Microwave assisted Pyrolysis of Food waste to Biochar and Biofuels

Abstract

Mangalore is one of the fast growing cities of India and situated on the west coast of the Indian peninsula covering an area of 132.45 sq-km. The city is generating approximately around 312 tons of MSW per day of which 40% is the food waste (125 tons per day). At present this MSW is land filled leading to serious environmental and health problems. The given research thesis aims to (i) quantify and characterize the food waste generated in commercial and residential complexes of Mangalore city, (ii) pyrolysis of food waste with the assistance of microwave irradiation, optimization of the process parameters for better yields and (iii) characterization of the pyrolysis products using ASTM standard methods. Preliminary pyrolysis experiments were carried out to decide about the operating ranges for the pyrolysis temperature, residence time and nitrogen flow rate. Based on the thermogravimetric analysis (TGA) and preliminary pyrolysis experiments, the operating ranges for the time, nitrogen flow rate and temperature to carry out pyrolysis experiments were 25 to 35 min., 40 to 60 mL min-1 and 350 to 450 ºC respectively to design the experiments by response surface methodology (RSM). Pyrolysis yields of 30. 24 wt. % (bio-oil), 60.03 wt. % (biochar) and 9.73 wt. % (biogas) were obtained under the optimum pyrolysis conditions of 400 ºC temperature, 30 min. residence time and nitrogen flow rate of 50 mL min-1 respectively. The actual values of the operating parameters namely temperature, time and nitrogen flow rate and the responses for twenty experiments were used for the prediction of bio-oil, biochar and fixed carbon models. The regression models with 95% confidence level resulted in the high value of R2 = 95.4% with R2 adjusted = 91.2% indicated a very good or excellent fit of the data to the bio-oil model, high value of R2 = 92.9% with R2 adjusted = 86.4% indicated a very good or excellent fit of the data to the biochar model and high value of R2 = 90.3% with R2 adjusted = 81.60% indicated a very good or excellent fit of the data to the fixed carbon content model respectively. Bio-oil model was analyzed statistically by using experimental data and analysis of variance (ANNOVA). The linear terms suchvii as temperature, time and nitrogen flow rate were having the positive effect to increase the bio-oil yield when these variables are increased, whereas, square terms were having negative effect and decreased the bio-oil yield. The predicted value of the bio-oil yield was 0.02 wt. % less than the experimental value. Main functional groups as detected by the Fourier transform infrared (FTIR) analysis are alcohols, alkenes, aromatic compounds, primary and secondary amines, carboxylic acid, esters and phenols. GC-MS analysis was carried out to find the major compounds present in the bio-oil. GC-MS analysis identified 11 major compounds out of more than 500 compounds those were present in the bio-oil. Compounds such as oxygenated and non-oxygenated compounds, nitrogenated compounds and other compounds such as phosphine, methyl-, propane, 2- fluoro-, (2-hydroxyethyl) trimethylsilyl methyl sulfide, and 1,3-bis(2-hydroxymethyl) urea were identified by the GC-MS analysis. Though the heating value of the bio-oil was 23.94 MJ kg-1 it cannot be used as a bio-fuel, as it contains more water as well as nitrogenated compounds. However, bio-oil obtained can be upgraded and blended with diesel to use as a fuel through further investigation. Biochar and fixed carbon content model were analyzed statistically by using experimental data and ANNOVA. Linear and square terms were significant to effect biochar production followed by the fixed carbon content whereas the interaction terms were less significant parameters. The predicted value of the biochar was 0.05 wt. % less than experimental value, whereas, the fixed carbon content was 0.03 wt. % less than the experimental value. Biochar obtained under the minimum pyrolysis conditions of 400 ºC temperature, 30 min time and 50 mL min-1 of nitrogen flow rate at a power level of 450 W was used for characterization. Higher heating value (HHV) of the biochar was 33.35 MJ kg-1. HHV as calculated by the bomb calorimeter (33.35 MJkg-1) was higher than that of the Dulong formula (27.79 MJkg-1) value as the latter did not include the dissociation effects. HHV of the biochar was more than that of the FWS due to reduction of some higher heating value hydrocarbons.