US3242241A - Method of controlling the pyrochemical bonding of a clay-carbon system - Google Patents

Description

United States Patent O The invention relates to processes in which moulded objects are tired in a continuous process, the required temperature being generated exclusively by burning out `the fuel present in the moulded objects.

The process described can be applied to the manufacture of ceramic objects and particularly of ceramic products of a porous structure.

It is known that moulded objects can be fired in a continuously operating furnace by supplying heat from the outside through the furnace wall or by generating the necessary heat in the furnace itself by combustion of gaseous, liquid or solid fuels supplied. When the necessary heat is generated in the furnace itself the required temperature is adjusted by Aregulating the amount of fuel supplied.

It has previously been proposed to fire moulded, combustible materials containing objects in a continuously operating furnace, the necessary heat being partially generated by combustion of the fuel present in the moulded objects. The adjustment of the required temperature then is achieved by supplying additional fuel from exterior sources sufficient to achieve the correct firing temperature (Silikat Technik, March 3, 1961, page 129 and onwards).

If moulded objects, containing suicient or more than suicient fuel to reach the desired temperature, are fired in a tunnel-furnace operating according tothe countercur-rent principle in which the moulded objects are supplied to one end of the furnace and the combustion air is supplied, counter-currently, to the other end, and in which too high a temperature in the furnace is avoided by reduction of the supply of air in respect of the amount of fuel present in the material, the firing zone tends to migrate to the outlet side of the furnace, and the moulded objects are almost entirely burned out with the result that the nal product has a bla-ck, carbon-containing core.

A counter-current process in which sufficient air is supplied, and the combustion zone temperature is controlled by adding an endothermically acting gasifying agent to the combustion air, such as water, carbon dioxide or a gas containing one of these components, has considerable similarity to the known process for the total gasification of solid fuel with air/steam mixtures, but' has the following drawbacks.

(a) The combustion Zone, which is very short, can only be located in a certain part of the furnace if the number of moulded objects and the amount of combustion agents which are supplied per unit of time are accurately adjusted in respect of each other, since such a process does not represent a stable, self-correcting system,

(b) The partially red moulded objects are too rapidly cooled by the great volume of cold air (and steam if supplied), and contain a black carbon-containing core.

(c) The combustible, toxic gases escaping from the furnace constitute a source of danger, In order to prevent air entering into these gases as a consequence of which an explosive mixture could be formed, it is necessary to maintain a slight pressure above atmospheric in the furnace as is common in gas-producer practice. This means that the inlet and the outlet of the tunnel-furnace rice must be provided with mechanical air-locks which not only involve additional expense, but moreover require man-power for their operation.

The present invention relates to a process for firing moulded objects containing combustible material in a continuously operating furnace, in which the moulded 0bjects are supplied at one end of the furnace and are discharged from the other end, and the required temperature is attained by combustion of the combustible material which is present in the moulded objects, and wherein an oxygen-containing combustion` agent is supplied at a place, situated between the charging end and the discharging end of the furnace, after which a part of the combustion agent is passed in the opposite direction and part in the same direction with respect to the direction of travel of the moulded objects through the furnace so as to form an ignition-zone and a burning-out zone.

An advantage of the present invention is that a stable system is created and the combustion zone is xed at a predetermined place. The combustion agents may be mixed with endothermically reacting gasification agents, and may be supplied at right angles to the direction of travel of the material to be fired and similarly the combustion products, if desired, are partially discharged at right angles to the direction of flow of the red products.

In order to ensure a complete burning out of the moulded objects it is of particular importance to draw-off a part of the combustion air and combustion gases co-currently with the iiow of the mouldedobjects being fired, creating a burning-out zone in this way.

