Effect of temperature and residence time on torrefaction of municipal sewage sludge: article

Malaysia, a developing country with a constant rise in population, recorded an increase in municipal and industrial wastes with approximately 23,000 tonnes of wastes were produced each day in Malaysia (Behzad, Ahmad, Saied, Elmira, & Bin, 2011). This resulted in an increase of total waste generated. With the common practice of waste disposal through landfill, and incineration, these contributes to another environmental issue; the pollution. To achieve a sustainable environment, new alternatives was found by researchers to treat wastes. Torrefaction is one of thermal process that is applied on biomass, to convert it into three useful products, char, bio-oil and condensable gases. In current research, char from lignocellulosic biomass was proven as the good alternative in the fuel industry. However, the characteristics of nonlignocellulosic biomass makes it possible for researchers to conduct a studies on it. Hence, the objective of this study is to produce char from municipal sewage sludge by using torrefaction method. In this study, municipal sewage sludge obtained from Indah Water Konsortium, located in Johor, Malaysia, has been used as the raw material. A characterization study was performed on the raw and dried sewage sludge. The study was conducted to record the moisture content, ash content and high heating value (HHV) of the raw sewage sludge. Next step is where biomass is being placed in a covered crucible and heated in a muffle furnace. The heating process operates under pressure of 1 atm and temperature of 200oC, 250oC and 300oC with residence time of 20, 30 and 60 mins for every temperature. The effects of torrefaction temperature and residence time on mass and energy yields, and HHV of torrefied sludge were investigated. Mass and energy yield of torrefied sludge were investigated to study the weight and energy loss of raw sewage sludge after torrefaction. HHV of the char was determined to study the energy value stored in torrefied sludge. From the results, the HHV, mass and energy yields were decreased as the torrefaction temperature and residence time increased. Highest peak was at temperature of 250oC and residence time of 60 mins. Thermogravimetric analysis was performed on high HHV of char. It was found that hemicellulose will degrade first at temperature of approximately above 200oC. An average reaction order of 0.59, 6.9852 kJ/mol of activation energy, Ea, and pre-exponential factor of 0.3772, were found to be the kinetics parameters of torrefaction on municipal sewage sludge.