## 2 Common Substrates Used in Anaerobic Digestion
### Lignocellulosic Biomass
Lignin structures, on the contrary, are scarcely affected during anaerobic digestion, hardly experiencing small changes in their native structure when extended digestion studies were performed [60]. Due to the recalcitrance of lignin structures, lignocellulosic biomass is usually used as a structuring agent during solid-phase fermentation under percolating leachate configurations. The poor degradation rate of lignocellulosicosides under anaerobic conditions here becomes an advantage since the porosity of the percolating bed is desirable to allow the circulation and homogenization of soluble compounds by leachate recirculation. However, if this type of biomass is added as a co-substrate, then pre-treatments are recommended to facilitate access to the microflora. Several value-added products can be obtained from the fractionation and conversion of this raw material by means of a concatenation of different processes capable of a sequential transformation, always keeping in mind the global efficiency of the production line. The coupling of different processes leads to new developments integrated into the biorefinery concept, where a set of conversion platforms are available for obtaining green chemicals and recovering energy. Second-generation biofuels, such as biogas from lignocellulosic biomass, have great potential because of their plentiful abundance, offering no interference with other commercial activities such as animal feed or crops for human consumption. Still, the heterogeneous structure of this material and its recalcitrant nature adversely affect its use as a substrate in biogas plants [61]. Pre-treatment stages considerably increase the energy demand of the installation. Careful analysis should be performed regarding improvement obtained in biogas production after pre-treatment application and the energy required in the process. Thermal pre-treatments are widely extended at an industrial scale due to the experience gained in pre-treating sewage sludge and the unique feature of recovering energy from high-quality lateral streams. The hydrolysis of hemicellulose produces oligosaccharides such as pentose (xylose and arabinose), hexose (glucose, mannose, and galactose), acids (acetic acid, formic acid, and levulinic acid) and furans (furfurral and 5-hydroxymethylfurral). Insoluble humins are also obtained as products under harsh hydrolysis conditions [62]. However, the high temperature and high pressure under which hydrolysis is carried out may release some compounds that can behave as inhibitors [63]. Recalcitrant inhibitory substances, such as furfurals and hydroxyl methyl furural can be produced at high temperatures [64; 65], thus introducing new complexities into the vaporization process due to the additional stages necessary for removing these toxic compounds. Another relevant fact that should be noted is the high installation and operating costs associated with pre-treatment units, which sum up to the already high capital investments of digestion plants. Table 1 reports on the different methane yield values found in the literature for a variety of substrates. Some of these results present a wide range of variability since methane yields are highly dependent on the characteristic of the substrate, experimental conditions and the presence of inhibitory compounds. Another parameter of relevance is the time needed  
for degrading the organic material, which translates into high retention times and therefore large digester volumes.