1. Prepared by: Alec Slungaard UW – Madison Chemical Engineering
Cellulosic Biofuels
Technoeconomic analysis of corn stover to transportation fuels via fast pyrolysis and enzymatic hydrolysis-fermentation
Prepared by: Alec Slungaard
UW – Madison
Chemical Engineering
Good morning! My name is Alec; I’m a senior studying chemical engineering. I want to talk to you today about cellulosic biofuels.

2. Biofuel production goal: 16 billion gallons/yr by 2022
Back in 2007, the United States government introduced a bill that mandated increasing the amount of alternative fuel sources. It was called the Energy Independence Act and it mandated the production of 16 billion gallons of cellulosic biofuels. As the chart shows current production levels are much lower than the required amount. In order to reach this goal biofuel production methods need to be optimized. Two such methods that have the capability to reach this goal are pyrolysis and enzymatic hydrolysis-fermentation. I want to stress the fact that these are gasoline-equivalent ethanol produced from cellulose-based materials and a starch based feedstock.

3. Key Points
Enzymatic hydrolysis-fermentation is the preferred conversion method
High production rates
Pyrolysis alternative – small-scale combustion sites with a centralized hydroprocessing facility
Collaboration is necessary for first cellulosic biofuel plant
Before I move forward I wanted to make it clear what my conclusions were. Out of the two conversion methods I researched, enzymatic hydrolysis-fermentation was the preferred process. This is due primarily to the fact that fermentation can produce a greater quantity of biofuel per year than pyrolysis. Now, research on pyrolysis should not stop, but it should shift its focus to smaller-scale operations. Lastly, I propose that in order to overcome the high costs associated with building a cellulosic biofuel plant for the first time, a few companies enter into a joint contract. These are the key points to keep in the back of your mind throughout the presentation.

4. Corn is composed of two separable sub-units
Corn Cob
Corn Stover
I know I showed this slide in my last presentation but I wanted to reiterate the point that my research only focused on using corn stover as the biomass. As you can see corn stover is everything but the cob, so this includes the leaves, husks, and stalks. Basically all the green parts of the corn plant.

5. No full-scale commercial plants
Before I dive into the description of each process, I wanted to one thing clear; currently, there are very few full-scale commercial facilities producing biofuels using corn stover as the exclusive feedstock. What this means is that any laboratory research that has been done is on a much smaller scale than what I’m proposing. Typically, what most researchers do is develop a model for each process and then use a computer software program called Aspen to run the mass and energy calculations. So, the data and analyses shown later on is from a computer-simulated process, not from a real plant. This is just something to keep in the back of your mind throughout this presentation.

6. Enzymatic Hydrolysis-Fermentation
Fermentation is a concept I’m sure you’re all familiar with. This is a process that is used extensively in the food and beverage industry. If you’ve ever had a beer or ate a loaf of bread, you’re consuming the products of fermentation. Basically, fermentation is process of converting sugar molecules into ethanol. However, that same basic process can be applied to corn stover to create gasoline-equivalent ethanol. This diagram that I have up illustrates this conversion process. First the feed is washed and mixed with sulfuric acid to sort of soften up the material. Then enzymes are added that work to break down those long cellulose chains into individual glucose molecules. The bugs that convert sugar to ethanol have a much easier time if the sugar molecules are broken down into individual components. Once the cellulose is hydrolyzed, the slurry is moved to the fermenters. Here, we add a specific type of bacteria which can convert those sugar molecules into ethanol. The ethanol is recovered using a distillation column and the solid waste is collected and burned to produce the electricity and steam. This is where the term enzymatic hydrolysis comes from.

7. Pyrolysis
An alternative to fermentation is using heat to convert the biomass into a liquid. One example of this is called pyrolysis – heating to high temperatures in the absence of oxygen. What’s cool about pyrolysis is that it can produce solid, liquid, and gaseous products by simply heating the biomass to high temperatures very rapidly. What we’re essentially doing is vaporizing the solid material and then cooling down the smoke that forms. Again, I put up the process flow diagram so you guys have a better idea of how this works. So, we bring in the biomass, and it goes through a little bit of a different preprocessing step; instead of adding acids and enzymes, here, all we have to do is grind up the biomass. That pulp is then fed into a reactor where the combustion takes place. We separate out the solid and gas products, and the liquid is just condensed out. Now, unlike ethanol, the liquid that is collected is very crude and requires extensive upgrading in order to be suitable for use in engines. This upgrading process is called hydroprocessing and what it does is it removes any inorganic impurities as well as lowers the oxygen content.

