| Abstract | Multi-stage air-blow and air-steam gasification processes were researched out in this study. Eucalyptus wood and solid waste were used as feedstock for both processes. The first law and the second law of thermodynamics were performed to analyze the efficiencies of these processes. In this study, thermodynamic equilibrium models, based on equilibrium constant and minimization of Gibbs free energy, were also developed and studied. The specific objectives of this study were: 1.) to develop the thermodynamic equilibrium model for multi-stage air-steam downdraft gasification, 2.) to investigate the gasification process in a multi-stage air-steam downdraft gasifier, and 3.) to analyze the performance of a multi-stage air-steam downdraft gasifier, based on first and second law of thennodynamics. In multi-stage air-blow gasification using wood and solid waste as feedstock, five cases of air supplies were studied. The primary and secondary air supply of these cases are 60 and 80 lit/min, 60 and 100 lit/min, 80 and 100 lit/min, 100 and 100 lit/min, and 100 and 120 lit/min. The result of the experiments showed that the heating value and the tar content reduced when air supply was increased. As for the carbon conversion, it presented an increasing trend with increasing equivalent ratio. The comparison of producer gas composition showed that the concentration of hydrogen was nearly the same in both wood and solid waste gasification. As for tar content in the case of solid waste gasification, it was found to be higher than that from wood gasification. The difference ranged from 21 to 31 mg/Nm³. For multi-stage air-steam gasification, this process can provide hydrogen-rich producer gas. In all cases of air supplies, the concentration of hydrogen increased while the concentration of carbon monoxide decreased when steam was added. This is because of the water-gas shift reaction effect. However, it was also observed that when the steam supply reached a specific quantity, the concentration of hydrogen and carbon monoxide showed only a little change. The steam supplied to the process even caused the temperature to drop and this affected the increase of tar content in the producer gas. For example, it was found that for hydrogen yield, the highest values were 28.63 g/kg feed and 30.04 g/kg feed for cases of air supplies 80, 100 lit/min and 100, 100 lit/min, respectively. This translates to 40.71% and 41.62% increase from multi-stage air-blow gasification at the same values of air supply. In multi-stage air-steam gasification, using wood and solid waste as feed stock expressed the same trend when steam supply was increased. However, the comparison between using wood and solid waste as feed stock showed that the concentration of hydrogen and carbon monoxide in producer gas from solid waste gasification were lower than those obtained from wood gasification when steam was supplied. Based on the first law and the second law efficiency analysis, it was determined that the hot gas and cold gas efficiencies of multi-stage air blow wood gasification were 79.53% and 68.88%, respectively. While for solid waste gasification, these were 76.57% and 65.19%, respectively. For the exergy efficiency of the same process, the average values of 63.61 % for the first and the third kinds of exergy efficiencies, and 60.92% for the second and the fourth kinds of exergy efficiencies were found in wood gasification, while in solid waste gasification, they were 62.18% and 58.32%, respectively. When wood was used as feedstock in multi-stage air-stearn gasification, the hot gas and cold gas efficiencies were 78.41 % and 70.28%, respectively. On the other hand, when solid waste was used as feedstock, these were 72.93% and 66.07%, respectively. The exergy efficiencies of multi-stage air-steam wood gasification were 65.06%, 63.11%, 64.83% and 62.86% for the first to the fourth kinds of exergy efficiencies, respectively. For the case of solid waste, the first to the fourth kinds of exergy efficiencies were 61.64%, 59.48%, 61.40% and 59.21 %, respectively. In the process of model development, two validations were done. The first was the validation of them10dynamic properties used in the models and the second validation was carried out in the comparison of the composition of producer gas with the experimental data of downdraft gasification obtained from literatures. The first validation expressed the maximum relative percentage difference of about 3%, but generally it is less than 1%. The JANAF table was used as the reference for this validation. For the second validation, the minimum and maximum RMS (root mean square) differences were 0.88 and 5.64, respectively. Finally, the minimization of Gibbs free energy model was used to predict the composition of producer gas from multi-stage air-blow and air-steam gasification. The result showed that the minimum and maximum RMS differences between the predicted and actual experimental values were 0.94 and 3.34, respectively. The divergence can be explained by the fact that all reactions in predicted cases were assumed to have taken place well and even reach equilibrium condition. However, in reality, the supplied steam cannot react well with other reactants. This occurrence made the concentration of hydrogen in the experiment to be not as high as in the model where concentration of hydrogen steeply increased when steam to biomass ratio increased |