L24. Biofuels
Introduction
In Lesson 23 we began our investigation into biorenewable energy resources. Our first example was using wood to produce electricity. Electricity generation plants are sometimes referred to as stationary energy sources because energy conversion is happening at a large stationary site. Finding alternative fuels for stationary sources is important, but it is also important to find alternative fuels for non-stationary energy users like cars, trucks, trains, buses, and airplanes.
Traditionally liquid fuels like gasoline and diesel are used for transportation. Petroleum-based liquid fuels make great transportation fuels because they have a high energy density (amount of energy per gallon or pound) and can be stored and pumped easily and are economical. Think about it - 10 gallons of gas can move a car a very long way and when the 10 gallons are gone, it takes only a couple of minutes to refuel and keep driving. This is not possible with any other type of fuel.
So if gasoline and diesel are such perfect fuels, why would we want to replace them with fuels from biomass? Great question!
Why Biofuels? The Environment.
The extraction, pumping, piping, refining, and use of petroleum can cause water quality and air quality issues. We all have seen or read about environmental damage caused by oil spills on land or in the ocean. There are also pollutants released when the crude oil is transformed into gasoline and diesel through the refining process. However, the main problem with petroleum as a transportation fuel is the greenhouse gas emissions produced when the fuel is combusted in the vehicles.
Currently, about 92% of our transportation energy in the United States is from petroleum in the form of gasoline and diesel fuel. This represents about 37% of our total energy use, and more importantly, about 28% of the total greenhouse gas emissions in the US. (Inventory of US GHG Emissions, EPA Links to an external site.). If we want to reduce greenhouse gas emissions we have to think about cleaner alternatives!
Unfortunately, there are not many good substitutes for transportation fuels. Electric vehicles are one option, but face the challenges of cost and limited energy storage capacity of batteries. Some advances are being made with bigger and better batteries and faster battery charging, but convenience and cost remain an issue. Other "clean" fuel options are hydrogen and natural gas, but both of these require significant changes in the vehicles or the fueling infrastructure. As such, biofuels (primarily ethanol and biodiesel) made from biorenewable resources seem to be the best available option. We have already learned about how ethanol and biodiesel are made (ethanol from fermentation and diesel production through transesterification), but now we will dig a bit deeper into the viability of biofuels as a replacement to fossil fuels.
Please take a look at this short video from studentenergy.org introducing biofuels.
Classifications of Biofuels
As you heard in the video, biofuels are grouped by categories - first generation, second generation, and third generation – based on the type of feedstock (the input material) used to produce them.
- First generation biofuels are produced from food crops. For ethanol, feedstocks include sugar cane, corn, maize, etc. For biodiesel, feedstocks are naturally occurring vegetable oils such as soybean and canola.
- Second generation biofuels are produced from cellulosic material such as wood, grasses, and inedible parts of plants. This material is more difficult to break down through fermentation and therefore requires pre-treatment before it can be processed.
- Third generation biofuels are produced using lipids from algae.
Although there is a lot of interest in second and third generation biofuels, commercial scale production is very limited due primarily to economics. In this lesson we will focus on 1st generation biofuels as those represent almost 100% of the biofuels sold today.
Ethanol
The US produces about 15 billion gallons of ethanol per year, nearly all of it from corn grain. (Only about 10 million gallons were produced from cellulose in 2017.) This is in contrast to the 137 billion gallons of gasoline consumed per year (Statisitca Links to an external site.). This means we are currently replacing about 10% of our petroleum with biofuels. Could we produce and use more ethanol? Yes. But is more ethanol a good idea? Let's take a look at some of the issues.
