- How much paraffins can be blended in diesel fuel?
- Blending HVO/XTL paraffins with FAME
- Compatibility with materials
- Storage and handling
Paraffinic HVO/XTL components that meet proposed EN 15940 standard are "premium grade" diesel components, which can be added by tens of percents into diesel fuel. This is possible without compromising fuel quality, exhaust emissions and engine operation. HVO/XTL paraffins as blending components in conventional diesel decrease aromatic content of diesel fuel, and when HVO/XTL meets EN 15940 Class A requirements, cetane number of diesel fuel increases leading to reduced exhaust emissions and better engine performance (see Emissions Chapter).
Diesel fuel standards such as EN 590 and ASTM D 975 are met even with high blending ratios without the technical problem of “blending wall”. Therefore these components are called "drop in fuels". In practice, some blending limitations exist. For example, about 30% HVO can be blended with diesel fuel with density higher than 0.845 kg/dm3 to meet lower density limit of EN 590 standard. In the U.S such restrictions for HVO/XTL paraffins do not exist, because ASTM D975 standard does not limit density. When high concentration ratio of paraffinic components is used, lubricity additive is needed in the final blend to protect fuel injection equipment against excess wear. Lubricity additive is needed also for conventional winter grade or sulfur-free diesel fuel.
Paraffinic components "upgrade" fossil diesel used in blending. For instance, cetane number and aromatic content limits can be met even with modest diesel blending stock. In addition, detrimental effects on the final boiling area do not occur. This offers economic benefits for refineries.
HVO/XTL paraffins can be used as such in fleet operations like city buses, taxis or mine vehicles in order to improve local air quality. Reduced engine-out emissions help exhaust after-treatment devices to perform better. Full benefits are achieved by recalibration of fuel injection system due to the lower density and higher cetane number of paraffinic fuel. (Mikkonen et al. 2012).
When blending diesel fuel, paraffins (HVO/XTL) and FAME, the following instructions should be followed: diesel fuel and paraffins are mixed first, and then FAME is added into blend. Paraffins and diesel fuel are chemically close to each other. A risk for precipitation of impurities increases when FAME is blended with any low aromatic fuel. Less than 7% FAME may be mixed with HVO, otherwise used the risk of precipitation increases. (Mikkonen et al. 2012).
Paraffins (HVO/XTL) in diesel fuel do not risk engine durability or require any additional measures in logistics. Paraffinic components have generally the same compatibility regarding parts and materials as petroleum diesel, for example, with seals, hoses, diaphragms, dry couplers and base swivel joints. Materials of construction may include carbon and stainless steel which are suitable for petroleum diesel fuel. The use of both welded and riveted tanks is acceptable. Tanks may have internal floating roofs made of aluminum. Nitrogen blanketing can be used.
HVO/XTL paraffins are compatible with nitriles, fluoroelastomer, PTFE, vinyl ester resins and epoxy resins. In principle, the lack of aromatic compounds may shrink elastomers that have already been swollen when used with aromatic containing fuels, but no fuel leakages have been observed during several large field tests with HVO. Mechanical seals of pumps can be considered to be compatible with HVO, if they are compatible with diesel fuel. (Mikkonen et al. 2012).
However, similarly to conventional diesel fuel, a performance additive package shall be considered at least for corrosion protection to prevent problems with possible water and precipitates in fuel logistics or vehicle fuel systems. (Mikkonen et al. 2012).
Paraffinic fuels, such as HVO and XTL, are fully compatible with the current logistic systems and practices. Their tendency to pick up and solve water is even lower than that of traditional diesel fuels. During long-term storage paraffinic fuels behave like traditional diesel fuels (no need for a “use before” date). Paraffinic fuels do not contain any risky impurities. No blending ratio causes risk of precipitations or phase separation. However, some precipitation of paraffins originating either from the fossil part or HVO/XTL may take place if temperature is below cloud point for a long period.
Paraffinic fuels as such or as a blending component in diesel fuels does not require any additional precautions compared to fully fossil diesel fuels as concerns microbial growth. However, good housekeeping is needed as microbes may grow all fuels in the presence of water and favorable conditions over an extended time period. Tests at low temperatures (< +10 °C) with paraffinic fuel showed no microbiological growth (Neste 2016).
The flash point of HVO/XTL that meets standard is above +55 °C meaning that it can be stored and transported like standard diesel fuel. When neat HVO/XTL (100%) is shipped with the shipping name "Alkanes (C10-C26), linear and branched" under MARPOL Annex II, Category Y and ShipType 3, cargo shall be carried on chemical tankers with prewash requirements. HVO (NExBTL Renewable Diesel) is included into the IMO Biofuel category, and therefore a blend containing HVO and minimum 75% of diesel fuel or gasoil is shipped as a MARPOL Annex I cargo. If a blend contains less than 75% of diesel or gasoil then a cargo is carried under MARPOL Annex II requirements, Category X and ShipType 2. This means that tank wash waters need to be unloaded to a shore facility. (Mikkonen et al. 2012).
Antistatic additive can be used in HVO to allow high pumping velocities, which otherwise would be limited due to the low electrical conductivity of HVO (comparable to that of sulfur-free diesel fuels).