I am not often caught off guard in matters of boiler efficiency, although I must admit there are still phenomenons in this respect I find difficult to explain. But at least it creates a sense of expectation to know there is still more to be explored and discovered.

A few years ago we were called out to service a boiler control panel for a client. The panel was equipped with frequency inverters for the fans and stokers, a plc and state of the art oxygen trim control module, complete with flue gas oxygen analyzer. But a short circuit in the panel (caused by the client’s artisan) blew the output cards of the majority of the VSD’s and we were requested to reset the plc and control parameters. Within a few hours the boiler was back in service and producing steam, and everything seemed to back to normal.

A week later I received a call from a somewhat irate plant manager. His energy monitoring records showed that since our repairs to the control panel his boiler consumed approximately 20% more electrical power than before. Now this was a completely new one to me. If he complained about higher fuel consumption I could immediately think of at least five possible causes. But electrical consumption – this was completely unexpected. And I had no ready answer, as a matter of fact, I was dumb struck! I requested more information and he forwarded me the appropriate power consumption trends of his boiler during the past month. It showed a clear increase in electrical consumption since the time of the boiler being back on range.

I stretched my mind as best I could to recall what was actually done on the day of the service. And then it slowly started to dawn on me that we adjusted the oxygen set point. I remember that initially there was excessive smoke from the stack and we adjusted the flue gas oxygen set point upwards from 7,5% to 8,5% to clear some of the smoke. With the smoke problem out of the way we continued to complete the service. But could this have had a 20% impact on the boiler’s power consumption? – a mere 1% increase in flue gas oxygen level?

It was back to the drawing board for us, back to the first principles of fan laws and combustion calculations. So here we go, in a nutshell: 1% increase of the flue gas oxygen level means 22,4% increase of excess air and 8% increase of combustion air. According to fan laws fan power is a function of (fan rpm)³, which is (1,08)³ = 1,26. Mystery solved! Our 1% upward adjustment of the oxygen set point increased fan speeds by some 8% and fan power consumption by some 26%.

This incident really was an eye opener to me in a number of ways:

  1. We tend to focus so much on combustion optimization that we often forget that electrical consumption of boiler motors does form part of the total energy equation.
  2. Adjustment of any combustion parameter must be done with full consideration of its effect elsewhere in the system. I somehow get the impression that it is often just too easy to increase excess air to eliminate problems with smoke or stoker temperatures and nobody cares to find more novel solutions, or realizes how much electrical energy may be wasted in the process.
  3. Very few users of steam boilers measure or manage their boiler’s electricity consumption. The one client who did so, did me a huge favour in bringing the correlation between flue gas oxygen and fan power consumption to my attention.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

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