Choosing a biochar reactor to meet your needs

Designing, Building or Buying a Kiln

This guide is designed to assist you in deciding whether you will buy a kiln from a vendor, build a kiln from a published design, or develop your own kiln.

We provide details of some innovative designs that have been tested, organized in a way that will help you evaluate designs and choose a biochar device that suits your needs.

It should be read after reading the guide Basic Principles and Practice of Biochar Production, and the guide Properties of biochar.

Choosing a reactor

Considerations:

  • Purpose of the reactor (provide biochar and/ or energy, and/or manage residues)
  • Amount and type of biomass available
  • Current agriculture and waste management practices
  • Acceptance of small reactors, which is dependent on alignment with socio-economic context (current methods of cooking, feedstock handling required, cost, availability of labour, spare parts)
  • Selection of industrial scale units must consider economic and regulatory environment, skills required to operate, reliability of feedstock supplies, control system (manual and/or automated operation)
  • Location and size of markets for products

Terminology

A Reactor (or more specifically a chemical reactor) is a vessel designed to contain and control (chemical) reactions.

A Pyrolyser is a reactor designed for thermal decomposition of biomass in a limited oxygen environment (= pyrolysis).

A Gasifier is a reactor in which air is intentionally injected into the feedstock. Part of the feedstock is burned to produce a relatively clean pyrogas. A gasifier usually operates at a higher temperature than a pyrolyser.

A Stove is an enclosed space in which fuel is burned to provide heating, either to heat the stove itself and the space in which it is situated, or to heat items placed on the stove.

A Kiln is a kind of oven, a thermally insulated chamber,that produces temperatures sufficient to complete some process, such as drying, or chemical change. A kiln may be internally or externally heated.

A Retort is an airtight vessel in which substances are externally heated, usually producing gases to be collected in a collection vessel, or for further processes.

Batch Pyrolysers are simple low-cost devices that are filled with biomass, run to completion and then emptied.

Basic batch stoves, retorts and kilns  are often used for small-scale manufacture of biochar, and also for larger scale production of fuel- or process-charcoal (eg for reducing metals).

Continuous Pyrolysers are devices where biomass is fed into one end while biochar is continuously discharged from the other.

Continuous devices are more complex and expensive, but can provide:

  • more production from a given amount of equipment and labour
  • more control over the process conditions of the biochar
  • lower emissions

Terminology

Pyrogas (or Pyrolysis gas): The gas and aerosols from pyrolysis or gasification comprising primarily combustible gases CO, H2and CH4along with CO2, steam and N2; also known as wood gas and syngas.

Primary Air (PA):  In pyrolysis PA refers to air supplied to the fuel bed, needed to partially combust the material resulting in emission of combustible vapoursand gases.

If pyrolysis is sustained by external heat, PA provides a fraction of the air required for first stage combustion of emitted gases.

Secondary Air (SA): (and in some instances tertiary air) refers to additional air injected to the combustion zone to complete combustion of the fuel gases.

Materials Handling: This refers to the equipment that moves the biomass to the pyrolyserand moves the biochar from the pyrolyser.

Materials Preparation Equipment: This includes machinery that reduces the size (e.g. grinders), compacts the biomass into pellets or briquettes, dries the biomass, or mixes ingredients (such as biomass and minerals) together.

Characteristics of Kilns

Varying with specific kiln design and cost, most kiln categories can be constructed:

  • to have internal or external heating
  • to have emissions ranging from low to high
  • to take wood or agriculture residues; some to process sludges and manures.

The physical and chemical properties of fresh biochars

Important physical properties of fresh and aged biochars include:

  • Bulk density and particle density, particle size, macro and microporosity, surface area, water holding capacity.

The chemical, electrical and magnetic properties include:

  • Macro nutrient content (N, P, K, S, Ca, Mg) and micro-nutrient content (Fe, Mn, Cu, Zn, Mo, B, Cl, Si, Na, Ni ). Measuring their solubility at different soil pH will provide an indication of their plant availability
  • Soluble organic compounds (soluble in water or in other solvents)
  • pH (proton activity), Eh (electron activity), EC (electrical conductivity), liming value, cation and anion exchange capacity (CEC and AEC), reducing and oxidising potential, magnetic susceptibility/paramagnetism
  • Adsorptivity of heavy metals and organic pollutants.

