The Ergosphere
Friday, June 03, 2005
 

Fertilize this!

Running around the blogosphere, you'll find all kinds of people dancing to apocalypso music by various artists.  One favorite song among the fans of the Energy Depletion Band is that agriculture is bound to collapse when supplies of fuel used for production of nitrogen fertilizer become tight.  Here's a typical example (including typos):
Modern food production is depends on oil. There is no way to get fertilizer from solar, wind or nuclear.
I'm no agro-scientist, but I know that this claim is both factually and historically false.

History:  Nitrogen depletion in soil was historically dealt with using legume crops or green manures in crop rotation.  Just where do you think a bean plant gets the energy to fix its nitrogen?  It comes from the sun, turned into carbohydrates which go to symbiotic bacteria which perform the actual work of fixation.  (External nitrogen supplies from guano deposits and then Haber-process ammonia later supplanted green manures for most farming.)

Legume crops don't fix nitrogen fast enough for modern intensive techniques, but it proves that it is indeed possible to make nitrogen fertilizer with solar energy.  The big question is, how hard is it?

Suppose for a moment that we keep using the Haber process, but decide to power it with solar inputs.  We get the hydrogen from green algae trick operating reliably at 1% efficiency.  An area getting as much sun as mid-Kansas would receive about 1550 kWh/m2/year, so a square meter of this algal hydrogen factory would yield 15.5 kWh worth of hydrogen; at 70600 cal/mole, that's about 380 grams of hydrogen per square meter per year.  It takes 3 grams of hydrogen to fix 14 grams of nitrogen, so each square meter of algae farm could make the hydrogen to fix roughly 1.75 kg of nitrogen, or 17.5 tons nitrogen/ha/year.

US farmers use roughly 12 million tons of nitrogen fertilizer per year over 140 million ha of cropland, or about 86 kg/ha/year.  At that application rate, a hectare of algae farm could produce the nitrate to fertilize about 20 hectares of crops; in other words, a 1% efficient sunlight-to-hydrogen process can make all agricultural nitrate from the sun with about a 5% land-use penalty.  The penalty goes down to 1% if the efficiency goes up to 5%, and 0.5% if it hits the 10% target that researchers believe is possible.

If 10% efficiency can be achieved, the hydrogen production goes up to 38 tons/ha/year (1.55 MWh/ha/yr) and it can become the basis of a general energy business.  If crop wastes such as corn stover and wheat/rice straw are used as carbon inputs and have a general chemical formula of (CH2O)n, addition of H2 is all that is necessary to produce methanol (CH3OH).  If the process can use the inputs with 100% conversion efficiency, 2 grams of hydrogen plus 30 grams of carbohydrate yields 32 grams methanol; 38 tons of hydrogen becomes 608 tons of methanol (about 203,000 gallons, holding the energy equivalent of 122,000 gallons of gasoline).  At this level of production, inputs of crop waste are probably the limiting factor; long before this level was reached, the fuel production would satisfy all needs for cultivation.

Conclusion:  it is not only possible to generate all required nitrogen fertilizer from solar energy using known processes or slight improvements, at the limit they could lead to large-scale production of biofuels from crop wastes.  All it requires is hydrogen.

UPDATE:  Advances sometimes come too fast to comprehend.  Researchers at UW-Madison have a process to convert plant carbohydrates to alkanes.  This doesn't address the fertilizer issue directly, but turning grain farmers into net producers of motor fuel eliminates that mode of system failure.

Related items:  Zinc:  Miracle metal? 
Comments:
Good stuff, EP. Keep it up.
 
why not just use the algae as fertilizer, instead of making hydrogen from it. This would require less land.
 
even fish poop will do. no extraction needed - just use water from fish-pound
 
Post a Comment



<< Home
Talk largely about energy and work, but also politics and other random thoughts


Mail Engineer-Poet

(If you're mailing a question, is it already in the FAQ?)

Important links

The FAQ
Glossary
The Reference Library

Blogchild of

Armed and Dangerous

Blogparent of

R-Squared




The best prospect for our energy future:
Flibe Energy

ARCHIVES
January 1990 / February 2004 / March 2004 / June 2004 / July 2004 / August 2004 / September 2004 / October 2004 / November 2004 / December 2004 / January 2005 / February 2005 / March 2005 / April 2005 / May 2005 / June 2005 / July 2005 / August 2005 / September 2005 / October 2005 / November 2005 / December 2005 / January 2006 / February 2006 / March 2006 / April 2006 / May 2006 / June 2006 / July 2006 / August 2006 / September 2006 / October 2006 / November 2006 / December 2006 / January 2007 / February 2007 / March 2007 / April 2007 / December 2007 / January 2008 / May 2008 / June 2008 / August 2008 / October 2008 / November 2008 / December 2008 / February 2009 / March 2009 / April 2009 / May 2009 / June 2009 / July 2009 / August 2009 / September 2009 / October 2009 / November 2009 / December 2009 / January 2010 / April 2010 / May 2010 / June 2010 / July 2010 / August 2010 / September 2010 / October 2010 / November 2010 / December 2010 / January 2011 / February 2011 / March 2011 / April 2011 / May 2011 / July 2011 / August 2011 / September 2011 / October 2011 / April 2013 / November 2013 / December 2013 / January 2014 / February 2014 / March 2014 / April 2014 / July 2014 / August 2014 / September 2014 / October 2014 / November 2014 / February 2015 / April 2015 / October 2015 / March 2016 / April 2016 / May 2016 / June 2016 / July 2016 / November 2016 / December 2016 / February 2017 / May 2017 / June 2017 / September 2017 / October 2017 / November 2017 / March 2018 / May 2018 / June 2018 / October 2018 / December 2018 / January 2019 / March 2019 / June 2019 / October 2019 / November 2019 / March 2020 / June 2020 / December 2020 / March 2021 / April 2021 / May 2021 / July 2021 / January 2022 / February 2022 /


Powered by Blogger

RSS feed

Visits since 2006/05/11: