Physical Aspect On Biomass Pyrolysis

From a thermal standpoint, we may divide the pyrolysis process into four stages. Although divided by temperature, the boundaries between them are not sharp; there is always some overlap :

-Drying (~100 oC). During the initial phase of biomass heating at low temperature, the free moisture and some loosely bound water is released. The free moisture evaporates, and the heat then conducted into the biomass interior.
-Initial Stage (100-300 oC). In this stage, exothermic dehydration of the biomass take place with the release of water and low-molecular-weight like CO and CO2.
-Intermediate Stage (>200 oC). This is primary pyrolysis, and it takes place in the temperature range of 200 to 600 oC. Most of the vapor or precursor to bio-oil is produced at this stage. Large molecules of biomass particles decompose into char (primary char),  condensable gases (vapors and precursors of the liquid yield), and noncondensable gases.
-Final Stage (~300-900 oC). The final stage of pyrolysis involves secondary cracking of volatiles into char and noncondensable gases. If they reside in the biomass long enough, relatively large-molecular-weight condensable gases crack, yielding additional char (called secondary char) and gases. This stage typically occurs above 300 oC (Reed, 2002, P.III-6). The condensable gases, if removed quickly from reaction site, condense outside in the downstream reactor as tar or bio-oil. It is apparent from figure below that a higher pyrolysis temperature favor production of hydrogen, which increase quickly above 600 oC. An additional contribution of shift reaction further increase the hydrogen yield above 900 oC, in which typically used in gasification process since biomass pyrolysis use in lower temperature than gasification (range 400-600 oC).

Shift Reaction

Releses of Gases During Pyrolysis of Wood

Temperature has a major influence on the product of pyrolysis. The carbondioxide yield is high at lower temperature and decrease at higher temperature. The release of hydrocarbon gases peaks at around 450 oC and then starts decreasing above 500 C, boosting the generation of hydrogen.

Hot char particles can catalyze the primary cracking of the vapor released within biomass particle and the secondary cracking occurring outside the particle but inside the reactor. To avoid cracking of condensable gases and thereby increase the liquid yield, rapid removal of the condensable vapor is very important. The shorter the residence time of the condensable gas in the reactor, the less the secondary cracking and hence the higher the liquid yield. There’s always see the market demand of biomass pyrolysis products to meet that need. To see how our continous pyrolysis plant can do this, please click here and to find what real application on biomass pyrolysis for South East region please read the all articles in this blog.