Expansion turbine

The expansion turbine converts the dynamic energy of the flue gas into mechanical energy. The recoverable energy is determined by the pressure drop through the expander, the expander inlet temperature, and the mass flow of gas (66). This power is then typically used to drive the regenerator air blower. 

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. 

Judson S Swearingen/ Ph D / Retired President, Rotoflow Corporation. (Expansion Turbines)... 

By common usage, the terms turboexpanders and expansion turbines specifically exclude steam turbines and combustion gas turbines, which are covered elsewhere in Sec. 29. 

Turboexpanders are expansion turbines, rotating maehines similar to steam turbines. Commonly, the terms expansion turbines and turboexpanders speeifieally exelude steam turbines and eombustion gas turbines. Turboexpanders (Figure 1-1) ean also be eharaeterized as modern rotating deviees that eonvert the pressure energy of a gas or vapor stream into meehanieal work as the gas or vapor expands through the turbine. If ehilling the gas or vapor stream is the main... 

In praetieal terms, the applieation of expansion turbines depends on the relationship between the possible gain of meehanieal energy and the required investment eost. Typieal eommereial applieations for turboexpanders inelude ... 

Figure 1-9. Two-stage process gas expansion turbine for a terephtalic acid plant. (Source GHH-Borsig.)...

Gas expansion turbines may embody different designs depending on the proeess media and assoeiated systems. Speeial requirements may pertain to duties sueh as sealing off toxie, flaimuable, eaustie, eoiTosive, erosive, and high-temperature media. These requirements may lead to sealing geometries tliat are eommon to mrbines and turboeompressors. 

The efficiency of expansion turbines (partial admission axial, full admission axial, and radial inflow turbines) is a function of the following four basic parameters. 

For the preliminary estimate of the expected efficiency of expansion turbines, in most cases it is sufficient to neglect Reynolds number effects (Rg > 10 ) and use the efficiency and specific speed correlations shown in Figure 2-12 for partial admission axial impulse, reaction radial inflow and full admission impulse and reaction axial turbines. Due to the economic advantage of the radial turbine, die radial inflow turbine is die best selection when operating in die specific speed range 20 < Nj < 140, whereby die optimum efficiency will be achieved at N, = 80. 

Figure 2-12. Efficiency of various expansion turbine types as a function of specific speed,...

As mentioned earlier, a variety of turbomaehines are required to produee nitrie aeid. In the ease of small plants, these maehines represent a substantial investment. A eombination of proven air eompressor modules with newly developed expansion turbines and nitrous gas eompressors on a single gearbox provides a new, low-eost alternative and gives the plant owner/eontraetor useful potential for faeulty optimization (Figures 4-36 through 4-38). 

Agahi, R. and Ershaghi, B., Expansion Turbine in Energy Recovery Applications, XVIIIth International Congress of Refrigeration, August 1991. 

The nitrogen is obtained as a byproduct of an oxygen production process (1,200 tpd of oxygen) in a dedicated oxygen plant. In diis plant, parallel expansion turbines are used dial were designed especially for die process. The first expansion turbine train (two... 

Although operating in a dry, hot, and dusty desert environment, eompressor and expansion turbines must perform reliably if the Chuqui mine is to remain eompetitive in eopper and molybdenum produetion. 

Expansion turbines—three in Phase 1 and two in Phase 2—cool the gas to -90°C, at which point more liquid drops out in another separator. In this system, the cold methane gas emerges from the expansion turbines at a reduced pressure of 325 psi and is recirculated to the original heat exchanger to reduce the temperature of the natural gas entering the plant. 

The methane warms to 10°C. It then passes through the booster compressors on the expansion turbine shaft, increasing in pressure from 325 psi to 375 psi before being introduced into other gas compressors tliat boost the pressure back up to 600 psi. This is the pressure needed for reintroduction of the natural gas back into the TransCanada pipeline. This 50 psi boost, which makes use of available energy from the expansion turbines, provides a significant savings in electrical power. 

Eollowing initial installation in 1971, the Phase 1 expansion turbines have proven extremely reliable. The redesign effort of Phase 2 in 1992 incorporated improved aerodynamics and changed inlet guide vane profiles in the expansion turbines. This redesign yielded an additional. 40 to. 50 Bcf of gas per year without any increase in recompression horsepower. This translates to an increase in propane and butane production of an additional 3,600 to 4,500 bbl of liquid without an increase in electrical power consumption. 

The Brayton cycle in its ideal form consists of two isobaric processes and two isentropic processes. The two isobaric processes consist of the combustor system of the gas turbine and the gas side of the HRSG. The two isentropic processes represent the compression (Compressor) and the expansion (Turbine Expander) processes in the gas turbine. Figure 2-1 shows the Ideal Brayton Cycle. 

An expansion turbine (also called turboexpander) converts gas or vapor energy into mechanical work as the gas or vapor expands through the turbine. The internal energy of the gas decreases as work is done. The exit temperature of the gas may be very low. Therefore, the expander has the ability to act as a refrigerator in the separation and liquefaction of gases. 

TTiere are two main types of expansion turbines axial flow and radial flow. Axial flow expansion turbines are like conventional steam turbines. They may be single-stage or multistage with impulse or reaction blading, or some combination of the two. Turbines of this type are used as power recovery turbines. They are used where flow rates, inlet temperatures, or total energy drops are very high. 

Radial-flow expansion turbines are normally single-stage, with combination impulse reaction blades and a rotor resembling a centrifugal... 

Expansion Turbine, 296-300 applications, 300 condensation, 299 operating limits, 297 power recovery, 297 refrigeration cycles, 298 reliability, 480 types. 206 Experience... 

A feature of this cycle is the reduction in compressor air flow for the same size of main expansion turbine. The figure shows air for the PO turbine taken from the discharge of the main compres.sor, but it may be taken straight from atmosphere. Note also that steam is raised for injection into the PO reactor and Newby et al. suggested that some of the steam raised in the HRSG may also be used to cool the PO turbine. The chemical reactions for the PO reactor of this case were described in Section 8.5.3. 

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