A comprehensive experimental and modeling investigation of walnut shell gasification process in a pilot-scale downdraft gasifier integrated with an internal combustion engine

In the present study, a small-scale downdraft gasification system integrated with an internal combustion engine was investigated for the sustainable conversion of biomass into energy in the form of electricity within heat recovery stages using walnut shell with a lower heating value of 18.07 MJ/kg. Different measurements and analyses, including biomass characteristics, syngas composition, bottom ash assessment, and flue gas emission, were performed to monitor and evaluate the quality of feedstock and products. The results indicated that with increasing electrical loads from 4 to 12 kW, the lower heating value of syngas decreased from 5.07 to 4.72 MJ/Nm(3), while the cold gas efficiency and carbon conversion efficiency increased from 66% to 71%, and 74% to 83%, respectively. The syngas composition was found to be 15.42-14.78 % H-2, 17.31-16.87% CO, 6.93-7.29% CO2, 3.41-2.77% CH4, and 56.93-58.29% N-2. From the bottom ash analyses results, it was observed that walnut shell biochar contained high concentrations of K2O and CaO that could have many effects on the gasification process, such as tar removal, CO2 reduction, and less agglomeration. In addition, a modified quasi-thermoequilibrium model has been proposed to predict the composition of syngas, yields of tar (five common biomass tar compounds), and char released during the gasification process. To improve the accuracy of the model, non-equilibrium correction factors were estimated and validated with 40 experimental data sets, including 25 various biomass feedstocks in the literature for equivalence ratio and reduction temperature of 0.2 to 0.45 and 740 to 1300 K, respectively. After all modifications, the average RMSE value was reduced from 5.02 to 2.19.