Char gasification

The importance of these concepts can be illustrated by the extent to which the pyrolysis reactions contribute to gas produdion. In a moving-bed gasifier (e.g., producer-gas gasifier), the particle is heated through several distinct thermal zones. At the initial heat-up zone, coal carbonization or devolatilization dominates. In the successively hotter zones, char devolatihzation, char gasification, and fixed carbon... 

Oxygen-Char Gasification in a Non-Slagging Fluid Bed., Joint Amer. Inst. Chem. Eng.-Gesellschaft Verfahrenstech. Chemieing. Mtg., Munich, September, 1974. 

Dasappa, S., Paul, P. J., Mukunda, H. S., and Shrinivasa, U., Wood-Char Gasification Experiments and Analysis on Single Particles and Packed Beds, 27th International Symposium on Combustion, The Combustion Institute, 1998, pp 1335-1342. 

Backman, R., Fredrick, W.J., Hupa, M. 1993. Basic studies on black-liquor pyrolysis and char gasification. Biores Technol 46 153-158. 

This review defines the thermochemical conversion processes of solid fuels in general and biofuels in particular that is, what they are (drying, pyrolysis, char combustion and char gasification) and where they take place (in the conversion zone of the packed bed) in the context of the three-step model. 

The thermochemical conversion of solid fuels is a complex process consisting of drying, pyrolysis, char combustion, and char gasification. 

The fuel bed (packed bed) is a two-phase system, also referred to as a porous medium [20]. Thermochemical conversion processes, such as drying, pyrolysis, char combustion and char gasification, take place simultaneously in the conversion zone of the fuel bed (Figure 16). They are extremely complex, and are reviewed more in detail in section B. 4. Review of thermochemical conversion processes. 

The solid char residue can also react in reducing atmospheres and this phenomenon is then referred to as char gasification. The char gasification reactions are also heterogeneous reactions [35]. In Table 11 are the most common char gasification reactions [35]. 

Table 11 The most common char gasification reactions...

The heat and mass transport phenomena of the char gasification is not described in the literature as much as for the char combustion [11,28,78]. There are good reasons to believe that it is quite analogous to the char combustion phenomenology [79]. However, the heterogeneous gasification reactions are overall endothermic which results in some differences with respect to the intraparticle heat transport [79]. 

The char gasification process is a consequence of reducing gaseous conditions in the bed. Reducing conditions is a result of complete consumption of oxygen in the interstitial gas phase [12]. 

The thermochemical conversion of solid fuels, in the context of PBC systems takes place, in the conversion system, and more precisely in the conversion zone. The thermochemical processes are drying, pyrolysis, char combustion, and char gasification. The conversion process is promoted by the exothermic char combustion reactions. 

When authors illustrate the subject of thermochemical conversion of solid fuels in the literature, the conversion zone in a packed bed is divided into different process zones (drying zone, pyrolysis zone, char combustion zone, and char gasification zone), one for each thermochemical conversion process. The spatial order of this process zones is herein referred to as the bed process structure or conversion process structure. The conversion process structure is a function of conversion concept. Even more important, the bed process structure can only exist in the diffusion controlled conversion regime when the conversion zone has a significant thickness. 

Laurendeau N.M., Heterogeneous Kinetics of Coal Char Gasification and Combustion , Prog. Energy Combust. Sci. 4, 221-270(1978). 

The kinetics of coal char gasification can usually be interpreted in terms of the following set of reactions ... 

A Pore Diffusion Model of Char Gasification with Simultaneous Sulfur Capture... 

A model Is presented for char gasification with simultaneous capture of sulfur In the ash minerals as CaS. This model encompasses the physicochemical rate processes In the boundary layer, In the porous char, and around the mineral matter. A description of the widening of the pores and the eventual collapse of the char structure Is Included. The modeling equations are solved analytically for two limiting cases. The results demonstrate that pore diffusion effects make It possible to capture sulfur as CaS In the pores of the char even when CaS formation Is not feasible at bulk gas conditions. The model predictions show good agreement with experimentally determined sulfur capture levels and reaction times necessary to complete gasification. 

The specific char surface area, Sc, the char porosity, e, and the effective diffusivities, De vary with char conversion and thus have to be determined from a model of the pore structure evolution Various models can be used for that purpose [10-12]. We chose to use Gavalas s random capillary model [12,13] to describe the widening of the pores and the eventual collapse of the char structure This model provides exact expressions for Sc, e, and De in terms of a local carbon conversion, q(r,t), which represents the length the pore surface has retreated at time t due to char gasification, i.e. the pore radius at the radial coordinate r and time t the initial pore radius + q(r,t). The conservation equation for this local carbon conversion takes the form ... 

Sulfur Capture During the Initial Stages of Char Gasification... 

Since a char particle typically contains < 2% Ca (w/w), while the char surface is > 200 m /g, the Thiele moduli for the calcium reactions are likely to be much smaller than those associated with char gasification even when the turnover numbers for the reactions are of the same order of magnitude. Thus, we will assume that the sulfur reactions are kinetically controlled while the gasification is diffusion limited. In that case HjS and 00S concentrations... 

The model and the results presented here illustrate the physicochemical processes involved in char gasification with simultaneous sulfur capture. In particular, they demonstrate that diffusion limitations in the gasification reactions enable the conversion of CaO to CaS within the char even though CaS formation is not feasible at bulk gas conditions. Furthermore, this first version of the model correctly predicts the trends observed experimentally. Future effort in this area will focus on quantitative comparisons of model predictions with results from carefully designed gasification experiments. 

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