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Renewable Natural Gas—Biogas
Cleaning and Upgrading 101
Nicholas Kennedy, Georgine Yorgey, Craig Frear, Dan Evans, Jim
Jen...
Outlines
1. Raw biogas and Upgrade
Standards
2. Constituents to Scrub
3. Additional Concerns
Photo: DVO
Raw Biogas and Upgrade
Standards
Photo: Jim Jensen
Raw Biogas from Dairy Manure
Composition
• Methane 54-70%
• Carbon Dioxide 27-45%
• Hydrogen Sulfide 100-3,000 ppm
• Hydro...
Upgraded Biogas for Renewable Natural Gas
Composition
• Methane > 75%
• Carbon Dioxide 3-4%
• Hydrogen Sulfide < 1 g/100 s...
Constituents to Scrub
Photo: Jim Jensen
Constituents to Scrub
Water Vapor
Moisture is typically the
first contaminant to be
removed and is
typically accomplished
...
Hydrogen Sulfide
In-Vessel Biological
• It is possible to dose in small quantities of air/oxygen
(2-6% O2) into the anaero...
Hydrogen Sulfide, Continued
Physical-Chemical
• A very common approach is called an iron
sponge. This takes advantage of t...
Carbon Dioxide
Water Scrubber
• Hydrogen sulfide and carbon dioxide have
lower solubility in water than methane, thus
elev...
Carbon Dioxide, Cont.
Pressure Swing Absorption
• At high pressures, the respective gas
impurities have different affiniti...
Carbon Dioxide, Cont.
Membranes
• At high pressures, the respective gas
impurities have different affinities to
chemical a...
Additional Concerns
Photo: Jim Jensen
Additional Concerns
If farm-based AD projects are to transition from primarily combined heat and
power business plans to R...
Additional Concerns, Cont.
• Can cost of biogas purification (~$1.5-2 MMBTU-1) be decreased at scale so
that in addition t...
Further Reading
• Ryckebosch, E., Drouillon, M., Vervaeren, H. 2011. Techniques for
transformation of biogas to biomethane...
This research was supported by funding from USDA
National Institute of Food and Agriculture, Contract #2012-
6800219814; a...
Contact Information
Craig Frear, PhD
Assistant Professor
Washington State University
PO Box 646120
Pullman WA 99164-6120
5...
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Renewable Natural Gas - Biogas Cleaning and Upgrading 101

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Full proceedings at: http://www.extension.org/72772 With depressed electrical prices for produced biogas, many projects are now moving towards business models predicated on production of renewable natural gas (RNG). In order to produce RNG, projects must first clean and upgrade raw biogas to pipeline and/or transportation fuel quality through the use of various engineering approaches. In this presentation, an overview of available and emerging biogas cleaning and upgrading technologies are discussed, highlighting positives, negatives and costs.

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Renewable Natural Gas - Biogas Cleaning and Upgrading 101

  1. 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. 2. Outlines 1. Raw biogas and Upgrade Standards 2. Constituents to Scrub 3. Additional Concerns Photo: DVO
  3. 3. Raw Biogas and Upgrade Standards Photo: Jim Jensen
  4. 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. 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
  6. 6. Constituents to Scrub Photo: Jim Jensen
  7. 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. 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. 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. 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. 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. 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
  13. 13. Additional Concerns Photo: Jim Jensen
  14. 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. 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. 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. 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. 18. Contact Information Craig Frear, PhD Assistant Professor Washington State University PO Box 646120 Pullman WA 99164-6120 509-335-0194 cfrear@wsu.edu

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