Effects of temperature and particle size on bio-char yield
from pyrolysis of agricultural residues
Ayhan Demirbas*
Department of Chemical Engineering,Selcuk University,42031Konya,Turkey
Accepted8July2004
Available online11September2004
Abstract
This article deals with slow pyrolysis of agricultural residues such as olive husk,corncob and tea waste at high temperature(950–1250K) in a cylindrical reactor batch reactor.The aim of this study was to experimentally investigate how different residues utilizing strategies affect the treatment conditions such as temperature,particle size,and lignin and inorganic matter contents on bio-char yield and reactivity.When the pyrolysis temperature is incread,the bio-char yield decreas.The bio-char yield incread with increasing particle size of the sample.A high temperature and smaller particles increa the heating rate resulting in a decread bio-char yield.The higher lignin content in olive husk results in a higher bio-char yield comparison with corncob.Bio-char from olive husk was m
ore reactive in gasification than bio-char from corncob becau of the higher ash content.
#2004Elvier B.V.All rights rerved.
Keywords:Agricultural residue;Slow pyrolysis;Bio-char;Char reactivity
1.Introduction
Bio-char can be obtained from biomass pyrolysis.For a high bio-char production from biomass pyrolysis,a low temperature and low heating rate process would be chon. The bio-char can be ud in the preparation of active carbon when its pore structure and surface area are appropriate.The starting materials ud in commercial production of activated carbons are tho with high carbon contents such as wood,lignite,peat,and coal of different ranks or low-cost and abundantly available agricultural by-products.Active carbons can be manufactured from virtually any carbonac-eous precursor,but the most commonly ud materials are wood,coal and coconut shell[1].The development of activated carbons from agricultural carbonaceous wastes will be advantageous for environmental problems.In water contamination,wastewater contains many traces of organic compounds,which is a rious environmental problem.
Active carbons are carbonaceous materials with highly developed internal surface area and porosity.Activated carbon is widely ud as an effective adsorbent in many applications such as air paration and purification,vehicle exhaust emission control,solvent recovery and catalyst support becau of its high specific pore surface area, adequate pore size distribution and relatively high mechan-ical strength.The large surface area results in high capacity for adsorbing chemicals from gas and liquids[2].
Pyrolysis can be ud for the production of bio-oil ifflash pyrolysis process are ud and are currently at pilot stage [3].Some problems in the conversion process and u of the oil need to be overcome;the include poor thermal stability and corrosivity of the oil.Upgrading by lowering the oxygen content and removing alkalis by means of hydrogenation and catalytic cracking of the oil may be required for certain applications[4].
Chemical additives(AlCl3,FeCl3,H3PO4,NH4Cl,KOH and ZnCl2)slightly affect thefirst step by inhibiting hemicellulos decomposition and accelerating cellulo decomposition through the dehydration reaction.Phosphoric acid exhibited the largest influence on the pyrolysis process. At concentrations higher than30%H3PO4,the two weight loss steps ascribed to hemicellulos and cellulo decom-position overlapped.Bio-char with an alkaline character of
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J.Anal.Appl.Pyrolysis72(2004)243–248
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doi:10.1016/j.jaap.2004.07.003
the surface,tar and gas products are obtained by steam pyrolysis of biomass(almond shells,nut shells,apricot stones,cherry stones,grape eds)[5].
The reaction mechanisms of biomass pyrolysis are complex but can be defined in three main steps: Biomass!WaterþUnreacted residue(1)
Unreacted residue!ðVolatileþGasÞ1þðCharÞ1(2)
ðCharÞ1!ðVolatileþGasÞ2þðCharÞ2(3) Pyrolysis proceeds in three steps:in the initial step moisture and some volatile loss(Eq.(1)).In the condary step occurred primary bio-char(Eq.(2)).The last fast step follows by a slower step including some chemical rearrangement of the bio-char.During the third step,the bio-char decompos at a very slow rate and carbon-rich residual solid forms.The formation of condary charring (Eq.(3))makes the char less reactive.
