Often it is only in retrospect that one appreciates the value of certain things which happened in your life, or the course your career has taken. One of the great opportunities life offered me was to have worked under the supervision of a very talented engineer. Not only was he an intelligent person, but he would always approach a problem from first principles. “If nothing changes, everything stays the same” was one of his favourite slogans, and often assisted us in a remarkable way to get to the root of many problems. But what I remember best was his way of critical thinking about situations. And to some extent I think this has greatly influenced my way of looking at and evaluating challenging situations crossing my path.

In my dealings with users of steam boilers critical thinking has often left me amazed at how easily persons, even management, believe the most ridiculous claims made by vendors and persons responsible for boiler plant in respect of boiler performance and efficiency. Claims like 98% boiler efficiency, 10 tons of steam per ton of coal and 30% fuel savings after a control system upgrade are not uncommon. However, if in doubt I go back to first principles to test the claims made and to formulate my own opinion on the matter.

In many circles the efficiency of a coal boiler is expressed in terms of a “steam to coal ratio” number, or SCR in short. A consistent SCR of 7,5 tons of steam per ton of coal is more or less the dividing line between good and exceptional. This of course depends in the first place on the quality of the coal used. With A and B grade coals the 7,5 SCR target is well within reach, in conjunction with a premium combustion control system. With lower coal grades the SCR will obviously drop, which renders the SCR not a true indicator of efficiency, as it uses coal usage, rather than energy input, as the basis of its calculation.

But let us go back to basics and investigate a specific scenario by assuming we burn 27,7 MJ/kg coal in a boiler operating at 10 bar gauge pressure with feed water at 80 ᵒC. Flue gas temperature is at 230 ᵒC and oxygen at 7% (50% excess air). A pretty efficient operating profile, rendering a SCR of 8,5 as per my combustion calculator. This is as close to best one can get with a premium combustion control system without flue gas heat recovery.

In view of this bench mark let us investigate a few other scenarios:

  1. In a “from and at” situation (popular with boiler manufacturers when specifying boiler capacity) the feed water temperature is assumed to be at saturated steam pressure, which normally is atmospheric or 100 ᵒC. Under these conditions, and assuming similar coal as with our bench mark model, SCR can reach 9,6. If the boiler operates at a hypothetical 100% efficiency (no energy losses) the SCR reaches a theoretical maximum of 12,27.
  2. Using D-grade coal (25,0 MJ/kg) with the same boiler as above, but with feed water temperature at 30 ᵒC and flue gas temperature at 250 ᵒC and 16% oxygen, the system only renders a SCR of 4,2. I believe this is as bad as it can get.
  3. Thus the worst of operating conditions can double the coal usage compared to best operating practices.
  4. But to be fair in one’s comparison of combustion control systems we must assume that we use the same coal and feed water in both instances, and that only certain relevant combustion parameters change because of a control system upgrade or replacement.
  5. This takes us back to our bench mark system which is based on a premium control system, with a SCR = 8,5 as calculated previously. If we assume this boiler was previously fitted with a manual control system and that operators could only manage 14% flue gas oxygen level (200% excess air), it would have rendered a SCR of 7,3. The overall improvement gained from worst to best would be 17,3%, based on the lower efficiency.

What do we learn from all of these numbers?

  • If it appears too good to be true, it probably is.
  • Do not believe everyone who claims a SCR in excess of 8,5 unless stack heat is being recovered. Anything approaching 9,0 must be treated with a good measure of suspicion, as this will only be possible if all heat losses, except stack losses, are ignored.
  • Do not believe everyone who claims to have saved in excess of 20% fuel as result of an upgraded control system. It just does not happen so easily in that way, unless other interventions took place at the time of the upgrade, such as boiler cleaning, upgrading of condensate return, burning of higher CV coal, etc. The norm for fuel savings due to upgrading a basic control system is between 6% and 15%.
  • Mostly excessively high efficiency is a matter of incorrect measuring of coal used and/or steam produced. With coal the accurate estimation of usage is normally a problem due to challenges with estimating opening and closing coal stock. With steam consumption it is customary to measure feed water flow as a substitute for steam flow. Any blow down or other water wastage after the flow meter counts for steam production. High levels of feed water wastage can significantly boost the SCR. And this wastage often goes unnoticed.
  • Normally short term determination of SCR is not accurate; one has to calculate this number on at least a daily basis, covering at least a 24 hour period.
  • It is the duty of the user of the boiler to be objective regarding efficiency claims made by vendors and even by operating staff. And there is only one way of doing this – revert to the first principles governing the matter. If the claims made do not tie up with first principles the red lights must start flickering. 

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about combustion control systems and 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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