Combustor

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

M. E. Lackey, Utilisation of Energetic Materials in an Industrial Combustor, AMXTHE-TE-R 85003, U.S. Army Toxic and Ha2ardous Materials Agency, Edgewood, Md.,June 1985. 

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

W. R. Seeker, W. S. Lanier, and M. P. Heap, Municipal Waste Combustion Study Combustion Control of MSW Combustors to Minimi Emissions of Trace Organics, EER Corporation, Irvine, Calif., 1987. 

In a combined power cycle operation, clean (sulfur- and particulate-free) gas is burned with air in the combustor at elevated pressure. The gas is either low or medium heat-value, depending on the method of gasification. 

The hot gases from the combustor, temperature controlled to 980°C by excess air, are expanded through the gas turbine, driving the air compressor and generating electricity. Sensible heat in the gas turbine exhaust is recovered in a waste heat boiler by generating steam for additional electrical power production. 

A significant issue in combustors in the mid-1990s is the performance of the process in an environmentally acceptable manner through the use of either low sulfur coal or post-combustion clean-up of the flue gases. Thus there is a marked trend to more efficient methods of coal combustion and, in fact, a combustion system that is able to accept coal without the necessity of a post-combustion treatment or without emitting objectionable amounts of sulfur oxides, nitrogen oxides, and particulates is very desirable (51,52). 

A fluidized bed is an excellent medium for contacting gases with sohds, and this can be exploited in a combustor because sulfur dioxide emissions can be reduced by adding limestone, CaCO, or dolomite, CaCO MgCO, to the bed. 

R. L. Patel and co-workers, "Reactivity Characteri2ation of SoHd Fuels in an Atmospheric Bench-Scale Fluidi2ed-Bed Combustor," presented at the 1988 Joint AS ME/IEEE Power Generation Conference, Philadelphia, Sept. 25—29, 1988 also as Combustion Engineering Tpuhlic tion TlS-8391. 

Hot Corrosion. Hot corrosion is an accelerated form of oxidation that arises from the presence not only of an oxidizing gas, but also of a molten salt on the component surface. The molten salt interacts with the protective oxide so as to render the oxide nonprotective. Most commonly, hot corrosion is associated with the condensation of a thin molten film of sodium sulfate [7757-82-6], Na2S04, on superaHoys commonly used in components for gas turbines, particularly first-stage turbine blades and vanes. Other examples of hot corrosion have been identified in energy conversion systems, particularly coal gasifiers and direct coal combustors. In these cases the salt originates from alkali impurities in the coal which condense on the internal... 

Cobalt aHoys may find appHcation ia a fluidized-bed process for the direct combustion of coal (qv). CoCrAlY-coated Haynes 188 has proven to be one of the most resistant materials to a fireside corrosion process encountered ia tubes coimected the fluidized-bed combustor to a steam turbiae. 

Starved-Air or Partial Combustion. To obtaia the temperatures aeeded for the pyrolysis reactioa to occur, a limited amouat of oxygea is allowed to eater the combustioa zoae. This oxygea reacts with the feed or pyrolysis products and releases the needed energy within the combustor. Both pyrolysis and combustion products are obtained. The products leaving the system contain a large amount of chemical energy. 

For central station power generation the open cycle system using electrically conducting coal combustion products as the working fluid is employed. The fuel typically is pulverized coal burned directly in the MHD combustor, although in some plant designs cleaner fuels made from coal by gasification or by beneficiation have been considered (8—10) (see Fuels, synthetic). 

At the high temperatures found in MHD combustors, nitrogen oxides, NO, are formed primarily by gas-phase reactions, rather than from fuel-bound nitrogen. The principal constituent is nitric oxide [10102-43-9] NO, and the amount formed is generally limited by kinetics. Equilibrium values are reached only at very high temperatures. NO decomposes as the gas cools, at a rate which decreases with temperature. If the combustion gas cools too rapidly after the MHD channel the NO has insufficient time to decompose and excessive amounts can be released to the atmosphere. Below about 1800 K there is essentially no thermal decomposition of NO. 