In order to ensure suiiciently rapid ignition of the fuelcontaining moulded objects, it is of `particular importance to lead another part ofthe combustion air and combustion gases in counter-current through `the furnace to the preheating and ignition zone, a Zonevbefore the combustion zone, in order to obtain sufficient pre-heating of the objects to realize ignition. j

Combustion air, mixed or notwith an endothermically reacting gasication agent, is supplied at. a xed place in the continuously operating furnace in cross-flow to the moulded objects. The waste gases are drawn oif partially in the same direction, and partially in the reverse direction and, if necessary partially in cross-flow in respect of the flow of moulded objects through the furnace.

Although the burning out of the moulded objects is carried out with an air/ gas mixture travelling in the same direction as the moulded objects, it is an advantage to cool the tired product in a counter-current of cold air. This cooling air, consequently, enters the furnace at the point where the iired and cooled moulded objects leave the system.

The cooling air leaves the cooling zone with a relatively high temperature and it is led through a simple system of ducts whereby this hot air and the hot gases which have passed through the burning-out zone of the furnace are drawn off from the system.

To achieve `ignition the moulded objects, containing combustible material, are pre-heated by hot combustion gases in counter-current iiow and combustible and toxic gases are formed by gasifying and degasifying reactions.

`In order to prevent accidents it is necessary to avoid contact of these gases with the outer air. Therefore, moulded objects are preferably supplied to the furnace via a mechanical airlock. Since even then the danger of formation of explosive mixtures is not yet. completely excluded, it is preferable to draw-off the combustible gases by means of short-circuit ducts which lead into the same channel(s) through which the hot oxygen-containing gases are drawn otf from the cooling zone and the burning-out zone, so that the combustible toxic gases are immediately burnedin the channels. In that case mixing of the combustible gases with air is not dangerous and the mechanical air-lock can be replaced by an aerodynamic air-lock.

In this embodiment of the process according to the invention, mouldings containing combustible material are fed continuously through a tunnel or shaft-shaped furnace, air being admitted at both open ends and waste gases being drawn off at two points, at one point oxygen-containing relatively hot gases and at the other point relaaively cool combustible gases. These gas streams are drawn off together by a central flue gas fan after the remaining combustible components have been burnt with the air, oxygen being present in excess, and the hot flue gases have been led through a boiler in which the steam necessary for the process is generated.

A drawback of the process described is that great amounts of air have to be admitted via the aerodynamic airlock which, otherwise, does not play a useful part in the process. The flue gas temperature and therewith the yield of the boiler are reduced by this cold air. It is an object of this invention to solve this problem. An eiiicient solution of this problem is to use the air necessary for the formation of an aerodynamic air-lock as the combustion air in the combustion zone. This can be realized by blowing the air from the aerodynamic airlock and a part of the gases from the ignition and preheating zone by way of a booster system, consisting of one or more fans or injectors or a combination of both, via a shunt line into the combustion zone.

When the gas velocities in the circulation system are sufficiently high, the gas and oxygen-containing mixture can be blown into the combustion zone. When these velocities are insuflicient, ignition is caused at the point where the combustible gases and the cold air, come together whereupon the hot ue gases are supplied to the lire hearth by an injection system.

The higher the fuel content of the moulded objects, the more combustion air has to be admitted and the more combustion and gasification products have to be drawn off. The amount of combustion air and recycled gas drawn from the ignition zone can be governed by control of the capacity of the said booster system. Moreover, the amount of combustion air can be controlled by varying the length of the aerodynamic air-lock, e.g. by the removal or replacement of individually removable roof vent-plates.

The withdrawal of the greater part of combustion and gasification products from the furnace is best performed by the use of short-circuit ducts, provided with exhaust valves which are mounted at each side of the furnace and debouch into burner channels and which can be opened and closed individually. When exhaust valves are opened the pressure in the combustion zone decreases by which means more combustion air is drawn in, less gas circulates through the ignition and preheating zone and less air is forced through the burning-out zone. Closing of exhaust valves has the result that the pressure in the combustion zone increases thereby less combustion air is drawn in and more gas is circulated through the ignition zone and more air is forced through the burningout zone.

