Types of Biofuel

The US Energy Information Administration (EIA 2022 another web page which gets updated regularly), remarks that “The terminology for different types of biofuels used in government legislation and incentive programs and in industry branding and marketing efforts varies,” and that “definitions for these biofuels may also differ depending on the language in government legislation and programs that require or promote their use and among industry and other organizations.” This makes it hard to be definitive.


Biofuels are generally made from specially grown biomass, as implied above and burnt releasing GHG emissions which are hopefully absorbed over time by regrowth.  The currently most common biofuel involves ethanol Ethanol is a fermentation product made from plants such as corn, sugarcane, sugar beets etc. with a high sugar content. Fermentation to make ethanol also releases CO2, whether it is possible to lower this release is possibly likely, but still difficult to predict. It is added to petrol to dilute the amount of petrol being used, but as stated previously still produces emissions.

If fermentation is not used, as in ethanol production, then the plant material has to be broken down. One family of methods involves high temperatures, which of course takes energy. If this energy is provided by fossil fuels or further biofuels, then there will be added emissions.

  • Pyrolysis: biomass rapidly heated in an Oxygen free environment at 500-700 degrees Centigrade. The char then needs to be removed.
  • Gasification uses higher temperatures still >700 degrees. It produces ‘syngas’ a mixture of CO and hydrogen.
  • Hyrdothermal liquefaction for wet biomass like algae uses water at 200-350 degrees C and high pressure.

The resultant product needs purification and upgrading.

Ethanol is usually less efficient for petrol engines than petrol, it has less energy density, and in Australia the fuel is lower octane than usual petrol. Some research has suggested that cars use ethanol diluted fuel require more refuelling than those which do not, which may lead to extra fuel burning, and hence reduce the emissions reduction. As far as I can see more research is needed.

Cellulosic ethanol

This kind of ethanol is made from the cellulose and hemicelluloses which are found in plant cell walls, and the fuel tends to be made from agricultural waste, or non-edible remnants of crops. It is considerably harder to ferment the glucose in cellulose than to ferment the sugar rich seeds of corn etc. A story from 2016 states

no company is currently selling microorganisms capable of fermenting sugars contained in hemicellulose to corn ethanol refiners.  Therefore, such ‘cellulosic ethanol’ originates from the cellulose sugars in the fiber or [in] the starch which adheres to it.

Almuth Ernsting Cashing in on Cellulosic Ethanol: Subsidy Loophole Set to Rescue Corn Biofuel Profits

Cellulosic fuels are sometimes called second generation biofuels. This biomass should be able to come from more marginal land or from waste (EPA 2022). However, there is still a risk of soil depletion from the plant material not being returned to the soil, and it appears the energy consumption in making it is high.

Biodiesel [unclear]

Biodiesel tends to be made from vegetable oils, and animal fats, both new and used. Some diesel engines appear to be able to run on pure biodiesel, but in most cases the vegetable oils have too high a viscosity and the oils require heating before they can be used, so they are temperature vulnerable. The NSW department of primary industry claims: “the Australian diesel fuel standard allows up to 5% biodiesel in pump fuel. Higher concentrations of conventional biodiesel can cause issues with current infrastructure and engines.”

When I began writing this, the US Office of Energy Efficiency stated that “Currently one commercial scale facility (World Energy in Paramount, California) is producing renewable diesel from waste fats, oils, and greases.” Presumably more companies have appeared.

One of the possible techniques used is hydrocracking which uses hydrogen to break carbon to carbon bonds, but it is not clear to me what this technique is applied to, or what kind of energy and chemical processes are involved.

Biodiesel is often distinguished from Renewable diesel. The NSW government states:

Renewable diesel is produced from a wider variety of feedstocks than conventional biodiesel including non-food biomass and feedstock such as straw, cotton trash and urban waste streams. It can also use purpose-grown crops such as grass, woody biomass or algae. [Or sewage vegetable oils and animal fats] Renewable diesel is compatible with existing infrastructure and vehicles, but commercial scale production has yet to occur in Australia, though some pilot scale plants are in operation.

NSW Department of Primary Industries Biodiesel, renewable diesel and bioethanol 7 June 2022

Again we have the problem of the pollution through manufacturing and agricultural processes. It also appears that the NSW government at least is currently more interested in Hydrogen power than in biodiesel, but hydrogen production requires excess green energy to produce clear hydrogen, or working Carbon Capture and Storage to make from methane.


