Catalytic Steam Gasification of Biomass Surrogates: A Thermodynamic and Kinetic Approach

Gasification of biomass is an environmentally important technology that offers an alternative to the direct use of fossil fuel energy. Steam gasification is getting increased attention as a potential source of renewable energy since it produces a gaseous fuel suitable for industrial applications in highly efficiently power/heat energy production, transport fuel, and as a feedstock for chemical synthesis. Furthermore, catalytic steam gasification has other advantages hence (i) it produces a gas with higher heating value; (ii) it reduces the diluting effect of N2 from air; (iii) it eliminates the need of cleaning, upgrading and/or conditioning the product gas for certain applications; and (iv) it eliminates the need for an expensive oxygen plant when both air and oxygen are used as gasification mediums. Catalytic steam gasification of biomass in fluidized beds is a promising approach given its rapid biomass heating, its effective heat and mass transfer between reacting phases, and its uniform reaction temperature. Moreover, fluidized beds tolerate wide variations in fuel quality as well as broad particle-size distributions. However, catalytic steam gasification is a more complex process resulting from: (i) the heat necessary to sustain the process is directly supplied by the partial combustion of the feedstock during the process, as it happens when air or oxygen is used, (ii) the rapid catalyst deactivation that occurs due to heavy coking, and (iii) tar formed during the process. This Ph.D. dissertation reports a research study on the steam gasification of biomass over a Ni/-alumina catalyst using model compounds. This research allows elucidating the factors inherent to this process such as thermodynamic restrictions and mechanistic reaction steps. The ultimate aim is to establish the chemical reaction engineering tools that will allow the design and operation of large scale fluidized bed units for biomass steam catalytic gasification. On this basis, a thermodynamic equilibrium model based on evaluations involving C, H and O elemental balances and various product species (up to C6 hydrocarbons) was developed. This model establishes the effect of biomass composition, temperature, and steam on the various gas product molar fractions. Based on the proposed equilibrium model and using glucose, as a model biomass species, an optimum gasification temperature close to 800°C and a steam/biomass ratio between 0.5 and 0.7 g/g is established. Experiments were carried out in the CREC fluidized Riser Simulator under gasification conditions using a) glucose as a model compound for the cellulose contained in biomass, and b) 2-methoxy-4-methyphenol as a model compound for the lignin that is found in biomass. The experimental data show that for reaction times longer than 30 seconds, chemical species are essentially equilibrated and that the proposed thermodynamic model does provide an adequate description of various product fractions. Data obtained also demonstrate the shortcomings of equilibrium models for gasifiers with reaction times shorter than 10 seconds and the need for non-equilibrium models to describe gasifier performance at such conditions. Taking the above into consideration, a reaction network and a kinetic model for biomass catalytic steam gasification were proposed. This kinetic model was developed using a coherent reaction engineering approach where reaction rates for various species are the result of the algebraic addition of the dominant reactions. It is also demonstrated that using an experimental-modeling procedure, where intrinsic kinetic parameters and adsorption constants are decoupled in their evaluation in the CREC Riser Simulator eliminates overparametrization with successfully parameter correlation. Numerical regression of the experimental data leads to kinetic parameters with narrow spans suggesting that the proposed kinetic model satisfactorily describe the catalytic conversion of glucose and 2-methoxy-4-methyphenol under gasification conditions