Food vs Fuel
Some argue that the current level of acreage used to grow corn for ethanol production impacts food prices. That is, food prices have increased because of ethanol production. This is not true in the United States. About 90 million acres of corn are grown in the US each year and about 39% of the corn grown goes to ethanol plants. These ethanol plants produce ethanol and what is left over from the process is used for animal feed. This means that ultimately, about 30% of all corn grown in the United States is used for ethanol (World of Corn Links to an external site.). But even with 30% of the corn going to fuel, the US still produces a surplus of corn every year. There is no real competition between food and fuel in the United States. In fact, corn prices are similar to what they were in 2006 despite the increase in ethanol production (Macro Trends Corn Links to an external site.). There may be issues of food vs fuel in other countries, but not in the US.
Ethanol and Engines
Ethanol is a much different chemical than gasoline. Ethanol is an alcohol and contains some oxygen. Gasoline is a mixture of hydrocarbons, having only carbon and hydrogen atoms. Indeed, they both burn but their chemical properties are quite different. This means that engines have to be designed for the use of ethanol. Most of the ethanol used today is mixed with gasoline just before it is delivered to the gas stations. You will see it listed at the gas pumps as E10 - meaning that up to 10% of the gasoline at the pump is ethanol. Automobiles built since 2001 can burn blends of up to 15% ethanol (E15). You will also see some cars called "flex fuel" vehicles. These are designed to run on up to 85% ethanol (E85). Small engines used for lawn and garden equipment are typically not designed for ethanol blends. So ethanol is not bad for engines - unless the engines are not designed for ethanol.
Ethanol also has a lower energy density than gasoline. The energy in one gallon of gasoline is about the same amount of energy in 1.5 gallons of ethanol. That means that fueling with ethanol results in less miles traveled per gallon. Because our fuel mix is E10, this difference is almost unnoticeable in our miles per gallon (about 3%), but it is there. However, this loss in energy density and miles per gallon traveled is offset by the lower cost of ethanol. The whole sale price of ethanol is about half that of gasoline.
EROI of Ethanol
Remember the discussion about energy balance with fuels? There is a significant amount of energy invested in the planting, growing, harvesting, processing, and fermentation of corn to get ethanol. The energy outputs are ethanol and the left over product that goes for animal feed called distillers grains. The figure below shows the relative amounts of energy outputs and energy inputs. The ethanol plant (biorefinery) uses about 65% of the energy and the production of the corn uses about 35% of the energy. Reports of ethanol EROI have been quite varied as the calculations are quite complex and many assumptions are made. A review of literature was done in 2011 and showed an EROI for ethanol (cradle to pump) of about 1.2 (Murphy et al 2011 Links to an external site.). This means that there is only a slight positive energy output. For every one BTU of energy input used in teh production of ethanol just 1.2 BTUs of energy are produced. This is much less than gasoline which has an EROI at the pump of 5 or 6. Gradual improvements to EROI come with more efficient corn production, powering the ethanol plant with cleaner fuels, and generally improving the technologies through the supply chain.
Figure 1. Relative energy output and input from ethanol (Murphy 2011 Links to an external site.).
Greenhouse Gas Emissions
As mentioned earlier, the problem with fossil fuels is greenhouse gases. To be a good alternative, biofuels must have fewer greenhouse gas emissions. As we just saw, there is a significant amount of energy invested in the production of biofuels. In addition to direct emissions from fossil fuel use, any ethanol LCA also considers land use changes. For instance, if forest acres were converted to acres of corn for ethanol this land use change would create additional GHG emissions attributed to ethanol.
Figure 2 shows the generally accepted values for GHG reduction from the use of ethanol. As we saw above, a 65% of the energy inputs are at the ethanol plant. The figure below shows three different energy sources to power these ethanol plants: conventional (coal and natural gas), 100% natural gas, and 100% biomass. Conventional energy inputs (mix of natural gas and coal) result in a 19% reduction of GHG emissions, but there are larger reductions if the plants use other energy sources. The figure also shows reductions in GHG emissions are much greater for ethanol from sugar cane and ethanol from cellulose. Reduction in GHG's is the main reason why there is a push for cellulosic ethanol production. Unfortunately, production of cellulosic ethanol continues to be very expensive.
Figure 2. Ethanol GHG emissions compared to gasoline.
