Briquetting and combustion of spring-harvested reed canary-grass: effect of fuel composition

Biomass and Bioenergy 20 (2001) 25–35

Briquetting and combustion of spring-harvested reed canary-grass: eect of fuel composition Susanne Paulrud ∗ , Calle Nilsson Laboratory for Chemistry and Biomass, Department of Agricultural Research for Northern Sweden,  Sweden Swedish University of Agricultural Sciences, P.O. Box 4097, SE-904 03 Umea, Received 10 June 1999; accepted 27 June 2000

Abstract The purpose of this study was to increase the understanding how spring-harvested reed canary-grass briquettes with various chemical compositions with respect to ash content in uence the formation of emissions during combustion in a 180 kW burner. Furthermore, an objective was to investigate possible ash problems during the combustion. Five fuels were used in the study consisting of three reed canary-grass samples with dierent ash contents from dierent growing sites in Sweden and additionally one of these materials was separated into a stem and leaf fraction. Flue gas emissions were measured and fuels and ashes were analysed. The variation in ash content did not aect the production of briquettes. Further, fuel containing only stem fraction showed the highest mechanical strength. The variation in ash content did not eect the ue gas emissions. The presented results showed low mean values for carbon monoxide ( ¡ 42 mg=MJ, except one experiment) and particles in the

ue gas ( ¡ 150 mg=Nm3 ) (no puri cation of ue gas). Emissions of nitrogen oxides were ¡ 110 mg=MJ. The ash formed light voluminous big chunks where small parts showed a tendency to be sintered. The combustion experiments imply that spring-harvested reed canary-grass can be burned with success in combustion equipment that is designed for the dierences c 2001 Elsevier Science Ltd. All rights reserved. in ash content. Keywords: Bioenergy; Reed canary-grass; Combustion; Briquettes; Ash composition; Emissions; Nitrogen oxides

1. Introduction Spring-harvested reed canary-grass (Phalaris arundinacea L.) is a new promising bioenergy crop in Sweden. Studies show that a delayed harvest system for reed canary-grass (RCG) is bene cial for the biological production as well as the quality of the harvested material for energy purposes [1,2]. The delayed

∗ Corresponding author. Tel.: +46-90-786-9466; fax: +46-90-786-9494. E-mail address: [email protected] (S. Paulrud).

harvest system gives a dry (85% dry matter content) and storable material that can be directly briquetted without any arti cial drying [3]. In addition, undesirable elements like Cl, K and Ca are reduced as compared to harvest in late summer, which is likely to present a reduced risk for the formation of persistent organochlorine compounds in the emissions [4] and less slagging and deposit problems [4,5]. The fuel and ash characteristics of RCG dier somewhat from wood fuels (wood shavings). Springharvested RCG demonstrates lower values for net calori c and for volatile matter, and higher values for elements like Cl, S and N [6,7]. For ash forming

c 2001 Elsevier Science Ltd. All rights reserved. 0961-9534/01/$ - see front matter PII: S 0 9 6 1 - 9 5 3 4 ( 0 0 ) 0 0 0 6 1 - 1

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S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

elements, RCG usually shows higher values for Si, K, Ca and P content [6,7]. The chemical composition of RCG varies considerably depending on soil type and fertilisation. Dierent soil types aect the uptake of ash components in RCG and this is most applicable for silicon [1,6,8], which is a major ash component, especially at high ash content levels. There is also a dierence in chemical elemental characteristics between stems and leaves. Higher concentrations of most nutrients exist in the leaves than in the stem, and the ash content is on average twice as high in the leaves [1]. Fuel composition impacts combustion behaviour. A higher content of N is likely to increase NO emissions [9,10] and production of a voluminous ash makes it more dicult to get an ecient combustion and to handle the ash produced. Knowledge of the behaviour and forms of occurrence of K, Ca, Na and Si, S, Cl during combustion is important for the understanding of deposit formation and sintering. Earlier combustion experiments with spring-harvested RCG briquettes, pellets and powder imply that the higher ash content might present problems like uy, not sintered, deposits in furnace and convection areas [11]. Other eects are enclosure of combustible material in the ash due to the fact that the ash will not fall together on the grate and also the bridging of ash in furnace and ash handling system (practical observations). The large volumes of ash from such high ash content fuels also require a continuous ashing system. The purpose of this study was to increase the understanding how spring-harvested RCG briquettes with dierent chemical compositions related to ash content (3–10% of dry matter) in uence the formation of emissions using a 180 kW burner and to investigate possible ash problems that may occur during the combustion.

