Resumen de Exergy analysis of biofuels
Industries use resources to produce goods, but also generate wastes and emissions. To study the environmental impact they cause, it is necessary to quantify inputs and outputs to the environment. Industrial Ecology has developed tools based on the mass conservation principle to evaluate the impact of processes, industries and also human activity in geographical areas. Resources and energy are limited, at least with present technologies, and part of the work to ensure their sustainable use is to evaluate them environmentally.
The evaluation of energy systems must be based on the first law and second law of thermodynamics. The first law states that energy is conserved and the second law that energy is degraded (quality lost) due to entropy generation. Exergy is defined based on those two laws as the potential work of a system. Based on exergy, different assessment tools such as Cumulative Exergy Consumption (CExC), Exergetic Life Cycle Assessment (ELCA) and Extended Exergy Accounting (EEA) have been developed.
This thesis includes four cases studies that show how exergy derived tools are used to quantify emissions and wastes, and resource use in the production of biofuels. The first case study uses exergy to account for wastes and emissions, and determine the exergetic efficiency of the production of biodiesel from used cooking oil (UCOME). The results show that the process has a low exergy loss (492 MJ) due to the use of potassium hydroxide and sulphuric acid as process catalysts. Such loss can be further minimised by improving the quality of the used cooking oil (UCO).
The second case study evaluates the Integrated Waste Management (IWM) system implemented in Catalonia for producing UCOME. The IWM includes the collection, pre-treatment and delivery of UCO and the production of biodiesel. The results show that the greatest exergy loss occurs during the transport stages (57%) and can be minimised to 20% by exploiting the full capacity of collecting vans and using biodiesel in the transport stages. The resource use (CExC) of biodiesel is 48.87 GJ/ton can be further reduced to 45.33 GJ/ton by using methanol obtained from biogas in the transesterification stage.
The third case study assesses the life cycle of biodiesel of UCOME by a life cycle assessment (LCA) and by an ELCA. The results show that the transesterification stage causes 68% of the total environmental impact. The exergy input is 148.20 GJ/ton. The major exergy inputs are uranium and natural gas that are the main sources used for electricity production.
The fourth case study evaluates the resource use of 1 ton of biodiesel from used cooking oil (UCOME) and from rapeseed crops (RME) by EEA. EEA calculates the total amount of primary exergy resources necessary to obtain a product or service, and includes capital, labor and environmental remediation costs. The results show that the extended exergy of UCOME is about half the extended exergy content of RME. UCOME requires 50.52 GJ less resources (materials and energy) and requires lower total investments and environmental remediation costs than that of RME.
During the development of the thesis, we concluded that it is necessary to define standard reference species and describe the methodology used for calculating exergy when studying systems. The discrepancy of results highlights the need to develop databases that apart of listing the inputs (in mass and energy units) required for producing a product also include exergy and CExC values. This is particularly important when studying biofuels, since inputs to agricultural systems vary according to location, land properties and agricultural practices. Including exergy and CExC in databases like Ecoinvent can help standardize exergy accounting and better the calculations of resource use in products.
