This paper provides an updated review on fast pyrolysis of biomass for production of a liquid usually referred to as bio-oil. The technology of fast pyrolysis is described including the major reaction systems. The primary liquid product is characterised by reference to the many properties that impact on its use. These properties have caused increasingly extensive research to be undertaken to address properties that need modification and this area is reviewed in terms of physical, catalytic and chemical upgrading. Of particular note is the increasing diversity of methods and catalysts and particularly the complexity and sophistication of multi-functional catalyst systems. It is also important to see more companies involved in this technology area and increased take-up of evolving upgrading processes. © 2011 Elsevier Ltd.

Review of fast pyrolysis of biomass and product upgrading

In this review, we summarize the knowledge of the production and properties of charcoal that has been accumulated over the past 38 millenia. The manipulation of pressure, moisture content, and gas flow enables biomass carbonization with fixed-carbon yields that approachor attainthe theoretical limit after reaction times of a few tens of minutes. Much of the heat needed to carbonize the feed is released by vigorous, exothermic secondary reactions that reduce the formation of unwanted tars by augmenting the charcoal yield in a well-designed carbonizer. As a renewable fuel, charcoal has many attractive features:  it contains virtually no sulfur or mercury and is low in nitrogen and ash; it is highly reactive yet easy to store and handle. Carbonized charcoal can be a good adsorbent with a large surface area and a semimetal with an electrical resistivity comparable to that of graphite. Recent advances in knowledge about the production and properties of charcoal presage its expanded use as a renewable fuel, red...

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The Art, Science, and Technology of Charcoal Production†

Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of organic species

Combustion and gasification rates of lignocellulosic chars

Alkanolamine solutions are frequently used as solvent for the removal of acid compounds from industrial gases (Kohl and Riesenfeld, 1979). Depending on the process requirements, e.g., selective removal of H2S, CO2-bulk removal, several options for alkanolamine based treating solvents with varying compositions of the solution have been proposed. In this paper an overview is presented of the mechanisms that have been proposed in literature and the kinetic data for the various reactions are critically evaluated. Conclusions on the applicability and restrictions are discussed along with perspectives. In addition white spots in the present knowledge are indicated. The reaction between CO2 and primary/secondary amines both in aqueous and non-aqueous solutions can be described over a wide range of conditions and amine concentrations with the zwitterion-mechanism as originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979). All published results, both non-aqueous and aqueous solutions, amine-prom...

On the kinetics between co2 and alkanolamines both in aqueous and non-aqueous solutions. an overview

Solubility and diffusivity of N2O and CO2 in water were determined as a function of temperature from the results published in the open literature, and new data were measured in the present work. The solubility of N2O in several aqueous alkanolamine (DEA, DIPA, DMMEA, and DIPA) solutions at various temperatures was measured and correlated over a wide range of conditions. For both the diffusivity of N2O and the alkanolamine in aqueous alkanolamine solutions a modified Stokes-Einstein relation was derived. With the aid also of the “N2O analogy” the diffusivity of CO2 in these solutions can be estimated.

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Solubility and Diffusivity of Acid Gases (CO2, N2O) in Aqueous Alkanolamine Solutions

In this paper, we have investigated the possibilities to steer the composition and, thus, the quality of pyrolysis liquids by the reactor temperature and the pyrolysis vapor condenser temperature. Pine wood was pyrolyzed in a 1 kg/h fluidized-bed pyrolysis reactor operated at 330 or 480 °C. The pyrolysis vapors produced were condensed using a condenser train of two counter-current spray columns arranged in series. In this paper, the temperature of the first condenser was varied between 20 and 115 °C, while the second condenser temperature was kept at 20 °C. To describe the composition of the oils, we have integrated several analytical techniques into a novel characterization scheme that can account for 77−82 wt % of the oils. The effects of the condensation conditions on fractions of light compounds in the oils can be predicted with a simple equilibrium stage condensation model. It has been observed that pyrolysis at 330 °C gives a light oil with a low amount of mid-boilers [normal boiling point (nbp) of 150−300 °C] and heavy compounds (water insolubles and mono- and oligosugars). Sugars, mid-boilers, and water-insoluble lignin-derived oligomers are more present in the oil obtained at 480 °C, while the yields of light organics are approximately the same for 330 and 480 °C. It can be concluded that fractional condensation is a promising cheap downstream approach to concentrate compounds (classes) and, thus, to control the quality of pyrolysis oils. For instance, operating the first condenser around 70−90 °C gives an aqueous liquid in the second condenser containing 40 wt % light organics, which are interesting for extraction (e.g., 10 wt % acetic acid) and supercritical water gasification to produce hydrogen. Under these conditions, the oils from the first condenser have a high content of sugars (20 wt %) and lignin-derived oligomers (40 wt %), which are attractive fractions for fermentation/sugar chemistry and gasoline production via fluidized catalytic cracking (FCC)/hydrotreatment, respectively.

