2010 Biochar From Biomass and Waste

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Biochar from Biomass and Waste

W. Kwapinski

C. M. P. Byrne

E. Kryachko

P. Wolfram C. Adley J. J. Leahy

E. H. Novotny M. H. B. Hayes

Received: 3 March 2010 / Accepted: 28 May 2010/ Published online: 12 June 2010 Springer Science+Business Media B.V. 2010

Abstract There is an increasing realisation that biomassand organic wastes are valuable feedstocks for secondgeneration biorening processes that give rise to platformchemicals to substitute for dwindling petrochemicalresources, and for pyrolysis processes that produce syngas,bio-oil, and biochar from biomass, organic wastes, and thebiorening residuals of the future. The experimental work described has focused on physical properties and compo-sitions of biochars produced from miscanthus ( Miscan-thus 9 giganteus ), willow (Salix spp) and pine (Pinussylvestris ) at 500 C and at 400, 500, and 600 C in the caseof the miscanthus. Although the morphologies of the cellstructures were maintained in the pyrolysis, the surfacearea of the miscanthus biochar was greatly increased byheating at 600 C for 60 min. Nuclear magnetic resonancespectra showed the disappearance of evidence for the car-bohydrate and lignin plant components as the pyrolysistemperature was raised, and the compositions of miscan-thus biochars after heating for 10 and for 60 min at 600 Cwere very similar and composed of fused aromatic struc-tures and with no traces of the aliphatic components inthe starting materials. In greenhouse and growth chamber

experiments the growth of maize ( Zea mays L) seedlingswas found to be inhibited by soil amendments with biocharfrom miscanthus formed at 400 C for 10 min, but stimu-lated by miscanthus char formed at 600 C for 60 min. Inthe course of discussion the relevance of the resultsobtained is related to the roles that soil amendments withbiochar can have on soil fertility, carbon sequestration, onthe emissions of greenhouse gases from soil, on fertilizerrequirements, and on waste management. It is clear thatbiochar soil amendments can have denite agronomic andenvironmental benets, but it will be essential to have clearguidelines for biochar production from various feedstocksand under varying pyrolysis parameters. It will be equallyimportant to have a classication system for biochars thatclearly indicate the product compositions that will meetacceptable standards. A case can be made for sets of standard biochars from different substrates that meet therequired criteria.

Keywords Biochar Biomass Waste Pyrolysis Thermal conversion Plant growth Carbon sequestration

Introduction

Soil amendment with biochar has attracted widespreadattention because it increases the sequestration of carbon insoils and thereby decreases the amounts of CO 2 that entersthe atmosphere [ 13]. Biochar is the carbon rich productobtained when biomass is thermally decomposed (pyroly-sed) in restricted air conditions at temperatures between350 and 700 C [4].

The use of biochar as a soil amender is not a newconcept. The Terra Preta da indio (TP), or the dark earthsoils of the Amazon region are areas of very fertile

W. Kwapinski C. M. P. Byrne ( & ) E. Kryachko

P. Wolfram C. Adley J. J. Leahy M. H. B. HayesDepartment of Chemical and Environmental Sciences,University of Limerick, Limerick, Irelande-mail: [email protected]

W. Kwapinskie-mail: [email protected]

M. H. B. Hayese-mail: [email protected]

E. H. NovotnyEmbrapa Solos, Rua Jardim Bota nico, Rio de Janerio 1024,Brazil

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anthropogenic soils interspersed with relatively infertileferrisols close to rivers. There are several theories regard-ing the processes that led to their formation. It is not clearwhether the soils resulted from intentional soil improve-ment processes, or were the products of the agricultural andhousehold activities of indigenous populations [ 5]. Thedwelling places in the pre-Columbian past gave rise toaccumulations of plant and animal debris, as well as tolarge amounts of ashes and of bonre residues (charcoal),and several chemical elements, such as P, Mg, Zn, Cu, Ca,Sr, and Ba, which represent a geochemical signature of human occupation [ 6]. The accumulation of such nutrientresidues is likely to be a major driver of the formation of the fertile soils.

Large amounts of biochar derived carbon stocks remainin these soils centuries and millennia after they wereabandoned. Typical Amazonian ferrisols have low fertilityand contain about 100 Mg carbon ha

- 1 m- 1 . The fertile

TP soils, though derived from similar mineral parentmaterials, contain around 2.7 0.5 times more organiccarbon (approximately 250 Mg ha

- 1 m- 1 ) than the corre-

sponding ferrisols [ 4]. Carbon stored in soils in this way farexceeds the potential sequestration of C in plant biomassshould bare soil be restocked with primary forest thatcould, at maturity, be expected to have about110 Mg C ha

- 1 above ground [ 7, 8].There is increasing evidence indicating that biochar not

only increases the stable carbon stocks in a soil but alsoincreases nutrient availability beyond a fertiliser effect [ 9],and is much more efcient at enhancing soil quality thanany other organic soil amendment [ 10]. Based on theseconsiderations the concept of Terra Nova has emerged, orof soils whose properties and fertility would be enhancedby modern variants of the management practises that cre-ated the TP [11].

