In much of west Texas, the iconic Prickly Pear cactus — with its plum-like fruit and forbidding spiked pads — is at best considered a nuisance, and at worst a downright hazard to livestock.
But in most of the rest of the semi-arid world — from Mexico and Chile, large swaths of India and South Africa, as well as Spain and Morocco — Opuntia ficus-indica (Prickly Pear) is used in dye-making, as feed for livestock, and, little by little, as feedstock for anaerobic biogas production.

Image: Cultivated Opuntia (prickly pear cactus)

The beauty of this hardy, drought-resistant cactus, which can tolerate surprising bouts of cold weather, is that it can be grown on veritable desert-like wastelands, where conventional crops would wither and die. 
“Opuntia pads have 8 to 12 percent dry matter which is ideal for anaerobic digestion,” said Axel Tarrisse, managing partner in Zoe Biotech, a two year-old Marseille, France-based agricultural and environmental tech company. 
Tarrisse notes that with a rainfed climate, there’s no need for extra irrigation or extra water to facilitate the anaerobic digestion process.  In fact, with only 300 millimeters of precipitation per year, he says, Opuntia can produce 12,000 kilograms of dry matter feedstock and still retain enough moisture to facilitate biogas production. 
By some estimates, Prickly Pear cactus pads degrade five to ten times faster than manure.  Thus, only 4 hectares of the Opuntia crop can produce an estimated 800 cubic meters of biogas per day.  Although the cactus is native to semi-arid regions with stifling hot temperatures, it can also survive and even thrive in mountainous areas that can have temperatures as low as minus 15 degrees Celsius. 
“The world has millions of hectares of land prone to drought and desertification,” said Tarrisse.  “Opuntia helps create a vegetative cover, which enhances soil regeneration and improves the infiltration of rainfall back into the soil.” 
The idea of using Opuntia feedstock to generate methane-based biogas first took root in Chile.  Although the process had been observed as early as 1984 in the lab, its commercial application was actually first realized by environmental engineer Rodrigo Wayland Morales, the owner and current manager of Elqui Global Energy in La Serena, Chile. 
As a student in Chile looking for a thesis project, Morales realized that throughout Chile’s centrally-located Coquimbo region, there were hundreds of tons of the cactus pads being discarded annually.  Such waste was primarily a byproduct of the cochineal insect, which feeds on the cactus and, in turn, is the source of commercial crimson dyes used in some food coloring. 
Morales secured $50,000 USD in financing for the first biogas cactus project in Chile’s Elqui Valley.  Morales says he’s been working on the idea for more than a decade.  Mexico produces hundreds of thousands of tons of Opuntia per year.  Thus, since 2009 has been consulting with Mexican industry in turning a portion of their rich cactus bounty into bioenergy.  
Morales notes that Opuntia digesters are not only much smaller, but also much less expensive than those used for anaerobic biogas production from manure.  He says his company currently has turnkey cactus biogas projects in both Mexico and India.  But for more than a decade, his company has been also consulting with current and potential Opuntia biogas clients in Chile, Brazil, Ecuador, and Colombia. 
Although Morales’ home country of Chile has a stated goal of generating at least 10 percent of its energy needs from renewable sources by the year 2022; at present, Opuntia is not a part of that mix. 
“Chile could desalinate sea water and grow the cactus in the desert,” said Morales.  “I imagine the Atacama Desert with cactus; producing the energy that our country needs.” 
Tarrisse says that it’s possible to garner as much as 2.5kwh of methane from 1kg of dry Opuntia. 
This bio-methane, he says, can be turned into electricity in combined heat and power systems where waste heat can be used in the heating or cooling process.  There’s also the option, he says, of using this bio-methane to charge electric vehicles; or compressing or liquifying it for use in natural gas vehicles.  It can even be purified for injection into existing natural gas grids.     
Depending on the planting density, Tarrisse says that Opuntia can grow to maturity in as little as one to three years.  It can then be mechanically harvested and chopped before being transferred to anaerobic digesters where methanogenic bacteria consume release a 50-50 biogas mixture of methane and carbon dioxide.
Tarrisse says the biggest challenge is finding the right investors who really appreciate the idea of creating dedicated biomass plantations on degraded land.   Northeast Brazil arguably has the most potential for Opuntia bioenergy production of any region in the world.  Today, some 5,000 square kms is already under cultivation in Brazil for use as a fodder for livestock.  
Increasing this same cultivated area to 75,000 square kms would by some calculations potentially generate enough liquid bio-methane to replace 15 percent of the European Union’s annual consumption of fossil natural gas.  But this would require an estimated investment of $10 billion and at least ten years of development.
Tarrisse says many farmers who had previously grown cactus for livestock feed are indeed willing to grow Opuntia for biogas.  He maintains that farmers currently growing Opuntia in South Africa would be more than willing to establish hundreds, even thousands, of hectares of the cactus for energy conversion.  But the trouble is, as Tarrisse admits, currently in South Africa, there are no investors willing to fund such a project. 
While the state of Texas is also not pursuing Opuntia as feedstock for biogas production, Wayne Hanselka, retired range manager with Texas A&M University’s Agri-Life Extension Service, says west Texas ranchers would likely welcome the harvest of the cactus on their lands, just to get it off their hands. 
Opuntia also currently grows freely across large areas of Australia. Even so, Andrew Lang, an Australian agricultural scientist and World Bioenergy Association board member, says it is generally considered a nuisance “Down Under,” a perception that still lingers from the early 1900s when Opuntia “spread over vast areas” of Australia’s inland farm country.
As Lang notes, Opuntia is already well established in Australia.  “So, nothing is stopping someone from establishing it at scale if they wanted to try,” said Lang.  “But we have other possible biomass sources, including eucalyptus, that produce well in lower rainfall zones.”   
But unlike a lot of current biomass feedstocks, sliced Opuntia pads can also easily be cooked up and folded in with the morning eggs.   “Saute it,” advises Hanselka, “and it tastes something like green beans.”

http://www.renewableenergyworld.com/rea/news/article/2014/01/prickly-pear-cactus-nuisance-or-bioenergy-opportunity