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Presented to 18thEuropean Biomass Conference and Exhibition, Lyon, 3-7 Mai 2010

COMBUSTION EVALUATION OF TORREFIED WOOD PELLETS ON A 50 KWTHBOILER

J.-B. Michel 1,4,C. Mahmed1, J. Ropp1, J. Richard2, M. Sattler3, M. Schmid31School of Business and Engineering Vaud, University of Applied Sciences Western Switzerland, 1401 Yverdon-les-Bains,

Switzerland2

HEPIA, University of Applied Sciences Western Switzerland, 1201 Geneva, Switzerland3Centre of Appropriate Technology and Social Ecology, Laboratories for Sustainable Energy Systems, Langenbruck,

Switzerland4Corresponding author Ph: +41265577594, Fax:+41265577579,

e-mail: [email protected]

ABSTRACT: Torrefied wood pellets are produced from torrefied chips by thermo-chemical pre-treatment of biomass at200-320C in the absence of oxygen during about 15-30 minutes. Overall, the torrefaction process efficiency has been

reported to be 90-95% % as compared to 84% for pelletisation. Torrefaction improves the biomass: 30% higher calorificvalue and 50% higher energy density resulting in much lower handling and transport costs. The fuel becomes

hydrophobic making long term outdoor storage possible. The purpose of this project was to compare the combustion andemission characteristics of torrefied vs. normal wood pellets.

With no modification to the feeding and the burner parameters, the ignition and combustion characteristics of

torrefied pellets are found very similar to those of normal pellets. Particulate emissions per energy output were foundvery close and directly related to the ash content in the feedstock. Using the Taguchi approach, it was possible toestablish a model of the boiler performance as a function of the input parameters. Further testing confirmed the validityof the model showing optimum performance with a defined value of primary and secondary air flow rates whichminimized particulate emissions for both the normal pellets and the torrefied pellets.

Keywords: biomass, torrefaction, combustion, boiler.

1 INTRODUCTION

Torrefied wood pellets are an attractive fuel for co-combustion in coal-fired power stations[ 1 ]. Except for

start-up, the process is autothermal (it generates its own

energy due to mild pyrolysis reactions) and the energy ofthe off-gases, which represent about 10% of the input

energy, is recovered. Overall, the process efficiency hasbeen reported to be 90-95% % as compared to 84% for

pelletisation in one given set of operating conditions)

[ 2 ].The purpose of this R&D project is to compare the

combustion and emission characteristics of torrefied

wood pellets with those of normal wood pellets.Although there are a large number of publications

regarding the torrefaction process itself, this is the first

comprehensive study on the combustion properties for

domestic heating applications and on a complete lifecycle analysis including the combustion part. The resultsare also relevant for cogeneration applications.

2 BIOMASS TORREFACTION REVIEW

Torrefied wood was used during the early years ofsteel production as a reducing agent in blast-furnaces and

was afterwards replaced by charcoal and coke [ 3 ].

The process is rather simple and involves anaerobicheating of dried biomass chips as shown in Figure 1.Several reactor types are used depending on the

proprietary process. The ECN BO2process uses a verticalmoving bed countercurrent with recirculated flue-gas.

The temperature is about 240C with a residence time

of sabout 20 minutes. Topell use a cyclone type swirling

flow (entrained flow) and temperatures up to 350 C witha much lower residence time (about 90 seconds) and fastquenching of the torrefied chips. Airless technology(Airless web-site) use a rotary drum reactor, a technology

that has evolved from the ceramic drying technology. Thedrying and torrefaction technology operates by creating

superheated steam generated solely from the moisture

contained in the biomass.The work of Prins [ 4 ] demonstrated that the mass

yield during torrefaction is typically contains 70% while

the energy yield is about 90% of the original energy

content. No moisture is left following torrefaction but thetorrefied biomass may uptake 6% of moisture from the

ambient air.In 1985, Pechiney built a 10000 t/y production plant,

to use torrefied wood instead of charcoal in electric

furnaces (5 Peguret, 1986).

