Coke burning problems

A third problem was that of fast coke. This coke also exhibited first-order intrinsic burning, but with a rate constant 17 times that of slow coke at 950°F (783 K). Again a fortunate simplification was found so that the kiln equations would not have to be solved for two kinds of coke burning... 

The radiation from a flame is due to radiation from burning soot particles of microscopic andsubmicroscopic dimensions, from suspended larger particles of coal, coke, or ash, and from the water vapor and carbon dioxide in the hot gaseous combustion products. The contribution of radiation emitted by the combustion process itself, so-called chemiluminescence, is relatively neghgible. Common to these problems is the effect of the shape of the emitting volume on the radiative fliix this is considered first. 

The separation of n-alkanes from a kerosene or gas oil fraction by a molecular sieve can be performed in a liquid phase or in a gas phase process. In the gas phase processes there are no problems of cleaning the loaded molecular sieve from adherent branched and cyclic hydrocarbons. However, the high reaction temperature of the gas phase processes leads to the development of coke-contaminated sieves, which have to be regenerated from time to time by a careful burning off of the coke deposits. 

Commercial Development of Fixed-Bed Process. From the above process considerations it became obvious that the capacity of a commercial unit, and its economic value, were closely related to its ability to burn coke. Indeed, most of the design problems associated with catalytic cracking have been centered around the question of catalyst regeneration. To obtain the most favorable economic return from a 10,000-barrel-per-day unit, it was designed to burn approximately 6000 pounds per hour of coke. This coke yield represents approximately 5% by weight of the charge. 

The coke deposited on the catalyst is burned off in the regenerator along with the coke formed during the cracking of the gas oil fraction. If the feedstock contains high proportions of metals, control of the metals on the catalyst requires excessive amounts of catalyst withdrawal and fresh catalyst addition. This problem can be addressed by feedstock pretreatment. 

Sulfur Content. Another important feedstock physical property related to delayed coking is the sulfur content. The sulfur present in the feedstock tends to concentrate in the coke, where the sulfur level is usually equal to or higher than that of the feedstock. Sulfur levels as high as 4 weight % in today s feedstocks can cause unacceptably high levels of sulfur in the coke product. The resulting coke may not be acceptable for metallurgical use and may be a problem when burned as fuel. 

Extensive coke deposition takes place during catalytic cracking, resulting in loss of activity. Typically, the catalyst loses 90% of its activity within one second. An elegant solution has been found for this problem. The clue to this solution is a combination of a reactor in which cracking takes place with a reactor used for regeneration of the catalyst by burning the deposited coke. In this set-up coke is... 

An examination of this problem was provided by Weisz and Cktodwin [11,12]. The pellets were silica-alumina cracking catalyst, and the coke resulted from the cracking of light gas oil and naphtha. Measurements of the burning rate were followed by oxygen consumption rates, as shown in Fig. I. 

However, underfeed stokers tend to exhibit incomplete combustion and can suffer serious screw-feeder erosion problems with abrasive coal (such as high mineral matter coal). These systems also exhibit the tendency to form coke trees in the bed. If the fresh coal becomes plastic and carbonizes on its way up to the combustion front, a central solid mass of coke forms. Since the coke is largely impermeable to air, and remote from the air tuyeres, a coke tree will not burn and has to be broken down manually. Therefore, caking coals are not attractive for the underfeed stoker. Because ash removal at best is only semiautomatic, low-ash fuel is preferred. The combustion temperature should be controlled so that the ash melts and fuses into large chunks rather than a powdery ash that causes grit and dust emission problems when carried over. 

Burners for Other Gases. Many industrial applications utilize coke-oven gas, blast-furnace gas, refinery gas, or other industrial by-product gases. With these gases, the heat release per unit volume of fuel gas may be very different from that of natural gas. Hence, gas elements must be designed to accommodate the particular characteristics of the gas to be burned. Other special problems may be introduced by the presence of impurities in industrial gases, such as sulfur in coke-oven gas and entrained dust in blast-furnace gas. 

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