At the same time the hot oxygen-containing gases from the cooling zone and the burning-out zone are discharging via the said burner channels by which means combustible and toxic gases drawn ofrr by the exhaust valves are irnmediately burnt in the hot, oxygen-containing gas mixture, and the danger of accidents is substantially reduced.

It appeared from experiments that the time necessary for a complete firing governs the production capacity of the described system. Therefore it is useful to make the length of the burning-out zone as long as possible, although cooling of the fired product is also time-consuming, A good cooling, however, is less important for the quality of the final product than a good firing. Therefore in general the burning out zone should be at least as long as the cooling zone. The ignition or pre-heating zone can be relatively short. It also appeared that the external surface of the moulded objects can be rapidly brought to the temperature which is necessary for smooth ignition. Also the aerodynamic air-lock may be short, even when less combustion air is necessary. By closing the exhaust valves over a greater length of the preheating and ignition zone and the combustion zone, less combution air can be admitted than the amount of air which is necessary for the cooling of the fired product.

It has also been found that good results from the process are promoted when the furnace at the place where the combustion agents are blown into the combustion zone is widened over a certain portion of its length. This increase in width causes the combustion agents to spread out resulting in the combustion Zone proper having a greater length, which has a favourable influence on the stability of the process.

The cold gases from -the ignition and preheating zones contain tarlike by-products. When these gases together with combustion air are blown into the furnace with the aid of a fan, tar is deposited on the fan which may give rise to interruption in 4the operation of the fan system. A system is therefore to be preferred in which only a part of the combustion air is drawn in by the fan and the other part of the combustion air and the recirculating gases are drawn from the preheating and ignition zone by means of an air-operated jet -pump in which the combustion air displaced by the fan is used as an operating medium.

The invention will now be described with reference to the drawings:

FIGURE 1 is a diagrammatic representation `of a tunnel-furnace operating according to the invention.

FIGURE 2 is a diagrammatic representation of a tunnel-furnace in which a preferred embodiment of the process according to the invention can be `carried out.

According to FIGURES 1 and 2, I indicates the preheating and ignition zone, II indicates the combustion zone, III indicates the burning-out zone and IV indicates the cooling zone.

The furnace 10 according to FIGURE 1 is provided at the charging end 11 with a lock 1, where the furnace carriages, on which the moulded, combustible material containing objects, which have to be tired (not shown), are stacked, and where they enter the furnace.

Via line 2 a mixture of air and steam is supplied to the combustion zone II. The gases move in the burningout zone III in the same direction A as the objects yto be red and in the reverse direction B in the ignition and preheating zone I. Hot gases from the burning out zone III and used cooling air from the cooling zone IV are drawn off by the ducts 3.

Relatively cold gases are sucked from the ignition zone I at 4, These are mixed with hot oxygen-containing gases which are withdrawn from the furnace along the ducts 3 and 12.

In the furnace 13 according to FIGURE 2 an aerodynamic air-lock S `is present. The combustion air is admitted via this air-lock. Part of this air is blown to the combustion zone II via fan 6. The rest is blown into the combustion zone, together with the gases from the ignition and preheating zone I, through the injector 7 to which, via line 8, steam is also supplied. Gases are also withdrawn from the combustion zone II and the ignition and preheating zone I through exhaust Valves 4 which can be operated individually. The provision of lsaid valves makes it possible to vary the amount of combustion air according to the fuel content of the moulded objects.

Dried, moulded objects containing combustible material stacked on furnace-carriages (not shown), are pushed, with a speed of 10 `metres/hour through a furnace 13 having a length of 72 metres which is open both at the inlet or charging end 14 and the outlet 15 end. The moulded objects are made of coal sludge, (a waste prod'uct derived from coal washeries), to which by Volume of clay, by volume of sawdust and 20% by volume of ily ash is added. The content of combustible constitutents of the dried moulded objects to be fired corresponds to 20% by weigh-t 0f carbon. At a distance of 8 metres from the charging end 14 of the furnace, a fan 6 is mounted which coupled to an injection system 16 mounted at 10 metres from the charging end, draws air in the same direction A as the travel of the moulded objects. The injection system 16 is so arranged that air from the inlet end 14 and gases from the ignition and preheating Zone I are combined by means of a steam injector 7 and by the air displaced by the fau and together are blown into the lcombustion zone II. The mixture of air, steam and gas is blown into the furnace via a shunt line 17 entering the combustion zone at a distance 18 metres from the charging end.