Fractional Condensation of Biomass Pyrolysis Vapors

A system to remove in situ char/ash from hot pyrolysis vapors has been developed and tested at the University of Twente. The system consists of a continuous fluidized bed reactor (0.7 kg/h) with immersed filters (wire mesh, pore size 5 μm) for extracting pyrolysis vapors. Integration of the filter system in the fluidized bed should overcome operational problems related to the increase in pressure drop across the filter in time and a decrease in oil yield as typically observed in downstream pyrolysis vapor filtration and lead to process intensification. In this study the effect of in situ hot pyrolysis vapor filtration has been studied with respect to process stability, product yields, and product quality. Oil obtained via a more conventional cyclone system placed in parallel to the filter system served as reference for the quality and yields of the filtered oil. Good process stability concerning temperature and pressure drop across the hot gas vapor filter was achieved during a 2 h run, even when using a reused filter. Particles (char/sand) were retained inside the filter pores located at the outside surface of the filter, while the inside of the filter remained clean apart from some deposits formed on the metal wire and small 1 μm particles which slipped through the filter. Mass balance closures higher than 94 wt % were obtained. Comparable yields (cyclone + filtered oil) were obtained as in the experiments carried out with only the cyclones. The filtered oil contained significantly less solids, alkali metals, and ash compared to cyclone oil. For the alkali metals, only a considerable amount of potassium (K) was still present in filtered pyrolysis oil, which most likely entered the filtered oil via the vapor phase. There were no significant differences in the elemental composition of the oil produced via the filter line and cyclone line. The molecular weight of the filtered oil obtained with nondried feed was always marginally lower compared to the cyclone oil. Results of the aging tests show that the reactivity of pyrolysis oil can already originate from the highly reactive components in pyrolysis oil itself and does not need the presence of char/ash. To show the intrinsically high reactivity of solids-free pyrolysis oil vapors, an external filter section (1 μm pore size) was placed additionally and in series with the filter inside the fluidized bed. The results show that char is formed from the reactive pyrolysis vapors.

Fast Pyrolysis of Biomass in a Fluidized Bed Reactor: In Situ Filtering of the Vapors

Gasification of pyrolysis oil was studied in a fluidized bed over a wide temperature range (523−914 °C) with and without the use of nickel-based catalysts. Noncatalytically, a typical fuel gas was produced. Both a special designed fluid bed catalyst and a crushed commercial fixed bed catalyst showed an initial activity for syngas (H2 and CO) production at T > 700 °C. However, these catalysts lost activity irreversibly and elutriation from the fluid bed occurred. The equilibrium catalytic activity suffered from incomplete reforming of hydrocarbons (CH4). In all the experiments the carbon to gas conversion was incomplete, which was mainly caused by the formation of deposits and the slip of microcarbonaceous particles. A two-stage reactor concept, which consisted of a sand fluidized bed followed by a fixed catalytic bed, was proposed and tested. This system uncouples the atomization/cracking of the oil and the catalytic conditioning of the produced gases, enabling protection of the catalyst and creating opportunities for energy efficiency improvements. In a bench scale unit of this reactor (0.5 kg oil/h), methane and C2−C3 free syngas (2.1 Nm3 CO + H2/kg dry oil, H2/CO = 2.6) with a low tar content (0.2 g/Nm3; dry, N2 free gas) was produced in a long duration test (11 h).

Catalytic and Noncatalytic Gasification of Pyrolysis Oil

The combustion kinetics of rapidly pyrolyzed wood have been investigated within the temperature range of 573−773 K and the oxygen concentration range of 2.25−36 vol %. These kinetics are, for instance, required for the design of a char combustion section in an integrated flash pyrolysis pilot plant. Two different experimental techniques were used: a standard thermogravimetrical technique (TGA) and flue gas analysis after combustion in a diluted packed bed. The pyrolysis char was prepared in a small screen heater reactor, enabling heating rates of over 300 K/s. The results of the TGA and the packed-bed measurements are in good agreement for temperatures above 648 K. Below that temperature, the reaction rate observed in the TGA appears to be (up to 1.5 times) lower than that in the packed-bed reactor. The combustion kinetics can be described by the following simple rate equation which has an average deviation of 20%: r = 5.3 × 105e-125×103/RTPO20.53(1 − X)0.49. Several pore models reported in the literature were tested against the experimental data, but they could not improve the accuracy of the fit. In comparison with available data, this specific type of char shows a notably higher rate of combustion.

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Wim P.M. van Swaaij

Papers

Solubility and Diffusivity of Acid Gases (CO2, N2O) in Aqueous Alkanolamine Solutions

Fractional Condensation of Biomass Pyrolysis Vapors

Fast Pyrolysis of Biomass in a Fluidized Bed Reactor: In Situ Filtering of the Vapors

Catalytic and Noncatalytic Gasification of Pyrolysis Oil

Combustion Kinetics of Char Obtained by Flash Pyrolysis of Pine Wood