Investigation of pollutant formation of Sweet Sorghum–lignite (Orhaneli) mixtures in fluidised beds

Biomass and Bioenergy 27 (2004) 277 – 287

Investigation of pollutant formation of Sweet Sorghum–lignite (Orhaneli) mixtures in !uidised beds M. Handan C ' ubuk∗ , Hasan A. Heperkan Heat and Thermodynamics Division, Mechanical Engineering Faculty, Yldz Technical University, Yldz, Istanbul 80750, Turkey Received 7 September 2002; received in revised form 6 October 2003; accepted 8 February 2004

Abstract Most of the Turkish lignites have undesired fuel properties and they are extremely pollutant. In this study, Sweet Sorghum was chosen as the energy plant. Combustion experiments of lignite and lignite–Sweet Sorghum mixtures were carried out in a !uidised bed system. The fuel-feeding ratio was set such that the thermal output of the system remained constant. Addition of Sweet Sorghum to the lignite reduces the pollutant concentration. The results were supported by experimental results. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Sweet sorghum; Sorgum bicolor; Lignite; Co-combustion; Fluidised bed; Pollutant

1. Introduction It is impossible to evaluate energy sources independent from their in!uence on the environment. CO2 , SO2 and NOx emissions from fossil fuel combustion, play an important role on atmospheric pollution. CO2 and SO2 emissions are directly proportional to energy consumption, whereas NOx and CO are highly dependent on the conversion technology. The emissions of hazardous air pollutant from coal combustion have become an important issue in light of the new environmental regulations in several developed countries and these pollutants have been the subject of important research activities in recent decades [1]. ∗ Corresponding author. Tel.: +90-212-259-7070; fax: +90212-261-6659. E-mail addresses: [email protected] (M.H. C'ubuk), [email protected] (H.A. Heperkan).

0961-9534/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2004.02.001

Turkey currently imports over 55% of its energy needs from other countries [2]. Turkey has to turn towards its own natural sources. The most important reserve is the lignites, unfortunately low-quality exerting high environmental pollution with high sulphur and ash contents. Scarcity of the primary energy sources should draw our attention more on renewable energy sources. To investigate the pollutant formation and reduction behaviour of a !uidised bed combustion reactor experimentally, Orhaneli lignite was burned alone, initially in the combustor (deCned as R = 0). In order to replace the fuel partially with a renewable source Sweet Sorghum (Sorgum bicolor), a biomass plant, was fed into the reactor at diEerent ratios, 5%, 10% and 15% and the emissions were monitored and compared. The ratios have been deCned as R = 1, 2 and 3, respectively, and their properties are given in Table 1.

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Table 1 Mixture ratio

Lignite (kg h−1 ) Sweet Sorghum (kg h−1 ) Total feeding rate (kg h−1 ) CaloriCc heating value (kJ kg−1 )

R=0

R=1

R=2

R=3

(100% lignite)

(5% SS)

(10% SS)

(15% SS)

2.702 0 2.702 15494.06

2.567 0.129 2.696 15527.99

2.432 0.258 2.690 15562.07

2.296 0.388 2.684 15596.44

2. Lignite and biomass 2.1. Turkish lignites and their characteristics Turkish coal reserves constitute 0.88% of the European reserves and 0.11% of the total world reserves. Analyses show that the ash, moisture and sulphur content of domestic lignites vary a lot [3,4]. The share of the lignites containing less than 20% ash among the reserves is only 3.73%. Therefore, nearly 96% of the lignites have high ash content. On the other hand, the share of the lignites containing less than 1% sulphur among the reserves is also around 3.7%. US standards for acceptable sulphur content for coal is 0.08%, which puts 97% of the domestic lignites in the high sulphur category. Lignites with less than 20% moisture content constitute 15.14% of the total reserves. Moreover approximately 69% of the lignites have caloriCc values under 8:4 MJ=kg. Fluidised bed combustion is generally considered to be an environmentally favourable combustion technology where control of emissions can be integrated into the combustion system. FBC system operating at low temperatures around 800–900◦ C not only prevent thermal NO formation but promote NO-reducing reactions during the combustion process [5]. CO2 emission, which is an unavoidable product of hydrocarbon combustion, however, together with the greenhouse eEect, cannot be eliminated. Estimated CO2 emissions in Turkey from fossil-fuel combustion are 410 and 550 million of tonnes for the years 2005 and 2010, respectively [6]. 2.2. Biomass energy and Sweet Sorghum Biomass has an important potential among new and renewable energy sources. Generally biomass,

