Making Sense of Emerging Biofuel Technology
Shana Lee McAlexander
What are biofuels?
Delving into the subject of biofuels with students provides opportunities to connect biological, chemical, and environmental topics. Students may be familiar with the debate over corn as a source of ethanol, but they may not be familiar with the significant advancements beyond corn kernels in the biofuel arena that have occurred in the last 10 years.
Biofuels are renewable alternatives to fossil fuel. Upon combustion, most biofuels generate less carbon monoxide, carbon dioxide, nitrogen oxides, and hydrocarbons than fossil fuels.
First generation biofuels rely on the cultivation of crops such as corn and sugar cane, substances that are also used as sources for food and feed. The economic competition (use as fuel or food), along with the associated media attention, quickly made the first generation of biofuels a hot-button issue. The next generation of biofuels came to rely on feedstocks that are not in direct competition with sources of food. The following sections discuss cellulosic ethanol, algal oil, and biomethane as developing biofuel sources.
Cellulosic ethanol with yeast
Cellulose is part of a plant cell, and sources of cellulose are readily available in plant or paper waste. It is a good polysaccharide source of carbon. The source of carbon from which the ethanol is made—in this case cellulose—is called a feedstock.
Selecting a feedstock requires special consideration of a number of factors, including the feedstock’s availability, its growing requirements (climate or region, its maturity rate, and the space needed for the crop), and the competition for it from other markets. Once scientists select an ethanol feedstock, they test various chemical and mechanical means to find the most efficient way to convert it to a source of sugar that is ideal for fermentation with yeast.
The first step in making cellulosic ethanol is to access the cellulose in plants by breaking down cell walls. This pretreatment process may include thermal (boiling or freezing), chemical (acid or base treatment), and mechanical (grinding or chopping) methods.
Ethanol is a by-product of the fermentation performed by certain yeasts as they break down sugars to fuel their cellular processes. The commonly used yeast Saccharomyces cerevisiae ferments some monosaccharides (e.g., glucose and xylose) and some disaccharides (e.g., sucrose and fructose), but it is unable to digest polysaccharides such as cellulose. To access the energy of cellulose using yeast, scientists must first break down this polysaccharide into its basic glucose units. Then they can isolate the ethanol that is produced and use it as fuel.
Algae are becoming a popular source of lipids used to generate biodiesel. Maintaining an alga culture requires temperature-regulated water, sunlight, and carbon dioxide. Growing algae for biodiesel can occur in open ponds, but for most commercial operations, the algae are grown in closed chambers called bioreactors.
Some varieties of algae contain a significant portion of lipids, or oil. For biodiesel production from algae, scientists extract the oil from the algae in a 2-step process. First, the algae are pressed (think of squeezing an orange). During pressing, up to 75% of the oil is removed. Scientists treat the leftover biomass with a solvent called hexane, which removes most of the remaining oil from the biomass. They then filter the remaining product to remove the hexane. Because of hexane’s toxicity, this procedure is not recommended in the classroom.
To produce biodiesel from the alga oil, scientists separate the components of the oil using a process called transesterification. A catalyst such as sodium hydroxide drives the reaction of oil and added methanol to produce biodiesel and glycerol. The glycerol is then removed, leaving refined biodiesel. See Figure 1 for details of the chemical reaction.
Figure 1 Transesterification. During transesterification, the 3 fatty acid chains (esters) separate from a triglyceride molecule and bind to the methyl groups from 3 methanol molecules in the presence of a catalyst, producing 3 methyl esters (the biodiesel). The hydroxyl (OH) groups from the methanol replace the esters removed from the triglyceride, producing glycerol, a by-product of biodiesel production.
The decomposition of organic waste such as manure, sewage, or other solid waste produces biomethane. While landfills and wastewater treatment centers regularly produce large amounts of biogas, it often escapes into the atmosphere. Capturing the biogas provides another biofuel source. The use of large chambers to contain the biomass, also called bioreactors or biodigesters, can increase the efficiency and volume of biogas production.
Several types of bacteria and archaea (single-celled prokaryotic microorganisms) contribute to the breakdown of organic material. There is a different type of bacteria for each step of the process. In the 1st step, called hydrolysis, bacteria convert carbohydrates, proteins, and fats into their basic units (simple sugars, amino acids, and fatty acids). In the 2nd step, fermenting bacteria break down the products of hydrolysis into even simpler molecules (organic acids, alcohols, carbon dioxide, and hydrogen gas). In the 3rd step, methanogenic archaea metabolize carbon dioxide and hydrogen to produce biogas rich in methane. The conditions for efficient biogas production depend on how well these 3 types of microorganisms can thrive in the bioreactor environment.
Biogas is rich in methane (55% to 80%) and carbon dioxide (20% to 45%). Other gases including ammonia, hydrogen sulfide, and water vapor may be present depending on the source of the organic waste and the digestion conditions. Scientists subsequently process the biogas to remove the nonmethane components.
Biomethane is chemically identical to natural gas, and it can be used in the same applications—such as generating electricity, heating water, heating space in homes and businesses, and fueling vehicles. The main difference between fossil fuel-derived methane and biomethane is that fossil fuel-derived methane results from the decomposition of buried organic matter, which is a limited resource. On the other hand, biomethane results from so-called “fresh” organic matter, which is an abundant and growing resource.
To help your students appreciate the exciting potential of biofuels, try one of Carolina’s biofuels kits (see products listed below).
*AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, these products.