Idea solar power & NH3 fertilizer economically

Discussion in 'Chemistry' started by Billy T, Apr 14, 2008.

  1. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    23,198
    I am not a chemist. I need a little help here. Thanks if you can.

    The preparation of natural gas (removal of sulfur, “reforming with steam”, addition of nitrogen from the air, etc.) for the commercial production of ammonia (fertilizer) is a complex process that precedes the exothermic Haber-Bosch synthesis of NH3. That preparation, as described by Wiki* includes:

    “…The air added during this step also serves as a nitrogen source for ammonia synthesis:
    CH4 + 1/2O2 → CO + 2H2
    CH4 + 2O2 → CO2 + 2H2O
    Then occur two ’’shifts’’ which convert CO to CO2 by reaction with steam, one at high temperature, then one at low temperature:
    CO + H2O → CO2 + H2 high temperature
    The catalyst here is a mixture of iron, chromium and copper
    CO + H2O → CO2 + H2 low temperature
    The catalyst here is a mixture of copper, zinc and aluminium …"

    My interest is to know if the “high temperature” step above I have made larger and bold above is endothermic or does it create the high temperature? What is that temperature range used, if energy must be supplied? Also is it reasonably insensitive to pressure so it could proceed at relatively high pressure? (I would think it is as two molecules make two molecules.)

    Reason for my interest is to know if an old invention**, using a hot quartz tube*** for efficient conversion of solar thermal energy could help produce both food and energy. I.e. Can the high temperature step bold above take place at high pressure, before a turbine that produces work (drive an electric generator) and the next low temperature process take place in the cooler low pressure gas after it has expanded thru the turbine?

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    * http://en.wikipedia.org/wiki/Haber-Bosch

    **For discription of invention see post: http://www.sciforums.com/showpost.php?p=1819507&postcount=36

    ***According to Wiki,**** fused quartz tube has a softening temp of 1665C, an annealing point of 1140C and a “strain point” of 1070C. Thus at 1000C or less a circular tube can support considerable external pressure. Quartz hemispheres have been used as windows for some of the deepest ocean dives ever made, I believe.

    **** http://en.wikipedia.org/wiki/Fused_quartz#Physical_properties
     
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  3. kevinalm Registered Senior Member

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    993
    Exothermic, but only about 6.8 kcal/mole. That's if I didn't mess up the calculation.

    Free energies of formation:

    H20 -54.64 kcal/mole
    CO -32.81 kcal/mole
    CO2 -94.26 kcal/mole

    From an old chem text of mine.
     
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  5. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Thanks. I was hoping it was endothermic, but that is so small it can not be the only "fuel" to drive the turbine. That is I think some of the energy (most of it probably) would be the high temperature solar input from my "mass flow solar absorber;" however, that is not its only use. There are lots of needs now for a solar thermal system that is cheaper to build (Just mirors, not expensive solar cells, collecting the energy and only a cheap pair of co-axial tubes, One of steel and the other of glass grading into quartz. Plus the turbine engine and flow pumps, of course - One of the common natural gas "peaking units" used by electric companies would be OK, I think.)

    For example, a good Stirling engine operating between 1200K and 400K or less has efficiency of (12-4)/12 = 66% or more than 4 times as efficient as the good commercial solar cells.

    Would the reaction made big and bold in the OP run well at 927C as assume in Carnot efficiency above?
     
    Last edited by a moderator: Apr 15, 2008
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  7. kevinalm Registered Senior Member

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    993
    I believe that temp is in the right 'neighborhood', but I'm not sure it has anything particulary to do with carnot effiency. I think it has more to do with getting over the H2 + 1/2 O2 -> H20 -54.64kcal energy barrier , but I could be wrong. Have you read this wiki article?

    http://en.wikipedia.org/wiki/Water_gas
     
  8. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    23,198
    No I had not. Thanks, but not much of value there for me. I also think tha carnot has little to say about chemical process.

    I knew it is nearly (I worry about a cosmic ray) 100% safe to mix hydrogen and oxygen in the ratio of two H2 to each O2 at room temperature. My interest is in knowing the temperature at which separate steams of these two gasses "instantly" make H2O as they mix. I am not well enough informed to guess.* Can you?

    As three molecules become two of water, I am pretty sure that pressure increase lowers the temperature at which the reaction goes to completion, but if you can say more about the P T relationship that would be great.
    ------------
    *Nor brave enough to discover experimentally.

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  9. kevinalm Registered Senior Member

    Messages:
    993
    I really don't know at what temp they spontaneously ignite. I do know that H2 has one of the widest explosive mixture ranges of any flammable gas (something like 10 to 95 per cent in air iirc), which is one of the reasons the idea of the "hydrogen" economy has never seemed that great to me.

    This may be of interest:

    http://en.wikipedia.org/wiki/Activation_energy
     
  10. ElectricFetus Sanity going, going, gone Valued Senior Member

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    18,140
    Look up water shifting reaction in gasification.
     
