Concentration of Nitrogen Oxides When Burning Wood Pellets of Various Origins

The paper examined the impact of wood pellets of different type on the emission of nitrogen oxides (NOx) formed during the combustion process in a low-power heating boiler. The study aim was to determine the dependencies of the NOx emission levels on the chemical composition of fuel. The test was carried out using a model of a combustion chamber equipped with a dedicated burner. The analysis of the results confirmed the relationship between flue gas and the formation of NOx. The obtained results prove the possibility of reorganizing the combustion process in order to reduce the NOx emission by selecting a proper composition of raw materials when producing wood pellets.


INTRODUCTION
The environmental aspect of combustion processes has been long analysed by the scientists studying the ecological properties of combustion processes [Fiedler and Persson 2009;Hardy et al. 2012; Musialik-Piotrowska et al. 2010; Zhang and Smith 2007]. The basic principle of reviewing a heating boiler in relation to thermodynamics is to consider it as an open thermodynamic system with a strong impact on the environment, which results from the conversion of the chemical energy contained in the fuel into the thermal energy that is partially exchanged with the environment [Khodaei et al. 2015]. An inherent element of these processes, both in industrial and local power section, is the emission of harmful substances during combustion, which are closely related to the technical parameters of boilers  [Juszczak 2016]. On the other hand, limiting the emission of toxic compounds is associated with a proper organization of the combustion process, which directly affects the characteristics of the emitted harmful substances related to the air-fuel equivalence ratio (λ) ].
Due to a limited amount of naturally occurring fossil fuels and higher concentrations of toxic compounds released when burning, efforts are currently being made to increase the use of renewable fuels. An essential benefit of the biomass fuel combustion is that the process emits trace amounts of sulfur dioxide and significantly less particulate matter. The importance of using different types of pellets for energy purposes in Poland and around the world has been described in the cited publi- The study was divided into three stages. The first stage involved determining the NO x emissions while burning the wood pellets made from deciduous tree waste -beech. The second part of the research examined the NO x emission levels during the pine pellet combustion, while the last stage involved burning the two previously mentioned fuels in the 50:50 ratio. The obtained test results were displayed in graphs as mass distributions of NO x and in tables containing the minimum, maximum and average measurements. Additionally, the relationship between the flue gas temperature changes and the NO x emissions for individual fuels was presented in the form of a chart.

METHODS
The test stand used in the study was a model of a cylindrical combustion chamber equipped with a burner assembly (a retort furnace and a blowing fan). The test system was connected to a water installation along with the required fittings. Three types of fuel were supplied to the combustion process. The first fuel was the beech wood pellets, followed by the pine wood pellets, and the third fuel was a 50:50 mixture of the beech and pine pellets. The comparison of chemical compositions as well as the calorific values and the heat of fuel combustion are presented in Table 1.
The combustion chamber is equipped with an electronic controller that allows programming individual components. In consequence, before the measurement started, the combustion process had been stabilized in order to maximize efficiency and minimize the emission of pollutants. Fuels were periodically delivered by a drive unit with a screw feeder. Additionally, the system was equipped with an additional blowing fan speed controller that steadily supplied the air needed for the combustion process.
The measuring probe of the TESTO 330-2 LL flue gas analyzer was placed in the openings of the exhaust gas installation. The device was used to measure the composition of the flue gas and NO x concentration changes analyzed in the article. The PT100 sensor was used to measure the temperature of the flue gas. The obtained values of the NO x contents in the flue gas were subsequently calculated for 10% residual oxygen and converted to mg/m 3 . The test method was based on the guidelines of the PN-EN 303-5: 2012 standard.

RESULTS
The results of the conducted test generated parameters of the combustion process and the NO x content in the flue gas for individual fuels. The results of the first test stage are presented in Table 2. The obtained flue gas temperature was in the range of 103.40-121.70 °C. The average value of the NO x concentration obtained through continuous measurement was 265.86 mg/m 3 , while the air-fuel equivalence ratio (λ) oscillated within 1.61 ± 0.30. The mass distribution of NO x emission for the first test stage is presented in Figure 2.
The results of pine pellets combustion in the second stage of the study are shown in Table 3. The obtained flue gas temperature was in the range of 79.00-92.60 °C. In comparison to the first stage of the study, the average NO x emission decreased by 30.5%. The obtained air-fuel equivalence ratio (λ) revealed a significant increase reaching value 2.54 ± 0.30. The mass distribution of NO x emission is presented in Figure 3.
The measurement results obtained in the third test stage are presented in Table 4. The mixed pellets achieved a flue gas temperature of 100.70-117.70 °C, an intermediate between the first and the second stage.
The NO x concentration was slightly lower than in the case of the first stage of the study and its average value   was 252.14 mg/m 3 . The air-fuel equivalence ratio (λ) was similar to the first stage and its average value also reached 1.61 ± 0.20. The NO x mass distribution for the third stage is shown in Figure 4 and the relationship between the flue gas temperature and NO x emissions for all test stages is presented in Figure 5.

CONCLUSIONS
This study investigated the impact of different pellet types on the NO x emissions during the combustion processes in a low-power heating boiler. The obtained results demonstrate lower NO x concentrations in flue gas when using the wood pellets originating from the pine tree waste. The higher content of elemental oxygen in this fuel achieved an increased airfuel equivalence ratio (λ) and, the lowest temperature of flue gas among all stages of the study. The flue gas temperature was higher while burning the beech pellets due to a higher calorific value of this fuel and the highest NO x content in flue gas. While studying mass distributions, it was observed that the NO x concentration increases noticeably along with an increase of the air-fuel equivalence ratio (λ). On the other hand, combustion of the mixed fuel shows a mild and small increase of emissions when the air-fuel equivalence ratio (λ) increased. Therefore, the presence of pine in fuel has a positive effect on the combustion process, because it reduces the NO x formation. From the point of view of thermodynamic parameters and chemistry of combustion processes, the conducted study confirmed the existence of a strong dependency between the changes of the flue gas temperature and the formation of NO x . The highest flue gas temperature during the beech pellet combustion went hand in hand with the most elevated level of the NO x generation. The study is the basis for further research on the reorganization of the combustion process in terms of the NO x reduction and it is also a good way to expand the knowledge in this area.