Chapter 5, Problem 5.67P

A small power plant produces 500 MW of electricity through combustion of coal that has the following composition on a dry basis: 76.2 wt% carbon, 5.6% hydrogen, 3.5% sulfur, 7.5% oxygen, and the remainder ash. The coal contains 4.0 wt% water. The feed rate of coal is 183 tons/h, and it is burned with 15% excess air at 1 atm, 80°F, and 30.0% relative humidity.

(a) Estimate the volumetric ?ow rate (ft³/min) of air drawn into the furnace.

(b) Effluent gases are discharged from the furnace at 625°F and 1 atm. Estimate the molar (lb-mole/min) and volumetric (ft3/min) ?ow rates of gas leaving the furnace.

(c) Injection of dry limestone (CaCO₃) into the furnace is being considered as a means of reducing the SO₂emitted from the plant. The technology calls for SO₂to react with limestone:

   ${\text{CaCO}}_{\text{3}}{\text{+SO}}_{\text{2}}\text{+}\frac{\text{1}}{\text{2}}{\text{O}}_{\text{2}}\to {\text{CaSO}}_{\text{4}}{\text{+CO}}_{\text{2}}$

Unfortunately, the process is expected to remove only 75% of the SO₂in the ef?uent gases, even though the limestone is fed at a rate 2.5 times the stoichiometric amount. What is the required feed rate of limestone? Since some of the SO₂is removed from the furnace ef?uent [in contrast to Pan (b)], recalculate the molar ?ow rate and composition of the ef?uent from the furnace.

(d) The gas leaving the furnace passes through an electrostatic precipitator, where particulates from ash and limestone are removed, and then enters a stack (chimney) for release to the atmosphere. What is the gas velocity at a point in the stack where the stack diameter is 25 ft and the temperature is 300°F? Does the gas discharged from the stack meet the new Environmental Protection Agency standard that emissions from such power plants contain less than 75 pans of SO₂per billion?