[Digestion] Biogas yields for tropical energy crops

Mon Sep 10 18:43:01 EDT 2007

Warren, thanks so much for these data. They're a great help.

Meanwhile, I found the following on sugarcane, and it puts us in front of an interesting question, I think:

=====================
English Title: Sugar cane (Saccharum sp.) juice energetic potential as substrate in UASB reactor.
Personal Authors: Colen, F., Pasqual, A.
Author Affiliation: Aluno do Programa de PÃ³s-GraduaÃ§Ã£o em Agronomia, Energia na Agricultura, FCA/UNESP, Botucatu/SP, Brazil.
Document Title: Energia na Agricultura, 2003 (Vol. 18) (No. 4) 58-71

http://www.cababstractsplus.org/google/abstract.asp?AcNo=20053101335

Abstract:

The energetic potential of sugarcane juice was studied in an upflow anaerobic sludge blanket reactor (UASB) of 7.78 litres useful volume, working at a temperature of 35Â°CÂ±1Â°C. Before being added to the reactor, the juice was naturally fermented for 24 hours, with addition of sodium bicarbonate, urea and diacid potassium phosphate [potassium dihydrogen phosphate?] to correct the pH (between 6.00 and 6.60) and carbon/nitrogen and carbon/phosphorus relations respectively. In the reactor, 3 hydraulic retention times (HRTs) were used: 20 days, 18.78 days and 15.30 days. For the HRT of 20 days, the affluent chemical oxygen demand (COD) was 260 304 mg.litre-1 and the effluent COD was 16 991.40 mg.litre-1, an efficiency of 93.47%. The gas volume was 136.33 m3 of biogas.m-3 of juice, its average composition including 72.43% methane. For the HRT of 18.78 days, the affluent COD was 272 779 mg.litre-1 and the effluent COD was 8612 mg.litre-1, an efficiency of 96.84%. The gas volume
 was 147.42 m3 of biogas.m-3 of juice, its average composition including 72.91% methane. For the HRT of 15.30 days, the COD was reduced by 71.49%. The gas volume was 103.58 m3 of biogas.m-3 of juice, with an average methane content of 63.30%. When calculating the energetic potential, the figures from the HRT of 18.78 days were used, and the amount of energy produced was found to be 2767.82 MJ (661.21 Mcal) per tonne of processed sugar cane. This value was extrapolated using the average Brazilian cane productivity (69.50 tonnes per hectare) to give a figure of 192.36 GJ (45.95 Gcal) per hectare; compared to anhydrous ethanol production, biogas production gave an efficiency 35.33% higher than that of ethanol. When the energy from sugarcane bagasse, which is used as energy for ethanol distillation, was included in the calculations, the energy output increased to 4738.30 MJ.t-1 of processed sugar cane, compared with 2045.27 MJ.t-1 of ethanol, not considering the bagasse input
 in distillation, and the amount of energy obtained using biogas increased to 131.67% more than the figure for ethanol.

Publisher: Faculdade de CiÃªncias Agrono~circumflex~micas, Universidade Estadual Paulista (UNESP)
===============

Apparently, digesting an entire sugarcane crop yields far more energy than converting it into ethanol, even if you take into account that bagasse is used as a power source for the ethanol plant. I assume that the same is true for such species as sorghum and other high yielding tropical grass crops.

Now why don't we see big biogas plants all over the developing world? Why ethanol when biogas would be so much more efficient? Are capital and O&M costs for anaerobic digesters so much higher than for ethanol plants? I don't think so. Is it because biogas cannot be transported as easily? 

Why isn't anyone digesting tropical grass species on a large scale to ship the biomethane out as an alternative to natural gas? CNG ships are already here, so this should be possible.

Why isn't anyone in the (tropical) developing world building dedicated biogas plants and energy plantations to fuel the poor's CNG-fleet? This must become a possibility, certainly in remote regions. 

I'm confused and very optimistic at the same time. 

Deducing a bit: 4750MJ per ton of sugarcane at 80 ton/ha comes down to a spectacular 10,000 cubic meters of methane per hectare (worth US$ 2300 at current NG prices).

Common, with that much energy you can power all CNG cars/tricycles in a typical small village in the developing world for an entire year.

Am I missing something?

Yours, Lou

[PS: sorry for sending this mail twice to you alone, Warren; forgot to check "reply to all"].

Warren Weisman <weiswar at yahoo.com> wrote: 
I think there's so little and so much conflicting data
is that digestion is not an exact science. A small,
well-maintained digester can out-perform a big, fancy
one. The second most important aspect of digestion,
after an airtight environment, is the proper carbon to
nitrogen ratio, which I did not see referred to in the
article. 

Here's the volumes from "A Chinese Biogas Manual",
which I consider the best source of information on
digestion and biogas, since its empirical data
gathered from making MILLIONS of digesters...

volumes are per tonne of material:

General Stable Manure 260-280 cu.m. gas  50-60% CH4
Pig Manure 561 cu.m. gas  
Horse Manure 200-300 cu.m. gas
Rice husks  615 cu.m. gas
Fresh grass   630 cu.m. gas   70% CH4
Flax stalks or hemp   359 cu.m. gas  59% CH4
Straw       342 cu.m. gas 59% CH4
Leaves from trees   210-294 cu.m. gas   58% CH4
Potato plant leaves & vine, etc.  260-280 cu.m.
Sunflower leaves & stalks  300 cu.m. 58% CH4
Sludge   640 cu.m.     50% CH4
Waste water from wine or spirits making 300-600 cu.m.
58% CH4

--- Lou Dobb  wrote:

> Hi all, I'm new to this mailing list. Thanks for the
> very interesting resources and discussions. 
> 
> I have been trying to find some data on biogas
> yields for tropical energy crops (such as sugarcane,
> sweet sorghum, cassava, sweet potato, grass species,
> tropical sugar beet, etc...). But I can't find much.
> 
> In my search I only found data on crops grown in
> temperate and even cold climates (Finland). 
> 
> E.g. Annimari Lehtomï¿½ki: "Biogas production from
> energy crops and crop residues", Jyvï¿½skylï¿½ Studies
> in Biological and Environmental Sciences 163, PhD
> Dissertation, Faculty of Mathematics and Sciences,
> University of Jyvï¿½skylï¿½, 2006.
> 
> http://dissertations.jyu.fi/studbiol/9513925595.pdf
> 
> Could anyone help me compile a list of yields for
> tropical crops?
> 
> We could use the list for a wiki, as a basic
> reference.
> 
> 
> How come there are so little data on this? I don't
> understand.
> 
> Best regards,
> Lou
> 
>        
> ---------------------------------
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