3.1. Natural Gas (NG) Combustion Experiments
This section presents the results of applying primary de-NOx methods to a natural gas-fired system. Denitrification efficiency of flue-gas recirculation, reburning, overfire air use, and combinations of these were evaluated at natural gas thermal inputs of Q = 1.5 and 1.8 m3/h and air excess coefficients of m = 1.1 and 1.2, respectively.
Figure 2 depicts the measured nitrogen oxide emissions and denitrification efficiency of individual primary methods and their combinations at Q = 1.5 m
3/h and m = 1.1. Reburning proved to be an efficient primary method; with a rising reburning ratio from 10 to 25%, the denitrification efficiency rose from 9 to over 60%. Overfire air use, whether applied separately or in combination with reburning, did not lead to a significant nitrogen emissions reduction.
The effects of flue-gas recirculation on nitrogen emissions and its comparison with reburning application at Q = 1.5 m
3/h and m = 1.1 are shown in
Figure 3. Flue-gas recirculation halved the NO
x emissions even at 10% recirculation, and the final NO
x emissions at 20% flue-gas recirculation dropped to roughly 20 mg/m
3, representing denitrification efficiency of almost 75%. A further increase in flue-gas recirculation was hindered by a significant increase in CO emissions, leading us to conclude that flue-gas recirculation of 20% is the upper limit for feasible combustor operation. Comparison with reburning shows that flue-gas recirculation is a more efficient NO
x emissions reduction method, as it reduced them by half at a reburning ratio of over 20% (compare
Figure 2 and
Figure 3), while 25% represents the upper limit of commonly applied reburning ratios.
The nitrogen oxide reduction values shown in
Figure 2 were calculated as relative nitrogen oxide emission differences resulting from measured nitrogen oxide emissions after primary methods application compared to those measured without any denitrification method applied, as follows:
where R refers to emissions reduction (%), C
WAPM stands for nitrogen oxide emissions without application of primary methods, and C
APM represents emissions with application of primary methods.
Figure 4 presents the trends in carbon monoxide content in flue gas with the application of individual primary methods and their combinations. Flue-gas recirculation applied alone helped in decreasing CO emissions, compared to the situation without the application of any de-NO
x method. Therefore, it can be concluded that the FGR method was able to significantly reduce both NO
x and CO emissions (see
Figure 3 for comparison) and could thus be considered as a very promising method when striving towards reduction of combustion processes’ environmental impact. All other de-NO
x methods and their combinations resulted in increased CO emissions by around 30 mg/m
3 (OFA + 10% reburning) to over 1100 mg/m
3 (20% reburning). Reburning itself proved to be a less effective de-NO
x method than FGR (see
Figure 3), yielding substantially higher CO emissions than FGR. Therefore, the application of reburning alone does not appear sensible. Its combination with overfire air, however, reduced the NO
x emissions significantly (see
Figure 2) and helped approach the emission limit for CO (200 mg/m
3). This results from the fact that overfire air introduction leads to oxidation of a major portion of the carbon monoxide formed in the reburning zone.
Figure 5 explains the reaction mechanisms leading to NO
x and CO formation in individual temperature zones of the furnace and allows monitoring of temperature trends resulting from primary de-NO
x methods application.
NO
x generated in the main combustion zone (
Figure 1 and
Figure 5, between T13–T14) reacted with fuel remnants injected into the reburn zone (
Figure 1 and
Figure 5, between T3–T4), which reduced it to molecular nitrogen. Reburning chemistry involves fuel radicals, which reduce NO to N
2 [
32,
46]. Reburning increases the flue-gas temperature from thermocouple T4 onwards, compared to OFA application or no de-NO
x-method use.
The addition of overfire air completed combustion in the burn-out zone (
Figure 1 and
Figure 5, between T4–T5), but the reaction heat released by CO and hydrocarbon fragments was not enough to compensate for the cold, fresh air introduced, and as a result the flue-gas temperature decreased from T5 onwards, compared to the application of reburning alone.
A similar series of measurements was conducted at natural gas thermal input Q = 1.8 m
3/h and air excess coefficient m = 1.2, with the results shown in
Figure 6 and
Figure 7. A comparison of the data shown in
Figure 2 and
Figure 6 revealed that the latter combustor operation conditions generally yielded higher NO
x emissions by around 75 to 100%, which is in accordance with current knowledge on nitrogen oxide formation in combustion processes. Considering the previous finding about FGR application effects, this method was combined with modest reburning (reburning ratio of 10%) and overfire air.
