Autumn 2017 EN

Biogas Journal | Autumn_2017 English Issue
ed for up to twenty years. For use as a food, the leaves
grow for about 30 days or, to use the plant for energy, for
up to four months. “But no longer than that; otherwise,
the methane yield decreases”. Castañeda breaks a lightgreen
leaf off of a plant. Contrary to expectations, the
spines are soft; later, they even fall right out.
The methane yield of the prickly pear cactus is 860
cubic metres per tonne of dry matter, which is equivalent
to 10 tonnes fresh weight. This means that with
respect to its weight, this prickly fellow does not have
an especially high yield. But in terms of the yield per
hectare it does. “In three harvests we get a total of 600
tonnes of fresh weight per year and hectare”. Moreover,
cactus is only in the biogas plant for 16 hours, a very
short period.
A look at the plant near the farm, though, makes it clear:
a great deal of mass has to be moved in order for it to
operate. The cactus leaves are chopped and are placed
in the digester. No water is added. Just 1 percent cow
dung is added to the mixture. Nopal is fermented in four
large containers of 1,000 cubic metres each. The containers
are four metres tall and are made simply of foil,
iron lattice and some concrete, stones and soil. “All of
the components can be bought locally and the work was
done by a Mexican company”, explains Miguel Angel
Perales de la Cruz, who planned the design, financing
and construction of the plant for the cooperative. These
hybrid constructions of a lagoon digester and a reactor
are not heated, however.
“When we’re at peak production, everything here is covered
in cactus leaves”, continues Perales. The plant
grounds cover an area as large as two to three football
fields. And the light-coloured concrete gleams, demonstrating
the involvement of project partner Cruz Azul.
The large Mexican concrete manufacturer utilizes the
electricity, more than seven million kilowatt hours, produced
by the Caterpillar generator, which has a capacity
of one megawatt. Cruz Azul also provided far more
than half of the investment costs of two million euros
(converted from pesos).
The rest came from the Mexican National Council of
Science and Technology (CONACYT). Room for expansion
is planned, but it will probably not occur quickly.
The plant, even at its current size, does not yet run at
full capacity is also because the state-operated provider
and network operator Comisión Federal de Electricidad
(CFE) allows feed-in only at certain times so that the
grid is not overloaded.
phFotos: Martin Egbert
Juan Manuel Castañeda Muñoz, a member of the Comite Estatal Sistema Producto
Nopal. 70 hectares of prickly pear cacti are grown for the biogas plant.
Cactus leaves are used as the fermentation substrate for the biogas plant of the
Comite Estatal Sistema Producto Nopal cooperative. A small front loader pushes the
leaves into the intake container.
Long approval phases
Indeed, the Mexican government has ended the CFE
monopoly by enacting an energy reform. However, the
commission is still tenacious as ever with regard to
some issues. For example, two years passed between
the approval for production of electricity and the approval
for feed-in for the Nopal biogas plant. The production
costs for electricity generated by cacti are
The solid constituents of the fermentation residues will be used for filling car seats.
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