Torrefaction of Biomass

Torrefaction

• An inert or limited oxygen environment is used.
• Biomass is gradually heated to a specific temperature range and held for a specific duration.
• The hemicellulose fraction is almost entirely degraded.
• Maximizes the solid product's mass and energy yield.
• The temperature ranges from 200 to 300 degrees Celsius.
• Extensive polymer devolatilization and carbonization occur above this temperature.
• High lignin loss occurs above 300 degrees Celsius, making pellet formation difficult since lignin binds solid particles.
• Cellulose's rapid thermal cracking produces tar between 300 and 320 degrees Celsius.
• The upper limit for torrefaction temperature is 300 degrees Celsius.
• Depolymerization occurs at this important stage.
• The degree of torrefaction depends on temperature and time.
  • Light torrefaction occurs between 200 and 240 degrees C (approximately 230 degrees C), affecting only hemicellulose.
  • Medium torrefaction occurs between 240 and 260 degrees C (approximately 250 degrees C), with a mild effect on cellulose.
  • Severe torrefaction occurs between 260 and 300 degrees C (approximately 275 degrees C), causing depolymerization of lignin, cellulose, and hemicellulose.
• Torrefaction products include:
  • Solids: Original and modified sugar structures, new polymeric structures, ash, and char.
  • Liquids: Water organic repeats
  • Gases: Toluene, benzene, and other gases
• Solid product formation is emphasized.

Torrefaction vs. Carbonization vs. Pyrolysis

• Torrefaction maximizes energy and mass yields while decreasing O/C and H/C ratios (oxygen-to-carbon and hydrogen-to-carbon, respectively).
• Carbonization maximizes the content of fixed carbon, while minimizing hydrocarbon content.
• Pyrolysis maximizes liquid production for bio-oil.
• Torrefaction retains most volatiles and drives away low energy-dense compounds and chemically bound moisture.
• Carbonization drives away the majority of volatiles.
• Pyrolysis is a complete devolatilization process.
• Torrefaction and carbonization require slow heating rates.
• Pyrolysis requires rapid heating.
• Torrefaction avoids oxygen and combustion.
• Carbonization occurs at higher temperatures with some oxygen.
• Pyrolysis takes place at a higher temperature and utilises the partial oxygen,
• Torrefaction takes place between 200 and 300 degrees Celsius.
• Carbonization is carried out at 300 to 600 degrees Celsius.
• Pyrolysis decomposes materials at 300 to 400 degrees Celsius, or up to 600 degrees Celsius.
• Carbonization produces more energy-dense fuel, but with a much lower energy yield than torrefaction.

Combustion

• A widely used process at a commercial scale to produce energy from biomass.
• Oxygen and combustible substances react, producing CO2, H2O, and heat.
• Carbon (C) reacts with oxygen to produce CO2 and energy; one mole of carbon burned produces around 393 kilojoules of energy.
• Stoichiometrically balanced, one mole of C reacts with one mole of oxygen to produce one mole of CO2.
• Hydrogen oxidation in fuel produces steam; one mole of hydrogen combusted produces H2O and energy.
• Reactions are balanced stoichiometrically to determine if the air to fuel ratio is correct.
• More air than is stoichiometrically required is needed for the oxidation reaction.

Combustion of Biomass

• Carbohydrate molecules are combusted to form stable oxidized compounds in an exothermic reaction between oxygen and hydrocarbons.
• During plant growth, CO2 and water are absorbed, along with solar radiation via chlorophyll, to convert into carbohydrate (glucose).
• Oxygen is produced during photosynthesis.
• Combustion releases chemical energy stored in carbohydrate molecules in the form of radiant and kinetic energy.
• Biomass carbohydrate molecule + oxygen + ignition temperature = x moles CO2 + y moles H2O + heat + other gases/char/ash.
• Incomplete combustion may produce CO and C in addition to CO2 and H2O.
• CO may react with oxygen to form CO2 and char may undergo oxidation to form CO2.
• Combustion reactions are balanced based on the oxidizing medium used.

Direct Combustion of Biomass

• Straightforward combustion of thermal energy into mechanical or electrical energy results in considerable losses, not raising ratio above 60%.
• Effective use of low-temperature waste heat for drying and heating purposes can increase overall efficiency.
• Fuel and air mixtures are burned in combustion units to produce heat energy, combustion products, and radiant energy.
• The air-fuel ratio is the mass of fuel to the mass of air.
• Fuel + oxidizing medium (oxygen or air) yields combustion product + energy.
• Combustible solid carbons are divided into volatile matter and combustible solid carbon.
• Wood has a high volatile share and low solid combustible metal; 80% of wood energy comes from combustion of volatile matter and 20% from solid carbon fuel.
• Burning wood in an oxidizing medium causes volatile components, consisting of aromatic hydrocarbons and long and short chain hydrocarbon compounds, to burn rapidly.
• Combustion forms flaming behavior, and radiant energy is transferred to the wood and surroundings, with some conductive heat transfer.

