ef200450h

Published: June 02, 2011

r 2011 American Chemical Society 3251 dx.doi.org/10.1021/ef200450h | Energy Fuels 2011, 25, 3251–3265

ARTICLE

pubs.acs.org/EF

Is Elevated Pressure Required To Achieve a High Fixed-Carbon Yield ofCharcoal from Biomass? Part 1: Round-Robin Results for ThreeDifferent Corncob MaterialsLiang Wang,† Marta Trninic,‡ Øyvind Skreiberg,§ Morten Gronli,† Roland Considine,|| andMichael Jerry Antal, Jr.*,||

†Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Kolbjørn Hejes vei 1B,NO-7491 Trondheim, Norway‡Department of Process Engineering, Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16,11000 Belgrade, Serbia§SINTEF Energy Research, Sem Saelands vei 11, NO-7465 Trondheim, Norway

)Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu,Hawaii 96822, United States

ABSTRACT: Elevated pressure secures the highest fixed-carbon yields of charcoal from corncob. Operating at a pressure of0.8 MPa, a flash-carbonization reactor realizes fixed-carbon yields that range from 70 to 85% of the theoretical thermochemicalequilibrium value from Waimanalo corncob. The fixed-carbon yield is reduced to a range from 68 to 75% of the theoretical valuewhen whole Waimanalo corncobs are carbonized under nitrogen at atmospheric pressure in an electrically heated muffle furnace.The lowest fixed-carbon yields are obtained by the standard proximate analysis procedure for biomass feedstocks; this yield falls in arange from 49 to 54% of the theoretical value. A round-robin study of corncob charcoal and fixed-carbon yields involving threedifferent thermogravimetric analyzers (TGAs) revealed the impact of vapor-phase reactions on the formation of charcoal. Deepcrucibles that limit the egress of volatiles from the pyrolyzing solid greatly enhance charcoal and fixed-carbon yields. Likewise,capped crucibles with pinholes increase the charcoal and fixed-carbon yields compared to values obtained from open crucibles. Largecorncob particles offer much higher yields than small particles. These findings show that secondary reactions involving vapor-phasespecies (or nascent vapor-phase species) are at least as influential as primary reactions in the formation of charcoal. Our results offerconsiderable guidance to industry for its development of efficient biomass carbonization technologies. Size reduction handling ofbiomass (e.g., tub grinders and chippers), which can be a necessity in the field, significantly reduces the fixed-carbon yield of charcoal.Fluidized-bed and transport reactors, which require small particles and minimize the interaction of pyrolytic volatiles with solidcharcoal, cannot realize high yields of charcoal from biomass. When a high yield of corncob charcoal is desired, whole corncobsshould be carbonized at elevated pressure. Under these circumstances, carbonization is both efficient and quick.

’ INTRODUCTION

Coal combustion is the largest source of carbon dioxideemissions in the U.S.A.1 Alternatives to coal-fired powerplants(e.g., wind, photovoltaics, solar thermal, natural gas, etc.) are nowbeing deployed, but cost-competitive substitutes for coal as areductant (i.e., coke) are lacking. CO2 emissions from the ironand steel industries represented 16% of energy-related coal CO2

emissions in 2000.2 During that year, coal use was responsible for8.7 Gt or 37% of global CO2 emissions from fossil fuels. In 2008,CO2 emissions because of coal grew to 12.6 Gt (i.e., 42% ofglobal CO2 emissions).

3 This growth of emissions was (in part)due to world crude steel production that increased from 848 Mtin 2000 to 1.3 Gt in 2008.4 Most of the CO2 emissions associatedwith conventional crude steelmaking result from the reductionprocess in a blast furnace,5 whereby coke made from hard coaland/or pulverized coal made from steam coal are used to convertiron ore into iron.

The substitution of biocarbon (i.e., charcoal) for coal in theiron and steel industry can reduce CO2 emissions6 if thebiocarbon is manufactured efficiently from sustainably grown

biomass. This use of biocarbon is not novel; before the dawn ofrecorded history, mankind employed charcoal to smelt tin forthe manufacture of bronze tools,7 and today in Brazil, blastfurnaces use charcoal produced from Eucalyptus wood that iscultivated nearby.8 Likewise, the Norwegian ferroalloy industrymakes heavy use of charcoal imports from the Pacific.9 Unfortu-nately, biocarbon is not produced efficiently by conventionaltechnology;10�12 consequently, greenhouse gas emissions asso-ciated with biocarbon production are unnecessarily large andworrisome.13�15 The goal of this work is to learn what reactionconditions offer the highest yields of biocarbon from biomass.

Anxiety about the efficient production of charcoal and itsresultant properties motivated one of the earliest publicationsconcerned with industrial chemistry research. In 1851, Violette,who was Commissioner of Gunpowder Production in France,the same post that was held earlier by Lavoisier, released the

Received: March 23, 2011Revised: June 2, 2011

3252 dx.doi.org/10.1021/ef200450h |Energy Fuels 2011, 25, 3251–3265

Energy & Fuels ARTICLE

first16 of two important papers16,17 concerning the productionand properties of charcoal. Violette observed very high yieldsof charcoal when carbonization was conducted at high pressure.His observations were confirmed by Palmer18 in 1914 andBergstrom19,20 in 1915. Almost 7 decades passed before Mokand Antal21,22 reported an increase from 12 to 22% in the yieldof charcoal from cellulose when the gas pressure was increasedfrom 0.1 to 2.5 MPa. Their findings concerning improvements incharcoal yields at elevated pressures were corroborated byBlackadder and Rensfelt,23 Richard and Antal,24 and Mok et al.25

Thus, there can be no doubt that elevated pressures enhancecharcoal yields.

In 1909, Klason and his co-workers used their experimentalresults to deduce a stoichiometric equation for the pyrolyticproduction of charcoal from cellulose26 and “wood”27 at 400 �C.For cellulose, they found

C6H10O5 f 3:75CH0:60O0:13 + 2:88H2O + 0:5CO2

+ 0:25CO + C1:5H1:25O0:38ð1Þ

where the first product is charcoal with a carbon content of81.7 wt %. If we employ the usual definition of charcoal yield:ychar = Mchar/Mbio, where Mchar is the dry mass of charcoal andMbio is the dry mass of the feedstock, then Klason et al. realizeda value ychar = 34.0% from cellulose as represented by eq 1. For“wood”, Klason et al. reported a charcoal yield of 36.7% with acarbon content of 68.1%.

The findings by Klason et al.26,27 (summarized above) illus-trate the fact that charcoal is not a well-defined chemicalcompound with an explicit chemical formula. To further empha-size this point, we note that Schenkel in his Ph.D. thesis10,12,28

reported a steady increase from 70 to 95% in the carbon contentof the charcoal product of beech wood pyrolysis as the pyrolysistemperature increased from 400 to 800 �C. This increase incarbon content was accompanied by a decrease in ychar from 35 to24%. The notion of charcoal yield as a “moving target” is furtherillustrated by our own experience. In 1990, Antal et al.29 co-authored a review that listed charcoal yields ranging from 27.9to 50 wt % together with proximate analyses when available.In 1996, Antal et al.30 speculated that corncob could offer acharcoal yield of 55 wt % and reported an experimental valueof the charcoal yield from kukui nut shell of 62.1 wt %with a fixedcarbon content of 78.1 wt %. These findings cause us to concludethat, although the value of the charcoal yield is a convenientmetric for qualitative discussions, it is meaningless as a quanti-tative measure of the efficiency of a carbonization process.

In our previous work,10,31�35 we introduced the fixed-carbonyield yfC as a meaningful metric of carbonization efficiency

yfC ¼ ychar% fC

100�% feed ash

� �ð2Þ

where % fC and % feed ash represent the percentage of fixed-carbon contained in the charcoal and the percentage of ash in thefeedstock, respectively. Note that the fixed-carbon content of acharcoal approximates the fraction of carbon that is effective as ametallurgical reductant. From a different perspective, the fixed-carbon content approximates the amount of pure carbon that canbe obtained by further thermal treatment of the charcoal and yfCserves as a measure of the efficiency of the pyrolysis process inconverting biomass into pure carbon.

