A mathematical model of fluidized bed biomass gasification

Abstract

A mathematical model of biomass gasification in a fluidized bed has been developed. It considers axial variations of concentrations and temperature in the bubble and emulsion phases. The mass balance involves instantaneous oxidation and equilibrium devolatilization of the biomass, kinetics of solid-gas gasification reactions as well as of gaseous phase reactions and interphase mass transfer and gas convection. The energy balance is solved locally for each vertical volume element, and globally on the reactor by iteration on the temperature at the bottom of the bed. Three parameters have been adjusted based on the experimental results: the heat transfer coefficient at the wall, the weighting of the kinetics of the water-gas shift reaction and the fraction of biomass carbon remaining as char after devolatilization. The model is used to simulate a pilot scale (50 kg/h) biomass gasifier, and its predictions compared to experimental measurements. The temperature and gaseous concentrations are estimated with good accuracy for the experiments using a wood feedstock, except for the concentration of hydrogen which is overestimated.Un modkle mathematique de gazbification de la biomasse en lit fluidisk a ktk dCvelopp6. Les variations axiales de concentrations et de tempirature sont considirkes, dans les phases bulle et Cmulsion. Le bilan massique considkre les apports suivants: oxydation et dkvolatilisation B I'kquilibre du combustible instantankes, cinktique des reactions de gazkification en phase gazeuse et gaz-solide, transfert de masse interphase et convection du gaz. Le bilan d'knergie est rksolu localement pour chaque Clkment de volume vertical, et globalement sur le rkacteur par itCration de la tempkrature B la base du lit. Trois paramktres ont 6tk ajust6s a partir de rksultats expkrimentaux: Le coefficient de transfert de chaleur B la paroi, la ponderation de la cinktique de la reaction de transfert eau-gas et la fraction du carbone de la biomasse formant le rCsidu carbonk h la dkvolatilisation. Le programme a Ctk appliquk h la simulation d'un gazogkne B 1'Cchelle pilote (50 kglh), et les prkdictions obtenues comparkes aux mesures expkrimentales. Les tempkratures et concentrations gazeuses sont estimees avec une bonne prkision pour les expiriences utilisant du bois comme combustible, sauf pour le cas de la concentration d'hydrogkne qui est surestirnee.luidized bed gasification of biomass has been widely F investigated since 1975. This generic technology provides high heat and mass transfer rates when compared to fixed beds, and requires relatively simple equipment. The transfer rates ensure efficient solid-gas contacting and favor gas phase and gas-solid reactions. The temperature and concentration gradients are also kept to a minimum, allowing efficient control of the operating conditions and good turndown ratios. The industrial application of biomass gasification has only been slowed down by the low cost of readily available energy sources (i.e. natural gas and petroleum).The high costs of municipal solid waste dis...

Transient mathematical modelling of a fluidized bed incinerator for sewage sludge

“…The solid material present in this zone originates from bursting of bubbles at the top of the dense bed [23]. The presence of sand and particles implies heterogeneous transfers and reactions that affect greatly the global conversion rate.…”

“…[23,24,28,29]. De Soete [24] has proposed the followed simplified model: Therefore, given the local values of the kinetics constants for thermal NO formation, one can write the local value of the NO chemical reaction rate:…”

Comparison of dual fluidized bed steam gasification of biomass with and without selective transport of CO2

“…The water-free (wf) composition of the fuel is presented in Table 4. According to former studies (Schuster et al, 2002;Bilodeau et al, 1993;Fiaschi and Michelini, 2001;Liliedahl and Sjöström, 1997;Wang and Kinoshita, 1993) as well as extensive measurements at the process development unit used in this work and at the biomass combined heat and power plant in Guessing Loeffler et al, 2003;Kaiser et al, 2003) it can be assumed that the char consists of pure carbon. The tar composition is measured for every experiment according to the method described in chapter 3.2.…”

Use of a transient model to simulate fluidized bed incineration of sewage sludge

“…1) are known to be very efficient for sludge incineration [2], this study focuses on the modeling of these devices. Such models have already been derived both for coal combustion or gasification [3][4][5][6][7][8][9][10] as well as for biomass and sludge incineration or gasification [11][12][13][14]. One of the main drawbacks of these models is that the reactor is often considered as isothermal and/or to operate at stationary conditions.…”