nr 578, 2014, 71–80
Corresponding author – Adres do korespondencji: Sławomir Obidziński, Politechnika Biało- stocka, Wydział Budownictwa i Inżynierii Środowiska, Zakład Inżynierii Rolno-Spożywczej i Leśnej, ul. Wiejska 45E, 15-351 Białystok, e-mail: s.obidzinski@pb.edu.pl
THE PRODUCING OF THE FUEL PELLETS FROM A MIXTURE OF OAT STRAW AND POTATO PULP Sławomir Obidziński
Politechnika Białostocka
Abstract: The aim of the research described in the present paper was to determine the best possible parameters for producing biofuel in the form of pellets from a mixture of oat straw and potato pulp. The research of the densifi cation process of mixtures were carried out on SS-4 stand whose main element is a P-300 pellet mill with a ‘fl at matrix-densifi cation rolls’ working system. The densifi cation process of mixtures tests were conducted for the working gap between the densifi cation roll and the fl at matrix, measuring 0.4 mm, and for the mass velocity of the mixtures of approx. 50 kg·h–1. The diameter of the openings in the matrix used in the tests equalled 8 mm, and the length of the openings in the matrix was 28 mm, while the rotational speed of the matrix was 280 rpm. During the measurements, the author assessed the infl uence of the content of potato pulp (10, 20, 30, and 40%) in the mixture with oat straw on the electric energy consumption of the motor which was driving the pellet mill and on the kinetic durability of obtained pellets. The kinetic durability of pel- lets was determined with the use of a Holmen tester. The pellets were kept in the chamber of Holmen tester for 60 s. Designating the materials moisture (potato pulp, oat straw and mixtures potato pulp and oat straw) was performed pursuant to Polish Standards with the use of a WPE 300S scale-dryer. Increasing the content of potato pulp in the mixture with oat straw from 10 to 40% brought about a reduction of the of the electric energy consumption of the pellet mill by approx. 66% (from 5.94 to 2.03 kW) with a simultaneous decrease in the kinetic durability of the pellets by approx. 40% (from 94.65 to 54.99%). The research results obtained in the working system of the pellet mill, made it possible to conclude that in order to produce pellets of satisfactory quality (density and kinetic durability), the content of potato pulp added to oat straw should not exceed 20%. The performed research has made it possible to develop a detailed technology of producing heating pellets, which resulted in fi ling a patent application.
Key words: waste, potato pulp, straw, pelletisation, kinetic durability
INTRODUCTION
A big source of energy from biomass is the agricultural-food industry which ge- nerates huge amounts of post-production waste (e.g. buckwheat hulls obtained during the production process of groats in grain processing plants, fruit pomace left over from juicing facilities, rapeseed pomace obtained in rapeseed oil production, or herbal wa- ste). One method of biomass waste management is producing solid fuels in the form of pellets or briquettes [Laskowski and Skonecki 1999, Hejft 2002, Shaw 2008].
Stelte et al. [2011] claim that fuel pellets were made from beech, spruce and straw, which represent the three most common classes of biomass used for fuel pellet pro- duction, i.e. hardwoods, softwoods and grasses, but for fuel pellet production, scientists used also other kinds of biomass: various cereal straw [Adapa et al. 2010], waste left after trimming olive trees [Carone et al. 2010], rice husk, coconut fibre [Chin and Sid- diqui 2000], different types of grass [Mani et al. 2006], compost created after mushro- om production [Ryu et al. 2008] or barley straw [Serrano et al. 2011].
Numerous experimental research studies of the densification process of different mixtures of biomass and waste from the agricultural and food industry, e.g. mixtures of various types of plant waste with different additives (also of plant origin) serving as binder, are conducted by various scientific centres in Poland and abroad. The additives significantly influences the energy consumption of the pelleting process [Nasrin et al.
2008, Chou et al. 2009, Mediavilla et al. 2009, Stahl and Berghel 2011, Mediavilla et al. 2012] and the mechanical (density and kinetic durability) of the obtained pellets [Finney et al. 2009, Miranda et al. 2009, Sotannde et al. 2010, Filbakk et al. 2011, Ser- rano et al. 2011, Celma et al. 2012, Miranda et al. 2012], causes changes of the heating value [Chou et al. 2009, Miranda et al. 2009], and ash content [Miranda et al. 2009, Miranda et al. 2012]. It also has an influence on the stability of the process [Mediavilla et al. 2012].
One of the types of plant waste that can serve as a binder additive is potato pulp.
