Page 39 - Energize February 2022

TECHNICAL

and ensure that all the waste is combusted. Waste is generally (sulphur dioxide, nitrogen oxides), carbon dioxide, particulate
delivered direct to the hopper, and shredding and removal of matter, dioxins and dibenzofurans (PCDD/PCDFs), volatile organic
non-combustibles is not practiced. MSW from the compacted compounds and heavy metals.
waste collection stream will be placed in garbage bags and large The flue gas emissions of WTE facilities depend on the
non-combustible items will have been removed at site and been composition of input waste, the type of incinerator and the
diverted to an alternative stream. Small non-combustible items combustion conditions. Flue gas is treated with a combination of
such as metal, ceramics and broken glass still exist in the waste electrostatic filters, catalytic reactors and wet scrubbers. Electro-
stream and remain in the ash from the furnace. An example of a filters remove the fly ashes. Lime is injected in a reactor to capture
mass burn grate incinerator is shown in Figure 5. chloride and sulphur components, while brown coal is added for
the removal of heavy metals and dioxins. The baghouse filters
separate the solid components. The final step in the cleaning of
the flue gases is the removal of the rest of the chloride and sulphur
components by means of wet washing.

Modular plant
A recent development is that of modular incineration plant
which provides standard sized plant that can be preassembled
and transported to site. The plant can incorporate all equipment
necessary for WtE operation, including sorting, shredding,
incineration, steam production and electricity generation. Figure 7
shows an example.

Figure 5: Mass burn grate incinerator (Rosatom)

Mass burn fluidised bed (FBC)
Fluidised bed incineration is a recent development that is
more efficient than grate combustion. The process requires a
small particle size to work and pre-processing to remove non
combustibles larger than the maximum size is required. Fuel
has to be within size limits to enable fluidisation to work and
requires shredding before combustion. Pre-sorting is required,
either mechanised or manual to remove larger non-combustible
components. FBC has a higher efficiency than grate type and is less Figure 7: Modular WtE plant (Woima Corporation)
sensitive to variations in fuel composition but has a higher running
cost (Figure 6). A representative modular plant is scalable with one to four
combustion lines, each capable of converting 30 000 to 50 000
MSW incineration emissions and flue gas scrubbing tonnes of non-recyclable waste annually and producing 3 MW of
WtE plants have a complex set of pollutants in the flue gas, due to electricity, or 2 MW of electricity and 10 MW of heat, or 17 t/h of
the natures of the material being combusted. The most significant steam at 400°C / 40 bar.
pollutants emitted from mass burning of MSW are acid gases
Refuse derived fuel
Refuse derived fuel (RDF) systems use mechanical methods to
shred incoming MSW, separate out non-combustible materials,
and produce a combustible mixture that is suitable as a fuel in a
dedicated furnace or as a supplemental fuel in a conventional boiler
system. Figure 8 shows the process. RDF has a CV higher than bulk
waste, with locally produced product approaching 27 MJ/kg.
SRF is a solid fuel derived from RDF that has well-defined
characteristics, properties and composition and can be traded as a
fuel for energy generation. SRF presents significant opportunities,
as the main pollution sources have been removed during the
mechanical pre-treatment process. It has a relatively high heating
value and can be standardised as a commercial fuel. SRF can
be co-utilised with several other solid fuels such as coal and/or
Figure 6: Fluidised bed waste incinerator (Indaver) biomass in co-combustion or co-firing processes. Since pelletisation

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