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Study on Catalytic Combustion ofBiomass Mixtures with Poor CoalsDespina Vamvuka a , Victoria Tsamourgeli a & Michael Galetakis aa Department of Mineral Resources Engineering , TechnicalUniversity of Crete, University Campus , Hania , GreeceAccepted author version posted online: 10 Oct 2013.Publishedonline: 18 Dec 2013.

To cite this article: Despina Vamvuka , Victoria Tsamourgeli & Michael Galetakis (2014) Study onCatalytic Combustion of Biomass Mixtures with Poor Coals, Combustion Science and Technology, 186:1,68-82, DOI: 10.1080/00102202.2013.846331

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STUDY ON CATALYTIC COMBUSTION OF BIOMASSMIXTURES WITH POOR COALS

Despina Vamvuka, Victoria Tsamourgeli, andMichael GaletakisDepartment of Mineral Resources Engineering, Technical University ofCrete, University Campus, Hania, Greece

The effect of catalyst addition on ignition and combustion characteristics of biomass fuels

from the Mediterranean region, one lignite and lignite/biomass blends, were investigated.

Transition metal oxides and alkali/alkaline earth compounds were used as catalysts. The

experiments were conducted in a thermogravimetric analysis system. The performance of

the process was evaluated in terms of combustion rate, ignition, and combustion indices.

The results showed that metal oxides enhanced the ignition and combustion performance

of lignite, showed no catalytic activity to burn off of biomass fuels, but influenced the

temperature sensitivity of their reaction in air. The relative activity was in the following

order: V2O5>Fe2O3>CaO>NaCl>MgO>CuO>TiO2>LiCl. The combustion characteris-

tics of the blends followed those of parent fuels in an additive manner. Blending lignite with

biomass fuels increased its thermochemical reactivity. In terms of burnout, catalyzed

combustion of blends was further improved.

Keywords: Biomass; Blends; Catalysts; Combustion; Lignite

INTRODUCTION

In view of the energy crisis worldwide, renewable energy fuels, such as residuesfrom agricultural production and urban wastes, are becoming important, as theyoffer a remarkable potential to reduce greenhouse emissions, replace part of conven-tional fossil fuels, and increase economic returns to rural communities.

Co-combustion of solid residues and wastes with coal in existing power plantsis a most interesting option, because apart from the environmental benefits, it offerstechnical and economic benefits (Vamvuka, 2009) by replacing part of conventionalenergy sources, while at the same time using existing infrastructures (EuropeanBioenergy Networks, 2003). The knowledge of the behavior of coal and biomassfuels during combustion, not only separately but also together, is essential for theeffective operation of the conversion units, as interactions may occur between them

Received 4 April 2013; accepted 16 September 2013.

Address correspondence to Despina Vamvuka, Department of Mineral Resources Engineering,

Technical University of Crete, University Campus, Hania 73100, Greece. E-mail: vamvuka@mred.tuc.gr

Color versions of one or more of the figures in the article can be found online at www.tandfonline.

com/gcst.

Combust. Sci. Technol., 186: 68–82, 2014

Copyright # Taylor & Francis Group, LLC

ISSN: 0010-2202 print=1563-521X online

DOI: 10.1080/00102202.2013.846331

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that may affect the overall efficiency of the process (Hupa, 2005; Senneca, 2008;Vamvuka and Sfakiotakis, 2011). The much higher volatile matter content of bio-mass fuels may require modifications of the combustion systems, along with otherfactors. Also, ignition performance of solid fuels determines the optimal locationfor their injection into industrial combustors, as well as the inlet air temperatureand, together with volatiles’ behavior, the handling and feeding of materials, in orderto avoid catastrophic events.

Catalytic combustion of coal, carbon, or soot has been extensively studied overmany years (McKee, 1970; Sui and Yu, 2008; Zhang and Zou, 2006). Alkali, alkalineearth, and transition metals in the form of salts or oxides have demonstrated highercombustion performance by decreasing ignition temperature and increasing combus-tion rates (Gong et al., 2010; Le Manquais et al., 2011; Wu et al., 1998). The catalyticeffect is considered to arise from the improved oxygen transfer behavior, which iscaused by the oxygen storage and redox properties of the metal oxides and the firstionization energy of the metal in the case of salts, during the combustion process(Farrow et al., 2013; Gong et al., 2010; Wu et al., 1998).

On the other hand, the catalytic combustion of biomass fuels is very limited inthe published literature. Alkali, alkaline earth, and some transition metals havebeen evaluated in previous investigations as catalysts for rice or wheat straw andmunicipal solid wastes (MSW) (Shen and Qinlei, 2006; Wang et al., 2011; Zhaoshenget al., 2009).

However, there is lack of information on the effect of different catalysts on theignition and combustion behavior of coal=biomass mixtures. Issues such as whethercatalysts that are suitable for coal combustion will be suitable for biomass combustionor their blends, have not been examined before. Therefore, the aim of this work was toinvestigate the ignition and combustion characteristics of different biomass materials,typical of South European countries, and one lignite (as an indigenous fuel of Greece,it is very important for the national economy), as well as mixtures of them in the pres-ence of catalysts. The thermogravimetric analysis technique has been used, as it hasbeen proved (Farrow et al., 2013; Shen and Qinlei, 2006; Vamvuka and Sfakiotakis,2011; Wang et al., 2011; Zhaosheng et al., 2009) as a valuable method for obtainingdetailed information about reaction rates, burning time, and temperatures and anyinteractions between individual blend constituents. The results should be useful forfurther design and operation of co-firing units, and may provide important infor-mation for promotion of biomass materials as co-fuels for energy generation.

EXPERIMENTAL

Materials and Equipment

Different biomass materials from the Mediterranean region, which are abun-dantly found at low or no cost, such as olive tree pruning, cardoon, and sewagesludge, and one lignite from Mavropigi power plant in Northern Greece were chosenfor this study. Olive tree pruning was provided by the Institute of Olive Tree and Sub-tropical Plants of Chania (Crete), cardoon from a field trial in the area of Thessalia,and sewage sludge from the waste water treatment plant of the city of Chania. Blendsof lignite with biomass samples at biomass percentages up to 30%wt, which are typical

CATALYTIC COMBUSTION OF BIOMASS MIXTURES 69

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of co-firing applications in European industry (Luts et al., 1999), were also prepared.After taking representative samples for initial moisture determination, the materialswere air-dried, homogenized, riffled, milled (olive tree pruning in a cutting mill, modelPulverisette 15 of Fritsch and sewage sludge and lignite in a ball mill, Sepor Inc.,USA), and sieved to a particle size of �250 mm.

Analytical, reagent-grade LiCl, NaCl, MgO, CaO, TiO2, V2O5, Fe2O3, and CuOwere used as catalysts (materials with catalytic effect, not synthesized catalysts) in theexperiments. Catalyst loading was 10%wt of each sample. In certain experiments, itwas varied between 2.5 and 10%wt. The incipient wetness method was used forimproved catalyst dispersion on the biomass surface. Impregnation was carried outby mixing each fuel with appropriate amounts of catalysts and adding de-ionizedwater, in such a way that all the material remained perfectly wetted. The mixturewas stirred for 24 h, and then it was dried in the oven at 110�C.

Characterization of the biomass fuels was performed according to the Europeanstandards drawn up by the Technical Committee CEN=TC335, while for lignite therelevant ASTM standards were employed. Proximate analysis was carried out usingprogrammable laboratory furnaces (EN14774, EN15148, EN14775, ASTM D5142),ultimate analysis using Leco type analyzers CHN-600 and S532-500 (EN15104,EN15289, ASTM D5373, D4239), and calorific value using a Leco AC-300 typecalorimeter (EN14918, ASTM D5865).

