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BIOCHAR, PYROLYSIS, AND THE PHASES OF PYROLYSIS

Updated: Aug 1



https://www.istc.illinois.edu/research/waste_utilization/biochar

What is Biochar, and how does it work?

Biochar is a fine-grained, high-carbon residue created nowadays by sophisticated pyrolysis techniques (direct thermal decomposition of biomass in the absence of oxygen and preventing combustion).

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It creates a combination of solids (biochar), liquids (bio-oil), and gases (syngas) as a result of the process. Biochar has the potential to improve soil fertility in acidic soils (low pH soils), boost agricultural production, and protect crops from foliar and soil-borne diseases.

Why has it grown into prominence?

The nitrogen and phosphorus levels in biochars made from manures and food wastes were greater.

Some biochars can boost soil fertility, water storage capacity, and crop yield.

The economic imperatives that drive a farmer’s choice to burn stubble are paramount.

Farmers delivering stubble will be rewarded with incentives from power generators and biofuel producers.

BIOCHAR IS A VIABLE SOLUTION

The Super SMS (straw management system) is an attachment for paddy combine harvesters that equally distributes residue over the field. It should be used when the stubbles are being chopped.

Biochar should be marketed as a nutrition, and commercial companies should be included in the infrastructure development.

Its advantages

Carbon Sink:

Biomass combustion and natural decomposition release significant volumes of carbon dioxide and methane into the atmosphere. Although the biochar-making process produces CO2, the carbon content of the leftover biochar remains constant indefinitely. Soil Improvement: Biochar is known to provide a variety of soil health advantages. Biochar has been discovered to be good in retaining both water and water-soluble nutrients due to its porous structure. Its existence on the planet has the potential to enhance water quality, soil fertility, agricultural production, and relieve strain on old-growth forests.

Biochar is hygroscopic, which means it holds water. As a result of its propensity to attract and hold water, it is a desirable soil material in many regions.

What exactly is Pyrolysis?

Pyrolysis is the thermal breakdown of materials in an inert environment at high temperatures. A change in chemical composition is involved. The most prevalent application is in the treatment of organic materials. It’s one of the steps in the charring process. It is regarded as the initial phase in the gasification or combustion processes.

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What is the mechanism behind it?

Pyrolysis of organic compounds yields volatile chemicals and a solid carbon-rich residue, char, in general.

Carbonization is a type of extreme pyrolysis that produces primarily carbon as a byproduct.

The technique is widely used in the chemical industry to make ethylene, various kinds of carbon, and other compounds from petroleum, coal, and even wood, as well as to make coke from coal.

Carbonisation stresses the carbon enrichment element of pyrolysis rather than the “breakdown” aspect. The terms “carbonization” and “pyrolysis” are frequently interchanged.

Conditioning refers to changes in the chemical and physical characteristics of biomass at temperatures between 110 and 180 degrees Celsius, when the biomass begins to soften and chemically bound water begins to evaporate.

Torrefaction is a chemical process that creates a higher energy dense, stable, sterile feedstock or soil amendment at a temperature of around 180-300oC.

Activation is the process of improving charcoal by chemical procedures and/or higher-temperature oxidation in order to generate activated carbon with a high microporosity and surface area.

Gasification is the process of converting biomass into a gas known as “producer gas” with the help of a little quantity of air or steam. The result is a gas rich in CO, CH4, CO2, and H2.

THE PHASES OF PYROLYSIS

  1. CONDITIONING AND DRYING

Depending on the source, most biomass is made up of five primary components: cellulose, hemicellulose, lignin, water, and minerals (ash).

Water adsorbed on the cellulose/lignin structure of “seasoned” wood ranges from 12–19 percent. Water content in freshly cut wood or agricultural leftovers can range from 40 to 60% wb (wet basis, i.e. expressed as a percent of the wet weight of the biomass).

As the biomass is heated over 100oC, the majority of the water is eliminated.

Biomass begins to decompose at temperatures over 150°C.

The biomass begins to break down and soften at a temperature of around 150oC. (referred to as conditioning). Water that has been chemically bound (because to the structure of biomass molecules) is released, along with minor quantities of carbon dioxide and volatile organic chemicals.

IMPORTANT NOTE

  • Driving on water needs a significant amount of energy.

  • To guarantee a high production, quality biochar, and minimal emissions, biomass should have a moisture content of 15% or less when it enters the pyrolysis kiln.

  1. TORREFACTION

It is the second step in the TORREFACTION process. Chemical bonds within the biomass components begin to dissolve as the biomass is heated to temperatures between 200 and 280oC. Endothermic means that heat is required to raise the temperature of the dry biomass and break the molecular bonds.

During this stage, methanol, acetic acid, and other oxygenated VOCs are emitted, as well as CO2 and CO2 emissions from the breakdown of hemicellulose and cellulose.

Torrefied biomass is more brittle than fresh biomass, allowing for simpler and less energy-intensive grinding (for example, for boiler fuel). It’s more resistant to biological destruction and water absorption, which means it’ll last longer.

Historically, the liquid condensate of low-temperature pyrolysis fumes was referred to as “wood vinegar” or “smoke water.” It’s also known as pyroligneous acid, and it’s utilised as a flavouring agent as “liquid smoke.” It may be used as a fungicide, plant growth promoter, seed germination aid, compostable stimulator, and to increase the performance of biochar depending on the dosage and temperature of synthesis.

  1. PYROLYSIS EXOTHERMIC

Thermal breakdown of biomass gets more intense depending on feedstock composition, releasing a flammable combination of H2, CO, CH4, CO2, various hydrocarbons, and tars.

Pyrolysis becomes exothermic when the big polymers in the biomass are broken apart, releasing energy. Some of the oxygen held within the biomass structure is freed, and it reacts with the gases and char to release energy.

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The heat generated by the energy released is used to break down more chemical bonds in the biomass. In theory, the process becomes self-sustaining and can run on its own up to 400 degrees Celsius, leaving an oxygen-depleted, carbon-rich charcoal-like residue.

Heat is lost from the pyrolysis zone in practise, hence additional heat is required to raise and then maintain the temperature during pyrolysis.

Although the maximum yield is attained before the completion of exothermic pyrolysis, the stable carbon content is minimal.

  1. ENDOTHERMIC PYROLYSIS

This is a kind of endothermic pyrolysis.There are still significant levels of volatile chemicals in the biochar that remain after exothermic pyrolysis.

By driving out and decomposing more of the volatiles, more heating is required to enhance the fixed carbon content, surface area, and porosity.

Wood biochar has a fixed carbon content of around 80-85 percent and a volatile concentration of about 12 percent when heated to 550-600 degrees Celsius.

At this temperature, the output of wood biochar is around 25-30% of the weight of the oven-dry feedstock.

IMPORTANT NOTE

Biochar properties are influenced by both the feedstock and the ultimate pyrolysis temperature.

The yield, characteristics, and volume of gas generated are also affected by the pyrolysis period and the amount of oxygen present.

  1. GASIFICATION AND ACTIVATION

Once the temperature has risen to 600°C, a tiny quantity of air and steam can be added to elevate the surface temperature of the biochar to 700-800°C, triggering two separate processes:

Activation. The surface of the biochar can be activated by air, steam, or heat, releasing additional volatiles. By adding acidic functional groups, the surface area of the biochar may be increased, as well as the cation exchange. The yield has been lowered.

Gasification. The process is known as gasification when a lot more air and/or steam is introduced. This can provide a pretty clean gas, which can be used to generate energy. The biochar output is poor (typically less than 20%) and the ash concentration is significant.

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