Key Point
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Driving off water requires a large amount of energy.
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Ideally biomass should have a moisture content of ~15% when it enters the pyrolysis kiln, to ensure high yield, quality biochar and low emissions.
Terminology
The products
Biomass: living or once-living material, which is the feedstock (starting material) for making biochar. Nearly all organic materials, such as bark, nutshells, crop residues, and manurescan be used as feedstock in appropriate devices.
Charcoal: the solid, carbon-rich residue left when biomass is heated in an environment with limited oxygen. Generally, charcoal is made from wood, and is intended for use as fuel. Charcoal can be further processed to produce “activated carbon”.
Biochar: a charcoal-like material made under suitable conditions from non-contaminated starting material, and crushed into small pieces for mixing in the soil. It is often enhanced with nutrients and microorganisms, intended to improve soil properties and plant growth.
Char: a general term for the solid product arising from thermal decomposition (pyrolysis) of any organic material.
Pyrogas (or Pyrolysis gas): The gas and aerosols from pyrolysis or gasification comprising primarily combustible gases CO, H2 and CH4 along with CO2, steam and N2; also known as wood gas and syngas.
Ash: Inorganic compounds in the biochar. (Also refers to material remaining after combustion, which includes a small percentage of carbon.)
The processes
Pyrolysis (from Greek roots pyr “fire” and lysis “loosening”) is the thermal decomposition (breakdown under heat), in a limited oxygen environment, of biomass into a carbon-rich solid residue (char), gases, and liquids.
Carbonisation emphasizes the carbon enrichment, as opposed to the “breakdown”, aspect of pyrolysis. “Carbonization” is often used interchangeably with “pyrolysis.”
Conditioning refers to changes in chemical and physical properties of biomass at temperatures of approx. 110-180oC, where biomass starts to soften and chemically-bound water starts to be driven off.
Torrefaction is a chemical process that takes place at a temperature of approx. 180-300oC which produces a more energy dense, stable, sterile feedstock or soil amendment.
Activation refers to further enhancement of charcoal via chemical processes and/or higher temperature oxidation to produce activated carbonwith high microporosityand surface area.
Gasification is the conversion of biomass into a gas commonly referred to as “producer gas”, using a limited amount of air or steam. A gas rich in CO, CH4, CO2 and H2 is produced.
Batch Roo pyrolyser produces high quality biochar with low emissions
Pyrolysis as a phase of combustion
Stages of Pyrolysis
1. Drying and Conditioning
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Most biomass consists of five main components: cellulose, hemicellulose, lignin, water, and minerals (ash), in proportions depending on the source.
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“Seasoned” wood contains 12–19% water adsorbed on the cellulose/lignin structure. Freshly cut wood or agricultural residues can have a water content of 40 to 60%wb (wet basis, i.e. expressed as a % of the wet weight of the biomass).
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Most of the water is removed as the biomass is heated above 100oC
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Above 150oC the biomass starts to break down.
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At a temperature of approximately 150oC the biomass begins to break down and soften (referred to as conditioning). Chemically-bound water (from the structure of the molecules of the biomass) is released along with small amounts of carbon dioxide and volatile organic compounds.
2. Torrefaction
Kon Tiki kiln used in the B4SS project in Peru
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As the biomass is further heated into the temperature range of 200-280oC, chemical bonds within the constituents of the biomass begin to break.
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This process is endothermic– heat input is required to increase the temperature of the dry biomass and break the molecular bonds.
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Methanol, acetic acid, and other oxygenated Volatile Organic Compounds are released during this stage, along with emissions of CO2 and CO from the breakdown of hemicellulose and cellulose.
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Torrefied biomass is more brittle than fresh biomass, making grinding (e.g. for boiler fuel) easier and less energy intensive. It is more resistant to biological degradation and water uptake, enhancing storability.
