Improving Inorganic Composition of Lignocellulosic Biomass by Water Leaching Pre-treatment for Thermochemical Applications

Abstract

Worldwide market for new and upgraded solid biofuels from low-quality lignocellulosic biomass residues is rapidly increasing for energy and chemical recovery. However, the inorganic composition of low-quality herbaceous and forest residues severely restricts their valorisation in thermochemical processes. Water leaching pre-treatment is an effective method for reducing inorganic content in lignocellulosic biomass. However, specific research on its implementation and optimization for industrial applications is missing from the available body of knowledge. The main goal of this doctoral work is to improve the basic understanding of water leaching pre-treatment and upgrade its technical feasibility for large-scale applications based on experimental findings.

The research work was divided into three steps. In the first step goal was to better understand the effect of basic leaching parameters on pre-treatment efficiency and evaluate the impact of pre-treatment on a range of feedstocks. The goal in the second step was to improve the pre-treatment efficiency by process modification, pilot-scale testing and comparing the results with laboratory findings. In the third step, the effect of leaching pre-treatment was evaluated on ash-fusion behaviour of different biomasses and stepwise pyrolysis product yields. Experimental results show that water leaching successfully removed about 30-100% of troubling elements (K, Na, Cl, N, S, Ca, Mg, P, Si) from different types of biomasses. Consequently, fewer ash- related issues can be expected from the leached biomass in combustion and gasification. Also, higher yields and quality of bio-oil and biochar were achieved after using pre-treated biomass in stepwise pyrolysis. A novel pre-treatment process, “step washing”, was also explicitly developed for practical application and tested on a pilot- scale. Step washing process showed better pre-treatment efficiency than the single step leaching (3-24 hrs) process in much shorter durations (4-15 min).

The major contribution of this doctoral work is about improving the understanding of the overall leaching process and modifying it for large-scale applications based on detailed experimental studies. The findings presented in this dissertation will benefit both researchers and practitioners working in the area of biomass pre-treatment, process engineering, and biomass valorisation by thermochemical conversion.