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 and even coal ash fuses into sizeable clinkers.
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.
When coal (containing mostly carbon) is burned in the presence of oxygen we know that 1 atom of carbon combines with 2 atoms of oxygen to form 1 molecule of carbon dioxide, and that heat to the tune of 32,8 MJ is liberated per kg of carbon so combusted. With bituminous coal the energy released varies, but let us assume that with a 26 MJ/kg coal the carbon content is close to 80%.
Our first challenge is to determine the adiabatic temperature of combustion, i.e. a combustion process without heat loss or gain and under stoichiometric conditions. So this is very much a test tube exercise. Air and fuel enters the test system at ambient temperature, the heat of combustion is released in the combustion reaction and the products of combustion (POC) are elevated to the flame temperature. In its most elementary state the equation looks something like this:
CV = POC*Cp*(Tf-Ta), where CV is the calorific value of the fuel, POC is the mass of the products of combustion (flue gas and ash), Cp is the specific heat of the POC, Tf is the flame temperature, and Ta is the ambient temperature of the fuel and air entering the combustion space. It is then possible to calculate the approximate flame temperature (Tf) from this formula.
By way of example: One kg of a certain coal of 26 MJ/kg CV requires 8,8 kg of air for stoichiometric combustion. The POC thus consists of 9,8 kg of combustion gas and ash (combustion calculations are not shown here). For the sake of simplicity we are going to assume the Cp of the POC to be 1,35 kJ/kg⁰C and ambient temperature to be 25 ⁰C. This results in an adiabatic flame temperature of 2054 ⁰C, assuming no heat loss from the combustion process. If we now add 60% excess air to the process the POC becomes 15,0 kg with corresponding adiabatic flame temperature of 1345 ⁰C. Pretty hot in any man’s language and able to soften and melt steel, but substantially lower than with stoichiometric combustion.
In terms of efficiency and heat transfer we can clearly see from the example above that excess air even has an influence on the furnace (flame) temperature, and if 60% of heat is transferred by means of radiation, it is so much more important to keep the flame temperature as high as possible and to exercise proper control over the air-fuel ratio of combustion.
I once witnessed the practical influence of excess air on furnace temperature. This was at a sawmill where wood chips were burned in a Dutch Oven (external furnace) to produce steam. The furnace temperature is monitored continuously and kept below 1100 ⁰C to prevent damage to the refractory. The engineer believed he could improve the efficiency of combustion by increasing the furnace temperature and reducing the stack temperature. This was easily achieved by regulating the air supply to the furnace through two dampers in the front wall. By partially closing the dampers the furnace temperature would increase rapidly, as one would expect in light of the explanation above: less excess air results in a higher flame temperature. Unfortunately this phenomenon cannot be so easily observed with a compact packaged boiler.
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 ⁰C!
This post was compiled by René le Roux for 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.
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