Abstract
Densification of biomass feedstocks, such as pelletizing, can increase bulk density, improve storability, reduce transportation costs, and ease the handling of biomass using existing handling and storage equipment for grains. In order to study the pelletizing process, compost pellets were produced under controlled conditions. The aim of the work was to investigate the effect of raw material properties and the die geometry on the true density of formed pellets and also find the optimal conditions of the densification process for producing pellets with high density. Compost was extruded into cylindrical pellets utilizing open-end dies under axial stress from a vertical piston applied by a hydraulic press. The effects of independent variables, including the raw material moisture content (35 to 45% (wet basis)), hammer mill screen size (0.3 to 1.5 mm), speed of piston (2 to 10 mm/s), and die length (8 to 12 mm) on pellet density, were determined using response surface methodology. A quadratic model was proposed to predict the pellet density, which had high F and R2 values along with a low p value, indicating the predictability of the model. Moisture content, speed of piston, and particle size significantly affected (P < 0.01) the density of pellets, while the influence of die length was negligible (P > 0.05).
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Effect of raw material properties and die geometry on the density of biomass pellets from composted municipal solid waste
Abedin Zafari and Mohammad Hossein Kianmehr*
Densification of biomass feedstocks, such as pelletizing, can increase bulk density, improve storability, reduce transportation costs, and ease the handling of biomass using existing handling and storage equipment for grains. In order to study the pelletizing process, compost pellets were produced under controlled conditions. The aim of the work was to investigate the effect of raw material properties and the die geometry on the true density of formed pellets and also find the optimal conditions of the densification process for producing pellets with high density. Compost was extruded into cylindrical pellets utilizing open-end dies under axial stress from a vertical piston applied by a hydraulic press. The effects of independent variables, including the raw material moisture content (35 to 45% (wet basis)), hammer mill screen size (0.3 to 1.5 mm), speed of piston (2 to 10 mm/s), and die length (8 to 12 mm) on pellet density, were determined using response surface methodology. A quadratic model was proposed to predict the pellet density, which had high F and R2 values along with a low p value, indicating the predictability of the model. Moisture content, speed of piston, and particle size significantly affected (P < 0.01) the density of pellets, while the influence of die length was negligible (P > 0.05).
Keywords: Solid waste; Compost pellet; Density; Die geometry; Raw material properties; Response surface methodology
Contact information: Department of Agrotechnology, College of Abouraihan University of Tehran, Tehran, Iran * Corresponding author: Kianmehr@ut.ac.ir
INTRODUCTION
Municipal solid waste (MSW) is produced in great quantity in Iran and its management has become a challenge, both economically and environmentally. More than 50% of the MSW generated by the Iranian population is organic waste (Mavaddati et al. 2010). Composting of MSW is considered a method of transferring organic waste materials from landfills to a product that is suitable for agricultural purposes at relatively low cost (Eriksen et al. 1999; Wolkowski 2003). Composting MSW reduces the volume of the waste, kills pathogens that may be present, decreases germination of weeds in agricultural fields, and destroys malodorous compounds (Jakobsen 1995). Converting the municipal waste to compost is very important, because useful materials like compost produced from waste has been widely used for agricultural and horticultural purposes (Mavaddati et al. 2010). Composting of MSW has the potential to become a beneficial recycling tool for waste management in Iran. One of the major barriers against the use of composts is their handling, application, and storage due to low density. Therefore, these bulky residues need to be densified into pellets.
Pelletizing is a method that increases the bulk density of biomass, which facilitates transportation and reduces the cost. Pellets have low moisture content for safe storage and a high bulk density for efficient transport and storage. The process of forming biomass into pellets depends upon the physical properties of ground particles and the process variables during pelletizing. The compaction (pelletization) process is a complex interaction between particles, their constituents, and forces. Tabil and Sokhansanj (1996) studied the effect of process parameters, such as steam conditioning, die geometry, L/D ratio, die speed, and particle sizes of the biomass, on quality of pellets. Mani et al. (2003) and Samson et al. (2005) reviewed the biomass pelletizing process and the effect of various process parameters on pellet density and durability. Tabil and Sokhansanj (1997) studied the bulk properties of alfalfa in relation to its compaction characteristics. Mani et al. (2006) studied the effects of compressive force, particle size, and moisture content on mechanical properties of biomass pellets from grasses. The results of their research showed that compressive force, particle size, and moisture content significantly affected the pellet density of barley straw, corn stover, and switchgrass. Also, Serrano et al. (2011) showed that there is an optimal range of moisture content to obtain straw pellets with high density. Tumuluru et al. (2010) studied the effect of process conditions, such as die temperature and moisture content of raw material, on the pellet properties. They observed that the physical properties of biomass pellets varied depending on the moisture content of the biomass and the die temperature. Knowledge of the fundamental compaction properties of particles of different biomass species, sizes, shapes, chemical compositions, bulk densities, and particle densities is essential to optimize densification processes (Tabil and Sokhansanj 1995; Tabil 1996; Thomas and van der Poel, 1996). It is also important to understand the compaction mechanisms in order to design energy-efficient compaction equipment and to quantify the effects of various process variables on pellet density and pellet durability. The objective of this study was to investigate the effects of moisture content, speed of piston, length of die, and particles size on the density of pellet from composted municipal solid waste.
EXPERIMENTAL
Sample Preparation
Compost samples were ground using a hammer mill with three different hammer mill screen sizes (0.3, 0.9, and 1.5 mm) in order to understand the influence of particle size on density. The ground feedstocks were stored at room temperature (25 ºC). The moisture content of the ground samples was determined following the procedure given in ASAE Standard S 269.4. The samples of compost were placed in an oven at 105±3ºC for 24 hours. The moisture content of the samples was determined by weighing and is expressed in percent wb. The moisture contents of ground feedstocks were adjusted to 35, 40, and 45% (w.b.) by adding water; they were equilibrated overnight to determine the effect of moisture content on pellet density.
Pellet Production
A hydraulic press and a single pelletizer were used to produce pellets (Fig. 1). The pelletizer consisted of a plunger and cylinder assembly. The cylinder had an internal diameter of 10 mm and a length of 100 mm. The dies placed at the end of the cylinder had a hole with a 6 mm diameter and different lengths. A hydraulic press was used to move the piston. This press had three important features: pressure control, piston speed control, and residence time control at the set pressure. The hydraulic press was equipped with a data recording system for displacement, force, and time. By moving the piston, material is compressed and converted to pellet form when emerging from the die. Produced pellets were dried at ambient temperature until their moisture content reached about 12% (wet basis).