Acid pretreatment of lignocellulosic biomass for energy vectors production: A review focused on operational conditions and techno-economic assessment for bioethanol production

The acid pretreatment process is the most employed technique used to disrupt the lignocellulosic matrix. Thus, this paper provides an overview of the acid pretreatment considering important aspects related to the operation conditions, yields, future improvements and the potential use to upgrade lignocellulosic biomass in energy vectors. Moreover, this paper is focused on technical and economic aspects of the use of the acid pretreatment in bioethanol production using olive tree biomass as a case study. This review will provide the framework of the acid pretreatment based on reported applications, experimental and theoretical data. The most well-studied applications of the acid pretreatment are in bioethanol and biogas production. Furthermore, this pretreatment has been improved using solid catalysts and by increasing the solids loading. These options have advantages compared to the traditional way. Even so, improvements in the technical and energy aspects must be performed to make the implementation of this process viable from an economic and environmental perspective. Instead, the kinetic modelling and statistical analysis of the acid pretreatment can be postulated as strong tools to predict the behaviour of different feedstocks under acidic conditions. Moreover, new alternatives to increase the mass and heat transfer in this process related to reactor design conform a new research field. Regarding the case study, the acid pretreatment is one of the most energy-consuming stages of bioethanol production. Low solids loadings are recommended to obtain production costs that are comparable with those reported on the industrial level. In contrast, a high solids loading allows the environmental impact of the process to be reduced in terms of liquid waste generation and carbon dioxide emissions. Finally, the acid pretreatment should be included as an effective method for the biotechnological conversion of lignocellulosic biomass to produce energy vectors once the technical, economic and environmental drawbacks have been overcome.