GB2091857A - Tunnel kiln - Google Patents

Abstract

The invention is concerned with a kiln for the treatment of payloads which contain therewithin one or more combustible materials capable of being volatilized at or up to kiln furnace temperatures. In order to make use of the heat content of such volatile materials driven off the payload, these materials are arranged to flow in the direction of payload movement into the furnace zone (F) of the kiln where combustion takes place. Hot gases are removed in a region (E) of the kiln and include not only the normal combustion products of the furnace (F) but also the hot combustion products of the volatilized materials. These hot gases are returned to the kiln at one or more upstream locations for pre-heating the payload. <IMAGE>

Description

SPECIFICATION Tunnel kiln The present invention is concerned with tunnel kilns of the type used, for example, in the manufacture of bricks.

In a conventional tunnel kiln, heat is generated principally at an intermediate furnace section of the kiln, for example by means of a plurality of gas or oil burners, the gaseous products of combustion being passed in a counterflow direction relative to the path of the payload or charge through the kiln so that heat is exchanged from these gas products to the payload as the latter progresses further into the kiln. The exhausted gases are usually discharged to atmosphere.

It is known to reclaim the heat remaining in the waste gases by preheating the incoming air to the burner system, particularly in the cases of gas and oil-fired burners, or by preheating the payload itself.

At the cooling end of the kiln where the payload is to be cooled at a prescribed rate, a counterflow of cooling air can be provided. Again, some heat can be reclaimed from the cooling airwhere it leaves the kiln since it has by then been heated by the cooling payload.

Many payloads or charges which are processed using the aforegoing known kiln are of a nature such that they contain combustible substances which are volatilised at the high temperatures encountered in the furnace sections of such kilns. It is an object of the present invention to provide a new kiln arrangemenu which uses this effect to achieve an improved level of efficiency.

In accordance with the present invention, in a kiln for the heat treatment of kiln payloads which contain therewithin one or more combustible materials capable of being volatilized at kiln furnace temperatures, volatilized materials are exhausted from the kiln and re-introduced to the kiln at one or more upstream locations, considered in the direction of travel of the payload through the kiln, whereby at least partial heating of the incoming payload is achieved.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings in which Figure 1 is a highly diagrammatic illustration of a tunnel kiln in accordance with the present invention; and Figure2 is a typical tunnel kiln firing curve showing the distribution of temperature along the kiln.

The basic structure of the kiln illustrated in Figure 1 is conventional and will not be described in detail herein. Objects to be heat treated are inserted at the left-hand side of the kiln as viewed in Figure 1. Their temperature is progressively raised to a maximum temperature reached at the point A (see the curve of Figure 2 whose abscissa corresponds to the longitudinal dimension of the kiln of Figure 1 ) from where a substantially constant temperature region is maintained until point B. The payload is then progressively cooled between the regions B and C before finally emerging from the right-hand side of the kiln.

In a conventional kiln arrangement, some form of heating devices, usually a plurality of gas or oil fired burners, are disposed in the region between the kiln inlet and point B. The hot gases from the burners are caused to flow counter to the direction of movement of the payload and are exhausted adjacent the kiln inlet. Cooling air is introduced adjacent the outlet end and is exhausted approximately at point B.

The arrangement in accordance with the invention illustrated in Figure 1 is designed specifically for use with a payload made of a substance which contains volatilizable combustible material. An example of such a substance is the lower Oxfordshire clays which contain 5 to 7% combustible compounds. As indicated in Figure 1, hot gases are exhausted through the sides of the kiln approximately in the region marked E and these gases are re-introduced into the kiln at one or more of a plurality of locations identified by double arrows. The exhausted gases can be commonly connected or a number of take-off locations can be maintained separate and reintroduced at separate locations. In each case, the point of re-introduction is upstream of the point of extraction, considered in the direction of movement of the payload through the kiln.

