Fuel 108 (2013) 656–667

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Fuel

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Dynamic Elemental Thermal Analysis (DETA) – A characterisationtechnique for the production of biochar and bio-oil from biomass resources

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.02.065

⇑ Corresponding author. Address: Department of Chemical Engineering, Univer-sity of Newcastle, University Drive, Callaghan, 2308 NSW, Australia. Tel.: +61 2 49216 108; fax: +61 2 49 216 521.

E-mail address: rohan.stanger@newcastle.edu.au (R. Stanger).

Rohan Stanger ⇑, Terry Wall, John Lucas, Merrick MahoneyDepartment of Chemical Engineering, University of Newcastle, Australia

h i g h l i g h t s

� A novel thermal analysis technique describes volatile evolution in terms of CHNOS.� The data was used to close the mass and energy balance for any process temperature.� An economic analysis of pyrolysis revenue streams was also performed.

a r t i c l e i n f o

Article history:Received 8 November 2012Received in revised form 15 February 2013Accepted 27 February 2013Available online 17 March 2013

Keywords:PyrolysisBiocharBio-oilThermal analysis

a b s t r a c t

A novel thermal analysis technique – Dynamic Elemental Thermal Analysis (DETA) – is demonstrated forthe characterisation of biomass. The technique provides a unique characterisation of the pyrolysisstreams by conversion into combustion products using a custom built downstream O2 lance. The resultis a continuous characterisation of evolving total volatiles, light gases and tars and solid char, in terms ofthe elements CHNOS, with pyrolytic heating. The condensed tars were also characterised as CHNOS withboiling point. The generated suite of integrated data provided quantitative mass yields and elementalcompositions that compares well with standard analytical methods. The results were used to estimatethe thermal properties of the biochar and the heat of combustion of the volatile streams (i.e. total vola-tiles and gas only), allowing a dynamic evaluation of the energy balance of a biochar generation process.At an illustrative temperature of 500 �C for a Spotted Gum biomass, 17% of the biomass carbon is retainedin the biochar. The results are used to estimate the potential revenue from each product of the heatingprocess. The economic assessment showed that bio-oil (being the largest fraction) would provide thegreatest income stream as either renewable electricity or when sold as crude liquid. The sequestrationof carbon as biochar would be the smallest income stream for a carbon price of $23/tonne CO2 (around4% total revenue). Overall, the DETA technique was shown to provide data suitable for assessment andoptimisation of a biochar production technology.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The rising level of CO2 in the atmosphere is considered the chiefcause of global climate change [1]. Anthropogenic emissions frompower generation, steel and cement manufacture and transportare the dominant sources of CO2, arising through the utilisationof fossil fuels. While efforts are underway to replace fossil fuels,it is clear that the expected population rise in the near future(e.g. by 50% in 2050 [2]) will create an unprecedented energy de-mand (200% of current capacity) which cannot be met without areliance on coal and gas. Given the inherent need to decouple eco-nomic growth from CO2 emissions, there exists a range of technol-

ogy options in various stages of demonstration (e.g. solar thermal/PV, carbon capture and storage, 2nd generation biofuels) howeverthere exists only one class of technologies that offers an ability toreduce atmospheric CO2 and produce renewable energy – the pro-duction of biochar. The world wide adoption of biochar productionhas been modelled to sustainably abate 12% (CO2 equivalent) ofglobal emissions (CO2, CH4, N2O) without endangering food secu-rity, habitat or soil conservation [3]. In Australia, it is predicted thatup to 50% emissions reduction may come from internationalsources as a method of cost reduction [4] and the utilisation of bio-char could be a relatively easy technology to be adopted in devel-oping economies, either as a directly deposited solid carbon orthrough production of renewable electricity. The overall benefitsand possible negative aspects of biochar are summarised inTable 1.

The production of biochar involves the harvesting of biomass,transport to plant, thermal processing and final sequestration.

Table 1Positives and negatives of biochar.

Positive benefits of biochar Negative aspects of biochar

� Actively removes CO2 from the atmosphere and sequesters it as stable carbon� Location not geographically important (i.e. can be done anywhere, potentially internationally) but

tied to biomass source� Enhances most soils, particularly moisture retention� Sustainably puts minerals back in soil (rather than ash)� Provides high surface area for micro-organism growth� Reduces the use of fertilisers� Use of volatile component for renewable energy generation� 1 tonne solid carbon is worth 3.7 tonnes CO2 equivalent in a carbon market

� Variability of biomass feed stocks (processing difficulties)� Variability of thermal processes� Fossil fuelled transport to/from processing facility (emits

CO2)� Biomass feed stock is low bulk density (high transport

cost)� Breakdown of biochar in soil (to reform CO2) not well

known� Dust from fine biochar particles� Carbon accounting monitoring difficult to determine in

the long term� Overall variability in negative carbon offset

R. Stanger et al. / Fuel 108 (2013) 656–667 657

These processes have been summarised in Fig. 1. For the Australiancontext, this ‘‘pre-processing’’ has been estimated at costing AU$33/tonne of biomass [5] and any revenue streams derived solelythrough the application of biochar for CO2 sequestration (througha carbon price or credit) must overcome this initial barrier. Typicallyhowever, revenue would be derived from (a) biochar for its soilenhancing features and (b) electricity for volatiles combustionand/or (c) sale of bio-oil as a liquid fuel replacement. Other biomassutilisation technologies have been reviewed elsewhere [6] for theAustralian context and can be seen to be uncompetitive with coaland gas fired utilities without a carbon price or legislation.

