Chemical looping gasification of high nitrogen wood waste using a copper slag oxygen carrier modified by alkali and alkaline earth metals

Abstract

Chemical looping gasification (CLG) can achieve clean and highly efficient use of high nitrogen wood waste (HNWW). In this work, the feasibility of CLG of HNWW is explored using a modified copper slag (CS1100) as an oxygen carrier (OC). The CS1100 sample modified by alkali and alkaline earth metals (AAEMs) was first prepared, and then the reactivity of different OCs was evaluated. The K-modified CS1100 (K-CS) sample showed the highest reactivity due to the formation of KFe2O4. Lattice oxygen was beneficial to char conversion, evidently promoting HNWW conversion. The K-CS sample showed the highest carbon conversion (74.65%) and valid gas yield (641 mL/g), followed by the Ca-CS and Na-CS samples, which exhibited a much higher carbon conversion than the inert SiO2 sample (40.45% and 327 mL/g). The K-CS sample can evidently promote the release of N and the subsequent oxidation of nitrogen-containing pollutants (i.e., NH3 and HCN), thus lowering the emissions of nitrogen pollutants. Char-N accounts for 22.54% of N during pyrolysis; however, char-N only accounts for 2.27% of N during CLG. Additionally, compared to pyrolysis, the emissions of nitrogen-containing pollutants decreased by approximately 20% in CLG. The Protein-N (N-P) and Urea-N (N-U) were decomposed to form Pyrrole-N (N-5) and Pyridine-N (N-6); and then formed Protonated pyridine N (N-Q) and Pyridine N-oxide (N-X) during thermal conversion of HNWW. Additionally, more N-X and N-6 remained in the residual char during CLG than during pyrolysis because of the presence of active OC. KFe2O4 acted as a catalyst rather than an oxidizer during CLG, and the K-CS sample can act as an OC for recycling.