Figure 1 - Simplified process description

This new type of fuel is very promising because it

alleviates a lot of the disadvantages of normal biomasspellets:

The volumetric energy density is 50% higher thanwith normal pellets resulting in the same reduction of

handling and transport cost per energy output. Grinding energy is reduced by 90% and overall, the

process efficiency has been reported to be 90-94%as compared to 84% for pelletisation)

Drying to about

20% moisture

Anaerobic heatingbetween 240-320C

Autothermal process

Flue gas recyclingand postcombustion

Raw biomass chips

TorrefiedBiomass

pellets

Mass yield ~70%Energy yield ~90%10% left is partly recoveredLCV increase by ~ 20%

grinding

pelletisation

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[ 2 ]. See Figure 2.

Torrefied biomass is hydrophobic and therefore notsubject to swelling and degradation allowing outdoor

storage and in the long term. Its greater calorific value should be beneficial for

combustion.

Figure 2 - Process efficiency comparison. Normal pellets(top), torrefied pellets (bottom) after Uslu et al. (2008)

Several large scale production plants are planned or

in construction in Europe and elsewhere, for the co-

combustion of torrefied wood in coal-fired powerstations: Energy Center of the Netherlands, BO2 process :

large demonstration plant foreseen (6 Kiel, J et al., 2008) Atmosclear (Switzerland) large projects planned from130 to 270 kt/y[ 7 ]

Integro Earth Fuels, Wyssmont process, USA, 84 kt/y

Roxborrow, NC[ 8 ] Topell, NL , Polow Torbed reactor technology,planned 60 ktons/y in Arnhem (NL) together with RWE[ 9]

4Energy Invest (B), 38 kt/y in Ambleve (B) and

Stramproy[ 10 ] Essent trading (RWE) and Stramproy : 90 kt/y inSteenwijk (NL)[ 11 ]

However, there was so far no project directly targeted

to domestic heating and cogeneration.

3 COST ANALYSESSeveral economic comparisons have shown the

benefits of using of torrefied pellets instead of normalpellets. The table below provides a comparison of the

cost of pellets for power generation with biomass fromCanada and from South-Africa shipped to Europe.

Hamelink [ 12 ] reported that feedstock costs

contribute around 2065% of the total delivery costwhereas pre-treatment and transport contribute 2025%and 2540%, respectively, depending on the location of

the biomass resources.

According to Uslu)

[ 2 ] TOP pellets can be delivered at costs as low as 3.3/GJ (73.5 /ton) with a biomass cost of 10 /ton as

compared to 3.9 /GJ (66.3 /ton) for normal pellets.

This is mainly due to higher energy density compared toconventional pellets, which lowers both the road and seatransport costs. This is also in agreement with the work of

Peng[ 13 ] for pellets processed in South-Africa with theECN process and transported to Europe. The comparisonwith pellets produced in Vancouver and processed in

Europe after Herold[ 14 ] is presented in Table I.

Similarly Kiel [ 15 ] reported delivery costs for

sawdust pellets supplied to North-West Europe: 4.7 /GJfor torrefied and 5.9/GJ for normal pellets which

confirms the economic advantage of torrefied pellets.

Table I:Pellet costs from various sourcesCost item

Source 2

[ 14 ]

VancouverEurope

Source 1[ 13]S-AfricaEurope

Sawdust case

Productioncapacity(ktons/y)

40 80 56

Product Pellets Pellets

Torrefied

pellets(ECN)

Costs in /ton product

Rawmaterial

23.6 11 15

Production 70 41 45

Transport 62.6 54 42

Margin 23.9

Total

(/ton )180.1 106 102

(/GJ) 11.2 6.61 4.99

4. BIOMASS PREPARATION AND COMPOSITIONAbout 1 ton of torrefied pellets have been prepared

for our tests by ECN on their 100 kg/h pilot facility,

using poplar as the feedstock.

Table II:Composition of raw and torrefied chips (ECNdata)

Parameter Unit

Rawpoplar

chips

TorrefiedPoplar

Chips

Length/width/height Mm 40/30/10 40/30/15

Water % (m/m)om 9,23 4,8

Ash % (m/m)om 0,51 0,56

Calorific value, upper MJ/kg dm 18,7 19,8Sulfur % (m/m)om Nm Nm

Nitrogen % (m/m)om < 0,1 < 0,1

Arsenic mg/kg dm < 2,5

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Type C1: a mixture of resinous and leafy trees.

