![THERMO-CHEMICAL REACTIONS OCCURRING
IN GASIFICATION
REDUCTION ZONE
Products of partial combustion – H2O, CO2 and uncombusted and partially cracked pyrolytic products undergo the
following chemical reactions in the red-hot charcoal bed
C + CO2 = 2CO (- 164.9 MJ/kg mole) [Boudouard reaction]
C +H2O = CO + H2 (- 122.6 MJ/kg mole) [water gas ]
CO + H2O = CO + H2 (+ 42 MJ/kg mole) [water shift reaction]
C + 2H2 = CH4 (+ 75 MJ/kg mole) [Methane reaction]
CO2 + H2 = CO + H2O (- 42.3 MJ/kg mole)
Low reduction zone temperature yields low calorific value of gas
Average temp is 850 0C](https://image.slidesharecdn.com/lec18-240407150048-21e2f08a/85/Gasifier-types-Fixed-and-Fluidized-bed-gasification-8-320.jpg)
Gasifier types - Fixed and Fluidized bed gasification
- 1. Gasifier–types By Er. T. AYISHA NAZIBA, Dr. D. RAMESH, Dr. S. PUGALENDHI
- 2. GASIFICATION Gasification is a partial oxidation process whereby a carbon source such as coal, natural gas or biomass is broken down into carbon monoxide (CO) and Hydrogen (H2) plus carbon dioxide (CO2) and possibly hydrocarbon molecules such as methane (CH4) This mix of gas is known as producer gas and the precise characteristics of the gas will depend on the gasification parameters such as temperature and also the oxidizer used The oxidizer may be air, in which case the producer gas will also contain Nitrogen (N2) or steam or oxygen
- 3. GASIFIER types FIXED BED oUPDRAFT GASIFIER (COUNTER CURRENT GASIFIER) oDOWN DRAFT GASIFIER (CO-CURRENT GASIFIER) oCROSS DRAFT GASIFIER FLUDIZED BED GASIFIER (Bubbling or circulating)
- 4. Updraft Down draft Cross draft TYPES OF GASIFIER
- 5. UP DRAFT GASIFIER Reaction zones in an up draft gasifier An up draft gasifier is characterized by a counter current flow of fuel and air or gas in the reactor. The producer gas exits from the top of the gasifier
- 6. FEATURES OF UPDRAFT GASIFIER The biomass enters the top of the reaction chamber while steam and air or oxygen enter from bottom through grate The fuel flows downward and up-flowing hot gases pyrolyze it. some of the resulting charcoal residue falls to the grate, where it burns, producing heat and giving off carbon dioxide and water vapor The CO2 and H2O react with other charcoal particles, producing carbon monoxide and hydrogen gases The gases exit from the top and ashes fall through the grate The updraft design is relatively simple and can handle biomass fuels with high ash and moisture content The gas contains 10-20 percent volatile oils making the gas unsuitable for use in engines or gas turbines
- 7. TYPICAL ZONAL CHEMICAL REACTIONS AND PRODUCTS IN A UP-DRAF GASIFIER
- 8. THERMO-CHEMICAL REACTIONS OCCURRING IN GASIFICATION REDUCTION ZONE Products of partial combustion – H2O, CO2 and uncombusted and partially cracked pyrolytic products undergo the following chemical reactions in the red-hot charcoal bed C + CO2 = 2CO (- 164.9 MJ/kg mole) [Boudouard reaction] C +H2O = CO + H2 (- 122.6 MJ/kg mole) [water gas ] CO + H2O = CO + H2 (+ 42 MJ/kg mole) [water shift reaction] C + 2H2 = CH4 (+ 75 MJ/kg mole) [Methane reaction] CO2 + H2 = CO + H2O (- 42.3 MJ/kg mole) Low reduction zone temperature yields low calorific value of gas Average temp is 850 0C
- 9. UPDRAFT GASIFICATION DESIGN • Simple design • Not sensitive to fuel selection • Has a long start time • Delay in response • Best used for large, long use applications
- 10. REACTIONS, TEMPERTURES AND GAS COMPOSITION IN UPDRAFT GASIFIER
- 11. Down draft gasifier is characterized by co-current flow of air or gas and the fuel. Fuel and air or gas move in the same direction Reaction zones in a down draft gasifier DOWN DRAFT GASIFIER
- 12. FEATURES OF DOWN DRAFT GASIFIER Gasifier requires drying of biomass fuel to a moisture content of less than 20 percent Fuel and air or oxygen enter the top of the reaction chamber Down-flowing fuel particles ignite, burning intensely and leaving a charcoal residue The charcoal which is about 5 to 15 percent of the mass of original fuel, then reacts with the combustion gases, producing CO and H2 gases These gases flow down and exit from the chamber below a grate The producer gas leaving the gasifier is at a high temperature Combustion ash falls through the grate The advantage of the downdraft design is the very low tar content of the producer gas
