Question

Based on the boiling curve why is boiling to 100% quality avoided in risers?

The number of natural gas fossil fuel power plants have now exceeded coal plants. Many of the coal plants are being converted to natural gas. Explain 4 items that would change at the plant as a result of this conversion.

Answer 1

The boiling regimes are best described in terms of a boiling curve showing heat flux or heat transfer coefficient as a function of ingot surface temperature. Below ~ 100 °C convection cooling occurs, above that nucleate boiling takes place where steam bubbles nucleate in non-wetted crevices on the ingot surface . The release of these bubbles into the water generates stirring in the water and reduction of the boundary layer and a high heat flux. These bubbles collapse in the water. If the ingot surface temperature is higher than some critical temperature there are so many steam bubbles that they start to agglomerate and form a periodically collapsing steam layer on the ingot. This boiling regime is known as unstable film boiling where the heat flux is reduced by the low thermal conductivity of the steam layer. At temperatures above the Leidenfrost temperature the steam layers become stable and fully stable film boiling occurs.

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changes in conversion of coal to gas plant

A conversion is much less costly than installing large Air Quality Control System Equipment such as SCRs, Fabric Filters, Flue Gas Desulphurization (FGD) systems, etc.
and has lower emission characteristics while burning a lower cost fuel.
Converting to natural gas also means getting rid of all the material handling and associated costs that go with burning coal
A conveyor carried the coal to the units, where it would be pulverized. All of that equipment is gone
Converting the plant's boilers involved installing natural gas piping and replacing the burners.
Just making the switch to natural gas cut carbon dioxide emissions by 45 percent, but the ability to crank down the boilers cut carbon dioxide emissions another 15 percent.
A pipeline would need to be built to carry natural gas to the boiler building because the plant doesn't currently use gas as a fuel source.