Combustion (or burning) is a rapid combination of oxygen with fuel, resulting in a release of heat. Air (the oxygen source) is about 21% oxygen and 78% nitrogen by volume. Most fuels contain carbon, hydrogen, and sometimes sulphur. A simplification of combustion could be listed in the following three processes.carbon + oxygen = carbon dioxide + heat
hydrogen + oxygen = water vapor + heat
sulphur + oxygen = sulphur dioxide + heat
These products of combustion are chemical compounds. They consist of molecules, combined in fixed proportions. Heat given off in any combustion process is excess energy which the molecules must release.
Stoichiometric combustion results when no fuel or air goes unused during the combustion process. Combustion with too much (excess) air is said to be lean or oxidizing. The excess air or oxygen plays no part in the combustion process. In fact, it reduces efficiency. Visually, excess air produces a short and clear flame. Combustion with too much fuel is called rich or reducing, producing incomplete combustion. This flame appears long and some- times smoky. The oxygen supply for combustion generally comes from ambient air.
Because air contains primarily (78%) nitrogen, the required volume of air is generally larger than the required volume of fuel. Primary air is air mixed with the fuel before or within the burner’s fuel delivery system. Secondary air is usually brought in around the burner’s fuel delivery system and spun through a diffuser or turning vane system in order to optimize air-fuel mixing. Tertiary air is used to control the shape of the flame envelope or to control flame temperature on low-NOx burners. It is brought in downstream of the secondary air.
Most fuels are mixtures of chemical compounds called hydrocarbons. When these burn, the by-products are carbon dioxide and water vapor (unless a shortage of oxygen exists when carbon monoxide, hydrogen, unburned hydro- carbons and free carbon may be produced). Heat available from fuels is measured in Btu, Kilocalories, watt-seconds, or joules. A flame is a zone within which the combustion reaction occurs at a rate that produces visible radiation. A flame front is the contour along which the combustion starts — the dividing line between the fuel-air mixture and the combustion process. In stable flames, the flame front appears to be stationary. The flame moves toward the burner-nozzle(s) at the same speed that the fuel-air mixture leaves the burner. A variety of feed ranges exist in a wide range of burner designs.
Common flame characteristics are as follows:
- Production of heat energy
- Expansion of gases
- By-product production.
- Radiation emission.
- Ionization within the flame envelope.
Natural gas fuel requires no special handling in filtering, drying, heating, etc. Efficiently using fuel oils largely depends upon the ability of the burner system to atomize the oil and mix it with air in the correct proportions. Heavy fuel oils are usually heated with steam. Tank heaters may raise the oil temperature sufficiently to reduce viscosity to facilitate pumping and straining. Steam atomization occurs when steam is tangentially projected across jets of oil at the oil nozzle. This results in a conical spray of finely divided oil after the mixture leaves the nozzle. Air atomizing occurs when air is used as the atomizing agent in a proportioning inside- mixing type oil burner using low pressure air.
Large capacity oil burners use two steps to combust the oil — atomizing and vaporization. Vaporization converts oil from the liquid to vapor by application of heat at the flame-front. By atomizing the oil into millions of tiny droplets, the exposed surface area is increased and the oil can vaporize at its highest rate. For good atomizing and vaporizing a large volume of air must be mixed initially with the oil particles.
Mechanical atomization. Atomization without the used of either air or steam is synonymous with pressure atomizing. The nozzle consists of a system of slots tangential to a small inner whirl chamber followed by a small orifice. When passing through the slots, the volume of liquid increases. The high velocity prevailing in the whirl chamber tangentially imparts a centrifugal effect that forces the oil against the walls of the nozzle. It passes through the orifices in the nozzle tip and into the combustion chamber, fanning out into a cone shaped spray of very small particles.
Coal burning in multi-burner applications uses pulverized coal. Boilers can be equipped with one or more pulverizing mills through which the coal passes on its way to the burners. Hot air from the preheater dries the pulverized coal and carries it through the burners and into the furnace (suspension firing). There are wide variations in fineness requirements. The lower the coal’s volatile content, the finer it must be milled. Generally four to six burners are fed by one mill.
The main fuel supply subsystem (consisting of the piping and/or ducts and associated equipment to deliver the fuel to the burners) connects to the main burner subsystem. A fuel supply system needs to be sized and designed to ensure continuous flow adequate for all operating requirements.
It needs to include the co-ordination of the main fuel control valve, burner safety shutoff valves, and associated piping volume to ensure against fuel pressure transients. This can result in exceeding burner limits for stable flame when burners are placed in and out of service.
Main burner subsystems (fuel trains) continuously supply burner inputs to the furnace with stable flame limits. Variations in the burning characteristics of a fuel introduce unreliability to the lower operating limits of a burner subsystem of any design. Class 1 or 2 igniters may be required to maintain stable flame.
To be continued in Part Three.
This blog post is excerpted from the paper titled "Flame Safeguard Controls in Multi-Burner Environments" by Willy Vandermeer and courtesy of Fireye. The entire document may be downloaded here.