Key points:

All biochar reactors should be designed to:

  • be easy to load and unload,
  • handle the high heat,
  • burn the released gases without emissions that exceed local environmental limits
  • be safe to operate.

Continuous devices are more complex and expensive, but provide:

  • higher production from a given amount of equipment and labour
  • more control over the process conditions  to produce a consistent biochar
  • lower emissions.

Batch pyrolysers

Traditionally, production of charcoal occurred in batch earth, brick or metal kilns and had very high emissions (particulates, methane and other unburnt hydrocarbons).  These polluting kilns are not acceptable today.

Modern batch devices achieve low emissions by efficient mixing of air and gas.

The advantages of modern batch kilns are the low cost, capacity to use larger feedstock (e.g. logs), ease of operation, portability and relatively low emissions when operated correctly.

The disadvantages can include:

  • excludes some feedstock (e.g. municipal solid waste (MSW), manures, sludges)
  • need for careful stacking, igniting, quenching and constant supervision
  • for many designs, the amount of biochar produced per unit kiln volume is small.

Batch pyrolysers: TLUD Gasifier Stoves – The Principles

  • Fire is ignited at the top of a column of biomass fuel, and a hot “pyrolysis front” moves down through the stationary fuel bed, converting biomass to biochar, and liberating pyrolysis gas.
  • Primary air (PA) enters at the bottom of the fuel chamber, and flows upward through the fuel bed.
  • PA supply is limited/controlled so only a portion of the liberated gases are combusted, generating just enough heat to sustain pyrolysis.
  • The upward-moving gas mixture has little remaining oxygen to oxidize the char—thus the char is preserved.
  • Hot pyrolysis gas/smoke, including tars, CO2& steam, rising through the hot char, are cracked into smaller molecules and emerge as combustible gas.
  • Excess secondary air (SA), introduced below a concentrator disk, blends with gas as they both channel through the hole, resulting in a clean burning flame.
  • A chimney creates draft for PA & SA.
  • Various more sophisticated “burner” arrangements have been developed to optimize clean combustion.

TLUD Gasifier Stoves – Operating Procedure

  • The stove chamber is almost filled with biomass fuel.
  • The fuel is lit from the top, typically by lighting a small amount of fuel that has been soaked in alcohol or lighter fluid and spread thinly on top of the load, and a chimney is placed on to begin normal operation.
  • Once alight the primary air (PA) is controlled so that the flame flows under the pot and does not smoke. Too much PA makes too much pyrogas, generating smoke. PA is turned down, to simmer.
  • It takes about 1 hour, varying with fuel and PA rate,  for the pyrolysis front to migrate to the bottom of a 20cm tall fuel bed in a small cook stove.
  • When the pyrolysis front has reached the bottom, and the flame goes out, leaving some smoke, the stove is extinguished and the char preserved.
  • This can be done by tipping out the char and wetting it fully, or by transferring it to a metallic or clay container with a tight lid, where the fire is snuffed and the charcoal cools in the absence of air.

Christa Roth, 2016. Paul Anderson, 2016

TLUD Gasifier Stoves – Operating Tips

TLUD Gasifier Stoves – High Performing Cookstove

Kirk Harris – Natural Draft TLUD stove

http://stoves.bioenergylists.org/content/author/%20kirk-harris

Vietnam: Combination of Rocket and TLUD: DK T5*

Basic Dimensions for DK T5*

Batch Pyrolysers: Retort Stove – The Principles

Operating procedure:

Fill pyrolysis chamber with biomass, place wood in main chamber and start fire. Heat from the fire is transferred to the pyrolysis chamber, releasing volatile gases. These gases are burnt in the wood flame.

Secondary air is provided to efficiently combust the pyrolysis gases.

Retort Stove – Plan

Batch Pyrolysers: Retort Stove – Operating procedure

  • Fill outer chamber with biomass (eg coffee husks, straw, twigs)
  • Start a wood stick fire in the inner chamber.
  • After 15 minutes biomass in the outer chamber pyrolysesand releases gas, which enters the inner chamber and burns above the fire.
  • High temperature biochar is made in the inner chamber & low temperature (~450C) in the outer.

Batch Kilns: Kon-Tiki Open flame-cover pyrolysis

The cone kiln was developed in Japan and has been used to make wood and bamboo biochar for over 200 years.  It consists of a steel cone that can sit on the ground. Pyrolysis occurs inside the kiln and burning occurs at the top of the kiln.

Why do you get efficient burning in a cone kiln?

