Gasification Lecture Notes

Introduction

• Continuation of discussion on thermochemical conversion processes, specifically gasification of biomass.
• Various technological options available for conversion of biomass to energy.

Biomass Conversion Technologies

• Direct Use of Biomass: Firewood for heating has limitations due to high moisture content.
• Denser Pellets: More energy-efficient alternative to firewood due to lower moisture content (4-5%).
• Biogas Production: Slurry digestion to yield biogas.
• Gasification vs. Incineration: Gasification as a method to produce gaseous fuel (syngas); incineration generates heat for steam generation.

Gaseous Fuel Production Methods

• Biochemical Method: Anaerobic digestion, yielding biogas (20 MJ/m³).
• Thermochemical Method: Gasification, yielding syngas (4-5 MJ/m³).

Importance of Gasification

• Flexibility: Gasification can use a wide variety of biomass feedstocks.
• High Efficiency: Thermochemical conversion efficiency of gasification is 70-90%.
• Suitable for Internal Combustion Engines: Gasifier outputs are appropriate fuels with capacities ranging from 15-30%.

History of Gasification Technology

• Established technology since 1830, initially using coal and peat.
• Significant shift in 1920s for synthetic material production.
• WWII saw the use of wood gas generators due to petroleum shortages.

Advantages of Gasification

1. Increases Heating Value: Removes non-combustibles (water and nitrogen).
2. Reduces Carbon to Hydrogen Ratio: Biomass has higher oxygen content; gasification leads to high-density fuel.
3. Clean Gas Production: Syngas can be utilized for various applications.

Gasification Process Overview

• Definition: Conversion of solid or liquid feedstock into gaseous fuel via thermochemical methods without carbon residues.
• **Two Stages:
  1. Primary Zone:** Biomass is partially combusted forming producer gas and charcoal.
  2. Reduction Zone: CO2 and H2O from the primary stage are reduced by charcoal to produce CO and H2.

Reaction Types in Gasification

• Heterogeneous Reactions: Carbon reactions involving solid phase with gases.
• Homogeneous Reactions: Gas phase reactions among gaseous products.

Equivalence Ratio in Gasification

• Definition: Ratio of air to fuel ratio needed for gasification to air supplied.
• Optimal Range: 0.2 to 0.4 for effective gasification; lower leads to incomplete gasification, higher leads to combustion.

Types of Gasifiers

1. Fixed Bed Gasifiers:
   • Classified as updraft, downdraft, and cross-draft based on air flow direction.
   • Issues include slagging and syngas exit blockage.
2. Fluidized Bed Gasifiers:
   • Bubbling and circulating types.
   • Benefits include good mixing and high conversion efficiency.
3. Entrained Bed Gasifiers:
   • Challenges include slag mobility and ash disposal.

Comparison of Gasification and Combustion

• Flue Gas Production: Higher in combustion.
• CO2 Emissions: Lower in gasification.
• Solid Waste Generation: Higher in combustion; gasification minimizes residues.
• Product Usage: Gasification allows for value-added products unlike combustion.

Conclusion

• Overview of gasification as a superior method for biomass conversion compared to combustion.
• Future discussions will focus on pyrolysis, liquefaction, and chemical conversion processes.