas and are most widely used in industrial heat applications. Combustion occurs almost simultaneously in large volumes of the mixture and its chief characteristic is the relatively short, sharp blue flame. It is obvious that with this type of gas burners, higher temperatures close to the burner ports are experienced and, consequently, lower temperatures at the flues. Therefore, it is commonpractice to apply a mul- I titude of smaller burners which arrangement insures a more uniform temperature together with uniform atmospheric conditions throughout the furnace. In all these applications air and gas are thoroughly mixed before the mixture leaves the burner. 'I'heseapplications are called the non-luminous or pre-mix type of combustion.

In luminous flame combustion no primary air is admitted to the burner, and the mixing of gas and 'air takes place entirely by the slow process of diffusion inside of the furnace. In principle, the chemical reactioncausing luminosity is the thermal breakdown of the hydrocarbon particles into carbon and hydrogen. A large excess of secondary air must beadmitted to insure complete oxidation of all carbon molecules. This procedure-necessarily decreases the flame temperature inasmuch as gas-air mixture and the products of combustion are to a great extent diluted, which retards the rate of flame propagation as the solid minute particles of carbon in the luminous flame are very slow to be heated up to incandescence.

Basically, it may seem that the higher the flame temperature, the greater is the efliciency of heat transfer to the material in the furnace and that it is only a matter of equalizing the temperature difference between material and source of heat energy. This, however, is problematical, since dissociation, an endothermic phenomenon, proceeds as the temperature rises and, consequently, the calorific value of'the gas will be lessened by the amount of dissociation. Excess air, in all cases, lowers the partial pressures of the dissociating substances and, hence,

the flame temperature.

Lower flame temperatures, as experienced with ordinary luminous flame burners, therefore affect materially the rate of heat transmission. To produce a merely luminous flame, certain fundamental combustion principles will be violated and the results are generally not justifled in up to date, economical furnace operation.

It is generally conceded that a great part of the liberated heat from flames is transferred by convection. As the velocity of the streams ofburning gases increases, the ratio of heat transfer increases by decreasing the relative thickness of the stagnant gas fllm on the surface of the object to be heated.' A very great portion of the heat of flames is, however, transferred by radiation, a factor in furnace operation whose magnitude varies as the fourth power of the absolute temperature. This factor is, therefore, both theoretically and practically important in industrial furnace work.

As stated in the foregoing, hydrocarbons break down thermally into carbon and hydrogen; but when this reaction takes place under controlled velocity. and pressure conditions, it is obvious that combustion will proceed at a constant rate over thefurnace hearth with uniform precipitation of free carbon. The rate of radiant heat emissivity is increased (although the flame temperature is lowered) and the actual furnace atmosphere can be subjected to a control by means of the free carbon reaction on the surface of the metal to be heated.

In one type of luminous flame combustion, as in patent to Burke 1,900,223 dated March '7, 1933, strata of gas and air are projected in laminar flow across the furnace hearth and the chemical reaction occurs only by inter-diffusion of gas and air at a slow rate. The excess of free carbon caused by this slow process of diffusion is, in many instances, a serious detriment to practical furnace operation. It, therefore, the highest possible economy of gaseous combustion is to be obtained, it is obvious that all inherent qualities of inter-difl'usion and premix should be combined, and it is the object of the present invention to provide ways and means for effecting this desirable result.

Referring to the accompanying drawing forming part of this speciflcation- Fig. 1 is a schematic sectional view of an improvedburner for permitting gas to be burned in accordance with the present invention, a portion of a furnace wall being also shown, and

Fig. 2 is a section on line 2-2 of Fig. 1 the burner being shown as located at one end of an elongate furnace such as a relatively small forge furnace. I

In the drawing, l indicates an air supply main provided with a shut-off valve I I and having two branches I2 and I3 each provided with a flow controlling means l4 and I5 shown as of the fixed orifice type. The branch l3 discharges into a chamber Hi from which extends a channel or passage I1 of slot form in cross section. The branch 12 discharges into lateral passages l8 and I9 which discharge into chambars 20 and 2|. Extending from these respective chambers are channels or passages.22 and 23 of slot form in cross section.

24 indicates a gas supply main provided with a cut-off valve 25 and shown as having three branches 26, 21 and 28 each of which is provided with a flow controlling means 29, 30 and 3| shown as of the fixed orifice type. Pipes 2B and 21 project into the passages l8 and IS, the

inlets Hand 34 of the passages being sufficiently large to form annular ports for the free flow of air from the branch I2. The air-gas mixtures thus resulting from the admission of both air and gas into the chambers 20 and 2| flow from the chambers by way of passages 22 and 23. The gas pipe 28 delivers to a chamber 35 from which extends a channel or passage 36 of slot form in cross section.

The channels [1, 22, 23 and 36 extend side by side and together form what I choose to call a multi-channelled burner nozzle 31. In Fig. 1, the nozzle is shown as projecting through a wall W of any furnace to be fired whereas in Fig. 2, it is shown as projecting into one end of a furnace F as wide as the nozzle. The furnace shown in Fig. 2 may be considered as'representing a forge furnace for heating rods and the like whereas the wall W in Fig. 1 may be taken as representing a normalizing or annealing furnace of a cross section much greater than the burner and which may require a plurality of burners.

