Biodiesel Cold Soak Filterability - ASTM D6751 Annex A1

New ASTM Biodiesel Test Specification for Cold Weather Operability

The American Society for Testing of Materials has recently added a new test requirement to the D6751 Biodiesel Specification.

This new requirement is referred to as ASTM 6217 or as Annex A1 of ASTM D6751– Cold Soak Filterability.

Cold Soak Filtration Analysis is defined as: The time in seconds that it takes for cold soaked biodiesel to pass through two 0.8 micron filters and the amount of particulate matter expressed in milligrams per liter (mg/l) collected on the filter.

What does this mean? When biodiesel is stored in temperatures below 40°F for extended periods of time, certain components will precipitate (fall) out of solution and fall to the bottom of the storage tank. This precipitate will build in a thickening layer at or near the tank bottom. In general the colder the temperature and the longer the biodiesel stays at a given temperature, the more material will fall out.

This material can very quickly plug filters and shut down engines, usually at the worst time.

What is this material? It can have to do with the feedstock from which the biodiesel is created. Certain feedstocks, particularly Used Cooking Oils (UCO), Waste Vegetable Oil (WVO), and Animal Fats (Tallow) will produce high levels of precipitate. The material can also be due to incomplete removal of glycerin during the transestrification process.

This new test is a positive step in making biodiesel a more consistent user friendly product.

Diesel Doctor

Copyright 2009© - William Richards

Biodiesel and Cold Weather meet in Minnesota

Biodiesel and Cold Weather meet in Minnesota

The Minnesota Department of Agriculture recently released (February 15, 2009) a report to the legislature regarding the states mandated biodiesel program and the cold weather issues it has caused.

Starting in September 2005 Minnesota required that virtually all diesel fuel sold in the state contain at least 2% biodiesel. This level is supposed to be raised to 5% in 2009, 10% in 2012, and finally 20% in 2015.

This report by the Governors “Biodiesel Task Force” created in 2003 shows that there a number of significant problems with using biodiesel blends, particularly in areas subject to long periods of cold weather.

Some of the issues noted in the report:

· Discussion indicated at least fifteen cases of unusual filter plugging in commercial trucks this winter for which the cause had not been determined. Discussion of possible causes included engine manufacturers’ reduction of truck filter sizes (from 10 to15 microns down to 2 to 5 microns), paraffin from diesel, glycerin from biodiesel, water contamination, biotic contamination as a result of ultra-low sulfur diesel levels plus water contamination, and vehicle designs in which the fuel filter is located away from the engine.

· Discussion that existing cold flow test procedures are not sufficiently predictive of the
cold temperature performance of diesel fuel with or without biodiesel. ASTM International and other organizations at a national level must develop new test methods that are more predictive of the cold weather performance of diesel fuel and biodiesel blends.

· Variations in seasonal availability of fuel were also discussed, specifically routine shortages of diesel fuel at terminals in the fall leading to the practice of bulk plants and fleets buying and storing fuel in the late summer for use in the fall and early winter months when shortages of diesel are anticipated. Such stockpiling of B10 or higher could result in problems in above ground tanks.

· The suggestion was made that the Task Force discussions should freely address the availability and quality of all winter fuel in the state instead of being confined only to biodiesel. Given recent changes in the diesel industry, including biodiesel, low-sulfur diesel fuel and smaller fuel filter pore diameters, a wide range of issues regarding equipment and diesel fuel with and without biodiesel must be addressed.

Laboratory testing has indicated that blending with number one (#1) diesel at 50/50 rate produces results similar to treating with additives but the #1 fuel is at a far higher cost.

There is also a concern over whether the current testing (e.g. Cold Point (CP), Cold Filter Plug Point (CFPP), Pour Point (PP), Low Temperature Flow Test (LTFT)) done to assess cold weather operability characteristics of diesel fuel are adequate for testing biodiesel blends.

Unfortunately this discussion has been complicated by many factors including the change from Low Sulfur (LSD) to Ultra-Low Sulfur diesel (ULSD), that diesel fuel changes seasonally, the fact that biodiesel derived from different base oils have far different characteristics, that transportation and storage affect quality, and that materials will tend to precipitate out of biodiesel when it cold for extended periods of time.

