**3. Hydrothermal pretreatments**

The main problems associated with acid hydrolysis relate to the formation of toxic com‐ pounds from biomass degradation and from equipment corrosion. Such toxic products en‐ tail inhibition in cell metabolism when biomass hydrolyzates are used for bioconversion. Steps to remove these inhibitory compounds have been employed to improve the yields in

Table 1 presents different conditions of acid pretreatment for different lignocellulosic mate‐ rials for enzymatic hydrolysis, as well as the cellulose conversion efficiency for hardwood, softwood, and sugarcane bagasse and straw. Historically, acid pretreatment has been the

> **H2SO4 (wt %)**

*Eucalyptus camaldulensis*<sup>2</sup> 165 8 1.4 15FPU/52.5BGU 38 (160 h) [17]

(10% birch and 90% maple)2 <sup>230</sup> 0.12 1.17 --- <sup>95</sup> [19]

Sugarcane bagasse 130 15 0.5 7FPU/3.5BGU 41.5 (72 h) [20]

Sugarcane straw 195 10 1.0 15FPU/10BGU 72.4 [21]

Pretreatment with diluted sulfuric acid has been reported as one of the most widely used processes due to its high efficiency [14]. This pretreatment removes and hydrolyzes up to 90% of the hemicellulose fraction, rendering the cellulose fraction more accessible to hydro‐ lytic enzymes. However, it presents important drawbacks related to the need for a neutrali‐ zation step that generates salt and biomass sugar degradation with the formation of inhibitors for the subsequent fermentation step such as furfural from xylose degradation. The removal of inhibitors from the biomass sugar syrups adds cost to the process and gener‐ ates a waste stream. Additionally, mineral acids are corrosive to the equipment, calling for the use of more sturdy materials alongside higher maintenance costs. Acid recovery is also costly. The availability of the biomass acid pretreatment and the knowledge that has been built up on this subject highlights its important and costly drawbacks. In addition, the envi‐ ronmental problems caused by its waste streams have called for the need for other options

Athel pine1 165 8 1.4 15FPU/52.5BGU 60 (160 h) [17]

Spruce1 180 30 5.0 15FPU/g solid 55 (24 h) [18]

**Enzyme loading (FPU/BGU)**

**Enzymatic digestibility (%)**

**References**

**Time (min)**

52 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

**Table 1.** Examples of sugarcane and woody biomass pretreated with diluted acid.

**2.1. Advantages and disadvantages of acid pretreatment**

for the pretreatment of lignocellulosic materials.

bioconversion processes.

Mixed wood

1Softwood; 2Hardwood

main choice for wood pretreatment [16].

**Feedstock T (°C)**