This communication reviews procedures for the pro-duction and characterisation of biochars, and describes,with some results, procedures that were utilised for thepreparation and characterisation of biochars. It reviewsaspects of how applications of biochar can inuence pro-cesses causing climate change, decrease emissions fromwaste and of NO x from soil, and provides evidence to showthat although biochar can enhance seed germination andplant growth there must be careful control of the prepara-tion processes because some preparations can inhibit plantgrowth.

Biochar Production and Characterisation

Pyrolysis is one of the many technologies that produceenergy from biomass and waste. The production of biochar,a carbon-rich, solid, by-product [ 1] distinguishes pyrolysis

from the other technologies. The major pyrolysis productsare pyrolytic oil (bio-oil), synthesis gas with differingenergy values (syngas), and biochar [ 12, 13]. A range of process conditions, such as the composition of the feed-stock, temperature and heating rate can be optimized toprovide different amounts and properties of products.Volatile products can be captured to provide energy, orupgraded to specic chemical products (e.g. wood pre-servative, meat browning, food avouring, adhesives, etc.)[1, 14].

Depending on the operational time and temperature,there are three subclasses of pyrolysis. These are conven-tional slow pyrolysis, fast pyrolysis, and ash pyrolysis[15]. Yields of liquid products are maximised in conditionsof low temperature, high heating rate, and a short gasresidence time, whereas a high temperature, low heatingrate and long gas residence time would maximise yields of fuel gas. Low operational temperatures and low heatingrates give maximum yields of biochar [ 16].

Feedstocks currently used on a commercial-scale, or inresearch facilities include wood chip and wood pellets, treebark, crop residues (straw, nut shells and rice hulls), switchgrass, organic wastes including distillers grain, bagassefrom the sugarcane industry, olive mill waste, chicken lit-ter, dairy manure, sewage sludge and paper sludge [ 11].The particle sizes and moisture contents of feedstock areimportant as wet feedstocks with large particle sizes willrequire more energy for pyrolysis. Some of the carbon inthe feedstock may need to be burned to supply the energyneeded, and so less feedstock is converted to biochar and tothe other products. Maximising the production of biocharrelative to the mass of the initial feedstock is always at theexpense of bio-oil and gas production.

The studies described here have involved the pyrolysisof willow (Salix species), miscanthus ( 9 giganteus ) andpine (Pinus sylvestris ) biomass and the characterisation of the biochars formed.

Materials and Methods

In the rst set of experiments 250 g samples of themiscanthus, pine and willow were chipped (maximum size1 cm) and heated in the absence of air to 500 C for 10 minin a 1 dm3 capacity pyrolyser. The pyrolysed materialswere then rapidly moved to an ambient temperature zone,still in a nitrogen atmosphere, and allowed to cool. In thesecond set of experiments 250 g samples of the miscanthuschips were heated for 10 min at 400 C and for 60 min at600 C. The data for the yields of biochar obtained (on a dryweight basis) from the three substrates; the elementalanalyses (Elementar); BET surface area (Gemini Microm-eritics apparatus); and high heating value (HHV) deter-mined using a Paar calorimeter (CEN TS 14918) are given

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in Table 1. Morphologies of biochars were observed usingScanning Electron Microscopy (SEM) (CaryScopeJcm5700).

Solid-state 13 C NMR experiments were carried out usinga VARIAN INOVA spectrometer at 13 C and 1 H frequen-cies of 100.5 and 400.0 MHz, respectively. A JACKOB-SEN 5 mm magic-angle spinning double-resonance probehead was used for Varied-Amplitude Cross-Polarisation(CP) experiments at spinning frequencies of 13 kHz.Typical cross-polarisation times of 1 ms, acquisition timesof 13 ms, and recycle delays of 500 ms were used. Thecross-polarisation time was chosen after variable contacttime experiments, and the recycle delays in CP experi-ments were chosen to be ve times longer than the longest1 H spinlattice relaxation time (T 1H ) as determined byinversion-recovery experiments.

Results

The yields of biochar obtained from the different sourcesare given in Table 1. The pyrolysis temperature typicallychanges the yield and properties of biochar. Withincreasing temperature, the yield of biochar commonlydecreases but the carbon concentration increases [ 17]. Thattrend is evident for the data in Table 1.

The data for Miscanthus (Table 1) show the very greatincrease in surface area for the Miscanthus biochar as thetemperature and time of heating were increased. The SEMpictures (Fig. 1) show that the plant cellular morphologieswere maintained in the biochars following heating of themiscanthus and willow feedstocks for 10 min at 500 C.

