Biomass Energy and Photosynthesis

Biomass Energy Resources

• Biomass is organic material that can be converted into energy through combustion or metabolic processes
• Main sources of biomass energy: wood, agricultural waste, urban waste, and forest waste

Photosynthesis

• Process by which plants convert solar energy into chemical energy (carbohydrates)
• Requires chlorophyll, carbon dioxide, and water
• Plants absorb red and blue light, but not green light, which is why they appear green
• Biomass production efficiency: 28% of used energy is converted into carbohydrates

Conditions for Photosynthesis

• Temperature: 20-30°C for maximum rate
• Carbon dioxide concentration: increases photosynthesis
• Oxygen concentration: reduces photosynthesis
• Water: increases photosynthesis
• Intensity and wavelength of solar radiation: increases photosynthesis with PAR (photo synthetically active radiation)

Biomass

• Can be used in its original form or transformed into solid, liquid, or gaseous fuels
• Examples of biomass fuels: fuel wood, charcoal, fuel pellets, bioethanol, biogas, producer gas, and biodiesel

Biofuels

• Fuel wood: most common source of biomass energy, energy density of 16-20 MJ/kg
• Charcoal: obtained from wood, energy density of 30 MJ/kg
• Fuel pellets: formed from crop residues, energy density of 16-20 MJ/kg
• Bioethanol: derived from sugarcane, starch, or cellulose, energy density of 26.9 MJ/kg
• Biogas: gaseous fuel produced from biomass, energy density of 23 MJ/m3

Biomass Resources

• Forests: source of fuel wood, charcoal, and producer gas
• Agricultural residues: straw, rice husk, groundnut shell, coconut shell, and sugarcane bagasse
• Energy crops: sugar plants, starch plants, and oil-producing plants
• Urban waste: garbage, sewage, and liquid waste
• Aquatic plants: water hyacinth, seaweed, algae, and kelp

Advantages and Disadvantages of Biomass Energy

• Advantages: renewable, storable, waste management, indigenous, economic development, sanitation, and fertilizers
• Disadvantages: low energy density, labor-intensive, and large land area required

Biogas

• Gaseous fuel produced from biomass through anaerobic digestion
• Composition: 50-60% methane, 25-35% carbon dioxide, and 5% hydrogen and other gases
• Energy density: 23 MJ/m3
• Uses: cooking, heating, lighting, and running small IC engines

Anaerobic and Aerobic Processes

• Anaerobic: process occurring in absence of oxygen, used in biogas production
• Aerobic: process occurring in presence of oxygen

Anaerobic Digestion

• Process of biogas production from biomass slurry
• Factors favoring digestion: wetness, warmness, and darkness
• Three stages: hydrolysis, acid formation, and methane formation
• Digester: airtight equipment used for anaerobic digestion

Advantages of Anaerobic Digestion

• Energy production from waste materials
• Fertilizer production
• Waste management
• Improved sanitation and hygiene
• Containment of odors

Raw Materials for Biogas

• Waste: industrial, agricultural, urban, and forest waste
• Cultivated materials: rice, wheat, and cereals
• Harvested materials: agricultural crops and residues

Factors Affecting Biogas Production

• Temperature: 20-65°C

• Pressure: 6-10 cm of water column

• Water: 9-10% solid content

• pH value: 6-7.5

• Feeding rate: uniform feeding rate

• Presence of nutrients: carbon, nitrogen, and other nutrients

• Seeding: adding digested slurry to fresh feed

• Mixing and stirring: mixing of biomass slurry

• Retention time: 50-60 days

• Toxic substances: presence of pesticides, detergents, and ammonia

• Type of biomass: cow dung, poultry manure, etc.### Advantages and Disadvantages of Fixed Drum Type Biogas Plant

• Lower cost

• No corrosion problem

• Better heat insulation

• No maintenance required

Disadvantages of Fixed Drum Type Biogas Plant

• Gas production per cubic meter of the digester is less
• Variable pressure of biogas
• More risk of leakage due to higher pressure of gas
• More risk of explosion
• Complex installation

