&EFK
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S7-81-035 May 1981
Project Summary
Environmental Effects of
In Situ Gasification of
Texas Lignite
T. F. Edgar, M. J. Humenick, W. R. Kaiser, and R. J. Charbeneau
A general survey of environmental
effects of in-situ gasification of Texas
lignite has been undertaken. The sur-
vey has emphasized the following
subjects:
(1) Identification of the location,
quality, and quantity of lignites
in Texas: An exploration model
for the various lignite-bearing
formations has now been devel-
oped, thus allowing evaluation
of the commercial potential of
various regions of Texas where
deep basin lignite can be found.
In addition, hydrological and
baseline environmental data have
been obtained for several areas
in Texas.
(2) Assessment of in-situ gasifica-
tion technologies: The effects of
various geological conditions,
coal characteristics, and operat-
ing conditions have been re-
viewed with reference to Texas
lignite. Recent field test data for
both air and oxygen injection
have been analyzed; it appears
that medium Btu gas (oxygen-
blown system) will be the most
attractive product from in-situ
gasification.
(3) Determination of possible ad-
verse environmental impacts
(air, land, and water) resulting
from underground coal gasifica-
tion (UCG): Air pollution for
UCG is essentially the same as
for surface gasification, and
overall air emissions are lower
compared to those from a con-
ventional solid coal burning fa-
cility. Subsidence is the major
land impact, but it is difficult to
predict the extent of subsidence
at a given site. The major impact
to be considered from UCG is
subsurface water pollution; field
test data from Texas and Wyom-
ing have been reviewed, both for
organic and inorganic compounds.
Applicable regulations are dis-
cussed.
(4) Evaluation of the dispersion of
pollutant species and the appli-
cation of mathematical models
to predict pollutant transport:
The literature on dispersion mod-
eling has been reviewed and
specific problems posed by typi-
cal groundwater flows in Texas
are discussed. Hydrological data
at UCG test sites in Texas are
reviewed.
(5) Assessment of existing water
pollution control technology to
counteract pollution caused by
UCG: The philosophy of site
restoration, given that some
natural renovation occurs under-
ground after a test is performed,
is discussed, and strategies for
site restoration have been identi-
fied.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
-------
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The future use of underground coal
gasification (UCG) in the state of Texas
to produce a synthetic fuel to replace oil
and gas appears to be both technically
and economically feasible. UCG has
some environmental advantages over
conventional extraction and conversion
technologies, in that strip mining is
avoided and sulfur emissions are more
readily controlled. However, one disad-
vantage of UCG is that subsurface water
pollution does occur, although the
short- and long-term water quality
impacts are difficult to predict.
The evaluation of UCG for Texas has
required analysis of engineering and
geological data available in the litera-
ture. Operating data from UCG tests,
both in the U.S. and U.S.S.R., and
geological, hydrological, and environ-
mental quality data have been accumu-
lated. Relevant data have been analyzed
in order to assess the environmental
(land, air, water) impact of UCG in Texas.
Due to an inadequate data base in the
above three areas, engineering analysis
must be employed to reach quantitative
and in some cases qualitative conclu-
sions. Therefore this report should be
considered as a preliminary evaluation
of UCG in Texas and serves to identify
those areas of research which should be
pursued in more detail.
The major alternatives for use of UCG
include on-site power production using
low Btu gas and manufacture of chemi-
cal products from medium Btu gas.
Oxygen injection and subsequent
processing of the medium Btu gas to
produce methanol or gasoline is a
promising near-term technological al-
ternative for UCG in Texas.
Resources:
A detailed analysis of well logs in the
state of Texas has led to the development
of a regional exploration model for
lignite. This model has allowed the
estimation of deep basin lignite re-
sources which might be recoverable via
underground gasification. The minimum
seam thickness for economic or techni-
cal feasibility has been selected as five
feet; approximately 35 billion short tons
of lignite lie in five distinct regions
under less than 2000 feet of cover.
These blocks have also been evaluated
in terms of their hydrology and baseline
water quality, although the data are
limited. Overall geological criteria for
selecting gasifier sites have been devel-
oped. Preferred hydrological factors
have also been identified. Figure 1 and
Table 1 summarize the deep basin
resources of lignite.
