Page 22 - Energy from Toxic Organic Waste for Heat and Power Generation

Toxic Waste From Municipality 11

generation. (iii) Waste in its initial form may be objectionable in nature,
containing decaying organic matter and whatever [3]. The by-product from
the incineration is used as supporting materials in the civil engineering
constructions. Their granularity nature and dense characteristics make them
a substitute for the aggregates used in road construction [4]. The residue
also finds its application as a precursor of alkali-activated materials, as an
adsorbent material for the removal of hazardous elements from wastewater
and landfill gases, as a soil replacement in agricultural activities, as partial or
complete substitute of raw materials for the manufacture of ceramic-based
products, as landfill cover, and as a biogas production enhancer. The residue
may contain hazardous components like lead, zinc, and other heavy metals.
The fly ash can be recycled by hydro-cyclonating them to remove fine
particles of such hazardous materials [5]. The residue from the incinerator is
mixed with cement and used in light weight constructions. The usage has
shown increased strength in the constructed item [6].
2.2.1.2 Pyrolysis
Of the disposal methods, pyrolysis of wastes, a thermal method of treatment
requiring the heating of wastes in an oxygen-free atmosphere, is of interest.
Various advantages claimed of the pyrolysis process are: (a) significant re-
duction in volume of the waste(<50%–90%), (b) production of solid, liquid,
and gaseous fuels from waste, (c) storable/transportable fuel or chemical
feedstock is obtained, (d) least environmental problem, (e) desirable process
as energy is obtained from renewable sources like municipal solid waste or
sewage sludge, (f) the capital cost is comparatively less than that of incinera-
tion process, and (h) once started, the process is self-sustaining. Pyrolysis is a
thermochemical decomposition process conducted under oxygen- deficient
conditions and typically at temperatures ranging between 300°C and 650°C
[1]. Various pyrolysis processes have been briefly described in this section.
The layout of the pyrolysis process is shown in Fig. 2.3 [7].
Conventional pyrolysis (slow pyrolysis) proceeds under a slow heating
rate with solid, liquid, and gaseous products in significant portions [8]. It is
an ancient process used mainly for charcoal production. Vapors can be con-
tinuously removed as they are formed [9]. But the fast pyrolysis is associated
with tar, at low temperature (850–1250 K), and/or gas, at high temperature
(1050–1300 K). At present, the preferred technology is fast or flash pyrolysis
at high temperatures with very short residence time. Fast pyrolysis (more
accurately defined as thermolysis) is a process in which a material, such as
biomass, is rapidly heated to high temperatures in the absence of oxygen
[10]. Table 2.1 shows the range of the main operating parameters for pyrol-
ysis processes [11].