SEVERE HYDROTREATING OF DIESEL CAN CAUSE FUEL-INJECTOR FAILURE
Martin Booth
Shell International Petroleum Co.
London
Peter E. Wolveridge
Shell Research Ltd.
Chester, U.K.
The U.S. and Europe are increasingly tightening diesel-fuel quality specifications, particularly with respect to sulfur content.
This trend is directing the oil industry toward progressively more severe types of hydrocatalytic manufacturing.
Although usually beneficial in emissions terms, the overall consequences of such processing routes are not necessarily universally beneficial. In fact, there is strong evidence that automotive diesel fuel manufactured by severe hydrotreating may increase the risk of premature mechanical failure of certain classes of automotive fuel-injection pumps.
Recent events in Sweden - where strict, environmentally driven fuel specifications have been introduced - have confirmed the existence of this risk. In response to these events, the Royal Dutch/Shell Group of companies has attempted to quantify and solve the problem of reduced diesel fuel lubricity in that market.
These circumstances in Sweden, however, have potential implications for other markets. In the U.S., for instance, comparable pressures are driving manufacturers toward highly processed fuels that may lead to similar problems.
LUBRICITY
Lubricity can be defined as a liquid's intrinsic ability to prevent wear on contacting solid surfaces in the absence of any hydrodynamic lubricating films. This property, more correctly known as boundary lubricity, is believed to be associated with polar species in the liquid, which adsorb onto the mating surfaces to form a protective low-friction layer.
Lubricity is an important property for transportation fuels because many critical fuel-system components rely on inherent fuel lubricity for their safe, long-term operation. Any fuel processing step, chemical or physical, that removes these natural boundary lubricants also increases the risk of promoting equipment failure by reducing the fuel's lubricity.
DIESEL FUEL QUALITY
The adequacy of the natural lubricity of traditionally manufactured automotive diesel fuels has never been called into question. The exception to this statement is, of course, very-low-viscosity grades used in arctic environments. In these situations, experience has resulted in the use of some pre-cautionary, measures, such as the addition of engine oil to the fuel.
When initial concerns about lubricity surfaced, they were in the context of a different fuel - jet kerosine. It was some 20 years ago when the increasing proportion of hydroprocessed kerosine, then finding its way into the aviation fuel pool, was implicated in catastrophic wear and premature failure in certain highly rated, jet-engine, fuel-system components such as pumps and governors.
Although, in general, automotive fuel-injection equipment (FIE) systems are less "overengineered" than their aviation fuel counterparts, parallels can be drawn between the two system types.
The class of diesel injection pump often used in lighter-duty vehicles (passenger cars, light commercial vehicles) relies on a high degree of natural fuel lubricity for trouble-free operation. Examples of these types of FIE are the so-called rotary or distributor pump and the more highly loaded types of fuel injectors (unit and electronic injectors).
Thus, both of these types of injectors might be expected to be vulnerable to diesel manufacturing technology developments that remove trace fuel species.
QUALITY ADVANCES
Environmental concerns in the developed world have focused recently, on exhaust emissions-especially particulates and NO'.
Although, for particulate emissions, these concerns are seen largely, as a matter for equipment manufacturers, fuel composition also is considered a potential contributor to particulates reduction.
The U.S. Environmental Protection Agency (EPA) has called for reductions in diesel fuel sulfur beginning in October 1993 (to a 0.05 wt %, maximum). The California Air Resources Board (CARB) has proposed similar limits, to begin in 1993, including restrictions on composition and aromatics content and tighter controls on boiling range (Table 1). Europe-wide specifications likewise have been tightened.
In January 1991, Sweden enacted legislation that resulted in especially stringent regulations (Table 1). The Swedish government called for three grades of "cleaner" diesel fuel-Classes 1, 2, and 3.
Even the least restrictive of these - Class 3 - bears comparison with pending European requirements enacted by Comite Europeen de Normalisation, or CEN. Classes 1 and 2 fuels, on the other hand, are more stringent than the 1993 EPA and CARB specifications.
To produce fuels with a sulfur content of, say, less than 50 ppm and an aromatics content of 5 vol % or less, it is necessary to apply more severe hydroprocessing conditions or invoke unconventional hydroprocessing technology.
To achieve these stringent specifications, the application of conventional, single-stage hydroprocessing would require hydrogen partial pressures as high as 100 bar and large catalyst volumes with space velocities as low as 0.5 tons/cu m-hr.
Even with these high-severity conditions, however, aromatics saturation to levels as low as 5 vol % cannot be achieved on the more refractive feedstocks. To reach such low concentrations of fuel sulfur and aromatics reliably, other approaches are necessary.
LUBRICITY IMPLICATIONS
Clearly, processing steps of such severity will remove polar species from the fuel. It might be expected, therefore, that Swedish Classes 1 and 2 fuels will exhibit much less intrinsic lubricity than more conventionally manufactured materials.