Air updrafts past the rim of the kiln, pushed from below by convection up the hot kiln wall, and pulled from above by the low pressure created by the updraft above the kiln.

  1. The fast moving air at the rim (measured at around 2m/s) creates a low pressure (by the Bernoulli Principle, or Venturi effect), which pulls air up the inside wall of the kiln.  Convection on the inner hot wall also drives air up the inner wall.
  2. The low pressure at the wall drafts air from the center of the kiln that drives an inwardly rolling vortex, like a rolling donut of air all around the rim.
  3. The vortex rolls air down into the center of the kiln, providing the primary air for pyrolysis. This can be seen in the smoke trace from a smoke stick.
  4. The vortex rolls pyrogas released from the pyrolysis zone out and up to the rim, where it meets fresh secondary air.
  5. The natural vortex convective dynamic of the Kon-Tiki is aided by enclosing the cone in a rim shield.

Emissions Data for Different Kilns and Stoves

Cornelissen et al. 2016 PLOS ONE Data for the TNUC: MacCarty et al. 2008. Energy for sustainable development.

Emission factors for CO2, CO, CH4, TSP (aerosols, derived from PM10), non-methane volatile organic carbon (NMVOC), nitrogen oxide and nitrogen dioxide (NOx), and the sum of all products of incomplete combustion (PIC). Error bars represent standard deviations from 50 to 250 individual measurements

Emissions from Kon-Tiki are well below other simple tech pyrolysers, even with difficult feedstocks like wet eupatorium shrubs and rice husks

Cornelissen et al. 2015

Operation of a Kon-Tiki

Firing
  1. Build an upside down fire on the bottom of the kiln to nearly the rim (so it is easy to light and sustain plenty of air), either by:
    • Partially filling the kiln with dry brushy material to near the rim
    • Building a crisscross pier of sticks, progressing to smaller sticks towards the top (don’t use big or moist sticks on the bottom – they may get buried and cooled before they are fully charred
  2. Insert some kindling and tinder into the pier or into the top of the brush
  3. If using liquid starter fuel, spray only on top, or use a gel. Do NOT  use highly flammable liquid like petrol.  It will fall into the lower kiln and be explosive in a confined space.
  4. Once flaming hotly, feed the flame to build a complete fire cap and a strong fire bed.

Method of Firing

  1. A fire is started inside the Kon-Tiki. Once the fire is burning well, and flaming across the bottom of the kiln, it provides the heat to dry and pyrolysethe next layer of wood, and the flame cover to burn the smoke.
  2. Each new layer of material in the kiln blackens and chars, and white ash may form on the outside of the biomass, indicating the material has dried out, reached pyrolysis temperature and has begun carbonizing. At this stage the wood will continue to pyrolyse (an exothermic process) even without oxygen, as long as the material stays in a heat field sufficient to overcome heat loss by conduction, convection and radiation. Ash formation is minimized by adding the next layer of wood early in that process so the flame cap is maintained.
  3. The flame cap consumes the smoke diffusing upwards off the pyrolysing wood, radiates heat down onto the wood to continue the pyrolysis, and protects the the biochar formed from oxygen by consuming it before it gets to the char.

During the run

The goal in operating the Kon-Tiki is to maintain a full flame cap in the cone to:

  • dry and heat the biomass,
  • burn the smoke,
  • shield the char from oxygen.

For the cleanest, fastest production add material at a rate that keeps the flames high and the smoke low. Material can be added more rapidly once the kiln is over half full.

  • If the flame is too small or smoky, feed dry thin material
  • If the flame is large, take advantage of the heat to feed heavier or moister material.
  • Do not leave the burn unattended for more than 10 minutes. A 20 minute break might result in the fire dying back and some char turning to ash. Even though ash may not be bad for the biochar, it will reduce the yield, and it may take effort and small material to build the intensity again.

Feedstock Size

  • The length is best kept shorter than the kiln diameter.
  • Don’t use material thicker than will char fully during the duration of the run (roughly 2 hours for a 1.2m kiln and 4-6 hours for a 1.6m kiln).
  • It takes about 1 hour per inch (2.5cm) of radius to fully carbonize wood, so a 4in piece will take about 2 hours to fully carbonize.
  • Don’t use large or moist wood at the start. It may become buried and cooled on the kiln floor before it is fully charred.
  • Similarly don’t let big wood or moist wood roll to the walls of the kiln.
  • Insert bigger wood into the middle of the kiln and of the run. Taper to finish with small wood.

