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I S D U S T R I A L A N D EiVGIiYEERIA'G CHEMISTRY
T'ol. 15, No. 9
Mechanical Stokers and the Chemical Industries By Walter H.Wood COMBUST~ON ENGINEERING CORP.,43 BROADS T , N E WYORE, N. Y.
T
HE use of mechanical stokers as a means of supply- given furnace to any stills other than the one served by its ing heat for the numerous and varied processes in stoker. One large chemical industry has done a considerable amount the chemical industries has not as yet been given very serious consideration. There may possibly be several of experimenting in the use of a stoker for obtaining the oxide of a metal from its ore by mixing the ore with anthracite reasons for this. First of all, competition among stoker manufacturers has fuel and burning the same on the stoker. There is not much been quite keen in the past, and the sales effortshave natu- information available, however, regarding this particular rally been directed along the line which offers the least stoker application. Efforts have been made to apply stokers for the production resistance. Since the use of stokers in steam generating plants has presented a favorable and fully developed field of carbon dioxide gas for use in certain industries. It appears to the stoker manufacturer, i t is hardly reasonable to suppose that the analysis of such gas should show a t least 16 per cent that a manufacturer would spend much time or effort in the carbon dioxide content to make the application worth while. development of a new and uncertain field in the face of This field might become attractive, provided the carbon dioxide content can be maintained sufficiently high and all existing competition. Another reason why stokers have not found general appli- the gas from the fuel bed made use of, and a t the same time cation among the chemical industries lies in the fact that insure against waste of the heat generated in the furnace many of the chemical processes are more or less intermittent, when the fuel is burned. I n the operation of a forced draft chain grate stoker, requiring the application of heat, a t a definite rate for a certain period of time, followed by a period of no heat demand. I n especially when burning anthracite coal as a fuel, the gases order that a stoker and its furnace may perform their func- rising from the fuel bed a t the point where ignition takes tions with the best economy, the stoker should operate place and for the first foot or two thereafter usually contain fairly uniformly and without frequent interruption. Other- a very high percentage of carbon monoxide. It might be wise,. the fuel on the stoker will a t one time be burning with possible to so arrange a furnace equipped with such a type an insufficient amount of air, resulting in loss due to the of stoker as to permit of withdrawal from the furnace of evolving of unburned combustible gases, and a t another gases running high in carbon monoxide, which gases could be period of time there will be more air supplied to the fuel bed used as a reducing agent while the remainder of the gases than is necessary for economical burning of the coal in order in the furnace could be directed into the boiler for the that the furnace temperature may be brought up to the purpose of generating steam. There is one use of stokers in what might perhaps be desired point quickly. For this reason oil and gas find a more ready application in the chemical processes than do termed a chemical industry, which has proved very suceessmechanical stokers. If it were possible to conceive of a ful. The great majority of the furnaces for heat treatment stoker and furnace operating continuously and delivering of steel in this country are equipped with stokers. 'Flus the products of combustion of the fuel through channels application has resulted in reducing quite materially $Be to the different places where heat application is required, labor required about the furnace, a t the same time giving the stoker might find a pretty general use in .chemical a much more uniform product than can be obtained by h m d industries. It would, of course, be necessary to provide firing. dampers for directing the heated gases through different STEAM GENERATION THE P R I N C I P A L FIELD FOR MECHANICAL channels and at the same time to regulate the amount of STOKERS heat delivered at any given point, and it would also be necesAny use to which the mechanical stoker may be gut sary to have a sufficient number of places where the heat would be used to enable the stoker to be operated a t a fairly in the chemical industries for furnishing heat to any of constant rate and a t the same time have the heat derived the processes may not result in enlarging the field for the from the fuel used without undue waste. There might be stoker manufacturer to any very considerable extent, but times during such method of operating when the heat gen- there is one very important application of mechanical stokers erated in the furnace would not be required in the various to almost every industry which uses steam-and that b in processes, and a t such times the heat might be applied in connection with boiler furnaces. I n the one hundred and fifty answers to a questionnaire heating water for use about the plant, or possibly as an auxiliary means of generating steam for use in engines and recently sent to as many plants of chemical industries, turbines. If such an application of stokers in the chemical approximately one-half of these plants reported that their industries is possible, it would, of course, involve the develop- furnaces were hand-fired. The total boiler-horse power ment of a