The water from the deaerator being returned to the boiler generally has a temperature of approximately105 °C. The water in the boiler at a higher pressure is at a higher temperature. The steam boiler is fed with water to replace system losses and recycle condensate, etc. Heat recovery is possible by preheating the feed-water, thus reducing the steam boiler fuel requirements.
The preheating can be done in four ways:
•using waste heat (e.g. from a process): feed-water can be preheated by available waste heat, e.g. using water/water heat exchangers
•using economisers: an economiser ((1) in Figure 3.7) is a heat exchanger which reduces steam boiler fuel requirements by transferring heat from the flue-gas to the incoming feed-water
•using deaerated feed-water: in addition, the condensate can be preheated with deaerated feed-water before reaching the feed-water container ((2) in Figure 3.7)). The feed-water from the condensate tank ((3) in Figure 3.7)) has a lower temperature than the deaerated feed-water from the feed-water container ((2) Figure 3.7)). Through a heat exchanger, the deaerated f eed-water i s cooled down further ( the heat is transmitted to the feed-water from the condensate tank). As a result, the deaerated feed-water forwarded through the feed-water pump is cooler when it runs through the economiser ( (1) in Figure 3.7)). It thus increases its efficiency due to the larger difference in “ and reduces the flue-gas temperature and flue-gas losses. Overall, this saves live steam, as the feed-water in the feed-water container is warmer and therefore less live steam is necessary for its deaeration
•installing a heat exchanger in the feed-water stream entering the deaerator and preheating this feed-water by condensing the steam used for stripping ( see Section 3.2.8 for details of deaeration).
The overall efficiency can be increased through these measures, that is, less fuel energy input is required for a certain steam output.
Possible disadvantages of these four possibilities are that more space is required and their availability for industrial facilities decreases with rising complexity.
In some existing plants, feed-water preheating systems can only be integrated with difficulty. In practice, feed-water preheating with deaerated feed-water is applied only rarely.
In high output plants, feed-water preheating through an economiser is standard. In this context, however, it is possible to improve the efficiency of the economiser by up to 1 % by increasing the temperature difference. Using waste heat from other processes is also feasible in most installations. There is also potential to use it in lower output plants.
The amount of energy savings potential by implementing economiser feed-water preheating depends on several conditions such as local system requirements, condition of the stack or flue- gas quality. The payback for a particular steam distribution system will depend on the operating hours, the actual fuel price and the location.
In practice, the possible savings from feed-water preheating amount to several percent of the steam volume generated. Therefore, even in small boilers the energy savings can be in the range of several GWh per year. For example, with a 15 MW boiler, savings of roughly 5 GWh/yr, some EUR 60000/yr and about 1000 tonnes CO2/yr can be attained. The savings are proportional to the size of the plant, which means that larger plants will see higher savings.
Boiler flue-gases are often rejected to t he stack at temperatures of more than 1 00 t o 150 º C higher than the temperature of the generated steam. Generally, boiler efficiency can be increased by 1 % for every 40 º C reduction in the flue-gas temperature. By recovering waste heat, an economiser can often reduce fuel requirements by 5 t o 10 % and pay for itself in less than 2 years. Table 3.7 shows examples of the potential for heat recovery.
一般而言，鍋爐煙氣以高於產出蒸汽100~150 º C的溫度從煙囪排放，煙氣溫度每降低40 º C，鍋爐效率就可提升1%。利用節煤器回收廢熱往往可減少5~10%的燃料需求，如此兩年內就可回收投入成本。
Energy Efficiency (2009) 3.2.5