Finishing

  • It takes about an hour to finish after the final fuel is added to a 1.6m Kon-Tiki.
  • Yellow flames die away indicating that most pyrogas has been evolved.
  • Quenching from below can be started during this process. Steam generated will be adsorbed by hot char, where it probably cracks and opens up the char.
  • As the flame cap dies fine material can be added that will fully carbonize before the end: fine sticks, bark, straw, leaves, grass clippings.  These keep flames going, to burn the remaining volatiles being released.  This new material also adds some different ash components that may complex with and enhance the biochar.
  • Remaining flaming embers can be raked into a pile to finish.
  • Apply water mist to the finished hot coals, avoiding remaining flaming areas, to slow ashing and generate steam, activating surfaces.
  • When you are losing char to ash faster than it is being created from remaining uncarbonized wood, flood the kiln fully to stop combustion of the biochar (quenching).
  • The top-most layer of red hot char will float on the water.  Quench it with a top  spray.
  • Drain kiln overnight, or for a few hours, saving quench/nutrient water if desired.

Quenching & Adding Nutrients

A special feature of the Kon-Tiki is the ability to quench the hot char with nutrient laden water right in the kiln at the end of the run. The rapid cooling of the hot char, and the steam that is flashed off and re-adsorbed in higher char layers, cracks the char, activates surfaces, and opens it to nutrients.

You can add to the quench water the minerals and nutrients your soil and plants need.  These could include rock dust, acids (phosphoric or acetic acid or wood vinegar ) to neutralize the pH, or manure slurries.  Only limited N will be lost from the nutrient rich water because volatilized N will be adsorbed by char. The char is quickly cooled from the bottom up in the first flood, so microorganisms in the quench water will attach to the cooled char and survive.

Quench water e.g. from 1000L bulk liquid tank (optionally with nutrients) is:

  • pumped into the kiln through the bottom drain
  • soaked overnight
  • drained back into the tank next morning

Smoke water/nutrients are recycled and used.

Alternatively, nutrient/quench water can be gravity-flowed from a high tank, and drained to a low tank, or nutrients can be stirred into the biochar slurry.

Drying the Biochar

  • Once thoroughly drained the biochar is easy to shovel out.
  • A 1.6m kiln in a cradle kiln is easily tilted and propped for emptying.
  • Biochar is raked out on tarpaulins and sundried for at least 2 to 4 days, covered at nights.
  • Moisture content of sundried biochar has ranged from 34 to 63% weight basis (wb)

Crushing the Biochar

  • The biochar can be crushed in various ways.
  • Here the sundried biochar is passed through a hammer mill. Too much moisture in the biochar can clog the mill when grinding, and too little causes dust.
  • The milled biochar, raked out on the tarpaulin, may be sprayed with phosphoric acid or organic acid to adjust pH and  then further sun-dried.
  • Moisture content of original biomass and sun-dried biochar can be determined by oven drying samples. E.G.
    • Wood feedstock = 15%, wb
    • Biochar = 15-63%, wb (sundried)

Running the Kon-Tiki with agricultural residues

In many places wood is not readily available or it has a high moisture content or the soils have low nutrient content.  To efficiently convert  high nutrient content biomass that has small particle sizes (e.gmanures , straw etc) simple retorts can be placed in the Kon-tiki to use the residual heat from the biochar

Operation: Find used drums (10-20l) that have a lid.

Load with straw, manure etcwith a moisture content of less than 25%. Loosely fix the lid onto the drum so that the pyrolysing gases can escape and burn in the flame from the pyrolysing wood. Keep adding wood until the kiln is full of biochar and the flame from the  drum has ceased. Then quench.

Using the residual heat in the biochar to pyrolyse a mixture of manure, grasses, compost and minerals

Quenching with Compost in Peru

Once the kiln has been half to ¾ filled with biochar it can be quenched with a mixture high in organic nitrogen and minerals to promote growth of beneficial micro-organisms.

