Page 37 - Energize February 2022

TECHNICAL

Waste to energy: Thermal treatment of

municipal waste leads to energy bonus

by Mike Rycroft, Energize

Disposal of municipal solid waste has always been a problem and is increasing as the amount of waste increases. The
traditional method of disposal in landfills is no longer environmentally acceptable and other means of disposal are
necessary. Recycling of paper, glass, metal and plastic, as well as the biodigestion of organic (food) waste has reduced
the volume of waste that enters landfills, but there is still a large volume of non-recyclable waste that must be dealt
with. Thermal waste to energy (WtE) is an option gaining in popularity.

There are several primary waste streams in urban areas: Energy content of MSW
• Municipal solid waste (MSW) Waste is a challenging fuel due to its heterogenous nature,
• Commercial industrial waste (CIW) low calorific value (6-10 MJ/kg, well below coal at roughly 25
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• Mixed municipal solid waste including non-hazardous industrial to 30 MJ/kg,) , high moisture content and potentially harmful
and commercial waste. emissions. The key is identifying the calorific value (CV) of the
waste as it determines the most feasible solution.
Commercial and industrial waste may be handled by the The CV will vary depending on the processing of the fuel as
municipality, but where hazardous waste is included, is generally shown in Figure 1.
handled by private waste collectors. MSW is the primary waste
stream that is disposed of in municipal landfills and this article
deals with that stream. Similar processes may be followed for CIW.
There are more than 860 landfills in the country, according to
the South African Waste Information Centre. In 2020, these landfills
had 48,5 Mt of waste (64% hazardous waste and 36% general Figure 1: CV of MSW with different processing methods
waste), of which 34% was recycled. These landfills pose a risk
through the emission of greenhouse gases. They also contaminate Determining the CV of MSW is important to the design and
the soil near the dump sites. Based on this, thermal WtE may success of any incineration project. Accurate determination
reduce the volume of waste entering landfills by 75 to 90%, while of the CV of MSW is difficult and influenced by the following
also helping to address South Africa’s power deficit. 5 factors:
It is estimated that South Africa produces 12,7 Mt of MSW per • The MSW is not homogeneous and the composition of samples
year , including paper, plastic and metal waste in two streams. from the same batch can vary. Accurate determination would
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Compacted waste is collected on a weekly basis and is generally require a large sample size.
contained in garbage bags. Non-compacted waste is collected daily • The composition of different batches delivered from different
from large waste sources such as hotels and is placed directly in areas can and will vary.
skips. • Composition of batches from the same site can vary with time,
Some SA municipalities have introduced separation at source depending on seasons.
of recoverable material such as paper, plastics, glass and metal
containers (cans), which are collected separately, but such The most accurate method of determining CV is using
programs are voluntary and recyclable material still finds its way calorimeters, but this involves complicated equipment and is
into the MSW stream. In addition, non-recyclable material such limited by the sample size required. Alternative methods, using
as contaminated paper and plastic also ends up in the recyclable complex modelling based on the composition of the waste, and
stream and will be diverted to the MSW stream. using artificial intelligence methods, have been developed by
Several alternative methods to landfill of MSW disposal have several sources and have been found to be reliable. Sampling
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been developed and are in use, including biodigestion of the of waste to determine composition and sort into components
organic components, sorting and recycling of suitable materials, is not technically challenging and can be performed using
and thermal treatment of waste to reduce the volume. Thermal unskilled labour, and this method is suggested as a preferable
treatment of waste is an environmentally acceptable alternative alternative to calorimetry.
method, and includes incineration, gasification and pyrolysis as well The composition of waste samples collected from the
as the production of refuse derived fuel (RDF) for sale to industry. Robinson Deep waste site in Johannesburg is shown in Figure 2. 7

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