Efficiency in boiler operations is serious business, and\nrightfully so. A coal steam boiler typically consumes 20 times the value of its\ncapital investment in the cost of fuel during it economic lifespan. So if one\nspends any amount of effort acquiring the right boiler for the job, one should\nby comparison spend 20 times as much effort in ensuring that every bit of fuel\nproduces its maximum contribution to the process of steam generation.\nConsidering too that the basic design of the packaged fire tube boiler has not\nundergone any significant change since the times when coal was generally\nplentiful, cheap and of excellent quality, making it so much harder for steam\nusers to improve efficiency with odds stacked against their reasonable efforts.<\/p>\n\n\n\n
It is therefore clear that achieving high boiler plant efficiency is no\nstroke of luck or something that falls into one\u2019s lap. It takes hard work,\nunderstanding, dedication and excellent management to continuously operate a\nsteam boiler at peak efficiency. And all for one objective: to produce steam at\nthe lowest cost per unit. But also understanding that assets must be preserved\nand the carbon footprint minimized in pursuit of peak efficiency.<\/p>\n\n\n\n
So let us jump straight into some of the definitions pertaining to\n\u201cefficiency\u201d, and there is a multitude of different interpretations of it; one\ncan easily get totally confused. But in essence efficiency expresses as a\npercentage the ratio of useful energy output from a system to the total energy\ninput into the system. In a boiler environment the energy input comes from the\nfuel, the energy output is found in the energy added to the produced steam.<\/p>\n\n\n\n
In a perfect system without any energy loss the efficiency will be 100%.\nUnfortunately we live in a broken world and energy loss in any thermal system\nis a reality. The real challenge is minimizing the energy loss from the system\non a continuous basis, remembering always that energy once lost from the\nsystem cannot be recovered!<\/em><\/p>\n\n\n\n Our first challenge is to clarify the value of energy input per unit of\nfuel. Some schools prefer gross calorific value (GCV, also called higher\nheating value), others net calorific value (NCV, also called lower heating\nvalue). The difference between the two is the latent heat of water vapour formed\nduring combustion. Thus NCV = GCV \u2013 Latent Heat. Using GCV renders a lower\nefficiency percentage than when NCV is used.<\/em> So next time you encounter\nboiler efficiency statistics and comparisons, clarify beforehand if it is based\non GCV or NCV. It can make a significant difference, particularly with oil and\ngas fired boilers where the hydrogen content of the fuel is relatively high.\nWith coal fired boilers it is customary to use GCV in efficiency calculations;\nwith oil and gas fired boilers it is NCV. <\/p>\n\n\n\n To complicate matters further efficiency can be calculated in two\ndifferent ways; according to the direct method or to the indirect method.<\/p>\n\n\n\n The direct method<\/strong> is based on energy added to the steam as a\npercentage of the fuel energy input. The equation below renders the system (or\noverall or thermal) efficiency (not the boiler efficiency!), where<\/p>\n\n\n\n The indirect method<\/strong> starts off with 100% of fuel energy input\nand then deducts individual calculated or estimated energy losses encountered\nwithin the combustion process, such as stack loss, carbon loss, blow down loss,\nshell loss, etc. The indirect method calculates energy loss across the boiler only\nand normally fails to address energy loss downstream of the boiler, i.e. along\nsteam and condensate lines and in steam application processes. These can\nhowever be approximated if the make-up water percentage and temperature is\nknown, since make-up water replenishes downstream steam and condensate losses.\nI use this approach in my combustion calculator; not absolutely accurate, but\nsufficiently useful for purposes of evaluating boiler performance under various\noperating conditions, or with coal of differing characteristics.<\/p>\n\n\n\n