- 13. REACTIONS, TEMPERTURES AND GAS COMPOSITION IN DOWNDRAFT GASIFIER
- 14. DOWNDRAFT GASIFICATION ANALYSIS The general idea behind this design is that the tarry oils and vapors given off in the distillation zone are highly unstable at high temperatures. In order to reach the gas outlet they must pass through the partial combustion zone where a high amount of tar will be cracked and reduced to non- condensable gaseous products before leaving the gasifier Although the general principle behind this seems convincing easily, in practice it requires some testing and high skill to come up with a downdraft gas producer capable of generating a tar free gas under equilibrium conditions
- 15. TYPICAL CHEMICAL REACTIONS AND PRODUCTS IN A DOWN-DRAF GASIFIER
- 16. DOWNDRAFT GASIFICATION Can use high tar fuels (wood, peat) • Quicker in response Has shorter start time Complex design Can’t use high ash content fuels Fuel pellet size must be uniform
- 18. Down draft gasifier without throat Down draft gasifier with throat is not suitable for gasification of agro- residues like paddy husk, groundnut shell etc. A down draft gasifier without throat can be used for the gasification of paddy husk, groundnut shell, maize cob, etc. Down draft gasifier with throat Gasifier has narrow section below air entrance point in the reactor, which is called throat. Due to decrease in the cross-sectional area at the throat, air velocity increases resulting in high and better temperature distribution in the oxidation zone . SIGNIFICANCE OF THROAT IN A DOWN DRAFT GASIFIER
- 19. This gasifier can operate with wide variety of fuels compared to an up draft or a down draft gasifier High gas exit temperature, higher gas velocity at the gas exit Poor CO2 reduction are certain characteristics of this type of gasifier This type of gasifier has been used for gasification of coal CROSS DRAFT GASIFIER
- 20. CROSS DRAFT GASIFICATION ANALYSIS Can’t use high tar fuels Quicker in response Has shorter start time Complex design Can’t use high ash content fuels Fuel pellet size must be uniform
- 21. FLUIDIZED BED GASIFICATION A fluidization bed is a chamber with a perforated floor having pressurized air flowing vertically where a particle medium usually sand, is contained. The pressurized and flowing air helps the medium allowing it to act as a fluid
- 22. PRINCIPLE OF FLUIDIZED BED GASIFICATION Can use most fuels (wood, peat and coal) including agriculture waste such as straw, corn stover and manure Has potential to use municipal waste such as garbage Quicker in response Has shorter start time Complex design Lends itself to complete combustion applications which would allow it to use liquid wastes such as used engine oil, non- recyclable plastics & old shoes, garbage for generation of heat
- 23. Fluidized bed gasifier is a homogeneous reactor bed of some inert sand material. The fuel is introduced in the inert bed material and air at the bottom of the bed in the reactor. This gasifier is characterized by high gas exit temperature, very high solid particulate matter in the gas and relatively low efficiency. The gasifier can operate with low bulk density materials such as agro- residues, leaves, etc. FLUIDIZED BED GASIFIER
- 24. REACTIONS, TEMPERTURES AND GAS COMPOSITION IN FLUIDIZED BED GASIFIER
- 25. Sr. No Gasifier Type Advantages Disadvantages 1 Updraft • Small pressure drop • Good thermal efficiency •Little tendency towards slag formation •Great sensitivity to tar •Moisture and moisture content of fuel • Relatively long time required for start up of IC engine • Poor reaction capability with heavy gas load 2 Downdraft • Flexible adaptation of gas production to load • Low sensitivity to charcoal dust and tar content of fuel • Design tends to be tall - not feasible for very small particle size of fuel 3 Cross-draft • Short design height • Fast response time to load • Flexible gas production • Very high sensitivity to slag formation • High pressure drop COMPARISON ON THE PERFORMANCE OF GASIFIERS
- 26. THANK YOU