James G. Speight - Encyclopedia of Renewable Energy

The expression tar sand is commonly used in the crude oil industry to describe sandstone reservoirs that are impregnated with a heavy, viscous black crude oil that cannot be retrieved through a well by conventional production techniques (FE-76-4, above). However, the term tar sand is actually a misnomer; more correctly, the name tar is usually applied to the heavy product remaining after the destructive distillation of coal or other organic matter. Thus, alternative names, such as bituminous sand or oil sand , are gradually finding usage, with the former name (bituminous sands) more technically correct. The term oil sand is also used in the same way as the term tar sand , and these terms are often used interchangeably.

On an international note, the bitumen in tar sand deposits represents a potentially large supply of energy. However, many of the reserves are available only with some difficulty and that optional refinery scenarios will be necessary for conversion of these materials to liquid products because of the substantial differences in character between conventional crude oil and tar sand bitumen.

Because of the diversity of available information and the continuing attempts to delineate the various world tar sand deposits, it is virtually impossible to present accurate numbers that reflect the extent of the reserves in terms of the barrel unit. Indeed, investigations into the extent of many of the world’s deposits are continuing at such a rate that the numbers vary from one year to the next.

The term bitumen , as used in oil shale technology is the lower molecular weight soluble, organic component of oil shale. The amount of bitumen is low, usually on the order of 0.5 to 5% w/w of the total weight of the oil shale. Thus, oil shale organic matter can be characterized as two materials, kerogen and bitumen, the latter being benzene soluble. Bitumen is also the product produced by the high molecular weight material thermal decomposition of kerogen.

The yield of bitumen (extractable material) increases with (i) increasing extraction temperature, (ii) with increasing polarity of the extraction solvent, and (iii) with the chemical reactivity of the solvent. Moreover, bitumen is generally richer in hydrogen (H/C may be on the order of approximately 1.6 with a molecular weight of approximately 1,200) and nitrogen, while it is lower in the proportion of aromatic constituents (and consequently richer in aliphatic constituents) than the corresponding kerogen.

Although the thermal decomposition kinetics is complex, the thermal decomposition of the bitumen (either extracted or produced from the thermal decomposition of kerogen) is often simply described (like the thermal decomposition of kerogen) by a kinetic mechanism involving a first order mechanism which involves the production of thermal bitumen:

Black liquor is the solution of lignin-residue and the pulping chemicals from the Kraft process which is used to extract lignin during the manufacture of paper. The black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process and contains more than half of the energy content of the wood fed into the digester. Black liquor ( Table B-19) is typically concentrated to 65 to 80% by multi-effect evaporators and burned in a recovery boiler to produce energy and recover the cooking chemicals.

The gasification of black liquor has the potential to achieve higher overall energy efficiency while generating an energy-rich synthesis gas. The synthesis gas can be burned in a gas turbine combined cycle system or converted through Fischer-Tropsch chemistry into chemicals or fuels such as methanol, dimethyl ether (DME), gasoline, or diesel.

See also: Dimethyl Ether, Fischer-Tropsch, Methanol.

Blended fuel usually refers to a mixture composed of automotive gasoline and another liquid, other than a minimal amount of a product such as carburetor detergent or oxidation inhibitor, which can be used as a fuel in a motor vehicle.

Typically, in the current sense of the term, a blended fuel is a mixture of traditional fuels (such as gasoline or diesel fuel) and alternative fuels (such as ethanol or biodiesel, respectively) in varying percentages. The lowest-percentage blends are being marketed and introduced to work with current technologies while paving the way for future integration. For example, B5 and B20 (diesel fuel blended with 5% or a 20%, receptively of a biofuel) can be pumped directly into the tank of any diesel-fueled vehicle. Ethanol is also blended (approximately 10% v/v) into much of the gasoline dispensed in many countries, especially in metropolitan areas, to reduce emissions.

The use of blended fuels is part of the transition to using more alternative fuels. Although the pure alcohol (methanol or ethanol) will burn independently, cold weather starting can be a problem. An engine has to be designed exclusively for a particular fuel to take advantage of all the characteristics of that fuel. Without the infrastructure in place to support pure alcohol fuels, flex-fuel vehicles (FFVs) have been designed to run on both alcohol and gasoline. The flex-fuel vehicles join the best characteristics of both methanol and gasoline (or ethanol and gasoline) and make it possible to utilize higher blend percentages such as E85 (ethanol) and M85 (methanol). Examples of blended fuels are presented below.

E10

E10 is a low-level blend composed of 10% ethanol and 90% gasoline that has been approved by the United States Environmental Protection Agency (EPA) for use in any conventional, gasoline-powered vehicle. The use of E10 was spurred by the Clean Air Act Amendment of 1990 (and subsequent laws), which mandated the sale of oxygenated fuels in areas with unhealthy levels of carbon monoxide. Currently, the E10 fuel is sold in every state.

E15

E15 is a low-level blend composed of 10.5 to 15% v/v ethanol and gasoline and is approved for use in newer light-duty conventional vehicles. Stations must adhere to several EPA requirements and regulations when selling E15.

E85

E85 (or flex fuel) is an ethanol-gasoline blend containing 51 to 83% v/v ethanol, depending on geography and season that qualifies as an alternative that can be used in flexible fuel vehicles (FFVs), which have an internal combustion engine and are designed to run on E85, gasoline, or any blend of gasoline and ethanol up to 83% v/v.

See also: Alcohol Blended Fuels, E85.

Blending is a process which, in the current context, mixes the constituents of the fuel in such a manner as to ensure that there is a homogeneous mixture of all of the ingredients. The process can be carried out numerous times within a fuel manufacturing process when new excipients need to be added to the blend. Fuels as produced at the end of the process are actually blends of different streams since no one unit can, for example, produce specification-grade gasoline or diesel fuel. Thus, blending is the final operation in fuel production.

Blending consists of mixing the products in various proportions to meet specifications such as vapor pressure, specific gravity, sulfur content, viscosity, octane number, cetane index, initial boiling point, and pour point. Blending can be carried out in-line or in batch blending tanks. Air emissions from blending are fugitive volatile organic compounds (VOCs) from blending tanks, valves, pumps, and mixing operations. During the blending process, not only the physical and chemical properties of each blending component have to be considered in order to produce the specification-grade fuel and ensure that instability or incompatibility do not occur.