2.1.3. Cooling System

Figure 2 shows a simplified scheme of the cold side of the cooling system. Heat is absorbed from the gas-side by two di fferent subsystems. Five water-cooled lances reduce the temperature in the furnace. The lances can be moved vertically into the reactor at varying depth. The immersion depth was fixed during the experiment also at part load (two lances at 4.5 m, three lances at 6.5 m immersion depth). The cooling lances are manufactured with a double-tube design. Water is flowing down through the inner tube and flowing up through the outer tube of the lances. The second cooling subsystem is located downstream of the cyclone, where the flue gas is cooled down in a two-path heat exchanger. In the first path, the flue gas streams downwards via a tube bundle heat exchanger. It is then redirected upwards via a second tube bundle heat exchanger. The walls of both paths are designed as membrane wall heat exchangers. The mass flow rate of water is measured before and after the cooling lances and after the flue gas cooler. The water temperature is measured before and after each lance. The mixing temperature after the lances is measured and the temperatures before and after each path of the flue gas cooler are measured. The hot water is cooled down in an air re-cooling system to 110 ◦C before it re-enters the cooling paths. The cooling system operates at 8–16 bar and the cooling liquid is always in a liquid state (no steam generation). During the experiment, the inlet temperatures and the mass flow rates of each cooling line are fixed. Thereby, the transferred heat from the gas side to waterside can be determined by the temperature di fference of the water between the inlet and outlet of the cooling subsystems. Heat losses in the subsystems are unavoidable in the pilot plant. The heat transfer calculation is therefore subject to uncertainties. The design parameters of the cooling lances and the flue gas cooler are shown in Table 2.

**Figure 2.** Simplified flow diagram of the cooling system (waterside).


**Table 2.** Design parameters of the cooling lances and the flue gas cooler.

#### *2.2. Experimental Conditions and Procedure*

Low-rank polish-lignite was combusted in the experiments in the pilot CFB furnace. The lignite has a lower heating value of 11.4 MJ/kg and a moisture content of around 49%. The sand was fed to the furnace to improve the fluidization properties and keep the bed particle size in a suitable range for CFB combustion. The proximate and ultimate analyses of the raw and dried lignite are shown in Table 3, while the properties of the sand are shown in Table 4. The average particle size of the sand particles is 200 μm.




**Table 4.** Properties of sand.

To evaluate the long-term steady-state behavior of the fuel, a 57-h test was carried out at the beginning of the test series with a thermal load of 845 kWth. Short-term fluctuations of the fuel mass flow rate and composition are compensated in such a long period, making this test point very suitable for tuning and validation of process simulations. After the steady-state test, load change tests were carried out to evaluate the dynamic behavior and the hydrodynamic conditions at low loads. Firstly, the load was slowly reduced to 648 kW. After a stabilization time, the load was increased stepwise to 914 kW and 1032 kW. The thermal load depends on the speed of the conveying screw of the fuel, but also on changing properties of the fuel such as bulk density, moisture content, particle size, and chemical composition. To keep the excess-air factor nearly constant, the air mass flow rates were increased to the desired set point and only the fuel mass flow rate was controlled to keep an oxygen excess of around 4–6 Vol-%wet. With the same approach, two load decreasing steps from 1032 kW to 921 kW and 699 kW were performed. After each load change, the pilot was operated in the same conditions for at least 50 min before the next load step was carried out. The large thermal inertia of the bed material acts as a bu ffer after a load change. Therefore, the stabilization time of ~50 min is not long enough to achieve steady-state conditions for each part-load test. However, the fluctuating behavior of wind and solar power requires quick and flexible load changes by power plants. In the energy market nowadays, it is necessary to perform load changes, when steady-state conditions are not achieved. The target of the experiments was to represent this behavior. Table 5 presents the conditions of the long-term steady-state test and the part-load tests. The experimental approach for the load changes is illustrated in Figure 3. As mentioned, the mass flow of the solid fuel fluctuates strongly, so that the figure only illustrates the average mass flow/setpoint and not the actual one. For the evaluation of the tests, mean values are calculated for the temperature, pressure, and other measured parameters for each part-load test. This evaluation period starts 15 min after each load change and ends 5 min before the next load change.



**Figure 3.** Schematic presentation of test conditions during part-load tests.

Part load conditions are part of the investigation in this paper. At lower loads, the velocity in the furnace decreases. To estimate the state of fluidization in the fluidized bed, it is helpful to compare the present velocity with the minimum fluidization velocity and the transition velocity. These two velocities can be calculated using various formulas. With the help of Grace et al. [23] *um f* is estimated to 0.015 m/s and *use* is estimated to 6.0 m/s. The velocity in the furnace depends on the flue gas flow rate, but it also depends on the position, as gaseous components are released along the reactor height and secondary air injection increases the gas flow rate. However, in this study, the velocity in the furnace is estimated according to the following equation, while *<sup>T</sup>*avg is the arithmetic mean of the measured reactor temperatures, the index N means normal conditions (0 ◦C, 1.01325 bar), and A is the reactor cross-section:

.

.

$$w\_{\text{avg}} = \frac{V\_{\text{Flux gas}}}{A\_{\text{Racator}}} = \frac{V\_{\text{flux gas},N}}{A\_{\text{Racator}}} \cdot \frac{T\_{\text{avg}} + 273.15 \text{ K}}{273.15 \text{ K}} \tag{1}$$

## **3. Model Description**

The model of the pilot CFB furnace is created with the software APROS (Advanced Process Simulation). APROS was developed by Fortum and the Technical Research Centre of Finland (VTT) [21,24–28]. Process components such as pumps, fans, heat exchangers, or piping can be selected from a component library of the programme to model the process realistically. The components are connected via mass flow and energy equations. Data of the process components can be inserted at a high level of detail. Geometries, material properties, and characteristic curves of electrical machines can be inserted into the model. APROS is often used in literature for the modeling of thermal processes, especially thermal power plants, such as pulverized-coal fired power plants [20,29–31], municipal waste incinerators [32], circulating fluidized beds [21,33], concentrated solar power plants [34–36], and nuclear power plants [37,38].

The test facility in Darmstadt is modeled in detail with APROS, using the design data of the pilot plant. The subsystems fluidized bed combustor, air-supply, cooling system, flue gas path, and several boundary conditions for the simulation are modeled in individual nets with high detail. Where no suitable standard components are provided by APROS library (e.g., air preheater, and bag-house filter), the components are implemented with an in-house code. The homogeneous flow model describes the fluid properties and behavior in the combustion air lines, the flue gas path and the waterside of the cooling system. After the model was built, it was tuned with the long-term steady-state experiment at 82% thermal load. The test duration for this test point was 57 h, so the experimental data is considered to be very reliable. According to the experimental test schedule, the load was decreased afterward to

63%. Then, the load increasing (63–88–100%) and load decreasing steps (100–89–68%) were performed according to the test procedure described in Section 2.2. After tuning, the model was validated with the experimental data of the transient operation. Pressures, and temperatures inside and after the furnace, as well as the flue gas composition, were compared to the measurement data for validation.
