Pelletization of Compost for Energy Utilization

Available online at www.sciencedirect.com

ScienceDirect IERI Procedia 8 (2014) 2 – 10

2014 International Conference on Agricultural and Biosystem Engineering

Pelletization of Compost for Energy Utilization Ondrej Zajonca*, Jan Frydrychb, Lucie Jezerskaa a

VSB-Technical University of Ostrava,17.listopadu 15/2172, 708 33 Ostrava, Czech Republic b OSEVA Development and Research Ltd., Hamerska 698, 756 54 Zubri,Czech Republic

Abstract The presented paper tested pelletization of seven compost samples from six composting plants. If the produced compost meets the legal requirements for application to agricultural land, it is suitable for application in the soil. Low-quality composts that cannot be applied to soils or for which there is no demand can be energy utilized for co-incineration with fuels of high calorific value. Compost pelletization represents a type of processing that makes the handling of the material easier and enables more accurate dosing. Mechanical properties were determined for the produced pellets. The average durability of pellets expressed as the Pellet Durability Index was 94.1 %. The average pellet hardness expressed as the load value was 17.5 kg. The average value of moisture resistance expressed as the Wettability Index was 18.0 %. These pellets are of lower quality than pellets produced from spruce wood (Picea abies L.). © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 Ondrej Zajonc, Jan Frydrych, Lucie Jezerska. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of Information Engineering Research Institute Selection and peer review under responsibility of Information Engineering Research Institute

Keywords: Compost, energy utilization, pelletization

1. Introduction Composting represents an increasingly popular method of processing biodegradable wastes. Compost should be preferentially applied to the soil in order to increase its quality [1], [2], [3], [4]. Agriculture in many European countries is facing an interrupted carbon cycle and a decrease of soil organic carbon (SOC) [5].

* Ondrej Zajonc. Tel.: +420-597-329-431 E-mail address: [email protected].

2212-6678 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of Information Engineering Research Institute doi:10.1016/j.ieri.2014.09.002

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Application of compost is one way to add total organic carbon to the soil [6]. During compositing, composts that do not meet the legal parameters for their use in agriculture and recultivation (content of harmful substances, insufficient quality) [7] are also produced or there is insufficient demand for the produced composts. If these composts meet the legal requirements, they may be used for energy utilization. Energy utilization of composts is not intended for small incinerators (household boilers). One possible method of energy utilization of composts is their use in the form of pellets. Pelletization consists of adjustment of the material to a more compact form. The advantage of pelletization is easier handling, lower costs for transport [8] and more precise dosing to combustion device [9]. 2. Material and methods Energy properties of composts have already been studied in [10], [11]. Seven compost samples have been taken from six composting plants. Raw materials for the production of composts consisted mainly of wastes from the city greenery maintenance, sludge from waste water treatment plants and wood chips and one compost was produced within home composting [10]. The most problematic parameter for compost processing is moisture; the average moisture in the sample composts from the composting plants was 57.3 % (sampling was performed during winter). [10]. In order to be able to utilize composts for energy, they need to be dried to a value suitable for treatment and pelletization (10 - 30 % depending on the used treatment equipment). Before the pelletization itself, composts were dried in a laboratory drier to approximately 6 %. In practice, composts do not need to be dried to such a low value. After drying, the compost was crushed in the biomass hammer crusher Green Energy 9FQ 50. The average grain size distribution of composts is listed in Table 2. The pellet mixture was moistened to the suitable value and then pelletized. Table 1. Entry raw materials of composts Compost sample labelling

Entry raw materials

Velka Polom

stabilized sludge from waste water treatment plants, wastes from plant production, wastes from animal production

Rymarov

1 - grass, leaves, branches - chipping (wastes generated from city greenery maintenance) 2 - grass, branches - chipping (wastes generated from city greenery maintenance) 3 - grass, leaves, branches - chipping (wastes generated from city greenery maintenance), sludge from waste water treatment plant

Trinec (energy compost)

wastes generated from city greenery maintenance, wood chipping

Vratimov

compost produced in households. Entry raw materials: straw, hey, rabbit and sheep dung, sawdust, oat remains, potatoes, chicken dung.

