Tall Hoppers for Pressurizing
Another option is to feed the coal via so-called dynamic hoppers (see Figure 6-4) (Visconty 1956). Pulverized coal is fed to the top of a high hopper and flows down through the hopper to a vessel that is pressurized. The height of the coal column in the hopper should be such that the pressure in the vessel is less or equal to the static height of the column. Moreover, the downward velocity of the coal should be higher or equal to the velocity of the gas from the pressurized vessel that wants to migrate through the interstices between the coal particles to the lower pressure space at the
PULVERIZED
COAL
jL
Figure 6-4. Tall Lock Hopper Feeding Device top of the column. When this is not the case, the gas has a chance to expand and the coal column would be blown out of the hopper. The relevant formulae are:
where Ap is the pressure required to overcome the difference between, for example, the pressure in the gasifier and the atmospheric pressure. The bulk density of the coal is p, the height of the column filling the tall bunker is h, the average vertical velocity of the gas in the interstices is v, the dynamic viscosity of the gas T|, the specific surface area of the pulverized coal is S, and the porosity of the coal/gas mixture in the column is £. The “5” is an empirical constant.
Taking SI units and the bulk density of coal as 1000 kg/m3, the minimum height of the bunker to overcome a pressure difference of 10 bar (= 106 Pascal) is 100 m. Taking h= 100 m, p as 1.5 x 10_6kg/s. m, S as 40000 m2/m3, and £ as 0.4, this results in vcoa]=0.13m/s. This means that for a 2000 t/d=0.023 m3/s coal gasifier, the bunker should have an internal cross-section of 0.023/0.13 = 0.18 m2, that is, 500 mm diameter.
The calculation shows that such bunkers are very thin and could advantageously be located near, or rather be incorporated in, a tall structure such as a stack. The coal can be either transported pneumatically to the top of the bunker or by mechanical means.
A problem with the use of tall hoppers is that the maximum pressure that can be obtained in a one-stage operation is about 20 bar, and even then a 200 m high structure is required. Proposals have been made to build them as a multistage machine in which much higher pressures can be reached. Were a 200 m structure to be built, a pressure of 40 bar could be reached with a two-stage operation. This would be sufficient for most applications (van der Burgt 1983).
Hang-ups in the bunker are not so likely to occur, as the bunker will operate near the point of incipient fluidization. Moreover, they can be avoided by building a bunker with an annular cross-section where the center part is turning very slowly to avoid any bridging or blocking.
The use of a small amount of inert gas in the column is probably mandatory.
The principle of these hoppers is the same as that of centrifugal devices that have been proposed (van der Burgt 1978, 1982). The difference is that instead of the centrifugal field, the gravitational field is used. In all these dynamic hoppers there will be hardly any contamination with the gas at the top of the hopper.