Gasification Kinetics

Generally, gasification kinetics research includes the study of rates and mechanisms of gasification reactions and the effect of factors on the reaction rates. Reactions in coal gasification are mainly heterogeneous. Besides the chemical reaction (heterogeneous and homogeneous), some physical processes such as adsorption, diffusion, heat transfer, and fluid mechanics also play important roles in this complicated process. 

In a gas-solid reaction, generally there are seven steps  

diffusion of reactant gas from gas phase to solid surface, 

diffusion of product from the internal surface to solid surface, 

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Gasification. The only study on gasification kinetics of Texas lignite has been performed by Bass (23). Using a differential reactor, he obtained rate data at 700°C and for pressures ranging from 61.6 to 225.9 kPa. Rate equations as a function of steam partial pressure and carbon conversion were developed. 

Kojima, T., Assavadakom, P. Furusawa, T. (1993).Measurement and evaluation of gasification kinetics of sawdust char with steam in an experimental fluidized bed. Fuel Processing Technology, Vol. 36, pp. 201-207. 

Sprensen, L.H. et al. (1997), Straw - H20 gasification kinetics. Determination and discussion, Nordic Seminar on Thermochemical Conversion of Solid Fuels, 3" December, 1997, Chalmers University of Technology, Sweden. 

The inhibition effect of CO is widely accepted and reasonably well documented for coal char gasification but not for biomass. The lack of extensive literature for wood char CO2 gasification kinetics including CO has strongly motivated this investigation. 

Henrich, E. et al. (1999). Combustion and gasification kinetics of pyrolysis chars from waste and biomass. Journal of Analytical and Applied Pyrolysis, Vol. 49, pp. 221-241. 

Several studies about pyrolysis and gasification of krafl black liquor from cooking of wood can be found in literature, but there are scarcely data for black liquor coining from alkaline pulping of straw. The aim of this research is to provide more infoimation about the thermochemical conversion of this residue. Studies [1] about CO3 gasification kinetics of alkaline black liquor from straw have been previously carried out in thermobalance by our research group. The present work is focus on the... 

Gasification Kinetics of Coke Deposited on Silica-Alumina. Within the temperature range 1400 to 1600°F and in the presence of excess steam, the gasification reaction of coke deposited on the silica-alumina cracking catalyst closely followed first-order kinetics with respect to unreacted carbon (Figure 1). First-order rate constants were calculated from the slopes of these plots (Table III), and yielded an activation energy of 55.5 Kcal/mole. 

After the devolatilization and rapid-rate methane formation stages are completed, char gasification occurs at a relatively slow rate various models to describe the gasification kinetics of this material for various limited ranges of conditions have been proposed. The differential rates of reaction of devolatilized coal chars are a function of temperature, pressure, gas composition, carbon conversion, and prior history. 

It is generally assumed that a carbon surface is mainly composed of two types of planes basal and edge. The edge planes include all types of defects that may be present in the structure (mainly potentially unsaturated sp2-hybridized carbon atoms bound to only two other carbon atoms) and thus constitute the so-called active sites or active surface area (ASA). During the last four decades, the concept of active sites has been very useful in the study of carbon reactivity [52,79-81] as well as to the gasification kinetics of coal chars [82-85]. 

Grzegorczyk, W., and Leboda, R., Controlled gasification of carbonaceous materials in a water vapour atmosphere Effect of Ca(Il) and Fe(III) ions on the gasification kinetics, Adsorpt. Sci. Technol., 14(3), 173-178(1996). 

What could be termed the classic exploratory experiments of the gasification kinetics of carbons established unequivocally the important role of hydrogen adsorption in the gasification (activation) of carbons by steam. More recent studies have added considerable detail to that basic understanding and these are summarized below in an attempt to create a movie of the dynamics of these gasification reactions at surfaces of carbons. 

Gasification kinetics during entrained flow gasification Part I - devolatilisation and char deactivation. Fuel, 103,... 

Development of gasification kinetic models is more complex than equilibrium models. Kinetic models include parameters such as reaction rates, residence times, reactor hydrodynamics, and mass and energy transfer phenomena. The biomass gasification process involves significant nmnbers of homogeneous and heterogeneous reactions between numerous reactants and products and the exact reaction pathways are dilfrcult to formulate (Patra and Sheth, 2015). 

Recent reviews on gasification kinetic models observed that most of the existing kinetic models either consider only the reduction zone of the gasifier or define the problem with separate submodels for devolatilization, oxidation, and reduction zones (Patra and Sheth, 2015 Puig-Amavat et al., 2010). An integrated kinetic model has not yet been developed. 

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