Written on: November 1, 2012 by Timmie McElwain
I am sorry to report that conversions to gas continue to climb in many cases, just for the sake of cost of fuel. That, in my estimation and experience, is not the primary reason customers should change fuels. You might think I, coming out of the gas industry, would be gung ho for gas all the way, but that is not really the case. I am a believer in fair competition and insuring that customers continue to have a choice.
We should all endeavor to point out to our customers the plusses and minuses of fuel and equipment choices. The truth is, we are in the energy business today and we need to look to diversity as our offering to customers. It is also important that we all tell the truth about the pros and cons of our fuels, equipment and the servicing of all that. I hope my articles here will help to unite as to a common cause, and that is the safety, comfort and efficiency of customer’s equipment and homes. So with that in mind, we’ll continue our presentation on gas fuels.
Physical Characteristics of Fuel Gases
Petroleum gases are hydrocarbons—a chemical structure of hydrogen and carbon. Natural gas is primarily methane (CH4) and ethane (C2H6), but it will typically contain a number of heavier hydrocarbons when it leaves the well. Among these are propane and butane (the prime constituents of LP-gas), pentane, hexane, septane, octane and decane (the natural gasolines).
Natural gas right from the well will also contain impurities such as water, carbon dioxide and helium. The volume of heavier hydrocarbons is called the “wet gas” content. These heavier hydrocarbons are normally removed from natural gas by condensation and are used in the manufacture of LP-gas and gasoline. Impurities are also removed during refinery processes and the resulting “dry” gas is the natural gas commonly marketed.
At temperatures above –260°F at atmospheric pressure, natural gas occurs as a gas. It is normally transported from the source to the consumer in a gaseous state by pipeline. Recently, methods have been developed to transport natural gas as a relatively low pressure, low temperature liquid by ocean-going tanker. To accommodate periods of peak demand, natural gas may be stored as a pressurized gas or as a pressurized low temperature liquid. Natural gas is costly to liquefy because very low temperatures are required. The use of liquefied natural gas as a motor vehicle fuel is possible only with thick-walled storage tanks, well insulated to maintain the fuel in a liquid state. As a practical matter, natural gas is only liquefied to make bulk storage and transportation possible.
The fuel is lighter than air, weighing from 56% to 79% of the weight of an equal volume of air. The specific gravity of natural gas is, therefore, within the range of .56 to .79. Released into the atmosphere, it will normally rise and mix readily with air.
When burned efficiently, one cubic foot of processed or “dry” natural gas will normally produce 900 to 1,200 British Thermal Units (Btus) of heat, depending on the exact specific gravity. This value is called the gross heating value of natural gas and is an important factor in the design and operation of heat producing appliances.
Natural gas is colorless, odorless, and nontoxic. Although nontoxic, it can displace oxygen in the air and cause unconsciousness and death through asphyxiation, due to lack of oxygen. To aid in the detection of leaks, odorants are normally added to natural gas before distribution. Mercaptans are sulfur compounds commonly used as odorants. Although added in small amounts, they provide an effective means of detecting gas leaks.
Propane (C3H8) and butane (C4H10) are chemical compounds of hydrogen and carbon (hydrocarbons) which occur in “wet” natural gas and in crude oil. These heavier gases are removed from “wet” natural gas along with other impurities in order to produce “dry” natural gas for distribution. In oil refineries, these gases are produced in the process of manufacturing gasoline and other petroleum products.
In 1971, approximately 74% of LP-gas marketed was produced at the well from “wet” gas; the remaining production came from refineries. The ratio of production from wells and refineries varies from time to time, but in general the newer gasoline-making processes are yielding a higher percentage of propane. At normal temperatures and atmospheric pressure, both butane and propane occur in a gaseous state. Pure butane will condense to form a liquid at temperatures below 32°F when it is at atmospheric pressure. Both gases, however, will condense to form a liquid at normal temperatures and relatively low pressures.
Processed butane is normally 93% butane and 7% propane; processed propane is normally pure. LP-gas is distributed as a liquid by pipelines, rail cars, tanker trucks and tanker ships and barges. It is normally stored in pressurized tanks. Transportation to the consumer is usually by bulk truck. Storage at the point of use is in small tanks or cylinders, as a liquid under pressure.