same eld. Materials B and C were a 5-year ley and material A was a 3-year ley. The soil types where the RCG was grown were a clayey silt with low humus content (2% DM) for material A, a humus-rich (13% DM) clayey sand for material B and a silty clay with medium humus content (5% DM) for material C. In addition a pelletized RCG (I–12 mm) was used in order to control the stability and repeatability of the experimental set-up. This material had an ash content around 5%.

2.2. Briquetting Materials A, B and C were cut in a Cormall Hach HMG (Cormall A=S, Denmark) 500=540 rpm with a 3 mm sieve. In addition, 2:7 tons of material A were separated into a leaf and stem fraction in a fractionation plant [12]. The fractionation process yielded a ne ( ¡ 2 mm) leaf fraction and a coarser (20 –40 mm) stem fraction. No further cutting was done on the leaf fraction. Half of the coarse stem fraction was cut in the Cormall Hach using the same procedure as the other materials (above). Consequently the stem fraction was used as two qualities, partly as coarser chips directly from the fractionation plant and partly as a more cut quality. For briquetting, a piston press, Bogma M75 (Bogma Maskin AB, Ulricehamn, Sweden) with a press cylinder diameter of 75 mm was used. The normal working pressure for this machine piston is approximately 1400 kg=cm2 . The machine production capacity can be up to 1000 kg=h depending on the raw material’s loose weight. The press was supplied with approximately a 12 m cooling line (for ambient air cooling of the briquettes).

2. Material and methods

2.3. Fuel characterisation

2.1. Raw material

2.3.1. Raw material After cutting, several samples were taken for particle size distribution and dry matter content analyses. For sampling, a peat drill type “Russian drill” was used. Ten grams of the material was sieved 5 min, using a sieving machine Fritsch analysette with DIN sieves, 2, 1, 0.5, 0.315, 0.25 and 0:1 mm.

The crops used were 1996 and 1997 springharvested reed canary-grass (RCG) from three different locations in Sweden, (termed A, B and C). Samples were chosen to give as large variation in ash content as possible based on earlier analysis on the

±10%

[24]

27

 Fig. 1. Illustration of Oko Therm Compact boiler, 150 kW for dry fuels.

◦

Heated to 550 ± 25 C (oxidising atmosphere).

2.3.2. Briquettes The bulk density was measured by weighing the material in a 20-l vessel. The briquette density was measured by volume calculation and weighing. According to Swedish standards [13,14], the mechanical strength was determined by using a rotating drum during 8 min. The material was then sieved using sieve size 45, 15, 5 and 3 mm.

Ash Content unburned matter a Values are for this lab.

Fusibility of ash

The external shape, i.e. deformation, shrinkage and ow, of a pyramidal pellet of ash during ◦ heating in a laboratory furnace (980 –1650 C) in oxidising atmosphere.

±3%

[23]

[21] [22]

±20% ±1; 2; 2; 3; 4; 4; 9; 3% ±3% Cl Si, Al, Ca, K, Mg, Mn, Na, P

[20] ±10% Total S

[15] [16] [17] [18] [19] ±1% ±5% ±5% ±2% ±5; 5; 20%

By bomb calori c method and calculation of net calori c value, (LECO AC 300). ◦ Sample dried in air at 105 ± 2 C. ◦ Heated to 550 ± 25 C in air. ◦ Heated at 900 C out of contact with air for 7 min. ◦ High-temperature combustion in oxygen (1050 C), with an IR detection procedure (LECO CHN 1000). ◦ High-temperature tube furnace combustion in oxygen (1350 C) with an IR detection procedure, (LECO SC 432). Using Eschka mixture, titration by the Mohr procedure. Melt with LiBO2 ; wet dissolving with HNO3 , nal determination using ICP-AES. Fuel Net calori c heat value Moisture content Ash Volatile matter C, H, N

