Elsevier

Bioresource Technology

Volume 95, Issue 1, October 2004, Pages 95-101
Bioresource Technology

An experimental study on biomass air–steam gasification in a fluidized bed

https://doi.org/10.1016/j.biortech.2004.02.003Get rights and content

Abstract

The characteristics of biomass air–steam gasification in a fluidized bed are studied in this paper. A series of experiments have been performed to investigate the effects of reactor temperature, steam to biomass ratio (S/B), equivalence ratio (ER) and biomass particle size on gas composition, gas yield, steam decomposition, low heating value (LHV) and carbon conversion efficiency. Over the ranges of the experimental conditions used, the fuel gas yield varied between 1.43 and 2.57 N m3/kg biomass and the LHV of the fuel gas was between 6741 and 9143 kJ/N m3. The results showed that higher temperature contributed to more hydrogen production, but too high a temperature lowered gas heating value. The LHV of fuel gas decreased with ER. Compared with biomass air gasification, the introduction of steam improved gas quality. However, excessive steam would lower gasification temperature and so degrade fuel gas quality. It was also shown that a smaller particle was more favorable for higher gas LHV and yield.

Introduction

Biomass is potentially an attractive feedstock for producing transportation fuels as its use contributes little or no net carbon dioxide to the atmosphere. Renewable biomass resources include short-rotation woody crops, herbaceous biomass, and agricultural residues. Thermochemical gasification of biomass is a well-known technology that can be classified depending on the gasifying agent: air, steam, steam–oxygen, air–steam, O2-enriched air, etc.

The technology of biomass air gasification seems to have a feasible application and has been developed actively for industrial applications. However this technology produces a gas with a low heating value (4–6 MJ/m3) and an 8–14 vol.% H2 content only (Delgado and Aznar, 1997). Biomass oxygen-rich air gasification is one effective way of producing medium heating value (MHV) gas, but it needs a large investment for oxygen production equipment and this disadvantage impedes its popularization. Extensive experimental studies reported in the literature (Delgado et al., 1996; Aznar et al., 1998; Gil et al., 1999; Rapagnà et al., 2000; Courson et al., 2000; Schuster et al., 2001; Mathieu and Dubuisson, 2002) show that fluidized-bed, steam-gasification processes (with or without O2 added) are also capable of producing a MHV (10–16 MJ/N m3) gas with a 30–60 vol.% H2 content. However, this technology requires that the temperature of steam be over 700 °C, which demands additional cost for steam generator of good performance (Wu et al., 1995).

Under this background, the technology of biomass air gasification with low temperature steam was put forward from the economic point of view. Since the steam gasification reactions are endothermic as a whole, the process must be supplied with energy. This can be done by partial combustion of biomass within the gasifier using a hypostoichiometric amount of air.

Sadaka et al., 2002a, Sadaka et al., 2002b, Sadaka et al., 2002c developed a two-phase dynamic finite element model to simulate a self-sustained biomass air–steam gasification process. They also performed a sensitivity analysis on this model and validate the model using the experimental results. A good agreement between the model predictions and experimental data was obtained under their operating conditions. Although the model given by Sadaka et al. is valuable, further studies are needed to explore the mechanism of thermodynamic reactions that occur in the process and analyze the tests results to supply useful information for better performance of biomass air–steam gasification.

In the present work, a small scale fluidized bed was developed and low temperature steam (154 °C) and air were used as the biomass gasification agent to explore the effects of some critical parameters on gasification performance.

Section snippets

Feed materials

With silica sand (particle size 0.2–0.3 mm) as bed material, the pine sawdust obtained from a local timber mill was used as the feedstock. Four size ranges of pine sawdust (0.6–0.9, 0.45–0.6, 0.3–0.45, and 0.2–0.3 mm, respectively) were selected for tests. The proximate and ultimate analyses of the biomass were reported in Table 1. The formula of CH1.7O0.6 was calculated from the ultimate analysis of biomass.

Facility

The experimental set-up, shown in Fig. 1, consists of six main parts: (i) a fluidized

Effect of reactor temperature

Temperature is crucial for the overall biomass gasification process. In the present work, reactor temperature was varied from 700 to 900 °C in 50 °C increments. The test results are presented in Table 2 and Fig. 2.

From Fig. 2, it can be seen that H2 concentration increased with temperature and the content of CH4 showed an opposite trend. According to Le Chatelier's principle, higher temperatures favor the reactants in exothermic reactions and favor the products in endothermic reactions.

Conclusions

Parametric tests, varying the temperature, equivalence ratio, steam to biomass ratio and biomass particle size, have been performed to determine their effects on product gas composition, gas yields, gas LHV, carbon conversion efficiency and steam decomposition. Over the range of operating conditions tested, gas yield varied from 1.43 to 2.57 N m3/kg biomass; gas LHV ranged between 6741 and 9143 kJ/N m3; carbon conversion efficiency varied from 68.67% to 95.10%; and steam decomposition ranged

Acknowledgements

The financial support received from the National Natural Science Foundation of China (project no. 20206031), Guangdong Province Natural Science Foundation (project no. 010876) and “One-hundred-scientist Programme” of Chinese Academy of Sciences (J. Chang) is gratefully appreciated.

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