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The liquid condensate of the vapors of low temperature pyrolysis was historically called “wood vinegar” or “smoke water.” It is also known as pyroligneousacid, and as “liquid smoke” used as a flavoring product. Depending on its concentration and temperature of production, it can be used as a fungicide, plant growth promoter, to aid seed germination, stimulate composting, and to improve effectiveness of biochar.
3. Exothermic Pyrolysis
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At 250-300oC, depending on feedstock composition, thermal decomposition of the biomass becomes more extreme giving off a combustible mixture of H2, CO, CH4, CO2, other hydrocarbons, and tars.
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Pyrolysis becomes exothermic because breakup of the large polymers of the biomass releases energy. Some oxygen contained within the biomass structure is liberated and enters into energy-releasing oxidation reactions with the gases and char.
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The energy released creates the heat required to break further chemical bonds in the biomass. In principle, the process becomes self-sustaining, and can continue on its own up to a temperature of about 400oC, leaving an oxygen-depleted, carbon-enriched charcoal-like residue.
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In practice heat is lost from the pyrolysis region, so external heat input is needed to increase and then maintain the temperature during pyrolysis.
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Maximum yield is obtained before the end of exothermic pyrolysis but the stable content of the carbon is relatively low. The ash content of a wood biochar is usually around 1.5–5%, the volatiles around 25–35% by weight, and the balance is fixed carbon at 60–70%.
4. Endothermic Pyrolysis
- The biochar remaining at the end of exothermic pyrolysis still contains appreciable amounts of volatile compounds.
- Further heating is required to increase the fixed carbon content, surface area and porosity by driving off and decomposing more of the volatiles.
- A temperature of 550-600oC gives a typical fixed carbon content of wood biochar of about 80-85% and a volatile content of about 12%.
- The yield of wood biochar at this temperature is about 25-30% of the weight of the oven-dry feedstock.
Key Point
- Properties of the biochar depend both on the feedstock and the final pyrolysis temperature.
- The yield, properties and the amount of gas produced also depend on the time taken for pyrolysis and the amount of air present.
5. Activation and Gasification
- Once the temperature is above 600oC adding small amount of air and steam can raise the biochar surface temperature up to 700-800oC, and initiate two distinct processes:
- Activation. Air, steam and heat can activate the surface of the biochar, and release more volatiles. This can increase the surface area, and also cation exchange of the biochar by adding acidic functional groups. Yield is reduced.
- Gasification. If a lot more air and/or steam is added the process is known as gasification. This can produce a gas that is relatively clean that can be use to generate electricity. The yield of the biochar is low (often less than 20%) and the ash content high.
Key Point
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If the feedstock has a high ash content, melting of the minerals/inorganic compounds can occur and pores in the biochar can be blocked
- Gasification biochar may not have the same positive plant response as slow pyrolysis biochar, and may contain toxic compounds such as polyaromatic hydrocarbons or crystalline silica.
Pre-processing to enhance biochar properties
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The rate at which the feedstock pyrolysesand the final properties of the biochar can be altered by pre-processing the biomass.
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Techniques can include:
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pre-treat the biomass with phosphoric acid to enhance functional groups, reduce pH and make a slow-release phosphate fertiliser
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pre-treat the biomass with alkali (e.g. potassium hydroxide) to “soften” the biomass i.e. break down ligno-cellulosic compounds
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pre-treat the biomass with Fe salts to make a magnetic biochar (eg to remove heavy metals from water)
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mix clay, salts (e.g. ferrous sulphate) or other minerals such as rock phosphate with the biomass to slow the rate of pyrolysis, enhance capture of N, and increase concentration of nutrient-rich nanoparticles on the surface
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pelletiseor briquette low density biomass to aid handling and increase biochar yield
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Key Point
- For some applications biomass that is pre-treated will produce biochar that is more effective in addressing specific soil constraints.
Pyrolysis of an Individual particle
Heat flows into (and out of) biomass and char slowly.
Tip: It takes about 1 hour for charring to penetrate 30mm (~1in) into wood.
Charring proceeds at about 0.5mm/min (the “charring rate” in fire science).
