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Andrew
C. Minden (aminden@alzeta.com) and David F. Bartz (ALZETA
Corp., Santa Clara, CA USA
William A. Plaehn, Tim Shangraw, and Mark Murphy (Parsons
Engineering Science, Inc., Denver, CO USA)
Lori T. Tagawa, Waste Management of Colorado, Inc., Lakewood,
CO USA
Dennis Bollmann, City and County of Denver, Denver, CO, USA
ABSTRACT: Thermally
enhancing (through steam, radio frequency, or electrical
resistance heating) soil vapor extraction systems improves the
effectiveness of the remediation effort. However, the elevated
and variable concentration VOC loads generated by these
systems pose unique challenges for off-gas treatment. At the
Lowry Landfill Superfund Site (Arapahoe County, Colorado), two
waste pits are being remediated using electric resistance
heating in combination with a modified soil vapor extraction (SVE)
system. The VOC-laden off-gases generated by this system are
being treated using two flameless thermal oxidation (FTO)
units produced by ALZETA. The two primary challenges of this
project were:
- Achieving up to 99.99%
Destruction Removal Efficiency (DRE) on all chlorinated and
non-chlorinated VOCs (primarily TCE, 1,1,1-TCA and BTEX);
and
- Continuous and complete
processing of VOC-laden off-gas during spike concentrations
that can occur during heating of the waste pits.
This paper presents the design,
performance and operation of a system intended to meet these
challenges. With a total capacity of up to 500 scfm, the
packaged skid-mounted treatment systems include: vacuum
extraction blowers and well gas conditioning equipment, an
integrated oxidizer and wet quench chamber, and a packed tower
acid gas scrubber. Key features of this design are a) parallel
paired sets of treatment skids, b) a quick response FTO
system, and c) a specially designed, variable-flow extraction
system.
 During
preliminary remediation efforts intended to remove the
principal threat wastes at the site, excavation was
attempted, but deemed inappropriate due to short-term risk
to workers from the resulting high levels of ambient VOCs.
As a result, thermally enhanced soil vapor extraction and
off-gas treatment was selected as an alternate technology to
complete the remediation. In addition to the subsurface
challenges resulting from this modified approach, the above
ground off-gas treatment system needed to be capable of
reliably handling elevated and variable concentration VOC
loads generated during subsurface heating while delivering
the following performance criteria at an acceptable capital
and operating cost:
- Delivering consistently
high levels of DRE (up to 99.99%), due to low 24-hour and
annual average ambient air concentration standards at the
property boundary,
- Generating minimal
secondary waste streams (HCl, waste water, other), and
- Maximizing system online
availability.
TECHNOLOGY SELECTION
 During the design phase of the project, carbon or resin
absorption, and conventional thermal or catalytic oxidation
technologies were considered as options for the off-gas
treatment system. Carbon or resin adsorption approaches were
removed from consideration because of the relatively low VOC
loading capabilities as well as the limited ability to handle
some of the light-end contaminants such as vinyl chloride.
Similarly catalytic oxidation was eliminated due to loading
capabilities, but also due to questions regarding its ability
to achieve the required DREs. Finally, while conventional
thermal oxidation could adequately handle the VOC loads, it
was removed from consideration due to questions regarding DRE
performance. Flameless Thermal Oxidation (FTO) was the only
identified technology considered that met all of the project
requirements. The ALZETA EDGE QR™ – “Quick Response” FTO (see
Figure 1) was chosen because of its ability to completely
destroy high concentration VOCs and it’s ability to reliably
and efficiently handle variable VOC loads.
