Topic 6.7 Notes – Energy from Biomass

What Biomass Energy Is

Biomass is organic material from recently living organisms that people use as fuel. The energy in biomass started as solar energy. Producers like plants captured sunlight through photosynthesis and stored it as chemical energy.

That is why biomass is often called renewable. But renewable only means it can be replaced on a human time scale.

• It is renewable only if new biomass grows back about as fast as people use it.
• Renewable does not mean sustainable.
  • A forest can regrow in theory.
  • If trees are cut faster than they regrow, the use is not sustainable.

Major biomass sources

• Wood, charcoal, and other forest products used for heating and cooking
• Crop residues such as corn stalks, rice husks, and sugarcane bagasse left after harvest
• Crops grown specifically for fuel such as corn or sugarcane
• Animal manure
• Organic portions of municipal waste
• Methane-rich biogas from landfills, sewage treatment, or anaerobic digestion
• Liquid biofuels, especially ethanol

Main uses

• Direct heat
• Electricity generation
• Transportation fuel

How Biomass Produces Energy

Some biomass is just burned directly. That gives heat for cooking, home heating, or industrial use.

Biomass power plants

A biomass power plant works a lot like a fossil fuel power plant. In the diagram, biomass is burned to heat water, make steam, and spin a turbine connected to a generator.

1. Biomass is burned in a combustion chamber
2. The heat boils water
3. The steam turns a turbine
4. The turbine drives a generator
5. Electricity is produced
6. Exhaust gases leave through a chimney or smokestack, and ash stays behind as solid waste

Biomass power plant

The energy changes are:

$$\text{chemical energy} \rightarrow \text{thermal energy} \rightarrow \text{mechanical energy} \rightarrow \text{electrical energy}$$

Biogas and ethanol

• Biogas is methane-rich gas. It can be burned directly for heat or used in engines/turbines for electricity.
• Ethanol is a liquid fuel. It is mainly used as a gasoline additive or substitute, not in steam-electric plants.

Environmental Effects of Biomass Use

Here is the main trade-off. Biomass can be low-cost, local, and storable, but the way it is burned and harvested can create major environmental problems.

Air pollutants from biomass combustion

• Carbon dioxide $CO_2$
  A greenhouse gas. Biomass is only called carbon neutral if regrowth captures the released carbon. If forests are cleared or harvest is unsustainable, that offset does not fully happen.

• Carbon monoxide $CO$
  Forms during incomplete combustion. It is dangerous because it lowers the blood’s ability to carry oxygen. This is a major indoor hazard with open fires or bad ventilation.

• Nitrogen oxides $NO_x$
  Produced during high-temperature combustion. They help cause photochemical smog, ground-level ozone, and acid deposition.

• Particulate matter
  Smoke and soot damage the respiratory and cardiovascular systems and reduce visibility. Wet fuel and inefficient burning make this worse.

• Volatile organic compounds (VOCs)
  Often released during incomplete combustion. With $NO_x$ and sunlight, they help form ozone and smog.

Land effects from harvesting biomass

If people use too much fuelwood, this causal chain can happen:

Fuelwood demand and deforestation

fuelwood demand → harvest exceeds regrowth → forest biomass declines → deforestation

Major results of deforestation:

• Habitat destruction and biodiversity loss
• Soil erosion
• More runoff and sediment transport
• Less carbon storage
• Disrupted nutrient and water cycling

Why source matters

• Waste and residue biomass usually has lower land-use impact.
• Dedicated fuel crops can compete with food production and natural habitat.
• Fuel crops may need land, water, fertilizer, pesticides, and transportation.

Ethanol and Why Its Trade-Offs Matter

Ethanol is an alcohol fuel made by fermenting plant sugars.

• Corn is the main feedstock in the United States
• Sugarcane is the main feedstock in Brazil

It can replace gasoline or be blended with it. When ethanol burns, it releases $CO_2$, but that carbon was recently taken from the atmosphere by plants.

That does not mean ethanol has no climate impact.

• It does not introduce additional carbon via combustion the way fossil fuels do
• It still does release $CO_2$
• Its overall greenhouse effect depends on the full life cycle

EROI

Energy return on energy investment compares energy out to energy in.

$$\text{EROI}=\frac{\text{usable energy obtained}}{\text{energy invested}}$$

Ethanol has a low EROI, which means the net energy gain is small because production takes a lot of energy.

Energy inputs include:

• fertilizer production
• farm machinery
• irrigation
• harvesting
• transportation
• fermentation
• distillation

How to Reduce Biomass Impacts

Better biomass systems depend on both the source and the technology.

• Sustainable harvesting
  • replanting
  • managed woodlots
  • rotation of harvest areas
  • avoiding old-growth forest removal
• Use wastes and residues instead of clearing forests or planting more fuel crops
• Improve combustion efficiency with better cookstoves and boilers
• Ventilation and chimneys reduce indoor exposure
• Pollution controls at large facilities can reduce particulate emissions
• Improve ethanol systems by cutting fossil-fuel inputs, using renewable energy in processing, and using nonfood or waste feedstocks

The best APES conclusion is simple. Biomass can help, but its environmental value depends on source, harvest rate, combustion method, and full life-cycle impacts.