Biogas Production & Biomass Energy Comprehensive Notes

Biomass & Bio-Energy Basics

• Biomass = any recent organic matter (plants, animals, organic wastes) that stores solar energy as chemical energy via photosynthesis.
• When this energy is extracted it is called bio-energy; the raw materials are feedstocks.
• Universal constituents: C, H ₂O, volatile organics; physical form varies (wood chips ↔ sewage sludge).

Major Biomass Feedstock Categories

• Agricultural residues: leaves, stems, stalks, seed pods left after harvest.
• Animal waste: dung, urine, bedding (hay, straw, shavings) – rich in nutrients & chemical energy.
• Wood/forestry waste: whole trees, pruned branches, stumps, sawdust; subdivided into primary/secondary forestry residues.
• Industrial waste: off-cuts, solvents, cafeteria garbage; must be pre-screened for toxic components.
• Municipal solid waste (MSW) & sewage sludge: everyday trash, wastewater solids.
• Aquatic & crop residues: algae, water hyacinth; straw, weed, cotton sticks, bagasse, bran.
• Human waste: “night soil”.

Biofuels Overview

• Definition: “inexhaustible, biodegradable fuels manufactured from biomass.”
• Forms: solid (direct combustion), liquid (bioethanol, biodiesel, biomethanol), gaseous (biogas/biomethane, hydrogen).
• Uses: cooking/heating, electricity generation, transport fuel.
• First-generation biofuels in wide use today: bioethanol & biodiesel.

Advantages

• Healthier population; cleaner environment; negligible CO & SOₓ; lower carcinogenic risk; mitigates global warming.

Disadvantages (implied)

• Land-use, feed vs. fuel debate, possible emissions from poor feedstock choice.

Biomass–to–Energy Conversion Pathways

• Direct combustion: burn biomass for space/process heat or steam turbines.
• Thermochemical:
  • Pyrolysis: $400\,^{\circ}C!−!500\,^{\circ}C$, near-zero O₂ → charcoal, bio-oil, renewable diesel, CH₄, H₂.
  • Hydrotreating: bio-oil + H₂ + catalyst (high T, P) → renewable diesel/gasoline/jet fuel.
  • Gasification: $800\,^{\circ}C!−!900\,^{\circ}C$, limited O₂/steam → syngas (CO+H₂) for engines, heating, power.
• Chemical: Trans-esterification → vegetable oil/fats + methanol → FAME (biodiesel).
• Biological:
  • Fermentation → ethanol.
  • Anaerobic digestion → biogas/biomethane.

Biomethane (Biogas)

Microbiology & Origin

• Produced by methanogenic archaea (e.g., Methanobacterium, Methanococcus jannaschii).
• Natural habitats: rumen of cattle, sewage sludge; hence dung is inoculum-rich.
• Gas mix ≈ 50–70 % CH₄, 25–45 % CO₂ + traces H₂, NH₃, H₂S.

Feedstock Availability & Scales

• Village/farm: agri & vegetable wastes.
• Household: animal dung, kitchen garbage.
• City: MSW, sewage.
• Industry: effluents from dairies, distilleries, breweries, food & chemical plants.

Stages of Anaerobic Digestion

1. Liquefaction / Hydrolysis (hydrolytic enzymes):
   $\text{Proteins} \rightarrow \text{polypeptides} \rightarrow \text{amino acids}$
   $\text{Fats} \rightarrow \text{glycerol} + \text{fatty acids}$
   \text{Starch (amylose)} \rightarrow \text{mono- & polysaccharides}
2. Acidogenesis: simple organics → acetic acid + H₂ + CO₂ (via hydrogen- & acid-producing bacteria).
3. Methanogenesis: $CH<em>3COOH \;\text{or}\; CO</em>2 + 4H<em>2 \rightarrow CH</em>4 + …$ carried out by strict-pH, temp-sensitive methanogens.

Generic Reaction

${\small \text{Organic matter} \;\rightarrow CH<em>4 + CO</em>2 + H<em>2 + NH</em>3 + H_2S}$

Fixed-Dome Biogas Plant Operation

• Slurry (biomass + equal water) fed via inlet → digester.
• Gas accumulates in dome, raising pressure → spent slurry displaced to outlet chamber → overflow tank → removed as manure.
• Continuous feeding → steady gas supply via pipeline & valve.

Landfill Gas Recovery

• Organic MSW decomposes in situ; pipe networks collect biogas → prevents CH₄ explosions & atmospheric loss.

Benefits

• Safe, cheap, renewable; cooking, lighting, engine fuel; improved sanitation; digestate = valuable fertilizer.

