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.
Maybe a word on “optimization” for starters. One of the objectives we strive for with a combustion control system is a consistently high efficiency of the steam raising plant. But because higher efficiency usually comes at a higher cost one has to strike a balance between the cost spent to improve efficiency, and the actual benefits derived from the higher efficiency. Striking this balance embodies the concept of optimization. It obviously does not always pay to invest vast amounts of capital in efficiency improving technology just to gain 1% or 2% in a reduced fuel bill, unless the quantity of fuel consumed warrants such capital expenditure.
Everything you ever wanted to know about combustion rests on three unshakable pillars of physics, namely fuel, oxygen and heat. Yes, I know we were taught the elements of fire in primary school, but if managing combustion under controlled conditions becomes one’s occupation, your approach to the matter will invariably take on a new dimension.
So let us look at combustion a bit closer. A very basic definition of combustion may be something like this: it is the process of burning something (super basic!). Or a more complex and detailed approach: combustion is any process in which a substance (fuel) reacts with oxygen to produce heat and light.
Keeping in mind also that heat (a source of ignition) is required to start and sustain the combustion process. Typically the spark plug in an internal combustion engine or the ignition arch of a coal fired boiler serves this purpose. In many instances the heat liberated by the combustion process is sufficient to sustain it.
Any fireman knows that removing only one of the three pillars of combustion will cause the process to collapse and the fire to be extinguished. Thus all fire fighting practices are based on removing either the oxygen from the fire (spray foam or inert gas), or by removing heat from the fire (spray water), or by removing the fuel (isolate the fuel source). Similarly, interfering with any one of these pillars with your boiler in operation will cause inferior combustion performance, or even entire loss of combustion.
Another aspect of combustion is that the generated heat invariably causes an increase in temperature. It is this high temperature (resulting in a temperature difference between combustion gases and heat exchange surfaces) that causes heat to flow and to perform useful functions and work, such as producing steam.
The chemical nature of combustion also produces by-products of combustion, 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 in the fuel will produce SO2 gas, etc.
But let us get back to the basics of combustion. Due to the chemical nature of combustion a certain amount of oxygen will always combine with a specific amount of a combustible substance during “perfect” combustion of that substance. For instance, 12 kg of carbon requires 32 kg of oxygen for its complete combustion and produces 44 kg of CO2 and some 390 MJ of heat (energy). Unfortunately these numbers are only achievable under conditions of perfect combustion where each atom of carbon finds exactly two atoms of oxygen to combine with, and at the end of the process there is no unburned carbon or oxygen left. This process is also known as stoichiometric combustion.
In the real world we find however that for complete combustion of the fuel more than the stoichiometric quantity of oxygen is required. Furthermore, because of inevitable imperfections in the combustion process (like lack of turbulence and intimate mixing of air and fuel particles) more than the stoichiometric oxygen requirements must be provided to make sure every atom of fuel finds the correct number atoms of oxygen for its complete combustion. This “more than the stoichiometric air requirement” is appropriately called “excess air” and is normally expressed as a percentage of the stoichiometric air requirement.
Because air is normally the carrier of the oxygen we must keep in mind that for every kg of oxygen a total of 4,32 kg of dry air needs to be delivered to the combustion process, consisting of 1 kg of oxygen and some 3,32 kg of nitrogen.
The excess air requirement depends totally on the fuel burned and the combustion environment. Typical excess air requirements are 50% to 80% for pea coal, 5% to 10% for gas and 10% to 20% for fuel oil.
But know for certain that excess air plays a major role in the combustion process, its efficiency and its control. In our next edition of Boiler Bits I will discuss the significance of excess air on the combustion process in more detail.
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.