Both butane and propane gases are heavier than air with specific gravities of 2.00 and 1.53 respectively. Neither propane nor butane gas will rise and disperse as quickly as natural gas when released into a nonventilated area. Because both gases are heavier than air, LP-gas will normally settle in calm air to form pockets, or it will hover around the source of the leak. With turbulence of ventilation, LP gas dissipates almost as quickly as natural gas.
Propane has a gross heating value of about 2,500 Btu/cubic foot; butane has a gross heating value of about 3,225 Btu/cubic foot. A given volume of LP-gas, therefore, will produce two and a half to three times the number of Btus as an equal volume of natural gas. This is an important consideration in the conversion of appliances from one fuel gas to another.
Butane and propane are colorless, odorless, and nontoxic. LP-gases also can displace oxygen in the air and cause unconsciousness and death through asphyxiation. Again, to aid in the detection of leaks, odorants are normally added to LP-gas before distribution to the consumer. In nonventilated areas, LP-gas leaks are more difficult to detect than natural gas leaks because LP-gas does not rise and disperse in air as readily as natural gas.
Petroleum gases utilized in heating appliances are natural gas and liquefied petroleum gas. Natural gas occurs naturally as a gas and requires minimal refinement. LP-gas occurs in “wet” natural gas and crude oil and must be extracted and refined before use. Natural gas is normally distributed in pipelines as a gas; LP-gas is usually transported under pressure as a liquid. Both gases are colorless, odorless, and nontoxic in their natural state. Natural gas is lighter than air and will rise and disperse. LP-gas is heavier than air and will settle and hover around the source of a leak.
Comparison of Fuels
When a comparison is made of fuels as to which one is the better fuel for heating, it has always been that oil comes out on top. The reason is the higher BTU per gallon as compared to the other fuels. The examples given are:
1 Gallon of #2 Fuel Oil = about 140,000 BTUs
1 Gallon of #2 Fuel Oil = about 1.4 CCF Natural Gas
CCF Natural Gas = about 0.7142 Gallon #2 Fuel Oil
Fuel Cost Conversion Formulas:
Cost per Gallon Fuel Oil x 0.7142 = about cost per CCF Natural Gas
Cost per CCF Natural Gas / 0.7142 = about cost per Gallon #2 Fuel Oil
If Fuel Oil costs $1.50 per gallon, the BTU equivalent cost of natural Gas is $1.50 x 0.7142 = $1.07 per CCF
If Natural Gas costs 1.00 per CCF, Fuel Oil would have to cost ($1.00 / 0.7142 = $1.40 per Gallon) to match the BTU cost of Natural Gas.
In the past, when comparisons were made of Natural Gas or Propane to oil relative to heating equipment, it was always atmospheric gas heating equipment. That equipment was compared to the oil power burner type equipment. When this comparison was made, oil always came out on top when efficiency numbers were discussed.
I am not a big fan of discussing efficiency as related to numbers such as 75% or 80% efficient. I will, as my articles continue, explain that. The reason oil was more efficient when looking at combustion efficiency numbers was that it was compared to gas equipment using room air for combustion (atmospheric). The oil power burner needed less excess air, usually around 25% as compared to the atmospheric gas equipment, which needed 40% to 50% excess air in order to burn safely and efficiently. Years ago, nobody was too concerned about that as fuel was cheap and most of our equipment on both sides was vented into chimneys. The interesting thing about all of that is that when the oil power burner was replaced by a gas power conversion burner, the gas efficiency numbers then ran right alongside the oil simply because not as much excess air was required. In some cases, the efficiency slightly exceeded the oil numbers.
Controlling excess air makes the difference when looking at this from a combustion perspective. In my many years in the gas industry, I have been involved in one way or another with over 3,000 conversions from either coal or oil to gas using a power gas conversion burner. In all cases, when the conversion was completed, we strived for a final efficiency number of 75% and in most cases, were able to accomplish that.
The chart below compares the average characteristics of common fuel gases. Notably, when we talk abo