Page 24 - Energy from Toxic Organic Waste for Heat and Power Generation

Toxic Waste From Municipality 13

various contaminants such as small char particles, ash, and tars [14]. If the
process does not occur with help of an oxidizing agent, it is called indirect
gasification and needs an external energy source Fig. 2.4 [13,15].
Steam is the most commonly used indirect gasification agent, because it
is easily produced and increases the hydrogen content of the combustible
gas [13]. An issue common between all developed countries is the produc-
tion of excessive amounts of waste per capita. As societies have developed,
the quantity of waste material generated has also increased to a level that
is unsustainable. Along with increasing awareness of the general public of
the damage caused to the environment, there is an urgent need to plan and
implement sustainable and integrated strategies for managing and treating
waste materials which has become a priority for many local authorities [15].
A gasification system is made up of three fundamental elements: (1) the gas-
ifier, helpful in producing the combustible gas; (2) the gas clean-up system,
required to remove harmful compounds from the combustible gas; (3) the
energy recovery system. The system is completed with suitable subsystems
helpful to control environmental impacts (air pollution, solid waste produc-
tion, and wastewater). A sufficiently homogeneous carbon-based material
is required for a correct and efficient gasification process. Therefore, several
types of waste cannot be treated in the gasification process and for certain
types an extensive pretreatment is required (refuse-derived fuel). Instead,
there are several types of waste that are directly suitable for the process;
these are: paper mills waste, mixed plastic waste, forest industry waste, and
agricultural residues [13]. The gasifier is a reactor in which the conversion
of a feedstock into fuel gas takes place. There are three fundamental types of
gasifier: (1) fixed bed, (2) fluidized bed, and (3) indirect gasifier. The gasifi-
cation process represents a future alternative to the waste incinerator for the
thermal treatment of homogeneous carbon-based waste and for pretreated
heterogeneous waste.

2.2.2 Biochemical Conversion
Biochemical conversion processes make use of the enzymes of bacteria
and other microorganisms to break down biomass. In most of the case,
microorganisms are used to perform the conversion process: anaerobic
digestion, fermentation, and composting. Biochemical conversion is one
among the few which provide environment-friendly direction for obtain-
ing energy fuel from MSW. Anaerobic digestion is helpful in lessening the
amount of organic solid waste and recovering energy. Basically, organic
fraction of MSW is a potential feedstock for anaerobic digestion [16–18].