Environmental Impacts
Water Pollution
Of the possible environmental impacts
of UCG, the effects on local ground-
water quality may be the most significant
concern in implementation of the
process. Documentation exists in the
literature and from current projects that
pollutants are released to groundwaters
when normal flow returns to a post
gasification zone. Above-ground
processing of product gas streams can
also present water pollution control
problems. Although there has been no
demonstration of large-scale gas
cleaning facilities in conjunction with
UCG field tests, the wastewaters
generated should be similar to those
produced by above-ground gasification
facilities.
Available data on inorganic and
organic water pollutants for UCG field
tests in Texas and Wyoming have been
accumulated and analyzed. The organic
compounds include light hydrocarbons,
phenols, oils, and tars. Heavier organics
include some polynuclear aromatic
hydrocarbons (PAH's) and hetero-cyclic
compounds. Other gaseous components
that condense or are absorbed in sur-
rounding groundwater are ammonia,
carbon dioxide, hydrogen sulfide, and
methane. Qualitatively similar pollutant
profiles are obtained.
Field data have indicated that the
pollutants tend to decrease both with
time and distance from the burn cavities.
There are natural means by which the
groundwater concentrations can be
reduced. Most interpretations of field
data attribute the improved water quality
to adsorption and ion exchange of
properties of surrounding strata, precip-
itation reactions, dilution and dispersion
by groundwater flow, and biological
conversion reactions. It should be noted
that only the last process, biological
conversion, is ultimately useful in the
final destruction or conversion of harm-
ful contaminants to nonharmful prod-
ucts. Inorganic pollutants are not as
susceptible to attenuation as are the
organic pollutants. The attenuation of
phenol is shown in Figure 2.
After gasification is completed, an
ash residue remains in the burn cavity
which yields soluble inorganic compo-
nents to reinvading groundwaters. The
soluble ash components can greatly
increase the total dissolved solids con-
tent of the groundwater. These soluble
materials include a wide array of ionic
species, mainly calcium, sodium, sulfate,
and bicarbonate. These components
yield increases in non-specific param-
eters such as conductivity and total
dissolved solids. The exact quantities of
these materials are a function of initial
coal composition, combustion tempera-
ture, water temperature, and ground-
water composition. There are, however,
many other inorganic materials leached
into the groundwater which are of
interest even though they are present in
lesser quantities. These include alumi-
num, arsenic, barium, boron, iron, zinc,
cyanide, selenium, and hydroxide. Again
the exact compositions depend on the
variables cited above.
The conceptual model of pollutant
generation and transport proposed in
the literature appears valid. Materials
having little affinity for solid surfaces,
such as many of the TDS components.
Table 1. Deep-Basin Resources in Texas (millions of short tons)
x 10s short
tons
Percent
Data
(E-logs)
Wilcox
east
central
10499
30
626
Wilcox
north
east
0
0
506
Wilcox
Sabine
Uplift
8814
25
396
Wilcox
south
4631
14
363
Wilcox
subtotal
23944
69
1891
Jackson
east
5600
16
587
Jackson
south
5275
15
547
Jackson
subtotal
10875
31
1134
Yegua
east
0
0
595
Total
34819
100
3620
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'exarkana
.••"•.... Drainage divide
4 Resource block number
'//? s/'ru gasification field test
Deep-Basin Lignite
i;;i|'-| lower Wilcox Group
Co/vert Bluff Formation (Wilcox Group}
undivided Wilcox Group
lower Jackson Group
Manning and Wellborn Formations
(Jackson Group)
40
80 mi
0 40 80 km
W.R. Kaiser 1979
Figure 1. Distribution of deep-basin lignite in Texas.
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600
100
10
o
0)
0.1
0.01
0.001
7 JO 100
Distance from burn boundary — ft
Figure 2. Concentration of phenolic
materials at Hoe Creek I
as a function of distance
from burn boundary and
time after gasification had
ceased.
will not be retained by surrounding
strata and will travel at the rate of
groundwater flow with little diminuation
of peak concentrations. Those materials
with medium to large affinity for solid
surfaces will travel much more slowly
and have significant decreases in peak
concentrations as time progresses.
