Why does complete combustion occur

By monitoring and regulating some of the gases in the stack or exhaust, it is easy to improve combustion efficiency, which conserves fuel and lowers expenses. Combustion efficiency is the calculation of how effectively the combustion process runs.

To achieve the highest levels of combustion efficiency, complete combustion should take place. Complete combustion occurs when all of the energy in the fuel being burned is extracted and none of the Carbon and Hydrogen compounds are left unburned. Although theoretically stoichiometric combustion provides the perfect fuel to air ratio, which thus lowers losses and extracts all of the energy from the fuel; in reality, stoichiometric combustion is unattainable due to many varying factors.

In practice, in order to achieve complete combustion, it is necessary to increase the amounts of air to the combustion process to ensure the burning of all of the fuel. The amount of air that must be added to make certain all energy is retrieved is known as excess air. In most combustion processes, some additional chemicals are formed during the combustion reactions.

Some of the products created such as CO carbon monoxide , NO nitric oxide , NO2 nitrogen dioxide , SO2 sulfur dioxide , soot, and ash should be minimized and accurately measured. The EPA has set specific standards and regulations for emissions of some of these products, as they are harmful to the environment. Combustion analysis is a vital step to properly operate and control any combustion process in order to obtain the highest combustion efficiency with the lowest emissions of pollutants.

The objective of combustion is to retrieve energy from the burning of fuels in the most efficient way possible. To maximize combustion efficiency, it is necessary to burn all fuel material with the least amount of losses.

The more efficiently fuels are burned and energy is gathered, the cheaper the combustion process becomes. 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.

To find and correct problems caused by excess gas flow it is important to check gas flow, gas pressure, orifice sizing, and carbon monoxide concentrations in combustion products.

Large holes in a heat exchanger are dangerous. Air forced through holes by the circulating fan increases the production of CO by disrupting the flame. The air flow increases the amount of combustion products spilling into the living quarters by disrupting the air flow through the appliance vent flue. In most natural draft and induced draft furnaces, high pressure on the furnace blower side of the heat exchanger prevents combustion products from flowing directly into the circulation air.

Usually combustion products enter the house because of vent failure or spillage. In power vent and pulse combustion furnaces high pressures on the combustion side can force combustion products directly through holes into the circulating air and the house.

Combustion products contain large amounts of carbon monoxide and water vapor, which can smother flames or lead to increased rust and corrosion of the heat exchanger.

Flame roll-out occurs in severe cases. Holes in heat exchangers can be found by direct observation, using either mirrors or by disassembling the unit.

Some direct vent sealed combustion units can be tested by sealing the inlet and outlet and pressure testing. Smoke bombs, odor tracing and salt sprays have been used, but have not been totally acceptable.

The American Gas Association has developed a heat exchanger test method using tracer gases which they have determined to be more reliable and accurate. Observation of flame disruption when the furnace blower turns on will reveal large holes. Measurement of carbon monoxide in flue products using CO analyzers reveals incomplete combustion caused by air flow through holes.

Combustion gases should be sampled before draft hood dilution. In natural draft appliances the flue passages chambers at the top of the appliance can usually be reached, with a probe, through the draft diverter or hood. For equipment with several burners and separate flue passages chambers , each burner must be checked separately by inserting the probe into the top of each chamber. It is important to insert the probe a sufficient distance into the flue chambers to ensure undiluted combustion products are being sampled.

Sealed combustion units can often be sampled most easily at the outdoor exhaust outlet. Locations that will not give an accurate reading of combustion gases include: the perimeter of the draft hood this will represent room air , directly above the flame sampling here may give an erroneous high reading , and at furnace air outlets which contain a mixed sample.

There are many potential causes of carbon monoxide production and spillage of combustion products into living areas. Many causes are accidental and unpredictable; i. To protect against accidental carbon monoxide poisoning, the U. Heating appliances should be inspected and maintained yearly by a qualified heating contractor. Because flame color is not a reliable indicator of carbon monoxide production from a burner the heating contractor must use a carbon monoxide analyzer to inspect and maintain burners.

Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen. Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only.

When they burn completely:. In general, for complete combustion:. Here are the equations for the complete combustion of propane , used in bottled gas:.