Optimization of the pyrolysis conditions for production of rice husk-based bio-oils as an energy carrier

Abstract

This study explored the production of bio-oil from rice husks as a strategy to solve energy poverty, rice husk disposal problems as well as improve farmers’ income in Uganda. Response surface methodology was employed to optimize pyrolysis conditions for production of bio–oil. Maximum yield, higher heating value, minimum water and minimum ash content were the response variables from rice husks obtained from NERICA (New Rice for Africa) rice variety. The pyrolysis parameters were temperature (400–650 °C), heating rate (6000–9750 °C h−1), and holding time (600-1800 s). The aim was to obtain bio-oil with more suitable properties as an energy carrier. Mathematical models were developed to explain the relationships between the experimental responses and pyrolysis parameters The bio-oil produced using the optimized conditions was further characterized for proximate compositions. Results from the analysis of variance (ANOVA) revealed that the linear model best fits the bio-oil yield and water content responses. The quadratic model best fits the higher heating value (HHV) and ash content responses. Pyrolysis temperature had the greatest influence on each of the responses, followed by holding time, and lastly heating rate. Optimum pyrolysis conditions were found to be temperature (650 °C), heating rate (9750 °C h−1), and holding time (1800 s), resulting in bio-oil yield, HHV, water and ash contents of 38.13%, 23.40 MJ/Kg, 18.27% db and 0.156 % db, respectively. These results fall in the range of standard quality values for bio oil in published literature where >15 MJ/kg, 20-30%, 0.15-0.25% are the recommended ranges for HHV, water and ash content, respectively. FTIR surface functional groups demonstrated that bio-oil contained alcohols, phenols, alkenes, ketones, alkenes, aromatics and esters a fact which was confirmed by the compounds identified through gas chromatography and mass spectrometry (GC-MS). The flash and fire point values were 98 oC and 103 oC, respectively which makes the bio-oil safer to handle due the reduced fire risk from accidental vapor ignition. The bio-oil production process was found to be viable with a benefits to costs ratio of 1.82 and an annual worth of $50,962.7. Proximate characteristics of the resultant char produced at the different runs ranged from 4.42 - 6.86%, 13.75 - 70.46% db., 6.65- 48.14% db., 17.04 - 57.28% wb for moisture content, volatile matter, ash and fixed carbon, respectively. The fixed carbon content of char increased with increase in temperature. Char produced at the predicted optimum pyrolysis conditions had fixed carbon of about 57.60% on dry basis making it a suitable precursor for activated carbon production.