Evaluation of the incidence of pellets and wood chips in the performance of a "downdraft" type gasifier

The size, geometry, density and mechanical properties of the biofuel particles are important factors in the performance of any biomass gasification process. They influence the mass transfer rates of the reacting species just as the drying and the thermal decomposition rates in the rest of the system. This work evaluates the significant differences between two different feedstocks, wood pellets and woodchips, on the performance and efficiency of a pilot scale "downdraft" gasifier unit with a combined heat and power system. When using woodchips as feedstock, on average, the temperature profile of the pyrolysis zone was 147 +/- 15 degrees C higher, while the pressure drop inside the reactor and its variability were lower. The Cold Gas Efficiency and Global Thermal Efficiency of the system were higher as well with the woodchips as feedstock, despite the fact that the calorific value of the synthesis gas produced from the wood pellets was 8.00 MJ/Nm(3) and the one obtained from the woodchips was slightly lower at 6.18 MJ/Nm(3). At a power generation of 6.2 kW, the syngas and electrical energy yields, were also higher on average for the woodchips at 1.75 kg/kg and 564.50 kWh/t, compared to the wood pellets as feedstock at 1.40 kg/kg and 487.35 kWh/t respectively. The relationship between the electrical energy yield and the calorific value of the syngas flow was stablished by means of the Motor/Generator Efficiency, and it was within typical range for this type conversion systems, at 21.41 +/- 3.23 % for the woodchips and 18.94 +/- 3.68 % for wood pellets. In conclusion, even though the transport of the woodchips within system is more prone to clogging, the higher efficiency and better overall process performance obtained when using woodchips, makes it the preferred feedstock for the system compared to the wood pellets.