*2.2. Experimental Apparatus and Procedure*

The experiments were conducted in the 12-kW bench-scale test apparatus under conditions similar to those experienced in a real CFB boiler at the Department of Thermal Machinery at the Czestochowa University of Technology. The apparatus consists of a combustion chamber (riser), a down-comer, a cyclone, and a loop-seal, and has been described in detail in references [21–25]. In this section, only the experimental conditions are briefly mentioned.

Mixtures of O2 and CO2 mixtures from gas cylinders were used to simulate the oxyfuel combustion conditions. The concentrations of oxygen varied from 21% to 40%vol. All tests were carried out at a temperature of 850 ◦C without the addition of a sorbent for the capture of SO2.

A 0.5 g sample of fuel was entered into the combustion chamber and burnt in the fluidized bed consisting of silica sand and ash particles. All fuel samples were prepared as pellets. The technique of pellets preparation is described in papers [23,24].

The composition of flue gases during the air and oxy-fuel combustion was measured by a portable FTIR spectrometer (Gasmet DX-4000) while O2 was measured using a zirconium sensor analyzer (AMS Oxitrace). The volume fractions of NO, NO2, N2O, SO2, CO, and other compounds were recorded every three seconds. The maximum error was less than 2%. Each experiment was repeated thrice minimum to guarantee a relative standard deviation below 5%.

In this study, temperature (850 ◦C), the superficial gas velocity (5 m/s), the mass of the fuel sample (0.5 g), and the mass of the inert material (0.3 kg) were constant. Only fuels and oxidizing atmospheres were different. Information collected during tests with a single fuel particle is sufficient to compare the influence of the combustion atmosphere on pollutant emissions. Our transient experiments with single biomass particles allowed us to collect data on instantaneous concentrations of emitted pollutants. These results are useful in assessing the behavior of biomass fuels in different combustion systems.
