United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-83-047 Sept. 1983
SEPA Project Summary
Theoretical Investigation of
Selected Trace Elements in Coal
Gasification Plants
A. H. Hill, G. L Anderson, and D. K. Fleming
Results of a theoretical investigation
of the disposition of five volatile trace
elements—arsenic, boron, lead, sele-
nium, and mercury—in SNG-producing
coal gasification plants are reported.
Three coal gasification processes—dry-
bottom Lurgi, Koppers-Totzek, and
HYGAS—were investigated to examine
the possible effects of gasifier operation
conditions on speciation of the volatile
trace elements. Dispositions of these
volatile trace elements in a general gas
processing scheme (consisting of gas
quench, acid-gas removal, and combined
shift methanation) were then projected.
A different acid-gas removal process
was used in each process. The Rectisol
process was used for the Lurgi projec-
tion; the Benfield, for the Koppers-
Totzek; and the Selexol, for the HYGAS.
Results of this theoretical investigation
suggest that none of the trace elements
considered in this study will be present
in the product SNG from a coal gasifica-
tion plant, but will be removed from the
fuel gas by various unit operations.
Results also suggest that the speciation
of these volatile trace elements is not
significantly affected by gasifier condi-
tions.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Research Triangle
Park, NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Results of a theoretical investigation of
the disposition of five volatile trace
elements—arsenic, boron, lead, selenium,
and mercury—in SNG-producing coal
gasification plants are reported. Three
distinct coal gasification processes—dry-
bottom Lurgi, Koppers-Totzek, and HY-
GAS—were investigated to examine the
possible effects of gasifier operating
conditions on the speciation of the
volatile trace elements. The dispositions
of these volatile trace elements in a
general gas processing scheme (Figure
1)—consisting of gas quench, acid-gas
removal, and combined shift-methana-
tion—were then projected. A different
acid-gas removal process was used in
each process. The Rectisol process was
used for the Lurgi projection; the Benfield,
for the Koppers-Totzek; and the Selexol,
for the HYGAS.
Results of this theoretical investigation
suggest that none of the trace elements
considered in this study will be present in
the product SNG from a coal gasification
plant, but will be removed from the fuel
gas by various unit operations. Results
also suggest that the speciation of these
volatile trace elements is not significantly
affected by gasifier conditions. Arsenic is
expected to appear in the raw fuel gas
from the gasifier as a combination of the
elemental forms and the hydride, arsine
(As Ha). Boron is expected to appear as
boric acid, B(OH)3. Selenium is expected
to exist as the hydride, hydrogen selenide
(H2Se). Mercury is expected to appear as
the elemental form. Contrary to expecta-
tions, the projection also indicated that
only a minor fraction of the lead is
expected to be present in the raw fuel gas.
The range of concentration for these
volatile trace elements in the raw fuel gas
from the gasifiers was projected to be
AsH3, 0.02 to 0.5 ppmv; As*, 0 to 4 ppmv;
H2Se, 0.02 to 1.4 ppmv; B(OH)3, 1 to 300
ppmv; PbS, <1 x 10~* ppmv; PbCI2, <2 x
-------
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gasifier ash, and the primary condensate
streams from quenching operations and
performing elemental balances. Varia-
tions in the concentration of the trace
element in the feed coal make this
approach imprecise. Based on IGT's
theoretical projections, the volatile trace-
element-containing compounds in the
quenched fuel gas may represent only a
small percentage (<\ 0%) of the feed. This
fraction is of the same order of magnitude
as the uncertainty in the elemental
balances.
The projected dispositions of the five
trace elements in commercial-scale
versions [7.082 million m3 (std)/d (250
million scf/d) of SNG] of the three
gasification processes are summarized in
Table 1.
C02(ll)
H2O
Figure 1. Typical gas processing configuration for producing SNG. (Arabic numerals in circles
are identified in Table 1.)