Figure 6 documents that the combination of 10% reburning ratio, overfire air, and 10% FGR more than halved the NO
x emissions compared to the situation without any primary de-NO
x method application. Further increases in denitrification efficiency can be followed in
Figure 7 with increasing FGR, with the highest efficiency value of over 70% being reached at 20% FGR. In contrast to this, application of either modest reburning (reburning ratio of 10%) or overfire air or their combination did not lead to significant denitrification, and the same could be observed at a lower burner load and lower air excess coefficient—see
Figure 2. In both situations the denitrification efficiencies reached below or around 20% maximally.
As
Figure 6 further shows, denitrification efficiency is closely coupled with combustion temperature. The decrease in combustion temperature resulting from the application of primary denitrification methods hinders the formation of thermal NO
x, which, together with the creation of a reductive environment, leads to effective nitrogen oxide emissions reduction. Test experiments aimed at verification of both temperature measurements’ stability and sensitivity to combustion conditions were performed and their results, shown in
Appendix C, prove that temperature changes of around 10 °C and higher can be clearly recognized and attributed to process condition changes. This justifies the conclusions drawn from the analysis of the temperature data and their trends presented in
Figure 5 and
Figure 6.
It can be concluded that OFA application should be part of any combined primary de-NOx method, as it is able to reduce both NOx and CO contents in flue gas on its own, and when applied together with reburning and OFA it should ensure very efficient denitrification while still meeting the CO emission limits.
3.2. Propane–Butane Gas (PBG) Combustion Experiments
This section presents the results of primary de-NOx methods application during propane–butane gas combustion, carried out in parallel with our natural gas combustion experiments. The combustion conditions involved a range of total fuel input Q = 16 to 22 kW and overall air excess coefficient m = 1.1 and 1.2, respectively. Having found that flue-gas recirculation was the most efficient denitrification method in natural gas combustion experiments, it was applied as the sole de-NOx method in our PBG experiments as well.
Figure 8 provides a comparison of NO
x emissions in PBG experiments as a result of different fuel input and air excess coefficient values, with no other denitrification method applied. As expected, and in line with our NG experiment results, increased fuel input and air excess coefficient led to an increase in emissions. Nitrogen oxide emissions values in our PBG experiments were around 20% higher than those observed in the NG experiments under identical conditions (i.e., Q = 1.5 m
3/h and m = 1.1; Q = 1.8 m
3/h and m = 1.2): compare
Figure 2 and
Figure 6.
The impact of FGR application on NO
x emissions reduction in PGB experiments at burner power input 16 kW and air excess coefficient m = 1.1 is shown in
Figure 9. The obtained results are similar to those in our NG experiments, yielding an NO
x content decrease with the introduction and increased share of flue-gas recirculation. Both
Figure 3 and
Figure 9 reveal that the application of 10% FGR decreased the NO
x emissions approximately by half, and a further emissions reduction was achieved with greater FGR increase. Denitrification efficiency of almost 65% was achieved at 20% FGR in our PBG experiments, which is somewhat lower than the almost 75% efficiency documented in the NG experiments under identical conditions.
Similar to the NG experiments, denitrification was found to be closely related to flue-gas temperature in the PBG combustion chamber. Its values for 10%, 15%, and 20% FGR are provided in
Figure 10. The more modest decrease in combustion temperature (thermocouples T13 and T14) of around 50 °C documented in
Figure 10 resulting from an FGR increase from 10% to 20% is lower than the over 100 °C combustion temperature decrease shown in
Figure 6 resulting from FGR increase in the same range in the NG experiments. This probably allows for a partial explanation of why the denitrification efficiency increase in our PBG experiments in the range of 10% to 20% FGR is lower than in the NG experiments.
For this reason, experiments with FGR application alone during both NG and PBG combustion are compared in
Figure 11 in terms of calculated NO
x emissions reduction efficiency.
The trend in nitrogen oxide emissions decrease with increasing FGR can be readily observed in both fuel-type experiments, while the previously commented steeper NOx emissions decrease in NG experiments is clearly confirmed. As a result of the NOx formation reaction mechanisms, the emissions were suppressed due not only to the lower combustion temperature but also to the lower oxygen partial pressure resulting from FGR implementation. The greater NOx emissions reductions observed in our PBG experiments compared to those in the NG experiments most probably resulted from slower combustion of PBG fuel and from its higher volumetric heating value. Higher PBG adiabatic flame temperature compared to NG can play a significant role in this respect too. It results from the observations above that combusted fuel type as well as combustion conditions play an important role in NOx formation and reduction processes. Combustion of PBG yields lower volumetric flow of recirculated flue gas compared to NG for the same burner heat input.