Biomass-Based Diesel Fuels

Hannu Jääskeläinen

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• Introduction
• Feedstocks
• Commercial Status
• Future Potential

Introduction

The use of biologically derived fuels in diesel engines is as old as the diesel engine itself. Rudolph Diesel reportedly considered vegetable oils as a potential fuel for use in his invention. While unprocessed vegetable oils are attractive for some diesel engine applications [1388][1389], their use generally requires fuel system and/or engine modifications to deal with the high viscosity, poor low temperature flow characteristics, and relatively poor stability at high temperatures.

Diesel fuels that contain pure or partially processed vegetable oils may lead to a variety of engine problems such as long-term engine deposits, piston ring sticking, fuel injector plugging, or lube oil gelling which can cause engine failure or require more frequent maintenance and shorter engine overhaul intervals. Exhaust from engines fueled with raw vegetable oils can also have more adverse health effects, as suggested by their higher mutagenic activity compared to exhaust from the same engine using diesel fuel [1390].

Pure and partially esterified vegetable oils, both in their neat form and in blends with petrodiesel, were the starting point for biofuel research conducted during the 1970s and 1980s. Pre-heating of the virgin oil was sometimes employed to lower its viscosity [1208]. A chemical modification of high molecular mass triglycerides—where some of the long fatty acid chains are replaced with a short acetyl chain—has also been suggested to lower the viscosity of virgin oils [2273]. Eventually, the alkyl ester chemistry was selected due to its superior properties—significantly higher cetane and much lower viscosity. In parallel, diesel-like liquids were being developed though the hydrogenation of vegetable oils and other plant and animal based fatty feedstocks. These two directions of research produced two classes of commercial, biomass-based diesel (BBD) fuels:

• Biodiesel—Historically the first type of mass-produced BBD fuel, biodiesel is a fatty acid alkyl ester and most commonly a fatty acid methyl ester, FAME. While biodiesel is compatible with compression ignition, it is an oxygenated fuel of properties that are significantly different from petroleum diesel, which can be a source of compatibility issues with existing diesel engines. Higher level biodiesel blends require that the elastomers in the engine fuel system be compatible with biodiesel. Higher level blends may also contribute to low temperature operability issues and are more challenging for long term storage and handling than petroleum diesel.
• Renewable diesel—A hydrocarbon based renewable fuel, most commonly manufactured via the hydrogenation of vegetable oils (hydrogenated vegetable oil, HVO). Renewable diesel is chemically similar—but not identical—to petroleum diesel, can meet several existing petrodiesel specifications, and is often considered a drop-in alternative diesel fuel. The hydrogenated diesel technology was pioneered by Finland’s Neste Corporation, which operates large renewable diesel refineries in Singapore and Rotterdam. The Neste process has been known as NEXBTL™ technology.

The above nomenclature has evolved over a few decades and is entirely customary. As both types of fuels are produced from the same biomass-based feedstocks, both can be described as bio-diesel as well as renewable diesel.

Due to its better compatibility with existing diesel engines and fuel infrastructure, renewable diesel has been preferred by many engine manufacturers and users over biodiesel. In some markets, including the United States, renewable diesel has been gaining market share—in spite of its higher manufacturing cost compared to biodiesel. In the general case, however, both alternative fuels depend on government policy and subsidies, which can be largely different in different parts of the world.

Feedstocks

Many vegetable oils and animal fats have been suggested, investigated, and—some of them—commercialized as feedstocks for the production of alkyl ester biodiesel and hydrocarbon-based renewable diesel. The common sources of oil include soybean, rape/canola, palm, sunflower, coconut, camelina, used frying oil, as well as a number on non-edible oils including those from jatropha or kanuga seeds.

The most common source of biodiesel in the USA is soybeans, while in Europe the main source of biodiesel is rapeseed. Other significant biodiesel and renewable diesel resources—estimated to be more abundant than soy—are greases and animal fats [1209]. These feedstocks are also potentially more sustainable than agricultural crops.

Oil from jatropha (Jatropha curcas) nuts was considered as a potential biomass feedstock in tropical climates in the early years of the 21 century because jatropha plants tolerate poor, degraded soils and are resistant to pest and disease. Plantations for biodiesel production were started in India and in some African countries [1159]. However, under marginal growing conditions, jatropha plants produce few of the nuts from which the oil is extracted and good yields typically require growing conditions that make it compete with food crops. Additionally, unlike other crops, the residuals from jatropha plants have no commercial value which makes revenue from its cultivation entirely dependent on marketing the nuts. Since interest in jatropha has waned, some hybrid varieties have been developed that make it a more appealing crop [3580][3581].

At one time, oils naturally produced by certain species of algae were considered another promising BBD fuel feedstock [495]. Later studies have shown that commercial cultivation and processing of algae would require costly infrastructure and vast amounts of energy—so much so that algal biofuel production might consume more energy than it produces [5729].

Palm (Elaeis guineensis) oil is one of the most cost effective feedstocks for the production of biodiesel and renewable diesel. Palm oil has been increasingly produced in Southeast Asia (Malaysia, Indonesia, Thailand) and in South America. From the commercial point of view, palm oil is particularly attractive because its yield per unit area of land is much higher than that of soy, rapeseed or other crops. Demand for palm oil is therefore likely to continue all over the world, in spite of the negative association of palm oil plantations with deforestation and environmental degradation of large areas of tropical forest.

Waste Oil Feedstocks. While waste oils such as used cooking oil (UCO) and tallow are considered to have some of the lowest carbon intensity of available biofuels, supply is limited. This has led to strong demand for these feedstocks and created incentives to fraudulently misrepresent some high carbon intensity feedstocks as UCO.

In 2024, it was estimated that China already exported more than half its UCO for biofuel production to Europe and the USA. Europe consumed about 130,000 barrels per day (b/d) of UCO, about eight times more than it collects, and the USA 40,000 b/d. Imports also come from Indonesia and Malaysia. As airlines increase their use of renewable diesel—a.k.a. sustainable aviation fuel, SAF—demand is expected to quickly outstrip what can be sustainably collected. Global SAF targets in 2030 would require at least twice the UCO that can be collected in the USA, Europe and China combined.

There is evidence to strongly suggest that fraudulent feedstock misrepresentation as UCO is significant. While collection capacity and export levels appear to match up in China, the existence of a large illegal gutter oil market means that the country is likely consuming significant volumes of UCO domestically. This suggests that the country uses and exports more than it collects, raising strong suspicions over virgin vegetable oil being mislabeled as waste oils. Also, Malaysia exports three times more used cooking oil than is collected in the country. With Malaysia being one of the world’s largest palm oil producers, it would strongly suggest that palm oil could be fraudulently represented as UCO.

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