Steam Gasification of Cottonwood (Branches) in A Fluidized Bed

Satish K. Singh, Walter P. Walawender, L. T. Fan, Wayne A. Geyer

Abstract


Cottonwood (branches) were gasified in the presence of steam in a 5.08 cm I.D. bench-scale fluidized bed reactor, over a temperature range of 850 to 1,075 K. The objective of the study was to investigate the effect of reactor temperature on the produced gas composition, yield, heating value, energy recovery, and carbon conversion. The results were compared with data obtained from alpha-cellulose in the same reactor to gain insight into the secondary gas phase reactions that occur in biomass gasification.

The major components of the produced gas were CO, CO2, H2, and CH4, which comprised over 90% of the gas. The gas yield, energy recovery, and carbon conversion all increased with increasing temperature: from 0.23 to 1.59 m3/kg dry ash-free feed, 15.0 to 96.8%, and 16.9 to 87.1%, respectively. The higher heating value of the gas ranged from 12.4 to 11.6 MJ/m3. The dependence of the gas composition on temperature indicated the existence of two regimes for the secondary gasification reactions with a transition occurring over the temperature range of 930-950 K. The first regime (below this temperature range) was dominated mainly by the cracking of cellulose tar. The extent of lignin tar cracking was small compared to that of cellulose tar in this regime. The second regime was dominated by the water gas shift reaction. Cellulose tar cracking was essentially complete in the second regime, and the relative amount of lignin tar cracking was small. Thus the gas composition was primarily determined by the shift reaction in the second regime. The observed behavior for wood gasification was similar to that for alpha-cellulose, although the transition from one regime to the other was not as sharp. Steam, when present in large amounts, was found to be an active gasification agent in the second regime.


Keywords


Cottonwood;steam gasification;fluidized bed;temperature dependence;tar cracking;shift reaction

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