Gasification description

The description given apphes to DR processes that are based on the use of gaseous reductants ia shaft furnaces, batch retorts, and fluidized beds. In the processes that use sohd reductants, eg, coal (qv), the reduction is accomphshed to a minor extent first by volatiles and reduciag gases that are released as the coal is heated and then by CO that is formed by gasification of fixed carbon contained ia the coal char with CO2. Reductioa by sohd carboa and coal volatiles ia kilns is insignificant. 

The Texaeo Gasifieation proeess is a eontinuous, entrained flow, pressurised, non-eatalytie partial oxidation process in whieh earbonaeeous solids, liquids or gases reaet with oxygen. Gasification breaks the polymer ehains and eonverts the hydroearbons to their simplest forms. A detailed description is given of the proeess and its eommereial applieation. The proeess is a eommereially... 

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. 

A computer model has been developed to provide numerical simulations of fluidized bed coal gasification reactors and to yield detailed descriptions, in space and time, of the coupled chemistry, particle dynamics and gas flows within the reactor vessels. Time histories and spatial distributions of the important process variables are explicitly described by the model. With this simulation one is able to predict the formation and rise of gas bubbles, the transient and quasi-steady temperature and gas composition, and the conversion of carbon throughout the reactor. 

The computer model is based upon a continuum description of fluidization in coal gasification reactors. In general, fluidized flows are dominated by specific physicochemical processes and, hence, require particular theoretical representations. For example, in the heavily loaded gas-particle regime appropriate to fluidization, the solid particles dominate the transport of momentum and energy. This aspect of fluidization is reflected in the mathematical descriptions which have been used in the fluidized bed model. 

These mathematical representations are complex and it is necessary to use numerical techniques for the solution of the initial-boundary value problems associated with the descriptions of fluidized bed gasification. The numerical model is based on finite difference techniques. A detailed description of this model is presented in (11-14). With this model there is a degree of flexibility in the representation of geometric surfaces and hence the code can be used to model rather arbitrary reactor geometries appropriate to the systems of interest. [The model includes both two-dimensional planar and... 

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... 

With uncatalysed gasification of charcoal, the functionality [l+(bt) ] appearing in Eq, (9) basically simulates the gradual emergence of new surface area (i.e., other than that already constituted by the charcoal pores) by the particle disintegration process, whereas the description of its decline with time by the gasification reaction is implicitly handled by the boundary conditions already set in the original derivation. The volumetric excess reactive surface area involved at time t can be estimated from... 

An important part of the description of the char bed gasification is the chemical reaction kinetics of the char. In this area limited attention has been paid to inhibiting effects on the reaction kinetics of H and CO in the gas. Experimental work has demonstrated that presence of, for instance, 10 % H2 in the reactant gas inhibit the char reactivity with about 90 % compared with no content of Hz- These effects are taken into account in the presented model. 

Figures 6 and 7 show the experimental PFBG results with respect to fuel nitrogen to Ammonia and Hydrogen Cyanide. These species are known precursors for NOx formation under e.g. gas turbine combustion conditions, which is a problem when dry, high temperature gas cleaning is applied, see e.g. Hoppesteyn [9], From the results it can be concluded that a major part of the fuel-bound nitrogen is converted to Ammonia. This has also been indicated before, in the description of the pressurised fluidised bed pinewood gasification using the DWSA test rig, although somewhat lower conversion values are observed in the PFBG tests.

A more detailed description of the gasification of solid waste is presented elsewhere [4]. It is possible to construct the gasifier simply from brickwork and steel, see figure 2, and it has no moving parts. 

Partial oxidation is another important method to convert the petroleum feedstocks to methane rich synthetic gas [20], There are three well-developed commercial processes available in the market the Shell Gasification Process (SGP), the Texaco Gasification Process (TGP), and the Ube Process. In the following text a description of the SGP is presented and this process is compared with the TGP. A description of the Ube process is not included. 

Brief descriptions of commercial gasification processes can be found in the literature 4-8 gasification with steam (Exxon process), gasification with oxygen or air (Koppers-Totzek and Prenflo processes), and gasification with steam/oxygen or air mixtures (Shell, Winkler and Texaco processes). 

Modelling of carbon gasification has been performed by a variety of methods [1, 2], but the adequate description of all the characteristics of this process is not usually achieved, in particular in the respect to the existence of induction periods, that are commonly observed, namely for gasifications performed at lower temperatures, and the decrease in gasification rates for high conversions. 

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