Kuncicky u Basky

grasses, wastes generated from city greenery maintenance

2.1. Compost pelletization A problematic parameter for compost pelletization is the moisture of the pelletized mixture. In the tested samples the optimum value for pelletization ranged between 25 - 30 % (vol.), from which it is clear that the samples needed to be dried before their energy utilization. The optimum humidity value depends on the compost composition. Composts were pelletized on the Green Energy JGE 120 pellet press. The pellets were then repeatedly dried in a laboratory drier. Finally, the monitored parameters were measured (durability,

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hardness, moisture resistance) In order to verify the impact of the pelletizing press on the quality of pellets, one compost sample was additionally pelletized on the KAHL 14-175 pellet press. 2.2. Determining parameters of pellets Durability of pellets expressed as the Pellet Durability Index (PDI) was measured. Durability represents the amount of shattering after mechanical or pneumatic shuffling during transport [12] PDI was determined on the Holmen NHP 100 device. A 100 g sample was put into the tester where the pellets circulated in a chamber shaped like an inverted pyramid with perforated walls for a determined period of time (60 s) where they striked to each other and to the chamber walls. The shattering from pellets fell outside the chamber. Then the final sample was sieved through a sieve with a mesh size of 3.15 mm. The sample weight was measured before and after the assessment. PDI is calculated as the percentage of the weight after assessment from the weight before assessment (1).

PDI

m2 u 100 m1

(1)

where m2 - sample weight after testing; m1 - sample weight before testing [13], [14]. The testing was repeated five times. The average value was calculated from the assessment results. Hardness represents the force necessary for crushing or deformation a pellet [12]. Pellets with higher hardness have better quality and higher mass per volume [15]. Hardness of the pellets was measured by the KAHL ak14 hardness tester. A pellet was inserted between compressing surfaces that simulated load on the pellet by gradually compressing it until it cracked. The load value expressed in kg is read from the scale. Moisture resistance is expressed as the so-called Wettability Index (WI). When determining the Wettability Index, the tested pellet is weighed and then submerged in water for 30 s. After testing the pellet is weighed again. The Wettability Index is calculated as the percentage of the difference in weight of the pellet before and after testing from the pellet weight before testing (2) [16].

WI

m2  m1 u 100 m1

(2)

2.3. Evaluated compost parameters The evaluated parameters in composts included the content of C, H, N, O, S, moisture, content of main biomass components (lignin, cellulose, hemicellulose), ash content and calorific value. These are the most important parameters for energy utilization of materials. Individual parameters are provided in Table 2, 3 and Graph 1 [10]. 3. Results and Discussion

3.1. Compost parameters Table 2. Grain size distribution percentage of compost before and after crushing for pelletization Moravian – Silesian Region composts’ particles distribution (average values of the samples)

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Ondrej Zajonc et al. / IERI Procedia 8 (2014) 2 – 10 Grain size [mm]

< 0.25

0.25 - 0.5

0.5 - 1.0

1.0 - 1.6

1.6 - 2.0

2.0 - 2.5

2.5 - 3.15

>3.15

Before crushing [%]

24.6

18.5

18.8

9.9

9.9

6.9

4.7

6.8

After crushing [%]

26.9

26.1

25.8

15.1

3.1

2.0

1.0

0

Table 3. Proximate and ultimate parameters of the composts [10] Sample

Moisture [%]

Ash [%]

Calorific value (d.m.) [kJ/kg]

Calorific value (w.m.) [kJ/kg]

C [%]

H [%]

O [%]

N [%]

S [%]

Velka Polom

72.25

44.13

9,571

994

26.3

3.4

23.9

2.2

<0.1

Rymarov 1

60.53

56.82

8,407

1,926

23.4

3.1

14.5

2.1

<0.1

Rymarov 2

57.33

64.15

5,706

1,116

21.0

2.8

10.1

2.0

<0.1

Rymarov 3

41.57

65.14

5,863

2,469

19.9

2.7

10.6

1.7

<0.1

Trinec (energy compost)

