Using alcohol as a working fluid in boilers

trevor borocz johnson

trevor borocz johnson

Registered Senior Member
Mixing alcohol with water as the working fluid of a boiler would reduce the amount of heat needed to get the same pressures from the boiler. A Organic Rankine Cycle is when they use a condenser to convert the steam from a boiler back into liquid and pump it back into the boiler. Obviously if you were to use 100% alcohol as working fluid you would want to use the Rankine cycle so you aren't allowing flammable gas everywhere. I found some information on experiments with differrent working fluids in boilers most around the turn of the century and no word the amount of energy burned to create pressures, just that they were generally low pressure systems. Here's a Rankine cycle that I invented:
thermometer power plant.jpg
 
trevor borocz johnson said:
Mixing alcohol with water as the working fluid of a boiler would reduce the amount of heat needed to get the same pressures from the boiler. A Organic Rankine Cycle is when they use a condenser to convert the steam from a boiler back into liquid and pump it back into the boiler. Obviously if you were to use 100% alcohol as working fluid you would want to use the Rankine cycle so you aren't allowing flammable gas everywhere. I found some information on experiments with differrent working fluids in boilers most around the turn of the century and no word the amount of energy burned to create pressures, just that they were generally low pressure systems. Here's a Rankine cycle that I invented:
View attachment 5084
Click to expand...
Yes, the Rankine Cycle applies to any closed cycle heat engine that works on converting liquid to to vapour and condensing it again.
 
trevor borocz johnson said:
Mixing alcohol with water as the working fluid of a boiler would reduce the amount of heat needed to get the same pressures from the boiler.
Click to expand...
Yep. Ammonia also works. There are a lot of working fluids out there you can use. Note that choosing a different working fluid may optimize temperatures and pressures, but will not get around basic issues of thermodynamic efficiency like the Carnot limit.
 
For most part they use the Rankine Cycle in power plants to recycle heat from WATER though, I have yet to read about any information on using alcohol or gasoline or any mixture. By turn down the heat I mean burn less coal to get the same results.
 
trevor borocz johnson said:
For most part they use the Rankine Cycle in power plants to recycle heat from WATER though, I have yet to read about any information on using alcohol or gasoline or any mixture. By turn down the heat I mean burn less coal to get the same results.
Click to expand...
You are making the mistake of assuming "lower boiling point" = "more efficiency." You can achieve exatly the same thing by operating at lower pressures with water. But it will not improve efficiency.
 
billvon said:
You are making the mistake of assuming "lower boiling point" = "more efficiency." You can achieve exatly the same thing by operating at lower pressures with water. But it will not improve efficiency.
Click to expand...

Lower boiling point and higher volatility. The pressure in the boiler would be the same while burning less fuel to obtain those pressures. I don't get what you're getting at about efficiency.
 
trevor borocz johnson said:
Lower boiling point and higher volatility. The pressure in the boiler would be the same while burning less fuel to obtain those pressures. I don't get what you're getting at about efficiency.
Click to expand...
You said "burn less coal to get the same results." That means an increase in efficiency.
 
billvon said:
You said "burn less coal to get the same results." That means an increase in efficiency.
Click to expand...
My thermodynamics gets hazy at time these days but isn't the latent heat input, to change from liquid at atmospheric pressure into vapour at atmospheric pressure, and which is subsequently rejected as waste heat in the condenser, something that is basically wasted from the viewpoint of the thermodynamic cycle?

In which case, using a working fluid with a lower latent heat of vaporisation should improve the efficiency of the cycle, shouldn't it?
 
exchemist said:
My thermodynamics gets hazy at time these days but isn't the latent heat input, to change from liquid at atmospheric pressure into vapour at atmospheric pressure, and which is subsequently rejected as waste heat in the condenser, something that is basically wasted from the viewpoint of the thermodynamic cycle?

In which case, using a working fluid with a lower latent heat of vaporisation should improve the efficiency of the cycle, shouldn't it?
Click to expand...
Well, keep in mind that any thermal plant is designed to work best with one working fluid. If, for example, you decrease the boiling point, you transfer less heat to the fluid in the boiler - and that has downstream effects (like less energy available later.)

Also note that no modern thermal plant works like an old fashioned steam engine as depicted in a science book. Superheat is usually used, as is heat recovery / economizers. All of that increases the efficiency of the engine, and all of that has to be carefully matched to the working fluid.
 
billvon said:
Well, keep in mind that any thermal plant is designed to work best with one working fluid. If, for example, you decrease the boiling point, you transfer less heat to the fluid in the boiler - and that has downstream effects (like less energy available later.)

Also note that no modern thermal plant works like an old fashioned steam engine as depicted in a science book. Superheat is usually used, as is heat recovery / economizers. All of that increases the efficiency of the engine, and all of that has to be carefully matched to the working fluid.
Click to expand...
OK but I'm not talking about the boiling point. I'm talking about the Latent Heat of Vaporisation: the amount of heat you have to add to convert a given amount of liquid to vapour at the same temperature and pressure. Water has a high latent heat of vaporisation, and all the heat put in to vaporise it has to be be extracted again and wasted in the condenser part of the steam circuit. I should have thought that a different fluid with lower latent heat would cut down on this waste.

However, thinking about this more, I see your point about pressure. Enthalpy of vaporisation is ΔU + pΔV, so if you are evaporating against the back-pressure of superheated fluid, it is a lot more than just the internal energy change. I also see that, for example, n-heptane which boils at 98C has an enthalpy of vaporisation (at 1bar) of 36kJ/mol whereas that of water is 41kJ/mol, so the difference is not that great. (I use the molar values because a mole produces the same volume of vapour.)

So maybe the difference is fairly negligible- and of course handling water and steam is less hazardous that some flammable organic fluid.
 
You must log in or register to reply here.
Back
Top