Properties of Biogas

• Composition: 60% methane, 40% carbon dioxide, and traces of hydrogen, hydrogen sulphide, and other gases
• Calorific value: 5,600 kcal/m3 or 4,250 kcal/kg
• Stoichiometric air-fuel ratio: 5.27 by volume
• Calorific value of mixture: 767 kcal/m3, equivalent to 85% of gasoline

Application of Biogas in IC Engine

• Biogas cannot be used in IC engine without modification
• Blending biogas with petrol and diesel can run IC engine with small modification
• A mixture of biogas and gasoline containing 5-10% biogas can be used as IC engine fuel, reducing gasoline consumption by 5-10% and fuel cost

Models of Biogas Plants

• Common circular digester with floating gas holder without water seal (KVIC design, India)
• Common circular fixed dome digester (China)
• Flexible gas type combined digester and gas holder
• Taper digester with floating gas holder (Nepal)
• Two-chamber rectangular digester with floating gas holder and water seal (Philippines)
• Jet digester with separate gas holder (Thailand)
• KVIC model
• PRAD model
• ASTRA model
• Ganesh model

Biogas Plant in Hilly Area

• Average temperature remains below 15°C in hilly regions
• Anaerobic bacteria grow and digest biomass in the temperature range of 25-75°C
• Methods to overcome temperature limitations:
  • Hot water circulation
  • Use of chemicals (e.g., urea and urine)
  • Solar energy system

By-Product of Digestion

• By-products of digestion are shown in Figure

Location of Biogas Plant

• Considerations for locating a biogas plant:
  • Distance from well or spring used for drinking water purposes
  • Proximity to biomass generation area
  • Availability of water source
  • Open and exposed site for better performance
  • Proximity to point of gas consumption

Size of Biogas Plant

• Considerations for deciding the size of the plant:
  • Waste generation and animal/people population
  • Gas requirements
  • Volume of digester tank needed
  • Size and shape of digestion tank
  • Size of floating drum of the digester

Community Biogas Plants

• Advantages of community biogas plants:
  • Efficient cooking fuel
  • Electricity generation for various purposes
• Suggestions for improving productivity and efficiency:
  • Using biogas in dual fuel engine to generate power
  • Using hot exhaust to operate biogas plants at higher temperature
  • Using additional materials (e.g., water plants with animal waste) to generate biogas
  • New methods of biogas generation using bacterial support structures and recycling of spent slurry

Biomass Conversion Technologies

• Basic technologies to convert biomass into:
  • Direct energy
  • More valuable or convenient products
• Technologies:
  • Incineration
  • Thermochemical (pyrolysis)
  • Biochemical

Incineration

• Burning or combustion of biomass to obtain useful heat
• Heat can be used for:
  • Space heating and cooking
  • Generating steam in boiler to run turbine with electric generator

Thermochemical (Pyrolysis)

• Conversion of biomass into more valuable and convenient fuels
• Process:
  • Heating biomass in absence of air or partial combustion
• Products:
  • Gaseous mixture (H2, CO, CO2, CH4, N2)
  • Liquid (acetic acid, acetone, methanol, oil, and tar)
  • Pure carbon char

Biomass Gasification

• Conversion of solid biomass into combustible gas mixture (carbon monoxide and hydrogen)
• Process:
  • Partial combustion of biomass in absence of sufficient air
• Types of gasifiers:
  • Fixed bed updraft gasifier
  • Fixed bed down draft gasifier
  • Cross draft gasifier
  • Fluidized bed gasifier

Energy Recovery from Urban Waste by Landfill Reactors

• Municipal solid waste (MSW) disposal by controlled sanitary landfilling
• Biodegradable urban wastes are segregated and compacted to generate biogas
• Preparing the landfill site:
  • Lining with plastic liner and clay to prevent groundwater contamination
  • Leachate collection system to collect and recycle leachate
• Optimum moisture content of garbage: 60%
• Biogas production using anaerobic bacteria
• Temperature rises to 60°C during digestion