28.88

40.45

12,092

7,936

26.9

3.6

26.9

2.2

<0.1

Vratimov

77.31

46.20

10,204

537

27.8

3.9

20.5

1.6

<0.1

Kuncicky u Basky

63.05

56.60

8,437

1,667

25.1

3.6

12.7

1.9

<0.1

The calorific value of evaluated composts ranged between 5,706 kJ/kg (d.m.) for composts produced from waste water treatment plant sludge and 12,092 kJ/kg (d.m.) for energy compost (compost with addition of wood chippings) [10]. The average calorific value was 8,611 kJ/kg (d.m.). These values were measured in dried composts. The calorific value of composts in the delivered state ranged between 537 (w.m.) to 7,936 kJ/kg [10]. The average calorific value of composts in the delivered state was 2,378 kJ/kg. Composts for energy utilization should be prepared in a sheltered area to prevent over-watering due to precipitation. Due to the lower average calorific value (d.m.), composts should be used in a mixture with a more calorific fuel. This can be achieved by pelletization of a mixture of compost and a more calorific fuel [11] or pelletization of only composts and then mixing them with a more calorific fuel (coal, wood chippings). The ash content of composts ranged between 40 - 65 % [10]. The average ash content for the compost samples was 53.4 %. Due to the high content of ash and low content of carbon, this fuel is of lower quality than phytomass or coal. That is why composts need to be co-incinerated with higher quality fuel. For comparison, the content of ash in black coal is 29.16 %, in pine wood it is 3.45 % and in corn it is 7.37 % [17]. The ash content in Central European energy grasses ranges between 5 - 10 % [18]. The composition of ash depends on the compost composition, since ash remains in composts from entry components of the compost. That is why the ash composition in different composts might vary. The ash content increases with the duration of the maturing period. The ash content in biomass is problematic due to the lower melting point of ash, which can cause sintering of the ash contents- [19], [20], [21], [22], [23]. This is caused by the content of alkaline metals K, Na and content of Si [24], [25], [26]. Sintering of ash can be suppressed just like in other types of biomass [27] by cocombustion of composts with coal. This variant is more suitable also with regards to the necessity to incinerate compost with fuel with higher calorific value. Main components of biomass are cellulose, hemicellulose and lignin [28], [29]. The lignin content ranged between 12 - 24 % [10]. The average lignin content was 18.8 %. For softwood the lignin content ranges between 25 - 35 %, for hardwood it is 18 - 25 % [30]. The lignin content in pine wood is 26.7 %, in hornbeam wood it is 20.1 % and in walnut wood it is 25.9 % [31]. The lignin content for grasses ranges between 14 - 19 % [32]. For energy grasses cultivated in Central Europe the lignin content ranged between 12 - 21 %

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[18]. Lignin works as a binder in biomass [33]. Lignin has a lower plastification value, ranging between 77 - 128 °C [34]. A temperature of about 100 °C is reached during pelletization due to friction and compression of materials. Lignin works as one of the binders during compost pelletization. Compost contains particles with easy deformability and it can be therefore pelletized completely without binders. The cellulose content in composts ranged between 15 - 24 % [10]. The average cellulose content was 19.1 %. The cellulose content in pine wood is 49.8 %, in hornbeam wood it is 48.9 % and in walnut wood it is 47.8 % [31]. The cellulose content in energy grasses cultivated in Central Europe ranged between 42 - 56 % [18]. The hemicellulose content of composts ranged between 5 - 16 % [10]. The average hemicellulose content was 8.9 %. The hemicellulose content in pine wood was 20.8 %, for hornbeam wood it was 23.3 % and for walnut wood it was 22.1 % [31]. The hemicellulose content in energy grasses cultivated in Central Europe ranged between 20 - 34 % [18].

Fig. 1. Ash and main components of biomass content in compost [10]

The nitrogen content of compost ranged between 1.55 - 2.23 % [10]. The average nitrogen of compost was 1.95 %. The stated content of nitrogen in wood is 0.1 - 0.5 %, for herbal biomass it is 0.5 - 4.0 % [35]. The stated content for Central European energy grass species is 0.6 -1.75 % [18]. The nitrogen content in fuel is one of the parameters that causes NOx emission [35]. Co-combustion of compost with coal is advantageous also due to the higher content of nitrogen in compost. 3.2. Mechanical properties of pellets The Pellet Durability Index ranged between 89.4 and 97.5 %. The average durability value was 94.1 %. Czech standard CSN EN 14961-6 for non-woody pellets lists a minimum value of 96.0 % [36]. Values required by this norm were met only by pellets produced from samples Velka Polom a), Vratimov and Trinec (energy compost). Lower durability was caused by the use of composts with a higher ash content. The higher the ash content, the lower the durability of pellets. None of the samples met the limit of 10% [36] for ash

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content stipulated in the Czech Standard CSN EN 14961-6. The durability of pellets depends on moisture of the pelletized mixture and on the compost composition.

