We’ve already discussed biofuels at length: what they are, what they’re made of, their advantages and disadvantages. In today’s post, we tackle the various types of biofuels and what they’re good for.
There are three main categories of biofuels, classed according to the complexity of the process through which they are obtained. The categories in question are called ‘generations,’ in the case of biofuels. First generation fuels are derived directly from the feedstock crops; the further down the line you go, the more complex the production process becomes.
Image source: EU Observer
In terms of the uses of biofuels, the situation may seem simple at first glance: in theory, fuels derived from biomass can be used for the same purposes as fossil fuels. Is that really so, though, in practice? Has technology actually evolved to the point where you can power jets with biomass fuels? Read on!
The three main types of biofuels
Like fossil fuels, there’s great diversity among types of biofuels, too, and each comes with its energy content. As a class, biofuels are divided into three generations, according to how they’re produced, how ‘green’ they are, and also according to their impact on food crops.
An important observation at this point is that biofuel has the same structure across all three generations—it’s just the source that differs.
First generation biofuels
First generation biofuels are the original biofuels in use today. Their source material is the feedstock crops—which, in all the cases explored below, is food crop. This, in turn, makes them less environmentally friendly than the biofuels in subsequent generations and ultimately unsustainable for the long term.
Some of the most common sources for first generation biofuels include wheat, corn, and sugar cane. The biofuels produced from these sources are derived from the sugars, animal fats, vegetable oils, and starches contained in the crops.
Let’s take a look at some of the most frequently used crops for first generation biofuels:
Most of the ethanol produced at global level comes from corn in general and U.S. corn crops specifically. By 2012, over 40% of the corn crops produced in the United States were used for making ethanol. Since ethanol is also the alcohol found in beverages, it’s important to note not all that ethanol was used for fuel.
In 2013, of the 36 billion gallons of biofuels produced in the United States, some 15 billion of those fuels comprised grain-based ethanol (which also includes corn).
Image source: Richard Cruver
Corn-based ethanol: pros and cons
- Corn has been harvested for a long time, and there’s a solid infrastructure already active for planting it, harvesting the crops, and then processing them.
- Indirect land use costs do not apply, when it comes to corn.
- The process of turning corn starches into ethanol is relatively uncomplicated.
- Ethanol can be produced from the unusable parts of the corn plant, too (the stalk, cob, etc.).
- Corn-based biofuels can replace about 25% of the gasoline consumed in the U.S.
- Compared to other, more efficient feedstock crops, the biofuel yield of corn is a mere 350 gallons/acre on average.
- It’s expensive to fertilize corn and protect it against pests, compared to other crops. Also, this increases the risk of contaminating the soil and water in the area where the crop is cultivated.
- Using corn crops for fuel has boosted food prices and increased the rate of world hunger.
- Corn ethanol has a barely positive energy yield of around 1.2.
All in all, at the moment, corn doesn’t seem to have the potential to take center stage in the world of biofuels. The main disadvantage to using corn crops for biofuel production is the effect it would have on food crops and prices around the world.
Until recently, sugar cane was the world’s main source of ethanol, because of its prevalence in Brazil. Brazil was the world’s top alcohol fuel producer, before the United States gained supremacy in the field.
During the 1970s, Brazil was the target of an oil embargo, which caused the country’s leaders to turn to sugar cane-based ethanol to supply its fuel needs. At the moment, Brazilian cars run on at least 22% ethanol-blended gasoline, but the population can also buy 100% pure ethanol. Each year, Brazil produces an average of 5 billion gallons (18 billion liters) of ethanol.
Image source: Hawaii Bioenergy Research Group
Sugar cane biofuel advantages and disadvantages
- As is the case with corn, the infrastructure for harvesting sugarcane (as well as for planting and processing) has already been set up.
- Plantation sizes or sugarcane do not vary since there are usually no land use costs in place for sugarcane.
- Sugarcane crops yield a higher amount of fuel (650 gallons/acre).
- In the case of crops with no land use costs in place, the level of CO2 emissions for sugarcane-based ethanol is up to 90% lower than that of gasoline.
- The net energy yield of sugarcane is relatively low (even though it’s higher than that of corn).
- Sugarcane can’t be as widely cultivated as other feedstock crops since it requires very specific climate conditions.
- In many Southern and Central American countries, sugarcane is a staple food; using it for fuel would deplete those food resources.
All in all, sugarcane is also not a scalable crop for biofuel. While cultivating it for this purpose is suitable in Brazil and several other countries, it cannot be used to provide energy to the entire world.
While corn and sugarcane are particular crops to a handful of regions, soybeans are widely cultivated for food in Asia, South American, and North America. The U.S. accounts for 32% of global soybean production and the world’s second largest producer is Brazil (28%).
Pros and cons of using soybeans for fuel
- Soybeans can be grown in most places around the world.
- Crops are low-maintenance, in terms of fertilization and other needs.
- Soybean crops have the lowest yield among all feedstock crops for biomass: no more than 70 gallons/acre (almost 10 times lower than some second generation biofuel crops).
- The net energy output of soybean crops is negative (i.e. they require more energy to grow than the energy they produce).
