Coal-steam electricity generation

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A coal-fired steam power plant operates on the ideal Rankine cycle. Feedwater from the

 

condenser at P1 = 10 kPa is pumped to the boiler at P2 = P3 = 20 MPa, where it is

 

superheated and exits the boiler at T3 = 550 °C.

 

The moisture content in the turbine is not to exceed 12% (i.e. steam quality is to be

 

maintained above x = 0.88 throughout the turbine). Up to two reheat stages can be added to

 

the steam cycle to satisfy this requirement.

 

If needed, one stage can be added to reheat the steam to T5 = 525 °C at P4 = P5 = 10 MPa. If

 

more reheating is required, a second stage can be added to reheat the steam to T7 = 500 °C at

 

P6 = P7 = 2.5 MPa. A process diagram for single reheat is shown schematically below.

 

(a). How many reheat stages are required for this power plant (no reheat, single reheat, or

 

double reheat)? Provide calculations in support of your answer.

 

(b).What is the thermal efficiency η th of the power cycle?

 

©. Determine the annual emission of sulfur dioxide, mSO2, in metric tons, from this power

 

plant, if the net generating capacity of the plant is 600 MW, and the boiler is 88%

 

efficient in transferring combustion heat into the feedwater superheat and steam reheat

 

stages. The boiler burns, to completion, Power River Basin coal that has a heating value

 

of 22,800 kJ/kg and contains 0.46 wt% sulfur.

 

Assume that all of the sulfur content of the coal is emitted as SO2 in the combustion flue

 

gas, and that the coal-burning plant has a capacity factor of 0.85; i.e. the boiler is

 

operational for 85% of the hours in one calendar year. (1 metric ton = 1000 kg)

 

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I completed the first 2 parts of the problem andI found out that one reheating stage is enough to active a steam quality morethan 88.2%. I also calculated the thermal efficiency of 38/6% of this powerplant.

My question is about part 3. How can I approach?

thanks

 

 

 

[DeltaVega]

I don't think the value of quality you found is correct...

stage 5 is defined by T = 525 C and P = 10 MPa this gives h = 3437.7 and s = 6.6777

Then when this is extracted through the turbine we have that at stage 6: P = 10 kPa and s = 6.6777 (which is the same entropy as stage 5)

if you look at 10 kPa: s_f = 0.6493 s_g = 8.1502 and so s = 6.6777 gives you a quality of 80.35% not 88% (so this means in this question you need another reheat)

 

now to answer your original question, follow these steps:

Given 600MW and the time the plant is functional in one year find the heat transferred to the steam

Then find the heat of combustion of coal

then find the mass of coal given the heating value

then use the wt% of sulfur to find the mass of sulfur

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Hi,

 

Thanks for the reply. Yes, I realized that I made a mistake calculating the first stage's steam quality. I did calculate the second stage and got a steam quality of 89% which satisfies the problem statement.

 

About part ©, should I divide the amount of sulfur (0.46%) by the molar mass of the sulfur (32.06) or should I directly multiply the amount of coal by 0.46?