The char gasification forms an important part of biomass gasification.The major thermochemical gasification reac-tions include the following:Carbon char to methane:
Cþ2H2@CH4(4) Carbon char to oxides:
CþO2!CO2and CþCO2@2CO(5) Carbon char to CO and H2:
CþCO2þH2O@COþH2(6) The hot combustion products(CO2and H2O)are further reduced by the char.The endothermic reactions generate synthetic gas(syngas):CO and H2(Eq.(6)),and the exit g
as can be utilized as a gaous fuel.The molecules in the biomass(primarily carbon,hydrogen and oxygen)and the molecules in the steam(hydrogen and oxygen)reorganize to form this syngas.The high reactivity of bio-char is higher when smaller biomass particle are subjected to pyrolysis. The reactions of CO2and H2O with the char to produce CO and H2are considerably slower than the drying,pyrolysis or combustion reactions.The bio-char samples obtained by rapid pyrolysis at higher temperatures are more reactive in steam gasification than tho obtained at lower pyrolysis temperatures.This result is of practical interest for utilization of biomass as a raw material for gasification.
The aim of this work is to study the effect of the treatment conditions such as temperature,particle size,and lignin and inorganic matter contents on bio-char yield.
2.Experimental
In this study,olive husk,corncob and tea waste from East Black Sea region in Turkey were ud as agricultural residues.The samples were ground and sieved to give particle size of between<0.5and>2.2mm.The particle size distributions of the samples are prented in Table1. The pyrolysis experiments were performed in a device designed for this purpo.The main element of thi
s device was a cylindrical reactor of height95.1mm,i.d.17.0mm, and o.d.19.0mm heated externally by an electric furnace with the temperature being controlled by a thermocouple inside the reactor.
The chemical analys of the samples were carried out according to the ASTM D1103-80and ASTM D1104-56 standard test methods.The standard test methods for biomass fuel analys are:particle size distribution(ASTM E828),moisture(ASTM E871),ash(ASTM D1102), volatile matter(ASTM E872),carbon and hydrogen (ASTM E777),nitrogen(ASTM E778),sulfur(ASTM E 775),chlorine(ASTM E776)and ash elemental(ASTM D3682,ASTM D2795,ASTM D4278,AOAC14.7).For structural analys,the wood samples were prepared according to TAPPI standard(TAPPI T11m-45).The ground sample was extracted with ethanol–benzene accord-ing to ASTM,and lignin was determined as the insoluble residue after hydrolysis with72%sulfuric acid.The ud materials are characterized by analytical methods.
The samples were subjected to pyrolize for obtaining bio-chars at high temperature(450–1250K)in a cylindrical reactor batch reactor.The pyrolysis process were carried out with10K/s heating rate for obtaining the bio-char products from the samples at different temperatures: 470550,650,750,850,950and1050K.
All yields were expresd on a dry and ash-free(daf) basis,and the average yields from three experiments were prented within the experimental error of<Æ0.5wt.%.
3.Results and discussion
The chemical analysis results of agricultural residues are given in Table2.From Table2,the corncob has the highest volatile matter content(84.6wt.%daf).The structural analysis results of biomass samples are shown in Table3.As en from Table3,the lignin content of olive husk was 50.6wt.%daf.
Fig.1shows the effect of temperature on the bio-char yield. The decrea for the olive husk was56.4%(from44.5to 19.4wt.%daf)for particle size between1.5and2.2mm when the temperature is incread from450to1250K.The decrea for the corncob is81.4%(from30.6to5.7wt.%daf)
A.Demirbas/J.Anal.Appl.Pyrolysis72(2004)243–248
244
Table1
Particle size distributions of agricultural residues
Particle size (mm)Dry olive
husk(%)
Dry
corncob(%)
Dry tea
waste(%)
<0.5 6.48.5 4.2
0.5–1.07.612.8 5.7
1.0–1.538.637.53
2.6 1.5–2.227.926.32
3.4 >2.29.51
4.934.1
at the same conditions.Corncob has very high cellulo(52% by weight)and hemicellulos(32.5%by weight)contents. The yield bio-char from corncob at lower temperatures was relatively high.However,the bio-char yield rapidly decreas with increasing of pyrolysis temperature.