The technology of thermally processing biomass (pyrolysis) isrelatively mature and has been employed for centuries, however asustainable process which puts a stable biochar back in the groundand produces electricity has yet to be demonstrated at large scale. Inparticular, the question of how much carbon is actually being re-moved from the atmosphere is vitally important for any biocharproject because the negative carbon offset can provide a valuablestream of income in a carbon market. In Australia, the methodologyof carbon offsetting for biochar and soil carbon is currently being as-sessed [7] under the Carbon Farming Initiative, however the applica-tion of bio-char to soil is on the list of positive sequestrationactivities, providing the project does not adversely affect its sur-roundings (e.g. destruction of native forests and water courses).

1.1. Carbon sequestration and biochar

There are many reasons to use biochar as a soil conditioner,with researchers pointing to the terra preta soils of South Americaas an example of highly fertile soil with a stable carbon content. It

Fig. 1. Processes involved in biochar and bio-oil product

has also been suggested that the use of biochar can also suppressthe natural carbon cycles in biomass mixtures which produceCO2 [8]. These benefits add to the overall appeal of utilising bio-char, but require significant long term field testing. From an eco-nomic perspective, the cost of harvesting/transport/processingmust be evaluated against the income derived from sequesteringcarbon as biochar and the generation of renewable electricity.Other factors such as soil conditioning in farming and landfillreduction in waste management will add additional value to theindividual business model. However, the primary motivation forutilising biochar is a price on CO2 and this baseline is commonfor all activities.

Internationally, the use of biochar is gaining interest and hasbeen rigorously reviewed [9–12]. The greatest uncertainties in thelife cycle analysis revolve around the variability of the feed materi-als and the breakdown of biochar within the soil. The loss of labilecarbon or the slow decomposition of biochar in the soil is an impor-tant variable as this affects the ability to sequester carbon on a longterm basis (100–1000 years). Such testing is currently under wayand as yet there is no agreed method of determining such long termcarbon loss. However, there is growing agreement that the rate ofdecomposition is far slower than the ability to add to biochar car-bon into the soil and thus can result in a net sequestration of car-bon. Thermal processing can introduce a number of variablessuch as the ability to use waste heat for preliminary drying andwhether air is introduced directly to aid in heating. Volatiles aretypically combusted to form heat for the process and electricity.To determine whether such a process is deemed carbon negative(i.e. removing CO2 from the atmosphere) or financially viable re-quires a mass and energy balance of the feed stock within a given

ion and their relation to potential revenue streams.

658 R. Stanger et al. / Fuel 108 (2013) 656–667

process and knowledge of the CO2 emissions during harvesting andtransport. Such a study is beyond the ability of farmers and land-owners and must be performed by biochar plant manufacturers,typically at considerable cost. As such, the potential carbon seques-tration of a biomass feedstock is not easily obtained by those thatare generating the material and may not be objectively evaluated.Current analytical standards for assessing combustion feed stocks(such as proximate analysis, ultimate analysis) are based on coalutilisation technologies and require heating to temperatures muchhigher than those expected in biochar plants (�500 �C) and do notnecessarily generate the data for a full assessment. In general, suchstandard analyses are useful for characterising the initial feed stockand products after thermal treatment, however the product yields(i.e. char, oils and gases) and their character are dependent on thetesting conditions (i.e. temperature of reactor, residence time). Assuch, a significant amount of testing is often required to determineoptimum conditions for any given feed stock. Furthermore, obtain-ing a mass balance across reactors such as a packed or fluidised bedis often hampered by the inability to collect and analyse the highmolecular weight condensable fraction. Such materials are often asignificant proportion of the pyrolysis products, are difficult toquantify using current analytical techniques [13] and are typicallyestimated using ‘‘semi-quantitative’’ methods [14]. Estimation ofthe ‘‘missing’’ non-volatile mass has ranged from 19% to 25% forfluidised bed reactors [15]. Torri et al. [13] used pyrolysis–gas chro-matography (Py–GC) fitted with a Atomic Elemental Detector (AED)to show that 23–29% of the biomass carbon is lost in the analysis,while Ischia and co-workers [14] used a thermogravimetric–massspectrometer (TG–MS) and a thermogravimetric–gas chromato-graphic–mass spectrometer (TG–GC–MS) to estimate the elementalbalance across the pyrolysis system to within +5.5% for carbon,+1.7% for hydrogen and �4.7% for nitrogen. The current analyticaltechniques such as GC and MS are incapable of measuring this lostfraction either because the high molecular weight material con-denses in the column, or because they are too high for the MS detec-tor. Both techniques suffer from an inability to properly quantifythe amount of this material. Attempts at cracking the high molecu-lar weight tars using a catalyst prior to the GC have shown that car-bon losses are still in the range of 15–40%, depending on catalyst[16], translating to around 50–70% unanalysed bio-oil material.De Jong and co-workers [17] worked around this problem by usingthermogravimetric analysis coupled with a Fourier transform infra-red analyser (TG-FTIR). The technique can measure mass loss andselected non-condensable gases, and by difference calculate themass loss of tars. However, while the technique enables closing ofthe mass balance, it is incapable of determining other tar attributes

Fig. 2. Diagram of furnace, heating chamb

which would enable either elemental or energy balance closure.Overall, the testing of biomass in pyrolysis systems often requirenumerous and expensive techniques to characterise thermalbehaviour. As observed above, these techniques can provide rigor-ous analysis of the biomass behaviour during thermal treatment,however the closure of a mass, elemental and energy balance is stillelusive. Given the inherent issues associated with the measurementof high molecular weight bio-oils, it is difficult to see how techno-economic studies can be derived from smaller scale testing. A re-cent report on research gaps for biochar [10] highlights the needfor a predictive capacity for biochar performance to optimise themultiple uses of a bio-resource. This is listed as the inhibiting stepin the application of biochar technology.

