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Posted by Gburns on November 11, 2003 at 09:25:00:
In Reply to: Combustion control of wood-waste & natural gas boiler posted by Cesar on November 10, 2003 at 10:44:20:
The natural gas and the waste-wood controllers should be separate, however, they should both be controlling off of the same pressure. The natural gas controller's setpoint should be set lower than the waste-wood. This will result in maximizing waste-wood combustion while minimizing natural gas usage while maintaining the natural gas controller in automatic to react to pressure swings if needed.
Your air system also sounds like it could use some updating. Do you have airflows for undergrate and overfire air? Do you measure excess oxygen at the boiler outlet/stack?
To support response times you would also need some feedforward in a couple areas.
1. A feed forward to the undergrate air (based on rate of change which goes to zero over time) can help the boiler react quicker on waste-wood alone. The inventory of waste-wood you have on the grates at any given time can be rapidly burned to help load by increasing overfire air. This gives the boiler a quick increase in steam flow until the wood feeders can react.
2. A feed forward to the natural gas so you don't have to wait for a drop in header pressure on large swings. It is important that these feed forward signals be based on rate of change and go to zero overtime.
You may want to consider brining in a boiler control specialist. There are several good control plans for boilers like yours but it is difficult to go over all details in this forum.
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