Assessment of agro-forest and industrial residues potential as an alternative energy source

Abstract

The generation of energy from alternative and renewable sources is of enormous importance for the sustainable development of the world bioeconomy, as its use mitigates environmental issues such as greenhouse gas emissions. Of the energy sources available, biomass has shown great potential for expansion due to existing reserves worldwide and its very versatile characteristics that can help to meet energy demands and reduce the accumulation of waste. Several countries aim to promote and support the bioenergy expansion, however to increase the share of renewables, policy goals, new technologies and sustainable biomass evaluation need to be explored. This thesis investigates a variety of different biomass-based residue streams, covering forest, agriculture and industrial processes. These residues may represent a serious source of environmental concern, if discarded inadequately and with poor management.

The considerable diversity of biomass sources and the differences in their chemical structure require detailed evaluation of the properties and the corresponding impacts on the chosen conversion processes. The quality of untreated biomass presents several challenges for its use and conversion into value-added products on a large scale. This thesis aims to characterize the residue recovery chain by studying the alternatives of energy conversion through a diverse thermochemical (hydrothermal carbonization, gasification, pyrolysis, direct combustion) and physicochemical (briquetting) utilization path. An extensive characterization of residual biomasses from the coffee production chain for energy purposes was evaluated. The results were not readily available in the literature before this study and are a fundamental tool to describe the impact of chemical components on thermal decomposition and further possible applications. Solid coffee residues showed high volatiles and cellulose and hemicelluloses content, suitable characteristics to thermal degradation. The HHV between 16-24 MJ kg-1 (db), analogous to biomasses commonly used in energy generation was also observed for coffee residues.

The performance of the conversion processes not only depends on feedstock characteristics but also on the process parameters. Thermochemical conversion technologies were studied through mass and energy balances. Results such as the alternative potential to generate heat and electricity to local areas trough gasification technology, which reported energy conversion rate of 84% for cherry coffee parchment, was found. Moreover, a hydrothermal carbonization (HTC) evaluation was extended to experimental procedures using several globally important woody and non woody waste biomasses. Since the process requires heat to be supplied, heat integration with combined heat and power (CHP) plants was simulated in order to analyze the potential of HTC treatment as an attractive process for a biorefinery using biomass residues. Integrating the HTC allowed a simpler process design, and an efficient benefit, provided that extraction steam is available at sufficient pressure levels. Additionally, sludge from pulp mill effluent treatment plants was studied through HTC technology, and integration with the pulp mill process was also evaluated in order to increase carbon capture alternatives. Physical, mechanical and chemical properties of densified biomass were also studied in this thesis, and agro-forest residue briquettes from coffee residues and pine produced a potential solid fuel of regular shape and high energy density and resistance, for use in local firing systems.