ARTICLE pubs.acs.org/EF

Physicomechanical Properties of Rice Husk Pellets for Energy Generation Bruna Missagia,* Cinthya Guerrero, Satyanarayana Narra, Yiling Sun, Peter Ay, and Hans Joachim Krautz Chair of Power Plant Technology, Brandenburg University of Technology, Cottbus, Germany ABSTRACT: The use of biomass for energy generation is a promising alternative due to its potential to satisfy environmental compatibility. Rice is one of the world’s most important food crops. Processing of paddy leads to the generation of crop and agroindustrial wastes, which are voluminous and difficult to handle. By upgrading this biomass into a uniform compressed fuel, namely pellets or briquettes, its density increases, improving the storage, transportation, and combustion characteristics. In this study, densification characteristics of Brazilian rice husks were studied using a laboratory compactor, Hosokowa Bepex, Type L200/50GpK, for the production of pellets. Experiments showed that their bulk density in raw form increased 6.3 times. The effects of different particle sizes (rice husks milled with 2 mm, 4 mm, and 6 mm sieve meshes) and moisture contents (17%, 19%, and 20%) on the durability of the rice husk pellets were assessed. It was concluded that rice husk pellets with 17% water content and made of raw material ground with 2 mm, 4 mm, and 6 mm sieves are durable and stable and comply with the standard quality values of agricultural pellets as specified in Agro and Agro+. Biomass conversion technologies, using pellets, might facilitate the development of decentralized energy systems in rural areas. Nevertheless, in order to be deployed in an efficient and responsible way, economic, technical, environmental, and social aspects over the whole supply chain should also be taken into consideration.

’ INTRODUCTION Biomass is one of the most promising and heavily subsidized renewable energy sources.1 Agricultural waste is becoming an increasingly important energy source due to its environmental benefits and because its use does not interfere with traditional agricultural practices or jeopardize food security. Countries carrying out intensive agricultural practices produce a vast amount of residues, which could have enormous energetic potential. The national rice production in Brazil for the 2009/2010 harvest was 12 million tons, the average national productivity being ca. 4 ton/ha.2 Processing of paddy leads to the generation of crop and agro-industrial wastes. Approximately 22% of the rice grain is constituted of husks. Loose rice husks have a low bulk density of about 85 110 kg/m3.6 The densification process of biomass into pellets or briquettes can increase bulk density up to 10 times.5 The pellet density is measured as the ratio of the mass of pellets in a predefined volume. The density of Brazilian rice husks was reported to be 98.52 kg/m3, while the density of raw rice husk pellets was 621.469 kg/m3 in previous experiments. The higher the pellet density, the higher is the energy density and the lower are the transportation and storage costs. Rice husk pellets are a solid fuel with uniform shape and size suitable for direct combustion or cofiring with coal, pyrolysis, or gasification. Moisture content affects the calorific value of the pellets. A low moisture content fosters more predictable combustion efficiency. Typically, pellet fuels have a moisture content of approximately 10%.11 The lower heating value of Brazilian rice husk pellets was reported to be 13 MJ/kg (5588.7 BTU/lb) with a moisture content of 8.5% in previous experiments.11 Rice husk pellets could meet the thermal and mechanical energy requirements of the rice mills themselves.3,4 In Brazil, briquetting machines were introduced in the 1980s. They were sold initially to rice mills, but afterward the wood r 2011 American Chemical Society

factories became the main users. The most common raw materials used are saw dust, rice husks, and sugar cane bagasse.7 The quality parameters of biomass pellets like density, abrasion, and durability are influenced by several factors, including process variables (die temperature, pressure, and geometry),; feedstock variables (moisture content, particle size, shape, and distribution), and feedstock composition (protein, fat, cellulose, hemicelluloses, and lignin).8 Some other process variables include feed rate, binder usage, and biomass preheating.5 Different biomass materials require different optimum conditions of fabrication. Water content is a key parameter influencing the solid fuel quality since it affects the combustion behavior and the stability of biomass pellets. Too high of a water percentage leads to poor combustion and increases in the risk of dust buildup in the chimney. When water content is too low, pellets are less stable. The objective of this paper is to assess some selected physicomechanical properties of rice husk pellets, namely abrasion and strenghth, and their variation due to different moisture contents and particle sizes.

’ MATERIALS AND METHODS The rice husks used for the experiments were collected, stored, and dried during the months of May and June, 2008 in the Federal State of Minas Gerais, Brazil before their transportation to the laboratory facilities in Cottbus, Germany.