This paper introduces a novel thermal characterisation tech-nique, Dynamic Elemental Thermal Analysis (DETA) and demon-strates its use as a method of determining the intrinsic pyrolysisyields (i.e. char, tar, gas), their elemental constituents and theirestimated thermal properties. The technique has previously beendemonstrated on a coking coal [18] and its vitrinite component[19]. This paper will also show how such information can be usedto estimate the potential revenue streams under different businesscases.

2. Methodology

2.1. Apparatus and conditions

The apparatus consists of four sections: furnace, heating cham-ber, heated oxygen lance and combustion gas analysis. Fig. 2 showsa diagram of the experimental set-up. The concept of the techniqueis to produce hot pyrolysis gases and combust them in situ duringdevolatilisation. The combustion products were then analysed andused to back calculate the evolution as a function of CHNOS. Thefurnace used was a Sinku Riko Infra-red Gold Image furnace, whichreflects radiation from four tungsten lamps onto a central axis. Ingeneral, the methodology does not strictly require such an ad-vanced furnace, however experimentally the IR furnace is capableof relatively rapid heating and cooling due to the lack of refractory.The heating chamber consisted of an outer quartz tube which al-lowed the radiation through to a central quartz tube surroundedby a graphite sheath. The graphite was used to imbed the controlthermocouple, and inner quartz tube was used to contain the sam-ple and lance. The biomass sample was a Spotted Gum (EucalyptusMaculata) milled to 63–90 lm. Around 250 mg of sample wasloaded into a custom made quartz crucible and slid down the cen-tral inner quartz tube. Two thermocouples were fitted to the gas

er, heated O2 lance and gas analysis.

Fig. 3. Heated O2 lance, diagram showing the sequence of operation as the volatiles are evolved and converted into combustion products.

R. Stanger et al. / Fuel 108 (2013) 656–667 659

inlet side of the chamber, measuring the gas temperature at the sideof the crucible and within the sample. The furnace was heated to1000 �C at 25 �C/min, with sample carrier gas set to 100 mL/min ofargon. The sample was analysed three times to determine the quan-titative experimental error for volatile yield, char yield and CHNOS.

2.2. Heated oxygen lance and gas analysis

The oxygen lance was constructed from alumina tubing, de-signed to be set downstream of the pyrolysing sample. It intro-duces hot O2 in order to combust the evolved volatiles into CO2

and H2O as the main products, with minor amounts of NO, NO2,SO2 and CO. The lance diagram is given in Fig. 3. The O2 flow ratewas set to 100 mL/min and the lance temperature was controlledat 950 �C using NiCr heating element. The combustion gases weremonitored using a infrared LiCorA CO2/H2O analyser and a Testo350XL Combustion Flue Gas analyser. A rotameter (not shown inFig. 2) was fitted to the CO2/H2O analyser and adjusted to allowa maximum of 1 L/min through the IR cell of the analyser, withthe remaining 0.5 L/min allowed to bypass. The CO2/H2O analyserwas capable of measuring concentrations up to 20,000 ppm CO2

and 60,000 ppm H2O, while the Testo measured O2 (0–25%), CO(0–500 ppm), H2 (0–500 ppm), NO (0–300 ppm), NO2 (0–300 ppm) and SO2 (0–3000 ppm). The hot combustion gases werequenched on furnace exit using a diluent stream of argon at1300 mL/min to produce a gas flow large enough for the Testo unit(�1350 mL/min required). Gas concentrations were measuredevery second. The total CHNS in the gas stream is calculated byCO2 + CO, 2 � (H2O + H2), NO + NO2, SO2, respectively. Oxygen iscalculated by a dynamic difference between oxygen contained inthe combustion products and the measured oxygen consumed.As such it must be considered to have a larger error than the otherelements.

2.3. Modes of operation

There are five modes of operation used with the apparatus,summarised in Fig. 4. These modes are: (1) Total Volatile combus-tion, (2) Char combustion, (3) gas only combustion, (4) Tar Onlycombustion, and (5) Residual Tar Burnoff. The first two modesare performed using the same sample of biomass, with the samplebeing initially heated to 1000 �C at 25 �C/min and held for 10 minbefore being allowed to cool. The Total Volatile stream (tars +gases) was evolved as a gas and combusted downstream by theO2 lance, exiting the furnace as combustion products. After cooling(when the sample reached below 100 �C) a second stream of O2

was introduced to the sample gas stream (30 mL/min). The samplewas then re-heated to combust the remaining char (mode 2). Thesecondary stream of O2 added to the front end combusted the charwith limited oxygen and hence prevented runaway combustion.Above 500 �C, the gas stream produces larger amounts of CO,

which are then combusted over the hot O2 lance. After combustion,the sample was allowed to cool and weighed back for a final ashcontent. The reason for using 1000 �C is to allow almost all of thevolatiles to be evolved from the char. The char is then combustedin a controlled way to allow for the total amount of each element(i.e. Total Volatile CHNOS + residual char CHNOS) to be integrated.This allows back calculation of the volatile and char yield and ele-mental composition for any temperature lower than 1000 �C (mostthermal systems). This provides a general data set that can be ap-plied to different thermal systems at operating at different temper-atures. A new sample was used for mode 3 gas only combustion.Mode 3 used a condenser at the outlet of the furnace prior to com-busting the pyrolysis gases in a second reactor (consisting of aquartz tube, insulated in kaowool). In mode 3 operation the tarsand gases were swept out of the IR furnace, where the tars (andwater) were condensed in a ice bath. The light gases travel out ofthe condenser and were then combusted across the lance in thesecondary reactor, set at 950 �C. This process introduces �10 s de-lay between the runs. By comparing the difference between thecombustion of total volatiles and the Gas Only streams, a mathemat-ical Dynamic Tar curve may be derived, which quantitatively showshow the condensable material is evolved from the biomass struc-ture. A small amount of tars were observed to evaporate after peaktar formation despite being at 0 �C, however in practice this amountis small (<2%). The condensed tars and water were collected usingan acetone solvent wash and a portion of the condensable phasewas placed back into the crucible by allowing the acetone to evap-orate. These tars were then reloaded into the furnace for mode 4 TarOnly combustion, allowing the characterisation of the tars in termsof CHNOS as a function of boiling point. After cooling, a final mode ofoperation (not shown in Fig. 4) introduced O2 to the gas inlet sideand reheated, producing a Residual Tar Burnoff. This mode is thesame as the Char combustion (mode 2), but with residual solid fromthe tars rather than the whole biomass sample.