Fig. 2. Influence of ash content on PDI

It appears that the optimum moisture value is between 25 - 30 %. This value might vary based on the compost composition. Higher values are not recommended due to higher moisture values in produced pellets. Moisture values below 25 - 30 % can be used for composts with higher content of main components of biomass (lignin, cellulose, hemicellulose). Based on literature, the optimum moisture for pelletization of spent mushroom compost is 20 % [37]. In the evaluated samples, mechanical resistance increase with higher contents of main biomass components in the compost. Pellet hardness expressed as pellet load in kg ranged between 10.1 to 27.4 kg of load. The average hardness value was 17.5 kg. For comparison, the hardness of spruce wood pellets is 21 kg [18] Pellets from compost are of lower quality than pellets produced from plant biomass. The Wettability Index values ranged between 8.9 - 29.2 %. The average Wettability Index value was 18.0 %. For comparison, the provided Wettability Index values of spruce wood pellets from two producers were WI = 59.6 % and WI = 83.2 % [18]. Table 4. Properties of the composts pellets

Compost labelling Kuncicky u Basky a)

Moisture of the pelletized mixture [%]

PDI (60s)

Hardness

WI

(not the optimum moisture)

[%]

[load in kg]

[%]

26.4

91.2

20.2

18.3

Kuncicky u Basky b)

39.9

95.8

17.9

8.9

Rymarov 1

29.4

95.7

18.4

13.8

Rymarov 2 a)

25.2

89.4

19.6

12.6

Rymarov 2 b)

27.8

90.1

10.2

26.1

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Rymarov 2 c)

36.2

94.3

10.1

29.2

Rymarov 3

23.0

92.8

14.5

18.6

Velka Polom a)

31.5

96.8

18.3

15.7

Velka Polom b)

28.9

94.9

18.1

20.9

Vratimov

43.38

97.5

17.8

9.3

Trinec(energy compost)

25.9

97.1

27.4

24.8

a),b),c) – different moisture values of pelletized composts

4. Conclusions

Composts that do not meet the legal quality requirements for application to the soil or for which there is insufficient demand can be used for energy utilization. Pelletization represents one possible form of utilization. In order to dry composts to the required value, waste heat can be used to reduce energy inputs necessary for compost pelletization. This problem can also be solved by composting biomass in a sheltered composting plant to prevent increase of moisture due to atmospheric precipitation. Due to the lower melting point of ash, it is suitable to co-combustion pellets with coal. That is why combustion of composts in small boilers is not recommended. Co-combustion is suitable also for increasing the calorific value; the average calorific value is 7,890 kJ/kg (d.m.). The calorific value ranged between 5,706 kJ/kg and 12,092 kJ/kg (d.m.). The optimum moisture of pelletization mixtures is approximately between 25 - 30 % (w./w.). The optimum moisture value for pelletization depends on the compost composition and might vary for different composts. The durability of pellets decreases when the ash content increases. The average durability expressed as the Pellet Durability Index was 94.1 %. The durability value determined in CSN EN 14961 -6 (96.0 %) was met by three pellet samples. None of the pellet samples met the ash content required by the standard. During pelletization of a mixture of compost and wood chips, the mechanical resistance value was 97.1 %. The average pellet hardness expressed as the weight load value was 17.5 kg. The average value of moisture resistance expressed as the Wettability Index was 18.0 %.

Acknowledgements

This paper has been elaborated in the framework of the project New creative teams in priorities of scientific research, reg. no. CZ.1.07/2.3.00/30.0055, supported by Operational Programme Education for Competitiveness and co-financed by the European Social Fund and the state budget of the Czech Republic. This paper was supported by research projects of the Ministry of Education, Youth and Sport of the Czech Republic: the Centre ENET CZ.1.05/2.1.00/03.0069. This paper was supported by the research projects of Ministry of Education, Youth and Sport of the Czech Republic: MSMT SGS SP2014/54 Research of the effects of a catalyst and technical parameters on properties and yield of process gases created in thermochemical degradation equipment.

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