Flue-gas temperatures

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

A PFBC boiler is visually similar to an AFBC boiler. The combustor is made of water-wall tubing, which contains the high-temperature environment, but the whole assembly is placed within a pressure vessel. Unlike an AFBC unit, there is no convection pass, as the flue-gas temperature must be maintained at boiler temperature to maximize energy recovery by the expansion turbine. There is an economizer after the turbine for final heat recoveiy. A simplified schematic is presented in Fig. 27-49. An 80-MWe demonstration plant, operating at 1.2 MPa (180 psia), began operation in 1989 with a power producdion intensity of 3 MWe/m (1 MWe/3.5 fU). By 1996, five units of this size had been construcded, and a 320-MWe unit is planned to commence operation in 1998. 

Step 2. System calculations over the entire range of possible operating conditions are required. The range must cover from the air blower minimum flow point to the expander bypass point for all reasonable variations in the applicable parameters of COj/CO ratio, fresh feed rate, flue gas temperature, ambient air temperature, and so forth. 

Actual flue gas temperature (°C) Figure 30.3 Flue gas losses - natural gas... 

The regenerated catalyst supplies enough energy to heat the feed to the riser outlet temperature, to heat the combustion air to the flue gas temperature, to provide the endothermic heat of reaction, and to compensate for any heat losses to atmosphere. The source of this energy is the burning of coke produced from the reaction. 

Increase the temperature of the combustion air from the blower discharge temperature to the regenerator flue gas temperature... 

Heat to raise the coke products from the regenerator dense temperature to flue gas temperature... 

It is important that combustion of the coke in the spent catalyst occur in the dense bed of catalyst. Without the catalyst bed to absorb this heat of combustion, the dilute phase and flue gas temperatures increase rapidly. This phenomenon is called afterburning. It is critical that spent catalyst and combustion/lift air are being introduced into the regenerator as evenly as possible across the catalyst bed. It is also important to note that vertical mixing is much faster than lateral mixing. 

Air heaters should not extract so much heat that the flue-gas temperature is reduced below its dewpoint because moisture will condense out of the flue gas and corrosion may occur. 

Economizers are one of several types of FW heaters, all of which are designed to provide thermodynamic gains in the steam cycle. They typically are located in the exit gas system, where their use improves overall boiler efficiency, which tends to increase by 1 % for every 40 to 50 °F (22-28 °C) reduction in flue gas temperature... 

Boiler operational management processes provide for various daily procedures, maintenance routines and checks, including attending to BD, ash removal (where solid fuels are used), and the monitoring and control of fuel and MU water consumption, steam production, operating pressures, air and flue-gas temperatures, and FW and CR flows. 

Recorders measure steam flow, air flow, FW flow, drum water level, steam temperatures, flue gas temperatures, flue gas analysis and smoke opacity. 

Fired heaters radiant rate, 12,000 Btu/(hr)(sqft) convection rate, 4000 cold oil tube velocity, 6 ft/sec approx equal transfers of heat in the two sections thermal efficiency 70-75% flue gas temperature 250-350°F above feed inlet stack gas temperature 650-950°F. 

Catalyst cost constitutes 15-20% of the capital cost of an SCR unit therefore, it is essential to operate at temperatures as high as possible to maximize space velocity and thus minimize catalyst volume. At the same time, it is necessary to minimize the rate of oxidation of S02 to S03, which is more temperature sensitive than the SCR reaction. The optimum operating temperature for the SCR process using titanium and vanadium oxide catalysts is about 38CM180oC. Most installations use an economizer bypass to provide flue gas to the reactors at the desired temperature during periods when flue gas temperatures are low, such as low-load operation. 

Also the thermohydrolysis of the urea solution after the injection into the hot exhaust gas upstream of the SCR catalyst has been investigated at the diesel test rig. Urea solution was atomized about 3 m upstream of the SCR catalyst into the hot exhaust equivalent to a residence time in the pipe section of 0.1 s at 440°C. As expected for the thermolysis reaction, ammonia and isocyanic acid were found at the catalyst entrance at all temperatures (Figure 9.3). The 1 1 ratio of both components shows that only the thermolysis but not the hydrolysis is taking place in the gas phase. It can also be seen that the residence time of 0.1 s is not sufficient for the quantitative thermolysis of urea, as appreciable amounts of undecomposed urea were always found. The urea share even raises with lowering the flue gas temperature, although the residence time... 

Yet thermal conductivity alone is not sufficient for the characterization of gaseous mixtures. Given a fixed air ratio and temperature, thermal conductivity of flue gases, resulting from the combustion of different fuels, does not vary by more than 1%. A changing air ratio has a smaller effect than a small rise in the flue gas temperature. Therefore the thermal conductivity alone is not suitable as reference value. Further information is required to identify fuel gases. 

The problem solved is to find the number of steps along the axis required to make the average conversion x - 0.5. Several values of jacket temperature and heat transfer coefficient are employed. Also investigated is the effect of taking the inlet temperature at the wall either the same as elsewhere at the entrance or the average with the flue gas temperature, that is. 

Reformers are fired to maintain a required process gas outlet temperature. Most modern reformers are top fired. In a top-fired reformer, the burners are located at the top of the furnace and fire downward. Process gas flows downward through catalyst-filled tubes. This flow of process gas and flue gas allows the highest flue gas temperature when the in-tube process gas temperature is lowest and the lowest flue gas temperature when the in-tube process gas temperature is highest. This results in tube-wall temperatures that are uniform over the tube s length and since the average tubewall temperature is lower this reduces tube cost and increases tube life. 

FIG. 17-68 Trends in resistivity of fly ash with variations in flue-gas temperature and coal sulfur content. °C = (°F — 32) x %. (Oglesby and Nichols, A Manual of Electrostatic Precipitator Technology, part II, Southern Research Institute, Birmingham, Ala., 1970.)... 

Flue gas temperature entering high-temperature side heat exchanger 3500°F... 

---