# Analysis of Sequential Pretreatments to Enhance the Early-Stage Biorefinery Designs
## 3 Results
### Heuristic Analysis
Different authors have proposed biorefineries based on the fractionation of lignocellulosic biomass for catalytic products, biochemical pathway products, platform molecules, and energy products [41]. However, increasing the rate of commercial products will also increase the complexity of the processing structure. For example, it is possible to pro  
Figure 3: Schematic mass balance of DA and WAO sequential pretreatment of rice husk. pose different products in a biorefinery based on wood biomass. By performing a DA pretreatment, the hydrolyzed liquor can be considered for the production of furfural, which has been widely used as a solvent or pesticide precursor. After detoxification, this first hydrolyzed liquor can also produce xylitol from xylose, whose nutritional properties of the sugar alcohol have been described [42]. After the second pretreatment with WAO, the soluble lignin can be used as a raw material for producing phenolic compounds or a precursor for lignin-based materials [43]. Meanwhile, the cellulose remaining from the pretreatment sequence could be used to produce biofuels or platform chemicals. In contrast, the complexity of the biorefinery can be decreased by contemplating the sequential scheme for the pentose platform as the LHW and DA sequence. The first hydrolysate would have a lower composition of inhibitory compounds [44], allowing its use in biochemical processes such as xylose fermentation. Moreover, after the second pretreatment with DA, the cellulose could be used to manufacture chemical products [45]. Although sequential pretreatments show promise in biorefinery schemes, some implications must be discussed. After a pretreatment, there is not only the lignocellulosic removal or formation of simpler molecules but also structural changes in the biomass that will affect the performance of a second pretreatment. Some authors have shown that after hydrothermal treatment, there are changes in the cell wall or the ultrastructure of the biomass, even degrading it significantly with increasing severity [46]. This phenomenon is also reflected in lignin, whose dispersity, molecular weight, structural organization, and properties change according to the pretreatment performed [24]. On the other hand, the formation of unwanted compounds or by-products could interact in the second pretreatment, decreasing the performance of the process. These undesired compounds will also interfere in future polarizations of the obtained undercurrents or hydrolysates. Therefore, studying the influence of by-products and biomass structural change could be a future work to improve biomass fractionation in sequential schemes. Moreover, this work focused on the techno-economic discussion, leaving aside the environmental assessment. Considering an integral sustainability analysis, environmental and social aspects must be adressed. Therefore, a study on the influence of the toxicity of reagents, the environmental impact of waste streams such as washing streams, and the transport of raw materials would help to complement the sustainability analysis.