The role of char and tar in determining the gas-phase partitioning of nitrogen during biomass gasification

Gasification is an attractive option for converting biomass into fuels and chemicals. Most biomass contains significant amounts of fuel-bound nitrogen (FBN), which partially converts into ammonia (NH3) and hydrogen cyanide (HCN) during gasification. These nitrogen compounds are problematic as they can lead to NOX emissions or catalyst poisoning in downstream applications of syngas. FBN can convert to other products as well, including diatomic nitrogen (N2), char-bound nitrogen (char-N), and tar-bound nitrogen (tar-N). Efforts to predict concentrations of NH3 and HCN have been hindered by a lack of accurate, comprehensive measurements of nitrogen partitioning among gasification products. The present study gasified switchgrass under allothermal, short residence time conditions and measured NH3, HCN, char-N, and tar-N as a function of temperature in the range of 650–850°C with diatomic nitrogen determined by difference. It was found that a major portion of FBN was retained in the char and tar products. As temperature was increased, char and tar were consumed, releasing nitrogen as gaseous NH3 and HCN. This increase in undesirable nitrogen compounds is contrary to the predictions of most gasification models, which overlook the presence of significant nitrogen in char and tar even if they include tar cracking and char gasification reactions. The results of this study demonstrate that gas-phase reactions alone are not sufficient to predict the fate of nitrogen during gasification. In order for modeling efforts to obtain more accurate predictions of nitrogen partitioning, the reactions through which gaseous nitrogen-bearing species are released from char and tar must be considered.