Fuel Balancing Systems
- Description
- Tower Distributors
- Coal Balancing Valves
Typically, pulverized coal delivery systems produce very poor fuel distribution among coal pipes within a mill. Imbalances of ±25-30% are not uncommon. These imbalances increase:
- CO, unburned carbon and NOx above the values that would result if balanced
- The potential for slagging and corrosion from fuel rich burners.
Siemens goal is to improve the balance between burners to within ±10% using aerodynamic principles that do not require on-line coal and air flow measurements and computer control of coal line orifices.
The problem of coal line imbalances is only partially a result of coal line differences such as length and different numbers of bends per pipe. Essentially, the problem results from coal laning inside the mill, and is most effectively addressed inside the specific mill, when possible, or by the use of external balancing devices.
Siemens has solutions for external balancing problems: Tower Distributors and Coal Balancing Valves.
The Tower Distributor, which replaces conventional distributors and rifflers, is applicable to all pulverizers (except vertical roller mills).
- Double-ended ball (tube-type) mills
- Exhauster mills
- Beater mills with multiple outlets
- Vertical ball and race mills
Figures 1 and 2 compare an exhauster mill with a standard riffler to the mill with a Siemens Tower Distributor. The Tower Distributor is designed, like all Siemens products, for long life and reliability. All surfaces in contact with pulverized coal are lined with erosion resistant material for long life. Further, since the tower isolates the burners from the mill, mill upsets do not unbalance the burners and therefore combustion efficiency and NOx control do not suffer.
The figure below shows, over a very broad range, the pipe-to-pipe deviation remains less than ±10%. Note that, at the design point:
- Flow deviation is 5.5%, an improvement of nearly 5:1
- Pressure drop is reduced from 5“ wc to 3.2“ wc.
Application of the Tower Distributor to beater mills and vertical ball and race mills is similar to the exhauster mill, with similar results. Another application is to resolve both fuel balance and auxiliary air flow control problems in double ended ball mills (tube type). Here, pipe-to-pipe coal flow imbalances occur that are exacerbated by imprecise aux air flow control and balancing. The Tower Distributor solves these problems.
Siemens novel coal balancing valve can be used in any coal line, with any type of pulverizer. The balancing valve allows on-line adjustment of primary air and coal flows to the burners, without the limitations of conventional fixed or adjustable orifices.
The balancing valve minimizes coal and primary air imbalances between coal lines to aid in control of both NOx and unburned carbon. Although useful to equalize pressure among coal pipes, the valve is most useful with vertical pressurized pulverizers for which there is not yet available an internal mill modification to eliminate internal coal laning that is the root cause of the imbalance.
The photo above shows a coal balancing valve nearly ready for shipment. Note adjusting mechanism, and flange on side which allows easy removal of internals for maintenance without breaking valve-to-pipe welds. Valve is fully lined with ceramic materials for long life.
The balancing valve has two major advantages as compared to standard fixed and adjustable orifices:
- It can be placed in horizontal or vertical orientation without forming fire-prone coal piles downstream of the orifice blade
- It does not generate coal eddies along the pipe that can eventually wear through the pipe.