Waste Heat Recovery General Considerations
Some general considerations are:
1. All waste heat recovery options, if installed, should be done with full consideration of:
– Simplicity of operations
– Ease of maintenance
– Prevention of corrosion
– Proven technology
2. Dewpoint and corrosion of waste heat recovery equipment.
The dewpoint of the flue gas in waste heat recovery is very critical to the efficient operation and maintenance of boilers, fired heaters, gas turbines, and heat recovery steam generators, (HRSG). At temperatures below the dewpoint, sulfuric acid condenses on surfaces and corrodes the metal. At temperatures above the dewpoint, corrosion is not a problem.
Accordingly, each project must select the optimum exit temperature and the appropriate alloys to maximize the efficiency and minimize corrosion.
A guideline showing the temperature at which the acid dewpoint is reached, based on percent weight of the sulfur in the fuel, is shown on Figure 3400-1. It shows the minimum metal temperatures recommended by economizer, air preheater, and boiler manufacturers.
3. Retractable soot-blowing equipment should be installed on all convective heat transfer areas where liquid fuel oils may be fired. Soot-blowing lanes should be appropriately located to be able to clean all tubes in all rows of convective sections.
4. To minimize sulfuric acid corrosion, the temperature of steel surfaces in direct contact with flue gases, should be maintained above the limits referred to in HTR-MS-1350.
5. On all waste heat recovery systems, baffles and bypasses can be used to control temperatures.
6. In all waste heat recovery systems, extended tubes are commonly used on gasside flows to increase the heat absorption.
7. Problems of reduced draft, increased pressure drop, and added pumping or blowing requirements on each of the waste heat recovery streams must be fully evaluated.
8. Year-round availability of the waste heat stream and the cold stream must be evaluated if they are in different plants. How does the second stream get cooled or heated while the other stream is in turnaround or shut down for some unforeseen event? Is there an operating plan that will permit this? Figure 3400-2 illustrates this situation. The heat source is in one plant and the stream receiving the heat is in another. Very few inter-plant exchanges are installed because of the complications caused by outage of one of the streams.
9. Along these lines, there are significant options of integrating process plants and the direct flow of hot feeds between units. This eliminates some intermediate tankage. However, it adds to the complexity and reduces operating
flexibility. For example, there is surplus heat in Isocrackers (exothermic heat of reaction) and Rheniformers (high process fluid heater exit temperatures). This surplus heat would be most useful in the Crude Unit which requires much crude feed heating.