Precision RSDa Method Parameter

Table 1 Methods used for fuel and ash analyses of reed canary-grass

Reference

S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

2.3.3. Chemical analysis The methods used for chemical analyses are shown in Table 1. Two fuel samples of 1 kg each were collected (from a moving stream) every hour during the combustion tests (5 times for each test). 2.4. Combustion equipment The combustion tests were carried out using a  boiler, Oko Therm compact type C2 with regula  tion system Oko Therm Lambdamatic MC 1.1. (Oko Therm, anlagenbau Fellner GmbH, Hirschau). The boiler has a capacity of 140 –180 kW depending on fuel quality and moisture content. The boiler can handle fuel with moisture content between 5 and 30%. It is equipped with an ash screw, a moving ash stoker and a slag scraper to handle ash-rich fuels and slag. The frequency on the ash pusher can be regulated depending on the ash content in the fuel. See Fig. 1.

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2.5. Chemical analysis of ue gases On-line measurements of the gaseous products O2 ; CO2 , CO and NO were performed during the entire test period. The concentration of O2 was measured using a paramagnetic instrument M&C PMA25 (Maihak AG, Hamburg, Germany). The concentrations of CO2 and CO were measured using an IR-instrument, type Maihak Finor and type Maihak Unor 600 (Maihak AG, Hamburg, Germany). The concentration of NO was measured using a chemiluminescence instrument ECO Physics CLD 700 EL (Maihak AG, Hamburg, Germany). The instruments were calibrated before each combustion test according to the manufacturers instructions. Particles in the

ue gas were determined by isokinetic sampling using STL-miniplus sampler (Metlab Miljo AB, Enkoping, Sweden) and the ue gas rate was calculated and measured with a type S pilot tube, Micatrone (Micatrone Regulator, Gotland Sweden). The sampling time was 1 h and two samples were taken consecutively for each fuel (A, A-stem, A-leaf and C) in experiment series 1. 2.6. Technical parameters The eciency in percent (%) was calculated using the formula; Eciency % = (fuel eect − total losses) ×100=fuel eect. The fuel eect was calculated by using registered fuel feeding data (amount fuel in weight every 20 s) and the net Calori c heat value (wet). The theoretical ue gas losses was calculated and the thermal radiation loss was estimated to 5 kW [25]. The ash losses were calculated when the content of unburned matter in the ash was higher than 15%. The oven temperature was measured using seven type K thermocouples (Pentronic, Gunnebo, Sweden), placed 40 cm above the grate at 25-cm intervals, starting from the fuel inlet to the end of the oven (Fig. 1). The oven temperature values, ue gases temperature, input and output water temperature, lambda value and secondary air were registered every 20 s. 2.7. Ash characterisation Two ash samples were taken directly from the oven just above the ash screw using an in-oven ash sampler every hour during the combustion test. A larger

sample was taken from the oven the day after the test. The analysis method for unburned matter is shown in Table 1. 2.8. Experimental procedure Since the purpose of the study was to see how the fuel composition eects the combustion result, most of the boiler parameters were pre-set for all fuels (see Table 2). Small adjustments were made between experiment series 1 and 2 and also when the dierences in ash content were large between the fuels (see Table 2). The boiler was in continuous operation with, respectively, test fuel 2–2 12 h before the measuring period, which lasted approximately 2 h. Before mea◦ suring, boiler water temperature (65 –70 C), returned ◦ water temperature (55 –65 C), ue gas temperature ◦ ◦ ( ¿ 160 C), oven temperature 1 ( ¿ 1000 C) and ◦ oven temperature 7 ( ¿ 500 C) were stable and equal between the dierent experiments. Before every experiment the boiler was cleaned to remove the old ash. Two series of experiments were performed each comprising of one burning experiment for each fuel in random order. Some smaller adjustments in the operating parameters were made in experiment series 2 except for fuel B. In addition, for determination of the repeatability of the combustion equipment, a pelletised RCG was used in triplicate combustion experiments. The tests were done according to the same experimental procedure as for the briquettes. The boiler adjustments can be seen in Table 2.

3. Results 3.1. Briquetting 3.1.1. Raw material The Cormall Hach HMG produced a material with a high content of nes, 20% less than screen size 0:25 mm and more than 90% of the material was ¡ 1 mm. The sieving analyses showed approximately the same result for all materials. The dry matter content of the ground material varied between 87% and 91%.

