2. Classification of thermochemical biomass conversion processes

Recently, the bio-refinery concept has emerged as an important option. A bio-refinery integrates several conversion and resource recovery processes with the aim of maximizing process efficiency, minimizing waste and improving profits [9]. An integrated bio-refinery may use additional feedstocks besides biomass and will produce multiple products including fuels, chemicals and thermal or electrical energy. The bio-refinery concept is still evolving, and has the potential to be an important biomass utilization option in the future that incorporates a wide range of options including biological and thermochemical processes to overcome the

Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals

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Hydrothermal conversion has been studied for more than a 100 years. Friedrich Bergius, who would later receive the 1931 Nobel Prize in Chemistry along with Carl Bosch, developed the Bergius process that produces liquid fuel through hydrogenation of crude oil derived from hydrothermal treatment of coal. The technology was also applied to peat and plant material [11]. Hydrothermal conversion converts biomass into "bio-crude" through thermal depolymerisation under high pressures and moderate temperatures and has since then been studied by several research groups. A comprehensive review of the hydrothermal conversion

Hydrothermal processes can convert all types of biomass, including wet organic biomass, and typically involves the use of a catalyst to improve conversion efficiency. Hydrothermal liquefaction study conducted by Yang et al., shows that the process produces larger amount of oil product compared to other methods [12]. The product liquid fuel is easy to separate, for once it is cooled downed to the room temperature the gaseous product is emitted very quickly [13]. The product bio-crude, can be further processed into high-quality diesel or kerosene. The fast pyrolysis process, discussed later, is a dry conversion process that produces a bio-crude (or bio-oil) from dry biomass feedstock. Hydrothermal conversion process has lower efficiencies

Hydrothermal conversion processes can be further divided into supercritical hydrothermal and subcritical conversion processes. A supercritical hydrothermal conversion process developed by Aalborg University and commercialized by Steeper Energy under the name of "Hydrofraction" converts organic wastes into a raw bio-crude under supercritical conditions in the presence of K2CO3 catalyst [14]. Another process, referred to as the "Catliq" process uses Zirconia catalyst under supercritical conditions to produce a bio-crude with less than 6% oxygen content [15].

Shell research group has demonstrated a subcritical process named Hydro Thermal Upgrading (HTU) that converts the biomass into bio-crude with and without a catalyst [16]. Research has shown that in the presence of a catalyst with adequate activity, conversion could be accomplished at conditions that are less severe than supercritical. A number of catalysts including ones based on Ru, Carbon, and Ni have been proposed with the ultimate goal of developing an

limitations of specific technologies.

3. Wet biomass conversion processes

process of the biomass is provided by Peterson et al. [12].

caused by the significant energy requirement of water evaporation.

optimal hydrothermal conversion process under subcritical conditions [17].

3.1. Hydrothermal conversion process

Biomass is the only renewable resource that can be directly converted in to concentrated energy products using thermochemical conversion [9]. Biomass consists of organic and inorganic matter and often significant amounts of moisture. Organic matter in biomass contributes to its calorific value. Organic matter can be further classified into cellulose, hemicellulose, and lignin. The inorganic matter is conventionally referred to as ash. Compared to conventional fuels, the oxygen content of biomass is typically very high, approximately ranging from 20 to 50% by weight. The moisture content plays a very important role in selecting the appropriate thermochemical conversion process. Heating value of biomass is heavily dependent on the moisture content. The LHV value is negative for biomass with a high moisture content (80%), since the heat released during the combustion process is not sufficient to evaporate all the water [10]. Therefore, biomass is usually dried under sunlight or through other methods, e.g., using recycled thermal energy as part of feed preparation. Solar and air drying in the production field is the preferred method of drying, primarily due to the lower cost. However, there are several conversion routes that use wet biomass feedstocks without the need for drying. Whether a conversion process uses wet or dry biomass as the feedstock is an important criteria for the classification of biomass conversion processes. A technology is categorized as a wet biomass process based on whether the moisture in the feedstock plays an important role in the process, either as a major reactant, or as physical media to maintain the reaction environment. In other words, wet biomass processes do not benefit from drying the feedstock, and often require the feedstock to contain certain quantity of moisture. Table 4 lists the wet and dry biomass feedstock processes, and these processes are discussed in detail in the rest of the chapter.


\* These processes are outside the scope of this chapter and are not covered.

Table 4. Classification of biomass conversion processes.

Recently, the bio-refinery concept has emerged as an important option. A bio-refinery integrates several conversion and resource recovery processes with the aim of maximizing process efficiency, minimizing waste and improving profits [9]. An integrated bio-refinery may use additional feedstocks besides biomass and will produce multiple products including fuels, chemicals and thermal or electrical energy. The bio-refinery concept is still evolving, and has the potential to be an important biomass utilization option in the future that incorporates a wide range of options including biological and thermochemical processes to overcome the limitations of specific technologies.