The HHV data indicate the relative values as fuels of thedifferent biochar products. There were little differences inthe values for willow, miscanthus and pine biochars pro-duced under the same conditions (at 500 C for 10 min). Thedata for miscanthus indicate that the more mature biocharformed at 600 C for 60 min was a slightly better fuel.

The NMR spectrum of the miscanthus used as a feed-stock for biochar production in this study is shown inFig. 2. This spectrum is similar to that obtained for

miscanthus by Brosse et al. [ 18] and is dominated by car-bohydrate signals in the resonance region 60110 ppm,representing the contributions from cellulose and hemi-celluloses, and the resonance at 105 ppm is assigned to theanomeric C. Contributions from lignin are indicated by theresonance at 56 ppm (methoxyl) and at 142164 ppm(O-aromatic) [ 5]. There are also some aliphatic contributionswith signals from alkyl C groups (046 ppm), probably

Table 1 Yields and some properties of biochars from different substrates and reaction conditions

Biochar source Willow Pine Miscanthus Miscanthus Miscanthus

Pyrolysis time, min 10 10 10 10 60Temperature, C 500 500 500 400 600Yield of char, wg.% 25.026.2 22.222.5 25.926.2 29.431.2 19.820.2Surface area, m 2 g

- 1 1.411.55 1.932.15 1.651.95 1.41.7 50.951.1

HHV, MJ kg - 1 31.031.5 30.431.3 29.930.7 29.430.3 31.532.5C, wt.% 79.9 81.4 76.3 74.8 85.1H, wt.% 3.34 3.38 4.26 4.33 2.40N, wt.% 0.97 0.47 0.40 0.39 0.55

Fig. 1 SEM of A miscanthus biochar and B willow biochar

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attributable to cutin and waxes, and the 2025 ppm can beattributed to terminal methyl groups.

13 C NMR is one of the most promising techniques forthe determination and characterisation of biochar in soils[19]. The 13 C NMR spectra for the biochars formed frommiscanthus by heating at different temperatures and timeintervals (Fig. 3) are vastly different from that for thefeedstock. The spectral patterns show that the structuralcompositions of the biochars change during pyrolysis. Thespectra for the miscanthus biochar at 400, 500 and 600 Care directly comparable because the pyrolysis time was thesame (10 min) for both reactions. Both spectra show

signicant aliphatic composition [ 3] at the 2535 ppmresonance, and for terminal methyl at * 15 ppm. Raisingthe pyrolysis temperature to greater than 400 C gave rise toa loss of aliphatic-C moieties and a transformation of Ccompounds to mostly poly-condensed aromatic-C typestructures dominated by a peak at 130 ppm [ 3]. There isstill some evidence in the spectrum at 400 C, and to alesser extent in that at 500 C for contributions from ligninwith a peak at 56 ppm indicative of methoxyl and ashoulder at * 150 ppm typical of the O-aromatic func-tionalities in lignin. When the temperature was raised to600 C the lignin signals disappeared. Increased heatingalso resulted in a signicant peak shift in the aryl regionfrom 131 to 126 ppm [ 3]. An up eld shift of the aryl peak to 126 ppm is typical of charred residues attributable topolycyclic aromatic structures [ 5]. That indicates that fusedaromatic structures were formed with low H (see Table 1)and O substituents [ 20]. These changes signify the trans-formations of labile compounds into environmentallyrecalcitrant forms. Recognisable fragments of feedstock biopolymers (lignin signals) in the chars produced at 400and 500 C would indicate that the reaction time wasinsufcient for complete conversion to char. On the otherhand, the char produced at 600 C in 10 min and 60 mincontained no recognisable feedstock components, indicat-ing complete conversion to char.

Biochar as a Fertiliser and Soil Ameliorant

By improving on existing best management practices,biochar can play a major role in expanding options forsustainable soil management. Biochar amendments will notonly improve soil productivity, but also decrease theenvironmental impact of agriculture on soil and waterresources. Biochar should therefore not be seen as an

Fig. 2 13 C DP/MAS NMRspectrum of miscanthus

Fig. 3 13 C DP/MAS NMR spectra of biochar resulting from thepyrolysis of miscanthus at various temperatures and durations


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alternative to existing soil management, but as a valuableaddition that facilitates the development of sustainable landuse [10]. Biochar can act as a soil conditioner and as afertiliser. It will enhance plant growth by supplying andretaining nutrients, and by improving soil physical andbiological properties [ 8]. However, it has been reportedthat biochar can have negative as well as positive effects onplant growth.

Materials and Methods

Greenhouse Experiments

The preparation, and some properties of biochars, preparedat 500 C for 10 min from willow, pine and miscanthus arehave been described, and relevant data are given inTable 1. Biochars were mixed at rates of 1% and 5% with aclay loam soil and cropped to maize ( Zea mays L.). Six

maize seeds, steeped in water for 2 h, were planted per pot(13 cm pot diameter). Eight replications for each experi-ment were incubated in the greenhouse during May 2009.Biochar was not added to the control. Pots were watered toeld capacity and allowed to drain. Subsequently the potswere watered (50 cm 3 ) every fourth day. After 10 days theweakest three plants were removed from each pot. Wholeplant and leaf growth were monitored every 3 days.

