Tar generation and decomposition in downdraft packed bed reactor for woody biomass gasification

Abstract

When the biomass gasifier is connected with a gas engine system directly, tar should be removed from the syngas to prevent the engine from breaking down. A downdraft packed bed gasifier has the advantage for low tar emission because the syngas passes through the char gasification zone downstream of the reactor, where tar compounds can be trapped and decomposed. Then, objective of this study is to confirm the tar decomposition behaviors inside the downdraft packed bed reactor. Woody biomass gasification experiments were carried out, using an auto-thermal downdraft packed bed gasifier. The reactor’s height and inner diameter were 1000mm and 100mm, respectively. Black pine pallets were continuously fed into the reactor from the top. The gasifying agent was air, which was introduced into the reactor at the air-fuel equivalent ratio of 0.49. The packed bed height was kept to be constant at 600mm. The reactor has eleven thermo-couples and eleven sampling ports at the wall along the flow direction. They were used for measurements of temperature profiles and gas compositions in the reactor. Micro-GC was used for the measurement of N2, O2, CO, CO2 and H2, and FIDGC was used for other hydrocarbons. In some ports among them, tar in syngas was also sampled via dichloromethane scrubbing in ice-bath, and analyzed for molecular weight distributions of tar compounds by TOF-MS. As a result, tar and larger hydrocarbons were confirmed to be generated in the upstream, and then decomposed downstream inside the downdraft reactor.

Keywords

• biomass gasification
• downdraft gasifier
• tar sampling
• tof-ms