The use of biodiesel-blended heating oil in oil-fired appliances is a trend that is slowly growing in the U.S. Some retailers of heating oil have been offering residential and industrial customers biodiesel blends for about 10 years at concentrations of typically 5 and 20 percent biodiesel. In fact, 5 percent biodiesel blends have been included into ASTM D396, a standard that states the industry-accepted grades of fuel oil for use in oil-fired appliances. While there are boilers and furnaces operating in the field at both concentrations, it has been shown that higher concentrations and even pure biodiesel can be burned in heating appliances.

Biodiesel is a renewable resource that can be made from both plant oil and tallow. In tests conducted in oil-fired appliances by Brookhaven National Laboratory (BNL), Upton, N.Y., biodiesel and biodiesel blends have been found to burn nearly as efficiently as petroleum-based heating oil, while producing less noxious emissions and less wear and tear on appliances. (See Table 1.) For example, BNL has found that the heating value of a 20-percent biodiesel to 80-percent heating oil blend is only 2 percent lower than that of a purely petroleum-based heating oil. Furthermore, biodiesel manufactured today is subject to much higher quality control than that which was available 10 to 15 years ago, when the fuel was still in its infancy. Biodiesel blends are subject to ASTM D6751, the industry standard that covers blending biodiesel with distillate fuels.

Cleaner Burn

Enlarge this picture Fig. 1. A Brookhaven National Laboratory study found that pure biodiesel combustion in a boiler resulted in NOx emission being reduced by approximately 25 percent and SO2 by 97 percent.

Biodiesel has very low sulfur content, which means less SO2 emission and less damage to the appliance. Tom Butcher, head - energy resources division for BNL, says that this low level of SO2 lessens corrosion, fouling and scaling in combustion chambers as well as the deposition of sulfuric acid on heat exchanger metal. Of course, in the case of a residential furnace or industrial boiler, the SO2 reduction is contingent on the concentration of biodiesel in the fuel; a 5-percent biodiesel blend would only yield a 5-percent benefit, for example.

In a 2001 test, BNL studied the combustion performance of 10-, 20-, and 30-percent biodiesel blends in petroleum-based heating oil. The effects of 50-percent biodiesel in kerosene and 100 percent biodiesel combustion were examined as well. The study used a cast iron residential boiler equipped with a conventional burner, as well as a large commercial cast iron boiler. In the residential boiler tests done to evaluate ignition quality, findings did not indicate a significant difference between the test fuels.

The residential boiler results indicated a 10-percent reduction in CO emissions for all biodiesel blends. The biodiesel blends also had a NOX reduction, which increased with higher biodiesel concentrations. With pure biodiesel a 20-percent reduction of NOX was reported for the residential boiler; the commercial boiler saw a 23-percent reduction in NOX.

In a more recent study by BNL, substantial reductions of SO2 and NOX in flue gases were achieved through the use of 100-percent biodiesel in a boiler. The boiler’s fuel was switched from petroleum-based heating oil to pure biodiesel during continuous, steady-state operation and flue gases were monitored. The study found that pure biodiesel resulted in the NOX emission being reduced by approximately 25 percent and SO2 by 97 percent. (See Fig. 1.)

Nonmetallic Parts

BNL has found that concentrations of up to 30 percent biodiesel can be used with oil-fired appliances without the need for modification. But, in some cases, it has been found that higher concentrations can be detrimental to an appliance’s nonmetallic components. In older appliances with seals made of nitrile or Buna N rubber it is more likely to find problems when a 20 percent biodiesel concentration is used; this blend ratio has a tendency to cause burner seals to swell and leak. This is a cause for concern, as some suppliers keep their ratio as high as 20 percent biodiesel. Today manufacturers can use components, such as fluorocarbon rubber (Viton) seals, that can withstand such concentrations.

However, Victor Turk, director of quality for R.W. Beckett, North Ridgeville, Ohio, says that there can be a performance tradeoff when one decides to retrofit an appliance with a fluorocarbon rubber seal. Turk explains that the seals in most burners rely on lip-seal technology, which was developed around nitrile rubber. And because of property differences between modern seal material and the older nitrile rubber, a new seal designed to replace an older counterpart may not seal as well as the one it is replacing under all conditions.

When supplying to residential customers, a 5 percent blend is the safest and most prevalent product on the market, as it practically eliminates the possibility of appliance damage or malfunction. However, the use of 20 percent biodiesel is not uncommon, sold to both residential and industrial customers. Sometimes older appliances will be retrofitted with new seals to eliminate compatibility issues.

Temperature and Light

Enlarge this picture Table 1. Comparison of Typical Fuel Properties: Heating Oil and Biodiesel.

High biodiesel blends also take issue with the cold. Cold temperatures can affect blending, handling and storage. For example, biodiesel has a higher cloud point temperature than petroleum-based diesel and therefore has a tendency to gel at relatively warm temperatures. Because of this, Turk says that the use of higher-concentration (B20) biodiesel blends has been more successful in moderate climates, such as in the Mid-Atlantic States and on the Pacific Northwest coast.

Butcher says that tallow-based biodiesel can gel at temperatures as high as 50 DegF. However, biodiesel made from soy is more resilient to the cold. And as most biodiesel blends are used at a low concentration of 5 percent, gelling is usually not an issue. One way to get around gelling with the use of a high blend would be to store fuel indoors. A very large facility, for example, may have the capacity to store a high-concentration fuel in a warm, indoor area. Another option, says Turk, would be to blend biodiesel with light petroleum fuels or mix cold-flow additives with the fuel.

Biodiesel also can affect flame sensor functionality. Turk says that a standard flame sensor in residential burners detects the yellow light emitted by oil-fire flames. Biodiesel contains 10 percent oxygen, which results in a more transparent flame. It has been observed that the flame of a very high biodiesel concentration can prevent a flame sensor from detecting a flame’s presence. This can cause nuisance lockouts of the primary safety control. With 5 percent biodiesel blends this effect is measurable, but not significant enough to cause appliance malfunction.

Steady at 5 Percent

As mentioned above, the 5 percent biodiesel blend has been incorporated in ASTM D396. At the time of writing, Underwriters Laboratories (UL) stated that it was finalizing protocols covering biodiesel blends of more than 5 percent, but would not comment further.

In the near term, it is expected that marketers of biodiesel blends will be holding steady at a 5 percent blend, says Turk. While progressive providers market 20-percent biodiesel blends, 5-percent biodiesel blends are accepted industry wide and have properties very similar to purely petroleum-based diesel. Mainstream acceptance of higher concentrations would require additional standards and extensive testing of new appliance components.

Furthermore, after the 20 percent blend point is attained — a blend that can have problems in itself — the fuel begins to lose some of its petroleum-based qualities. At a 20 percent blend, the fuel begins to show more of the instability of pure biodiesel. Also, Turk points out that while a high biodiesel blend sits idle in the boiler during the summer months, instability could become a problem. Additionally, bacteria could grow at the interface between the fuel and condensate water in the storage tank. The bacterial solids could eventually make their way to the intake and clog the burner.

Blending biodiesel with petroleum-based diesel has been successful at the low-blend 5-percent ratio, and, in some cases, blends of up to 20 percent are commercially viable. However, today’s biodiesel, while of higher quality than that of the past, does have its drawbacks, and it remains to be seen if biodiesel blend stock and appliance components will advance to the point where higher blends can be used commercially.

For more information, visit:
Brookhaven National Lab - www.bnl.gov/est/erd/biofuel
R.W. Beckett - www.beckettcorp.com