Gas high-temperature flue

Another very important quality characteristic is the combustion efficiency (fuel efficiency) of radiant tubes. The efficiency strongly depends on the flue gas outlet temperature of radiant tube systems. In order to achieve maximum efficiency, air preheating systems use the enthalpy of the flue gas for preheating the combustion air simultaneously reducing the flue gas outlet temperature of such systems. High efficiencies with less flue gas losses result in lower fuel consumptions and lead to a reduction of the CO2 emission per ton of product. 

A2. Because fuel costs are much higher in high-temperature furnaces than in lower temperature furnaces as a result of the higher flue gas exit temperature causing higher stack loss. 

Tat flue gas exit temperature will always be higher than the furnace temperature at the flue because otherwise heat would not flow from the furnace gases to the walls and loads. Accurate measurement of flue gas exit temperature can be difficult. A high-velocity thermocouple with several radiation shields is essential. Figure 5.3 helps estimate the temperature elevation of the exiting gases above the furnace temperature. The sum of the furnace temperature and this elevation is the temperature that should be used to enter the bottom scale of available heat charts 5.1 and 5.2 to determine the %available heat. 

The loss caused by sensible heat in the flue gases (stack loss) can be evaluated as the %net heating value (90% for natural gas) minus the %available heat at the flue gas exit temperature, from Figure 5.1. At high temperature, the loss becomes excessive, especially with high excess air thus, such cases give payback by using heat recovery. 

Example 7.1 Given A car furnace (batch) 10 x 20 x 9 high inside is to heat 40 tons of steel loads from 60 F to 2250 F at a rate of 250°F per hour. Specific heat of steel, fromp. 275 of reference 52 is 0.165 Btu/lb F. Average flue gas exit temperature will be 2200 F. The fuel will be natural gas with 10% excess air. Average losses, in Btu/ft hr are roof 900, walls 500, door 1100, and car 600. 

The Alkalized Alumina process was developed by the U.S. Bureau of Mines and carried through the pilot-scale testing phase (Bienstock et al., 1964 1967). It has not been applied commercially. The process uses dawsonite [NaAl(C03)(OH)2]/sodium aluminate [NaA102] as the sorbent. Ihe material is activated at 1,200°F to form a high surface area, high porosity, dry solid, which removes sulfur dioxide from flue gas at temperatures between 300° and 650°F. This process is no longer being pursued due primarily to an excessive sorbent attrition rate. 

High preheat temperatures for feed and combustion air lead to reduced fuel consumption and thereby to savings, especially when the steam production in the plant must be minimized. Reduction of the flue gas stack temperature reduces the heat loss to the atmosphere. A similar effect has, perhaps more importantly, been obtained by improved insulation in the reformer. 

Example 6.4 The process in Fig. 6.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C, and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume = 10°C for process-to-process heat transfer but = 30°C for flue-gas-to-process heat transfer. A high value for for flue-gas-to-process heat... 

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. 

Some PFBC boiler designs incorporate high-temperature, high-pressure (HTHP) filter devices in the flue-gas stream. These are installed primarily to protec t the gas turbine from erosion damage by the fine particles that escape the cyclones, but as the filters remove virtually all the suspended particulates, they also eliminate the need for back-end removal. The commonest HTHP filter elements used are rigid ceramic candles. 

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. 

Prevention and Control Hardware Equipment and technology descriptions Dry and wet control hardware NO, control and application of pollution prevention technologies Flue gas scrubbing and control technologies Incineration and high temperature technologies... 

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