Combustion, gaseous fuels

The burning of fuels involves a combination of thermal decomposition (dominant when combusting solid fuels) and hydroxylation (dominant when combusting gaseous fuels), as part of the overall process. 

Let us consider ways of determining the detonation concentration limits in binary combustible gaseous fuel + air mixtures. Calculation of the detonation limits in HCO mixtures can be performed using the one-dimensional Zeldovich model that takes into account energy losses caused by friction and heat-exchange with the duct walls [27]. 

Fuel switch. Since formation is fuel-dependent, switching fuel can reduce formation. The general trend is that from solid to liquid to gaseous fuel, the formation decreases. However, it should be emphasized again that this is also very much dependent on the combustion device. 

Combustion. Coal combustion, not being in the strictest sense a process for the generation of gaseous synfuels, is nevertheless an important use of coal as a source of gaseous fuels. Coal combustion, an old art and probably the oldest known use of this fossil fuel, is an accumulation of complex chemical and physical phenomena. The complexity of coal itself and the variable process parameters all contribute to the overall process (8,10,47—50) (see also COLffiUSTION SCIENCE AND technology). 

Partial Oxidation. It is often desirable to augment the supply of naturally occurring or by-product gaseous fuels or to produce gaseous fuels of well-defined composition and combustion characteristics (5). This is particularly tme in areas where the refinery fuel (natural gas) is in poor supply and/or where the manufacture of fuel gases, originally from coal and more recently from petroleum, has become well estabHshed. 

Combustion. The burning of soHd, Hquid, and gaseous fuels as a source of energy is very common. Using sufficient and reHable combustion controls, this process seldom causes serious problems. However, some combustion processes are deHberately carried out with an inadequate oxygen supply in order to obtain products of incomplete combustion. Explosive mixtures sometimes occur, and then flashback is a serious problem. 

New units can be ordered having dry, low NO burners that can reduce NO emissions below 25 ppm on gaseous fuels in many cases, without back-end flue-gas cleanup or front-end controls, such as steam or water injection which can reduce efficiency. Similar in concept to low NO burners used in boilers, dry low NO gas turbine burners aim to reduce peak combustion temperatures through staged combustion and/or improved fuel—air mixing. 

Spent Acid or Burning. Burners for spent acid or hydrogen sulfide are generally similar to those used for elemental sulfur. There are, however, a few critical differences. Special types of nozzles are required both for H2S, a gaseous fuel, and for the corrosive and viscous spent acids. In a few cases, spent acids maybe so viscous that only a spinning cup can satisfactorily atomize them. Because combustion of H2S is highly exothermic, carehil design is necessary to avoid excessive temperatures. 

Flame Types and Their Characteristics. There are two main types of flames diffusion and premixed. In diffusion flames, the fuel and oxidant are separately introduced and the rate of the overall process is determined by the mixing rate. Examples of diffusion flames include the flames associated with candles, matches, gaseous fuel jets, oil sprays, and large fires, whether accidental or otherwise. In premixed flames, fuel and oxidant are mixed thoroughly prior to combustion. A fundamental understanding of both flame types and their stmcture involves the determination of the dimensions of the various zones in the flame and the temperature, velocity, and species concentrations throughout the system. 

Surface combustion devices are designed for fully premixing the gaseous fuel and air and burning it on a porous radiant surface. The close coupling of the combustion process with the burner surface results in low flame temperatures and, consequently, low NO formation. Surface materials can include ceramic fibers, reticulated ceramics, and metal alloy mats. This approach allows the burner shape to be customized to match the heat transfer profile with the application. 

Fuel-Staged Burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain little or no fixed nitrogen. Figure 27-36 illustrates a fuel-staged natural draft refineiy process heater burner. The fuel is spht into primaiy (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirciJated by the primaiy... 

When considering the possibility of using internal combustion drivers, evaluate process requirements and costs. If a low-cost, gaseous fuel is available, gas engines and gas turbines may surpass other drivers in economical installation and operation. In the initial process design stage, a method of establishing the cost of purchase, installation, and operation for drivers is needed. 

The following provides a calculation method for determining the amount of air needed for perfect combustion of one cubic foot of any gaseous fuel. The following expression provides an estimate of the ratio of the volume of air needed to the volume of fuel (i.e., the air to fuel ratio, 0) ... 

For a combustion process burning gaseous fuel (which may have been compressed from state 0 to state l ), the left-hand side of the exergy Eq. (2.41) may be rewritten as... 

Matsui, H., and J. H. S. Lee. 1979. On the measure of relative detonation hazards of gaseous fuel-oxygen and air mixtures. Seventeenth Symposium (International) on Combustion, pp. 1269-1280. Pittsburgh, PA The Combustion Institute. 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

Internal combustion engines can be operated by a number of liquid and gaseous fuels. Table 3-2 gives the high value heat of combustion for various field available liquid and gaseous fuels [5J. 

Net Heat of Combustion (High Heat Value) for Various Liquid and Gaseous Fuels [4]... 

Approximate Gaseous Fuel Consumption for Multicylinder Internal Combustion Engines [4,5] ... 

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