3.4. Energy consumption for drum drying
In this investigation, a comparative evaluation of energy consumption for drum-dried biomass at 80, 90, and 100 °C of drum surface temperature with varying drum speed was thoroughly conducted. The energy consumption of the drum dryer used in this study for drying 1 kg of biomass at 80 °C for 0.3 rpm was 7.328 kWh. By decreasing the rotational speed of the drum, the time required for obtaining 1 kg of dried biomass increased, which resulted in greater energy consumption. At 90 °C, the energy consumption was 8.4 kWh/ kg dry biomass for 0.4 rpm, whereas it was 9.511 kWh/ kg dry biomass at 100 °C for 0.5 rpm. Thus, considering the lipid recovery above 90% of the bone-dried materials, the minimum energy consumption to get 1 kg of dried biomass was at 80 °C for 0.3 rpm followed by 90 °C for 0.4 rpm and 100 °C for 0.5 rpm amongst all the other conditions (Table 5). Moreover, the difference in the time consumption for drying 1 kg of biomass at 80 °C for 0.3 rpm as compared to 90 °C for 0.4 rpm and 100 °C for 0.5 rpm was found to be marginal. Thus, considering the lipid recovery, energy consumption, and time required to obtain 1 kg of dried biomass, drum surface temperature of 80 °C with drum speed of 0.3 rpm can be concluded to be the best fit for large-scale drying of the harvested algal biomass in a single rotary drum dryer.
A study conducted by Bagchi, Rao, & Mallick (2015) revealed thatScenedesmus sp. biomass dried at 80 °C in a laboratory-scale hot air oven recorded an energy consumption of 5.231 kWh/ kg of dry biomass, whereas it was 11.227 kWh/ kg of dry biomass at 80 °C in a tray dryer with lipid recovery of >90% as compared to the bone-dried biomass (Bagchi, 2017). A significant reduction in drying time was observed in drum drying as compared to oven drying and tray drying (Table 6). The lipid recovery with the drum-dried biomass was at par with oven- and tray-dried biomass. The energy consumption was found to be 40% higher in drum drying as compared to oven drying. However, as compared to tray drying, energy consumption was reduced by 35% for 1 kg of dried biomass. Moreover, a profound reduction in drying time was evident with drum drying. Therefore, considering the drying time, lipid recovery, and energy consumption, the drum dryer carries immense potential and could be recommended for drying microalgal slurry at an industrial scale.