SPI Physics · Energy Sources
Biomass and Biogas
Biomass is fuel from the bodies and waste of plants and animals, and biogas is the gas that bacteria make from it when they break it down without air.
Why this matters
Three questions in the bank, all EASY: how biogas forms, what a biogas plant needs, and which fuel is not biomass. Each turns on one word, so learn why air is kept out and why coal and nuclear fuel are not biomass.
Concept 1 of 2: What counts as biomass, and how biogas forms
Wood, cow dung and crop waste all come from living things that stored the Sun's energy as they grew. Burning them, or letting bacteria turn them into gas, gets that energy back. That is biomass energy.
Definition
- Biomass is the material of plants and animals used as fuel: wood, cow dung cakes, crop waste and the gobar gas (biogas) made from them.
- Coal and petroleum are fossil fuels. They formed from plants and animals buried for millions of years, so they are not counted as biomass.
- Nuclear fuel (uranium) has nothing to do with living matter, so it is never biomass.
- Biogas forms when bacteria break down dung and plant waste without oxygen. This breaking down by microbes is called fermentation (here, anaerobic fermentation). It is not aerobic respiration, which needs oxygen.
| Fuel | Biomass? | Why |
|---|---|---|
| Wood | Yes | Comes from trees |
| Cow dung cakes | Yes | Animal waste |
| Crop waste | Yes | Plant material |
| Gobar gas (biogas) | Yes | Made by bacteria from dung and plant waste |
| Coal | No | A fossil fuel, buried for millions of years |
| Petroleum | No | A fossil fuel |
| Uranium (nuclear fuel) | No | Not from living matter at all |
Practice this conceptself-check · 3 quick reps
The same idea in a practice question:
Practice questionEasy
Example 1 · Energy Sources · Biomass and Biogas
Biogas is formed through
Calling coal biomass
Coal did come from plants, but it is a fossil fuel formed over millions of years. Biomass means fresh plant and animal material such as wood and dung.
Choosing aerobic respiration for biogas
Biogas forms by fermentation without oxygen. Aerobic respiration needs oxygen, which is exactly what the biogas plant keeps out.
Concept 2 of 2: The biogas plant: what goes in and what comes out
A biogas plant is a closed tank where dung and water sit together with no air. Bacteria eat the dung slowly and give off gas that collects at the top, ready for a stove.
Definition
- Dung (or plant waste) is mixed with water to make a slurry, which goes into a closed tank called the digester.
- The digester is sealed so that no air gets in. The bacteria need water but not air.
- The gas is mostly methane (up to about three quarters), with some carbon dioxide, hydrogen and hydrogen sulphide.
- The slurry left behind is a good manure, rich in nitrogen and phosphorus.
- Biogas burns without smoke and is used for cooking and lighting.
| Part of the process | What it is |
|---|---|
| Goes in | Slurry of dung (or plant waste) and water |
| Conditions | Water present, air kept out |
| Closed tank | The digester |
| Main gas made | Methane |
| Other gases | Carbon dioxide, hydrogen, hydrogen sulphide |
| Left behind | Slurry used as manure, rich in nitrogen and phosphorus |
| Uses of the gas | Cooking and lighting |
The two things a biogas plant needs: water in the slurry, and no air.
Practice this conceptself-check · 3 quick reps
The same idea in a practice question:
Practice questionEasy
Example 2 · Energy Sources · Biomass and Biogas
The conditions for producing biogas is
Thinking the digester needs fresh air
Air must be kept out of a biogas plant. The bacteria that make methane work only without oxygen, so the digester is sealed.
Forgetting the water
Dry dung does not work. It is mixed with water into a slurry, so a biogas plant needs water but not air.
Calling carbon dioxide the main gas
Biogas has some carbon dioxide, but most of it is methane, which is the part that burns.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
What counts as biomass, and how biogas forms7 rows
| Fuel | Biomass? | Why |
|---|---|---|
| Wood | Yes | Comes from trees |
| Cow dung cakes | Yes | Animal waste |
| Crop waste | Yes | Plant material |
| Gobar gas (biogas) | Yes | Made by bacteria from dung and plant waste |
| Coal | No | A fossil fuel, buried for millions of years |
| Petroleum | No | A fossil fuel |
| Uranium (nuclear fuel) | No | Not from living matter at all |
The biogas plant: what goes in and what comes out7 rows
| Part of the process | What it is |
|---|---|
| Goes in | Slurry of dung (or plant waste) and water |
| Conditions | Water present, air kept out |
| Closed tank | The digester |
| Main gas made | Methane |
| Other gases | Carbon dioxide, hydrogen, hydrogen sulphide |
| Left behind | Slurry used as manure, rich in nitrogen and phosphorus |
| Uses of the gas | Cooking and lighting |
The two things a biogas plant needs: water in the slurry, and no air.
Watch out for (5)
- Calling coal biomass→ What counts as biomass, and how biogas forms
- Choosing aerobic respiration for biogas→ What counts as biomass, and how biogas forms
- Thinking the digester needs fresh air→ The biogas plant: what goes in and what comes out
- Forgetting the water→ The biogas plant: what goes in and what comes out
- Calling carbon dioxide the main gas→ The biogas plant: what goes in and what comes out
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