In composing the specifications, Swedish legislators concentrated exclusively on the resultant environmental benefits and, in particular, on the reduction in health hazards thought to be associated with diesel particulates. No attention was given to the possibility of FIE durability problems
The Swedish government offers attractive tax incentives to promote the manufacture and sale of these environmentally benign fuels (for Class 1, about $100/metric ton and for Class 2, about $50/metric ton). In view of these incentives, significant market penetration was expected for these products from the outset.
Shell accordingly concentrated its efforts on addressing whether such fuel grades were fit for the purpose for which they were intended. In seeking to establish those areas in which the performance of these fuels might be questionable, Shell determined that, based on earlier aviation-fuel experiences, study was needed in the area of lubricity.
Class 1 fuel (originally 10 ppm sulfur) was almost certain to cause problems. On the other hand, Class 2 (initially a 200 ppm but modified to a 50 ppm), was thought to be possibly problematic, with the most vulnerable area being the rotary/distributor pump.
To ensure the most rapid resolution of these concerns, a research strategy was adopted. This strategy, included controlled field trials, durability, tests (performed jointly, with leading FIE manufacturers), and fundamental lubricity studies using a laboratory "rig."
FIELD TRIALS
Preliminary, field trials were carried out in a mixed fleet of passenger cars, trucks, and buses, using Classes 1 and 2 fuels, free of any additional components that could potentially enhance lubricity.
No wear problems were reported for heavy-duty vehicles. This is not surprising because these vehicles typically use in-line injector pumps, which are separately lubricated with engine oil.
The vehicles fitted with rotary/distributor pumps, however, all experienced failures. Of the nine passenger cars tested, those running on Class 1 fuel experienced either catastrophic mechanical failure or unacceptable loss of performance at 2,000-8,000 miles. Those burning Class 2 fuel experienced these problems at 3,000-19,000 miles.
These tests revealed new pumps to be more susceptible than those previously run on conventional fuel. Irrespective of history, however, all pumps failed with unacceptably low mileage. (Service lives for such pumps are typically much greater than 60,000 miles or 100,000 km.)
These finds were subsequently corroborated by further field experience.
TEST-BENCH RESULTS
The results from the field trials were supported by test-bench pump durability evaluations carried out in conjunction with two leading European fuel pump manufacturers. Although subjected to somewhat different test methodologies, both grades of fuel produced unacceptable pump lives (Table 2).
Incidentally, subsequent testing by a leading U.S. manufacturer has confirmed these findings.
In seeking to explain the rig and field durability data in a systematic manner and, most importantly, to identify a potential solution to the problem, it was necessary to be able to evaluate automotive diesel fuel lubricity in the laboratory.
There are, unfortunately, no generally accepted specification tests for diesel fuel lubricity. The ASTM D5001 ball-on-cylinder lubricity evaluator (Bocle) method - developed for jet fuel lubricity determination-although studied by several researchers, suffers from several shortcomings.
(An International Standards Organization task force [ISO TC22/SC7/WG6] is seeking to identify a standard lubricity test, with the support of the Society of Automotive Engineers, Coordinating Research Council, and the Coordinating European Council [CEN].)
In particular, when used in its standard form, the Bocle does not reproduce the adhesive-wear mechanisms characteristic of most fuel pump failures.1 The test also can be overresponsive to particular fuel chemistries (i.e., naturally occurring compounds or additives) to the extent that its performance predictions are either strongly optimistic or pessimistic.
In this last context, certain classes of additive known to improve lubricity or reduce wear in the field are rated as being significantly "pro-wear" by the Bocle test.
Lastly, the test lacks any body of data capable of correlating its predictions with actual vehicle FIE field performance.
Faced with this dilemma, Shell was able to draw on its resources and experience, accumulated during some 20 years of direct involvement in the area of jet fuel lubricity. And, as mentioned previously, this bears strong parallels with automotive diesel fuel lubricity.
In particular, Shell's experience included work with a test rig designed specifically to evaluate adhesive wear. This rig - called the Thornton aviation fuel lubricity evaluator (Tafle) - was developed jointly with the British Ministry of Defence as a tool for determining the intrinsic lubricity of kerosines and evaluating the effectiveness of lubricity-improving additives.
The Tafle had been successful in both these roles, but its broader application to diesel fuels had not been established . 2 Measurements made on the prototype Classes I and 2 fuels, however, confirmed that the evaluator can indeed discriminate among fuels in a manner that agrees with field-trial data (Fig. 1).
LUBRICITY IMPROVEMENT
In seeking to remedy deficiencies in the intrinsic lubricity of highly processed diesel fuels, various options are open to the fuel supplier, including:
- Tailoring process conditions to minimize the removal of desirable components
- Backblending with a stream rich in natural boundary lubricants
- Using additive technology.
The scope of this article includes only the additive route, insofar as the severe requirements of the Swedish specifications exclude the other possibilities.
Although, in principle, many possibilities exist for diesel fuel lubricity-improving additives, in practice, the choice is significantly narrower.
Most of the wear-reducing additives used in crankcase and industrial lubricant formulations are too aggressive for the fuel environment. For instance, the additives may be too corrosive to FIE metallurgy or cause water separation problems.