S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

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Table 2 Boiler settings during combustion experiments with reed canary-grass Parameter

Pellets experiment

Fuel feeding time, feeding, pause 2, 12 s 95% Primary air a Secondary air (range) 30 –100% Lambda value 1.70 Ash pusher A, A-leaf, C, Pellets Every 20 min A-stem B a In experiment series 2 the primary air was changed to 80% for

Briquette experiment series 1

Briquette experiment series 2a

2, 8 s 95% 30 –100% 1.70

2, 8 s 85% 30 –100% 1.70

Every 20 min Every 20 min Every 30 min the stem briquettes.

Every 15 min Every 10 min Every 30 min

Table 3 Briquette density and bulk density for reed canary-grass briquettes Fuel briquettes

Briquette density (kg=dm3 )

Bulk density (kg=m3 )

A A-stems ( ne) A-leaves B C

1.0 1.2 1.1 1.1 1.0

550 620 470 600 470

3.1.2. Briquettes At the rst briquette trial for fuel A-stem, the coarser raw stem material 20 – 40 mm was used. The briquette production worked well with 100 –150 kg before the material got stuck in the feed screw silo and intermediate bin before the press. The stem material was then cut to the same size, with a 3 mm sieve as the other materials and the briquetting then went well. The briquette production succeeded well with all the other materials. The bulk density and briquette densities are shown in Table 3. Fig. 2 shows how much coarse and ne material the briquettes contained after the mechanical strength test (rotating drum). The stem briquettes ( ne cut) showed the highest value for both bulk density and mechanical strength. 3.2. Fuel characterisation The pellet fuel that was used in order to control the repeatability was of even quality (Table 4). Chemical composition for the briquettes is presented in Table 5. The stem fraction briquettes showed lower concentrations of most nutrients compared to the leaf fraction

Fig. 2. Mechanical strength analyses for reed canary-grass briquettes. Sieve analysis of the material remaining after tumbling test.

briquettes from the same growing place. The most signi cant ash forming elements were Si, Ca and K and the largest dierences between the fuels from dierent growing sites were seen in their Si content. 3.3. Combustion experiments 3.3.1. Experimental set-up stability The three experiments with RCG pellets resulted in good repeatability. Relative standard deviation (RSD) between runs was 1.2% for CO2 , 0.8% for O2 and 5.6% for NO. The combustion was stable with the exception that when the ash pusher moved, short (20 –120 s) high CO-peaks were produced (Fig. 3). These peaks could reach up to almost 20 000 ppm. Between these peaks the CO-value went down to ¡ 100 ppm. The geometric mean (not normally distributed) for CO without peaks for test 1–3 were 86, 46 and 56 ppm, respectively (geometric standard deviation =1:4–1.7). Emissions of CO2 ; O2 and NO showed small within

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S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

Table 4 Fuel analysis of reed canary-grass pellets. Moisture content and ash content are mean values of 5 samplesa Parameter

Pellets test 1

Pellets test 2

Pellets test 3

Net calori c heat value (MJ=kg) 16.3 16.2 16.2 Moisture content (%) 7.2 (0.1) 8.2 (1.1) 8.6 (1.0) Ash content (% DM) 5.4 (0.9) 5.2 (3.9) 5.2 (4.0) ◦ 1390 1380 1370 Initial deformation temperature, IT ( C) ◦ 1480 1480 1460 Softening temperature, ST ( C) ◦ 1520 1530 1520 Hemispherical temperature, HT ( C) ◦ Fluid temperature, FT ( C) 1630 1640 1610 a Relative standard deviation (RSD) can be seen in parenthesis. Other analyses are general samples combined from 5 samples. Table 5 Fuel characteristics of reed-canary grass briquettes useda Parameter

