**7. Additional prospects for biorefinery applications**

Prospects in other biomass conversion pathways such as thermochemical, mechanical and chemical cannot be completely ignored and in some cases entirely replaced with the biochemical processes proposed (**Figure 1**). Thermochemical processes such gasification which involves the application of heat to biomass at high temperatures (> 700°C) in the presence of low oxygen concentrations can be used to obtain syngas (mixture of methane, hydrogen, carbon dioxide and carbon monoxide) [77]. Syngas can be used as a standalone fuel or a platform chemical for the production of alcohols and organic acids. Alternatively, biomass can be subjected to a pyrolysis process which involves the use of temperatures between 300 and 600°C in the absence of oxygen to convert the biomass to a liquid bio-oil with biochar and light gases as by-products [78]. Such thermochemical processes could be considered as downstream processes after lignocellulosic and algal biomass fermentation where large non-cellulose fractions are generated as side-streams. A variant thermochemical process is hydrothermal treatment or upgradation. It involves the use of high temperature (200–600°C) and pressure (5–40 MPa) liquids often in the form of supercritical water to produce various liquid fuels [33].

Mechanical processes which do not typically change the composition of biomass but tend to reduce sizes or separate impurities or other components are usually applied in most biorefinery processes. It is particularly popular when handling and pre-treating lignocellulosic biomass [79]. However, there are mechanical processes that are considered complete standalone processes which generates their own useful products. A typical example is briquetting. Briquettes are often in the form of relatively evenly sized pellets produced by the compression of carbon-rich biomass. They are known to burn longer and produce a lower net greenhouse gas emissions which promotes their use as good substitutes to coal, charcoal and raw firewood [80]. Such a process could be used as a downstream process after lignocellulosic and algal biomass fermentation to minimise waste generation and add more value to residual materials.
