When fuel doesn’t burn completely in a coal-fired power plant, the following products of partial or incomplete combustion are formed:
When a fuel burns completely the products of combustion come from the fuel and the combustion air. The major products of complete combustion include:
Incomplete combustion occurs in a coal-fired unit’s furnace when insufficient oxygen (O2) is available during fuel combustion. “Excess Air” is the amount of O2 available for combustion. Coal requires a relatively high amount of excess air to burn completely, quickly, and in the furnace compared to other types of fuel.
Carbon monoxide (CO) forms when there is not enough excess air to burn the fuel or the available air is not mixed properly with the fuel. CO is not undesirable in itself, but its presence indicates inadequate excess air or incomplete combustion. Carbon monoxide is also a harmful gas and can be fatal.
Unburned fuel is a more dangerous result of incomplete combustion. When there is not enough oxygen to burn the fuel completely, unburned or still-burning fuel particles travel through the flue gas system. This is a dangerous situation. If ash has been collecting in the ducting or the economizer, the smoldering fuel can cause it to ignite, and a fire can result. Unburned fuel also results in a loss of efficiency.
Since oxygen is the major gas resulting from excess air in the products of combustion, it is used to measure the amount of excess air to ensure complete combustion. The percentage of oxygen in the flue gas is monitored by plant instrumentation. This percentage is a reliable indicator of the amount of excess air. By maintaining a set amount of oxygen in the flue gas using the combustion air and flue gas system fans speed and damper positions the amount of excess air can be controlled.
Another method of checking the amount of excess air is to measure the amount of CO2 in the flue gas. This is not as accurate a method of determining excess air as measuring the amount of oxygen.
Also note, that when insufficient excess air fills the furnace, black smoke can result, also referred to as opacity. The black smoke is unburned carbon.
Other than a lack of excess air, unburned fuel and carbon particles can also result from:
Plant operators and management should ensure that proper excess air always exists when operating their units. Doing so may reduce emissions, increase efficiency, and avoid a dangerous condition. FCS has extensive experience training personnel on the effect that excess air, too high or too low, can have on your unit. If your facility is having difficulty in this area, please contact us for more information.
References:
Improve Your Boiler’s Combustion Efficiency. US Department of Energy. https://www.energy.gov/sites/prod/files/2014/05/f16/steam4_boiler_efficiency.pdf
The Importance of Operating with Proper Furnace Draft. Fossil Consulting Services. https://www.fossilconsulting.com/2020/08/26/the-importance-of-operating-with-proper-furnace-draft/