Type C2: poplar, in order to get a more representative

comparison with BO2pellets made from poplar.The compositions of the raw and torrefied chips

(ECN data) are given in Table II. The analysis of C1

pellets and ECN pellets was also carried out by alaboratory in Germany according to DIN standards.

Results are given in Table III.

We can observe some discrepancy between themeasurements of BO2 material (chips and pellets),especially for the ash and chromium content. Accordingto ECN, since the chips are not 100% homogeneous, it is

very difficult to get representative samples. Differencesin composition can therefore be explained by the

inhomogeneous character of the biomass. The ash contentof C2 pellets was found to be 3.2%, i.e. three timeshigher than the other feed-stocks.

Table III:Composition of C1 Swiss pellets and torrefied

ECN pellets

Parameter Unit

Rawpellets C1(best

pellets)

Torrefied

pellets

Length Mm 19,5 18,5

Diameter Mm 6,0 6,7

Gross density kg/dm3om 1,18 1,13

Water content % (m/m)om 7,4 / 8,2 5,6 / 5,9

Volatiles % (m/m)om 17,3 21,8

Ash content % (m/m)om 0,97 1,14

Calorific value,upper MJ/kg dm 18,91 19,82Abrasion

(lingo tester) % (m/m)om 2,3 2,8

Sulfur content % (m/m)om 0,014 0,011

Nitrogen content % (m/m)om

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sample, one can reconstruct a three-dimensional

representation of the object.The images obtained are shown onFigure 5.It is here

confirmed that torrefied pellets are more homogeneouswith a more regular assembly of wood fibers. This

explains the higher density of torrefied pellets.

Figure 5 Confocal Microscopic images of the samples,

left: C1 pellets, right: BO2 pellets

4. COMBUSTION TESTS4.1 Test set-up

Combustion tests were carried-out on a 50 kW pelletboiler of the company Hoval shown schematically inFigure 3.

Figure 6:Schematic of the 50 kW Hoval Biolyt boiler

A forced draught burner is used on this boiler (and

not a grid or a drum), allowing a rather accurate controlof primary and secondary combustion. A scanningmobility particle sizer (SMPS) was applied to determine

the size distribution and the total number concentrationsof particles in the range from 0.01-0.400 m. Exhaust gasis taken with a probe, which is also fed with particle free

air. The resulting dilution factor is adjusted by the flow

rate of the diluting air and the total flow. To preventcondensation of water onto the particle surface, thedilution factor is chosen high enough, to achieve a dewpoint below ambient temperature[ 16 ].

The design of experiment method from Taguchi wasused to reduce the number of tests to a minimum while

exploring the complete space of variables. In this case we

defined 4 variables with a 9*4 test matrix (Table IV).

Figure 7:photograph of the sampling system

Table IV:Taguchi test matrix (L9)

Test

N

Pellets

Type

Secondary

airSetting

Primary air

setting

Screw

setting

1 C1 35% 35% 30%

2 C1 45% 40% 50%

3 C1 60% 45% 65%

4 T 35% 40% 65%

5 T 45% 45% 30%

6 T 60% 35% 50%

7 T 35% 45% 50%

8 T 45% 35% 65%

9 T 60% 40% 30%

N.B. full load with C1 pellets, 50kW is obtained with a

screw setting of 65%

Gas sampling was done with three measurements perminutes during an average 16 minutes period (48 samplesper measurement). Total particulate matter emissions(PME) were sampled using a disk filter following the EN

13284-1 standards. TPE samples were extracted

isocinetically from the flue gas duct by a 90 offsetstainless steel probe. The main filter and backup filterwere heated at 120C during PME sampling. Particulate

sampling was done on a 16 minutes period. Themeasurements were repeated three times for each test.

This first series of experiments demonstrated that a

secondary air level of 55% and a primary air level of 45%

(of the fan range) were optimum in terms of combustion

efficiency and emissions. These levels were fixed in alatter series of experiments with a variation of the loadand of the excess air. Particle size distribution of the fly

ash was measured for both C2 and BO2 pellets.

4.2 Combustion test resultsAt first, the combustion behavior of the torrefied

pellets was found very similar to that of the normal

pellets:

The warm-up period was slightly reduced The mass flow of the torrefied pellets had to bereduced by about 10% to achieve the same energy input

The optimum settings of primary & secondary airflows in terms of emissions were identical (55% / 45%).

