Choice of Combustion Technology
The choice of combustion technology depends on the operating conditions and the nature of the waste. According to today’s knowledge, fluidized bed combustion is the most suitable for burning sludges and rejects. In fluidized bed combustion neither fluctuations of heating value of the fuel nor changing proportions of non¬combustible components such as sand, metals or fillers have a negative influence on the combustion efficiency.
For fuels with a low heating value, such as deinking sludges, the stationary fluidizing is probably a more suitable option. Fuels with a higher heating value such as wood and bark are incinerated more effectively in a circulating fluidized bed. Due to the mechanical flow requirements of the fuel, producing a consistent grain size distribution by mechanical processing such as extraction of metals, shredding and grinding may be necessary. This requires consideration, especially when co-burning rejects with sludges.
For sludge and reject incineration in paper mills the following conclusions con¬cerning flue gas emissions are possible:
. • Emissions of solid particles, sulfur dioxide, hydrogen chloride, hydrogen fluo¬ride and carbon monoxide are low compared with legal standards. Due to the high calcium carbonate content of deinking sludge this sludge should not re¬lease all sulfur as sulfur dioxide. State-of-the-art flue gas purification guarantees that no significant environmental impact arises.
. • The emission of heavy metals is very low compared with the legal standards, especially for emission of cadmium and mercury.
. • The emission standard of nitrogen oxides can usually be met by improved com¬bustion control (temperature and oxygen level). Sometimes, the application of specific measures to remove NOx could be necessary.
. • The stringent emission standard of polychlorinated dioxins and furans can be met without any additional measures in flue gas purification.