At this point the roof of the furnace 18 lies approximately cm. higher and extends over a distance of 10 metres, `to be measured from a point 19 located 14 metres Ifrom the charging end to a point 20 located 24 metres from the charging end, in order to spread the combustion agents over the combustion zone II. The burning out zone III extends over about 24 metres measured from 24 metres from the charging end to 48 metres from the charging end. At 48 metres from the charging end 4the gases are drawn off from the burning out zone III together with the hot air from the cooling zone IV, which has a length of 24 metres, via two longitudinal channels 21 and 22 lying one at either side of the lower edge of the furnace extending over a length of 38 metres measured from 10 metres from the charging end to 48 metres from the charging end. Channel 22 is now shown in FIGURE 2. It is exactly parallel to channel 21 as obscured by it in the drawing. At 10 metres from the charging end both longitudinal channels enter a collecting channel 23 which at that point lies transversely beneath the furnace and from which the hot gases are drawn off to the boiler 24 (not shown). In each of the two longitudinal channels, 16 exhaust valves 4 open which are arranged at a distance of 75 cm. from each other over a length of 14 metres, measured between a point l0 metres from the charging and 24 metres from the charging end. By operating these exhaust valves, more or less gas can be withdrawn at will from the combustion zone Il or the ignition and preheating zone I and be discharged together with the hot air from the cooling zone IV, and the llue gases still containing oxygen from the burning-out zone, after any remaining combustible constituents have been burned in the hot oxygen-containing mixture. The hot flue gases are drawn off via a boiler 24 with the aid of a flue gas fan (not shown). The boiler also produces the steam necessary for the steam injector system 16.

The higher is the content of combustible material in the moulded objects the more combustion air that is required. To be able to draw this in to a sufficient degree a number of exhaust valves are opened and the requisite number of roof vent plates (not shown) are removed from the aerodynamic air-lock (not shown).

Alternatively the same furnace as already described is used, the moulded objects, however, contain considerably less fuel, namely 7% by weight calculated in respect of pure carbon calculated on the dried moulded objects. In this case no exhaust valves require to be opened and no roof vent-plates need to be removed from the aerodynamic air-lock.

What we claim is:

1. A process for ring molded objects containing combustible material which comprises the steps of passing said objects from the inlet to the outlet of `a kiln sequentially through a pre-heating and ignition zone, a combustion Zone, a burning-out zone, and a cooling zone, passing lan oxygen containing combustion gas into said combustion Zone, causing said rgas to -ow countercurrently wit-h respect to t-he movement of said objects in said preheating and ignition zone .and concurrently with respect to the movement of said objects through said burning-out zone, withdrawing said concurrently owing gas from said kiln at the outlet end of said burning-out zone, and withdrawing said countercurrently flowing gas from said kiln at .a point adjacent the Iinlet end of said pre-heating and ignition zone.

2. The process of claim 1 in which the temperature required to tire the molded objects is obtained by providing sufficient combustible material in said molded objects, and in which an endot-hermic gas is combined with said combustion gas.

3. The process of claim 1 which includes the steps of passing atmospheric air` into the outlet end of said kiln countercurrently with respect to the movement of said objects through said cooling zone, withdrawing said atmospheric air from `the inlet end of said cooling zone, combining and burning said withdrawn air and said Withdrawn concurrently flowing gas to form lproducts of combustion, combining an endother-mic gas with said combustion gas, yand heating said endothermic gas by said products of combustion.