consuming CO2 in the air during its growth shows an interesting character among other fuels and is easy to produce. Biomass is an organic fuel, which does not originate from fossils. The total energy equivalent of biomass as a renewable energy source is around 65:376 Gtoe, which is approximately 8 times the total global energy consumption for the year 1997 [7–9]. Only 8% of this potential is being currently used. During combustion of a biomass, CO2 is released, making a closed cycle with CO2 Cxation by the plant. Globally, it results in no net increase of CO2 in the atmosphere [10]. In developing countries, over 2 billion people depend on biomass as their primary source of energy; 70% of the population live in rural areas, and biomass accounts for about 43% of total energy used [11]. Energy production utilising biomass is an important source for Turkey due to her vast farming areas and her suitable climate. Sweet Sorghum (S. bicolor) used in this study is known to be an annual C4 plant of tropical origin with a caloriCc value between 16.744 and 17:580 MJ=kg on a dry basis. It is well adapted to sub-tropical and temperate regions, being highly biomass productive and water eLcient. Recently, a great deal of research has been undertaken in the EU and other countries to explore its biomass productivity and energy potential under various environmental conditions and cultural practices [10,12]. It grows rapidly, widely, is easily cultivated over a wide range of climates and its cost per ton is less than the other biomass [13,14]. It does not need much fertiliser, pesticides or irrigation, has high photosynthetic eLciency (about 2–3%) and high productivity [12]. It has been estimated that the net CO2 savings are 90%. Compared with equivalent energy from petroleum, this represents a saving of 25–40 tonnes

M.H. C + ubuk, H.A. Heperkan / Biomass and Bioenergy 27 (2004) 277 – 287

279

Table 2 Fuel characteristics of some Turkish lignites and Sweet Sorghum varieties Type of fuel

Ultimate analysis (wt % dry basis)

HHV (kJ/kg)

Ash (%)

C

H

N

S

Lignite source [16] YataOgan Kemerburgaz SeyitQomer Soma-1 C'an Orhaneli [18] Soma-2 Elbistan [17]

49.00 51.10 55.20 48.10 59.50 50.68 64.50 47.36

4.80 4.90 5.20 4.90 4.80 4.17 5.00 3.57

0.70 0.80 1.20 1.20 1.30 0.50 1.30 1.78

3.90 3.60 1.10 4.10 6.10 1.65 0.60 5.82

13897.94 13864.87 17665.34 16320.38 20107.87 15494.06 21235.58 12084.15

18.47 17.50 13.11 24.00 09.87 15.11 11.97 28.19

Sweet Sorghum Brandes Wray Rona Rio M81E MN1500 Dale Theis Keller

44.61 43.51 42.56 44.40 44.23 44.77 43.64 44.96 43.64

6.25 6.10 6.19 6.22 6.25 6.27 6.33 6.45 6.31

0.20 0.12 0.16 0.39 0.18 0.09 0.16 0.50 0.19

0.13 0.04 0.05 0.11 0.09 0.06 0.04 0.13 0.05

16711.77 16923.16 17013.58 16746.51 17206.97 17070.93 16268.89 16878.79 16779.99

03.17 01.78 01.58 02.20 01.59 01.63 01.80 03.70 02.20

of CO2 per hectare [12]. SO2 emissions are reduced, leading to microclimate control, landscape preservation, dust emission control, etc. Results of analyses of Turkish lignites and of nine varieties of Sweet Sorghum are summarised in Table 2 [15]. As may be seen from Table 2, the sulphur and ash contents of all varieties of Sweet Sorghum were found to be lower than those of lignites by factors of about 50 and 8 times, respectively [16–18]. 3. Method and material 3.1. Experimental set-up Experimental measurements were carried out in the !uidised bed laboratory at TUBITAK Marmara Research Centre (MRC). The reactor was designed to sustain stable combustion with continuous coal feed (see Fig. 1). The combustor consists of a 150 mm internal diameter with 300 mm high active bed and 210 mm internal diameter with 900 mm high free

