Should the oxygen-coal ratio be controlled automatically?
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Our plant has an automatic function for the oxygen-coal ratio: when I increase the load, with this automatic control of the oxygen-coal ratio, oxygen is automatically added to maintain it at a stable level. When I reduce the oxygen level, the coal slurry flow rate is automatically reduced as well. This is originally a good thing. But I heard that setting the instrument to automatic mode can cause significant fluctuations. The second Huaihua plant doesn’t use automatic mode, and they also haven’t installed any interlocks for stopping the HH unit in such situations. I’d like to ask everyone: what do you do in these cases? (Our factory hasn’t started operating yet, so we haven’t had a chance to test it; it’s all just theoretical knowledge*, hehe.)
I forgot to mention that this one is from Texaco, while the Huaihua model operates entirely in manual mode; that is, the oxygen flow rate and the slurry flow rate are controlled manually. By the way, what is your oxygen-to-coal ratio? Is the slurry flow rate taken as the median value of the various flow rates? Should only one of the flow meters be used as the reference for the slurry flow rate in terms of the oxygen-to-coal ratio? I heard that the flow rate calculated based on the rotation speed of the slurry pump is relatively stable. I wonder what everyone does. Thank you!
Automatic operation involves significant fluctuations; in fact, it’s not a simple form of automatic operation either – it’s a type of cascade control. Currently, operations are also mostly carried out manually; whether the high-level interlock is engaged or not has little impact. During normal production, in order to reduce fluctuations and facilitate operation, the oxygen-coal ratio is generally determined based on speed and flow rates, which means that the variation in this ratio is usually very small.
In instrumentation terminology, this type of control is called “coupled control”. The speed-flow rate is based on empirical values derived from the rotation speed of the slurry pump; when the rotation speed fluctuates little, the flow rate remains stable. However, if one cylinder of the slurry pump does not function properly and yet the speed-flow rate is considered accurate, serious problems will arise.
Whether to put it into operation or not depends mainly on the conditions of your process – whether there are too large fluctuations
This design represents an excessive gap between idealism and reality; the numerous practical experiences to date show that manual operation is better than automatic one. We do it manually too.
It is recommended to switch to automatic mode when the operating conditions are stable; in case of fluctuations, oxygen or coal supply can be adjusted automatically in a timely manner (even if the operator does not notice it). When the conditions are unstable, manual control is used, which makes it easier to stabilize them, but it requires certain operational experience.
How can such a critical controller be set to automatic mode? It has to be in manual mode; if it’s set to automatic, any fluctuations that occur during the transport of coal slurry will cause the controller to act immediately to eliminate those disturbances, and such fluctuations can be quite significant
The slurry flow rate is taken as the median value to control the oxygen-coal ratio; at low loads, due to large fluctuations in the electromagnetic flow meter, the oxygen-coal ratio also fluctuates significantly and frequently, so manual operation is relatively safer. When operating at high loads, if the slurry flow rate is stable with little variation, then automatic control is the best option. Whether to administer it or not depends on the fluctuation of the readings from your electromagnetic flowmeter.
If the oxygen-coal ratio is calculated based on the average flow rate, the fluctuations in this ratio are relatively large; in such a case, should an interlock be activated?