Combustion tests were performed in the range 25–850�C, in a Perkin Elmerdifferential thermogravimetric analyzer (TGA-6=DTG), as described in a previouswork (Vamvuka and Sfakiotakis, 2011). The air flow rate was 45mL=min, and theheating rate was 10�C=min. Preliminary tests with different sample masses and sizesand gas flow rates were carried out in order to check the influence of heat and masstransfer. Small masses (�20mg) of each material, thinly distributed in the cruciblewith particle sizes of �250 mm were found to be best for the experiments, in orderto eliminate the effects of eventual side reactions and mass and heat transfer limita-tions. In addition to combustion experiments, pyrolysis tests under the same con-ditions, using nitrogen as the reactant gas, were performed in order to investigatethe behavior of volatiles and char combustion. The experiments were replicated atleast twice to determine their reproducibility, which was found to be very good.

Measurement of Combustion Parameters

The thermograms were analyzed to determine the relevant combustion para-meters, such as the ignition and burnout temperatures and the corresponding times,as well as the maximum temperature and rate of weight loss and the correspondingtime. The ignition temperature was defined according to Li et al. (2006).

In order to evaluate ignition and combustion performance of the fuels, anignition index (D) and a combustion index (S) were calculated according to thefollowing equations:

D ¼ Rmax=ðtm � tiÞ ð1Þ

where Rmax is the maximum combustion rate, tm is the time that corresponds tomaximum combustion rate, and ti is the ignition time that corresponds to ignition

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temperature Ti (Li et al., 2006) and

S ¼ Rmax �Ra=ðT2i � TbÞ ð2Þ

where Ra is the average mass loss rate over the whole temperature range and Tb isthe burnout temperature (Luo et al., 2009).

RESULTS AND DISCUSSION

Combustion Characteristics of Individual Fuels

Table 1 compares the proximate and ultimate analysis results, as well as the cal-orific value of the fuels. As can be observed, olive tree pruning and cardoon displayhigher amounts of volatiles than sewage sludge and Mavropigi lignite, ranging from�73–78%wt and low ash contents 0.8 and 8%wt, respectively. Sewage sludge andMavropigi lignite have very similar volatile contents (�58%wt) and high ash contents(�33%wt and 20%wt, respectively). The carbon and hydrogen contents of olive treepruning and cardoon are higher than those of lignite, while ash content is lower,resulting in a higher calorific value. Also, sulfur and nitrogen concentrations of thesefuels are very low, indicating that SOx and NOx emissions during combustion are notof concern. However, the percentages of sulfur and nitrogen of sewage sludge arehigh, which might be a problem if combustion of this material alone is considered.

The TGA=DTG profiles of the samples in air are illustrated in Figures 1 and 2.As can be seen, the DTG curves in Figure 1 are separated into two stages, with thefirst one in the range of 200–400�C and the second stage in the range of 400–550�C. Olive tree pruning and cardoon present bimodal DTG curves in the lower tem-perature region, reflecting the existence of structures of different reactivity. Accordingto previous work by the authors (Vamvuka and Sfakiotakis, 2011) and others, it isgenerally agreed that the mass loss during the first stage is attributed to the totaldecomposition of hemi-cellulose, almost total decomposition of cellulose, and partialdecomposition of lignin, the thermal degradation of which is known to take placeover a wider temperature range (Vamvuka et al., 2009; Vamvuka and Sfakiotakis,2011). The second stage corresponds to the decomposition of the remaining ligninand combustion of the residual volatiles and char. The most reactive fuel duringthe low-temperature regime is olive tree pruning, while the most reactive fuel duringthe high-temperature regime is cardoon, taking into consideration that the peak

Table 1 Proximate analysis, ultimate analysis (%dry), and calorific value of the fuels (dry basis)

Sample

Volatile

matter

Fixed

carbon Ash C H N O S

HHV

(MJ=kg)

Olive tree pruning (OL) 77.9 21.3 0.8 48.5 5.3 0.7 44.7 – 19.1

Cardoon (CAR) 72.6 17.2 8.2 41.3 6.1 0.8 41.5 0.1 21.5

Sewage sludge (SS) 57.6 9.7 32.7 35.0 6.1 4.5 19.8 1.9 14.8

Mavropigi lignite (MLIG) 57.7 22.3 20.0 42.7 3.3 1.7 31.4 0.9 15.9

HHV: higher heating value; OL: olive tree pruning; CAR: cardoon; SS: sewage sludge; MLIG:

Mavropigi lignite.