Modern thermochemical equilibrium software (e.g., StanJan36

software or the NASA computer program Chemical Equilibrium

with Applications37) enables calculations of the equilibriumyields of the products of biomass pyrolysis as a function ofthe reaction temperature and pressure. Figure 1 displays“theoretical” equilibrium yields of the products of cellulosepyrolysis at 1.0 MPa as a function of the temperature and at400 �C as a function of the pressure. Although carbon yieldsincrease somewhat at temperatures below 400 �C, the reactionrate at low temperatures is very slow. In Figure 1, the yield of H2

is represented, despite its small value that caused us to omit itsdisplay (but not its presence) in our previous publications.10,32

Although the mass fraction yields of H2 are small, hydrogen playsan important role in the equilibrium chemistry, as evidenced byits yield on a mole basis. Figure 1 shows that the maximum yieldof carbon from cellulose is 28 wt % at 400 �C (i.e., 62 mol % ofcellulose carbon is converted into biocarbon). Note that thepyrolytic conversion of cellulose into carbon is an exothermicreaction with a large increase in entropy when equilibrium isreached; consequently, the carbonization process is irreversible,and pressure has little effect on the yield of carbon at 400 �C, whilehigher temperatures evoke a small decrease in the carbon yield.32

Ten years ago, we published comparisons for many differentbiomass feedstocks of experimental values of yfC with theoreticalvalues of yfC obtained using StanJan software and the elementalcompositions of the feedstocks.31 The experimental values of yfC

Figure 1. (a) Effects of the temperature on the products of cellulosepyrolysis, following the attainment of thermochemical equilibrium at1 MPa. (b) Effects of the pressure on the products of cellulose pyrolysis,following the attainment of thermochemical equilibrium at 400 �C.



were obtained at both atmospheric and elevated pressures usingelectrically heated carbonization retorts. In all cases, the valuesof yfC obtained at elevated pressure exceeded those obtained atatmospheric pressure, and in some cases, the yfC values ap-proached the theoretical, limiting, equilibrium values calculatedusing StanJan. Three years later, a flash-carbonization (FC)reactor at the University of Hawaii (UH), which effectedcarbonization by air oxidation at elevated pressure (see below),realized comparably high fixed-carbon yields using variousbiomass feedstocks. Furthermore, the flame speeds at elevatedpressure in the FC reactor were very high; consequently, the timerequired for carbonization of a bed of feedstock was very short(see below). Together, these findings indicate that charcoal canbe produced very efficiently and quickly at elevated pressure inpractical equipment.

Nevertheless, pressurized equipment is costly to purchase andoperate and demands special operational expertise. Also, it is acurious fact that most fundamental studies of biomass pyrolysisat atmospheric pressure have employed small samples or thethermal ablation of a large sample. Could the relatively low fixed-carbon yield obtained at atmospheric pressure be a result of smallparticles and facile mass transfer of volatiles away from the hot,pyrolyzing solid? This question is not new; in 1991, Hancoxet al.38 proposed studies of the systematic effect of a reduction inparticle size on pyrolysis chemistry, with the goal of detailing thevapor�solid secondary reactions and their impact on char yields.However, to the best of our knowledge they did not accomplishsuch studies. Could large particles provide high fixed-carbonyields of charcoal at atmospheric pressure? This question isespecially important because tub-grinders are increasingly usedby industry to shred waste biomass. Is shredded biomass suitablefor carbonization? Similarly, some carbonization processes em-ploy fluidized-bed reactors that require particulate feedstocks.Can fluidized-bed reactors realize high fixed-carbon yields ofcharcoal? The aim of this paper is to elucidate the effects ofparticle size on fixed-carbon yields at atmospheric pressure andelevated pressure in FC equipment.

We hypothesize that increasing particle size substantiallyimproves fixed-carbon yields. This improvement in yield is aresult of increasing heterogeneous interactions between thepyrolytic vapors and the solid charcoal together with its mineralmatter, both of which may be catalytic for the formation ofcharcoal. Also, we hypothesize that elevated pressure enables thecarbonization of liquid bio-oil before it can vaporize and escapethe solid matrix. In this paper, we examine the validity of thesehypotheses using corncob samples sourced from three differentlocations. Corncob is particularly well-suited for this workbecause of its widespread availability and complementary resultsthat are now becoming available from other laboratories.39�42

Subsequent papers will examine other feedstocks of interest toindustry. The use of chemical dehydration agents to producecharcoal is not a focus of our work but was discussed in a previousreview.29

’APPARATUS AND EXPERIMENTAL PROCEDURES

Grab samples of corncobs were obtained from Surcin, Belgrade’smunicipality in Serbia (Scob; ZP Maize Hybrid, ZP 505), Pioneer Hi-Bred International (Pcob), Oahu, Hawaii, and the Waimanalo farm ofthe UH College of Tropical Agriculture and Human Resources, Oahu,Hawaii (Wcob). There are two varieties of Wcob: red and white. All ofthe work described in this paper employed red Wcob, except the FC

experiment labeled 101014 that used white Wcob. No pretreatments(except for drying and grinding) were applied to the feedstocks toenhance carbon yields.

At NTNU, each sample of corncob was prepared for carbonizationtests in two different ways: either the cob was ground in a cutting millmounted with a 1 mm sieve, or using a sharp knife, a thin cross-sectionwas sliced from a whole cob, as shown in Figure 2. This cross-sectionincluded representative amounts of pith, woody ring, and chaffmaterial.Also, in some cases, cubic samples were cut from the woody ring ofPcob and Scob cross-sections. The cubic particles weighed 5, 10, 20, and40 mg, corresponding to cube sizes from 2 to 6 mm. At NTNU, all cobsamples were dried in an oven at 105 �C for 24 h prior to carbonization.At the UH, the cobs were used as received, following storage in theopen air.

Three atmospheric pressure, thermogravimetric analyzers (TGAs)were employed in this work: models TA Q5000 and TA Q600 of TAInstruments and aMettler Toledomodel TGA/SDTA 851e. Table 1 andFigure 3 summarize the geometry and depth of the crucibles and thepans used with each TGA. All TGA runs employed nitrogen (99.999%pure) as purge gas with a flow rate of 100 mL min�1. Prior to eachexperiment, a measured amount of corncob material (5, 10, 20, and40mg in single particle or powder form) was loaded into the appropriatecrucible/sample pan. Each experiment was initiated with a 30 min purgeat room temperature, followed by 30 min of drying at 105 �C. Then, thesample was heated from 105 to 950 �C at a heating rate of 10 �Cmin�1.This temperature program is summarized in Table 2. For some experi-ments conducted in the TA Q600 and Mettler Toledo model TGA/SDTA 851e instruments, a lid with a small pinhole was used to coverthe crucible with loaded sample. These runs are identified as “closedcrucible” experiments. The char yield ychar was calculated by dividing the

Figure 2. Cross-sectioned view of (a) Pcob and (b) Scob.

Table 1. Specifications of Instruments and Their Crucibles/Pans

instruments

crucible/

pan number

crucible/pan

volume (μL)

crucible geometry

(d � h, mm)

TA Q600 1 crucible (90) 6� 4

Mettler Toledo TGA 851e 2 crucible (150) 7� 4.5

TA Q5000 3 pan (100) 10� 1

Figure 3. Crucibles/pans used in pyrolysis experiments.



final sample mass by the mass measured at the end of the drying periodat 105 �C.A temperature controllable muffle furnace (approximately 0.009 m3),

a stainless-steel retort (approximately 0.004 m3), and two ceramiccrucibles (approximately 200 mL) with lids were employed by NTNUto determine the maximum charcoal and fixed-carbon yields that can berealized at atmospheric pressure from untreated whole corncob samples.Cob samples were placed in each crucible and thereafter covered with alid. Then, the crucibles were placed in the retort that was covered with ametal lid prior to insertion into themuffle furnace. The retort was purgedwith nitrogen for 30 min before heating as well as during the run toensure carbonization in an inert atmosphere. The furnace was heatedfrom room temperature to 950 �C with a heating rate of 5 �C/min.A thermocouple was placed in the retort to monitor the temperaturehistory during the experiment.At NTNU, all corncob feed samples were subjected to proximate

analysis according to American Society for Testing and Materials(ASTM) E 871 and 872; however, the ash content of the feedswas determined according to ASTM D 1102. Both NTNU and UHemployed the ASTM D 1762-84 procedure for proximate analyses ofthe charcoal products. However, at UH, the charcoal volatile mattercontent was determined by preheating the covered crucible with samplefor 2 min on the outer ledge of the furnace and 3 min on the edge of thefurnace with the furnace door open and then heating the coveredcrucible with sample at the rear of the furnace for 6 min at 950 �C withthe furnace door closed. At NTNU, the covered crucibe with sample wasimmediately placed at the rear of the furnace and heated for 6 min at950 �C. The greater overall heating time used by UH suggests that theUH values for volatile matter content may be somewhat larger thanthose of NTNU.We offer our readers the following explanation for the differences in

our procedures for proximate analysis. The NTNU researchers alteredthe standard procedure because of the practical difficulty in following itexactly. In the standard procedure, the measurement of volatile contentrequires the muffle furnace to be heated to 950 �C. Then, with thefurnace door open, the crucible with the sample is set on the outer ledgeof the furnace (300 �C) and then for 3 min on the edge of the furnace(500 �C). Then, the sample is moved to the rear of the furnace for 6 minwith the muffle furnace closed. There are two practical difficulties thatmay cause misleading volatile content measurements: (1) It is hard toensure the temperatures on the outer ledge and edge of the furnace are300 and 500 �C. (2) When the furnace door is open, the temperature inthe furnace drops quickly and requires a relatively long time to return to950 �C. This means that, after the crucible with the sample is placed inthe back of the furnace with its door closed, during the first severalminutes, the temperature in the furnace is lower than 950 �C. It ispossible that the temperature is still lower than 950 �C after 6 min. Thiswould mean that the sample is not heated at 950 �C for 6 min.To perform the volatile content measurement more efficiently and to

ensure that the sample is heated at 950 �C for 6 min, NTNU places thecrucible with the sample directly in the rear of the furnace withoutpreheating on the outer ledge and edge of the furnace.

