US1958671A - Method for enriching and burning gaseous fuels of low heat value - Google Patents

Description

Patented May 15, 1934 PATENT OFFICE METHOD FOR ENRICIHNG AND BURNING GASEOUS FUELS F LOW HEAT VALUE Walther Mathesius, Chicago, 111.

No Drawing. Application April 17, 1930,

Serial No. 445,231

Claims.

The invention relates to a new and improved method of furnace operation and has reference more particularly to a method of enriching gaseous fuels of low heat value for utilization in 5 apparatus requiring high temperatures.

Gaseous fuels of relatively low heat value are produced, for example, in large quantities by blast furnaces which form an integral part of the average steel plant of today. These blast furnace gases, except for the quantities burned in hot-blast stoves for preheating the air blast, were in the earlier days of the industry largely wasted. Later they were more generally used as steam boiler fuel and also, in a smaller de gree, for driving internal combustion engines. ,In

recent years the superior economic value of this fuel has been recognized for various heating purposes, such as for under-firing coke ovens, in ingot soaking pits, rolling mill heating furnaces,

etc., which by practical necessity require a gaseous fuel.

Thus today considerable quantities of blast furnace gas are taking the place of high-grade gaseous fuels such as coke-oven gas or natural gas,

or of producer gas formerly generated from solid fuel, while solid fuel replaces, if need be, the blast furnace gas at the steam boilers.

In the application of such gases of low heat value to heating purposes, due regard has been given to the possibilities of increasing the thermal effect of combustion by the preheating of gas and air in either regenerative or recuperative apparatus, and the development in this direction up to the practical limits prescribed by the heat resistivity of the regenerator and recuperator materials has opened uprmany new fields of application. Nevertheless, it appears today to be generally conceded, that the utilization of such low heat value gases in high temperature work,

such. as steel melting in openhearth practice, cannot be expected to bring economic advantages even if the highest practical gas and airpreheating temperatures are obtained. For such use, in addition to the relatively low flame temperature obtainable, the non-luminous character of the resulting flame offers a further disadvantage, because it minimizes the most important means of heat transfer from flame to bath by direct radiation and must, therefore, rely for this object on convection only and, on indirect radiation, from flame to furnace roof and walls, and back again to the bath, while gases flowing through the furnace are a hindrance rather than a help to the heat transfer by such indirect radiation.

To overcome these obstacles, various means have been employed, all aiming for either an enrichment in heat value, or an increase in flame luminosity, or both. v Of such means the simplest would seem to be the admixture of richer gases, such as for instance producer gas, cokeoven gas and natural gas. This procedure has been practiced effectively and by various methods of operation, but is obviously restricted to' plants where such gaseous enriching fuels are available at a sufficiently low cost. The earlier method of this type provided for admixture of the gaseous enriching fuel to the low heat value gas at the point of combustion or immediately preceding it, the latter gas alone being conducted through the regenerator or recuperatorchambers in order to avoid the breaking up of the hydro-carbon contents of the enriching fuel and the corresponding lowering of the theoretical heating value of the latter.

More recentlythe fact, however, has been recognized that the breaking up of the hydrocarbons during the passage of the mixed gas through the preheating apparatus and, the resultant finely divided carbon, which is suspended in and carried' along by thegas, increases the luminosity of the combustion flame and thereby aids in the direct transfer of heat from flame to bath. The ultimate thermal effect of this procedure is most favorable for high temperature work, in spite of the .above described theoretical loss in the heating value of the enriching fuel.

Similarly, attempts have been made to enrich low heat value gaseous fuels for high temperature work by the admixture of liquid fuels. However, to date such arrangements have all been of the type as first described above for gaseous enriching methods, that is, the liquid enriching fuel has always been introduced into the preheated low heat value gas at or near the point of combustion. Such arrangements have been moderately successful, but have remained somewhat inferior in economy to operation on preheated producer gas of a heat value equal to the average B. t. u. value of the low heat value gas including the enriching fuel. It appears that the principal causes for this disadvantage are the endothermic effect of vaporization and of decomposition of the liquid fuel at the point of injection and the consequent lowering of the flame temperature, the difiiculties of uniformly mixing and distributing the two fuels so as to obtain a uniformly radiant and properly directed flame and the consequent inferiority of direct heat transfer.

