Biochar is pyrolized biomass where the various gasses are off gassed at high heat, in an oxygen free environment, the gasses are burned, and the carbon left over can be used as a soil amendment. Albert Bates wrote an excellent book about biochar called “The Biochar Solution” which I recommend to anyone interested in soil and agriculture.

The below is an article written about two of the the biochar stoves we built. The original appears on the Permaculture Research Institute web page. We have a new design which incorporates both a rocket stove and a biochar stove, which we are fine tuning. A picture of that is to the left.


Maya Mountain Research Farm (MMRF) is a small NGO and working farm located in southern Belize. The farm has about 20 acres of managed land, with the remaining 50 acres managed for limited extraction of timber, fuel wood and medicinals and as a wildlife corridor between the Columbia River Forest Reserve and the Columbia river. We are a working demonstration farm, focusing on agroforestry and the intersection between agriculture and ecology. One thing we have done is to provide a working example of an alternative to raising pigs with corn, which is a local practice amongst Kekchi and Mopan Maya farmers, and combine that with the making and applying of biochar while cooking the pig food.

Corn is a staple food amongst the Maya of Belize, but can have a high environmental cost. Clearing land and burning it off gives the farmer a 3 to 5 year cycle of production, before fertility drops to the point where further use of that land for annual crops gives such a diminished return that the land is fallowed. Fallow periods for the soil types in the foothills of the Maya Mountains are usually 12-15 years, until certain indicator species, like Cecropia, Schizolobium parahybum and Bursera simaruba reach a certain size. This indicates the land has recovered fertility, and can be cleared again, and the cycle repeated. Because of pressure on land use tied to the expanding population, this fallow period is frequently shortened to five years, or less, resulting in an even smaller window of productivity, and a need to clear more land.


MMRF has an agroforestry system that, in places, is over 20 years in age. Much of the farm is devoted to cacao production. These mature areas are very productive, providing food, timber, fuel, medicinals, marketable crops, like cacao and vanilla, while also providing ecological services like carbon sequestration, soil and soil moisture retention and habitat creation. The agroforestry system, where mature, requires minimal energy to maintain. In 2009, Kevin Mascarenhas, a UK permaculture diploma student, spent three months with us as an intern. He was the energy behind the design and implementation of the piggery.


Kevin and I collaborated and built on the vision to complex the mature agroforestry system while demonstrating a culturally appropriate and repeatable system for local people. Much of the area around the piggery was planted to pioneer species, like banana, cassava, plantain, cocoyam, pineapple, sugar cane, mulberry, and alley cropped with vetiver planted on contour to control erosion. This gave food within a few months, and now is on its way to being a stacked polyculture dominated by coconuts, with mango, avocado and sour sop. We have expanded that to about a half acre. The pigs are fed part of their diet every day from this area.


The piggery was created to take advantage of our surplus food due to seasonality. The farm has a seasonal abundance of breadnut (Artocarpus camansi), breadfruit, (Artocapus Altilis), Guava, banana, coco yam, cassava, mangos, avocados and other food. During pineapple season, for example, when we can pineapple, we end up with many buckets of skins and hearts, which we use to make vinegars, and for chutneys, or feed to the pigs. We do not butcher the pigs, but sell piglets to other farmers, or exchange piglets for goods and services.

We donate some of our surplus food to an elderly feeding program in nearby Punta Gorda Town, where we work with Helping Older People Equally (HOPE), a local community-based organization run out of the Red Cross building that works with 22 elderly people. The balance we cycle through our pigs. This deals with seasonal abundance. Frequently, when we have something, for example avocados or mangos, everyone in the area has them too. This makes selling them difficult, and there are times when farmers have not sold their produce at market by noon, when they must catch buses back to their villages. However, with our ever hungry pigs, we always have a use for excess foods, and the pigs act as a way to convert our surplus foods into energy and marketable products.


Pigs are an important part of local agricultural and cultural practices. Like the proverbial piggy bank, when a farmer needs a substantial amount of cash, they can sell a pig. Unlike perishable materials, pigs can wait to be sold until the market demands it. Pigs also bind communities together. Pigs are used for weddings, confirmation, graduation, funerals and for labour exchange, when farmers trade days of labour. If a farmer has 20 men come help him to clear, plant or harvest his corn field, or thatch his roof, or any task that requires a lot of hands, he must feed the men who come. Serving a pig is a way to show appreciation.


We avoided the high yielding Large White pigs, which were offered to us by a school we work with, and instead purchased the “local” pigs, pigs that have been raised by Kekchi Maya farmers for centuries. They are sturdy pigs, can eat a variable diet, are not prone to getting sick, and do not have the reputation of the Large White pigs of being aggressive. I did not want any 900 lb mean pigs. Our biggest pig, Romeo the Boar, weighs about 300lbs. He is well mannered.


I, personally, do not eat pork, but I love the place pigs hold in the farm ecology. Pigs eat things that we do not want — spoiled food, skins of fruit, surplus food — and in return provide us with manure, and a highly prized food source. Pork meat is considered a delicacy here. Additionally, pigs’ intrinsic behavior, rooting, can be used to plow land. We have two rotational paddocks for the pigs to root around and do their piggy thing. When one paddock has been denuded and the soil turned, we can sow it to grains or beans, and move them to the next pen, where they will plow and fertilize that for us.