Wood has better have better energy density and higher EREI than most other plant materials but it is less energy dense and has higher moisture levels than fossil fuels and produces more particulate pollution. As said previously deforestation or monoculture trees tend not to be good for resilient ecologies.


Algae is essentially an experimental venture, even though it has been worked with since the oil crisis of the 1970s. Often called the third generation of biofuels. In theory algae should be wonderful. It is much quicker growing than other biomass (even when compared to burning time). It is rich in lipids and this, and growth rates, could possibly be boosted even further by genetic engineering. However, the record does not match the enthusiasm.

From 2005 to 2012, dozens of companies managed to extract hundreds of millions in cash from VCs in hopes of ultimately extracting fuel oil from algae [and failed]

 In 2015, EnAlgae, an EU-funded coalition of 19 research bodies, concluded (p2) that “it now looks highly unlikely that algae can contribute significantly to Europe’s need for sustainable energy,” although the research had helped algae be useful for “food, nutraceuticals, etc.” and help cut back fishing.

Similarly, in 2017, the International Energy Agency made the ambiguous comment that:

• The single biggest barrier to market deployment of algae remains the high cost of
cultivating and harvesting the algal biomass feedstocks, currently a factor of 10-20
too high for commodity fuel production…

• Algae-based production to produce bioenergy products like liquid or gaseous fuels
as primary products is not foreseen to be economically viable in the near to
intermediate term and the technical, cost and sustainability barriers are reviewed
• Macroalgae have significant potential as a biogas, chemicals and biofuels crop in
temperate oceanic climates in coastal areas. Their commercial exploitation also
remains limited by cost and scalability challenges

IEA 2017 State of Technology Review – Algae Bioenergy

By 2012, Shell had ended its algae biofuel research and development program, news had dried up of BP’s $10 million deal with bioscience firm Martek, and Chevron’s five-year partnership with the government-funded National Renewable Energy Laboratory had produced no significant breakthroughs. By early 2018, Chevron’s website had gone from promising that algae biofuel development was “still in the research stage” to openly admitting its work was unsuccessful.

Joseph Winters 2020 The Myth of Algae Biofuels. Harvard Political Review 26 January

Apparently Exxon are still interested in algal fuels and genetic modification as the solution.

Genetically engineered high reproduction rate algae is ecologically risky, as the chances are high, that some will escape, and if they can breed in the wild, which given the reproduction rates and lack of predators that often lead to algal blooms is likely, they could produce massive damage. Other problems include co-products, waste, nutrients, harvesting, drying and conversion technology.

In 2017 Exxon announced that:

Using advanced cell engineering technologies at Synthetic Genomics, the ExxonMobil-Synthetic Genomics research team modified an algae strain to enhance the algae’s oil content from 20 percent to more than 40 percent.

Exxon Newsroom 2017 ExxonMobil and Synthetic Genomics report breakthrough in algae biofuel research 19 June

Later they moved to outdoor testing of.

naturally occurring algae in several contained ponds in California…

ExxonMobil anticipates that 10,000 barrels of algae biofuel per day could be produced by 2025 based on research conducted to date and emerging technical capability.

Exxon Newsroom 2018 ExxonMobil and Synthetic Genomics algae biofuels program targets 10,000 barrels per day by 2025 6 March

Finally in late 2018 they declared:

algal biofuels will have about 50 percent lower life cycle greenhouse gas emissions than petroleum-derived fuel…

producing algae does not compete with sources of food, rendering the food-vs.-fuel quandary a moot point

Because algae can be produced in brackish water, including seawater, its production will not strain freshwater resources the way ethanol does.

Algae consume CO2, and on a life-cycle basis have a much lower emissions profile than corn ethanol given the energy used to make fertilizer, distill the ethanol, and to farm and transport the latter.

Algae can yield more biofuel per acre than plant-based biofuels

Exxon Newsroom 2018 Advanced biofuels and algae research: targeting the technical capability to produce 10,000 barrels per day by 2025. 17 September

There seems to be no record of progress since then. The US EPA simply remarks in 2022: “algae biofuels are not yet produced commercially”. However the U.S. Department of Energy’s (DOE’s) Office of Energy Efficiency and Renewable Energy Bioenergy Technologies Office (BETO) states it is “working to build the algae bioeconomy of the future, where fossil fuels could be replaced with a renewable, abundant, and flexible source of energy.” It is offering awards to students for advances in algal tech.