The following are alternatives by which the scrubbed flue gas can be reheated:

(1) Bypassing the scrubber with a fraction of the flue gas leaving the air preheater and mixing this stream with cleaned flue gas;

(2) Burning natural gas and blending the combustion products with the scrubbed flue gas;

(3) Using steam from the power cycle either to heat air that is blended with the scrubbed flue gas or to heat the flue gas in a heat exchanger.

a. Give a reason for rejecting Alternative 1.

b. In evaluating Alternative 2, assume that the natural gas consists entirely of methane at 25°C and that it is burned adiabatically with 10% excess air that has the same conditions as the air fed to the furnace. If the combustion products are blended with the cleaned flue gas, at what rate would methane would have to be burned to raise the stack-gas temperature to the desired value?

c. How much more coal (kg dry coal/h) would have to be burned for Alternative 3 if the amount of heat released by burning coal (kJ/kg dry coal) is the same as determined in Problem 14.7? Suggest two process alternatives by which the heat can be transferred to the flue gas.