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

Greenhouse gases reduction by carbon storage in agricultural soils Carbon Stored (lb/acre/yr)

2000 1800 1600 1400 1200 1000 800 600 400 200 0 Pyrolytic Char

No-Till Switchgrass

No-Till Corn

Plow-Tilled Corn

Char from pyrolyzing one-half of corn stover

Traditional Charcoal Making Pit kiln

Mound kiln 

Traditional Charcoal Making Brick kiln 

TPI* transportable  metal kiln 

*Tropical Products Institute

Traditional Charcoal Making Missouri‐type  charcoal kiln 

Continuous multiple hearth kiln 

Charcoal yields (dry weight basis) for  different kinds of batch kilns Kiln Type Pit Mound Brick Portable Steel (TPI) Concrete (Missouri)

Charcoal Yield* (%) 12.5-30 2-42 12.5-33 18.9-31.4 33

*ηchar = (mchar/mbio) x100 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.

Charcoal Yield Corrected for Ash  Content of Biomass Charcoal yield on the basis of ash‐free organic mass into ash‐ free carbon is calculated according to: ηfc = (mchar/mbio)[cfc/(1‐ba)] x 100 where: mchar = dry mass of charcoal from the kiln mbio = dry mass of biomass loaded into the kiln cfc = fixed C content of biochar as measured by  ASTM Standard D 1762‐84  ba = ash content of the dry biomass 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 Uncontrolled batch Low control batch Controlled continuous

CO (g kg-1)

CH4 (g kg-1)

NMHC1 (g kg-1)

TSP2 (g kg-1)

160-179

44-57

7-60

197-598

24-27

6.6-8.6

1-9

27-89

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 Fast

Conditions Liquid Char Gas Moderate temperature ~ 500°C 75% 12% 13% short vapor residence time ~ 1 s moderate temperature ~ 500°C Moderate 50% 20% 30% moderate vapor residence time ~ 10-20 s moderate temperature ~ 500°C Slow 30% 35% 35% very long vapor residence time ~ 530 min Gasification high temperature > 750°C 5% 10% 85% moderate vapor residence time ~ 10-20 s

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 diluent gas flow on char production endothermic

exothermic

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.

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  Combustion of  750 C ratio infiltrated air

Few minutes

Scanning Electron Micrographs Switchgrass Feedstock

Fast Pyrolysis Char

Slow Pyrolysis Char

Gasification 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 C orn S tover F eedstock & C har

Arbitrary Units

C orn S to v er F ee d stoc k

S low P yro lysis C h ar

F a st P yro lysis C h ar

G asific atio n C ha r

4 00 0

30 00

20 00

W a ve n u m b e r (cm -1 )

10 0 0

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.