Boiler combustion controls for multifuel and biomass power plants

The main control loops and logics developed for a fluidized bed boiler island

• Solid fuel controls (coal, biomass, waste wood and other)
• Oil pressure / flow control (start-up burners, load burners)
• Air flow controls and division
• Furnace pressure control
• Steam pressure and temperature controls
• Start-up valve control
• Drum level / feedwater flow control
• Bed pressure and temperature controls
• SO₂ and NO_x control
• Boiler protection and burner management
• Controls of flue gas cleaning processes
• Start and stop sequence programs for subprocesses
• Automatic boiler and power plant start-up sequences

Fuel power compensator

Solid fuel flow or the fuel heat input cannot be measured accurately especially when the fuel mixture or the quality is constantly changing. Big variations in the fuel quality cause disturbances to the stable and efficient combustion process and load control. For this reason, Valmet has developed a special control application, Valmet DNA Fuel Power Compensator.

The DNA Fuel Power Compensator calculates the fuel heat input according to boiler balance calculation (steam) and oxygen consumption (flue gas) and compensates the changes in fuel quality or amount by controlling the fuel feeding.

DNA Fuel Power Compensator calculates the fuel heat using indirect measurements and provides faster and more stable load control compared to typical steam pressure control or boiler master control. Also, changing the main fuel with very different heat values (hard coal vs. biofuel) can be done quickly and automatically without disturbances in the boiler process. The continuous calculation of the fuel heat power also improves the performance of the air flow controls and steam temperature control.

Advanced live steam temperature controller

Advanced live steam temperature control greatly improves the desuperheater spray’s ability to respond to combustion disturbances. Whereas traditional cascade control of superheated steam temperature control needs to be tuned to react relatively slowly to combustion disturbances and to ensure control stability, the advanced model-based steam temperature control can react rapidly to both steam-side and combustion disturbances.

This is possible as the model-based controller uses just one PI controller instead of the cascade control. The superheater model is used to predict the effect of spraying the temperature after the superheater. This enables tuning the PI controller based on a dynamic response from the spray to the steam temperature right after the spray. It results in at least one order of magnitude faster controller response than what is possible with traditional cascade control.

Moreover, to cover all possible boiler loads, the controller is equipped with adaptive models. Such an approach guarantees that spray usage can be properly adapted to typical boiler loads, as there are different dynamics when the boiler operates on low load versus a high load, or during sootblowing.

Advanced live steam temperature controller

Fuel allocator

The fuel allocator is a control solution developed for multifuel boilers for handling the fuel feeding with fuels of different quality. This maintains a good performance of the boiler load control, even during limitations or disturbances in the fuel feeding. The purpose of the allocator is to divide the fuel demand between the fuels and the fuel feeding points that are in use at the time, according to the division decided on by the operator. The fuels in use can be solid, liquid or gas.

The allocator takes into account fuel heat values, minimum and maximum limitations and the change rate of each feeder or burner. In case of some limitation or disturbance in fuel feeding, the fuel load is allocated to the other feeders of the same fuel and/or for other fuels that are in use and selected for automatic control.

Fuel allocator structure