Chemistry of Reaction Process in Gasification

Gasification is a complex process that involves four distinct zones, each characterized by different chemical and thermal reactions.

1. Drying Zone

The primary function of the drying zone is to remove moisture from the biomass.
Process: As the temperature increases, moisture evaporates, resulting in dry biomass. During this phase, some organic acids are also released, which can contribute to corrosion in the gasifier.

2. Pyrolysis Zone

As the dry biomass moves into the pyrolysis zone, tar and volatile compounds are released.
Temperature Ranges and Reactions:
- Up to 200°C: Only water is driven off.
- 200°C – 280°C: Gases released include carbon dioxide (CO₂), acetic acid, and water.
- 280°C – 500°C: Significant production of tars and gases occurs, including carbon dioxide. Light tars and methyl alcohol are also formed. ( primary pyrolysis phase)
- 500°C – 700°C: Gas production is reduced, with hydrogen being the primary component.

3. Combustion (Oxidation) Zone

This zone facilitates the combustion of carbon present in the biomass.
Chemical Reaction: The carbon reacts with oxygen to produce carbon dioxide (CO₂):

Thermal Dynamics: The heat generated from this exothermic reaction raises the temperature, leading to a balance between heat supplied and heat lost in the gasifier.

4. Reduction Zone

This zone transforms carbon dioxide and other gases into combustible gases, particularly carbon monoxide (CO).

Key Reactions:
- Reduction of Carbon Dioxide:

This reaction is endothermic and occurs at temperatures exceeding approximately 1,000°C. Carbon monoxide is a crucial flammable component produced in this zone.

Water-Gas Shift Reaction:

This reaction also contributes to the production of combustible gases.

Exothermic Reaction: If excess water is present, carbon monoxide may react with water vapor, producing carbon dioxide and hydrogen:

This reaction is exothermic, reducing the heating value of the producer gas.

Temperature Dynamics: The endothermic nature of the reduction reactions generally results in temperatures ranging from 800°C to 1,000°C. Lower temperatures in this zone lead to lower calorific values of the gas.

Gas Composition and Efficiency

Gas Composition: The producer gas typically contains measurable amounts of particulate material and tar, with a heating value ranging from 4,000 to 5,000 kJ/m³. This value is relatively low compared to other gaseous fuels like natural gas.
Conversion Efficiency: The conversion efficiency of a gasifier, defined as the ratio of the heat content in the producer gas to the heat content in the biomass supplied, is typically around 75%.

The different zones of gasifiers (updraft, downdraft, and cross-draft) illustrate the varying temperature profiles and the chemical processes occurring in each zone.