Ethanol has other benefits. It is biodegradable and poses no environmental risk if spilled on the ground or in lakes or streams. Ethanol is also seen as helping with energy security because the US is now less reliant on petroleum from other countries. Ethanol is also good for local economies in locations where corn is grown and ethanol produced.
Ethanol from Cellulose
Many would like to see ethanol produced from cellulose because it would produce less greenhouse gas emissions than ethanol from corn (Figure 2). We also have an abundance of cellulosic feedstocks that could be used. A report called the Billion Ton Report, first completed in 2005 Links to an external site. and then updated in 2011 Links to an external site. and 2016, Links to an external site. shows that there is a potential for over one billion tons of lignocellulosic biomass to be used for energy. About 25% of this is from wood and wood waste and about 75% from agricultural residue such as corn stover. Unfortunately, the inability to find economical ways to convert cellulose to sugar has limited the use of this biomass for ethanol. For now, lignocellulosic biomass is largely used to generate electricity (as we learned in the last lesson).
Biodiesel
Biodiesel can be produced from almost any oil through transesterification. The major feedstocks (raw material) for making biodiesel in the United States in 2017 were:
- Soybean oil—52%
- Canola oil—13%
- Corn oil—13%
- Recycled feedstocks, such as used cooking oils and yellow grease—12%
- Animal fats—10%
Just like ethanol, biodiesel is not a "drop in" fuel - meaning that it is chemically different than petroleum-based diesel. These different chemical properties limit its use in cold weather and require it to be blended with petroleum diesel. As with ethanol, the mixture is noted as a percent and typically blended in the range of 2% to 20% (B2 or B20). In the US, we have 98 biodiesel plants that produced a little over 2 billion gallons in 2017, which represents only 3% of total diesel production. (We used about 45 billion gallons of petroleum diesel in 2017 (Statisitca Links to an external site.)). As you can see from Figure 3, there has been a significant increase in production in the last 15 years because of the interest in low GHG fuels and subsequent subsidies for production.
Figure 3. History of US biodiesel production. (EIA)
Environmental Concerns
There is much less controversy over the environmental impacts of the production and use of biodiesel. This is in part because of the much lower volume of biodiesel produced and fewer cropland acres involved, but also because biodiesel has more environmental benefits.
Just like ethanol, biodiesel is biodegradable so concerns about spills on land or in waters are negligible. The burning of biodiesel in engines results in generally less pollutant emissions. Most importantly, biodiesel has a better EROI than ethanol and much lower GHG emissions compared to petroleum diesel. A 2017 study found a 76% reduction in total GHG emissions using soybean oil (Chen et al 2017 Links to an external site.). As you may guess, these EROI and GHG results vary based on the type of feedstock (soybeans vs waste vegetable oil).
Engines and Biodiesel
One of the limitations of biodiesel is that it does not work well in cold weather. This means that the blends must change seasonally or additives are required. For instance, in Minnesota the regulations mandate a B20 blend in the summer (April 1-Sept 30) and B5 in the remaining winter months (Minnesota Depart Agriculture Links to an external site.). Unlike ethanol, biodiesel actually has some lubrication properties that are helpful to engines.
Regulations and Incentives
Biofuels would not likely not be used if there were not government policies and regulations in place. Here is the technical jargon just to demonstrate the history of biofuel regulations.
In 2005 the first Renewable Fuel Standard (RFS) was put in place. It required a volume of 4 billion gallons of renewable fuels to be used in the US in 2006. Over time the rules have changed with the last revision, completed in 2017 named RFS 2, sets the renewable fuel standards for each year until 2022 as shown in Figure 4.
Figure 4. RFS 2 production goals (EPA RFS Overview Links to an external site.)
