**3. Materials and Methods**

#### *3.1. Fuel Tested*

Wheat straw (agricultural biomass), willow (*Salix viminalis*, energy crop biomass) and Scots pine (woody biomass) were used as primary fuels, and a Polish bituminous coal, combusted in CFB boilers, was chosen as the reference fuel.

The moisture content in biomass fuels was determined with the use of the oven-drying method, based on the PN-EN ISO 18134. The ash content of the biomass samples was determined in accordance with PN-EN ISO 18122, whereas PN-EN ISO 18123 standard was used to assess the volatile matter content. Elemental analyzers Truespec CHN Leco and SC-144DR Leco were used to determine the hydrogen, carbon, sulfur and nitrogen contents. The proximate and ultimate analyses and higher heating value (HHV) of the tested fuels are presented in Table 2.


**Table 2.** Proximate and ultimate and higher heating value of the tested biomass and coal.

#### *3.2. Experimental Setup and Procedure*

Conventional and oxy-fuel combustion experiments were carried out in a 12-kW, electrically heated lab-scale CFB combustor, which has been described in detail in references [21–23]. Here, only the experimental conditions are briefly mentioned. All experiments were carried out at temperature 850 ◦C in air (base case) and mixtures of O2/CO2 with O2 concentrations in the range from 21% to 40%vol. The names given to the different oxy-fuel atmospheres, oxy-21, oxy-30 and oxy-40, are related to the O2 concentration in the oxy-fuel mixture fed to the combustor, i.e., 21, 30 and 40%vol., respectively.

Silica sand (particles below 400 μm) to a mass of 0.3 kg constituted the fluidized bed. The gases (O2 and CO2) to make up gas mixtures are supplied from cylinders to a mixer and then transferred via a preheater directly into the combustion chamber. Flow rates of gases are controlled by valves and measured by rotameters. During air and oxy-combustion experiments, the superficial gas velocity was kept at a constant level of about 5 m/s.

A small sample of fuel (0.5 g) was fed into the combustion chamber (riser) for each test. Sorbent was not used to capture SO2 during all tests. The biomass and coal samples were in the form of spherical pellets. The detail method of pellet making is explained by Kosowska-Golachowska et al. [24] and Kijo-Kleczkowska et al. [25]. Concentrations of NO, NO2, N2O, SO2, CO and other compounds (HCN and NH3) in the flue were measured online (with frequency of 1 Hz) by a FTIR spectrometer (Gasmet DX-4000). The maximum error of these measurements was less than 2%. The time period for each test was set 3 s. Each test was repeated minimum three times to guarantee a relative standard deviation of less than 5%. This methodology has been successfully used in our previously studies on pollutant emissions during combustion of sewage sludge [21] or lignite and bituminous coal [22].

#### **4. Results and Discussion**

Biomass combustion both in air and oxy-fuel atmospheres proceeds through several stages, namely drying, devolatilization, ignition and volatile matter combustion, char combustion and agglomeration of ash [7,22,23]. A detail report on all these stages, temperature profiles and visualization during oxy-combustion of wheat straw and *Salix viminalis* can be found in [22,23], respectively.

Volatile matter and oxygen contents in the biomass samples are significantly higher than those in the bituminous coal whereas the ash yield and HHV are lower than those in coal (see Table 1). The contents of S, N and Cl in biomass are lower than those in coal, which indicates that the tested biomass fuels are ideal renewable energy resources with a minor potential for environmental pollution.

In this section, emissions of NOx (NO, NO2, N2O and their precursors, such as NH3 and HCN), SO2 and CO during conventional and oxy-fuel combustion of three kinds of biomass and a reference coal are presented and discussed.

#### *4.1. Conventional Combustion*

4.1.1. NO, NO2 and N2O Emissions

Nitrogen oxides (NOx), such as NO, NO2 and N2O, are main contributor to air pollution [26]. The mechanism of NOx formation in conventional air-fired combustion includes thermal, prompt and fuel-N NOx. Thermal and prompt NOx formation occurs at high temperatures, usually above 1400 ◦C, that are typical for pulverized fuel combustion. In fluidized-bed combustion, contributions of thermal and prompt mechanisms to the total NOx emissions are negligible.

Fuel nitrogen oxides are formed through oxidation of N-containing species in the fuel (fuel-N), which are released during devolatilization (volatile-N) and char oxidation (char-N) stages. At typical fluidized-bed combustion temperatures (below 900 ◦C), fuel-N is the dominant source for NOx formation. Although char is an important intermediate for NOx formation, the behavior of volatile-N is a dominating factor in the case of burning fuels with high-volatile matter contents such as biomass [27].

Figure 1 shows the time-resolved emissions of NO and N2O during air-CFB combustion of all tested fuels. Nitrogen oxide was the dominant N-containing pollutant and no NO2 was detected during tests in air. This finding is consistent with the results obtained by Liu et al. [28]. The highest NO concentrations (Figure 1a) were detected during the volatile matter combustion. The highest NO concentrations were approximately 75 ppm and 60 ppm for wheat straw and *Salix viminalis*, respectively. The lowest NO emissions were measured for the reference coal despite its highest nitrogen content. This observation can be attributed to the lower volatile matter content in coal and lower temperature during the volatile matter combustion compared to biomass fuels.

**Figure 1.** Impact of fuel type on the instantaneous NO (**a**) and N2O (**b**) concentrations during air combustion.

Nitrous oxide emissions (Figure 1b) were about four times lower than NO emissions, which indicates that fuel-N was more inclined to undergo conversion to NO than to N2O during combustion in air. The N2O formed, compared to NO, was also more readily reduced to N2 by char or CO [27,28]. The highest N2O concentrations were approximately 15 ppm for wheat straw, while the lowest for Scots pine what was attributed to the nitrogen content in biomass fuels.

Hydrogen cyanide, HCN, and ammonia, NH3, are major precursors for the formation of NOx during combustion of solid fuels containing nitrogen. There is a consensus that the conversion of fuel-N to nitrogen oxides proceeds through these intermediates [29]. The instantaneous concentrations of HCN were below 3 ppm and NH3 was not detected during air combustion of tested fuels.

Figure 2 shows the influence of fuel type on the total emissions of NO and N2O during combustion in air. In the case of whet straw, NO accounts for approximately 75% of the total NOx emitted and N2O for remaining 25%. For *Salix viminalis* and Scots pine, these figures are 85% and 15%, and 72% and 28%, respectively. In the case of bituminous coal, the fraction of NO in the total NOx is much lower, approximately 40%, and N2O accounts for remaining 60%. As coal contains less volatile matter than biomass fuels, high emissions of N2O can be attributed to oxidation and reduction reactions at the surface of char particles.

**Figure 2.** Effect of fuel type on the total NO and N2O concentrations during air combustion. Vertical bars represent standard deviation.

The split of NO and N2O emissions between volatile-N and char-N oxidation during air-combustion of wheat straw is shown in Figure 3. Oxidation of volatile matter accounts for approximately 80% of total NO formed and char oxidation contributed only 20%. In the case of N2O, contributions of volatile-N and char-N are similar and are approximately 51% and 49%.

**Figure 3.** The instantaneous concentrations of NO and N2O during air-combustion of wheat straw.

Oxidation of volatile-N was responsible for 75% of the total NO formed for *Salix viminalis*, 85% for Scots pine and 65% for coal (Figure 4). Volatile-N contributed 48% to the total N2O formed during combustion of *Salix viminalis* and 32% during combustion of Scots pine and coal.

**Figure 4.** Effect of fuel type on NO (**a**) and N2O (**b**) concentrations during air combustion. Vertical bars represent standard deviation.