Growth Chamber Experiments

A shallow calcareous brown earth loam soil (20% clay,

pH 4.9) from the Kinvara series in the Burren area(N.Clare-S. Galway, Ireland) was amended with biochar(5 wg.%). Two types of biochar were prepared frommiscanthus chips (max. size 1 cm) in a lab-scale pyrolyser(1 dm3 ) at: (a), 400 C for 10 min; and (b), 600 C for60 min. Ten maize ( Zea mays L.) seeds were planted perpot (13 cm pot diameter). Four replications for eachexperiment were incubated in the growth chamber. Thepots were watered to eld capacity and allowed to drain.The system was incubated at 25 C in a growth chamber for21 days and articial light was provided for 13 h/day. Thepots were watered (50 cm 3 ) every fourth day. After

10 days the weakest ve plants were removed from eachpot. Twenty-one days after planting all plants were cut atsoil level, weighed, and oven dried at 60 C until a constantweight was attained.

Microbial Experiments

Tissues of plant roots were examined to determine fungalpresence by staining with Chlorazol Black [ 21].

Results

Seedling emergence in the greenhouse was rst observedafter 15 days in the cases of the controls; however, at thesame time plant stems were 10 cm in height and had threeleaves per stem in the biochar amended pots. The bestgrowth was in pots amended with 5 wt% miscanthus bio-char (Fig. 4 and Table 2). Biochar from willow and pinegave similar growth (though less than that for miscanthus).The difference in growth in the control and char-amendedsoils became less with time. Better growth was obtained forhigher amendments in the cases of the willow and pinebiochars, but even relatively small amendments (1 wt.%)gave growth results similar to those at higher concentra-tions in the cases of the miscanthus biochars. The green-house experiments showed that the biochar amendmentshad the best effects in the rst stages of germination andplant growth.

Microbiological analyses showed greater enrichments of microorganisms on the root tissue of the plants grown insoils amended with biochar. Most of the root surface fromsoil amended with miscanthus biochar was covered withlarge fungal colonies whereas colonies were barely visibleon roots from the control pots (Fig. 5).

When plant roots were taken from the control and charamended soils it was seen that the roots proliferated wherebiochar enrichments occurred (Fig. 5).

The data in Table 3 for the growth chamber experimentindicate that favourable plant responses were not obtainedfor all of the biochar samples. Best growth was obtained forthe biochar heated at 600 C for 60 min. The product fromheating for 10 min at 400 C suppressed plant growthmeasured after 21 days. Material from the lower temper-ature and shorter heating time had a lower surface area, anda lower C content (indicating lesser transformation to sta-ble biochar product) than for the product at the highertemperature.

Fig. 4 Growth of plants in greenhouse experiments after 21 days insoil amended with 1 and 5 wg.% biochar from willow, pine, andmiscanthus


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Discussion and Considerations of Implicationsof Biochar Applications to Soil

It is important to establish the preparation criteria that giverise to biochars that will have optimum value when addedto soil. Properties that will inuence biochar applicationsfor promoting plant growth and the sequestration of carbon

in soil will include: nutrients content, and composition and availability; elemental composition, ash content, and volatility; capacity to adsorb a wide range of organic and

inorganic molecules; surface area, porosity, and particle size; and bulk density.

A classication system for biochar is needed forpotential users and researchers. Various characterisation

techniques can be utilised in order to establish a set of standards needed to develop a classication system.

The two properties of biochar that are most important

for carbon sequestration and for soil amendment are: (1),a high resistance to microbial degradation; and (2), anability to retain nutrients that will equal or surpass that of other forms of soil organic matter [ 1]. The ability of biochar to store C and improve soil fertility will dependon its physical and chemical properties. Properties of biochar relevant to soil fertility and carbon sequestrationcan be varied in the pyrolysis process and through thechoice of feedstock [ 3].