Combustion Process

• Volatile matter in solid fuels like wood enables flaming combustion and fixed carbon enables glowing combustion
• Solid fuel is ~80% volatile matter and ~20% solid carbon
• Radiant energy transfers heat to the fuel and surroundings during combustion, along with heat conduction in solid fuels
• Wood combustion produces heat, light, and radiation energy

Fuel Properties

• Key fuel properties include density, moisture content, volatile matter, fixed carbon content, sulfur content, ash, and calorific value
• Biomass typically has high volatile matter content compared to lignite and anthracite coal
• Coal has a significantly higher fixed carbon content than wood
• High volatile matter in coal aids ignition and combustion and releases low levels of NOx
• Volatile matter causes flaming combustion, whereas fixed carbon causes glowing combustion

Flaming vs Glowing Combustion

• Flaming combustion occurs when there's high volatile matter, producing flames
• Glowing combustion happens when fixed carbon content is high, lacking flames because of less volatile matter

Biomass Combustion

• Reaction time depends on fuel size, properties, temperature, and conditions
• Biomass combustion includes heterogeneous (solid-gas) and homogeneous (gas-gas) reactions
• Incomplete combustion releases CO, which oxidizes to CO2 in a homogeneous reaction
• Solid and char combustion with an oxidizing medium is a heterogeneous reaction
• Main combustion steps include drying, devolatilization, gasification, char combustion, and gas phase oxidation
• Incomplete combustion leads to pollutant generation

Complete vs Incomplete Combustion

• Reactions involve rearrangement of atoms to form oxidized products (CO2, H2O)
• Complete combustion with sufficient oxygen gives non-toxic products like CO2, H2O, and NOx
• Incomplete combustion produces CO, polycyclic aromatic hydrocarbons (PAH), soot particles, and unconverted carbon
• Reaction equations show initial and final results but don't indicate intermediate steps

Combustion Reactions

• Sulfur in fuels like coal leads to SO2 gas
• Air combustion is complex due to nitrogen and various fuel elements (carbon, hydrogen, nitrogen, sulfur, oxygen)
• Fundamental reactions of combustion include CO2 and H2O formation
• Combustion with oxygen produces CO2 and H2O; air combustion produces CO2, H2O, and nitrogen
• Incomplete combustion forms uncombusted products
• Air comprises ~21% oxygen and ~78.1% nitrogen
• Approximations: molar volume of oxygen is 21% and nitrogen is 79%
• Each mole of oxygen is accompanied by ~3.76 moles of nitrogen

Stoichiometry

• Theoretical air is required for complete combustion
• Stoichiometric mixture combustion: flue gas has no fuel or oxygen
• 6 moles of air mixture is required for combustion of biomass carbohydrate, forms 6 moles of CO2 and number of moles of water
• In practice, excess air ensures complete solid or liquid fuel combustion

Air-Fuel Ratio

• Air-fuel ratio is the mass of fuel to the mass of air
• Stoichiometric combustion is complete burning based on the quantity of fuel or oxidizer
• Stoichiometric oxidizer (oxygen or air) to fuel ratio is determined via mole balance equations, assuming ideal product state
• Complete combustion of hydrocarbon fuel with the oxygen produces CO2, H2O, and significant heat
• Hydrocarbon fuel (CxHy) stoichiometric relation: CxHy + nO2 -> yCO2 + zH2O, where, for 1 mole of hydrocarbon fuel, specific moles of oxygen ensures complete combustion

Combustion and Oxygen Supply

• Surplus oxygen leads to complete combustion but may leave excess oxygen in the products
• Insufficient oxygen in the combustion chamber leads to incomplete combustion
• The stoichiometric equation is used to calculate the exact amount of oxygen needed for complete combustion through mole balance of the fuel

Air to Fuel Ratio

• Equivalence ratio compares stoichiometric air to fuel ratio to the actual air to fuel ratio in combustion
• Stoichiometric oxygen allows calculation of excess air needed, expressed as the air to fuel ratio

Air Composition

• Air for combustion contains 21% oxygen and 79% nitrogen
• Each mole of oxygen in air is accompanied by 3.76 moles of nitrogen

Hydrocarbon Fuel Combustion

• Yields CO2, H2O, and nitrogen.
• Hydrocarbon fuel combustion with air (oxygen and nitrogen) requires a balanced equation to account for all the elements.

Stoichiometric Air to Fuel Ratio Calculation

• Air to fuel ratio is calculated stoichiometrically as the mass of air to the mass of fuel, converting moles of air and fuel to mass for accurate ratio calculation.

Actual vs. Stoichiometric Air

• Actual oxygen needed may differ from stoichiometric amounts based on the fuel properties
• The equivalence ratio represents the ratio of stoichiometric air to fuel ratio to the actual air to fuel ratio

Equivalence Ratio Significance

• Fuel-lean mixtures have excess oxygen; Fuel-rich mixtures have a lack of oxygen.
• The equivalence ratio is the ratio of stoichiometric air to fuel ratio to the actual air to fuel ratio
• An equivalence ratio of 1 indicates the actual fuel to air ratio matches the stoichiometric ratio
• Oxidizer-rich is an alternate term for fuel lean condition
• Oxidizer-lean mixture is an alternate term for a fuel-rich mixture

Combustion of Hydrocarbons with Oxygen Content

• Consideration of the oxygen already present in the fuel is a must when balancing the equation
• Hydrocarbons containing oxygen produce CO2, H2O, and nitrogen during combustion

Balancing Equations with Oxygen in Fuel

• Balance moles of carbon, hydrogen, and oxygen to determine the required oxygen from the air mixture

Air to Fuel Ratio Application

• Electricity generation, steam engines, steel industries: needs high temperatures for steel, domestic heating, and Brick kilns.
• Combustion has many practical applications, and is common in:
  • Internal combustion engines
  • Boilers for heat production