Ash melting temperature

Unlike conventional pulverized-coal combustor, the circulating fluidized-bed (CFB) combustor is capable of burning fuel with volatile content as low as 8%-9% w/w (e.g., anthracite coke, petroleum, etc., with minimal carbon loss). Fuels with low ash-melting temperature such as wood and biomass have been proved to be feedstocks in CFB combustor due to the low operating temperature of 850°C-900 C... 

Extensive field experience has shown the 50 Cr/50 Ni and 60 Cr/40 Ni alloys to offer the best answer to controlling fuel oil ash corrosion. Type 446 stainless steel also shows acceptable corrosion rates but must be used judiciously due to its low strength at elevated temperatures and weldability. Since components of 50 Cr/50 Ni in contact with vanadium-sodium fuel ash melts still suffer high corrosion rates, they should be designed to minimize the amount of surface area available where ash may accumulate. 

Particle size distributions of smaller particles have been made using electrical mobility analyzers and diffusion batteries, (9-11) instruments which are not suited to chemical characterization of the aerosol. Nonetheless, these data have made major contributions to our understanding of particle formation mechanisms (1, 1 ). At least two distinct mechanisms make major contributions to the aerosols produced by pulverized coal combustors. The vast majority of the aerosol mass consists of the ash residue which is left after the coal is burned. At the high temperatures in these furnaces, the ash melts and coalesces to form large spherical particles. Their mean diameter is typically in the range 10-20 pm. The smallest particles produced by this process are expected to be the size of the mineral inclusions in the parent coal. Thus, we expect few residual ash particles smaller than a few tenths of a micrometer in diameter (12). 

A comparison of flame temperatures encountered in both the 11 and 12 boilers and the ash fluid temperatures exhibited by the fly ashes indicates that the flame temperatures are some 500°F higher than that required to melt the ash. 

In the entrained-bed gasifier, very fine coal (< 1 nun), sometimes as a water slurry, enters the gasifier and is mixed with steam and oxygen. Combustion is immediate and a very high temperature is achieved (1,800°C). Most of the ash melts and forms a slag over the inner wall of the vessel. The slag runs down the walls into a collector. Some ash is entrained in the synthesis gas which typically exits the top of the reactor. Heat is recovered by a waste heat boiler and the ash is then removed by means of a cyclone. 

Dry Mineralisation Dry mineralisation techniques include incineration (ashing), low-temperature mineralisation in oxygen plasma, mineralisation in oxygen, and melting. 

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