Article pubs.acs.org/EF
Mechanistic Investigation into Bed Agglomeration during Biomass Fast Pyrolysis in a Fluidized-Bed Reactor Alan Burton and Hongwei Wu* Department of Chemical Engineering and Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia ABSTRACT: This paper demonstrates that during the pyrolysis of mallee leaf (355−500 μm) in a fluidized-bed reactor (bed materials: silica sand, 125−355 μm) at 300−700 °C, bed agglomeration takes place due to the formation of char−char and/or char−sand agglomerates connected by carbon-enriched necks. There are two types of bed agglomeration: one formed due to solvent-soluble organic matter which dissembles upon solvent washing and the other due to solvent-insoluble organic matter produced from biomass pyrolysis. The yield of each type of bed agglomeration is broadly proportional to the yield of the corresponding type of organic matter in the bed samples. The total yield of bed agglomeration decreases with increasing pyrolysis temperature, from 16.5% at 300 °C to 9.5% at 500 °C and 1.8% at 700 °C. The distribution of the two types of bed agglomeration is also strongly temperature dependent. At low temperatures (e.g., 300 °C), bed agglomeration is dominantly contributed by those formed by solvent-insoluble organic matter. As pyrolysis temperature increases, bed agglomeration due to solvent-soluble organic matter becomes increasingly important and reaches a maximum at 500 °C. At pyrolysis temperatures above 600 °C, there is a drastic reduction in the bed agglomeration formed by solvent-soluble organic matter due to thermal cracking so that bed agglomeration is again dominantly formed by solvent-insoluble organic matter. Overall, bed agglomeration during biomass pyrolysis in a fluidized-bed reactor is due to the production of sticky agents, including both partially molten pyrolyzing biomass particles and the organic matter (both solvent- soluble and -insoluble) produced from biomass pyrolysis reactions.
1. INTRODUCTION Renewable energy sources, particularly biomass, are becoming increasingly important for future supply of renewable liquid fuels and green chemicals, energy security, and sustainable development.1−4 Fast pyrolysis,5−11 e.g. those deploying fluidized-bed reactors, is an attractive thermochemical technology for converting biomass into bio-oil and biochar. In recent years, there has been also extensive research activities on bio-oil related topics, e.g. bio-oil characterization,12−21 selective condensation of bio-oil,22−24 bio-oil derived high-energydensity fuels,25−29 bio-oil upgrading,30−37 and bio-oil refining,38−44 etc., just to list a few. During pyrolysis, it was observed experimentally that cellulose/biomass particles may experience a melting process upon rapid heating.45,46 Such a phenomenon may have significant implications for the reactor design and operation of a fluidized-bed biomass pyrolysis reactor because the melted particles may likely act as sticky agents that potentially lead to bed agglomeration. Unfortunately, while there has been extensive research on ash agglomeration during biomass combustion or gasification (typically at temperatures >700 °C),47−50 there has been little research work focusing on investigating bed agglomeration during biomass pyrolysis in the open literature. Therefore, it is the objective of this paper to carry out a series of experiments to investigate bed agglomeration during biomass fast pyrolysis in a laboratory-scale fluidized-bed reactor. In this study, mallee biomass, which is known to be an important second-generation biomass feedstock in Australia,51−55 is selected for experiments while silica sand is used as bed materials in the fluidized-bed pyrolysis reactor. The research © 2012 American Chemical Society
program is designed to obtain the direct experimental evidence on bed agglomeration and the essential data on the nature and characteristics of bed agglomerates formed under various pyrolysis conditions. On the basis of the experimental evidence and data, the paper further discusses the fundamental mechanisms responsible for bed agglomeration during biomass pyrolysis in a fluidized-bed reactor.
2. EXPERIMENTAL SECTION 2.1. Materials and Samples. Green mallee trees were harvested and collected from the field in Narrogin, Western Australia, then separated into individual components of wood, leaf, and bark. The leaf component was air-dried, cut and then sieved to prepare a size fraction of 355−500 μm that was used in this study. The properties of the biomass sample (mallee leaf) are given in Table 1. Silica sand was purchased, sieved to the size fraction of 125−355 μm, which was then used as the bed material in biomass pyrolysis experiments in a fludizedbed pyrolysis reactor. Chemical compositions analysis (followed by a previous method56) shows that the silica sand has a high purity (>99.5%). 2.2. Biomass Pyrolysis in a Fluidized-Bed Pyrolysis Reactor. Biomass pyrolysis experiments were conducted using a laboratory-scale quartz fluidized-bed pyrolysis reactor (ID: 40 mm), which was housed in an electrically heated furnace. The pyrolysis temperature is controlled by a thermocouple inserted into the bed of the pyrolysis reactor. Briefly, 20 g of silica sand (size: 125−355 μm) was used as the bed material and argon gas was used as fluidization gas for the fluidized-bed reactor. Biomass particles (size: 355−500 μm) were fed Received: March 6, 2012 Revised: May 7, 2012 Published: October 17, 2012 6979
dx.doi.org/10.1021/ef300406k | Energy Fuels 2012, 26, 6979−6987
Energy & Fuels
Article
Table 1. Properties of the Biomass Sample (Mallee Leaf) Used in This Study proximate analysis, wt % ar
a
moisture
ash
VMa
5.2
3.8
69.7
ultimate analysis, wt % daf FCb
H
N
S
Oc
21.3 55.3 7.2 contents of inorganic species, wt % db
2.3
0.9
34.3
C
Na
K
Mg
Ca
Si
Al
Fe
Ti
P
S
Sr
Ba
0.655
0.534
0.138
0.767
0.053
0.032
0.014
0.002
0.120
0.065
0.009
0.001
Volatile matter. bFixed carbon. cBy difference.
Figure 1. Particle size distribution of total materials in the bed after biomass fast pyrolysis (pyrolysis temperature 300−700 °C; feeding time 4 min; holding time 15 min; sand size 125−355 μm; biomass particle size 355−500 μm): (A) before solvent washing; (B) after solvent washing. argon continuously flowing through the bed until the bed temperature was below 50 °C. All materials in the bed were then collected from the reactor for subsequent characterization, referred to as “bed samples” hereafter. 2.3. Sample Characterization. To study the bed agglomeration phenomenon, the mass particle size distributions were determined for biomass feed, silica sand, and the bed samples collected after biomass pyrolysis experiments under various conditions. Each sample was sieved into a series of nine size fractions, i.e. 500 μm in the bed samples collected after biomass pyrolysis must be due to bed agglomeration. Indeed, Figure 1A clearly demonstrates that depending on pyrolysis temperature, there are various amounts (1.8−16.5 wt %) of the materials in the samples collected after biomass pyrolysis have particle sizes >500 μm. There are also very small but appreciable quantities (500 μm must be deemed as bed agglomerates and (b) there are minimal amounts (
