Fast Pyrolysis of Biomass in a Circulating Fluidised Bed

CFB biomass pyrolysis produces mostly bio-oil. Reaction rates are fast (k > 0.5 s). Yields exceed 60 wt% of bio-oil at 500 °C and at a residence time for oil and char τ (Utr + 1) m/s and G > 200 kg/m2 s. INTRODUCTION Biomass is a renewable energy source with high potential (1,2). Its pyrolysis mainly forms storable bio-oil and solid char; bio-oil also contains value-added chemicals (1). Important design issues are kinetics, modelling and reactor hydrodynamics (3,4). Pyrolysis proceeds in the absence of O2 and takes seconds only at moderate temperatures (~ 500 °C). Char is a cracking catalyst for bio-oil and must be removed from the vapour. After condensation, a brown, low viscosity liquid is obtained. High oil yields (1,5) are achieved at: (i) very fast particle heating; (ii) temperature of + 500 °C; (iii) short τ and (iv) fast char separation and vapour condensation. Fluidised beds achieve the fast heat transfer. Both bubbling fluidised beds (BFB) and circulating fluidised beds (CFB) can therefore be used for fast pyrolysis, each with its known advantages and drawbacks. Whereas a BFB will operate at a gas velocity to mix biomass and sand, albeit with a controlled carry-over of light char particles, the CFB will rely on the controlled co-existence of char and sand in the riser, followed by a selective separation of sand (return loop) and char (product). At present, both BFB (e.g. Wellman, University of Hamburg) and CFB (e.g. ENEL, VTT/Ensyn, CRES) are being investigated. The present research also focuses on CFB. Only further comparative studies between both fluidised beds will confirm or contest the tentative status of pyrolysis reactors as presented in Fig. 1. The main advantage of the CFB is the possibility to achieve a short and controllable τ for char. Both its success in coal combustion and minerals’ processing, and its general advantages over bubbling fluidised beds have moreover confirmed its technological strength and market potential (Fig. 1). The paper (i) reviews the kinetics, conversion and modeling; (ii) studies the particle movement and τ in a CFB by PEPT; and (iii) proposes a process design and tentative economics. 1 Van de Velden et al.: Fast Pyrolysis of Biomass in a Circulating Fluidised Bed (CFB) Published by ECI Digital Archives, 2007 VAN DE VELDEN et al. 898 Figure 1. Status of the pyrolysis reactors (5) KINETICS AND ENDOTHERMICITY Theory and experiments have been published (6). Differential Scanning Calorimetry (DSC) determines the endothermic heat at 210 (eucalyptus) to 430 kJ kg (sawdust). The kinetics of the pyrolysis are commonly determined by thermogravimetric analysis (TGA). Pyrolysis produces a solid residue or char, i.e. minerals and the organic coking-residue of the biomass, representing 25 to 35 wt%, with the exception of corn (only 10 wt%) and sunflower residue and sludge (40 to 50 wt%). The reaction is of the first order in biomass, with an Arrhenius-dependent reaction rate constant, k. The activation energy (Ea) is function of the biomass type. The pre-exponential factors (A), and thus k, depend on the heating rate, and achieve a maximum value at a heating rate of 100 K/min, where the reaction is kinetically controlled rather than by heat transfer. Such heating rates are easily achieved in a CFB. Values of k at 500 °C (the optimum temperature for pyrolysis) exceed 0.5 s (except poplar and sludge): a high conversion can thus be achieved in short reaction times, limiting side reactions. CONVERSIONS, BIO-OIL YIELD AND MODELLING The yield of bio-oil, gas and char is measured in a lab scale batch reactor and in a pilot CFB (Centre for Renewable Energy Sources, CRES), illustrated in Fig. 2. The riser has an I.D. of 80 mm and is 3,8 m high. The bottom bubbling fluidized bed burns the separated char. The char combustion gas is used as fluidization gas in the riser. Dry biomass (< 300 μm) is fed (up to 12 kg/hr) at a height of 1.4 m from the bottom. At start-up, the riser is electrically preheated. A nearly constant temperature is obtained above the biomass injection point. Both batch and CFB experiments were performed in the same temperature range. Fig. 3 depicts the experimental oil-yields, literature data and model predictions. Despite differences in reactor types, procedures and biomass used, all results show the same yield of bio-oil with a maximum (60 – 65 wt%) around 500°C. CFB hydrodynamics link the conversion of an individual particle to the overall conversion of all particles (13). The basis of the model (14,15) includes: • the use of the Waterloo concept (16), with primary and secondary reactions; • the possibility of suppressing (but never completely avoiding) the secondary reactions by a short and nearly constant residence time for the biomass particles and the vapours; Technology strength strong average weak Market potential high low Entrained bed Circulating fluidized bed Bubbling fluidized bed Ablative pyrolysis Rotating cone 2 The 12th International Conference n Fluidization New Horizons in Fluidization Engineering, Art. 110 [2007] http://dc.engconfintl.org/fluidization_xii/110 FLUIDIZATION XII 899