Chemical Looping Combustion Of Sewage Sludge And Nitrogen-Phosphorus Transformation

Sewage sludge is a product in wastewater treatment. It contains organic, inorganic and toxic substances,especially pathogenic or disease-causing micro-organisms. The growing global urbanization and increasingly stringent sludge disposal regulations demand sludge generators to recover valuable compositions in sludge, such as phosphorus and heat value. Phosphorus is an essential element for all living organisms. The declining of phosphate rock reserves necessitates the phosphprus recycle from sewage sludge. Chemical looping combustion (CLC) is a novel combustion technology based on continual oxygen transfer from air to fuel by oxygen carrier. Compared to traditional combustion process,CLC is of a higher energy conversion efficiency. Additionally, CLC can achieve inherent CO2 separation and low NOx emission. CLC can use sewage sludge as fuel, and the sewage sludge fueled CLC will shows its benefits in waste recycle, low NO. emission as well as phosphorus recovery.In this paper, the application of sewage sludge in CLC was conducted in a 1 kWth continuous unit. The combustion performance of sewage sludge was investigated by varying fuel reactor temperature and steam content.It shows that there are no char particles bypassing to air reactor. The rise of fuel reactor temperature and steam content can both intensify sludge gasification and oxygen carrier reactivity, leading to an increase in combustion efficiency of sewage sludge. In 10 h continuous operation, hematite shows a slightly decrease reactivity, indicating its robust reactivity in multi-cycles. To further improve the reactivity of hematite, cement was added in the oxygen carrier. It was found that cement-modified hematite possessed a higher reactivity, and a higher combustion efficiency as compared to hematite alone. However, the cement-modified hematite is prone to agglomerate after deep reduction in long-term operation.The transformation of nitrogen in CLC of sewage sludge was conducted in a continuous CLC unit. Compared to NOx formation in sludge air incineration, NOx formation in the fuel reactor was much lower in CLC of sewage sludge. In CLC, only 0.1%-0.3% of nitrogen exiting from the fuel reactor was as NO, while 1.8%-4.5% of nitrogem existed as NOx (NO, N2O and NO2) in sludge air incineration. In order to further investigate the fate of nitrogen in CLC, experiments were carried out in a batch fluidized bed reactor using both cement-hematite and Cu-Fe oxygen carrier. With regard to cement-hematite oxygen carrier, the addition of cement can not only increase sludge combustion efficiency, but also enhance NO emission as compared to hemetite alone. As regards Cu-Fe oxygen carrier, the addition of CuO can give significant improvement in sludge combustion in comparison to hematite alone.The release and transformation behavior of phosphorus in CLC of sewage sludge with hematite was conducted in a continuous CLC unit. The results show that the increase of fuel reactor temperature has a marginal influence on phosphorus transformation. Approximately 63-70% of phosphorus in sewage sludge was concentrated in fly ash during CLC of sewage sludge. The fraction of phosphorus in flue gas is relatively constant, since nearly all of the organic phosphorus volatized. Moreover,the available phosphorus in CLC fly ash is vastly superior to that in sludge incineration ash. The higher available phosphorus content can fulfill the quality parameters for a P-fertilizer.Furthermore, the effect of phosphorus on hematite was investigated in a reducing atmosphere (8 vol.% CO) at 900?. The binding of the different compounds in hematite have a complex effect on the affinity with phosphorus.With respect to the effect of sludge ash on hematite, the reaction mechanism of the interaction between sludge ash and hematite was evaluated. The active component (Fe2O3) in sludge ash can function as oxygen carrier,resulting in the increase of reactivity of hematite by adding proper content of sludge ash. Simultaneously, the presence of sludge ash can also cause sintering and agglomeration.