Direct conversion of LB to biofuels is liquefaction. Typical products include biodiesel and heavy oils that are typically very viscous. Adding an alkali to the con­version will enhance the liquefaction process (Itoh et al., 1994; Demirbas, 2005). Hydrothermal liquefaction is an application where thermal depolymerization, or hydrous pyrolysis, is accomplished using superheated water under pressure.

Drying feedstock in unnecessary when using hydro­thermal liquefaction. Consequently, it is suitable for con­verting any biomass regardless of its moisture content. Aquatic biomass, garbage, organic sludge as well as LB are all good feedstock candidates for hydrothermal liquefaction.

At the boiling point of water, 100 °C, extraction of aqueous soluble components is possible. At tempera­tures above 150 °C hydrolysis begins and biomass polymers, such as cellulose, hemicellulose, proteins, and so on, degrade into monomers. Then at 200 °C and 1 MPa solidlike biomass changes into a slurry, a process called liquidization. At higher temperatures below the critical point of water, around 300 °C and 10 MPa, liquefaction takes place and oily product is obtained. If one changes the reaction conditions such as reaction time or the catalyst, the main product can be changed to char, a process referred to as hydrother­mal carbonization. Finally, at a temperature around the critical point and in the presence of a catalyst, the biomass will gasify.

CONCLUSION

Utilizing LB for energy and fuel production is as old as mankind. However, modern sources of fuel are more cost-effective and convenient to drive and meet our contemporary energy demands. Liberating carbon that has accumulated over millennia in such an unnatu­rally condensed period of time threatens to alter current climate conditions. Our economic engine has created de­pendencies on fossil fuels and has encouraged unhealthy relationships between highly industrialized societies like the United States and China and some energy suppliers around the world. The cycle of growing and using LB for fuel and energy is a closed and therefore, sustainable sys­tem that consumes as much carbon dioxide as is liber­ated. Finding technologies that can return modernized societies around the globe to more sustainable and self-reliant sources of energy is critical to reduce the envi­ronmental impact and improve national security and sovereignty. To that end, this chapter has presented and reviewed a wide array of biochemical and thermo­chemical LB conversion options. These methods have developed into capable methods of converting LB into fermentable intermediates such as sugars or products. Much development has already taken place and there is still much more needed until utilizing LB for energy and fuel can compete with and replace more convenient and less expensive sources of fuel.

Industrializing the process of converting LB into valuable materials is the function of a biorefinery and it employs a complex and diverse set of conversion tech­nologies to accomplish its tasks. There are several pre­treatment options available that either exploit thermal, mechanical and chemical mechanisms or use biological and chemical mechanisms. Ultimately the treatments chosen will depend on the desired product and desired specifications. These products may include liquid bio­fuels, biochemicals as well as steam, heat and electricity. Regardless, the objectives of the pretreatment are the same: to break down the strong crystalline lignin and cellulosic structures such that the biomass becomes vulnerable to conversion processes, such as hydrolysis treatment that yields fermentable sugars that are free from microbial growth inhibitors, or a thermochemical conversion such as gasification or liquefaction. No treat­ment option is ideal. There are often trade-offs between competing features and liabilities such as high yield and long retention time, low concentration of inhibitors and high cost, significant environmental and safety concerns and high efficacy.

The biochemical and thermochemical pretreatments and conversions have developed and improved over time independently and in conjunction with a series of pretreatment methods. The goal of the development of these pretreatments is to improve the quantity and qual­ity of materials available to conversion, whether it is bioconversion or thermochemical conversion. In order to compete with contemporary and convenient sources of energy it is important to maximize the pretreatment and conversion processes as well as to eliminate and recycle waste and energy.

The biorefinery holds great promise to enable the us­age of biomass as a sustainable and reliable source of en­ergy and fuel. The biochemical and thermochemical conversion options described here are by no means an exhaustive representation of all the studied methods. Many more exist in the literature and maybe the best methods are yet to come.

[1] Strengths, Weaknesses, Opportunities, and Threats

Source: Reproduced with the permission oflEA Bioenergy Task Leader, Dr. Ed de Jong.

[2]These requirements are in addition to the major research projects underway regarding the influence of various biochar feedstocks and conversion technologies on the characteristics of the final biochar product.