A

Experiment series 1 A-leaf A-stem C

B

A

Experiment series 2 A-leaf A-stem C

Net calori c heat value (MJ=kg wet) 15.8 14.6 16.0 15.4 16.3 15.8 14.9 Moisture content (%) 7.2 (0.3) 10.9 (0.4) 7.2 (0.2) 7.5 (0.2) 8.2 (0.2) 7.4 10.3 Ash content (% DM) 7.4 (2.7) 10.2 (1.8) 5.3 (2.2) 9.1 (2.6) 3.1 (4.9) 7.1 9.9 Volatile matter (% DM) 77 74 80 76 81 77 75 C (% DM) 45 44 46 44 46 45 44 H (% DM) 5.4 5.3 5.5 5.3 5.9 5.4 5.2 O (% DM) 41.5 39.9 42.8 40.7 43.9 41.5 40.2 N (% DM) 0.3 0.5 0.2 0.4 0.6 0.4 0.5 S (% DM) 0.07 0.09 0.05 0.07 0.08 0.07 0.08 Cl (% DM) 0.04 0.04 0.05 0.03 0.04 0.05 0.04 Si (% DM) 3.0 3.9 2.1 3.8 0.91 2.9 3.9 Al (% DM) 0.02 0.09 0.01 0.01 0.03 0.02 0.09 Ca (% DM) 0.13 0.23 0.09 0.13 0.20 0.13 0.23 K (% DM) 0.20 0.27 0.21 0.15 0.18 0.20 0.28 Mg (% DM) 0.05 0.08 0.04 0.05 0.06 0.05 0.08 Na (% DM) 0.1 0.03 0.01 0.01 0.01 0.01 0.03 P (% DM) 0.09 0.12 0.07 0.07 0.08 0.09 0.12 ◦ Initial deformation, IT ( C) 1630 1570 1590 ¿ 1650 1150 1610 1540 ◦ Softening temperature, ST ( C) 1640 1580 1620 ¿ 1650 1260 ¿ 1650 1550 ◦ 1630 ¿ 1650 1360 ¿ 1650 1610 Hemispherical temperature, HT ( C) ¿ 1650 1630 ◦ ¿ 1650 1650 1650 ¿ 1650 1370 ¿ 1650 1650 Fluid temperature FT ( C) a Moisture content and ash content are mean values from 5 samples. Relative standard deviation (RSD) can be analyses are a general sample combined from 5 samples. Samples are taken during experiments.

16.0 15.4 7.5 7.6 5.5 9.2 79 76 46 44 5.5 5.2 42.7 40.9 0.2 0.4 0.06 0.08 0.04 0.03 2.1 3.9 0.02 0.01 0.11 0.12 0.23 0.15 0.05 0.05 0.01 0.01 0.08 0.07 1530 ¿ 1650 1590 ¿ 1650 1600 ¿ 1650 ¿ 1650 ¿ 1650 seen in parenthesis.

B 16.4 8.2 3.0 81 47 5.7 43.9 0.6 0.07 0.04 0.92 0.03 0.16 0.18 0.06 0.01 0.07 1150 1290 1360 1390 Other

run variation, i.e. less than a RSD of 9% for CO2 , 16% for O2 and 9% for NO.

Fig. 3. Variation of CO-emissions during combustion of reed-canary grass pellets test 3.

3.3.2. Briquette combustion In Table 6, mean values for fuel eect, losses, eciency, ue gas temperature, lambda value (set to 1.70) and secondary air (vary between 30 and 100%) are presented for the briquette experiment; whereas, the mean values for the chemical ue gas analyses, oven temperatures and dust analyses are summarised in Table 7. The fuel with lowest ash content gave the highest eect due to higher net calori c

S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

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Table 6 Combustion of reed-canary grass briquettes. Technical parameters, (average values) from experiments 1 and 2 Fuel

Fuel eect (kW)

Losses (kW)

A1 161 21 A2 161 24 A-leaf 1 138 19 A-leaf 2 149 23 C1 167 23 C2 167 24 A-stem 1 165 21 A-stem 2a 144 22 B1 178 25 B2 189 27 a Briquettes contain a coarser fraction.

Eciency (%)

Flue gas ◦ temperature ( C)

Lambda value

Secondary air %

87 85 86 85 86 86 87 85 86 86

172 188 170 186 183 195 173 159 194 202

1.70 1.77 1.73 1.82 1.68 1.73 1.70 1.78 1.68 1.67

95 38 47 31 72 45 54 32 80 91

Table 7 Combustion of reed-canary grass briquettes. Flue gas parameters and oven temperatures expressed as mean values and relative standard deviation Experiment series 1 A