The company PEPEES S.A. in Łomża, processes approx. 150 thou. tons of potatoes annually during the potato campaign, provided that the potato yield is good. It means that waste in the form of approx. 22,500 tons of pressed potato pulp is generated [Obi- dziński et al. 2008]. The heat of combustion for dry potato pulp (with moisture content of 4.96%) amounts to 16.33 MJ·kg–1, the calorific value is 15.41 MJ·kg–1, and moisture levels, exceeding 88% [Obidziński 2012]. Such moisture content is a serious problem in using potato pulp as (raw) material for producing solid fuel in the form of pellets or briquettes. Therefore, potato pulp needs to be additionally dried before it is pelletised and mixed with low-moisture materials. Another important aspect in favour of using potato pulp as solid fuel are its anti-corrosion properties. Król [2012] claims that if chlorine is in the fuel in the form of alkali metal salts or during the combustion process creates them, then it results in lowering the softening temperature of ash, which in turn leads to enduring boiler furnaces, defluidization of fluidized bed and high temperature corrosion.
The purpose of the research described in the present paper was to establish the best parameters for producing biofuel in the form of pellets from a mixture of oat straw and
potato pulp. The basic aim of the research was to determine the influence of the potato pulp content in the mixture with oat straw on the energy consumption of the pelletising process as well as on the quality of the obtained pellets, with respect to using potato pulp as heating fuel.
MATERIALS AND METHODS Research material
The research materials subjected to the densification process were oat straw (came from a private agricultural farm located near Białystok) and potato pulp. The oat straw was delivered to the laboratory in the form of a rectangular compressed block measuring 40 × 40 × 80 mm. The straw. was pulverised until approx. 5 mm long elements remained using a ‘BĄK’ H 111/1 hammer mill.
The pulp under research is created as post-production waste in the process of produ- cing potato starch in the company Peepees S.A. in Łomża. The pulp is quite a troubleso- me type of organic waste which is difficult to manage. It is left over after starch and most of juice waters have been washed from potatoes, and is mainly composed of raw fibre, starch residues, and mineral compounds. Pulp collected from a few areas of the pile in which it was stocked in the storage yard of the Peepes S.A. processing plant in Łomża, was used in the research.
The potato pulp was pulverised using a laboratory shaker equipped with a sieve with perforations 4 mm in diameter. Shaking the thick mass in the shaker resulted in obtaining potato pulp particles of approx. 4 mm in diameter and smaller, which made it possible to mix them properly with the pulverised straw.
Evaluating the moisture content of the raw materials
Designating the materials moisture (potato pulp, oat straw and mixtures potato pulp and oat straw) was performed pursuant to PN-ISO 6496:2002 with the use of a WPE 300S scale-dryer exact to 0.01%. Each time, the moisture of five samples was determined. In the measurement, samples weighing 5g each were taken and dried at the temperature of 105°C until three consecutive readings of the scale-dryer. The mean values obtained as a result of this procedure were adopted as the final result of determining the moisture.
Tests on the densification process of mixtures in the pellet mill
Tests on the densification process of mixtures of potato pulp with oat straw in the wor- king system of the device were carried out on an SS-4 stand (Fig. 1) whose main element is a P-300 pellet mill with a ‘flat matrix-densification rolls’ working system, according to the methodology presented in the paper [Obidziński 2014]. The SS-4 stand was equipped with a universal meter (6) for measuring the electric energy consumption of the device, and with a Spider 8 recorder (7) connected to a computer (8). Signals from the sensor (6) were transmitted to the Spider 8 recorder (7) in the form of binary files which were further processed using the Microsoft Excel and Statistica 10.0PL software.
The tests on the densification of the mixtures of straw and potato pulp were conducted for the working gap between the densification roll and the matrix, measuring 0.4 mm, and for the mass velocity of the mixtures of approx. 50 kg·h–1. The diameter of the openings in the matrix used in the tests equalled 8 mm, and the length of the openings in the matrix was 28 mm, while the rotational speed of the matrix was 280 rpm. During the measure- ments, the author assessed the influence of the content of potato pulp (10, 20, 30, and 40%) in the mixture with oat straw on the electric energy consumption of the motor of pellet mill.
Determining the kinetic durability of pellets
After 24 hours from the moment the pellets under research had left the working sy- stem, their kinetic durability was determined with the use of a Holmen tester, pursuant to the PN-R-64834:1998 Standard and according to the methodology presented in papers [Thomas at al. 1996, Obidziński 2014].
Statistical methods
In order to determine the significance of differences between the energy consumption of the pellet mill and the kinetic durability of the obtained pellets at increasing contents of potato pulp, one-way analysis of variance (one-factor Kolmogorov-Smirnov significance test) at a significance level of P = 0.05 was performed.
