Advantages and Disadvantages of Combustion Control Schemes

For the basis of this discussion I’ll be covering industrial  boilers of 10MMBtu/hr and higher

Jackshaft Control

jackshaft

The most basic combustion control scheme is a “Jackshaft” control.

Jackshaft position is a single actuator that adjusts a jackshaft based on the demand of the master control output. The fuel and air devices are connected to the jackshaft by a series of cams and linkages. When the jackshaft moves, it moves the fuel valve and air damper to set the fuel/air ratio at various loads. The system is set up during the commissioning of the burner at various firing rates by setting the set screws on the fuel valve and linkage adjustments on the air damper. This achieves desirable excess air levels.

Advantages:

  • Jackshaft controls don’t require any measuring equipment (airflow or fuel flow transmitters)
  • Jackshaft controls are mechanically linked, which allow very fast load changes
  • Jackshaft controls are simplistic with minimal hardware, which demands less maintenance and upfront costs
  • Jackshaft controls are inherently safe because the fuel and air are mechanically linked together. This decreases the likelihood of a problem occurring with mechanical failure significantly.

Disadvantages:

  • Jackshaft controls don’t have feedback and/or measurements, so the excess air is often setup higher than optimal efficiency
  • Jackshaft controls have no way of compensating for ambient changes in temperature or heating value for fuel
  • Jackshaft controls allow adding O2 trim as an option; however, this is usually complex and costly with higher maintenance costs and reduces the inherent safety of the system (another point of failure)
  • Jackshaft controls have Limitations with firing different fuels because only one fuel can be fired at one time and only one fuel can be set optimally.

Parallel Control

Parallel Control

Parallel control systems have separate controllers for the fuel and air. Feedback is provided to ensure that the system is at its predetermined positions. The control scheme uses cross limiting that prevents the feedback of the fuel from being greater than the feedback of the air. This system allows more complex control scheme, especially in a multi-fuel burner like natural gas and diesel fuel.

Advantages:

  • Parallel controls can adapt to O2 more efficiently than Jackshaft controls can
  • Parallel controls have a more complex control scheme that allows burners to fire multi-fuel at their optimal excess air.
  • Parallel controls give the end user more flexibility in the fuel/air ratio.
  • Parallel controls allow more sophisticated burners can be used (Low NOx, Ultra Low NOx, specialty fuels).

Disadvantages:

  • Parallel controls don’t measure the fuel and air. Therefore, end users must have further training and operate at higher levels
  • Parallel controls cannot compensate for ambient changes in temperature or heating value of the fuel.
  • Parallel controls require more sophisticated and demanding maintenance and tuning to ensure that the feedback signal matches the demand signal
  • The higher cost of installing a parallel control systems are more costly to install than jackshaft control systems, and the parallel control system doesn’t have a substantive efficiency gain compared to jackshaft control system with O2 trim
  • Parallel controls have a higher risk of failure due to the more complex system

Fully Metered control

Full Metered Control Scheme

Fully metered control schemes use fuel flow and airflow transmitters to control the excess air of the burner at all firing rates. This control scheme addresses the shortcomings of the parallel control scheme. Fully metered control schemes use cross limiting to prevent the burner from becoming fuel rich.

Advantages:

  • Fully metered controls provide precise fuel/air ratios at all firing rates for optimal efficiency
    • Fully metered controls can correct fuel/air ratio for ambient changes in air temperature and fuel heating values
    • Fully metered controls give the end user more flexibility over the fuel/air ratio
    • Fully metered controls allow multi-fuels can be fired at their optimal efficiency
    • Fully metered controls allow simultaneous fuel firing can be done at the optimal efficiency
    • Fully metered burners fuels offer flexibility by giving the end users a wide range of choices of burner selections
    • Fully metered controls have  a slightly higher cost than parallel controls

Disadvantages:

    • Fully metered controls have a high upfront cost, especially compared to jackshaft controls
    • Fully metered controls require higher levels of maintenance and setup because of adding transmitters and selection of equipment (fuel valves, dampers, etc.)
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