c/EPA
United States
Environmental Protection Agency
430F98081
Air and Radiation
6202J
Draft
April 1998
EPA Coalbed Methane Outreach Program Technical Options Series
ENRICHMENT OF MEDIUM QUALITY COAL MINE GAS
Gas Enrichment Using Pressure Swing Adsorption at the Abandoned Nelms No. 1 Mine, Ohio
(Process development co-funded by Northwest Fuel Development, Inc. and U.S. DOE)
SOME FACTS CONCERNING ENRICHMENT OF MEDIUM QUAUTY GAS.
+ Technology for enriching gas containing as little as 50% methane is now feasible
4 Coal mine gas enrichment can be profitable, especially for large projects
* Many mines have gob gas flows that would support financially attractive projects
* Recovery and use of gob gas reduces emissions of methane, a greenhouse gas
-------
gas enrichment
requires rejection of
nitrogen, oxygen,
carbon dioxide, and
water vapor
Gas processing
vendors soon expect
to offer integrated
gob gas enrichment
systems
At natural gas prices
of $2.00 per mmBtu,
enrichment projects
that self upgraded
gob gas may be cost-
effective if feed gas is
available in flows of 5
mmcffd or higher
Why Consider Enrichment of Medium Quality Cool Mine Gos?
Gas drained from gob areas (collapsed rock over mined-out areas) typically
contains 30-90 percent methane. Although gob gas is a potentially valuable fuel
source, many mines vent it to the atmosphere, primarily because it does not meet
quality specifications for injection into natural gas pipelines, which typically require
a minimum of 95 percent methane. The coal mine methane industry has been
searching for a proven and affordable solution to upgrading gob gas so they can
take advantage of new markets now accessible through deregulation of the
natural gas industry.
For years, enrichment facilities have been successfully upgrading medium-quality
gas from natural gas wells, but none had been able to economically remove
nitrogen, oxygen, carbon dioxide, and water vapor in the same integrated facility.
Gob gas enrichment has made great progress in recent years, however, and a
full-scale integrated gob gas enrichment project is currently in operation in
Pennsylvania.
In 1997, EPA's Coalbed Methane Outreach Program prepared a report titled
Technical and Economic Assessment of Potential to Upgrade Gob Gas to Pipeline
Quality. The report examined average costs that enrichment projects would incur
in a typical mine setting for a variety of feed gas qualities and daily flows. The
most critical and expensive component of any enrichment system is the nitrogen
rejection unit (NRU). Suppliers of three major nitrogen rejection technologies affirm
that a gob gas enrichment plant is technically feasible and free of unacceptable
risks. Below is a brief overview of the major nitrogen rejection technologies whose
suppliers are willing to make firm proposals for integrated enrichment plants.
Cryogenics Process. The cryogenics process pressurizes and flashes the feed
gas stream and then uses a series of heat exchangers to liquefy the gas mixture.
A distillation separator vents a nitrogen-rich stream, leaving the methane-rich
stream. BCCK Engineering supplied the cryogenics technology used in the
integrated gob gas enrichment project underway in Pennsylvania.
Pressure Swing Adsorption (PSA) systems repeatedly pressurize the gob gas and
use various adsorbents to selectively adsorb nitrogen and methane in different
concentrations and/or rates. During successive cycles, the process preferentially
adsorbs methane in favor of nitrogen until the output attains the desired methane
proportion. BOC Gases demonstrated a PSA process on gob gas and reported
good methane recovery; however, in order to achieve the required gas quality, it
was necessary to use feed rates lower than the nominal rating of the system.
Selective absorption uses specific solvents that have different absorption
capacities with respect to different gas species. The petroleum refining industry
commonly uses this method to enrich gas streams. One firm rnai offers selective
absorption to reject nitrogen from methane, Advanced Extraction Technologies, is
ready to offer the system to interested mine owners.
Other nitrogen rejection technologies are under development, as discussed on
the following page.
-------
Key Conclusions from Technical and Economic Assessment of Potential to Upgrade Gob Gas to
Pipeline Quality...
EPA's report included a technical assessment of cryogenic, PSA, and selective absorption NRU processes as
the key components of their respective integrated gob gas enrichment facilities. The report also estimated
capital and operating costs for a range of feed gas flows (3-6 mmcf/d) and qualities (50-90%). Following is
a summary of some of the report's key findings:
• All of the three nitrogen rejection techniques evaluated could successfully operate as the principal
component of an integrated plant to enrich gob gas. In some instances, a cryogenics unit could be
risky, given the presence of oxygen and carbon dioxide and the compositional and flow rate variations
inherent with gob gas. In the selective absorption process, oxygen removal must be the first step.
Systems that carry oxygen through the process units (such as PSA) would have to provide designs that
remove the risk of explosion in certain combinations of oxygen and methane.
• Several companies are developing
other nitrogen rejection technologies
that are potentially applicable to an
integrated gob gas cleanup system.
These include alternative PSA systems
(improved adsorbents, continuous
PSA), alternative absorption technolo-
gies, and membrane units. Northwest
Fuel Development Inc. has successfully
demonstrated both PSA and CPSA
nitrogen rejection units at the Nelms
Mine in Ohio (see cover photo). These
systems appear to be economic with
flow rates as low as 1 mmcf/d.
• Enrichment projects that sell upgraded gob gas into the natural gas transmission or distribution market
may be cost-effective relative to current natural gas prices if 80% methane feed gas is consistently
available in daily gas flows of 5 mmcf/d or higher. This conclusion is based on conservative estimates
of capital and operating costs for typical plants operating under an assumed set of conditions.
Comparison of Nitrogen Rejection Enrichment Units in integrated Systems
Vendor UOP Nitrotec BOC AET Darnell Schedule A
Enrichment may work well with a broader,
integrated strategy that includes one or more
of the following: 1) improving gas recovery
systems to enhance gas quality; 2) blending
gob gas with higher quality gas; and 3) spiking
gob gas with propane. EPA has prepared a
user-friendly computer program that helps gas
project developers identify cost-effective
combinations of these various upgrade
options.
Technology
Phase Change
Methane Recovery
First Stage
Deoxygenation
Ready to design and
build an integrated
gob gas plant?
PSA*
No
Up to
95%
No
Yes
PSA
No
Up to 95%
No
Yes
PSA
No
98%
No
Yes,
possibly
Selective
Absorption
No
96-98%
Yes
Yes
Cryogenic
Liquefy
98%
Yes
Yes
Cryogenic
Liquefy
98%
Yes
Yes, after trials
Vendor list is not complete; includes only vendors that supplied technical and cost details of their systems.
Contact information for these and other suppliers is available in the EPA report described above.
Minimum plant size available from most vendors is 3 mmcf/day of feed gas; all are capable of processing at least 6
mmcf/day of feed gas.
-------
For More Information.
The EPA report Technical and Economic
Assessment of Potential to Upgrade Gob
Gas to Pipeline Quality provides more
detailed information on enrichment
technologies and average costs that
enrichment projects would incur.
To obtain a copy of the report, and a
computer program that helps gas project
developers identify cost-effective combina-
tions of enrichment, blending, and spiking
options, contact:
Coalbed Methane Outreach Program
U.S. EPA
401 M Street, SW (6202J)
Washington, DC 20460 USA
(202) 564-9468 or (202) 564-9481
Fax: (202) 565-2077
e-mail: fernandez.roger@epamail.epa.gov
schultz.karl@epamail.epa.gov
http://www.epa.gov/coalbed
M HAN
OUTREACH
1 R O C. R A M
The mention of products or services in this case study does not constitute an endorsement by EPA.
------- |