The Ergosphere
Monday, December 12, 2005
 

Meet you halfway

SHEC Labs (h/t:  GCC) is touting a process to convert landfill gas (mainly methane and carbon dioxide) to hydrogen using a two-step process:

CH4 + CO2 + 65.5 kcal/mol1  →  2 CO + 2 H2           (1)
CO + H2O(gas)  →  CO2 + H2 + 11.6 kcal/mol2           (2)

SHEC claims that their process increases the useful energy of the fuel by 14% (based on the lower heating value, no doubt; if it is assumed that the water's heat of evaporation can be recovered, it's about 29%).  But that's not the end.  Step (2) loses energy.  Why not just take the process halfway?

The final conversion to hydrogen is only necessary if hydrogen is the desired product or to sequester the carbon from the process; without that, it's just throwing energy away.  Keeping more energy in the product gas would be useful for stretching natural gas, getting more out of what has become a very expensive fuel.  With that in mind, I propose a modified process:

 Thermochemical process:  CH4 + CO2 + 65.5 kcal/mol  →  2 CO + 2 H2           (1)
 50% CO flows through,   (2a)
 50% process to CO2 CO + H2O(gas) →  CO2 + H2 + 11.6 kcal/mol           (2b)
 Recycle CO2 to step 1, H2 to output.            (3)
 Net reaction:  CH4 + H2O →  CO + 3 H2           (4)

The heat of combustion of CO is 68.56 kcal/mol, while the LHV of the hydrogen is 56.93 kcal/mol.  This implies that a thermochemical process can convert 185.45 kcal (LHV) of methane into 239.35 kcal of gas mixture (plus leftover solar heat and steam), an increase of 29%.  If the natural gas for a generating or heating plant was processed this way, the gas demand would be reduced by up to 22.5%.

Is it worthwhile to do this?  Perhaps not; the cost of the hardware might not be justified compared to getting the same amount of energy from something else.  Or maybe it's justified by its versatility; the concentrating mirrors could be used with anything at the collector, including solar Stirling engines.  Whichever way the conclusion goes, it's worth thinking about.

Footnotes:

  1. SHEC claims 917 kJ/mol (219 kcal/mol).  I can't reproduce this figure.  The numbers herein are based on the heat of formation of water (steam) as 56.93 kcal/mol, CO as 25.40 kcal/mol, CO2 as 93.96 kcal/mol, and the HHV of methane (25 C) as 212.79 kcal/mol; the corresponding LHV of methane would be 185.45 kcal/mol.
  2. SHEC's figure is 40.6 kJ/mole (9.70 kcal/mol).  The same caveat applies.

 
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