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Biochar, carbon sequestration and water quality
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Posted 9/13/2021 16:18 (#9218374)
Subject: Biochar, carbon sequestration and water quality

Why is there no interest?

Soil Organic Carbon (SOC) comes in a wide variety of forms with different half-lives (i.e., the time it takes for half of it to be degraded to CO2 by soil microbes). Crop residue on the surface of the soil will have degradation times of days to months. Research has shown conclusively that very, very little of the organic carbon in above-ground residue will be incorporated in SOC. If I remember correctly, the number for corn stalks was 0.03% using corn stalks labeled with C-14 as the tracer. If one incorporates the residue by plowing it under, the incorporation percentage increases significantly, but plowing increases the oxidation of pre-existing SOM resulting in the loss of more SOC than was gained by plowing the corn stalks under.

Biochar is a unique form of carbon with a long-er half-life ranging from months to centuries. Biochar has been used in agricultural production for well over 1000 years and probably much longer. It is an inherent part of the Black Carbon soils in Europe, the Terra Preta soils of South & Central America and the Black Prairie soils of North America. Investigations of the chemical structures of biochar in these soils shows that they are all the same – just charcoal from biomass.

From a physical perspective, biochar is pretty unique material. Properly made, it looks like a tiny piece of very porous charcoal where the pore spaces are lined with positive and negative ions that can bind to both negative and positive nutrients, strongly enough to keep them out of ground water without binding them so tightly the nutrients can’t be used for growth.  At the same time, particle porosity changes bulk density, water infiltration, resistance to compaction, etc.

Biochar is created by heating biomass in a low oxygen environment and exists in many forms dependent on how it was created (open burning, controlled combustion, etc.) and the type and form of biomass being burned. The half-life of biochar, which is defined by its physical structure, ranges from months to a few 1000’s of years making it an ideal carbon sequestration (capture) approach. There is no question about application, amount incorporated, longevity, etc.

Biochar is created by burning biomass. With lots of oxygen, one gets lots of CO2 and a little ash (mineral) residue -and no biochar. Controlling the combustion process can change that. Big burns of wood is good, but big burns of wood stacked and covered with dirt is significantly better and wood burned in kilns is even better, but still not very efficient. This, slow pyrolysis, is the process used today to create the charcoal used for grilling.

A derivative of that approach, called fast pyrolysis, quickly burns (say, 5-10 seconds) dry (10% moisture is ideal), finely ground(<1mm) biomass in a controlled low oxygen environment to produce biooil and biochar. The yield of oil (i.e., biooil) and biochar is dependent the source of the biomass and burn conditions.

A century ago, before railroads crossed the Appalachian Mountains, engineers were looking for ways to pump coal to market. First, they tried to figure out how to pump a coal slurry hundreds of miles and failed. Then they developed a process to liquify coal in a process called hydrothermal liquefaction (HTL). HTL heats a slurry of coal and water under high temperatures and high pressure to create an oil (biocrude) that could easily be pumped. Unfortunately, railroads came into being with railcars becoming the lowest cost approach for moving coal out of the mountains.

HTL became a scientific curiosity and sat on the shelf until the first oil crisis in 1973. It was pulled from the shelf by multiple groups who quickly learned that one could turn any carbon-based organic material (including coal) into biooil and biochar when heated in a water slurry under pressure. The organic material could be anything – crop residue, animal manure, sewage, ground up trees, garbage – anything.

But the Oil Crisis was resolved and HTL went back on the shelf. It stayed there until people started to understand the role of CO2 in global warming.

There has been an immense amount of research on HTL in the last decade, but virtually all of the research has focused on bench scale systems in academic institutions which are tough to work with. Small batch systems (which are easy to operate) and continuous flow systems (hard to operate but needed for commercial viability) produce different results. A few groups are building pre-commercial scale, continuous flow systems, but their primary interest at this time isn’t in agriculture. On the biochar side, there is a biochar organization calling for universal use.

The opportunity exists for farmers to control this market and their future. An acre of 200 bpa corn will produce ~5 tons of above ground harvestable corn stalks which is equivalent to 5 barrels of petroleum crude on a BTU basis. This leaves ~6.5 dry tons of cornstalk biomass underground and 1.5 dry tons on the surface to manage wind erosion (a little more than soybeans would leave.

The petroleum crude can be transported in any oil pipeline and converted into renewable diesel (or refined into value-added chemicals) by most all refineries. The biochar and recovered nutrients goes back to the farm to enhance production (or sold). And the water gets recycled.

Corn fodder is the single largest, annually renewable, biomass resource in the US. And almost all gets converted to CO2 via microbial decay in the months following harvest. So, harvesting corn stalks for the production of renewable diesel while simultaneously creating a biochar side-stream which goes back to the land to enhance its productivity and simultaneously recycling nutrients removed during biomass harvesting biomass makes for an attractive economic picture.

Historically, the limiting factor in HTL has been high pressure pumps capable of pumping suspended solids. The development of fracking which is based upon pumping sand into rock formations has solved that problem.

One would think that corn producers would be jumping on this opportunity but that isn’t the case.  The situation gets even stranger since animal manure can be co-processed.

Here is an ignored opportunity for farmers to address a climate/global warming issue, Gulf of Mex water quality issues and local water quality issues, enhance the productivity of their farmed land – all while enhancing existing and creating additional income streams.

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