Figure 1 summarises the available waste-to-energy conversion technologies. The
conventional approach for energetic valorisation of waste is direct combustion or
incineration. Besides incineration more advanced thermochemical approaches, such
as pyrolysis, gasification and plasma-based technologies, have been developed
since the 1970s.5 In general these alternative technologies have been applied to
selected waste streams and on a smaller scale than incineration, and attempt to
control process temperatures and pressures in specially designed reactors (see
Table 1). Each conversion technology gives a different range of products, sets
different requirements for the input, and employs different equipment configurations,
operating in different modes.
Both pyrolysis and gasification differ from incineration in that they may be used for
recovering the chemical value from the waste, rather than its energetic value. The
chemical products derived may in some cases then be used as feedstock for other
processes or as a secondary fuel. However, when applied to wastes, pyrolysis,
gasification and combustion based processes are often combined, usually on the
same site as part of an integrated process. When this is the case the installation is, in
total, generally recovering the energy value rather than the chemical value of the
waste, as would a conventional incinerator do.
In a first step the waste is converted into a secondary energy carrier (a combustible
liquid, gas or solid product), while in a second step this secondary energy carrier is
burned (in a steam turbine, gas turbine or gas engine) in order to produce heat and/or
electricity. The conversion of solid wastes to secondary energy carriers allows for a
cleaner and more efficient process. Smaller flue gas volumes allow reduced gas
cleaning equipment sizes. Furthermore, it enables a greater market penetration since
these secondary energy carriers are compatible with gas turbines and gas motors,
characterised by a high electrical efficiency.
The following sections discuss the main available thermochemical conversion
technologies for calorific waste (RDF) treatment: