In my encounters with boiler operators, and even engineers, I still find a general lack of awareness among the operating and supervisory staff as to the significance of excess air. Considering that anything from 10% to 50% of fuel energy input may be wasted to atmosphere because of excess air, it is essential that everybody responsible for boiler performance has a solid understanding of its nature and impact.

In my vocabulary I like to refer to excess air (most unscientifically) as the necessary evil of combustion, or to put it more mildly, the air which hitches a ride through the combustion system without making a direct contribution to the combustion process. But unfortunately excess air is required just to make sure sufficient oxygen is going around to enable every carbon atom to find an oxygen mate (actually two of them!) to combine with and to produce heat and carbon dioxide (CO2) gas.

But maybe we should just refer back to Boiler Bits 5 to explain where we are heading.

The problem with excess air as an energy waster is of course aggravated when burning pea coal fuel. The more atomized the fuel is, the more intimately does it mix with the combustion air and the more rapidly combustion takes place. And the less excess air is required for complete combustion. For example, combustion in a petrol engine only requires some 5% excess air. And when firing a steam boiler with fuel oil or pulverized coal the excess air required is approximately 15%.

With burning of pea coal on a chain grate stoker the situation changes dramatically. The fuel consists of coarse material and the fire has to burn its way through the individual coal pieces for oxygen to reach the innermost fuel atoms, and even more so with less reactive coal. This process takes anything from 30 to 40 minutes at best before all fuel is completely burned out. And this also takes its toll in terms of excess air requirements – anything upwards from 50% to burn the coal to ashes. With less reactive coals excess air may be as high as 80% and more for complete combustion of the coal within a reasonable time on the grate.

Although high excess air levels promote complete combustion of the fuel, as well as cool stoker grates and (almost) no visible smoke from the stack, it wastes fuel energy and may add significantly to the fuel bill.

Let us consider how excess air wastes energy. Assume that 50% excess air is required for the complete combustion of coal on a stoker grate. As we have mentioned earlier the excess air does not take part in combustion and it does not give up any of its oxygen to combine with fuel atoms. It merely enters the furnace at ambient temperature, absorbs the heat liberated by the combustion process, its temperature rises to combustion temperature, it gives up some of the absorbed heat in a heat exchange process to produce steam and exits the boiler with the flue gas at flue gas temperature, which may be anything between 160 ⁰C and 260 ⁰C with a fire tube boiler. Thus there is a net energy loss to excess air which is a function of the excess air percentage and the temperature difference between flue gas and ambient air.

Stack energy loss (with coal fuel) normally constitutes between 50% and 70% of the total energy lost in the steam production process. The excess air contribution to this loss can be estimated if its percentage is known. With coal combustion it can be between 20% and 35% of the total energy loss. No wonder users of steam boilers spend millions on the optimization of excess air control in an endeavour to curb the fuel bill. Of course, and unfortunately so, excess air will always be part of the combustion of fuels and users of steam plant will just have to tolerate the energy loss that goes with it. But having said that, at the current cost of energy even one or two percent loss can run up significant costs over a period of months and years.

Minimizing excess air percentage (and flue gas stack temperature) addresses the single greatest energy loss from the steam boiler.

It is therefore vital that persons responsible for steam plant operations are well aware of the impact of excess air on the cost of steam generation, and are equally knowledgeable about its control and optimization. Achieving this is a topic for a next discussion.

This post was compiled by René le Roux 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.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

Leave a Reply