CHARACTERISTICS OF BIOMASS PELLET AS FUEL

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1 CHARACTERISTICS OF BIOMASS PELLET AS FUEL *Widodo Wahyu Purwanto; *Dijan Supramono; **Yulianto S. Nugroho; *Dwi Endah Lestari * Department of Chemical Engineering, **Department of Mechanical Engineering Faculty of Engineering, University of Indonesia, Kampus UI Depok 16424, INDONESIA, Fax , Ph widodo@che.ui.ac.id Abstract Biomass is a potential alternative energy replacing oil and gas. The experiment goals are preparing and testing pellets in order to create fuels with good combustion performance and low emission. Pellets were shaped cylindrical. The variations are pellets raw materials (rice husk, straw, rubber wood, camphor wood), pellets size (10 20 mm in diameter), moisture content (4-20%). The experiment result showed that good combustion performance occurred to pellets with low moisture content and small size. Camphor pellets with diameter 1 cm and moisture content 4% have the highest heating value and bulk density. Highest combustion temperature occurred to straw wood with diameter 1.5 cm and moisture 11% while highest CO 2 emission occurred to rubber wood pellets with diameter 2 cm and moisture content 11%. Keywords: combustion; biomass; heating value; temperature. 1. Introduction Nowadays, some regions in the world have passed peak point of oil production as the oil reserves decline. The world is trying to reduce its reliance on the use of oil as energy source. The needs of using energy which reduce CO 2 emission and is sustainable way encourage people to consider biomass as an alternative energy. Contrary to the oil, the use of biomass as an energy source can be sustained by replanting the biomass and therefore, biomass is regarded as a renewable energy source. Biomass as a fuel can be used for many kinds of applications such as producing electricity, producing heat by utilizing it such as for cooking. In Indonesia, biomass is potential to be used as a fuel considering that large amount of biomass waste production has not yet been used as a fuel. Rubber wood waste is highly produced in Sumatera, Kalimantan, and Java [1]. However, conventionally biomass is mostly used as fuel for cooking in rural areas by direct combustion in open fire. This gives low combustion ISBN : I-348

2 efficiency and high emission. The emission consists of smoke and gas dangerous for our health and can lead to Acute Respiratory Infections (ARI), lungs cancer, and eye disease [2]. Those problems may be analysed and later overcome by some pre-treatment to the wood and analysis of all factors that have impacts to the combustion process. Some important factors are the raw material of biomass, moisture content, size and shape of the biomass fuels. Raw material of biomass may be of wood, branches or farm residues. Many countries have produced biomass fuels in the shape of pellet or briquette. Many researches have undertaken some experiments to increase the heating value of some biomass fuels and decrease the emission. For example, the heating value of biomass pellets has been increased to reach about 17 MJ/kg [3]. Based on this problem, the objective of this research is to design biomass pellet fuel which has high heating value comparable that of other biomass pellet, and high combustion temperature. 2. Methods 2.1 Pellets Designing Designing pellets considered the following parameters, the biomass raw material, moisture content size and shape of pellets. These parameters have been examined on their effects on the combustion performance of the biomass pellets. Raw materials chosen in this research were camphor wood, rubber wood, rice straw, and rice husk, with pellet moisture content varied between 4% to 20% and pellet shape being cylindrical of various diameters between 1-2 cm and length of 3 cm. 2.2 Pellets Preparation The following procedure was applied to every type of biomass material. Firstly the biomass material was fed into a hammer mill. After it became smaller in size, it was fed to a smoothing machine. Then die was installed to the machine and pressing on the powder was carried out under pressure of about 2 ton/kg. One minute after pressing, pellets were unloaded from the die. Pellets were kept in a plastic bag with sealer to avoid contact with air. In order to make moisture content variation in pellets, laboratory ISBN : I-349

