Gasification stages

The Carnot temperatures of the first stage of the processes (gasification stage) correspond to a high-temperature process, whereas the second stage Carnot temperatures (synthesis stage) are lower. 

Let us first consider the implications of not operating the gasification stage at the Carnot temperature, TCarnot . Usually the gasification stage is run at temperatures (T > 600°C) much higher than TCarnot... 

If the gasification stage operated above its Carnot temperature (i.e., T, > TCamot see Figure 17.5), this would imply that the heat added to the gasification stage carried more than the required work with it. This excess work would need to be rejected from the reformers, and if the excess work was not recovered, it would lead to irreversibility of the process and, hence, losses and higher carbon dioxide emissions. 

It is clear from Table 17.5 that in terms of the gasification stage, both enthalpy and Gibbs energy values are significantly smaller than those of the conventional CTL route. The lost work associated with this first stage is also comparatively smaller. In terms of the synthesis section, we note that it operates close to its Carnot temperature (TCarnot = 480 K), and thus the lost work from this process is reduced significantly. Overall, the lost work amounts to 19 kJ/mol, as compared to the 112 kJ/mol for the conventional route. 

The D.M.2 gasifier is also based on an indirectly heated gasification process. The heat needed for the gasification stage is introduced into the process by a heat carrier (e.g., spheres of corundum). One possible process layout for hydrogen production based on the staged-reforming process is shown in Fig. 10.4. 

Inhibition effects induced by chlorine and reactivation by hydrolysis have been reported in the literature, but mainly from a phenomenological point of view in alkali metal catalysed steam gasification studies, However, a description of the charcoal reactivity in the presence of chlorine over the entire gasification stage is lacking. This study utilises the capability of acid washing to remove mineral matter from charcoal to separate structurally from catalytically determined contributions to the charcoal reactivity. 

In the pore model developed by Bhatia and Perlmutter, the rate of the gasification reaction per unit pore surface area is characterised by the reaction rate constant, K,. As the original work addresses structurally based effects only, Kj may well be assumed constant throughout the gasification stage and, under kinetic control, the char reactivity is then a direct measure of the available surface area. To allow the description of additional (i.e., non-porous) phenomena, we follow a semi-empirical approach in which we assume that Kj can vary with time, the cause of which can either be structural or catalytic in nature. Accordingly, we define Ks(t) = KsoucnirtCt) Strictly... 

The value of a in the above expression is ca. 0.97 for tests done in pure hydrogen or in hydrogen-methane mixtures and is approximately equal to 1.7 for a variety of gas compositions containing steam and hydrogen. This parameter is discussed in greater detail in a later section on the low-rate gasification stage. However, for the case where a = 0.97, then M(Xr) - - ln(l - XR). 

The dependence of the conversion rate, dX/dt, on conversion fraction, X, shown in Equation 10 is the same as that used in correlations presented in a later section which were developed to describe gasification in the low-rate gasification stage. With the models assumed, the term (1 — X)2/3 is proportional to the effective surface area undergoing gasification, and the term exp( — X2) represents the relative reactivity of the effective surface area which decreases with increasing conversion level for positive values of a. 

The correlations developed in this study to describe kinetics in the low-rate gasification stage are summarized as follows ... 

Ammonia Wood or high moisture Gasification staged air/or oxygen blown SRI... 

Likely sources of hazard Large volumes of raw gas that is not flammable xmtil mixed with oxygen. Oxygen is used in the upstream gasification stage, hence there is a possible risk of producing flammable or explosive gas mixtures. Potential for internal explosions or fires, external explosions or toxic gas releases. 

S Interactions with other EUCs Gas holder volume (level) rises and falls in response to mismatch between upstream and downstream plant rates. Gas holder contents must be isolated if either upstream gasification stage or downstream processing stage is shutdown. 

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