Rotary Kiln Analysis and Operational Improvement and Control

  • by Dr. Albert Vaillant of APS

    Methods of improving product quality have been simulated in our Numerical Kiln program that shows the temperature profiles within the Rotary Kiln, including the rotating wall’s internal and external temperature profiles.

    Dr. Vaillant’s Ph.D. dissertation is entitled "Thermal Analysis of Rotary Kilns."

    Cytec flies a large powder blue flag from President Jimmy Carter, for their improvements in fuel consumption that APS accomplished.

    Rotary Kiln Simulation (Counter or Parallel Flow):

    • The Rotary Kiln Simulator, may be used in a Rotary kiln Feed-Forward Rotary Kiln control scheme.
      • The APS simulator develops the temperature profiles of materials being heated up to 3200 deg F, as in Periclase operations. More normal levels of product temperatures are under 2700 deg F, as in needle coke operations, while anode coke calciners run at under 2400 deg F, and Lime kilns usually run under 2000 deg F. In counter-flow units the product rate needs to be specified. In parallel flow, the feed rate needs to be specified. Given a laboratory Gravimetric Analysis, and some chemical, physical characteristics of the feed and product, this simulator determines the:
        • temperature profiles and composition of processed material, burner gases, and gases introduced through the shell. For a given kiln length and product temperature, out model computes the kiln’s exit gas temperature.
        • Shell temperature along the shell’s length, for comparison with measured shell temperatures. A fluctuation of shell temperature at a fixed point along the kiln, however, indicates encrusted or lost refractory partway around.
        • rotating refractory’s temperature, since it heats the material by contact and by radiation. . .for various qualities and thicknesses of refractory along the kiln’s length. Although low conductivity reduces heat loss, some refractories reduce the heat transfer rate from brick to material due to their low thermal diffusivity. Most Refractory suppliers are unaware of this "inefficiency" caused by a refractory’s wide temperature swings, DU, as the kiln rotates (DU=HiTemp into bed minus LoTemp out of bed), at a position along the kiln. This DU may be large at the hot end, requiring that more fuel be used.
        • Heat required from the burner, as well as the effect of burner size
        • Dust loss and fuel required, as well as how much of the material (such as coke) is burned up within the kiln at the bed’s surface, as dust swept up into the flowing gases, and what dust loss takes place at the kiln’s feed end.
        • The simulator puts out a comprehensive Heat/Mass balance with a summary of the data used and details about the operation. Burning bed and dust is included.

    Indirect Fired Rotary Kilns:

    • For the indirect kilns, it reports the Heat supplied by each external burner and the corresponding exit loss from the kiln section handled by that burner.
    • This model includes a physical configuration of Indirect firing by multiple external burners, combined with one internal burner firing longitudinally with gases such as Methane, or Hydrogen as for catalyst kilns, or with just inject-ed superheated steam or an inert gas. This kiln has the obvious name of Direct-Indirect Kiln.
    • The Simulator is able to simulate indirect kilns with external radiant heaters instead of burners. This may save energy since fuel combustibles have a large exit gas loss due to excess air needed for the protection of the external burners. Saving fuel may become important again.

    Rotary Kiln Operation Simulation:

    • APS has simulated the Kiln/Exhaust system as an electric analog circuit with Blowers and ID Fan as batteries, Tramp air as resistance to ground, burner and release of volatiles as current sources, Firing hood leakage as battery (for chimney effect), plus a resistance, scrubber H2O as a current source, dampers as resistances, and atmosphere as a ground. The corresponding electrical analog circuit ties all portions of the plant into one computation, involving the solution to only four, simultaneous, non-linear equations. A solution of this type showed one client (Engelhard) that we could increase his kiln production rate by 40% once we improved his fuel consumption, modified the combustion chamber, corrected his seals and put a larger motor on his existing ID fan.