05 Gasification of biomass

• For wood-like substances not able to ferment with bacteria.

• For gaseous and liquid fuels more working machines are available than for solid fuels.

• Working machines for gaseous and liquid fuels are there even in small power range(decentralized energy supply) with high efficiencies

1. Drying and heating

2. Pyrolysis

3. Gasification

Directly provided

—> By partial combustion

• Medium: Air, oxygen

• Low heating values

Indirect provided

—> By heat exchangers or circulating bed material

• Medium: Water vapor

• High heating values but technical problem heat input

There are two types of gasification

1. Autothermal - required heat is acquired internally (medium: air, O2)

2. Allothermal - required heat is acquired externally (medium: water vapor)

   
   

• It is usually the identification number for the evaluation of gasification processes

• It rates the chemical bonded energy in the product gas

Calorific value

—> is a measure of the amount of energy released when a substance undergoes complete combustion

• Maximum (theoretical) calorific values at the gasification of wood chips

Problem

• In the product gas always higher hydrocarbons

• Condese in pipe and on valves

Solutions

• Low tar gasifier

  • Thermal cracking of tars

  • Allothermal steam gasification

• Removing of tars

  • Gas scrubbing with water

  • Gas scrubbing with organic solvents(gasoline,RME)

  • Catalytic cracking of the tars

  • Electrostatic filter(polluted dusts)

• Concepts with hot gas cleaning and use of tars

  • System with gas turbines

  • Systems with high temperature fuel cells

Reaction equations can be reduced to this 4 divided into 2 groups:

• Heteregenous reactions (gas-solid reactions) —> Boudouard and shift reaction

• Homogeneous reactions (gas phase reactions) —> Oxidaion of carbon monoxide and refomation

Exothermal = positive gradient

• Oxidation of carbon monoxide

• Shift reaction

Endothermal = negative gradient

• Reforming / methanation

• Boudourd reaction

—> Influence of the gas composition by the temperature

Temperature

• Strongly exothermic reactions (e.g. combustion) can only reversed at very high temperatures

• Exothermic reaction balance shifts to the educts at higher temperatures

• Hydrogen production can be increased with lower temperatures

• No pressure dependence

Educts

• Surplus of one educt shifts the balance to the products

• Disadvantage

  • Heat supply for the gasification increasing

  • At autothermal gasification higher water content difficult

  • High waste heat reduces cold gas efficiency

• Reaction speed is essential for designing the plant (residence time)

• reactions of the homogeneous processes are much quicker than the reactions of the heterogeneous processes and especially the pyrolysis

• speed-determining phases in biomass gasification are drying and pyrolysis