Research paperPhysical properties of rice husk and bran briquettes under low pressure densification for rural applications
Introduction
Worldwide, up to three billion people cook using fuelwood on a daily basis [1]. Biomass represents 80% of the energy supply in developing countries [2]. There is an increasing need to source alternative fuels, especially for cooking so as to reduce deforestation as a result of trees being cut for fuelwood. More than 14 MT of rice paddy is produced in West Africa every year [3] along with a significant amount of by-products. Rice paddy contains about 25% husk and 10% bran by mass [4]. Rice husk and bran are mostly considered as waste and are left to accumulate in big piles, which represent a handling issue and a source of water contamination [5]. When too much accumulates, the piles are usually burned in open fires and become a major source of air pollutants and greenhouse gases [6]. The majority of the rice mills in the developing world involve a one-stage process, with both the husk and bran being removed simultaneously. Otherwise, a two-stage mill separates the by-products. These by-products could be managed as a new domestic fuel to replace fuelwood. Rice husk does not compete with food sources [5] and receives stable supply as rice is the third crop in importance in the world in terms of tonnes harvested [7]. With the majority of rice husk produced in developing countries, it represents an opportunity for greater energy sustainability in this part of the world [5], [6], [8], [9].
Different studies have recognized the qualities of rice husk (and bran which has 10–23% oil content by mass [4]) as a combustion fuel. Despite its high ash content (up to 25% dry mass) [8], rice husk has low moisture content [10], satisfying calorific value and low sulphur content [5], [11]. However, agricultural residues, including rice husk, have a low bulk density usually under 200 kg m−3 [12]. Densification into pellets (less than 2.5 cm in diameter) or briquettes (2.5–10 cm in diameter) is required to overcome this challenge. The advantages of densified biomass include: increased volumetric energy density, ease of handling, transportation and storage, improved combustion, lower particulate emissions, reduced volatility, and uniform size, density and quality [12], [13].
Energy can be obtained from biomass through direct combustion, gasification, pyrolysis, anaerobic digestion, hydrolysis, hydrogenation or fermentation [14]. Lim et al. [5] provided an overview of technologies available to produce heat, electricity or value added products (ethanol composites, absorbents) from rice husk and rice straw. However, such processes are energy and equipment intensive. Briquetting remains the most applicable technology to produce energy in the form of solid fuel for cooking at the household level in rural settings [15], [16].
The characteristics of briquettes made of grass [17], wood [18], fibres [19], straws [20], leaves [21], stalks [7] and husks, including rice husk alone [10], [15], [22] and in combination with other biomass [23], [24], have been previously studied under high pressure. Limited studies have been conducted on rice bran, and mostly in combination with rice straw [25]. The quality of densification can be determined by testing the physical properties of the biomass briquettes [26]. Kaliyan and Morey [13] discussed how the final product is affected by the pre-treatments and the initial characteristics of the biomass, namely particle size, moisture, temperature, and binder content. The high quality of a briquette is an indicator of the effectiveness of inter-particle bonds [27]. Particle size reduction is usually recommended in order to reduce inter-particle space and create stronger bonds during compression [10], [26], [28]. High temperature and pressure are widely agreed to enhance binding mechanisms as well [25], [28], [29]. Yet, these different processes require important energy input.
Few studies have been conducted on the densification of rice husk at low pressure and room temperature. Low pressure densification of rice husk presents an opportunity to reduce energy input although it requires using a binding agent to create sufficient cohesion and briquette durability. Binders have been used for low pressure densification of biomass. Taulbee et al. [30] investigated over 50 different binders for coal and sawdust briquettes. Water is also suggested to play a role in reducing friction and void space during compression [26], [31]. Carbohydrates, proteins and lignin are considered excellent organic binders, but proteins and lignin only melt and bind under high temperature densification (above 140 °C) [12]. Also, binders should ideally consist of waste by-products. For instance, West Africa produces 84 MT of cassava annually and the subsequent processing discharges about 250–300 L of wastewater per tonne of tuber, which affects the water streams with high biological oxygen demand (25,000–50,000 mg L−1) [32]. The cassava wastewater is rich in cassava starch as the tubers present a starch content typically between 20 and 30% by mass [33], [34]. Another waste by-product consists of rice dust. Rice kernels are often milled to produce flour. Large amount of rice dust accumulates on the floor of milling facilities and is then swept and discarded. Finally, the okra fruit is known for its high mucilage content used in pharmaceuticals because of its binding properties [35], [36]. The roots and stems of the okra plant also contain mucilage [35] but are barely used, though the stems may be collected sometimes for fibre or rope.
The objectives of this study were firstly to develop a village scale low pressure densification method to use rice husk for briquette production for cooking purposes; secondly, to investigate the impact of bran and water addition on low pressure and temperature densification of rice husk, and thirdly to identify high quality briquettes based on a range of combinations of rice bran, water addition, types of binder and binder content.
Section snippets
Rice husk
Approximately 50 kg of dried two-stage rice husk grinds (imported from USA) was purchased from Moût International (Montreal, QC, Canada). The experiments took place at the McGill's Macdonald campus engineering shop. The rice husk was ground in a hammer mill (Thomas no. 4275-210, PA, USA) fitted with a 2 mm mesh to simulate the one-stage grinds usually found in the area of interest targeted by this study. Grinding is also recommended when two-stage rice husk is to be used for low pressure
Results and discussion
The initial moisture content of the rice husk (RH) before grinding was 9.2% wet basis (moisture content in this paper is expressed as wet basis). The HHV of the RH alone before densification was 16.08 MJ kg−1 dry basis (db). The two-stage RH in this study had a particle size distribution similar to a two-stage RH sample collected in Nigeria (Table 1). This similarity was also obtained between the ground RH and a one-stage milling sample from Nigeria (Table 1). This validated the use of a 2 mm
Conclusion
In conclusion, rice husk is a suitable biomass for low pressure densification to produce briquettes as an alternative cooking fuel. The briquettes presented adequate characteristics namely physical integrity (durability and compressive strength) as well as low moisture content (below 7.5%) and high calorific value (16.08 MJ kg−1 db). The RD was found to provide the highest compressive strength and durability, though CSW offered the highest density and similar durability to RD. The use of RD as
Acknowledgments
This study was made possible with the collaboration of Africa Rice and the financial support of the Canadian International Development Agency (CIDA) and the Natural Sciences and Engineering Research Council of Canada (NSERC).
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