Advanced, or second generation (2G) biofuels represent an important contribution to a renewable society. Liquid fuels have many advantages, they have a high energy density, and can be distributed and used in existing infrastructure and equipment. The transport sector has a significant contribution to CO2 emissions (around 25%), and elements of this sector is difficult to electrify. Biomass can be transformed to liquid fuels by gasification to syngas followed by catalytic fuel synthesis. The gasification process is performed at high temperatures, and the gas formed (a mixture of hydrogen, carbon monoxide, CO2 and water) contains undesired components, including sulfur species. These components need to be removed from the gas because they poison the catalysts used in the fuel synthesis process. This is usually proposed to be done in scrubbing processes, where the pollutants are removed using organic solvents at low temperatures. This is an expensive part of the process, and technical improvements are necessary.
The goal of this project is to improve this part of the process through the introduction of a chemical sorbent, a solid material reacting with the sulfur component and thus cleans the gas. Afterwards the sorbent is regenerated, and the pollutant, in this case sulfur, is removed and the sorbent can be reused in numerous cycles. This is beneficial because it allows gas cleaning at a higher temperature, which saves investment costs and gives improved efficiencies. Since the sorbent is regenerated the reactor volume and the amount of sorbent needed is reduced.
The original idea of the project was to develop a new reactor for this process, based on cycling the solid sorbent between taking up sulfur species, and by this cleaning the gas, and then regenerating the sorbent and recovering the sulfur in a separate step. Instead we propose that the simpler swing-reactor principle is applied. Our results show that it is possible to use the same reactor for both process steps, and switch between reactors where one is cleaning the gas while another reactor is regenerated. Then it is not necessary to transport the sorbent material between the reactor and the regenerator, and the requirements for mechanical strength are less demanding. For this process we have developed new sorbent materials with a unique composition, and with this composition we have developed pellets that can be used in larger (commercial) reactors without significant pressure drop.
We are investigating manganese as the active material in the sorbent. Manganese is one of many metals that can be used for this purpose, with properties that are well suited for medium- and high temperature syngas cleaning, significant in developing processes with high efficiencies. The initial work has been directed towards developing the chemistry of the support material. We support the manganese on a high surface area carrier in order to maximize the effect. A range of promoters was investigated, these are additives that improve the properties, both related to how clean the gas can get and the stability over time (repeated cycles of cleaning and regeneration).
We have developed improved methods to study the properties of the sorbents. We can now also measure with high precision the lowest attainable level over a sorbent, and the effect of steam in the gas. This is achieved using a precise sulfur analyzer capable of quantifying the sulfur concentrations in the gas-phase below 1 ppm, the range necessary to use the syngas in a catalytic synthesis. A key result is that we have developed a new sorbent with improved properties both in terms of capacity, the lowest attainable level and stability over repeated cycles. This was achieved by supporting manganese promoted with molybdenum on alumina support. Furthermore, larger sorbent particles (pellets), suitable for application in larger (commercial) reactors were developed. This requires careful design of the radial distribution of the active phase. We have developed pellets with ?egg-shell? structure, where the active phase is concentrated close to the external surface of the pellet, maintaining the properties developed using fine powder. A solid dataset for further process development is important, for this purpose we have developed models for the system, both a model that describes the capacity, and a kinetic model that can be used to design a gas cleaning step.
The main outcomes of this work is new knowledge and technology in the field of gas cleaning. The main technology is a new material for regenerable sorption of sulfur compounds from syngas from biomass, to be applied in new processes for advanced biofuels. An important additional outcome is new and improved experimental methods for the investigation of sorbent materials. This new method includes the ability to measure sulfur concentrations down to the sub-ppm level as well as to investigate the impact of water (steam) in the syngas, known to inhibit sulfur sorption. The impact will be on the efforts to design plants and equipment for advanced biofuels synthesis. This includes simpler and cheaper (in terms of investment cost) gas cleaning with less loss in efficiency.
In this project, we aim to study sulfur removal from the raw syngas from biomass gasification using Mn-based high temperature solid sorbents and develop a novel reactor system for high temperature desulphurization. The chosen technology represents an efficient concept regarding thermal efficiency and process economics versus conventional sorbent processes with the potential for step change improvement. The current study includes development of chemically- and mechanically stable high temperature Mn-based solid sorbent spherical pellets, reactor modeling and experimental work of a novel reactor concept for high temperature desulphurization, which consists of a combination of a riser and moving bed reactor (MBR). The Mn-based spherical pellets will be developed by applying a high shear granulator, and will have high S capture capacity. In the development of the novel reactor concept, a multiscale modeling and simulation approach will be employed to effectively combine reliable kinetic models, design, testing, model validation as well as optimization of a cold-flow circulating reactor, which can be directly used for future scaling-up of the reactors and for a reliable evaluation of the process. The modeling work will focus on the description of the coupling between flow phenomena and chemical kinetics, and mass- and heat transfer processes of the novel reactor system. This will provide the feasibility of high temperature desulphurization at larger scale using Mn-based spherical pellet and the novel reactor system. The potential of this process will be studied through a techno-economical process evaluation. The labscale reactor is included in the plans for the 2nd phase of NorBioLab infrastructure programme (NorBioLab II). The work in this project will be closely coordinated with efforts in Bio4Fuels, the new FME center on bioenergy about to start activities.