The mixing is achieved by burner parts designed to create high turbulence If

The mixing is achieved by burner parts designed to

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The mixing is achieved by burner parts designed to create high turbulence. If insufficient tur- bulence is produced by the burner, the combustion will be incomplete and samples taken at the stack will reveal carbon monoxide as evidence. Since the velocity of air affects the turbulence, it becomes harder and harder to get good fuel and air mixing at higher turndown ratios since the air amount is reduced. Towards the highest turndown ratios of any burner, it becomes necessary to increase the excess air amounts to obtain Figure 1.3: Relation Between Residual Oxygen and Excess Air Figure 1.4 Burner Head FUEL AIR AIR AIR
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1. Fuels and Combustion 22 Bureau of Energy Efficiency enough turbulence to get proper mixing. The better burner design will be one that is able to properly mix the air and fuel at the lowest possible air flow or excess air . An important aspect to be considered in selection of burner is turndown ratio. Turndown ratio is the relationship between the maximum and minimum fuel input without affecting the excess air level. For example, a burner whose maximum input is 250,000 kCals and minimum rate is 50,000 kCals, has a ‘Turn-Down Ratio’ of 5 to 1. 1.8 Combustion of Coal Features of coal combustion 1 kg of coal will typically require 7–8 kg of air depending upon the carbon, hydrogen, nitrogen, oxygen and sulphur content for complete combus- tion. This air is also known as theoretical or stoi- chiometric air. If for any reason the air supplied is inadequate, the combustion will be incomplete. The result is poor generation of heat with some portions of car- bon remaining unburnt (black smoke) and forming carbon monoxide instead of carbon dioxides. As in the case of oil, coal cannot be burnt with stoichiometric quantity of air. Complete combustion is not achieved unless an excess of air is supplied. The excess air required for coal combustion depends on the type of coal firing equipment. Hand fired boilers use large lumps of coal and hence need very high excess air. Stoker fired boilers as shown in the Figure 1.5 use sized coal and hence require less excess air. Also in these systems primary air is supplied below the grate and secondary air is supplied over the grate to ensure complete combustion. Fluidised bed combustion in which turbulence is created leads to intimate mixing of air and fuel resulting in further reduction of excess air. The pulverized fuel firing in which powdered coal is fired has the minimum excess air due to high surface area of coal ensuring complete combustion. Clinker formation Clinker is a mass of rough, hard, slag-like material formed during combustion of coal due to low fusion temperature of ash present in coal. Presence of silica, calcium oxide, magnesium oxides etc. in ash lead to a low fusion temperature. Typically Indian coals contain ash fusion temperature as low as 1100°C. Once clinker is formed, it has a tendency to grow. Clinker will stick to a hot surface rather than a cold one and to a rough surface rather than a smooth one.
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  • Spring '17
  • Jerome Ramos

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