Char Gasification Reactions

The main reactions that take place in a gasification system are discussed below, emphasizing the heterogeneous gas-char reactions. 

All of these reactions are highly exothermic, providing heat for the subsequent endothermic gasification reactions. Because gasifiers operate under fuel-rich conditions, the 02 is completely consumed before these particular reactions have consumed all of the carbon. Reaction R-4.3 achieves thermodynamic equilibrium quickly enough at the high temperature conditions in entrained flow gasifiers so that chemical equilibrium can be assumed. 

This reaction is called the reverse Boudouard reaction and is an endothermic reaction. The reaction rate of the reverse Boudouard reaction is several orders of magnitude lower than reaction rates of R-4.1 and R-4.2 (at the same temperature in the absence of a catalyst). Also, the reverse Boudouard reaction is inhibited by the presence of products, namely carbon monoxide. 

Reactions R-4.5 and R-4.6 are main reactions to generate H2 and CO. Both reactions are endothermic and have high activation energies. This means that they are slow compared with reactions R-4.1 and R-4.2, and slow down as they proceed since the temperature decreases with extent of reaction. The reaction rates are proportional to the partial pressure of the steam in the system. 

Reaction R-4.7, the water-gas shift reaction, is an exothermic reaction. The water-gas shift reaction has influence on the CO/H2 ratio in the gasification product, which is very important when the gas is used for synthesis purpose. Therefore, the shift process can be found in almost all the ammonia plants and hydrogen generation process in gas plants. The shift reaction can generally be taken into account using thermodynamic chemical equilibrium, since gas-phase temperatures are high. 

---

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...

Hydrogen inhibits the steam gasification reaction. The char gasification reaction with steam and hydrogen can be modelled based on Langmuir-Hinshelwood kinetics. The model fits well the results. 

These authors consider that a system of good kinetic equations for the biomass-char gasification reactions under the true gasifier atmosphere do not exist yet. This conclusion can be discussed and even criticised, of course. The authors are open to a discussion and experimental checking with the person(s) who considers that holds a system of kinetic equations to correctly describe the network of reactions 4a-4d. These authors do not know such system of kinetic equations and have again to look for an empirical and easy-to-use approach. 

Roberts, D.G. and Harris, D.J. A kinetic analysis of coal char gasification reactions at high pressures. Energy Fuels, 2006, 20, 2314. 

Because pyrolysis (step 1) produces less than 30% by weight char for most biomass materials, the char gasification reactions (step 3) play a less important role in biomass gasification than steps 1 and 2. 

The empirical nature of these expressions is apparent and thus a major goal of our work was to attempt to derive rate expressions more typical of what would be expected for char gasification reactions based on the coal literature( 7). Another goal was to be able to predict make-gas compositions and thus a separate determination of the water-gas shift reaction rate was also undertaken. Finally, because of evidence that the iron present in the shale acted to catalyze the shift reaction, a number of oxidation/reduction experiments were run in order to assess the ability of the reacting gases to affect the oxidation state of the iron. 

The COGAS process involves the gasification of the COED char to produce a synthesis gas composed of carbon monoxide and hydrogen. The heat for the char gasification reaction is provided by the combustion of part of the char. 

The char combustion is exothermic, whereas the char gasification reactions are endothermic. 

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... 

Fig. 3 Plot of the conversion fraction of the gingko nut shell char vs. the reaction time (s) in the gasification reaction. (H2O 0.4 atm).

Therefore, the steam gasification reaction rate of the gingko nut shell-char can be represented by the following kinetic equation as ... 

Gasification is the result of chemical reactions between carbon in the char and steam, carbon dioxide, and hydrogen in the gasifier vessel as well as chemical reactions between the resulting gases. Gasification reactions can be represented by ... 

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 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. 

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

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. 

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. 

Reactions 17.5, 17.6, and 17.7 illustrate the gasification of char by reaction with various gases. The carbon-steam Reaction 17.5 is an endothermic reversible reaction. Steam undergoes a side reaction, Reaction 17.8, called the water-gas shift reaction. This reaction, which is very rapid, is catalyzed by various impurities and surfaces. The carbon-C02 reaction, Reaction 17.6, is favored at high temperatures and low pressures, whereas the carbon-H2 reaction, Reaction 17.7, is favored at low temperatures and high pressure. Since only three of Reactions 17.5-17.9 are independent, if the equilibrium constants for Reactions 17.6, 17.7, and 17.8 are known, the... 

Because coal chars are highly microporous, most of the gasification reactions take place inside the char particles. Therefore, diffusion of gas into, and products out of, porous particles is required. The overall diffusion process can be described by the following steps (1) diffusion of the reactant from the bulk gas to the solid surface (film diffusion) (2) diffusion of the reactant from the particle s surface to its interior (internal diffusion) (3) diffusion of the product from the interior to the particle s surface (internal diffusion) and (4) diffusion of the product from the surface to the bulk gas (film diffusion). 

This coal gasification reaction attains appreciable rate for sustaining the reduction reaction only at about 1000 °C for most of the reactive chars (devolatilised coal). Those who have access to very highly reactive coals (coals which give on devolatilisation very highly reactive chars), operate their rotary kilns below 1000 C. 

---