In this production run in Australia the biochar produced from wood was covered in a mixture of

  • 52 kg compost
  • 14 kg soil
  • 63 kg of red clay
  • 23 kg of lump biochar
  • 20kg animal manure

We covered the biomass clay mixture with biochar to capture any smoke that was emitted but no smoke was observed

How pyrolysis of the quenched material takes place

Example: Biochar-clay complex from wet feedstock

Vietnam, May 2016

Biochar-clay complex from wet feedstock

Vietnam, May 2016

Waste heat capture from Kon-Tiki in Nepal

Example: Capturing heat from Kon-tiki for distillation of essential oil

Batch Pyrolyser TLUD Oven: The Principles

Vietnam TLUD with inner secondary air for Straw Pyrolysis

Using TLUD with inner Secondary Air with Residues

Vietnam Hybrid TLUD: Large Brick Kiln Design

  • Kiln was easy to load and to ignite
  • No visible emissions after 5 minutes
  • When operated properly, relatively even firing with temperature around 425oC-475oC
  • Easy to Unload
  • No smell of tar
  • Important to load wood vertically with straw and husk and to have at least 10kgs on top.

Method of operation

  1. Place layer of husks and then straw on the bottom of the kiln
  2. Insert wood and bamboo vertically and 3 Cover with 3 layers of straw and husk
  3. Light a fire in the fire box at the bottom of the kiln (see previous slide)
  4. When there is smoke coming from the chimney ignite biomass at top
  5. When the flame stops shut the air holes at the bottom and top of the kiln (see previous slide)
  6. Allow to cool and remove biochar from the bottom of the kiln

Choosing a Kiln to Manufacture or Purchase for Larger scale Applications

For larger scale applications many companies and organisationsmay want to either purchase the kiln and associated materials handling and biochar processing equipment or manufacture all of some of the the plant.

It is recommended that a functional specification of both the whole plant and the plant components is prepared to ensure that the unit is fit for purpose.

Write a functional specification

Example of process diagram

A functional specification can have the following contents:

  1. The Scope– describe the goal for the overall process, and the performance, product quality and design objectives for the plant.
  2. Process Diagram with all major components and materials flows.
  3. Major Components (Unit Operations)

    Include a detailed specification of the equipment items that comprise the whole plant, from feed reception to discharge of solids, liquids and gases.

  4. Components Supplied by Other Manufacturers; For example
    • Materials Handling Equipment including: e.g. Equipment to size (shred, grind, sieve), clean and possibly dry the feedstock and post-process e.g. by adding nutrients and pelleting or granulating
    • Burners (startup, secondary, flare)
    • Instruments
    • Operating controls
  5. Detailed Component SpecificationEach of the major components will have a number of sub-components that need detailed specification. For the kiln this could include chimneys, emergency vent, char exit screw, steam injection system, control and electrical system.

Roberts et al, 2009…

Purpose to Specification of Biochar

 The desired biochar properties will  determine the type of plant that  will be purchased (or designed and manufactured)

  • Key biochar properties are
    • persistence, porosity, adsorptivityand hydrophobicity of biochar, and percentage of ash (affects pyrolysis temperature and time)
    • size of biomass and biochar particles (affects residence time, features of materials handling equipment)
    • pre-processing will affect both of the points above
  • There are complexities and trade-offs required in the design of pyrolysis systems, and desired goals may not all be achievable in any particular pyrolysis plant. It may be necessary to prioritize desired properties of the end product.

Functional Specification for the Biochar

Start by defining the desired qualities and properties of the biochar. These could include:

  • Water holding capacity, surface area, pore volume
  • Mineralisable and persistent carbon content
  • Liming ability, pH, available N, P, K
  • Total macro- and micro-nutrient content
  • Cation and Anion exchange capacity
  • Ability to adsorb heavy metals and other toxic compounds
  • Average particle size, bulk and particle density
  • Polyaromatic hydrocarbons and dioxin content

See guide “Properties of Fresh and Aged Biochar” for more detail

Functional Specification for the reactor

Specify the following

  1. The amount, moisture content and type of feedstock coming into the pyrolyser per hour
  2. Number of operating hours per week
  3. Specify the desired temperature range that the biochar should reach, based on desired application
  4. Consider the balance between biochar and energy required for your application and  (determines yield of biochar vs pyrogas)
  5. How will the pyrogas be used/treated? (flared, utilised for drying biomass or heating the kiln, or as an energy source for external applications)
  6. Estimate the amount of pyrogas and biochar produced
  7. Specify the temperature of the water and air coming into the reactor
  8. Calculate the approximate mass flow
  9. Calculate the energy balance over the whole plant (see example in Basic Principles guide)
  10. Specify whether there will be a wet or a dry quench of the biochar and estimate the amount of liquid or solid required to quench the biochar to less than 100oC, based on mass and energy balance
  11. Estimate the amount of air needed to burn the syngas efficiently, and the heat produced
  12. Estimate the particle carry over from the combustor, to determine whether a particle management system is needed
  13. How the post-processing equipment will be integrated with the kiln
  14. Determine if the equipment can be manufactured and repaired appropriately and economically under local conditions.

