Kinetic study on co-gasification of coal and biomass
Thermal co-processing of coal and biomass has been increasingly focused for its environmental and economic benefits. In the present work, the experimental and kinetic study on co-pyrolysis and co-gasification of Rhenish brown coal (HKN) and wheat straw (WS) was made. The pyrolysis behavior, especial...
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| Format: | Doctoral Thesis |
| Language: | English |
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Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola"
2014
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| Online Access: | http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-154403 http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-154403 |
| Summary: | Thermal co-processing of coal and biomass has been increasingly focused for its environmental and economic benefits. In the present work, the experimental and kinetic study on co-pyrolysis and co-gasification of Rhenish brown coal (HKN) and wheat straw (WS) was made.
The pyrolysis behavior, especially for co-pyrolysis, was investigated in a thermogravimetric analyzer (TGA) and a small fixed bed reactor (LPA). In TGA, the mass loss and reaction rate of single and blend samples were studied under various experimental conditions, and their effects on synergy effects. The synergy effects on products yield and properties of chars were studied in LPA. The kinetics of pyrolysis was obtained based on data from TGA by using the Coats-Redfern method. For gasification with CO2, a small fixed bed reactor (quartz glass reactor), equipped with an online GC to monitor the gas composition, was used. The effects of processing conditions on gasification behavior and synergy effects for mixed chars and co-pyrolysis chars were investigated. The volume reaction model (VRM), shrinking core model (SCM) and random pore model (RPM), were applied to fit the experimental data. The model best fitting the experiments was used to calculate the kinetic parameters. The reaction orders of gasification reactions with single chars are also investigated.
The pyrolysis study showed that a small amount of wheat straw added to the brown coal promoted the decomposition better and showed more significant synergy effects. The synergy effects varied with increasing heating rates and pressures, especially at 40 bar. The kinetic parameters were inconsistent with experimental behavior during co-pyrolysis, since the reaction was also affected by heat transfer, contact time, particles distribution and so on. The gasification study on single chars showed that Rhenish brown coal chars had higher reactivity; chars pyrolyzed at higher temperatures showed lower reactivity; and higher gasification temperatures and CO2 partial pressures led to higher reactivity. For co-gasification process, there was no significant synergy effect for mixed chars. However, negative synergy effects (reactivity decreased compared to the calculated values based on rule of mixing) were observed for co-pyrolysis chars, caused by properties change by co-pyrolysis process. For kinetics, the reaction orders of chars ranged from 0.3 to 0.7. Only random pore model fitted most experiments at low and high temperatures. Synergy effects were also observed in kinetic parameters. The values of activation energy E and pre-exponential factor A for mixed chars and co-pyrolysis chars were lower than expected. The negative synergy effects showed the pre-exponential factor A had more effects. However, the higher reactivity of mixed chars than co-pyrolysis chars showed that the reaction was affected more by activation energy E. Therefore, only investigating E or A value was not enough. In addition, a marked compensation effect between activation energies and pre-exponential factors was found in the present study. The isokinetic temperature for the present study was 856 °C. This was close to the temperature at which the gasification reaction transforms from the chemical controlled zone to the diffusion controlled zone for most chars.
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