Visualization of the 5 Stages of Pyrolysis
Interaction between pyrolysis time and temperature
Biochar can be made at different temperatures in seconds, or over many days depending on size and type of feedstock
The outcome of pyrolysis depends on many factors
Inside the pyrolysis reactor
- Pyrolysis results in massive physical and chemical transformation. The chemical dynamics is very complex involving hundreds of molecular species, reaction energies and rates.
- The outcome is that biochars can have a large range of properties.
Pyrolysis Methods & Process Conditions
- Biochar can be made at different speeds.
- At a high temperature, the material can be pyrolyzed in only seconds if the particles are small enough for the heat to quickly penetrate. This is often termed fast pyrolysis.
- If the biomass particles are large (e.g. log of wood) then it may take days or weeks to completely carbonize. This is typical of fuelwood charcoal kilns.
TYPES OF PYROLYSIS PROCESSES
Slow pyrolysisis is carried out in an oxygen-starved environment in kilns or retorts, in a batch process or with a slow auger feed.
Peak temperatures are relatively low, heating rates are relatively slow, and residence times of the char in the reaction are long.
The term “biochar” was originally associated with this type of production.
Fast pyrolysis converts finely ground feedstock into bio-oils, gas and char, in seconds. It is likely there will be higher condensed volatiles present in the char, which could affect its performance and desirability as biochar for soil amendment.
Fast pyrolysis tends to be used by commercial biochar/bio-oil producers.
Gasification includes a combustion and reduction stage after drying and pyrolysis. It is designed to produce a gaseous fuel mostly of H2, CH4, CO. Gasification processes happen over a wide range of temperatures, with combustion and tar-cracking often occurring above 1000°C.
The gas can be used in heating, or (after cleaning) to run engines, or as a feedstock for conversion to liquid fuels, chemicals and fertilizers.
Basic Methods of Pyrolysing Biomass
1. External Heating of the Biomass
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A separate source of heat is applied to the vessel containing the biochar.
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Liberated volatiles can be condensed and captured (wood vinegar) and/or burned with the combustible gases in excess air in an afterburner.
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Heat can be recycled to continue the process.
- Disadvantages:
- Heat takes a long time to penetrate a large retort.
- If built and operated carelessly a retort can explode.
Basic Methods of Pyrolysing Biomass
2. Internal heating of biomass by flaming, often known as “Flaming Pyrolysis”
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Biomass, preferably partially or fully enclosed in a container, is ignited.
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Sufficient air to maintain combustion is introduced above and/or below the biomass.
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Heat from the flame pyrolyses neighboring biomass liberating more gases to sustain the process.
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Flame around the charred biomass consumes the oxygen and protects the char from oxidation.
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Excess secondary air is introduced to fully and cleanly combust liberated gases above, or separated from, the biomass.
Basic Methods of Pyrolysing Biomass
3. External Heating of the Biomass
- Part of the pyrolysis vapors are combusted in an external combustion chamber.
- The hot combustion gases are directed into the reactor, where they make direct contact with the biomass.
- Liberated volatiles can be condensed and captured (to make “wood vinegar”) or combusted with gases in excess air in an afterburner.
Mass and energy balance in a simple kiln
Mass Balance:
Mass of wood – mass char = Mass pyrogas + mass bulk water
Energy Balance
Energy in moist biomass
–
Energy in Biochar
=
Energy for Drying Bulk Moisture
+
Energy in the Exhaust Gas
+
Heat energy lost from the kiln
Mass and Energy Balance
NOTE
HHV is the higher heating value, a measure of energy content, including the latent heat of vaporisation of water in the biomass.
Getting efficient combustion of the Pyrolysis Gases
Every 100kg wood (20% MC) produced 20kg of steam and 60kg of Pyrogas (plus 20kg of BC).
To burn the gas cleanly, with low emissions, requires:
- One cubic metre(=1000L) of combustion volume for every Megawatt (MW or MJ/s) of pyrogas combusted, to allow for the air and gas to mix fully.
