Feed heating

We describe a parametric point calculation of the efficiency of a simple CCGT plant, firstly with no feed heating. It is supposed that the main parameters of the gas turbine upper plant (pressure ratio, maximum temperature, and component efficiencies) have been specified and its performance (t)o)h determined (Fig. 7.3 shows the T,s diagram for the two plants and the various state points). 

This is essentially the approach adopted by Ruffi [9] in a comprehensive set of calculations, but he assumed that the economiser entry water temperature 7b is raised above the condenser temperature by feed heating, which was specified for all his calculations. The T,s diagram is shown in Fig. 7.6 the feed pump work terms are neglected so that h.j, = hy and h. = /ib. 

The reason for using feed heating to set the entry feed water temperature at a level T, above the condenser temperature is that Tb must exceed the dewpoint temperature Tjp of the exhaust gases. If is below Tjp then condensation may occur on the outside of the economiser tubes (the temperature of the metal on the outside of the tubes is virtually the same as the internal water temperature because of the high heat transfer on the water side). With Tb > Tjp possible corrosion will be avoided. 

For (a), calculations showed that the presence of feed heating made little difference to the overall efficiency. Essentially, this is because although feed heating raises the thermal efficiency %, it leads to a higher value of and hence a lower value of the boiler efficiency, tjb- The overall lower cycle efficiency (t o)l = Vb Hl expected to... 

For a comprehensive discussion on feed heating in a CCGT plant, readers may refer to Kehlhofer s excellent practical book on CCGTs [2] a summary of this discussion is given below. 

This efficiency can be improved by the use of a feed heating cycle whereby bled steam can be taken from the turbine after certain stages of expansion and then used to raise the feedwater temperature via use of feed heat exchangers. By such means the feedwater temperature. 

Large power stations use complex feed heating systems before the boiler feed pumps (LP) and after the boiler feed pumps (HP), which can give high overall thermal efficiencies of 39 per cent. However, for the smaller machine, it becomes uneconomic to consider multiple bleeds from the turbine, and the final choice is dictated by the extra cost for the additional complexity against lower running costs due to increased efficiency. As a minimum, a contact type de-aerator is often employed which would extract a small bleed of around 2-3bar from the turbine. 

Table 15.2 gives performance data for typical industrial type schemes using thermal power plant in a condensing steam cycle. These do not operate strictly in the simple cycle mode as varying degrees of feed heating are employed. However, overall they convey the basic cycle conditions that the industrial user would encounter and give efficiencies that can be expected. 

For watertube boilers it is necessary to maintain low O2 levels, and for this purpose a de-aerator in the feed line is required, which will also provide a degree of feed heating. The steam supply can be taken down from the low-pressure process steam main. 

Alternatively, the option that a steam plant offers of the provision of process steam coupled with power generation may be the key element in the selection of generating plant. The turbo-generators and their auxiliaries for use in such applications tend therefore to be relatively unsophisticated, with no feed heating, except for probably the provision of a deaerator. Again, in the turbine itself, machine efficiency tends to be sacrificed for robustness and the ability to accommodate varying load conditions. 

Condensate The water returning from the steam user(s). Feedwater The water entering the boiler feed heating system, which will normally be a blend of treated water and condensate. 

Water at ambient temperature in contact with air contains about lOmg/1 of oxygen. To avoid corrosion, boiler water must contain no oxygen, and have an excess of oxygenscavenging chemical (usually catalyzed sodium sulfite) to ensure this. Ideally, sulfite is dosed into the feed heating system with the feedwater to give the hot well, etc. some protection, with a second dosing point into the boiler itself to ensure that the residual is actually maintained. 

Oxygen can also be removed from feedwater by thermal de-aeration, or partially removed by skilful design of the feed heating system and blowdown recovery. These processes run without cost to the operator, but save chemicals, and, by reducing the required dose of sulfite into the system, decrease the amount of non-volatile solids added into the boiler. 

Cold gas conversion efficiency is the ratio of the gasifier fuel gas total heating value [i.e., (heating value)(mass flow)] to that of the coal feed, [(heating value)(mass flow)]. 

The disadvantages of a discontinuous process are reduced to a minimum by automating both methods. At the end of each step (feeding, heating, esterification, and topping) the next step is started. 

Product- to-Feed Heat Interchange. Heat exchange is commonly used to cool the product of a thermal process by preheating the feed to that process, thus providing a natural stabilizing, feed forward type of process integration. Product-to-feed interchange is common on reactors as well as distillation (qv) trains. 

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