board sections. Both sections were equipped with ports to serve as probe inlets for temperature and concentration measurements, ash removal and coal feed. The free board was designed to minimise particle transportation. Fluidisation air was provided from a compressor and the !ow rates were measured using three rotameters at diEerent ranges. Flue gases were collected and removed through a fan system and were analysed using a Flue Gas Analyser, taking measurements every 40 s. O2 , CO, SO2 and NOx gas concentrations were monitored. The !ue gas !ow rates (volume) have been measured using the same Gas Analyser during combustion. The measured !ue gas !ow rates and the mass entering the system are given in Table 3. Temperature measurements were taken using NiCr– Ni (type K) ceramic-shielded thermocouples probed at 6 diEerent levels, 25, 65, 110, 185, 450 and 1160 mm from the distributor plate in the reactor. The initial bed material was foundry sand having a particle size distribution between 0.5 and 1 mm with a density of  = 2:64 g=cm3 . The static bed height was 70–80 mm and the dynamic bed height

280

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Fig. 1. Experimental set-up : 1. Compressor, 2. Pressure Regulator, 3. Rotameters, 4. LPG , 5. LPG !owmeter, 6. Air plenum, 7.Distributor plate, 8. Fluidised bed, 9. Coal feeding system, 10. Ash cyclone, 11. Exhaust fan, 12. Thermocouples, 13. Flue gas measurement port. Table 3 The measured !ue gas !ow rates, average value of the surface temperature and the mass entering the system

V(m3 =h) Nm3 =h kg/h Mass (kg/h) The average value of the surface temperature (◦ C)

R=0

R=1

R=2

R=3

33.0 12.797 16.350 16.416 728.5

32.6 12.769 16.267 16.418 728

32.9 12.597 16.088 16.419 728

33.5 12.755 16.281 16.421 729

approximately 200 mm. The feeding of mixture was carried out 20 mm above the distributor plate directly into the foundry sand. The coal particles entering the feed-pipe entrance were conveyed with the secondary air stream, which was metered by a separate rotameter. Biomass was also fed utilising the same system, mixed into the coal before loading into the fuel entrance bin. The bed region has not been insulated during the experiments. Agglomeration occurred with insulated beds. Since there are no heat absorbing elements, like water or steam coils in the bed, access heat was removed through the surface of the bed without insulation. To determine the heat loss from the bed outer surface, temperatures were measured using a pyrometer; the average value of the surface temperature has been used in the calculations. The values are presented in Table 3.

3.2. Experiments The bed was kept at around 850◦ C during the experiments using only coal or coal/biomass mixture. The mixture-feeding rate can be controlled through the helical feeding mechanism. To maintain the dynamic bed height at a constant level, ashes were removed from the bed through an over!ow pipe connected to the reactor. Ashes were collected in a separate container to determine their composition. Sweet sorghum and coal were pre-mixed in a RETCH brand blender for 30 min, stored in a bin under controlled atmosphere to be used in the experiments. As indicated previously the emitted SO2 as a result of combustion of Orhaneli lignite in a !uidised bed reactor has been planned to be reduced utilising diEerent percentages of biomass, namely 5%, 10%

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281

Table 4 Analysis of the experimental materials

Proximate analysis (wt%; as received) Moisture Volatile matter Fixed carbon Ash CaloriCc heating value (kJ kg−1 ) Ultimate analysis (wt%; daf) Carbon Hydrogen Oxygen Nitrogen Sulphur Combustible sulphur

