This invention relates broadly to material processing plants in which particulate material is heat treated in a kiln or similar equipment and is thereafter passed to a material cooler. This invention is particularly applicable to cement and lime manufacturing facilities.
In a cement manufacturing plant cement raw meal is precalcined and then calcined to cement clinker in the sintering zone of a rotary kiln. The cement clinker is thereafter directed to a clinker cooler wherein it is cooler prior to further processing. The clinker cooler's primary function, therefore, is to quench the hot clinker as it discharges from the kiln. Additionally, the hot gases exiting the clinker cooler are recovered by the kiln hood and tertiary air duct and utilized as “super heated” secondary air for combustion in the kiln and tertiary air for use in the precalciner that is upstream, based on material flow, from the kiln. A portion of the hot gases can also be used to dry the raw materials entering a cement plant's raw mill. This system of recovering cooler exhaust air forms a very important heat recovery system contributing greatly to the overall energy efficiency and productivity of the modern cement plant.
The temperature of the clinker leaving the rotary kiln is about 1400° C. and between about 80-140° C. when discharged from cooler. The cooling air directed to the upper or front, i.e. recuperative, end of cooler is heated to a temperature from 750-1300° C., and this hot air is recycled back into the production line, normally into the kiln and calciner of the pyroprocessing tower via the so-called tertiary air duct. Typically, the heat lost by the clinker in the lower, or non-recuprative, end of the cooler, when it cools from about 400-700° C. to below about 80-140° C. is not recovered or is recovered via comparatively inefficient co-gen systems.
It would therefore be advantageous to have a method and an apparatus to recover such heat to thereby make the cement manufacturing facility, or any plant that utilizes a kiln and material cooler, more energy efficient.
This invention integrates one or more waste heat recovery heat exchangers in a cogeneration system with a material cooler whose primary purpose is to cool particulate material that was previously heat treated in a high temperature oven. The preferred embodiment of this invention is to incorporate the waste heat recovery economizer, boiler, and superheater of a steam Rankine cycle, Kalina cycle, Organic Rankine, or Brayton cycle after the recuperative section of a material cooler at a cement or lime plant. By placing the heat recovery heat exchanger in the clinker cooler, in the case of a cement plant, to absorb the energy in the clinker instead of, or example, in the cooler vent duct, more energy at a higher quality (i.e. at a higher temperature) is available which can be converted into electricity more efficiently and thereby also increase the electric power generated by 25% to 300%. This invention also improves the economics of a cogeneration system at cement or lime plants without significantly affecting plant layout.
It should be understood at the outset that identical reference numbers on the various drawing sheets, either of the prior art or of the invention, refer to identical elements. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the invention. With reference to
At the end of the recuperative section the clinker temperature ranges between 400 to 700° C. and therefore contains about 25 to about 55% of its initial thermal energy. The clinker in the non-recuperative section 16 is cooled by air to ˜100° C. The heat transferred to this air in the non-recuperative section exits through the cooler vent duct 17. The cooler vent duct 17 air temperature ranges from ˜250 to 300° C. The air then is drawn by ID fan 21 to, respectively, heat exchanger 18 to cool the air before it enters dust collector 19, from which clinker fines are removed via outlet 20, and finally stack 22, with the air exiting the stack ranges from ˜110 to 150° C. Clinker exiting the cooler can be first sent to crusher 50 prior being directed to a downstream cement mill (not shown).
Although
Calcined raw material is burned into cement clinker in rotary kiln 30. The clinker is thereafter cooled in clinker cooler 31. Super heated “secondary” air is thereafter directed from the cooler into the kiln via kiln hood 32 and heated “tertiary’ air is directed to the precalcining tower (not shown) via duct 33.
It is a feature of the present invention that a shortened clinker cooler 31 then is typically employed is utilized to cool the cement clinker. In a typical cement kilns the material is cooled to below about 80-140° C., which is about 4%-7% of the temperature at which the material enters the cooler, or less. The clinker cooler of the present invention is shortened to the point when the clinker has been cooled to about 25% to about 55%, of the temperature at which the material enters the cooler, or from about 400° C.-700° C. At such a point, the energy saving features of the cooler as exemplified by the transfer of the secondary and tertiary air, have been addressed, but the clinker still contains substantial heat energy that in prior art systems would have been wasted.
In one embodiment of the invention the shortened cooler is approximately the same size as the recuperative section of a normal sized cooler so that secondary and tertiary air are still provided to other areas of the plant and therefore the energy efficiency of the plant remains the same as in prior art systems. In any event, the size of the clinker cooler can be varied to adjust the heat consumption of the cement plant versus the power produced from the downstream cogeneration system to meet the needs of the end user. The downstream cogeneration system is utilized recover much of the heat that would have been removed in the non-recuperative end of the clinker cooler and thereafter lost.
In a less preferred embodiment of the invention, it is possible to reduce the size the conventional clinker cooler so that it contains only a portion of the recuperative section or in fact completely eliminate the conventional clinker cooler which would increase the quantity and quality of the heat going to the cogeneration system at the cost of increasing the heat consumption and decreasing the thermal efficiency of the cement plant.
If desired, a crusher 34 can be placed after the cooler and before (or in between heat recovery heat exchangers 37 when more than one heat exchanger is utilized), to reduce the material's particle size to thereby increase the heat transfer coefficient which will enable the end user to reduce the size of heat exchangers 37 without sacrificing the amount of electricity produced.
The clinker, whether or not a crushing step is employed, is thereafter transferred to bucket elevator (or any other vertical material conveyor) 35 after which it is transferred to the top material inlet of 36 of vertical heat exchanger 37 that is typically, but not always, integral with the cooler, that is, in the same interior environment or within the same overall housing, as the cooler. As indicated, more than one heat exchanger 3′7 may be employed. Gravity moves the clinker vertically downward across the heat recovery heat exchanger(s) as the first step in a cogeneration process which, with regard to the system depicted in
Clinker, at a temperature of approximate 65° C. above ambient, is discharged from the cooler and is directed to the next stage in the process, typically a finish grinding mill (not shown).
The steam output of heat exchanger 37 is connected to an input of one or more steam turbines 38, each of which is coupled by a shaft to one or more electric generators 39. The outputs of steam turbines are connected to the gas inputs of a condensor 40, condensed at constant temperature and pressure to liquid form and reinserted into heat exchanger 37 as part of a closed loop.
In another embodiment shown in
The steam generated in the heat exchanger 42 is directed to steam turbine 38 in the same manner as specified above. The main advantages of the configuration of
The fluidization air, depicted by cross-hatched arrows 3, can be vented through the clinker cooler to become part of the secondary air.
The present invention has advantages over the prior art cement plant having a cogeneration system as depicted in
Although this invention has been described in detail by reference to the drawings, this detail is for illustration only, and it is not to be construed as a limitation upon the invention as described in the appended claims.