load curve for a given process which would insure installed in these hand-fired plants amounted to approxiagainst unnecessary waste, and enable the stoker and furnace mately 300,000 boiler-horse power. From this it is evident to operate a t a point of maximum efficiency. Such an that in the chemical industries a t large there must be a vast arrangement would result in good economy in the use of number of steam generating plants which are being fired by the fuel and might afford a means of reducing to some extent hand, many of which no doubt are of sufficient size to make the use of stokers well worth while. Up to the present time the labor about the plant. Stoker application somewhat along this line has been made far too little thought has been given by the managemen$ of with very satisfactory results in some of the oil refineries. such industries concerning their power plants. In many The stokers have been used for furnishing heat to oil stills, of the industries, such as paper mills, where much steam Es but no attempt has been made to direct the gases from a needed, the power plant has in the past been t h o u g h of
September, 1923
I N D U S T R I A L A X D E-YGINEERILVGCHEMISTRY
only as a necessaiy evil, and little consideration has been given to it by the management except a t times when for some reason the power plant fails to perform its function and as a result the production of the mill is stopped. Usually a t such times the management is none too complimentary
FIG.1
in the remarks or criticisms of the power plant and the men operating the same. These power plants are, as a matter of fact, the heart of the industry, for when the power plant is forced to shut down the rest of t h e b l a n t is about ready trs do the same. The management of any industry should realiae that the power plant in that industry is one of its several important departments, which should be given just as much consideration as to its design, equipment, and operation as any other department in that industry. Such a position is the only sane one for a management to take, became when the power plant as a department of the industry is properly designed and equipped and properly operated, it does its share along with the other departments in turning out a product a t the lowest possible cost. In selecting stokers for use in any industry, whether that stoker is to be used for steam generation or in connection with some special process, there are several points which s h d d be given careful consideration. Chief among these may be mentioned the selection of a stoker best suited to bum the coal which is available a t the plant in question. In case more than one variety of fuel is available, a stoker should be selected which will give the lowest steam production c o s b when the prices of the coals are taken into consideration. In many instances the plants are so located that both anthracite and bituminous fuels can be had at reasonable prices, and where this is the case it is well to consider the use of the stoker which will burn one of the fuels economically and i l l be able to make use of the other fuel with at which w least a fair degree of success in the erent that the regular
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fuel supply should be interrupted on account of strikes and other conditions, or if the price differential between these varieties of fuels varies enough to warrant changing from one fuel to another. GEOGRAPHICAL LOCATION OF
PL.4NTS AND
FUELS AVAILABLE
Different sections of the country are naturally supplied with different kinds of fuels, and usually there is one type of stoker best suited for burning a given fuel. For example, the anthracite coals are burned far more economically on the forced draft chain grate than on any other type of firing machine, and with this machine -the highest possible rates of combustion of that fuel are obtainable. The reasons for this lie in the facts that anthracite coal must not be agitated mechanically or disturbed while it is burning, and i t should be burned in thin and very even fuel beds. The chain grate stoker is practically the only type of stoker that fulfils these conditions. The natural draft chain grate does not answer the requirements of an anthracite stoker, because when the anthracite fuel bed is once ignited, the fuel should be burned as rapidly as possible, and this condition can be accomplished only by supplying air for combustion to the fuel bed from beneath the same, a t the same time maintaining practically atmospheric pressure in the furnace above the fuel bed. This can be done with the forced draft chain grate, but not with a natural draft chain grate for in order to get the necessary air through the fuel bed on a natural draft stoker, the pressure in the furnace would be so far belolv atmospheric that ignition of the fuel would be quite impossible to maintain. practically, no better results can be had with anthracite fuel on any stoker, other than the forced draft chain grate, than can be secured by hand firing on a suitable grate with an ash-pit blast. The chemical industries in sections of the country where anthracite coal can be had at reasonable prices, should, therefore, give due consideration to this type of stoker. These territories include Eastern Pennsylvania, New Jersey, S e w York, the New England States, and parts of Canada. It might be said here that the forced draft chain grate stoker has in the past four years become quite a fa;crorite stoker for use with the bituminous fuels which do not coke to any great degree,
FIG 2