Microbial Hydrogen Production

• H₂ calorific value ≈ 30,000 cal g⁻¹ vs. gasoline 11,000 cal g⁻¹, coal 8,000 cal g⁻¹.
• Clean (no CO₂ on combustion); versatile (liquid/gaseous storage, fuel cells).

Biological Routes

1. Photolysis (bio-photolysis)
   • Photosynthetic algae/cyanobacteria (Chlorella, Chlamydomonas, Oscillatoria).
   • Water split by light, enzyme hydrogenase releases H₂; oxygen sensitivity limits large-scale use.
2. Dark Fermentation
   • Anaerobic bacteria convert glucose → H₂ + by-products; low yields, uneconomical.
   • Rhodospirillum can use organic wastes.
3. Nitrogenase-dependent (Legume nodules)
   $N<em>2 + 8H^+ + 8e^- \xrightarrow{\text{nitrogenase}} 2NH</em>3 + H_2$
   • Up to 30 billion m³ H₂ per ha of soybean annually—currently unharvested.

Microbial Fuel Cells (MFC)

Principle & Core Reactions

• Anaerobic microbes oxidize substrate releasing e⁻/H⁺.
• Anode (anaerobic):
  $C<em>6H</em>{12}O<em>6 + 6H</em>2O \rightarrow 6CO_2 + 24H^+ + 24e^-$
• Electrons travel through external circuit → electricity; protons migrate via exchange membrane.
• Cathode (aerobic):
  $O<em>2 + 4H^+ + 4e^- \rightarrow 2H</em>2O$

Architectures

• Mediator-free MFC: electro-active bacteria (e.g., Aeromonas hydrophila, Shewanella putrefaciens) form biofilm → direct electron transfer via outer-membrane cytochromes.
• Mediator-type MFC: electro-inactive microbes; soluble mediators (e.g., humic acid) shuttle electrons.

Components

• Anode chamber (stainless mesh + graphite rods/plates) – anaerobic.
• Cathode chamber – aerobic, O₂ bubbled.
• Exchange membrane (PEM/CEM) – proton selective.
• Electrical circuit – external load.
• Substrates: acetate (preferred), proteins, volatile acids, wastewater, cellulose.
• Microbes: Geobacter sulfurreducens, Rhodoferax ferrireducens, Clostridium beijerinckii.

Advantages

• Simultaneous wastewater treatment & power generation.
• Direct chemical-to-electrical energy conversion.
• Potential for bioremediation of toxic compounds.

Biomass-Based Steam Power Plants (Conventional Thermal)

• India generates >500 Mt agri/agro-industrial residue yr⁻¹ (≈175 Mt oil equivalent).
• 150–200 Mt economically available → 15,000–25,000 MW potential.
• Additional dedicated energy plantations on wastelands/roadsides could exceed 70,000 MW; total potential ≈100,000 MW.
• Technology analogous to coal plants: biomass → boiler → steam → turbo-alternator.
• Advantages: load-matching via stored feed, rural grid support, technology familiarity, multi-fuel flexibility.

Biomass Gasification

• Partial combustion (air < stoichiometric) converts solid biomass → producer gas.
  • Typical composition: CO 18–20 %, H₂ 15–20 %, CH₄ 1–5 %, CO₂ 9–12 %.
• Applications:
  • Fuel for IC engines coupled to generators (diesel substitution).
  • Industrial heating (oil replacement).
  • Cleaner emissions vs. raw biomass burning; economic diesel savings.

Electricity Generation Process Summaries

1. Combustion Route
   1. Biomass burned → heat.
   2. Water in boiler → steam.
   3. Steam drives turbine → alternator → electricity.
   4. Cogeneration: part of steam reused for heating; condensate recycled.
2. Gasification Route
   • High-T (800–1400 °C) transforms biomass→biogas; gas used in IC engines/turbines or as chemical feedstock.

Combined Cycle Power Plants (CCPP)

• Integrate Brayton (gas turbine) + Rankine (steam turbine) cycles.
• Gas turbine produces electricity & exhaust heat; HRSG recovers exhaust heat → steam → steam turbine → extra electricity.
• Main parts: Gas turbine, Heat Recovery Steam Generator, Steam turbine, condenser.
• Higher efficiency than single-cycle plants due to heat recovery.

Cogeneration (Combined Heat & Power, CHP)

• Single plant produces electricity + useful heat (hot water/steam).
• Fuels: natural gas, diesel, biogas, biomass, vegetable oil.
• Efficiency 70–90 % (vs. ~30 % in separate generation where ~60 % heat is wasted).
• Types:
  • Gas turbine CHP (use flue-gas heat).
  • Biofuel engine CHP.
  • Biomass CHP (uses industrial/MSW biomass).
• Benefits: energy savings, reduced carbon, on-site reliable energy for C&I users.