**2. Experimental**

The following chapter describes the test facility including the pilot plant, the measurement equipment, and the cooling system. Afterward, the experimental procedure is presented with the most important boundary conditions of the experiment.

#### *2.1. Pilot Plant Description*

Experiments were carried out in a 1 MWth CFB furnace for the combustion of low-rank Polish lignite. The flow diagram of the CFB reactor (CFB600) and its subsystems is shown in Figure 1. The main components of the facility are the air-supply system (primary air, secondary air, and burner air), the reactor with a hot-loop circulation (cyclone and loop seal), the solid handling systems (solid feeding and ash extraction), the cooling system and the flue gas line (heat exchanger, bag filter, induced-draft fan).

**Figure 1.** Simplified flow diagram of the 1 MWth pilot plant.

#### 2.1.1. CFB Furnace and Auxiliary Systems

The furnace of the CFB600 reactor includes the riser and a hot-loop circulation for the recirculation of solids. The reactor itself has an inner diameter of 0.59 m and a total height of 8.6 m. It is fully refractory-lined and the outer diameter of the refractory is 1.3 m. Table 1 shows selected design parameters of the furnace. See Figure A1 in the Appendix A for a detailed geometry of the CFB combustor.

Solids that leave the riser, enter a cyclone, which separates them from the flue gas. The flue gas and fly ash enter the flue gas line with a small amount of fly ash. The main portion of the solids is recirculated to the furnace by a standpipe and a loop seal. Thereby, the residence time of char particles inside the furnace increases and the burn-out of the fuel improves. Air is injected into the loop seal at 25 ◦C via two nozzles to maintain fluidization of particles and to ensure a continuous recirculation. Besides the purpose of hot solid recirculation, the loop seal also provides pressure sealing between riser and standpipe/cyclone. Five water-cooled lances can be immersed vertically into the reactor at varying depth to control the combustion temperature.


**Table 1.** Design parameters of the furnace.

The combustion air is injected into the furnace at several locations and heights. The primary air is entering the furnace via a nozzle grid at the bottom of the riser. It is electrically preheated to around 300 ◦C before entering the reactor. A primary air fan controls the mass flow rate to a certain set point. Secondary air enters the riser at two di fferent heights (2.74 m and 6.0 m) with two oppositely arranged nozzles at each elevation. A fan provides the desired mass flow of secondary air. The third major portion of the air is injected via the start-up burner at a height of 0.70 m. During start-up, this burner is fired with propane. During the tests, the air is supplied here to prevent backflow of particles and cool the burner components. The secondary and the burner air are not preheated and enter the furnace at an ambient temperature of around 25 ◦C.

A screw conveyor feeding system feeds solid fuel to a rotary valve, which is located above the return leg. The return leg connects the loop seal and riser. Thereby, the fuel is rapidly flowing to the bed of the furnace at a height of 0.48 m. The rotary valve guarantees pressure sealing between the reactor and the feeding systems. Another screw conveyor feeding system feeds sand to the same rotary valve. A water-cooled conveying screw extracts bed material via a downpipe in the middle of the nozzle grid. The particles are extracted batch-wise to keep the inventory in a suitable range. The target is to keep the bed pressure between 50 and 60 mbar.

The flue gas and fly ash leave the reactor through the cyclone. Afterward, it flows to the flue gas heat exchanger and cools down for further treatment. The heat exchanger is water-cooled and is arranged in two vertical paths. After cooling down, the flue gas is separated from the fly ash in a fabric filter. The fly ash is collected in a hopper and transported to a barrel by a rotary valve. A downstream induced-draft (ID) fan ensures a constant pressure of around 1 mbar below ambient pressure after the cyclone. After leaving the ID fan, the flue gas leaves the system through the stack at a temperature of 130–150 ◦C.