8. < Process Efficiency
It is 10 times cheaper to use coal or natural gas to produce steam than burn the solid waste products
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One thing to notice about each of these process designs is that they are set-up as a closed-loop system. This means that the fuel used to produce steam and electricity comes from the solid by-products. In both conversion methods, the boilerhouse is designed to burn the solid waste. While this is an attractive option based on reducing carbon emissions, it has a significant impact on the cost. A natural gas or coal fueled boilerhouse costs about ten times less than a biomass-burning boilerhouse. The take home point from this slide is that if companies are looking to reduce capital costs, the first thing to go will be the biomass-burning boilerhouse. This is something that many of my sources neglected to point out.

9. Production Analysis Table 2—Product analysis of pyrolysis and EHF
Process
Product Yield
Fuel Production
Operation time
Enzymatic Hydrolysis-fermentation (EHF)a
89.8 gal/ton
69.3 Mgal/yr
350 d/yr
Pyrolysisb
88.2 gal/ton
58.2 Mgal/yr
330 d/yr
Sources: a) Aden; b) Anex
This table shows the mass efficiency of each process. As we can see the product yields are nearly identical. More importantly though, annual fuel production rate for fermentation is nearly 20% higher than pyrolysis. This is due primarily to the time each plant spends on-line. Pyrolysis plants require greater downtime because the process is more complex. The combustion step creates these very fine particles which have a tendency to foul up equipment and makes cleaning much more difficult and time-consuming. This is mainly why we see a difference in operating times.

10. Nth plant analysis and product value
Nth plant: benefits from the experience gained through construction and start-up of the first plant
Product Value (PV): combines operating costs and product yield into a quantitative assessment
Before I go into the economic analysis section I want to define two important concepts. The first is this concept of an nth plant. This refers to the idea that these facilities should be evaluated under the assumption that they are not the first plants being built. This means that the technology has been employed elsewhere and is relatively well understood. What we’re saying is that somebody else was the guinea pig and worked out all the bugs. This is a key assumption, as you’ll see in the next slide where we compare the difference in capital costs between a pioneer plant and an nth plant. The second term I want to define is the product value. The product value is way to incorporate operating costs and product yield into a quantitative analysis. As engineers, any optimization project we work on will impact operating costs so this is a useful tool for comparing different processes. A good rule of thumb is that the lower the product value, the lower the operating costs.

11. Economic Analysis
Table 3—Total capital investment and product value for biomass-to-liquid fuel
Process Scenario
Total capital investment ($ millions)
Product value ($/GGE)
EHF
380
5.05
EHF (pioneer plant)
886
8.75
Pyrolysis
200
2.11
Pyrolysis (pioneer plant)
585
3.41
Source: Anex (2010)
This table compares the total capital investment needed to start-up a biofuel production plant. According to this table implementing a fermentation process plant would cost almost double the amount of a pyrolysis plant. This is mostly due to the extra materials involved in the fermentation process. The extra cost comes from the purchasing of enzymes, the sulfuric acid, and the evaporation equipment. This is also reflected in the product value. A good takeaway from this chart is the fact that pioneer plants cost nearly triple the amount of an nth plant. Again, this is due primarily to a lack of infrastructure related to cellulosic biofuel production.

12. Recommendation
Enzymatic hydrolysis-fermentation is better suited to achieve the goal of 16 billion gallons/yr
Pyrolysis alternative – small-scale combustion sites with a centralized hydroprocessing facility
Pioneer plants are prohibitively expensive
Collaboration could prove to be successful
I want to return to the points I made at the beginning of this presentation. My recommendation is that enzymatic hydrolysis-fermentation is better suited to achieve the goal of producing 16 billion gallons of cellulosic biofuel per year by This is evidenced by the fact that the fermentation production rate is nearly 20% greater than that of pyrolysis. Now, I’m not advocating for the elimination of pyrolysis, but I do feel that the research focus needs to shift. Instead of designing large-scale processes, a better option would be to consolidate the hydroprocessing step into one central location and have numerous smaller combustion sites. This has the potential to succeed because the energy density of liquid bio-oil is times greater than for the dry corn stover. Thus, this represents a more efficient way to transport and upgrade the biofuel. In order to achieve the stated goal of 16billion gals of biofuel per year about 230 plants would have to be built. This corresponds to a total investment cost of nearly $90 billion. Because the pioneer plant cost is almost prohibitively expensive, I propose that the first plant is started using a project charter shared between a few companies. In this scenario, the initial capital investment, operating responsibilities and fiscal damages associated with the first plant are shared between a few different companies instead of being focused on one single company. This could also prove to be beneficial for optimization because each company can devote their resources to one specific process step. Just to recap: enzymatic hydrolysis-fermentation is the better option based on annual production rates, pyrolysis research should not be eliminated but the focus needs to change, and because the first plant is going to be so expensive, it makes sense for companies to collaborate in order to share responsibilities.

13. Questions?

14. Discussion Questions
What are some other possible alternatives to pyrolysis?
Does anyone have any first-hand experience with this industry?
With the 2022 deadline approaching, does it make sense to explore other options?