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height is directly proportional to the reactivity, while the temperature correspondingto peak height is inversely proportional to the reactivity (Zheng and Kozinski, 2000).

Figure 3, which compares pyrolysis and combustion profiles of cardoon,clearly indicates that combustion of fuel volatiles occurs during the first mass lossstage. TGA data showed that �80–90% of volatile matter of all biomass materialswas burned up to 400�C, i.e., during the first stage.

On the other hand, sewage sludge and lignite exhibit different combustion beha-vior to the woody biomass residues, as shown in Figure 2. For sewage sludge, which isa more heterogeneous material in nature, it is evident that the second stage,mentioned above, extends up to 750�C. Combustion of residual volatiles and charoccurs with a high rate between 400�C and 550�C and a slow rate between 550�Cand 750�C. Mavropigi lignite consists of two components with different reactivity,a high combustibility material between 300�C and 500�C, where the majority of

Figure 1 TGA=DTG profiles of (a) olive tree pruning and (b) cardoon in air.

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volatile constituents were released and burned and a low combustibility materialbetween 500�C and 750�C. Therefore, it is interesting to investigate whether biomassmaterials could increase the combustion rate of the lignite upon blending.

Table 2 summarizes the combustion characteristic parameters of the fuelsunder study. From these results, it is clear that the ignition temperature of biomasssamples is lower than that of lignite. This is attributed to the greater amount of vola-tiles and the energy released by them, as well as the higher H=C ratio (Table 1) ofthese biomass fuels. The earlier ignition of sewage sludge than any other fuel canbe explained by the fact that this material contains more easily decomposed compo-nents and has the highest H=C ratio. Similar correlations have been found by otherinvestigations (Jeguirim et al., 2010; Muthuraman et al., 2010; Varol et al., 2010).

Figure 2 TGA=DTG profiles of (a) sewage sludge and (b) Mavropigi lignite in air.

CATALYTIC COMBUSTION OF BIOMASS MIXTURES 73

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Considering that ignition of biomass samples occurs soon after the weight loss (seealso Figure 3), and that the particles are small with an average particle size of100 mm, the reaction mechanism seems to be heterogeneous, where gaseous and solidmaterial burn together at the surface. The ignition index D in Table 2 agrees with theresults of ignition temperature. The higher value of ignition index is indicative of bet-ter ignition performance. With respect to combustion parameters, such as maximumcombustion rate (Rmax), temperature corresponding to peak rate (Tmax), and burn-out temperature (Tb), it can be concluded that olive tree pruning and cardoon, whichburn at lower temperatures in a shorter time (tb), are the most reactive fuels, whereasMavropigi lignite the least reactive among the species studied.

Combustion Characteristics of Mixtures

The combustion profiles of the lignite=sewage sludge mixtures are shown inFigure 4. Peak positions are intermediate to those corresponding to each constituentof the mixture and remain close to that of lignite. The curve of the blend lies betweenthe curves of the individual fuels, and the contributions of sewage sludge and ligniteto these profiles are apparent. A similar behavior was obtained for the lignite=olivetree pruning and lignite=cardoon blends. To further investigate any additivity effect,

Figure 3 Pyrolysis and combustion DTG profiles of cardoon.

