Comparison of gasification and pyrolysis methods for

preparing biochar from corn stover and wheat

straw

Steve PetersonUSDA – ARS – NCAUR

Applications of biochar

Uses beyond carbon sequestration◦ rubber composite filler – particle size a problem◦ filtration media

for filtration applications, particle size is not as important as surface area

Applications of biochar

Uses beyond carbon sequestration◦rubber composite filler◦filtration media

for composite fillers, large particles = poor

reinforcement

as filtration media, large, porous particles OK as long as they’re permeable to the medium

Applications of biochar

Uses beyond carbon sequestration◦rubber composite filler◦filtration medium◦peat moss substitute

Later in the program…

Pyrolysis vs. gasification

pyrolysis gasification

feedstock

Pyrolysis: pros and cons

heat

biochar(solid)

bio-oil(liquid)

+O2

(gas)(H2, CH4)

biomass

Cons:• batch method limits throughput• controlled environment = $$ • bio-oil can be problematic during processing

Pros:• oxygen is omitted, increasing the carbon yield• temperature control is accurate and variable

Gasification: pros and cons

open air system is cheaper and easier to run

can facilitate higher thoughput

scale up is easier and more cost-effecttive

side products are burned off

Pros:

Cons: no temperature control,

high temps are limited biochar typically has higher

ash contentimage courtesy of www.cleanstove.org

TLUD = Top Lit UpDraft

secondary air

primary air“AVUD” design by Paul Anderson

Feedstocks used

corn stover (CS) wheat straw (WS)

• both feedstocks are cheap and plentiful

• our collaborator has provided us with both glycerin and glycerin-free pelletized forms of WS

Feedstocks used

wheat straw + glycerin (WS+G)

corn stover (CS) wheat straw (WS)

Experimental design

Gasification (TLUD)Pyrolysis (retort oven)

400, 500, 600, and 700°C

temperature is not controlled;subject to gasification process

Feedstocks: CS, WS, WS+G

Biochar production method:

Temps:

Monitoring TLUD temperature

T1

T2

T3

T4

6:14:24 6:43:12 7:12:00 7:40:48 8:09:36 8:38:24 9:07:120

100

200

300

400

500

600

700

800

900T (

°C)

global time

Surface area/porosity

400 500 600 700 TLUD0

200

400

600

800

CS

pyrolysis temperature (deg C)

surf

ace a

rea (

m^

2/g

)

400 500 600 700 TLUD0

100200300400500600700

WS

pyrolysis temperature (deg C)

surf

ace a

rea (

m^

2/g

)

400 500 600 700 TLUD0

100200300400500600700800

WS+G

pyrolysis temperature (deg C)

surf

ace a

rea (

m^

2/g

)

total surface area

micropore surfacearea

(micropore pore withd < 2 nm)

Surface area/porosity

400 500 600 700 TLUD0

200

400

600

800

CS

pyrolysis temperature (deg C)

surf

ace a

rea (

m^

2/g

)

400 500 600 700 TLUD0

100200300400500600700

WS

pyrolysis temperature (deg C)

surf

ace a

rea (

m^

2/g

)

400 500 600 700 TLUD0

100200300400500600700800

WS+G

pyrolysis temperature (deg C)

surf

ace a

rea (

m^

2/g

)

• CS surface area with T

• WS samples peak below 700°C

• micropore % roughly 55-70%

• WS+G TLUD markedly higher surface area

Observations

Water sorption trends

up

down

0 10 20 30 40 50 60 70 80 90 1000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Moisture Sorption

Relative Humidity (% )

Mois

ture

Conte

nt

(g

of

wate

r/g o

f sam

ple

)

CS, 500°C

Water sorption trends

CS WS WS+G

g water per g sample

400°C 0.13 0.065 0.063

500°C 0.17 0.071 0.054

600°C 0.20 0.091 0.083

700°C 0.17 0.077 0.117

TLUD 0.14 0.069 0.050

CS significantly more water-sorptive than WS and WS+G

For CS and WS, water-sorption peaks at 600°C

Water-holding capacity is highest at 400°C and decreases with increasing temperature (not shown)

Ash content

CS WS WS+Gash content (%)

400°C 18 12 20

500°C 17 15 24

600°C 16 14 29

700°C 21 15 30

TLUD 35 27 25

Ash is an undesired component of biochar consisting of metal oxides; tends to dilute the effects of carbon

Assume limiting oxygen in the process will help reduce ash; retort > TLUD

Conclusions/Summary

Higher surface area & micropore SA with retort methods vs. TLUD

Lower ash content with retort method (except for WS+G sample)

Appears that the addition of glycerin to WS increases the biochar ash content

CS much more water sorptive than WS and WS+G

Bottom line: is the “lower quality” char from gasification a deal-breaker with the given

applications?

Acknowledgements

Paul Wever, Chip Energy

AJ Thomas & Ashley Maness

Mike Jackson

Steve Vaughn

HydroStraw, LLC

Jason Adkins

Nancy Holm, IBG, and ISTC