Thermochemical conversions of biomass residues to valuable fuels, chemicals and materials

Abstract

As the pressure to reduce greenhouse gas emission grows, more interest towards sustainable biofuels and bio-chemicals has emerged. Fast pyrolysis is a robust liquefaction technique which can be used to turn biogenic solid wastes and residues into liquid intermediates (bio-oils). This is advantageous as liquids are much more easily utilizable in various chemical processes compared to solid waste. Ideally, sustainable non-edible and renewable lignocellulosic waste streams or industrial residues are used as feedstock for liquefaction to maximize the emission reductions, but the qualities of such feedstocks may be problematic from the scope of the processing. In this dissertation, fast pyrolysis of various lignocellulosic residues was studied in industrially relevant fluidized bed fast pyrolysis units. Target was to better understand the behavior of different residual feedstocks in the production of the fast pyrolysis bio-oils (FPBO), study the effect of feedstock pretreatment with low quality feeds and to improve understanding of the potential of FPBO and its further valorization into chemicals and materials. Most studied applications for FPBO are upgrading routes into transportation fuels but the focus on this thesis is in FPBO gasification to syngas and fractionation into chemicals, and materials.

Pretreatment of the lignocellulosic feeds was found to have both positive and negative effects depending on the starting feedstock and pretreatment severity. Alkali removal via mild acid leaching was found to significantly increase the organic bio-oil yield with high-alkali feedstocks. However, feeds, where alkali content was reduced below detection limits, were difficult to pyrolyze due to the bed agglomeration. The absence of alkali metals which are active in pyrolysis reactions may result in operational problems, while too large content of these alkali metals results in suboptimal bio-oil yield. Similar operational difficulties were observed also when fast pyrolysis of different hydrolysis lignins, by-products from the lignocellulosic ethanol production process, were studied. Hydrolysis lignin has gone rather severe pretreatment where a part of hemicelluloses and cellulose are removed from the feedstock prior to fast pyrolysis. The carbohydrate content of hydrolysis lignin had a clear correlation to its processability. More challenges were observed with lignin feedstocks having lower carbohydrate content. Increased amount of lignin caused problems through bed agglomeration. Additionally, rapid secondary reactions in the vapor phase resulted in deposit formation and pressure buildup in product gas lines, underlining the aspect that technical feasibility and readiness of lignin pyrolysis is still immature.

Regarding the FPBO valorization, the pathways studied in this dissertation were FPBO gasification into syngas and FPBO fractionation with subsequent use of obtained fractions in the phenolic resin synthesis. Results showed that the combination of fast pyrolysis with subsequent FPBO gasification provides a technically feasible and feedstock flexible solution to produce synthesis gas which can be used in the synthesis of fuels or various chemicals. FPBO fractions were also found to be potential substitutes for fossil phenol in phenolic resin production. All the produced resins performed well in dry conditions, but in wet conditions resins with the highest replacement ratio of 50 wt% had somewhat reduced strength. Although technical potential is promising, better understanding of techno-economic aspects of these routes will be needed.