The present invention is concerned with plants for burning granular or pulverulent material, particularly for burning cement raw materials to cement clinker, and of the kind comprising a preheater with associated precalciner, a kiln, e.g. a rotary kiln, and a cooler for cooling the burnt material by means of air subsequently utilized as combustion air.

Various constructions of such plants for burning and sintering of ores, lime and cement raw materials are known. Most frequently the preheater includes a string of cyclones through which the raw material passes in one direction from the inlet to the-outlet in a heat exchange process with the exhaust gas from the rotary kiln passing in the opposite direction through the preheater to a dust precipitator, e.g. an electrostatic precipitator, positioned after the preheater and before a chimney.

Coolers for cooling the material burnt in the rotary kiln by means of cooling air are likewise well known, for instance in the form of a planetary cooler associated with the rotary kiln or of a stationary grate cooler into which the rotary kiln discharges the hot material.

The air utilized in the cooler is, through the heat exchange in the cooler, brought to a temperature of between 600^u and 900^u depending on the type of cooler, so that the heated cooling air has a high heat content, which it is advantageous to recover. The heated cooling air is therefore usually utilized partly as secondary combustion air in the burning process performed in the rotary kiln, and partly as combustion air in the precalciner to which it may be fed through a separate pipe directly from the cooler. Further, the heated spent cooling air together with exhaust gases from the kiln and/or the precalciner may be used in the preheater for preheating the treated material. Such plants are known from instance from British Patent Specification Nos. 1,433,109, 1,463,124 and 1,478,246.

Known single string preheater systems with associated precalciners work with two calcination stages in series, a precalcination stage and a postcalcination stage, one of these often in the form of the lowermost riser pipe in combination with the lowermost preheater stage. Combustion air may be fed separately to each of the two calcination stages either as spent cooling air or in-kiln exhaust gas. The use of two calcination stages is intended to ensure the best possible precalcination but includes the drawback of a more complicated construction and regulation system. Such systems are known for instance from GB 1,406,965 and US 4,183,762.

A single string preheater-precalciner of hitherto known type does not therefore offer the same advantageous technical and economical working conditions as a multi string preheater where the precalciner forms part of a separate calciner string and is fed entirely with spent cooling air, while the preheater string(s) are fed with kiln exhaust gas, and where an optimal control of the ratio of air drawn through the strings is obtained by fans in each string. Contrary hereto, the presence of the precalciner as an integral part of the single string preheater has made it difficult to avoid compromising the air-fuel balance both in the kiln and in the appertaining calciner due to the problems of regulating the two burning processes together and obtaining, at the same time, an optimal precalcination of the treated material.

It is therefore the object of the invention to devise an apparatus for burning granular or pulverulent materials in a kiln plant with a single string suspension preheater-precalciner which is not encumbered with the above disadvantages and which in addition offers the favourable working conditions of multi-string preheaters, but is considerably less expensive to construct, work, and maintain and far more space saving than the latter.

In accordance with the present invention, in a kiln plant for burning granular or pulverulent raw material and comprising a kiln, a single string multi stage suspension preheater with an associated suspension precalciner for pretreating material prior to burning of the material in the kiln, and a cooler coupled to the kiln for cooling the burnt material; a lowermost riser pipe is arranged to convey kiln exhaust gas to a gas inlet of the lowermost preheater stage; a gas outlet of the lowermost preheater stage is connected via a gas duct to the gas inlet of the penultimate preheater stage; a conduit conveys spent cooling air to the precalciner to provide substantially the entire combustion air supply to the precalciner; a precalciner exhaust gas outlet is connected to a separator for separating the precalcined material and for feeding the separated material into the kiln; the precalciner exhaust gas outlet is also connected to the gas duct whereby both the- kiln exhaust gas and the precalciner exhaust gas combine in the gas duct and pass together up through the penultimate and any higher stages of the preheater; and a throttle is provided in the gas duct upstream of the connection from the precalciner exhaust gas outlet whereby the gas flows through the lowermost riser pipe and lowermost preheater stage and through the precalciner respectively are controlled to provide the desired combustion conditions in the precalciner.

The two gas flows, the one from the cooler through the precalciner, and the other from the kiln through the lowermost preheater stage, would inevitably unite under the same pressure before being fed into the penultimate preheater stage. However the provision of the throttle in the gas duct upstream of the point where the two flows. unite, imposes an extra pressure drop in the path of the kiln exhaust.gas and compensates for the fact that the pressure drop across the precalciner should preferably be of the order of twice that otherwise provided through the kiln and lowermost preheater stage. The united gas flows can then pass up through the upper parts of the single preheater string together.

