|Fig. 1: World energy Usage where all available manure is burned.|
While it may be the product of very humble origins, some news sources have proposed that the use of manure for fuel may help to resolve the world's energy crisis.  Using manure as an energy source is nothing new, as dried manure has been burned as long as man has had agriculture (and possibly before), but it has never been utilized on so large of a scale before, which will present a variety of unique issues.
For the entire course of human history, manure has been primarily used as fertilizer for crops. This is still very true today, and while it is difficult to pinpoint an exact percentage of the manure that will be utilized for agricultural purposes, it is a safe estimate that a large majority of the 3 × 1012 kg dry weight of manure around the world is used in some agricultural capacity. [2,3] If a large chunk of that manure was taken away for energy production, then something would need to fill the fertilizer gap or the resulting famine could decimate populations in broad regions across the world, particularly in less developed countries. 
With an energy density of 2.0 x 107 J/kg, manure could account for 6 x 1019 J, which is over a tenth of the world's annual energy budget of 4.99 x 1020 J.
Looking purely at the theoretical number of joules that could be produced, manure would seem to be an ideal energy source due to its cheapness and abundance. The problem is that all of manure used for energy would require the use of additional artificial fertilizers, which to say nothing of cost, would be very energy intensive to produce. Most artificial fertilizer is produced using the Haber-Bosch process, which reduces nitrogen gas to useful ammonia. The Haber-Bosch process currently produces 1.0 x 1010 kg of nitrogenous fertilizer every year, and accounts for about 1% of the world's total energy consumption.  While manure generally ranges from 25-60% nitrogen, artificial fertilizers are more commonly in the 10% range.  That means that for every kilogram of manure used as fuel, anywhere between 3 and 6 kg of artificial fertilizer would be needed to replace it.
If we assume that all the world's manure could somehow be used as fuel, we would produce:
This would, of course, necessitate the production of an enormous amount of artificial fertilizer to make up for the manure, and even assuming that the nitrogen content of manure is only 25%:
This would require a 750 fold increase in the world's ammonia production! As the Haber-Bosch process already account for approximately 1% of our total energy usage, this would increase the world's power consumption over 7 times.
The graph of energy usage by type would look something like Fig. 1. The red slice for all intents and purposes is equal to our current energy demand of 4.99 x 1020 J, and the blue slice represents 3.78 x 1021 J!
Given the fact that the net loss of energy would be greater than the world's current energy supply, it is laughable to consider burning manure a viable energy alternative. While excess manure may be capable of powering some small scale operations in regions with excess manure, as the scale grows larger the project would grow work more and more poorly until it would ultimately serve as an energy sink, not an energy source.
© 2009 Sam Birer. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. MacDonald et al., "Manure Use for Fertilizer and for Energy: A Report to Congress," U.S. Department of Agriculture, Administrative Publication No. AP-037, June 2009).
 J. Sheffield, "Farm Animal Manure is an Important Sustainable Renewable Energy Source," Joint Institute for Energy and Environment, University of Tennessee, Research Paper 99-01, 1 May 99.
 A.G. Chalmers, "A Review of Fertilizer, Lime and Organic Manure Use on Farm Crops from 1983 to 1987," Soil Use and Management 17, 254 (2001).
 J. Twidell and A.D. Weir, Renewable Energy Resources, 2nd Ed. (Taylor & Francis, 2006).
 A. D. M. Glass. "Nitrogen Use Efficiency of Crop Plants: Physiological Constraints upon Nitrogen Absoption," Crit. Rev. Plant Sci. 22, 5 (2003).
 C. R. Frink, P.E. Waggoner and J. E. Ausubel, "Nitrogen Fertilizer: Retrospect and Prospect," Proc. Natl. Acad. Sci. 96, 4 (1999).