It is the purpose of the invention to overcome furnace.

these disadvantages and to provide a method and means for the enriching of gaseous fuels with liquid fuels in a manner which permits of pass-,

ing the mixture effectively through the preheating equipment of the furnace, of decomposing thereby the hydro-carbon content of the enriching fuel, creating finely and uniformly distributed carbon particles in the preheated gaseous fuel and achieving in that way a flame luminosity and aneffectiveness of direct heat transfersuperior to that obtainable by former practices and equal, in every respect, to that obtained throughthe use of gaseous enriching fuels according to the second and newer method of application as described above. 1

To accomplish this objectthe invention provides a separate primary gas preheater for the low heat value gas, prior to its entrance into the gas preheating chambers of the furnace proper. This preheater, which should preferably be of the metallic type and which may operate on either the regenerative or recuperative principle, is so arranged and dimensioned as to permit of raising the temperature of the low heat value fuel to a temperature somewhat above that of producer gas as commonly used in open hearth practice, or to approximately 1200 degrees F. or higher. For the next step the invention provides for the introduction of the liquid enriching fuel into a suitably designed chamber by means of suitable atomizing or distributing means. The chamber functions as a conduit through which the preheated low heat value gas is conducted on its way to the Through the effect of atomizing and of the high temperature of the gas stream, the liquid fuel is finely divided and vaporized, and is then carried along as a part of the gaseous fuel, having been completely and uniformly imparted to the latter.

The enrichedhot gaseous fuel then passes through suitable valves and flues to the gas preheating means of the furnace proper, for instance to the gas checker chamber of an open hearth furnace, where the temperature of the enriched hot gaseous fuel is further increased to an extent which will insure proper flame temperature through maximum preheating, and also will effectively break up or crack the hydro-carbons and thus, through the high luminosity and radiating power of the resultant flame, will promote an efficient heat transfer from flame to bath, together with its various well known practical as well as economical advantages.

In the practice of the method described the heat requirements of the primary gas preheater may be provided for in various ways, depending upon local conditions, Separate extraneous firing of the preheater with gaseous, liquid or solid fuel may be arranged, although it is contemplated that as a rule a heat supply, adequate as to temperature as well as to volume, will be available for theprimary gas preheater in the form of waste gases, mainly from the high temperature furnace, to which its gas conduit is connected. Such would normally be the case, for instance, in steel melting in open hearth furnaces.

As applied to regenerative furnaces of this type, the method of the present invention is not necessarily limited to the functions of the primary gas preheater, as described above, but may include the use of this apparatus for the purpose of equalizing the temperatures maintained in the gas checker chambers and the air checker chambers respectively. As used for this purpose, the application of the primary gas preheater is not confined to the above described practice with low heat value gas enriched by liquid fuel, but it may be employed with equal advantage to low heat value gas enriched with gaseous fuel, or to the ordinary producer gas; in short to any gas used after preheating in high temperature work as carried out in regenerative furnaces.

As is well known, in modern furnaces of this type, it is not customary to provide free areas in the gas and air ports relative to each other, which would be proportional to the gas and the air volume as introduced respectively through these ports. Instead it iscustomary to restrict materially the gas port area, to convey the gas through itat a pressure somewhat higher than that of the combustion air entering through the corresponding air port, and to utilize the so created nozzle velocity of the gas stream entering the furnace for the purpose of controlling the flame direction and of promoting the intimate mixture of gas and air, which results in a higher speed'of combustion.

