Biochar Preparation

Increasing the value and performance of biochar after pyrolysis

The term "biochar" has come to be used very broadly to refer to any type of char used for nearly any purpose; to enhance soil fertility, as an animal feed supplement, a filtration medium, a fuel for a cooking stove or a byproduct of gasification, a building material, char for steel production and for supercapacitors. The suggestions given below refer to biochar used as a soil amendment.

As outlined on our Rationale page, if we are going to use thermal decomposition of biomass instead of biological decompostion to produce a soil amendment, then the thermal process should be as gentle as possible to produce a molecular structure that plants and soil ecosystems are adapted to.

A piece of biochar
What's next?

The primary benefit of biochar that is properly prepared to increase soil fertility is a significant increase in cation exchange capacity (CEC)1, the ability of soil to retain plant nutrients. Biochar has been shown to have 6 to 7 times more CEC than humic matter of the same mass. Secondary benefits that flow from the increased CEC include increased aggregation.2 Soil aggregates will retain moisture and provide better aeration to root systems, and in conjuction with the biochar provide an ideal habitat for soil microbial and mycorrhizal populations.

The ability of a soil to retain plant nutrients and release them when required (CEC) is a very important component of soil fertlity. Without it, rain leaches nutrients into deeper soil layers, out of the reach of plant roots and soil microbiota, and the loss of soil organic matter is accelerated, soils become "highly weathered", yellow or orange in color. Areas of the world with soils low in organic carbon tend toward poverty, a lack of development and societal disfunction. Soil organic carbon, particularly its ability to retain plant nutrients, is an essential foundation of wealth and well being in our world.

Follow these guidelines to optimize the soil fertility benefits of biochar:

Proper biochar preparation is essential to optimize its fertility benefit in soils. The primary benefit of properly prepared biochar is a significant increase in the ability of soil to retain plant nutrients, called cation exchange capacity or CEC. Soil organic carbon, more precisely cation exchange capacity, is an essential foundation of wealth and well being in our world.

Low Temperature Pyrolysis

As discussed on our Rationale page, pyrolysis temperatures should be low, in the range of 300° C to 550° C.2 3 One objective is to retain as much oxygen and hydrogen in the molecular structures of the char as possible to facilitate the formation of OH functional groups on the char surfaces. Practically, this means that the biomass cannot be exposed to air during pyrolysis. Hence it must be in a closed vessel that prevents air from getting in, or more precisely, prevents oxygen from contacting the biomass. The moment the particles are exposed to oxygen, as occurs in all flaming pyrolysis and many gasification methods, their surfaces reach combustion temperatures of ~ 1000° C, which immediately breaks the more fragile oxygen and hydrogen molecular bonds, releasing them as gases that oxidize (burn) in the flame.

A second objective to keeping pyrolysis temperatures low, particularly with non-woody feedstocks such as poultry manure, is to retain as much mineral nutrients in the biochar as possible. Nitrogen is particularly sensitvie to volatilization at temperatures over 300° C. 4

Of course, closed vessel reactors can also exceed 550° C. This is particularly the case in certain auger or rotary designs that require higher temperature pyrolysis to prevent the recondensation of tars on the feedstock being processed. Our Horizontal Bed Kiln is designed to provide precise control over process conditions of pyrolysis temperature and residence time, in a production device, that until now has only been achieved in a laboratory.

Biochar produced at high temperatures will eventually form OH functional groups on its surfaces as it oxidizes, but only over decades to centuries. Waiting that long isn't economically or practically viable. That said, under the right conditions, particularly in a temperate climate with plentiful organic matter available to the soil, such as when the soil is often amended with compost, a film of decomposing organic matter may form on high temperature char particles that exhibit some CEC. But this is from the compost rather than the biochar.

