Hydrocarbon Processing - November 2021 - 11

P. O'BRIEN, Opportune LLP, Houston, Texas
Business Trends
Battle of the biofuels: Renewable diesel vs. biodiesel
Fuel standards programs, such as the U.S. Renewable Fuels
Standard (RFS-2) and California's Low-Carbon Fuels Standard
(LCFS), have incentivized fuel producers to innovate and
create new renewable fuels like biodiesel and renewable diesel.
As new low-carbon diesel fuels and blends become more readily
available, it is important to understand the differences between
them and how each impacts engine performance.
Diesel engine basics. Before jumping into the different fuel
options available, the following provides the basics of a diesel
engine and how it differs from gasoline engines. In the U.S.,
diesel engines are commonly used by trains, boats, barges,
heavy-duty trucks, public buses, and farming, construction
and military equipment.
Diesel engines are compression ignition, meaning they
use compression to create enough heat and pressure that the
injected fuel spontaneously combusts. In most modern diesel
engines, this is accomplished by using a turbocharger to
compress the air going into the cylinder, directly injecting
fuel just before the piston reaches its peak (top-dead-center)
and relying on compression to trigger combustion.
Meanwhile, gasoline engines are spark ignition that use
spark plugs to ignite a compressed mixture of air and fuel in
the cylinder. The order and timing of when each spark plug
receives a pulse of electricity is primarily dependent on the rotation
of the crankshaft, which connects the pistons of all the
engine's cylinders, and the camshaft, which controls the intake
and exhaust valves.1
Diesel engines have the highest thermal efficiency of any
internal combustion engine. Typically converting around 40%
of the energy stored in the fuel into useful work, diesel engines
nearly double the thermal efficiency of gasoline engines, which
averages around 20%. This is largely due to the higher compression
ratios of diesel engines (ranging from 14:1-25:1) vs.
gasoline engines (ranging from 8:1-12:1).1
Fuels, blends and energy. Although new low-carbon fuels
and blends are continuously being created, two primary groups
of renewable fuels in the diesel pool have emerged: biodiesel
and renewable diesel.
Biodiesel-sometimes referred to as fatty acid methyl ester
(FAME)-is typically produced by reacting vegetable oils or
animal fats with methanol or ethanol in a transesterification
process to create mono-alkyl esters of long-chain fatty acids.
Biodiesel contains oxygen atoms, which makes it chemically
distinct from regular ultra-low sulfur diesel (ULSD). Like
ethanol blending limits in gasoline engines, most diesel engine
manufacturers only support diesel blends up to B20, with
higher blends requiring fuel system modifications.
Conversely, renewable diesel uses the same feedstocks as
biodiesel; however, it undergoes hydrotreating, thermal conversion
or biomass-to-liquid production processes. The result is a
fungible renewable fuel chemically identical to ULSD that can
be used as a drop-in replacement for all current diesel engines.
The differences between ULSD, biodiesel and renewable diesel
are best illustrated in TABLE 1, but there are a few properties
worth discussing. Unfortunately, neither renewable diesel nor
biodiesel can match the energy density of ULSD. This value is
calculated by multiplying the energy density per unit of mass
and the volumetric density and unit conversions. However, the
energy density is not the only important property related to the
energy and volumetric densities. Biodiesel's higher volumetric
density is also responsible for its higher cloud point, poorer cold
flow properties and improved lubricity vs. ULSD. Conversely, renewable
diesel's improved cloud point and cold flow properties
can both be attributed to its lower volumetric density.
The different chemical properties of the three diesel fuel
types significantly impact the resulting greenhouse gas (GHG),
nitrous oxide (NOx
) and particulate matter emissions.3
For example,
FIG. 1 compares proprietary blends of biodiesel and renewable
diesel offered by Renewable Energy Group (REG), a
leading producer of biodiesel and renewable diesel.
Biodiesel decreases nearly every emissions category as the
percentage blended in ULSD increases; however, recent studies
have proven it increases NOx
emissions compared to ULSD.3
Although both REG Ultra Clean RD B50 and RD B20 may still
reduce NOx
diesel's ability to reduce NOx
Based on this information, California's Air Resource Board
(CARB) announced a change within its Alternative Diesel FuTABLE
1. Diesel fuel properties2
Properties
Cetane No.
Energy density, Mj/kg
Density, g/ml
Energy content,
BTU/gal
Sulfur, ppm
NOx
emissions
Cloud point, °C
Oxidative stability
Cold flow properties
Lubricity
Petrodiesel
40-55
43
083-0.85
129 K
< 10
Baseline
-5
Baseline
Baseline
Baseline
Biodiesel Renewable diesel
50-65
38
75-90
44
0.88
118 K
< 5
+10
20
Poor
Poor
Excellent
0.78
123 K
< 10
-10-0
-10
Excellent
Excellent
Similar
Hydrocarbon Processing | NOVEMBER 2021 11
emissions vs. ULSD, this is a result of renewable
emissions.
Hydrocarbon Processing - November 2021

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