Lignocellulose is an element of the plant cell wall, and it majorly composes of hemicellulose, cellulose and lignin.

Lignin is a natural complex polymer and the chief noncarbohydrate constituent of wood binds to cellulose fibers providing mechanical strength and structural support of plants where it can be found extensively in the cell walls of all woody plants. Lignin is the most abundant natural source after cellulose, and between 40 and 50 million tons of lignin per annum are produced worldwide [30], constituting one-fourth to one-third of the total dry weight of trees. As the chemical composition of lignin has a certain variation, it is not possible to define the precise structure of lignin.

Due to the mechanical strength of lignocellulose supported by lignin, lignocellulose is known to be recalcitrant carbon pools. Lignocellulose is very slowly bioconvertible in anaerobic environments due to its rigid structure, as lignin is non-degradable [31] and the lignin suppresses degradation of lignocellulosic fibers such as hemicellulose and celluloses [10]. For this reason, it is often pointed out as the main cause of low BM in plant biomass. Many studies reported that lignin content and the efficiency of enzymatic hydrolysis have an inverse relationship. [23,29,32] In this text, pretreatment of substrate to increase biogas productivity usually focuses on improving hydrolysis by releasing lignocellulosic bindings, occasionally degrading lignin polymers. To the contrast of the critical role of lignin for anaerobic digestion, a larger amount of lignin is preferable to obtain energy from combustion, as higher heating values of biomass positively correlate with lignin content [33,34]. The higher heating value is the absolute value of the specific energy combustion, when solid biofuel burns in oxygen in a calorimetric bomb under specific conditions.

Lignocellulose is namely most abundant for plant biomass, likewise it’s often called lignocellulosic biomass. High concentration of lignocellulose can also be found in animal slurry, since animals are fed plants i. e. grass, straw, etc. Bruni et al.[35] reported that the concentration of lignocellulose in DM ranged 40-50%. Lignocellulose in animal slurries has different characteristics compared to plants, whose structure is broken down during animal digestion. The concentrations of each lignocellulosic fibrous fraction are shown in Figure 7.

Lignocellulose fraction in VS in animal slurry ranged 30 — 80%. Relatively lower lignocellulose was found in pig and mink slurry, whereas it was higher for cow slurry, which seems to be due to a different animal diet. The concentration of lignin in VS for most animal slurries was larger than 10% except for pig fattening slurry. Within the pig slurry, the lignin was highest for sow slurry. In detail, lignin was 8.6(±6.0)% for piglet, 4.8(±5.5)% for fattening pig, 12.5(±1.2)% for sow and 10.6(±1.1) for mixture of sow and piglet slurry, respectively. In case of cow manure, dairy cow had most abundant lignin at 18.0 (±2.1)%, whereas cattle and calf contains 13.1(±2.1)% and 10.1(±2.2)%, respectively. The concentration of lignin in mink slurry was 10.8%. The concentration of hemicellulose was similar to the lignin concentration, ranging 8.1% to 26.3%. However, the larger amounts were found in the slurry of young animals and in pig slurry, whereas high concentration of lignin was found cow in manure. The highest concentration of lignocellulose in VS with larger amount of cellulose and lignocellulose of calf slurry seems to be due to straw used for the bedding materials. In case of mink slurry, as can be seen in figure 6, the concentration of lignocellulose in VS and distribution of each fibrous fraction is similar to piglet slurry. The results of lignocellulose charaterisation and BMP clearly demonstrate that pattern of inverse relationship between lignin and BMP, which is in accordance to literatures [10,23,29,32].

In case of plant biomass, Triolo et al. [36] reported lignocellulose concentration in VS to be in the range of 49.0 — 82.8%, and lignin concentration in VS was 3.6 — 10.5% for grass and crop residues, whereas the concentration of lignin was larger for woody biomass, that is, 13.9 — 24.0%. In comparison with lignocellulosic characteristics of plant biomass from Triolo et al. [36], the concentrations of lignocellulose seem to be at approximately the same level, except pig fattening slurry. It is interesting that lignin of grass and pig slurries are relatively similar while the concentration of lignin in cow manure seems to be close to woody biomass to some extent. These results seem to be because lignin in straw and grass, which is cow diet, is up-concentrated up to the level of woody biomass, while relatively easily degradable organic pools are degraded. This result highlights that cow manure has critically high concentration of lignin that is the same level with woody biomass which is known as critical digestibility. Likewise, the difference between lignin and lignocellulose concentrations between pig and cow slurry seems to be more dependent on animal diet than management method, except calf manure.