Thermo-chemical Biomass Conversion

Thermo-chemical Biomass Conversion

• Thermochemical biomass conversion yields solid, liquid, and gaseous fuels.
• Technologies directly release energy as heat or electricity, or create liquid/combustible gas products.
• Processes encompass combustion, paralysis, gasification, and liquefaction.
• Torrefaction transforms biomass into a carbonized solid fuel.
• Carbonization and torrefaction have slight distinctions.
• Process conditions like temperature and oxygen levels greatly influence biomass conversion.
• Thermochemical conversion shifts biomass components into carbon, hydrogen, and oxygen.
• Slow paralysis maximizes biomass conversion to solid char.
• Rapid paralysis mainly produces hydrocarbons.
• Feedstock oxidation can yield combustible gas, CO2, H2O, and heat.
• Hydro gasification enhances the hydrogen content of gas products.

Impact of Process Conditions

• Biomass conversion into higher-value products relies on process conditions like combustion, gasification, or pyrolysis.
• Released gas has a heating value of 4-6 MJ/kg.
• Produced liquid has a heating value of 17-20 MJ/kg.
• Char has a heating value of 16-18 MJ/kg.
• Variability stems from the carbonaceous content in raw biomass.
• The final product relies on technology and oxygen supply.

Thermochemical Processes

• Heat is the dominant mechanism in thermochemical conversion.
• Processes include combustion, gasification, pyrolysis, and liquefaction.
• These processes occur at high temperatures, sometimes with high pressure.
• Excess air results in combustion, producing CO2, H2O, and heat.
• Partial air supply leads to gasification, creating fuel gas (H2 and CO).
• No air supply leads to pyrolysis or hydrothermal liquefaction, producing liquids, solid carbon, and gases.
• Hydrothermal liquefaction (HTL) is used when a liquid product is desired.

Gasification

• Gasification turns carbonaceous material into useful gas or chemical feedstock.
• Unlike combustion, gasification produces gas with heating value.
• Gasification packs energy into chemical bonds in the product gas; combustion releases energy.
• Gasification needs a reduced environment and energy input.
• Combustion oxidizes material and releases heat.
• Gasification and pyrolysis yield energy, chemicals, or valuable products.

Torrefaction

• Torrefaction transforms residual biomass into high-carbon solid fuel.
• It is used in co-firing biomass in coal power stations.
• Pyrolysis converts biomass into transportable clean fuel.
• It creates liquid/gaseous fuels, but the gas has low molecular weight and heating value.
• Gasification converts heavy oil residues into syn gas.
• Hydro gasification uses steam to produce gas with a high hydrogen-to-carbon ratio.

Supercritical Water Gasification

• Supercritical water gasification uses wet biomass as a feedstock.
• Biomass is converted into a slurry before gasification.
• Oil yield is less than pyrolysis but the oil quality is higher.

Thermochemical Conversion Advantages

• Processes efficiently convert various biomass types.
• They address seasonal and regional variations in biomass.
• They utilize entire biomass (e.g., corn plant).
• Biochemical conversion processes are well-established but may take longer than thermochemical processes.
• Thermochemical conversion offers high throughput on various biomass types.
• It offers wide fuel opportunities, including ethanol, mixed alcohols, and green diesel.

Pre-processing Stages

• Lignocellulosic biomass needs pre-processing such as pre-treatment, drying, size reduction, and densification.
• Densification increases energy density, making it suitable for combustion and gasification.
• Combustion occurs above 700°C with excess oxygen, producing heat, CO2, and H2O.
• Heat can be used for drying, steam generation, or electricity production.
• Gasification of dried biomass uses catalytic or non-catalytic processes.
• Catalytic gasification requires lower temperatures.
• The product is syngas, which can be used directly or converted to methane or methanol.

Pyrolysis

• Pyrolysis converts biomass into gas, liquid, and solid char.
• It occurs at 300-700°C without external oxidizing agents.
• When the goal is only liquid, it's called liquefaction.
• Gas can be combusted for process heat.
• Liquids can be upgraded to fuels or chemicals.
• Char can be used as a sorbent or soil enhancer.

