Provided the combustion is complete, all the hydrocarbons will burn with a blue flame. However, combustion tends to be less complete as the number of carbon atoms in the molecules rises. That means that the bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame. Incomplete combustion where there is not enough oxygen present can lead to the formation of carbon or carbon monoxide.
As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over! The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colorless poisonous gas.
Oxygen is carried around the blood by hemoglobin. Unfortunately carbon monoxide also binds to exactly the same site on the hemoglobin that oxygen does. It is important to strive for complete combustion to preserve fuel and improve the cost efficiency of the combustion process. There must be enough air in the combustion chamber for complete combustion to occur.
The addition of excess air greatly lowers the formation of CO carbon monoxide by allowing CO to react with O2. The less CO remaining in the flue gas, the closer to complete combustion the reaction becomes. This is because the toxic gas carbon monoxide CO still contains a very significant amount of energy that should be completely burned. Stoichiometric combustion is the theoretical point at which the fuel to air ratio is ideal so that there is complete combustion with perfect efficiency.
Although stoichiometric combustion is not possible, it is striven for in all combustion processes to maximize profits. The higher the carbon in the fuel the more air is required to achieve complete combustion. There are many varieties of coal being used in combustion processes around the world; the most widely used are anthracite, bituminous, sub-bituminous, and lignite.
When burning coal a considerable amount of carbon dioxide is generated given the extremely high levels of carbon in coal; since carbon requires more oxygen to burn, more combustion air is needed to burn coal that other fossil fuels. In addition to the carbon dioxide emissions, coal burning creates some other pollutants including NOx, sulfur dioxide SO2 , sulfur trioxide SO3 , and particle emissions. Sulfur dioxide chemically combines with water vapor in the air to produce a weak form of sulfuric acid, one of the main causes of acid rain.
Oil fuels are mostly a mixture of very heavy hydrocarbons, which have higher levels of hydrogen than those found in coal. At the same time, oil contains less carbon than coal and therefore requires less combustion air to achieve complete combustion.
Therefore, burning oil releases less carbon dioxide than burning coal, but more carbon dioxide than burning natural gas. Most of the pollutants produced when burning coal are also a byproduct of burning oil. Natural gas requires much less air in combustion because of its relatively low amounts of carbon and high amounts of hydrogen.
The burning of natural gas is cleaner than the burning of oil and coal. When gas is burned with insufficient combustion air some volatile hydrocarbons can be created, which could become a safety hazard; care should be taken to avoid dangerous conditions. The burning of natural gas produces less greenhouse gases, which are believed to be one of the main sources for global warming.
In addition to the carbon dioxide emissions, gas burning creates NOx emissions, while the emissions of sulfur dioxide SO2 and Particles are negligible. Other fuels including wood, diesel, gasoline, propane, butane, bio fuels such as ethanol, etc. Maintaining appropriate airflow during combustion is fundamental to ensure safe and complete combustion. The total airflow includes combustion air, infiltration air, and dilution air.
Combustion Air Combustion air is the air that is used to actually burn the fuel. Without combustion air, which is normally forced into the furnace, combustion is impossible. Infiltration Air Infiltration air is the outdoor air that is not deliberately in the boiler. Sources of infiltration air maybe cracks or leaks. Dilution Air Dilution air is the air that combines with the flue gases and lowers the oncentration of the emissions. There are two types of dilution air, natural and induced artificially created.
The combustion process is extremely dependent on time, temperature, and turbulence. Time is important to combustion because if a fuel is not given a sufficient amount of time to burn, a significant amount of energy will be left in the fuel. Fossil fuels contain carbon C and hydrogen H. During complete combustion carbon and hydrogen combine with oxygen O2 to produce carbon dioxide CO2 and water H2O. During incomplete combustion part of the carbon is not completely oxidized producing soot or carbon monoxide CO.
Incomplete combustion uses fuel inefficiently and the carbon monoxide produced is a health hazard. A properly designed, adjusted, and maintained gas flame produces only small amounts of carbon monoxide, with parts per million ppm being the maximum allowed in flue products.
Most burners produce much less, with between 0 and 50 ppm being typical. During incomplete combustion, carbon monoxide concentrations may reach levels above 7, ppm. Even a small amount of spillage into occupied structures from appliances producing large amounts of CO is a health risk and can be a threat to life.
Incomplete combustion occurs because of:. The following are recommended additional steps for servicing and inspecting gas heating appliances:. Visually inspect the burner and flame, looking for rust, soot, discoloration, and abnormal flame color or pattern. Visually check the heating appliance for evidence of flame roll-out, downdrafting, and spillage.
Check the vent system for proper design, integrity, and draft. Check adequacy of combustion air and make-up air. Check flue passages in appliance for blockage or restriction— clean if necessary. Visually inspect heat exchanger for integrity.
Check manifold gas pressures using a manometer and adjust if necessary. Check gas flow rate to appliance on metered appliances. Measure CO in flue products using an electronic CO analyzer with digital display. Additional steps which help determine the cause of a heating appliance carbon monoxide problem include:. Verify proper combustion using a combustion analyzer with capability to measure carbon monoxide and oxygen content.
Determine leakage areas and pressure differences in the structure and vent system using a blower door and micromanometer. Check for proper gas orifice size.
Continuously monitor carbon monoxide concentrations in the structure. Visual inspection of the burner will reveal obvious problems including rust, scale, or soot. Obvious flame pattern disruptions or improper color indicates a problem with combustion.
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