Our piggery is comprised of four pens and has housed 4-20 pigs. At the time of this writing, we have four mature pigs, and six piglets, now being weaned to allow us to market them.The piggery has two rotational paddocks for the pigs to get out, root around and do piggy things. We strictly bring food for them to their pens, so they are trained to return to their pens to be fed. The piggery is designed for easy collection of the manure, with an eventual goal of building a biogas plant, which will add an additional harvest cycle to the system, burnable methane, which can be used in a modified propane stove, while the effluent will still be useful as a nitrogen rich soil amendment. In the mean time, we use the manure generated by the pigs for composting, and use the compost for our tree crops, especially coffee and cacao.


While much of the food we provide for our pigs can be eaten raw — ripe banana, ripe plantain, guava, all fruits, amaranths, sugarcane, inga edulis pods, to name a few — some of the food we prepare for the pigs needs to be cooked for palatability. Breadnut and breadfruit, banana and plantain (when green), cocoyam and cassava, all benefit from cooking. Like most households in rural areas of Belize, we cook with fuel wood, mostly small diameter wood collected from the 20 acre agroforestry system, or trees we have thinned out. We have a wood burning cook stove, a few hearths and a cob oven for pizza, which we fire up every Saturday, in the kitchen, and a biochar stove located near the piggery.



Placing the biochar stove near the piggery gives us a short distance from where we prepare the food, to where we feed the pigs. Additionally, it allows us to transport the biochar directly to the pig pens.


Albert Bates is a good friend of mine and co teacher at our annual Permaculture Design Courses here at MMRF. He wrote a book about biochar, called “The Biochar Solution“. It is a very good book for explaining what biochar is and its value. It gives more detail than I can provide in this article about the benefits to soil and the environment from making biochar. After reading it, inspired, we set out to build a biochar stove out of local materials for both pig food and human food.


Biochar is produced by heating biomass to high temperature, in a low or no oxygen environment. As the biomass heats up, it starts to off-gas, leaving behind the carbon. This is called pyrolisis. When making biochar the feed stock gets hot, and if the environment is well designed, the oxygen is depleted, and the feed stock goes from combustion to pyrolisis, as the biomass off-gasses. The gasses that come out of the pyrolisis chamber are flammable, and they are burned by the fire as they exit the inner tank, or retort, and meet the flames inside the container in which the inner tank is set. We burn those gasses to make more heat. Initially, there is some smoke, but very soon almost no smoke is visible, as the flames are burning very hot.

Biochar is a rapidly expanding field, and offers a way to create energy in the form of heat, while creating a stable form of charcoal that can be then stored in the soil for hundreds of years. This is a valuable tool to remove carbon from the atmosphere and lock it up while improving the porosity and fertility of soil.


Many biochar stoves do not take advantage of the heat generated by the stove, which, to small holders in the developing world, especially rural households dependent on wood for fuel, makes little sense. Here in Belize many rural people have to walk considerable distances from village centers to find fuel wood. The idea of generating charcoal to bury in soil has been met with skepticism. However, many people cook their pig food, or cook their home food, and, for them, being able to take advantage of the heat generated by the biochar stove makes the concept of making biochar work for them.


There are some very large biochar kilns, quite impressive, that have been brought into Belize, and most of them have only been fired once or twice. One farmer who had one placed in his cacao grove said he did not know how to use it. They look like they cost thousands of dollars to build, and involved a massive amount of energy to create. They were imported from UK or the EU at great expense, with a lot of petroleum miles on them, and sit in the various villages, rusting. While they look like they could make a lot of biochar, because of the time and energy spent to prepare and operate the stove, they have not been used often, and their design is such that when they are used, the heat generated will be wasted. With a large sterling engine, or an ammonia absorption system, you could make a lot of electricity or ice from them.

While this model looks promising, because of its size and cost it is unlikely they will be used much, and they are not accessible by most farmers without massive capital investment. We wanted to make biochar, and we wanted to design and build a biochar stove that was inexpensive, and created from locally available materials, that took advantage of the massive amount of heat generated. Most of all, we wanted to make a stove that was easily replicable by smallholders without access to capital or to traditional avenues of credit.


We looked around for material to work with. Talking with Scott Forsyth, a friend and beekeeper who lives in Guatemala, we discussed various designs, and looked for freely available material to work with. We looked at all the available options and decided to try using the common 55 gallon drum, available anywhere, and old retired propane tanks, also available anywhere.