A-leaf

A-stem

O2 % ppm 9.2 (11) 9.7 (10) 9.2 (11) CO ppma 64 (1.4) 44 (1.6) 78 (1.6) 35 25 42 CO mg=MJa CO2 % 10.5 (8) 10.5 (9) 11.3 (8) NO ppm 163 (10) 176 (11) 117 (9) NO mg=MJ 95 109 68 ◦ 1064 (3) b 1063 (3) Oven temp 1 C ◦ Oven temp 2 C 1026 (7) 983 (7) 1038 (4) ◦ 898 (7) 818 (5) 869 (5) Oven temp 3 C ◦ 785 (6) 699 (6) 792 (6) Oven temp 4 C ◦ 743 (6) 681 (6) 739 (6) Oven temp 5 C ◦ 713 (5) 653 (5) 691 (6) Oven temp 6 C ◦ Oven temp 7 C 603 (5) 601 (6) 613 (6) 163 147 Particles mg=Nm3 117 (13% CO2 ) a The geometric mean value is used for CO since data b Data are missing. c n:a: = not analysed.

Experiment series 2

C

B

A

A-leaf

A-stem

C

B

9.1 (12) 28 (2.4) 15 11.3 (9) 190 (9) 110 1106 (6) 1075 (4) 809 (4) 787 (5) 753 (5) 729 (5) 647 (7) 137

9.5 (11) 63 (1.4) 35 10.8 (8) 164 (9) 97 1015 (2) 956 (4) 891 (4) 811 (5) 765 (5) 737 (4) 700 (6) n.ac

9.9 (11) 71 (1.5) 41 10.2 (9) 174 (7) 108 1083 (3) 1004 (3) 879 (7) 798 (8) 586 (5) 687 (3) 739 (6) n.a.

9.9 (11) 59 (1.5) 25 10.2 (9) 166 (8) 102 1083 (3) 1004 (3) 879 (7) 798 (8) 586 (5) 687 (3) 739 (6) n.a.

10.6 (9) 274 (1.4) 168 10.0 (9) 116 (10) 76 956 (4) 891 (5) 741 (5) 675 (5) 654 (4) 630 (4) 543 (4) n.a.

9.6 (11) 43 (1.5) 24 10.6 (8) 190 (9) 113 1116 (3) 1048 (2) 889 (6) 808 (7) 761 (5) 707 (4) 647 (5) n.a.

9.4 (10) 38 (1.4) 35 10.8 (7) 174 (8) 101 1028 (2) 934 (4) 842 (5) 796 (5) 777 (5) 744 (4) 721 (5) n.a.

are not normally distributed. Geometric standard deviation is given in parenthesis.

heat value. Furthermore, the ue gas losses were quite high because of high ue gas temperatures and high excess air. In experiment series 2 the ue gas temperature and lambda value slightly increased and the secondary air decreased. The ue gas analyses resulted in low CO values, low values of particles in the ue gas and high oven temperatures. There were no problems with high CO-peaks like in the experiments with pellets (Fig. 4). It was noticed that the ash pusher eected the CO value in the beginning of the experiments but when

the ash bed was high enough (after 2 h combustion) the CO value stabilised. There were hardly any dierences in emission levels of the measured components between briquettes with low and high ash content. It is shown in Table 7 that the experiment with the stem briquettes in series 2 diered somewhat from the other results by exhibiting higher CO emissions and lower oven temperatures. Figs. 5 and 6 shows the eect of fuel nitrogen content on NO emissions. The amounts of NO in the ue gas were fairly similar for fuel A–L, C and B (fuel-N content 0.4 – 0.6 wt%) and

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S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

Fig. 4. Variation of CO-emissions during combustion of reed-canary grass briquettes, experiment series 1, fuel A.

Fig. 6. The conversion of percent fuel-nitrogen to NO vs. fuel-N content. Fuel-N 0.2– 0.6%: () A-S; ( ) A; (•) C; (4) A-L; ( ) B.

Fig. 5. The concentration of NO mg=MJ vs. fuel-nitogen content. Fuel-N 0.2– 0.6%: () A-S; ( ) A; (•) C; (4) A-L; (◦) B.

slightly lower for fuel A and A-stem (fuel-N content 0.2– 0.3 wt%). Good correlation was found when percent conversions of fuel-N to NO were plotted as a function of fuel-N content (Fig. 6). The high ash content in the fuels resulted in large amounts of ash in the oven, which were removed by the continuously operating ashing system. The oven contained more ash in experiment series 1 as compared to experiment series 2, where the ash pusher operated more frequently except for the experiments with the low ash content, fuel B. The ash formed light voluminous big chunks, (see Fig. 7). Small parts showed tendency to be sintered. For sample B large parts of the ashes were melted. The ash analyses showed low contents of unburned matter in the ash for all materials except for the stem fraction in series 2 that showed slightly higher values (Table 8).