Table 2 Characteristic combustion parameters of individual fuels

Sample Ti (�C) Tmax (

�C) Rmax (102=min) Tb (�C) tb (min) D� 105 S� 1011

OL 258.0 320.0 8.8 517.0 50.1 6.8 5.6

CAR 261.0 416.0 11.0 525.0 50.9 6.8 5.8

SS 237.8 515.0 5.4 732.0 71.6 6.3 1.8

MLIG 297.3 368.0 5.5 752.0 73.6 3.0 0.68

OL: olive tree pruning; CAR: cardoon; SS: sewage sludge; MLIG: Mavropigi lignite.

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theoretical DTG curves were calculated, representing the sum of the individualcomponents’ behavior in the mixture:

ðdm=dtÞsum ¼ x1 ðdm=dtÞlignite þ x2 ðdm=dtÞbiomass ð3Þ

where (dm=dt)lignite and (dm=dt)biomass are the normalized rates of mass loss as foundfrom the individual experiments, and x1 and x2 are the mass fractions of lignite andbiomass in the mixture, respectively. Representative calculated and experimentalDTG curves for lignite=sewage sludge mixtures are compared in Figure 5. As can

Figure 4 DTG combustion profile of Mavropigi lignite=sewage sludge mixture.

Figure 5 Comparison between experimental and calculated DTG combustion curves for Mavropigi

lignite=sewage sludge 70:30 mixture.

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be seen from the similarity of these curves, the combustion behavior of the blendsunder study can be predicted to a great extent from the parent fuel constituents.

Proximate analysis and combustion parameters, which were derived from theco-combustion of biomass fuels with Mavropigi lignite, are presented in Table 3.From these results, it is evident that the ignition temperature of the blends is almostthe same as that of biomass, indicating that the two fuels ignite independently whenmixed. The ignition and combustion indices were improved with biomass additionand remained close to weighted average values. The same trend was followed forburnout temperature and time. Therefore, a higher reactivity biomass fuel couldimprove the burnout of the lignite, and the fuels showed more or less an additivebehavior upon blending.

An improvement in the burnout performance for co-combustion of biomasswith coal, as well as both additivity or synergy, have been reported by other authors(Li et al., 2006; Muthuraman et al., 2010).

Combustion Characteristics of Individual Fuels with Catalysts

The influence of the different catalysts on the ignition and combustion ofMavropigi lignite is represented in Table 4. As can be seen, all catalysts reducethe ignition temperature of lignite to a different extent, varying from 31�C to43�C. This effect can be explained by the enhancement of the volatiles’ emission.It is considered that catalysts prevent the formation of stable chemical structures,speed up degradation of hydrocarbons, and by an oxygen transfer mechanism theyweaken the C-C bond, thereby decreasing the activation energy for the complexpyrolysis reactions (Rustamov et al., 1998; Shen and Qinlei, 2006). Thus, in thepresence of catalysts, the concentrations of volatile gases around the lignite particleare higher, so that ignition can occur earlier.

Furthermore, Table 4 shows and Figure 6 clearly illustrates when a catalyst wasadded the peak and burnout temperatures were decreased. With the exception oflignite=CuO and lignite=MgO tests, the location of the maximum temperature wasobtained during the second stage, i.e., at the high temperature regime. Also, the com-bustion rate of the second stage was doubled or more. As a consequence, for mostcatalysts, D and S indices were considerably improved (up to 38% and 154%, respect-ively). Therefore, catalysts intensify the oxygen transfer to the unburned fixed carbonsurface when evolution of volatiles diminishes and the effect of ash on retardationof combustion becomes important. A suggested mechanism is that basic ions in the

Table 3 Characteristic combustion parameters of Mavropigi lignite=biomass blends (70=30)

Sample

Volatile

matter

(%)

Fixed

carbon

(%)

Ash

(%)

Ti

(�C)Tmax

(�C)Rmax

(102=min)

Tb

(�C)tb

(min) D� 105 S� 1011

MLIG=OL 63.1 23.0 13.9 256.3 365.7 5.4 740.0 72.4 3.8 (3.9) 1.1 (1.3)

MLIG=CAR 61.0 22.8 16.2 262.4 365.0 4.6 728.0 71.2 3.1 (3.7) 0.99 (1.2)

MLIG=SS 56.8 19.7 23.5 237.9 384.6 4.4 750.0 73.4 3.4 (3.7) 1.1 (0.9)

Values in brackets show weighted average values. OL: olive tree pruning; CAR: cardoon; SS: sewage

sludge; MLIG: Mavropigi lignite.

