Biochar Production Technology Robert C. Brown Center for Sustainable Environmental Technologies Department of Mechanical Engineering Iowa State University
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.