**6. Conclusions**

LCB, the most abundant biomass, represents a major carbon source. It is a sustainable and eco-friendly alternative renewable energy source. LCB is a complex heterogeneous mixture of three main biopolymers (cellulose, hemicellulose, and lignin) and

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

a small amount of inorganic compounds and organic extractives. Biomass conversion breaks down the complex structure of biopolymers into smaller molecules. The selection of the biomass conversion process depends on several factors. Thermochemical conversion processes usually offer many advantages, but processes such as pyrolysis, gasification, or combustion, generally require dry biomasses. High moisture content is a major barrier and significantly impacts the biomass conversion process as it requires a large amount of energy to evaporate moisture.

In recent years, more attention has been given to HT processing as a method suitable for processing high moisture content biomass. It converts biomass with high moisture content in the presence of water, eliminating drying, and reducing energy costs. Water has multiple roles in HT conversions as a solvent, reactant, catalyst, and/ or catalyst precursor. There are three HT processes, HTC, HTL, and HTG, based on the temperature, pressure, and residence time. The main products of HTC, HTL, and HTG are solid (HC), liquid (bio-oil/water), and gaseous (non-condensable) fractions, respectively. The conversion mechanisms of HTC differ from pyrolysis in that hydrolysis is the determining first step of HTC. Under HT conditions, the onset of carbonization takes place around 180°C.

Hydrolysis initiates the biomass degradation process in HTC as it has the lowest activation energy. Water catalyze and facilitates hydrolysis reactions. Degradation and depolymerization of hydrolyzed products of hemicellulose and cellulose produce a wide range of fragments, oligomers, and monomers; lignin fragments produce phenolic compounds. Subsequent dehydration and decarboxylation reduce oxygen and hydrogen content, leading to condensation reactions. Smaller molecules produced by dehydration and decarboxylation undergo condensation and polymerization, producing larger molecules; further, polymerization and aromatization produce highly aromatized fragments. Condensation and polymerization of these fragments form HC. The intermediate compounds, 5-HMF and furfural, are promising platform chemicals that can produce various chemical products. The process conditions and the biomass feedstock primarily determine the product distribution and characteristics.

The primary conversion mechanisms of HTL are depolymerization, decomposition, and recombination (repolymerization). Long-chain biopolymers undergo depolymerization and decomposition into short chains. Recombination of these highly reactive short-chain molecules produces liquid, gaseous, and solid products. High temperatures and physicochemical properties of water (acts as a catalyst) facilitate the depolymerization process. The three steps of decomposition are dehydration, decarboxylation, and deamination. Subcritical water at HTL conditions breaks down hydrogen bonds to form monomers. These are rapidly degraded to reactive intermediates by isomerization, dehydration, hydrolysis, reverse aldol defragmentation, rearrangement, and recombination. Recombination and repolymerization of shortchain compounds yield long-chain compounds. Under HTG conditions, biopolymers are converted to simple sugars. The simple sugars are gasified in different routes to produce a mixture of gases (CO, H2, CH4, and CO2), the composition of which depends on the conversion route.

Further, research is required to investigate the reaction mechanisms, reaction kinetics and chemistry, integration of processes to make it more energy efficient, uses of process water, environmental implications of liquid and gas fractions, the effect of HC on soil applications, recovery of nutrients, heat transfer, energy, and heat recovery, etc.

*From Biomass to Biobased Products*
