Waste Heat Recycle in Cement Plant
Cement production has been one of the most energy-intensive industries in the world. At present, cement production by dry preheater/precalciner /rotary kiln process invariably results in three waste heat streams, on an average: Preheater exhaust gases (19.15 per cent), clinker cooler exhaust (5.61 per cent) and combined radiative and convective heat transfer from kiln surfaces (15.1 per cent) of total heat input, resulting in system efficiency of around 46 per cent. Instead of waste heat recovery, the innovative approach would be "waste heat recycle" in various process /unit operations to increase the system efficiency to higher levels.
One of the most accessible and in turn the most cost effective heat losses available are the kiln exhaust gas and cooler exhaust air. The exhaust gases from the kilns are, on average, around 315 degree. And the temperature of the air discharged from the cooler stack is 215 degree Celsius. Commonly, both these air streams is directed into a waste heat recovery generator, which will convert the available energy into electricity either by waste heat boiler set up or by employing organic Rankine cycle system.
Waste heat recycle from clinker cooler exhaust air for raw material drying while grinding: At present, in most of the plants, preheater exhaust gases are diverted to raw mill circuit be it ball milling or vertical roller grinding, for drying during grinding of raw materials. This is not beneficial... On the contrary, the obvious choice should be use of clinker cooler exhaust air: "duct" the same to Raw material grinding plant. In view of comparatively lower temperature of hot air (compared to preheater exhaust gases); minimal hot exhaust from kiln inlet chamber may be mixed as required.
Waste heat recycle from preheater exhaust gases: Cemtherm heat pipe heat exchanger: In majority of the plants, at least a major part of the preheater exhaust gases is utilised in drying of raw materials while grinding. As raw meal is blended and stored in silos prior to feeding the preheater, its temperature will be almost ambient or at the most a few degrees higher than ambient. What is not used, viz., remaining preheater exhaust gases are conditioned before de-dusting and exhausted to atmosphere. Generally, preheater cyclone are increased to five or six stages to reduce the temperature of exhaust gases, or conditioned by water spray, but, this would increase the pressure drop and the power draw required. This is not optimal.
On the contrary, the obvious choice should be to employ "dosed raw meal" as the medium to absorb heat available in preheater exhaust gases, preferably limiting to four stages, to "warm" the meal by installing a specially designed Cemtherm Heat Pipe Heat exchanger in the down comer gas ducting for counter current indirect heat exchange. The specific heat of preheater exhaust gases is 0.22 + 0.00005t. The specific heat of raw meal is 0.21 +0.00007t. Heat pipe heat exchanger would provide a better solution for cooling of exhaust gases and simultaneous heating of raw meal as a compact unit with least system pressure drop and of modular construction. Cooling exhaust gases down to around 150 degree Celsius will result in higher clinker production while maintaining the same volume of preheater exhaust gases. If required we may install a higher prime mover to drive the preheater fan.
Waste heat recycle from kiln shell surface: Heat is transferred from the inside of the kiln where the burning process takes place to the kiln's outside shell surface. As a result, a large temperature differential exists between the kiln's shell and the ambient, resulting in significant heat losses through convection and radiation. It is estimated that 15 per cent of the input energy is lost in the form of convection and radiation heat transfer from the kiln's shell. Heat transfer calculations based on natural convection and radiation indicate that the heat lost to the ambient from the kiln's shell is around 5 kW/m2 ( 1 kW/m2 = 860kcals/h m2).
The kiln shell, between supports, will be "double walled" by wrapping an externally insulated aluminum sheet supported over perforated hallow steel pipes forming annulus passage for flow of ambient air. Alternately, curved solar thermic panels can be assembled in place of Aluminium sheet envelope. The inlet end of external shell is fitted with an Air blower mounted over the kiln to blow air into the annulus, and the other closed end has air injection tubes protruding inside the kiln for axial flow.
Loosely packed hallow Aluminum beads inside the annulus form the heat recipient and heat the air flowing through. The hot air is thus injected to act as axial hot air flow thus "recycling" the convection and radiation heat back to process.
The innovative measures, as suggested above, can recycle most of the heat from the waste heat streams in at least capital costs, besides the operational and maintenance costs will be lowest. Moreover, these measures are custom built to suit every installation to result in higher clinker production with substantial saving in fuel and power consumption.
About the author:
The article is authored by DS Venkatesh, who is a freelance industrial consultant
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