3. Results

3.1. Thermal analysis of volatile evolution

The evolution of volatile products during pyrolysis is perhapsthe most difficult to characterise due to the wide range of speciesand the number of analytical instruments required. The DETAtechnique allows quantification of pyrolysis streams in terms ofelemental constituents, by converting volatile species into com-bustion products. Fig. 5 shows the main combustion productsCO2 and H2O produced during combustion of total volatiles (mode1 combustion of tars + gases) and the accompanying decrease in O2

exiting the furnace. The benefit of this technique is that the tars arecombusted and can be accounted for in terms of mass balancing.The combustion products can be then used to back calculate theelemental contents of the volatiles as they are evolved, rather than

Fig. 4. Modes of operation using the DETA apparatus. Note that mode 4 uses the same set-up as mode 1, however the sample (tar) is derived from the condensing stage inmode 3. Not shown is mode 5 Residual Tar Burnoff, which is the same as mode 2 but uses the left over tars in mode 4.

0

1

2

3

4

0 100 200 300 400 500 600 700 800 900 1000Temperature, C

Prod

uct G

as C

once

ntra

tion,

%

0

1

2

3

4

5

6

7

O2 C

oncentration, %

CO2H2OO2

Fig. 5. The main combustion species produced from controlled downstreamoxidation of total volatiles (tars + gases) evolved from biomass during pyrolysis(mode 1).

660 R. Stanger et al. / Fuel 108 (2013) 656–667

by analysis of sample after heat treatment. Fig. 6 shows that flow oforganic elements CHON. The presence of SO2 was detected at verysmall levels (1–2 ppm) and is not shown here, but is included inthe integrated results further down. The units of measurementare given as ppm/g biomass, which are the raw measured combus-tion gas concentrations divided by the starting mass. Since the flowrate of carrier gas is constant (i.e. the volatile streams are small),the units of ppm/g biomass can be equated back to a molar basis,rather than mass basis. Peak volatile release occurred at 400 �Cfor all elements. Fig. 7 demonstrates the difference between TotalVolatile combustion and gas only combustion for carbon evolution.This difference can be mathematically subtracted to produce theDynamic Tar evolution. This curve is the amount of material thatis condensed out in the ice bath between the two runs and includesa number of higher molecular weight organic species and water.One particular benefit of this dynamic method is that it enables

an assessment of how the collected crude bio-oil product willchange with different process temperatures without performingmultiple experiments. This information can be used to predictyields, heating value and potential revenue as will be demon-strated in the next section. A small amount of overlap is observableabove 700 �C and this is considered to result from evaporation oflighter tars species after peak volatile release. The water was pro-duced during the initial drying stage (observed below 200 �C) andafter peak volatile release. Fig. 7 compares the mathematically de-rived Dynamic Tar for each element. With the gas only results sub-tracted from the Total Volatile evolution curves, it can be observedthat the evolution of condensable phase carbon from the biomassis completed by 470 �C, while the evolution of hydrogen and oxy-gen continues to 700 �C. Without the presence of carbon, the evo-lution of hydrogen and oxygen above 470 �C must come from theformation of pyrolysis water (see Fig. 8).

3.2. Thermal characterisation of collected bio-tars

Fig. 9 gives the results of the tar analysis (modes 4 and 5). Thefigure has been modified to show the initial time during purgingas this is significant in terms of H2O evolution. The H2O has two dry-ing peaks, the first prior to the lance being turned on and the secondduring the small amount of back heating (up to 50 �C). This evapo-rated water consists of the combined amount of drying water andpyrolysis water and makes up 21.6% of the total condensed sample(determined by integration) and can be resolved into a total of 8.6%from drying and 5.1% from devolatilisation (wt% biomass). The tarvaporisation shows a peak in C and H distribution at 200 �C, fol-lowed by a shoulder at 450 �C finishing at about 650 �C. This indi-cated that a significantly wide variety of species are present inthe bio-tar. A very small amount of material appears to be removedafter 800 �C and this is thought to arise from polymerisation(resulting in light gas species) rather than the vaporisation of heavy

0

20000

40000

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80000

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0 200 400 600 800 1000 1200Temperature, C

CH

O C

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tion,

ppm

/g b

iom

ass

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200

300

400

500

600

700

800

900

1000 N C

oncentration, ppm/g biom

ass

C total volatilesH total volatilesO total volatilesN total volatiles

Fig. 6. The calculated evolution of carbon, hydrogen, oxygen and nitrogen contained in the Total Volatile stream (mode 1).

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20000

40000

60000

80000

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0 100 200 300 400 500 600 700 800 900 1000Temperature, C

Car

bon

(as

CO

2+C

O),

ppm

/g b

iom

ass

Total VolatilesGas OnlyDynamic Tar

Fig. 7. Comparison of the carbon evolved from the total volatile (mode 1) and gas only (mode 3) results. The difference between the run experiments represents the DynamicTar evolution.