- Between 20%-100% excess air, depending on the burner system used, for complete combustion.
- The air should be injected at different points in the combustion chamber to allow for progressive combustion.
Mass balances in the pyrogas combustor for 20% and 100% excess air
- Computed by stoichiometry and combustion gas equation, normalized to:
- 20% moisture content on wet basis
- 25% biochar yield on dry basis, for biochar and wood
- HHV of dry wood = 18MJ/kg
Techniques for achieving Clean Combustion
Kilns need to be designed to meet limits for emissions of nitrogen oxides and unburned carbon compounds such as CO.
Flameless combustion: The pyrogas is introduced on the outside of the air jets, and air and fuel are premixed with hot exhaust gases. Conditioned fuel and air burn cleanly, at lower temperature, without a flame.
FLOX (Flameless Oxidation) Burner
http://www.bine.info/fileadmin/content/Publikationen/Projekt-Infos/Zusatzinfos/2006-07_Flameless_Combustion.pdf
Conventional flame (with no air preheat)
Flameless Oxidation (with air preheat)
Clean Combustion in a Batch Closed Biochar Kiln
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Air comes in the bottom of the kiln around the biomass and mixes with the pyrolysis gas. The flaming air-fuel mixture then passes pipes where secondary and tertiary air is added through holes.
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Spraying water into the flaming region results in a very clean gas. Sprayed water also activates the hot surface of the biochar.
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Carbon monoxide and nitric and nitrous oxides (NOx) are less than 50ppm and there are virtually no other unburned hydrocarbons.
Post-processing to enhance biochar properties
- The effectiveness of the biochar can be altered by post-processing the biochar.
- Techniques can include:
- treat with phosphoric acid to enhance functional groups, reduce pH and make a slow-release phosphate fertiliser,
- treat the biomass with alkali (e.g. potassium hydroxide) to increase pH and increase K content
- Infuse with organic or inorganic nitrogen compounds e.g. urine to enhance N content
- Mix with nutrient-rich organic material eg manure. May need to heat, to sterilize and dry the mixture to eliminate biohazard and facilitate handling
- Add minerals e.g. rock phosphate, gypsum, dolomite, iron oxides, lime to address specific soil constraints
- Add chemicals e.g. urea, diammonium phosphate to make a compound fertilizer
- Granulate or pelletise to aid handling and biochar application
- Add steam or oxygen. (Note that this is not creating activated carbon, which occurs in the reactor.)
Key Point
- For some applications post-processing biochar will produce a biochar that is more effective in addressing specific soil constraints.
Tips for Biochar Production
Drying and torrefaction processes require external energy.
- Wet feedstock needs more energy:
- use dry fuel (moisture content= 10% to 20%).
- do not over dry – it reduces yield and quality
- Screen feedstock into uniform particle size, 5-15mm if possible.
- Easily combustible feedstocks like straw should be pelletised, or mixed/coated with clay before pyrolysis, to
- reduce loss of nutrients
- increase yields
- For a continuous pyrolysis process, minimise the air entering with the feedstock, such as by purging with a low oxygen gas or vapour (e.g. with cooled flue gas).
- Preheating the feed with steam, combustion gas, or exhaust gas from an engine, helps to:
- reduce air entering with the feed
- optimise energy use
- increase yield.
- For industrial applications there must be no emissions of syngas to the atmosphere. The composition of the exhaust gas must meet local air emissions regulations.
Tips for Biochar Production – Safety
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Pyrolysis gas is toxic. Do not breathe or allow to touch skin.
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The gas can explode if air mixes with it in a closed vessel in presence of an ignition source.
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Avoid breathing in fine biochar particles.
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Biochar that is incorrectly produced (e.g. not fully pyrolysed) or sufficiently cooled, can catch fire. Make sure that the biochar is all cooled in absence of air to ambient temperature, or filled with water, before allowing to stand in air.