R=0

R=1

R=2

R=3

22.32 54.19 30.70 15.11 15494.06

55.34 29.95 14.71 15527.99

56.50 29.19 14.31 15562.07

57.67 28.42 13.90 15596.44

59.70 4.91 32.85 0.59 1.94 1.10

59.03 4.95 33.56 0.60 1.85 1.05

58.37 4.99 34.26 0.62 1.76 1.00

57.70 5.03 34.97 0.63 1.67 0.95

and 15%, keeping the heat output power of the combustor constant. The lignite used in the study was supplied from Orhaneli Thermal Power Plant and sized between 1–2 mm. Sweet sorghum was grown at TUBITAK-MRC, harvested, chopped into pieces between 0.5 and 1 mm in special grinders, pressed under a pressure of 15 Mpa to remove its structural water and left to be dried at ambient conditions. Proximate and elemental analyses of both materials have been carried out to determine their characteristics [18]. The results are presented in Table 4 for Orhaneli lignite and Sweet Sorghum. All loose Sweet Sorghum and lignite analyses were made according to ASTM Standards [18]. A Fisher 490 model proximate analysis system, LECO-SC-32 model sulphur analyser and LECO AC-200 model calorimeter were utilised within this frame. During combustion calculations the total combustible sulphur has been used instead of the total sulphur content of the mixture. The air !ow rate has been taken to be 12 m3 =h [18]. 4. Results and discussion Sweet Sorghum has 82% volatile matter on dry basis while lignites contain around 45% volatiles depending on the region. In general lignites have lower amounts of volatile matter, which suggests that the design of the combustor is extremely important to

achieve high combustion and thermal eLciencies, the access air coeLcient should be selected properly. Lignites have considerably higher ash contents than Sweet Sorghum, the heating values are approximately the same for the mixture under investigation, however, there are many low-quality Turkish lignites with even lower heating values. Lignites generally contain more sulphur compared to other fossil fuels and contribute to pollution by emitting considerable amounts of SO2 during combustion. It is well known that these emissions are highly health hazardous. SO2 emission is usually directly proportional to the sulphur content of the fuel. It reacts with water molecules to form sulphuric acid. These products are highly soluble in water; they dissolve in rain to return to earth as acid rains. Sulphur oxides are absorbed by plants, soil and other materials even when they are dry. Their concentrations in air are limited through standards. Exceeding the limits could cause health problems or even promote mass deaths. Emissions from combustion systems are also limited through the Air Quality Control Regulation published on November 2, 1986, in Turkey. To establish the combustion characteristics of lignite–Sweet Sorghum mixtures, they were burned in a !uidised bed combustor and the !ue gas concentrations were measured together with temperatures at diEerent locations in the system. The Flue Gas Analyser measures O2 , CO, SO2 and NOx concentrations and calculates CO2 and H2 O

282

M.H. C + ubuk, H.A. Heperkan / Biomass and Bioenergy 27 (2004) 277 – 287 3400

750

3300

725

3200

CO (ppm)

SO2 (ppm)

700

675

3100 3000

650

2900

625

2800 2700

600 1

0

2

0

3

1

2

3

R (mixing ratio)

R (mixing ratio)

Fig. 4. Experimental CO emissions.

Fig. 2. Experimental SO2 emissions.

15.2 225 15.1 15

220

215

CO2 (%)

NOx (ppm)

14.9

210

14.8 14.7 14.6 14.5 14.4

205

14.3 200 0

1

2

3

R (mixing ratio)

14.2 0

1

2

3

R (mixing ratio)

Fig. 3. Experimental NOx emissions.

Fig. 5. Experimental CO2 emissions.

concentrations. Since the analyser calculates these values from a pre-assumed fuel-type, CO2 and H2 O were calculated based on the original fuel composition and these results were used in the analysis. The !ue gas emissions were measured every 40 s for 2 h during the experiments; the average values are presented on the graphs in Figs. 2–6. N2 O emission cannot be measured by this device. To estimate the N2 O concentrations, results from

the literature have been used. In the study by Liu and Gibbs a model for NO and N2 O emissions from biomass (Crewood chips) has been developed and evaluated [19]. This model gives the desired variation for NO and N2 O formation as a function of the fuel N-content (%) and the bed temperature on diagrams (Figs. 7 and 8, respectively). The measurements obtained during our experiments were compared and evaluated using these results to estimate the N2 O

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283

3.1

3

O2 (%)

2.9

2.8

2.7

2.6

2.5 0

1

2

3

Fig. 8. Predicted eEect of bed temperature on NO and N2 O emissions and the measured eEect of bed temperature on NO emissions.