Humic acid model
OH functional groups, the COOH, OH and HO branches shown in the humic molecule above, create a slight negative electrical charge on the surfaces of decomposed organic carbon molecules, including biochar produced at low temperatures. This negative charge attracts the positively charged plant nutrients in the soil solution, causing them to cling to the organic carbon. The scientific term for this attraction is "adsorption".

It is for this reason that OH functional groups are a critical molecular component of biochar to be used as a soil amendment. Without them, the char is more or less an inert bystander to the biochemical processes of soil fertility.
Reduce Particle Size

For use as a soil amendment, biochar should be ground into small, sub-millimeter particles. Why? Because the fertility benefit of biochar is largely a function of exposed surface area. Increased surface area "increases the rate of reaction with minerals and soil organic matter".5 Biochar particles made from woody feedstocks will have a large internal surface area because of their pore structure. However, the larger the particle is, the less its internal pore surfaces will be available to the biochemical processes that make nutrients available to plants. While capillary forces during heavy rain may initially draw soil solution into deep pores, free circulation is inhibited, hence oxygen and organic matter.2 A shallow pore will be more available to the circulating soil solution, plant enzymes, root hairs and mycorrhizae.

Another equally important reason to reduce biochar particle size is that sub-millimeter particles, of low temperature biochar containing aliphatic carbon, will be most likely to form soil aggregates.6 Aggregation will enhance the effects of biochar, retaining water and nutrients, often providing islands of habitat for microbial life, and it will make the biochar stable.7 8 9

Current research indicates aggregation is most likely the primary mechanism of soil organic carbon stability. "Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate."10 "Physical protection and interactions with soil minerals play a significant part in black-carbon stability over long periods of time."10 11

A recent study showed that one third of the ground biochar particles added directly to temperate forest soil had formed aggregates within 10 months, demonstrating that aggregation occurs quickly under the right conditions. The biochar used in this study was produced at 450° C, and virtually none of it was lost to decomposition during that time period.7

The biochemical processes plants use to absorb mineral nutrients from soil occur at a microscopic scale. Soil bacteria are from 0.5 to 1 μm (micrometer) in diameter. There are one thousand micrometers in a milimeter, and for reference a human hair is about 70 μm thick. Mycorrhizal hyphae, the thin, hollow tubes of fungi that many plants form a symbiotic relationship with to obtain minerals, are from 2 to 20 μm in diameter. The dilute minerals plants rely on are absorbed as sub-micrometer sized particles.

To a soil bacterium, the chunk of biochar in the hand shown above is roughly as large as the earth is compared to you and me.

While there is speculation and some evidence that biochar pores may be a refuge for beneficial soil bacteria, bacterial populations will only survive where they have access to sufficient nutrients. The soil solution isn't under pressure in such a way that it can continually flush decomposing biomass or root exodates deep into those pores. Research has demonstrated that bacterial populations mineralizing biogenic material can only survive near the surface of a biochar particle, generally in the range of 10 micrometers deep.

Root hairs and mycorrhizal hyphae do at times wander into the pores of a biochar particle, but a given root hair's ability to absorb nutrients will be limited to those that are within the pore it inhabits. It is only via the soil solution surrounding the biochar particle that minerals can be freely exchanged.

Root hairs penetrating biochar
Root hairs penetrating into a piece of biochar that has been in soil for 20 years. Photo by Nikolaus Foidl.

A quick calculation may help in understanding the importance of reducing particle size. Assume we have a biochar particle that is a cube 1 centimeter in all dimensions, to make the math easy. It has 6 faces, so the exposed surface area of our 1 cm cube is 6 square centimeters.

If we reduce our particle size in half, by cutting our cube in half along all its axes, we will have 8 cubes 5 millimeters in all dimensions. Each cube has 6 faces with a surface area of 0.25 cm2 ( 0.5 x 0.5 = 0.25 square centimeters) 0.25 cm x 6 faces x 8 cubes = 12 square centimeters in total.