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catalyst can react with the oxygenic base on the surface of the char, to form complexchemical substances. When oxygen is absorbed on the surface of the catalysts,activated oxygen spills will be released to react with the fixed carbon at a lower tem-perature (Shen and Qinlei, 2006). The surface of the catalysts provides reaction sitesfor combustion and supply of oxygen to the char facilitated by catalysts, resulting incomplete combustion.

Although the influence of catalyst type on ignition temperature was not sogreat, the influence on combustion rate and temperature was much more pronounced.Transition metal oxides V2O5 and Fe2O3 enhanced to a higher degree the combustionprocess than the alkali and alkaline earth compounds. The most active catalyst wasV2O5; however at about 650

�C, V2O5 particles became molten, causing problems tothe experimental apparatus. This finding is supported by previous studies (McKee,1970). Among alkaline earth oxides, CaO proved to be more active, while amongalkali chlorides, NaCl showed a higher catalytic activity than LiCl, which agrees with

Figure 6 DTG combustion profiles of Mavropigi lignite and Mavropigi lignite with Fe2O3 as catalyst.

Table 4 Characteristic combustion parameters of Mavropigi lignite with catalysts

Sample

Ti

(�C)T1max

(�C)T2max

(�C)Rmax

(102=min)

Tb

(�C)tb

(min) D� 105 S� 1012

MLIG 297.3 368.0 533.0 5.5 752.0 73.6 3.0 6.8

MLIG-V2O5 254.3 341.0 510.0 7.7 736.0 72.0 4.2 17.4

MLIG-Fe2O3 258.6 344.5 504.0 6.2 746.0 73.0 4.1 13.5

MLIG-TiO2 263.0 349.4 518.0 4.2 750.0 73.4 2.3 8.5

MLIG-CuO� 261.0 357.7 521.0 4.0 751.0 73.5 3.1 8.8

MLIG-CaO 259.8 348.4 514.0 5.5 765.0 74.9 3.1 9.8

MLIG-MgO� 266.1 361.6 524.0 4.7 745.0 72.9 3.1 9.7

MLIG-LiCl 262.0 376.0 491.7 4.8 751.0 73.5 2.7 9.7

MLIG-NaCl 259.3 358.5 501.0 5.0 750.0 73.4 2.9 9.8

Maximum temperature obtained during second stage. MLIG: Mavropigi lignite.

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earlier investigations (Wu et al., 1998). With respect to ignition temperature andcombustion characteristics, the relative activity of the different catalysts studiedwas in the following order:

V2O5 > Fe2O3 > CaO > NaCl > MgO > CuO > TiO2 > LiCl

Besides catalyst type, catalyst amount is also important for combustion.Table 5 shows that when the percentage of catalyst increased from 2.5 to 10%wt,the catalyzed efficiency was higher, since ignition temperature, maximum peak tem-perature, burnout temperature, and time were progressively decreased, whereas thecombustion rate was increased. Therefore, catalyst loading for the most active oxidesof iron and calcium was selected to be 10%wt for subsequent tests, taking also inconsideration that for this catalyst quantity the ignition temperature of Mavropigilignite was closer to that of the biomass materials under study. In addition, ironand calcium oxides are not expected to contribute to operational problems in boilers,such as slagging, fouling, or corrosion (Farrow et al., 2013; Vamvuka and Kakaras,2011).

