Resumen de Life cycle assessment of biomass wastes valorization through thermochemical and biochemical processes
Environmental problems are represented by any alteration that can cause an environment imbalance, affecting it negatively. Nowadays, the main factor of the environmental deterioration is the human activity, specifically those actions which imply the use of conventional energy and involve the accumulation of solid waste and toxic emissions. The primary measures to mitigate part of the environmental problems is to generate energy and other valuable products from renewable resources, such as biomass. The integration of life cycle assessment, for the study of the renewable energy processes, can offer relevant information related to hotspots that appear throughout the entire production chain.
In Spain, the olive industry generates during the olive oil extraction approximately 3 tons of olive pomace annually. On the other hand, in Mexico, about 300 million tons/year of castor husk, coffee pulp, pine sawdust, sugarcane bagasse, and agave bagasse are generated during castor oil extraction, coffee production, wood manufacturing, sugar extraction and bacanora production, respectively. Due to the socio-economic importance of these agro-food industries in Spain and Mexico, it is essential to find alternatives to valorize these wastes and, therefore, reduce the environmental impact they generate.
The main objective of this Ph.D. thesis consisted in studying the environmental performance of the valorization of agroindustry biomasses through thermochemical (pyrolysis, combustion and gasification) and biochemical (fermentation) processes. In this sense, the selected biomasses as raw material for the study were olive pomace (Spanish biomass), castor husk, coffee pulp, pine sawdust, sugarcane bagasse, sugarcane molasses, agave bagasse and agave juice (Mexican biomasses). The olive pomace sample and the inventory data were provided by the olive mill Aceites García de la Cruz located in the region of Castilla-La Mancha, particularly in Madridejos (Toledo, Spain). On the other hand, the Higher Technological Institute of Perote (Mexico), due to a collaboration agreement between this Institute and the University of Castilla-La Mancha where this research work was developed, provided the Mexican biomasses.
Chapters 1, 2 and 3 correspond to the general information and the methodology, respectively. On the other hand, Chapters 4, 5, 6 and 7 provide the results and the main conclusions obtained during this research. These chapters are explained in more detail below.
Chapter 1 provides general information on current environmental problems and presents different alternatives to reduce these problems, especially focusing on the use of biomass as renewable energy. This information facilitates the understanding of the motivations and objectives of this research.
Chapter 2 shows the theoretical background, describing in detail the biomass and conversion processes that were used in this research. In addition, it offers theoretical information about life cycle analysis and economic analysis, respectively. This chapter also establishes the reasons and objectives to carry out this study.
Chapter 3 summarizes the methodology related to this work, describing the biomass characterization techniques and the software that were used to perform the simulations of the conversion processes and the life cycle analysis.
Chapter 4 presents the results obtained using the life cycle analysis methodology, focusing on the evaluation of the potential environmental impact of the pyrolysis process to obtain tar and char. The biomass used as raw material were olive pomace, castor husk, coffee pulp and pine sawdust, respectively.
For the olive pomace pyrolysis, the environmental analysis was carried out considering the following stages: olive cultivation (organic farming versus conventional farming), olive oil extraction and olive pomace pyrolysis. In addition, the environmental impacts of each equipment involved in the pyrolysis plant were analyzed. The obtained results revealed that the olives and olive oil production through the ecological system offered better environmental performances than the conventional system. Comparing each equipment involved in the pyrolysis plant, it was observed that the cyclone had the least environmental impact, while the dryer, crusher and pyrolizer had the highest impact value.
On the other hand, in the case of castor husk, coffee pulp and pine sawdust, only the environmental evaluation of the pyrolysis process was investigated. Additionally, an economic analysis was carried out. Comparing the castor husk, coffee pulp and pine sawdust, it was determined that the pine sawdust had the least environmental impact. Moreover, the negative environmental impacts of each equipment involved in the pyrolysis process were evaluated. In addition, the separator was the equipment that most affected the environment. Finally, the results showed that the pyrolysis process using the coffee pulp as a raw material could be considered a beneficial tool to reduce the amount of this biomass due to its economic viability.
Chapter 5 summarizes the results obtained through the life cycle assessment of the combustion and gasification processes of olive pomace, castor husk, coffee pulp, pine sawdust, sugarcane bagasse and agave bagasse to obtain energy. The combustion and gasification processes occur at high temperatures under an oxygen atmosphere (combustion) or gasifying agents presence (gasification).
Firstly, the environmental impact generated by the combustion and gasification of olive pomace was analyzed. To carry out this study, the following stages were evaluated: cultivation, olive oil extraction and combustion/gasification process. For the last stage, all the equipment involved in both processes were evaluated. The life cycle assessment results showed that, for the olive pomace valorization, the olive oil extraction stage had the highest impact values on almost all the categories evaluated.
In the second part of this chapter, the life cycle assessment was only performed for the combustion and gasification processes of castor husk, coffee pulp and pine sawdust. For a complete perspective, the environmental impacts of each equipment involved in the conversion processes were analyzed. The comparative study of castor husk, coffee pulp and pine sawdust concluded that the pine sawdust was the best biomass from the environmental point of veiw.
In the last part of the chapter, the environmental impacts associated with the energy production from sugarcane and agave bagasse were analyzed, considering the following stages: cultivation, sugar/bacanora production and combustion/gasification of the biomass. Comparing the sugarcane and agave bagasse scenarios, it was observed that agave bagasse scenario showed less environmental damage. Analyzing the energy production from sugarcane bagasse, it was observed that the stage that most negatively affected the environment was the sugarcane cultivation. On the other hand, for the agave bagasse scenario, the stage that most damaged the environment was the bacanora production.
In addition, it was observed that the combustion process was more economically and environmentally viable than the gasification one. On the other hand, analyzing each equipment involved in the thermochemical processes, Rankine Cycle was key contributor to the environmental damage. Finally, it could be concluded that both combustion and gasification processes are feasible options to produce energy. However, the combustion process provided a better response in terms of environmental and economic results.
Chapter 6 compares the environmental performance associated with the production of bioethanol from sugarcane molasses and agave juice, using life cycle assessment methodology. The alcoholic fermentation is an anaerobic biochemical process that is used to transform the feedstock into alcohols. In this research, the following stages were considered: sugarcane and agave cultivation, sugar and agave juice extraction and bioethanol production process. The results showed that for the sugarcane scenario, the cultivation stage had the highest environmental impacts, while for the agave scenario, the stage with the highest impact values was the bioethanol production process. On the other hand, comparing the two scenarios, it was observed that the agave scenario had less environmental impact values than sugarcane scenario. Finally, the economic analysis of the bioethanol production process showed that using only grid energy the process was not economically feasible. However, using grid energy in combination with renewable energy could be economically profitable scenarios.
Finally, Chapter 7 specifies the main conclusions and recommendations derived from this research work.
It can be concluded that all the analyzed processes (pyrolysis, combustion, gasification and fermentation) are viable options to obtain added-value products from solid waste generated in agroindustry. Additionally, the contamination caused by inadequate management of these wastes can be reduced.
