Chemical Engineering July 2021 - 41

To pipeline
CH4
To sulfur
removal/TOX
Adsorber
Regenerator
From TSA/
compressor
LFG
Cool rich solvent
Hot lean solvent
FIGURE 4. A solvent-circulation step treats the LFG's acid-gas content
stack should also be designed with
burner tips that can be plugged, as
well as adjustable air dampers, which
maintain the DRE at the end of the
landfill's life.
It should be noted that in most locations,
removal of sulfur and nmVOCs
is not required prior to flaring or direct
use [7]. The requirement for removal
is a function of the concentration of
these substances, direct-user gas
specifications and local regulations.
Direct-use methods and LFGTE
Combustion of LFG provides a great
deal of heat duty for boilers, steam
and electricity cogeneration and gas
turbines, among other technologies.
Most of the world's LFG direct use is
electricity generation (LFGTE) [3].
LFG generator efficiencies are typically
less than 40% because of the
lower heating values of LFG compared
to natural gas (around 550
Btu/std. ft3 versus around 1,000 Btu/
std. ft3). High-efficiency generators
can produce up to 400 kW per 100
std. ft3/min of LFG.
LFG generator motors are susceptible
to increased wear because,
unlike natural gas, many LFG components
can create hard deposits on
the engine's pistons, cylinder heads,
spark plugs and other motor parts.
Siloxane, sulfur and halogenated hydrocarbons
all can have a negative
impact on the motor, as well as possibly
exceed local regulations.
Pretreatment of the LFG and
regular maintenance will prevent
decreased engine life and performance.
However, the process engineer's
capex versus opex analysis
could consider that more frequent
engine maintenance may be acceptable
as a substitute for a siloxane
removal system.
Siloxane and water removal
Siloxanes are a type of silicone
polymer, and when present in LFG,
cause severe mechanical problems
in the internal combustion engines
of the generators. Silicon dioxide is
the main siloxane combustion product,
and it causes deposits of hard
scale. Worse yet, landfills with metal
content (such as iron or aluminum)
will form hard, glass-like silicate deposits.
These deposits significantly
reduce the engine's life and significantly
increase downtime due to
maintenance. A common technology
to remove siloxanes is temperature
swing adsorption (TSA). In some
cases, TSA units can be designed by
process engineers, but they are usually
vendor-supplied packages. Ref.
6 details the complete procedures
for designing TSA units.
TSA systems (Figure 3) comprise
two or more vessels (TSA beds) in
parallel (one online
for adsorption
and the
other offline for
high-temperature
regeneration),
a regeneration
blower and
a heater. LFG
passes through
the adsorbent
bed of the online
vessel in downflow
during the
adsorption step.
At the end of the
adsorption step,
the valves switch
PSA
beds
LFG
from
TSA
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2021
to set the offline vessel to online for
adsorption and set the online vessel
offline for regeneration.
In the regeneration step, hot regeneration
gas (usually heated air)
is blown through the adsorbent
bed in an upflow pattern to swing
the bed temperature, thus releasing
the adsorbed siloxanes and water.
Depending on local environmental
regulations, the regeneration gas
is vented, preferably to a thermal
oxidizer (TOX). Industry experience
suggests heating the adsorbent to
about 400°F with 450°F gas. After
the adsorbent has been regenerated,
cool gas passes over the adsorbent
to swing down the temperature to
100°F. The preferred adsorbent temperature
is about 75°F or lower.
TSA adsorbents. The top adsorbent
layer of the TSA bed is typically activated
alumina or a molecular sieve
for water removal, and the lower
layer is silica gel for siloxane removal.
Activated carbon was used for many
years for siloxane removal. Now,
however, the industry is using silica
gel adsorbent because it has been
demonstrated as the better siloxane
adsorbent [8]. Activated alumina or
molecular sieves are typical adsorbents
for moisture adsorption. These
allow silica gel (an excellent desiccant)
to adsorb siloxanes selectively
over water.
Adsorbent vessels are specified
with a low length-to-diameter
ratio to operate within a balance
between a low pressure drop and
minimum mass flux. The standard
maximum pressure drop should not
exceed 0.33 psi/ft across the bed.
A low overall ∆P across the bed enCH4
compressor
Pulsation
dampener
Adsorption
Adsorption
Pressurization Depressurization
CO2
compressor
FIGURE 5. PSA units are used to absorb carbon dioxide from the LFG stream
41
http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2021

Table of Contents for the Digital Edition of Chemical Engineering July 2021
