Biochar can be defined as a charcoal produced for some biological purpose; to enhance soil fertility, as a livestock feed additive, for soil remediation in case of contamination, or to reduce nitrate and phosphate levels in agricultural water runoff. For such uses, particularly for soil fertility, the molecular structure of the char matters.

Plants have evolved over hundreds of millions of years to obtain their nutrients from topsoil, which is made of decomposed biomass ( humic matter) and minerals. The minerals are mostly positively charged, while the humic matter is negatively charged. Hence they bind together to form fertile topsoil. This happy marriage makes these minerals available to plant roots, particularly the more shallow roots of agricultural plants.

If we want to produce a biochar that optimally enhances soil fertility, we might start with the assumption that it should be as similar to humic matter in molecular structure as possible. Why? Because soil biochemistry, and the plant life it supports, will likely respond to such a biochar in the same way as they respond to humic matter. Soil, soil microorganisms and plants have evolved symbiotically over eons. This complex, mutually supportative ecosystem will not adapt to any novel idea simply because it seems good to us. Rather we must ensure our agricultural innovations are adapted to soil ecosystems.

Scientific research supports this simple assumption. The cation exchange capacity (CEC) of biochar, its ability to retain plant nutrients, depends on its molecular structure. Biochar particles produced at low temperatures will have high CEC levels because of the negatively charged OH functional groups on its molecular surfaces.1

Humic acid model
Model humic acid molecule showing OH functional groups

The molecular structure of char is largely determined by pyrolysis processing temperature and residence time. Biological decomposition, which produces humic acid, is a gentle process. If we want to mimic humic acid's molecular structure in biochar, we need precise control over the temperature and residence time of each biomass particle to orchestrate a gentle thermal biomass decompostion. Precise, low temperature pyrolysis retains the more delicate, negatively charged OH, COOH and HO groups on the carbon backbone. If we want to produce commercial quantities of biochar, we need that control at scale.

This is why we have developed a novel, multi-purpose kiln to produce biochar at production scale, as shown below. The horizontal bed design, essentially a wide, slow conveyor belt moving the biomass particles through 3 compartments, allows us to heat biomass particles in distinct stages, separate the gases produced in each stage, and most importantly, control the essential process conditions of residence time and temperature so that each particle’s molecular transformation is precisely determined and has an identical signature across all particles produced.

Biochar production with horizontal bed kiln
3d model of the horizontal bed kiln

Most production scale pyrolysis kilns are of 2 types, auger and rotary. Both tumble the feedstock to distribute heat, which is fine for a clothes dryer, but energy intensive and prone to expensive mechanical breakdowns when processing many tonnes of biomass per day. In an auger kiln, residence time can be controlled, but temperature is kept quite high to prevent tar build up within the auger. In a rotary kiln, temperature can sometimes be kept in a lower range, but residence time cannot be controlled. Hence both are unsuitable to produce a biochar with optimal soil fertility characteristics.

Our horizontal bed kiln distributes heat via hot, oxygen deprived exhaust gases drawn through the bed of biomass particles. Our approach is designed to lower capital investment, operating and maintenance costs. Horizontal bed ovens are used in a very wide variety of industries to heat everything from glass to medical equipment, and to bake a wide variety of foods. So the core design principles have a very reliable foundation.

While our design objective was to provide a precise means of low temperature pyrolysis at scale, the kiln can be precisely operated at a range of temperatures and residence times to accurately produce thermally decomposed biomass for a wide variety of purposes. Our kiln also extracts pyroligneous acid, (wood vinegar) from the second stage gas stream, and has an adaptable post pyrolysis processing system that can be used for quenching, impregnation, or steam activation.

All in all, the primary goal of the horizontal bed kiln is to maximize its potential for finanical profitability in that it can accurately produce multiple types of thermally decomposed biomass end products, including torrefied biomass, wood vinegar, a range of chars for soil fertility and remediation, animal feed, and activated carbons. Precise thermal decomposition also serves as the first stage in our process to produce humic and fulvic acid directly from biomass.