1. Renewable Natural Gas—Biogas
Cleaning and Upgrading 101
Nicholas Kennedy, Georgine Yorgey, Craig Frear, Dan Evans, Jim
Jensen, and Chad Kruger
Center for Sustaining Agriculture and
Natural Resources
Washington State University
Photo: Andgar
2. Outlines
1. Raw biogas and Upgrade
Standards
2. Constituents to Scrub
3. Additional Concerns
Photo: DVO
4. Raw Biogas from Dairy Manure
Composition
• Methane 54-70%
• Carbon Dioxide 27-45%
• Hydrogen Sulfide 100-3,000 ppm
• Hydrogen 1-10%
• Nitrogen, Oxygen 0-3%
• Water Vapor Varies
Biogas from dairy manure is typically low
or non detectable in siloxanes, a
potentially harmful contaminant to engines
and downstream processing equipment,
although use of co-digestion can impact
that.
GTI 2009 and Rutledge 2005
EPA
5. Upgraded Biogas for Renewable Natural Gas
Composition
• Methane > 75%
• Carbon Dioxide 3-4%
• Hydrogen Sulfide < 1 g/100 scfm
• Hydrogen 0%
• Oxygen < 1 ppm
• Nitrogen 3-4%
• Water Vapor 0%
• Siloxanes 0%
The above are for entry into a gas
pipeline, whereas composition will differ if
directly used as CNG without entry to
pipeline.
GTI 2009 and Rutledge 2005
cng-tank.com
iea.gov
7. Constituents to Scrub
Water Vapor
Moisture is typically the
first contaminant to be
removed and is
typically accomplished
by chillers that drop
the temperature of the
biogas to the point
where the vapor
condenses to liquid
Hydrogen Sulfide
Three general methods
are used to remove
hydrogen sulfide.
These are: in-vessel
biological, out of
vessel biological and
physical-chemical via
agents such as iron
sponges, activated
carbon, and water.
Carbon Dioxide
Carbon dioxide is often
the last to be cleaned
as its removal is only
needed for RNG
whereas the others are
sufficient for engine
and electricity systems.
Typical systems found
on dairies are water
scrubbers, pressure
swing absorption and
membrane
separations.
Greenboxchillers.com
Iron Dosed Material for Use in Iron
Sponge (MV Technologies)
8. Hydrogen Sulfide
In-Vessel Biological
• It is possible to dose in small quantities of air/oxygen
(2-6% O2) into the anaerobic digester so as to induce
aerobic bacteria to consume the produced hydrogen
sulfide, converting it to elemental sulfur that leaves with
the effluent.
• Typically can reduce levels from 3,000 ppm to < 1,000
ppm in a very inexpensive manner. Sometimes not
enough for engine manufacturers, local air boards and
not enough for RNG, so will often need additional
systems.
Out-Vessel Biological
The same sulfur-consuming bacteria are utilized in these
system to remove nearly all of the hydrogen sulfide.
Requires aeration and supply of minerals for sustaining
the bacteria as well as associated pumps, etc.
Energy Cube, LLC
9. Hydrogen Sulfide, Continued
Physical-Chemical
• A very common approach is called an iron
sponge. This takes advantage of the chemical
affinity between iron and sulfur, sequestering
the sulfur in the iron as iron sulfide.
• Another approach is to use activated carbon,
which absorbs the gas on its high surface area
• Water scrubbers can also remove the
hydrogen sulfide, but more details on this as
they also are effective at removing carbon
dioxide.
• The iron sponge and activated carbon
eventually become saturated and will need to
be replaced adding to cost.
Iron Sponge, MV
Technologies (Pixlie
Biogas, CA)
10. Carbon Dioxide
Water Scrubber
• Hydrogen sulfide and carbon dioxide have
lower solubility in water than methane, thus
elevations in T and P can create a system
where methane stays out of solution but the
impurities go into solution. In a regenerative
step, the T and P are released, allowing for the
impurities to leave the solution and allow for
continued re-use.