To read the complete Minnesota Report to the Legislature please click here: Report to the Legislature February 2009

Please post your comments, questions, ideas, and thoughts.

Diesel Doctor

Copyright 2009© - William Richards

Ultra-Low Sulfur Diesel Cold Weather Information

Ultra-Low Sulfur Diesel Fuel

Cold Weather Information

The Ultra-Low Sulfur Diesel (ULSD (S-15)) that we started to receive in mid 2006 has shown some dramatically different cold weather characteristics from the earlier High Sulfur (HSD (S-5000)) and Low Sulfur Fuels (LSD (S-500)).

These new characteristics including higher temperature gelling, wax dropout, icing, and difficulty in treating have in the first year and will continue into the foreseeable future to provide some significant challenges to distributors and end users during cold weather.

Due to these new characteristics users in areas of the US where they have not seen cold weather problems in the past, are now and will continue to see serious issues with gelling, wax dropout, and icing.

Here are the main issues known today:

Wax in diesel fuels – Paraffin wax is a natural and important part of diesel fuel. This wax provides several beneficial characteristics including high energy content (Btu’s), lubricity, stability, and viscosity. The negative characteristics mainly revolve around cold weather operation and include gelling and something new we refer to as wax dropout.

In HSD and LSD the wax characteristics were relatively well understood and consistent. For example the “Rule of Thumb” used for adding kerosene (#1 diesel, Jet A) to #2 fuel to lower Cold Filter Plug Point (CFPP) was that for every 10% kerosene added to #2 diesel you would lower CFPP by approximately 5°F. An example would be that a 50% blend would have improved CFPP by about 25°F.

However the new ULSD has had several important characteristics changed by the new refining processes. The catalytic cracking and hydrodesulfurization processes remove some of the wax, it alters the size and shape of the wax seed crystals in the fuel, lowers the aromatic content of the fuel, removes a significant amount of the Lubricity, and lowers the fuels ability to dissipate static electricity by as much as 100 times.

The result of this is that the ULSD fuel actually will gel at a higher temperature than the old LSD and HSD. This problem is made more difficult because we can no longer use regular kerosene (#1 diesel, Jet A) for cold weather blending. These fuels are considered High Sulfur and their use would cause the end fuel to have sulfur content higher than the allowable 15 ppm. So refiners have had to create an ULSD #1 specifically for winter blending purposes.

There are a number of problems with this new fuel. First, it is currently very expensive, ranging anywhere from $.30 to $1.00 more than regular kerosene, second it is not available in all areas, and third this new ULSD #1 is not as effective at lowering the Cloud Point (CP) and CFPP (gel point) of the fuel. For example; ULSD #2 when blended with 10% ULSD #1 will lower the CFPP by only 2°F or maybe 3°F. This means that a 50% blend would only improve CFPP by 10°F.

To make this problem even more difficult, many of the diesel fuel anti-gel additive products that have been on the market for last 5 to 30 years have little or no effect on ULSD. The change in fuel chemistry brought about by changes in the Catalytic Cracking processes and the addition of Hydrodesulfurization have rendered many of the most popular products nearly useless in ULSD.

There is a new cold weather problem that the industry has not adequately defined
as of today. We are calling this issue “Wax Dropout”. Wax Dropout occurs when diesel fuel is “cold saturated”. This where the fuel reaches a given temperature and stays at or below that temperature for a given period of time. This time period is usually between 48 and 72 hours or longer and the temperature can vary with different batches of fuel. This past winter we saw this problem at between 5°F and 10°F.

When the fuel gets to the Wax Dropout temperature, say for example 8°F and stays there for 48 to 72 hours, the wax will suddenly agglomerate and fall to the bottom of the container. This wax plugs filters and fuel lines until it is removed or until the fuel temperature is raised to a point where the fuel will reabsorb the wax.

Again there is a further complication, in that the “old” HSD and LSD wax would gradually start to reabsorb as the fuel temperature rose. With ULSD when wax dropout has occurred the wax does not begin to reabsorb until the fuel reaches fairly high temperatures, often above 40°F, 50°F or even higher. This can make the process of getting an engine with gelled fuel to run properly far more challenging than we have ever seen before.