Addition of Fe2O3 and CaO catalysts to the biomass fuels did not practicallychange the ignition temperature, as indicated in Table 6 (tests with different catalystshad also the same effect). However, peak combustion temperature was shifted to sig-nificantly lower values, between 14�C and 123�C, and the shift was remarkable in thecase of cardoon. Nevertheless, the maximum reaction rate, with the exception ofolive tree pruning catalyzed by Fe2O3, was lowered with incorporation of catalysts.Therefore, it seems that the catalysts used increased the temperature sensitivity of thereaction in air for biomass fuels, as compared to that without the presence of cata-lysts. Fe2O3 showed a better performance than CaO for the combustion process. Thepeak at 725�C in Figure 7 corresponds to the decomposition of some CaCO3, whichwas formed by the reaction of CaO with CO2 in the gas, and it was not observedduring the combustion of cardoon. In the case of sewage sludge, this peak was moreapparent (see also Figure 2), because this biomass fuel contained �35% of CaCO3 inits ash (Vamvuka et al., 2011). Concerning the ignition and combustion indices, fromTable 6 it can be observed that (with the exception of olive tree pruning catalyzed byFe2O3) these indices were worsened with catalyst addition to biomass fuels, due tothe drop in combustion rate.

Consequently, Fe2O3 and CaO showed no catalytic activity to burn off ofbiomass fuels, but influenced the temperature sensitivity of the combustion process.

Table 5 Effect of catalyst loading on combustion parameters of Mavropigi lignite

Sample

Catalyst

loading (%wt)

Ti

(�C)Tmax

(�C)Rmax

(102=min)

Tb

(�C)tb

(min) D� 105 S� 1012

MLIG-Fe2O3 2.5 266.5 515.4 4.3 752.0 73.6 2.1 8.0

5.0 264.3 506.6 5.0 749.0 73.3 2.6 9.6

10.0 258.6 504.0 6.2 746.0 73.0 4.1 13.5

MLIG-CaO 2.5 262.7 516.0 5.3 769.0 75.3 2.7 9.7

5.0 261.1 514.0 5.4 767.0 75.1 2.8 9.9

10.0 259.8 514.0 5.5 765.0 74.9 3.1 9.8

MLIG: Mavropigi lignite.

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Previous studies, in which these oxides have been used as biomass catalysts, reportthat they reduce the ignition temperature and intensify the combustion of wheator rice straw and MSW, but their catalytic activity is not great (Shen and Qinlei,2006; Zhaosheng et al., 2009).

COMBUSTION CHARACTERISTICS OF MIXTURES WITH CATALYSTS

By comparing the DTG profiles of the lignite=olive tree pruning mixture withor without the presence of Fe2O3 as catalyst in Figure 8, it can be observed that peakheights of catalyzed mixture were somehow higher and their position was obtained ata lower temperature. Thus, the reactivity of the blends increased when Fe2O3 wasadded and it was higher than that of Mavropigi lignite. A similar behavior wasobtained for lignite=cardoon mixture and for CaO catalyst, as represented inTable 7 (repeatability of tests was very good). Sewage sludge and its mixture with

Figure 7 DTG combustion profiles of cardoon and cardoon with CaO as catalyst.

Table 6 Characteristic combustion parameters of biomass fuels with catalysts

Sample Ti (�C) Tmax (

�C) Rmax (102=min) Tb (�C) tb (min) D� 105 S� 1011

OL 258.0 320.0 8.8 517.0 50.1 6.8 5.6

OL-Fe2O3 246.1 305.5 9.2 502.0 48.6 8.0 2.9

OL-CaO 245.0 308.0 5.2 754.0 73.9 4.5 1.1

CAR 261.0 416.0 11.0 525.0 50.9 6.8 5.8

CAR-Fe2O3 263.1 292.6 7.5 510.0 49.4 6.0 2.1

CAR-CaO 256.3 291.6 5.7 748.0 73.2 4.8 1.1

SS 237.8 515.0 5.4 732.0 71.6 6.3 1.8

SS-Fe2O3 232.9 487.3 5.2 725.0 70.9 3.5 1.6

SS-CaO 237.0 498.0 4.7 766.0 75.0 3.0 1.2

OL: olive tree pruning; CAR: cardoon; SS: sewage sludge.