3 oven was used. Moisture content was measured by using Infrared Moisture Determination Balance FD-240 KETT. 2.3 Biomass Characteristics and Combustion Test Biomass characteristic test includes the determination of biomass morphology using Scanning Electron Microscope (SEM), biomass compositions using proximate and ultimate analysis, heating value using adiabatic bomb calorimeter, and bulk density by calculating ratio of the pellet mass to its volume. Combustion test include the measurement of combustion temperature and emission. The combustion temperature was logged using ADAM 4018 which read temperature sensed by thermocouples (type K). Furnace temperature was set at 250 o C. For emission test, syringe was used to suck 1 ml flue gas sample then the gas was injected to the Shimadzu Gas Chromatography. 3. Results and Discussions 3.1 Biomass Characteristics Figure 1Figure 1Figure 2. Biomass morphology of (a) straw (b) rubber wood (c) camphor wood (d) rice huskfigure 2 (a) (b) (c) Figure 1. Biomass pellets of different diameters: (a) 2 cm, (b) 1.5 cm and (c) 1 cm Figure 2 shows the morphology of different biomass raw materials used in this research. The figure shows that straw and camphor wood has more porous structure than others. This affects the pellets bulk density. (a) (b) (c) (d) ISBN : I-350

4 Figure 2. Biomass morphology of (a) straw (b) rubber wood (c) camphor wood (d) rice husk 3.3 Biomass Compositions Proximate analysis of different biomass raw materials is presented in Table 1 while their ultimate analysis is presented in Table 2. Table 1. Proximate analysis Moisture Volatile Ash Biomass content matters Fixed carbon Calorific value %, adb %, adb %, adb %, adb Cal/g, adb Rice husk Straw Camphor wood Rubber wood Table 2. Ultimate analysis Biomass Carbon Hydrogen Nitrogen Sulfur Oxygen (%, adb) (%, adb) (%, adb) (%, adb) (%, adb) Rice husk Straw Camphor wood Rubber wood Trace Rice husk have the lowest moisture content. Its silica concentration is so high and has been shown to be in the outside layer of it [4], which forms hard surface and hardly absorbs water [5]. 3.4 Heating Value By using proximate and ultimate analysis data, the theoretical heating value can be calculated by the following equation [6]: HHV (kj/g) = C H O A S N (1) ISBN : I-351

5 Where C, H, O, A, S and N respectively are mass compositions of carbon, hydrogen, oxygen, ash, sulphur and nitrogen. Theoretical heating value was calculated to compare with that of experiment and the result is shown in Figure 3. This comparison was aimed to verify the proposed equation. The difference obtained is 4±2 %, which is still tolerable. Figure 3. Comparison between experimental and theoretical heating values The effect of moisture content on heating value of different biomass raw materials is presented in Figure 4. The figure shows that the heating value increases when the pellet moisture content increases. Water inside biomass pellets vaporised when it was heated. The vaporisation absorbed part of the exothermic combustion reaction. The higher the moisture content, the higher would be the heat absorbed for vaporisation. The rice husk pellets have low moisture content because its structure is non-porous (as shown by SEM analysis in Figure 2) and therefore they hardly absorb water. Comparison of different biomass materials shows that camphor and rubber woods have the highest heating value ( cal/g). It results from their higher amount of volatile matter, carbon, and oxygen compared to those in the rest materials. ISBN : I-352

6 Figure 4. Effect of moisture content on heating value Heating value was also influenced by the biomass composition. Biomass with high % carbon produced higher heating value. Table 2 shows that both rubber and camphor woods have higher % carbon compared to that of the rest materials. This relates to higher heating value as shown in Figure Bulk Density Bulk density is defined as the ratio of the pellet mass to its bulk volume. It has been found that the bulk density decreases when the pellet diameter increases (see Figure 5a). It means that pellets of smaller diameter can be stored in larger mass than those of larger diameter in the same volume. In the relation to moisture content, the authors found that the lower the bulk density the higher is the moisture content (see Figure 5 (b)). The figure infers that the amount of water in raw material seems to affect the amount of water in its pellets where higher moisture content in raw material produced higher moisture content in its pellets. Because the density of moisture is less than that of solid materials, the higher the moisture content in the raw material, the lower is the bulk density of the corresponding pellets. These results are similar to those achieved by M.D. Shaw [7]. Too much water in the biomass material inhibits lignin to function properly as a natural binder which produces porous pellets. In a stove, it is desired to have a compact pellet bed and high heating value, which correspond to low moisture content as shown in Figures 4 and 5 (a). ISBN : I-353

7 (a) (b) Figure 5. Effect of bulk density on: (a) diameter (b) moisture content Figure 5 shows that straw pellets have the highest bulk density. This result is closely related to the fact that the structure of straw raw material is more porous than others. The higher porosity enabled the straw to be pressed more easily to produce higher bulk density. Higher bulk density is desired to achieve high efficiency storage and transportation. 3.6 Combustion Temperature Furnace and pellet temperatures profiles are presented in Figure 6. Furnace temperature was kept constant and not influenced by the rising of pellet temperature. It was possible because volume of pellets was much smaller than that of furnace. Besides, heat produced by the combustion reaction was not so high. ISBN : I-354

