Advanced Bioenergy Lab - Plant design via process simulation

Abstract

In the ABL1 project, a R&D infrastructure (“ABL plant”) for experimental investigation and demonstration of biomass conversion into sustainable energy carriers will be erected. Therefore, a full process chain for conversion of solid biomass to a product gas (PG) and subsequent gas cleaning to a syngas (SG) will be built. Subsequently, the synthesis to the desired energy carrier will be performed with this SG. Due to high availability of woody biomass in Austria and quite easy handling to be expected in the process, forestry residues were selected as a feedstock. The plant for SG production consists of a fuel handling system, a biomass dryer for the use of feedstocks with water contents >> 20 wt.%, a dual fluidized bed (DFB) gasification system, several heat exchangers for PG and flue gas cooling, a system for coarse gas cleaning and for further fine gas cleaning, and several sub-systems, e.g., for heat management. The removal of char, dust and some of the ammonia and organic compounds like tar and BTEX is performed by the coarse gas cleaning units. For further purification, the remaining impurties harmuful for syntheses, especially ammonia, tar, BTEX and sulphuric compounds are then removed via fine gas cleaning. The designed plant must be capable of being operated with both high-quality and low-quality woody biomass. Therefore, the feedstock composition for process simulation was set with values in the range of the standard EN ISO 17225-4:2021 [1], but with elevated values concerning the ash content (3 wt.%) and the water content (55 wt.%). This way, the plant can be designed for extreme conditions, e.g., maximum heat required for biomass drying. Similarily, the PG impurities are set to maximum expected values for DFB gasification for a fail-proof design of the gas cleaning system. Subsequently to SG production, the SG is used for synthesis purposes. The two synthesis routes for synthetic natural gas (SNG) and Fischer-Tropsch (FT) diesel proposed in the feasibility study [2] conducted prior to this project in 2020, were investigated via process simulation. For evaluation of a continuously operated SNG plant, a plant scale with a nominal load of 8 MWLHV fuel input (chemical energy input to the gasifier) was selected for process simulation, as this scale offers economy of scale benefits to some extent. The plant is assumed to be operational in the range of 4-10 MWLHV. This would offer the opportunity to erect the FT route in smaller scale (2-4 MWLHV SG input) for test operation only, potentially parallel with the SNG route. The process simulation results show promising process efficiencies even for worst case operation, e.g., high tar and BTEX contents, as > 60% of energy input can be converted into SNG. Additionally, internal heat requirements, e.g., for biomass drying, can be met by internal heat management in the ABL plant alone, without the necessity of external heat input. The results also show that the chemical energy bound in tar and BTEX components can significantely influence the process efficiencies, especially when high concentrations of these substances are present in the PG. Therefore, these components should be utilized after separation from the PG, e.g., for combustion in the DFB system instead of valuable PG recycling. More detailed results and thoughts for a process chain layout will be presented for SNG and FT synthesis.