calculate the amount of oxygen reduced to burn 120 kg of Carbon completely
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Answer:
This section explains the principles of combustion, how fuel performance can be evaluated using the stochiometric calculation of air requirement, the concept of excess air, and the draft system of exhaust gases.
Principles of Combustion:
Combustion process
Combustion refers to the rapid oxidation of fuel accompanied by the production of heat, or heat and light. Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen. Oxygen (O2) is one of the most common elements on earth making up 20.9% of our air. Rapid fuel oxidation results in large amounts of heat. Solid or liquid fuels must be changed to a gas before they will burn. Usually heat is required to change liquids or solids into gases. Fuel gases will burn in their normal state if enough air is present. Most of the 79% of air (that is not oxygen) is nitrogen, with traces of other elements. Nitrogen is considered to be a temperature reducing diluter that must be present to obtain the oxygen required for combustion. Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases. This reduces the heat available for transfer through the heat exchange surfaces. It also increases the volume of combustion by-products, which then have to travel through the heat exchanger and up the stack faster to allow the introduction of additional fuel-air mixture. This nitrogen also can combine with oxygen (particularly at high flame temperatures) to produce oxides of nitrogen (NOx), which are toxic pollutants. Carbon, hydrogen and sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and sulphur dioxide, releasing 8,084 kcals, 28,922 kcals and 2,224 kcals of heat respectively. Under certain conditions, carbon may also combine with oxygen to form carbon monoxide, which results in the release of a smaller quantity of heat (2,430 kcals/kg of carbon). Carbon burned to CO2 will produce more heat per unit of fuel than when CO or smoke are produced.
Three T’s of combustion
The objective of good combustion is to release all of the heat in the fuel. This is accomplished by controlling the "three T's" of combustion which are (1) Temperature high enough to ignite and maintain ignition of the fuel, (2) Turbulence or intimate mixing of the fuel and oxygen, and (3) Time, sufficient for complete combustion.
Commonly used fuels like natural gas and propane generally consist of carbon and hydrogen. Water vapour is a by-product of burning hydrogen. This removes heat from the flue gases, which would otherwise be available for more heat transfer.
Natural gas contains more hydrogen and less carbon per kg than fuel oils and as such produces more water vapour. Consequently, more heat will be carried away by exhaust while firing natural gas. Too much, or too little fuel with the available combustion air may potentially result in unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and some additional (excess) air is required for ensuring complete combustion. However, too much excess air will result in heat and efficiency losses. Not all of the fuel is converted to heat and absorbed by the steam generation equipment.
Usually all of the hydrogen in the fuel is burned and most boiler fuels, allowable with today's air pollution standards, contain little or no sulphur. So the main challenge in combustion efficiency is directed toward unburned carbon (in the ash or incompletely burned gas), which forms CO instead of CO2.
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