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Gasification

• • Gasification means the conversion of solid or liquid feedstock into useful and convenient gaseous fuel or chemical feedstock that can be burned to release energy or used for production of value-added chemicals.   

Difference between gasification and combustion

• Gasification packs energy into chemical bonds in the product gas; but combustion breaks those bonds to release the energy.
• The gasification process adds hydrogen to and strips carbon away from the feedstock to produce gases with higher hydrogen-to-carbon (h/c) ratio, while combustion oxidizes the hydrogen and the carbon into water and carbon dioxide.

Gasification steps

1.Drying.

2.Thermal decomposition or pyrolysis.

3.Partial combustion of some gases, vapors, and char.

4.Gasification of decomposition products.

Difference between pyrolysis and gasification

Gasifying mediums / agents

1-Oxygen

• If oxygen is used as the gasifying agent, the conversion path moves toward the oxygen corner. Its products include co for low oxygen and co2 for high oxygen.
• When the amount of oxygen exceeds a certain amount (stoichiometric), the process moves from gasification to combustion, the product is the flue gas instead of the fuel gas.
• A move towards the oxygen corner leads to a lowering of hydrogen content and an increase in carbon-based compounds such as co and co2 in the product gas.

2-Steam

• If steam is used as the gasification agent, the path is upward towards hydrogen corner.
• The product gas contain more hydrogen per unit of carbon, resulting in a higher h/c ratio.
• Some of the intermediate reaction products like co and h2 also help to gasify the solid carbon.

• The choice of gasifying agent affects the heating value of the product gas.
• If air is used instead of oxygen, the nitrogen in it greatly dilutes the product.
• We can see that oxygen gasification has the highest heating value followed by steam and air gasification.

The Gasification Process

1-Drying

• The typical moisture content of freshly cut wood ranges from 30% to 60% and for some biomass it can exceed 90%.
• Every kilogram of moisture in the biomass takes away a minimum of 2260 kj of extra energy from the gasifier to vaporize water and that energy isn’t recoverable.
• For the production of a fuel gas with a reasonably high heating value, most gasification systems use dry biomass with a moisture content of 10-20%.
• The final drying takes place after the feed enters the gasifier, where it receives heat from the hot zone downstream.
• The typical moisture content of freshly cut wood ranges from 30% to 60% and for some biomass it can exceed 90%.
• Every kilogram of moisture in the biomass takes away a minimum of 2260 kj of extra energy from the gasifier to vaporize water and that energy isn’t recoverable.
• For the production of a fuel gas with a reasonably high heating value, most gasification systems use dry biomass with a moisture content of 10-20%.
• The final drying takes place after the feed enters the gasifier, where it receives heat from the hot zone downstream.

2-Pyrolysis

• In pyrolysis no external agent is added.
• IN slow pyrolysis process, more char is formed.
• In fast pyrolysis, more liquid hydrocarbon is produced.
• Pyrolysis which precedes gasification, involves the thermal breakdown of larger hydrocarbon molecules of biomass into smaller gas molecules (condensable and non-condensable) with no major chemical reaction with air, gas, or any other gasifying medium.

3- Char gasification reactions

• Gasification of biomass char involves several reactions between the char and the gasifying mediums.
• The following is a description of some of those reactions with carbon, carbon dioxide, hydrogen, steam, and methane.
• Char+O2→CO2 and CO
• Char+CO2→CO
• Char+H2O→CH4 and CO
• Char+H2→CH4
• The speed of char reactions:
• Rc+o2 >> Rc+h2o>> Rc+co2 >> Rc+h2

4-Char combustion reactions

• Most gasification reactions are endothermic.
• Heat is required for:

1.Heat for the reaction.

2.Heat required for heating.

3.Heat required for drying.

4.Heat required for pyrolysis.

• Certain amount of exothermic combustion reaction is allowed in gasifier.
• Combustion reactions are generally faster than gasification reactions under similar.
• • When carbon comes in contact with oxygen two reactions may takes place as follow:
  • C+O2→CO2
  • C+1/2 O2→CO
  • A partition coefficient, ß, may be defined to determine how oxygen will partition itself between the two and written as follow:

  ßc+o2→2(ß-1)co+(2-ß)co2

  The value of the partition coefficient ß lies between 1 and 2 and depends on temperature.

  ß=[co]/[co2]= 2400 exp(-6234/t), where t is the surface temperature of the char.

Carbon dioxide reforming of methane

• Carbon dioxide reforming of methane in not widely used commercially as steam reforming.
• It has special attraction of reducing 2 greenhouse gases (CO2 AND CH4) in one reaction.
• CH4+CO2→2CO+2H2+4226.67KJ/kg

Nickel based catalysts are also effective for dry-reforming reaction