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ENERGIX-Stort program energi

FLASH-Predicting the FLow behavior of ASH mixtures for production of transport biofuels in the circular economy

Alternative title: Strømningsegenskaper til askeblandinger ved produksjon av biodrivstoff i den sirkulære økonomien

Awarded: NOK 9.9 mill.

The need for advanced biofuels is emphasized at both the national and international level. Biofuel can be produced from sustainable sources such as wood and waste from forestry, agriculture, industry, and household. It is important to develop an energy effective process that addresses the challenges associated with the conversion of biomass into biofuel. The first step in the production of biofuels from biomass is known as gasification. In this step, the biomass is heated and converted into a synthesis gas consisting of carbon monoxide and hydrogen. This gas is further converted into biofuels. Ash is a by-product in the gasification process. There are major challenges associated with the ash, and the key to success is to identify and solve these problems. Ash in gasification reactors can cause various problems depending on the type of reactor used. Ash melting and formation of agglomerates can reduce the availability in some reactors, while in other reactors there is a dependency on conversion to molten ash which can run out of the reactor. In the latter case the ash may become too viscous and result in poor flowability and accumulation of ash in the reactor. The ash has different properties depending on the composition of the biomass, and in this project the impact of ash from different types of biomasses is investigated. Synthetic ash has been used in the project, and the focus has been to find ash compositions that are representative for biomass from forestry and agriculture. A 3-component phase diagram representing the main chemical composition of ash from woody biomass was used to select representative ash compositions. Based on calculations and simulations it was decided to use three different ash compositions representing the ash composition in wheat straw, grass, and waste from forestry. Synthetic ash was prepared and analyzed with respect to chemical composition and mineral phases. The results were further used in numerical studies to estimate the viscosity of the ash. A computational model which extracts images from wettability furnace experiments and calculates the flow behaviour of the sample inside the furnace was developed. The viscosity model was optimized so that the simulated sample matched the experimental one. Ash viscosity measurements of 3 different synthetic ash mixtures were performed at Aalto University. Further tests were carried out at Montan-University Leoben. Viscosity in ash as a function of temperature was compared with results from the viscosity model, and it was found that the model agrees well with the experiments for ash with a low alkali content, but there some deviations were observed for ash with a higher alkali content. Experiments were carried out in a lab-scale fluidized gasification reactor, where the problem with ash is melting and formation of agglomerates. The experiments were performed with wood pellets, wood chips and grass at different temperatures. The results indicated that under given operating conditions, the ash was melting, and agglomerates were formed. The size and the number of agglomerates depend on the temperature and the fraction of ash in the biomass. Agglomerates were formed even at temperatures below the ash melting temperature for the different types of biomasses. Pressure and temperature in the reactor were observed during the experiments, and based on these data, the onset of agglomeration was predicted. Analyses were performed to study the composition of ash and agglomerates. Experiments have also been carried out in a cold fluidized bed to study the flow behaviour in the bed with and without agglomerates. The results showed that the fluidization conditions change when agglomerates are present in the gasification reactor. The experimental results were used for validation and improvement of a computational model which was further used to simulate an up-scaled gasification reactor. The simulations clearly showed that with increased number of agglomerates, the fluidization properties changed, and under given conditions, the reactor was defluidized. It is therefore important to get an overview of the impact of ash behaviour in gasification processes. Experimental tests were also run in a microscale fluidized bed reactor to visualize the agglomeration process and predict the onset of defluidization. The experiments were performed with wood, bark, grass, and straw at different temperatures. The results were used to develop a mathematical model that can predict defluidization based on the ash composition and the operating temperature. FLASH has so far resulted in a completed PhD thesis, two completed master's theses, 8 published scientific articles and 2 articles under review in international journals. In addition, there are 3 scientific papers in process, planned to be published in 2022. Overall, FLASH has made a valuable contribution to accelerating the commercialization of conversion of biomass to biofuels via gasification.

The project has succeeded in increasing the fundamental understanding of ash properties and behaviour in thermal systems and the results will contribute to unlocking gasification of biomass for production of biofuel. New scientific tools to determine critical ash properties analytically and tools to determine critical operation conditions have been developed. The project has clear utility values to the field of research and competence development and will bring the utilization of Norwegian biomass one step closer to commercialization. The project has contributed to increase the national and international cooperation. The project results are available through one PhD and two master theses, and several published papers. Gasification of biomass has been implemented in courses on both Master and Bachelor level. The outcome of the project is a contribution to a better exploitation of biomass and lower emission of greenhouse gases.

The need for advanced biofuels produced from sustainable sources, is stressed on both national and international level. The key to unlocking gasification and downstream catalytic upgrading as a viable route for biomass and waste to biofuels is solving the challenge related to the ash, and this is the main motivation for the project. The project will develop a novel method to determine ash viscosity combined with chemical analysis. Using reference ash materials, the project aims at quantifying the flowability of selected ash samples. In parallel, the project will focus on thermodynamic equilibrium modelling to assess the phase distribution and composition of the ash. The work will consist of establishing a new thermodynamic database for ash reactions for main, minor and trace ash elements followed by parametric studies. The results will be compared with the ash microscope results for further improvements and validation. Combined with viscosity measurements generated by rheology tests and experimental tests performed in a pilot fluidized bed gasifier, a solid basis for ash behaviour prediction will be attained. Experimental study in a pilot fluidized bed (FB) gasifier will be performed with ash coated bed material and different types of bed material, fluidization agents and biomass ash composition will be used to study the ash behaviour and agglomeration in FB gasifiers. Combination of new ash chemistry, a predictive model for ash related problems for biomass mixtures, in-depth experimental studies of the gasification process will give the basis for developing an validate a computational fluid dynamic (CFD) model to predict ash related problems in biomass gasification processes. Altogether, the project will give a valuable contribution to improve the gasification efficiency and accelerate the implementation of biomass to biofuels via gasification by mitigating ash-related challenges.

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Funding scheme:

ENERGIX-Stort program energi