Fig. 7. Ash from combustion of reed canary-grass briquettes. The ash chunk has a diameter of approximately 12 cm.

4. Discussion 4.1. Briquetting There are several parameters that might have affected the mechanical strength of the briquettes in this study (Fig. 2). A number of variables aect the briquetting process and the properties of the briquettes produced. These include raw material properties like particle size, moisture content, chemical composition as well as variables in the briquetting process (pressure, temperature) [26,27]. Although briquetting is a well-established technology, the mechanism by which compacted biomass attains self-bonding is not clear and several authors have dierent opinions about the

S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35 Table 8 Combustion of reed-canary grass briquettes. Analysis of content unburned matter in the ash. The mean of 4 samples and relative standard deviation (RSD, %) Fuels

Content unburned matter Experiment 1

A 2.0 (19) A-leaf 1.4 (21) A-stem 2.9 (45) C 4.4 (90) B 2.1a a One sample from oven.

Content unburned matter Experiment 2 8.2 (75) 1.3 (10) 15 (9) 3.7 (38) 5.1 (31)

parameters role for the briquette product [26,27]. In this context the explanation for the low mechanical strength compared to wood briquettes [3] is because of a too high content of nes and the material being too dry. A higher mechanical strength for the stem briquettes is probably due to a higher content of lignin and=or hemicellulose. Briquetting of newly harvested material can be expected to result in briquettes with a higher mechanical strength since DM of such material is typically 85%. 4.2. Fuel characterisation The fuel data obtained in this study and presented in Table 5, tend to show the same trend as earlier studies [6,8]. The lowest ash content (3%) was found in the crop from a humus-rich soil and the highest ash content (9%) in clayey silt. The concentrations of inorganic materials in the stem were about half of those in the leaves, except for K and Cl which were about the same for stem and leaves. There seems to be a correlation between high Si content and high ash content and the ratio Si : Ca + K (6 –9) are similar for all fuels with the exception of material B and C. Fuel B shows higher content of Ca and K (ratio Si : Ca + K = 2) and fuel C shows lower content of Ca and K (ratio Si : Ca + K = 14). As presented earlier [6,8], the increase of the initial ash fusion for RCG seems to be correlated to the ratio Si : K. The fusion temperature is increased by Si and lowered by K. The N-content in the fuels was slightly lower than earlier studies have shown [28]. An explanation is that the RCG used is taken from elds that were fertilised with small amounts of N.

33

4.3. Combustion experiments The initial tests with pellets showed for all parameters a good stability and repeatability adequate for the study. The short time CO-peaks observed are probably caused by a compact ash bed, more sensitive to mixing by the ash-pusher. Such peaks were considerably less pronounced when later using briquettes. In Table 7 it can be seen that a higher ash content did not have an eect on the combustion result. The presented data showed low mean values for CO and particles in the ue gas for all fuels. Fuel NO is probably the main source of emissions from these combustion experiments since thermal NO is of minor importance where the combustion chamber tem◦ perature is ¡ 1300 C. The NO emissions increased non-linear with increasing fuel-N content since the percent conversion of the evolved nitrogen decreased at the higher nitrogen levels (Figs. 5 and 6). This is in good agreement with previous studies [9,10,29,30]. There are several other parameters that can in uence the NO formation such as ame temperature and excess air ratio [10,31,32]. The excess air was relatively high in the experiments and it should be possible to decrease the NO values by optimisation, thus lowering the excess air. The content of particles in the ue gas is considered low since the analyses were done on raw ue gas. A simple multi-cyclone is probably enough to obtain levels below 100 mg=nm3 which is the recommended value from the Swedish supervision authorities. In Table 6 it can be seen that there are slight differences between the results in experiment series 1 and 2. In experiment series 2 the ash pusher worked more frequently, except for fuel B, which decreased the need for secondary air and thus increased the combustion intensity and the temperature. The higher ue gas temperature for fuel B is explained by a higher fuel eect. Also material B was used in the last experiments where the convection area contained slightly more ash which might have reduced heat transfer and thus resulting in a higher ue gas temperature. The most divergent result was seen in material A-stem (coarse fraction) in experiment 2. The coarser fraction (20 – 40 mm) resulted in briquettes with lower bulk density and consequently a lower fuel rate per hour. The lower fuel eect gives lower oven temperature and ue gas temperatures, which might explain the increased CO