The positioning of the throttle in the gas duct downstream of the lowermost preheater stage is the most advantageous position as the kiln exhaust gas passing this point is significantly cooler than for example that passing up the riser pipe between the kiln and lowermost preheater stage. The throttle will therefore suffer minimal thermal stress. This is particularly important if the throttle is an adjustable damper or valve involving moving parts.

The uniting of the two gas flows may take place in a number of different ways. For example a gas outlet from the calciner separator may lead into the gas duct which in turn leads directly from the lowermost to the penultimate preheater stages. Alternatively, the gas duct may pass from the lowermost preheater stage, through the top of the precalciner and thence into the calciner separator and onto the penultimate preheater stage. As a further alternative, the gas duct may lead from the lowermost preheater stage, through the calciner separator and to the penultimate preheater stage.

The avoid overloading of the material feed from the penultimate preheater stage to the lowermost riser pipe and hence into the lowermost preheater stage, the plant may further include means for leading the separated material stream, or at least part of it, from the penultimate preheater stage directly to the precalciner.

By use of a splitting gate at the material outlet of this preheater stage, it may thus be possible to regulate the material feed to the lowermost riser pipe in accordance with the suspension abilities of the exhaust gases in that pipe.

Three examples of plant constructed in accordance with the invention are illustrated in the accompanying diagrammatic drawings, in which:-

• Figure 1 is a side elevation of one plant where the precalciner exhaust gas is connected to the conventional riser pipe between the lowermost and penultimate preheater stages;
• Figure 2 is a side elevation of a similar plant where the kiln exhaust gas suspension from the lowermost preheater passes through the top of the precalciner; and,
• Figure 3 is a side elevation of a similar plant where the kiln exhaust gas suspension is led to a separator common to the kiln and precalciner exhaust gas suspensions.

The same references are used for similar units in the different figures. The plant has a rotary kiln 1, a grate cooler 2, a conduit pipe 3 leading part'of the spent cooler air from the cooler to a suspension precalciner 4 with a separator 5,5'_?5'', for separating the precalcined material from the suspension, and a riser pipe 15 connecting the separator with a penultimate preheater stage 13. A riser pipe 6 connects the kiln 1 with a suspension cyclone preheater with four suspension stages 11,12,13 and 14, connected to one another by riser pipes 16,17, and 18. Pulverized or granular raw material is fed to the preheater through a inlet 23. While being preheated in suspension the material passes down through pipes 19,20 and 21 from stage to stage eventually reaching the riser pipe 6. The exhaust gas leaves the preheater through a pipe 24 leading to a filter which is not shown. The material outlet of the pipe 21 feeding raw material from the penultimate preheater stage 13 to the riser pipe 6 is positioned near to the kiln inlet.

The riser pipe 6 may have a burner 9 in the lowermost part of the pipe for supplementing the pre- calcining conditions in the riser pipe. Preheated raw material separated from the suspension in the lowermost preheater stage 14 is fed to the suspension precalciner 4 through a pipe 22. The precalciner is fired by means of a burner 10, and precalcined material separated from the suspension in the separator 5,5',5" is led to the kiln inlet through a pipe 26.

The avoid the overloading of preheated raw material from stage 13 into the riser pipe 6, which may cause difficulties as regards suspending the raw materials in the kiln exhaust gas, the plant may have an alternative pipe 21¹ making it possible via a splitting gate 27 placed at the material outlet of stage 13 to lead part of the material stream directly to the precalciner 4.

In Figure 1 riser pipe 16 forms a gas duct connecting preheater stage 14 with stage 13. In Figure 2 the corresponding, riser pipe l6' forms part of a gas duct leading from the preheater stage 14, through the top of the precalciner 4 and separator 5', and through the pipe 15 to the preheater stage 13. In Figure 3 pipe 16" forms part of a gas duct leading through the top of the separator 5" and the pipe 15 to connect the preheater stage 14 with the stage 13. In each case pipes 16,16' and 16'' is provided with a respective movable damper or valve 25, 25', 25" positioned in the pipe immediately upstream of the meeting point of the kiln exhaust gas and precalciner exhaust gas. This damper or valve allows for regulation of the pressure over the lowermost riser pipe 6 and preheater stage 14, so that the kiln exhaust gas meets the precalciner gas under a pressure which is appropriate to provide a satisfactory pressure drop across the precalciner 4 for satisfactory operation of the precalciner.