James G. Speight - Encyclopedia of Renewable Energy

Also, the product is of interest as a possible complement (eventually a substitute) to crude oil to produce specification-grade fuels ( Table B-26).

Table B-26Representation of the conversion of biomass into fuels.

Bio-oil is not a product of thermodynamic equilibrium during pyrolysis but is produced with short reactor times and rapid cooling or quenching from the pyrolysis temperatures. Bio-oils are multi-component mixtures of different size molecules derived from depolymerization and fragmentation of cellulose, hemicellulose, and lignin. Bio-oil is a liquid mixture of oxygenated compounds containing carbonyl, carboxyl, and phenolic functional groups. One of the main drawbacks of the bio-oil is that the composition of the pyrolytic oils is similar to that of the original biomass and is different from crude oil-derived fuels and chemicals (Maher and Bressler, 2007).

Hydrocarbon moieties are predominant in the product, but the presence of varying levels of oxygen (depending upon the character of the feedstock) requires testament (using for example, hydrotreating) during refining. On the other hand, the bio-oil can be used as a feedstock to the Fischer-Tropsch process for the production of lower-boiling products, as is the case when naphtha and gas oil are used as feedstocks for the Fischer-Tropsch process. In summary, the Fischer-Tropsch process produces hydrocarbon products of different molecular weight from a gas mixture of carbon monoxide and hydrogen (synthesis gas) all of which can find use in various energy scenarios.

In the hydrothermal upgrading process (HTU process), biomass is treated with water at high temperature (300 to 350°C, 570 to 660°F) and pressure 1,770 to 2,650 psi) to produce bio-crude. This can be separated by flashing or by extraction to a viscous oil that is suitable for co-combustion in coal power stations and low-density crude oil, which can be upgraded by hydrodeoxygenation (HDO) to biofuels.

The rapid thermal processing process (RTP™ process) occurs at a temperature of approximately 500°C (930°F), when a turbulent stream of hot sand flashes the biomass into a vapor. The vapor is then rapidly condensed into a liquid. This process occurs in less than two seconds, yielding high quantities of bio-oil (typically 65 to 75% w/w yield of pyrolysis oil from dried lignocellulosic biomass).

Bio-oil is a dark brown viscous liquid that bears some resemblance to fossil crude oil. However, bio-oil is a complex oxygenated compound comprised of water, water- soluble compounds, such as acid derivatives, ester derivative, and water-insoluble compounds, usually called pyrolytic lignin because it comes from the lignin fraction of the biomass. The elemental composition of bio-oil is similar to that of the parent biomass. Because of its high oxygen content, the heating value (Btu per gallon) of bio-oil is lower than fossil fuel, typically only approximately half the heating value of fossil crude such as high-boiling fuel oil. However, it contains less nitrogen and only traces of metals or sulfur.

Bio-oil is acidic with a pH in the 2-4 range, making it highly unstable and corrosive – the acidity can be lessened by the addition of readily available base compounds. It, therefore, presents transportation/piping and storage challenges including the tendency to corrode most metals. Hence, it is usually transported and stored in stainless steel containers. Generally, the properties of bio-oil are variable ( Table B-27) and dependent on the feedstock. The specific gravity of the liquid is approximately 1.10 to 1.25, which means it is slightly heavier than water, heavier than fuel oil, and significantly heavier than the bulk density of the original biomass. The viscosity of the oil varies from as low as 25 cP up to 1,000 cP, depending on the water content and the original feedstock.

Table B-27Typical properties and composition of bio-oil *.

Although freshly made bio-oil can be pumped and transported through pipelines, its viscosity increases with time. Unprocessed bio-oil cannot be readily mixed with crude oil-derived fuels.

Despite the above-mentioned shortcomings, bio-oil has great potential. It can be used as heating oil if proper furnaces can be designed to do so; nitrogen oxide emissions are low when combusted. Additionally, it can be potentially upgraded (or refined) to produce liquid transportation fuels and organic chemicals.

Due to large amounts of oxygenated components present in bio-oil, the oil tends to be polar (like water) and, therefore, does not mix readily with hydrocarbon derivatives or with biodiesel. The degradation products from the biomass constituents include organic acids (like formic and acetic acid), giving the oil its low pH, typically between 2 and 4. Water is also an integral part of the single-phase chemical solution (water-soluble fraction).

The water content of bio-oil is typically 15 to 35% v/v, and the oil has the tendency to phase-separate when the water content reaches the 30 to 45% v/v range. The heating value (i.e., the higher heating value, HHV) is below 11,175 Btu/lb compared to 18,052-18,911 Btu/lb for conventional fuel oils. The high heating values of bio-oil (dry-ash free) of switchgrass (cave-in-rock), corn cob, corn stover (no cobs) and alfalfa stems at early bud are 10,164, 11,249, 10,448, and 14,249 Btu/lb, respectively.

See also: Biomass, Bio-oil Upgrading, Fischer-Tropsch Process, Hydrothermal Upgrading Process.

Bio-oil produced by the pyrolysis of lignocellulosic materials is among the most complex and inexpensive raw oils that can be derived from biomass and required upgrading prior to use. Typically, bio-oil consists of five major fractions: (i) water, 15 to 30% w/w, (ii) low-boiling oxygenated compounds, 8 to 26% w/w, (iii) phenols derivatives, 2 to -7% w/w, (iv) water-insoluble oligomers derived from lignin, 15 to 25% w/w, and (v) water- soluble products, 10 to 30% w/w.

Upgrading bio-oil to a conventional transport fuel such as gasoline, diesel, gasoline, methane, or liquefied petroleum gas (LPG) requires water removal, deoxygenation (by hydrocracking and/or hydrotreating), followed by product recovery (typically by distillation), which can be accomplished by integrated conventional refinery processes ( Table B-28).