It will be noted that, as shown, the lower nozzle channel 36 is supplied with raw gas and the top channel l1 with raw air whereas the intermediate channels 22 and 23 are supplied with independent mixtures of air and gas. Ordinarily, the

mixture supplied to the lower channel 23 will be richer in gas than the mixture supplied to the next upper channel 22 and ordinarily the deficiency of air .in the mixtures will be made up by the-flow of air through the uppermost channel l1. Ordinarily the flow of air through the uppermost channel I1 will also include suflicient air to supply the air requirements of the raw gas discharged from the lowermost channel 36. Generally speaking, the various strata discharged from the nozzle 31 will be progressively richer in gas from the top stratum down, the top stratum ordinarily being raw air, 1. e. air which contains no fuel gas, and the bottom stratum ordinarily being raw fuel gas. Generally speaking, the individual strata should be 9 horizontally. stratified stream is progressively less luminous from bottom up or what amounts to the same thing the flame will be progressively shorter from the bottom up. The reason for this will be readily appreciated when it is remembered that an explosive mixture of air and fuel gas burns with great rapidity and with a blue transparent flame whereas when raw fuel gas is burned in the presence of air it burns relatively slowly and with a yellow flame.

Ordinarily it is preferred that the various strata flow from the nozzle 31 with relatively low and substantially the same velocities in order. to prolong the straight-away flow from the nozzle and thus to obtain the maximum length of flame. The air and /or gas or mixture may be preheated in any suitable manner thus increasing the velocity of flame propagation. Velocities of gas within the range of 5 to 40 ft. per second may be successfully employed, depending, of course, upon the dimensions of the furnace, travel of flame desired and nature of work to be heated.

In forging furnaces, the rate of scaling and control of decarbonization are of utmost importance. In the application of my invention to such furnaces, not only was a substantially higher efficiency obtained but the scale formation was negligible for practical work and did not in the least harm the forging dies. For example, a large continuous pusher type forging furnace could be brought up to forging heat (2500 F.) within one hour and forty minutes whereas premix system and preheated 011 fuel required four to five hours. This furnace is in.

daily operation and the gas consumption is only 0.75 cu. ft. per 1b. of steel, or 56% of the theoretical heat requirements usually computed for this type of work in ordinary, well insulated furnaces. The chief reason for these low figures is the novel combination of the radiant power of an incandescent flame with the effects afl'orded by means of rich premix layers under controlled velocities. layers nearest to the metal to be heated possess the highest radiating power, whereas the leaner top layers serve only as a vehicle for combustion speed and also provide sufficient turbulence in their controlled laminar flow to direct the majority of the radiant heat towards the furnace hearth. An excess of free carbon formation is, therefore, in the spirit of this invention impossible and any danger of overheating burner nozzles prevented. 4

In heat treating operations, such as performed in annealing furnaces, formation of free, or at least uncontrolled, carbon reacts with the material usually as discoloration of sheet steel or surface distortion. Results obtained with my invention have shown that with intelligent arrangement of mixtures and thereby flame propagation, those detriments can be overcome. The reverse is also readily obtained; for

example, in the hot mill process of rolling cop- It is apparent that the oxide which always forms to some degree in copper that has been melted. If the heating furnace atmosphere is sufliciently oxidizing, so that excess air can reach the hot metal surface, it will combine with the oxygen to form its oxide. However, if the amount of free oxygen is limited or cannot reach the surface of the metal because a quiet flame is projected closely over it, oxygen already in the copper as oxide feeds the flame and the oxide is reduced to metallic copper. Another danger to guard against is the water vapor formed in the combustion products. Cracking of the surface, noticeable on the rolled sheets, is usually attributable to the turbulence of the heating gases, The average gas consumption is in this case 0.26 cu. ft. per lb. of copper. Velocity of the strata applied is held extremely low, air and gas pressures at the burner inlet are kept at 2 in. of water.

What I claim is- 1. In the operation of a gas-fired furnace the method herein described of maintaining combustion which consists in projecting into the combustion chamber a horizontally stratified stream of the two components, air and gas, of a combustion mixture, the stream including an intermediate stratum of mingled air and gas, overlaid by a stratum of greater air content and underlaid by a stratum of greater gas content, with the consequence and eifect that the flame produced is of decreasing luminosity from the lower portion of the combustion chamber upwardly.

2. In the operation of a gas-fired furnace the method herein described of maintaining combustion which consists in projecting into the combustion chamber a horizontally stratified stream of the two components, air and gas, of a combustible mixture, the stream including an intermediate portion in which are found two intermediate strata of mingled air and gas of which the inferior is richer in gas than the superior, such intermediate portion being overlaid by a stratum of greater air content and underlaid by a stratum of greater gas content.

3. In the operation of a gas-fired ,furnace the method herein described of maintaining combustion which consists in projecting into the combustion chamber a horizontally Stratified stream of the two components, air and gas, of a combustible mixture, the stream including an intermediate stratum of mingled air and gas, a superior stratum of unmingled air, and an interior stratum of unmingled gas.

HARVEY QWELLER.

Images