- Soy is a major food source in all the regions where it’s currently being cultivated.
- While it’s not difficult to maintain, soy as a crop is rather prone to infestations and diseases.
At the end of the day, soybeans are possibly the poorest alternative to producing organic mass for biofuels.
Image source: University of Idaho
Interestingly enough, depending on the process through which it is derived, vegetable oil can be classed both as a first and as a second generation biofuel. When it’s ‘virgin’ oil, i.e. produced directly from vegetables, it falls under the first generation category; when it’s used cooking oil, it’s a second generation fuel (but more on that later).
Advantages and disadvantages to using vegetable oils for biofuel
- It’s easy to convert to fuel.
- Since it can be derived from a wide range of vegetables, it’s available throughout the world.
- In most cases, it can be used directly in diesel engines, without needing to be modified too much.
- Vegetable oils are major food staples around the world.
- In some cases, it does need to be modified, since using it as is can cause carbon deposits because the fuel does not burn completely. This, in turn, can damage diesel engines.
- The deforestation of old growth forests, in order to replace them with palm trees grown for oil, has caused massive biodiversity problems and also increased carbon emission levels.
Throughout time, several other food crops have been used as feedstock for biofuel. These include wheat, sugar beet, peanuts, rapeseed, and many others. However, they all stumbled onto the same issue: they’re all major food crops and using them for fuel threatened the food chain.
Several attempts were made at growing such crops outside the existing settings for agriculture. However, this came with higher carbon emissions and high maintenance needs. Finally, most minor first generation biofuels have been abandoned or are being researched for alternatives.
Meanwhile, some of the biofuels mentioned above (most notably ethanol) will continue to be used in the future. Yet they are also quite clearly losing ground while more efficient and sustainable alternatives are being developed.
Second generation biofuels
Second generation biofuels are the more developed version of first generation types, in the sense that they’re typically not derived from food crops. Hence, they pose less of a risk to the food chain.
These are substances only used as biomass after they’ve been used for their primary purpose. Perhaps the best example in this sense is waste vegetable oil (WVO), which, in contrast with virgin vegetable oil, is only turned into biofuel once it is no longer fit for human consumption.
Some second generation fuels, like Switchgrass, for instance, are burned directly from biomass. The technology used to extract the energy from them is different than in the case of first generation fuels. And several such fuels are cultivated specifically for biomass harvesting purposes.
Image source: Wix.com
How are second generation biofuels extracted?
By and large, extraction technologies are the main difference between first and second generation biofuels. For instance, feedstock harvested for lignocellulose is processed several times before it reaches the point of fermentation. Fermentation into ethanol is classed as a first generation process.
Here are some of the main procedures used in processing second generation biofuels:
- Conversion through thermochemical reactions
Several such technologies are in use today:
- This technology, which was first developed for fossil fuels, has since been adapted to suit biomass. In this process, materials based on carbon are turned into CO, H, and CO2. Some of the more frequently used feedstock are black liquor, brown liquor, and wood. While this may strike you as similar to burning (combustion), it’s different in the sense that it requires less oxygen. The end-result of this process is called synthetic gas (or syngas, in short). Syngas is turned into power, energy, and heat.
- Another technology initially designed to accommodate fossil fuels, pyrolysis requires no oxygen, but is typically carried out with the aid of halogen, or other inert gases. Wood is usually the go-to energy crop for producing bio-oil via pyrolysis. The by-products of fuels obtained as such are tar and char.
- This is essentially the same as pyrolysis; only it occurs in colder temperatures. This produces better fuels, which then go on to be used through gasification or combustion. The main purpose of this technology is to convert feedstock for biomass into forms that are easier to store and carry.
- Biochemical conversion
At the moment, researchers are looking into biological and chemical process that can help turn second generation feedstock into biofuels. As of this writing, one of the most popular such processes involves fermentation, with the use of genetically edited (or otherwise unique) bacteria. This process has been successfully employed to turn landfill gases and municipal waste into biofuels.
The main types of second generation biofuels
First off, it’s important to establish that, in order to qualify, second generation fuels cannot be fit for eating by humans. A second, albeit unspoken rule, is that such feedstock should never be cultivated on agricultural land, but on marginal land. This is land that could not be used for arable crops (i.e. food).
Derived from this unspoken rule is also the fact that such feedstock should be as low-maintenance as possible, in terms of water and fertilizer requirements. In practice, many of the feedstock tested until now has been rather high-maintenance, leading to further questions regarding their sustainability.
Waste vegetable oil
The use of WVO as biofuel is not new, given that some of the world’s first diesel engines ran on peanut oil (alongside other vegetable oils). However, in order for vegetable oil to qualify as a second generation biofuel, it needs to have been previously used—to the point where it’s no longer usable for human consumption.
Using vegetable oil that has no value as food is actually a sound environmental decision since it help lower its impact on the environment.
- Turning it into biofuel actually comes with a positive impact for the environment.
- VWO can be found around the world.
- Some diesel engines are designed to use it as such, with no prior blending or refining necessary.
- It doesn’t produce significant sulfur emissions.
- Since this is a recycled product, it doesn’t disturb the food chain.
