| Show advantages OF pulverizing COAL FOR FIRING SMELTING FURNACES ETC manufacturers and operators of the larger coal fired furnaces cannot aiford afford to disregard the possible advantages of pulverizing their coal before burning it states john blizard fuel enc engineer of the department of interior through the bureau of mines powdered coal has been successfully applied and is commonly used in open hearth furnaces burnac es and fur furnaces nac es continuous heating furnaces for blooms and billets furnaces for heating reheat re heat ing and forging annealing furnaces for malleable iron and steel castings and plates sheet and pair and annealing furnaces and tin pots galvanizing pots soaking pits ore oa casting Sting and volatilizing copper ore roas roasting ting and si sm elting the zinc industry the gold and silver industry calci ning kiln kalns lime burning refractory materials and a also aso in the fertilizer industry it is used more than any other fuel in ill the cement industry and has been successfully applied for steam raising whenever powdered coal has displaced hand firing the coal consumption has been reduced considerably by the term terin powdered p powdered coal is meant coal subdivided so that it may be burned in suspension when mixed with the necessary supply of air and may be conveyed easily by means of a screw conveyor conveyer by compressed air or suspended in a stream of low pressure air to the furnace since the coal has to be pulverized it is usually better to purchase slack coal coal which is usually cheaper and costs less to pulverize the requisite corn composition position of the coal depends upon many factors practically all coals from lignite to anthracite and even coke breeze require more energy to pulverize them than softer coals and low volatile coals are difficult to i ignite nite and must be burned in specially designed furnaces one of the principal difficulties in burning powdered coal lies in the disposition of the ash and for this reason it is desirable also to use a coal which contains little ash and that melting at a comparatively high temperature crushing and firing advantages the power used to pulverize arid and convey similar coals to the burner is approximately proportional to the weight of the coal pulverized and it is clear that the pulverizing and conveying costs will therefore be greater per heat unit delivered the lower the calorific value of the coal A powdered coal plant consists of apparatus which converts raw coal into powder and conveys it to the furnace into which it is delivered as required and burned in suspension the systems used to accomplish this may be divided into two classes 1 the unit system in which one machine prepares and de liveres the coal to the furnace with the necessary air for combustion and 2 the multiple system in which the coal is prepared in one building and transported to another building wherein is situated the furnace in which coal is to be burned the principal advantages over hand and stoker firing lie in the comparative ease of convey eing coal to furnaces and in the practically complete combustion of the coal with little excess air in close contact with the material to be heated thus avoiding the convection radiation and excess air losses which accompany hand or stoker fired furnaces placed outside and many other furnaces for this reason the most successful field of use of pulverized coal installations has been for those purposes where they have replaced externally fired furnaces for purposes such as steam raising where the burning coal can give give up heat directly by radiation to the boiler heating surface there is therefore less opportunity for reducing the fuel consumption by burning powdered coal instead of burning coal on a grate since the losses which may be reduced by substituting powdered coal fired for hand firing or stoker firing are those only which are due to incomplete combustion and using excess air these losses however are not inconsiderable drawbacks to the use of powdered coal certain drawbacks to the use of powdered coal are cited by the author of the bulletin before powdered coal firing can compete successfully with grate tiring firing it is obvious that the gain due to the smaller consumption of powdered coal must offset the cost of preparing conveying and burning it there is a further disadvantage with powdered coal in grate firing the ash is left on the grates and in the ash pit but with powdered coal the ash is blown into the furnace out through the stack and with some badly designed furnaces out through openings in the furnaces it may also form a troublesome slag and fill up the flues so as to impede the draft on oil the whole powdered coal plants can not be said to be clean there are fairly clean powdered coal plants but generally though not universally a plant using powdered powder ej coal is dirtier than a grate rat fired plant 1 powdered coal is better adapted for firing b stat stationary ionar water tube boilers than other boilers with these boilers furnaces of sufficient size and of the correct shap emay be constructed and the gasses pass through no tubes wherein ash may settle to obstruct the draft and shield the heating surface it has been found difficult to burn powdered coe in locomotive and cylindrical marine boilers because the combustion space is too small to permit the coal to be burned completely although men have been killed by explosions and fire fira in powdered coal plants the causes of such accidents are known and precautions may be taken that they do not oc cur greater precautions are required with some system than with others for instance dangerous fires and have occurred more frequently with the direct lowa low pressure air system of transport than with the indirect screw conveying or compressed air transport sy systems stenis although the indirect transport system has not been entirely free from disasters the possibility of a dangerous fire or explosion in a well designed well managed powdered coal plant is remote and should not influence the prospective user of powdered c abal 1 oal against installing it |
