Supercritical water gasification of lignocellulosic biomass materials for hydrogen production

The primary aim of this research is to optimize supercritical water gasification process operating conditions and develop a cost effective heterogeneous catalyst to reduce reaction temperature and improve H2 yield. Furthermore, a detailed techno- economic feasibility study was performed to evaluate the economic feasibility of SCWG process. The work plan is divided into five phases. In the phase one, model lignocellulosic biomass comprising of cellulose, hemicellulose and lignin were selected as the feedstock for SCWG process optimization. The objective of the study was to optimize the process conditions, propose a detailed reaction pathway for model compounds and understand how each intermediate product behaves under subcritical and supercritical conditions. The response surface methodology using the Box-Behnken design was applied for the first time to optimize the process parameters during subcritical and supercritical water gasification of cellulose. The process parameters investigated include temperature (300-500 °C), reaction time (30-60 min) and feedstock concentration (10-30 wt%). Temperature was found to be the most significant factor that influenced the yields of hydrogen and total gas yield. Among the three model compounds, hydrogen yields increased in the order of lignin (0.73 mmol/g) < cellulose (1.95 mmol/g) < xylose (2.26 mmol/g). Based on the gas yields from these model compounds, a reaction pathway of model lignocellulosic biomass decomposition in supercritical water was proposed. The results from the first phase raised several research questions, for example, does biomass heterogeneity have an effect on product yield? How far are the batch experimental results from equilibrium values when considering a real feedstock? Lignocellulosic biomass is heterogeneous in nature and it comprises of several molecules of different compounds including cellulose, hemicellulose, and lignin along with extractives. Lignocellulosic biomass (soybean straw and flax straw) were gasified under similar conditions as those of the model compounds in Phase two. Soybean straw exhibited superior H2 yield (6.62 mmol/g) and total gas yield (14.91 mmol/g). Similarly, the gaseous products from soybean straw showed improved lower heating value (1592 kJ/Nm3). The experimental results showed slight deviations from the thermodynamic models which could be as a result of temperature gradient and absence of agitation in the batch reactor. In the third phase, several Ni-based catalysts were screened and tested for SCWG of soybean straw. The aim is to develop a cost-effective heterogeneous catalyst that could improve the gas yields towards equilibrium values and lower the reaction temperature. All experiments were performed at the desired operating conditions identified in Phase 2. A comprehensive screening of different support materials ranging from activated carbon (AC), carbon nanotubes (CNT), ZrO2, Al2O3, SiO2 and Al2O3-SiO2 was performed at 10 wt% Ni loading. The effectiveness of each support in improving H2 yield and selectivity was in the order: ZrO2 > Al2O3 > AC > CNT > SiO2 > Al2O3-SiO2. The effect of three promoters (i.e. Na, K and Ce) added to the supported Ni/ZrO2 catalysts was evaluated. Ce promoter was found to be the best for ZrO2 supported Ni catalysts. The performance of Ce was attributed to its high capacity for storing oxygen species which have the ability to react with the carbon deposits on the surface of the catalysts thereby preventing carbon deposition. The objective of the fourth phase was to study the kinetics of Ni - Ce/ZrO2 catalyzed SCWG of soybean straw. The lumped parameter kinetics method was employed with several reactions resulting from the experimental results in Phase three and the proposed reaction pathway in Phase one. The pathways were used to develop the kinetic equations. Kinetic model results were found to correlate with experimental results. Furthermore, the kinetic model was used to predict experimental yields for long residence time. The kinetics results are also in agreement with thermodynamic predictions. In the last phase, a detailed techno-economic evaluation and sensitivity analysis was performed for a conceptual design for hydrogen production from soybean straw gasification in SCW. The economic feasibility of hydrogen production was evaluated based on a discounted cash flow analysis. Economic analysis suggested a minimum selling price of U.S. $1.94/kg for hydrogen. The cost is relatively low when compared with that of hydrogen produced from other biomass conversion processes. Besides, the net rate of return (NRR) estimated was 37.1%. A positive NRR value indicates that the project is profitable from an economic perspective. Sensitivity analysis indicates that the minimum selling price of hydrogen is affected by the feedstock price, utility cost, tax rate and labor cost