The 36 billion gallons might be an easy target to meet but corn based ethanol is limited to 15 billion gallons. We have already met this requirement and this 15 billion on corn ethanol is actually a limit. We cannot meet the RFS2 target by producing more corn ethanol. This means the remaining 21 billion gallons must come from the other renewable fuel options. This goal was set to promote fuels with better GHG reductions than corn ethanol. The other three categories of renewable fuels: biodiesel, cellulosic, and advanced, all must have 50-60% reductions in GHG emissions as opposed to the 20% reduction from corn based ethanol (Figure 5). This is a good plan, but not very feasible. In 2017 the target for cellulosic ethanol was 5.55 billion gallons but only 252 million gallons were produced (about 4.6% of the target) (CRS Source).
(For those of you that want to dig deeper into this rather complex regulatory standard check out this overview by the Congressional Research Service written for policymakers. (Renewable Fuel Standard : An Overview Links to an external site. Congressional Research Service, 2018)
Figure 5. GHG Emission reductions for various biofuels. (Source EPA Links to an external site.)
Incentives and Subsidies
RFS2 does not require production of these fuels but rather mandates the use of these renewable fuels as part of the production and sale of petroleum fuels. In the past there were direct payments for buying these renewable fuels. These direct subsidies are now gone, but there are still tax credits given to those companies that are required to purchase the renewable fuels.
Microalgae to Biodiesel
We need to take one step back now. One of the renewable fuels mentioned in the regulatory discussion was Advanced and biodiesel fuels. A potential source of these fuels that has been getting a lot of attention is the production and use of microalgae. This is because microalgae can theoretically produce up to 300 times more oil per acre than conventional crops like soybeans.
Algae Growth
Like any living plant, algae use sunlight, water, carbon dioxide and nutrients to produce biomass and oxygen. Unlike most plants, algae float in the water instead of growing tall in the soil. This means that algae don't require as much lignin and cellulose for plant structure, and instead produce oils that we can harvest. Some species of algae have up to 70% of their biomass as oil!
The process is very basic:
- Get some water
- Find some algae and put them in the water
- Feed the algae the food they need (primarily nitrogen and phosphorus like any other growing plant)
- Keep the algae at the right temperature
- Algae multiply
- Harvest the algae
- Extract the oil
- Convert the oil to fuel
Of course, nothing is simple. It turns out algae need lots of sunlight, so the water they grow in can't be too deep. As a result, researchers grow algae in shallow open ponds that are about 4-12 inches deep. In order to feed and harvest the algae the water needs to circulate through the system, so production is in a serpentine raceway system with pumps to move the water and algae.
The shallow raceways systems work, but there is a problem. Just like yeast that specialize in ethanol production, researchers develop algae for the best oil production, and the high-producing algae are seeded (planted) into the system. However, with open raceways the systems can be easily contaminated with other algae which compete for sun and nutrients, sometimes outgrowing the specialized strain of algae. It is also hard to control temperatures in these systems and the water evaporates quickly.
To solve this problem, researchers have developed closed tube systems. This controls sunlight and contamination but is a much more expensive system to build and operate.
In both systems there is also the challenge of collecting the oil from the algae in an economical manner so it can be refined. Figure 5 outlines the many steps of algae production along with the challenges that must be overcome to make algae fuel economically viable.
Figure 5. Algae Fuels Production Chain (Hannon 2010 Links to an external site.)
Currently, biofuels from microalgae are in the the development phase. Research has been done in the lab but consistent production of large quantities of biofuel have been challenging technically and economically. Most of the current emphasis on microalgae production is in higher value products such as foods, medicines, or cosmetics.
(Primary references for algae section: Hannon 2010 Links to an external site., Han 2014 Links to an external site.)
Biofuel Summary
This class is focused on the use of biorenewable resources. It is clear that we can produce liquid fuels from bioresources to replace the use of petroleum. Currently, we are producing liquid fuels from corn ethanol, soybeans, other oil plants and oil waste. These biofuels are replacing about 10% of our petroleum consumption. This is good! Taking that next step to offset more petroleum use is the bigger challenge. Ethanol from wood or corn stover would be great, as would be biodiesel from algae. Unfortunately, neither are currently economically viable.