Hydrocarbon fuels combustion

Following the conclusions of Bowman [1], then, from the definition of prompt NO, these sources of prompt NO in hydrocarbon fuel combustion can be identified (1) nonequilibrium O and OH concentrations in the reaction zone and burned gas, which accelerate the rate of the thermal NO mechanism (2) a reaction sequence, shown in Fig. 8.7, that is initiated by reactions of hydrocarbon radicals, present in and near the reaction zone, with molecular nitrogen... 

Combustion of aluminum particle as fuel, and oxygen, air, or steam as oxidant provides an attractive propulsion strategy. In addition to hydrocarbon fuel combustion, research is focussed on determining the particle size and distribution and other relevant parameters for effectively combusting aluminum/oxygen and aluminum/steam in a laboratory-scale atmospheric dump combustor by John Foote at Engineering Research and Consulting, Inc. (Chapter 8). A Monte-Carlo numerical scheme was utilized to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. These results provide some of the basic information needed for realistic aluminum combustor development for underwater propulsion. 

Exploration of new avenues to reduce emissions to comply with environmental regulations has always been a priority in the past decade. As regulations get more and more stringent, innovative concepts are researched and applied to real engines. Soot control has been an issue with hydrocarbon fuel combustion, both from emission and signature points of view. Certain signatures are of concern for military weapon and platform propulsion. Recently, nonthermal plasma techniques for remediation of emissions have been found to be a vi-... 

E. Ranzi, A. Sogaro, P. Gaffuri, G. Pennati, C.K. Westbrook, and W.J. Pitz. A New Comprehensive Reaction Mechanism for Combustion of Hydrocarbon Fuels. Combust. Flame, 99 201-211,1994. 

Bittner, J.D. and Howard, J.B., "Role of Aromatics in Soot Formation" Alternative Hydrocarbon Fuels Combustion and Chemical Kinetics (C.T. Bowan and J. Birkeland, eds.) Progress in Aeronautics and Astronautics, Vol. 62, American Institute of Aeronautics Astronautics, New York, 1978... 

CC is the other dominant product of hydrocarbon-fueled combustion and its CARS flame spectrum (23)is displayed in Figure 1A. The spectrum is complicated by Fermi resonance and the fact that the rotational transitions are closely overlapped. This precludes treating them as independently broadened and so-called collisional narrowing may need to be taken into account. Computer modelling is currently in progress. Since all hydrocarbon fuels are Raman-active,CARS should ultimately be capable of monitoring total hydrocarbon concentrations during combustion as well. 

The furnace is first heated by hydrocarbon fuel combustion. Combustion is then stopped, and dehydrated air is passed through the furnace into the catalyst beds (repeated several times). Once the catalyst beds are hot enough to avoid H20(g) condensation, hot hydrocarbon combustion offgas is passed directly through the beds. 

Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

Natural gas is attractive as a fuel ia many appHcatioas because of its relatively clean burning characteristics and low air pollution (qv) potential compared to other fossil fuels. Combustion of natural gas iavolves mixing with air or oxygen and igniting the mixture. The overall combustion process does not iavolve particulate combustion or the vaporization of Hquid droplets. With proper burner design and operation, the combustion of natural gas is essentially complete. No unbumed hydrocarbon or carbon monoxide is present ia the products of combustioa. 

The third characteristic of interest grows directly from the first, ie, the high thermal conductance of the heat pipe can make possible the physical separation of the heat source and the heat consumer (heat sink). Heat pipes >100 m in length have been constmcted and shown to behave predictably (3). Separation of source and sink is especially important in those appHcations in which chemical incompatibilities exist. For example, it may be necessary to inject heat into a reaction vessel. The lowest cost source of heat may be combustion of hydrocarbon fuels. However, contact with an open flame or with the combustion products might jeopardize the desired reaction process. In such a case it might be feasible to carry heat from the flame through the wall of the reaction vessel by use of a heat pipe. 

