Feed Forward Control

[maypot]

Hi,

We have a 80 t/hr bagasse fired boiler operating at 30 Bar 450 deg C. Actually, the output of the master pressure controller feeds the ID fan Inverter through a ratio controller. We have been advised to use feed forward control instead. Basically, what is a feed forward control ?

Thanks.

Bob

 

[dbaird]

FEEDFORWARD CONTROL - A method of control that compensates for a disturbance before its effect is felt in the output. It is based on a model that relates the output to the input where the disturbance occurs. In distillation, the disturbances are usually feed rate and feed compositions. Steady-state feedforward models are usually combined with dynamic compensation functions to set the manipulative variables and combined with feedback adjustments (trim) to correct for control model-accuracy constraints.



David Baird
mrbaird@hotmail.com
Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[davefitz]

I am not sure this is the correct interpretation, but most drum pressure controllers will adjust firing rate based on deviation of drum pressure from setpoint.

Bagasse is a slow burning fuel , and it is possible that the current air flow control is primarlily based on feedback to hold Oxygen in the exhaust at 3% O2. If so, then one problem with feedback control of air flow is that you will get spikes in CO if there is a large increase in bagasse input ( following a demand for more fuel due to low drum pressure).

If the above is a correct interpretaion of your problem, then you will need to modify the air flow control logic to something similar to what is used for faster burning fuels ( such as oil, nat gas). That is , when the drum pressure drops below setpoint and increases demand for more bagasse, you should increase airflow in parrellel with increases in fuel flow without waiting for O2 to drop. For firign rte decreases, though, you must lag the decrease in air flow to avoid CO spikes or flame out. This may be mandated by NFPA.

 

[25362]

Quoting from Shinskey's Process Control Systems (McGraw Hill):

It is the intent of a feed forward system to force the process to respond as it was designed, to follow the set points as directed without regard to load upsets.

The book brings a variety of examples of feed forward control worth reading.

Paul W, Murrill's Application Concepts of Process Control (ISA) brings a conceptual definition:

"Feedback control worked to eliminate errors bur feed forward control operates to prevent errors from occurring in the first place."

 

[rmw]

To put it in terms simplistic to your bagasse boiler, the feedback control would react when drum pressure dropped on a load increase, and then react, adding fuel and combustion air, smoke, etc. whereas a feed forward controller would look at steam flow, and react to the increased flow that it sees before (and while) the drum pressure moved off of set point.

rmw

 

[fzacamw]

ID fan is most probably taking charge of furnace pressure/draft control, by taking gases out of the furnace. A change in fuel and air input will also mean a change in the mass flow of combustion gases the ID fan has to handle. By feed-forwarding the boiler master output (i.e., the load demand), the ID fan will modify the flow of gases at the same time they change, before the error in the furnace pressure has developed.