Waste heat, available

Energy Fuel consumption is lower for the ACO process relative to a steam cracker. However, steam import is higher since there is less waste heat available for recovery. 

Advanced Reforming Technology. The advanced reforming technology (ART) reformer concept (31) developed by KTI SpA (now Technip) is a convective reformer and is characterized by utilizing up to the maximum extent of the waste heat available in the steam-reforming process for the process itself and not for the production of export steam. This results in a reduced operating and investment costs. 

Z. Evaporation. If the wastewater is in low volume and the waste material involatile, then evaporation can be used to concentrate the waste. The relatively pure evaporated water might still require biological treatment after condensation. The concentrated waste can then be recycled or sent for further treatment or disposal. The cost of such operations can be prohibitively expensive unless the heat available in the evaporated water can be recovered. 

The inability to produce high-quality effluents is one significant disadvantage. Another disadvantage is that anaerobic processes must be maintained at temperatures between 35 and 40°C to get the best performance. If low-temperature waste heat is available from the production process, then this is not a problem. 

Vapor-Compression Evaporation and Waste Heat Evaporation. Both of these processes remove water from contaminants rather than contaminants from water. They are better suited for industrial installations where excess energy is available. The water thus produced is of high quaUty and can be used directly. An important advantage is the concentration of waste-residue volume with attendant economies of handling and transportation... 

The waste heat recovered in the boilers is in the form of steam and used either to generate electricity or to produce compressed air. The latter is used in the converter tuyeres. If electricity is generated, it is in turn used to produce compressed air for the converters. In practice, the energy made available in the waste heat boilers is almost equal to the energy required for producing the converter air. 

When a large flow of gas is reduced from a high pressure to some lower pressure, or a high-temperature process stream (waste heat) is available at moderate pressures, turboexpanders should be considered to recover the energy (see Figure 7-22). The turbine can drive a compressor... 

Lower exhaust temperature reduces available waste heat. 

I. Substantial exhaust heat available for waste heat recovery. 

These delicate engines provide value as educational tools, but they immediately inspire curiosity into the possibility of generating power from one of the many sources of low temperature waste heat (less than 100°C) that are available. A quick look at the Carnot formula shows that an engine operating with a hot side at 100°C and a cold side at 23°C will have a maximum Carnot efficiency of [((373 K—296 K)/373 K) X 100] about 21 percent. If an engine could be built that achieved 25 percent of the possible 21 percent Carnot efficiency it would have about 5 percent overall Carnot efficiency. 

As with the electrical load profile, it is also necessary to analyze the heat load over the daily and annual cycles. Ideally, the heat load will match the available heat from the electrical generator (however, this is rarely the case). There will be periods when supplementary output will be necessary which can be achieved by, say, supplementary firing the waste heat gases of a gas turbine, or heat output reduction is necessary by the introduction of bypass stacks. For a steam turbine installation bypass pressure-reducing valves will be necessary to supplement steam output, while a dump condenser may be needed at low-process steam demands. The nature of the electrical and heat load will obviously have significant influence in the development of the scheme and scope of equipment. 

Figure 15.27 Total heat load. The heat available from the diesel waste gases of 13 x 10 kWh will provide only a proportion of the heat required for the absorption-type chiller...

Gas turbines are available with power outputs of 1 MW upwards, and the exhaust is used to fire waste-heat boilers. The high oxygen content of the exhaust enables supplementary firing to be used to increase the heat/power ratio as desired. 

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. 

If low-cost natural gas is available, a gas turbine can be used to generate power. In this case, the waste heat in the exhaust gas is used to produce steam in a heat recovery boiler (HR boiler). This approach also is used with some gas turbine plants (as in some high-speed navy vessels). Where an HR boiler is employed, if steam demand exceeds power demand, the boiler is fitted with auxiliary burners. 

Avoid using unlagged condensate lines because much of the heat available will simply be wasted to the air rather than returned to the boiler FW system. 

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