The destructive reaction of cellulo is started at temperatures lower than325K and is characterized by a decreasing polymerization degree.Thermal degradation of cellulo proceeds through two types of reaction:a gradual degradation,decomposition and charring on heating at lower temperatures,and a rapid volatilization accompanied by the formation of levoglucosan on pyrolysis at higher tempera-tures.The degradation of cellulo to a more stable anhydrocellulo,which gives higher bio-char yield,is the dominant reaction at temperature<575K[7].At temperatures>575K,cellulo depolymerizes,producing volatiles.If the heating rate is very high,the residence time of the biomass at temperatures<575K is insignificant. Thus,a high heating rate provides a shorter time for the dehydration reactions and the formation of less reactive anhydrocellulo,which gives higher yield of char[2].The result is that the rapid heating of the biomass favors the polymerization of cellulo and the formation of volatiles and suppress the dehydration to anhydrocellulo and char formation[6].Hence the effect of heating rate is stronger in the pyrolysis of biomass than in that coal.
The initial degradation reactions include depolymeriza-tion,hydrolysis,oxidation,dehydration,and decarboxyla-tion[7].The isothermal pyrolysis of cellulo in air and milder conditions,in the temperature range623–643K,was investigated[8].Under the conditions,the pyrolysis reactions produced62–72%aqueous distillate and left10–18%charred residue.After the pyrolysis,the residue was found to consist of some water-soluble materials,in addition to char and undecompod cellulo.
The hemicellulos undergo thermal decomposition very readily.The hemicellulos reacted more readily than cellulo during heating.The thermal degradation of hemicellulos begins above373K during heating for 48h;hemicellulos and lignin are depolymerized by steaming at high temperature for a short time.The metoxyl content of wet meals decread at493K[9].
The stronger effect of the heating rate on the formation of bi-char from biomass than from coal may be attributed to the cellulo content of the biomass[10].It is well known that heating rate has a significant effect on the pyrolysis of cellulo.Heating rate has a much greater effect on the pyrolysis of biomass than on that of coal.The quick devolatilization of the biomass in rapid pyrolysis favors the formation of char with high porosity and high reactivity[11]. The decread formation of char at the higher heating rate was accompanied by an incread formation of tar.The net effect is a decrea i
n the volatile fuel production and an incread yield of bio-char cellulo converted to levoglu-cosan at above535K temperatures[12].
The inorganic properties of biomass samples are prented in Table4.The different amount of inorganics may also affect the results.
The yield of bio-char was calculated according to the equation(Eq.(7))below[11]:
Bio-char yieldðwt:%dafÞ
¼
ðA b=A cÞÀðA b=100Þ
1ÀðA b=100Þ
Â100(7)
where A b is wt.%ash in dry biomass and A c is wt.%ash in dry bio-char.
Fig.2shows the effect of particle size on bio-char yield in the conditions lected for the study.In the e
xperiments with olive husk at950K the bio-char yield decreas45.5% (from35.6to19.4wt.%daf)when the particle size reduced from2.2to0.5mm.An increa in particle size from0.5to
A.Demirbas/J.Anal.Appl.Pyrolysis72(2004)243–248245 Table2
Chemical analysis results of agricultural residues(wt.%dry basis)
Sample C H N O(diff.)Ash V olatile matter(wt.%daf)Fixed carbon(wt.%daf) Olive husk50.2 6.4 1.038.4 4.172.527.5
Corncob49.0 5.60.543.8 1.184.615.4
Tea waste48.2 5.50.544.3 1.583.816.2
Table3
Structural analysis results of agricultural residues(wt.%dry,ash and extractive free)
Sample Hemicellulos Cellulo Lignin Olive husk24.225.250.6 Corncob32.552.015.5 Tea waste23.333.2
43.5
2.2mm for corncob at 950K increas the solid residue from 5.7to 16.6wt.%after total a 65.7%increa in amount of bio-char.
Fig.3shows the effect of temperature on carbon content in char.Fig.4shows the effect of temperature on oxygen content in char.Fig.5shows the effect of temperature on hydrogen content in char.The results of the elemental analysis (Figs.3–5)indicate that contents of carbon increa with pyrolysis temperature while the corresponding to hydrogen and oxygen decrea.Loss in hydrogen and oxygen correspond to the scission of weaker bonds within the bio-char structure favored by the higher temperature [13].