The major components of biomass are cellulose, hemi-cellulose and lignin. The proportions of these three com-ponents determine the ratios of volatile carbon (in bio-oil

and gas) and stabilised carbon (biochar) in the pyrolysisproducts. Fast pyrolysis favours feedstocks with high cel-lulose and hemicellulose contents as bio-oil and gas are themain products, and with lesser yields of biochar. Feed-stocks with high lignin contents produce the highest bio-char yields when pyrolysed at moderate temperatures(approx. 500 C) [17, 22, 23]. Slow pyrolysis yields highquantities of biochar, and therefore favours feedstocks withhigh lignin contents. In the future, selection of feedstock may be dictated by the desired balance between pyrolysis

Table 2 Growth of maize seedlings (as a percentage of the controls where biochar was not added) in soils amended with biochars (5 wt.%) ingreenhouse experiments

Biochar source Control Willow Pine Miscanthus

Yield after 21 days (as growth % of control) 100 ( 9.12) 393 ( 33.9) 402 ( 32.2) 437 ( 40.3)Yield after 28 days (as growth % of control) 100 ( 6.51) 128 ( 9.82) 135 ( 9.41) 153 ( 11.9)

Biochar was made for 10 min in 500 C. ( standard deviation)

Fig. 5 Root tissues A coveredwith fungal colonies and B rootsfrom control pots

Table 3 Mass of maize seedlings after 21 days of growth (as apercentage of the control where biochar was not added) from soilsamended with miscanthus biochar (3 wt.%) ( standard deviation)

Preparation method Control 400 C, 10 min 600 C, 60 minYield of dry matter

(as wt.% of control)100 ( 16.1) 76.6 ( 7.00) 165 ( 19.8)


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products (gas, oil and biochar), and whether or not slowpyrolysis is being used [ 11].

Solid state 13 C NMR provides an excellent tool for thecharacterization of biochars. The spectra in Fig. 3 allowdetermination of the optimum conditions for the prepara-tion of chars with different compositions. These spectraand the elemental analyses data (Table 1) give indicationsof the extents to which the biomass feedstocks are con-verted to the fused aromatic structures. Such structuralarrangements may be considered to be optimum for longterm C sequestration, and may be an ideal biochar char-acteristic for the sequestration of soil C for millennia.Biochars produced at higher temperatures do not provideion exchange functional groups due to dehydration anddecarboxylation. That would of course limit its usefulnessin retaining soil nutrients by ion exchange mechanisms.

As seen in Table 1, biochars produced at the highertemperatures have high surface areas, and will have highporosities. Such porosity will allow the retention of water,enhance the soil sorption properties [ 2427], and providerefuge for microorganisms.

Earlier studies have shown that biochar with appropriatesurface areas and pore sizes will provide a refuge for Arbuscular mycorrhizal fungi (AMF) [ 13]. The addition of biochar to soils has been found to promote higher coloni-zation rates of host plant roots by AMF [ 28]. These fungiform a symbiotic association with plant roots, effectivelyextending the roots and enabling the uptake of additionalplant nutrients. In return the plant provides the organicenergy that the fungi need. Recent research suggests thatAMF might be an important component of the soil organicpool, in addition to facilitating carbon sequestration bystabilizing soil aggregates [ 29]. Glomalin, a glycoproteincontaining 30-40% carbon, is produced by all AMF [ 30]. Ithas been found to be involved in the aggregation and sta-bilization of soil particles.

Higher nutrient retention and nutrient availabilityproperties have been found after charcoal additions to soil[24]. These properties could arise from the oxidation of peripheral aromatic groups. However, that would be a longterm effect. It is reasonable to conclude that the prolifer-ation of microbial species in the environs of biochar willgive rise to signicant amounts of biomass which will betransformed to humic substances with high ion exchangeproperties.

Signicance of Biochar for the Sequestration of Carbon

The global carbon cycle is made up of ows and pools of carbon in the Earths system. The important pools of car-bon are terrestrial, atmospheric, ocean, and geological. Thecarbon within these pools has varying lifetimes, and owstake place between them all. Carbon in the active carbon

pool moves rapidly between pools [ 1]. In order to decreasecarbon in the atmosphere, it is necessary to move it into apassive pool containing stable or inert carbon. Biocharprovides a facile ow of carbon from the active pool to thepassive pool.

Biomass (non biochar) materials have varying degreesof resistance to biodegradation to CO 2 and water. The morereadily biodegradable components of the biomass decom-pose quickly over one to 5 years, the more stable compo-nents transform over decades to centuries, and the mostrefractory components can last for several centuries to afew thousand years. The organic matter in long termgrassland or forest reaches a steady state that will notchange substantially until the management is changed.Then there will be a slow decline in the organic C contentuntil eventually a much lower steady state is reached [ 31].The carbon released in that way must be regarded as fossilcarbon. Additions of organic matter during cultivation willlessen the losses of indigenous organic matter. However, itis estimated that less than 1020% of the added carbon willremain in agricultural soils after 510 years (depending onthe carbon quality and the environment) [ 8]. The net resultis that the bulk of carbon added as biomass is rapidlyreleased back to the atmosphere as CO 2 .

Adding biochar to soils can be regarded as a means of sequestering atmospheric CO 2 [10]. The conversion bypyrolysis of biomass organic matter to biochar signicantlyincreases the recalcitrance of the carbon in biomass.Because biochar is inherently stable in soil, applications of biochar inevitably gives rise to far more soil C sequestra-tion than would result from applications of biomass of similar C contents [ 8]. Biochar can thus be considered toharness the natural carbon cycle.