Fig. 1. View of SS-4 stand: 1 – working system of pellet mill with fl at matrix, 2 – electric motor, 3 – raw material inlet, 4 – pellet outlet, 5 – vibrating feeder, 6 – universal meter for measuring electric power demand, 7 – Spider 8 recorder, 8 – PC computer
Rys. 1. Widok stanowiska SS-4: 1 – układ roboczy granulatora z matrycą płaską, 2 – sil- nik elektryczny, 3 – zasyp surowca, 4 – wysyp granulatu, 5 – dozownik wibracyjny, 6 – uniwersalny miernik do pomiaru zapotrzebowania na moc, 7 – rejestrator Spider 8, 8 – komputer PC
RESULTS AND DISCUSSION
Figure 2 illustrate the research results on the influence of the content of potato pulp (in mixtures with straw) on the electric energy consumption of the pellet mill during gra- nulating (densifying) the mixtures in the working system of the pellet mill, using a flat matrix.
The observed results of the research on pelleting mixtures in the working system of the pellet mill, using a flat matrix (Fig. 2) allowed to conclude that increasing the content of potato pulp from 10 to 40% in the mixture with oat straw resulted in reducing the elec- tric energy consumption of the pellet mill by 65.83% (from 5.94 to 2.03 kW). The obser- ved values of the electric energy consumption of the pellet mill show that the addition of potato pulp very favourably influences the reduction of the electric energy consumption which is much lower than the analogous demand in the case of granulating straw alone.
The performed one-factor ANOVA variance analysis (one-way Kolmogorov-Smir- nov significance test) at a significance level of P = 0.05 allowed to observe significant differences between the values of the energy consumption of the power mill obtained at increasing contents of potato pulp.
Fig. 2. The infl uence of the content of potato pulp in mixtures with oat straw on the energy con- sumption of the pellet mill during the pelleting of mixtures of straw and potato pulp (the average values in graph marked by the same letter do not differ signifi cantly at the level of P = 0.05)
Rys. 2. Wpływ zawartości wycierki ziemniaczanej na zapotrzebowanie na moc granulatora w trakcie granulowania mieszaniny słomy owsianej i wycierki ziemniaczanej (warto- ści średnie na wykresie zaznaczone taką samą literą nie różnią się istotnie na poziomie P = 0,05)
Potato pulp content Zawartość wycierki ziemniaczanej [%]
10 20 30 40
Demand of the pellet mill for energy Zapotrzebowanie granulatora na moc [kW]
6,5 6,0 5,5 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5
Średnia – Average
Średnia ± Odchylenie standardowe Average ± Standard deviation 5,95a
4,76b
3,09c
2,03d
Numerous experimental research studies [Kaliyan and Morey 2009, Mediavilla et al.
2009, Mediavilla et al. 2012] confirmed that an addition of the binder during the pelleti- sation process reduces the power demand.
The influence of the content of potato pulp zw on the electric energy consumption of the pellet mill Ng was described with a general equation of the linear function:
Ng = –0,134 zw +7,31 (1)
Figure 3 present the results of the tests on the influence of the content of potato pulp zw (in mixtures with oat straw) on the kinetic durability of the pellets obtained from mix- tures of oat straw and potato pulp in the working system of the pellet mill. The observed test results (Fig. 3) made it possible to conclude that increasing the content of potato pulp from 10 to 40% in mixtures with straw led to reducing the kinetic durability of the pellets by approx. 39.66% (from 94.65 to 54.99%). The observed test also shown that during the process of preparing mixtures with the additive of potato pulp, special care needs to be exercised in mixing the components evenly. This can be facilitated by pulverising potato pulp properly before the mixing process.
Fig. 3. The infl uence of the content of potato pulp in mixtures with oat straw on the kinetic dura- bility of pellets obtained during the pelletting in the working system of the pellet mill (the average values in graph marked by the same letter do not differ signifi cantly at the level of P = 0.05)
Rys. 3. Wpływ zawartości wycierki ziemniaczanej w mieszaninie ze słomą owsianą na wytrzy- małość kinetyczną granulatu otrzymanego w trakcie granulowania w układzie roboczym granulatora (wartości średnie na wykresie zaznaczone taką samą literą nie różnią się istot- nie na poziomie P = 0,05)
Potato pulp content Zawartość wycierki ziemniaczanej [%]
10 20 30 40
Kinetic durability of pellets Wytrzymałość kinetyczna granulatu [%]
100 95 90 85 80 75 70 65 60 55 50
Średnia – Average
Średnia ± Odchylenie standardowe Average ± Standard deviation 94,65a
78,37b
61,26c
54,99d
A positive effect of the addition of binder to biomass in the pelletisation process is confirmed by numerous experimental research studies [Filbakk et al. 2011, Sotannde et al. 2010, Razuan et al. 2011, Serrano et al. 2011].
