Loads fuel-fired

Gas transport properties for the products of combustion of the common fuels, fired at normal excess air at or nearfull boiler load, may be obtained from Tables 23.1-23.4. Non-luminous gas radiation has a small overall effect in the convective section, typically 2-5 per cent of total convection. It may therefore be neglected for a conservative calculation. 

In 1977, two fully loaded Boeing 747 commercial aircraft crashed into each other on a foggy runway in the Canary Islands. This accident was then the worst in aviation history and took 583 lives. An inquiry concluded most of the deaths in the Canary Islands accident resulted from the aviation fuel fire that lasted for more than 10 hours. G. Daniel Brewer, who was the hydrogen program manager for Lockheed, stated that if both aircraft had been using liquid hydrogen as fuel instead of kerosene, hun-... 

If a load is heated electrically— by actually using the load as a resistance in a circuit or by induction heating—the flux lines will concentrate just inside the surface. In fuel-fired heating processes, heat enters the load through its surface (by radiation or convection) and diffuses throughout the piece by conduction. This heat flow requires a difference in temperature within the piece. Steady heat flow through a flat plate is described by ... 

Rg. 2.21 The many concurrent modes of heat transfer within a fuel-fired furnace. Some refractory surfaces, r, and charged loads, c, are convection-heated by hot poc flowing over them. Triatomic molecules of the combustion gases, g, and soot particles, p, radiate in all directions to refractories, r and loads, c. The surfaces of r and c in turn radiate in all possible directions, such as r to r, r to c, c to c, and c to r. 

The soot particles are confined within the visible flame. The triatomic molecules are everywhere within the furnace, but can absorb and emit radiation only within narrow wavelength bands. Interference among the several modes of heat transfer can make calculation of net heat transfer in a fuel-fired furnace difficult. Some of the many variables that must be considered are composition, velocity, temperature, and beam thicknesses of the poc and well as emissivities, absorptivities, conductivities, densities, and specific heats of the refractory wall and load materials. 

In fuel-fired furnaces, heat release rate is usually expressed in heat units liberated per unit of furnace volume in unit time, commonly in Btu/ft hr or MJ/m hr. Closely related to rate of furnace heat release is the combustion volume or flame volume. Generally, the furnace volume should be at least equal to the sum of the maximum flame volume and the maximum load volume. The volume of the flame is a function of the combustion intensity condition discussed with table 3.1 subsequently, and where Fc is a configuration factor to assure that all of any one flame s volume is contiguous. 

With good design and operation, fuel-fired furnace efficiencies of 60% or higher can be had, depending much on process temperature. Efficiency here is the ratio of heat input into the load/hr to the gross heat released by the fuel used/hr. The Glossary compares efficiency terms. When comparing costs, always ask for clarification as to what is meant by efficiency. ... 

Regenerative burners and oxy-fuel firing lack mass flow to load bottoms in pits, therefore increasing top-to-bottom temperature differentials from 40°F to 100°F (22°F to 56°C). (See sec. 7.4.6.) B = batch. C = continuous. He = hot charge. Hr = heat recovery. Rec = recuperative. Reg = regenerative, longs = billets, blooms, pipe, rails, and structurals (but not rounds or short pieces). 

Example (a) In a one-way, top-fired soaking pit without spin, control of its poc will have an end-to end temperature difference of about 175°F (97°C) at the time when the load is expected to be reliable, but with oxy-fuel firing and its lower mass circulation, the corresponding end-to-end temperature difference might be 400°F (222°C) or more. 

AFBC is a commercial technology that is widespread across the world. Coal-fueled AFBC competes with conventional stoker-fired boilers, with practical capacity of 20,000 to 300,000 MJ/h fuel energy input, all the way up to the lower end of base-load pulverized fuel-fired boilers, having practical capacities of 250,000 to 9,000,000 MJ/h. Among the many claimed practical advantages of AFBC are the following ... 

At partial loads on aU units, regardless of the fuel fired, it is necessary to increase the excess air as the load is reduced. Burner dampers are designed not to close tightly in order to permit the air to protect the idle bumer(s) from overheating by radiant heat from nearby operating burners. 

NO, emissions from power stations vary with the type of boiler as well as with other factors such as fuel type, boiler heat release rate, excess air, and mode of operation (e.g., cyclic vs. base load). Coal-fired boilers typically produce more NO, than oil- or gas-fired units. Wall-fired wet bottom and cyclone-fired boilers usually have the highest NO, emissions, while tangentially-fired and arch-fired boilers have the lowest. 

Electric power generation using biomass as a fuel is economic in situations where the cost of the fuel is competitive with that of fossil fuels. The cost of a commercially available biomass steam—electric power plant is about 1500/kW for a wood-fired facility. If wood can be obtained at a cost of 2.00/GJ ( 2.10 X 10 /Btu), the total cost of power for base-load operation would be about 0.05/kWh. If wood or agricultural wastes are available at... 

The above simple analysis has to be modified for a supplementary fired CHP plant such as that shown in Fig. 9.3c, meeting a unit electrical demand and an increased heat load A. The reference. system fuel energy supplied is now... 

Electrical power output Heat output (normal load) (with supplementary firing) Gas fuel energy supply Thermal efficiency... 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Since the early 1960s, advanced steam conditions have not been pursued. In the 1960s and early 1970s there was little motivation to continue lowering heat rates of fossil-fired plants due to the expected increase in nuclear power generation for base-load application and the availability of relatively inexpensive fossil fuel. Therefore the metallurgical development required to provide material X for advanced steam conditions was never undertaken. 

Due to interruptible gas tariffs, it is often necessary to adopt gas as the primary fuel and burn oil in periods of peak loads. This means that the economizer has to be arranged so that when oil firing, the flue gases are bypassed around the economizer. The bypass duct must also contain a damper to simulate the economizer gas resistance so that the burner back pressure remains the same for both fuels. Figure 25.6 shows a typical installation layout. 

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