{"id":94,"date":"2020-05-12T16:12:01","date_gmt":"2020-05-12T14:12:01","guid":{"rendered":"http:\/\/leroux.mobi\/blog\/?p=94"},"modified":"2020-05-12T16:43:33","modified_gmt":"2020-05-12T14:43:33","slug":"boiler-bits-11-coal-boilers-how-light-on-fuel","status":"publish","type":"post","link":"https:\/\/leroux.mobi\/blog\/2020\/05\/12\/boiler-bits-11-coal-boilers-how-light-on-fuel\/","title":{"rendered":"BOILER BITS 13: COAL BOILERS – HOW LIGHT ON FUEL?"},"content":{"rendered":"\n

Often it is only in\nretrospect that one appreciates the value of certain things which happened in\nyour life, or the course your career has taken. One of the great opportunities\nlife offered me was to have worked under the supervision of a very talented\nengineer. Not only was he an intelligent person, but he would always approach a\nproblem from first principles. \u201cIf nothing changes, everything stays the same\u201d\nwas one of his favourite slogans, and often assisted us in a remarkable way to\nget to the root of many problems. But what I remember best was his way of\ncritical thinking about situations. And to some extent I think this has greatly\ninfluenced my way of looking at and evaluating challenging situations crossing\nmy path.<\/p>\n\n\n\n

In my dealings with users of\nsteam boilers critical thinking has often left me amazed at how easily persons,\neven management, believe the most ridiculous claims made by vendors and persons\nresponsible for boiler plant in respect of boiler performance and efficiency.\nClaims like 98% boiler efficiency, 10 tons of steam per ton of coal and 30%\nfuel savings after a control system upgrade are not uncommon. However, if in\ndoubt I go back to first principles to test the claims made and to formulate my\nown opinion on the matter.<\/p>\n\n\n\n

In many circles the\nefficiency of a coal boiler is expressed in terms of a \u201csteam to coal ratio\u201d\nnumber, or SCR in short. A consistent SCR of 7,5 tons of steam per ton of coal\nis more or less the dividing line between good and exceptional. This of course\ndepends in the first place on the quality of the coal used. With A and B grade\ncoals the 7,5 SCR target is well within reach, in conjunction with a premium\ncombustion control system. With lower coal grades the SCR will obviously drop,\nwhich renders the SCR not a true indicator of efficiency, as it uses coal\nusage, rather than energy input, as the basis of its calculation.<\/p>\n\n\n\n

But let us go back to basics\nand investigate a specific scenario by assuming we burn 27,7 MJ\/kg coal in a\nboiler operating at 10 bar gauge pressure with feed water at 80 \u1d52C. Flue gas\ntemperature is at 230 \u1d52C and oxygen at 7% (50% excess air). A pretty efficient\noperating profile, rendering a SCR of 8,5 as per my combustion calculator. This\nis as close to best one can get with a premium combustion control system\nwithout flue gas heat recovery.<\/p>\n\n\n\n

In view of this bench mark\nlet us investigate a few other scenarios:<\/p>\n\n\n\n

  1. In a \u201cfrom and at\u201d 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 \u1d52C. 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.<\/li>
  2. Using D-grade coal (25,0 MJ\/kg) with the same boiler as above, but with feed water temperature at 30 \u1d52C and flue gas temperature at 250 \u1d52C and 16% oxygen, the system only renders a SCR of 4,2. I believe this is as bad as it can get.<\/li>
  3. Thus the worst of operating conditions can double the coal usage compared to best operating practices.<\/li>
  4. But to be fair in one\u2019s 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. <\/li>
  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.<\/li><\/ol>\n\n\n\n

    What do we learn from all of these numbers?<\/p>\n\n\n\n