There is significant growing interest in the production of renewable hydrocarbon fuels, with the most recent announcement being the Department of Energy’s investment of almost $78 million in two consortia targeting the development of such biofuels from algae and from biomass. (Earlier post.)

Non-ester fuels derived from triglycerides—as opposed to fatty acid methyl ester (FAME, or biodiesel) produced via transesterification—can be drop-in replacements for petroleum fuels. Benefits of the non-ester fuels can include higher energy content than alcohols or ester-based fuels; excellent combustion quality, similar to Fischer-Tropsch fuels (low soot and high cetane); good low-temperature properties (viscosity, freeze point, pour point, and cloud point); and superior thermal stability, storage stability, and materials compatibility.

One approach to converting plant oils to hydrocarbon fuels that has gained visibility and project traction over the past months has been the UOP/Eni process of hydrotreatment to achieve deoxygenation and decarboxylation (e.g., earlier post, earlier post).

However, the ARA authors note:

...the UOP/Eni Ecofining process is intrinsically associated with a monotonic spectrum of paraffinic products and high consumption of hydrogen. The former results in jet fuels of inferior quality, and the latter increases product costs. In addition, hydrogen used in the conventional petroleum hydrotreating process is derived from non-renewable sources, such as natural gas steam reforming.

To overcome these shortcomings, a novel technical approach based on high-temperature water chemistry, known as catalytic hydrothermolysis (CH), has been proposed. The presence of water serves as a reactant, catalyst, and solvent for typically acid- or base-catalyzed reactions. Hydrogen is supplied in part by water for hydrocarbon cracking, as well as hydrolysis of triglycerides followed by decarboxylation. High-temperature and high-pressure water reduces the formation of gaseous products and minimizes the formation of chars. Water also functions as an effective heat-transfer and catalytic reaction medium.

—Li et al.

The study used triglycerides from crop oils—all containing more than 30% polyunsaturated fatty acids—that can vary significantly in their fatty acid compositions: corn oil, cottonseed oil, linseed oil, peanut oil, safflower oil, soybean oil, sunflower oil, camelina oil, hemp oil, tung oil, and jatropha oil. Soybean oil was used as the baseline feedstock for this study.

The process involves three main production steps:

1. The CH process that converts triglyceride to biocrude. The CH reaction is the key conversion step in the triglyceride to biofuel process.
2. Decarboxylation and hydrotreatment of the biocrude resulting from step 1.
3. Fractionation of the resulting non-ester biofuel into JP-8, naval distillate, and gasoline cuts.

Reactions under hydrothermal conditions may involve cracking, hydrolysis, decarboxylation, isomerization, and cyclization that convert triglycerides and/or modified triglycerides to a mixture of straight chain, branched, and cyclic hydrocarbons. High-temperature water catalyzes and attenuates these reactions with minimal formation of gaseous products or char. High-temperature water has been widely used in green chemistry and biofuel processes primarily dealing with hydrolysis, liquefaction, and gasification of biomass.

In comparison to conventional catalytic cracking and pyrolysis typically operated at 400-650 °C, the temperature for the CH process is significantly lower (240-470 °C). Lower operating temperatures and the presence of water minimizes the degradation of the oil into less valuable gaseous byproducts and coke, respectively.

—Li et al.

In this study, the CH conversion was carried out at temperatures ranging from 450 to 475 °C and a pressure of 210 bar in the presence of water with and without a catalyst. The organic phase (biocrude) from the CH process underwent post-treatment involving decarboxylation and hydrotreating. Among the findings were:

• High-grade non-ester biofuels were produced by refining the CH biocrude, with a total yield conservatively ranging from 40 to 52% for JP-8, naval distillate, and gasoline fractions.

• The temperature, water/oil ratio, heating rate of oil, pressure, and catalyst were key control parameters for the CH process to produce desired molecular structures and distributions of hydrocarbons and their isomers.

• The biofuels produced from soybean, tung, and jatropha oils indicated that the CH process is applicable for converting a wide range of triglycerides containing fatty acids from highly polyunsaturated to highly saturated types.

• The identified biofuel components appear to support the proposed reaction pathways, leading to the formation of cyclic and aromatic compounds in the CH process.

• Tung oil is a unique feedstock for producing biofuels with greater than 60% aromatic content. Therefore, tung-oil-derived CH biofuel will have added value as a blend stock for existing FT jet fuels and emerging biofuels such as those produced from the UOP/ENI Ecofining process to meet fuel specifications by increasing density and aromatic content. (Earlier post.)

Resources

• Lixiong Li, Edward Coppola, Jeffrey Rine, Jonathan L. Miller and Devin Walker (2010) Catalytic Hydrothermal Conversion of Triglycerides to Non-ester Biofuels. Energy Fuels, Article ASAP doi: 10.1021/ef901163a