R (mixing ratio) Fig. 6. Experimental O2 emissions.

Fig. 7. The conversion of fuel-N to NO and N2 O emission.

formation. NO formation as a function of the bed temperature is also available in this study. The outcome is summarized in Tables 5 and 6. As can be followed from Table 5, the NO conversion percentages as a function of the fuel-N content for diEerent mixture ratios agree well with the NO percentages obtained from the experiments. Based on this agreement the same diagrams have been used to estimate the N2 O formation during combustion of the Sweet Sorghum–lignite mixture.

Another study [20] on the modelling of N2 O and NO formation gives results in the form of non-dimensional functions (Eqs. (1) and (2)). The calculated NO and N2 O conversion rates according to the bed temperature were given on graphs for diEerent particles sizes (Fig. 9). The functions indicated on the graphs are deCned as using the following relations: fNO = [NO]={(N=C)([CO] + [CO2 ])};

(1)

fN2 O = (2[N2 O])={(N=C)([CO] + [CO2 ])}:

(2)

The values taken from the graphs for a particle size of 1–2 mm have been compared with the experimental results in Table 7. Table 7 indicates that the NO conversion ratios calculated from Eq. (1) for the bed temperature and the experimental results are in good agreement. This allows us to calculate the N2 O conversion ratio from Eq. (2), to determine the N2 O concentration during combustion. The results also agree well with the study by Liu and Gibbs. From the above observations it was concluded that the N2 O emission for our case was in the order of 0.5 to 0:6 g=h. This value was used later in the analysis. As indicated before the bed zone was not insulated during the experiments. There are convective and radiative heat losses from the surface. Using average surface temperatures (measured) and the ambient

284

M.H. C + ubuk, H.A. Heperkan / Biomass and Bioenergy 27 (2004) 277 – 287

Table 5 EEect of the fuel-N content on NO formation [19] R

0 1 2 3

NO

N2 O

Experimental NO

(%)

(%)

(%)

(gr/h)

5 5 5.4 5.5

44 43 42.8 44

4.83 4.87 4.93 5.20

22–40 22–40 24–39 24–39

Estimated N2 O (gr/h) ∼ 0:55 ∼ 0:56 ∼ 0:62 ∼ 0:65

Table 6 EEect of the bed temperature on NO formation [19] R

NO (ppm)

N2 O (ppm)

Experimental NO (ppm)

Estimated N2 O (ppm)

0 1 2 3

125–195 125–195 125–200 123–197

4.7–6.2 4.7–6.2 4.1–5.9 4.4–5.9

184 186 191 199

∼5 ∼5 ∼5 ∼5

Fig. 9. Result of parametric study of the net char-nitrogen conversion to NO and N2 O.

temperature, the heat transfer coeLcients have been calculated from correlations for the Nusselt number. The total enthalpy of the !ue gases has been determined and Sankey diagram for R = 0 has been prepared to show the !ow of energy for the process (Fig. 10). This distribution is characteristic and applies to the other fuel mixtures. The results are presented in Table 8. The components in the fuel and air have also been compared with the measured !ue gas emissions in Table 9. Results show a slight increase in the NOx , CO and O2 concentrations. The NOx increase is due to the local increase in temperature around the Sweet Sorghum particles. A better mixing of the fuel will reduce this increase. However, the variation is within acceptable limits and could be regarded as unaEected (Fig. 3). Increase in O2 concentration (Fig. 6) is due to the O2 present in the plant composition. CO emissions however, show around 700 ppm increase (Fig. 4). Sweet Sorghum has a lower density compared to coal. Since the bed air velocity is adjusted according to the coal and ash particles present in the bed, Sweet Sorghum is conveyed to the free board region. Biomass completes its combustion in the free board. The !ue gas concentrations were taken at a point within the region and therefore the combustion of the biomass particles are not completed. The feeding point of the mixture

