Gas-Fired Systems

Air preheat temperature requirements of 2250—2300 K are anticipated for natural gas-fired systems, and about 2000 K for oil or coal-fired systems (11). Use of 32—40% oxygen enrichment lowers the preheat temperature requirement to a moderate 900—1000 K, which can be attained with conventional metal-type tubular heat exchangers. Depending on the cost of oxygen, this is a viable alternative to the use of separately fired high temperature preheaters. 

For gas-fired systems the state-of-the-art is represented by the preheater described in Reference 69. A pebble bed instead of a cored brick matrix is used. The pebbles are made of alumina spheres, 20 mm in diameter. Heat-transfer coefficients 3—4 times greater than for checkerwork matrices are achieved. A prototype device 400 m in volume has been operated for three years at an industrial blast furnace, achieving preheat temperatures of 1670 to 1770 K. 

Figure 12. In other words, the conversion gas of a PBC system is equivalent to the gas fuel of a gas-fired system. Consequently, the mass flow and stoichiometry of the conversion gas are key quantities in the calculation of the correct excess air number (see Paper I), the latent heat flow of combustion, and the conversion efficiency, and the combustion efficiency of a PBC system.

Figure 12 The difference and similarities between a gas-fired system and a PBC system...

Finally, the mass flux of conversion gas is primarily controlled by the volume flux of primary air and conversion concepf, whereas for a gas-fired system the mass flux of gas fuel into the combustion chamber is limited by the gas fuel fan capacity and the burner design. 

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. 

Increased pressure drop. Could require conversion to FD burners, or installation of an ID fan. Precipitation can still occur in the catalyst bed because of capillary action that raises the dew point of the ammonium salts. This is negligible for natural gas and refinery fuel gas fired systems but can be significant for applications with high sulfur in the fuel. 

In Germany iron oxide/chromium oxide catalysts have been used since the 1960s. Flockenhaus [99] describes an application for SCR downstream in a natural-gas fired system NO.v conversions of about 70% for a NH3 NO ratio of 0.8 were obtained. 

Gas turbine-based power plants, particularly natural gas-fired cogeneration and combined-cycle faciUties, have proven to be highly rehable, efficient, and environmentally attractive. Advances in machine design, more efficient plant integration, and optimistic forecasts for the availabiUty of affordable natural gas worldwide have boosted the appeal of these systems for both base-load and peaking service. 

Motor-driven, multistage reciprocating compressors have reportedly been the most popular choice for aeroderivatives. Motor-driven, oil-fiooded screw compressors are also used in some cases. High horsepower, multistage centrifugal compressors, similar to those used at many pipeline compressor stations, may be required for the newer heavy-duty units if the distribution pipeline pressure is insufficient (see Pipelines). Gas turbines have more stringent fuel-gas specifications in terms of cleanliness than do gas-fired boilers. Thus oil- and water-knockout systems, coalescing filters, and fine-mesh filters are used. 

AH gas-fired power plants require oxygen analy2ers to ensure that air has not been drawn into the piping system. Oxygen intake can lead to the presence of an explosive mixture in the pipeline before the fuel reaches the burner or combustor 2one. When gas-fired units are located in an enclosed area, multiple ultraviolet flame detectors are used to shut down equipment and flood the area with CO2 or a chemical fire suppressant whenever a spark or flame is detected. 

Metafile arsenic can be obtained by the direct smelting of the minerals arsenopyrite or loeUingite. The arsenic vapor is sublimed when these minerals are heated to about 650—700°C in the absence of air. The metal can also be prepared commercially by the reduction of arsenic trioxide with charcoal. The oxide and charcoal are mixed and placed into a horizontal steel retort jacketed with fire-brick which is then gas-fired. The reduced arsenic vapor is collected in a water-cooled condenser (5). In a process used by Bofiden Aktiebolag (6), the steel retort, heated to 700—800°C in an electric furnace, is equipped with a demountable air-cooled condenser. The off-gases are cleaned in a sembber system. The yield of metallic arsenic from the reduction of arsenic trioxide with carbon and carbon monoxide has been studied (7) and a process has been patented describing the gaseous reduction of arsenic trioxide to metal (8). 

Flue Ga.s Desulfuriza.tion. Citric acid can be used to buffer systems that can scmb sulfur dioxide from flue gas produced by large coal and gas-fired boilers generating steam for electrical power (134—143). The optimum pH for sulfur dioxide absorption is pH 4.5, which is where citrate has buffer capacity. Sulfur dioxide is the primary contributor to acid rain, which can cause environmental damage. 

The total emissions of hazardous air pollutants from a CGCC plant having wet cleanup are expected to be at least an order of magnitude lower than those achievable from a modem coal-fired steam plant (41). Metals removal in hot-gas cleanup systems is still under development. 

Drying rotary oil- or gas-fired Particulates, SO2, NO, VOC, CO, and smoke Proper combustion controls, fuel-oil preheating where required local exhaust system, cyclone and a scrubber or baghouse... 

The OREDA Offshore Reliability Data Handbooks covers a variety of components used in offshore oil drilling and platforms, including gas/fire detection systems, process alarm systems, firefighting systems, pressure relieving systems, general alarm and communication systems, evacuation systems, process systems (vessels, valves, pumps, heat exchangers, and compressors), electrical and utility systems, and drilling equipment. 

Figure 1.4 represents an open-loop eontrol system and is used for very simple applieations. The main problem with open-loop eontrol is that the eontrolled variable is sensitive to ehanges in disturbanee inputs. So, for example, if a gas fire is switehed on in a room, and the temperature elimbs to 20 °C, it will remain at that value unless there is a disturbanee. This eould be eaused by leaving a door to the room open, for example. Or alternatively by a ehange in outside temperature. In either ease, the internal room temperature will ehange. For the room temperature to remain eonstant, a meehanism is required to vary the energy output from the gas fire. 

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