A model of the flue gas and particulate emissions as afunction of the three input variables (load, primary and

secondary air settings) was established so that resultscould be interpolated and plotted for the same settings.The model was found very accurate as shown inFigure 8.

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Figure 8: Comparison of measured data and calculatedmodel data.

As a result of this interpolation, the comparison of theflue-gas emissions of torrefied (T = BO2) and classical(C1) pellets is given inFigure 9 where data are plotted as

a function of the load and for NOx as a function ofsecondary air as well.

Particulate emissions are about 30% higher and this isdue to the lower ash content of C1 pellets. However, one

should normally expect similar fly ash emissions for thesame load input as with the original poplar from ECN

(not tested) since the ash content, expressed per MJ

(LCV) should not be increased by torrefaction. On thecontrary, it should be decreased with an increase of LCV.

One can also see that torrefied pellets can producesignificantly less NO than classical C1 pellets.

Figure 9: Compared flue-gas emissions of C1 and T(BO2) pellets

However one cannot draw a definite conclusion from

this except to state that fuel nitrogen content wascertainly much lower in the BO2 pellets than in the C1

pellets. One could expect lower NOx emissions

depending on the amount of fuel nitrogen that has beenreleased during torrefaction.

Similarly to previous, the comparison of the flue-gas

emissions of torrefied (T) and poplar C2 pellets is giveninFigure 10 where, in that case, both interpolated model

data and measured data are represented. One can see thegood agreement between the two sets of measurements

and the reproducible burner operation (O2 = f(P)). Asexpected, C2 pellets produce three to four times more fly

ash emissions than C1 pellets at high load conditions, due

to their three times higher ash content.Also, torrefied pellets can potentially produce less

CO than classical pellets making it possible to reduce the

excess air, thereby increasing the thermal efficiency.

Figure 10: Compared flue-gas emissions of C2 and T(BO2) pellets

4.3 Particle size measurementsSMPS data were collected for both C2 and BO2

pellets. Overall results are shown in Table V.

Table V: SMPS data for BO2 and C2 pellets

Typ

e

Pin

kW

Conc.mg/Nm

13% O2

Number/

cm

Modal

sizenm

Av.

sizenm

Std.

dev.nm

T46,6

60 4,36E+08 57,860,0

1,54

T47,7

55 3,89E+08 59,359,8

1,56

T26,1

87 3,20E+08 57,360,5

1,58

T26,

184 3,15E+08 52,8

58,

8

1,5

8

T54,8

58 4,07E+08 55,857,0

1,52

T55,5

59 4,36E+08 58,856,8

1,54

C248,3

97 3,59E+08 67,066,0

1,57

C248,

5100 3,56E+08 61,8

67,

0

1,5

6

C221,1

208 3,17E+08 63,567,5

1,54

C222,9

208 3,22E+08 59,866,5

1,55

C242,

3138 3,36E+08 66,3

69,

0

1,5

6

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C253,

3140 3,72E+08 67,5

70,

5

1,5

6

One can see that, although the total particulateemissions from C2 pellets are much higher than those of

BO2 pellets, their sizes are finer and there number largerthan those of BO2 pellets.

This is also illustrated in Figure 11: there are morefiner particulates coming from the torrefied pellets,

contributing to an overall greater number of particulates.

Figure 11: comparison of the size distribution of C2

pellets (white) and BO2 pellets (red) for an input powerof 48 kW

5. LIFE CYCLE IMPACT ASSESSMENT

The comparison of the overall environmental impactsof the two biomass fuels was performed using the Impact2002+ life cycle impact assessment (LCIA) method[ 17 ]

This method was developed by O. Joliet and his team

and it considers 14 mid-point categories of impact and

four damage categories (human health, ecosystemquality, climate change, resources) as shown in Figure12.

Figure 12: Overall scheme of the IMPACT 2002+framework. By courtesy of Prof. O. Jolliet,Environmental Health Sciences Associate Director,

University of Michigan Risk Science Center

The functional unit was the MJ of heat produced by the

boiler. Results are summarized in the following tableshowing an overall gain of 50% mainly due to the

improvement of the overall process efficiency.