The tests carried out in the working system of the pellet mill allow to conclude that the content of potato pulp of below 20% is the most favourable content with respect to the pellet quality, and this content makes it possible to obtain pellets of satisfactory quality. There are no definite criteria concerning the kinetic durability of pellets, which would unambiguously determine pellets as being of high quality. However, according to research results obtained by many authors investigating biomass densification for energy purposes, pellets whose kinetic durability exceeds 80% should be considered as pellets of satisfactory quality.
The performed one-factor ANOVA variance analysis (one-way Kolmogorov-Smir- nov significance test) at a significance level of P = 0.05 allowed to observe significant differences between the values of the kinetic durability of pellets obtained at increasing contents of potato pulp.
The influence of the potato pulp content zw on the kinetic durability of pellets Pdx, made from a mixture of oat straw and potato pulp, was described with the following ge- neral equation of logarithmic function:
Pdx = 95.75 – 67.91 – log10(zw) (2)
The results of the research was affirmed that, in order to produce pellets of satisfac- tory quality (in terms of their density and kinetic durability) from a mixture of oat straw and potato pulp, no more than 20% of potato pulp should be added to oat straw. On the basis of the conducted research, details of the course of the particular operations in the technology of producing heating pellets were developed, which resulted in filing a patent application [Obidziński 2012a].
CONCLUSIONS
On the basis of the conducted research was affirmed:
1. Adding potato pulp to oat straw results in increasing the susceptibility of the mix- ture to densification, which is confirmed by reduction of the electric energy consumption of the pellet mill. Increasing the content of potato pulp from 10 to 40% in mixtures with straw led to reducing by approx. 65.83% (from 4.76 to 2.03 kW).
2. Increasing the content of potato pulp from 10 to 40% in mixtures with straw led to reducing the kinetic durability of the pellets by approx. 39.66% (from 94.65 to 54.99%).
3. The results of the tests observed in the working system of the pellet mill with a flat matrix (on the SS-4 stand). To produce pellets of satisfactory kinetic durability amounting to over 78% from a mixture of oat straw and potato pulp, no more than 20% of potato pulp should be added to oat straw.
4. The performed research has made it possible to develop a detailed technology of producing heating pellets, which resulted in filing a patent application.
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WYTWARZANIE GRANULATU OPAŁOWEGO Z MIESZANINY SŁOMY OWSIANEJ I WYCIERKI ZIEMNIACZANEJ
Streszczenie. Celem badań podjętych w pracy było określenie najkorzystniejszych parametrów wytwarzania biopaliwa w postaci granulatu z mieszaniny słomy owsianej i wycierki ziemniaczanej. Badania procesu zagęszczania mieszanin przeprowadzono na stanowisku SS-4, którego głównym elementem był granulator P-300 z układem „płaska matryca – rolki zagęszczające”. Proces zagęszczania badanych mieszanin prowadzony był przy szczelinie roboczej między rolką zagęszczającą a matrycą równą 0,4 mm oraz przy masowym natężeniu przepływu mieszanki 50 kg·h–1. Do badań użyto matrycy o średnicy otworów 8 mm i długości otworów w matrycy 28 mm. Prędkość obrotowa matrycy wy- nosiła 280 obr·min–1. W trakcie pomiarów określono wpływ zawartości wycierki ziem- niaczanej (10, 20, 30 i 40%) w mieszance ze słomą owsianą na zapotrzebowanie na moc silnika napędzającego granulator oraz na wytrzymałość kinetyczną otrzymanego granula- tu. Wytrzymałość kinetyczną uzyskanego granulatu określono z wykorzystaniem testera Holmena. Granulat przebywał w komorze testera przez 60 s. Oznaczenie wilgotności su- rowców (wycierki ziemniaczanej, słomy owsianej i mieszanin słomy owsianej i wycierki ziemniaczanej) wykonano zgodnie z Polskimi Normami, używając wagosuszarki WPE 300S. Zwiększenie zawartości wycierki ziemniaczanej od 10 do 40% w mieszaninie ze słomą owsianą spowodowało obniżenie zapotrzebowania na moc silnika granulatora o ok. 66% (od 5,94 do 2,03 kW) z jednoczesnym obniżeniem wytrzymałości kinetycznej
granulatu o ok. 40% (od 94,65 do 54,99%). Badania w układzie roboczym granulatora pokazały, że aby otrzymać granulat o zadowalającej jakości (gęstości, wytrzymałości ki- netycznej) do słomy owsianej należy dodawać nie więcej niż 20% wycierki ziemniacza- nej. Na podstawie przeprowadzonych badań zostały opracowane szczegóły technologii wytwarzania granulatu opałowego, czego owocem było zgłoszenie patentowe.
Słowa kluczowe: odpady, wycierka ziemniaczana, słoma, granulowanie, wytrzy małość
