Fundamentals of Charcoal Production

Stefan CzernikNational Bioenergy Center

IBI Conference on Biochar, Sustainability and Security in a Changing Climate

September 8-10, Newcastle, U.K.

Outline• Introduction

CharcoalBiomassPyrolysis

• Charcoal formationStoichiometric and thermodynamic potentialBiomass pyrolysis pathways

• Optimum process conditions for charcoal production

• Technologies for producing charcoal• Conclusions

What is Charcoal?Charcoal is a solid product of pyrolysis of biomass carried out at temperature above 300ºC.

Is black in color, retains morphology of original feedstock, burns without flame.

Is not a pure carbon or a single compound. Elemental composition: C, H, O, N, S, ashProximate analysis: fixed carbon >70% , volatiles, ash

Has been produced for thousands years.Picture from Wikipedia

Charcoal

Emerging use as a soil amendment and a carbon sequestrating material. 5.5 Gton carbon released annually by combustion of fossil fuels can be offset by 7.5 Gton of charcoal used as soil amendment

Cost of charcoal: $100- 400/tonApplications: fuel, metallurgy, activated carbon

Global charcoal consumption: 45 Mton/yearAfrica 23 Mton/year South America 17 Mton/yearWEC 2007 Survey of Energy Resources FAOSTAT-Forestry

(Ethanol global production: 60 Mton/year)

BiomassBiomass is a plant matter, renewable product of photosynthesis. Includes trees, grasses, agricultural crops and residues, animalwastes and municipal solid wastes.

World terrestrial biomass resources 120 Gton/year.Field, C. B. et al., (1998) Science 281, 237-240.

Potential for 30 Gton/year charcoal.

Food supplies<20% of total biomass.

Starch, fat, and protein richbiomass

Lignocellulosic biomass:trees, grasses, agricultural residues

Non food>80% of total biomass.

Lignocellulosic Biomass

Extractives: 1% - 5%

Average elemental composition: CH1.4O0.6

Lignin: 15% - 25%Complex aromatic structurep-hydroxyphenylpropene building blocks

Cellulose: 38% - 50%Most abundant form of carbon in biospherePolymer of glucose

Hemicellulose: 23% - 32%Polymer of 5- and 6-carbon sugarsXylose is the second mostabundant sugar in the biosphere

Pyrolysis of BiomassThermal decomposition occurring in the absence of oxygen.

At temperature above 300ºC biomass polymeric building blocks undergocrosslinking as well as partial depolymerization and fragmentation to form smaller molecules which are released as gases and vapors that can react with residual solids producing more condensed structures.

Pyrolysis always produces solid (charcoal), liquid (water and organics), and gaseous (CO, CO2, CH4, H2) products at proportions and composition dependent on feedstock and on process conditions.

Slow heating of biomass

Temperature Solid Phase Gas Phase

<200ºC Drying H2O

230ºC-250ºC Retification Acetic acid, MeOH

250ºC-280ºC Torrefaction Extractives

300ºC-500ºC Devolatilization Organics, H2O, gas

>500ºC Carbonization Tars, H2O, gas

Charcoal YieldsStoichiometric:CH1.4O0.6 CH0.2 + 0.6H2O

53.0% (100% C)

Thermodynamic:Cellulose:C6H10O5 3.74C + 2.65H2O + 1.17CO2 + 1.08CH4

27.7% (62.4% C)

Antal, M,J. and Gronli, M, Ind.Eng.Chem.Res 2003, 42, 1619-1640

Practical:

CH1.4O0.6 charcoal + gas + liquid10-35% (15-60% C)

Biomass Pyrolysis Pathways

Pyrolysis Severity

Primary Processes Secondary Processes Tertiary Processes

VaporPhase

LiquidPhase

SolidPhase

Low

P

HighP

Low

P

HighP

Biomass Charcoal Coke Soot

PrimaryLiquids

PrimaryVapors

Condensed Oils(phenols, aromatics)

CO, H2,CO2, H2O

PNA’s, CO, H2, CO2,

H2O, CH4

Olefins, AromaticsCO, H2, CO2, H2O

CO, CO2,H2O

Evans, R.J. and Milne, T.A., Energy & Fuels 1987, 1, 123-137.

