Combustion Performance And No Formation In Chemical Looping Combustion Of Sewage Sludge

Sewage sludge is the major solid wastes generated from the municipal wastewater treatment. With the rapidly increasing quantity in recent years, sewage sludge has become an increasingly important residue needing effective management. Compared to the conversional method for the treatment of sludge, chemical looping technology is an appropriate solution for the conversion of sludge to energy with no environmental impact. Chemical looping technology is a novel conversion technology based on transfer oxygen from air to fuel by oxygen carrier, which is continuously circulating between air and fuel reactor. When sewage sludge is utilized as fuel, the high nitrogen content could cause a high NO emission. However, there are few researches on the NO emission in chemical looping combustion of sewage sludge. In this context, the combustion performance and NO emission in chemical looping gasification (CLG) and chemical looping combustion (CLC) of sewage sludge were evaluated. The experiments were conducted in a batch fluidized bed reactor.Because of the low carbon conversion efficiency and low gasification rate in sludge gasification process, the gasification performance of sludge in CLG process was investigated in this study. The effect of gasification temperature, steam content, O/C molar ratio, and cycle number on sludge gasification characteristics and NO emission were evaluated. The results indicated that the addition of hematite significantly promotes the gasification rate and carbon conversion efficiency. The increase of gasification temperature, steam content and O/C molar ratio can intensify gasification step, leading to the increase of carbon conversion, gasification rate and the syngas volume. Additionally, the rise of gasification temperature led to the increase of NO conversion. The increase of steam content produced a significant decrease in NO conversion.In this part, the combustion performance and NO emission in CLC of sewage sludge were investigated in a batch fluidized bed reactor with hematite as oxygen carrier. Different variables affecting the process were analyzed, such as fuel reactor temperature, the mass ratio of steam and sludge, and cycle number. Both the rise of fuel reactor temperature and the mass ratio of steam and sludge can significantly increase the carbon conversion and CO2 concentration. NO emission mainly results from the volatile-N in sewage sludge. The rise of fuel reactor temperature leads to the increase of NO conversion. The increase of the mass ratio of steam and sludge intensified the water gas shift reaction, leading to the decrease of NO conversion. Moreover, hematite shows slightly decrease reactivity during 10 redox cycles.As the carbon conversion of sewage sludge is relatively low using hematite as oxygen carrier, cement was added to promote the reactivity of hematite. The effects of fuel reactor temperature, the mass ratio of steam and sludge,and cycle number on sludge combustion performance and NO emission were investigated. The results show that the addition of cement can vastly promote the reactivity of hematite. The mixture of cement and hematite can give significant improvement in carbon conversion and gasification rate in comparison to only hematite. Nevertheless, the NO conversion increased with the addition of cement to hematite. Moreover, the effect of CaO on NO emission in CLC of sewage sludge with hematite as oxygen carrier was investigated. CaO can intensify the production of NO and NH3. With the increase of CaO/hematite mass ratio, NO conversion increased. The increase of fuel reactor temperature leads to the rise of NO conversion. The mass ratio of steam and sludge on NO emission has an opposite trend. The rise of the mass ratio of steam and sludge contributes to enhance reducing atmosphere. There are no defluidization and sintering problems after multiple redox reactions. It can be concluded that the cement-hematite has a significantly catalytic effect on the steam gasification of sewage sludge with a good reactivity and stable cyclic performance.