Example: mass flow

Health and Safety

Consider the risks for operators and the public, along with all relevant regulations, for each phase of the process including loading, start-up, operation, shut-down, unloading and storage.
Risks could include:

  • Fire and explosion (including dust explosion on hot surfaces, combustion during storage of feedstock or biochar)
  • Particulate and gaseous emissions
  • Gas leakage (particularly CO)
  • Noise pollution

Produce a Checklist For Buying or Building a Kiln

A check list should include the following topics:

 Suitability of the equipment for purpose & location

 Regulatory requirements for:

 Health and Safety

 Air and water emissions

 Application of biochar to soil

 Control system for plant operation

  Cost benefit analysis

  Training, Staffing & Commissioning requirements

  Warranties and support offered

Checklist – Additional safety Considerations

What pressure relief system is provided to prevent/minimize explosions?

What deviation from the specification is permissible without causing safety issues?

What warranties are offered on sub-contracted items and what level of support can be expected and for how long?

Have the materials, components and manufacturing process been appropriately selected to withstand temperature, pressure and weight stresses during operation?

Checklist – Environmental Considerations

Does the feedstock come from a sustainably produced source?

Are there risks associated with a particular feedstock? For example carcinogens may be produced when pyrolysing certain feedstocks under certain conditions.

Does obtaining the feedstock have any other negative environmental implications (on humans, animals, plants, biodiversity, water, land-use)? Does the biochar have any components which are toxic to certain organisms or plants growing in the soil?

Are there any solid or liquid (e.g., tar) wastes to be disposed of?  What are the relevant regulations and how will this/these material/materials be effectively dealt with?

Checklist – Additional Regulatory Considerations

Have all relevant standards been complied with?

Gaseous and particulate emissions standards, obtained from the applicable Environmental Protection Agency (or equivalent).

Manufacturing standards (e.g. CE certification mark).

Ratings for regional classifications such as the ATEX rating for explosive atmospheres in the EU.

Noise and odour standards beyond the site boundary.

Standards for water disposal.

Standards for application of biochar to soil.

Checklist – Control and instrumentation Considerations

Does the plant need to be fully automated? If so, what level of sophistication is required?

Does the plant instrumentation provide sufficient data on the process so that a life cycle assessment and cost benefit analysis may be performed to measure the energy, greenhouse gas and economic impacts of the process?

.

Checklist – Training, Staffing, Commissioning

What level of training is required for plant operation?

How many shifts are required to operate the plant?

How will staff be trained and certified that they are proficient in its operation, with the required awareness of the nature of the process, operating conditions, safety issues and potential hazards?

Who is responsible for plant commissioning, start-up and handover?

Are long term operation and maintenance contracts available from the plant provider (if purchased)?

Larger transportable Pyrolysers

There are now a number of companies producing pyrolysers that can be taken to the biomass in the field to convert it to biochar. These have been designed to have minimal emissions.  There are two types of machines: batch and continuous.

Batch and continuous kilns are transported on a truck, or integrated into a trailer.

Emissions are minimised through careful management of air introduced into the flaming combustion zone.

Water must be available to quench the biochar and prevent fires.

Temperature of the biochar can be controlled with water sprays.

The BIG ROO Mark 2 INFIELD PYROLYSER

LARGE INFIELD BATCH PYROLYSER: BIG ROO

Fixed BATCH PYROLYSER

In operation in Peru B4SS project

Transportable container-based pyrolyser

Commercial Batch Units

Example: Exeter Kiln

Wood is placed in the inner chamber and the inner door is shut.  A wood fire is started under the  inner chamber.  The water vapour and the volatile gases come out of the inner chamber and pass through the wood fire where they are burnt.  Once the temperature reaches 300-350°C then the retort does not need any external heat.