A key component of the ALZETA
FTO technology is an inwardly fired radiant surface
combustion burner (see Figure 2). This burner is a
premixed burner with no visible flame and a uniform heat
release over the entire surface. It consists of a
cylindrical shaped, low-density ceramic fiber layer
measuring approximately ½ inch thick. Oxidation reactions
takes place on the inside surface of this cylinder, with a
characteristic uniform orange glowing appearance and a
temperature of approximately 1700°F. Premixed VOC-laden
air and natural gas flows through the porous ceramic
layer, where all VOCs are exposed to the uniform
temperature oxidation zone. The ALZETA technology achieves
destruction removal efficiencies of over 99.99%, even when
processing difficult-to-treat chlorinated compounds. There
is no refractory lined combustion chamber, and the
“thermal mass” of the ceramic fiber firing surface is low.
The unit responds very quickly to changes in the heat
content of the VOCs being treated, allowing the
temperature control system to rapidly adjust supplemental
fuel firing rates and maintain a stable operating
temperature
SYSTEM DESIGN
In addition to the selection of
the basic off-gas treatment technology, an integrated
treatment train was chosen to provide optimum emissions
control. The system includes redundant off-gas treatment skids
for operational reliability, and independent, variable flow
soil vapor extraction blower packages to accommodate VOC
concentration spikes.
In thermally enhanced soil
vapor extraction systems, underground heating raises the
temperature of the soil resulting in an increase in vapor
pressure generated by the liquids within the soil. In the
event of a nuisance trip (shut down) of the off-gas treatment
system, residual heat within the soil makes it difficult to
rapidly cut back on the generation of soil vapors. This
compares to low temperature soil vapor extraction systems,
where off-gas flow rates can be turned “off” simply by
shutting down the positive displacement vacuum extraction
blower. To account for this requirement, two parallel FTO
units treat 100% of well field gas flows in a 50/50 split. In
the event of a single unit shutdown, the treatment system
diverts the well gases to a single unit, reduces the vacuum
extraction rate, and shuts down the subsurface heating system.
While the vapor generation rate from the hot well field is not
immediately stopped, vaporization rates are significantly
reduced and the system maintains its ability to treat all of
the soil gases generated without a build up of soil gas vapor
pressure. The basic
treatment system (see Figure 3) is a skid-mounted,
factory-assembled package including three basic
sub-assemblies: a) a variable flow soil vapor extraction
blower with an inlet air/liquid separator with automatic
level controls and liquid transfer pump, b) a flameless
thermal oxidizer with an integral quench chamber, and c) a
packed tower acid gas scrubber with a chemical
neutralization feed and pH control system. The scrubber
removes acid gases from oxidizer effluent generated by the
oxidation of chlorinated VOCs. In addition, the
quench/scrubbing water recirculation system reduces water
consumption and sewage discharge, while eliminating build
up of salt solids (generated by caustic – acid
neutralization reactions). The packaged skid-mounted
assemblies allow for moving the treatment system to
multiple sites as well as assuring simplicity and cost
effectiveness in the installation, checkout and start-up
of the units. Figure 4 shows a schematic of the basic flow
pathways and equipment supplied.
click image to enlarge
FIGURE 4:
Process Flow Diagram of EDGE QR-QS-250 SVE System.
In a typical oxidizer, the
oxidation temperature is controlled using an automatic
Temperature Indicating Controller (TIC) with relatively
constant flow rates of well gases. The flow rate of the
supplemental fuel gas is adjusted by the TIC to maintain a
constant exhaust temperature while the flow rate of well
gases are fixed by a constant speed, positive displacement
vacuum extraction blower. Depending on the energy content
of the VOCs contained in the well gases, more or less fuel
is added to maintain the desired operating temperature of
the oxidizer. This controls the oxidizer at a stable
operating temperature, provided the energy content of the
well gases is less than a maximum capacity for the unit
specified. At VOC energy levels above the maximum capacity
of the oxidizer, the temperature will rise above the high
temperature safety alarm causing a system shutdown.
During normal operation of a
thermally enhanced vacuum extraction system, VOC
concentrations can readily spike when a pocket of
organic-rich soil is heated or if a sealed drum of
solvents should suddenly burst underground. Operational
reliability of a larger oxidizer is less impacted by these
spikes, but cost more to operate. Optimal sizing of the
unit is therefore based on a combination of factors
including the capital and operating costs of the unit
compared to the magnitude and frequency of the expected
VOCs concentration spikes and the sophistication of the
control system used to accommodate these spikes.
