A two-step approach to hydrothermal gasification

Varsha Reddy Paida is a PhD student in the department of Sustainable Process Technology (SPT). Her supervisors are prof.dr. S.R.A. Kersten and dr. D.W.F. Brilman from the faculty of Science and Technology (TNW).

Aside from fossil fuels being a finite source of energy, they present other significant disadvantages including anthropogenic CO2 emission concerns held responsible for global warming, as well as energy security concerns leading to political tensions. This has motivated the search for renewable sources of energy in order to minimise, and hopefully eliminate, society’s dependence on fossil fuels. Along with other renewable sources of energy, biomass plays a vital role as a renewable feedstock, with potential to replace fossils for the production of electricity, heat and transportation. Additionally, the replacement of fossil fuels by renewable biomass can contribute to stabilising atmospheric CO2 concentrations.

This thesis deals with the design and development of a process for the conversion of wet biomasses, primarily to H2. Wet biomasses contain over 80% moisture, such as bio-wastes and wastewaters produced in the food and agro-industries. In order to overcome the energy-intensive drying step for traditional biomass conversion technologies, a newer route for conversion involves using the water as a reaction medium. Such processes are hydrothermal in nature. The process under consideration is a two-step catalytic process for hydrothermal gasification. In Chapter 1, relevant background information into wet biomass processing is highlighted, following which the current challenges faced in the field of hydrothermal gasification are presented. The chapter closes with the scope and outline of the thesis.

The first part of the thesis (Chapters 2 and 3) includes studies focussed on obtaining a more fundamental understanding of the two main catalytic processes under consideration: stabilisation and gasification. The concept of stabilisation, described in Chapter 2, was applied to convert highly reactive aqueous sugars and carbohydrates to more stable molecules that would subsequently minimise, if not eliminate, coke production upon further processing. Regardless of the choice of feed, hydrolysis was found to be the limiting rate under the experimental conditions considered. Stabilised samples were gasified with negligible coke formation on the catalyst, producing similar carbon gasification efficiencies and H2 yields as the gasification of stabilised sucrose, or stabilised glucose. The success of stabilisation was based on the absence of oligosaccharides obtained from the incomplete or partial hydrolysis of starch detected using HPLC and is supported by the colouring tendency of the liquid product. A simple mathematical model was developed to describe the kinetics of sucrose stabilisation.

The gasification step was the focus of work presented in Chapter 3. Sorbitol, a C6 sugar alcohol derived from glucose was selected as a model compound in this study. The hydrothermal gasification of sorbitol was studied using a 5 wt% Pt on γ-Al2O3 catalyst and N2 as a sweep gas. The main requisite of the process was to optimise H2 production at high carbon gasification efficiencies. This is challenging because of the low selectivity that sorbitol presents towards H2 production. The use of N2 was found to enhance H2 yields without affecting the total carbon gasification. A comprehensive reactor model taking into account both mass transfer and reaction kinetics was developed. The complex reaction mechanisms were described through a path-lumped kinetic scheme. The reactor model was used to demonstrate the feasibility of the process on an industrial scale. It was found that increased yields of H2 could be achieved by considering reactors that show improved mass transfer (high kLa’s as in slurry reactors) in combination with the in-situ separation of H2 as soon as it’s produced (through the use of a sweep gas, or a catalytic membrane reactor).

The second part of this thesis (Chapters 4 and 5) includes work focussed on process development with an aim to identify whether the process is ready for implementation on an industrial scale. A sequential combination of the studied gasification catalysts (Pt and Ru) provided a two-fold advantage to the hydrothermal gasification process. The high selectivity for H2 production using Pt, combined with the high reactivity of Ru, was found to be promising at increased WHSVs. On an industrial scale, faster WHSVs translate to the requirement of smaller reactors with lower catalyst loadings. The Pt-Ru sequential combination, reported in Chapter 4, therefore presents an industrially attractive route for increased productivities of H2 and carbon gasification, introducing opportunities not only as a technology of H2 production from renewable feeds, but also as a technology for the treatment of wastewaters.

The developed kinetic models for stabilisation and gasification were used to design the process for industrial scale, reported in Chapter 5. An energy balance demonstrated that the H2 product and off-gases each contained roughly 30% of the energy of the feedstock. The rest of the energy produced (~40%) was used to meet the process demands of electricity, steam and fired heat, therefore giving the process an energy efficiency of 60%. An economic analysis was conducted in order to determine the minimum H2 selling price from the process. The price of H2 obtained was comparable to H2 prices obtained from other renewable technologies. However, it was found to be significantly influenced by the concentration, quantity and price of feedstock. This implies that prior to selection of this process, the developed cost model should be used to evaluate the application and suitability of the technology for the feedstock considered.

The key conclusions of this work are presented in Chapter 6, along with a perspective on the potential application of the technology to relevant waste and wastewater industries.