Supercritical Water Gasification of Wet Biomass

Abstract

In the following decades, biomass will play an important role among the other renewable energy sources globally as it is already the fourth largest energy resource after coal, oil and natural gas. It is possible to obtain gaseous, liquid or solid biofuels from biomass via thermochemical or biochemical conversion routes. Among them, gasification is one of the most favorable options as the products can serve all types of energy markets: heat, electricity and transportation. Conventional gasification is an excellent method for dry ligno-cellulosic biomass feedstocks. However, in case of wet biomass with a high moisture content, it results in a negative impact on the energy efficiency of the gasification process due to the fact that drying costs more energy than the energy content of the product for some very wet biomass types. An alternative method applied for the conversion of wet biomass such as sewage sludge, cattle manure and food industry waste is anaerobic digestion. This process is however characterized by a slow reaction rate and typical residence times are almost 2–4 weeks. Besides, the fermentation sludge and wastewater from the reactors should further be treated. The supercritical water gasification (SCWG) process is an alternative to both conventional gasification as well as the anaerobic digestion processes for conversion of wet biomass. This process does not require drying and the process takes place at much shorter residence times; a few minutes at most. Supercritical water gasification is therefore considered to be a promising technology for the efficient conversion of wet biomass into a product gas that after upgrading can be used as substitute natural gas. The main reason why supercritical water gasification is a promising technology is due to the favorable thermo-physical properties of water and the way they change in the supercritical region which causes water to act as a solvent as well as a catalyst. Furthermore, through hydrolysis reactions, water also acts as a reactant. Gasification of biomass is mainly influenced by the density, viscosity and dielectric constant of water. Above the critical point, physical properties of water drastically change and water behaves as a homogeneous fluid phase. In its supercritical state, water has a gas-like viscosity and liquid-like density, two properties which enhance mass transfer and solvation properties, respectively. Besides, when water enters its supercritical phase, the dielectric constant drastically decreases. Water thus starts to behave like an organic, non-polar solvent which results in poor solubility for inorganics, and complete miscibility with gases and many hydrocarbons. Due to its miscibility, phase boundaries do not exist any-more. This absence leads to fast and complete homogeneous reactions of water with organic compounds. This dissertation focuses on the three aspects of SCWG of biomass systems: i) thermodynamic equilibrium modeling, ii) experimental approaches and iii) process modeling. In Chapter 2, the state of the art of the supercritical water gasification technology starting from the thermophysical properties of water and the chemistry of reactions to previous studies on modeling and experimental approaches, and the process challenges of such a biomass based supercritical water gasification plant is presented. In Chapter 3, the thermodynamic equilibrium modeling of SCWG of biomass is presented in two sub-chapters. In the first sub-chapter, commercial software packages are dealt with to model the gasification process of a pig–cow manure mixture in supercritical water. The phase and compound behavior of elements, behavior of gas products and water are investigated. Besides, the influence of pressure and dry biomass concentration on the gas yields are reported. In the second sub-chapter, a multi–phase thermodynamic equilibrium model is described. The model is validated by comparing the predictions with the various experimental results. A case study concerning microalgae gasification in supercritical water was performed. The phase and compound behavior of elements and behavior of gas products are investigated. Additionally, the influence of pressure and dry biomass concentration on the gas yields as well as the phase behavior of elements are studied and reported. In Chapter 4, constrained equilibrium model is tested for SCWG of biomass systems in order to model systems which do not reach to their thermodynamic equilibrium state. Additional constraints are introduced into the developed multi–phase thermodynamic equilibrium model, which is described in Chapter 3, and the importance of the additional constraints are tested by comparing the predictions of the model with the experimental results available in the literature. In Chapter 5, the experimental methods used in this work are described. A new and novel type of experimental setup which incorporates a fluidized bed reactor is designed with and manufactured by Gensos B.V. The setup has a capacity of 50 l/h and allows for clogging-free conditions for the experiments. In Chapter 6, the results of the experiments for starch are given. The influence of reactor temperature and feed flow rate are investigated. The results include not only the gas composition but also the temperature profile along different process units and the velocity profile along the reactor for different process conditions. The observed process challenges are also reported in this chapter. In Chapter 7, process modeling of SCWG of biomass is presented in two sub-chapters. In the first sub-chapter, SCWG of biomass process is modeled with an assumption of partial conversion in the pre-heater and a thermodynamic equilibrium in the reactor. Constrained equilibrium modeling, described in Chapter 4, is used to model pre-heater and the multi-phase thermodynamic equilibrium model described in Chapter 3, was used to model the reactor. The influence of the inorganic content of the biomass on the final products and thermal behavior of the process is investigated and the results are reported in detail. In the second sub-chapter, based on the existing literature data, an integrated kinetic model consisting of decomposition and gasification reactions of cellulose, hemi-cellulose, lignin and protein is developed for the modeling of pre-heater and reactor of such a SCWG of biomass plant. The influence of biomass feedstock type, temperature and reactor residence time is investigated and the results are reported in detail. Finally, in Chapter 8 main concluding remarks are provided, as well as recommendations for future research.