Advantages of Supplementary Firing in HRSGs

Advantages of Supplementary Firing in HRSGs

As discussed in Chapter 1, there is lot of oxygen in turbine exhaust gases that can be utilized to fire additional fuel to the exhaust gas and thus increase its temperature. Increase in inlet gas temperature generates additional steam in the evaporator, which increases the capac- ity of the heat sink, which is the economizer. Hence, the exit gas temperature beyond the economizer will be lower in the fired mode compared to the unfired mode. Since additional air is also not added, the lower exit gas temperature helps to increase the fuel utilization sig- nificantly. It can be shown through performance calculations that supplementary firing is an excellent way of generating additional steam in HRSGs and fuel utilization is nearly 100% or even more. That is, one can generate additional steam with nearly the same amount or lesser amount of fuel. In conventional steam generators, the LHV efficiency is about 93%, while in HRSGs, it can be close to 100% or even more. A layman’s explanation for this is as follows: when we increase excess air in steam generators, the efficiency decreases. In gas turbine HRSGs, we decrease the excess air by firing fuel into the oxygen-rich exhaust gas and hence efficiency is higher. In addition, the exit gas temperature from the economizer exit is lower when inlet gas temperature is higher and that contributes to the higher efficiency as well. The economizer acts as a larger heat sink in the fired mode due to the larger flow of water through it and hence is able to cool the exhaust gases to a lower temperature. Chapter 5 on simulation gives examples of improved fuel utilization with supplementary firing.

In Example 4.1, one may check the fuel utilization also. The difference in energy recovered between the unfired and fired cases for vendor A is (22.51 − 8.71) = 13.80 MM kcal/h, while only 13.0 MM kcal/h is fired on LHV basis. That means the fired case is more than 100% efficient. In the case of steam generators, the efficiency on LHV basis was shown to be about 93.5%. That is, by firing in an HRSG, we straightaway get more than 100% fuel utilization. If we look at vendor B, the additional energy from steam = 22.51 − 8.21 = 14.3 MM kcal/h, while

14.0 MM kcal/h is fired! Here again, it is more than 100% efficient. Thus, plant engineers may analyze the HRSG performance in both fired and unfired cases and understand how well the fuel energy is utilized. (The HRSG should have an economizer to be more efficient in the fired

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mode if it is a single-pressure unit. If we did not have an economizer, the exit gas temperature will be higher without an economizer and hence efficiency in fired mode will be lower.)

Thus, supplementary firing is one way to improve the efficiency of gas turbine HRSGs. That is because we are not adding any combustion air while firing but utilizing the oxygen in the exhaust. In effect, we are reducing the excess air. The heat losses from the boiler are also lowered due to the lower exit gas temperature in the fired case. This is due to the fact that a larger heat sink is available in the form of the economizer in the fired mode with the gas flow remaining the same. Plant engineers while planning future projects with HRSGs in cogeneration plants may consider supplementary- or furnace-fired HRSGs as they are more efficient. Since the cross section of an HRSG is determined by the gas flow, it does not matter whether the steam output is 15 t/h or 60 t/h, as the same exhaust gas flow with dif- ferent temperatures and analysis flows through the HRSG. The size of drums may be larger for the higher capacity, but the overall cost should not be much more than an unfired unit.