About four times more organic carbon is stored in theEarths soil than in atmospheric CO 2 . The annual uptake of CO2 by plants through photosynthesis, 120 Gt CO 2 C[32], is 8 times greater than anthropogenic greenhouse gas(GHG) emissions at this time [ 10]. Switching even a smallamount of the carbon cycling between the atmosphere andplants into a much slower biochar cycle would make aworthwhile difference to atmospheric CO 2 concentrations.It is estimated, for example, that diverting as little as 1% of net annual plant uptake into biochar would mitigate almost10% of current anthropogenic carbon emissions [ 10]. Thatdoes not take into consideration the additional multipliereffects that lock up additional carbon, such as increasedplant growth, increased root biomass, and increased soilmicrobial biomass.

Based on what has been stated above, for biochar to actas a carbon sink it must remain in the soil for a very longtime and it must have a high level of resistance to bio-logical and chemical degradation. Radiocarbon dating of TP soils indicate origins of 500 to 7,000 years BP [ 33, 34].


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Mean residence times of 13002600 years for biochar fromAustralian savannah soils were estimated by matchingannual production of biochar from the savannah soils withmeasured carbon stocks for soils in Northern Australianwoodlands [ 35]. At a mean annual temperature of 10 C themean residence time of biochars was estimated to be1,335 years [ 36]. Although there is some uncertainty aboutthe decomposition rates of biochar in soils, the great age of biochars found in soil studies and in archaeological sitesprovides evidence for its stability [ 34].

Biochar creates a real increase in terrestrial carbonstocks because it gives rise to a stable form of biomasscarbon and signicantly delays its decomposition [ 37].Lehmann [ 38] estimates that 20% of all the carbon inbiomass could be captured by conversion to biochar.Theoretically that could amount to 24 9 109 tonnes of carbon, or 88 9 109 tonnes of CO 2 annually. However, inpractical terms much of that biomass would not be avail-able for conversion to biochar and the actual potentialwould depend on the ability to access biomass feedstocksin an economically viable and environmentally responsiblemanner [ 37]. It is estimated that 10% of the annual USfossil-fuel emissions could be captured if biochar wasproduced using existing forest residues, or crop wastes, orfast growing biomass established on idle cropland [ 38].

As already mentioned, the ability of biochar to storecarbon and improve soil fertility will depend on its physicaland chemical properties, and these can be varied in thepyrolysis process, or through the choice of feedstock.Hayes et al. [ 39] reviewed the potential feedstocks avail-able in Ireland for biorening technologies. These feed-stocks include wastes and biomass residues (biodegradablemunicipal solid waste (MSW), poultry litter, spent mush-room compost (SMC), straws, forest residues, and sawmillresidues that potentially could be utilised for biocharproduction.

Converting biomass carbon to biochar carbon enables upto 50% of the initial carbon to be sequestered in a relativelystable form. The proportion of the carbon in the biomasswhich is converted to biochar is highly dependent on thetype of feedstock [ 8]. Downie et al. [40] examined biocharproduction from different feedstocks (wastes, sludge andwood) and estimated a 63% retrieval of the biomass carbonin the biochar. These values, along with measurementsmade in the laboratory, were used to estimate the potentialcarbon sequestration of biochar produced from the avail-able wastes and residues in Ireland (Fig. 6). These calcu-lations do not evaluate emissions associated with the wholelife cycle of biochar production but they aim to give aninsight into the potential of biochar. Approximately 2.4%of Irelands 2007 GHG emissions, 1.67 Mt of CO 2 , couldbe sequestered by the biochar produced from slow pyro-lysis of wastes and residues available under Scenario 1.

That uses the minimum available straw resources, theminimum collectable forest residues, all available biode-gradable MSW, poultry litter, SMC, and the availablesawmill residues. In Scenario 2, when all the maximumavailable straw resources for energy purposes and themaximum collectable forest residues are included for bio-char production, the amount of CO 2 eq that can besequestered increases to 2.25 Mt of CO 2 or 3.25% of thetotal GHG emitted in 2007. When all straw resources,including those currently used for animal bedding and soilamendments, are utilised, with the maximum collectableforest residues, Scenario 3 becomes operational. Theresulting biochar would enable the sequestration of 2.79 Mt of CO2 or 4.03% of the total GHG (CO 2 eq)emitted in 2007.

Biochar for a More Appropriate Utilisation of Waste

Wastes and residues from animal, crop, and forest pro-duction are valuable resources that can be used directly andindirectly for energy, including pyrolysis energy and bio-char [41]. In addition to providing energy and biochar, thepyrolysis process will decrease the volume and the weightof the waste material [ 10]. Many waste streams offereconomic opportunities for energy recovery particularlywhen a signicant reliable source of feedstock is generatedat a specic location. Biochar as a waste managementstrategy decreases GHG emissions associated with tradi-tional strategies. Land lling of organic waste, e.g. gardenwaste, results in the release of signicant quantities of methane. Anaerobic digestion of animal wastes releasemethane and nitrous oxide, and these gases are 25 and 298times, respectively, more potent as GHGs than CO 2 .

