A Comparison of the Pyrolysis of Olive Kernel Biomass in Fluidised and Fixed Bed Conditions
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The use of thermogravimetric analysis to describe biomass kinetics often uses bench top thermogravimetric analyser (TGA) analysers which are only capable of low heating rates. The aim of this research was to compare experimental fast pyrolysis of Olive kernels in a bespoke laboratory thermogravimetric fluidised bed reactor (TGFBR) characterised by rapid heating rates at high flow rates, compared to a smaller bench scale fixed bed TGA system. The pyrolysis in the TGFBR was analysed by using the isothermal kinetic approach and it was theorised that the pyrolysis decomposition reactions occurred by two mechanisms depending on the temperature, resulting in an activation energy of 67.4 kJ/mol at temperatures below <500 °C and 60.8 kJ/mol at temperatures >500 °C. For comparison, a bench scale TGA was used to look at the thermal behaviour in different fixed bed thermal conditions giving a higher activation energy of 74.4 kJ/mol due to the effect of external particle gas diffusion. The effect of biomass particle size (0.3–4.0 mm) on the conversion of biomass at different temperatures, was investigated between 300 and 660 °C in the TGFBR. The results suggested inhibition of internal gas diffusion was more important at lower temperatures, but in comparison had no significant effect when measured in the fixed bed TGA at lower heating rates. Bench top TGA analysis of pyrolysis is a rapid and valuable method, but is limited by smaller sample sizes and lower heating rates. In comparison, the conditions encountered with the laboratory scale TGFBR are more likely to be relevant to larger scale systems where heat distribution, heat transfer and mass diffusion effects play major roles in the reactivity of biomass.
KeywordsBiomass Pyrolysis Processes Fluidised bed Olive kernel Renewable
Olive kernels are a co-product residue of agricultural activity in the Mediterranean region. Greece has the third largest Olive oil production industry in the world. About 400,000 tons of Olive kernels are produced annually . Olive kernel is already exploited as a low cost solid biomass fuel (0.046 £/kg), and is mostly utilized for conventional combustion. However, Olive kernel has not yet utilised its full potential as alternative biofuel . There is limited scientific research concerning the comparison of experimental fast pyrolysis of Olive kernels in fluidised bed and fixed bed systems, hence experimentation with this kind of biomass is of great interest.
Biomass pyrolysis is a viable route to produce renewable bio-oil and includes fundamental chemical reactions that are precursors of other thermal conversion technologies, such as combustion and gasification. Therefore, the study of the pyrolytic characteristics of biomass covers a key issue in demand for an advancement of biomass thermal conversion technologies.
There are two processes for biomass pyrolysis, slow pyrolysis and fast pyrolysis. Slow pyrolysis has been used for the production of charcoal, while fast pyrolysis has generally been used to obtain liquid products. Fast pyrolysis is described by high heating rates and rapid quenching of bio-oil products to terminate the secondary conversion of the products .
The pyrolysis characteristics of biomass, in particular measuring and deriving reaction kinetics, has been carried out using thermogravimetric analysis (TGA) by other researchers. This is normally undertaken via the measurement of variation of mass loss with time of a sample held in the TGA cell at a preset heating rate [4, 5, 6, 7]. Other work has involved the development of reactors at a larger scales to determine the kinetics of biomass pyrolysis with larger sample sizes, determining conversion through the analysis of the yield of gas [8, 9, 10, 11].