or which, if they do so, form a coke which is soft and friable enough while it is burning to break down of its own weight. Such fuels burn out quite completely on this type of stoker. But there are certain fuels of the bituminous variety, especially those semibituminous coals from the Clearfield and
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Cumberland regions-in fact, most Pennsylvania semibituminous coals lying east of the Pittsburgh seam-which are not burned without excessive ash-pit losses, owing to the nature of the coke formed. Coke breeze makes as satisfactory fuel for the forced draft chain grate as does the anthracite coal. The extensive graphitic anthracite de-
FIG.3
posits in Rhode Island will probably be developed a t an early date as a source of fuel, experiments being under way at the present time in using this fuel on forced draft chain grate stokers, and in pulverized fuel furnaces. Such a development will be of great benefit to the industries of Xew England especially. A typical forced draft traveling grate stoker is shown in Fig. 1. The semibituminous fuels mined in Pennsylvania to the east of the Pittsburgh district in the Connellsville section, and in Cambria, Clearfield, and Indiana Counties, as well as the New River and Pocahontas and Cumberland coals from West Virginia and Maryland, are burned to the best advantage on the multiple retort stokers. While this type of stoker (Fig. 2) is particularly well suited to the semibituminous fuel, it is used quite extensively for burning the better grades of bituminous. The semibituminous coals as a rule are low in ash content with volatile matter ranging between 16 and 26 or 28 per cent, and usually show a higher temperature of fusion of the clinker in the ash than do any other coals; therefore, high combustion rates are obtainable with this kind of coal, and a t the same time little difficulty is experienced in cleaning fires. This variety of coal is used principally east of Pittsburgh and in the n'ew England States. The low ash content of most of the coals of this kind makes their shipment on the long hauls to New England and elsewhere more economical than the shipment of low heat value, high ash fuels. The bituminous coals are quite widely distributed in groups over the country, being found principally in Alabama, West Virginia, Pennsylvania and Ohio, in Illinois and Indiana, Kansas, Oklahoma and Iowa, in Texas and Arkansas, and in Colorado and New Mexico. The generally accepted reason for classing the fuels from these different groups as bituminous coals, is that their volatile combustible content ranges between the semibituminous and the sub-bituminous fuels, or from, say, 28 to 35 or 40percent. Thebituminous coals often contain more ash than the semibituminous coals, and as a class they have a somewhat lower heat value and a Iower fusing temperature of the clinker in the ash. Owing to the greater amount of ash in bituminous coals and to the lower
VOl. 15, No. 9
fusibility of the same, the multiple retort type of stoker sometimes has difficulty in handling these coals successfully. This type of stoker has a dump grate running the full width of the furnace, and in some installations, because of narrow aisles, etc., it is impossible to get a chance to dislodge clinkers from the bridge walls, which may adhere to the same. For this reason fuels of this kind are vwy often handled with greatest success on the underfeed type of stoker, such as illustrated in Fig. 3. I n most underfeed stokers the ash can be dumped a t the sides of the furnaces, and if clinkers adhere to the brickwork, this occurs along the side walls where the clinkers can easily be dislodged and removed. The underfeed stoker is generally a forced draft stoker, and high capacities and efficiencies are obtainable with it when burning bituminous coals. The natural draft chain grate stoker (Fig. 4) has also been used quite extensively in burning bituminous coals, especially those having very low fusing temperatures of clinker in the ash, such as the Illinois, Ohio, Iowa, and Kansas coals. It might be said here that the forced draft chain grate stoker is us&d quite extensively now as a firing machine for these fuels, and very high rating and efficiencies are being obtained in some of the most important plants in the Middle West with this stoker. The sub-bituminous coals of Wyoming, Montana and other of the western states can be burned with only a fair degree of success with most stokers, except with the forced draft chain grate stoker, which is showing remarkably high ratings and efficiencies with these fuels. The low temperature at which the clinker fuses, rilnnifig often as low as 1800" F., does not interfere with the results either as to efficiency or capacity. Lignite has not as yet become one of the principal fuels of the country, although there are very extensive deposits of this fuel, especially in Texas and North Dakota. In the one state, oil is too strong a competitor as a fuel, and in the other, industries are not developed to such an extent as to require large power plants. The forced draft chain grate stoker burns lignite, with perhaps as good results as can be obtained with any of the other stokers, and it will in time probably be the principal stoker for this kind of fuel.
FIG.4
In concluding, it might be added that the puiverized fuel furnace is now developed to such a degree that it can be used with practically any coal mined, and the capacities and efficiencies obtainable with these furnaces are higher than those which can be secured with any stoker. It may not be impossible that this method of burning coal will find application in many of the chemical industries, as a means of supplying heat for all kinds of process work.