Fig. 6 The potential percentage of Irish 2007 GHG emissions thatcould be sequestered by the biochar produced from slow pyrolysis of varying amounts of available wastes and residues


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Management strategies that avoid these emissions cantherefore contribute signicantly to mitigation of climatechange [ 37].

The production of biochar rather than composting ismore effective in locking up carbon. Carbon contained incompost will be released by microbial transformationswithin 1020 years [ 37]. In contrast, the carbon seques-tered in the pyrolysed wastes would be stable in the soil,and also simultaneous emissions of GHGs, such as meth-ane, would be decreased [ 27].

The combination of biorening and pyrolysis can pro-vide protable industries that will decrease waste disposalcosts and provide cost effective energy services that protagriculture and industry [ 42]. Substrates with signicantcarbohydrate contents, including the lignocellulose com-ponents of straws, waste wood and forestry thinnings,provide appropriate materials for biorening operations.One example is the production by acid hydrolysis underconditions of high temperature and pressure of levulinicacid, furfural, and formic acid from the hexose componentsand furfural from the pentoses of hemicelluloses of bio-mass and of lignocellulosic and other waste materials.These components provide excellent platform chemicalsfor the production of a wide variety of products, includingliquid fuels and fuel additives such as ethyllevulinate,gammavalerolactone (GVL), and methyltetrahydrofuran(MTHF) [ 43]. In the cases of lignocellulose materials theresiduals from the biorening process will amount to about50% of the starting materials. These can be pyrolysed togive syngas, bio-oil, and biochar.

Biochar and Decreases in Nitrous Oxide and MethaneEmissions from Soil

There are limited studies to indicate that biochar can sig-nicantly decrease nitrous oxide and methane emissionsfrom soil. In 2004 agriculture contributed 42% of the totalof the 8% GHG emissions attributable nitrous oxide [ 44].Nitrogen fertilisers, organic fertilisers, nitrogen xation bybacteria, the urine and the excreta of grazing animals arethe major sources of nitrous oxide emissions from soil.Methane made up about 14% of global GHG emissions in2004 [44], and methane emissions from soil are associatedwith anaerobic conditions, typical of those found in wet-lands, paddy elds, and landlls.

Additions of biochar to the soil have been shown todecrease nitrous oxide emissions [ 45], and some by up to50% [46]. Biochars from municipal biowaste also caused adecrease in emissions of nitrous oxide in laboratorysoil chambers [ 47]. Additions of 15 g kg

- 1 of soil to agrass sward and 30 g kg

- 1 of soil to a soil cropped with -soybeans completely suppressed methane emissions [ 46].

Such information emphasises the need for further stud-ies to aid the development of biochar as a tool fordecreasing non-CO 2 GHG emissions from soil. Moreresearch is needed to understand the interactions betweenbiochar, site specic soil, climatic conditions, and man-agement practises that alter the sink capacity of soils.

In addition to climate change mitigation, additionalcomplementary and often synergistic objectives maymotivate biochar applications for environmental manage-ment. Soil improvement, waste management, and energyproduction, will individually, or in combination have eithera social or a nancial benet, or both [ 10].

Role of Biochar in Decreasing Atmospheric CO 2and Fertiliser Requirements

The increased plant growth from the fertiliser effect willincrease carbon sequestration in the growing biomass andthe organic carbon in soil as the result of increased rootgrowth and microbial proliferations, and increased addi-tions to soil of plant residues. Because biochar increasesthe soils nutrient retention capacity, it can increase theefciency with which fertiliser is used. This will decreaseemissions from articial fertiliser, both in terms of thoseassociated with the manufacture and the uses of fertilisers.Research has demonstrated that the use of biochar pluschemical amendments (N, P, K fertiliser and lime) onaverage doubled grain yield over four harvests comparedwith the use of fertiliser alone [ 48]. Another study foundthat biochar plus chemical fertiliser increased growth of winter wheat and several vegetable crops by 2550%compared with chemical fertilization alone [ 49]. Lehmannand Joseph [ 50] have presented a case study that examinesthe production of biochar from poultry litter in West Vir-ginia, USA. Biochar was sold for US $480 per tonne at thefarm gate for application to soybean crops and hay elds.Since the poultry litter biochar is rich in nutrients, 20% lessnitrogen fertiliser was required, and no applications of phosphorus and potassium were needed. Such savingsincrease the economic value of biochar. For example, inIreland approximately 320 k tonnes of nitrogen in syn-thetic fertiliser is used annually [ 51]. The average price of one tonne of nitrogen is approximately 1,800 (based oncalculations from [ 52]), and so the annual spend on syn-thetic nitrogen fertiliser in Ireland amounts to 576 million.However, poultry litter biochar could provide up to 20% of the total amount of nitrogen fertiliser needed savingfarmers 115 million annually. Biochar could also be for-tied with nitrogen, for example in a composting step, bymixing with poultry litter. It will be important to carry out adetailed study of the availability to plant needs of thenutrients in biochar. In the making of biochar, nitrogenmay be sorbed to the biochar in an available form or


