Requirement to control Lime Rotary Kiln output

Requirement to control Lime Rotary Kiln output

 

The lime kiln mainly consists of kiln body, loading device, burning deice, unloading device, electronic appliance, duct collection, etc

The lime kiln operation in a kraft pulp mill is one of the most challenging to control.

First of all, it is a complicated mixed-phase chemical reaction process. Quickly moving combustion gases provide the heat input to chemically change the slowly moving solid-phase lime mud into lime.

Secondly, the many different types and degrees of process delays are very significant and difficult to manage manually. Adequate, but not optimum, manual control takes a well developed feel for the process and careful manual management of the individual control loop targets. In a process with such long time delays, it is common for operators to overreact to changes, resulting in long periods of instability and variable lime quality. The result of a change in operation is often seen by the operators in the next 8-hour shift. And any attempt at optimizing energy consumption is futile if the process is not stable.

The challenges to the operators of a lime kiln are considerable. The availability of the lime kiln can be reduced significantly by the buildup of rings which plug the kiln and need to be physically removed. Availability is also reduced by overheating of the refractory bricks, causing the bricks to crumble and fall.

Specific energy consumption (GigaJoules / ton of CaO) is increasingly an issue, as energy costs are rising rapidly and the emission of greenhouse gases needs to be reduced. Variable lime mud quality (residual carbonate) can cause instability in the causticizing process, resulting in overliming and unstable white liquor quality.

In the kiln, the temperature profile from the inlet to the outlet is the single most important variable, which, if properly controlled, will result in even lime quality and a lower tendency to form rings, which can cause significant process downtime.

The flue gas temperature profile is managed by:

The fuel feed, which regulates the temperature level.

The primary air flow, which affects the flame shape and provides effective heat transfer.

The flue gas fan rpm, which affects the kiln temperature profile. Oxygen and carbon

monoxide measurements modify the draft flow control to maintain a proper level of excess

oxygen in the flue gas and low carbon monoxide emissions.

The kiln rpm, which affects the lime mud depth (degree of filling).

 

 

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