Elemental analyses of the samples by NTNU were conducted byuse of an elemental analyzer (Vario MACRO Elementar) accordingto standards ASTM E 777 (carbon and hydrogen), ASTM E 778(nitrogen), and ASTM E 775 (sulfur). The oxygen content wasdetermined by the difference of 100% and the sum of the ash, C, H,N, and S contents. Also, elemental analyses were obtained from twocommercial laboratories in the U.S.A. The microstructure and surfacetopography of the char particles were investigated using a Zeiss Supra-55variable-pressure field emission scanning electron microscope (LVFE-SEM). Samples were mounted on carbon tape without furtherpreparation and scanned by a SEM. The SEM is equipped with anenergy-dispersive X-ray spectroscope (Bruker Quantax) that enables thedetection of the elemental compositions of selected spots.

At UH, the biomass feed was placed in a canister that was subse-quently loaded into the top of a pressure vessel (the FC reactor) that wasthen pressurized with air to 0.8 MPa (100 psig). Electric heating coils atthe bottom of the pressure vessel ignited the lower portion of thebiomass. After the specified ignition time, compressed air was deliveredto the top of the pressure vessel and flowed through the packed bedof feed to sustain the carbonization process. The pressure within thereactor was continuously monitored and maintained at 0.8 MPa by avalve located downstream of the reactor. After sufficient air was deliveredto carbonize the corncob, the airflow was halted and the reactor cooledovernight. The charcoal was removed from the reactor and allowedto equilibrate under a fume hood for 2 days before proximate analysis(i.e., ASTM D 1762-84) was performed. For moisture content determi-nation, the charcoal samples were dried in a Fisher Scientific Isotempmodel 282A vacuum oven evacuated below 0.015 MPa (4 in. Hg). Thevolatile matter and ash analyses were performed using a Thermolyne1300 muffle furnace.

’RESULTS

Fixed-Carbon Yields. In Table 3, we see that all three cobsenjoy similar proximate analyses. The proximate analysis proce-dure can be viewed to be a type of carbonization process. Fromthis perspective, we can calculate the fixed-carbon yields that areoffered by proximate analysis: 18.2 wt % (Pcob) versus 17.7 wt %(Scob) versus 18.0 wt % (Wcob). In this context, the proximateanalysis offers a benchmark value for the fixed-carbon yield thatcan be obtained at atmospheric pressure. Throughout this paper,we will be comparing fixed-carbon yields obtained under differ-ent conditions with the values obtained from the proximateanalysis procedure.Table 4 displays ultimate (i.e., dry, elemental) analyses of the

cobs used in this work as determined by three different labora-tories, together with published analyses of cobs used in our earlierstudies. Considering the fact that the Pcob and Wcob bothoriginated in Hawaii, whereas the Scob originated in Europe,their elemental analyses are remarkably similar. Note that bothNTNU and Hazen determine oxygen content by difference (i.e.,they normalize their analyses to 100%), whereas Huffman doesnot. Because of the normalization, we lack a metric of the

Table 2. Temperature Programs

pyrolysis method

step dynamic isothermal

time

(min)

heating rate

(K/min) temperature (�C)

1 � 30 25

2 � jump 25 f 105

3 � 30 105

4 � 10 105 f 950

Table 3. Proximate Analysis, Heating Value, and Fixed Car-bon Yield of Feed Materials

proximate analyses (wt %)

feed VM fC ash yfC (wt %) HHV (MJ/kg)

Pcob 79.64 17.75 2.61 18.23 18.87

Wcob 80.32 17.64 2.04 18.01 18.43

Scob 81.08 17.47 1.45 17.73 18.63



absolute accuracy of the two measurements. Values of C, H, andO listed in Table 4 for the Hazen analysis of Avicel microcrystal-line cellulose suggest an accuracy of at least two significantfigures, but the Hazen values for N, S, and ash are high (unlessthe sample was somehow contaminated). The large range invalues of the C, H, and O contents reported by the threelaboratories for the same cob material was unexpected.When the nitrogen, sulfur, and ash contents of the cobs are

neglected, these elemental analyses can be used to calculate theyields of the pyrolysis products as a function of the pressure whenthermochemical equilibrium is achieved at 400 �C using StanJansoftware. Figure 4 displays these yields for the Wcob ultimateanalysis as a function of the pressure at 400 �C. Note that thevalue of the fixed-carbon yield is largely independent of thepressure. Returning to Table 4, we see that the theoretical fixed-carbon yield of Pcob ranges from 32.4 to 36.5 wt %, that thetheoretical fixed-carbon yield of Wcob ranges from 33.1 to36.4 wt %, and that the theoretical fixed-carbon yield of Scob is32.8 wt % based on only the NTNU analysis. Note the impact ofthe higher oxygen content (43.23 wt %) in Huffman’s analysisof Wcob on the theoretical value of yfC (34.8 wt %), relative toHuffman’s 2003 value (41.48 wt %) for Wcob and the resultant

value of yfC (36.1 wt %). The oxygen measurements by Huffmandisplayed in Table 4 were direct measurements, whereas theHazen and NTNU measurements were by difference. Clearly,the value of the theoretical fixed-carbon yield is quite sensitive tothe accuracy of oxygen determination of the feed.We note that, in Table 4, we correct an error in our earlier

publication32 of the analysis of Wcob (i.e., the “Wcob (2003,Huffman) values”). In the 2003 laboratory report, Huffmanincluded the moisture content (11.01%) of the sample in theelemental analysis (i.e., its elemental analysis was on a wet basis),but this was not clearly declared in its laboratory report. For thisreason, the 2003 values of the H and O contents of the Wcob,which we published under the impression that they were drybasis values,32 were high and the C content was low relativeto dry basis values. In our paper, we noted these anomaliesbut incorrectly ascribed them to natural variations in the cobcomposition. The low value of C together with the high valuesof O and H resulted in a low value of the thermochemicalequilibrium “theoretical” fixed-carbon yield and caused us tobelieve that our experimental value of the fixed-carbon yield wasequal to the theoretical value. Table 4 presents the corrected,

Table 4. Ultimate Analyses of Avicel Cellulose, Waimanalo (Wcob), Pioneer (Pcob), and Serbian (Scob) Corncobs and theCalculated Theoretical Fixed-Carbon Yield yfC

ultimate analysisa (wt %)

MCb (wt %) C H O N S ash total yfC (wt %)

Avicel cellulose (Hazen) 5.20 44.50 6.02 49.07 0.25 0.01 0.15 100.00 28.7

Avicel cellulose (Huffman) 5.48 44.37 6.15 49.23 0.01 0.00 0.01 99.77 28.4

corncob (2000)c 48.22 6.20 42.92 1.57 0.13 3.48 102.52 34.1

Pcob (NTNU)d 6.40 46.98 6.39 43.38 0.54 0.10 2.61 100.00 32.4

Pcob (Hazen)d 7.37 49.66 5.74 41.82 0.44 0.05 2.29 100.00 36.5

Pcob (Huffman) 6.69 46.79 5.80 44.80 0.40 0.04 1.88 99.71 32.8

Wcob (NTNU)d 4.18 47.79 6.37 43.19 0.52 0.09 2.04 100.00 33.1

Wcob (2003, Huffman)c 11.01 48.79 5.72 41.48 0.75 0.08 1.31 98.13 36.1

Wcob (Hazen)d 9.69 50.27 5.58 43.12 0.43 0.00 0.60 100.00 36.4

Wcob (Huffman) 10.64 48.55 5.81 43.23 0.56 0.04 1.61 99.80 34.8

Scob (NTNU)d 5.18 47.61 6.27 43.89 0.55 0.23 1.45 100.00 32.8aDry mass basis. bMoisture content on a wet mass basis. c From ref 32. dBy difference.

Figure 4. Effects of pressure on Waimanalo cob pyrolysis following theattainment of thermochemical equilibrium at 400 �C.

Figure 5. Influence of different instruments on one Pcob single particlesample char yield in an open crucible.



dry-weight elemental analysis values for the theoretical fixed-carbon yield of the 2003 data (i.e., 36.1 wt %). When theexperimental value is compared to this corrected value, theexperimental value obtained by the FC process in 2003 was78% of the theoretical value (see below).We emphasize that the chief point of Table 4 is the surprising

inefficiency of conventional pyrolysis procedures (e.g., proximateanalysis); the theoretical fixed-carbon yield values are double theactual fixed-carbon yields obtained by the proximate analysisprocedure! This disparity between practice and theory indicatesthe improvements in yield that can potentially be realized byinformed chemical reaction engineering of the carbonizationprocess.The availability of three different TGA instruments enabled us

to conduct an internal round-robin study of char and fixed-carbon yields from two of the three cob samples. Figures 5 and 6display the effects of particle size on char yields at 950 �C asmeasured by the three instruments. These results representTGA runs of a single, nominally cubic “particle” with varying

sizes (to achieve the indicated mass) cut from the woody ringannulus of each cob. Because the composition of the woody ringis not representative of the composition of the whole cob, thechar and fixed-carbon yields displayed in these figures cannot bedirectly compared to those of whole cobs (see below). Never-theless, the trends displayed in these figures are meaningful. Forall of the cobs in all of the instruments, the char yield increaseswith an increase in particle size. Furthermore, the TA Q600instrument realizes a significantly higher char yield than the otherinstruments. As noted previously, the TA Q600 employs anarrow, deep crucible that isolates the sample from the flow ofpurge gas and thereby enhances secondary reactions. We expandupon this finding below.Figures 7 and 8 display similar results representing the effects

of sample size with two of the three cob powders (not cubes) ontheir respective char yields. In this case, the sample size repre-sents the amount of powder loaded into the open TGA crucible/pan. In agreement with Figures 5 and 6, higher char yields areobtained from larger samples and the TA Q600 instrumentprovides the highest char yields. However, in these figures, thechar yields from the powders are lower than the comparable yieldfrom single cubes. Because the powder is representative of thecomposition of the whole cob, whereas the cubes are not, thelower yields may reflect compositional differences, as well asthe reduced dimensions of the particles (see below).The char sample remaining in the TGA from these runs was

too small to ash. To obtain an estimate of the fixed-carbon yield,we employed the volatile matter and ash contents of charcoalsheated to the same final temperature in the N2-purged mufflefurnace (see below; Table 6). With this additional information,values of the estimated fixed-carbon yields for the TA Q600 as afunction of the sample size with open and closed crucibles arelisted in Table 5. All estimated values exceed the comparable yfCobtained by the proximate analysis procedure. In all cases, largersample sizes offered enhanced charcoal and estimated fixed-carbon yields. In all cases, the closed crucible increased theestimated fixed-carbon yield by about 15�20%; nevertheless,even the closed crucible yields are much lower than the theore-tical fixed-carbon yield. Note that, in some cases, the crucible/pan was not able to accommodate the largest sample; conse-quently, the measurement was not made.