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O , p

pm/g

bio

mas

s

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500

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700

800

900

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N C

oncentration, ppm/g biom

ass

C tar H tar O tarN tar

Fig. 8. Comparison of elemental evolution for the Dynamic Tar (and water) derived by mathematically subtracting the gas only results from the total volatile results (i.e.Mode 1 results – mode 3 results).

R. Stanger et al. / Fuel 108 (2013) 656–667 661

molecules. After cooling, the secondary stream of oxygen was at-tached to the gas inlet (similar to mode 2 Char combustion) andthe remaining tar was combusted. No species other than carbon

was present and this material was considered to be the coked resid-ual, which represented 7.8% of the re-analysed tar fraction (withoutwater). Of the minor species, NO was detected at a maximum

0 200 400 600 800 1000Temperature, C

Gas

Con

cent

ratio

n, p

pm/g

CHOCO2 Residual burnoff

Initial purging at 25 C

0

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3000

3500

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4500

11:51:22 11:52:48 11:54:14 11:55:41 11:57:07 11:58:34 12:00:00 12:01:26 12:02:53

Fig. 9. Bio-tar analysis using DETA (mode 4) followed by a controlled burn-off ofresidual carbon (mode 5).

662 R. Stanger et al. / Fuel 108 (2013) 656–667

concentration of 7.6 ppm at 220 �C and is not shown in Fig. 9, but isincluded in the integration. The elemental distribution ofbio-oil boiling points and the residual carbon could potentially beused as an indicator for performance and fouling within acombustion engine or boiler applications. If further refinement ofthe crude oil is considered, the use of boiling point distributioncould also serve as a method of estimating yields in a distillationprocess.

3.3. Calculated composition change in bio-char during pyrolysis

The dynamic evolution curves provide an advanced picture ofthe devolatilisation process, the production of tars and light gasand a characterisation method of the condensable fraction. Thisdynamic data can also be re-calculated to produce the changingelemental composition of the char fraction. Fig. 10 shows the dy-namic partition of elements reporting to the char fraction (of origi-nal biomass) while Fig. 11 shows the instantaneous elemental charcomposition (as a fraction of char at any temperature). Fig. 10shows that the proportion of carbon reporting to the char fractionis stable after 500 �C, while oxygen continues to be evolved (prin-cipally as H2O). Fig. 11 confirms that the evolution of oxygen after500 �C produces an increasingly different char product. Spokas [20]found that the stability of biochars in soil was related to O:C ratio.

0

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0.5

0 100 200 300 400 5Temper

Cha

r Ele

men

tal P

artit

ion,

g/g

bio

mas

s

Fig. 10. Partition of elements reporting to the biochar fraction as a fra

He found that a maximum O:C ratio of 0.2 was necessary to pro-vide a minimum of 1000 years half-life. Figs. 10 and 11 show thatthe O and C portions of the biomass diverge at around 500 �C, how-ever a process temperature of 685 �C would be required to producean O:C of 0.2. This provides an important criteria if biochar stabilityis considered to be more favourable over its soil enhancingproperties.

3.4. Overall mass and elemental balance

An important aspect of the generated data is that it can be usedquantitatively to produce an overall mass balance by integratingthe elemental streams and reconstituting the information. Table 2gives an overall summary of the pyrolysis yields from modes 1 to 4and compares them to measured values (determined by weighingback the sample after treatment) and the standard proximate anal-ysis. It can be seen that the total sum of volatile matter, char yieldand H2O adds to 105.9%, rather than 100% and this discrepancy isconsidered to be due to the re-adsorption of moisture from theatmosphere between the weighing of sample and loading intothe apparatus (sometimes �30 min). The integrated char yield(from gas analysis) compares very well with the measured charyield (obtained after the mode 3 gas only run) as does the mea-sured ash content (obtained after mode 2 Char combustion) withthe standard proximate analysis. The difference between the prox-imate fixed carbon and the DETA char (and indeed the measuredchar value) is considered to be due to the difference in final tem-peratures in each test (i.e. 1000 �C in DETA, 900 �C in proximateVM) and the difference in moisture content.

It should be noted that there remains a small difference be-tween the total volatile yield (80.3%) and the sum total of tar andgas yields (79.3%). This difference arises due to the evaporationof light tar species and possibly water in the condenser (mentionedearlier, above 500 �C) and was calculated at 0.9% of the total bio-mass sample. In this case the tar yield is defined as the differencebetween the total volatile and gas only run and is integrated untilthe point of intersection. As the light tar species are evaporatedwhen the concentration of tar begins to fall (after peak tar release),the gas only results will be marginally higher than the Total Vola-tile results. However, by not including the higher temperature(negative difference) results in a more accurate tar yield and allowsthe evaporated tar yield to be obtained. Rather than be counted asa drawback, the ability to precisely account for the phenomena is

00 600 700 800 900 1000ature, C

CHON (x100)

ction of original biomass weight. Calculated from modes 1 and 2.

0

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1

1.2

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Cha

r Ele

men

tal

Frac

tion

(inst

anta

neou

s)g/

g ch

ar

CHON (x100)

Fig. 11. Instantaneous elemental composition of the biochar fraction during pyrolysis. Calculated from modes 1 and 2.

Table 2Summary of pyrolysis yields from experiments to 1000 �C (from modes 1 to 4).

Sum Total yields Measured Standarda Errore

wt% ad % daf wt% ad wt% ad ±wt% ad

H2O 8.6 6.4Total volatiles 80.3 82.5 79.3 0.8Biocharb 17.0 17.5 17.29 13.7 0.1Gas 15.7 16.1Tar 63.6 65.4Ash 0.61 0.6sum V + C + H2O 105.9Tar onlyc 11.7H2Od pyrolysis 5.1Residual Tar burn off 0.17

a Taken from [13].b Organic combustible fraction, does not include ash.c Indicates portion of collected tars re-analysed.d Generated during heating above 200 �C, collected with tar fraction.e Determined by three repeat analyses of DETA modes 1 and 2.

