Biochar Production Technology

Robert C. BrownCenter for Sustainable Environmental Technologies

Department of Mechanical EngineeringIowa State University

Purported Properties of Biochar

• High soil organic matter

• Enhanced cation exchange capacity (nutrient holding capacity)

• Improved water retention

• Beneficial soil microbial activity

• Enhanced fertility

• Stable (“aromatic”) carbon structure

Car

bon

Stor

ed (l

b/ac

re/y

r)

0200400600800

100012001400160018002000

Pyrolytic Char No-Till Switchgrass No-Till Corn Plow-Tilled Corn

Greenhouse gases reduction by carbon storage in agricultural soils

Char from pyrolyzing one-half of corn stover

Pit kiln Mound kiln 

Traditional Charcoal Making

Brick kiln  TPI* transportable metal kiln 

*Tropical Products Institute

Traditional Charcoal Making

Missouri‐type charcoal kiln 

Continuous multiplehearth kiln 

Traditional Charcoal Making

Kiln Type Charcoal Yield* (%)

Pit 12.5-30Mound 2-42Brick 12.5-33Portable Steel (TPI) 18.9-31.4Concrete (Missouri) 33

Charcoal yields (dry weight basis) for different kinds of batch kilns

Kammen, D. M., and Lew, D. J. (2005) Review of technologies for the production and use of charcoal, Renewable and Appropriate Energy Laboratory, Berkeley University, March 1, http://rael.berkeley.edu/files/2005/Kammen-Lew-Charcoal-2005.pdf, accessed November 17, 2007.

*ηchar = (mchar/mbio) x100

Charcoal Yield Corrected for Ash Content of Biomass

ηfc = (mchar/mbio)[cfc/(1‐ba)] x 100

where:mchar = dry mass of charcoal from the kilnmbio = dry mass of biomass loaded into the kilncfc = fixed C content of biochar as measured by 

ASTM Standard D 1762‐84 ba = ash content of the dry biomass

Charcoal yield on the basis of ash‐free organic mass into ash‐free carbon is calculated according to:

A perfect kiln would have fixed‐C yield equal to the solid C yield predicted by thermodynamic equilibrium.  For example, the pyrolysis of cellulose at 400° C and 1 MPa should have a fixed‐C yield of 27.7%.  

Air emissions per kilogram biomass from different kinds of charcoal kilns

CO(g kg-1)

CH4(g kg-1)

NMHC1

(g kg-1)TSP2

(g kg-1)Uncontrolled batch

160-179 44-57 7-60 197-598

Low control batch

24-27 6.6-8.6 1-9 27-89

Controlled continuous

8.0-8.9 2.2-2.9 0.4-3.0 9.1-30

1 NMHC – non‐methane hydrocarbons (includes recoverable methanol and acetic acid)

2 TSP – total suspended particulates

Shafizadeh, Fred, 1982, Chemistry of pyrolysis and combustion of wood, in Sarkanen, K.V., Tillman, D.A., and Jahns, E.C., eds., Progress in biomass conversion: London, Academic Press, p. 51–76.

Typical product yields (dry basis) for different modes of pyrolysis

Mode Conditions Liquid Char GasFast Moderate temperature ~ 500°C

short vapor residence time ~ 1 s75% 12% 13%

Moderate moderate temperature ~ 500°Cmoderate vapor residence time ~ 10-20 s

50% 20% 30%

Slow moderate temperature ~ 500°Cvery long vapor residence time ~ 5-30 min

30% 35% 35%

Gasification high temperature > 750°Cmoderate vapor residence time ~ 10-20 s

5% 10% 85%

Thermogravimetric analysis of the pyrolysis of plant components

Constant heating rate (10° C/min) with N (99.9995%) sweep gas at 120 ml/min

Yang, H., Yan, R., Chen, H., Lee, D. H., and Zheng, C. (2007) Characteristics of hemicellulose, cellulose and lignin pyrolysis Fuel 86, 1781-1788.

Reaction pathways for cellulose decomposition

Mok, W. S. L.; Antal, M. J. Effects of Pressure on Biomass Pyrolysis. II. Heats of Reaction of Cellulose Pyrolysis. Thermochim. Acta 1983, 68, 165.

