POWER September 2016 - 67

FUELS
2. Variety is the spice of life,
and the bane of waste-to-energy
(WTE) projects. Within each of these
broad categories of municipal solid waste
(MSW) lie innumerable unique subtypes of
products. For example, " paper " includes
everything from plain white bond paper to
rough kraft packaging paper saturated with
color inks. " Wood " could contain pine sawdust
as well as construction debris featuring
CCA-treated wood with galvanized nails. WTE
facilities require either a highly flexible design
or focused source fuel segregation and sorting
to operate successfully. Source: U.S. Environmental
Protection Agency
■ Paper ■ Food ■ Yard waste ■ Plastics ■ Metal
■ Rubber, leather, textiles ■ Wood ■ Glass ■ Other
3.3
4.5
6.2
27.0
9.1
9.0
14.6
12.8
13.5
organic waste to produce a low-energy
gas that can be directly used for heating or
power production.
As a result, fuel flexibility has never had
greater importance than for WTE facilities.
Utilization Techniques
dust and wood waste from a furniture factory,
plastic fluff and trimmings from a
consumer products manufacturer, or grain
and plant waste from a cereal processor.
■ Sewage sludge is sometimes used for energy
generation, especially in the European
Union. Burned somewhat dried, or
processed into pellets, sewage sludge can
sometimes contain a surprising amount of
chlorides and heavy metals, which can be
an environmental concern.
■ Tires and similar rubber products are often
easy to separate, and thus amenable
to being made into tire-derived fuel. Key
concerns with burning these fuels include
accommodation of the steel belts in the
tread, and sulfur compounds in the rubber.
■ Landfill gas is formed from the anaerobic
decomposition of organic materials in a
landfill and is typically about 50% methane,
with the balance being a mix of CO2
,
nitrogen, and other volatile compounds.
Because methane is a much more powerful
greenhouse gas than CO2
, capturing
and burning landfill gas provides direct
environmental benefits.
■ Refuse-derived fuel is formed by heavily
processing MSW to remove inert materials
and moisture to produce a higher-quality
fuel that can be used with conventional
combustion technologies.
■ Biogas is typically formed by gasifying
September 2016 | POWER
Three general methods for utilizing waste
fuels are active biological treatment, passive
biological utilization, and thermal treatment.
Active biological treatment typically
involves using an anaerobic digester to break
down the waste into a biogas or (rarely) a bioliquid.
Passive biological utilization is best
exemplified by extracting landfill gas from
existing waste storage piles or basins. Thermal
treatment consists of many processes,
which can include pyrolysis, gasification,
and direct incineration.
Which utilization technique is selected depends
upon many parameters, with fuel quality
being the most important. Each of these
techniques requires different levels of capital
and operations and maintenance investment
as well. For example, although mixing
a modest amount of wood chips with coal
might be a straightforward and low-cost utilization
method, building a dedicated gasifier
to convert those same wood chips to biogas
can result in significant capital costs and may
be a " first-of-a-kind " engineering project.
The end-use power conversion technology
may also regulate the ability to utilize
a waste fuel. For another example, though
cofiring landfill gas in a standard boiler
may be a low-technology modification, using
landfill gas in a gas turbine may require
significantly more plant modifications. As a
result, each WTE project must be carefully
assessed to determine the best solution for
the situation.
WTE projects typically are considered to
be greenhouse gas neutral through a variety
of accounting factors. Although the combustion
of biomass-derived waste is a net benCIRCLE
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67
3. Waste segregation is a key to
success. Engineering efforts can be focused
on improved waste processing before
combustion, or the exploration of more fuelflexible
combustion technologies. At this Hartford,
Conn., plant, substantial effort is spent
separating MSW into recyclable components,
inert waste, and combustible waste. Courtesy:
Black & Veatch
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POWER September 2016

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