4.2.3. Dynamic Evaluation

Besides the evaluation of the part-load tests with mean values for the temperature and pressure, also the dynamic behavior during load change is relevant to validate the process model. In this chapter, the dynamic response of the furnace is evaluated and the model prediction of this response is assessed. Figure 12 shows the development of the temperature in the bed and at 8.2 m, the pressure at several heights, and the flue gas composition. The fuel mass flow is varying, due to differences in the fuel properties such as particle size, shape, density, or moisture. In the simulation, these fluctuations are not modeled. This results in fluctuating values for the flue gas composition and the temperatures.

**Figure 12.** Temperature, pressure, and fuel mass flow during load change tests.

At the starting point at 63% load, the bed temperature of the numerical analysis and the experiment agree very well, while the temperature at the top of the reactor is significantly higher in the simulation. It is also apparent that the pressure above the bed (2.1 m) is overestimated, while the pressure in the bed and at the top, as well as the flue gas composition agree well with the experiment. The reasons for the deviations are discussed in detail in Section 4.2.1. After 47 min, the load is increased to 88%. In the simulation, the temperatures oscillate shortly after the load changes, especially in the dense bed region. PI-controllers control the mass flows of combustion air in the model. These controllers

cause a temporary excess/shortage of air and thus they are responsible for the temperature oscillation. Further optimization of the controller is necessary to match the air-flow of the experiment at all times. There is a very good agreemen<sup>t</sup> for the overall duration of the load change. In the experiment and the simulation, 75% of the absolute change in temperature is completed after 3–4 min. After 97 min, the load is increased to 100%. The load change duration is similar to the first load transient. Despite the oscillation after the load change in the numerical model, the furnace response is predicted well by the model and 75% of the new temperature level is reached after 2–3 min. As already discussed in Section 4.2.1, the model is more accurate at higher loads with regard to the average pressure and temperature in the furnace.

The first load decreasing step (100–89%) is performed after 182 min. Due to the fuel mass flow fluctuations, shortly after the load change, there is a higher oxygen excess in the flue gas in the experiment compared to the simulation. When the fuel mass flow decreases slightly over time, the average oxygen excess decreases and is closer to the model prediction. This leads to a lower temperature level at the beginning of the 88% load test and an increasing temperature to the end of this test. For both, experiment and process model, the temperature di fference between the 100% and 89% load test is low compared to the other tests. Therefore, and due to the fuel mass flow fluctuations, it is di fficult to assess the duration of the temperature change in detail. After 264 min, the load changes from 89% to 68% load. The temperature response directly after the load change is reproduced well by the model. However, in the experiment, there is a long period (20–25 min), where the temperature converges to the final value, which is not observed in the simulation.

Compared to the partly slowly changing temperatures, the pressures are changing very fast (a few minutes) in all load transient tests—for the experiment and the numerical model. This is because the hydrodynamic condition mainly depends on the gas velocity and the solid particle properties. It depends only indirectly on the temperature—mainly by changing gas density and viscosity. On the other hand, the temperatures do of course depend strongly on the hydrodynamic conditions. However, the new temperature levels do not adjust as quickly as the pressure, due to the heat storage capacity of the bed material and the refractory. The large fluctuations of the pressure in the experiment in areas with high solid content (bed and splash zone) are typical for a CFB. The fluctuations decrease with the height of the reactor. The fluctuations are not given in the simulation, due to its one-dimensional nature.

Concluding this, the response of pressure and temperature on positive load changes can be predicted well by the model. It is not clear if this is also valid for negative load transients, but the first response after the load change seems to be predicted with good accuracy. In future studies, the fuel mass flow including the fluctuations should be modeled with higher details to further assess the dynamic properties of the model. However, with some improvements, the model seems to be suitable to predict the load transient response of a boiler very well. Therefore, core annulus models could be used to test novel concepts to increase the maximum load ramps of CFB combustors.