5 CONCLUSIONS

From this study, it becomes clear that torrefaction is aninteresting solution if one considers to use pellets for

power and heat production: this can lead to more valuefor the effort invested in terms of resources (primary

energy, finance, costs) and a reduced environmentalimpact. Also, torrefaction makes it possible to use

alternate form of woody biomass residues.

However, despite the projects announced for largeplants, no plant operation data has been made availableyet. Further experimental work at pilot size will be

needed to better characterize and optimize the wholeprocess operating with a variety of biomass residues.

6. REFERENCES

[ 1 ] Maciejewska A. et al. Co-firing of biomass withcoal: constraints and role of biomass pre-treatment.

European commission report, DG JRC, Institute for

Energy, EUR 22461 EN (2006)[ 2 ] Uslu A., Faaij A.P.C., Bergman P.C.A. Pre-

treatment technologies, and their effect on

international bioenergy supply chain logistics.Techno-economic evaluation of torrefaction, fast

pyrolysis and pelletisation. Energy, Volume 33,Issue 8, August 2008, Pag 1206

[ 3 ] Annales des Mines, Troisime Srie, Tome XII.Recueil de mmoires sur lexploitation des mines et

sur les sciences et les arts qui sy rapportent, chezCardillan-Goery diteur libraire. Paris (1857).Available at http://books.google.com

[ 4 ] Prins M.J.. Thermodynamic analysis of biomass

gasification and torrefaction. Ph.D. EindhovenTechnical University, (2005) The Netherlands.

[ 5 ] Peguret A. Le bois torrfi: cots et position parrapport aux autres combustibles. Rapport AFME

85-91-1001, (1986) N INIST 10128404

[ 6 ] Kiel, J et al.BO2-technology for biomass upgradinginto solid fuel pilot-scale testing and marketimplementation. 16th European Biomass

Conference & Exhibition. (2008), Valencia, Spain

[ 7 ] Atmosclear web site: www.atmosclear.com(accessed 22.03.10)

[ 8 ] Integro Earth Fuels web site:

www.integrofuels.com (accessed 22.03.10)

[ 9 ] Maaskant, E. Topell on torrefaction. IEABioenergy Task 32, New Biomass Co-firingConcepts. Hamburg, (2009)

[ 10 ] 4Energy Invest web site: www.4energyinvest.com

(accessed 22.03.10)

[ 11 ] Essent trading web site: www.essent.eu (accessed22.03.10)

[ 12 ] Hamelinck CN, Suurs RAA, Faaij APC. Techno-

economic analysis of international bio-energy trade

chains. Biomass Bioenergy. 2005;29(2) pag. 114

[ 13 ] Peng J, et al. Study of Torrefaction for theProduction of High Quality Wood Pellets. CSBE

50th Annual Conference. North Vancouver, B.C.,

Canada (2008).

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[ 14 ] Herold, I. Biomass and Waste to Energy: Trends in

Investment in the EU. Biomass Industry Day.Hamburg (2009).

[ 15 ] Kiel, J. Torrefaction for biomass upgrading intocommodity fuels, IEA Bioenergy Task 32

workshop, Fuel storage, handling and preparationand system analysis for biomass combustion

technologies, Berlin (2007).

[ 16 ] Wieser U. and Gaegauf C.K., 2000. Nanoparticleemissions of wood combustion processes.1stWorldConference and Exhibition on Biomass for Energy

and Industry, June 2000. Available atwww.oekozentrum.ch/files/nanoparticles.pdf(accessed 22.03.10)

[ 17] Jolliet O, Margni M., Charles R., Humbert S. ,Payet J. , Rebitzer G. and Rosenbaum R. IMPACT

2002+: A new life cycle impact assessmentmethodology, The International Journal of Life

Cycle Assessment, Vol. 8, N 6 / Nov. 2003, Pages324-330

7. ACKNOWLEGEMENTS

The authors gratefully acknowledge the financial support

from the University of Applied Sciences WesternSwitzerland, the provision of torrefied pellets fromEnergy Center of the Netherlands and the supply of the50 kW Biolyt boiler from Hoval.

8. LOGO SPACE
http://www.oekozentrum.ch/files/nanoparticles.pdfhttp://www.oekozentrum.ch/files/nanoparticles.pdfhttp://www.oekozentrum.ch/files/nanoparticles.pdf

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