Main Principles of Fast Pyrolysis of biomass
Main principles of fast pyrolysis
Thermal decomposition of biomass results in the production of char and non-condensable gas (the main slow pyrolysis products) and condensable vapors (the liquid product aimed at in fast pyrolysis). It is realized by rapid heat transfer to the surface of the particle and subsequent heat penetration into the particle by conduction. For fast pyrolysis conditions, meant to maximize the oil yield, the temperature development inside the particle, and the corresponding intrinsic reaction kinetics dominate the conversion rates and product distributions. Principally, biomass is decomposed to a mixture of defragmented lignin and (hemi)cellulose, and fractions derived from extractives (if present). The intention of fast pyrolysis is to prevent the primary decomposition products i) to be cracked thermally or catalytically (over char formed already) to small non-condensable gas molecules on the one hand, ii) or to be recombined / polymerized to char (precursors) on the other. Such conditions would then lead to a maximum yield of condensable vapors and include the rapid heating of small biomass feed particles. Besides, it is also essential to create a short residence time for the primary products, both inside the decomposing particle and in the equipment before the condenser. For biochemicals or biofuel application, an additional target would be to control the chemical composition of the condensables, for instance by applying catalysts.
First reactor developers adopted the concept of flash pyrolysis in which small particles (< 1 mm) were used to achieve high oil yields. Later research showed that the oil yield is much less dependent on biomass particle size and vapor residence times than originally assumed. The composition of the oil, however, is sensitive for these parameters. High external heat transfer to the biomass particles can be realized by mixing the cold biomass feed stream intensively with an excess of preheated, expectedly inert, heat carrier (e.g. hot sand). A number of reactor designs have been explored that may be capable of achieving high heat transfer rates, such as fluidized beds and mechanical mixing devices. For an efficient heat transfer through the biomass particle, though, a relatively small heat penetration depth is required, which limits the ‘size’ of biomass particles to, typically, 3 mm. ‘Size’ here reflects the actual (heat) penetration depth of the particle. For such particles the decomposition rate is controlled by a combination of intra-particle heat conduction and the decomposition kinetics. Oil yield values observed in continuously operated laboratory reactors and pilot plants, for wood as a feedstock material, are usually in the range of 60 to 70 wt.% (dry-feed basis). Although generally reported in reviews, oil yields over 70% are exceptional and only for well-defined feedstocks as cellulose. Energetic yields are a bit lower, approx. 55 to 65%. It is obvious that the energy left in the by-products should be used as well, e.g. for drying the feedstock and / or steam - electricity production. If the objective is to derive chemicals from the pyrolysis liquid, it is essential to operate the process at the proper conditions (temperature, residence time, feedstock type and feedstock pre-treatment) in order to maximize the yield of the specific component aimed at. When fuels are required, less stringent criteria must be met; the conversion of as much as possible biomass energy to the liquid product is then decisive. Until recently, most RTD work has been focused on maximizing the overall oil yield, without paying sufficient attention to the product composition and quality. Next to water, the major components of biomass are:
- cellulose (mostly glucans), with a composition roughly according to (C6H10O5)n, and n = 500 to 4000,
- hemi cellulose (mostly xylans), with an average composition according to (C5H8O4)n, and n = 50 to 200,
- lignin, consisting of highly branched, substituted, mononuclear aromatic polymers, often bound to adjacent cellulose and hemicellulose fibers to form a lignocellulosic complex.
The total mass loss rate is plotted versus the temperature on the left hand side, while on the right hand side the TGA data are interpreted in terms of cellulose (almost 30%), hemicellulose (25%), and lignin (20%). The differential plot for these fractions is given on the left hand side against the original biomass data. Hemicellulose is the first component to decompose, starting at about 220oC and completed around 400oC. Cellulose appears to be stable up to approx. 310oC, where after almost all cellulose is converted to non-condensable gas and condensable organic vapors at 320 - 420oC. Though lignin may begin to decompose already at 160oC, it appears to be a slow, steady process extending up to 800 - 900oC. At fast pyrolysis temperatures of around 500oC, the conversion of lignin is probably limited to 40%. In general, for fast pyrolysis a solid residues remains , which is then mainly derived from lignin and some hemicellulose fractions, respectively 40 and 20 wt-% of the original sample. A conclusion from such TGA data can be that the oil is derived mainly from cellulose, and only partially from hemicellulose (depending on the heating rate up to approx. 80% conversion to oil and gas) and lignin (roughly 50% conversion to oil and gas). The explanation is that in the biomass structure lignin and hemicellulose are linked through covalent bonds (ester and ether) and cannot be released that easily upon pyrolysis, while cellulose and hemicellulose are linked by much weaker hydrogen bonds. Indirect evidence for this hypothesis is given by the composition of the pyrolysis-derived char, which has an elemental composition close to that of the lignin. The pyrolysis of biomass can be both endothermic or exothermic, depending on the temperature of the reactions and the type of feed. For (holo)cellulosic materials, the pyrolysis is endothermic at temperatures below about 450oC, and exothermic at higher temperatures. As argued already, vapors formed inside the pores of a decomposing biomass particle are subject to further cracking, leading to the formation of additional gas and/or (stabilized) tars). Especially the sugar-like fractions can be readily re-polymerized, increasing the overall char yield (mostly ex-bed of the pyrolysis process). This may be the purpose of slow pyrolysis but should be avoided in fast pyrolysis. For the small particles used in fast pyrolysis, secondary cracking inside the particles is relatively unimportant due to a lack of residence time. However, when the vapor products enter the surrounding gas phase, they will still decompose further, if they are not condensed quickly enough.Although other mechanisms have been proposed as well, the Figure right shows a possible reaction pathway for biomass pyrolysis.  and starts with a reaction that is first order in the decomposing component. Unfortunately, there is a wide variety in results of reaction rate measurements, even for a “single” biomass type like wood. Published rate and selectivity expressions maybe useful in describing trends, but they can hardly be used for reliable quantitative predictions. It should be realized here that biomass is a natural material, with widely varying structural and compositional properties. Despite all such uncertainties in the required input data, many scientists still propose single particle models based on fundamental chemical and physical phenomena taking place inside the particles. Kinetic data are proposed for the pyrolysis of wood, but a small variation in ash content seriously affects these reaction rates, and probably the pyrolysis pathways themselves. Already stated in 1991, ‘it should not be expected that any simple one-step kinetic scheme can account for all the facts concerning the pyrolytic behavior of [just] carbohydrates’. Although the predictive power is, consequently, limited, modeling is still useful to create a better understanding.
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