The elemental and chemical structure of biorefinery raw materials differs from that on which the current fossil refinery and chemical industry is based. Chemical and elemental composition of petroleum is compared with some lignocellulosic biomass feedstocks in

FIGURE 2 World average composition of the above ground standing biomass.

feedstock is that, unlike biomass, it is very low in oxygen content. The most important chemical products currently derived from oil and natural gas refinery are shown in Figure 3.

This figure shows that today’s chemical industry processes fossil resources into a limited number of bulk chemicals from which a wide spectrum of secondary commodity chemicals are produced. These commodity chemicals have many applications in almost all the sectors of our society as textiles, plastics, resins, food and feed additives, and others. The bulk chemicals from which the majority of commodity chemicals can be produced are ethylene, propylene, batanes/butadiene, and the aromatic benzene, toluene, and xylene (BTX).

The composition of biomass is less homogeneous than petroleum. The share of biomass components in the feedstock can change and the elemental composition is a mixture of C, H, and O (plus other minor components such as N, S, and other mineral compounds). If com­pared to petroleum, biomass generally has less hydrogen, more oxygen, and a lower fraction of carbon. The compositional variety in biomass feedstocks is both an advantage and a
drawback. An advantage is that biorefineries can make more classes of products than can petroleum refineries and can rely on a wider range of raw materials. A drawback is that a relatively larger range of processing technologies is needed, and most of these technologies are still at a precommercial stage (Dale and Kim, 2006). Another difference with petroleum resources concerns the seasonal changes which biomass suppliers have to face, since harvesting is usually not possible throughout the year. A switch from crude oil to biomass may require a change in the capacity of chemical industries, with a requirement to generate the materials and chemicals in a seasonal time frame. Alternatively, biomass may have to be stabilized prior to long-term storage in order to ensure continuous, year-round operation of the biorefinery (Clark et al., 2009).

More difficult is to adapt chemical processes to act on nonhomogeneous substrates, since the chemical industry has been built largely on the use of uniform and consistent raw materials (Hatti-Kaul, 2010). It is unlikely that this will change, so technologies will need to be developed to precondition biomass feedstocks to make their properties and reactivity patterns more stable, consistent, and uniform. One concept that may be of value is to separate the different biomass components early in biorefinery operations, so to make a distinction between those which are subject to energy uses (whose quality can be degraded) and those destined to chemical applications (which need high degree of purity and should be subject to milder process conditions to conserve the original structure).