Fires fuel-rich

Combustion behavior differed in some respects between continuous and instantaneous spills, and also between LNG and refrigerated liquid propane. For continuous spills, a short period of premixed burning occurred immediately after ignition. This was characterized by a weakly luminous flame, and was followed by combustion of the fuel-rich portions of the plume, which burned with a rather low, bright yellow flame. Hame height increased markedly as soon as the fire burned back to the liquid pool at the spill point, and assumed the tilted, cylindrical shape that is characteristic of a pool fire. 

Reburning is a process involving staged addition of fuel into two combustion zones. Coal is fired under normal conditions in the primary combustion zone and additional fuel, often gas, is added in a reburn zone, resulting in a fuel rich, oxygen deficient condition that converts the NO, produced in the primai y combustion zone to molecular nitrogen and water. In a burnout zone above the reburn zone, OFA is added to complete combustion. 

If (f> < 1, we have burned all of the available fuel and have leftover oxygen. This state is commonly called fuel-lean . On the other hand, if > 1, there is unbumed gaseous fuel and all of the oxygen is consumed. This state is commonly called fuel-rich . When we have a fire beginning within a room, 0 is less than 1, starting out as zero. As the fire grows, the fuel release can exceed the available oxygen supply. Room fires are termed ventilation-limited when > 1. 

Alternatively, vacuum within equipment could lead to ingress of air into inerted or fuel-rich systems, posing a fire or explosion hazard within the equipment. 

Even comprehensive mechanisms, however, must be utilized with caution. The GRI-Mech fails, for instance, under pyrolysis or very fuel-rich conditions, because it does not include formation of higher hydrocarbons or aromatic species. Its predictive capabilities are also limited under conditions where the presence of nitrogen oxides enhances the fuel oxidation rate (NO f sensitized oxidation), a reaction that may affect unbumed hydrocarbon emissions from some gas-fired systems, for example, internal combustion engines. 

Prompt NO Formation A second source of NO in gas firing is prompt NO. This formation pathway can be the dominating source of NO under conditions characterized by lower temperatures, fuel-rich conditions, and short residence times. This route, which is also called Fenimore NO, was first proposed by C. P. Fenimore [125]. Prompt NO formation is initiated by attack of CH, radicals on N2, forming cyanide species. The most important initiation step is the reaction... 

Figures 4 and 5 present model calculations for a Montana Rosebud coal-fired, potassium carbonate seeded combustor operated under slightly fuel-rich conditions (equivalence ratio = 1.09). Note that KPO2 and KPO3 are the dominant neutral phosphorus species at all temperatures. Negative ion chemistry is dominated by PO2 and PO3 below 2000 K, phosphorus species negative ions outnumber free electrons. The only negative ion which Is comparable in concentration to PO2 is Fe02 and then only at the upper temperature range. The sharp temperature falloff of Fe02 Is caused by the stability of condensed Iron containing species.

A K-Tech burner, shown in Figure 7.59, was fired at about 0.64 MW (2.2 x 10 ) Btu/hr. During the experiments the oxygen/natural gas volumetric ratio was varied between 1.8 (fuel rich) and 2.2 (fuel lean). 

Combustion processes can create pollutant emissions other than carbon monoxide and oxides of nifrogen. Unbumed hydrocarbons (UHC) is a term describing any fuel or partially oxidized hydrocarbon species that exit the stack of a furnace. The cause for these emissions is typically due to incomplete combustion of the fuel from poor mixing or low furnace temperature. A low temperature environment can be created by operating the furnace at a reduced firing rate or turndown. Particulate matter (commonly called soot) is often produced from fuel rich regions in diffusion flames. Soot becomes smoke if the rate of formation of soot exceeds the rate of oxidation of soot. Oxides of sulfur are formed when sulfur is present in the fuel. 

The Albright 3 boiler is a single furnace unit with 4 elevations of burners or injectors in each comer. Sawdust injectors or burners were added to the unit in 2 comers between the B and C rows of coal burners or injectors. The sawdust injectors can follow the burner tilts associated with the coal burners. Sawdust is fed to the unit at up to 10 percent on a mass basis, or about 4.5 percent on a heat input basis. It is injected directly into the center of the fireball with a very high fuel/air ratio in the transport system. The stoichiometric ratio of the sawdust/transport air mix, when firing at 10 percent sawdust, is fuel-rich. 

Oxygen-Rich versus Fuel-Rich Fires The distinction between an oxygen-rich or fuel-rich fire is used to describe the state of the fire relative to a nominal or stoichiometric combustion process. In the oxygen-rich state there is sufficient oxygen to allow for, theoretically, complete combustion of the available fuel. Remember that the combustion process is a series of competitive equilibrium chemical reactions therefore, it is fair to assume that not all of the intermediate products will be fully converted in the reaction chain. Growth of a fire in the oxygen-rich state is marked by direct heat transfer from the flame to the additional fuel supply. The exhaust gases mix readily with available fresh air and are cooled and diluted. 

Loomis, I.M. 199S. Application of water mist to fuel-rich fires in model coal mine entries. Master s thesis. Blacksburg, VA Virginia Polytechnic Institute and State University. 

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