I. INTRODUCTION

The conversion of solar radiation into chemical energy via photosynthesis results in the growth of woody, herbaceous, and aquatic biomass and the formation of many organic compounds in situ, each of which has an intrinsic energy content. The lower the oxygenated state of the fixed carbon in these compounds, the higher the energy content. As discussed in previous chapters, a-cellulose, or cellulose as it is more commonly known, is usually the chief structural element and principal constituent of many biomass species, particu­larly woody biomass, but is not always the dominant carbohydrate, especially in aquatic species. The lignins and hemicelluloses comprise most of the remaining organic components. In addition, other polymers and a large variety of nonpoly­meric organic solids are formed naturally, although not equally, in biomass. Many of these chemicals are or have been used in specialty applications such as pharmaceuticals and industrial formulations. Natural products continue to be discovered, and many have been found to have useful applications. Hundreds of biomass species have also been found to produce low-molecular-weight organic liquids, several of which are used or proposed for use as transportation

fuels for vehicles driven by spark — or compression-ignition engines. These liquids are glycerides and terpenes. The glycerides, which are the primary members of a group of organic compounds called lipids, are mainly triglyceride esters of long-chain fatty acids and the triol glycerol. Lipid is a general name for plant and animal products that are structurally esters of higher fatty acids, but certain other oil-soluble, water-insoluble substances are also called lipids. The fatty acids are any of a variety of monobasic acids such as palmitic, stearic, and oleic acids. Among more than 50 fatty acids found in nature, almost all are straight-chain acids containing an even number of carbon atoms. A few biomass species produce esters of fatty alcohols and acids. Certain glycerides are essential components of the human diet and are obtained or derived from animal fats and vegetable oils. In addition to cooking and food uses, many natural glycerides have long been used as lubricants and as raw materials for the manufacture of soaps, detergents, cosmetics, and chemicals. Some are directly useful as motor fuels as formed or can be converted to suitable fuels after relatively simple upgrading using established processes. The derivatives formed on transesterification (alcoholysis) of several natural glycerides with low-molecular-weight alcohols are useful as neat diesel fuels or in diesel fuel blends or as diesel fuel additives. An example is the ester formed on methanol — ysis of soybean oil.

The term terpenes originally designated a mixture of isomeric hydrocarbons of molecular formula Cl0Hi6 occurring in turpentine obtained from coniferous trees, especially pine trees. Today, the term refers to a large number of naturally occurring hydrocarbons that can be represented as isoprene adducts having the formula (C5H8)n, where n is 2 or more, and to an even larger number of derived terpenoids in various states of oxidation and unsaturation. Terpenes are widely distributed in many biomass species and are often found in biomass oils, resins, and balsams. They are classified according to the number of iso­prene units contained in the empirical formula: for example, Ci0Hi6, monoter — penes; C15H24, sesquiterpenes; C20H32, diterpenes; C30H48, triterpenes; and (C5H8)X, polyisoprenes. The terpenes thus range from relatively simple hydro­carbons to large polymeric molecules. The lower molecular weight terpenes are usually liquid at room temperature at n = 2 or 3 and are mainly alicyclic structures. Terpenes are monocyclic, bicyclic, tricyclic, etc.; open-chain acyclic terpenes are also known. Examples of terpenes are the dienes limonene (mono­cyclic monoterpene) and cadinene (bicyclic sesquiterpene), the monoenes a — and /З-pinene (bicyclic monoterpenes), the triene myrcene (acyclic monoter­pene), the hexaene squalene (acyclic triterpene), and natural rubbers, which are high-molecular-weight polymers of isoprene.

In this chapter, the sources of natural biochemical liquids potentially suit­able as motor fuels, their basic properties and conversion chemistry, and their process economics are examined.