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Roger Arnold's picture
Director Silverthorn Institute

Roger Arnold is a former software engineer and systems architect. He studied physics, math, and chemistry at Michigan State University's Honors College. After graduation, he worked in...

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  • Jun 27, 2021
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The sources of the CO2 are interesting: production of ethanol and scrubbing of CO2 from biogas anaerobic digesters of dairy cow manure for RNG. I was also surprised by the scale of the project revenue cited in the article. The figure was $500 million of revenue per year from sequestration of 2 million tons of CO2. That would correspond to $250 per metric ton of CO2. Both the IRS 45Q tax credit rules and the California LCFS rules are complex, but I don't think they total up to anywhere close to $250 per ton of CO2. Either it's an error in reporting, or there's a lot of leveraging going on. 

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Matt Chester's picture
Matt Chester on Jun 27, 2021

biogas anaerobic digesters of dairy cow manure for RNG

I wonder what the entire environmental footprint looks like for this vs. creation of fertilizer? Or is fertilizer one of the byproducts of this process (I'm admittedly no agricultural expert!)

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Roger Arnold on Jun 27, 2021

Fertilizer is definitely a byproduct of anaerobic waste digesters. "Fertilizers" are nitrates, phosphates, and potassium (NPK). Anaerobic digestion releases carbon dioxide and methane from the waste biomass, while leaving its NPK content behind in a wet solution. Biogas as it comes from the digester is combustible, but its high CO2 content precludes selling it as natural gas. The big thing that Aemetis is doing is building a network of small pipelines to transport raw biogas from dairies to a central scrubbing facility where most of the CO2 content is removed. The scrubbed product meets specs and can be sold as RNG.

The nutrient-rich liquid from the digesters can be (and usually is?) applied locally to fields and pastures on the farms where it's produced. It's sufficient to sustain plant growth there, but I believe it's too bulky and low grade to be sold commercially.

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