Figure 6. Influence of different instruments on one Scob single particlesample char yield in an open crucible.

Figure 7. Influence of different instruments on Pcob powder samplechar yield in an open crucible/pan.

Figure 8. Influence of different instruments on Scob powder samplechar yield in an open crucible/pan.



To further explore the effects of the particle size on char yield,we sieved 10 g samples of the ground Pcob and Scob andmeasured the char yield from each of the sieved samples (eightdifferent particle sizes) using the MTT851e instrument. Figure 9displays the particle size distributions obtained from the twoground cob samples, while Figure 10 displays the char yields. Theresults are startling; both cobs provide evidence of nearlyidentical behavior, with a steady increase in the char yield from15.2 to 23.5 wt % as the particle size increased from 0.063�0.125to 2.5�3.0mm. Clearly, the particle size has a strong effect on thechar yield. We remark that smaller (<90 μm) particles of twoenergy crops (switchgrass and reed canary grass) are known tohave higher concentrations of minerals than larger (90�600 μm)particles because of ash speciation by grinding, and the catalyticnature of the mineral matter caused the pyrolysis temperature ofthe smaller particles of the grasses to decrease.43 These observa-tions might cause one to attribute our observations of particlesize effects on char yields to mineral speciation. However,minerals catalyze the formation of charcoal (see below); conse-quently, a putative increase in the mineral content of the smallerparticles in our work would cause an increase in their char yieldand not a decrease. Plainly, smaller particles offer lower yields ofcharcoal.Our observations concerning the influence of crucible/pan

geometry on char yields are not new; earlier work (see theDiscussion below) used open versus closed crucibles to accent-uate the impact of vapor-phase conditions on char yields.

Figures 11�14 display the influence of closed crucibles with asmall pinhole opening on char yields for Pcob and Scob cubesand powder samples in the TA Q600 and MT T851e instru-ments. In virtually all cases, the closure of the crucible substan-tially enhances the char yield. Nevertheless, the yields remainsignificantly below the theoretical fixed-carbon yields. Theseobservations corroborate earlier work and reveal the importanceof secondary reactions involving vapor-phase species in theformation of charcoal. Conditions that improve or prolongthe contact of vapor-phase pyrolysis species with the solid serveto enhance the char yield. As the thermodynamic equilibriumcalculations suggest, fragmentary compounds (e.g., levoglucosan,glycolaldehyde, 5-hydroxymethyl furfural, etc.) born by pyrolysisof biomass are not stable species at elevated temperatures. In ahot environment, they quickly decompose into carbon and gases,especially in the presence of catalytic mineral matter or solidcarbon.As noted above, the char yields from the cob woody ring are

not representative of yields obtained from the whole cob.Figures 15 and 16 are weight loss curves to 550 �C (not950 �C) for the inner (pith), middle (woody ring), and outer

Table 5. Charcoal and Fixed-Carbon Yields Realized atAtmospheric Pressure (0.1 MPa) in the TAQ600Micro-TGA

ychar (wt %)a yfC (wt %)b

open crucible closed crucible open crucible closed crucible

PCobc 20.43 24.51 19.27 23.12

PCobd 21.64 24.87 20.41 23.46

PCobe 22.80 25.66 21.50 24.20

SCobc 20.52 24.46 19.14 22.81

SCobd 21.28 25.14 19.84 23.44

SCobe 22.56 25.62 21.04 23.89

PCobf 27.31 25.76

SCobf 27.12 25.29

WCobf 26.47 24.73a Percent of dried feed material. b yfC = charcoal yield � (100 � %volatile matter � % char ash)/(100 � % feed ash). Here, the volatilematter for char produced at 950 �C and ash content measured from themuffle-furnace-produced charcoal is used. cA total of 5 mg of powdersample. dA total of 10mg of powder sample. eA total of 20mg of powdersample. fA 180�190 mg thin cross-sectioned sample.

Table 6. Charcoal and Fixed-Carbon Yields Realized atAtmospheric Pressure (0.1 MPa) in a N2-Purged MuffleFurnace

proximate analysis (wt %)

VM FC ash ychar (wt %) yfC (wt %)

Pcob charcoal 4.61 91.85 3.54 26.77 25.25

Wcob charcoal 4.85 92.07 3.08 26.45 24.86

Scob charcoal 4.69 91.90 3.41 26.52 24.73

Figure 9. Particle size distributions of ground Pcob and Scob samples.

Figure 10. Influence of the particle size on Pcob and Scob char yield(sample mass of 10 mg). (a) Representing char yields from Pcob andScob as single particle samples with a particle size of 2.5�3 mm.



(fine chaff) parts of Pcob and Scob compared to the relevantpowder sample. The results are consistent with earlier findings;the Pcob and the Scob woody rings give the highest char yields,whereas the powders give average yields between those of thewoody rings and the inner piths that offer the lowest yields.Table 5 displays char and fixed-carbon yields from thin cross-

sections of each cob as measured by theMTT851e. Note that thelarge cob cross-section significantly augments the fixed-carbonyield relative to the comparable powder yield; nevertheless, thehighest fixed-carbon yield (25.66 wt % from Pcob) is much lowerthan theoretical values predicted using thermodynamics.Table 6 displays comparable charcoal yields obtained from

whole cobs in closed crucibles under nitrogen in a muffle furnaceat 950 �C. In Table 6, all values represent the mean of triplicatemeasurements. Table 6 also displays proximate analyses of themuffle furnace charcoals that allow us to calculate the fixed-carbon yields obtained from whole cobs at atmospheric pressure.Note the good agreement of the muffle furnace results with thoseof the cob cross-sections (see Table 5). TheWcob values given inTable 6 can be compared to those for Wcob displayed in Table 7

that were obtained by the FC process at elevated pressure.The atmospheric pressure value of 24.86 wt % is less than theFC fixed-carbon yields that range from 25.5 to 28.0 wt %. Inagreement with our previous work,31 pyrolysis at elevatedpressure representing practical conditions offers a nominal 10%increase in the fixed-carbon yield above that which can beobtained under nitrogen at atmospheric pressure using anexternally heated electrical furnace. We remark that Scob wasnot tested in Hawaii because of importation difficulties. On theother hand, we have extensive experience with Pcob feedstocks;unlike any other biomass which we have tested, Pcob ignitesprematurely at elevated pressures in FC equipment. Specialprocedures are needed to deal with its high reactivity, and theseprocedures are beyond the scope of this paper.The parity plot displayed in Figure 17 summarizes our

findings. Ordinate values of the parity plot represent the theore-tical fixed-carbon yields predicted by thermodynamics for each ofthe four Wcob ultimate analyses listed in Table 4. Abscissa valuesrepresent experimental measurements of the fixed-carbon yields

Figure 11. Effects of open versus closed crucible on one Pcob singleparticle sample char yield.

Figure 14. Effects of open versus closed crucible on Scob powdersample char yield.

Figure 12. Effects of open versus closed crucible on one Scob singleparticle sample char yield.