R. Stanger et al. / Fuel 108 (2013) 656–667 663

considered significantly more accurate than other methods. Table 3summarises the elemental constituents of each pyrolysis stream,while Table 4 provides the same information as an elemental par-tition (e.g. percentage carbon reporting to char or tar). The carboncontent appears to be lower than the standard analysis by �3.5%while the oxygen appears higher. It should be clarified here thatthe standard oxygen is defined as a difference between startingmass and the integrated sum of CHN. With the DETA technique,this difference is between the measured O2 consumed and the inte-grated oxygen contained in CO2/CO/H2O/NO/NO2/SO2. Also of noteis that this method does not seek to replace the standard methodsof proximate and ultimate analysis but rather to show the relativeaccuracies. Given that all of the DETA results are from integratedcombustion gas analysis, the similarities between the tests areremarkably close.

3.5. Utilisation of dynamic elemental thermal analysis for energybalance

The DETA technique has been demonstrated to produce aneffective means of balancing the mass and elemental compositionsof the various pyrolysis streams during heating. This section de-scribes how such information can be used to estimate the thermal

properties of the char fraction and the heat of combustion of thevolatile matter. The method suggested by Merrick [21] has beenused to convert the changing elemental composition of the char(shown in Fig. 11) into specific heat. This method is shown in thefollowing equations:

Cp ¼Ra

g380

T

� �þ 2g

1800T

� �� �ðJ=kg KÞ ð1Þ

1a¼ C

12:01þ H

1:008þ O

15:999þ N

14:1þ S

32:06ð2Þ

gðxÞ ¼ expðxÞðexpðxÞ�1

x

h i2 ð3Þ

where R is the gas constant (J/kmol K), T is temperature (�K), a isthe mean atomic weight of the char, CHNOS are the mass fractionsof the respective elements and g(x) is the function to define solidoscillation with temperature in two dimensions. Merricks worksuggests that the use of two dimensions, where one dimension isstrong and the normal plane is weak (i.e. x = 380/T and x = 1800/T, respectively), provides better accuracy to the standard Einsteinform of the equation with only one common frequency.

Fig. 12 shows the calculated result for the modelled Cp using theinstantaneous elemental composition of the char. Also on Fig. 12 isthe original biomass elemental composition as it would changewith temperature (i.e. without decomposition), and the specificheat on a original biomass basis as a comparison. There is a signif-icant difference between the ‘‘instantaneous biochar’’ basis and‘‘original biomass feed stock’’ basis, solely due to the change in bio-char yield (calculated from DETA). The modelled Cp (on a per bio-char basis) remains similar to the modelled Cp withoutdevolatilisation up to around 400 �C because the elemental ratiosremain similar, i.e. are evolved at the same rate. The main differ-ence is due to large gain in carbon content, which increases themean molecular weight of the biochar from 7.8 to 11.3 g/mol (froma in Eq. (2) above). This difference is largely due to the removal ofmolecular hydrogen and becomes significant after 500 �C. Not in-cluded in this calculation is the reactive heat of devolatilisation,however this has been shown to be relatively small [22]. Integra-tion of the specific heat curve (on an original biomass basis) pro-duces the amount of energy required to heat the biomass fromambient temperature to process temperature. Typically this heat

Table 3Elemental constituents of pyrolysis streams from experiments to 1000 �C.

Ultimate analysis for each fraction (%)

C H O N S

Total volatiles 35.8 6.9 57.2 0.08 0.00Biochar 99.4 0.5 0.0 0.06 0.00Gas 38.9 6.7 54.4 0.01 0.00Tar by diff 36.8 9.2 53.8 0.10 0.00Tar only 30.0 6.4 63.5 0.16 0.00Tar residual burnoff 100.0 0.0 0.0 0.00 0.00Total biomass 46.9 5.8 47.2 0.08 0.002Standard analysis 50.5 5.9 43.4 0.10 0.01Errora ±1.0 ±0.5 ±0.5 ±0.01 ±0.003

a Determined by three repeat analyses of DETA modes 1 and 2.

Table 4Partition of elements into pyrolysis streams from experiments to 1000 �C.

Elemental partition (%)

C H O N S

Total volatiles 38.3 44.1 51.8 46.6 50.0Biochar 22.5 0.7 0.0 6.7 0.0Gas 8.1 8.4 9.6 0.6 0.0Tar by diff 31.1 46.8 38.6 46.1 50.0

664 R. Stanger et al. / Fuel 108 (2013) 656–667

is derived from combustion of volatiles, though could also comefrom partial combustion of the char itself. To estimate the grosscalorific value (i.e. heat of combustion) of the total volatile andGas Only streams, the second correlation suggested by Merrick[21] was used (shown in Eq. (4)). This estimation method is typi-cally used for calculating the calorific value of coals/coke/chars,however for simplicity it has also been used with the elementaldata of the total volatile and Gas Only streams.

