This course is designed to introduce students to the issues of energy in the 21st century – including food and fuels – which are inseparably linked – and will discuss energy production and utilization from the biology, engineering, economics, climate science, and social science perspectives. This course will cover the current production and utilization of energy, as well as the consequences of this use, examining finite fossil energy reserves, how food and energy are linked, impacts on the environment and climate, and the social and economic impacts of our present energy and food production and use. After the introductory lectures, we will examine the emerging field of sustainable energy, fuel and food production, emphasizing the importance of developing energy efficient and sustainable methods of production, and how these new technologies can contribute to replacing the diminishing supplies of fossil fuels, and reduce the consequences of carbon dioxide release into the environment. This course will also cover the importance of creating a sustainable energy future for all societies including those of the developing world. Lectures will be prepared and delivered by leading UC San Diego and Scripps Institution of Oceanography faculty and industry professionals across these areas of expertise.
Corn Ethanol Part 1
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Из курса от партнера University of California San Diego
Our Energy Future
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Plant Biofuel
Discusses various plant-based fuels including corn ethanol, cellulosic ethanol, and jatropha biodiesel. It will cover the different technologies used, the pros and cons of each source, and current production around the world.
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Dr. Stephen MayfieldProfessor of Molecular Biology
University of California San Diego
[MUSIC]
Hello, my name is Laurie Smith and I'm a professor
in the Section of Cell and Developmental Biology, UC San Diego.
And I'm here today to talk to you about corn ethanol.
The most widely used bio fuel in the United States today.
In 2013.
So, I'm going to be focusing today on the questions that are outlined here.
Why and how is ethanol used as a fuel?
Why and how is ethanol made, predominantly from corn, in the United States?
And how does corn ethanol compare to other
types of fuel in terms of greenhouse gas emissions?
So, as I'm sure most of you will
already know, ethanol is a clear, colorless liquid.
It's a simple organic molecule whose formula is
shown here and whose structure is, is illustrated here.
And here, ethanol is made by microbes when they metabolize
sugar in the absence of oxygen through a process called fermentation.
And this process has been exploited by humans for hundreds
of years by humans to brew a variety of tasty beverages.
And is also used now in a very large scale in
the United States to produce gasoline for use as a transportation fuel.
So, first I'd like to talk about why ethanol is used.
It's used as a gasoline additive and it's
added to gasoline to raise its octane levels.
Okay, so you can see that the octane rating
of pure ethanol here is considerably higher than pure gasoline.
Raising the octane rating of gasoline is important because it allows the gasoline
to be burned more efficiently, reducing
emissions, such as carbon dioxide and soot.
So raising, octane rating is important for pollution control, and, ethanol now
replaces other additives that have been used for this in the past.
Such as MTBE.
So, in principle, adding a biofuel to gasoline will also
reduce the carbon dioxide emissions
associated with transportation fuel consumption.
As I'll talk more about a little bit later, in
the case of corn ethanol, that reduction is actually very modest.
however, I'm sure it was at least with with that thought in mind
among others, that the Federal Government
created renewable fuel standards in 2005, that
introduced mandates as well as tax
incentives that encouraged the expansion of ethanol,
use of ethanol as a gasoline additive for, for in, increasing the octane rating.
So, another question would be okay, if we accept that we're going to
use ethanol as a gasoline additive, why are we producing it from corn?
In principle there are many other plants, so called feed-stocks,
that could be used as a starting material for ethanol production.
Well, there's a couple of answers to that question.
One is that we already grow a lot of corn in the United States.
It's by far our largest crop in terms of tonnage of grains per year.
for, for quite some time we're producing, been
producing more than 300 million metric tons per year.
Corn production is concentrated in the upper Midwestern United States.
Demand for corn has not always been as strong as farmers would like, and the U.S.
government has always looked for ways to support
farm incomes, including the incomes of corn farmers.
So, creating federal policies that created incent, incentives to
expand ethanol production made from corn seemed like a good
way to, to help farmers out while also achieving
other valuable objectives such as using bio-fuels for transportation use.
Another important answer to the question of why we're using corn is that
the technology already existed in 2005 to produce ethanol from corn seeds on a
large scale at a cost that's comparable to gasoline, so it was feasible
to produce a lot of ethanol from corn seed and we had the corn.
Okay.
So these federal incentives, the renewable fuel standards were
very effective in increasing ethanol production in the United States.
You can see that ethanol production is shown here by
these purple bars, did start to increase even before 2005.
But after 2005, when these standards came into
play, the rate of ethanol production dramatically increased,
bringing us up to where we are now
in 2013, at about 13 billion gallons per year.
Gasoline is now added, excuse me, ethanol is
now added to gasoline everywhere in the United States
to a final concentration of either 5% or
10%, with a national average in 2013 of 7%.
You can also see from this graph though, that ethanol production
from corn is expected to level off in the near future.
Close to the point where we're at now.
So, why is that?
Well the answer is that the, the
Federal Renewable Fuel Standards were changed in 2012.
Changed in a way that basically caps the total amount
of ethanol that can be used as a gasoline additive.
Close to where we are now.
And also eliminates the tax incentives that encourage this growth.
So, why, if ethanol is such a great thing
as a fuel additive, why are we leveling this off?
Well basically, it's because ethanol now consumes
a very large proportion of our corn crop.
So in 2009, when this pie chart was, was put together,
ethanol production from corn was about 34% of the total corn crop.
Traditionally, the major consumers of our corn crop have
been livestock, and they still are the largest single consumer.
