From Waste to Wealth: Transforming Biorefineries into Platforms to Valorize Low-Quality Residues

Abstract: The transition to a bioeconomy, in which resources from biowaste could be reclaimed for the production of chemicals, materials or fuels, could contribute to the establishment of a sustainable society by replacing fossil fuels as raw materials. The core of this bioeconomy would be biorefineries, which are similar to petroleum-based refineries, but are facilities where biowaste is processed and refined through different chemical and/or biochemical processes. There are already some commercial-scale biorefineries that utilize edible biomass, such as corn or sugarcane, to produce mainly fuels. However, edible biomass can hardly be considered a biowaste, and there is an ethical conflict between the use of such resources for food and the production of energy. An alternative strategy has been developed to utilize non-edible materials such as forest and agricultural residues in biorefineries, but the relatively high price of these residues has prevented biorefineries from being economically viable.This thesis presents a framework for the redesign of current biorefineries so that they can use low-value waste as feedstock. This shift in feedstock would improve not only the economics of biorefineries, as feedstock cost accounts for almost one third of the total production cost, but also their environmental performance, as it would be possible to valorize residues that are otherwise incinerated or landfilled. The framework was developed based on research on animal bedding, i.e. a mixture of manure and straw, and cotton-based waste textiles.The first step involves the adaptation of analytical methods so that they can be applied to these high-complexity and variable low-value residues. Since low-value waste is usually unsorted, fractionation will be required to separate the different materials so that each fraction can be analyzed separately with conventional methods. An example of such fractionation is presented for the case of animal bedding, where manure and straw were separated by washing with water. Secondly, conversion tech¬nologies must be adapted to the special characteristics of low-value residues to avoid compromising the efficiency of the operation. This was demonstrated by the redesign of steam treatment for the pretreatment of animal bedding, and concentrated-acid hydrolysis for the depolymerization of waste textiles. In both cases, the overall efficiency of the biorefinery would be around 70%, which is similar to that achieved with higher quality residues. Finally, process concepts must be redesigned to transform complex low-value residues into opportunities to further improve biorefineries. This was demonstrated by the exploitation of synergies between simultaneous biogas and bioethanol production from animal bedding, and the possibility of establishing industrial symbiosis between the depolymerization of waste textiles and pulping for paper production.The superior economic performance of a low-value waste biorefinery, after appropriate adaptation, was illustrated through a biorefinery based on animal bedding. Such a biorefinery could deliver bioethanol to the market 40% cheaper than a biorefinery based on wheat straw; the higher quality counterpart of animal bedding. This example showed that the conditioning cost associated with low-value residues could potentially be compensated for by savings in the cost of feedstock, leading to an economic advantage in the biorefinery. Thus, low-value waste biorefineries could provide early market opportunities for the transition to a bioeconomy.