8 Figure 6. Furnace temperature and pellet temperature profiles vs time Combustion characteristics from different biomass materials vary and were influenced by chemical reaction, physical characteristics, and ash characteristics of the biomass. According to the combustion theory, combustion reaction of biomass pellets occurs in four steps. First step is the release of vapour and volatile gases (CO, CO 2, CH 4 and H 2 ), which occurs until pellet temperature reaches about 100 o C. Second step is the release of heat when volatile matters start to burn occurring at temperature between 100 o C to 400 o C. In the third step, the pellet temperature rises to 500 o C and the pellets start to produce fire. In the last step, carbon was burned at temperature around 500 o C. The pellet temperature is influenced by the furnace temperature and its magnitude depends on the thermocouple location inside the pellet. Hence it is important to set up a furnace temperature and the thermocouple location inside the pellet in every experiment Effect of Pellet Diameter on Combustion Temperature In this experiment, the highest maximum combustion temperature occurs in the burning process of pellets of diameter 1 cm. One possible cause was that smaller pellets, which has smaller diameter, absorb less conduction heat from the combustion and therefore release more remaining heat detected by the thermocouple. Comparison of maximum combustion temperature produced by variation of pellet diameter in every type of biomass pellet is presented in Figure 7 (a). ISBN : I-355

9 (a.1) (a.2) (a.3) (b.1) (b.2) (b.3) Figure 7. Comparison of combustion temperatures produced by various biomass types: (a.1) rubber wood, (a.2) camphor wood, and (a.3) straw and in variation of pellet diameter: (b.1) 1 cm, (b.2) 1.5 cm, and (b.3) 2 cm. Figure 7 (b) shows comparison of maximum combustion temperature produced by various biomass types of 3 different pellet diameters. The figure shows that the highest maximum combustion temperature occurs in the combustion of straw pellets. ISBN : I-356

10 The temperature is influenced by two factors, i.e. pellet bulk density and the mass ratio of hydrogen to carbon in pellet material. Straw has the highest bulk density. The higher bulk density allows more mass per surface area of the pellets contacting with air to produce heat and therefore higher maximum combustion temperature. Observation on different diameters gives similar effect. In terms of the ratio of hydrogen to carbon, Table 2 shows straw gives the highest ratio. Equation (1) indicates that the higher ratio of hydrogen to carbon results in more heat of combustion which concurrently gives more maximum combustion temperature. Hence, the straw would give the highest maximum combustion temperature Effect of Pellet Moisture Content on Combustion Temperature Figure 8 (a) shows effect of pellet moisture content on the combustion temperature measured on pellets of all biomass materials chosen. As explained before that pellets with more moisture content produces less exothermic heat during combustion which consequently results in more maximum combustion temperature. Beside that, more moisture content incurs more time to release more vapour. Therefore, pellets with higher moisture content requires more time for pellets to burn. ISBN : I-357

11 (a.1) (a.2) (a.3) (b.1) (b.2) (b.3) Figure 8. Comparison of effect of types of biomass material on the combustion temperature produced by various of biomass types: (a.1) rubber wood, (a.2) camphor wood, and (a.3) straw and various pellet moisture contents: (b.1) 4%, (b.2) 11%, and (b.3) 20%. Figure 8 (b) shows the comparison of combustion temperature produced by various biomass types in 3 different moisture contents. At higher moisture contents, the result for maximum combustion temperature is the same, which is straw pellet have the highest combustion temperature. 3.7 Combustion Emission Effect of Pellet Diameter on CO 2 Emission Figure 9 (a) demonstrated that increasing pellet diameter incurs the increase of CO 2 emission. Larger pellets expose more surface area to be burnt. This makes combustion process take longer and consequently produces more CO 2 emission. ISBN : I-358

