[Gasification] reducing temp. of gasification

Shah, Naresh naresh.shah at uky.edu
Mon Sep 12 10:25:29 CDT 2011

What you are describing are two different processes.
 When you heat biomass in absence of oxygen, you are in a purely pyrolysis zone and all the gases produced - CO, CO2, H2, H2O, CH4, C2H6, etc. are results of breaking of bonds in the biomass.  As the temperature increases, there is more energy available to break more bonds and you get more gas.  A closed vessel pyrolysis will produce large amount of char as it is very difficult to break C-C bonds.

When you blow oxidant (air) in the reactor, it can react with the biomass.  This reaction has two effects: (1) Since oxidation is exothermic reaction, temperature of the charge increases, and (2) oxidant (air) also chemically reacts reaction with  biomass and it is quite effective in breaking C-C bonds to produce additional CO and CO2 gases at high enough temperatures (>700 C) CO2 can also react with biomass to produce additional CO.  This is the main reason for increasing the temperature of gasification. Low temperature gasification, even with the use of catalysts, cannot overcome this thermodynamic limitation.

To minimize nitrogen dilution in the product gas stream, the easiest solution is to feed oxygen instead of air.  This solution can be a big challenge and impractical in small scale gasifiers.  To control the process temperature of a closed vessel and to supply heat from outside, one can use either externally heated pyrolyzer like coke oven or externally/internally heated fluidized bed reactors.

Use of catalysts in chemical reactions is only to reduce activation energy barriers.

-Naresh Shah

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