Coal burned in power plants in the United States has a energy content of approxi
ID: 1828079 • Letter: C
Question
Coal burned in power plants in the United States has a energy content of approximately 24 kJ/g and an average carbon content of about 62 percent (by wt). For almost all new coal plants, Clean Air Act emission standards limit emissions of sulfur dioxide (SO2) to 260 grams per million kJ of heat input (fuel value). They also restrict particulate
emissions to 13 g/106 kJ.
Suppose the average plant burns fuel with 2% sulfur content (by wt) and 8% unburnable minerals called "ash." About 70% of the ash is released as "fly ash" and about 30% settles out of the firing chamber and is collected as "bottom ash." Assume that this is a typical coal plant with 3 units of heat energy required to deliver 1 unit of electrical
energy.
a) Calculate the emissions of sulfur dioxide, particulates (fly ash), and carbon dioxide per kilowatt-hour of electrical energy produced. Assume that all of the carbon in the coal is released to the atmosphere.
b) Calculate the efficiency (percent removal) of the sulfur emission control system required to meet the sulfur emission limitations.
c) Calculate the efficiency of the particulate emission control system required to meet the particulate emission limitations.
Explanation / Answer
a)
Heat Input = 3 unit of Electrical unit
Electrical unit = KW-hr = 1KJ/s * 60*60 s = 3600 KJ
Heat Input = 3*3600 KJ = 10800 KJ
SO2 conc = 260 gms / million KJ of heat input
so
SO2 emitted = (260/10^6)*10800 = 2.808 gms/ KW-hr
Particulate emmision = (13/10^6)*10800 = 0.1404 gms/KW-hr
1 g cola provides 24 KJ
so 10800 KJ would be provided by 10800/24 = 450 g of coal
coal has 62 % carbon ;
so Carbon emitted = mass of coal *0.62 = 0.62*450 = 279 g /KW-
b)
Coal has 2% by weight of sulfur
so sulfur = 0.02*450 = 9 gms/KW-hr
allowable emission = 2.808/2 gms/KW-hr = 1.404 gms/KW-hr
Efficiency = 1-1.404/9 = 84.4 %
c) 10 % of coal is ash ;
so ash = 0.1*450 = 45 gms/KW-hr ;
70% of ash becomes fly-ash
so fly-ash = 0.7*45 = 31.5 gms/KW-hr
allowable emission = 0.1404 gms/KW-hr
effficiency = 1-0.1404/31.5 = 99.55 %