Control of SO is intrinsic to the MHD process because of the strong chemical affinity of the potassium seed in the flow for the sulfur in the gas. Although the system is operated fuel-rich from the primary combustor to the secondary combustor, the predominant sulfur compound in the gas is sulfur... 

The potassium combines with the sulfur to form potassium sulfate, which condenses as a soHd primarily in the electrostatic precipitator (ESP) or baghouse. The recovered potassium sulfate is then deUvered to a seed regeneration unit where the ash and sulfur are removed, and the potassium, in a sulfur-free form such as formate or carbonate, is recycled to the MHD combustor. It is necessary also to remove anions such as Cf and E which reduce the electrical conductivity of the generator gas flow. These are present in the coal ash in very small and therefore relatively harmless concentrations. As the seed is recycled, however, the concentrations, particularly of CF, tend to build up and to become a serious contaminant unless removed. 

A diagram for one implementation of this process (61,62) is shown in Eigure 11. Recovered potassium sulfate is converted to potassium formate [590-29 ] by reaction with calcium formate [544-17-2] which is made by reacting hydrated lime, Ca(OH)2, and carbon monoxide. The potassium formate (mp 167°C), in hquid form, is recycled to the combustor at about 170°C. Sulfur is removed as soHd calcium sulfate by filtration and then disposed of (see... 

The resulting overall energy balance for the plant at nominal load conditions is shown in Table 3. The primary combustor operates at 760 kPa (7.5 atm) pressure the equivalence ratio is 0.9 the heat loss is about 3.5%. The channel operates in the subsonic mode, in a peak magnetic field of 6 T. AH critical electrical and gas dynamic operating parameters of the channel are within prescribed constraints the magnetic field and electrical loading are tailored to limit the maximum axial electrical field to 2 kV/m, the transverse current density to 0.9 A/cm , and the Hall parameter to 4. The diffuser pressure recovery factor is 0.6. 

The oxidant preheater, positioned in the convective section and designed to preheat the oxygen-enriched air for the MHD combustor to 922 K, is located after the finishing superheat and reheat sections. Seed is removed from the stack gas by electrostatic precipitation before the gas is emitted to the atmosphere. The recovered seed is recycled by use of the formate process. Alkali carbonates ate separated from potassium sulfate before conversion of potassium sulfate to potassium formate. Sodium carbonate and potassium carbonate are further separated to avoid buildup of sodium in the system by recycling of seed. The slag and fly-ash removed from the HRSR system is assumed to contain 15—17% of potassium as K2O, dissolved in ash and not recoverable. 

Combustor. In the majority of MHD plant designs the MHD combustor bums coal directly. Because MHD power generation is able to utilize pulverized coal in an environmentally acceptable fashion, there is usually no need to make cleaner fuels from coal, eg, by gasification or by beneficiation. A discussion of combustion techniques for MHD plants is available (70). 

The combustor is designed to operate at a pressure of 600 kPa (6 atm) with 1867 K preheated air. First stage heat loss of the 250 MW combustor is about 4.3% and the total heat loss is about 6%. The relative pressure drop is 3%. More complete discussions of the design and scale-up of the combustor are available (75). 

The combustor is assembled of flanged, spool-shaped water-cooled metal components, each with its own water-cooling circuit and pressure shell. No ceramic linings are used. Gas pressure is contained by stainless steel outer shells and the internal surfaces subject to high heat fluxes are lined with low alloy water-cooled panels. 

Assessments of control, operabiHty and part load performance of MHD—steam plants are discussed elsewhere (rl44 and rl45). Analyses have shown that relatively high plant efficiency can be maintained at part load, by reduction of fuel input, mass flow, and MHD combustor pressure. In order to achieve efficient part load operation the steam temperature to the turbine must be maintained. This is accompHshed by the use of flue gas recirculation in the heat recovery furnace at load conditions less than about 75% of fiiU load. 

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