Particle Size Distribution and Moisture Content of Rice Husks. Prior to pelleting, biomass can be milled to a specific particle size. The milling partially breaks down the lignin, increases the specific surface area of the material, and improves binding.5 The rice husks were Received: August 24, 2011 Revised: November 10, 2011 Published: November 10, 2011 5786

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Figure 2. Compression stress principle (own source). Figure 1. Pellet machine (BEPEX 2000).

milled with 2 mm, 4 mm, and 6 mm sieve meshes. The biomass was groud with a cutting mill (SM 2000 Retsch). The particle size distribution was obtained with the Scanning Mobility Particle Sizer (CAMSIZER). Thereafter, the mass median diameter (d50) was obtained. The moisture content was determined using ASAE Standard S358.29 by drying the samples in an oven for 24 h at a temperature of 105 °C.9 Each experiment was repeated five times. The average water content of the raw rice husks assessed was 9.5%. Past experiments with rice husk pelleting showed that the ideal water content to ensure agglomeration was 17 20%; therefore, the rice husks had to be further moistened.11 Water as moisture is one of the most useful binder agents.10 Pelleting and Abrasion. Pelleting was performed with the Hosokowa Bepex, Type L200/50GpK laboratory compactor. The production capacity of the compactor is approximately 20 kg pellets per hour. The principle of the compactor is comparable to that of a hollow roller press (Figure 1). The feed material is auger fed toward the die where it is pressed, and the materials passes through the openings. The densification of the biomass takes place in the dies, which are 25-mm-long and have a diameter of 6 mm, resulting in pellets with these dimensions.12 The pressure applied to the material produced an increase of temperature in the device. The heat generated by friction softens the natural lignin, which acts as resin binding the surface of the pellet. After compaction, the resulting pellets were spread for indoor drying for seven days in order to achieve stabilization of the water content. The quality of pellets is perceptible by a remarkably even surface without fissures. The rice husk pellets were tested for abrasion as described in ASAE standard S269.4.13 Abrasion is the loss of particulate material due to handling of pellets, like transportation and storing. Stronger pellets release less particulate matter during handling.12 The abrasion tests were carried out with the three samples of pellets (made with the raw material groud with the 2 mm, 4 mm, and 6 mm sieves). The samples were placed in the test rotating machine (Erweka AR 401) at a rotation speed of 50 r/min for 10 min. The sample was then sieved with a sieve having an aperture of 0.8  pellet diameter mm.12 The difference in the weights of the pellets before and after the abrasion test gives the abrasion value. Each experiment was repeated five times. Hardness: Compression and Bending Strength. The ability of pellets to withstand forces such as compression, impact, and shear can be measured with durability experiments. Three groups of tests were carried out to investigate the hardness of pellets against compression and bending strengths. These experiments were selected, as such pressures might occur during handling, transportation, and storage of the pellets.12 Compressive resistance is defined as the maximum crushing load a pellet can withstand before cracking or breaking.14 The compressive stress behavior for rice husk pellets was obtained with a device (Zwick Roell-ZMART.PRO). As shown in Figure 2, the rectangular

Figure 3. Three point pressure (left) and point pressure (right) principle (own source).

Figure 4. Particle size distributions of the three groups of samples. compression die A is moved at a constant rate from top to bottom. The pellet sample C lies on the fixed plane B. When the die A reaches the pellet C, the compression force would act on the pellet, increasing the load gradually. The compression die A continues moving downward until the test specimen fails by cracking or breaking. The load at fracture is read off a recorded stress strain curve, which is reported as force. The bending strength avoids pellets’ breakage when they experience a bending load. The “three point pressure” technique was used to measure the bending strength of the pellets. One load acts on the middle of the sample, and another two forces act on the each side of sample to support it. Another bending strength test is the “point pressure” test in which the load acts on the middle of the material on the top and a plane supports the material from below (Figure 3).

’ RESULTS Particle Size Distribution and Moisture Content. Figure 4 shows the particle size distribution of each group of raw material samples. For the rice husks ground with the 2 mm sieve, the mass median diameter (d50) was 0.55 mm. For the ones ground with 5787

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the 4 mm sieve, d50 was 0.75 mm, and for the ones using the 6 mm sieve, it was about 1.25 mm. The mass percentage was distributed among seven intervals, which represent the different size classes. The rice husks milled with the 2 mm sieve mesh presented the most particles in the intervals from 0.25 mm to 0.5 mm. The rice husks milled with the 4 mm sieve mesh presented over 60% of the particles of the whole sample in the size range from 0.5 mm to 2 mm. The rice husks milled with the 6 mm sieve mesh presented over 60% of the particles of the whole sample in the size range from 1 mm to 2 mm. The water content for all of the experiments was 9.5%. Pelleting and Abrasion. The preprocessing of the rice husks by grinding improves pellets’ stability by reducing air spaces between particles during the compression, allowing closer surface-to-surface contact for a given volume of feed. Nevertheless, the results showed that the stability of the pellets was only slightly affected by the particle size. Discrepancies in the abrasion from particle sizes ranging from 2 to 6 mm grinding with a 19% moisture content were observed. However, the abrasion of the pellets made of raw material ground with the 2 mm sieve was smaller than the pellets with an average particle size of 4 mm and 6 mm with three different moisture contents. On the other hand, the water content had a considerable influence on the abrasion of rice husk pellets. The higher moisture content (for the three particle sizes) resulted in lower pellet durability (Figure 5). The abrasion percentage values of the pellets with a 17% moisture content were 4% (for the pellets made with the raw material ground with the 2 mm sieve), 5% (for the 4 mm ones), and 4.6% (for the 6 mm ones). The abrasion percentage of these three samples complies with the standard quality values of agricultural pellets as specified in the Agro and Agro+ standards from France (g8, g5 correspondingly; Table 1). On the other hand, the samples analyzed would not satisfy the norms of the € Onorm M7135 (0.60 kg/dm3

g650 kg/m3

g650 kg/m3

moisture content