Bio energy

Biogas is a mixture of about 55–65 % methane, 35–45 % carbon dioxide and traces of H₂S, H₂ and N₂, produced when organic matter decomposes without oxygen. Its calorific value is about 20–24 MJ/m³.

Stages of anaerobic digestion

1. Hydrolysis: extracellular enzymes of bacteria break complex polymers (cellulose, proteins, fats) into soluble sugars, amino acids and fatty acids.
2. Acidogenesis (fermentation): acid-forming bacteria convert these into volatile fatty acids, alcohols, CO₂ and H₂.
3. Acetogenesis: the acids are converted into acetic acid, H₂ and CO₂.
4. Methanogenesis: methane-forming bacteria produce methane: $CH_3COOH \rightarrow CH_4 + CO_2$ and $CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O$.

The first two stages are done by acid formers; the methanogens are slow growing and sensitive.

Factors affecting gas production

A biogas plant has an inlet (mixing tank), a digester where bacteria act on the slurry, a gas holder and an outlet for digested slurry.

Floating drum plant (KVIC / Indian type)

        gas out
          |
      ___ | ___ <- steel drum (moves
     |  gas   |     up/down on guide)
  ---|--------|---
  inlet  \   /  outlet
  --->|  slurry  |--->
      |__________|
       digester well

Slurry is fed into an underground cylindrical digester. The gas collects in an inverted mild steel drum that floats on the slurry and rises or falls with the gas stored, so the gas pressure stays constant (about 8–10 cm of water). A partition wall helps slurry flow.

Fixed dome plant (Chinese / Deenbandhu type)

        gas pipe
           |
        _.-'-._   <- fixed brick dome
       /  gas   \
  --->|  slurry  |---> outlet/displacement
       \_______ /      chamber
        digester

The digester and gas holder are one masonry (brick or concrete) structure. Gas collects under the fixed dome and pushes slurry into the outlet (displacement) chamber, so gas pressure varies with the amount of gas stored (up to 1 m of water column).

Biomass is organic matter of recent plant or animal origin (wood, crop residues, animal dung, sewage, energy crops, municipal organic waste) that stores solar energy by photosynthesis. It is renewable and nearly carbon-neutral.

Conversion routes are thermochemical (combustion, gasification, pyrolysis) and biochemical, where microbes or enzymes act on wet biomass at low temperature.

1. Anaerobic digestion (biogas)

Bacteria decompose wet biomass such as dung and sewage sludge in absence of air giving methane-rich biogas (see the four stages: hydrolysis, acidogenesis, acetogenesis, methanogenesis). The digested slurry is a good fertiliser.

2. Alcohol fermentation (ethanol)

Sugar and starch crops (sugarcane, molasses, maize, cassava) are fermented by yeast. Starch is first converted to glucose by enzymes (hydrolysis), then:

$$C_6H_{12}O_6 \xrightarrow{\text{yeast}} 2C_2H_5OH + 2CO_2$$

The dilute product (8–12 % ethanol) is distilled up to about 95 % and dehydrated to anhydrous ethanol for blending with petrol (E10). Cellulosic biomass needs pre-treatment and cellulase enzymes (second-generation ethanol). Theoretical yield is 0.51 kg ethanol per kg glucose.

3. Other biochemical routes

• Biodiesel by transesterification of vegetable oil with methanol and a catalyst (a chemical process, often grouped with these).
• Biohydrogen by dark fermentation or photo-fermentation.
• Composting to produce soil conditioner.

Biochemical routes suit biomass with moisture above 50 %, which would waste energy in drying for thermochemical routes.

Data

Dung per day $= 6 \times 10 = 60$ kg.

(a) Gas production

$$V_g = 60 \times 0.036 = 2.16\ \text{m}^3/\text{day}$$

(b) Digester volume

Slurry = dung + equal water $= 60 + 60 = 120$ kg/day. Taking slurry density about 1000 kg/m³, the feed is 0.12 m³/day.

$$V_d = \text{feed rate} \times \text{retention time} = 0.12 \times 40 = 4.8\ \text{m}^3$$

About 10 % extra may be added for gas space and scum, giving about 5.3 m³ in practice.

(c) Gas holder volume

$$V_h = 0.60 \times 2.16 = 1.30\ \text{m}^3$$

(d) Persons served

$$\frac{2.16}{0.34} = 6.35 \approx 6\ \text{persons}$$

Energy content (at about 22 MJ/m³): $2.16 \times 22 = 47.5$ MJ per day.

Answer: gas = 2.16 m³/day; digester = 4.8 m³ (about 5.3 m³ with allowance); gas holder = 1.30 m³; cooking for about 6 persons.