Continuous Pyrolysers

Larger Continuous pyrolysers – Trough Pyrolysers

Examples

The Kansai kiln uses a screw feeder that sits inside a trough.  The pyrolysis gases are combusted above the trough and the heat is  reflected down onto the moving biomass.  The biomass must have a diameter less than 10mm. The reactor  biochar temperature is approximately 450°C. The yield varies between 25 and 40% depending on feedstock. The kiln has a biomass feed input of 300-1000kg/hr.

Biomass Tech & Equip Pty Ltd is manufacturing a trough pyrolyser.  The principle used is similar to the Kansai kiln. Water is injected to give flameless very low emissions combustion.  The input is 300kg/hr of poultry litter.

  • A burner is used to preheat the chamber.
  • Fuel comes in from an external hopper.
  • Primary air enters through holes in flat steel sheets beside the troughs & through 2 jets on the end wall.
  • Pyrogas from pyrolysing biomass in the trough and Primary Air mix and partially combust above the bed.
  • Radiant flux above the bed causes biomass to dry, torrefy and pyrolyseas the material moves along the trough.
  • Paddles stir charring biomass and sweep it to the bottom of the trough mixing with the drying biomass.
  • Biochar drops over a weir at the end of the trough.
  • Temperature of the biochar can be controlled by changing the feed rate, the flow rate of water mist onto the charring bed and the amount of primary air.
  • The temperature of the burning gas is maintained above 800°C to ensure complete combustion.

PYREG DOUBLE AUGER PYROLYSER

Screw Pyrolyser 500kg/hr – Rainbow Bee Eater

Continuous Screw pyrolyser

Biogreen-energy.com

Continuous Pyrolysers: Screw Gasifier; CoalTech

coaltecenergy.com

Fixed Continuous Carbon Terra Retort

Carbon-terra.eu

Larger continuous In Field Pyrolysers: Rotary Hearth

  1. Biomass enters the top deck where it contacts hot gases from lower decks.
  2. As biomass pieces are dragged across perforated decks by centrally driven rakes, smaller pieces drop through. Larger pieces continue around until they drop through a larger hole onto the deck below. Thus larger pieces have longer residence times.
  3. Biomass chars and breaks up, dropping through the decks. The nominal residence time in a 3 deck hearth is 120s.
  4. Biochar is discharged from the lowest level, and
  5. Quenched immediately.
  1. Air is drawn in at the bottom, constrained by   the design of input and discharge ports.
  2. Biomass pyrolysesin hot flue gases, releasing flammable gases, which consume the limited oxygen, typically 25-35%  of stoichiometric air for biomass combustion.
  3. Rising hot gases counter-flow to the falling fuel.
  4. Remaining flammable gases are burned in a thermal oxidizer.

Leakage of gases is prevented by a negative interior pressure, maintained by draft & process controls.

Fixed Continuous Pyrolysers: Rotating drum

  • The majority of kilns operating today have been designed and manufactured in China.
  • Most popular are rotating drums where the outside of the drum is heated by combusting pyrogas.
  • The pyrogas is first cleaned with water and the particulates and the condensate recovered.
  • The condensate is then used to quench the biochar.
  • The biochar is produced at about 450oC, although they can run at higher temperatures.
  • Yields vary between 20 and 35% depending on feedstock.
  • Feed input of 500-2000kg/hr.
  • Cost from USD$200,000 to $1,000,000.

Fixed Continuous: San Li New Energy Systems

  • Open Core gasifiers have been converted to produce biochar.
  • Air is drawn down through the top of the unit and biochar is produced at a temperature around 480oC
  • The feed input is ~500kg/hr and the pyrogas is quenched and filtered before taken to an engine to generate power.
  • A plant with 4-5 gasifiers, with feed input of 2000kg/hr, costs between USD$1,000,000 and $2,000,000 in China

Gasifier for electricity production:

All Power Labs Power Pallet

  • Power Pallet PP30: 25kW + 50kW heat CHP
  • Powertainer PT150: 150kWe + 150kWt         in 20ft shipping container. Efficiency 55%.
  • Takes woody chips in size range 12-50mm
  • 1 ton of dry biomass à1MWh of electricity + 50kg Biochar
  • Future machines will target 15-20% biochar.

Larger Continuous Reactors


Example: ICM Auger Kiln Gasifier

Drying as well as gasification takes place in the kiln.  To achieve this, air is introduced along the kiln to cause some of the biomass to burn. At the front of the kiln the water vapor is removed and taken to the exit. An auger moves the biomass along the kiln where it is pyrolysed and gasified.  A gas rich in CO, hydrogen and methane is produced.