• The regenerative release leads to release of
hydrogen sulfide and carbon dioxide tail gases
(methane losses).
• Heat recovery from the T and P differences can
be captured, but not nearly as much heat is
recovered as with engine/electricity systems,
potentially impacting operations.
• High electrical and processing costs an issue
Regenerative Water
Scrubber, Greenlane (Fair
Oaks IN)
11. Carbon Dioxide, Cont.
Pressure Swing Absorption
• At high pressures, the respective gas
impurities have different affinities to
chemical absorbents as compared to
methane, thus sequestering the
impurities on the absorbents while
methane passes through. Here as well,
pressure can be altered to regenerate
the absorbent.
• Again tail gases will be released during
the regenerative step, both good and
bad.
• Electrical costs can be high for this
system, dry gas is required and
hydrogen sulfide can be a corrosive
problem limiting the lifespan of the
absorbents.
PSA system at Hilarides
Dairy, Lyndsay CA (OWS)
12. Carbon Dioxide, Cont.
Membranes
• At high pressures, the respective gas
impurities have different affinities to
chemical absorbents as compared to
methane, thus sequestering the
impurities on the absorbents while
methane passes through. Here as well,
pressure can be altered to regenerate
the absorbent.
• Again tail gases will be released during
the regenerative step, both good and
bad.
• Membranes susceptible to corrosion
and colloidal solids interference.
Hydrogen sulfide in particular can be
corrosive. Cost also an issue.
Membrane System,
American Biogas Council
14. Additional Concerns
If farm-based AD projects are to transition from primarily combined heat and
power business plans to RNG business plans, the following concerns must be
considered:
• Will the historic de-coupling of diesel and natural gas prices continue, with
diesel staying considerably higher than natural gas? If yes, RNG can ride the
CNG wave.
• Will CNG continue to expand allowing for much needed development in fueling
station and car/tractor-trailer CNG/hybrid vehicles? If yes, CNG/RNG wave
will intensify.
• Can RNG compete with CNG or put another way, will the federal government
stand by the RFSII standard and the pricing and classification of biogas
Renewable Identification Numbers (RINs) needed to compete with CNG? If
yes, RNG projects will piggy-back off of CNG project development and
compete for a smaller percentage of total project development and
production.
15. Additional Concerns, Cont.
• Can cost of biogas purification (~$1.5-2 MMBTU-1) be decreased at scale so
that in addition to the cost of AD processing (~$4-7 MMBTU-1) positive
business plans can develop from bulk purchase prices offered by wholesalers
(~$8-10 MMBTU-1). If yes, without focus on niche markets and extra eco-
credits then large possible adoption.
• Can access to gas pipelines (standards, access fees, tap-in costs,
public/private partnerships on gas spurs to farms, etc.) be smooth or will they
be problematic, which is already the case historically with electrical projects. If
yes, then greater project development.
• If the earlier does not allow for extensive enough of cost/revenue difference,
then what is role/extent of niche markets and unique eco-credits? (i.e.
California, low carbon fuel standard (LCFA), stacking of credits,
municipal/state/federal green standards, etc.). The greater the state policy
drivers, the greater for expanded RNG development.
• Maximize biogas/RNG production, so co-digestion will be a driver, can one
simultaneously resolve emerging nutrient loading concerns on farms?
Unknown but developing.
16. Further Reading
• Ryckebosch, E., Drouillon, M., Vervaeren, H. 2011. Techniques for
transformation of biogas to biomethane. Biomass and Bioenergy, 35(5), 1633-
1645.
• Krich, K., Augenstein, D., Batmale, J., Benemann, J., Rutledge, B., Salour, D.
2005. Biomethane from dairy waste: a sourcebook for the production and use
of renewable natural gas in California.
17. This research was supported by funding from USDA
National Institute of Food and Agriculture, Contract #2012-
6800219814; and from the WSU Agricultural Research
Center
Acknowledgements
18. Contact Information
Craig Frear, PhD
Assistant Professor
Washington State University
PO Box 646120
Pullman WA 99164-6120
509-335-0194
cfrear@wsu.edu