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Mavropigi lignite were the least reactive fuels among the species studied even when acatalyst was used.

On the other hand, from Table 7 it is obvious that the ignition temperature ofthe blends with catalysts is close to that corresponding to the biomass fuel, but thetrend nearly coincides with the ignition temperature of Mavropigi lignite used withcatalysts. In other words, in terms of ignition, these catalysts show activity only forthe lignite. The effect is better illustrated in the case of sewage sludge, the ignitiontemperature of which is much lower than the rest of the fuels (Tss¼ 237.8�C,TMl=ss¼ 237.9�C, TMl=Ca¼ 259.8�C, TMl=ss=Ca¼ 254.5�C).

The above results explain the improved ignition and combustion indices ofnearly all blends with catalysts, as compared to the blends where no catalysts wereadded. The increased reaction rates, and the lowering of peak and burnout tempera-tures are indicative of better combustion performance.

Figure 8 DTG combustion profiles of Mavropigi lignite=olive tree pruning mixture and Mavropigi lignite=

olive tree pruning mixture with Fe2O3 as catalyst.

Table 7 Characteristic combustion parameters of Mavropigi lignite=biomass blends (70=30) with catalysts

Sample Ti (�C) Tmax (

�C) Rmax (102=min) Tb (�C) tb (min) D� 105 S� 1011

MLIG=OL 256.3 365.7 5.4 740.0 72.4 3.8 1.1

MLIG=OL-Fe2O3 257.2 349.0 5.8 735.7 72.0 4.2 1.2

MLIG=OL-CaO 257.3 354.2 5.4 752.8 73.7 3.9 1.2

MLIG=CAR 262.4 365.0 4.6 728.0 71.2 3.1 0.99

MLIG=CAR-Fe2O3 261.3 344.0 4.8 730.6 71.5 3.5 1.0

MLIG=CAR-CaO 262.6 355.5 4.7 751.6 73.6 3.2 0.98

MLIG=SS 237.9 384.6 4.4 750.0 73.4 3.4 1.1

MLIG=SS-Fe2O3 245.0 374.4 4.4 747.0 73.1 3.3 1.2

MLIG=SS-CaO 254.5 370.7 4.3 766.0 75.0 3.1 1.1

OL: olive tree pruning; CAR: cardoon; SS: sewage sludge; MLIG: Mavropigi lignite.

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The present findings prove that the examined solid biofuels could support thecombustion of poor coals and provide useful information for practical applicationsfor power production. For lignite=biomass mixtures, the higher quantity of volatilecompounds liberated from biomass at a low temperature could mean an advantagewhen attempting to initiate the process; however, it might need some extra equipment.

CONCLUSIONS

. Biomass fuels improved the ignition and burnout of Mavropigi lignite, and thefuels showed more or less an additive behavior upon blending.

. The addition of a catalyst to Mavropigi lignite reduced the ignition temperatureand intensified the combustion process, due to enhancement of volatiles emissionand increased supply of oxygen to the char. Transition metal oxides showed ahigher catalytic activity than alkaline earth compounds. The relative activitywas in the following order: V2O5>Fe2O3>CaO>NaCl>MgO>CuO>TiO2>LiCl.

. The metal oxides studied showed no catalytic activity to burn off of biomass fuels,but influenced the temperature sensitivity of the reaction in air.

. The catalysts studied increased the reactivity of lignite=biomass blends in air andimproved combustion by increasing the reaction rate and lowering the peak andburnout temperatures. However, in terms of ignition they showed catalyticactivity only for the lignite.

The results of this study provide a useful basis for further application of poor coal=biomass mixtures in energy production.

ACKNOWLEDGMENT

The authors kindly thank the laboratories of Inorganic and OrganicGeochemistry and Toxic and Hazardous Waste Management of the TechnicalUniversity of Crete for the chemical analysis of the fuels.

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