The Ergosphere
Wednesday, June 15, 2005
 

June biomass roundup

The news lately has had a couple of new and interesting (to me) schemes for conversion of waste matter to energy.  These appear to have considerable potential.  Unfortunately, they also seem to have built-in losses which seriously limit the efficiency and thus the useful outputs.

First up:  a biomass to biodiesel process from the University of Wisconsin.  This process proceeds in 5 steps:
  1. Convert plant matter to sugars in water (by means unspecified).
  2. Strip hydrogen from the sugar/water solution using a nickel/tin catalyst.  (Removing hydrogen from the mixture would appear to produce CO2 as a byproduct, but this is not specified.)
  3. Treat the non-hydrogen fraction with acids to dry it.
  4. Convert to alkanes using a solid alkali catalyst.
  5. Convert to light carbon chains using the hydrogen from step 2 and another catalyst.
The fuel product is claimed to be equivalent to conventional biodiesel.  It is immiscible with water and separates by gravity, eliminating distillation steps required with alcohol fuels.

The issue I can see here is efficiency.  Plant matter is largely carbohydrates, which have the general chemical formula of (CH2O)n.  Alkanes have the general formula (CH2)nH2, and have no oxygen; the oxygen in the feedstock has to come out either as water or as CO2.  A process which extracts hydrogen will leave carbon and oxygen, so the obvious byproduct is CO2.

How much?  Consider the production of pentadecane, C15H32.  Each CH2O carbohydrate subunit can yield one atom of carbon plus a molecule of water, or one-half atom of carbon, one-half molecule of CO2 and one molecule of H2.  Pentadecane requires 16 molecules of H2 which consumes 16 CH2O subunits and yields 8 atoms of carbon and 8 molecules of CO2; the remaining 7 atoms of carbon requires 7 CH2O subunits and emits 7 molecules of H2O:

23 CH2O   ->   C15H32 + 8 CO2 + 7 H2O

Assuming none of the feedstock is used to provide process energy, the net carbon loss is 8 atoms out of 23, or 35%.  The theoretical dry-carbohydrate-to-fuel efficiency is 212/690, or 30.7%.  Based on the heats of combustion of sugars vs. decane and assuming no mass losses the energy efficiency of the process is roughly 88%, but the first step (conversion of biomass to sugars) is a huge unknown.  Then there are all the different catalysts.  This looks tricky.

Second is conversion of gasified biomass to ethanol by bacteria from a company called BRI.  This process partially burns waste (of most any sort) to a syngas of CO, CO2 and H2; this syngas is fed to Clostridium ljungdahlii bacteria which ferment it to ethanol, which presumably has to be distilled.  The liquid-fuel yield is 75 gallons of ethanol per ton for municipal solid waste (MSW), rising to 150 gallons per ton for used tires or hydrocarbons (I presume that plastics are equivalent to those feedstocks also).  The process equipment would be divided into modules, each consuming 85,000 (presumably dry) tons of MSW per year, producing 7 million gallons of ethanol and 5 megawatts of electricity as a byproduct.

This process has two steps which massively increase entropy, one in the gasifier and the second in the bacteria.  Its biomass-to-fuel efficiency is correspondingly low:  75 gallons of ethanol is a mere 494 pounds, containing the energy of about 310 pounds of hydrocarbons.  This 15.5% effective biomass conversion efficiency (45% energy efficiency) is about half that of the University of Wisconsin process, but it also yields electricity with an efficiency of about 11%.  This 56% overall efficiency, combined with the lack of catalysts and flexible variation between the fuel and electric products, might be a winner due to low capital costs and ability to tune output to market conditions and maximize total value.

This analysis wouldn't be complete without a reference to a previous headliner of biofuels, Changing World Technology's thermal depolymerization process.  CWT claims a 70% conversion of plastic to alkanes, or 44% conversion of tires to alkanes plus 42% carbon and metal solids.  This is about three times the conversion efficiency of the BRI process, but there's an interesting quirk:  the carbon portion of the product of the TDP process could be gasified in the BRI system and used to produce additional fuel and electricity.  The byproduct heat of the BRI gasifier might also be sufficient to run a TDP converter, and the TDP output gas might be suitable to feed the Clostridium cultures directly (certainly after gasification).  Total efficiency could go way up.

If there's any chance of CWT and BRI "swapping spit" and cross-licensing their patents, this could get a lot more interesting.  The sudden absence of landfills in the countryside could be the least of the outcomes. 
Comments:
http://www.che.wisc.edu/JAD/index2.htm
 
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