API PUBL 4312
NET ENERGY ANALYSIS OF ALCOHOL FUELS
|Publication Date:||1 November 1979|
Obtaining an adequate supply of energy is one of the most important issues facing the nation today. Of particular concern is the rising cost of petroleum imports and the high dependence of the United States upon imported oil to balance its energy budget. One of the methods proposed to alleviate this problem is the manufacture of liquid fuels from domestically grown biomass, i.e. plant material.
There has been considerable debate in both the popular and technical press regarding the merits of manufacturing liquid fuels from biomass. Much of this controversy has centered on the manufacture of ethanol (or ethyl alcohol) from grain. It is hoped that the results of this study will help resolve some of the controversy surrounding the question of the feasibility of manufacturing liquid fuels from biomass.
The objective of this study is to determine the net fuel consumption for the production of liquid fuels from biomass. Consideration of the relative combustion efficiencies of gasoline vs. alcohols, beyond the scope of this report, has been addressed elsewhere.*
Five different technologies were considered in this study. Three of these technologies manufacture ethanol, while two have methanol (and fuel gas) as their primary product. The three ethanol processes considered consume sugar cane, corn, and corn stover as raw materials.
The manufacture of ethanol from com was selected for study because this process is frequently proposed and because its merits are most frequently debated in the press. The technology is well established and is commercially practiced. The manufacture of ethanol from com is similar to its manufacture from other grains (i.e., wheat, barley, and milo). Com is the least expensive of the grains suitable for ethanol manufacture in the United States.
Sugar cane was selected as an ethanol feedstock because (1) a commercially established technology for its conversion to ethanol exists and (2) it can be grown in the United States. The process energy balance differs significantly from the grain-based process. Although sugar cane can be grown in only a limited part of the United States, the technology and energy balance for making ethanol from sweet sorghum would be similar. Sweet sorghum could be grown in much of the U.S. agricultural area.
Corn stover is the residue consisting of leaves, husks, stalks, and cobs which are left behind in conventional com harvesting. At present, there is no commercially practiced manufacture of ethanol from corn stover. Rather, this technology is still being developed under sponsorship of the U.S. Department of Energy. The research is still in the early stages, and the energy analysis was made on a conceptual design based on research findings to date. On the other hand, energy analyses from corn and sugar cane are based on proven technology.
Manioc or cassava, which is used in Brazil for manufacture of ethanol. is not grown in the United States and was not considered in this study.
Two process schemes were investigated for the manufacture of methanol from wood. One scheme uses the Purox gasification system developed by Union Carbide to convert the wood to a synthesis gas. This is followed by a conventional gas clean-up (Benfield hot carbonate system), water gas shift reaction, and finally by the ICI low-pressure methanol process. The second process scheme substitutes a Battelle fluidized-bed gasification process for Purox. The Purox process has been demonstrated on a pilot scale in the gasification of municipal solid waste. A plant to manufacture methanol was designed by Seattle, Washington, but was never built. Municipal waste is chemically similar to wood. The Battelle process has been demonstrated on a bench scale for the gasification of wood. Benfield, shift, and the ICI methanol process are commercially practiced in numerous locations.
Purox was selected as the most advanced wood gasification technology, while the Battelle process was selected as representative of the several improved technologies under development.
* American Petroleum Institute, Alcohols-A Technical Assessment of Their Application As Fuels. API Publication 4261 (July, 1976).