Page 38 - Energize February 2022

TECHNICAL

Incineration
Incineration is the main thermal treatment method adopted
worldwide. The worldwide number of waste-to-energy (WtE)
facilities for MSW reached a total of 1618 plants in 2016. The main
1
advantage of incineration is the capacity to handle large volumes
of unsorted waste and the fact that it uses standard thermal
power components, from the boilers to the turbines to the flue
(a) Weekly collected (b) Daily collected gas management systems. A power plant fuelled by municipal
Figure 2: Composition of weekly (compacted) and daily collected
waste collected at Robinson Deep 7 waste emits between 35% and 60% less pollutants than a fossil
fuel power plant, after considering the equivalent of methane
Waste sorting to improve CV emissions that would have been released in a landfill.
Waste sorting is employed to ensure that only those items Early plants were inefficient and produced high levels of
with the highest calorific value are incinerated. To increase the pollution and were closed in many countries. New highly efficient
calorific value of waste and improve the profitability of the plant with low levels of air pollution and noise are proving to be
plant, it may be worthwhile to implement simple measures an acceptable alternative to other methods of waste disposal in
such as separating organic waste and non-combustibles from many countries, and the systems now available offer a solution to
other waste at the source. The calorific value will depend on South African and African waste problems in general. Modern plant
the final composition after sorting and removal of recyclable itself is designed to have an acceptable architectural and aesthetic
materials and materials suitable for organic biodigestion. appeal. (Figure 4).
These include:
• Recyclable plastic
• Dry paper
• Wood
• Glass
• Metal
• Organics such as food waste, which can be biodigested.

Organic waste can be processed in a biodigester, while non-
combustibles may be recycled. Once organic waste is separated,
the average calorific value of incinerated waste will increase,
improving the efficiency of waste-to-energy plants.

Thermal treatment of waste
The thermal utilisation processes can be divided into three Figure 4: Waste incineration plant Lucerne Switzerland (Rosatom)
categories (combustion, gasification and production of refuse
derived fuel (RDF)), as shown in Figure 3. Combustion takes Technology
place in an environment with excess air, and gasification is a At the one end of the sector is the mass burn process where
process of partial oxidation, requiring excess air slightly below the unseparated waste is burned in a large moving grate furnace.
stoichiometric level. RDF production requires sorting, drying and At the other is the refuse derive fuel (RDF) system where waste
compacting of the combustible portion of the waste. material is separated out and the combustible material is dried
and compacted into pellets, which are burned in smaller controlled
furnaces, or used for other processes. Incinerators producing
electricity only have a conversion efficiency of up to 30%.
For plants burning RDF, a greater efficiency than mass burning
plants is achieved due to the more homogenous and controlled
nature of the feedstock. Energy recovery for district heating
schemes recover around 70% of energy released, whereas
combined heat and power schemes, which utilize the residual
heat after generation of electricity, achieve an overall conversion
Figure 3: Three categories of thermal treatment of waste efficiency of around 70 to 90%. 1

Refuse derived fuel production involves separating, drying Mass burn grate system
and compressing the combustible portion of the waste, resulting The mass burn system has been in use for many years and uses
in a product which can be used as feedstock for any of the three a large moving grate furnace in a surplus oxygen environment.
thermal processes, or combusted in an industrial application. Air flow from below the grate is used to agitate the burning mass

energize | February 2022 | 36