Fig.6shows relationships between the content of lignin and the bio-char yield from the samples.The higher lignin content in olive husk (Table 3)gives a higher bio-char yield comparison with oak wood and wheat straw.Lignin gives higher yields of charcoal and tar from wood although lignin has 3-fold methoxyl than that of wood [14–16].
If the purpo were to maximize the yield of liquid products resulting from biomass pyrolysis,a low tempera-ture,high heating rate,short gas residence time process would be required.For a high char production,a low temperature,low heating rate process would be chon.If the purpo were to maxi
mize the yield of fuel gas resulting from pyrolysis,a high temperature,low heating rate,long gas residence time process would be preferred.
Phenolics are derived from lignin by cracking of the phenyl –propane units of the macromolecule lattice.Pyrolysis
ems to produce the most substituted phenols on a lective basis.This phenomenon can be explained by the fact that the syringyl –propan units are not so linked to the lignin skeleton as the less substituted:gaiacyl –propane and phenyl –propane [17].It has been compared to the DTA curves of different lignin preparations in vacuo and concluded that the degradation pattern was virtually the same,an endotherm extending from about 373–453K and followed by an exotherm at about 675K,as the results of thermal analysis of individual types of lignin made by many other investigators [18,19].Lignin is broken down by extensive cleavage of b -aryl ether linkages during steaming of wood less than 488K [20].It has been found that on analysis of the metoxyl groups after isothermal heating of dry distilled wood,lignin decomposition begins at about 550K with a maximum rate occurring between 625and 725K and the completion of the reaction occurs at 725and 775K [9].
A.Demirbas /J.Anal.Appl.Pyrolysis 72(2004)243–248
246Table 4
Inorganic properties of biomass samples (wt.%of the ash)Sample
Si 2O Al 2O 2Fe 2O 3CaO MgO Na 2O K 2O SO 3P 2O 5
Olive husk 32.98.4 6.314.5 4.226.4 4.30.6 2.5Corncob 52.99.1 6.89.4 3.3 1.6 5.3 4.9 6.6Tea waste 44.47.2
5.2
12.6 3.8
1.8
18.8 1.4
4.8
Fig.3.Effect of temperature on carbon content in bio-char.Particle size:1.5–2.2
mm.
Fig.4.Effect of temperature on oxygen content in bio-char.Particle size:1.5–2.2mm.
The formation of char from lignin under mild reaction conditions is a result of the breaking of the relatively weak bonds,like the alkyl–aryl ether bond(s),and the conquent formation of more resistant condend structures,as has already been noted by Domburg et al.
[21].One additional parameter,which may also have an effect on the char formation is the moisture content of the kraft lignin ud.It has been found that the prence of moisture incread the yield of char from the pyrolysis of wood waste at temperatures between660and730K,while Stray et al.[22]found only a slight effect for water added on the hydrogenolysis of both hardwood and softwood lignins at temperatures between470and675K.The thermolysis reactions of the kraft lignin is comprid mainly of breaking the most reactive bonds like the methyl C–O bond of the methoxyl group and the condensation reactions to high molecular weight products(char)that follows.4.Conclusion
Bio-char can be obtained from biomass pyrolysis.If the purpo were to maximize the yield of bio-char resulting from pyrolysis,a low temperature,low heating rate process would be chon.When the
pyrolysis temperature incread the bio-char yield decread.The bio-char yield incread with increasing particle size of the sample.
The higher lignin content in olive husk results in a higher bio-char yield comparison with corncob.Bio-chars from olive husk are more reactive in gasification than bio-chars from corncob becau of the higher ash content.
Biomass converts by pyrolysis into liquid(bio-oil),bio-char and gass by heating the biomass to about850K in the abnce of air.The process can be adjusted to favor bio-char, pyrolytic oil,gas,or methanol production with a95.5%fuel-to-feed efficiency.The heatflux is proportional to the driving force,the temperature difference between the particle and environment.At higher temperature,the heatflux is higher. The size of the particles affects the heating rate.The heat flux and the heating rate are higher in small particles than in large particles.The higher heating rate favors a decrea of the char yield[2].
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Fig.6.Effect of lignin content on yield of bio-char at950Kfinal tem-perature.Particle size:1.5–2.2mm.
