Dry Feed Systems

A typical operating range is indicated, around 350kg/m and below 50 bar [1], 

Given availability on site, CO2 offers a technically proven alternative to N2 with respect to its critical behavior ( crit = 31 °C, = 73.8 bar). The main 

There are considerable ongoing research and development projects targeting the replacement of the lock hoppers to pressurize the coal by so-called solid feed pumps. Such technology is developed by Stamet (now GE, see Section 6.1.4.3) [17], Pratt Whitney Rocketdyne (now Aerojet Rocketdyne) [18], and TK Energi [19]. 

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When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

Due to the dry feeding of the Shell gasifier, the 02 consumption is reduced, and the efficiency is increased by -2% compared with the CWS feeding system. However, the maximum gasification pressure is limited to about 5 MPa due to the dry-feeding system, as opposed to 8 MPa for some CWS feeding systems. 

The dry-feeding system of the Shell gasifier requires the water content in the bituminous coal to be less than -2 wt %. For high-moisture, low-rank coal, the required water content could be less than 10wt%. 

It is also believed that nozzle life is affected by feedstock characteristics. Although the dry-feed system may be more difficult to operate at high pressures, nozzle life may be longer in the absence of large amounts of evaporating water. Deeper understanding is needed to identify the effect of feedstock preparation. 

The feed system is an important choice for entrained-flow gasifiers. GE and E-Gas use a coal-water slurry compared with the dry-feed systems of Shell and Future Energy, which require lock hoppering. 

The various gasifiers dififer in method of feeding and in waste heat recovery [230], The GE (Texaco) gasifier feeds coal in a waste slurry (paste) which means that oxygen is used for evaporation of water. However, the slurry feed system allows a much higher pressure (up to 70-80 bar) than possible for gasifiers with dry feeding systems. 

In the second section of Table 5.11, the stream properties used are summarized. In the case of dry-feed systems, coal is conveyed into the reactor by means of nitrogen. The coal/nitrogen mixture is set to 25 °C, while a solids loading of 350kg/m (eff.) is assumed, representing the mean value of the typical range... 

The highest overall cold gas efficiency can be achieved by reducing heat loss in a dry-feed system, which might be realized by refiractory lining. 

Given the feasibility of dry-feed systems, elevated moisture does not considerably affect cold gas efficiency as long as moderator steam is replaced. Any excess leads to a significant increase in oxygen consumption and a drop in cold gas efficiency. 

The carbon conversion is 99.9%, the same as to other dry-feed systems. 

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