1. 2011 http:// www1.eere.energy.gov/biomass/pdfs/billion_ton_ update.pdf
2. 2009 http://www.bts.gov/publications/ national_transportation_statistics/html/table_04_05.html
3. 2009 http:// www.bts.gov/publications/national_transportation_statistics/html/table_04_09.html
4. http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1290
5. A review of short residence time cracking processes;Hulet;International Journal of Chemical Reactor Engineering,2005