Two pilot-scale gasification test campaigns conducted at VTT’s Pilot Centre Bioruukki – process conditions in the gasifier optimized for co-production of biochar and synthesis gas from woody residues

One of the core activities in the FlexSNG project is to develop a new gasification process that can co-produce biochar and good-quality raw synthesis gas and can also switch between two operation modes: (1) co-producing biochar and synthesis gas that is further converted to biomethane, and (2) maximizing the fuel conversion to biomethane. The preliminary gasification experiments conducted in WP4 aim to identify optimal conditions for the gasification and gas clean-up process for both operation modes and various biomass residues and organic waste feedstocks.

So far, VTT has successfully carried out two three-day gasification test campaigns using two different fluidised-bed gasifier designs and crushed bark pellets as feedstock. The work has focused particularly on the co-production mode and optimizing the quality of biochar discharged from the gasification process. A number of operational parameters have been screened, such as gasification temperature, steam-to-fuel feed ratio, steam-to-oxygen feed ratio, fluidising velocity and bed material (sand/dolomite), to investigate their influence on biochar yield, characteristics and partitioning between bottom ash and fly ash. Based on the preliminary tests, gasification temperatures below 700 °C and low fluidising velocities at the bottom part of the gasifier were best suited for biochar co-production, resulting in high carbon content in the biochar product (>70 %). The biochar produced in the gasification test campaigns have been shipped to EIFER for further characterization to determine its suitability for end use applications, such as soil amendment, construction material, steel making industry etc. The low gasification temperature employed in co-production mode evidently results in increased tar loading in the raw synthesis gas – potentially creating further challenges in hot gas filtration and reforming (e.g. soot formation, clogging by tars). These challenges have been overcome by partially decomposing the tars in the upper section of the gasifier by injecting secondary oxygen (mixed with nitrogen) and elevating the gas temperature to 820-850 °C.

The next step in the gasification process development is to study the gasification performance of the more challenging raw materials: high alkali agrobiomass (e.g. straw) and waste-derived feedstocks (e.g. Solid Recovered Fuel, SRF). One of the key aspects will be to investigate how their gasification behavior can be improved (e.g. ash sintering prevented) by using biochar as co-feed in the gasifier.

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