**4. Biomass conversion**

Biomass conversion is the key step to produce heat, biofuels or chemicals from LCB. Biomass can be converted to these products through chemical, biochemical and thermochemical conversion processes. Selection of the conversion process depends on number of factors, including the desired form of end products (heat, biofuels or chemicals), biomass feedstock (type, quantity and characteristics) available, environmental standards, policy, economic conditions and specific factors related to the project. In most situations, the selection is based on two main factors, i.e., desired form of end products and the available feedstocks [5, 21, 52]. The moisture content of biomass primarily determines the conversion process for the selected biomass. Thermochemical conversions like pyrolysis, gasification or combustion are ideal for dry biomasses such as wood or straw. These processes are often unsuitable for wet biomass because of low energy density due to high moisture content. The high moisture content significantly increases the transportation cost and energy cost for drying. Wet conversion processes such as hydrothermal processing and biochemical processing (fermentation and anaerobic digestion) have gained growing attention and are more suitable to process high moisture content biomass like aquatic biomasses, sewage sludge, food waste and manures. Despite consuming less energy, biochemical conversion processes require more time compared to thermochemical conversion processes. Subsequently, much attention has been placed on hydrothermal processing, which is more cost-effective than conventional thermal drying followed by thermochemical conversion. The additional parameters (like cost and feasibility of drying etc.) need to be considered when selecting the correct conversion process, primarily if the moisture content lies between wet and dry regions [6, 21, 53, 54]. Biomass has substantial quantities of free and bound water. Wet biomass requires drying before pyrolysis, gasification or combustion; hence, additional energy and facilities are needed. Drying biomass outside the pyrolysis reactor is beneficial (produce pyrolysis vapor with high calorific value and bio-oil with low water content) [23].

The important biochemical conversion processes include anaerobic/aerobic digestion, fermentation and enzymatic or acid hydrolysis. In biochemical conversion, biomass molecules are broken down into smaller molecules by bacteria or enzymes. This process is much slower than the thermochemical conversion process but does not require much external energy. In anaerobic digestion, bacteria take oxygen from the biomass itself instead of atmospheric oxygen. The products of anaerobic digestion are biogas (a mixture of methane, carbon dioxide) and solid digestate. Only 5–10% of the feed into the digester is digested by the anaerobic bacteria. The digestate consists of remaining indigestible material. Aerobic digestion, commonly known as composting, takes place in the presence of oxygen. It uses different types of microorganisms that access oxygen from the air, producing carbon dioxide, heat and solid residue (compost). In fermentation, starch is converted into sugars using acids or enzymes. Then sugar is converted into ethanol or other chemicals with the help of yeast. The fermentation of lignocellulosic feedstock requires additional pretreatment (hydrolysis) to breakdown the cellulose and hemicellulose into simple sugars. Hydrolysis can be achieved by the use of acids, enzymes or hydrothermally [5, 21, 52].

The lignin is not converted and is left for thermochemical conversion. Major thermochemical conversion processes include combustion, gasification, pyrolysis, torrefaction and hydrothermal processing. These are further discussed in the following sections. Thermochemical conversions generally offer many advantages over biochemical conversions, such as handling a wide variety of feedstocks, better conversion efficiency, high energy efficiency and shorter reaction times. As a result, in recent years, thermochemical conversions have received greater attention for biofuels production [3, 21].