Traditional TGA can be considered to be a fixed bed technique with a relatively low heating rate compared to larger scale systems where biomass is added directly in the reactor at the reaction temperature so the particle heating rate is significantly greater. Meanwhile, the chemical processes in TGA are affected by the interfacial gas diffusion between the reactor space and the solid sample inside the TGA cell . Other authors have noted the effect of the heating rate on the reaction kinetics in a TGA, which limits how comparable these results are with high heating rate systems such as fluidised bed or circulating bed gasifiers . The kinetic parameters represented by the order of reaction or the activation energy may be so misleading that, if used in scaling up, it may result in problems with plant operation. It is therefore essential to make a careful study of the interaction to eliminate physical effects from purely chemical processes . The pyrolysis of biomass involves the transport of gas and heat from the external bulk gas phase to the internal particle surface, where the chemical reactions take place. Moreover, the intrinsic rate, i.e. the rate of the chemical reaction step, free from heat and mass transfer limitations, is of considerable importance. Therefore, it is valuable to develop an apparatus for the kinetic study of biomass pyrolysis that has the same principle of measurement, but it is dealing with a fluidised bed and operating under a fast heating rate regime.
The thermogravimetric fluidised bed reactor (TGFBR), designed and fabricated in the School of Engineering at Cardiff University, used in the present study was capable of using larger sample sizes up to 60 g per run, compared with up to 20 mg in the bench scale TGA. Furthermore, the reactor operates under isothermal conditions by using impervious alumina porcelain (IAP) as a heat transfer medium in the preheater. This paper compares the effects of particle size and temperature on pyrolysis kinetics under fixed bed conditions using a conventional bench scale TGA and under fluidised bed conditions using a novel thermogravimetric fluidisation system (TGFBR) equipped with built-in load cells for the dynamic measurement of biomass conversion. The aim of this work was to investigate the influence of heating rates and heat/mass transfer effects on the kinetic analyses of the results obtained in these different systems to describe and understand the importance of the bed conditions on the effect of biomass pyrolysis.
The kinetic study attempts to demonstrate how the thermal decomposition occurs by finding the best kinetic model that fits and describes the mechanism of the reaction to determine the kinetic parameters. This is crucial to the design, build and operation of a large scale industrial reactor for the Olive kernel biomass.
Typical reaction mechanisms for heterogeneous solid–state reaction
One-dimensional diffusion, 1D
Two-dimensional diffusion (Valensi)
[−ln(1 − x)]−1
x + (1 − x)ln(1 − x)
Three-dimensional diffusion (Jander)
1.5(1 − x)2/3[1 − (1 − x)1/3]−1
[1 − (1 − x)1/3]2
Three-dimensional diffusion (G–B)
1.5[1 − (1 − x)1/3]−1
1 − 2x/3 − (1 − x)2/3
Three-dimensional diffusion (A-J)
1.5(1 + x)2/3[(1 + x)1/3 − 1]−1
[(1 + x)1/3 − 1]2
Nucleation and growth (n = 2/3)
1.5(1 − x)[−ln(1 − x)]1/3
[−ln(1 − x)]2/3
Nucleation and growth (n = 1/2)
2(1 − x)[−ln(1 − x)]1/2
[−ln(1 − x)]1/2
Nucleation and growth (n = 1/3)
3(1 − x)[−ln(1 − x)]2/3
[−ln(1 − x)]1/3
Nucleation and growth (n = 1/4)
4(1 − x)[−ln(1 − x)]1/3
[−ln(1 − x)]1/4
x(1 − x)
ln[x/(1 − x)]
Mampel power law (n = 1/2)
Mampel power law (n = 1/3)
Mampel power law (n = 1/4)
Chemical reaction (n = 3)
(1 − x)3
[(1 − x)−2 − 1]/2
Chemical reaction (n = 2)
(1 − x)2
(1 − x)−1 − 1
Chemical reaction (n = 1)
1 − x
−ln(1 − x)
Chemical reaction (n = 0)
3(1 − x)2/3
1 − (1 − x)1/3
2(1 − x)1/2
1 − (1 − x)1/2
Thus the terms, ln (G(x)/T2) versus 1/T, were plotted to give a straight line with a slope −E/R since ln (AR/βE), allowing the calculation of the activation energy. The integral model G(x) was substituted from Table 1 into Eq. (9) to determine if it described the reaction depending on its linearity fit using the coefficient of determination (R2).