DhcðLHVÞ ¼ 32:76Cþ 121:54H� 14:88O� 1:8Nþ 4:42S

þ 4:42H0:3 ðMJ=kgÞ ð4Þ

The energy comparison between the heating requirements forthe conversion of biomass to biochar are compared against theheat released from both volatile gas streams in Fig. 13. This figureshows that the minimum temperature required to sustain thepyrolysis process is 325 �C. Below this temperature, the amountof energy produced from combusting the volatile matter is insuffi-cient to heat the biomass. If bio-oils are removed as product, and

0

500

1000

1500

2000

2500

3000

0 100 200 300 400Tempe

Spec

ific

Hea

t, J/

kg.K

instantaneous biochar basis

Fig. 12. Estimation of specific heat of the biochar through the combined DETA results (i.e.of original biomass CHNOS chemistry is also shown as a function of temperature (i.e.J/kgbiomass K).

the process heat is derived from gas combustion only, then theminimum sustainable temperature is 505 �C. Both temperaturesare marked on Fig. 13. Of course this temperature window is spe-cific to the amount of liquid moisture present in the sample (8.6%,Table 1), however such additional energy can easily be added usingthe heat of vaporisation of water. This difference is illustrated be-tween dry and wet biomass, shown in Fig. 13. A problem with thepyrolysis yields and elemental composition in Tables 2–4 is thatmost biochar plants would not run at 1000 �C, but at a much lowertemperature. To incorporate the data generated into a more spe-cific process, an example set of process parameters are given in Ta-ble 5. Here the process temperature is set to 500 �C (a typicalbiochar temperature) and the DETA results for total volatile, charand gas only combustion are integrated up to the desired processtemperature to derive the product yields for char, tar and gasstream. The char and bio-oil yields are 28.0% and 60.6% respec-tively, and the elemental compositions and heating requirementsare re-evaluated. The re-evaluated yield of biochar compares wellto other slow heating pyrolysis studies [23,24] of woody material,however the elemental composition calculated at 500 �C shows alower biochar-C and higher biochar-O than in other works[25–27]. This may be a result of using a heating rate of 25 �C/min, rather than specific agricultural biochar studies which areusually lower (e.g. 5–10 �C), which would tend to kinetically shiftthe evolution of volatiles to higher temperature. Figs. 10 and 11show that 500 �C marks a temperature at which significantchanges begin to occur to the biochar elemental composition. Thissuggests that elemental composition may be more sensitive toheating variations than char yield, and should be considered as akey variable in future work. Using the heat of combustion equationabove and the integrated CHNOS results, the predicted calorific va-lue for the bio-oil is 13.95 MJ/kg, which compares favourably withcommercial bio-oil 15.83 MJ/kg [28] and other laboratory studies17.6–19.2 MJ/kg [29]. In this case the energy derived from lightgas combustion is not enough to heat the biomass and some ofthe bio-oils must be used for combustion or produced fromanother source, such as natural gas or through partial Char com-bustion. Table 5 also shows this type of analysis for lower temper-atures as a comparison.

3.6. Economics of biochar and bio-oil

This section evaluates the potential revenue from a biocharplant. It is beyond the scope of this work to estimate a full

500 600 700 800 900 1000rature, C

original biomass basis, (includes char yield)

no chemical change

with chemical change

instantaneous biochar basis with devolatilisation). For comparison, the specific heatwithout devolatilisation) as is the specific heat on an original biomass basis (i.e.

0

2

4

6

8

10

12

0 200 400 600 800 1000 1200Temperature, C

Spec

ific

Ener

gy B

alan

ce, M

J/kg

bio

mas

s

Dry Biomass HeatingRequirements

Wet Biomass HeatingRequirements

Drying Energy

Calorific Value of Tar& Gas

Calorific Value ofGas

Excess energy for electricity generation

Insufficient energy liberated in volatiles for heating biomass

Excess energy for bio-liquids production

Only gases used for heating

Tars+ gases used for heating

Fig. 13. Evaluation of energetics surrounding pyrolysis; heating requirements of biochar production and heat released from volatile combustion.

Table 5Pyrolysis flow sheet and possible revenue streams.

Process temperature 400 450 500 �C

Char yield 51.0 34.9 28.0 wt% biomassC 49.7 49.3 55.7 wt% biocharH 6.2 6.2 5.3 wt% biocharO 44.0 44.5 38.9 wt% biocharN 0.083 0.085 0.085 wt% biocharS 0.002 0.000 0.000 wt% biochar

Crude bio-oil yield 43.7 56.6 60.6 wt% biomassC 36.7 41.0 38.6 wt% tarH 6.8 7.1 7.7 wt% tarO 56.4 51.8 53.6 wt% tarN 0.063 0.072 0.079 wt% tarS 0.002 0.003 0.003 wt% tar

Specific energy balanceBiomass heating 0.81 0.86 0.91 MJ/kg biomassCV total volatiles 5.39 8.61 9.32 MJ/kg biomassCV gas only 0.16 0.45 0.86 MJ/kg biomassCV tars 5.23 8.15 8.46 MJ/kg biomassCV bio-tar 11.97 14.41 13.95 MJ/kg tars

Excess fuelGas �0.65 �0.41 �0.05 MJ/kg biomassGas + Tars 4.58 7.74 8.41 MJ/kg biomass

Revenue streamsCarbon retained in biochar 0.25 0.17 0.17 kg Carbon/kg biomassBiochar CO2 equivalent 0.93 0.63 0.62 kg CO2 sequestered/kg biomassCO2 pricea 23 23 23 $/tonne CO2

Revenue from biochar 21.4 14.5 13.2 $/tonne Biomass

Engine efficiencya 10–30 10–30 10–30 %Electricity costa 0.1 0.1 0.1 $/kW hRevenue from volatile derived electricity 165–495 279–836 302–908 $/tonne biomass

Heavy fuel oil pricea 600 600 600 $/tonneEnergy differenceb 28.2 33.9 32.8 % of heavy fuel oilRevenue from crude bio-oil 73.9 115.1 119.1 $/tonne biomass

a Assumption for calculation.b Taken as 42.5 MJ/kg.

