Chemical Engineering July 2021 - 40

PVC SS
LFG
Condensate
knockout
LFG
underground
collection
header
LFG
underground
well
Pump
Landfi ll
FIGURE 2. Condensate collection and disposal is an important consideration in LFG processing
LFG header based on the differential
pressure between atmospheric
pressure and the vacuum blower.
On high-pressure days, more LFG
is collected. Typically, the process
engineer will design the LFG collection
system to provide about 40 to
60 in. H2O vacuum at the suction of
the blower [4]. The vacuum blower
should be robust enough to handle
gas with solid particles. A slidingvane
or liquid-ring blower is usually
selected. Liquid-ring type blowers
are much more expensive than
sliding-vane, but they also provide
improved reliability. Blower selection
should consider capex versus opex.
LFG condensate knockout. LFG is
collected at 100% relative humidity
at about 140°F [4]. The warm LFG
cools to about 90°F in the collection
header, where some of the vapors
condense. The condensate
is separated in the LFG knockout
vessel. LFG condensate is considered
a hazardous waste because it
contains halogenated organic compounds,
sulfur compounds, aromatics
and other aggressive chemicals
that lower the pH to a range of about
4.2 to 5.0. The condensate collection
header is usually designed with
polyvinyl
chloride
(PVC), and the
pipes and vessel metallurgy are usually
stainless steel [5].
LFG condensate must not be
drained to the sewer because it is a
hazardous waste. It is either pumped
back to the landfill or transferred to
a hazardous-wastewater treatment
facility. Condensate that is pumped
back to the landfill is absorbed by
the solid waste at the beginning of
LFG production. Toward the end of
its life, the landfill is saturated and
the condensate must be treated as
hazardous waste. At that time, the
condensate is pumped to a polyethylene
tank for storage and peri40
odically
transferred to a hazardous
wastewater-treatment facility. Condensate
contains both water and
non-methane volatile organic compounds
(nmVOCs). The nmVOCs
could be as high as 0.6 mol% [1], or
roughly 1 gal/h of hydrocarbon condensate
per 100 std. ft3/min of LFG.
Water condensate is calculated as
100% relative humidity LFG cooled
from 140°F to 90°F. The mass humidity
ratio (x) is the ratio of the mass
of water (mw) to the mass of dry LFG
[6] (mLFG), and is used to calculate
mass flowrate (ṁ) of condensate as
defined in Equation (1):
ṁcond = ṁLFG(x140ºF - x90ºF) (1)
This shows that the condensate
mass flowrate is roughly 7 gal/h of
water for every 100 std. ft3/min of
LFG. The pumps are specified as lowflow
positive-displacement pumps
at continuous operation. These
pumps are typically diaphragm-type
with non-metallic
internals.
LFG from
blower
LFG flaring
The minimumeffort
form of
LFG disposal is
to incinerate the
gas [7]. Smaller
landfills do not
produce enough
high-value LFG
to justify the
investment for
more expensive,
yet potentially
profitable,
processing.
There are two
types of flares:
simple candletype
flares and
the more expenVacuum
blower
(~8 gal/h HC/H2O
per 100 std. ft3/min LFG)
Condensate
storage
process
sive enclosed flares. Typically, environmental
regulations require 95%
or greater destruction and removal
efficiency (DRE) of CH4. The industry
is moving away from candle flares
because the temperature cannot be
controlled enough to achieve the required
DRE. An enclosed flare is a
large vertical cylinder with multiple
burners, temperature monitoring, air
and fuel controls and heat conservation
from refractory-lined walls. This
design develops the maximum possible
heat within the flare column.
Several factors (age of landfill,
quantity of organic material, moisture,
temperature and others) affect
the quantity of LFG produced, as
well as the composition of the LFG.
At the beginning of the landfill's life,
the composition is approximately
3 mol% CH4 and 97 mol% CO2, at
30 Btu/std. ft3. Toward the end of its
life, less LFG is produced, but at a
better lower heating value (LHV) of
550 Btu/std. ft3 (corresponding to
55 mol% CH4 and 45 mol% CO2).
Usually, enclosed flares are vendor-supplied
and not designed
by the process engineer. However,
the process engineer should
specify a maximum and minimum
LFG flowrate, as well as a range of
LFG compositions.
The process engineer should also
ensure that flare systems are designed
with auxiliary fuel injection to
maintain the required DRE at the beginning
of the landfill's life. The flare
To ATM or
TOX/fl are
Adsorption
Regeneration
TC
LFG
blower
Regeneration
heater
Regeneration
blower
FIGURE 3. Temperature-swing adsorption (TSA) beds are used to remove water
and siloxane content from LFG
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Chemical Engineering July 2021

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