Our biochar stove is multifunctional, making biochar while also cooking pig food. The initial design was a 55 gallon drum as an outer chamber, with an inner chamber made from an old propane tank. This worked well. The propane tank is inverted, so the bottom of the tank is now the top. The original hole for the gas valve was welded shut. It is open at the top, and the fuel wood is stacked around the tank, inside the drum. Air inlets are located at the bottom of the outer drum. Once lit, the flames rise, and the two air inlets create a venturi effect, concentrating the movement of oxygen rich air into the bottom of the drum. This is similar to a rocket stove. The heat from the outer fire heated the contents of the open topped propane tank, resulting in the material off-gassing through the open top. At the top of the drum, below the lid, we installed some holes to allow the flames coming up to meet more oxygen, and assist in burning off the gasses exiting the top of the tank. This worked well, and an advantage is that we could add more feed stock to the pyrolisis chamber as space opened up. However, I wanted something that would use less fuel, and make a larger batch of biochar. Total cost, for the first biochar stove, with welding by Mr. Baki in Big Falls, about US$50.

After firing this stove, it took about an hour and a half to do a complete burn. It left about eight gallons of biochar when done. If not careful, though, the biomass in the inner chamber could burn if not quenched in time, or fully. This only happened once, but it was a good lesson. We wanted to try a better design.

Our next design was more of a traditional retort model. Again, we used a 55 gallon drum, with a lid of metal, with a hole for the heat to escape, and an inner chamber made of an old propane tank. This time, we used a larger propane tank, which allowed us more feed stock, and less space for fuel. We made a lid for the inner tank, so that when we inverted the tank it would be open where the original gas valve was, to allow heated gasses to escape. The top of the tank is closed. With a lid on top of the tank, where the bottom of the tank had been, the only exit for hot gases from the feed stock is through the bottom. This system works much better, getting a much cleaner burn, more quickly.

The action of the stove is similar to the original model we developed. An outer chamber, filled with fuel, and with air intakes at the bottom of the stove, allows heat to pyrolize the material in the inner chamber, which has no oxygen. The big difference between the two models is that the second model inner chamber is closed on the top, and had a hole on the bottom, using the part of the tank where the gas valve had been attached to the tank in its previous life as the exit for the hot gasses. The gasses put off from the heat in the inner chamber escape through the bottom, where they are met by the oxygen rich environment and the flames of the burning fuel in the outer chamber. The heat from this also allows better combustion, requiring less fuel, and giving more char. Again, like the first model, we use primarily rice hulls, dried cacao pods, bamboo, cahune seed, coconut husks and shells, and other small bits of biomass with lots of surface area in relation to core mass. The disadvantage of this design is that it is not possible to add more feed stock to be pyrolized. However, of the two designs, this one is preferable as it is more tolerant of my wandering off to other places on the farm. The second design, with welding and material, set us back about US$75. Thanks to Jose Baki of Big Falls for doing such a good job and sourcing the material for us.


The resulting char has a lot of pores on it. Those pores act as habitat for fungi and bacteria, which, in turn, create good humus. Biochar is a fantastic soil amendment, but before you add it to soil you want to fill up all those pores with bacteria or fungi. One gram of biochar has several hundred square feet of surface area. This is massive edge effect, excellent habitat for soil biota. Consider the char to be like condos for soil microorganisms, or like a reef. The microorganisms move in, colonize the space, move nutrients, store moisture, breed, die, eat each other, breed more. Biochar, once inoculated, is teeming with life. If you simply add charcoal to the soil, many of the bacteria and fungi available in the soil will migrate into the charcoal, to colonize all of that surface, making this a temporary loss of soil biota to the areas where the biochar has been applied. One way to make immediate gains in fertility to the target soil is to add the char to your compost, first, where your active fungi and bacteria will quickly colonize the char, and then apply the compost with char to your soil, later.


With either design, once the char is made, we need to apply it to the soil. Instead of applying it directly to the soil, we add it into the bedding of the piggery, where the pigs stomp it into smaller pieces, saving us the time needed to break it into smaller pieces, and defecate and urinate on it, which makes for the perfect mixture for composting. Depending on whether or not we want to fertilize annual crops like corn or sesame, or tree crops, especially cacao, we add green manure, for example tithonia leaves and glyricidia leaves, to make bacterially dominated compost, or dry material, leaf litter from below our agroforestry system, or bamboo leaves, to make fungi dominated compost. We have mixed the charred rice hulls with pig food, which they eat, and then defecate. In addition to helping deal with parasites, it also gets the char completely inoculated when the feces meets the oxygen rich compost pile.


We are applying the composted pig manure mixed with biochar to all of our trees. We are still in the early stages of applying the biochar, and look forward to seeing the results. One of the things we hope will happen is that the fungal communities in the soil will be enhanced, helping to retain soil moisture in our dry season.


We are looking at ways to make more use of the wasted heat. Copper pipe coiled around the outer drum for a thermosiphon hot water system would be one way to harvest some of the excess heat. Designing a sterling engine to create electricity, is another. The idea I am excited most about is an ammonia absorption ice maker, which would turn heat into ice.


The stove is easy to use, and we use it three or four times a week. It can be operated by anyone, alone, requiring no special strength. It needs to be carefully sited as it gets very hot. This is a regenerative design, which can be replicated easily. The biochar stove can be made with freely available materials, recycled, and can be assembled by any welder. It could be adapted to many purposes, including house heating.


If you use wood for fuel, consider making a biochar stove. Your soil biota will love you for it!

For a detailed look at biochar, a good place to start is with Albert Bates book, “The Biochar Solution”.