Agricultural residue biochar bio-composite briquettes as sustainable domestic energy sources
Agricultural residue biochar bio-composite briquettes as sustainable domestic energy sources
In this study bio-composite briquettes were developed from rice husks, coffee husks and groundnut shells. The briquettes were developed under low pressure (≤ 7 MPa) after carbonization and application of starch binder. Determined calorific values and physical properties, mechanical strength and integrity of the developed briquettes. Results for drop strength for the developed composite briquettes were all above 86 %, indicating satisfactory characteristics. Bio-composite briquettes developed using coffee and rice husks bio-chars took less time to boil water compared to all the other bio-composite briquette combinations. Heat transfer was enhanced when no binder was present and coffee and rice husks were sequentially placed in the briquette composition. This study showed the advantages of producing bio-char bio-composite briquettes over single constituent briquettes. Bio-composite carbonized briquettes produced from rice husks, coffee husks and groundnut shells are a suitable and sustainable alternative to firewood and charcoal use in sub-Saharan Africa.
Materials
Agricultural waste raw materials used in this work were obtained from specific agricultural zones where each crop is grown in Uganda. Coffee husks were obtained from Kanyiginya Coffee factory. Rice husks were obtained from Eastern Uganda. The agricultural waste samples were first cleaned thoroughly to remove dirt. They were then sun-dried for 8 hours to reduce their moisture content to about 13 %. The agricultural waste raw materials were then carbonized into bio-char and crushed for biocomposite briquette development. To form bio-char, the dried/cleaned agricultural waste raw materials were fed in a carbonizer made of a 200 L volume capacity steel drum (2:1 height: 24 diameter ratio) with openings of 0.02 m diameter for air regulation. Ignition of the agricultural waste raw material took place from the top of the carbonizer drum after which the top of the drum was covered. During the carbonization process the holes were covered with mud in order to limit the amount of air available for complete combustion in the carbonizer as the agricultural waste raw material reduced due to pyrolytic processes.
Briquette development
Bio-chars obtained from the carbonization process were ground to particles sizes of less than 15mm particles and sieved in order to increase their surface area for binding with starch. This was done to enhance the binding ability and eventual drop strength of the developed biocomposite briquette. The particles were then mixed with cassava starch binder in different proportionality from 10 % – 40 %. The combination of the bio-char and binder were then poured in a metallic mold of inner diameter 5cm and a height of 8cm. The mixtures were compressed in the mold to take up the shape of the mold. Four categories of bio-composite briquettes were developed namely: (1) rice and coffee husk bio-composite briquettes; (2) groundnut shell and coffee husk bio-composite briquettes; (3) rice and groundnut shell bio-composite briquettes; and (4) Rice, coffee and groundnut shell bio-composite briquettes. In development of low cost carbonized briquettes, human energy dominates as contributions from electrical, chemical and thermal energy are negligible. Additionally, no thermal energy was needed to dry the raw materials because they were sufficiently dry (< 13 %) using solar radiation. Typical applied pressure of ≤ 7 MPa is sufficient for producing low-cost briquettes.
Mechanical properties
Mechanical integrity of the briquettes under impact loading was determined using drop strength tests. Developed bio-composite briquettes were elevated up to 2 m and then were dropped onto a thick steel metal plate. The ratio of weight after dropping to weight before dropping was recorded as the drop strength. Particle densities of the developed bio-composite briquettes were obtained by standard procedures involving ratios of mass and volume for each bio-composite briquette developed.
Conclusions
Low pressure technique and cassava starch binder was employed in the briquette development process. The physical properties of the developed composite briquettes were determined by using thermogravimetric analysis (TGA). Bomb calorimetry was used to determine the calorific values of the briquettes. TGA and bomb calorimetry provided the combustion properties of the developed briquettes.A water boiling test was carried out with each set of the developed briquettes. The carbonization process significantly improved the fixed carbon contents. Rice husks had higher moisture and ash contents compared to groundnut shells and coffee husks. The developed briquettes yielded higher volatile matter and ash contents compared to their constituent raw materials. The calorific values of the developed briquettes ranged between 16.6 MJ/kg and 22 MJ/kg. Drop strengths ranged between 85.4 % and 100.0 % while particle densities ranged between 476.14 kg/m3 and 578.1 kg/m3. This means that the developed briquettes will withstand forces during transportation and storage more than briquettes developed from individual bio-chars. TGA confirmed that the developed briquettes have high residuals and thus high fixed carbon contents. The lowest ignition and boiling time was obtained when composite coffee husks and rice husks bio chars were used in ratios of 90:10 respectively. The lowest total percentage weight losses at the highest combustion temperature was 36.04 %, obtained by the 90R10G briquette while thehighest total percentage weight loss was 53.69 % obtained by the 70C30G briquette, showing that the former yielded the best thermal stabilities, owing to its lowest volatile matter content (24.61 %). Heat flux ranged from 0.052 W/m2 to 12.887 W/m2. Briquettes developed with rice and coffee husks bio-chars had higher heat fluxes due to the fact that these two have the highest thermal conductivities compared to that of groundnut shells. This work showed that calorific values have no linear relationship with water boiling time, drop strength and particle density, even though it varies with them. The results provide vital information that shows composite briquettes have higher potentials for use as fuel for cooking purposes compared to briquettes developed from individual residues.
Reference:
Lubwama, M., Yiga, V.A., Muhairwe, F. and Kihedu, J., 2020. Physical and combustion properties of agricultural residue bio-char bio-composite briquettes as sustainable domestic energy sources. Renewable energy, 148, pp.1002-1016.