  11. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    23,198
    Thanks. Here is what I found:

    “Gasification relies on chemical processes at elevated temperatures >700°C, …”
    FROM: http://en.wikipedia.org/wiki/Gasification

    And FROM: http://en.wikipedia.org/wiki/Water_gas_shift_reaction I Found:

    “… The water gas shift reaction is an inorganic chemical reaction in which water and carbon monoxide react to form carbon dioxide and hydrogen (water splitting):
    CO + H2O → CO2 + H2
    The water gas shift reaction is part of steam reforming of hydrocarbons[1] and is involved in the chemistry of catalytic converters. … The reaction is slightly exothermic. {Kevinalm's post 2 seems correct.} While this reaction could be used to produce hydrogen, the high temperatures required make it cost-prohibitive.* The generation of hydrogen itself has significant promise as a replacement clean burning fuel itself however this reaction is usually done via the byproducts of fossil fuel combustion. …”

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    *As the OP suggested, this need to heat the reactants to ~700C seems an ideal application for the highly efficient thermal solar invention. I will try contacting some organizations that are trying the destructive distillation route to “cellulosic alcohol” instead of the lower temperature enzymatic route.
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    If anyone has comments on the idea of the OP post please make them, but initially it seems to me that switch grass growing in the field with the simple mirrors and fields near them could be the carbon source.** Those mirrors are very much cheaper than solar cells that collect the sunlight and concentrate it on the mass flow solar absorber for more than twice the solar cell conversion efficiency. (Half as large a mirror area as solar cells area required for same electric power output is a big economy even if the mirror were as expensive as the solar cells.)

    Both mirrors and solar cells should “track the sun” for optimum energy collection and that cost can avoided only in the case of solar cells; however solar cells require frequently cleaning if in a series string for higher voltage outputs, as they always are to keep the copper wire etc. cost reasonable. When one cell in the string is partially in the dark, because of leaf or bird sitting on it etc, it is with high internal resistance but others in the string can drive current thru it and thermally destroy it. Installation and maintaince costs can thus be much higher for solar cells than for simple mirrors.

    In the OP’s chemical application, the Carnot limit should not apply. This should give at least another factor of two advantage and possible even "carbon credits" add still more to the economic advantage.**
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    **This chemical application removes CO2 from the air as the only carbon source and is a sustainable energy system. Which, if it runs at high pressure and quickly expanded and cooled thru a turbine / generator, makes electricity, syngas into hydrogen and /or even ammonia for fertilizer (see remaining reactions at wiki in link of the OP.) out of rain, air and sunshine!
     
    Last edited by a moderator: Apr 17, 2008
  12. kevinalm Registered Senior Member

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    993
    Last edited: Apr 18, 2008
  13. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Thanks. The carbon would be required for making hydro-carbon fuels with the high temperature solar heat but not for H2 or NH3 etc production. The second ref's Iodine cycle needs 850C and looks attractive. Some of the other sub links of that ref are just "stubs" - not much infro. In the original patent there is some discussion of the reversible reaction SO2 SO3 and O2, mainly as chemical storage for night time (Use the O2 to oxidize the SO2 back to SO3), but that was just "stolen" from an article of that era as an illustration of a chemical application. As a Physicist I tend to think of generation of power, not valuable chemicals, but as one of your references notes there is no Carnot limit on transforming feed stock chemicals into others with more value. Even with the high temperatures my absorber can produce (900C or 1200K) the Carnot limit makes about 1/3 of the solar energy remain as "waste heat" if used for power production.

    Clearly driving some chemical process it the most efficient use of the energy, but as always, economics will rule.
     
  14. kevinalm Registered Senior Member

    Messages:
    993
    I agree that chemical reactions would be the most effective use of your idea. A few additional thoughts:

    The most important aspect is to procuce H2, as once you have free hydrogen, you can make lots of usefull compounds. CH4 from CO2 and H2 for example.

    H2 and CH4 are particularly attractive, as power transmition via pipeline is preferable to lossy electric transmition lines. Also, power generation/load matching is solved.

    I personally like the high temp electrolysis. No inventory of nasty chemicals on hand. A worst case accident would release steam, H2 and O2.
     
  15. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Perhaps a hybrid of my efficient solar thermal system to heat the steam up to near the compressive failure temperature of the central mirror tube with the surrounding steel* tube only slightly thicker than needed to resist the tensile stresses in it, plus some solar cells to supply the electric power required. Could also consider using part of the H2 and O2 produced to run a cheap gas turbine that electric utilities use as peaking units. I think one of your references said something about burning part of what is produced. I do not know which would be more efficient or cheaper.
    -----
    *It may need some coating of nickel etc to not be corroded by the hot steam.
     
  16. kevinalm Registered Senior Member

    Messages:
    993
    You could probably manage to use the partially cooled steam out of the eletrolysis cell, or maybe tap off part of the high temp steam, to run a turbine generator. Either way, the overall system should be much more efficient than direct generation. One of the links mentioned that in the temp range we're talking about, in the neighborhood of 2/3 of the energy to produce H2 can come from heat. I doubt any carnot cycle generating plant could come close. Of course there is the carnot loss at the point of use of the H2, but that holds for any chemical fuel.
     
  17. photovoltaik Registered Member

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    3
    @kevinalm; your instructions are not only useful for Billy T, and also useful for me......
     

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