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S. Paulrud, C. Nilsson / Biomass and Bioenergy 20 (2001) 25–35

emissions. The coarser fraction may also decrease the ability for the ash to fall o the briquettes and thus increase the content of unburned matter inside the briquette. This is observed as an increase of unburned material in the ash from that experiment (Table 8). This increase has also been reported from the eld. An alternative explanation may be the frequency of the ash pusher which worked 5 min more frequently compared to the other fuels in experiment 2 (Table 2). A more frequently operated ash pusher is likely to effect the size of the ash bed and dwell time for the fuel, which also might explain the lower temperature and higher amount of unburned matter in the ash. A lot of ash was formed during the combustion but it was no problem for the boiler. However, the ash bed produced using the high ash content fuels tended to be substantial in experiment series 1. This could be a problem in long run tests and thus, it is of importance to optimise the frequency of the ash pusher. The combustion result was not better using RCG with 3% ash content as compared to 10% ash content but since the purpose of the study was to see how the ash content aects the combustion result the combustion eciency could be improved by optimising the boiler for each fuel. In this study the boiler parameters were set equally the same or nearly for the dierent fuels. Further, it is also important to study the fusibility of the ash since parts of the dierent ashes showed a tendency to melt, especially sample B. The initial deformation temperature was also lower for sample ◦ B (1150 C) due to lower ratio Si : K. However, the method used for the fusibility of ash analysis in this study is criticised in the literature [33,34]. One criticism is that the melting starts at far lower temperature than what the analysis shows. More detailed studies on these RCG ashes are therefore in progress. 5. Conclusions The results from the briquetting of RCG imply that the variation in ash content did not aect the briquette product. Furthermore, RCG that contained more stem fraction gave briquettes with a higher mechanical strength. However, it should be considered that RCG that contains more silicon may aect the wear and tear of the die and pressure parts when briquetting large volumes of fuel.

The emission analyses imply that variation in ash content does not aect the result. By optimisation and using a multi-cyclone, the excess air, NO emissions and the particle in the ue gas should be possible to lower further. The combustion experiments imply that springharvested reed canary-grass can be red with success in combustion equipment that is designed for the differences in ash content and that can handle high ash contents. However, even if no severe deposit build-up or sintering was detected, extended time test runs are required in order to study this subject in more detail. Acknowledgements The sta at Energy Technology Centre, Pitea, Sweden and MBAB energi, Robertfors, Sweden are gratefully acknowledged. This study was nancially supported by the National research programme on energy grass (grants from SLF, NUTEK and Vattenfall AB). References [1] Landstrom S, Lomakka L, Andersson S. Harvest in spring improves yield and quality of reed canary grass as a Bioenergy crop. Biomass and Bioenergy 1996;11(4): 333–41. [2] Hadders G, Olsson R. Harvest of grass for combustion in late summer and in spring. Biomass and Bioenergy 1997;12(3):171–5.  [3] Burvall J, Orberg H. Brikettering av ror en-teknik och ekonomi. (Fuel briquettes from reed canary grass-technical and economical aspects. Robacksdalen meddelar, Swedish University of Agricultural Sciences, vol. 10, 1994 (in Swedish). [4] Obernberger I, Biedermann F, Widmann W, Riedl R. Concentrations of inorganic element in biomass fuels and recovery in the dierent ash fractions. Biomass and Bioenergy 1997;12(3):211–24. [5] Miles R, Miles JR, Baxter L, Bryers R, Jenkins B, Oden L. Boiler deposits from ring biomass fuels. Biomass and Bioenergy 1996;10(2–3):125–38. [6] Burvall J. In uence of harvest time and soil type on fuel quality in reed canary grass (Phalaris Arundinacea L.). Biomass and Bioenergy 1997;12(3):149–54. [7] Nordin A. Chemical elemental characteristics of biomass fuels. Biomass and Bioenergy 1994;6:339–47. [8] Burvall J, Hedman B, Landtrom S. Standortens inverkan pabranslekvalitet hos strabranslen (In uence of site class on

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