Mathematically it works out that every time we halve the particle size, we double the exposed surface area. If we keep halving the size of our particles until we have sub-millimeter particles ( 0.250 cm cubes will have 24 cm2 surface area, 0.125 cm cubes ( 1.2 mm ) will have 48 cm2 surface area, 0.0625 cm cubes will have 96 cm2 surface area, we significantly increase the effectiveness of the biochar we can produce.

Smaller particles will also be distributed more widely within the topsoil horizon, coming in closer proximity to the bacteria, minerals, fungi, decomposing organic matter that all contribute to soil fertility, and many more of the root hairs and mycorrhizae.

Most biochar particles found in Amazonian terra preta are between 10 and 20 μm (micrometers). If we start with our 1 cm cube of char and reduce the particle size by half 10 times, we will have over a billion 10 μm particles with a total of 6,144 square centimeters exposed surface area.

As noted above, the largest fraction of biochar found in Amazonian terra preta are particles in the 10 to 20 μm range that are aggregated with clay and wide variety of other minerals. Terra preta is both very stable and has remained very fertile, over thousands of years.

Table demonstrating the increase in total exposed surface area and the quantity of particles if a 1 centimeter cube of biochar is reduced in size by half 10 times. The majority of biochar particles in Amazonian terra preta are between 10 and 20 μm.
Size Quantity Total Surface Area cm2
1 cm 1 6
5 mm 8 12
2.5 mm 64 24
1.25 mm 512 48
625 μm 4,096 96
312 μm 32,768 192
156 μm 262,144 384
78 μm 2,097,152 768
39 μm 16,777,216 1536
20 μm 134,217,728 3072
10 μm 1,073,741,824 6144
Compost Biochar with Nutrient Dense Biomass

There are 2 possible drawbacks to reducing biochar particle size. One is that if it directly applied to the surface of a field before it can aggregate with other soil particles, much of it can wash or blow away. It can also percolate down into the sub topsoil layer if it is not yet aggregated. The second is that if the above 2 practices are followed and the finely milled biochar is incorporated directly into soil, it can rapidly adsorb most of the available nutrients in the soil solution, depriving the crop and soil biota of minerals and nitrogen, particularly at the beginning of the growing season when epigentic growth patterns are being set.

The simple solution is to compost the biochar with nutrient rich biomass (plus perhaps some clay if it is not available in the soil to which it will be added). Any manure would be a good choice, for instance. Adding biochar to a compost heap will help retain more of the nutrients in the compost, rather than losing them to leaching. It will also help to retain more of the organic carbon and nitrogen in the finished compost, rather than losing them to carbon dioxide, methane and nitrous oxide. Once our finely milled biochar particles are saturated with plant nutrients and the process of aggregation has begun, the biochar compost can be added to soil and the potential benefits will manifest themselves immediately.

An option to consider is biochar vermicomposting. In this approach, a finely milled biochar / manure mix is precomposted with (optimally) plentiful aeration for a short period of time until the temperature begins to come down, and then this mixture is used as feed for composting worms. Worms need fine sand-sized particles of grit to grind up the biogenic material they consume in their gizzard-like digestive systems, and they prefer particles of biochar, perhaps because they tend to be coated with bacteria. In a worm's gut, biochar particles will likely be ground to finer particle sizes, and certainly combined with minerals, bacteria, fungal spores, fulvic and humic acids, and residual organic matter. A worm's digestive tract is the ideal environment to accelerate the deep integration of small biochar particles into the soil matrix.

Evidence suggests that worms may have played a vital role in the formation of highly fertile, highly stable Terra Preta soils.12 It is remarkable to note that these soils were created, by local tribes, in a region of the world that typically has very infertile soils. The heat and humidity in the Amazon basin promotes the complete decomposition of organic matter before it can be stabilized in soils, in a matter of weeks. And these tribes did so without modern technology or scientific research techniques. With careful observation and experimentation, and the technology and knowledge we have at our disposal today, we can optimize the creation of biochar based substrates that are highly fertile and resilient.