Rotary kilns are broadly categorized as dry- and wet-process kilns, depending on how the raw materials are prepared. Wet-process kilns are fed raw material slurry with moisture content ranging between 30% and 40%. A wet-process kiln needs additional length to evaporate the water contained in the raw material feed. Nearly 33% additional kiln energy is consumed in evaporating the water in the slurry.
Dry-process kilns are fed dry powdered raw materials. Three designs of dry-process kiln systems are in operation: long dry (hereafter referred to as "dry"), preheater, and precalciner. In preheater and precalciner kilns, the early stages of pyro-processing occur before the materials enter the rotary kiln. These kiln systems have greater fuel efficiency compared to other types of cement kilns. Table 8.2 shows the specific heat input requirements for various types of rotary kilns . As these data reflect, preheater and precalciner kilns operate with greater fuel efficiency. As such, replacement of wet and certain dry process kiln capacity with modern kiln processes can yield substantial reductions in fuel use.
Over the recent years, the trend in the cement sector has been replacement of smaller, inefficient, wet, and long-dry kilns with larger more efficient kilns. This trend is expected to continue.
Between 1995 and 2004, the number of kilns in the U.S. decreased by 11%, but total clinker production capacity increased by 18.6%. The Portland Cement Association (PCA) data show that average kiln capacity also increased by 27%, from 367,000 to 504,000 metric tons per year, in the same period . The trend in kiln designs and average kiln capacity is shown in Fig. 8.3. Note that "dry" in this figure includes dry, preheater, and precalciner kilns.
Table 8.2 Heat input requirements of cement kiln types
Kiln type (106 Btu per short ton of clinker)
225 200 175
225 200 175
i=i Dry Kilns ^m Wet Kilns a Average Kiln Capacity
Fig. 8.3 Trends in cement kiln type and capacity in the United States (1995-2008)
The PCA projects a capacity expansion of 27 million metric tons between 2008 and 2013, an 18% increase in existing capacity from 2006. PCA's projected capacity expansions are to come from 23 kilns . The investment in the projected capacity expansions is projected at $6.9 billion.
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