James R. Stringer
(Reference: Power Engineering Third Class Edition 2.5; Part A2; Chapter 3: Fuels, Combustion, Flue Gas Analysis)
Combustion involves a fuel source and air which produces useful heat. Air is mainly made up of oxygen (21% volume, 23% mass) and nitrogen (78% volume, 77% mass), with trace amounts of other gases. The main elements of combustion of a fuel are carbon and hydrogen (or hydrocarbons) and will combine with the air to form carbon dioxide (CO2). If sulphur is present in the fuel, sulphur dioxide (SO2) can also be formed but is undesirable in a fuel. Listed below are the different types of combustion:
Perfect combustion, also known as stoichiometric combustion, will take place if the perfect theoretically correct amount of air and fuel are completely burned. Perfect combustion is considered impossible in any commercial practice because of the difficulty of mixing each oxygen molecule (O2) and fuel combustible.
Complete combustion occurs when all fuel combustibles have reacted completely with the air but there is an excess amount of air supplied above the theoretical air needed (natural gas needs 5-10% excess air; coal needs 25-50% excess air). Having excess air means that the flue gases will have an excess of oxygen and nitrogen (N2). The temperature must be adequate for ignition, the air must have turbulence to ensure the oxygen molecules contact all combustibles, and the combustion gases remain in the hot zone for a sufficient length of time to allow for complete combustion.
Too much excess air can mean a reduction in furnace temperatures (lower heat transfer), reduction in efficiency (less time in the combustion zone), and extra oxygen and nitrogen carries away sensible heat.
Too little excess air can mean incomplete combustion, deposits of unburned carbon, reduced heat transfer and efficiencies, and gray/black flue gases leaving through the stack.
The products of combustion should be carbon dioxide, water, nitrogen, and potentially sulphur dioxide or ash (depending on the fuel source). Nitrogen Oxides (NOx) are another possible product and are being increasingly more regulated as they are considered a pollutant.
Incomplete combustion is when carbon and hydrogen from fuels do not completely react with oxygen and pass through the stack. The products of incomplete combustion can also produce carbon monoxide (CO), hydrogen (H2), Carbon (C; soot), and methane (CH4) or other hydrocarbons.
Incomplete combustion is not desirable as it lowers the efficiency of the boiler, wastes fuel, as well as produces atmospheric pollutants.
Analysis of boiler flue gases is very important as it is to better control combustion efficiency and to monitor the operations to be within environmental standards. Flue gas analysis is to ensure proper and complete combustion, while stack emission monitoring is meant to find those products of combustion that are undesirable and have limits imposed by the government regulated standards.
Flue gas analysis generally measures carbon dioxide, carbon monoxide, and oxygen. In essence, the higher the CO2 value means more complete combustion; CO is the product of incomplete combustion and thus if present, the furnace is lacking air or the flame is not proper; O2 means there is excess air and too much means heat is being carried off which lowers boiler efficiency. Ideally the measurement should have maximum CO2, minimum O2, and a zero amount of CO. Overtime, boiler burners may need to be calibrated and adjusted as the fuel/air mixture becomes skewed. This is why adding boiler combustion analysis to regular preventative maintenance work should become common practice, ensuring the health of the equipment as well as our environment.