The focus of this lecture, Making Biofuels from Energy Crops, is feedstock composition and biochemical conversion. I will also discuss feedstock quality, and the related process and issues in converting feedstocks into ethanol.
Here we see the major components in corn and soybean grains. The starch comprises approximately 70-75% of the corn grain. It's readily hydrolyzed, meaning it's broken down and converted into smaller molecules that can be distilled into ethanol. There's also a portion of the remains, the dry distillers grain (DDG) that can be used as animal feed. The oil content in corn is 4 - 7% and is 18-20% in soybeans; much of this oil is used in food, although we also use soybean oil for biodiesel production. And then finally we look at the protein content in these two crops, so 20-25% in corn, 80% in soybean meal. Again, these are primarily used as feed and also as feedstocks for biochemicals and other bioproducts.
We've seen portions of this slide in the past. Remember that the three main components of biomass stems are lignin, cellulose, and hemicellulose. We are interested primarily in cellulose in the conversion of stems into ethanol.
Here is a a rough breakdown of the percent of lignin, hemicellulose, and cellulose in biomass. Lignin is a significant component of woody and herbaceous plants; it gives stems strength and water and rot resistance and is a very complex molecule that binds cellulose fibers together. Separating cellulose from lignin is one of the major steps in converting feedstocks to ethanol. Lignin has high energy content, but this is not used for the production of ethanol. Hemicellulose comprises approximately 23-32% of the stems that we work with. It's major component is xylose, a 5-carbon sugar that's the second most abundant sugar in the biosphere. There are also other sugars in hemicellulose including the 6-carbon sugars, mannose and galactose. Thus, it contains a number of different 5- and 6-carbon sugars. It is readily hydrolyzed or broken down as we convert the stems into ethanol. Finally we get to cellulose, the portion of the stem that is important in the conversion of feedstock to ethanol. Cellulose comprises about 38-50% of the stems. Again this number will vary depending on the year, the plant species we're dealing with, and where it's grown. It's the most abundant form of carbon in the biosphere, and it's comprised of large chain carbon molecules that can be broken down into 6-carbon glucose molecules that can then be distilled into ethanol. Cellulose contains a great amount of energy, and the large carbon molecules resist breakdown into the 6-carbon glucose molecules that are converted into ethanol.
Here are the results of research conducted with different feedstocks that present the variability of the amounts of cellulose, hemicellulose, and lignin, as well as other materials, in biomass. There are a number of things that can affect the quality of the feedstocks or the amounts of these components in the feedstocks. Feedstock chemical makeup can vary based on the species or the cultivars of the plants being grown, the soils, and the climate in which the crops are grown. Harvest date can also cause some variability. As you recall, early harvests in the normal harvest cycle result in more biomass, but the quality is usually lower than if we harvest late. With later harvests , biomass quantity is typically reduced, but the quality is improved. Finally, the chemical makeup can also change depending on how it's stored. If it's stored dry and for a short time the feedstock will typically have higher quality than if it's stored for long periods of time or if stored in unfavorable conditions such as wet conditions.