12 (a.1) (a.2) (a.3) Comparison of CO 2 emission produced by variation of biomass type pellet in every diameter Comparison of CO 2 emission produced by variation of biomass type pellet in every diameter Comparison of CO 2 emission produced by variation of biomass type pellet in every diameter Figure 17. Comparison of CO 2 emission produced by variation of biomass type pellet in every diameter Comparison of CO 2 emission produced by variation of biomass type pellet in every diameter Comparison of CO 2 emission produced by variation of biomass type pellet in every diameter(b.1) (b.2) (b.3) Figure 9. Comparison of CO 2 emission produced by various biomass types: (a.1) rubber wood, (a.2) camphor wood, and (a.3) straw and by various pellet diameters: (b.1) 1 cm, (b.2) 1.5 cm, and (b.3) 2 cm. Figure 9Figure 9Figure Effect of Pellet Moisture Content on CO 2 Emission Figure 10 (a) gives information that as the pellet moisture content increases, the CO 2 emission increases as well. This occurs because pellets of higher moisture content don t have compact structure and therefore more porous than those of lower moisture content. This porosity enables oxygen to diffuse easier through the pellets. ISBN : I-359

13 (a.1) (a.2) (a.3) (b.1) (b.2) (b.3) Figure 10. Comparison of CO 2 produced by variation of biomass type: (a.1) rubber wood, (a.2) camphor wood, and (a.3) straw and in variation of pellet moisture content: (b.1) 4%, (b.2) 11%, and (b.3) 20%. Figure 10 (b) shows effect of biomass type of pellets at 3 different moisture contents on CO 2 emission. The highest and the lowest CO 2 emissions are different in terms of biomass type. As we can see that CO 2 emission from burning biomass is ISBN : I-360

14 produced by combustion reaction of volatile gases. Biomass has high composition of volatile matters and it is hard to predict the pattern of the curves. 4. Conclusions The experiment gives results: 1. Biomass pellet heating value is influenced by biomass compositions given by proximate and ultimate analysis and also by moisture content. Heating value increases with the increase of the ratio of hydrogen to carbon contents, while it decreases with the increase of moisture content. The highest heating value is 4556 cal/g from camphor wood pellet with moisture content about 7.52%. 2. Bulk density is influenced by biomass morphology, pellet diameter and moisture content. Bulk density increases with the increase of biomass porosity, the decrease of biomass pellet and moisture content. The highest bulk density of 6006 kg/m 3 prevails in straw pellet with diameter 1 cm and moisture content 4%. 3. Combustion temperature is influenced by pellet bulk density, diameter and moisture content. Combustion temperature increases with the increase of bulk density, and it decreases with the increase of diameter and moisture content. The highest combustion temperature of 788 o C is achieved by burning straw pellets with diameter 1.5 cm and moisture content 11%. 4. CO 2 emission is influenced by pellet diameter and moisture content. The highest CO 2 emission of 72 x 10-8 gram/ml of flue gas results from the burning of rubber wood pellet with diameter 2 cm and moisture content 11 %. 5. Acknowledgement Authors would like to express their gratitude to the Directorate General of Higher Education, Ministry of Education Indonesia for its financing support to this research through Hibah Penelitian Nasional Strategis. References ISBN : I-361

15 [1] Abdullah, Kamaruddin. Biomass Energy Potentials and Utilization In Indonesia. Laboratory of Energy and Agricultural Electrification, Department of Agricultural Engineering, IPB. [2] Sukar, Agustina Lubis. (1996). Pengaruh Kualitas Lingkugan dalam Ruang (Indoor) terhadap Penyakit ISPA-Pnemonia di Indramayu Jawa Barat. Badan Litbang Kesehatan, Jakarta [3] Suyitno. (2008). Pengolahan Sekam Padi Menjadi Bahan Bakar Alternatif Melalui Proses Pirolisis Lambat. Jurusan Teknik Mesin Fakultas Teknik UNS. Surakarta. [4] DeDatta, S. K. (1981). Principles and Practices of Rice Production. John Wiley, New York. [5] Houston, D.F. (ed). (1972). Rice, Chemistry and Technology, American Association of Cereal Chemists.Inc., Minnesota. [6] Channiwala S. A. (1992). A Correlation for Calculating Elemental Composition from Proximate Analysis of Biomass Materials, PhD thesis. The Indian Institute of Technology, Bombay. [7] M.D. Shaw, et al. (2009). Physicochemical Characteristic of Densified Untreated and Steam Exploded Poplar Wood and Wheat Straw Grinds. Department of Agricultural & Bioresource Engineering, University of Saskatchewan, Canada. ISBN : I-362