Partitioning of fuel bound nitrogen in biomass gasification

Gasification of biomass can produce process heat, electricity, liquid fuels, and chemicals without the use of fossil fuels. Most biomass feedstocks contain small amounts of fuel bound nitrogen (FBN) which converts during gasification to mainly hydrogen cyanide (HCN), ammonia (NH3), char bound nitrogen (char-N), tar bound nitrogen (tar-N), and diatomic nitrogen (N2). Of these five products, HCN and NH3 are most problematic. They lead to NOX air pollution when the syngas is combusted. They also poison the catalysts used for conversion of syngas to fuels or chemicals. Yields of NH3 and HCN from FBN vary broadly in response to both feedstock properties and gasification conditions. Efforts to predict their yields via kinetic modeling have been hindered by lack of reliable experimental data, as nitrogen products are relatively difficult to measure. The differing physical and chemical properties dictate that separate analysis equipment and procedures are required for each one. There is even disagreement regarding whether char-N, tar-N, and HCN are significant products of FBN. To study FBN evolution, switchgrass was gasified in a 25 kg/h pilot scale fluidized bed gasifier. Equivalence ratio (ER) was varied from 0.21 to 0.38 while measurements of NH3 and HCN were taken. Switchgrass gasification experiments were also conducted with a smaller 100 g/h gasifier. Temperature and ER were varied independently from 650 to 850°C and zero to 0.4. Measurements of NH3, HCN, char-N, and tar-N were taken. N2 was found by difference. For all tests conducted, HCN yields remained on the same order of magnitude as NH3 yields. The methods of researchers reporting near-zero HCN yields were replicated, and their results were found to be the result of flawed sampling methodologies. Large amounts of nitrogen were found in char and tar. As temperature and ER were increased, char and tar conversion also increased, leading to increased release of nitrogen to gas phase. This tended to increase yields of NH3 and HCN. These results suggest that the performance of kinetic models could be improved by including char-N, tar-N, and reactions for their conversion to gaseous nitrogen products