R. Stanger et al. / Fuel 108 (2013) 656–667 665

economic model for a biochar plant, principally because such mod-els must be location and process specific (i.e., operating costs). Thebiochar yield and its carbon content are combined to estimate theamount of carbon (and thus CO2 equivalent) sequestered. To deriveincome from biochar for carbon sequestration, the process mustfirst qualify for carbon credits which can then be traded as a com-modity, however for this exercise a price of AU$23/tonne CO2 hasbeen assumed, corresponding to Australia’s planned carbon priceset for implementation in 2012. Table 4 shows the amount of

income is AU$14.3/tonne biomass. This income is well below thepre-processing cost of $33/tonne calculated by Stucley [5]. To illicitwide scale adoption of biochar production a breakeven carbonprice of $58/tonne CO2 would be required, though this does notcover operating costs. However, the data contained in this workhas shown that the most significant pyrolysis stream is not biocharbut bio-oil. This can either be combusted in an engine or collectedfor re-sale. Table 4 gives a simple evaluation of both business cases.To produce electricity, the volatile streams must be thermally

666 R. Stanger et al. / Fuel 108 (2013) 656–667

treated to crack large tar species prior to combustion (as indicatedby the residual carbon in the tar, Fig. 9). For this scenario, the en-ergy required for gasifying volatiles has not been specifically ac-counted for but an engine efficiency range has been assumedinstead. Producer gas (gasified wood gas) has been shown to workin both spark ignition and diesel engines (with efficiency rangesbetween 10% and 30%) depending on operating conditions [30]and the income derived from renewable electricity has been calcu-lated at $0.1/kW h (typical Australian power prices). Further in-come could be derived from the sale of renewable energy credits(currently traded at $40/MW h in Australia) however the currentlegislation requires that only Large Scale Renewable Energy pro-jects above 100 kW can be considered and thus have not been in-cluded here. Overall, the income derived from the sale of electricityproduced from volatile combustion is between $303–909/tonnebiomass depending on engine efficiency. As a further comparison,the sale of crude bio-oil has been estimated using the heavy oil fuelprice of $600/tonne (based on 2009 prices in the US [31] correctedfor energy content. Dynamotive, a leading bio-oil producer claimsthat its product can be substituted for heavy fuel oil applications[28]. The income produced from sale of crude bio-oil is $119/tonnebiomass. This income is approximately half the revenue from elec-tricity production under the low efficiency engine conditions. Afurther income stream could be derived from the soil enhancingproperties of biochar, which could be as high as $200–600/tonnebiochar [32] (equivalent to $57–169/tonne biomass for this exam-ple). However, the large scale adoption of bio-char as a soil enhan-cer is yet to be achieved but would almost certainly result in lowermarket prices as biochar production capacity increases. Furtherstudy is needed on the addition of biochar to soils at a larger scaleto better quantify the ‘‘permanence’’ (i.e. stability) of material laiddown and its effect as a fertiliser, particularly if income is to be de-rived from the carbon sequestration and/or agricultural benefit.Under the currently proposed Carbon Farming Initiative in Austra-lia, the level of biochar stability must be estimated up to 100 yearsand a full life cycle analysis is not required because the carbonemissions from harvesting and transport would already accountedfor through the direct implementation of a carbon price (i.e. in-creased power and diesel costs).

Overall, the economics are clearly in favour of the utilisation ofbio-oil over the use of bio-char for sequestration. A recent article ofa commercial biomass processing plant in the USA concluded sim-ilar results [32]. Of all the possible income streams derived fromthe thermal processing of biomass, the sequestration of carbon asbiochar is the least affluent. This should not be considered a reasonnot to produce biochar, but rather that the economics are heavilydependant on volatiles released and thus an accurate method fortheir characterisation is critical to implementation. A recent tech-no-economic analysis of slow and fast pyrolysis found that bio-oil was the dominant economic pathway using fast pyrolysis, asopposed to slow heating biochar production [33]. However, thestudy did not consider electricity production from volatile release,instead assuming a natural gas equivalent price (which is currentlylow in the US). The results presented in this paper have shown thatthe temperature of the process and the treatment of volatiles has asignificant impact on the mass, energy and hence economic out-comes. As such, extreme care must be taken in making generalassumptions regarding feedstock behaviour in specific processconditions, particularly in the conversion from mass to energy.

4. Conclusion

A novel thermal analysis method has been demonstrated thatcan characterise the evolution of volatiles during pyrolysis intoquantitative elemental stream. Dynamic elemental thermal

analysis uses a downstream heated oxygen lance to convert thewide variety of tars and light gases into simple combustion prod-ucts which can be used to calculate the changing elemental com-position with time and temperature. This continuous analyticalmethod has been used on a Spotted Gum biomass sample to char-acterise the total volatiles, light gas, char and tar fractions bothdynamically and integrated to final yields. As such, it enabled themass and energy balance to be closed at any temperature.

The method gives the composition of biochar formed at differ-ent temperatures. For this particular biomass sample, the carbonof the biochar reached a limiting value at about 500 �C, represent-ing 17% of the carbon in the biomass.

The generated data was then utilised to predict the changingheat balance during thermal processing and the relative incomegenerated from the value of the biochar (based on a carbon price),crude liquid bio-oil or electricity production from gases. The reve-nue from biochar generated at a process temperature of 500 �C wasthe product of lowest value by an order of magnitude. The overallfinancial viability of pyrolysing biomass appears to be based on theutilisation of the volatile matter, be it from electricity generation orbio-oil production, and thus an accurate method for their charac-terisation and quantification is critical to implementation.

The DETA apparatus and methodology has proven to produce abench scale suite of highly accurate data that can be utilised inassessing the viability of a biochar production process.

Acknowledgements

The authors would like to acknowledge the Australian ResearchCouncil for providing funding for the development of this tech-nique. Jim Wilson and Lonn Cooper of the University of Newcastleare thanked for providing the electrical and mechanical support forits demonstration. We would also like to thank Professor John Bur-gess from the University of Melbourne and David Lau from PacificPyrolysis for their valuable input.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2013.02.065.

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