ARTICLE pubs.acs.org/EF
Changes in Char Structure during the Gasification of Mallee Wood: Effects of Particle Size and Steam Supply Shu Zhang,†,‡ Zhenhua Min,† Hui-Ling Tay,† Yi Wang,† Li Dong,† and Chun-Zhu Li*,†,‡ † ‡
Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia ABSTRACT: The purpose of this study is to investigate the effects of the biomass particle size on the changes in char structure during the gasification in steam of mallee woody biomass. The experiments with continuous or sudden supply of steam were conducted using a fluidized-bed/fixed-bed quartz reactor. The structural features of chars were then characterized using Fourier transform (FT)-Raman spectroscopy. Our results indicate that the effects of the biomass particle size on char structure were minimal when the biomass was heated slowly while being in continuous contact with steam. On the contrary, when the biomass was first pyrolyzed in argon and then suddenly came into contact with steam, the biomass particle size could have a profound effect on the subsequent evolution of char structure during gasification. For the large biomass particles (>5 mm), the ratio between small and big aromatic ring systems in the char showed a maximum value after around 30 s of contact with steam.
1. INTRODUCTION The changes in char structure are an important aspect of the evolution of char reactivity during the gasification of biomass and coal.1 10 However, the characterization of char structure has always been a challenge because chars do not normally contain well-defined graphite crystals11 but consist mainly of amorphous carbons and fused aromatic ring systems of various sizes.11 Therefore, X-ray diffraction (XRD) would give very limited direct information about char structure. Attempts12 18 to characterize coal/char structure using Raman spectroscopy based on the procedure proposed by Tuinstra and Koenig19 have not been very successful because the procedure originated from the study on graphite. We have recently developed a new Raman spectroscopic approach11 to characterize the chars from coal and biomass. The broad spectra for all chars could be successfully deconvoluted/ curve-fitted, with 10 bands representing the typical structures in the chars.1,3,4,11 This method provides a new way to evaluate the structural features of char by considering the special features of char structure without the need to assume the presence of graphite-like structures in the chars produced from the thermal treatment of (low-rank) fuels at low temperatures. With our Fourier transform (FT)-Raman spectroscopic method, we have shown6 that contacting the chars from the pyrolysis of cane trash with steam for a very short period of time (20 s) could result in drastic changes in char structure. This is believed to be due to the rapid penetration of H radicals from the char surface deep into the char matrix.1 The H radicals then initiated/enhanced the condensation of aromatic ring systems within the char matrix. Our study thus indicated that biomass char could be extremely reactive and susceptible to changes in reaction conditions. The feed to a practical gasifier always consists of particles of a wide range of sizes and shapes. The gasification rates of larger particles are always the prime concern in the design and operation of a commercial gasifier. Understanding the gasification behavior of large particles is therefore an important aspect of gasification technology development. An important aspect of this r 2011 American Chemical Society
gasification behavior is the evolution of char structure. Insufficient data are available in this area. In particular, the penetration of H radicals across the char matrix cannot be predicted on the basis of the existing theory of mass transfer, e.g., gas diffusion. It remains unclear how the biomass particle size might affect the penetration of H radicals into the char matrix. The penetration of H radicals would also likely be affected by the char structure itself. The conditions under which a char is prepared are clearly important factors influencing the structure of the resulting char. In particular, the presence of reactive gases during the preparation of char (i.e., the pyrolysis in inert gas versus the gasification in a reactive atmosphere) would be a key factor determining the char structure. Little is currently known about the possible differences in the evolution of char structure during gasification between the chars prepared under pyrolysis conditions and the chars prepared under partial gasification conditions. This study aims to examine the evolution of char structure during gasification in steam as a function of the biomass particle size and presence/absence of steam when the char is prepared. Following the pyrolysis and gasification of mallee wood at a slow heating rate, the char structural features were characterized with our new FT-Raman spectroscopic method.11 Our results revealed some complicated effects of the biomass particle size on the char structural features during gasification.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Mallee woody biomass grown in Western Australia to combat the dryland salinity20,21 was used in this study. The mallee wood was debarked and then crushed to obtain particles in Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: October 20, 2011 Published: October 20, 2011 193
dx.doi.org/10.1021/ef2011589 | Energy Fuels 2012, 26, 193–198
Energy & Fuels
ARTICLE
the size range between 4.75 and 5.60 mm. After drying at 105 °C overnight under the atmosphere of nitrogen, this sample of 4.75 5.60 mm particle sizes was further pulverized in a cutter and sieved to obtain eight samples of different particle size ranges. This procedure of sample preparation would have minimized the property differences among the samples of different particle size ranges. All of the particle sizes used in this experiment were as follows: 0.90 0.18, 0.18 0.43, 0.43 0.60, 0.60 1.00, 1.00 2.00, 2.00 3.35, 3.35 4.00, 4.00 4.75, and 4.75 5.60 mm. It should be emphasized that the particle shapes were far from being spherical and that the sizes given here refer to the openings of the sieves used to prepare these samples. The average of the upper and lower limits for each particle size fraction was used as the average particle size to plot the data in this paper. The proximate and ultimate analyses of the wood sample22 used in this study were 0.9 wt % (dry) ash and 81.6 wt % [dry and ash free (daf)] volatile matter yield together with 48.2 wt % C, 6.1 wt % H, and 0.2 wt % N (daf).
