Brayton cycle

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Graph Brayton Cycle

Figure 10.4 Temperature vs entropy for Brayton cycle. (On this graph areas represent energy.)


Figure 10.4 Temperature vs entropy for Brayton cycle. (On this graph areas represent energy.)

and operational costs can be quite expensive. The main pieces of equipment, boilers, steam turbines and eventually huge condensers are very expensive in terms of equipment itself and installation costs. Operational costs range high also, because of manpower and chemicals, since each energy unit produced requires a significant amount of personnel and additional materials. Brayton cycle cogenera-tion systems, with the same power capacity of an equivalent Rankine, may have lower capital and maintenance costs. Boilers and steam turbines will be built to order while gas turbines are normally package equipment and although with a reasonable standardization, final arrangement and installation are much likely to be specific for each location.

Concerning aspects of flexibility between heat and power demand, depending on the design and operational conditions of each industry, neither a pure Rankine nor a pure Brayton cycle may comply with its energy requirements, across the full range of possibilities, at maximum efficiency. An option to be considered is the use of a combined cycle, using the high temperature exhaust heat from the Brayton cycle gas turbine by sending it into a heat recovery steam generator to produce steam for a Rankine cycle (Figures 10.5 and 10.6). This combined cycle can achieve higher efficiencies than each cycle alone, allowing a much greater flexibility over the type of fuel used and heat-to-power ratio.

Waste Heat Recovery Using Brayton Cycle
Figure 10.5 Schematic combined cycle.

Compared with purchased power and the operation of on-site boilers to provide heat, cogeneration can be considered the biggest energy efficiency opportunity for any industry. Typically, a plant with cogeneration will require 25% less primary energy compared with separate heat and power supplies. This reduced fuel consumption is the main economical and environmental benefit of cogeneration, because a plant' s energy requirements are attained more efficiently with fewer emissions. The possibility of using residual products and waste materials as energy source, increases cost effectiveness while reducing the need for waste disposal or treatment.

Cogeneration is certainly not a low cost energy efficiency option, especially considering capital costs, but it can be very cost effective, particularly in the case of a grassroots project or a system replacement. Accurate planning is necessary due to capital and operational costs, design and operational complexity of cogen-eration. From this approach, any medium-to-large-scale industry, that has significant power and thermal energy demands, should consider and investigate cogeneration.

Small-scale plants cannot be discarded from this consideration, and other cycles based on internal combustion engines like Otto and Diesel should be taken into account, particularly if power demand is much higher than heat requirement. These cycles have not been addressed here because of the limited power output they provide, but the general concepts apply just as in Rankine and Brayton cycles.

Combined cycle

Rankine Brayton Cycle

Figure 10.6 Temperature vs entropy for combined cycle. (On this graph areas represent energy.)


Figure 10.6 Temperature vs entropy for combined cycle. (On this graph areas represent energy.)

Main Pieces of Equipment Boilers

The average equipment called a boiler is composed of a specialized radiant and / or convection tube heat exchanger linked to a drum that accumulates water and steam. Separation between steam and water occurs inside this drum that has accessories to provide humidity removal from steam. The heat source is placed internally to the tube heat exchanger and, for most designs it is a furnace, where combustion heat is to be transferred to water until it becomes heated water or steam.

The configuration may vary depending upon energy source, steam pressure and load, fuel availability, water quality, emissions restrictions, reliability etc. Concerning water passage, it can be a fired tube boiler where water is in the drum and the heat source is immersed in it, passing inside the tubes. On a water tube boiler, the heat source is wrapped in water tubes, in order to direct released heat to the water, obtaining maximum efficiency. Alternatively, the energy source may be waste heat from another process, like flue gas from a gas turbine or a process heater, configuring a recovery boiler.

For mechanical reasons, fired tube boilers have restrictions on maximum allowable pressure and load, being generally used for relatively small steam demands and low to medium steam pressures. For economic reasons, most fired tube boilers are 'packaged' type equipment, being supplied in standard sizes and ready to be mounted and connected. Water tube boilers have almost no load or pressure restrictions, being selected when the steam demand and pressure requirements are high. Usually, these boilers have some basic design standardization, but capacity, maximum operational pressure and water treatment requirements end up determining the final project customization for each application.

Normally, in the average boiler, the heat source is a furnace, where a fuel is being burned, with air supplied by a forced, induced or balanced draft system. Similarly to process fired heaters, mentioned in the previous chapter, the same warnings for energy efficiency apply. Among these points of attention are emphasized:

• Reducing flue gas losses by keeping proper air-to-fuel ratio, aiming to minimize excess air, but providing enough air to avoid unburned fuel.

• Assuring sealing in balanced draft system to avoid air infiltration or leakage. Leakage implies direct heat loss to the atmosphere while infiltration may cause flue gas oxygen content misreading, jeopardizing air-to-fuel ratio adjustment.

• Diminishing convection and radiation losses by maintaining and improving boiler thermal insulation.

• Improving heat recovery by preventing heat transfer surface fouling through soot blowing and combustion control, either in economizers and air preheaters.

On the water-side, best practices for efficient boiler design and operation are linked mainly to water treatment quality. Assuring a water quality compliant to the boiler specification reduces scale in water-side, maintains heat transfer efficiency and helps to keep surface blowdown in an acceptable range. Recovery of surface blowdown heat by exchangers and flash steam is a good option. Flash steam is explained later in this chapter.

Just reprising a issue stressed in the previous chapter, monitoring and tracking mass and energy balance around a boiler is basic, not only to identify losses. Since these are the individual main energy converters for the industry, boiler efficiency tests tell a lot about energy efficiency in the whole complex. Any inefficiency in a boiler is passed to all other energy consumers in the site and nothing can be done by process equipment to compensate these losses.

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