Experimental Study On Nitrous Oxide Formation During Char Combustion Under Fluidized Bed Conditions

Nitrous oxide (N2O) is a pollutant gas which has very strong destruction influence on atmosphere environment. Coal combustion in fluidized beds is an important resource for N2O. N2O is formed by nitrogen in coal under fluidized bed conditions. The mechanism of N2O formation during coal char combustion is not understood. There is two main hypothesizes about N2O formation. One is homogeneous formation mechanism that N2O is produced by HCN oxidation in gas phase. The other is heterogeneous formation mechanism that N2O is produced by heterogeneous reaction of NO with char. Experimental study on N2O formation during char combustion has been carried out on a small-scale fluidized bed combustion reactor. The roles of homogeneous mechanism and heterogeneous mechanism have been investigated. HCN is produced during combustion of chars from different rank coals. That means HCN oxidation is a pathway for N2O formation during char combustion. But the instantaneous concentration profile for N2O is different from that of HCN. After HCN concentration decreases to zero, N2O is still formed and the concentration increases. It can be concluded that there is another pathway to form N2O that is different from HCN oxidation.HCN is only produced in the beginning phase of char combustion and disappear under certain burnout level of char. So the resource of HCN is slow devolatilization of char during combustion, not char gasification. The content variation of carbon, hydrogen and nitrogen in chars under different burnout level shows the ratio of hydrogen to carbon and nitrogen to carbon decrease very fast in the beginning phase of char combustion and then remain unchangeable. That means slow devolatilization exist in the char combustion that produces HCN. The effect of char preparation temperature on N2O formation has been investigated. When char preparationtemperature increases, the amount of HCN produced decreases and the role of homogeneous pathway become slight. The reaction of NO with char is dominant for N2O formation. The direct evidence for heterogeneous formation mechanism has been gained. Char after slow devolatilization is used as fresh char for combustion experiment. No HCN is formed and N2O is entirely is produced by the heterogeneous reaction of NO and char.So the conclusion is both homogeneous mechanism and heterogeneous mechanism play an important role in N2O formation during char combustion.N2O formation mechanism is also researched by numerical calculation simulation. The results on three different case show that the simulation result is most close to experiment results when both homogeneous mechanism and heterogeneous mechanism are considered. It is in agreement with experimental results.