Biomass Energy

why use biomass

reduces GHG emissions

biomass feedstocks and biofuels allow energy to be stored

biomass/bioenergy economics may be attractive if fossil fuel prices increases

technical challenges

Low energy density (costly storage of large volumes; transport)

Relatively high water content (requires energy to remove)

Biomass is degradable

stakeholder challenges

potential impacts on agriculture and agro-industry, forestry, waste management

biomass as a solar energy store

in photosynthesis, plants use (sun)light to convert carbon dioxide and water into plant matter

Energy is released:

From plant matter converted to CO2 and H2O by combustion

From biofuels, on combustion or when further converted

sources of biomass

Source (plant or animal)

Purpose (crop or residue/waste)

Physical state (solid, liquid or gas)

types of biomass for bioenergy

traditional biomass - wood, peat

energy crops - wood for charcoal, sugar cane, maize, plants with oily seeds

plant residues - wood, straw, husks, bagasse, vege oil

animal residues

domestic waste - mixed solid waste, sewage

types of biomass fuels

solid fuels - wood, crop residues, charcoal

biogas - methane, syngas

liquid biofuels - bioethanol, bio-oil, biodiesel, methanol

types of liquid biofuels

Bioethanol - Fermentation → added to gasoline

Bio-oil - Pyrolysis (volatile products condensed) → burned or refined

Biodiesel - Pressed from seeds (rape, sunflower) → used directly or converted

Methanol / longer-chain fuels - Chemical conversion of bio-syngas (CO + H₂)

conversion methods

thermochemical

physiochemical

biological

thermochemical route

high temp chemical degradation of biomass

- combustion

- gasification

- pyrolysis

- torrefaction

physiochemical route

Extraction and chemical modification of oils from crops.

Oil extraction: From rapeseed, soybean, etc.

Transesterification: Converts oils → biodiesel.

biological route

Microorganisms/enzymes degrade feedstock to fuels.

Fermentation: Sugars/starch/lignocellulose → ethanol.

Anaerobic digestion: Wet biomass → biogas (CH₄ + CO₂).

Bio‑photochemical: Algae → hydrogen.

Pelletisation

converting loose biomass (like sawdust or wood chips) into dense, uniform fuel pellets.

1. Sawdust compressed through a die under high pressure + temperature.

2. Lignin (natural polymer in wood) softens and acts as a glue → pellets hold shape.

3. Pellets: 6-12 mm diameter, 10-30 mm length.

advantages and disadvantages of pelletisation

A:

higher energy density

uniform size and quality

D:

hygroscopic (absorbs water)

torrefaction

Temperature: 200-300 °C.

Biomass undergoes partial thermal decomposition.

result: dark, brittle, hydrophobic material resembling coal.

advantages of torrefaction

higher density than raw wood

stable during storage

compatible with coal-fired power plants

combustion

biomass is burnt in the presence of oxygen, producing CO2, H20 and residue

the energy released depends on the heats of formation

gassification

uses partial oxidation at a high temperature of a carbon-rich feedstock to produce a syngas which can then be burnt in gas turbines or reformed

advantages:

Flexible: electricity generation or fuel synthesis.

Higher efficiency than direct combustion at moderate scales.

Lower emissions compared to burning raw biomass/waste.

diff between combustion and gassification

combustion uses the complete oxidation of biomass to release heat directly, while gasification partially oxidizes the biomass to produce syngas which can then be used for electricity or converted into biofuels

environmental impacts of bioenergy

negative:

1. particulate and gaseous emission, ash disposal

2. resource strain: increased demand for water, noise, traffic and odour

positive:

Low carbon energy compared to fossil fuels

Waste disposal through energy use.

Potential increase in biodiversity.

agricultural impacts

Water requirements may be high

Fertilizer will be needed

Perennial energy crops may help avoid erosion and nutrient run-off