Energy Recovery from Paper Sludge
In the pulp and paper industry there is a long tradition of waste incineration. For example, consider the combustion of bark and wood residues. Recent years have seen a growing interest in the use of other types of waste such as sludges and rejects for energy production purposes. The reasons are:
. • increased costs of fossil fuels and purchased power
. • reduced landfill capacities and increased landfill costs
. • more stringent environmental regulations governing the use of this waste in agriculture
. • development of new combustion technologies with highly effective flue gas cleaning technologies.
Due to their heating values and low content of harmful substances, most types of waste from paper mills are suitable for energy recovery. Sludges and rejects are burned mainly in grate and fluidized bed combustion facilities. Burning of sludges is also carried out in multiple hearth incineration plants.
10.2.3.1.1 Grate Combustion
Grate combustion systems are especially suitable for lumpy fuels but not for most, paste-like, and finely grained fuels. With moist sludges for example, the retention times on the grate can be insufficient for complete combustion. Nevertheless, grate combustion systems are used in the paper industry primarily for co-firing of sludges with coal, bark or wood residues. Grate combustion with a stationary grate is shown in Fig. 10.13. Long reaction times for control processes and the difficult combustion control on the grate by the addition of air presuppose that the mois¬ture content and ash content of the sludge vary only within narrow tolerances.
10.2.3.1.2 Fluidized Bed Combustion
Fluidized bed combustion results in good heat transmission and intimate mixing of air and fuel. Combustion is, on the whole, better than in grate furnaces, and emissions are therefore lower. Figure 10.14 shows the scheme of a stationary flui¬dized bed furnace. In fluidized bed boilers the combustion conditions can be con¬trolled through regular temperature and pressure measurements, which is not the case with grates. Stationary fluidized bed furnaces as well as circulating fluidized bed furnaces offer very good operating conditions for low emissions of air-borne pollutants. The emissions of nitrogen oxides are especially low compared with grate combustion. Direct desulfurization is possible by the addition of basic sor¬bents such as limestone or dolomite into the body of the furnace.
10.2.3.1.3 Multiple Hearth Combustion
Multiple hearth combustion plants, as shown schematically in Fig. 10.15, are espe¬cially suitable for the combustion of moist and paste-like waste. They have been used for decades in the paper industry for energy production from primary and biological sludges, often together with bark. The flue gas purification plant down¬stream from the multiple hearth furnaces usually consists of wet scrubbers to remove dust and sulfur compounds. The flue gas finally enters the stack via a mist eliminator.