We've seen this slide in an earlier lecture. On the left we see an image of how corn grain is converted into ethanol. On the right we see an image of biomass is converted into ethanol. Looking at corn, we start with corn grain that is ground up, heated, and liquefied. It produces a starchy mash, which is the component we're looking for. Enzymes break the starch down into fermentable sugars. Yeast is added to the sugars, which is distilled into ethanol. This process also results in dry distillers grain (DDG), which is used as animal feed. Generally about 2.77 gallons of ethanol per bushel of corn is created and about 18 pounds of dried distillers grain result, as well (a bushel of corn weighs 56 pounds or roughly 25 kilograms). There are additional steps in converting biomass into ethanol. First, the biomass needs to be chopped. It's then treated with heat and different chemicals - sometimes acid, sometimes alkaline materials - that break the cellulose free from the lignin and hemicellulose. Enzymes are then added to break the cellulose molecules into the 6-carbon sugars. Microbes such as yeast, are used to ferment the sugars into ethanol. At present, we can produce approximately 75-85 gallons of ethanol per dry ton of biomass, and we will use 80 gallons per ton as our number. It's hoped that as our technology improves, the process of converting feedstock into ethanol can produce as much as 120 gallons of ethanol per dried ton of biomass, a significant improvement over the 75-85 gallons we're able to produce at the present. We can convert corn into ethanol on a commercial scale - we're doing more than 13 billion gallons a year. Converting biomass to ethanol is a much more of a challenge (the first couple of refineries will come on line this year), and it's not as easy as the corn starch conversion. Conversion in laboratories works well using small quantities of biomass in individual batches. To be commercially acceptable, we need to not only develp a much larger scale of conversion, but we also need to do it on a continuous basis so we're not starting and stopping. The ideal process will have biomass fed in at one end and through continuous processing, ethanol will be the resulting product. That's proven to be a much greater challenge than it has making corn-based ethanol. Biomass variability is another concern that we need to address. Plant biomass can vary from crop to crop, from year to year with the same crop, and from site to site. Biologically originating enzymes and yeasts that work across a broad spectrum of biomass having different compositions is difficult. Corn grain is fairly consistent from crop to crop and it's not necessary to release cellulose from lignin when making ethanol from corn grain. Going from from batrches at lab scale to a continuous commercial scale and also the biomass variability are two issues that require attention when moving cellulosic ethanol production forward.
This slide offers another view at the process of converting cellulosic feedstock into ethanol. Starting with cellulosic feedstocks, such as energy crops or corn stover or other plant residue, it requires pretreatment in order to make the cellulose more accessible to enzymatic breakdown or hydrolysis, and solubilize the hemicellulose sugars. In the hydrolysis step, the cellulose is broken down into the component 6-carbon glucose sugars molecules. This is initially done using enzyme preparations, and then microbes, such as yeast, convert the sugar into ethanol.
Prior to processing the feedstock, it generally has to be milled or ground into small-sized particles by crushing, grinding, or by some other method of breaking the feedstock into small particles. Pretreatment includes using thermal pretreatment from hot water, steam or heat. Chemical pretreatments such as acids, alkalis, or solvents are also used. Finally, in the hydrolysis step that converts cellulose into 6-carbon molecules, an enzyme called cellulase is used to break the large carbon molecules down into the smaller, 6-carbon glucose molecules.
One of the greatest challenges in converting feedstock into ethanol is separating the lignin from the cellulose and hemicellulose. We call this cell wall recalcitrance. Recalcitrance is the natural resistance to degradation or breakdown. The lignin molecules, as you recall, form a protective coating to shield the cellulose and hemicellulose from the enzymes that we want to use to break them down. In addition lignin is a source of chemical by-products that can inhibit the conversion of sugar into ethanol. As I indicated, this is a great challenge to attaining a viable cost effective cellulosic biofuel industry.
Here are two proposed methods of improving the process of converting lignin cellulosic feedstocks into ethanol. In simultaneous scarification and fermentation, you combine the C5 and C6 sugars into fermentation in a single process. At this point they're done separately. In sequential hydrolysis and fermentation, the hydrolysis and fermentation steps are combined, thus reducing a step in the process and making it more efficient.
This summarizes the barriers to efficient and cost effective conversion of lignocellulosic feedstocks into ethanol. First, recall that the cell wall recalcitrance and separating lignin from cellulose and hemicellulose is one barrier. The fact that we're dealing with a small-scale batch system is a second barrier to this process. As we examine the feedstocks, the cost of producing feedstocks and particularly the cost of transporting and storing the feedstocks is a barrier. Determining which pre-treatment, whether chemical, such as acid or alkaline materials, or thermal using heat is a barrier. In the hydrolysis and scarification steps, enzyme costs and enzyme activities are a concern. Again, we're dealing with a variety of feedstock and the enzyme tends to be specific. And then finally in the fermentation step, converting C5 sugars from the xylose in the hemicellulose into ethanol is difficult. Overcoming these barriers of converting lignocellulosic feedstocks into ethanol in an efficient, cost-effective manner will speed commercial adoption.
