Why Consider Additional Technologies with Anaerobic Digestion?
Some dairy farms have experimented with “add-on” technologies to enhance the value of the products generated from anaerobic digesters to improve economics and address other environmental and management concerns. This effort has intensified in recent years, as prices paid for electricity continue to fall. This trend is making it more difficult to justify the installation of new digesters or maintain active anaerobic digestion (AD) projects based on electricity sales alone.
What did we do?
Based on ten years of research and extension within the field of dairy digesters, we are proposing that the concept of a dairy manure biorefinery can be useful to focus ongoing research and commercialization efforts (Figure 1). A biorefinery integrates a core biomass conversion process (in this case, AD, converting manure and in many cases other organic substrates) with additional downstream technologies. These combined technologies generate multiple value-added products including fuels, electricity, chemicals, and other products (NREL, 2009). Most add-on technologies relevant to dairy facilities have been modified from technologies used in the wastewater treatment and oil and gas industries.
What have we learned?
Ongoing research and commercialization efforts by our team and others aim to:
- Adapt technologies to fit the economic and other constraints of dairy digesters.
- Increase efficiency and reduce costs by maximizing the complimentary nature of technologies (e.g. waste heat from one process is used in another process).
Specific add-on technologies that are continuing to evolve within the biorefinery context include:
Biogas Upgrading to remove impurities from biogas (primarily carbon dioxide, hydrogen sulfide, and water vapor).
Output: Purified biogas that can be used as a transportation fuel (e.g. liquefied natural gas) or injected directly into natural gas piplelines.
Additional social and economic benefits: Renewable fuel can reduce demand for fossil fuels, and can often receive economic credits (e.g. renewable identification numbers, low carbon fuel standard)
Fiber Upgrading to process the fiber that is removed from AD effluent.
Output: Upgraded fiber can be sold as a higher-value soil amendment in the horticultural industry
Additional social and economic benefits: Fiber can replace use of non-renewable resource (peat moss) by horticultural industry
Nutrient Recovery to strip nitrogen (N) and phosphorus (P) from anaerobic digester effluent.
Outputs: Soil amendment products that can be sold offsite where nutrients are needed
Additional social and economic benefits: Reductions in N and P applied to nearby fields, and reduced effluent hauling distances/costs for land application due to lower nutrient concentration in effluent
Water Recovery to generate “recycled” water using advanced technologies
Output: Water that can be used for animal drinking, or as dilution water for the AD facility
Additional social and economic benefits: Reduces consumption of fresh water, a limited resource, and reduces costs for land-application of AD effluent
Overall Potential Impact. Improving economics and addressing other critical issues for dairy producers (e.g. nutrient issues) has the potential to advance farm-based AD adoption significantly beyond its current 244 farms. It has been estimated that a mature bio-refinery industry based on AD on large U.S. dairy farms could create an estimated bio-economy of nearly $3 billion that complements the production of milk and dairy products (ICUSD, 2013).
Georgine Yorgey (presenting author)a, Craig Frearb, Nick Kennedya, Chad Krugera, Jingwei Mab, and Tara Zimmermana
a Center for Sustaining Agriculture and Natural Resources, Washington State University
b Department of Biological Systems Engineering, Washington State University
An extension document describing this concept and the add-on technologies in additional detail is being prepared. This document is part of a series of extension documents on Dairy AD Systems, being prepared by the authors and other colleagues at Washington State University. In addition, ongoing work and collaborations by our team are seeking to investigate, evaluate, and improve individual technologies and the linkages amongst them.
ICUSD, 2013. National market value for anaerobic digestion products. Report to Innovation Center for US Dairy, August 2013.
This research was supported by USDA National Institute of Food and Agriculture, contract #2012-6800219814; and Biomass Research Funds from the Washington State University Agricultural Research Center.
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