Figure 13. Effects of open versus closed crucible on Pcob powdersample char yield.



obtained in this work. The dashed diagonal lines indicate thepercentage attainment of the theoretical yield. The highest yieldsobtained in this work were delivered by the FC process operatingat elevated pressure, realizing fixed-carbon yields ranging from 70to almost 90% of the theoretical limit. Note the impact of therange of uncertainty in the ultimate analysis values on thepercentage attainment results. Somewhat lower fixed-carbonyields were realized from whole cobs in the muffle furnace undernitrogen, and these yields were nearly identical to the yieldsobtained from cob cross-sections under nitrogen in the micro-TGA. Still lower yields were obtained from powders in closedcrucibles under nitrogen. Even lower yields were obtained from

powders in open crucibles under nitrogen. The lowest yieldswere delivered by the proximate analysis laboratory procedure;these were about 2/3 of those obtained at elevated pressure inpractical equipment. There is some irony here; practical equip-ment employing air at elevated pressure realized higher pyrolysisfixed-carbon yields than can be obtained under the best ofcircumstances under nitrogen in laboratory instruments at atmo-spheric pressure.SEM Micrographs and EDX Analyses. Recall that the FC

process behaves like a downdraft gasifier; air flows downward asthe flaming pyrolysis front moves upward. Tarry vapors born byflaming pyrolysis are carried downward through the hot charcoalbed where they suffer further pyrolysis and deposit secondarycarbon onto the charcoal formed earlier in the process. Also,mineral vapors can be condensed and deposited in the lowerparts of the bed. The intensity of the flame increases as it movesup the bed; consequently, temperatures toward the top of the bedare usually higher than those near the bottom. In what follows, wecompare representative SEM photos of samples of FC charcoaltaken from the top and bottom sections of the bed to photos ofatmospheric pressure charcoals, together with semi-quantitativeEDX analyses of various interesting locations visible in theSEM photo.Figure 18a displays a SEM micrograph of a grab sample of

Wcob charcoal particles taken from the top of the canister. Thearrow indicates the point of increased magnification given inFigure 18b, likewise in 18c. The surface of the pore shown inFigure 18d is littered with tiny “balls” of various shapes. Similarballs were displayed in the SEM photos by Bourke et al.44 andunpublished photos by Dr. Jim E. Amonette (Pacific NorthwestNational Laboratory). This prior work caused us to think that theballs were commonplace in FC charcoal, but in fact, they wereobserved only in the one particle shown in Figure 18a. Figure 18ddisplays numbered points where EDX analysis was accom-plished. The results of the EDX analyses are given in Table 8under “top section”. This sample typifies charcoal with low ashcontent. Analyses of the cell wall (e.g., points 2�4) were high(>80 wt %) in carbon content, whereas analyses of the balls (e.g.,points 1, 9, 10, and 11) were lower in carbon content (<80 wt %)but still rich in carbon. All of the points were rich in oxygen. Theash content (K and Mg) was higher in the tiny balls, and a verysmall amount of Cl was detected at all points.

Figure 15. Pyrolysis and char yield behavior of different constituents ofa Pcob thin cross-section in an open crucible.

Figure 16. Pyrolysis and char yield behavior of different constituents ofa Scob thin cross-section in an open crucible.

Figure 17. Parity plot displaying the experimental FC, muffle furnace(MF), proximate analysis (PA), and thermogravimetric analysis (TGA)fixed-carbon yields for open and closed crucibles (o and c) with varioussample sizes in mg versus the theoretical values calculated using theultimate elemental analyses (4 and2, Wcob;] and[, Pcob;� and +,Pcob TGA; 0 and 9, Scob; � and /, Scob TGA).

Table 7. Charcoal and Fixed-Carbon Yields Realized at1 MPa in the FC Reactor

proximate analysis (wt %)

ID VM FC ash ychar (wt %) yfC (wt %)

Wcob 90303 15.25 80.62 4.13 30.55 25.52

Wcob 90224 10.87 84.74 4.39 30.40 26.70

Wcob 101014 7.11 89.06 3.83 30.66 28.01



In Figure 19, the particle of Wcob from the bottom of thecanister typifies higher ash charcoals; its oxygen content is verylow at all points (see Figure 19b and Table 8 under “bottomsection”), and it is littered with crystals that are rich in K and Cl(points 1�3, 5, and 8). In this case, often the Cl content is closeto that of K, suggesting that these are crystals of KCl. Thesecrystals may appear to be the tiny balls of Figure 18; however, thecrystals have a white color (not the gray color of the balls inFigure 18), and their high mineral content as indicated by theEDX analysis is quite different from that of the balls. The cellwalls in Figure 19 appear thinner and more fluid compared tothose of Figure 18. These differences relative to Figure 18 mayresult from the position of the cob in the canister, or the particlesmay be derived from different parts of the cob.

Representative surfaces of the pores of Scob charcoal particlesproduced at atmospheric pressure in open and closed cruciblesare shown in Figures 20 and 21. No tiny balls are found in theopen crucible sample (Figure 20), but balls with high carbon andlow oxygen contents are visible in the closed crucible sample(Figure 21). The bright crystals present in the open cruciblecharcoal (Figure 20) are relatively low in C and high in K withlittle accompanying Cl (points 1, 3, 5, and 6 in Table 9 under“open crucible”). These crystals may be K2O and/or K2CO3.

45,46

The high carbon content (see Table 9) of the closed cruciblesample (Figure 21) may result from carbonization of the tarryvapors that could coat the surface with carbon. Note the low Clcontent relative to the K content of all of the measured points.Remarkably, the surfaces of both Scob particles produced at 1 atm

Figure 18. SEM micrographs of various charcoal samples. (a�d) 20�40 mesh Wcob charcoal from the top section of the FC reactor.

Table 8. SEM�EDX Analyses of Wcob Charcoal from the FC Reactor (Elemental Compositions in wt %)

top section bottom section

spot element 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8

C 76.94 85.14 83.63 84.48 80.09 76.24 81.47 77.67 74.18 77.66 74.67 76.59 82.01 80.35 73.64 86.99 76.69 87.15 84.57 81.60

O 20.28 13.60 15.65 14.57 18.66 21.88 16.92 20.28 23.30 20.28 22.72 21.53 6.35 5.12 7.18 8.87 5.76 10.02 9.47 5.43

K 0.70 0.89 0.52 0.71 0.67 0.59 0.62 0.66 0.65 0.65 0.59 0.55 5.83 7.79 10.33 2.18 9.15 1.86 3.95 6.89

Cl 0.05 0.06 0.06 0.05 0.07 0.06 0.06 0.04 0.04 0.04 0.04 0.07 5.30 6.37 8.61 1.24 7.60 0.63 0.93 5.71

Mg 1.29 0.00 0.00 0.00 0.00 0.60 0.74 1.06 1.35 1.06 1.47 0.94 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ca 0.30 0.20 0.07 0.13 0.10 0.12 0.16 0.23 0.33 0.23 0.32 0.19 0.10 0.07 0.08 0.13 0.15 0.11 0.31 0.11

Na 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.00 0.00 0.01

P 0.10 0.06 0.02 0.02 0.03 0.03 0.02 0.06 0.12 0.06 0.11 0.09 0.20 0.13 0.05 0.31 0.23 0.17 0.62 0.12

S 0.01 0.02 0.02 0.01 0.02 0.01 0.01 0.00 0.01 0.00 0.01 0.02 0.10 0.06 0.04 0.06 0.12 0.01 0.06 0.05

Si 0.33 0.04 0.03 0.02 0.35 0.47 0.00 0.00 0.00 0.00 0.07 0.01 0.11 0.10 0.05 0.22 0.28 0.05 0.10 0.08



appear more rounded than flash-carbonized particles shownpreviously, as though the Scob particles experienced a brief periodof partial liquidity prior to carbonization. The question of liquidityduring carbonization is among the most controversial in the fieldof biomass pyrolysis. We discuss this question further below.

’DISCUSSION

It is well-known that the char yield from different purecellulose samples can vary substantially, even when the samples

are heated under identical conditions. For example, Antal et al.47

reported char yields of 6.8, 7.0, 9.1, and 10.2 wt % for smallsamples of four, different, pure, ash-free celluloses heated to520 �C at 1 �C/min. Consequently, any comparison of charyields must involve identical substrates, and this constraint limitsour ability to make comparisons among yields reported in theprior literature. Nevertheless, earlier workers have reportedfindings similar to ours. For example, Varhegyi et al. observeda decrease from 3.3 to 2.8 wt % in the char yield at 400 �C(5 �C/min) when the Avicel cellulose sample size decreasedfrom 0.94 to 0.11 mg.48 Stiles in his Ph.D. thesis49 described asharp fall in char yields for particle diameters below 300 μm.Likewise for whole biomass materials, the gasification ofMaritime pine wood chips produced more char than that ofthe pine wood sawdust50 and more char was obtained fromhazelnut shells51 and almond shells52 than their respectivepowders. Even in the case of fast pyrolysis, large particles werereported to produce more charcoal than small particles.53

Why is more charcoal formed from larger particles? More than20 years ago, Varhegyi et al.54 described an increase in the charyield from 5 to 19 wt % when the pyrolysis of 1 mg samples ofAvicel cellulose was conducted in “covered” (with pinhole)versus open crucibles. This perspicuous finding revealed the roleof secondary reactions involving the interactions of pyrolyticvolatile matter with the solid sample in the formation of charcoaland confirmed the speculation of Bradbury et al. that “theresidence time of the volatiles in the cellulose during the pyrolysisreaction largely influences the extent of char formation”.55 Otherevidence corroborating the importance of secondary reactions incharcoal formation includes (i) the reduction in char yield whenpyrolysis is conducted in vacuum,56 (ii) the reduction in charyield when gas flow is increased,21,22,24,57 and (iii) the increase inchar yield with increasing pressure.10,24,25,30�32,58 In particular,an important recent study by Shen et al.59 described an increasein the particle size of Australian Eucalyptus loxophleba woodparticles from 0.18 to 1.5 mm that caused an increase in thecharcoal yield from 14 to 20 wt %. The preponderance of thisevidence evoked the following conclusion in a review paper:“Vapor�solid interactions (secondary reactions) are effectivelythe only source of char formed during the pyrolysis of purecellulose. These heterogeneous reactions alone can increase thechar yield from 0% to more than 40%.”60 However, in the case ofwhole biomass (not pure cellulose), it is likely that the primary,solid-phase pyrolysis reactions contribute to the formation ofcharcoal.

