[Stoves] Jatropha and its future (and biochar)

Wed Aug 17 03:00:39 CDT 2011

Dear Ron

I am replying to this message not another one with similar content because it has the calculations in it.

Biochar and how long it takes to create it with a stove:

>…  5.  What the Biochar folk want is to not sell the char back, but to place it in the stove-user's garden.  Each year there would be in this example about 1/2 kg per day of char or 180 kg in a year.  

½ a kg of char will (according to what I read) mean burning about 2 kg of fuel (25% recovery) or 2.5 (20% recovery). This is in line with an improved stove’s thermal performance.  Perhaps we can use 2.5 kg per day and a 20% yield as realistic with a TLUD stove, yes? Say, 2.5 kg of wood pellets or seeds or macerated wood. The Char yield is 2.5 x 0.2 = 0.5 kg per day. The fuel comes from a forest of some kind where it grows at a rate of 100 kg per m2 per 30 years, or 3 kg per m2 per year. That means it takes 2.5/3*365 = 304 m2 of forest to supply the fuel for the stove for a year. [Numbers obviously subject to correction and localisation.]

>At 10 tons per hectare loading (same as 1 kg/sqm)…

First I made my own calculations based on the photos of Terra Preta (TP) soils in Amazonia (which is where all this started apparently). The black soil is very deep – certainly more than 4 feet. The amount of carbon in it is large, perhaps 20% by mass. It is by all accounts very high.  With a density of about 1.6 it means a cubic metre of TP soil weighs 1600 kg and has maybe 320 kg of crushed char in it. Let’s say it is only 10% to be generous to the method. That is 160 kg of char per cubic metre, and we will not make the deep TP Amazon soils that stimulated the field of study, just 1 metre.

Organic carbon maximises at about 2% of soil mass automatically limiting itself to that level. That is not TP.  While it is often said that biochar lasts ‘for thousands of years’ in the soil (other say at least 100 years – local conditions decide) there is reason to doubt this because carbon is an active part of the upper soil activity, but let’s suppose it accumulates.

Application rates of 10 to 20 tons per hectare http://www.carbonedge.com.au/docs/CarbonEdge-CE2_Special_Report-biochar.pdf are suggested [with it noted that some people are saying it can be dosed per plant].

I will stick with that application rate because it is something close to both my estimates and reported application rates. http://www.biochar-international.org/bocashi reports 1.5-3 tons per hectare but it was also heavily fertilised which is probably not going to happen, generally.

>… this stove would cover180 sqm, if used twice a day.  

At 0.5 kg char per day and a 10-20 kg/m2 application rate it is 20 to 40 days of cooking per m2 or 365/[20,40] = 9 to 18 square metres of TP soil per year, not 180. It may be that you are talking about applying 1 kg per sq meter, but that is not going to create Terra Preta. The Japanese, often cited as practising this, use much higher rates.

There is support for lower rates: “However, there are substantial benefits related to soil biology at rates well below 1 kg/m2” http://biochar.pbworks.com/FrontPage 

Nat of WorldStove notes very much higher rates: “Glaser et al. (2002) have shown that adding 45%

volume of biochar to sandy soils increases their water holding capacity significantly.”  http://worldstove.com/wp-content/uploads/download/biochar_for_climate.pdf 

Yeah, well, we are not going there…

One stove, 2.5 kg of fuel per day, 20% yield of char, 10-20 tons per ha, means the stove will produce biochar/TP soils at a rate of 9 to 18 square metres of TP per year fed by 304 square metres of forest. On a 30 year cycle, you will need 304 x 30 = 9120 sq metres of forest (about 1 ha). Harvesting this wood and cooking and burying the biochar will create [9,18] x 30 = 270 to 540 sq metres of true TP soil every 30 years.

Let’s say the forest is stable and harvested sustainably. It is 1 hectare. The arable land left is (typically) 5 hectares (12.5 acres) per family in some counties like Swaziland – not sure what figure to use here because it is much more in other places, much less in Rwanda. 

To develop 5 TP hectares (50,000 sq m) would take 2,770 to 5,555 years, assuming none of the carbon in the sequestered char was digested or burned or converted to methane and so on. The carbon trading value of that char is presently about $100 or 3 to 4 cents per year. Not much of an incentive.

Suppose you halved the application rate to 5 tons per hectare. It would take 1,400 years to prepare 5 hectares using a stove and the cooking of a family. 1 ton per hectare is still going to take 277 years.

>6.   But the Biochar keeps coming and giving, so at the end of five years of putting char in new ground,  we are not deaing with a linear .36 km, but 5 times that or 1.8 km of vegetable row, and the number of row-years is up to 1+2+3+4+5 = 15 row-years (10 more than the previous 5) .  If one row year gave a new annual garden income of $36. and the 5 year total income was $550, then the new total would be $550+10*36 = $910.

This is speculation. There seems to be great enthusiasm about char-amended soils but wide disagreement on how much makes a difference, and some say there is no gain at all: It only applies to certain soil types with certain crops and as no one knows why it works, it is difficult to make broad predictions.

   7.  But, the Biochar is still working after 5 years, so, now using a discount factor, maybe we can convince the potential stove buyer that there is another $100  available should the land (now up to (a postulated) loaded 900 sq m, [or 0.09 hectare - approaching a quarter of an acre] be sold.  

900 square metres at 10 kg per sq meter is going to take (900 x 10)/0.5 18,000 days to service with char. That is 49 years. Roughly it is 50 years per 1/10th of a hectare. 5 hectares is about 2500 years – again close to the figure calculated above.

Let’s take a different tack: take the land out of production and grow trees on it 100%, the add char to someone else’s farm of 5 ha and enlist 4 urban families to contribute their char. That is 5 times the growing rate. 5 hectares can then be amended at 10 (not 20) kg per ha in 500 years or 100 years per family per hectare. It is starting to look realistic but half the land is permanently in forest now. Maybe not so good if the food supply does not double on the worked land.

If people really want to convert their lands to TP, they should harvest all the biomass they can find and char it in a kiln locally and put it on the land. It is not looking practical to hold that biomass stoves making ½ a kg of char per day (which I think is optimistic) is going to create much of an agricultural revolution. People are not willing to wait 20 to 50 generations to make the conversion.

Now, if it were to be shown that char amendment at 0.5 tons per ha creates a huge gain in production, at least with one crop in one place, one could support using it there. To hold that agriculture is going to be ‘saved’ by char producing stoves seems a bit of a stretch.

With reference to how the TO soils were created in the first place, it was the result of rotational slash-and-burn farming for thousands of years. Well, it works, but you have to be patient.

Regards

Crispin

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