My first four Boiler Bits articles primarily focused on the potential of boiler automation technology to support Management in achieving their objectives and mission. From this issue of Boiler Bits onward I think it appropriate to shift our attention to the more technical side of steam plant operations. My passion is with optimized boiler performance and I would like to share some insights and experiences gained in this respect over many years. Unfortunately this requires some academic background, which should not be unfamiliar territory for combustion engineers. But just to make sure we all start off from the same page our readers will have to master some initial scientific principles pertaining to the art of combustion.<\/p>\n\n\n\n
Maybe a word on \u201coptimization\u201d for starters. One of the objectives we\nstrive for with a combustion control system is a consistently high efficiency\nof the steam raising plant. But because higher efficiency usually comes at a\nhigher cost one has to strike a balance between the cost spent to improve\nefficiency, and the actual benefits derived from the higher efficiency.\nStriking this balance embodies the concept of optimization. It obviously does\nnot always pay to invest vast amounts of capital in efficiency improving\ntechnology just to gain 1% or 2% in a reduced fuel bill, unless the quantity of\nfuel consumed warrants such capital expenditure.<\/p>\n\n\n\n
Everything you ever wanted to know about combustion rests on three\nunshakable pillars of physics, namely fuel, oxygen and heat. Yes, I know we\nwere taught the elements of fire in primary school, but if managing combustion\nunder controlled conditions becomes one\u2019s occupation, your approach to the\nmatter will invariably take on a new dimension. <\/p>\n\n\n\n
So let us look at combustion a bit closer. A very basic definition of\ncombustion may be something like this: it is the process of burning something\n(super basic!). Or a more complex and detailed approach: combustion is any\nprocess in which a substance (fuel) reacts with oxygen to produce heat and\nlight.<\/p>\n\n\n\n
Keeping in mind also that heat (a source of ignition) is required to\nstart and sustain the combustion process. Typically the spark plug in an\ninternal combustion engine or the ignition arch of a coal fired boiler serves\nthis purpose. In many instances the heat liberated by the combustion process is\nsufficient to sustain it.<\/p>\n\n\n\n
Any fireman knows that removing only one of the three pillars of\ncombustion will cause the process to collapse and the fire to be extinguished.\nThus all fire fighting practices are based on removing either the oxygen from\nthe fire (spray foam or inert gas), or by removing heat from the fire (spray\nwater), or by removing the fuel (isolate the fuel source). Similarly,\ninterfering with any one of these pillars with your boiler in operation will\ncause inferior combustion performance, or even entire loss of combustion.<\/p>\n\n\n\n
Another aspect of combustion is that the generated heat invariably\ncauses an increase in temperature. It is this high temperature (resulting in a\ntemperature difference between combustion gases and heat exchange surfaces)\nthat causes heat to flow and to perform useful functions and work, such as\nproducing steam.<\/p>\n\n\n\n
The chemical nature of combustion also produces by-products of\ncombustion, typically CO and CO2 if the fuel contains carbon. If the fuel contains hydrogen (H2), water (H2O) may be produced as a by-product; sulphur\nin the fuel will produce SO2 gas, etc. <\/p>\n\n\n\n
But let us get back to the basics of combustion. Due to the chemical\nnature of combustion a certain amount of oxygen will always combine with a\nspecific amount of a combustible substance during \u201cperfect\u201d combustion of that\nsubstance. For instance, 12 kg of carbon requires 32 kg of oxygen for its\ncomplete combustion and produces 44 kg of CO2 and some 390 MJ of heat (energy). Unfortunately these numbers are only\nachievable under conditions of perfect combustion where each atom of carbon\nfinds exactly two atoms of oxygen to combine with, and at the end of the\nprocess there is no unburned carbon or oxygen left. This process is also known\nas stoichiometric combustion.<\/p>\n\n\n\n
In the real world we find however that for complete combustion of the\nfuel more than the stoichiometric quantity of oxygen is required. Furthermore,\nbecause of inevitable imperfections in the combustion process (like lack of\nturbulence and intimate mixing of air and fuel particles) more than the\nstoichiometric oxygen requirements must be provided to make sure every atom of\nfuel finds the correct number atoms of oxygen for its complete combustion. This\n\u201cmore than the stoichiometric air requirement\u201d is appropriately called \u201cexcess\nair\u201d and is normally expressed as a percentage of the stoichiometric air\nrequirement.<\/p>\n\n\n\n
Because air is normally the carrier of the oxygen we must keep in mind\nthat for every kg of oxygen a total of 4,32 kg of dry air needs to be delivered\nto the combustion process, consisting of 1 kg of oxygen and some 3,32 kg of\nnitrogen.<\/p>\n\n\n\n
The excess air requirement depends totally on the fuel burned and the\ncombustion environment. Typical excess air requirements are 50% to 80% for pea\ncoal, 5% to 10% for gas and 10% to 20% for fuel oil. <\/p>\n\n\n\n
But know for certain that excess air plays a major role in the\ncombustion process, its efficiency and its control. In our next edition of\nBoiler Bits I will discuss the significance of excess air on the combustion\nprocess in more detail.<\/p>\n\n\n\n
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This post was compiled by Ren\u00e9 le Roux<\/strong> for Le Roux Combustion, all rights reserved. Do you want to know more about boilers and optimization of combustion? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.<\/em><\/p>\n\n\n\n Kindly note that our posts do not constitute professional\nadvice and the comments, opinions and conclusions drawn from this post must be\nevaluated and implemented with discretion by our readers at their own risk.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" My first four Boiler Bits articles primarily focused on the potential of boiler automation technology to support Management in achieving their objectives and mission. From this issue of Boiler Bits onward I think it appropriate to shift our attention to the more technical side of steam plant operations. My passion is with optimized boiler performance … <\/p>\n