Biorefinery refers to the conversion of biomass feedstock into value-added chemi­cals and fuels with minimal waste and emissions. Some current aspects of biore­finery research and development since the early 1990s are presented, revealing that integrated processes, biomass upgrading technology, and biorefinery technol­ogy have become objects of research and development. Many of the currently used biorenewable-based industry products are the results of direct physical, catalytic, or chemical treatment and processing of biomass feedstocks.

The primary objective refining of pyrolysis products is to obtain valuable fuels like gasoline, diesel and jet fuel, and chemicals from biomass. Figure 7.7 shows the products from biomass by pyrolysis-based refining.

In vacuum pyrolysis, biomass is heated in a vacuum in order to decrease the boiling point and avoid adverse chemical reactions. In flash vacuum thermolysis

(FVT), the residence time of the substrate at the working temperature is limited as much as possible, again in order to minimize secondary reactions.

Vacuum pyrolysis is the thermal degradation of a feedstock in the absence of oxygen and under low pressure to produce a bio-oil and char as main products, to­gether with water and noncondensable gases. Both the bio-oil and char have a high energy content and may be used as fuels. An incredible number of chemical com­pounds are also found in the bio-oil, and these compounds can be extracted and sold as high-value chemicals.

Vacuum pyrolysis is a relatively new variant of pyrolysis with many recycling applications. During vacuum pyrolysis of biomass, the biorenewable feedstock is thermally decomposed under reduced pressure. The quick removal of the vapors reduces the residence times of the macromolecules and hence minimizes secondary decomposition reactions such as cracking, repolymerization, and recondensation, which occur during atmospheric pyrolysis. Temperatures between 675 K and 775 K and pressures of about 0.15 atm are typically used. Table 7.8 compares pyrolysis and gasification processes.

Bio-oil contains the thermally cracked products of the original cellulose, hemi- celluloses, and lignin fractions present in biomass. It also contains a high percentage of water, often as high as 30%. The total oil is often homogeneous after quenching but can easily be separated into two fractions, a water-soluble fraction and a heav­ier pyrolytic lignin fraction. The addition of more water allows the pyrolytic lignin fraction to be isolated, and the majority of it consists of the same phenolic polymer as lignin but with smaller-molecular-weight fragments. Bio-oils are composed of a range of cyclopentanone, methoxyphenol, acetic acid, methanol, acetone, furfural, phenol, formic acid, levoglucosan, guaioco, l and their alkylated phenol derivatives.

Products, wt%

Pyrolytic lignin is a better feedstock for liquid fuel production than the water-soluble fraction because of its lower oxygen content, and therefore the study focused on evaluating it as a potential feedstock for the production of highly aromatic gasoline (Demirbas 2000).

The pyrolysis of biomass is a thermal treatment that results in the production of charcoal, liquid, and gaseous products. Among the liquid products, methanol is one of the most valuable. The liquid fraction of the pyrolysis products consists of two phases: an aqueous phase containing a wide variety of organooxygen compounds of low molecular weight and a nonaqueous phase containing insoluble organics of high molecular weight. This phase is called tar and is the product of greatest interest. The ratios of acetic acid, methanol, and acetone of the aqueous phase are higher than those of the nonaqueous phase.

The bio-oil formed at 725 K contain high concentrations of compounds such as acetic acid, 1-hydroxy-2-butanone, 1-hydroxy-2-propanone, methanol, 2,6-dimeth — oxyphenol, 4-methyl-2,6-dimetoxyphenol, 2-cyclopenten-1-one, etc. A significant characteristic of bio-oils is the high percentage of alkylated compounds, especially methyl derivatives (Demirbas 2007).