Effect of reaction pressure and diluentgas flow on char production

Mok, W. S. L.; Antal, M. J. Effects of Pressure on Biomass Pyrolysis. II. Heats of Reaction of Cellulose Pyrolysis. Thermochim. Acta 1983, 68, 165.

endothermic

exothermic

Secondary Charcoal Generation

Some specific goals for advanced biochar manufacture

• Continuous feed pyrolyzers to improve energy efficiency and reduce pollution emissions associated with batch kilns

• Exothermic operation without air infiltration to improve energy efficiency and biochar yields

• Recovery of co‐products to reduce pollution emissions and improve process economics

• Control of operating conditions to improve biochar properties and allow changes in co‐product yields

• Feedstock flexibility allowing both woody and herbaceous biomass to be converted to biochar

Concepts for Advanced Charcoal Kilns

• Slow pyrolyzers (drum pyrolyzer, rotary kiln)

• Flash carbonizer

• Fast pyrolyzers (fluid bed, screw reactor, entrained)

• Biomass gasifiers (fluid bed, downdraft)

• Hydrothermal processing reactors

• Wood‐gas stoves

Preliminary Studies to Compare Chars from Different Thermal Processes

Process Air filtration Heat Source Temperature Time

Slow pyrolysis

None External 500 C 30 minutes

Fast pyrolysis

None External 500 C Few seconds

Gasification 20% equivalence ratio

Combustion of infiltrated air

750 C Few minutes

Scanning Electron MicrographsSwitchgrass Feedstock

Gasification CharSlow Pyrolysis Char

Fast Pyrolysis Char

Effect of Feedstock and Thermal Process on Char Properties

Feedstock Process Higher Heating Value(kJ/kg)

  BET Surface Area (m2/g)

Corn Stover Slow Pyrolysis 21,596 4.1

Switchgrass Slow Pyrolysis 12,799 22.8

Corn Stover Fast Pyrolysis 13,833 4.5

Switchgrass Fast Pyrolysis 16,337 17.7

Corn Stover Gasification 15,290 43.6

Switchgrass Gasification 15,864 39.2

Fourier Transform Spectra of Feedstock and Resulting Chars

Corn S tover Feedstock & Char

W avenumber (cm-1)

1000200030004000

Arbi

trary

Uni

ts

Corn S tov er Feedstock

S low Pyrolysis Char

Fast Pyrolysis Char

G asif ication Char

Cation Exchange Capacity (CEC) of Chars

Feedstock Process Reactor type CEC (cmol/kg)

Corn stover Fast pyrolysis PDU fluidized bed 29.89

Switchgrass  Fast pyrolysis PDU fluidized bed 16.3

Loblolly pine Fast pyrolysis Lab scale fluidized bed 14.21

Corn stover Fast pyrolysis Lab scale free fall reactor 12.23

Switchgrass  Gasification  PDU fluidized bed 11.34

Corn stover  Gasification (cyclone 1)  PDU fluidized bed 31.4

Corn stover Gasification (cyclone 2)  PDU fluidized bed 17.21

Hardwood  Slow pyrolysis Lab scale fixed bed 19.04

Switchgrass  Slow pyrolysis Lab scale fixed bed 12.35

Woodwaste Gasification Large pilot‐scale 12.11

Used modified Compulsive Exchange Method (Gilman & Sumpter 1986, Laird & Fleming 2008)

Conclusions

• Traditional charcoal kilns are unsuitable for biochar production (too inefficient and polluting)

• Modern processes will produce several co‐products (biochar, bio‐oil, syngas)

• Opportunities for controlling yields of co‐products and properties of biochars in an environmentally sustainable manner

Acknowledgments

This presentation is based on a chapter to appear in the book “Biochar for Environmental Management: Science and Technology,” edited by Johannes Lehmann and Stephen Joseph, and to be published early next year by Earthscan Publishers Ltd.   Some of the materials presented are the result of research performed by ISU graduate students Catie Brewer, Randy Kasparbauer, Cody Ellens, A.J. Sherwood Pollard, and Jared Brown and assisted by undergraduate students Hernan Trevino  and Daniel Assmann.  Drs. Justinus Satrio and Sam Jones also contributed to this research.  Frontline Bioenergy provided some of the charcoal samples evaluated in this study.