Main principles

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Main Principles of Fast Pyrolysis of biomass

Thermal decomposition of biomass is realized by heating the feed material. To maximize oil yields, 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 minerals) to small non-condensable gas molecules on the one hand, ii) or to be recombined / polymerized to char (precursors) on the other. 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 bio based chemicals or biofuel application, an additional target would be to control the chemical composition of the condensables, for instance by applying catalysts.

Where initially 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 (Wang et al., 2005). The composition of the oil, however, is sensitive for these parameters. High external heat transfer to the biomass particles can be realized by mixing biomass feed stream intensively with an excess of a heat carrier (e.g. hot sand). A number of reactor designs have been explored that are capable of achieving high heat transfer rates, such as fluidized beds and mechanical mixing devices. For an efficient heat transfer inside 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 such as cellulose. Energetic yields are slightly lower, approx. 55 to 65%. 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 aimed at, less stringent criteria must be met; the conversion of as much as possible biomass energy to the liquid product is then decisive.

The major components of lignocellulosic biomass, viz. cellulose, hemi cellulose and lignin all have a different thermal decomposition behavior, and each individually depends also on heating rates and presence of contaminants. A typical temperature dependence of the decomposition through thermo-gravimetric analysis (TGA) for reed is given on the right side
TGA.png
. 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 right hand side. Hemicellulose is the first component to decompose, starting at about 220C and completed around 400degrees centigrade. Cellulose appears stable up to approx. 310 degrees centigrade, where after almost all cellulose is converted to non-condensable gas and condensable organic vapors at 320 - 420degrees centigrade. Though lignin may begin to decompose already at 160degrees centigrade, it appears to be a slow, steady process extending up to 800 - 900 degrees centigrade.

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.

On the right side an indicative reaction pathway for biomass pyrolysis is shown.[1] Schemes like these, including three lumped product classes assume reactions that are first order in the decomposing component.
Reaction pathway.png
As there is a wide variety in results of reaction rate measurements, even for a “single” biomass type like wood, published rate and selectivity expressions may be useful in describing trends but cannot be used for reliable quantitative predictions. While biomass is a natural material, with widely varying structural and compositional properties, the validity of proposing such single particle models is thus rather uncertain.

References

  1. Venderbosch, R. H. W. Prins, 2010, Fast pyrolysis technology development, Biofuels, Bioproducts and Biorefining 4(2): 178-208.