10 5 ppmv; Pb, <6 x 10 " ppmv; and Hg,
0.001 to 0.1 ppmv. The ranges of these
projections reflect the entire range of con-
centrations at which these trace elements
exist in U.S. coals as reported by Ruch, et
a/.* The concentration ranges assumed
for this study were 0.5 to 14 ppmw ar-
senic, 0.45 to 7.7 ppmw selenium, 2 to
224 ppmw boron, 1 to 200 ppmw lead,
and 0.02 to 1.6 ppmw mercury.
Projections on the probable disposition
of these volatile trace-element-containing
compounds in the gas quenching opera-
tions indicated that essentially all the
BfOHJs, As*, and lead-containing com-
pounds, and a major fraction of the H2Se
will be removed from the fuel gas during
quenching. The B(OH)3 and the H2Se
should be recovered in the aqueous
condensate. The lead-containing com-
pounds should precipitate. In the Koppers-
Totzek process, the arsenic should also
precipitate, but in the HYGAS and Lurgi
processes, where aromatic oils and tars
are produced, the arsenic may be dissolved
in these by-products, reaching concen-
trations as high as 2000 ppmw. Only
AsH3, mercury, and part of the H2Se were
projected to remain in the quenched fuel
gas. The projected concentration range
for these constituents was 0.03 to 0.83
ppmv, 0.002 to 0.14 ppmv, and 0.003 to
1.35 ppmv, respectively.
Ruch, R. R. Gluskoter, H. J., and Shimp, N. F,
Occurrence and Distribution of Potentially Volatile
Trace Elements in Coal. Environmental Geology
Note Series 72, Illinois State Geological Survey,
Urbana, IL, 1974.
The disposition of the mercury and
HaSe in the three acid-gas removal proc-
esses was projected to be identical to
that of H2S. The disposition of arsine was
projected to be similar to that of C02 in the
Selexol and Rectisol processes, but the
AsHa was projected to remain with the
treated gas in the Benfield process. How-
ever, these projections have considerable
uncertainty associated with them: no
solubility data exist for these compounds
in these acid-gas removal solvents. If the
above projections are indeed correct, two
potential problems can be cited. First, the
CC>2-rich gas from the Selexol and
Rectisol processes may have environ-
mentally significant levels of AsH3. The
projected concentration range for AsH3 is
0.1 to 2.5 ppmv before additional treat-
ment. Second, arsenic, selenium, and
mercury may significantly contaminate
any elemental sulfur produced from the
H2S-rich acid-gas stream from these
three acid-gas removal processes. The
potential concentrations range from 1 to
1000 ppmw for selenium, 0.01 to 30
ppmw for arsenic, and 2 to 200 ppmw for
mercury. This degree of contamination
could make the elemental sulfur unsuit-
able as a chemical feedstock.
These projections indicate the need for
direct determination of the concentrations
of volatile trace-element-containing
compounds in all major gas streams of an
integrated coal gasification process. The
methodology used in previous environ-
mental assessment studies has consisted
primarily of analyzing the feed coal, the
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Table 1. Pro/ected Disposition flares for Selected Trace elements In Commercial-Scale Lurgi. Koppers-Totzek, andHYGAS Plants
Stream
No"
1
2
3
4
5
6
7
8
9
10
Stream
No-"
J
2
3
4
5
6
7
8
9
10
Description
Gasifier Residue*
Quench So/ids'
Wastewater
Oil/ Tar"
H2S-Rich Acid Gas
COi-Rich Acid Gas Hi
Methanation Condensate
CDs-Rich Acid Gas (ll/
Glycol Cooler Condensate
SNG
Description
Gasifier Residue"
Quench Solidsc
Wastewater
Oil/Tarc
HzS-Rich Acid Gas
COrRich Acid Gas (1)
Methanation Condensate
COz-Rich Acid Gas (II)
Glycol Cooler Condensate
SNG
L"
4-850
0
03-30
04-740
04-39
38-38
0
0
0
0
L
4 1-70
0
3.8-65
<0.005
03-4.5
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
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Agency
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Penalty for Private Use 5300
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