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Do you think this waste-to-energy plant will work?

  1. Jan 9, 2017 #1
    Some people say it's ambitious, some people say it's crazy and some people say it just won't work. What do you think?
     

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  2. jcsd
  3. Jan 9, 2017 #2

    anorlunda

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    It would cost a fortune and almost certainly be less efficient than using all the energy to make steam to run a steam turbine.

    A very important principle in good engineering is called KISS (keep it simple stupid). That picture you posted is the opposite.
     
  4. Jan 9, 2017 #3
    So are you saying it doesn't work? A complete one-stop-shop that will take all city waste and return electricity, gas and water is something worth considering I think. Electricity isn't only generated from steam, there is a GTE that governs/assists a bootstrap which also provides additional water heating.
     
  5. Jan 9, 2017 #4

    russ_watters

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    #3 needs to be a pump, not a turbine (otherwise you can't make the water/steam circulate) and the idea glosses over what is actually needed to filter hazardous waste out of the boiler exhaust. Burning the hazardous waste is the last thing you want to do to it. Also, thermoelectrics are really inefficient and expensive, so I wouldn't bother with that part.

    Also, I don't see what the "bootstrap/gas turbine" is doing.
     
  6. Jan 9, 2017 #5
     
  7. Jan 9, 2017 #6
    The idea of #3 as a water turbine at the base of the Water boiler/heater is that it will generate some electricity via gravity and pressure feeding/regulating heated water into the the incineration chamber for super heating, eventually reaching super-critical levels. As for the bootstrap, it is in essence a self-sustaining system that takes advantage of the temperature differential with a turbine and compressor to heat a larger volume of water, it will be governed by a Gas Turbine Engine (GTE) for control and start-up assitance. Also the GTE is used for overall power-plant start-up and additional power-plant electricity generation, obviously the exhaust will run through the water boiler for additional heating. Thermoelectrics might be a bit far away but still worth investigating as there are some commercially available technologies.
     
  8. Jan 9, 2017 #7

    russ_watters

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    If you use gravity to spin the turbine, you end up with water at the bottom of the boiler at very low pressure. You need something to generate the high pressure to spin the turbine at the top of the boiler. That something is a pump. I suppose you could use a pump to pressurize the feedwater from the waste, but then you are just using a pump to spin a turbine, which is pointless.

    If you attach values to the pressure it will become more clear (they are more important than the temperatures, which actually tend to follow from the pressures).
    If you intend to use the heat of the boiler to drive a steam cycle, it doesn't make any sense to also power a gas turbine -- one will be better than the other, so only use the better one, since you can't use the same energy twice.
     
  9. Jan 9, 2017 #8
    Thank you for your response,

    The head of pressure at the height of the boiler will be quite significant considering that super-critical vapour will be directed to the top of the boiler once it has passed through the incineration chamber. This will give some pressure directed towards the base of the boiler along with gravity that will ensure water flow through the water turbine base, however most of the high pressure steam will run through the Steam Turbine. The temperature will be around ~100 C at the base of the boiler before being feed into the incineration chamber.The water boiler and incineration chamber are essentially strapped together in a closed system with the only exit through the Steam Turbine. The feedwater into the boiler will need to be pumped in, although it will have some gravity pressure assistance.The hope is that the water turbine will offset the energy requirement for the feedwater into the boiler and more.

    As for the bootstrap, it will be self-sustaining. Hot air rises and becomes less dense, it will pass through a compressor where it cools and becomes more dense before passing through a turbine, then repeating the same process all in a closed system with the compressor and turbine rotating on the same spool and governed by the Gas Turbine Engine (GTE). In this process, there is an obivious transfer of heat from the incineration chamber to the water boiler. The system is a closed loop and is mostly self-sustaining, commonly and similarly seen in aircraft air-conditioning and refrigeration systems. The GTE governs and assists through a transfer drive with minimal energy efficiency loss and significant gain from heat transfer to the water boiler. At the same time the GTE provides its own electrical supply to the plant and also the waste heat runs through the water boiler, which is commonly seen in combined gas cycle system power-plants.

    I agree that further values need to be supplied, perhaps if a government wants to grant a subsidy? :-), Although I wouldn't hold my breath in Australia!
     
