Pyrolysis studies show how pre-treatment can improve the quality of bio-oil

As part of the LowCoBio project, biomass pre-treatment has been carried out at VTT, whilst small-scale pyrolysis experiments and subsequent analyses of bio-oil have been conducted at CNRS in France. 

“The pyrolysis studies show that extracting hemicelluloses from biomass can reduce the acidity of the resulting bio-oil,” says Elmeri Pienihäkkinen, a researcher at VTT. Although this leads to a slightly lower oil yield, this trade-off may be acceptable if the quality is improved. Furthermore, utilising the extracted hemicelluloses in value-adding applications can further enhance the process economics. 

Pre‑treatment impacts bio‑oil quality and yield

The results from the CNRS are being used to select the most promising feedstocks and pre-treatment strategies ahead of upscaling tests at VTT. The aim is to verify that the results from the micro-scale can be replicated on a larger scale, whilst deepening our understanding of the bio-oil’s composition and properties. 

The screening has covered several types of biomass, including bark, forest residues and straw, which have been pre-treated at VTT. Initial results suggest that the pre-treatment strategy is of great importance. Water-based extraction of hemicelluloses, for example, can significantly reduce the acidity of the bio-oil, which is important for subsequent processing and for managing corrosion. At the same time, overly aggressive pre-treatment can reduce the total oil yield, which underlines the need to find an optimal balance. 

Towards scale‑up and full value‑chain evaluation

Once the most promising pre-treatment methods have been identified, bio-oils from both untreated and pre-treated biomass will be produced and upgraded into fuels. This will make it possible to evaluate processability, as well as yield and quality, throughout the entire production chain. 

"The studies show that different raw materials behave differently, as their composition varies," says Elmeri Pienihäkkinen. "For example, straw appears to require more intensive pre-treatment conditions compared with forest residues and bark."

In the upcoming upscaling phase at VTT, the aim is to further clarify how pre-treatment affects the quality and yield of bio-oil. Larger-scale trials will also enable more comprehensive quality assessments and an evaluation of the technical performance of the raw materials in continuously operated pyrolysis plants.