For the study, the biodiesel (Fatty Acid Methyl Ester) consisted of a mixture of 80% palm-based methyl ester (PME) and 20% biodiesel derived from waste cooking oil—a typical blending ratio of commercial biodiesel in Korea.

HVO comprises alkanes obtained from triglycerides resulting from the hydrotreating of vegetable oils; HVO and iso-HVO have molecular components similar to that of petroleum diesel.

Very broadly, HVO entails the hydrogenation of triglycerides, which results in their breakdown into various intermediates, mainly mono-glycerides, di-glycerides, and carboxylic acids. These intermediates are then converted into alkanes by three different pathways: decarboxylation, decarbonylation (both removing a carbon atom from the initial intermediate), and hydrodeoxygenation (with no carbon removal) at the temperatures above 300–360 °C and pressure of at least 3 MPa.

Differences of emissions and fuel efficiency among 10% blended fuels. Kim et al. Click to enlarge.

Straight HVO shows poor low-temperature performance compared to that of petro-diesel, noted Dr. Soo-Young No of Chungbuk National University in a 2013 paper. (Dr. No is the corresponding author of the current study.) One method of avoiding the low-temperature properties of HVO is the introduction of a catalytic isomerization process. As an example, Neste Oil’s NExBTL process includes an isomerization unit for improving cold properties even down to arctic diesel fuel grades.

With higher levels of n-paraffin than biodiesel, HVO has a higher cetane number than biodiesel, which in turn has a cetane number similar to that of petroleum diesel. HVO also has a higher heating value than biodiesel; however, the heating value per volume is lower than that of conventional diesel, due to the lower density of HVO.

The 16 tested fuels included petro-diesel and 2%, 10%, 20%, 30%, 50% of BD, HVO, and iso-HVO blended diesel. Testing evaluated maximum power, fuel consumption, and emissions, especially PM and NO_x.

The engine was not modified or tuned; duration of each test was 60 minutes, and all tests were conducted 3 times. The testers used the NEDC: 4 cycles of ECE15 urban driving plus 1 cycle of EUDC extra-urban driving.

Among the results were:

• Power. Petro-diesel showed the highest power level; the higher the blend ratio with BD, HVO or iso-HVO, the more power decreased. On the power decrease at maximum torque conditions, BD blended diesel shows the worst power loss, while iso-HVO shows the least.

• Fuel consumption. HVO blended diesel showed lower fuel consumption than BD blended diesel. HVO fuel consumption showed little variance as blending volume increased, while fuel consumption levels of BD-blended diesel increased according with the blend ratio. Iso-HVO showed a slight decrease in fuel consumption as blending ratios rose.

• Emissions. All biofuel blended diesels showed a PM mass reduction compared to petro-diesel. Total particle numbers were the highest at lower loads, decreased at mid-load, and increased again at full loads.

  The amount of NO_x was larger at higher loads than those of other conditions, regardless of the fuel. All three biofuel-blended diesels emit much lower levels of THC than petro-diesel; HVO shows the lowest THC. HVO and iso-HVO blended diesel emit relatively lower levels of CO than petro-diesel.

Iso-HVO has better quality as fuel than BD. Iso-HVO blended diesel has higher cetane value, caloric value, and oxidation stability than BD or HVO. Iso-HVO and HVO are more environmentally friendly than BD and petro-diesel as shown emission test results.

On the engine performance aspects, iso-HVO and HVO blended diesel have better fuel efficiency and lower fuel consumption (compared by BSFC) than BD blended diesel. On the emission, iso-HVO and HVO blended diesel emit less THC and CO than BD, even though they emit similar levels of NO_x and PM to BD blended diesel. … All three kinds of 50% blended biofuels showed that the decrease of particle concentrations at all size range than petro-diesel. The amount of NO_x was larger at the higher loads (100% and 80%) than those of other conditions, with regardless of kinds of biofuels and the differences of blending ratios.

Consequently, iso-HVO is one of the promising alternatives of advanced biodiesels, as the results of not only engine performances but also emission test.

—Kim et al.

Resources

• Duckhan Kim, Seonghwan Kim, Sehun Oh, Soo-Young No (2014) “Engine performance and emission characteristics of hydrotreated vegetable oil in light duty diesel engines,” Fuel, Volume 125, Pages 36-43 doi: 10.1016/j.fuel.2014.01.089

• Soo-Young No (2013) “Application of hydrotreated vegetable oil from triglyceride based biomass to CI engines – A review,” Fuel, Volume 115, Pages 88-96 doi: 10.1016/j.fuel.2013.07.001