Which Grade of Oil?There are several standard grades of oil on the market, but in most localities, your choice will come down to two -- distillate oil and residual oil.
Distillate oil, known as #1 and #2 grades under ASTM specifications, is what most of us think of as heating oil. It’s a transparent, amber fluid with a consistency a little thicker than water. It’s very similar to diesel fuel.
Residual oil, also known as #6 oil or Bunker C, is a thick, dark-brown-to-black substance that is semi-solid at room temperature. It must be preheated before it can be pumped out of its storage tank, and it must be kept hot all the way to the burner. Against the advantage of lower cost, you have to weigh its higher air emissions, inferior operating flexibility, a smaller selection of equipment to burn it and a more expensive, complex preparation and handling system. With the present-day supply situation, you may not even be able to get it unless you’re situated near a seaport or major waterway. It’s really not suited to occasional use and is best left to boilers, aggregate dryers and other “heavy” applications.
Compatibility with Your Process. Can you even fire your process with fuel oil? Direct-firing with oil is taboo on some food applications, and many finishing operations gave up oil because of concerns about color contamination. Many products passing through today’s ovens didn’t even exist when oil was last in wide use -- there may be no history on how well they’ll tolerate its combustion products. This doesn’t mean you can’t use oil, but you may have to convert a direct-fired system to indirect firing.
Fuel Supply. Before committing to oil-burning equipment, be sure of an adequate supply infrastructure in your area. Much of it withered away when natural gas spread and achieved dominance. At the least, you need a local distributor able to supply the quantities you need on schedule.
Equipment Supply. Let’s say you clear that hurdle. Is there oil-burning equipment suitable for your process? Another consequence of natural gas’ widespread acceptance was that it discouraged manufacturers from continuing to develop oil-firing combustion equipment in recent years. Many even dropped existing products from their catalogs. Your customary supplier may not have the equipment you need, and when you find it, you’ll be looking at technology at least 20 to 30 years old. This may force you to consider performance concessions, among them:
Turndown to Extremely Low Firing Rates. Many modern natural gas burners designed for low temperature applications can be turned down to extremely low inputs -- 20,000 or 30,000 BTU/hr. A typical small capacity oil burner with a low pressure air atomizer will be hard-pressed to operate below about 0.5 gal of oil per hour -- that’s 70,000 BTU/hr.
Emissions. Burning fuel oil will produce higher levels of NOX than natural gas, a consequence of oil’s higher inherent NOX emissions and the lack of incentive to develop low emission oil burners in recent years. Depending on your application and the firing equipment, you may also have to contend with higher carbon monoxide levels. Will your local air-quality regulations allow this?
Round Peg in a Square Hole Syndrome. Many burners used on oven and dryer applications are line types -- rectangular burners, often assembled into larger arrays. Nearly all oil burners are coaxial -- basically cylindrical -- designs. Compared to line-type burners, they have a more concentrated heat release pattern, and this is accentuated by the luminosity of the flame. They probably won’t be suitable for direct firing into air ducts. A separate combustion chamber or heater box may be necessary.
Excess Air Limitations. Gas burners used in ovens, dryers and other low temperature applications tend to have very high excess air capability -- it’s a key ingredient in their exceptional turndown range. Most oil burners can’t stay with them on this count. Although they may be capable of operating at fairly high amounts of excess air, they can’t do it cleanly. The chilling effect of excess air on the flame may produce white smoke (actually, fine oil droplets) that contaminates products.
Your Infrastructure. If you opt to burn oil, you have also committed to go into the fuel storage and preparation business. You’ll need a storage tank large enough to keep you going between deliveries, and you’ll have to decide whether to place it above or below ground. You’ll need supply piping from the tank(s) to each point of use, and that system will need the pumps, filters, valves and regulators required to ensure a clean, well-regulated supply of oil.
If winter temperatures in your area regularly drop to 10 to 20oF (-12 to -6oC) or lower, you’ll have to provide your system with insulation or auxiliary heating to keep the oil from gelling and choking off flow to the burners.
Getting and Keeping It Running. There aren’t many operating and maintenance people left with first-hand oil experience. Oil supply and control equipment operates similar to gas equipment, but there are numerous subtle ways they differ, and these can be stumbling blocks to reliable operation. Training will be essential.The Bottom Line.Can you use fuel oil as a standby fuel? Assuming supply access and environmental regulations don’t get in the way, probably yes, but it won’t be a simple plug-it-in-and-run exercise. Significant equipment alterations may be necessary, and you may have to resort to clever engineering and design work to accommodate some of oil’s operating limitations.