In the fuel distribution and fleet operations businesses, we have relied on CFPP as
a measure of winter fuel quality for many years. CFPP is a fairly complicated test involving using a vacuum to draw a sample of fuel through a 45 um (micron) screen within a given period of time.

When the HSD and LSD were most prevalent and most fuel filters were 10 um there was a good correlation between CFPP and the temperature at which a standard fuel filter would plug. For example you could be relatively certain that a fuel testing for CFPP of -25°F would provide trouble free operation to -15°F to -20°F.

However the relationship is much different with ULSD. A ULSD fuel testing
-25°F CFPP might have filter plugging problems at between -5°F and -10°F. Also CFPP does not seem to be directly related to Wax Dropout. A fuel can test for
-15°F and still have Wax Dropout at 8°F.

Furthermore, OEM engine manufacturers have changed the media size of their fuel filters. Where 10 um has been almost an industry standard, we now see 7 um, 5 um, and even 2 um filters today. This throws the whole relationship between CFPP and winter operability out the window. For example fuel that is at the CP can have filter plugging problems with a 2 um fuel filter.

The industry has not yet agreed on or developed testing methods to measure cold weather operability with the new fuels and filters.

Until such time as the industry develops a test method for determining the relationship between CFPP, PP, Wax Dropout, and filter media size for ULSD, we suggest the following: For 10 um filters; Take the midpoint between PP and CFPP, for 7 um filters, take the midpoint between PP and CFPP, then take the midpoint between that number and the original CP, for 5 um and 2 um use the CP.

Water is more of a problem than ever before. Diesel and biodiesel fuels hold
water dissolved in them. The amount of water that ULSD is able to hold is greater than that of HSD or LSD. One of the characteristics of fuel is that its ability to hold water in solution diminishes as the temperature decreases. Fuel delivered at 70°F with 200 ppm of dissolved water will as the temperature drops begin to push that water out of the fuel into droplets. These droplets can be seen floating in the fuel and as temperatures reach and go below 32°F those droplets freeze becoming ice crystals.

As a result many of the cold weather problems where people believe they have fuel gelling problem are actually a fuel icing problem. If you have operability issues in temperatures above 0°F you should check to be sure that you aren’t dealing with ice.

Customers are regularly reporting situations where they have no water in storage tanks, no water in vehicle or equipment tanks, but they constantly have water in filters and separators. This is due to the dissolved water falling out of solution due to temperature changes.

Diesel Fuel Mileage Decrease in Winter

Why Diesel Fuel Economy Drops in the Winter

Diesel fuel, particularly in the northern tier states changes rather significantly from season to season. In the cold weather months generally starting in September or October refiners begin to alter the chemical composition of diesel fuels to improve cold weather operability characteristics to meet ASTM, Pipeline Operator, and Customer requirements and specifications.

Refiners talk about the components that come out of the refining process as “streams”. In a typical refinery today there can be over 180 “streams” coming from the refining of crude oil. The addition of lighter product streams are known by names such as “aromatic chemicals”, “naptha’s”,” kerosene’s” and others to #2 diesel (whether Ultra-Low Sulfur Diesel (S-15) or Low Sulfur Diesel (S-500)) will lower (improve) the Cloud Point (CP), Cold Filter Plug Point (CFPP) commonly referred to as the gel point, and Pour Point (PP) depending on how much of those components are added to the base fuel. Refiners have a lot of latitude in determining how much of and what components are used to make these improvements.

The issue from a fleet operators standpoint is that these changes lower energy (Btu) content of the fuel. It is normal for fuel economy to decrease from one to as much as five percent seasonally. This decrease can be further exacerbated by fuel racks and or distributors further cutting with kerosene to try and improve cold weather operability. The normal energy content of #2 ULSD ranges between 138,000 and 140,000 Btu’s, kerosene is much lower ranging between 130,000 and 135,000 Btu’s, whereas gasoline is about 124,000 Btu’s per gallon.

As you can see the more lighter components added to fuel, the lower the energy content. Note: ULSD has 1%-3% lower Btu content than the LSD. This is primarily due to reduction in wax content in ULSD.

So if you put all of this together in a time line, you can see that you begin using additive to improve cold weather performance at the same time the refiners are blending the fuel in a way that reduces Btu content which lowers your fuel economy, then in the spring you stop using additive at the same time the refiners are going to a “summer” blend which increases the Btu content and so your mileage goes up.

