Fuel Additives condition the fuel in order to maintain combustion efficiency. They reduce sludge formation in the fuel and improve atomization thus promoting proper combustion of fuel. Special additives modify slag and improve coal combustion. Because of improved combustion, fouling of heat exchange surfaces is reduced.
Fireside treatment chemicals are also used to condition the already formed deposits in boilers during operation and prevent further deposition. Such online treatment systems have proved to be very useful for preventive maintenance of boiler.
Balanced fuel additive program/chemical includes;
Dispersant helps in distributing the active chemical ingredients evenly throughout the boiler. It enables the chemical to reach even the inaccessible parts in the boiler, ensuring that all parts in a boiler are treated.
Catalysts reduce the ignition temperature of carbon and other combustible complexes of Sulphur-carbon, etc. This enables the combustibles to burn off at much lower temperature and at much fast rate. The organic foulants are thus removed by the combustion catalyst.
Oxidizers aid combustion of organic material in the deposits in presence of the catalyst. Nascent oxygen can modify the inorganic clinker structure by changing it to a higher oxidation state.
They neutralize acid deposits formed in boilers and reduce the formation of Sulphur trioxide by poisoning the vanadium and iron catalysts. They raise the dew point of acid and reduce cold end corrosion.
These chemicals raise the ash fusion temperature. Thus molten mass is not easily formed. A devitrification catalyst further modifies the slag making it amorphous.
Cold end corrosion in boilers is due to precipitation of sulphuric acid from flue gases. In the presence of excess air and high flame temperature, sulphur in the fuel is oxidized to sulphur trioxide. Impurities in the fuel like vanadium and corrosion products catalyze the reaction. Sulphur trioxide reacts with the water vapour in flue gas and forms sulphuric acid and starts precipitating below dew point. Approximately 130-160oC is the dew point range for commonly used fuels. Such corrosion is generally observed in heat recovery units, air pre-heaters, economizers, chimneys, etc.
Fireside deposits are generally poor conductors of heat. Accumulation of such deposits on metal surface reduces the heat conduction to a large extent. As little as 1 mm thick deposition can result in about 2% loss of efficiency. The build up of such deposits depends on the operating conditions, fuel characteristics and boiler design.
The reduced heat transfer in boiler is reflected in higher flue gas temperature. Increased stack losses reduce the overall boiler efficiency. In order to maintain peak boiler efficiency, it is necessary to clean the deposits regularly, which need a shut down. Periodic cleaning is troublesome and energy is wasted in cooling and reheating the boiler. Also, the boiler is not available during the shut down period.
It is therefore essential to remove or prevent fireside deposition in any combustion equipment.