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immobilized in the heterocyclic structures characteristic of biochar. Detailed 15 N NMR studies are needed to fullyascertain the nature of the nitrogen associated with biochar,and hence its nutrient capabilities. The limited evidence wehave suggests that biochar will prevent the leaching of nutrients, and allow these to be available to plants.

The fundamental mechanisms of biochar functions insoil are poorly resolved and consequently our abilities topredict the different roles that biochar can have in the soilenvironment are inadequate [ 27]. Research is also neededto determine the most suitable biochars for different soilapplications, and the most suitable application rates. Forexample, some studies [ 53, 54] have shown that additionsof biochar can temporarily decrease the soil fertility. Onesuch study found that amendment of soil with biochar mayin some situations lead to low nitrogen availability to crops[53] while another study [ 54] found that it increasednitrication.

Modern agriculture is dependent on phosphorus (P)derived from phosphate rock. This is a non-renewableresource and current global reserves may be depleted in50100 years. While P demand is projected to increase, theexpected global peak in P production is predicted to occuraround 2030. Although the exact timing of peak P pro-duction might be disputed, it is widely acknowledgedwithin the fertiliser industry that the quality of theremaining phosphate rock is decreasing and productioncosts are increasing [ 55]. Thus it is important to optimisethe use of P. Several studies have demonstrated enhanced Puptake by plants in the presence of biochar but very littlework has focused on the variety of mechanisms throughwhich biochar may directly or indirectly inuence thephosphorus cycle [ 56]. Pyrolysis can greatly enhance Pavailability by breaking bonds resulting in soluble phos-phorus salts associated with the biochar. In addition, bio-char modies soil pH and exchange capacity and thegeneral consensus is that it makes phosphorus moreavailable in the soil.

It is claimed that biochar might be fortied with nitro-gen in an energy production process by ue gas scrubbing[25]. Should that be so signicant environmental benetscould arise for such applications of biochar in powergeneration.

Biochar for Energy Production

Biochar products are often gasied to provide energy, orused in high value products such as activated carbon [ 11,57]. The addition of biochar to soil, instead of using it as afuel, decreases the energy efciency of pyrolysis bioenergyproduction. However, the decreases in the emissionsassociated with biochar additions to soil would be greaterthan those that would be attained had the biochar been used

as a fuel [13]. Biochar can contribute signicantly tosecuring a future supply of green energy [ 10] because it caneffectively retain nutrients and promote an environment inthe soil that will enhance plant growth.

Role of Biochar in Waste Management

In the future materials that are now regarded as organicwastes are likely to be seen as resources for biorening andbioenergy initiatives. In many agricultural and forestryproduction systems, waste is produced in signicantamounts from crop residues such as: (i), forest residues(logging residues, dead wood, excess saplings, pole trees);(ii), mill residues (lumber, pulp, veneers); (iii), eld cropresidues; or (iv), urban wastes (yard trimmings, siteclearing, pallets, wood packaging) [ 10]. Animal and cropwastes, for example, can present signicant environmentalburdens that lead to the pollution of ground and surfacewaters. Thus, strategies that will utilise such materials areecologically and economically attractive. All of these res-idues contain signicant amounts of carbohydrate thatprovide the raw materials for biorening processes [ 43],and those with high lignin contents will enhance the yieldsof residuals that, on pyrolysis can be expected to yieldsyngas, bio-oil, and high yields of biochar [ 10, 41, 58, 59].Such wastes can provide a signicant reliable source of feedstock generated at a single point location [ 60] therebydecreasing the energy used in transportation, and decreas-ing methane emissions that would result when such mate-rials are landlled.

Long Term Prospects for Biochar and Issuesto be Addressed

Biochar can be regarded as an important wedge, con-tributing to an overall portfolio of strategies for climatechange [ 10]. However, considerations must be given touses for biorening and for biochar production of biomassresources that would otherwise full ecosystem services orhuman needs. Possible conicts have to be consideredbetween producing energy and biochar versus food as aconsequence of any future adoption on a large scale of biochar technologies [ 61]. Such considerations need notapply when wastes and non-food biomass crops and resi-dues are used.

Biochar has the potential to have enormous economicand environmental benets in comparison to current con-ventional methods for the utilisation of biomass and wastematerials, such as combustion, co-ring of biomass, landlling, and incineration in the case of wastes. Biocharshould be seen as a part of an integral approach to theutilisation of biomass and waste materials and it will have


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