The variable flow vacuum
extraction system supplied for this application was custom
designed using a combination of manual and automatic
control loops to maximize system robustness, while
minimizing overall costs by:
- Limiting the flow rate
of well gases to the oxidizer in response to spiked
concentrations of VOCs and the resulting high energy
content,
- Maintaining a stable
total flow rate of process gases to the oxidizer for
operational stability, and
- Providing means to
manually adjust the maximum flow rate of well gases
relative to ambient dilution air in order to maintain a
minimum oxygen concentration in the oxidizer feed
stream.
The standard ALZETA FTO is
flexible in its ability to adjust to variable flow rates and
concentrations of VOCs in the off-gases being treated. For
this thermally enhanced soil vapor extraction project where
extremes in concentration spikes were anticipated, additional
controls features were required. To accomplish this objective,
the controls design includes the addition of a split range
control scheme for temperature control. At low values of VOC
concentration, the temperature controller modulates the fuel
gas valve to maintain a fixed oxidizer operating temperature
of approximately 1700F. Then at higher VOC concentrations,
that would otherwise exceed the oxidizer capacity, the
temperature controller restricts the well gas flow rate by
reducing the speed of the vacuum blower using a variable
frequency drive (see Figure 5). The addition of this feature
significantly broadens the operating envelope of the system
while processing VOC contaminated off-gases more efficiently.
As a consequence of the
variable flow vacuum extraction system, potentially wide
swings in the flow rate of well gases are expected. To
mitigate the impact of wide and rapidly changing variations in
the VOC load to the FTO unit, an ambient dilution air blower
was added down-stream of the SVE system, and a second control
loop was added to stabilize the total flow rate of process
gases delivered to the oxidizer. The flow rate of the combined
flow streams (well gases and dilution air) is measured using a
venturi flow element and a differential pressure transmitter,
and the flow rate of the dilution air is modulated using a
conventional flow control damper.
During initial pilot
studies, low oxygen concentrations (1% to 5%) were
measured in the well gases as a result of in-situ
degradation of the soil contaminants. Since a minimum
oxygen concentration (approximately 12%) is required in
the unit to complete the oxidation of the VOCs as well
as provide sufficient oxygen for the combustion of
supplemental fuel, additional ambient dilution air is
required. The FTO units are designed to treat a total
composite flow of 250 scfm, of which, between 100 and
140 scfm would be well gases containing initially only
1% to 5% O2. Eventually as more ambient/clean air is
pulled through the target volume, the O2 concentration
will eventually approach values of 10 to 15%. As this
point is approached, the unit will be able handle up to
250 scfm of well gases. Due to the slow, monotonic shift
in the well gas oxygen concentration expected, the
system was designed with a manual adjustment to the
maximum well gas to total process gas flow rate.
click image to enlarge
RESULTS
AND PROJECT STATUS
Preliminary operation and
source test results are presented. Initial FTO DRE
performance testing was conducted in conjunction with
pilot well testing in which no soil heating was
conducted and limited VOC extraction was achieved. No
attempt was made to optimize the FTO performance or SVE
system extraction rates. In short-term tests, the units
responded well to the variable VOC loadings as designed
and delivered high DRE performance (see Table 1).
For the two dominant species
during pilot testing, inlet concentrations were in the range
of 370 – 953 ppmv for 1,1,1-Trichloroethane and 94 – 269 ppmv
for Trichlorotrifluoroethane, and DRE results were
consistently 99.99% or better. Outlet concentrations were
below 0.01 ppmv in all cases, approaching detection limits for
the tests. For two other species, Chloroform and 2-Butanone,
lower levels of DRE were sometimes measured (in some cases in
the range of 90%). Under these conditions, inlet
concentrations were again near the detection limits (0.007 –
0.019 ppmv), so that almost any non-zero measurement would
result in a low DRE calculation. Despite these lower DRE
values, the outlet concentrations measured during these tests
were two orders of magnitude lower than the maximum allowable
concentrations per ambient air quality standards at the site.