Tars

Light HCs,Aromatics,& Oxygenates

Biomass Pyrolysis Processes

Char Liquid Gas

CARBONISATION 35% 30% 35%low temperaturelong residence time

FAST PYROLYSIS 12% 75% 13%moderate temperatureshort residence time

GASIFICATION 10% 5% 85%high temperaturelong residence time

How to Enhance Charcoal Formation?

Charcoal is a product of both primary (char) and secondary (coke) reactions

Increasing charcoal yields requires minimizing the carbon losses in the form of gases and liquids andpromoting the desired pathways:

• primary solid-phase dehydration, decarboxylation, and decarbonylationreactions

• secondary conversion of pyrolysis vapors to solids

Charcoal YieldsCharcoal yields depend on feeedstock and on process conditions:• Cellulose, hemicellulose, lignin and ash

content• Pyrolysis temperature• Process pressure• Vapor residence time• Particle size• Heating rate• Heat integration (biomass burn off).

TGA of Biomass Polymersheating at 10ºC/min to 600ºC in nitrogen

Beech wood xylan

Aspen lignin

Cellulose (cotton linters)

TGA of Poplar Woodheating at 10ºC/min to 600ºC in nitrogen

Beechwood xylan

Aspen lignin

Cellulose (cotton linters)

Increasing Charcoal Yield

The charcoal yields will increase with:• High hemicellulose, lignin and ash content in biomass• Low pyrolysis temperature (<400ºC)

(but also lower fixed carbon content)• High process pressure (1 MPa)

(higher concentration of pyrolysis vapor increases rate of secondary reactions)

• Long vapor residence time (extended vapor/solid contact promotes secondary coke forming reactions)

• Low heating rate (slower formation and escape of organic vapors)

• Large biomass particle size (low thermal conductivity of biomass results in slow heat and mass transfer rate within particles)

• Optimized heat integration (minimized biomass burn off)

Heat for Charcoal Production

<280ºC Endothermic (drying, depolymerizationand devolatilization)

300ºC-500ºC Exothermic (char formation)

>500ºC Endothermic (char carbonization)

Heat for the process can be provided:

• directly as the heat of reaction

• by flue gases from combustion of by-product and/or feedstock directly to the reactor

• by flue gases through the reactor wall

Charcoal ProductionBatch processes: Yield

Earth pits and mounds >10%

Brick, concrete, and metal kilns 20-25%

Retorts 30%

Continuous processes:Retorts (Lambiotte) 30-35%

Multiple hearth reactors (Herreshoff) 25-30%

Novel processes:Flash carbonization 40-50%

Earth Mound Charcoal Production

Release of pyrolysis gas and vapor to atmosphere

No heat recovery; significant wood burn off for process energy

Low yield; environmental pollution

A - fuelwoodB - carbonisation zoneC - charcoal zone

Swedish earth kiln with chimneySimple technologies for charcoal making

FAO Forestry Paper 41, Rome 1987.

Lambiotte RetortContinuous operationWood moves down in countercurrent with hot flue gases from combustion of pyrolysis gasCool charcoal is retrieved at the bottom through a lock mechanismPyrolysis liquid are condensed and used as by-productPyrolysis gas is heated in the bottom section then burned

FAO Forestry Paper 63, Rome 1985

Flash Carbonization

Batch operation; 10 tons/day charcoal.Biomass loaded to a canister then heated up to 350ºC at 0.7 MPa for 30-90 min.Charcoal yield 40-50%(70-80% fixed carbon).Catalytic afterburner for tars eliminates smoke from reactor effluents.Capital cost $200,000.

HNEI Flash Carbonization™Demonstration Reactor

Conclusions• Charcoal is a product of primary and secondary

reactions occurring during pyrolysis of biomass.• High yields of charcoal are favored by:

high lignin content and large particle size of biomass,low temperature and high process pressurelow heating rate and long vapor residence time in the reactor,heat integration of the process

• Production processes have to include by-product recovery or use for process energy to decrease detrimental environmental impact of traditional methods.