I am often\nquestioned by operating staff as to the approximate temperature of the fire in\nthe furnace of their boiler. It is clear that people perceive the fire to be\nquite hot and are often inquisitive to know exactly how hot. Especially if the\nguillotine door suffers heat damage, steel components warp and melt and even\ncoal ash fuses into sizeable clinkers. <\/p>\n\n\n\n
Good news is that there is a scientific way of calculating or estimating the flame temperature in absence of a suitable temperature measuring device. So we will be taking the scientific route to determine the flame temperature with coal as our source of fuel. <\/p>\n\n\n\n
When coal (containing mostly\ncarbon) is burned in the presence of oxygen we know that 1 atom of carbon\ncombines with 2 atoms of oxygen to form 1 molecule of carbon dioxide, and that\nheat to the tune of 32,8 MJ is liberated per kg of carbon so combusted. With\nbituminous coal the energy released varies, but let us assume that with a 26\nMJ\/kg coal the carbon content is close to 80%. <\/p>\n\n\n\n
Our first challenge is to\ndetermine the adiabatic temperature of combustion, i.e. a combustion process\nwithout heat loss or gain and under stoichiometric conditions. So this is very\nmuch a test tube exercise. Air and fuel enters the test system at ambient\ntemperature, the heat of combustion is released in the combustion reaction and\nthe products of combustion (POC) are elevated to the flame temperature. In its\nmost elementary state the equation looks something like this:<\/p>\n\n\n\n
CV = POC*Cp*(Tf-Ta), where CV is\nthe calorific value of the fuel, POC is the mass of the products of combustion\n(flue gas and ash), Cp is the specific heat of the POC, Tf is the flame\ntemperature, and Ta is the ambient temperature of the fuel and air entering the\ncombustion space. It is then possible to calculate the approximate flame\ntemperature (Tf) from this formula.<\/p>\n\n\n\n
By way of example: One kg of a\ncertain coal of 26 MJ\/kg CV requires 8,8 kg of air for stoichiometric\ncombustion. The POC thus consists of 9,8 kg of combustion gas and ash\n(combustion calculations are not shown here). For the sake of simplicity we are\ngoing to assume the Cp of the POC to be 1,35 kJ\/kg\u2070C and ambient temperature to\nbe 25 \u2070C. This results in an adiabatic flame temperature of 2054 \u2070C, assuming\nno heat loss from the combustion process. If we now add 60% excess air to the\nprocess the POC becomes 15,0 kg with corresponding adiabatic flame temperature\nof 1345 \u2070C. Pretty hot in any man\u2019s language and able to soften and melt steel,\nbut substantially lower than with stoichiometric combustion. <\/p>\n\n\n\n
In terms of efficiency and heat\ntransfer we can clearly see from the example above that excess air even has an\ninfluence on the furnace (flame) temperature, and if 60% of heat is transferred\nby means of radiation, it is so much more important to keep the flame\ntemperature as high as possible and to exercise proper control over the\nair-fuel ratio of combustion. <\/p>\n\n\n\n
I once witnessed the practical\ninfluence of excess air on furnace temperature. This was at a sawmill where\nwood chips were burned in a Dutch Oven (external furnace) to produce steam. The\nfurnace temperature is monitored continuously and kept below 1100 \u2070C to prevent\ndamage to the refractory. The engineer believed he could improve the efficiency\nof combustion by increasing the furnace temperature and reducing the stack\ntemperature. This was easily achieved by regulating the air supply to the\nfurnace through two dampers in the front wall. By partially closing the dampers\nthe furnace temperature would increase rapidly, as one would expect in light of\nthe explanation above: less excess air results in a higher flame temperature. Unfortunately\nthis phenomenon cannot be so easily observed with a compact packaged boiler. <\/p>\n\n\n\n
By the way, have you ever considered firing your boiler with coal and pure oxygen? POC is reduced to 2,85 kg and calculated adiabatic flame temperature increases to a staggering 6780 \u2070C! <\/p>\n\n\n\n
This post was compiled by\u00a0Ren\u00e9 le Roux<\/strong>\u00a0for 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.<\/em><\/p>\n\n\n\n Kindly note\nthat our posts do not constitute professional advice and the comments, opinions\nand conclusions drawn from this post must be evaluated and implemented with\ndiscretion by our readers at their own risk.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" I am often questioned by operating staff as to the approximate temperature of the fire in the furnace of their boiler. It is clear that people perceive the fire to be quite hot and are often inquisitive to know exactly how hot. Especially if the guillotine door suffers heat damage, steel components warp and melt … <\/p>\n