Designing coal feeders for a powerplant.

[Yander]

I was given this problem as an excercise in thermodynamics and to better understand powerplant systems. I am having difficulties because as a civil engineering student I haven't taken a lot of thermodynamics. The one course I did take was several semesters ago and wasn't that advanced.

Homework Statement

We are building a 600 MegaWatt (gross) peaking unit coal plant.
The plant burns weatern coal assumed to have 8200 BTU/lb

Turbine Conditions

• Turbine has High pressure, Intermediate Pressure and 2 Double Flow low pressure turbines.
• Main Steam at 2400 psig, 1000 degress F
• Reheat Steam at 1050 degrees F
• Exhaust Condenser at 2.0 in Hga
• No extractions for feedwater heaters

Boiler Data

• Western coal is assumed to have 5% ash
• 50 psig drop through the reheater
• 75 psig drop through the econmmizer, waterwalls, drum, and superheater.
• Stoichiometry in combustion zone is 0.97
• Furnace exit gas temp is 600 F

Boiler Losses, in % of full load

• 4.4% dry flue gas
• 3.3% as fired fuel moisture
• 4.4% H2 moisture
• 0.7% Unburned carbon
• 0.2% radiation and convection
• 0.1% moisture in air
• 0.5% unaccounted for losses

Turbine Data

• HP exhaust pressure of 600 psig
• HP effeciency of 0.88
• IP/LP efficiency of 0.88

Use motor driven boiler feed pump

Do not consider the following:

• Bearing and generator mechanical losses
• Shaft seal leakages
• Radiation and convective turbine losses
• Station power loads
• Atemperating sprays

The unit can reach full load with 5 coal mills. Design the neccesary coal feeders (Tons/Hr) to supply these mills to sustain the powerplant.

The Attempt at a Solution

I am not looking for someone to solve this for me, I am just trying to figure out where to get sarted. All the infromation is overwhelming. Can someone tell me where in the system I am supposed to start and maybe point to where online I can see good examples of Rankine cycle problems being worked?

 

[Zryn]

You have a lot of information there, but do you have a picture or otherwise of the flow of a Power Station from 'Coal In' to 'Power Out' ?

Creating separate modules for each section and identifying the characteristics (inputs outputs & processes) of each of these modules will be helpful in translating the overall input units of Tons/Hr to the overall output units of MJoules/Second (MW).

Make sure your units of measurement are consistant! I would use SI all throughout, but that is my learning bias and you may be happy/familiar with whatever those units listed are.

 

[Yander]

Good tip. I am currently drawing the system and writing the information and the useful equations I know for each component.

 

[Yander]

Here is a crude drawing I made of the problem with some of the relevant information.

I was wondering if anyone knows the typical state of the exhaust steam exiting a High Pressure turbine before it goes to the reheater. Is it superheated or saturated? How do I find the enthalpy there? I have the pressure but can't find the temperature if it is superheated. If it is saturated, the temperature is dependent of the pressure but I can't find the enthalpy without knowing the quality of the steam (x).

My current plan of attack is to find the enthalpy drop across the HP and the enthalpy drop across the IP and LPs.

Using the above equation and knowing the efficiencies of the turbines are .88 and the power plant produces 600 MW I could find the mass flow and go from there.

Any suggestions or comments would be appreciated. Thanks

 

[Zryn]

If the steam at any point in the system becomes saturated, then you begin to get small droplets of water in the system correct?

In order to get the most energy out of the steam, the steam needs to be at the point just before it becomes saturated, because at those temperatures and pressures, you can easily cut/puncture/pockmark steel (both the turbine blades and the turbine itself) with water droplets.

 

[Yander]

Thanks, that makes sense, but how do I get the enthalpy after the hp turbine or after the lp turbines. Am I supposed to be working from somewhere else in the system?

My other idea is maybe tabulating the solutions to the system at different massflows until I find a value where everything jives, but I still don't think I have all of the neccesary values.