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Combustion control of wood-waste & natural gas boiler

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Posted by Cesar on November 10, 2003 at 10:44:20:

Hi everyone, I am in charge of improving the strategies of control of the thermal plant of my company. The thermal plant consists of two boilers for the steam generation of process in 12 saturated bar. The main boiler is 50 ton/hr and uses wood-waste (saw dust, wood chips and bark) as main fuel (up to 40 metric ton/hr of steam) and in addition has a natural gas burner to achieve max capacity of steam generation (50 t/hr)and to support in the response forward to sudden demands. For the wood-waste combustion it has primary air intakes (with the fuel), air under grate and over fire. I have detected two problems with the control of this boiler:

1. Reviewing the present control system of the boiler, we found that that the natural gas and wood-waste burning control is independent the one of the other. Both controls act on the flow of each fuel measuring the pressure in the steam dome. This situation entails to the undesirable situation of which the gas burner elevates the steam production restraining the wood-waste combustion (our company has wood-waste to very low cost).

2. The wood burning control is based on a controller PID who measures pressure in the dome. The exit (0-100%) of this controller is sent to both wood-waste feeders and air dampers (primary, under grate and over fire). The control output is multiplied by a factor (one for each feeder and damper) before being sent to the actuator, then

(a) The air/fuel ratio for wood waste combustion is determined by the operator by setting these factors.

(b) The ratio between feeder's speed and % opening of dampers is linear, which added to the non-linearity of the curve air flow versus % opening of damper obtains that the relation between mass fuel flow and air mass flow is non-linear, obtaining an more air excess than necessary when the boiler operates to half load.

I really appreciate any suggestion, discussion and commentaries with respect to the control of wood combustion and master control of both fuels wood and natural gas. Thanks in advance

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