Fuel Oil News is pleased to run this two-part White Paper from John E. Batey, PE, president of Energy Research Center, Inc. Emissions of key air pollutants from oil heating equipment have been substantially reduced over the past three decades to levels that are comparable, and in some cases lower than, natural gas equipment. Notwithstanding that fact however, many misperceptions about air emissions from oil equipment remain. This report summarizes the results of extensive research related to air emissions from numerous, highly credible sources, all of which clearly show that heating oil is one of the cleanest fuels in both residential and commercial applications. In fact, as the use of lower sulfur fuel oil and biodiesel/heating oil blends expands, residential and commercial oil heating equipment could readily become the best option for lowering annual air emissions…including greenhouse gases.
Part One left off during a discussion of specific emissions, and this resumes with sulfur oxides.
The sulfur oxides emitted by a fuel are directly related to the sulfur content of the fuel. In recent years, the average sulfur content of home heating oil has been steadily decreasing. Twenty to 30 years ago, average sulfur content was approximately 0.5 percent and even higher in some areas. In recent years, the average sulfur content of home heating oil has been near 0.225 percent (3).
Figure 9 shows recent changes in fuel sulfur content for distillate oil. A low sulfur product with only 0.05 percent (500 ppm) of sulfur has been required for on-highway use for some time and is also being used by some fuel oil dealers for their residential customers. The oil heat industry is moving to make this the standard fuel for all homes, given that it results in an almost 80 percent reduction in sulfur oxide emissions. A new ultra-low sulfur diesel fuel is now required for on-highway use with only 0.0015 percent (15 ppm) sulfur. [Figure 9] Shown on the far right hand side of the chart, this fuel is approaching zero sulfur content and zero emissions, inasmuch as it represents a 99 percent reduction in both. As the use of this product expands over time to residential heating, overall sulfur oxide emissions will decrease towards virtually non-existent.
This dramatic reduction in sulfur oxide emissions is similar to the particulate matter reductions, which occurred from the 1960s to the 1990s. In this case, however, the improvement is not attributable to burner design advances, but rather is the result of using improved fuels with a lower sulfur content. No changes to the heating equipment are needed in order to use the cleaner-burning, lower sulfur fuel, although service intervals can be extended, thus lowering maintenance costs.
Figure 10 presents sulfur oxide emissions in lbs/MMBtu of fuel burned for a range of combustion sources evaluated. It shows that oil and gas emissions are both very low and approach the limit of zero emissions when compared to other common combustion sources.
While No. 2 oil is visible on the chart, it is a miniscule fraction of both coal and residual oil emissions; low and ultra-low sulfur oil products, with or without blended bio-fuels, represent trivial emission levels, as does natural gas and 100 percent biodiesel, which are the lowest. When compared to home oil burner SO2 emission rates, No. 6 residual oil is 10 times higher, and coal with 3 percent sulfur content is more than 17-1/2 times higher. If all of the combustion sources in the U.S. emitted the same low sulfur oxide levels as residential oil and gas heating equipment, acid rain and its related problems would be virtually eliminated.
Residential oil burners produce very low carbon monoxide emissions. Figure 11 shows that No. 6 fuel oil, No. 2 fuel oil, low sulfur No. 2 oil, ultra-low sulfur No. 2 oil, biodiesel blends, and natural gas all emit carbon monoxide at very low rates. Residential oil burners emit only 0.026 lbs/MMBtu, which is lower than natural gas burners at 0.039 lbs/MMBtu. Oil and gas emissions are all much lower than either coal or diesel engines.
It is important to note that the carbon monoxide emission rates shown here are for properly adjusted residential burners. Carbon monoxide emissions from heating equipment, however, can increase by several orders of magnitude if the burner is not properly adjusted or is not supplied with adequate air for complete combustion of the fuel; too much combustion air can also increase CO levels. In extreme cases, high concentrations of carbon monoxide released into the home can produce hazardous and life-threatening conditions that present an immediate health and safety danger to occupants. Residential fuel oil burners typically produce smoke before carbon monoxide levels increase; this serves as a rudimentary, but nonetheless reliable ‘warning signal” that the system is not running properly. In contrast, gas burners can produce very high CO levels before any smoke is observed. The emission values reported in Figure 11 are for properly adjusted burners that have adequate combustion air supply and are properly vented.
Figure 12 presents carbon monoxide emissions in lbs/MMBtu of fuel burned for the range of combustion sources that were evaluated. When wood burning fireplaces and gas engines are added, the CO emissions from residential oil and gas burners are so small they are no longer visible on the chart.
Other common combustion sources emit significantly greater levels of carbon monoxide: Coal burning emits 7 times more; diesel engines emit 28 times more; wood fireplaces emit 580 times more; and industrial gasoline engines emit 2400 times more CO than home oil or gas heating systems.
Given that modern oil burners produce very low levels of carbon monoxide compared to many other combustion sources, it is certainly safe to say that if all combustion equipment produced the same low levels, carbon monoxide would not be a serious problem in the U.S.
Figure 13 shows that coal, No. 2 fuel oil, low sulfur No. 2 oil, ultra-low sulfur No. 2 oil, and biodiesel blends all emit very low rates of unburned hydrocarbons. Residential oil burners emit only 0.0017 lbs/MMBtu based on BNL tests, which is considerably less than natural gas burners at 0.011 lbs/MMBtu. No. 2 oil emissions are lower than both No. 6 residual oil and natural gas.
