Heat-recovery applications

Expanders have not been the essence of reliability. It is not that the expander design in itself has any significant problems. The problems for the most part seem to be related to the application. Most of the failures have been the result of the expander ingesting foreign substances, such as the catalyst in a catalytic cracking unit heat recovery application. Unlike the expansion section of the gas turbine, the inlet temperature is not as high, therefore, temperature is not a significant factor in reliability reduction. 

Boilers may be used for domestic hot water heating, space heating, waste heat, or chemical recovery. They also may be used for mechanical work, electrical power generation, cogeneration, and innumerable industrial process applications using direct (live) steam or indirect steam (e.g., coil heated) processes. Both FT and WT designs are commonly employed for heat-recovery applications. 

Heat Recovery Equipment. Factors that limit heat recovery applications are corrosion, fouling, safety, and cost of heat-exchange surface. Most heat interchange utilizes shell and tube-type units because of the mgged construction, ease of mechanical cleaning, and ease of fabrication in a variety of materials. However, there is a rich assortment of other heat exchangers. Examples found in chemical plants in special applications include the following. 

In heat recovery applications there can be a large number of feasible plant configurations. After the configuration is optimized based on steady state considerations (which may not be an easy problem), the evaluation of the effectiveness of various control schemes can be performed. The dynamic plant operation must be evaluated in terms of economics, regulation, reliability, and safety over a broad range of operating regimes. 

Clathrates have attracted interest as potential TE materials over the last two decades. ZT as a function of temperature for selected state-of-art clathrates is shown in Fig. 6.2. The relatively high ZT values make clathrates relevant for TE power conversion applications, competitive with other state-of-art materials in terms of their TE performance [8]. The flexibility in synthesizing clathrates with different compositions potentially allows for peak efficiencies over a broad range of temperatures of interest for waste heat recovery applications [1, 5, 7]. 

Axial flow units perform well as natural circulation calandrias. Spiral-plate units are also effective for condensers and for heat recovery applications. 

Figure 32.14. Aerogel-encapsulated TEG for integration into waste-heat recovery applications.

Incineration with heat recovery Energy in the form of steam None Markets for steam required proven in numerous full-scale applications air-quality regulations possibly prohibiting use... 

The cycles in diese power recovery applications are relatively simple. They involve the removal of solids or liquids ahead of the expander, and often the incoming stream is heated so its temperature will not reach its frost point at the expander discharge. This heating also increases the amount of available power. Some examples of this... 

In chemical plants, turboexpanders are used to produce refrigeration for cold box installations. In all except energy recovery applications, there are gas-to-gas heat exchangers downstream of the turboexpander. Figure 6-14 recaps a process flow diagram of a natural gas processing gas plant. 

Plant Type. The determination to have an aero-derivative type gas turbine or a frame-type gas turbine is the plant location. In most cases if the plant is located off-shore on a platform then an aero-derivative plant is required. On most on-shore applications, if the size of the plant exceeds 100 MW then the frame type is best suited for the gas turbine. In smaller plants between 2-20 MW, the industrial type small turbines best suit the application, and in plants between 20-100 MW, both aero-derivative or frame types can apply. Aero-derivatives have lower maintenance and have high heat-recovery capabilities. In many cases, the type of fuel and service facilities may be the determination. Natural gas or diesel no. 2 would be suited for aero-derivative gas turbines, but heavy fuels would require a frame type gas turbine. 

Production of heat in furnaces and boilers Recovery of furnace heat Other Applications... 

Simple-cycle efficiency does not usually mean as much to process users as total-cycle efficiency, because the gas turbine is not usually economic in process applications without some type of heat recovery. Total-cycle efficiency is most important in any economic evaluation. In a cycle with heat recovery, the only major loss that is charged to the cycle is the heat exhausting from the boiler stack. With the good comes the bad. Gas turbine maintenance is generally somewhat higher in cost and should be included in the total evaluation. 

Meunier, F., Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles application to the zeolite-water pair. Heat Recovery Systems, 1985, 5, 133 141. 

Corrective Action Application Fluidized bed incineration has been used to incinerate municipal wastewater treatment plant sludge, oil refinery waste, some pharmaceutical wastes, and some chemical wastes including phenolic waste, and methyl methacrylate. Heat recovery is piossible. 

The advantages of thermal incineration are that it is simple in concept, has a wide application, and results in almost complete destruction of pollutants with no liquid or solid residue. Thermal incineration provides an opportunity for heat recovery and has low maintenance requirements and low capital cost. Thermal incineration units for small or moderate exhaust streams are generally compact and light. Such units can be installed on a roof when the plant area is limited. = The main disadvantage is the auxiliary fuel cost, which is partly offset with an efficient heat-recovery system. The formation of nitric oxides during the combustion processes must be reduced by control of excess air temperature, fuel supply, and combustion air distribution at the burner inlet, The formation of thermal NO increases dramatically above 980 Table 13.10)... 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

The diesel and gas turbine with waste heat recovery are limited in terms of fuel application being suitable for gas and oils only. Also when considering oil fuel firing... 

The present vertical boiler is generally used for heat recovery from exhaust gases from power generation or marine applications. The gases pass through small-bore... 

A recent development in heat recovery has been the heat tube. This is a sealed metal tube which has been evacuated of air and contains a small quantity of liquid which, for boiler applications, could be water. When heat from the flue gases is applied to one end of the heat pipes the water in the tube boils, turning to steam and absorbing the latent heat of evaporation. The steam travels to the opposite end of the tube which is surrounded by water, where it gives up its latent heat, condenses and returns to the heated end of the tube. Batteries of these tubes can be arranged to form units, usually as a water jacket around a section of a flue. 

Apart from general industrial applications, FT boilers also are sometimes used for special process purposes such as waste heat recovery. 

Merrigan, M., Dunwoody, W., and Lundberg, L., Heat Pipe Development for High-Temperature Recuperator Application, ,/ Heat Recovery Sy St., 2(2) 125-135 (1982)... 

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