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285

Table 7 Comparison of the experimental data with the values taken from literature for a particle size 1–2 mm [20] R

T (K)

Experimental fNO (%)

fNO from Eq. (1) (%)

fN2 O from graCc

N2 O (g/h)

0 1 2 3

1107 1106 1111 1119

22 21 22 22

22 21 22 22

3.5 3.5 3.5 3.5

0.567 0.583 0.575 0.609

Fig. 10. The !ow of energy for R = 0.

is also near the top of the bed, which enhances the carry-over. Pelletizing Sweet Sorghum before feeding into the bed will solve this problem. Addition of Sweet Sorghum into the lignite results in a non-homogenous mixture, which eEects the temperature distribution in the bed. Combining

the O2 present in the plant composition creates high-temperature spots locally. This process is thought to be the slight increase in the NOx concentrations. The same fact causes a better combustion locally reducing the CO levels, however. Both tendencies could be followed from the results. The decrease in the overall ash content of the mixture and the oxygen content of the energy plant, results in higher thermal eLciencies. Ash collected from the cyclone and the bed over!ow have been analysed for diEerent mixture ratios and the thermal eLciencies have been calculated and plotted (Fig. 11). As seen from the Cgure, the eLciency of the system increases with the mixture ratios. This is due to the improvement in the combustion and the reduction of the ash content [18]. 5. Conclusions Turkey must Cnd new sources of energy besides using its own resources in order to overcome the energy

Table 8 The !ow of energy for the process R=0

R=1

R=2

R=3

From fuel (kJ/h) From air (kJ/h)

39856 282

40000 282

40028 282

40082 282

Total energy (fed in) (kJ/h)

40138

40282

40310

40364

Losses heat (kJ/h): Radiative (kJ/h) Convective (kJ/h) Free board zone (kJ/h)

30550.8 2863.4 841.508

30834.6 2862.5 797.124

30686 2856.58 819.316

30723.1 2857.93 752.74

34255.7 5842.37

34494.2 5767.1

34361.9 5924

34333.8 6010

Total heat losses (kJ/h): Flue gas heat (kJ/h) DiEerence (%)

0.09

0.05

0.06

0.05

286

M.H. C + ubuk, H.A. Heperkan / Biomass and Bioenergy 27 (2004) 277 – 287

Table 9 Comparison of the components in the fuel and air with the !ue gas emissions R=0

R=1

R=2

R=3

Fed (in)

Measured

Fed (in)

Measured

Fed (in)

Measured

Fed (in)

Measured

H C Nfuel

154.65 1063.78 10.54

154.24 1062.76 11.05

152.4 1019.65 2.42 as NO 0.56 as N2 O

153.93 1062.01 11.29

152.10 987.85 2.47 as NO 0.62 as N2 O

153.55 1060.98 11.81

Oxygen Ofuel OW Oair

152.66 1040.67 2.39 as NO 0.55 as N2 O

151.73 994.53 2.62 as NO 0.65 as N2 O

585.52 535.98 3227.1

Total O Sfuel Scomb Unburned S

4348.6 34.58 22.98 11.6

607.95 517.88 3227.1 4346.12

4352.93 33.16 21.95 11.21

18.15

630.27 499.74 3227.1 4347.35 20.53

96

Efficiency (%)

95.8

95.6

95.4

95.2

95 0

1

2

3

R (mixing ratio) Fig. 11. Thermal eLciency.

shortage. As indicated in the previous chapters, Turkish lignites are of low quality and have high sulphur content. Our theoretical and experimental results have yielded that addition of Sweet Sorghum at diEerent percentages into lignites decrease the net sulphur oxide emission since the energy plants contain considerably lower sulphur in their composition. Biomass energy generated from selected energy plants have a potential electric power production ca-

4357.11 31.47 20.90 10.57

652.75 481.21 3227.1 4365.97 20.22

4361.06 30.06 19.86 10.20

4372.93 17.72

pacity exceeding the present total capacity obtained from lignites (39:8 Mtoe) and coal (1:44 Mtoe) [2,7]. It is obvious that exploitation of this important energy source is essential to decreasing the negative environmental eEect of lignites as well as itself for making it a clean energy source. The study indicates that Orhaneli lignite–Sweet Sorghum mixtures show better emission characteristics compared to lignite combustion alone. SO2 , NOx and CO emissions per unit amount of heating value can be improved. The decrease in the overall ash content of the mixture and the oxygen content of the energy plant, results in higher thermal eLciencies. Due to the oxygen–carbon dioxide cycle during the growth of the plant, green house eEects can be reduced and the contribution to the global CO2 increase can then be considered zero.