Hydrocarbon emissions are generally considered to be as undesirable because some components can directly cause adverse health effects, depending on their chemical composition. Hydrocarbon emissions also contribute to the formation of secondary pollutants in the atmosphere. Furthermore, they interact with nitrogen oxides in the presence of sunlight to form ozone and other oxidants in a series of very complex chemical reactions. These secondary pollutants have been shown to produce various health impacts and can even damage vegetation.
Figure 14 presents hydrocarbon (HC) emissions in lbs/MMBtu of fuel burned for a range of combustion sources evaluated. When diesel engines, gas engines and wood stoves are added, the HC emissions from residential oil and gas burners are so small they are no longer visible on the chart.
Other common combustion sources emit much higher levels of unburned hydrocarbons: No. 6 oil emits 4 times more; natural gas emits 6.5 times more; diesel engines emit 200 times more; gasoline engines emit 1200 times more; and finally, wood fireplaces emit 7,800 times more HC than home oil burners.
Compared to most other combustion sources, modern oil burners release very low levels of hydrocarbons and, thus, are an insignificant source of HC in the U.S.
Greenhouse Gas Emissions: Carbon Dioxide and Methane
Carbon dioxide (CO2) and methane are two of the gases that are emitted by fuel transport and combustion and are suspected of contributing to global warming by absorbing solar radiation in the atmosphere. Carbon dioxide is a non-toxic gas that is a primary exhaust constituent whenever hydrocarbon fuels are burned. Methane is the main component of natural gas, some of which is inadvertently released during natural gas production, transmission, storage and distribution by pipeline to end users. Methane is an extremely powerful greenhouse gas ‘ in fact it is between 30 and 70 times more powerful than CO2 in producing global warming (8,9). Both carbon monoxide and methane emissions must be considered in order to fully evaluate the total cumulative impact of air emissions from various combustion sources.
The rate of carbon dioxide emissions from various combustion sources are shown in Table 1. Heating oil used in homes has a USEPA emission rate for CO2 of 159 lbs/MMBtu. This is higher than natural gas, but lower than residual oil, coal and wood stoves. Figure 15 summarizes the results for CO2 emissions. Biodiesel is much lower because it is a renewable fuel, which re-absorbs carbon during the re-growing process. It is projected that in the very near future, the use of heating oil with biodiesel blends will actually reduce heating oil carbon dioxide emission rates far below even the level emitted by natural gas combustion.
Carbon dioxide is, however, only one part of total greenhouse gas emissions; the impact of methane releases must also be taken into account to fully evaluate the impact of a fuel’s greenhouse contributions. A new term, global warming potential (GWP), calculates the global warming impact of methane released, in terms of its equivalent pounds of carbon dioxide. The combined effect of carbon monoxide and methane emissions can then be evaluated to determine their net impact on the environment.
Natural gas (primarily methane) release rates during storage and transport are difficult to measure and estimates of total emissions vary. The World Resource Institute estimated that methane emissions from natural gas pipelines in 1987 totaled 53 million metric tons. A study by the United Kingdom House of Commons Energy Committee calculated average gas leakage rates worldwide of 4.7 percent of throughput, with 3 percent in North America. Greenpeace reviewed independent studies and found that a 3.5 percent methane release rate from natural gas systems worldwide may, in fact, be a conservative estimate.
Publications by the Gas Research Institute (GRI), (based on joint USEPA and GRI studies), identify some 800,000 gas leaks annually in the U.S. Annual methane emissions in the U.S. from natural gas leakage were calculated to be 314 billion cubic feet, or approximately 1.4 percent of gross gas production (10). The U.S. DOE publication ‘Natural Gas Annual” shows that from 1994 through 1998, the average ‘lost and unaccounted-for natural gas” figure represents approximately 2.6 percent of total gas consumption (11). While the concept of ‘unaccounted-for natural gas” is not directly equivalent to a leakage rate, the figure calculated by U.S. DOE is of the same magnitude as previously-cited estimates of natural gas system leakage rates. This research conservatively indicates that a leakage rate of between 1.4 and 3.5 percent annually is expected in the U.S., which is, therefore, used in this analysis.
Figure 16 presents the impact of these various methane release rates on the GWP of natural gas use. At zero percent leakage, the GWP is equal to the carbon dioxide emissions shown in Table 1. However, as the methane release rate increases, so does the GWP. At a 1.4 percent leakage rate (NG 1.4percent) the global warming potential of natural gas use increases by 14 percent. At a 2.6 percent methane release rate, the GWP increases by 26 percent. At this leakage rate, the GWP of natural gas is rapidly approaching that of home heating oil; if the actual methane release rate rises to 3.5 percent, the GWP of natural gas is higher than home heating oil.
This was noted in a report by Dr. Dean Abrahamson entitled ‘Relative Greenhouse Effect of Fossil Fuels and The Critical Contribution of Methane” (8), which concludes that heating oil produces less global warming than natural gas if more than 1 or 2 percent of the natural gas leaks to the atmosphere during transmission, storage and distribution.
When biodiesel, a renewable fuel increasingly available to home owners, is blended with heating oil, its GWP can be much lower than natural gas. (See Figure 17).
Figure 17 compares the emissions of greenhouse gases from common energy sources, increasing from the lowest to the highest global warming potential; as detailed below: