Comparison of gasification and pyrolysis methods for preparing biochar from corn stover and wheat straw Steve Peterson USDA – ARS – NCAUR

Applications of biochar Uses beyond carbon sequestration ◦rubber composite filler Advantages rrenewable & not petroleum-based vvariety in available feedstocks ffeedstocks are commonly lower-value products Disadvantages eessentially carbon black with varying degrees of impurities vvery heterogeneous and feedstock dependent vvery little production infrastructure in place

Applications of biochar Uses beyond carbon sequestration ◦rubber composite filler Advantages  renewable & not petroleum-based  variety in available feedstocks  feedstocks are commonly lower-value products Disadvantages  essentially carbon black with varying degrees of impurities  very heterogeneous and feedstock dependent  very little production infrastructure in place  Biochar works well as rubber composite filler only at low filler concentration  Problem is particle size – “top down” methods can only reduce to ~ 600 nm cost effectively (CB commonly < 50 nm)  Impurities add cost & reduce effectiveness as well

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 pyrolysisgasification feedstock

Pyrolysis: pros and cons heat biochar (solid) bio-oil (liquid) + O2O2 (gas) (H 2, CH 4 ) 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 content image courtesy of 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 T (°C) global time

Surface area/porosity total surface area micropore surface area (micropore  pore with d < 2 nm)

Surface area/porosity 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 CS, 500°C

Water sorption trends CSWSWS+G g water per g sample 400°C °C °C °C TLUD  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 CSWSWS+G ash content (%) 400°C °C °C °C TLUD 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

SEM images CS 400 CS 600 CS TLUD Mag = 1000x

SEM images Mag = 10,000 CSWSWS+G 400 TLUD 600