Ratio, oxygen/coal

Carbon Conversion. Carbon conversion on a once-through basis is a function of the coal composition and is strongly influenced by the oxygen/coal ratio. For some coals, the conversion pattern is also affected by the level of steam in the blast. Another factor is fly slag recycle, which raises the carbon conversion by recycling the unconverted carbon, most of which resides on the fly slag. This results in an overall carbon conversion greater than 99%. 

Volume and Heating Value. The volume of dry gas produced less the volume of coal carrier gas fed to the reactor is shown in Figure 11 as a function of the oxygen/coal ratio. The volume produced increases uniformly with this ratio. The corresponding heating value of the dry, carrier-free gas is shown in Figure 12. 

Figure 12. Variation of heating value of dry, carrier-free product gas with oxygen-coal ratio...

The volume of gas produced per pound of coal increases uniformly with the oxygen/coal ratio. However, this increase may be simply the result of increased reactor temperature. At the ratio corresponding to maximum hydrocarbon gas yield the volume produced is 22 scf/lb of coal. 

The Koppers-Totzek (K-T) gasifier produces a medium-Btu gas (in the general range of 300 Btu/scf) and has been commercially employed in many different syngas applications, with particular emphasis in the area of ammonia synthesis. The process is carried out at just over atmospheric pressure but at very hi temperatures of over 1870°C. The data in Table 3 [16] give the expected K-T gasifier product composition for an Illinois coal (62% C, 19.1% ash, 4.4% H2, and 5% S plus 02 and H20) that has been ified with a steam-coal ratio (wt/wt) of 0.27 and an oxygen/coal ratio (wt/wt) of 0.7. K-T units vary in size between those that convert about 300 t coal per day and those that convert over 750 t coal per day. 

Many reactions occur simultaneously in coal gasification systems and it is not possible to control the process precisely as indicated here. But by careful selection of temperature, pressure, reactant and recycle product feed rates, reaction times, and oxygen-steam ratios, it is often possible to maximize certain desired products. When high-energy fuel gas is the desired product, selective utilization of high pressure, low temperature, and recycled hydrogen can result in practically all of the net fuel gas production in the form of methane. 

The most important operating parameters are coal concentration in the CWS, gasification temperature, gasification pressure, residence time, and oxygen/carbon ratios. These effects are discussed below. 

The anomalous behaviour of No. 26 coal may be related to the high oxygen sulphur ratio, in this coal (Table I) and this is being investigated. In addition, it must be recognised that this SEM work was carried out on selected particles, which are not necessarily indicative of the bulk. 

The effect of potassium in the form of potassium carbonate or potassium silicate on reduction of NO, on coal chars was also investigated [108-111]. The best materials were prepared by pyrolysis of coal at 1300 K with high KOH/coal ratio [108]. On these adsorbents, at temperature smaller than 473 K, physical adsorption is predominant while the true NO, reduction by char occurs at T> 473 K with formation of Nj and COj. The results indicated that a material with the high surface area should be used to promote adsorption of NO, and potassium remaining in chars catalyzes NO reduction in the presence of oxygen [109]. 

In test 917 the same size coal was similarly heated and then dropped through the treater at a rate of 40 lbs. of coal/hr. Steam containing 9.4 mole % oxygen flowed countercurrent to the coal at 605°C. and 300 p.s.i.g. The steam - and oxygen coal weight ratios were 1.4 and 0.26, respectively. The treated coal did not cake when exposed to hydrogen at 600°C. It had a FSI of 1.5, and its volatile matter content was 26.7%. 

Combustion Equivalence Ratio. The effect of varying the equivalence ratio of combustion hydrogen to combustion oxygen was tested by operating the reactor with coal feed rate, oxygen-to-coal ratio, and carrier gas rate constant, and varying the combustion hydrogen feed rate. 

Figure 13. Effect of varying combustion gas equivalence ratio. Coal feed rate was 2 lbs/hr, and oxygen-to-coal ratio was 0.51.

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

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