Table 1. Combustion Properties for Common Hydrocarbon Fuels ...

An alternative method of produciag hydrocarbon fuels from biomass uses oils that are produced ia certaia plant seeds, such as rape seed, sunflowers, or oil palms, or from aquatic plants (see Soybeans and other oilseeds). Certain aquatic plants produce oils that can be extracted and upgraded to produce diesel fuel. The primary processiag requirement is to isolate the hydrocarbon portion of the carbon chain that closely matches diesel fuel and modify its combustion characteristics by chemical processiag. 

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

H. C. Barnett and R. R. Hibbard, Basic Considerations in the Combustion of Hydrocarbon Fuels with Air, NASA Technical Report, 1959, p. 1300. 

Fig. 3. Theoretical mole percent of the principal combustion products of hydrocarbon fuels for fuel hydrogen carbon ratios from 1, eg, to 4, eg, CH, ...

Unburnt hydrocarbon (UHC) and carbon monoxide (CO) are only produced in incomplete combustion typical of idle conditions. It appears probable that idling efficiency can be improved by detailed design to provide better atomization and higher local temperatures. CO2 production is a direct function of the fuel burnt (3.14 times the fuel burnt) it is not possible to control the production of CO2 in fossil fuel combustion, the best control is the increasing of the turbine efficiency, thus requiring less fuel to be burnt for the same power produced. 

In the conventional gas turbine plant, a hydrocarbon fuel (e.g. methane CH4) is burnt, usually with excess air, i.e. more air than is required for stoichiometric combustion. 

Alternatives to coal and hydrocarbon fuels as a source of power have been sought with increasing determination over the past three decades. One possibility is the Hydrogen Economy (p, 40), Another possibility, particularly for secondary, mobile sources of power, is the use of storage batteries. Indeed, electric vehicles were developed simultaneously with the first intemal-combustion-cngined vehicles, the first being made in 1888. In those days, over a century ago, electric vehicles were popular and sold well compared with the then noisy, inconvenient and rather unreliable peU ol-engined vehicles. In 1899 an electric car held the world land-speed record at 105 km per hour. In the early years of this century, taxis in New York, Boston and Berlin were mainly electric there were over 20000 electi ic vehicles in the USA and some 10000 cars and commercial vehicles in London. Even today (silent) battery-powered milk delivery vehicles are still operated in the UK. These use the traditional lead-sulfuric acid battery (p. 371), but this is extremely heavy and rather expensive. 

The most commonly used fuels for combustion are hydrocarbons, materials that are compounds of only hydrogen and carbon. Occasionally, fuels such as alcohols, that contain oxygen, are burned. Wlieti hydrocarbon fuels with or without oxygen arc burned in air (combusted) to completion, the products are water, from the hydrogen part of the fuel, and carbon dioxide, from the complete conversion of the carbon part. If oxygen is present m the fuel, it shows up in the final product as part of either the water or carbon dioxide. 

Carbon dioxide has been implicated as a contributing factor in global warming. Increased global warming has been associated with increased release of carbon dioxide into the atmosphere attributed in part to an increase in the combustion of hydrocarbon fuels. Carbon dioxide is an inevitable consequence of the complete combustion of hydrocarbons in air. If combustion devices are made more efficient, less fuel is required and less carbon dioxide is released into the atmosphere. 

A hydrogen fuel cell is environmentally friendly, but H2 is much more difficult to store than liquid fuels. The production, distribution, and storage of hydrogen present major difficulties, so researchers are working on fuel cells that use liquid hydrocarbon fuels. One such fuel cell is composed of layers of yttria-stabilized zirconia (YSZ), which is solid Zr02 containing around 5% Y2 O3. This cell uses the combustion of a... 

Volume of effluent gas Small volume because of absence of nitrogen and hydrocarbon combustion products Large volume because of presence of air and fuel combustion products... 

---