6.7 - Energy from Biomass

Learning Objectives

I. Biomass vs. Biofuels

• Biomass: Any organic matter used as fuel, including wood, charcoal, dried animal waste, or dry brush and grass meant to be burned.
• Use: Primarily used in developing nations to heat a home or cook food. It works as a subsistence fuel because it is easy to harvest and often free or very cheap.
• Electricity: Biomass can also be burned in power plants, where wood pellets, crop waste, or other plant material boil water for a steam turbine, just like coal. For example, the Drax power station in England burns wood pellets shipped from the U.S. and Canada. But bioenergy makes only a few percent of the world's electricity, because biomass holds less energy per kilogram than coal and has to be gathered and shipped in huge amounts.

• Biofuels: Liquid fuels like ethanol or biodiesel. They are made mostly from biomass such as corn, sugar cane, or palm plants that get planted, harvested, and converted into a liquid fuel. Biofuels work well as a replacement or supplement for liquid fuels like gasoline in transportation.

II. Modern Carbon vs. Fossil Carbon (Carbon Neutrality)

• Fossil carbon: Burning fossil fuels releases fossil carbon, which was stored underground and out of circulation for millions of years. That effectively adds new carbon to the atmosphere that was not previously circulating.

• Modern carbon: Burning biomass releases modern carbon, which was recently in the atmosphere as $CO_2$ and recently taken into a plant through photosynthesis.

• Carbon neutrality: Burning biomass is considered carbon neutral because the $CO_2$ released was recently captured from the atmosphere, which creates a relatively balanced cycle. It is not perfectly carbon neutral, but it does not raise atmospheric carbon levels the way burning fossil fuels does.

• But burning biomass still releases $CO_2$: Burning wood releases about as much $CO_2$ per unit of heat as burning coal. It only evens out if new plants grow back and reabsorb that carbon, which takes decades for trees. If forests are cut faster than they regrow, burning biomass adds $CO_2$ to the atmosphere.

III. Environmental and Human Health Consequences of Biomass Burning

• Air pollutants and respiratory illness: Burning biomass releases air pollutants including carbon monoxide, NOx, particulate matter, and VOCs (volatile organic compounds).
• These are all respiratory irritants and a major cause of respiratory disease globally.
• Indoor burning traps these pollutants and worsens problems like asthma, bronchitis, COPD, and eye irritation. About 2.1 billion people still cook over open fires or simple stoves burning wood, dung, crop waste, charcoal, coal, or kerosene, and household air pollution caused an estimated 2.9 million deaths in 2021 (WHO, 2025).
• NOx, VOCs, and particulate matter all contribute to smog formation.

• Ecological consequences (deforestation): Biomass burning, driven by the lack of environmental laws and the inability to afford other fuels in developing nations, leads to:
• Deforestation.
• Habitat loss from cutting down trees.
• Soil erosion, since there are fewer roots to stabilize the soil.
• Decreased ability of the forest to sequester carbon through photosynthesis.

IV. Biofuels: Ethanol and Biodiesel

A. Ethanol Production (from corn or sugarcane)

• Process: Corn and sugarcane get harvested and milled to make the sugars available (cellulose is hard to digest), then yeast is added for fermentation. Fermentation converts the sugar into ethanol, which is then separated by distillation and mixed with gasoline, as in E85 or flex fuel.

• Energy density: Ethanol is less energy dense than petroleum, so you need more E85 to travel the same distance.
• A gallon of ethanol holds only about two thirds of the energy in a gallon of gasoline (about 76,000 BTU vs. about 114,000 BTU).

• Carbon from burning ethanol: The carbon in ethanol came from $CO_2$ that the corn or sugarcane pulled out of the air as it grew, so burning ethanol doesn't add new carbon to the atmosphere the way burning gasoline does. But growing and processing the crop (tractors, fertilizer made with natural gas, and heat for fermenting and distilling) usually burns fossil fuels, so ethanol isn't carbon free over its whole life cycle.

• Low energy return on energy investment (EROI): EROI = energy you get out ÷ energy you put in. The higher the number, the better a fuel pays back the energy spent making it.
• Corn ethanol's EROI is low, often estimated at about 1.3 (studies range from about 1 to 2). It takes about 1 unit of fossil energy to make 1.3 units of ethanol energy, so the net gain is small.
• Brazilian sugarcane ethanol does much better, around 8 to 10, because sugarcane makes more sugar per acre and its leftover stalks (bagasse) are burned to power the distillery.
• For comparison, oil and gas have historically returned roughly 15 to 20 units of energy for every unit invested.

• Unsustainability risk: Ethanol is renewable because corn and sugarcane can be replanted, but it is only as sustainable as the agricultural methods used to produce it. Unsustainable practices include intensive tilling, heavy fertilizer use, and monocropping.

• Environmental consequences: Ethanol production carries all the same consequences as big agriculture from Unit 5: soil erosion, habitat loss from clearing land, greenhouse gas release from soil, machinery, and fertilizer, and heavy water consumption. Growing corn for fuel can also compete with corn needed for human or animal consumption, which can push corn prices up.

B. Biodiesel Production (from plant oils)

• Process: Biodiesel is a liquid fuel made specifically from plant oils, like palm. It involves no fermentation.

• Major drawback (palm biodiesel): An EU-commissioned study (Globiom, 2016) estimated that the emissions from clearing land for palm plantations alone are about 2.5 times the full life-cycle greenhouse gas emissions of regular diesel.
• That high emission rate comes mainly from clearing tropical forests for palm plantations, often on carbon-rich peat swamps in Indonesia and Malaysia.
• Ecological losses include $CO_2$ release (from decay and lost sequestration potential), habitat loss, soil erosion, and the loss of ecosystem services like air and water purification.

C. Algae as a More Sustainable Option

V. Practice Calculations: Ethanol