Understanding Biodiesel Key to Determining Application in School Buses

Part 1 of 2

By Denny Coughlin,
Special to School Transportation News

A decade ago we began hearing about how much pollution was emitted from diesel school buses, and we heard the cry for major reductions. Alternate fuels that stepped up to the plate included compressed natural gas (CNG), liquid natural gas (LNG) and propane. They burned much cleaner than diesel engines of the time, and certain areas of the country soon began converting fleets to utilize these cleaner burning energies. There are fleets that are well pleased with the alternate fuels and the operating costs associated.

But this involved great expense to install fueling stations, purchase new vehicles or convert the existing school buses with fuel tanks and engine modifications, and re-train personnel. This fleet cost usually had the assistance of grant money.

In 2005, we have faced diesel fuel costs of over $3 per gallon, and myriad supply problems. We all question what the future will bring. Will we be paying $5 or more a gallon next year? Will sufficient amounts of fuel be available? The same can be asked of alternative fuels. So what is the answer to our future fuel needs?

While there are no easy solutions to this dilemma, biodiesel appears to be a viable, renewable candidate. Biodiesel refers to 100 percent pure product derived from animal fats, or plants such as soybeans, rapeseed and mustard seed. The nation's farmers can grow the feed stock, which can also be converted to biodiesel. These processing plants are located in various parts of the country.

While biodiesel can be used in pure form, it is most commonly blended with diesel fuel at some designated mixture. A 20 percent biodiesel - 80 percent diesel fuel blend is referred to as B20, a 2 percent biodiesel - 98 percent diesel blend as B2, and so on. Any combination can be blended.

Biodiesel supplies a feast for bacteria. If you have water in your system, there is a potential for bacteria to grow at the bottom of your tank between the water and the fuel on top of the water. This bacteria factor has always been a concern, but with biodiesel the bacteria may grow at a faster rate. Unfortunately, bacteria may be detected when a fuel filter is plugged. Fuel filters should be cut apart on an occasional basis to check for a black slime that may be present. This is an indicator of bacteria. Water can be removed from the bottom of the storage tank, the tanks can be cleaned, and a chemical treatment can be introduced to kill the bacteria. This can be very expensive and time consuming for a fleet. With this item, "An ounce of prevention is worth a pound of cure." - D.C.

The quality of biodiesel is critical. Most biodiesel plants are continuously testing their product against a number of standards. There are about 35 separate tests that are run on the final product. All biodiesel must meet ASTM D6751.

Blending the products is an important process. An ideal method of mixing is for the biodiesel to be introduced within the pipeline at the transport loading facility. Another process is to introduce the blend into the fuel transport tank, and allow it to mix on the way to your facility. One product dropped on top of another in the storage tank creates a questionable mixture. Once the biodiesel is mixed thoroughly with diesel fuel, it will remain so for a considerable amount of time. If your fleet is not using fuel through the summer months, you may want to consult your fuel supplier about separation concerns.

Characteristics of Biodiesel

Biodiesel has a relatively high cetane rating, the ignition quality for diesel (octane is a similar term used for gasoline). This provides better combustion in the cylinder.

Biodiesel also has good detergent characteristics, and it will work to clean the varnish and deposits from the fuel injectors. It will clean sediments from the school bus fuel tank and fuel lines and will clean sludge from your storage tanks. While this all sounds good, we must pay special attention to fuel filter replacement. An extra filter change may be considered normal, but filter plugging and signs of bacteria in the filters may indicate larger problems with your fuel storage.

With a relatively high cloud point, biodiesel may gel at colder temperatures and not flow through the fuel filter. This is not a factor in the southern states where the temperature is always warm; in northern states this is a serious consideration and should be resolved with proper additives.

Diesel fuel by nature is a lubricant and, in this area, biodiesel excels. By June 1, all diesel fuel must undergo further refinement to lower the sulfur content to 15 parts per million (PPM) down from the current 500 parts per million (PPM). But this process will also lower the lubricity. Biodiesel adds a lubricating factor to help counter these effects. Fleets that are not using biodiesel shouldn't worry; the refiners will be required to introduce an additive package that brings the lubricity up to the applicable ASTM standard.

While emissions from most quality biodiesel blends will decrease, oxides of nitrogen (NOx) will slightly increase. One of the ways the engine manufacturers lower NOx emissions is to delay the combustion of the fuel in the cylinder. The higher cetane rating allows the biofuel to explode sooner, increasing the NOx. While this increase is minimal, I feel it is fair to mention.

Biodiesel has somewhere in the range of 5 to 8 percent less power than No. 2 diesel fuel. There are factors within the biodiesel that will determine this number.

So what are the engine manufacturers looking at? They must produce an engine that meets stricter pollution standards, something very difficult to accomplish. New more stringent standards will be in effect in 2007 and again in 2010. When the consumer makes the slightest change, it may no longer be a (pollution) compliant engine. A B2 blend should have a minimal effect, but the engine manufacturers may be consulted when a stronger blend is used. Older engines have less pollution restrictions than newer engines.

It must be realized that all of these mentioned characteristics are affected somewhat by the percent of the mixture of the biodiesel. For example, a B2 blend will have virtually the same power rating as regular diesel fuel. The difference with B20 may be measurable, and it may actually be noticeable with B100.

It is evident that biodiesel may create some challenges as the mixtures become stronger.

In future years, when the engines that meet the stricter pollution standards dominate our fleets, lower emissions will not be a reason to use biodiesel. The fact this fuel is renewable, non fossil, will be the driving force for its increased use.

Coughlin is the fleet operations manager for the Minneapolis Public Schools. He can be contacted at dennyc@mpls.k12.mn.us. Next month he will examine the viability of applying biodiesel to the school bus industry.