The modern motor fuel grade ethanol industry is only 18 years old. Early plants were very inefficient. Indeed, in 1980 a typical
ethanol plant all by itself consumed more energy than was contained in a gallon of ethanol. Some plants used as much as 120,000
BTUs to produce a gallon of ethanol that contained only 84,100 BTUs of energy.
In the last decade many ethanol
plants have become much more energy efficient. In 1980, for example, ethanol plants used 2.5 to 4.0 kWh of electricity per
gallon of ethanol produced. Today they use as little as 0.6 kWh. The majority of ethanol producers still purchase electricity
from outside sources, but newer facilities generate electricity from process steam within the plant.
In the late
1970s, ethanol plants did not recover waste heat. Today they do. Old energy intensive rectification and solvent extraction
systems required 12,000 BTUs per gallon of ethanol produced. Newer molecular sieves need only 500 BTUs.11
have been using molecular sieves for several years. Now smaller plants (20 million gallons per year and less) are starting
to incorporate them.
Best-existing and state-of-the-art ethanol plants can achieve energy reductions through a
combination of these technological innovations. Molecular sieves reduce distillation energy significantly; low cost cogeneration
facilities produce process steam and electricity; and semi-permeable membranes efficiently remove co-products from the process
water to reduce the energy requirements of drying.
Wet mills, which account for 63 percent of all ethanol currently
produced, extract higher value co-products than dry mills. Co-products from wet mills include corn oil, 21 percent protein
feed, 60 percent gluten meal, germ, and several grades of refined starches and corn sweeteners. In dry milling, co-products
can include corn oil and distillers dry grain with solubles (DDGS), which is used as animal feed. Carbon dioxide is a fermentation
by-product of both milling processes.
Dry mills derive the DDGS co-product from the process water after fermentation
occurs. It then requires a significant amount of energy to dry this co-product into a saleable form. Wet mills derive the
majority of the co-products before fermentation through mechanical separators, centrifuges, and screens. All told, wet mills
require 60 percent more electrical energy than dry mills on average, while requiring 10 percent less thermal energy. These
differences are related specifically to the processing of the co-products, and are illustrated in the "Average"
column in Table 3.
An integrated, relatively small-scale dry mill could avoid drying energy requirements for co-products.
Reeve Agri-Energy in Garden City, Kansas, operates a 10 million gallon per year plant that feeds wet DDGS to its cattle. This
operation uses only about 33,000 BTUs to produce a gallon of ethanol. However, a limited number of locations exist with a
sufficient number of nearby livestock to justify such an operation, and it would probably not be economical for larger dry
milling operations to adopt such practices.
A wider number of wet mills, on the other hand, may be able to achieve
the energy use levels noted in the best existing wet mill category in Table 3.
We conclude that the ethanol industry,
on average, uses 53,956 BTUs per gallon to manufacture ethanol. The best existing plants use 37,883 BTUs per gallon. Next
generation plants will require only 33,183 BTUs per gallon of ethanol produced.
3. How do we divide the energy
used among the products produced?
If we add the amount of energy currently used in growing corn on the average
farm to the amount of energy used to make ethanol in the average processing plant today, the total is 81,090 BTUs per gallon
(Table 1, Column 1). Under the best-existing practices, the amount of energy used to grow the corn and convert it into ethanol
is 57,504 BTUs per gallon. Ethanol itself contains 84,100 BTUs per gallon. Thus even without taking into account the energy
used to make co-products, ethanol is a net energy generator.
But an analysis that excludes co-product energy credits
is inappropriate. The same energy used to grow the corn and much of the energy used to process the corn into ethanol is used
to make other products as well. Consequently, we need to allocate the energy used in the cultivation and production process
over a variety of products. This can be done in several ways.
One is by taking the actual energy content of the
co-products to estimate the energy credit. For example, 21 percent protein feed has a calorie content of 16,388 BTUs per pound.
The problem with this method is that it puts a fuel value on what is a food and thus undermines the true value of the product.
Another way to assign an energy value to co-products is based on their market value. This is done by adding up the
market value, in dollars, of all the products from corn processing, including ethanol, and then allocating energy credits
based on each product's proportion of the total market value. For example, Table 4 shows the material balance and energy allocation
based on market value for a typical wet milling process. Here the various co-products account for 43 percent of the total
value derived from a bushel of corn, and thus are given an energy credit of 36,261 BTUs per gallon of ethanol.
an average efficiency corn farm and an average efficiency ethanol plant, the total energy used in growing the corn and processing
it into ethanol and other products is 81,090 BTUs. Ethanol contains 84,100 BTUs per gallon and the replacement energy value
for the other co-products is 27,579 BTUs. Thus, the total energy output is 111,679 BTUs and the net energy gain is 30,589
BTUs for an energy output-input ratio of 1.38:1.
In best-existing operations, assuming the corn is grown on the
most energy efficient farms and the ethanol is produced in the most energy efficient plants, the net energy gain would be
almost 58,000 BTUs for a net energy ratio of 2.09:1. Assuming state-of-the-art practices, the net energy ratio could be as
much as 2.51:1. Cellulosic crops, based on current data, would have a net energy ratio of 2.62:1.
There are circumstances
where ethanol production would not generate a positive energy balance. For example, one could assume corn raised by the least
energy efficient farmers, those who use continuous corn planting and irrigation, being processed by ethanol plants that do
not use cogeneration and other energy efficient processes. In this case ethanol production could have a negative energy balance
of about 0.7:1. However, a relatively small amount of ethanol is produced in this manner, possibly less than 5 percent. We
think it reasonable to look at least to columns one and two for the answer to our initial question. Based on industry averages,
far less energy is used to grow corn and make ethanol than is contained in the ethanol. Moreover, we think it is a safe assumption
that as the ethanol market expands, new facilities will tend to incorporate state-of-the-art processing technologies and techniques
so that each new plant is more energy efficient than the one before. It is less certain that farmers will continue to become
more energy efficient in their operations because of the many variables involved. Nevertheless, it does appear that growing
numbers of farmers are reducing their farm inputs and that this trend will continue.
A final word about cellulose.
If annual ethanol sales expand beyond 2 billion gallons, cellulosic crops, not starch, will probably become the feedstock
of choice. The data in the last column suggest a very large energy gain from converting cellulosic crops into ethanol. Cellulosic
crops, like fast growing tree plantations, use relatively little fertilizer and use less energy in harvesting than annual
row crops. The crop itself is burned to provide energy for the manufacture of ethanol and other co-products. A major co-product
of cellulosic crops is lignin, which currently is used only for fuel but which potentially has a high chemical value. Were
it to be processed for chemical markets, the net energy gain would be even greater.
1 The difference
between high and low heat values represents the heat contribution of the condensation of water during combustion. When ethanol
is burned, for example, it produces heat and water vapor. As the water vapor condenses it gives off additional heat. Ethanol
has a low heat value(LHV) of 76,000 BTUs/gallon, an estimate which more accurately represents the heat content of the fuel
in conventional combustion engines. Ethanol has a high heat value of 84,000 BTUs/gallon. In the United States the energy content
of fuels conventionally is expressed on a high heat value(HHV) basis. Interestingly, in Europe LHVs are used. The use of either
basis does not affect the conclusions of our analysis such as long as the same heat values are used for all inputs and outputs.
2 The estimate of the net energy gain from cellulosic crop-based ethanol is considered conservative. We believe that
as this industry develops, the same learning curve that occurred in the starch based ethanol industry will occur in the cellulosic
based ethanol industry, fostering a much more positive net energy gain for ethanol production from cellulose.
Chemical Usage: Field Crops Summary. U.S. Department of Agriculture. Economic Research Service. Washington, D.C. 1992-1994.
4 Bosch, D. J., K. O. Fuglie, and R. W. Keim, Economic and Environmental Effects of Nitrogen Testing for Fertilizer
Management, U.S. Department of Agriculture, Economic Research Service, 1994.
5 Alternative Agriculture. Committee
on the Role of Alternative Farming Methods in Modern Production Agriculture. Board on Agriculture. National Research Council.
National Academy Press. Washington, D.C. 1989.
6 Research conducted by the Department of Agricultural Economics.
University of Missouri-Columbia, Columbia, Missouri.
7 Testing indicates that one acre of corn absorbs approximately
90 lbs of nitrogen fertilizer in one growing season. All of the estimates for fertilizer usage in this report assume synthetic
fertilizer inputs. The difference between corn's nitrogen requirements and the fertilizer requirements indicated represent
the reductions possible via the alternative growing strategies mentioned specifically in the text. These include rotations
with leguminous crops, and the use of naturally occurring forms of nitrogen, such as animal waste.
8 Previous studies
have included other components in the on-farm analysis. One included the amount of solar energy used in photosynthesis. Another
included the embodied energy of farm machinery, that is, the energy used to make the machinery. We have decided not to include
energy inputs which are acquired at no cost, like sunlight. Also we have not included embodied energy because the estimates
are subject to a very high degree of uncertainty.
9 Personal conversation with Richard Thompson, November, 1992.
10 About 95 percent of the motor fuel grade ethanol in the United States is produced from 10 million gallon per year
facilities or larger. Although there are a number of facilities of smaller scale, the vast majority of those will quickly
expand production, if commercially successful.
Our conclusion is that under the vast majority of conditions, the
amount of energy contained in ethanol is significantly greater than the amount of energy used to make ethanol, even if the
raw material used is corn.
The full report, "How Much Energy Does It Take to Make a Gallon of Ethanol?"
can be ordered from Institute for Local Self-Reliance, National Office, Washington, DC office. Cost of the hard copy is $8.75
including shipping and handling.
Questions can be directed to firstname.lastname@example.org <email@example.com>.
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