According to actual measures and specifications of the reference plant, the insertion of the recovery system into the off-gas cleaning section can be analysed. We suppose to provide the settling chamber with water jackets; then a boiler and a steam turbine are taken into account for energy recovery and properly sized.
The boiler has a maximum steam flow rate of 49 t/h, an average steam temperature of 400 °C, and a pressure of 40 bar at turbine inlet; a 0.08 bar pressure is recorded at the condenser. The boiler represents the second step to affect off-gas properties; after smoothing the temperature profile, in fact, it is possible to affect also off-gas speed and fouling factor. The first boiler section (see Figure 3.26), characterized by a low off-gas speed (8-10 m/s) and a high fouling factor, includes the screen superheater (very rarefied tubes), two superheaters, and the evaporator at high temperature. Downstream, two
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81 parallel cyclones drastically reduce the dust content of the off-gas with an overall efficiency of 75% and allow a speed increase around 12-14 m/s. Therefore, the second section of the boiler can involve a low temperature evaporator and an economizer with very densely distributed tubes and consequently high thermal performance with reduced space requirements.
Figure 3.26 The boiler structure. SH (superheater); HT (high temperature); LT (low temperature); EVAP (evaporator)
When the off-gas enters the boiler with the maximum temperature allowed by the smoothing system (~720 °C, as shown in Figure 3.25), its outlet temperature is reduced to 199 °C, while water/steam temperature is increased from 105 °C at the economizer inlet to 420 °C at the superheater outlet (see Figure 3.27a). When off-gas, instead, enters the recovery system at its lowest smoothed temperature (~447 °C, see Figure 3.25), its outlet temperature is reduced to 216 °C; steam exits at 385 °C from the superheater (see Figure 3.27b). In both situations, the steam produced allows the turbine to work near nominal conditions.
To ensure a proper heat exchange, the off-gas temperature should be higher than the temperature of the superheated steam along the whole TTT cycle. While this is assured when the smoothing system is installed, in a traditional system a heat backflow from the working fluid to the off-gas occurs during charging and stand-by phases, when off-gas temperature decreases below the working fluid one. This leads to a decrease of thermodynamics properties of the produced steam, with a negative impact on turbine electrical efficiency.
The total energy released during a TTT cycle (68 min in our case) by the off-gas exiting the boiler at near 200 °C is approximately 33 MWht, for an average thermal power of
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about 29 MWt (see power duration curves in Figure 3.28). This value can be considered reasonably equal in both conditions, i.e. when the plant is provided with the smoothing module and when it is not, since the PCM-based module affects thermal power profile, not the energy content of off-gas significantly. However, the off-gas thermal power profile plays a major role when the steam turbine is sized, since the latter should be able to convert the maximum thermal power.
(a) (b)
Figure 3.27 Off-gas and water/steam temperatures when smoothed off-gas is at its a) maximum and b) minimum input temperature.
In our case, the thermal power released by the off-gas at the boiler when exiting the smoothing module at the maximum temperature of 720 °C is 35.5MWt, while it grows to 52.8MWt for off-gas at 950 °C when the smoothing system is not installed. By considering a full-load turbine efficiency of 0.3, these values lead to select a turbine of 10.65MWe for the plant with the smoothing system and of 15.85 MWe for the traditional plant, to convert quite the same amount of thermal energy.
If the actual thermal power released by off-gas for a given thermal power profile is divided by the maximum potential power that off-gas would release if it could persist at its highest temperature for the whole TTT cycle (e.g. 35.5 MWt for 720 °C), we can obtain
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83 a measure of the energy loss due to off-gas temperature variability, in comparison to the nominal power of the turbine. In our case, this measure is equal to 0.55 for the traditional system and to 0.81 for a plant with the PCM-based smoothing system.
Figure 3.28 Off-gas thermal power duration curves.
Therefore, an actual power of 8.6MWecan be supposed to be generated by the turbine in both systems, but with very different turbine sizes and investment costs. It should be noticed, however, that in a traditional plant several technological issues related to the strong temperature variability of the off-gas should be addressed to effectively recover its energy. Furthermore, given the reduced variability of steam properties at the turbine inlet in the case of the smoothing system plus the proposed boiler installation, a greater electrical efficiency of the turbine can also be expected, thus increasing the benefits on final energy recovery. Taking into account efficiency loss due to turbine partial load operations, fouling of exchange surfaces, and auxiliary equipment requirements, we can estimate to generate a net electrical power of about 6MWe, if the smoothing module plus the described recovery system are installed.
Referring to steel production, a final amount of 57 kWhe/ton can therefore be recovered. Given an electrical energy consumption of 396 kWhe/ton for the EAF of our reference case, 14.4% saving on electrical energy supply can be gained.
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