4. The process of claim 1 which includes the steps of passing atmospheric air into the inlet end of said kiln concurrently with the llow of objects, withdrawing said atmospheric air from said kiln at a point adjacent the inlet end of said pre-heating and ignition zone, combining said with-drawn air and said withdrawn countercurrently flowing gas and introducing same into said combustion zone.

5. The process of claim 1 in which the temperature required to tire the molded objects is obtained by providing suicient combustible material in said molded objects, and which includes the steps of combining a heated endother-mi-c gas with said combustion gas introduced into said combustion zone, passing a portion of said combined .gases in a cross llow relationship with respect to the movement of said molded objects through said combustion zone, and thereafter withdrawing said portion from said combustion Zone separately from said concurrently and countercurrently flowing gases.

6. A process for firing molded objects containing a suicient amount of combustible material required to attain the ring temperature of said objects which includes the steps of passing said objects from the inlet to the outlet end of a kiln sequential-ly through a pre-heating and ignition zone, a combustion zone, a burning-out zone and a cooling zone, passing an oxygen containing combustion gas and a heated endothermic gas into said combustion zone and causing said gases to ow countercurrently with respect to the movement of said objects in said pre-heating and ignition zone and concurrently with respect to the movement of said objects through said burning-out Zone, withdrawing said concurrently owing gases from the outlet end of said burning-out zone, withdrawing said countercurrently iiowing gases from the inlet end of said pre-heating and ignition zone, passing first atmospheric air into the outlet end of said kiln countercurrently with respect to the movement of said objects through said cooling zone, withdrawing said 4first air from the inlet end of said cooling zone, passing second atmospheric air into the inlet end of said kiln concurrently with the flow of objects, withdrawing said second air from said kiln at a point adjacent the inlet end of said pre-heating and ignition zone, combining and burning said withdrawn rst air and said withdrawn concurrently flowing gases to form rst products of combustion, combining said withdrawn second air and said withdrawn countercurrently owing gases and introducing same into said combustion Zone, and heating said endothermic gas with said first products of combustion.

7. The process of claim l6 which includes the steps of passing a portion of said combustion .and endothermic gases in a cross flow relationship with respect to the movement of said objects through said combustion zone, withdrawing said portion from said combustion zone separate- References Cited by the Examiner UNITED STATES PATENTS 10/1899 Boeing 25-150 5/1901 Boeing 25-150 Hay-how 264-44 Grunzweig 264-44 Holtcroft 25-157 Dressler 25-157 ROBERT F WHITE, Primary Examiner.

ALEXANDER H. BRODMERKEL, Examiner.

Claims (1)

1. A PROCESS FOR FIRING MOLDED OBJECTS CONTAINING COMBUSTIBLE MATERIAL WHICH COMPRISES THE STEPS OF PASSING SAID OBJECTS FROM THE INLET TO THE OUTLET OF A KILN SEQUENTIALLY THROUGH A PRE-HEATING AND IGNITION ZONE, A COMBUSTION ZONE, A BURNING-OUT ZONE, AND A COOLING ZONE, PASSING AN OXYGEN CONTAINING COMBUSTION GAS INTO SAID COMBUSTION ZONE, CAUSING SAID GAS TO FLOW COUNTERCURRENTLY WITH RESPECT TO THE MOVEMENT OF SAID OBJECTS IN SAID PREHEATING AND IGNITION ZONE AND CONCURRENTLY WITH RESPECT TO THE MOVEMENT OF SAID OBJECTS THROUGH SAID BURNING-OUT ZONE, WITHDRAWING SAID CONCURRENTLY FLOWING GAS FROM SAID KILN AT THE OUTLET END OF SAID BURNING-OUT ZONE, AND WITHDRAWING SAID COUNTERCURRENTLY FLOWING GAS FROM SAID KILN AT A POINT ADJACENT THE INLET END OF SAID PRE-HEATING AND IGNITION ZONE.

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