Gasification rate processes

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. 

If the pore diameter and process conditions are well defined, the rates of internal and film diffusion can be calculated. The temperature dependency of the rate can be presented in the form of an Arrhenius plot, that is, log rate vs. reciprocal absolute temperature. Gasification rates can be divided into three zones, depending on whether reaction rate is controlling,... 

FIGURE 7.2. Schematic illustration of the pressure dependence of the mass burning rate when the gasification process is an unopposed rate process. 

Char Gasification Rates. Despite the impurities and general complexity of the inorganic components, it is still important to compare quantitatively the solid and gas phase conversions achieved relative to calculated theoretical equilibria and the relative rates of char gasification, under various operating conditions, as function of the feed material. This should be useful at least for pyrolytic-spent liquor gasification processes... 

Kroger (3) found that metallic oxides and alkali carbonates or mixtures catalyzed the carbon-steam reactions. Lewis and co-workers (4) stated that if reactive carbons are catalyzed with alkali carbonates, reasonable gasification rates are attainable at temperatures as low as 650 °C. A process which uses molten sodium carbonate to catalyze as well as to supply heat for the carbon-steam gasification has been described (5). 

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. 

The addition of a catalyst to the carbon activation process influences both the gasification rate and the adsorption capacity of the activated carbon allowing to reduce reaction temperature and to select tailoring of the activated carbon pore size distribution. 

Recently (ref. 3) the use of calcium as a catalyst of the carbon-CO, reaction, was investigated in the preparation of activated carbons. Two different porous carbon precursors were used and the porosity of the activated carbons obtained with and without calcium were compared. It was found that the addition of calcium to the CO,-carbon activation influences the gasification rate and the adsorption capacity of the resulting activated carbons. It was proposed that catalytic activation may be used to tailor the pore size distribution in a way which is not possible by the usual uncatalyzed activation process. 

The addition of calcium to the CO -carbon activation process influences noticeably the gasification rate reducing considerably the time needed to prepare activated carbons (see Table 2). 

In this study detailed fault trees with probability and failure rate calculations were generated for the events (1) Fatality due to Explosion, Fire, Toxic Release or Asphyxiation at the Process Development Unit (PDU) Coal Gasification Process and (2) Loss of Availability of the PDU. The fault trees for the PDU were synthesized by Design Sciences, Inc., and then subjected to multiple reviews by Combustion Engineering. The steps involved in hazard identification and evaluation, fault tree generation, probability assessment, and design alteration are presented in the main body of this report. The fault trees, cut sets, failure rate data and unavailability calculations are included as attachments to this report. Although both safety and reliability trees have been constructed for the PDU, the verification and analysis of these trees were not completed as a result of the curtailment of the demonstration plant project. Certain items not completed for the PDU risk and reliability assessment are listed. 

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