Conversions of char nitrogen

There have been a limited number of studies which indicate that the conversion of char nitrogen to N0X can make a significant contribution to the total N0X emissions. The oxidation of char nitrogen... 

Effect of Temperature on Conversion of Char Nitrogen to N0X. The ef-fect of oxidation temperature upon the proportional release of N0X is tabulated in Table II for pure acridine and phenanthridine chars at... 

Table II. Effect of Temperature on Conversion of Char Nitrogen to N0X...

It was also found that the conversion of released nitrogen to N0X decreased with increasing reaction temperature, increasing char nitrogen content (for both acridine and phenanthridine-based chars) and with increasing sample weight (bed height). 

The practical motivation for understanding the microscopic details of char reaction stem from questions such as How does the variability in reactivity from particle to particle and with extent of reaction affect overall carbon conversion What is the interdependence of mineral matter evolution and char reactivity, which arises from the catalytic effect of mineral matter on carbon gasification and the effects of carbon surface recession, pitting, and fragmentation on ash distribution How are sulfur capture by alkaline earth additives, nitric oxide formation from organically bound nitrogen, vaporization of mineral constituents, and carbon monoxide oxidation influenced by the localized surface and gas chemistry within pores ... 

The dramatic decrease in char nitrogen to N0X conversion for the acridine char cannot be attributed only to a 50 K increase in temperature. The char ignited at 873 K but not at 823 K. The bed temperature during oxidation at 873 K was probably considerably higher than 873 K. The bed temperature during oxidation of the phenanthridine char at the furnace temperature of 873 K is shown in Figure 9. A maximum temperature of 1006 K was reached within 3 minutes and the bed temperature remained 50 K higher than the furnace temperature even after 30 minutes. 

In figure 4 the conversion of carbon to tar is shown as a function of X. Since the char content of the bed material was not determined for each individual experiments it is not possible to make a complete carbon balance. Based on tbe gas analysis and the tar conversion a rest fraction, which could be regarded as the carbon conversion to char, was instead calculated. The result as a function of X is plotted in figure 5. With the assumption that the nitrogen content of the char is similar to the nitrogen content of the fuel it is clear that a significant amount of nitrogen is maintained in the char fraction, especially at low X. 

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

The nitrogen conversions reported in Table II were measured with the chemiluminescent N0X analyzer. During the oxidation of the phenanthridine char at 873 K, no differences were seen in the instru-mentally measured concentration of NO and N0X. This indicates that NO and N are the principal nitrogen species present. Both NH3 and HCN and can be converted to NO by the stainless steel catalyst in the analyzer (7). If either of these two species were present, the measured N0X value would be higher than the NO value. 

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