drop-in fuels. These fuels closely resemble fossil-based fuels and offer better engine compatibility than traditional biodiesel. To avoid first-generation biofeedstocks, producers are increasingly turning to waste materials including animal fats, used cooking oils, and greases. However, potential supply limitations have resulted in extending the use of wastes to include MSW and sewage sludges. An additional benefit of using these waste products is that they minimise landfill costs and offer financial benefits. However, maintaining consistent quality and supply can be challenging. New regulations are emerging that require a minimum use of such third- and fourth- generation feedstocks, driving further innovation in this area. Production processes and corrosion challenges The production of SAF and renewable diesel involves significant modifications to existing refinery processes along with investment in advanced chemical processes that differ significantly from traditional petroleum refining. Most waste feedstocks require a pretreatment step, followed by hydrodeoxygenation (HDO) to remove oxygen and other contaminants, isomerisation or hydrocracking to achieve the desired fuel properties.
However, these new processes and feedstocks introduce novel challenges, particularly in terms of corrosion risk. The high acidity of some renewable feedstocks, combined with high temperatures and pressures in the refining process, can accelerate corrosion rates in equipment (see Figure 2 ). The main corrosion risks in biofuel production include: • Acidic corrosion: Renewable feedstocks often contain a combination of fatty acids, long carbon chains with single or multiple double bonds, and branched acidic components like resin acids. These feedstocks typically have a much higher total acid number (TAN) compared to fossil fuel feedstocks, ranging from 0 to 200 mg KOH/g feed. This high acidity leads to localised thinning of metal surfaces, which intensifies with increased process temperatures and flow rates. While temperatures above 230°C pose a risk for fossil feeds, this threshold drops to 150°C for renewables due to their elevated acidity. The corrosion products, soluble iron salts, lack protective scales and can accumulate in catalyst beds, causing pressure drops and clogging. This type of corrosion can lead to significant equipment damage, potentially causing leaks or even catastrophic failure if left unchecked. • Microbiologically influenced corrosion (MIC): MIC is caused by the metabolic byproducts of living organisms such as bacteria, algae, or
Recycle H to feed
Amine corrosion Ammonium bisul ph ide, ammonium chloride corrosion, errosion and errosion corrosion Ammonium chloride corrosion
Air cooled exchanger
E
E F
Water injection
C
Lean amine
G
LPS bottoms
H plant + recycle
Products
Amine absorb.
B
Light ends
KO pot
Fractionator
High press. separator
Feed
G
Stabiliser
Feed heater
Rich amine
A
O -gas recycle & are
D
HDO reactor
ISOM reactor
Sour water
Reboiler
Low press. separator
Acidic corrosion (fatty acids) HTHA, H /H S corrosion
A B C
Reactor feed
F
Ammonium bisul phi de, ammonium chloride, HCL corrosion, errosion and errosion corrosion Ammonium bisul phi de, errosion
Sour water
D
Figure 2 A basic overview of a biofuel production process, with the main corrosion mechanisms and areas of concern highlighted
www.decarbonisationtechnology.com
33
Powered by FlippingBook