Imagine how convenient it will be to have an operational parameter which is representative of the percentage excess air of combustion, especially if this parameter can be used to control excess air accurately towards maximizing combustion efficiency. The good news is that such a parameter indeed exists and is being used in many combustion applications, not only with steam boiler operations, but also and in particular in the automotive industry.
As we have mentioned previously excess air does not actively take part in the combustion process and it does not give up any of its oxygen to combine with fuel atoms. It merely enters the furnace as ambient air and travels along through the system with the flue gas. But it leaves an oxygen footprint in the flue gas which tells us quite accurately the percentage excess air supplied to the combustion system.
Unknowingly many of us are using this oxygen footprint for excess air control on a daily basis. Getting behind the steering wheel of your fuel injected motor vehicle you are about to activate oxygen based combustion control which regulates the air and fuel mixture as part of the engine management system. A so called lambda sensor located in the exhaust gas stream continuously monitors the oxygen level of the combustion gas and by this means determines if the mixture is too lean (excessive excess air), or too rich (deficiency of excess air), or just right. It regulates the fuel injection system to continuously adjust the excess air (air-fuel ratio) to keep the engine performing at optimum levels of power, economy and emissions quality.
Well, why not use this technology to enhance boiler performance? Oxygen trim control is widely used with oil, gas and pulverized coal boilers. But how effectively does it perform with chain grate stokers firing pea coal fuel? In my dealings with users of steam boilers I often encounter widely differing responses to oxygen trim control for pea coal firing, ranging from ignorance and apathy to total awe at the perceived superior efficiency of this technology.
But first, back to the classroom. Let us assume that a combustion process firing pea coal requires 50% excess air for optimum combustion of the fuel. The stoichiometric air requirement for the relevant fuel feed is an arbitrary 100 units and a further 50 units of excess air are added, giving us a total combustion air supply of 150 units. The 100 units of stoichiometric air give up all of its oxygen in the combustion process and produce flue gas consisting of CO2 and nitrogen. The 50 units of excess air on the other hand pass through the system unchanged and exit the system containing 79% nitrogen and 21% oxygen on a volume basis. Thus on a volume basis the total flue gas mixture contains (50 units * 21%) = 10,5 units of oxygen. If we do a bit more number crunching (not shown here) we find that the 10,5 units of oxygen represent approximately 7% of the total volume of flue gas discharged through the stack. Or to sum it all up: 7% oxygen in the flue gas is indicative of 50% excess air. [Please note that this is a somewhat simplified approach, as the effects of hydrogen, moisture, sulphur, carbon monoxide, etc. are being ignored. But do not stress, we are not so much into the chemistry as into the principles of combustion and its control, as well as of its practical application.]
This relationship between excess air and flue gas oxygen is a very handy one to know. It means that if the oxygen content of the flue gas can be measured and controlled, the excess air percentage can also be controlled to provide for optimized air-fuel ratios, provided of course one knows the optimum excess air requirement of the fuel being burned.
A well designed combustion control system will often incorporate a flue gas analyzer (oxygen sensor/transducer) to accurately adjust (trim) the combustion air supply in accordance with the fuel feed rate. Theoretically then oxygen trim should enable one to operate the boiler at best efficiency under all load conditions.
But is this a practical reality? Maybe it is easier said than done? In our next edition of Boiler Bits I will explore the Achilles heel of oxygen trim control in greater detail.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about combustion control systems and combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
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