Wastes and sludges
Besides to conventional fuels, it is important to consider that it is possible to obtain energy from the disposal of residuals coming from different human activities, leading at the same time to solution of an environmental problem and valorization in terms of energy of the potentially critical material.
The principal types of wastes that can be considered in this direction are :
Municipal Solid Wastes as they are collected, or the more energetically interesting part of them, named Residue Derived Fuel;
Animal wastes with an high solid content (chicken waste, as an example)
Wood or cellulosic products coming from forestry, or from wood or pulp and paper industry
Industrial wastes coming from chemical or oil industry
Hydrocarbon muds or sludges residual from petrochemical activity
Dried sludges resulting from wastewater treatment plants
Commercial wastes, deriving from packaging or transport of goods
A preliminary aspect that must be considered in case of thermal valorization of these wastes is a clear characterization of the material: together with thermotechnical parameters (dry content, ash, calorific value, volatility), it is required to know if in the material can be observed the presence of precursors of atmospheric pollutants that could be generated during the thermal treatment; this knowledge is important to establish the right operation scheme of the treatment process.
From the technological point of view, different systems of thermal valorization can be conceived: it is possible to operate with a direct combustion, a co-combustion in process plants (chiefly cement kilns and thermoelectric plants), or it possible to obtain the transformation of the solid waste in a combustible fluid, like a syn-gas or a liquid, by means of operations of pyrolysis or gasification. The choice between direct combustion or transformation in a secondary fluid derives from energetic balances, potential formation of pollutants, maturity of the technology that can be adopted, cost of operation.
b)
a)
Fig. 7 – Example of direct combustion of waste (a) , and transformation in syngas (b)
The aspect of the energy balance of the operation is very important: in some case, when a dry, high calorific value material is used (as for example in case of rubber, or plastic residuals, or a well selected fraction of MSW, or wood), it is possible to obtain from the thermal valorization of the waste an important production of energy; on the contrary, with sludges from wastewater systems, or with wet agricultural residues, or with un-selected municipal wastes of underdeveloped areas the energy balance is more or less neutral, with the consequence that it is only possible to obtain with the thermal process the compensation of the energy costs required to obtain the disposal of the residue.
In account of the heterogeneity of the wastes, and the potential presence of toxic components in the admitted waste flux, the atmospheric emission that is generated or from direct combustion or from indirect process of pyrolysis/gasification and subsequent fluid thermal treatment leads to potential environmental impact (presence of metallic components, chlorine, volatile toxic organic compounds, halogenated micro-pollutants); the need to treat in a suitable way this atmospheric flux leads to a strong increase in complexity of installation, in comparison with conventional fuel utilization, and also a substantial increase in the costs of operation are consequent.
Lastly, it is important to consider the need to properly destinate the fluxes of residual ashes obtained from the thermal valorization, and chiefly the solids that are obtained during the treatment of atmospheric emissions: in these fluxes there is a strong concentration of the most dangerous components, and so the careful consideration of a controlled destination of them is fundamental.
Fig. 8 - Treatment line after thermal treatment of waste