Title: A heuristic for improving clustering in biomass supply chains

Authors: Ioannis T. Christou, Fragkoulis Psathas, Athanasios Rentizelas, Athanasios Papadakis, Paraskevas N. Georgiou, Despina Anastasopoulos & Pantelis Lappas

Publication Year: 2024

Proposed by: NTUA partners

Project Areas: Clustering; weighted MSSC; supply chain; biomass; heuristics

Abstract

Clustering is commonly used in various fields such as statistics, geospatial analysis, and machine learning. In supply chain modelling, clustering is applied when the number of potential origins and/or destinations exceeds the solvable problem size. Related methods allow the reduction of the models’ dimensionality, hence facilitating their solution in acceptable timeframes for business applications. The weighted minimum sum-of-square distances clustering problem (Weighted MSSC) is a typical problem encountered in many biomass supply chain management applications, where large numbers of fields exist. This task is usually approached with the weighted K-means heuristic algorithm. This study proposes a novel, more efficient algorithm for solving the occurring weighted sum-of-squared distances minimization problem in a 2-dimensional Euclidean surface. The problem is formulated as a set-partitioning problem, and a column-generation inspired approach is applied, finding better solutions than the ones obtained from the weighted version of the K-means heuristic. Results from both benchmark datasets and a biomass supply chain case show that even for large values of K, the proposed approach consistently finds better solutions than the best solutions found by other heuristic algorithms. Ultimately, this study can contribute to more efficient clustering, which can lead to more realistic outcomes in supply chain optimization.

Title: Investigating Salt Precipitation in Continuous Supercritical Water Gasification of Biomass

Authors: Julia Dutzi, Nikolaos Boukis, Jörg Sauer

Publication Year: 2030

Proposed by: Rainer Janssen

Project Areas: Supercritical water, biomass, gasification, process design

Abstract

The formation of solid deposits in the process of supercritical water gasification (SCWG) is one of the main problems hindering the commercial application of the process. Seven experiments were conducted with the grass Reed Canary Grass with different preheating temperatures, but all ended early due to the formation of solid deposits (maximum operation of 3.8 h). The position of solid deposits in the lab plant changed with the variation in the temperature profile. Since the formation of solid deposits consisting of salts, coke, and corrosion products is a severe issue that needs to be resolved in order to enable long-time operation, inner temperature measurements were conducted to determine the temperature range that corresponds with the zone of solid formation. The temperature range was found to be 400 to 440 °C. Wherever this temperature was first reached solid deposits occurred in the system that led to blockage of the flow. Additional to the influence of the temperature, the influence of the flow direction (up-flow or down-flow) on the operation of the continuous SCWG plant was examined. If salts are not separated from the system sufficiently, upflow reactors should be avoided because they amplify the accumulation of solid deposits leading to a shortened operation time. The heating concept coupled with the salt separation needs to be redesigned in order to separate the salts before entering the gasification reactors. Outside of the determined temperature zone no deposition was visible. Thus, even though the gasification efficiency was low it could be shown that the operation was limited to the deposits forming in the heating section and not by incomplete gasification in the reactor where T > 600 °C.

Title: Screening of ten different plants in the process of supercritical water gasification

Authors: Julian Dutzi, I. Katharina Stoll, Nikolaos Boukis, Jörg Sauer

Publication Year: 2024

Proposed by: Rainer Janssen

Project Areas: Supercritical water, biomass, hydrogen, homogeneous catalyst

Abstract

It is important to know the limitations of the supercritical water gasification (SCWG) in terms of behavior of different biomasses, especially when determining whether SCWG is a suitable conversion process for a certain biomass. Ten different biomasses (eight different plant species, of which two were grown in two different sites) were processed to evaluate this aspect. Moist and dry, woody, and grassy biomasses were gasified in the same experimental setup under similar conditions. Only small differences could be seen in the gasification experiments. The carbon gasification efficiency was 60.3 ± 5.1 %, the gas compositions were very similar. Solid deposits formed in all experiments in the same temperature zone of the reactor containing coke, salt building elements and heavy metals, sometimes leading to plugging. Nevertheless, an experimental duration of 6 h could be achieved for the dry biomasses. The experiment with the moist biomass Reed Canary Grass was ended early due to plugging of the feed tubing which is due to the different size reduction procedure for moist biomasses resulting in bigger biomass particles. This emphasizes the importance of sufficient size reduction prior to the experiment. Potassium addition as a homogeneous catalyst, in form of potassium hydroxide, has proven to be beneficial regarding gasification efficiency, but poses a threat regarding plugging due to salt deposits in the system.

Title: Energetic Assessment of SCWG Experiments with Reed Canary Grass and Ethanol Solution on Laboratory and Pilot Scale

Authors: Julian Dutzi, Nikolaos Boukis, Jörg Sauer

Publication Year: 2023

Proposed by: Nikolaos Boukis

Project Areas: Supercritical water, Biomass, Gasification

Abstract

The energetic assessment of biomass conversion processes is important for evaluating their application potential. Process energy efficiency is often evaluated based on simulation results for processes under idealized conditions. The present work uses data from a laboratory plant to perform an evaluation of the supercritical water gasification (SCWG). For this purpose, experiments were conducted with two types of feedstock, Reed Canary Grass and ethanol. Under the present lab-plant configuration, a cold gas efficiency of up to 79% could be reached, which is comparable to the gasification of biomass in fluidized-bed gasifiers or entrained-flow gasifiers. Based on the obtained data on the produced substances and their distribution, a scale up to pilot plant size was conducted. A model was derived from the available data to energetically assess different SCWG plant sizes based on real laboratory results. This model can be transferred to other feedstocks and other process designs to approximate the optimal size for the used biomass feedstock. The importance of heat recuperation in this process is described in detail based on pilot-scale data.

Title: Process Effluent Recycling in the Supercritical Water Gasification of Dry Biomass

Authors: Julian Dutzi, Nikolaos Boukis and Jörg Sauer

Publication Year: 2023

Proposed by: Nikolaos Boukis

Project Areas: Biomass Resources, Process design, Technologies

Abstract

The influence of process water recycling during the Supercritical Water Gasification (SCWG) of dry biomasses was investigated. Dry biomass has to be diluted with water to a dry matter content of approximately 10 wt.% to gasify it in the process of supercritical water gasification. The treatment of wastewater in the SCWG process is cost intensive due to organic contaminants; therefore, the recycling of the process effluent is attractive. Salt separation is needed to avoid accumulation of salts in the effluents, since salts enhance corrosion rates and might cause blocking of the flow when the effluent is recycled. The grass Reed Canary Grass and grapevines were gasified. The recycling of the process effluent did not influence the composition of the product gas. In both cases the carbon efficiency decreased by 4% when wastewater was used to dilute the biomass. An increase in organic carbon and potassium in the reactor effluent was observed after gasification of the biomass with recycled process effluent. The addition of potassium hydroxide to the feed as a homogenous catalyst needs to be closely monitored and adjusted according to the potassium content of the reactor effluent. Insufficient salt separation proved to be an issue regarding formation of solid deposits in the reaction system.