Olive Kernel Biomass
Olive kernel biomass is a residue of Olive production received as coarse particles less than 5 mm and ground using a laboratory disc mill, model Lm1 pulverising mill. The ground biomass was screened to sizes ranges of 300–500, 500–710, 710–1180 and 1180–1400 µm using sieves.
Proximate analysis and higher heating value of Olive kernel
Particle size (µm)
Proximate analysis (wt%)
Fixed Bed Thermogravimetric Analysis (TGA)
Fluidised Bed Thermogravimetric Pyrolysis Reactor (TGFBR)
The fluidised bed was positioned on a bespoke platform load cell designed by Coventry Scale Company. The balance tolerance was ±0.5 g with a weighing range up to 25 kg. A multifunction weight indicator model DFW06XP was connected to a load cell and through a computer to record the mass change continuously at 1 s time intervals.
Fixed Bed Thermogravimetric Analysis (TGA)
Fluidised Bed Reactor Thermogravimetric Analysis (TGFBR)
The experimental measurements using the TGFBR were achieved at pre-set steady-state temperatures between 300 and 660 °C, covering the chemically controlled regime area of thermal decomposition illustrated in Fig. 3. Prior to pyrolysis, the experimental work was started by heating the reactor to the required temperature ensuring that good bed fluidisation was achieved as determined by the measurement of the bed pressure drop against the superficial velocity of the gas flow. Silica sand was used as the inert fluidised bed material with a diameter of 500–600 µm giving a measured minimum fluidisation velocity (umf) of 0.06 m/s . After that, the air stream was stopped and the nitrogen stream flowed at the minimum fluidisation velocity (umf)  until steady state temperature conditions inside the reactor were obtained. Olive kernel biomass was fed from the top of the reactor through a pipe into the hot fluidised bed as shown in Fig. 1. The amount of biomass used in each test was 40 g representing 10 wt% of the total bed material weight. The weight variation in TGFBR during the pyrolysis process was recorded online with the weighing indicators at 1 s time intervals. According to Choi et al. , bed particles should have terminal velocities larger than the superficial gas velocity to prevent elutriation loss of bed material. Therefore, during this study the superficial velocity was kept much lower than the terminal velocity (0.89 m/s) and accordingly no significant losses of bed material were measured (<0.1 %) with the load cell.
Influence of Nitrogen Flow on Pyrolysis Conversion Rate
A fundamental issue in pyrolysis is the interaction of evolving nascent, hot pyrolysis vapours with the surrounding decomposing solid. The residence time of the vapour phase of pyrolysis products is affected by the nitrogen flow used for fluidisation, which alters the extent of secondary reactions such as cracking and char formation  and improves the heat transfer from fluid gas to the particle.
At the higher temperature of 500 °C, the rate of reaction determined from the slope of the conversion line showed a wide variation up to a velocity of 0.12 m/s (40 l/min), after which a much smaller variation occurred. This critical gas flow velocity represents the flow required to minimise the external diffusion inhibition on the reaction rate . By operating the gas–solid reaction system at sufficiently high gas flow velocity and rate, the mass transfer effects could be minimised so that any further increase in the gas flow rate did not produce an increase in the overall reaction rate . Therefore, a flow velocity of 0.12 m/s (40 l/min) was chosen as the basis for all experimental work, representing the minimum gas velocity required to limit external diffusion.
Effect of Particle Size
Effect of Temperature
There are two types of reaction through which the thermal degradation occurs: a comparatively slow decomposition and charring on heating at lower temperatures <300 °C and a rapid devolatilization accompanied by the formation of levoglucosan from pyrolysis at higher temperatures. At temperatures >302 °C, cellulose and hemicellulose depolymerizes producing volatile products  as shown in Fig. 3. For this reason the significant weight percent change occurring between 300 and 350 °C is likely to be due to the increased devolatilization rate of hemicellulose and cellulose. The char formation decreases with increasing temperature due to further decomposition of biomass and there was little difference observed for the different classifications.
