PrintTwo Kinds of Coke
There are two kinds of coke produced by delayed coking of VDR: high-density shot coke, and porous sponge coke. Figure 6.7 shows the appearances of shot coke, consisting of aggregates of ~5 mm diameter spherical particles (resembling buckshots) and sponge coke (with a porous structure resembling a sponge). The formation of shot coke is usually troublesome because of difficulties in removing the coke from the drums and problems with grinding, although shot coke has some niche applications, such as in titanium dioxide (TiO2) production. Sponge coke is used as solid fuel, and manufacturing anodes for aluminum production, if its sulfur and metal concentrations are sufficiently low.
Among the delayed coking products, needle coke is a specialty coke produced mostly from coking of a highly aromatic FCC decant oil. The major properties of the needle coke include a low coefficient of thermal expansion, a low puffing (sudden volume expansion) tendency during graphitization because of lower nitrogen and sulfur contents, and high mechanical strength. The anode coke has limits on metal contaminants, requiring less than 500 ppm of Ni and V in the coke. The price of fuel coke depends on its carbon purity (S, N, and metal contaminants); however, the fuel coke is traded at a price comparable to that of coal.