Lime kilns play a vital role in pulp and paper mills as they convert calcium carbonate (a byproduct of the pulping process) to calcium oxide (lime). The lime returns back to the pulping process completing the loop utilized by mills employing the Kraft-pulping process. To facilitate this reaction, the calcium carbonate must be heated to temperatures over 1000°C to drive off carbon dioxide gas. Rotary style lime kilns are generally used as they represent the highest thermal efficiency presently available, given the production quantities required at a typical pulp mill. An ID fan is required to induce flow through the rotary kiln to facilitate the chemical reaction. This flow is normally heavily laden with dust (5-20gr/ft³), requiring scrubbing/cleaning equipment, which is often downstream of the ID fan.
The heavy dust load passing through the ID fan represents a major problem for mills, as the dust sticks to the fan blades. Over time this build-up accumulates and eventually a piece breaks off putting the rotor out of balance. Operators are forced to shutdown the entire lime kiln to clean the fan. Because lime kilns represent the completion of the pulping process cycle, this interrupts the entire mill operations. Unscheduled shutdowns are very time-consuming and have a significant impact on mill production.
There is a 12-24 hour time period required for kiln to cool down, before fan can be cleaned. Also, there is a 12-24 hour time required for the kiln to achieve stable production after start-up. This amounts to approximately 3-4 days of lost production for each unscheduled shutdown.
The frequency of unscheduled shutdowns range from every 2-3 weeks to 2-4 months. Some mills plan their scheduled shutdowns to coincide when operators know the fan will require cleaning, before vibration reaches an unacceptable threshold.
The inconvenience of unscheduled shutdowns (or planned shutdowns that revolve around the cleaning schedule of the fan) cannot be overstated. Yet, this ID fan build-up problem is endemic to the pulp and paper industry due to poorly designed fans. Often the suppliers of this poorly designed fan equipment will place the blame on the “system” (such as fuel type, amount of dust load, etc). However, the mill cannot modify the system to potentially reduce fan build-up, as market forces determine the fuel type or raw material quality.
These poorly designed fans are often straight bladed radial or radial tip blade types that represent 100+ year old fan technology. This design causes dust to impact at steep angles on rotor surfaces, causing build-up. These fans are also low-efficiency type; results from many site performance tests verify these fans operate in the 40-60% efficiency range.
We’ve successfully solved severe build-up problems in many lime kiln applications with our high-efficiency rotor upgrade approach. Good fan design involves ensuring dust impingement is minimized on rotor surfaces. In addition to solving build-up, we’ve been able to provide significant power savings, which is a great secondary benefit. Efficiency improvements of 20-30% are common. Project payback periods are very low, as we re-use existing housing, motor, bearings, and coupling.