Waste-heat recovery

The policy for waste heat recovery from the flue gas varies between incinerator operators. Incinerators located on the waste producer s site tend to be fitted with waste heat recovery systems, usually steam generation, which is fed into the site steam mains. Merchant incinerator operators, who incinerate other people s waste and... 

Owing to the cycHc nature of the TBRC operation, waste heat recovery from the off-gases is not practical and the SO2 content of the gas varies with the converter cycle. In order to supply a relatively uniform flow and strength SO2 gas to a sulfuric acid plant, a system has been installed at RonnskAr whereby the SO2 from fluctuating smelter gases is partially absorbed in water. During smelter gas intermption, SO2 is stripped with air and the concentrated gas deflvered to the acid plant. 

Noncatalytic partial oxidation of residual fuel oil accounts for the remainder of world methanol production. Shell and Texaco ate the predominant hcensors for partial oxidation technology (16) the two differ principally in the mechanical details of mixing the feedstock and oxidant, in waste heat recovery, and in soHds management. 

Many units have waste heat recovery systems that generate low pressure steam from reaction heat. Such steam is often employed to drive adsorption refrigeration units to cool the reactor feed stream and to increase polymer conversion per pass, an energy-saving process that reduces the demand for electrical power. 

Multiple-hearth roasting offers ease of operation, abiUty to handle a wide variety of ores or blends, and Httle downtime. On the other hand, these furnaces are no longer being built because of their high capital and labor costs, relatively low sulfur dioxide off-gas, need for added fuel, and marginal opportunity for waste-heat recovery. 

The first stage assures high carbon conversion and optimum slag removal. The second stage reduces the raw product gas temperature to about 1000°C. This latter step helps to improve cold gas efficiency relative to other slurry fed processes and to lower waste heat recovery costs (15—18). 

FIG. 27-57 Diagram of a metallic radiation recuperator. (From Goldstick [Pg.2408]

Use energy-efficient measures such as waste heat recovery from process gases to reduce fuel usage and associated emissions. 

For PM control from combustion sources, the tlue gas enters a coagulation area (e.g., ductwork, a chamber, or a cyclone) to reduce the number of ultrafine particles, and then a gas conditioner to cool the gas to a suitable temperature and saturation state. This is generally accomplished by means of a waste heat recovery heat exchanger to reduce the temperature of the flue gas or by spraying water directly into the hot flue gas stream. 

The design of heaters and waste heat recovery units is beyond the scope of this book. Sizing and design are best left to manufacturers. However, the concepts discussed in this chapter and in Chapter 2 can be used to verify the manufacturer s proposals. 

Turbine drive easily adapted to waste-heat recovery for high fuel efficiency. 

I. Substantial exhaust heat available for waste heat recovery. 

Etliylene production involves liigh temperatures (1500°F) in tlie pyrolysis section and cryogenic temperatures in tlie purification section. The feedstocks, products, and by-products of pyrolysis are flaimnable and pose severe fire liazards. Benzene, wliich is produced in small amounts as a byproduct, is a known carcinogen. Table 21.7.1 summarizes some of the properties of etliane (feedstock) and tlie product gases. Figure 21.7.1 shows a simplified schematic diagram of the pyrolysis and waste heat recovery section on an etliylene plant. 

Heat recovery tor steam generation, pre-heating combustion air, and high efficiency burners Adjustable speed drives, heat recovery coke oven gases, and dry coke quenching Efficient hot blast stove operation, waste heat recovery for hot blast stove, top gas power recovery turbines, direct coal injection... 

Figure 11 -59. Representative expander refrigeration systems for temperature requirements and/or power recovery. (A) Expander coupled to a generator recovers pressure-loss energy for conversion to electric power. (B) System for power recovery from reactors in chemical processing plants. (C) Typical waste-heat recovery system using expansion turbines to generate electrical power. (Used by permission Bui. 2781005601. Atlas Copco Comptec, Inc.)...

Dual pressure For comparison, a combined cycle scheme with dual pressure is shown in Figure 15.13. In this case, the waste heat recovery boiler also incorporates a low-pressure steam generator, with evaporator and superheater. The LP steam is fed to the turbine at an intermediate stage. As the LP steam boils at a lower temperature than the HP steam, there exists two pinch points between the exhaust gas and the saturated steam temperatures. The addition of the LP circuit gives much higher combined cycle efficiencies with typically 15 per cent more steam turbine output than the single pressure for the same gas turbine. 

Figure 15.19 Diesel with waste heat recovery...

As discussed earlier (Section 15.3), steam turbines providing low-pressure steam are more established for CHP. However, for lower heat loads the diesel and gas turbine with waste heat recovery offer an attractive alternative. 

For flexibility, supplementary firing of the oxygen rich exhaust gases can give additional heat. The following summarizes the potential of both the diesel and GT for waste heat recovery ... 

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... 

Furnaces are large users of energy, and in order to reduce costs, such equipment should be well insulated, used to maximum capacity and most of the waste heat in both the flue gases and product recovered. It should be possible to recover the waste heat in the flue gases down to at least 200°C. Specialist equipment for such waste heat recovery is available in the form of recuperators and regenerators. 

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

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