(617hq) Calcium Oxide As in-Situ Catalyst for Steam Gasification of Bio-Char

Authors: 
Weiss-Hortala, E., Mines Albi, CNRS, Centre RAPSODEE, Univ. Toulouse
Dong, J., Mines Albi, CNRS, Centre RAPSODEE, Université de Toulouse
Nzihou, A., Mines Albi
Ducousso, M., Mines Albi, CNRS, Centre RAPSODEE, Université de Toulouse
With increasing demand for energy and the environmental impact from fossil fuel use, energy recovery from waste biomass such as forestry residue and agricultural waste has attracted increasing attentions. Thermal treatment is the most commonly used conversion method to upgrade energy quality from biomass, among which pyrolysis is regarded as an economical and promising process to reach high efficiency and low emission. By pyrolysis, biomass can be converted into bio-oil, bio-char and gaseous fraction. As a by-product of biomass pyrolysis, bio-char can be burned for energy recovery, prepared as activated carbon, or soil amendment. Recently, gasification of bio-char has gained increasing interest. With steam introduced as gasification agent, bio-char exhibits effective ability to produce syngas with a high hydrogen content, which is regarded as a clean, low carbon future source of energy.

Calcium oxide (CaO) has been proven to be an effective and economical in-situ catalyst in biomass gasification, in order to further improve hydrogen production from syngas. The loaded CaO to the gasifier can take effect via several pathways. Firstly, CaO can act as sorbent for CO2 absorption to form CaCO3. Following the decrease in CO2 partial pressure, the water gas shift reaction is enhanced towards hydrogen production. Secondly, since the CaO carbonation reaction is exothermic, the releasing heat could also provide necessary energy to facilitate the endothermic gasification process. Besides, tar formation is a main issue during biomass gasification due to the fact that it may decrease the system efficiency and even block and damage downstream gasifier. Another potential advantage of the CaO additives is that it could catalytically contribute to tar cracking and accelerate the reaction between tar and H2O, therefore significantly reduce the tar content of the product gas from a gasifier. The tar cracking could result in gas yield increase.

Accordingly, in this study, steam gasification of bio-char in a fluidized bed reactor has been investigated, using CaO as in-bed catalyst. The purpose of the study is to evaluate the catalytic gasification performance of bio-char for hydrogen-rich gas production and tar removal. The influence of CaO/wood mass ratio, steam flowrate and reaction temperature is studied to search for an optimal working condition. Results reveal that increasing the ratio of CaO/wood is effective to enhance H2 concentration and yield in the syngas. Simultaneously the tar yield is reduced remarkably due to the enhanced reforming reactions. Increasing the steam flowrate is found to be beneficial to shift the water gas shift reactions and improve the reactivity of CaO towards H2 production. An excessive steam injection leads to an opposite effect, and a maximum H2 output is achieved at steam flowrate of 160 g/h. Temperature also plays a great role on the yield and product composition. A higher temperature enhances the H2 production and tar reduction. In conclusion, the results indicate that there is a strong potential for producing hydrogen-rich gas from bio-char by catalytic steam gasification with inexpensive and abundant CaO as catalyst. The data obtained can be served as leverage for future development on biomass to energy field.