Chemical Engineering July 2021 - 39

Engineering Practice
Landfill Gas Processing:
Plant-Design Considerations
There are several technologies used in the disposal of landfill gas (LFG). Here
are some key elements that can guide engineers in designing an optimized LFG
processing plant
Babak Firoozi
A/C Control, Inc.
P
eople generate trash. Trash
is stored in landfills. Landfills
contain bacteria, which
decompose the trash to
produce combustible landfill gas
(LFG) [1]. LFG is a hazardous, flammable
greenhouse gas containing
mainly methane (CH4), along with
many other components, including
siloxanes, CO2 and mercaptans, as
well as asphyxiants like hydrogen sulfide
(H2S) and volatile organic compounds
(VOCs). It also can be a valuable
source of renewable energy.
If not disposed of properly, LFG
can become an explosive and hazardous
threat to nearby communities
[2]. The minimum required disposal
for LFG is incineration. This is typically
used by smaller landfills, which
do not generate enough LFG to
produce a valuable volume of product.
Direct-use methods, which are
typically employed by medium- to
large-size landfills, combust the LFG
to take advantage of its heat content
[3]. Electricity generation, cogeneration
(cogen) of steam and electricity,
boilers, kilns, iron forges and
other thermal applications are typical
direct-use applications.
Another method is conversion of
LFG to pipeline-grade natural gas,
which is sometimes used at large
landfills. Pipeline-grade conversion
is the most complicated of these
methods, but could yield the greatest
profit, depending on the price of natural
gas. This article covers general
design criteria and disposal methods
used in the LFG industry. Various
equipment and processing technologies,
such as temperature-swing
adsorption (TSA), pressure-swing
adsorption (PSA), sulfur removal
and thermal oxidization (TOX) are
also discussed.
Consider economics
Economic analysis is the
first step to select the
best option from the available
LFG disposal technologies
(Figure 1). Plant
economics are based on
total LFG production during
the peak production
years of the landfill's life
(when the produced gas
is around 55 vol.% CH4),
usually from its fourth
year to its fortieth year.
Economic analysis must
consider initial capital investment
(capex) versus
continued operational
expenditures (opex). The
size of the landfill is one
factor to consider. Nearness
to gas users is another
factor. There may
be little economic incentive
to invest in a large,
high-producing landfill if it
is far away from energyuse
infrastructure.
LFG wells
Condensate
knockout
Direct use/
fl aring
Sulfur/nm VOCs
removal
Optional per
regulations/
maintenance
Technology
selection
Flaring
LFGTE/
cogen
Thermal
use
Pipeline-grade
natural gas
Amine
absorption
Siloxane
removal
(TSA)
Acid gas
treating
Sulfur
removal
TSA/
PSA
CH4
compression
TOX
FIGURE 1. Landfill gas (LFG) can be handled using a number of
technologies, depending on landfill size and economics
Flaring is usually considered by
landfill operators who do not wish
to risk capital investment. The economic
break-even point is usually
around 200 std. ft3/min of LFG
during peak production. Landfills
with high sulfur production may require
additional investment to meet
environmental regulations.
Many combustion-based directuse
technologies require a moderate
capital investment. LFG used in
kilns, boilers, LFG leachate evaporators
and other burning technologies
may not require any additional
treatment if the operator is willing
to spend more money on continued
maintenance. LFG-to-electricity
(LFGTE) production represents over
50% of the world's direct use of LFG
[3]. LFGTE projects typically generCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2021
ate 0.8 to 3 MW of energy, corresponding
to approximately 200 to
750 std. ft3/min of LFG.
The profitability for purification of
LFG to pipeline-grade natural gas is
directly related to local spot prices.
LFG collection
LFG collection and condensate disposal
design is the same for all LFG
processes (Figure 2). LFG is drawn
under low vacuum through underground
wells and passes through a
common header to a moisture separator.
A blower transfers the LFG
to the process, and a pump transfers
the condensate to storage or
back to the landfill.
LFG header and blower. LFG is collected
through gas wells into a common
header. The LFG is drawn to the
39
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Chemical Engineering July 2021

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