Of particular concern is the deposition
of tars and oils in the surrounding
strata. Some of the materials comprising
the tars and oils are toxic chemicals. It
appears that the solubility of these
components are very low and they have
high adsorption affinity for surrounding
solid strata. Thus, aqueous concentra-
tions of these components should be
low in the region of the tar deposits, and
their high affinity for solid surfaces
should prevent rapid migration in the
water phase. However, considering
solubility and adsorption only, it should
take a very long time before the source
of pollution is dispersed, but little
quantitative information is available.
Dispersion of potential groundwater
pollutants has been analyzed and found
not to be very significant, due to low
groundwater velocities (typically one
meter/year or less). Disturbance of
aquifer flow patterns and local hydrology
may, however, result from roof collapse
after gasification is completed. An
important consideration in dispersion
analysis is the ability of the subsurface
strata to adsorb potential contaminants,
thus gradually improving water quality
after gasification.
The general mathematical models
used for prediction and simulation of
flow and transport are deemed to be
adequate. The science and art of numer-
ical modeling is highly developed, and
there are few physical problems which
defy analysis. The limitation of numeri-
cal models lies in the difficulty in finding
the appropriate parameter values to use
in the models. Fractures within the
porous media present a number of
problems for modeling. There is no
generally accepted model of the hydrau-
lics of flow in fractured media. Also,
development of fractures in the over-
burden presents an avenue for increased
cross-formation flow between neigh-
boring water bearing units.
Air Pollution
While the water pollution impact from
underground coal gasification is localized
in nature, the air quality considerations
in UCG have much broader geographical
ramifications. This is because in-situ
gasification offers the possibility of
actually reducing regional air quality
impacts, when compared to the alterna-
tive of strip mining and burning solid
fuel in a conventional steam-electric
plant.
If, however, the power generation
plants are located at the lignite deposits
themselves and the power is transmitted
to the consuming centers on the Gulf
Coast, there may be shifts in the state-
wide air quality patterns. While the
consuming areas will experience little
or no adverse environmental impact,
the predominantly rural areas where
lignite deposits exist may experience
some problems. This of course will also
be tied to medium/ and long/range
transport of pollutants in the atmosphere.
A large number of lignite-based power
plants are indeed planned for the future
(up to 1989) at various locations along
the Texas lignite belt. These are shown
in the full report along with the plants
which will use western (non-Texas)
coals. All the lignite-based plants are
presently scheduled to use conventional
pulverized coal boilers. Table 2 summa-
rizes the projected effect of underground
coal gasification on the emission rates.
Subsidence from Underground
Coal Gasification
The controlled operation of any under-
ground coal gasification plant requires
the ability to predict thein-situ behavior
of the relevant rock strata, which in turn
presupposes a knowledge of the me-
chanical and physical rock properties.
Without this ability unforeseen opera-
tional and environmental problems may
arise. Of particular importance in this
context is the ability to predict roof
stability and subsidence during and
after the productive life of the operation.
Significant fracturing, deformation,
or collapse of the rocks surrounding the
cavity can affect the overall operation in
many ways. As discussed in Section 3 of
the full report, roof collapse can be
beneficial by minimizing void space
underground. Excessive void space
encourages gas phase oxidation of CO
to CO2, which lowers the heating value
of the product gas significantly and
causes excessive gas outlet tempera-
tures. The ultimate sweep efficiency of
the process depends upon the complete-
ness of the roof collapse. Moreover, if
the immediate cavity roof is coal, as it
may well be early in the development,
collapse of this roof will be advantageous
in giving a "self-fueling" effect.
On the other hand, extensive fractur-
ing of the surrounding rocks, or collapse
of the roof, could cause serious opera-
tional and environmental problems.
Roof collapse could lead to excessive
gas losses and excessive water influx,
possibly causing problems of pollution
in overlying aquifers. Finally, any defor-
mation or collapse of the roof rock will
ultimately be reflected on the surface as
subsidence.
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Table 2.
Comparative End Use Air Pollutant Emissions from Combustion of Medium Btu Gas (UCG) and Direct Lignite/Coal
Combustion-Houston-Galveston Air Quality Region (1985)
Incremental emission rate, tons/year'
Relative increase.
Air
pollutant
Paniculate
matter
Sulfur
oxides
Nitrogen
oxides
Direct
combustion
20,000
33,6002
(224.00O3)
172,000
UCG5
1,800
100
57,000
Regional
total
(1973)4
224,000
195.000
299,000
Direct
combustion
9.1%
17.2%
(1 14.6%3)
57.6%
UCG
0.8%
19.2%
Reduction
obtained by
gasification
90.8%
99.7%
66.6%
Notes: 'Based on additional 19X106 tons coal/year required in 1985 to meet energy demands.