  10. Jan 9, 2017 #9

    russ_watters

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    The boiler tube is just a tube; the pressure doesn't change from the bottom to the top except for the head (which means it is even lower at the top).
    ....using the boiler as the source of the energy.
    The "boiler" is the place where water boils: that's the thing on the right, not the thing on the left. In either case, yes, you are using the energy from the boiler to preheat the feed water, while siphoning some off to drive another turbine. Again: unnecessarily complicated since you can only use the energy once.
    Unlikely. This is something you should be able to develop better as part of an introductory thermodynamics class. No need for a grant.
     
  11. Jan 9, 2017 #10

    What is your background, are you a thermo or power-plant engineer? You seem to be getting mixed up here, just so I can provide you with a more simple explanation. You do understand the laws of thermodynamics right?
     
  12. Jan 9, 2017 #11

    russ_watters

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    I'm a mechanical engineer, specializing in HVAC. What is your background?
    I don't think so.
    Your explanation is already too simple: you need to develop it further with more details*. That's what is tripping you up. However, one simple principle: do you recognize that a boiler does not increase the pressure of the working fluid that flows through it?

    Anyway, the usual way to describe a thermodynamic cycle is to label, specifically, the four states/processes in the cycle. Can you do that with each of the cycles in your idea?

    *[edit] Or, alternately, start with a complete description of the core thermodynamic cycle, then build on it. The core cycle you have there is a simple steam power cycle.
     
    Last edited: Jan 9, 2017
  13. Jan 9, 2017 #12
    Mine is Aircraft Maintenance Mechanical Engineering.

    I agree further details are needed, however surely as an engineer you would recognize the challenge in doing the math for such an integrated waste-to-energy plant design. Are you saying this will not work?

    Good to hear that it isn't too simple and that you understand what a bootstrap is. The pressure, density and temperature relationship of water and its transition to a vapour in such a system indicates that high pressure will build towards the top of the water boiler (the blue thing on the left) once the water has been feed into the incineration chamber and reached super-critical vapour. As I said the water boiler and incineration chamber a closed system with the only out being through the Steam Turbine. The Water Turbine at the base of the water boiler (the blue thing on the left) is to provide additional energy recovery and regulate water inflow (which the water is already at ~100 C) into the incineration chamber. Not sure how you are getting mixed up here.

    Your comments about an undergrad in thermodynamics being able to do the math for such a design is amusing. 3rd year Mech students design little fans and wheels, not waste-to-energy plants LOL.
     
  14. Jan 9, 2017 #13

    russ_watters

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    I don't think it is very difficult and yes, as drawn, at least your primary cycle will not work. There is nothing in the cycle that can generate pressure to drive the cycle.
    I'm sorry, but that just isn't the case. With only gravity available, the pressure is highest at the bottom and lowest at the top. It does not increase as the water/steam flows up through the boiler (the thing on the right).
    Sorry, I but that just isn't the case. Third year mechanical engineers can analyze multi/combined-cycle thermodynamic plants.

    At this point, I'm going to need to insist on making this thread more rigorous. This isn't a place for pissing matches. Please provide the details of your primary cycle; the states/process and the pressure/temperature at each. Do you know the four states/processes of a Rankine cycle?
     
  15. Jan 10, 2017 #14
    Not sure if you are aware of temperature and how it affects on air/gas. Bootstraps are a proven system. Obviously you are not experienced with such systems.

    The pressure in the boiler (the blue thing on the left), will naturally have a higher pressure towards to top end of the boiler as much more heat is concentrated there. Heat rises doesn't it?

    3rd years might analyze and attempt to understand, but not design. Are you are 3rd year undergrad?

    Insist on what you like, however I will not be presenting the math on here. I am confident it works. If you believe otherwise, find a thermal, chemical, electrical and power-plant engineer to state their case on the record. Not some 3rd year undergrad.
     
  16. Jan 10, 2017 #15

    russ_watters

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    By the principle of hydrostatic pressure, the pressure at the bottom of the water tank is higher than the pressure at the top:
    http://www.edinformatics.com/math_science/hydrostatic_pressure.htm

    "Heat rises" does not mean pressure increases as you go up -- heck, it doesn't even mean the temperature is higher at the top.
    Fair enough; Thread locked. If you change your mind and want help, PM me with more details and I'll re-open the thread so we can analyze them. I suggest, however, you start simpler with learning some of the basic principles of physics and thermodynamics.
     
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