Digester Effiency

Discussion in 'Renewable Energy' started by Somerset Farmer, Nov 8, 2018.

  1. Somerset Farmer

    West Somerset
    I am now surrounded by crops grown for a digester, fodder beet, maize, and ryecorn(cut wholecrop) and I was thinking how much energy each of these crops produce an acre/year. There appears to be huge machinery costs in both growing and haulage before these crops even reach the digester (which is 10 miles away) and then the digestate has to be hauled back again. Can it ever be economic if this energy is paid for with the wholesale price of electricity or gas ?
  2. Don't know about economic but it certainly isn't green energy.
  3. thesilentone

    thesilentone Member

    There is a sustainability criteria to ensure it is green energy.
    wilber and Will Wilson like this.
  4. I assume that's a joke.
  5. thesilentone

    thesilentone Member

  6. Somerset Farmer

    West Somerset
  7. thesilentone

    thesilentone Member

    We need to break down into two areas, one being the field and crops grown = yield in tons/per/hectare.

    We then calculate the Biogas potential per ton,followed by the utilisation of the Biogas.

    Grass, maize and whole crop have a similar output of gas per ton, with Beet and Rye (whole) giving less due to the reduced DM level.

    This can be offset by the yield per hectare, so of course as your question suggests, the real figure is m3 biogas/hectare then cost per ton (which is why maize is so popular)

    Once we have produced biogas, we can use it to produce electricity, heat or methane which can be sent to the gas grid. We also produce digestate that returns the nutrient to whence it came.

    There are many references on the internet to gas yield/ton, kwhr production/m3 biogas and the benefits of digestate, I'll let you enjoy the research.
    wilber likes this.
  8. Suffolk Serf

    Suffolk Serf Member

    Easy answer. One I know is 2.25 MW/h gas to grid. Uses about 3000 acres of Maize and rye.
    Nearby is a 13MWh solar farm occupying 55 acres, but being solar probably averages 3 or so.
  9. thesilentone

    thesilentone Member

    Different efficiency levels for each technology, however Biogas is far and away the most efficient.
    Will Wilson likes this.
  10. Marsh roots

    Marsh roots New Member

    There are many variables at play but lets take some benchmark figures:

    1 acre of Maize silage will yield 16T per acre @ 30% DM (Ill add I believe this to be a very conservative yield)
    1 T of Maize will produce +/- 200m3 of biogas or +/- 104m3 of CH4 (Methane)
    there for 16T of maize will equate to 200x16= 3200m3 biogas or 1664m3 of CH4

    Now converting that to KW ( Energy produced per acre )

    Firstly CHP only Biogas plants - using Biogas to power a combined heat and power unit. At 38% efficiency electrically 1 acre of maize silage will produce +/- 6400 KW & 7040 KW of thermal KW.

    Upgraded Biogas to CH4 98% +/- - 1m3 of upgraded biomethane has an energy value of about 9.9KW there for 16640 KW of gas to grid energy per acre for maize silage.

    The interesting part is where we bring is back to the contentious subject of carting silage.

    1m3 of Biomethane is roughly equal to 1L of Diesel when considering base energy values for combustion.

    So lets say a field of maize is 15 mins away from the Clamp including Loading and tipping. A tractor that is carting 16 T of silage ( 1 acre to make the numbers easy ) using 30L of diesel per hour will be able to do 2 trips in the hour - essentially 2 acres re 32 T maize silage. 30 L diesel is the equivalent of using 300 KW per hour.

    In this using only CHP biogas plants as an example that are agreeably less energy efficient we can derive the KW used vs KW being produced per acre.

    1 acre maize silage = +/- 6400 KW of electric through a CHP. So 2 acres will produce 12800 KW divide this by Fuel used to cart Silage (300 KW equivalent ).

    This gives us our figure of 42.66 KW produced for every 1KW used to cart silage back to the farm gate.
    wilber and thesilentone like this.
  11. thesilentone

    thesilentone Member

    However, then we must also look at the cost v benefit of returning nutrient, as well as the CO2 offset of total energy produced v other methods.

    There are also ' green ' credentials for the Environment and Land from using organic fertilizer and soil conditioner.
    Marsh roots and wilber like this.
  12. Lincsman

    Lincsman Member

    Try researching woodchip from Canada to Drax, which ironically had coal underneath it!.

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