Figure 19. SEM micrographs of various charcoal samples. (a and b) 20�40 mesh Wcob charcoal from the bottom section of the FC reactor.

Figure 20. SEMmicrographs of a Scob charcoal sample from themicro-TGA (TA Q600) with an open crucible.

Figure 21. SEMmicrographs of a Scob charcoal sample from themicro-TGA (TA Q600) with a closed crucible.



The foregoing observations concerning the effects of particlesize and secondary reactions are in part a reflection of thecatalytic nature of charcoal and its mineral matter content. Sincethe 1800s, it has been known that downdraft reactors deliver atar-free gas because the tar-laden gas born by pyrolysis flowsthrough the hot bed of charcoal at the bottom of the reactorthat catalyzes the decomposition of the tars to more gas andchar.61 Recently, this insight has been employed to enhance theremoval of tar from the effluent of supercritical water biomassgasifiers,62�64 as well as more conventional systems.65�68 In hisPh.D. thesis, Abu El-Rub et al.69 showed that biomass chars gavethe highest naphthalene conversion among low-cost catalystsused for tar removal from biomass gasifiers. Very recently, AbuEl-Rub’s findings were questioned by Gilbert et al.,70 whose workshowed that the main mode of tar conversion in the presence ofcharcoal at 800 �C was homogeneous vapor-phase cracking. Theimportance of the homogeneous cracking chemistry should beno surprise, because its role in tar conversion was elucidated3 decades ago.71�76 In particular, competitive vapor-phasereactions71�76 (largely ignored by the research community) playa key role in the formation at 800 �C of the heavy, refractory,condensable phase described by Gilbert et al.70 In any case, thetemperature range for charcoal formation of 250�450 �C ismuch lower than that studied by Abu El-Rub et al.69 and Gilbertet al.;70 consequently, true, low-temperature primary tars are thereactants, and their sensitivity to the catalytic action of charcoalor mineral matter is not well-understood.

The catalytic action of the charcoal results (at least in part)from its mineral matter content. Themetal ions K, Li, Ca, Fe, andCu, typically present as mineral matter in biomass, greatlyenhance the formation of char from both cellulose and wood.60

In the case of corn straw, the removal of K+ and Ca2+ ions bywater or acid washing lowers the yield of charcoal obtained fromthe straw.77 Larger particles enhance the retention of alkali andalkaline earth metal species, thereby retaining catalytic speciesthat enhance charcoal yields.78

It is well-known that charcoal maintains the morphology of itsprecursor.10 It is also well-known that fast pyrolysis causes thebiomass to pass through a liquid phase prior to the formation oftarry vapors.60,79�81 Offering insight into this apparent contra-diction, Haas et al.82 report that some parts of the cell wallbecome molten at temperatures between 350 and 500 �C duringthe pyrolytic contraction of the cell wall structure. Nevertheless,Haas et al.82 conclude that the overall three-dimensional tissuestructure of the biomass is preserved following pyrolysis (i.e., the

char structure closely resembles that of its parent material). Theyalso note that some pyrolysis products become trapped withinthe cell walls. These observations are in harmony with our SEMphotos.

’CONCLUSION

(1) The “yield” of charcoal from biomass is not a meaningfulmetric of the efficiency of a carbonization process. Instead, thefixed-carbon yield should be used to characterize carbonizationefficiency. (2) When an elemental analysis of the feedstock isavailable, it can be used to calculate the yield of pure carbon thatcan be realized when thermochemical equilibrium is reached in acarbonizer. This theoretical yield of pure carbon can be com-pared to the experimental value of the fixed-carbon yield andthereby used as a meaningful metric of the efficiency of thecarbonization process. (3) The standard proximate analysisprocedure offers a very low fixed-carbon yield of charcoal fromcorncob. The fixed-carbon yields of chars produced by theproximate analysis procedure are about 1/2 of the theoreticalvalue. (4) The fixed-carbon yields of char obtained from thepyrolysis of corncob in analytic thermogravimetric analyzers islow but somewhat higher that that of the proximate analysisprocedure. A deep TGA sample pan/crucible improves the yield.(5) Ordinarily TGA studies employ small particles. Largeparticles offer significantly higher fixed-carbon yields than smallparticles within TGA instruments. This signature of the impor-tance of secondary reactions to charcoal formation has not beengiven adequate emphasis in prior work, especially in researchinvolving TGA. (6) The fixed-carbon yield is improved whencarbonization occurs in a vessel with a lid. Any restriction of theability of the pyrolytic vapors to escape from the vicinity of thechar product increases the fixed-carbon yield. (7) Relatively highfixed-carbon yields are obtained from whole corncobs heatedunder N2 in closed vessels in a muffle furnace. This is not apractical way to manufacture charcoal; electrical heat is tooexpensive to be used for carbonization. (8) With operation atelevated pressure with whole corncob, the FC process realizedthe highest fixed-carbon yields observed in this work, approach-ing 90% of the theoretical limiting values. This satisfying resultnevertheless offers opportunities for improvement. (9) Increas-ingly, tub grinders and other size-reduction equipment are beingused to handle biomass. If the goal is to maximize biocarbonproduction, size reduction of this sort is worse than uselessbecause it substantially reduces biocarbon yields while needlessly

Table 9. SEM�EDX Analyses of Scob Charcoal from the Micro-TGA (Elemental Compositions in wt %)

open crucible closed crucible

spot element 1 2 3 4 5 6 7 1 2 3 4 5 6

C 82.17 85.85 74.51 82.71 68.26 82.85 87.66 80.89 86.70 88.66 85.96 86.12 88.86

O 11.34 12.71 13.83 14.28 5.37 8.99 9.13 13.84 10.40 9.16 10.81 10.92 9.61

K 5.10 1.20 8.76 1.94 19.02 5.44 2.63 2.74 2.62 2.06 2.86 2.65 1.39

Cl 0.07 0.01 0.11 0.01 0.39 0.15 0.03 0.10 0.04 0.01 0.04 0.02 0.03

Mg 0.60 0.11 1.06 0.39 1.58 0.63 0.31 0.67 0.05 0.01 0.13 0.05 0.04

Ca 0.28 0.08 0.43 0.09 1.00 0.48 0.14 0.17 0.13 0.09 0.15 0.13 0.06

Na 0.17 0.04 0.33 0.45 0.76 0.31 0.09 0.37 0.04 0.01 0.04 0.08 0.01

P 0.11 0.00 0.34 0.02 1.51 0.34 0.00 0.03 0.00 0.00 0.00 0.01 0.00

S 0.08 0.00 0.18 0.04 0.78 0.35 0.01 0.03 0.00 0.00 0.00 0.00 0.00

Si 0.08 0.00 0.45 0.07 1.34 0.46 0.00 1.18 0.00 0.00 0.00 0.00 0.00



consuming energy and occupying capital. (10) Likewise, flui-dized beds and transport reactors,12 which demand fine particlesas feedstocks, cannot give high fixed-carbon yields of charcoal.(11) If biocarbon is the desired product, carbonization equip-ment that does not require size reduction is best suited tomaximize the biocarbon yield. Biomass is not easy to grind,shred, sliver, or chip. Size reduction demands considerablecapital investment and wastefully consumes power. The fact thatbiocarbons are produced most efficiently without size reductiongives carbonization processes a considerable advantage overother technologies that convert biomass into higher value fuels.(12)We emphasize that our TGA studies employed fine particlesbecause fine particles are increasingly used in commercialpractice (see 9�11 above). Our goal was to open industry’s eyesto the detriments of their use of fine particles in carbonizationequipment. These detriments are unequivocally illustrated bythe state-of-the-art TGA data displayed in this paper. (13) Ourfindings show that secondary reactions involving vapor-phasespecies (or nascent vapor-phase species) are at least as influentialas primary reactions in the formation of charcoal. Conditions thatimprove or prolong the contact of vapor-phase pyrolysis specieswith the solid enhance the fixed-carbon yield of charcoal. Asthermodynamic equilibrium calculations suggest, fragmentarycompounds (e.g., levoglucosan, glycolaldehyde, 5-hydroxy-methyl furfural, etc.) born by pyrolysis of biomass are not stablespecies at elevated temperatures. In a hot environment, theyquickly decompose into carbon and gases, especially in thepresence of catalytic mineral matter or solid carbon.

’AUTHOR INFORMATION

Corresponding Author*Telephone: 808-956-7267. Fax: 808-956-2336. E-mail: [email protected].

’ACKNOWLEDGMENT

Michael Jerry Antal, Jr. acknowledges support by the NationalScience Foundation (NSF, AwardCBET08-28006), theOffice ofNaval Research under the Hawaii Energy and EnvironmentalTechnologies (HEET) initiative, and the Coral IndustriesEndowment of the University of Hawaii. He thanks Dr. MariaBurka (NSF) and Ms. Bonnie Thompson (NSF) for their con-tinuing interest in biocarbons, Dr.Woraphat Arthayukti for alertinghim to the need for improving the yield of charcoal from biomass,30

andProf. JimBrewbaker and PioneerHi-Bred for providing neededcorncob. The Norwegian team acknowledges support by theBioenergy Innovation Centre (CenBio), which is funded by theResearch Council of Norway, 19 Norwegian industry partners and7 research and development institutes. Finally, we thank fivereviewers for their insightful comments on our work.