235% S02 removal.
3Uncontrolled SO2 emissions.
'Regional emission totals for Region 7 courtesy of Texas Air Control Board.
s Based on emissions from combustion of natural gas (EPA AP-42, "A Compilation of Air Pollutant Source Emission Factors,'
EPA Office of Air Quality and Standards, Research Triangle Park, N.C., 1977)
Conclusions
An extensive subsurface study of
lignite in Texas has led to the develop-
ment of a fairly complete regional
exploration model for lignite. New
resource maps for the Wilcox and
Jackson Groups in South Texas have
been completed. This model has allowed
the estimation of deep basin lignite
resources which might be recoverable
via underground gasification. The mini-
mum searrt thickness for economic or
technical feasibility has been selected
as five feet (1.5 m), and the maximum
seam depth is considered to be 2000
feet (600 m); approximately 35 billion
short tons of lignite, found in five
regions, meet these criteria. Resource
blocks have also been evaluated in
terms of their hydrology and baseline
water quality, although the data are
limited. Overall geological criteria for
selecting gasifier sites have been
developed.
The technical factors which are con-
ducive to application of UCG for Texas
lignites have been identified, including
coal properties, seam thickness, water
influx, and roof collapse. Water influx
appears to be a crucial factor in Texas
both in linking and gasification and is
expected to be more of a problem in the
Wilcox Group than the Jackson Group.
Field test data from Texas sites, all in the
Wilcox Group, have shown very high
water influxes; mathematical model
calculations for gas composition indicate
that the thermal efficiency of the process
suffers greatly for large water influx
rates. This suggests that dewatering of
a site may have a beneficial effect on
heating value and thermal efficiency.
Calculations on design and operation of
dewatering wells could provide some
insight as to the economic trade-offs of
such wells. The role of production
pressure in UCG to reduce water influx
is also poorly understood. Prior esti-
mates of water influx for a given site are
desirable but are difficult to calculate
with present knowledge.
Field data have indicated that the
pollutants tend to decrease both with
time and distance from the burn cavities.
There are natural means by which the
groundwater concentrations can be
reduced. Most interpretations of field
data attribute the improved waterquality
to adsorption and ion exchange proper-
ties of surrounding strata, precipitation
reactions, dilution and dispersion by
groundwater flow, and biological con-
version reactions. It should be noted
that only the last process, biological
conversion, is ultimately useful in the
final destruction or conversion of harm-
ful contaminants to nonharmful products.
The impact from air emissions from
UCG has much broader geographical
ramifications than the water quality
problem, which is more site-specific in
nature. Air emissions from a UCG
facility plus conversion plant are ex-
pected to be lower than those resulting
from conventional coal burning steam-
electric plant. The gas cleanup opera-
tions for UCG should be the same as
those needed for surface gasification
plants. For a given region, the lower
emissions would mean that the available
PSD increments in pollutant concentra-
tions in ambient air are consumed to a
lesser extent by each new source or
major modification. This could promote
a greater industrial growth in the region
while minimizing adverse environmental
effects on air quality. A qualitative
analysis of existing emission sources in
those regions where UCG might be used
reveals few sources which affect air
quality. However, a more detailed study
involving meteorological considerations
should be performed.
The prediction of roof collapse and
subsequent surface subsidence is not
possible given the state of knowledge
for large-scale rock mechanics. The
surface subsidence is site-specific, and
available field data do not allow any
specific conclusions regarding predicted
subsidence at Texas sites for a commer-
cial operation. Only one U.S. field test
(Hoe Creek III) has experienced measur-
able subsidence. The relatively thin
seams of Texas lignite do appear to be
advantageous for minimizing surface
movement.
-------
T. F. Edgar. M. J. Humenick, W. R. Kaiser, and R. J. Charbeneau are with the
University of Texas, Austin, TX 78712.
Robert Thurnau is.the EPA Project Officer (see below).
The complete report, entitled "Environmental Effects of In Situ Gasification of
Texas Lignite." (Order No. PBS 1-171 654; Cost: $14.00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
> US. GOVERNMENT ntlNTINaOFnCC:tH1-757-012/?093
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