These advantages are, however, in a measure offset at the exit end of the furnace, where no appreciable pressure differential exists betweenon through the air preheating chamber. Thus a condition is created whereby upon reversal of the flame direction in the furnace, and continuing so in its operation, the temperature of ducted through the relatively larger air port and;

the preheated air entering the furnace is higher;uo

by a considerable margin than the temperature of the preheated gas coming from the gas checker chambers. While in furnaces operating withproducer gas this tendency is somewhat counteracted by the fact that the producer gas enters the gas checker chamber at a temperature materially higher than that of the air upon its entry into the air chamber, and that therefore the amount of heat to be imparted during preheating of equal quantities of gas and air is less fur o the former, the general tendency toward relatively lower gas preheating temperatures prevails nevertheless. Naturally this tendency becomes more pronounced where cold gases, suchas blast furnace gas enriched with coke-oven gas, areused. In some cases the economic advantage 1 of using such mixed gases has been seriously impaired through the fact that gas preheating temperatures could not be maintained sumciently high to result in an adequate breaking up of thehydro-carbons, the resulting low luminosity flame necessitating a substantial increase in the proportion of enriching fuel used, as well as entailing a materially higher rate of total fuel consumption. a

For all such instances the use of a primary gas preheater, as contemplated by the present invention, provides a convenient and effective means for raising the final gas preheating temperature to the desired degree and for equalizing the temperatures of the gas and the air-checker-chambers respectively, by permitting the absorption of the excess waste gas temperature and waste heat quantity from the air-checker chambers and the air fiues, and by enabling the transfer of the heat so absorbed to the gas just prior to the entry of the latter into the gas checker chamber.

In this manner the disadvantages as described above and as frequently found in present day operation of high temperature regenerative furnaces are avoided and a high degree of flame luminosity, effective heat transfer by direct radiation, rapid heat generation, low rate of fuel consumption and improved operating economy are obtained. My method is capable of a number of variations to meet different conditions and it is my intention to cover all modifications coming within the spirit and scope of the appended claims.

I claim:

1. The method of producing and using furnace fuel which comprises heating a gaseous fuel, discharging into said heated gaseous fuel atomized liquid enriching fuel, increasing the temperature of the enriched hot gaseous fuel by passing the same through preheating means, and delivering said fuel to the furnace for combustion.

2. The method of producing and using furnace fuel which comprises first heating a gaseous fuel to substantially raise its temperature, discharging into said heated gaseous fuel a liquid enriching fuel in atomized form to vaporize the said liquid fuel and to thoroughly mix the resultant vapor with the gas, increasing the temperature of the enriched hot gaseous fuel by passing the same through preheating means, and delivering the mixture to a furnace for combustion.

3. The method of producing and using furnace fuel which comprises heating a gaseous fuel of low heat value to about 1200 Fahrenheit, discharging into said heated gaseous fuel atomized liquid enriching fuel, further increasing the temperature of the enriched hot gaseous fuel by passing the same through preheating means, and delivering the fuel to the furnace for combustion.

4. The method of producing and using furnace fuel which comprises heating a gaseous fuel of relatively low heat value to substantially raise its temperature, vaporizing a liquid enriching fuel of a higher heat value and discharging the same into said hot gaseous fuel, increasing the temperature of the mixture sufficiently to break up at least a portion of the hydrocarbons derived from the enriching fuel into carbon and gases, and delivering the resultant highly preheated mixture of gaseous fuel and finely divided carbon particles to a furnace for combustion.

5. The method of combustion of fluid fuels which comprises heating a gaseous fuel of relatively, low heat value to substantially raise its temperature, injecting atomized liquid enriching fuel into said heated gaseous fuel, heating the enriched gaseous fuel to a substantially higher temperature, delivering the highly preheated fuel mixture to a furnace and there causing it to burn by bringing it in contact with air preheated to a temperature substantially the same as that of the fuel mixture.

WALTHER MATHESIUS.