’REFERENCES

(1) Deyette, J.; Freese, B. Burning Coal, Burning Cash; Union ofConcerned Scientists: Cambridge, MA, 2010; pp 1�57.(2) International Energy Agency (IEA). Prospects for CO2 Capture

and Storage; IEA: Paris, France, 2004.(3) U.S. Energy Information Administration (EIA). International

Energy Outlook 2010: Energy Related Carbon Dioxide Emissions; EIA:Washington, D.C., 2010.(4) World Steel Association. Steel Statistical Yearbook 2009; World

Steel Association: Brussels, Belgium, 2010.

(5) Orth, A.; Anastasijevic, N.; Eichberger, H. Low CO2 emissiontechnology for iron and steelmaking as well as titania slag production.Miner. Eng. 2007, 20, 854–861.

(6) MacPhee, J. A.; Gransden, J. F.; Giroux, L.; Price, J. T. PossibleCO2 mitigation via addition of charcoal to coking coal blends. FuelProcess. Technol. 2009, 90, 16–20.

(7) Agricola, G. De Re Metallica; Dover Publications: New York,1950; pp 354�355.

(8) Castro, L. F. A.; Tavares, R. P.; Seshadri, V. Process controlmodel for charcoal blast furnaces in Brazil.Mater. Manuf. Processes 2005,20, 287–298.

(9) Monsen, B.; Gronli, M. G.; Nyugaard, L.; Tveit, H. Use ofbiocarbon in the Norwegian ferroalloy production. Proceedings of theINFACON 9 Conference; Quebec City, Quebec, Canada, 2001.

(10) Antal, M. J.; Gronli, M. G. The art, science, and technology ofcharcoal production. Ind. Eng. Chem. Res. 2003, 42, 1619–1640.

(11) Gronli, M. G. Charcoal Production—A Technology Review;SINTEF: Trondheim, Norway, Nov 1999; TR F5061 (in Norwegian).

(12) Gronli, M. G.; Antal, M. J.; Schenkel, Y.; Crehay, R. The Scienceand Technology of Charcoal Production; Norwegian University of Scienceand Technology (NTNU): Trondheim, Norway, 2003; pp 1�38.

(13) Lacaux, J.-P.; Brocard, D.; Lacaus, C.; Delmas, R.; Brou, A.;Yoboue, V.; Koffi, M. Traditional charcoal making: An important sourceof atmospheric pollution in the African tropics. Atmos. Res. 1994,35, 71–76.

(14) Smith, K. R.; Pennise, D. M.; Khummongkol, P.; Chaiwong, V.;Ritgeen, K.; Zhang, J.; Panyathanya, W.; Rasmussen, R. A.; Khalil,M. A. K. Greenhouse Gases from Small-Scale Combustion Devices inDeveloping Countries: Charcoal-Making Kilns in Thailand. Office of Airand Radiation and Policy and Program Evaluation Division, UnitedStates Environmental Protection Agency (U.S. EPA): Washington, D.C.,1999; EPA-600/R-99-109.

(15) Pennise, D. M.; Smith, K. R.; Kithinji, J. P.; Rezende, M. E.;Raad, T. J.; Zhang, J.; Fan, C. Emissions of greenhouse gases andother airborne pollutants from charcoal making in Kenya and Brazil.J. Geophys. Res. 2001, 106, 24143–24155.

(16) Violette, M.Memoire sur les charbons de bois. Ann. Chim. Phys.1853, 32, 304.

(17) Violette, M.Memoire sur les charbons de bois. Ann. Chim. Phys.1855, 39, 291–342.

(18) Palmer, R. C. Effect of pressure on yields of products in thedestructive distillation of hardwood. J. Ind. Eng. Chem. 1914, 6 (11),890–893.

(19) Bergstrom, H. Handbok for Kolare; Almqvist and Wiksell:Uppsala, Sweden, 1947.

(20) Bergstrom, H.; Wesslen, G. Om Trakolning; Jernkontoret:Stockholm, Sweden, 1915.

(21) Mok, W. S. L. The effects of pressure on biomass pyrolysis andgasification. M.S.E. Thesis, Princeton University, Princeton, New Jersey,1983.

(22) Mok, W. S. L.; Antal, M. J. Effects of presure on biomasspyrolysis. II. Heats of reaction of cellulose pyrolysis. Thermochim. Acta1983, 68, 165–186.

(23) Blackadder, W.; Rensfelt, E. A pressurized thermo balance forpyrolysis and gasification studies of biomass, wood, and peat. InFundamentals of Thermochemical Biomass Conversion; Overend, R. P.,Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science: London, U.K.,1985; pp 747�759.

(24) Richard, J.-R.; Antal, M. J. Thermogravimetric studies ofcharcoal formation from cellulose at elevated pressures. In Advances inThermochemical Biomass Conversion; Bridgwater, A. V., Ed.; BlackieAcademic and Professional: London, U.K., 1994; Vol. 2, pp 784�792.

(25) Mok, W. S. L.; Antal, M. J.; Szabo, P.; Varhegyi, G.; Zelei, B.Formation of charcoal from biomass in a sealed reactor. Ind. Eng. Chem.Res. 1992, 31, 1162–1166.

(26) Klason, P.; Heidenstam, G.; Norlin, E. Untersuchungen zurholzverkohlung. I. Die trockene destillation der cellulose. Z. Angew.Chem. 1909, 25, 1205–1214.



(27) Klason, P.; Heidenstam, G.; Norlin, E. Untersuchungen zurholzverkohlung. II. Die trockene destillation des Holzes von Kiefer,Fichte, Birke und Buche. Z. Angew. Chem. 1910, 1252–1257.(28) Schenkel, Y. Modelisation des flux massiques et energetiques

dans la carbonisation du bois en four cornue. Ph.D. Dissertation,Gembloux, Belgium, 1999.(29) Antal, M. J.; Mok, W. S. L.; Varhegyi, G.; Szekely, T. Review of

methods for improving the yield of charcoal from biomass. Energy Fuels1990, 4, 221–225.(30) Antal, M. J.; Croiset, E.; Dai, X. F.; DeAlmeida, C.; Mok,

W. S. L.; Norberg, N.; Richard, J. R.; Majthoub, M. A. High-yieldbiomass charcoal. Energy Fuels 1996, 10 (3), 652–658.(31) Antal, M. J.; Allen, S. G.; Dai, X.; Shimizu, B.; Tam, M. S.;

Gronli, M. G. Attainment of the theoretical yield of carbon frombiomass. Ind. Eng. Chem. Res. 2000, 39 (11), 4024–4031.(32) Antal, M. J.; Mochidzuki, K.; Paredes, L. S. Flash carbonization

of biomass. Ind. Eng. Chem. Res. 2003, 42, 3690–3699.(33) Nunoura, T.; Wade, S. R.; Bourke, J. P.; Antal, M. J. Studies of

the flash carbonization process. 1. Propagation of the flaming pyrolysisreaction and performance of a catalytic afterburner. Ind. Eng. Chem. Res.2006, 45, 585–599.(34) Antal, M. J.; Wade, S. R.; Nunoura, T. Biocarbon produc-

tion from Hungarian sunflower shells. J. Anal. Appl. Pyrolysis 2007,79, 86–90.(35) Yoshida, T.; Turn, S. Q.; Yost, R. S.; Antal, M. J. Banagrass vs

Eucalyptus wood as feedstocks for metallurgical biocarbon production.Ind. Eng. Chem. Res. 2008, 47, 9882–9888.(36) Reynolds, W. C. STANJAN Thermochemical Equilibrium Software,

3.91 ed.; Stanford University: Stanford, CA, 1987.(37) National Aeronautics and Space Administration (NASA).

Chemical Equilibrium with Applications (CEA2) 09/28/2005; NASA:Washington, D.C., 2005; http://www.grc.nasa.gov/WWW/CEAWeb/ceaHome.htm.(38) Hancox, R. N.; Lamb, G. D.; Lehrle, R. S. Sample size

dependence in pyrolysis: An embarassment or a utility? J. Anal. Appl.Pyrol. 1991, 19, 333–347.(39) Fu, P.; Hu, S.; Xiang, J.; Sun, L.; Li, P.; Zhang, J.; Zheng, C.

Pyrolysis of maize stalk on the characterization of chars formed underdifferent devolatilization conditions. Energy Fuels 2009, 23, 4605–4611.(40) Fu, P.; Hu, S.; Sun, L.; Xiang, J.; Yang, T.; Zhang, A.; Zhang, J.

Structural evolution of maize stalk/char particles during pyrolysis.Bioresour. Technol. 2009, 100, 4877–4883.(41) Branca, C.; DiBlasi, C.; Galgano, A. Pyrolysis of corncobs

catalyzed by zinc chloride for furfural production. Ind. Eng. Chem. Res.2010, 49, 9743–9752.(42) Branca, C.; Galgano, A.; Blasi, C.; Esposito, M.; Di Blasi, C.