The hot gases extracted from the kiln in the region E comprise in addition to the normal products of combustion from one or more burners F, the products of combustion of the volatilized materials driven off from the payload as it is heated. Such hot gases move to the points of extraction in the same direction as the direction of movement of payload.

The re-introduction of the hot gases into the kiln serves to provide pre-heating of the payload upstream of the normal furnace location. In the event, however, that enough combustible materials are present originally in the payload, the burners F may be rendered reduntant, except for the original starting-up purposes.

Air curtains are provided at the inlet and outlet ends of the kiln. Cooling air is injected at the outlet end and flows counter to the payload direction to exhaust in the region of point B.

The section of suitable points for extraction of the gases to be recirculated and the position of the re-introduction points can be determined in dependence upon the nature of the gases to be recirculated.

Thus, the volatile materials generated as a result of the heat treatment process can be passed through the higher temperature ranges of the kiln under an atmosphere containing an excess of oxygen (e.g.

from the cooling air) to fully oxidise the generated volatiles. In passing through these oxidising regions the heat potential of the volatiles will be released.

The oxygen level of the recirculated gases can thus be used to promote combustion or, conversely, the absence of oxygen can be used as a means of retarding combustion. In this connection, the gases from the furnace zones (zone B to C) could be expected to have a low content of free oxygen and to be high in carbon dioxide whilst the gases from the cooling zones could be expected to be high in oxygen and low in carbon dioxide. This situation can be used to promote combustion or retard same dependent on the oxygen level of the recirculated gases together with the gases liberated from the kiln payload.

Thus, by extracting and recirculating the heat from the exhaust gases at selected points, the kiln can be made to make maximum utility of the inherent heat content of the payload, render fumes generated from the kiln payload odourless, and carry out the necessary heat treatment of the kiln payload.

CLAIMS (filed 25 Jan 1982) 1. A kiln for the treatment of kiln payloads which contain therewithin one or more combustible materials capable of being volatilized at or up to kiln furnace temperatures, wherein the combustion products of volatilized materials from the payload are exhausted from the kiln and re-introduced to the kiln at one or more upstream locations, considered in the direction of travel of the payload through the kiln, whereby at least partial heating of the incoming payload is achieved thereby.

2. A kiln as claimed in claim 1 wherein the volatilized materials driven off from the payload are caused to flow in the direction of the payload through the kiln prior to being combusted and subsequently exhausted from the kiln.

3. A kiln as claimed in claim 1 or 2 wherein the exhaustion of said combustion products takes place adjacent the downstream end ofthefurnace region of the kiln, considered in the direction of payload passage through the kiln.

4. A kiln as claimed in claim 1,2 or 3, including air curtains at the inlet and outlet ends of the kiln.

5. A kiln constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. dependent on the oxygen level of the recirculated gases together with the gases liberated from the kiln payload. Thus, by extracting and recirculating the heat from the exhaust gases at selected points, the kiln can be made to make maximum utility of the inherent heat content of the payload, render fumes generated from the kiln payload odourless, and carry out the necessary heat treatment of the kiln payload. CLAIMS (filed 25 Jan 1982)

1. A kiln for the treatment of kiln payloads which contain therewithin one or more combustible materials capable of being volatilized at or up to kiln furnace temperatures, wherein the combustion products of volatilized materials from the payload are exhausted from the kiln and re-introduced to the kiln at one or more upstream locations, considered in the direction of travel of the payload through the kiln, whereby at least partial heating of the incoming payload is achieved thereby.

2. A kiln as claimed in claim 1 wherein the volatilized materials driven off from the payload are caused to flow in the direction of the payload through the kiln prior to being combusted and subsequently exhausted from the kiln.

3. A kiln as claimed in claim 1 or 2 wherein the exhaustion of said combustion products takes place adjacent the downstream end ofthefurnace region of the kiln, considered in the direction of payload passage through the kiln.

4. A kiln as claimed in claim 1,2 or 3, including air curtains at the inlet and outlet ends of the kiln.

5. A kiln constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.