Poor Combustion Waste Money
Proper combustion is safer and saves you money. In this article we will review the basics of combustion, explain the difference between good and poor combustion and share an example of the economic impact for your plant. Proper boiler service and routine maintenance ensures your boiler is operating at peak efficiency, with good and safe combustion.
Combustion, simply defined is; an act or instance of burning, a usually rapid chemical process (as oxidation) that produces heat and usually light. For boilers and their burners, this typically involves carbon-based fuels, such as Natural Gas or Light Oil (#2). During this process the fuel (here comprised primarily of carbon and hydrogen) react with oxygen. This reaction produces heat (useful), light, and products of combustion and byproducts of combustion (these make up your flue gas). There is also “excess air”, which we will talk more about in a moment.
The heat generated, is what is used to add energy to the boiler and create hot water or steam. The products of combustion are CO2 (an indicator of complete combustion – each fuel has a maximum % CO2 indicating most complete and efficient combustion), H2O (water vapor) and CO (an indication of incomplete combustion). The byproducts are Nitric Oxide, Oxides of Nitrogen (NOx), and depending on the fuel there may be some sulfur-based or other compounds. Any air (comprised mostly of Oxygen and Nitrogen), in the flue gas, not used by the combustion process is called excess air. This is typically expresses as the % of O2 present in the boiler stack.
There is a minimum, required amount of oxygen needed, to complete the combustion process for a given amount of fuel. If you were to burn a fuel at this exact ratio, this would be called stoichiometric combustion. This is the “ideal” combustion, where the air and fuel are completely consumed and there is no excess. This type of combustion is only practical in a laboratory environment. In the real world, where mixing of the air and fuel is never perfect, excess air is needed to ensure complete combustion. For a typical industrial burner, ~3% of excess stack O2 is required, to ensure complete combustion. Note: this ~3% is based on high fire and typically through a “reasonable” turn-down range. At higher turn-down ratios, the amount of excess air needed for complete combustion rises.
Excess air is good, in that it provides a margin of safety and ensures complete combustion. But, it also results in lower efficiency, if excess, excess air is used. Excess air is comprised mostly of Oxygen and Nitrogen. This air simply carries heat out of the boiler and does not contribute to the combustion process. This raises the stack temperature and reduces boiler efficiency.
With too little excess air, you will see incomplete combustion, indicated by rising CO. If insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide are exhausted from the boiler. The results is heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion or backfire. With too much excess air, you will have complete combustion, but a diluted CO2 percentage in the products of combustion and higher stack temperatures.
The percentage of CO2, Stack O2, and NET stack temperature are either read from appropriate combustion/fuel curves or directly interpreted via a combustion analyzer. The result of these is the boiler’s Combustion Efficiency (some use the term Thermal Efficiency) – this is defined as 100% efficiency – Stack Losses.
A 3% change in stack O2, accompanied by a ~40oF increase in NET stack temperature would reduce boiler efficiency by ~ 2%. That doesn’t seem like much, but for a typically heavily loaded industrial boiler, this can be thousands of dollars per year in wasted fuel! For example; an 800HP boiler, loaded to average of 80% of full rate, operating 4,000 hours per year, the fuel wasted due to the lower efficiency would roughly equal $15,000 per year!
Remember for a typical, industrial boiler, the fuel bill will quickly outstrip the purchase price of the equipment. Small improvements in efficiency can result in very notable savings! One way to realize these fuel savings is through consistent, quarterly combustion tune ups. The fuel savings can easily pay for the services, in a single visit (not even taking savings from improved up-time, due to the preventative maintenance, into account). Beyond the fuel savings due to tune-ups, other control upgrades or heat recovery strategies can be employed to provide even greater savings.
Our boiler experts can help define the scope and best apply the boiler room services and equipment that will help you realize the highest efficiency possible, for your boiler plant. If you would like more information, or help, in establishing a Quarterly Preventative Maintenance Plan or reviewing efficiency improving upgrades for your boiler room (with payback estimates), or with any other boiler room needs – we can help! Call us at 336-299-3035 or contact us by clicking here, for your next boiler room need, put our Boiler Experts to work for you – available 24/7/365! Offices near you in Greensboro, Charlotte, Wilmington, and Columbia South Carolina.