Evaluation of an Energy Production System from Sewage Sludge Using a Pilot-Scale Downdraft Gasifier

Abstract

Presently, sewage sludge from wastewater treatment systems has become a critical problem in many regions of the world because it can inflict harm on human beings and the environment. Gasification technology is widely held to be a suitable and convenient approach to convert waste materials to energy with minimal greenhouse gas emissions. In a pilot-scale experiment on sewage sludge gasification ultimately aimed at generating electricity, the reactor temperature profile, syngas characteristics, and performance of the syngas in the production of electricity were tied to the equivalence ratio (ER) and syngas flow rate (i.e., 100, 150, or 180 N m3 h–1). An increase of the ER resulted in an increase of the temperature inside the gasifier, causing a variation in syngas characteristics and the level of tar and dust contamination. Calorific values at different syngas flow rates were found to vary from 4.20 to 4.87 MJ N–1 m–3. The syngas obtained at flow rates of 150 and 180 N m3 h–1 could be used in an engine–generator set to generate 21 or 47 kW of electrical power, respectively, whereas at the flow rate of 100 N m3 h–1, the syngas could only run the engine without electrical load. The specific sewage sludge consumption, which is the amount of feedstock required to generate electricity, decreased with an increase in the syngas flow rate. The performance evaluation of the sewage sludge gasification system, i.e., gasification efficiency, engine–generator set efficiency, and electrical efficiency, showed that these were in the range of those obtained from biomass, such as agricultural residues. Overall, sewage sludge can serve as a feedstock for electricity generation using a pilot-scale downdraft gasification system.