For the overall composite system performance, the low VOC
concentrations result in DRE measurements approaching 99.99%,
with the highest levels of inlet concentrations below 0.3 ppmv
for the composite of all species.
The first phase of this
project began in mid-2001 with pilot studies conducted
using a single FTO unit. After the unit’s basic
performance characteristics were verified, a second unit
was delivered. Currently, shake down tests are being
conducted on the redundant oxidizer system design while
treating the south waste pit. Full implementation of the
complete remediation system including electrical
resistance soil heating is expected in spring-2002. Once
remediation work is completed on the south waste pit
(anticipated by the first quarter 2003), the units will
be moved to the north waste pit. Complete treatment of
the north waste pit is anticipated by the fourth quarter
2003.
click table to enlarge
CONCLUSIONS
Thermally enhanced methods
of soil vapor extraction provide the opportunity to
remediate contaminated soils more efficiently and more
completely than traditional approaches. However, the
off-gasses generated can pose challenges to the above
ground treatment systems due to the high levels and
highly variable concentrations of the VOCs generated.
High inlet concentrations require abatement equipment to
perform at high levels of DRE to meet allowable ambient
air concentration standards, and variable concentrations
poses added controls challenges to ensure system
reliability. The
ALZETA EDGE QR Flameless Thermal Oxidizer, which uses a
unique flameless combustion technology, meets these
challenges by responding quickly to peaking VOC loads
without over-heating. Combining the ALZETA FTO
technology with a variable flow vacuum extraction system
with automatic feedback control, extends the operating
range of a given sized unit. When exceptionally high VOC
load spikes are encountered that would otherwise exceed
the capacity of the unit, the flow rate of soil
off-gases is reduced thus avoiding over-heating and a
system shutdown. Finally, when integrated into a
redundant treatment train of dual parallel oxidizers,
near 100% online reliability is assured. The net result
is a system, which maximizes the treatment of soil vapor
off-gasses when low-to-moderate concentrations of VOCs
are present and automatically attenuates off-gas
production rates when concentration spikes are
encountered. In initial pilot tests with high and low
inlet VOCs concentrations, the ALZETA EDGE QR FTO
technology demonstrated its ability to treat both
chlorinated and non-chlorinated VOCs with up to 99.99%
DRE, well beyond the requirements to meet allowable
ambient air concentration regulations at the site.
REFERENCES
Bartz, David F., Bruce N.
Marshall, John D. Sullivan, Kevin Bruce and Anthony Lombardo,
1996. “Destruction of Halogenated VOCs Using Premixed Radiant
Burner.” 15th International Conference on Incineration and
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California, Irvine, CA.
Bartz, David F. and Fred E. Moreno, 1993.
“Rapid-Response, High Effectiveness Destruction of
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Processes.” A-1543, Semiconductor Safety Association,
McLean, VA Bartz,
David F., Fred E. Moreno, and S. Peter Barone, 1992.
“High VOC Destruction with Low NOX in Adiabatic Radiant
Combustors.” 11th International Conference on
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Hasback, Ann, 1998.
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Waste Pit Remedy Design Investigation Work Plan.
prepared for City and County of Denver, Chemical Waste
Management, Inc., and Waste Management of Colorado, Inc.
by Parsons Engineering Science, Inc., Denver, CO.
Parsons, 2001. Draft FTPA
Waste Pit Remedy Design Investigation Report.
prepared for City and County of Denver, Chemical Waste
Management, Inc., and Waste Management of Colorado, Inc.
by Parsons Engineering Science, Inc., Denver, CO.
Parsons, 2001. Draft FTPA
Waste Pit Remedy Pilot Study Work Plan. prepared for
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