Other cold weather considerations are more idle time, slower transit speeds, more time in traffic, and even driving through snow all, of which can have a significant negative impact on fuel usage.

Cold Filter Plug Point versus Cloud Point

CFPP (Cold Filter Plug Point) vs. CP (Cloud Point)
Cold Weather Operability in Diesel Fuels including ULSD

Traditionally the two main considerations for diesel fuel have been Cloud Point (CP) and Cold Filter Plug Point (CFPP).

Let’s start by defining the terms:

Cloud Point (CP) ASTM D2500 – This test determines the point where wax becomes visible in a fuel sample. This wax first appears as a floating cloudiness in a transparent fuel.

Cold Filter Plug Point (CFPP) ASTM D6371 – This test is a more complicated procedure involving using a vacuum to draw a 20cc fuel sample through a 45 micron screen within a 60 seconds.

There is usually but not always a spread between CP and CFPP of 2°F to 8°F.

CP is a first indicator of cold weather operability temperatures for diesel fuels. It is a visible indication of paraffin wax in diesel fuels. Prior to the introduction of Ultra-Low Sulfur Diesel (ULSD, S-15) into the US market, the importance of CP was often discounted by many due to fact that diesel engines could generally successfully operate at temperatures many degrees below the CP.

Up until the introduction of ULSD many if not most operators used CFPP to provide a reference temperature for cold weather operability with diesel fuels. This is however a complicated and imperfect test. As mentioned above, CFPP uses a vacuum to draw a sample of the fuel through a 45 micron screen within a given time. The point at which the sample fails to go through the screen in 60 seconds is the CFPP.

The main issue is that up until recently most fuel filters used a 10 micron filtering media. The significant difference 10 microns and 45 microns caused a disparity between the test and real world operations. However many in the industry felt that this differential was consistent and that provided a reliable guide for cold weather operability.

For example if you had a CFPP of -30°F, you could feel reasonably confident that you could operate to -20°F.

However three new factors need to taken into account due to changes in fuels and engines.

1. The new ULSD fuel does not appear to provide the same consistent differential between CP and CFPP as we had come to expect with High-Sulfur Diesel (HSD, S-5000) and Low-Sulfur Diesel (LSD, S-500).

2. The new phenomenon of Wax Drop Out (WDO) where under periods of extended “Cold Soak” (48-72+ hours) the wax in the fuel suddenly drops out of the fuel can happen at temperatures that can be above the CP. This problem appears at this time to be independent of CP or CFPP.

3. As diesel engines have become more sophisticated there has been a rise in fuel injection pressures. In order to obtain these higher pressures OEM’s have had to manufacture pump and injector parts to ever closer tolerances. Today many injectors have tolerances in the 2 micron range. These tight tolerances and the very high cost of making and replacing these components have caused manufacturers to use fuel filters with smaller media to protect these components. Where in the past fuel filters typically were 10 microns, today we are seeing filters of 7, 5, and even 2 microns.

This makes the problems associated with ULSD even more difficult. Cloudy fuel that would easily pass through a 10 micron filter can often plug a 5 or 2 micron filter. This makes correcting the cold weather operability issues of ULSD like hitting a moving target. Today you need to adjust your fuel treatment to reflect the engines and filter arrangements in your fleet.

We are now suggesting a formula based on both CP and CFPP. Take the difference between CP and CFPP, divide by 1.5 and add to the CFPP to get a safe operability number.

Example: CP = 8°F, CFPP = 3°F

The difference between 8 and 3 = 5, 5 x .75 = 3.75, Take the CFPP of 3 and add the 3.75 to it equaling 6.75°F. You could expect to reliably operate that fuel in an engine with a 7 to 5 micron filter at 6-7°F.

For those operating 2 micron filters we suggest using the CP of the fuel.

For those still able to operate with 10+ micron filters, we are suggesting a number half way between CP and CFPP.

It is important to remember that the traditional method of using Kerosene or Jet A to “cut” or blend with HSD or LSD to lower the CFPP and Pour Point (PP) is not as effective or reliable as it was in the past when using the new ULSD #1 to cut or blend with ULSD #2.