H2SO4-Catalyzed pyrolysis of corncobs. Energy Fuels 2011, 25, 359–369.(43) Bridgeman, T. G.; Darvell, L. I.; Jones, J. M.; Williams, P. T.;

Fahmi, R.; Bridgwater, A. V.; Barraclough, T.; Shield, I.; Yates, N.; Thain,S. C.; Donnison, I. S. Influence of particle size on the analytical andchemical properties of two energy crops. Fuel 2006, 86 (1�2), 60–72.(44) Bourke, J. P.; Manley-Harris, M.; Fushimi, C.; Dowaki, K.;

Nunoura, T.; Antal, M. J. Do all carbonized charcoals have the samechemical structure? 2. A model of the chemical structure of carbonizedcharcoal. Ind. Eng. Chem. Res. 2007, 46, 5954–5967.(45) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B.

Experimental investigation of the transformation and release to gasphase of potassium and chlorine during straw pyrolysis. Energy Fuels2000, 14, 1280–1285.(46) Yuan, J.-H.; Xu, R.-K.; Zhang, H. The forms of alkalis in the

biochar product from crop residues at different temperatures. Bioresour.Technol. 2010, 102, 3488–3497.(47) Antal, M. J., Jr.; Varhegyi, G.; Jakab, E. Cellulose pyrolysis

kinetics: Revisited. Ind. Eng. Chem. Res. 1998, 37 (4), 1267–1275.(48) Gronli, M.; Antal, M. J., Jr.; Varhegyi, G. A round-robin study of

cellulose pyrolysis kinetics by thermogravimetry. Ind. Eng. Chem. Res.1999, 38 (6), 2238–2244.

(49) Stiles, H. N. Secondary reactions of pyrolytic tars. Ph.D.,Diploma of the Imperial College (DIC), Imperial College, London,U.K., 1986.

(50) Herguido, J.; Corella, J.; Gonzalez-Saiz, J. Steam gasification oflignocellulosic residues in a fluidized bed at a small pilot scale. Effect ofthe type of feedstock. Ind. Eng. Chem. Res. 1992, 31, 1274–1282.

(51) Haykiri-Acma, H. The role of particle size in the non-isothermalpyrolysis of hazelnut shell. J. Anal. Appl. Pyrolysis 2006, 75 (2), 211–216.

(52) Rapagna, S.; Latif, A. Steam gasification of almond shells in afluidized bed reactor. The influence of temperature and particle size onproduct yield and distribution. Biomass Bioenergy 1997, 12 (4), 281–288.

(53) Beaumont, O.; Schwob, Y. Influence of physical and chemicalparameters on wood pyrolysis. Ind. Eng. Chem. Res. Process Des. Dev.1984, 23 (4), 637–641.

(54) Varhegyi, G.; Antal, M. J.; Szekely, T.; Till, F.; Jakab, E.Simultaneous thermogravimetric-mass spectrometric studies of thethermal decomposition of biopolymers. 1. Avicel cellulose in thepresence and absence of catalysts. Energy Fuels 1988, 2, 267–272.

(55) Bradbury, A. G.W.; Sakai, Y.; Shafizadeh, F. A kinetic model forpyrolysis of cellulose. J. Appl. Polym. Sci. 1979, 23, 3271–3280.

(56) Broido, A.; Nelson, M. Char yield on pyrolysis of cellulose.Combust. Flame 1975, 24, 263–268.

(57) Mok,W. S.-L.; Antal,M. J. Effects of pressure on biomass pyrolysis.I. Cellulose pyrolysis products. Thermochim. Acta 1983, 68, 1–165.

(58) Antal, M. J.; DeAlmeida, C.; Mok, W. S.-L. A new technologyfor manufacturing charcoal from biomass. In Proceedings of the IGTConference on Energy from Biomass and Wastes XV; Klass, D. E., Ed.;Institute for Gas Technology: Chicago, IL, 1991; pp 521�530.

(59) Shen, J.; Wang, X.-S.; Garcia-Perez, M.; Mourant, D.; Rhodes,M. J.; Li, C.-Z. Effects of particle size on the fast pyrolysis of oil malleewoody biomass. Fuel 2009, 88, 1810–1817.

(60) Antal, M. J.; Varhegyi, G. Cellulose pyrolysis kinetics: Thecurrent state of knowledge. Ind. Eng. Chem. Res. 1995, 34, 703–717.

(61) Reed, T. B.; Das, A. Handbook of Biomass Downdraft GasifierEngine Systems; Solar Energy Research Institute (SERI): Golden, CO,1988; Vol. SERI/SP-271-3022.

(62) Matsumura, Y.; Xu, X.; Antal, M. J. Gasification characteristicsof an activated carbon in supercritical water. Carbon 1997, 35, 819–824.

(63) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J. Carbon-catalyzed gasification of organic feedstocks in supercritical water. Ind.Eng. Chem. Res. 1996, 35, 2522–2530.

(64) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, R. J. A.; Prins, W.;van Swaaij,W. P.M.; van deBeld, B.; Elliott, D. C.;Neuenschwander, G.G.;Kruse, A.; Antal, M. J. Biomass gasification in near- and supercritical water:Status and prospects. Biomass Bioenergy 2005, 29, 269–292.

(65) Chembukulam, S. K. Smokeless fuel from carbonized sawdust.Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 714–719.

(66) Brandt, P.; Larsen, E.; Henriksen, U. High tar reduction in atwo-stage gasifier. Energy Fuels 2000, 14, 816–819.

(67) Zanzi, R.; Sjostrom, K.; Bjornbom, E. Rapid high-temperaturepyrolysis of biomass in a free-fall reactor. Fuel 1996, 75, 545–550.

(68) Ekstrom, C.; Lindman, N.; Pettersson, R. Catalytic conversionof tars, carbon black and methane from pyrolysis/gasification ofbiomass. In Fundamentals of Thermochemical Biomass Conversion;Bridgwater, A. V., Ed.; Elsevier Applied Science: London, U.K., 1982;pp 601�618.

(69) El-Rub, Z. A.; Bramer, E. A.; Brem, G. Experimental compar-ison of biomass chars with other catalysts for tar reduction. Fuel 2008,87, 2243–2252.

(70) Gilbert, P.; Ryu, C.; Sharifi, V.; Switenbank, J. Tar reduction inpyrolysis vapours from biomass over a hot char bed. Bioresour. Technol.2009, 100, 6045–6051.

(71) Antal, M. J. Synthesis gas production from organic wastes bypyrolysis/steam reforming. In Energy from Biomass and Wastes; Klass,D. L., Ed.; Institute of Gas Technology (IGT): Chicago, IL, 1978;pp 495�523.

(72) Antal, M. J., Jr. The effects of residence time, temperature, andpressure on the steam gasification of biomass. In Biomass as a Nonfossil



Fuel Source; Klass, D., Ed.; American Chemical Society (ACS):Washington, D.C., 1981; ACS Symposium Series, Vol. 144, Chapter16, pp 313�334.(73) Antal, M. J. Biomass pyrolysis: A review of the literature. Part

1—Carbohydrate pyrolysis. In Advances in Solar Energy; Boer, K. W.,Duffie, J. A., Eds.; American Solar Energy Society (ASES); New York,1982; Vol. 1, pp 61�112.(74) Antal, M. J. Effects of reactor severity on the gas-phase pyrolysis

of cellulose- and Kraft lignin-derived volatile matter. Ind. Eng. Chem.Prod. Res. Dev. 1983, 22, 366–375.(75) Antal, M. J. A review of the vapor phase pyrolysis of biomass

derived volatile matter. In Fundamentals of Thermochemical BiomassProcessing; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; ElsevierApplied Science Publishers: London, U.K., 1985; pp 511�537.(76) Antal, M. J. Biomass pyrolysis: A review of the literature. Part

2—Lignocellulose pyrolysis. In Advances in Solar Energy; Boer, K. W.,Duffie, J. A., Eds.; Plenum Press: New York, 1985; Vol. 2, pp 175�255.(77) Yang, C.; Yao, J.; Lu, X.; Yang, X.; Lin, W. Influence of K+ and

Ca2+ on mechanism of biomass pyrolysis. Taiyangneng Xuebao 2006,27 (5), 596–502.(78) Asadullah, M.; Zhang, S.; Min, Z.; Yimsiri, P.; Li, C.-Z.

Importance of biomass particle size in structural evolution and reactivityof char in steam gasification. Ind. Eng. Chem. Res. 2009, 48, 9858–9863.(79) Lede, J.; Li, H. Z.; Villermaux, J. Fusion-like behaviour of wood

pyrolysis. J. Anal. Appl. Pyrolysis 1987, 10, 291–308.(80) Lede, J.; Li, H. Z.; Villermaux, J. Pyrolysis of biomass evidence

for a fusionlike phenomenon. In Pyrolysis Oils from Biomass; Soltes, E. J.,Milne, T. A., Eds.; American Chemical Society (ACS): Washington, D.C., 1988; ACS Symposium Series, Vol. 376, Chapter 7, pp 66�78.(81) Narayan, R.; Antal, M. J. Thermal lag, fusion, and the compen-

sation effect during biomass pyrolysis. Ind. Eng. Chem. Res. 1996, 35,1711–1721.(82) Haas, T. J.; Nimlos, M. R.; Donohoe, B. S. Real-time and

post-reaction microscopic structural analysis of biomass undergoingpyrolysis. Energy Fuels 2009, 23, 3810–3817.

ef200450h

Documents

xedcarbon yield of charcoal

lowest xedcarbon yields

high yields of charcoal

formation of charcoal

high yield of corncob

higher yields

atmospheric pressure

pressure of0