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Saturday, September 22, 2018

BUNKER SUPPLEMENT No. 11- November 2000

IGNITION QUALITY - CALCULATED CARBON AROMATICITY INDEX (CCAI)

With the blending practices being undertaken for the preparation of bunker fuels to meet the current ISO 8217 specification, Ignition Quality of a fuel could become a significant issue especially for vessels operating in confined waters and seaways. 

Although Ignition Quality is not a formal requirement in ISO 8217 : 1996 (E), nevertheless, it is included as Annex B (Informative) in this internationally recognised marine fuel specification.  It is clearly stated in the Annex that ignition quality is "primarily determined by engine type and, more significantly, by engine operating conditions”.  As such CCAI needs to be carefully assessed.

In many locations, density is the overriding factor in blending of marine fuels which meet the ISO 8217 Specification.  This is due to a continuing trend to “deeper” refining and to the ready availability of the so-called “cat cracker bottoms”.  These fluid catalytic cracking slurry oils, widely used as blend stocks for residual fuels, are very aromatic in nature – meaning that they are very deficient in hydrogen content.  Although they have excellent solubility characteristics, they also have some inherent drawbacks – they contain the very abrasive “cat fines” and have very poor ignition quality.  Thus, when used as cutter stocks to blend high viscosity residua, the resultant products often have high densities (close to the allowed maxima in ISO – 8217) and relatively low viscosities.  This combination – high density and low viscosity – is the least desirable from an ignition quality viewpoint.  This problem is also compounded by the fact that no minimum viscosity is specified in ISO – 8217.

CCAI is an indication of the Ignition Quality of the fuel and can be calculated using the formula outlined in Annex B:

where

          D = Density at 15oC (kg/m3);  V = Viscosity (mm2/sec); T = temp (oC) at which viscosity is determined. 

CAI can also be determined, using the nomogram in Annex B; by extending a line connecting the viscosity and density values of the fuels under observation to the CCAI column.  The number thus obtained is the CCAI value.

CCAI can be considered as something akin to the “cetane” value of the fuel but with one very important exception.  In the case of cetane number/index, the higher the value the better the ignition quality of the product.  However, the higher the CCAI value, the worse the ignition quality of the fuel.  Thus, for residual products, the lower the CCAI value, the better the ignition quality.  The nomogram below shows how the CCAI values of four fuels of different viscosity from ISO 8217 : 1996 (E) are derived.  The fuels, with their respective viscosities and densities, follow:

 

GRADE

VISCOSITY (cSt/100oC)

DENSITY (kg/m3)

CCAI

RMA – 10

10

975

862

RMD – 15

15

985

862

RME – 25

25

991

857

RMG – 35

25

991

851

As can be seen, the highest viscosity product (35 cSt/100oc) affords the lowest CCAI, indicative of the best ignition quality, and several points better than the corresponding RM 25 grade.  Invariably, lower viscosity products afford higher CCAI values and that is the case for RMA - 10 and RMD – 15 grades shown above. 

Engine type is particularly critical with respect to ignition quality.  Since CCAI is the only readily available tool to predict ignition quality it can be stated that in general, medium speed engines are more susceptible to high CCAI (i.e., lower ignition quality) fuels whereas low speed cross-head engines appear to be able to better handle higher CCAI level fuels.  As indicated in ISO 8217 : 1996 (E), Annex B, “further guidance on acceptable ignition quality values should be obtained from the (specific) engine manufacturer.”

CCAI does, in many instances, correlate with Ignition Quality experience in the field.  However, across-the-board correlation is certainly not achieved.  Much work remains to be done.  The constant volume bomb ignition analyser appears promising.  DNVPS considers such instruments as good research tools and, to this end, they have equipped all of their labs with the FIA – 100.  DNVPS are in the process of carrying out research on this very important topic.