POWER August 2021 - 37

WASTE TO ENERGY
separated through the use of bag filters
and sometimes ceramic filter candles.
Ammonia is often used to react
with the NOx
produced during the combustion
process to greatly reduce the
NOx
Otherwise, NOx
aged via combustion control, such as
combustion air staging. Additionally,
efforts have been made in the area of
CO2
capture through the approaches
of post-combustion capture, pre-combustion
capture, or oxyfuel combustion
capture.
The combustion of MSW with higher
plastic content has been shown to
have no impact on emissions. Additionally,
combustion of MSW emits about
the same amount of CO2
compared
2. Emissions data from a U.S. waste-to-energy plant in 2020 compared to the plant's air permit
limits (expressed as a percentage of each limit). The actual emissions limits are included along
the top of the graph. Source: Onondaga County Resource Recovery Agency
market drivers.
Unlike recycling techniques, WTE facilities
can process all forms of plastic
resins (#1 through #7) and engineered
resins including multi-layer plastics.
Recycling facilities can only utilize a
select few types of resins (usually #1
and #2). The WTE thermal conversion
technique does not depend on the material
properties or manufacturing additives
(fillers, multilayers, colorants,
and flame retardants). Therefore, WTE
facilities can dispose of those non-recycled
plastics that contain insufficient
properties for reprocessing to produce
virgin resins.
Several chemical recycling techniques
of plastics have been proposed but none
have successfully demonstrated commercial
viability. The full-cost accounting
for the collection of plastics from
generators and processing the material
(separately from MSW) plus the added
energy to wash, clean, and chemically
process presents significant economic
drawbacks. Meanwhile, the alternative
of disposing plastics in a landfill is not
a good option, because those plastics
could take up to 600 years to decompose-and
that does not offer the benefit
of energy or material recovery.
The enrichment of plastic in MSW
streams increases the calorific value of
the refuse. However, WTE facilities are
not economically incentivized to source
plastics as a feedstock for combustion.
Taking additional or bulk quantities of
high-heat-content materials, like paper
and plastics, reduces the amount of
waste that a typical WTE facility can
August 2021 | POWER
process. Because most WTE revenues
come from waste tip fees, revenues
would decrease from taking in large
amounts of paper and plastics.
According
to
research
conducted
by Demetra A. Tsiamis and Marco J.
Castaldi at City College of New York,
the average higher heating value of
mixed plastics is 16,492 Btu/lb. Therefore,
if all landfilled plastic in 2017
alone was combusted (at an MSW average
of 20% efficiency), it would provide
1.77 x 108 MMBtu, if the proper
combined heat and power production
facilities were available to do so. This is
the energy equivalent of the consumption
of 32 million barrels of oil and a lot
of energy to bury.
Managing Emissions
Modern WTE facilities utilize advanced
emission controls. The EPA upholds
specific WTE emission limits on particulate
matter (PM), cadmium, carbon
monoxide, dioxins/furans, hydrogen
chloride, lead, mercury, nitrogen oxides,
opacity, and sulfur dioxide. The
typical U.S. WTE facility emits all
monitored species at a rate far below
the EPA limits. For example, Figure 2
shows 2020 emissions data compared
to the EPA limits from one U.S. WTE
facility. Notably, the PM, cadmium, and
lead emissions were lower than allowable
limits by factors of 57, 152, and
193, respectively.
Air pollution control systems reduce
the emissions of acid gases and other
byproducts through lime and activated
carbon injection, while PM is usually
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to combustion of methane-1,294 lb/
MWh vs. 1,176 lb/MWh, respectively-
according to a paper published by the
Energy Recovery Council.
Policy Considerations
There have been 17 closures of WTE
facilities since 2001 in the U.S. due to
policy obstructions. This is a shame
considering the use of WTE capacities
throughout the rest of the world are increasing.
From 2011 to 2015, the WTE
capacity of China has more than doubled
to 339 plants with a total capacity
of 7.3 GW. The amount of waste going
to WTE plants also doubled in Europe
between 1995 and 2015. The key reason
for the U.S. closures is economics,
mainly stemming from the depressed
price of electricity in competition with
natural gas.
The pronounced goal of instituting
policies surrounding plastics is to develop
a circular economy. It is clear that
a significant portion of recyclable material
remains in the waste stream. Efforts
to extract as much recyclable material
as is feasible to process must continue,
but the upper limit with the current state
of recycling technologies needs to be
recognized. WTE facilities should be regarded
as a method to deal with materials
left after recycling, and does not take
away from sustainability and increased
recycling efforts. ■
-Devin Peck is a Chemical Engineering
PhD candidate at University of Louisiana,
Lafayette, and Jeff LeBlanc, PhD is a
product design engineer with Advance
Products and Systems. This article was
reviewed by Nathiel Egosi, PE president
of RRT Design & Construction, and
Marco Castaldi, PhD associate
professor at City College of New York.
37
and produce nitrogen and water.
emissions are man

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POWER August 2021

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