And even many of the milder of these additives contain elements such as sulfur that are undesirable in an environmentally clean fuel, or elements such as phosphorous that may be detrimental to exhaust after-treatment devices. Among the potentially suitable materials, however, are those fatty-acid corrosion inhibitors approved as lubricity additives for jet fuels.
In selecting an effective and environmentally benign additive solution for Sweden, Shell adapted a strategy that involved only those additives capable o( giving Classes 1 and 2 fuels a lubricity at least as good as that exhibited by typical current European automotive diesel fuel (about 0.2 wt 17, sulfur and 25% aromatics). To this end, the Tafle provided the essential methodology for preliminarily selecting promising additives.
Fig. 2 indicates the measured ability of various fuels to resist adhesive wear-expressed in terms of failure load-in the Tafle.
It can be seen that the Swedish fuels exhibit an intrinsic wear-resistance that is markedly inferior to that of a standard current-quality European diesel fuel. In fact, the Class 1 fuel has an even lower lubricity than does the highly refined hydrocarbon White Spirit 98, used as a low reference for aviation fuel lubricity studies.
(White Spirit 98 is a very stable mixture of paraffins, cycloparaffins, and aromatic hydrocarbons [aromatics
When additives were chosen using these techniques and criteria (see Additive X in Fig. 2), the resultant pump-durability test data and field trial results exhibited performance levels that would be expected from a standard automotive diesel fuel (Table 2, Figs. 1 and 2).
By way of confirmation of the Tafle's ability to predict lubricity, Fig. 1 includes field data for Additive Y which is shown in Figs. 1 and 2 to have only intermediate lubricity-improving performance. This also holds true for fuels exhibiting "good" and "bad" performance.
This last point illustrates the critical need for a laboratory lubricity test that predicts unequivocally the field performance of a fuel, and particularly a fuel containing an additive.
Based on the strength of these and similar data, Shell identified additives enabling the fullest advantage to be taken of the environmental and commercial benefits promised by Classes 1 and 2 fuels, with no risk of attendant FIE durability problems.
In fact, a dedicated 1,500 metric ton/day plant, commissioned in August 1992, is now supplying the Swedish fuels market with wear-resistant fuel additives.
GLOBAL PICTURE
So far, this article has addressed the identification of fuel lubricity problems and their solutions, specific to the introduction of Swedish clean fuels. But the legislation behind the experience in Sweden is but one example, although an extreme one, of this worldwide trend toward low-emissions diesel fuels.
It is therefore instructive to look at the broader picture.
Initially, in the U.S. and Europe and increasingly throughout the developed and developing world, highly processed (deeply catalytically hydrotreated) diesel fuels will become more common, in response to environmental concerns.
This scenario, however, is by no means inevitable. Based on recent experiences in Sweden, the justification on purely environmental grounds of such extremely low concentrations of fuel sulfur and aromaticity, and thus the extent of manufacturing severity demanded, legimately can be questioned. The cost-effectiveness of such harsh solutions is also questionable.
Work in progress within the Shell Group, and by others, is beginning to indicate that sulfur contents significantly less than 0.05 wt % will give little additional benefit in meeting emissions limits for exhaust particulates. In addition, total aromaticity levels apparently play no direct role in determining the total quantity of particulates generated.
Similarly, a number of straightforward economic factors affect the scenario. Cost[benefit analysis shows that, as fuel quality approaches such extremes, the incremental manufacturing costs rise disproportionately, with inevitable market consequences.
Thus, a strong technical possibility and commercial rationale exist for the introduction of environmentally acceptable diesel fuels manufactured by processing routes that are not so severe as to remove all inherent polar species responsible for intrinsic lubricity.
It is to be hoped, therefore, that growing mechanistic insights of this type, and the parallel activities of ISO and others to develop quantitative methods for evaluating lubricity, will lead to the adoption of future specifications that yield optimum emissions benefits while ensuring adequate FIE performance.
A concern remains, however, where severely treated fuels are marketed for other reasons. Here, the extent of the problems that may confront fuel suppliers and customers need to be identified and quantified.
These problems are, today, a matter of immediate relevance to those applications using vulnerable models of rotary/distributor pumps. As for the future, informed industry debate indicates that the new generation of electronically controlled fuel injectors used in low-emissions engines may be even more susceptible to early failure.
In the longer term, FIE manufacturers have the scope to redesign their equipment to make it less lubricity sensitive-much as their aviation colleagues have done. In the interim, the use of additives and backblending provide options that should offer relief when problems occur.
REFERENCES
- Hadley, J.W., and Blackhurst, P., "An appraisal of the ball-on-cylinder technique for measuring aviation turbine fuel lubricity," STLE Annual Meeting, MaN 1990, ]Denver.
- Hadley, J.W., "A method for the evaluation of the boundary lubricating properties of aviation turbine fuels, "Wear, Vol. 101, No. 3, 1985, pp. 219-53.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.