Figure 1. Typical example of Raman spectrum fitted with 10 bands.5,11,24 The char was prepared from the largest particle (5.175 mm) pyrolysis with a 15 min holding plus a 30 min gasification in steam at 800 °C.
2.2. Gasification in the Continuous and Sudden Supply of Steam. Two types of gasification experiments were carried out, differing in the way steam was supplied as the gasifying agent. The first type was the gasification of biomass in the continuous supply of 15% steam in a modified one-stage quartz fluidized-bed/fixed-bed reactor23 heated with an external furnace at a slow heating rate of 10 K min 1. Briefly, the reactor differs from a normal fluidized bed in that a frit was installed in the freeboard. The frit allowed the gas (including volatiles) to pass through but retained the solid char particles within the reactor for in situ gasification. Around 5 g of biomass particles (accurately weighed) were loaded into the sand bed before the reactor was heated. Once the reactor reached 300 °C, water was pumped continuously with a highperformance liquid chromatography (HPLC) pump into the reactor (underneath the gas distribution frit supporting the fluidized bed of sand) to generate steam at a steam concentration of 15% (by volume) of the total income gas flow. The 15% steam in argon was continuously present in the reactor during the subsequent heating and holding time (15 min) at the target temperature. After the holding time, the steam supply was terminated and the reactor was lifted out of the furnace immediately to be cooled naturally with argon continuously flowing through the reactor. The second type of experiment was also carried out in the same reactor. The reactor preloaded with ∼5 g of biomass (accurately weighed) was first heated to the prescribed temperatures of 700 °C (or 800 °C) in the atmosphere of argon (pyrolysis condition) at 10 K min 1. The reactor was then held at the desired temperature for 0 or 15 min before a stream of steam was suddenly introduced. The experiment was also terminated in the same way, i.e., by lifting the reactor out of the furnace once the reaction time with steam was reached. The error for the reaction time could be well within 2 s. At the end of each experiment, the char was collected for further analysis by FT-Raman spectroscopy. 2.3. Characterization of the Char Structure. The Raman spectra of chars were acquired using a Perkin Elmer spectrum GX FTIR/Raman spectrometer equipped with an excitation laser of 1064 nm.11 Briefly, the chars were mixed with spectroscopic-grade KBr (0.5 wt %) and then co-ground into very fine particles. A back-scattering configuration with an InGaAs detector was used to collect the spectra. The spectra were then deconvoluted into 10 Gian bands,5,11,24 as shown in Figure 1. Of the 10 bands, G, GR, VL, VR, D, and S bands were always the key bands for all chars investigated. The total peak area and the band area ratios between different bands were used to trace the changes in char structure during the gasification in steam.
Figure 2. Biomass conversion as a function of the biomass particle size and final temperature as the biomass particles were heated in 15% steam at 10 K min 1 to the final temperature indicated with a 15 min holding time.
biomass particle size and final temperature for a holding time of 15 min. As expected, for each given particle size, the gasification conversion increased with the final temperature. At 900 and 950 °C, all biomass was practically gasified completely, regardless of the particle size. For the final temperatures of 600, 700, and 800 °C, biomass conversion decreased by about 5 wt % as the average biomass particle size increased from 0.135 to 5.175 mm. The wood biomass sample used in this study has a rather high volatile matter yield of >80 wt %. Substantial amounts of volatiles must have been released as the biomass particles were heated. For a large biomass particle, the volatiles released from the inner portion of the particle must pass through a porous medium of pyrolyzing biomass particle to be released. Part of volatiles could condense and/or polymerize onto the pore surface of pyrolyzing biomass/char particles, particularly considering that the pyrolyzing biomass/char would contain rich radical sites. Because the major volatile release would take place at relatively low temperatures25 27 (5 mm), the ratio of small to large aromatic ring systems went through a maximum with 30 s of contact of char with steam at 800 °C. No such maximum was observed for the small biomass particles (