- Moreover, it comes with no costs for land use.
- WVO can cause damage to diesel engines if it hasn’t been properly refined before use.
- It can be difficult to collect, since it’s spread around the world, in homes, restaurants, and other locations where humans prepare food.
When you draw the line, VWO is one of the best biodiesel sources in the world—especially because using it as such doesn’t require any complex processing. The only main challenges to using this excellent biofuel source is collecting it from the many points where it is located.
Grass choice depends on the location where it is cultivated. For instance, Southeast Asia uses Myscanthus, whereas Switchgrass is the most popular choice in the United States.
- They only need to be planted once, since they’re perennial.
- They grow fast and yield crops several times a year.
- They don’t need much fertilization.
- They can be used as biomass, with no need for further processing.
- Their net energy yield is an impressive 540%.
- They can actually grow on marginal land.
- While the use of direct biomass is good, grasses can’t be turned into biodiesel.
- Turning grasses into alcohol is also complicated, in terms of the processing they require.
- While they’re easy to plant, their seedlings need to be constantly protected from the much stronger species of weeds around them.
- They are largely suitable for climates with substantial humidity levels and can’t grow on arid soils.
- Crops are not dense enough within the first few years.
The United States have been using Switchgrass for biomass with some success, but the watering requirements of grasses in many areas around the globe is keeping them from becoming popular on a wider scale.
Exotic seed crops
Seed crops made substantial waves early on in the 3rd millennium when Jatropha became very well regarded amongst advocates of biodiesel. Compared to soybeans, whose yield is a paltry 15%, here was a crop with a net energy value of 40%. Best of all, it could be grown on marginal land.
Further research went on to prove that, when grown on marginal land, Jatropha isn’t anywhere near as efficient, in terms of energy yield, as previously believed. Further tests were undertaken on rapeseed, Cammelina, palm oil—and the results were similar.
Nowadays, due to the difficulties in growing such crops on farmland, their popularity has substantially dropped.
This type of fuel, which comprises all kinds of solid waste matter, can either be used as fuel or let to go to waste. It includes anything, from human waste to grass and leaf clippings, landfill gas, etc.
As an energy source, it’s comparatively less clean than solar or wind power. However, it still comes with a low level of carbon emissions and has already been put to good use in cogeneration plants. Thermal and electrical power can both be produced from solid waste.
Third generation biofuels
Third generation biofuels can, in fact, be ascribed to a single term: algal fuel. In the past, algae biofuel was part of the second generation of biofuels. However, when researchers realized that they are much more efficient and scalable than the other fuels, they also decided that algae deserve a category of their own.
So, then, why haven’t algae taken over the biofuel world? As you’ll see below, they come with several major advantages, but at least one major disadvantage. It is this weak point that needs to be overcome before algal biodiesel is declared a runaway success on a global scale.
Image source: University of Michigan
The advantages of third generation biofuels
They’re far more efficient than other feedstock. In short, algae can produce more biofuel output, with lower resources. Generally speaking, byproducts obtained from algae are easier to refine into diesel and even ingredients of gasoline. One such ingredient is butanol, which is almost as energetically dense as gasoline itself, but produces lower emissions. To boot, it doesn’t cause the kind of engine damage that biofuel ethanol does.
By and large, algae can produce ten times more gallons of biofuel than other crops. That’s 9,000 gallons of biofuels per acre, compared the typical 900 of other feedstocks. Scientists estimate that algae could one day yield 20,000 gallons of fuel per acre. The U.S. Department of Energy says that the country could fulfill all its fuel needs by only cultivating 0.42% of its land surface.
They’re very genetically diverse. Not only are there many different species of algae on Earth, but scientists have recently discovered that it’s very easy to genetically manipulate algae into obtaining better biofuels and similar byproducts.
This is especially true when it comes to butanol production—since the advent of these discoveries, several butanol-producing facilities have opened around the world. Before the genetic manipulation of algae was common knowledge, obtaining butanol was difficult.
They can be cultivated just about anywhere where it’s warm enough. Third generation biofuels can be cultivated in open ponds, closed-loop systems, and photobioreactors. None of these systems require farm land and only need a certain warm temperature to thrive. Closed loop systems and photobioreactors have even been successfully set up in deserts.
They can be grown with waste water. Not only do algae crops not require additional farm land, but they can actually be cultivated by reusing waste water from municipal systems.
They can make use of a wide range of carbon sources. Some researchers have pointed out that algae plants should be set up next to traditional power plants, as they could directly convert their carbon emissions into fuels.
The disadvantages of third generation biofuels
After all is said and done, third generation biofuels only come with one essential drawback. Though singular, it is important enough to constitute a deal breaker:
The water-based waste production of algal biofuel requires non-scalable amounts of water and fertilizers (nitrogen and phosphorus, specifically).
Why is this such a big problem? Because the greenhouse gas emission levels of producing the needed amount of fertilizer would basically offset all the advantages of using biofuels. In 2013, Exxon Mobil concluded its R&D program on algal biofuel by stating this type of fuel would not be viable for another 25 years. At the end of that program, the company had invested $600 million into it.