Kinetic Analysis of Pyrolysis of Olive Kernel
Reaction model for olive kernel decomposition during fixed bed non-isothermal pyrolysis
Non-isothermal (TGA), X = 0.2–0.8
Table 3 revealed that the two dimensional diffusion model (G2) was the best fit. The high coefficient value (0.986) demonstrated a good fit and the activation energy of Olive kernel (1180–1400 µm) measured 74.4 kJ/mol.
Reaction model for olive kernel decomposition during fluidised bed isothermal pyrolysis
Comparing the result obtained from fixed bed TGA (non-isothermal) to the fluidised bed (isothermal) in the TGFBR, both exhibit the same mechanism at <451 °C, and three dimensional diffusion control at ≥500 °C. However, the activation energy obtained from the TGA was higher and may be due to the effect of external gas diffusion at lower heating rates . The behaviour of three dimensional diffusion may be associated with greater degradation of hemicellulose and cellulose at high heating rates leading to higher volatility of the main biomass components. In addition, pore lattice defects are considered a significant factor because these defects promote reactivity and diffusion of material . The phenomena of two and three dimensional diffusion has been noticed by Li ; where during the study the kinetic mechanism of the reduction reactions of Ferrum niobate were quantified. In addition, the pyrolytic reactions of oil-palm shell at low and high temperature regimes were found to be based on two mechanisms according to Guo and Lua . In comparison to thermogravimetric pyrolysis methods other researchers have also reported different mechanisms and sequences involved in the formation of gas species, for example three dimensional diffusion was found responsible for the production of hydrogen and methane during the pyrolysis process [12, 17].
A laboratory scale thermogravimetric fluidised bed reactor (TGFBR) was developed to measure the reaction kinetics of Olive kernel biomass pyrolysis with fluidising sand mixtures over temperature ranges from 300 to 660 °C and the results compared with fixed bed pyrolysis in a typical bench top TGA.
It was shown that above 500 °C, the time taken to fully react a 40 g sample in a bed of 400 g of sand was less than 10 s. Furthermore, the fast pyrolysis exhibited in the TGFBR provided a uniform temperature inside the reactor supressing external diffusion effects confirmed by little variation in the reaction time above 40 l/min flow rate of the fluidising gas.
In the TGA apparatus, particle size had no measurable effect on the reaction rate, whereas a clear dependence of reaction rate on biomass particle size was demonstrated in the TGFBR. In both apparatus, at low heating rates (<451 °C) the reaction time was unaffected by the biomass particle size over the ranges tested. However, for the TGFBR there was a dependence of reaction rate on particle size above 500 °C when it was observed that the reaction time increased with larger particle sizes.
The pyrolysis reaction kinetics were studied under non-isothermal conditions in the TGA and isothermal conditions in the TGFBR. A two dimensional diffusion model was the controlling mechanism identified with the best fit for the fixed bed TGA with an activation energy of 74.4 kJ/mol. In comparison, 2-dimensional and 3-dimensional reaction mechanisms gave the best fits to describe the reaction kinetics of the biomass particles over 2 temperature ranges in the TGFBR which could be divided into two stages: the two dimensional diffusion reaction mechanism from 320 to 451 °C with an activation energy of 67.4 kJ/mol; and the three dimensional diffusion reaction mechanism from 500 to 660 °C with an activation energy of 60.8 kJ/mol.
Bench top TGA analysis of pyrolysis is a rapid and valuable method for comparing the behaviour of biomass reactivity, but the small sample sizes tested and low heating rates places limits on the relevance of results. In comparison, the larger laboratory scale TGFBR fitted with load cells allows detailed measurements at conditions likely to be more representative of those encountered on larger scale systems where heat distribution, heat transfer and mass diffusion effects play a major role in the reactivity of biomass.
We express our sincere thanks to the Ministry of Higher Education/AL-Nahrain University for their financial support under Contract Number 6052.
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