Our engineers were among the pioneers at developing and applying overfire air systems to cyclone-fired boilers.

The B&W cyclone is an oddity among the various firing systems used in electric utilities. Back in the 1950s and 1960s the cyclone was thought to be the final answer for fuel flexibility with minimum conditioning, and B&W was selling a lot of them. Then NOx regulations appeared and the cyclone, which fired extremely hot, produced so much NOx that it seemed to be doomed. Indeed, not many were built after about 1970 because of the new regulations. The loss of interest was accompanied by a loss of cyclone experts, as the inventors retired and no one really replaced them.

Today we have found new ways to drastically reduce NOx produced by the cyclone, with the primary method being overfire air. Our engineers were among the pioneers at developing and applying overfire air systems to cyclone-fired boilers, beginning in the early 1990s. Currently we have either designed, tuned, or participated in the design of approximately 14 cyclone units overfire air systems.

In addition to overfire air systems, we have the experience and expertise to correctly tune, control and operate cyclone-fired boilers. We have tuned or redesigned cyclones with vortex burners and scroll burners as well as the much more common radial burner. Other projects include LOI reduction, improving slagging, CO reduction and damper automation. Techniques include both primary and secondary air kinetic energy minimization technique via CFD modeling and tuning, independently controlling primary air pressure and flow for more precise velocity control, adding OFA to reduce cyclone air throughput and re-entry velocity, coal grind correction, split damper application and tuning, and blanking plate design. We use Reaction Engineering International (REI) for all cyclone CFD modeling.

Typical First Principles Program for Cyclone-Fired Units

  • Obtain a full range of expected fuel analyses as well as necessary drawings, dimensions and data on the boiler, burners, classifiers, coal conduits, mills, feeders and fans.
  • Perform boiler performance tests. Include boiler exit gas grid sampling and analysis.
  • Develop Heat Input Chart for the subject boiler. The chart helps the team decide which direction the program will take.


  • Correctly sample coal feed and sieve for grind. Also obtain several other analyses such as ultimate, HHV, T250, 8-point ash fusion temps, and ash mineral analysis.
  • Correct grind if necessary.
  • During an outage, perform an internal cyclone inspection. Part of the inspection will be to step secondary air dampers from close to open, and back, taking measurements at each stop. Data will be used to identify hysteresis, and for DCS velocity calculations.
  • Implement secondary air damper velocity calculations in the DCS.
  • Tune cyclones, starting with primary air and tertiary air, followed by secondary air. Secondary air tuning to be influenced by DCS velocity calculations as part of kinetic energy minimization. Velocity may be adjusted if necessary via modulating windbox pressure.
  • Load-following units may require modulating primary air control to follow load.
  • Evaluate the unit O2 curve and adjust if necessary. This involves consideration of slagging, fouling, steam temperatures and sprays.
  • Determine total air flow.
  • Evaluate FD and ID fan sizes, if necessary.