But you can see that this is now a much smaller proportion of the total.
This number has grown further, so by 2012, we're up to about 40%.
A very large proportion of our corn crop being used to make ethanol.
So, the thinking is that this is one of
the factors that's contributing to the increase in corn prices
that we've seen since 2005, which then puts pressure on
consumers who are buying products that have corn in them.
And especially putting a lot of pressure on livestock farmers who
now have to pay more for the, for their animal feed.
So, the thinking is, let's stop where we are now and not increase this any further.
So, the new re, renewable fuel standards still
have incentives to encourage the development of ethanol production
from other sources as well as other bio-fuels, but
leveling off the ethanol production where it is now.
So, how do we make ethanol from corn?
Well, corn kernels are packed full of starch, which is
a carbohydrate that can be easily broken down to, to glucose.
So, here is a diagram showing starch.
It's just a chain of glucose subunits, and on an industrial
scale you can break that down into glucose in one of
two ways, either through acid hydrolysis, or by adding an enzyme
called amylase, which just clips off these glucose units one by one.
So, to make ethanol from corn, the first step is to mash up the corn kernels.
And mix them amylase in these giant fermentors,
which are also have yeast added to them
and the yeast can then metabolize the, the
glucose that's released, to produce ethanol as shown here.
So, the, the glucose is going to metabolize
this, the yeast are going to metabolize the glucose in
the absence of oxygen producing ATP, which is a
form of energy that the yeast cells can use.
Breaking it down into these smaller or molecules and releasing,
in the process, two waste products, carbon dioxide and ethanol.
So, the yeast are not producing the ethanol because it's useful to them.
It's essentially a by-product of this metabolic
pathway that's actually toxic to the yeast.
So, this process can only go so far before the ethanol
builds up to a concentration that would kill off the yeast.
So, in practice, fermentation can only produce so much ethanol.
And, and in, in industrial ethanol production here we end up
with a sort of a beer-like liquid containing about 10% ethanol.
So, that's still mostly water, at that point.
And a lot more work is needed here to get this up
to a level of purity that makes it useful as a gasoline additive.
Basically, the next step is accomplished by a fermentation, okay?
So basically, you're taking this fermentation
slurry with a 10% or so ethanol.
Heat, and heating it up, taking advantage of the fact
that the boiling point for ethanol is lower than water.
So, you heat this up to the point
where the ethanol vaporizes but the water doesn't.
The ethanol vaporizes, and is collected here in this
tube, that's then cooled down so the ethanol returns
to a liquid state and is collected in a
separate compartment here, leaving most of the water behind.
Okay?
So that gets the ethanol up to 92
to 95% purity, and it's purified further by passing
it over these molecular sieves that mostly absorb
water, bringing this up to something like 99% purity.
So, an important thing to understand about, ethanol production from
corn then, is that this process requires a lot of energy.
This is the most energy intensive part of
corn ethanol production, is heating up this fermentation slurry
of enormous volume of liquid that's mostly water,
takes a lot of energy to heat that up.
So, it's because of the amount of energy that's
needed to produce corn ethanol that what I said earlier
is true, that the reduction in carbon dioxide emissions
resulting form using ethanol as a fuel, compared to gasoline.
There's not that much reduction.
This is a study that was done many years ago, trying to analyze this.
First, I just want to point out that when
you consider the carbon dioxide, greenhouse gas emissions from
gasoline, what you see is that most of that
CO2 emission comes from combustion of the fuel itself.
Okay.
Now, in comparison, when, ethanol was used as a fuel source,
we don't have any net CO2 release as a result of
combustion because all the CO2 that's released when the ethanol was
burned, comes from the carbon that that plant fixed through photosynthesis.
So, so there's not net release of CO2 associated with combustion in itself.
However, a lot of other fuel has to be, burned in order to make that ethanol.
First of all, a lot of fuel is used just to grow the corn.
So, corn production in this country is a very energy-intensive process.
It uses mechanized equipment that has to burn fuel to prepare
the soil, plant the seeds, harvest the corn, and so on.
In addition there's fertilizers that are used on
corn require a lot of energy to produce.
So, so we use quite a bit of fuel to produce the corn, and then,
as I said before, we use a lot more fuel to heat up the fermentation slurry.
If coal was used as a fuel source for that, for that heating, we use more.
And if natural gas is used, then we use less.
Okay, so that's what this refers to
here, is energy consumption at the ethanol plant.
Now, this is analysis that was done many years ago
that made the assumption in, in coming up with these
numbers that what's left over at the end of the
fermentation process is thrown away and not used for anything.
And that's actually not true.
So, the, the, the, what remains here in the fermentation slurry after
the, after, after distillation is actually dried down and used as animal feed.
So, if you take that into account, the picture here does look better than this.
But it's, remains true that the deduction in greenhouse gas
emissions that we get when we burn ethanol as a
fuel the reduction is very small compared to gasoline because
of how much energy is used to produce corn ethanol.
So, if natural gas is used, as it mostly is now in the United
States, we get something like a 30%
reduction in CO2 emission, compared to gasoline.
So, looking at this another way, you can see
that, the amount of energy that comes out of
that ethanol when it's consumed is only about 50%
more than what was put in to generate that ethanol.
And what we can expect with other types of
ethanol production, such as production of ethanol from cellulose.
If this can be achieved on an industrial scale, at a
reasonable cost, we can expect to see a much greater yield here.
Something more like five times as much energy coming out.
As was put in to generate that ethanol.
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