References [1] Yan R, Zhu H, Zheng C, Xu M. Emissions of organic hazardous air pollutants during Chinese coal combustion. Energy 2002;27:485–503. [2] Republic of Turkey Ministry of Energy and Natural Resources Energy Report, 2001, Ankara. [3] Ak'cura F, Gerger M. Characteristics of relevant Turkish lignite. MTA Publication, Ankara; 1982.

M.H. C + ubuk, H.A. Heperkan / Biomass and Bioenergy 27 (2004) 277 – 287 Q [4] ArVkol M, KVnayyiOgit G, OzdoO gan S, Uyar TS, Vural H. Coal transportation and utilization technologies. TQubitak-MAE Publication, Gebze-Kocaeli, Turkey; 1984. [5] Mastral AM, Callen M, Murilla R, Garcia T. Assessment of PAH emissions as a function of coal combustion variables in !uidised bed 2. air excess percentage. Fuel 1998;77(13):1513. [6] Alta's M, Fikret H, C ' elebi E. Energy Statistics. Sixth Turkish Energy Congress, ˙Izmir, 1994:17–22. [7] AcaroOglu M. The present status of biomass energy in Turkey, research and development, politics and suggested measures to be taken. Special Report. 11, Energy Council-Turkey, ˙Istanbul, December 7–9, 1998. [8] Dreier T, Geiger B, Wagner U. Environmental impacts and system analysis of biofuels. Tenth European Conference and Technology Exhibition, Biomass for Energy and Industry. WQurzburg, Germany; 1998. p. 544–8. [9] Hall DO, Rosillo-Calle F. The role of bioenergy in developing Countries. Tenth European Conference and Technology Exhibition, Biomass for Energy and Industry. WQurzburg, Germany; 1998. p. 52–5. [10] Hall DO, Grassi G, Scheer H. Biomass for energy and industry. Seventh E.C. Conference. Bochum, Germany: Ponte Press; 1994. p. 350–529. [11] BQoer KW. Advances in solar energy, vol. 3. New York: Plenum Press; 1986. p. 439–71.

287

[12] Grassi G, Moncado P, Zibetta H. Promising industrial energy crop: Sweet-Sorghum. Elsevier, Oxford: Commission of the European Communities Publication; 1992. [13] Hallam A, Andersen IC, Buxton DR. Comparative economic analysis of perennial, annual and intercops for biomass production. Biomass and Bioenergy 2001;21:407–24. [14] Chiaramonti D, Grimm HP, Bassam N, Cendagorta M. Energy crops and bioenergy for rescuing deserting coastal area by desalination: feasibility study. Bioresource Technology 2000;72:131–46. [15] TQure S, Uzun D, TQure IE. The potential use of Sweet Sorghum as a non-polluting source of energy. Energy 1997;22(1): 17–9. [16] Urkan MK, Arikol M. Fuel 1994;73:768. [17] ErgQudenler A, I'siOgigQur A. Renewable Energy 1994;5:786. [18] C'ubuk MH. Investigation of the environmental pollution characteristics of Orhaneli lignite–biomass mixtures in Q Graduate Institute, Ph.D. thesis, ˙Istanbul, !uidised beds. YTU 1998. [19] Liu H, Gibbs BM. Modelling of NO and N2 O emissions from biomass-Cred circulating !uidized bed combustors. Fuel 2002;81:271–80. [20] Kilpinen P, Kallio S, Konttinen J, Barisic V. Char-Nitrogen oxidation under !uidized bed combustion conditions:single particle studies. Fuel 2002;81:2349–62.