Hydrocarbon Ratio

• The hydrocarbon ratio doubles when biomass converts to methane/methanol.
• The ratio remains nearly unchanged in catalytic liquefaction.
• It is increased by using steam in gasification or by removing char from pyrolysis.

Torrefaction vs Carbonization

• Torrefaction retains volatiles, driving away low-energy compounds and moisture.
• Carbonization drives away most volatiles.
• Torrefaction avoids oxygen supply and combustion, while carbonization uses them.
• Torrefaction is thermal decomposition at 200-300°C.
• Carbonization occurs at 300-600°C and is regarded as destructive distillation.
• Carbonization produces more energy-dense fuel but with a lower energy yield than torrefaction.

Pyrolysis Process Detailed

• Pyrolysis is thermal conversion below 600°C without oxygen.
• It produces high energy density materials, including solid carbon, liquid, and gas.
• Irreversible degradation starts at 150-200°C, converting higher hydrocarbon molecules into lower molecular weight compounds.

Combustion Process Detailed

• Biomass combustion accounts for 97% of the world's bioenergy production.
• Denser pellets or dried biomass can be combusted for higher efficiency.
• The feedstock is subjected to temperatures over 700°C with excess air.
• It produces gaseous products (CO2, H2O) and significant heat.
• Oxidation releases energy during combustion of the biomass.

Gasification Process Detailed

• Gasification uses restricted oxygen to convert carbonaceous material into carbon monoxide.
• Carbon produces 72% less heat than in complete combustion.
• Partial oxidation generates combustible gas (CO) and energy.
• The CO can be combusted with adequate oxygen to release the remaining 72% of the heat.

Use of Steam

• Steam can be a gasifying agent.
• It results in product gas with H2 and CO (syngas or producer gas).
• Requires heat input during reduction.

Liquefaction Process Detailed

• Liquefaction converts solid biomass into liquid fuel through pyrolysis, gasification, or hydrothermal processes.
• Hydrothermal processes use water at 300-350°C and 12-20 MPa to produce oily liquid.
• Pyrolysis yields pyrolytic liquid oil, gas, and solid char.
• With emphasis on oil production instead of gas, it's termed liquefaction.
• Pyrolysis liquid oil has high water content and low pH, making it corrosive.

Liquefaction via Gasification

• This involves methanol production from H2 and CO.
• H2 and CO come from gasification of biomass.
• The reaction takes place at 330°C and 150 atmospheric pressure.
• Unwanted gases are removed before the methanol reactor.
• Methanol yield: 480-568 liters per ton of biomass.
• Methanol has an energy density of 23 MJ/kg.

Hydrothermal Liquefaction Detailed

• Hydrothermal liquefaction (HTL) uses biomass and water slurries.
• It's well-suited for high-moisture material, including MSW.
• HTL bio-oil is higher quality and has less oxygen content than pyrolysis oil.
• Oil yield is lower, but still higher than crude oil.
• It operates in a wide temperature range: 200-600°C.
• At 200-275°C, it produces solids.
• At 275-350°C, it produces liquids.
• Above 400°C, it produces gas.
• Requires pressure of around 5-40 MPa to maintain a liquid state.
• It can handle wet biomass, so drying and size reduction are not required.

Thermochemical Processes Comparison

• The processes include liquefaction, paralysis, combustion, and gasification.
• Liquefaction:
  • Requires relatively lower temperatures.
  • Requires sufficiently high pressure.
  • Needs a catalyst for conversion but does not require drying.
• Paralysis:
  • Requires 300 to 600 degrees Celsius.
  • Operates at a really low pressure range.
  • Does not require a catalyst.
  • Requires drying material.
• Combustion:
  • Requires significantly high temperatures.
  • Pressure is not high.
  • Does not require a catalyst.
  • Drying is not essential, but it helps if dried.
• Gasification:
  • Requires temperatures from 500 to 1300 degrees Celsius.
  • Pressure is not essential.
  • Does not require a catalyst.
  • Drying is absolutely necessary.