Hydrothermal liquefaction for energy recovery from high-moisture waste biomass
Hydrothermal liquefaction is promising technology for converting high-moisture waste biomass into energy dense “biocrude” oil that can be used for direct combustion or refined for transportation grade fuels. This study investigates the influence of biomass composition on the chemical characteristics of hydrothermal liquefaction biocrude oil, and the potential of algal biomass as a feedstock for combined wastewater treatment and bioenergy production. Hydrothermal liquefaction was examined for converting various types of waste biomass into biocrude oil to determine the effect of biomass composition on product yields and biocrude oil chemical characteristics. Feedstocks tested included Spirulina algae, swine manure, and digested anaerobic sludge. Bulk biocrude oil properties (e.g., oil yield, elemental analysis, higher heating value) and physico-chemical characteristics (e.g., molecular constituents, functional group allocation, proton and carbon speciation, molecular weight distribution, boiling point distribution) were compared for the varying feedstocks as well as with published results for petroleum crudes and tar sand bitumens. Results demonstrated that although the biocrude oils displayed similar higher heating values (32-35 MJ/kg), widely varying biocrude oil yields were observed, ranging from 9.4% with digested sludge to 33% with Spirulina. High total nitrogen and oxygen content was observed in the biocrude oils (19-23%), greatly differentiating them from petroleum crude oils which are typically less than 4% total. Detailed characterization also revealed significant differences in biocrude oil chemistry. Feedstock composition influenced the individual compounds identified in the low-boiling fraction of the biocrude oil as well as the functional group chemistry. Biocrude oil molecular weights tracked with the feedstock obdurate carbohydrate content and followed the order of Spirulina < swine manure < digested sludge. A similar trend was observed in the biocrude oil boiling point distributions and the long branched aliphatic contents. After examining the hydrothermal liquefaction of various waste biomass sources, a detailed analysis of the hydrothermal liquefaction of Spirulina and Scenedesmus algal biomass was undertaken. Algal biomass was analyzed due to its potential integration into wastewater treatment plant operations for nutrient removal, carbon dioxide capture, and bioenergy production. Recently, much work has focused on extracting high value chemicals (e.g., nutraceuticals) and energy-dense lipids (e.g., for biodiesel) from algae, but effective utilization of residual “defatted” algal biomass will be necessary to achieve favorable energy balances and production costs. Therefore, hydrothermal liquefaction product yields, biocrude oil properties, and energy balance for raw and defatted Scenedesmus biomass were compared against slow pyrolysis, an alternative thermochemical process. Biocrude oil chemistries from both processes were then evaluated against Illinois shale oil as a representative low-quality petroleum crude oil. While both thermochemical conversion routes produced energy dense biocrude oil (35-37 MJ/kg), biocrude oil yields and physico-chemical characteristics were highly influenced by the conversion route and algal biomass composition. Biocrude oils derived from hydrothermal liquefaction and slow pyrolysis of Spirulina and raw and defatted Scenedesmus displayed similar elemental compositions and chemical functionalities; however, pyrolysis biocrude oils were lower in molecular weight and boiling point distribution compared to hydrothermal liquefaction biocrude oils. The high lignin and cellulose content of raw and defatted Scenedesmus biomass also significantly affected the hydrothermal liquefaction biocrude oils properties, and increased the molecular weight and boiling point distribution compared to hydrothermal liquefaction biocrude oil derived from Spirulina. Analysis of the energy consumption ratio revealed that for wet algal biomass (e.g., 80% moisture content), hydrothermal liquefaction is more energetically favorable compared to slow pyrolysis due to water volatilization required in the latter technique. The high heteroatom content of the biocrude oils greatly differentiated them from Illinois shale oil, and imparts negative properties such as poor storage stability, high viscosity, increased boiling point range, and undesirable emissions during combustion. In summary, this study demonstrated that hydrothermal liquefaction is effective for recovering energy from high-moisture waste biomass in the form of energy-dense biocrude oil. Biomass composition was shown to greatly influence biocrude oil chemical properties, and algae hold potential for combined wastewater treatment and bioenergy production. However, the high heteroatom content of biocrude oil is problematic and will likely require removal prior to downstream applications. Further efforts are needed to examine the upgradation of biocrude oils and examine the integration of hydrothermal processing with algal cultivation and wastewater treatment.
hydrothermal liquefaction, pyrolysis, algae, swine manure, digested sludge
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign