#
Analysis of Oscillating Combustion for NO_{x}−Reduction in Pulverized Fuel Boilers

^{1}

^{2}

^{3}

^{*}

^{†}

## Abstract

**:**

_{X}emissions, downstream reduction processes (Selective Non-Catalytic Reduction, SNCR or Selective Catalytic Reduction) are applied, which use of operating resources (essentially ammonia water) thereby increase. By the means of an experimentally validated process, by which pulverized fuel is fed by oscillation through a swirl burner into a pilot combustion chamber with a thermal output of 2.5 MW, nitrogen oxides can be reduced without further activities, for instance from 450 mg/m

_{N}

^{3}in non-oscillation operation mode (0 Hz) to 280 mg/m

_{N}

^{3}in oscillation operation mode (3.5 Hz), normalized to an O

_{2}–content of 6% each. These findings were patented in EP3084300. Particularly promising are the experiments which utilize oscillation of a large portion of the burn out air instead of the fuel in order to minimize the fatigue of the pulverized fuel oscillator, amongst others. Thereby, the nitrogen conversion rate, which describes the ratio of NO

_{X}to fuel nitrogen, including thermal NO

_{X}can be reduced from 26% for non-oscillation operation mode down to 16%. The present findings show that fuel oscillation alone is not sufficient to achieve nitrogen oxides concentrations below the legislative values. Therefore, a combination of different primary (and secondary) measures is required. This paper presents the experimental results for oscillating coal-dust firing. Furthermore, an expert model based on a multivariate regression is developed to evaluate the experimental results.

## 1. Introduction

_{2}are combined as NO

_{X}and play a special role in the assessment of pollutant emissions.

_{3}and thus lead to acid rain. In addition to the formation of ground-level ozone, NO

_{X}also contributes to the chain reactions of ozone depletion in the stratosphere. This desired ozone protects the earth from increased ultraviolet radiation from the sun. In addition, NO

_{X}acts as a pure substance or in high concentrations as a strong respiratory poison and leads to damage to the respiratory organs.

_{X}concentration in the flue gas of industrial plants is reduced by two different measures. The so-called primary measures are based on the combustion fundamentals of nitrogen oxide formation. These formation mechanisms can be inhibited by clever air guidance in the combustion chamber. Subsequent denitrification of the flue gas is one of the secondary measures and includes processes such as Selective Catalytic Reduction (SCR) and Selective Non Catalytic Reduction (SNCR). Here, the nitrogen oxides are reduced to ammonia by means of a catalyst or a reducing agent such as urea. The subsequent treatment of the flue gases is usually associated with higher investments and a decrease in the efficiency of the plant, which is why primary denitrification is more economical and ecological [2].

_{x}formation. The rate of reduction could reach 50% and more compared to non-oscillating combustion, depending also on temperature, residence time and mixing conditions.

_{X}emissions. The reason for this is the changing stoichiometry that occurs in the flame range, as a result of which the reaction mechanisms of nitrogen oxide formation lead to less NO

_{X}. The investigations carried out up to date have largely been limited to the oscillating addition of natural gas [4], which is easier to oscillate in the process. Due to the high abrasive properties and the associated process engineering challenges in the oscillating combustion of coal dust, there are hardly any investigations in this area. For the investigations presented in this paper the burner was equipped with a valve to initiate the oscillating combustion.

## 2. Basic Principles

#### 2.1. Fuel-NO Formation

_{3}, NO and N

_{2}[8]. References [7,8,9,10] give detailed literature overviews on the fuel-N mechanism, whose simplified scheme in Figure 1 is shown in green with the other two formation mechanisms, thermal in orange and prompt in blue.

_{i}molecule then reacts either by oxidation to nitric oxide or under reducing conditions with NO to molecular nitrogen. The NO-recycling reaction of nitric oxide to HCN takes place under CH

_{i}radicals and air deficiency effects and allows the formed nitrogen monoxide to be returned to the fuel N reaction path. In this way, NO that has already been formed can be returned to molecular nitrogen depending on the air ratio λ.

_{3}and HCN in the flue gas as Total Fixed Nitrogen (TFN) and the shift of their ratios as a function of the air ratio.

_{3}are also converted to NO in the atmosphere and are therefore classified as harmful emission components [7]. Starting from a low air ratio of λ = 0.6, the proportion of the NO precursor species HCN and NH

_{3}decreases significantly and the NO component increases steadily. For low air ratios of λ < 0.7, this can be explained by the sub-stoichiometric ratios and low concentration of oxidizing components. With an increasing air ratio for λ > 0.7, the proportion of the NO

_{X}component increases steadily due to the available oxidizing radicals, whereas the concentrations of the two precursor species are close to zero. The minimum TFN is reached at an air ratio of λ ≈ 0.7 and forms the basis for staged combustion for NO

_{X}reduction, since the conditions for NO recycling are met here Figure 1, a increased conversion to molecular nitrogen [6].

#### 2.2. Oscillating Combustion as Primary Measure

_{X}emissions from furnaces for gaseous fuels.

_{x}-reduction (SCR, SNCR), due to less formation of NO

_{x}. The disadvantages include higher burner requirements (development of special valves, bigger pipe diameters, abrasion, resonance), as well as lower average burner output at the same burner size.

_{X}emissions of 31–67% and an improved heat transfer of the flame of up to 13%. The results were confirmed by [3,4,12,13] for other combustion systems.

_{1}CO, NH

_{3}and NO

_{X}are increasingly produced while in the fuel-poor periods of time ∆t

_{2}CO reacts to CO

_{2}and NO

_{X}with NH

_{3}to N

_{2}and H

_{2}O [3].

_{X}reduction potential is investigated by a oscillating addition of a gaseous energy carrier such as natural gas. Although [3] shows in his work that a oscillating addition of secondary air during the combustion of chipboard cubes in a fixed-bed reactor also leads to a significant reduction in NO

_{X}emissions, none of the previous investigations shows the effect of the oscillating combustion of a solid on NO

_{X}emissions. The challenge lies in the high abrasion of the valve during the oscillating conveyance of coal dust. In this paper the results of the oscillating combustion of pulverized coal with a coal burner are investigated and complement the previous work of [3].

## 3. Methods

#### 3.1. Pilot-Scale Power Plant BRENDA

^{3}/h of natural gas which is required for safety reasons of operation.

_{X}emissions of the pulverized coal burner, the NO

_{X}concentration is measured at level 0 and level 2. Level 0 characterizes the flue gas from the rotary kiln burner which enters the coal burner section. The NO

_{X}emission resulting from the pulverized coal firing is determined by the difference between the NO

_{X}load of the dry flue gas from level 2 and level 0. Since the air ratio at the coal burner was always greater or equal to 1 the NO- recycling mechanism (see Figure 1) is inhibited. Thus, it is assumed that the nitrogen oxides entering level 0 are inert and don’t react anymore with other gas compounds. For the discussion of the impact of oscillation on the NOx, in level “0” and “2” NO

_{x}, CO, O

_{2}and CO

_{2}are measured. The emissions of SO

_{2}, HCl etc. are not relevant for this purpose, of course they are analyzed after the boiler and the stack with regard to the emission levels.

_{0}, c

_{1}and c

_{2}consider the burner geometry whose profile can be seen in Figure 5.

_{p50}was about 50 µm.

#### 3.2. Screening Campaign A

_{x}−concentration. With the model tests, the previously determined influences of the oscillation frequency and the height of the volume flows in R1 and R2, the volume flow of the combustion air from the rotary kiln, the quantity of coal supplied and the volume flow conveying coal were examined. The data from the screening campaign was used to create a multivariate regression model considering expert knowledge further called Expert model.

_{X}emissions. That’s why in this campaign the effect of oscillating volume flows in R1 and R2 without fuel were investigated in 4 levels. Due to safety limitations at certain test settings, it was not possible during operation to set a minimum volume flow of the air in R2 to 50 m

_{N}

^{3}/h ein. In these cases, the volume flow in R2 could only be reduced to 65 m

_{N}

^{3}/h and led to a subsequent adaptation of the test plan.

#### 3.3. Expert Model

_{X}concentration as a function of the main influencing parameters. As additional interactions, the two interactions between the oscillation frequency and the level of the oscillated air flow are included in the model. In addition to the varied parameters, the swirl number S according to Equation (1) and ${\mathsf{\lambda}}_{burner}$ according to Equation (4). were added to the model. Table 2 shows the influencing variables selected for the model and the corresponding abbreviation.

#### 3.4. Test Campaign B

_{X}reduction in the flue gas. In the first part the influence of the height of the oscillated air flow by keeping the air ratio ${\mathsf{\lambda}}_{burner}$, the incoming air and coal flow in Z constant was investigated. The second part focussed on proving the reproducibility of the test settings from the past, where a high air flow was oscillated. The test settings of both investigations are described in the following paragraphs.

_{N}

^{3}/h due to the minimum air volume technically required for burner cooling. This resulted in a slight deviation from ${\mathsf{\lambda}}_{burner}$ of 1.12 instead of 1.11.

## 4. Results and Discussion

^{3}/h) and coal, the measurements start according the parameters shown in Table 1, Table 3 and Table 5.

_{x}values are not normalized to 6 Vol.-% of oxygen.

^{3}; NO

_{x}: 79.7 mg/m

^{3}; O

_{2}11.9 Vol.-%; CO

_{2}: 6.7 Vol.-%; gas temperature: 842 °C.

^{3}; NO

_{x}: 315 (182) mg/m

^{3}; O

_{2}10.0 (9.8) Vol.-%; CO

_{2}: 8.3 (8.4) Vol.-%; gas temperature: 949 (946) °C; total carbon (TC) in the fly ashes: 1.44 (2.37) wt.-%.

#### 4.1. Screening Campaign A

_{x}concentrations, so additional calculations with the EDI-Hive-model as well as the expert model are required to get more specific answers for the question: which are the main influencing parameters for the oscillation combustion?

_{X}concentration at a set oscillation frequency of 3 Hz.

_{X}concentration at a oscillation of 3 Hz in the right part of the figure shows a clear reduction of the NO

_{X}concentration with increasing volume flow. The point at which the effect of the volume flow dependence reverses in R2 could be determined with the model generator and is at a oscillation frequency of 0.8 Hz.

_{X}concentration. At a oscillation of 3 Hz the increase of the volume flow leads to a significant reduction of the nitrogen oxide concentration.

_{i}species from Figure 1 react directly to NO under these conditions and the reaction to molecular nitrogen is inhibited here. In the case of an increase in volume flow at a oscillation of 3 Hz, a similar result should also be obtained. But apparently a more concise oscillating profile of the local stoichiometry can be generated by oscillation of higher volume flows. This allows the reaction of the formed NO to molecular nitrogen in the time gaps of the interruption of the volume flow. It should be mentioned that a higher volumetric flow has a larger impulse than a lower volumetric flow due to the equation $\dot{I}=\dot{m}\ast v$. The higher the impulse of air (by an increase of the volumetric flow and corresponding to a higher velocity) the better the interruption of the coal mass flow is possible. Not to the extent that the oscillation of the fuel flow would cause it, but to the extent that favourable reaction conditions prevail for the reduction of nitrogen oxide emissions.

#### 4.2. Expert Model

_{X}emission data, the calculated predictive accuracy of the model for training and test data and the F-value of the model are shown in Figure 8. For the tabular F-value with the degrees of freedom f

_{1}= 11 and f

_{2}= 36, an F-value of F

_{11,36,0.95}= 2.07 is obtained. The model has a higher F-value and therefore contains variables describing the problem more significantly.

_{N}

^{3}. The unit and the value of the coefficients are given in the table below. For the non-standardized regression coefficients, no comparison is possible regarding the weighting due to the different orders of magnitude of the parameters. For this reason, the standardized, dimensionless beta coefficients are also shown. With them the coefficients can be compared regarding their weighting because of the previously carried out standardization of the data.

_{norm}to g

_{norm}and the interaction terms ae

_{norm}and bf

_{norm}is given by the following pattern.

_{X}emissions should increase accordingly. The effect of the stoichiometric air ratio can be explained by the anti-proportionality to the coal mass flow (see Equation (3)). Similarly, the linear influences of the oscillation frequency a and b have a positive sign and the interaction terms of the two variables ae and bf have a negative sign. Thus, according to Equation (5), the effects are contrary related to the target quantity. Such dependencies can be used to map the interactions for the oscillation frequency and the volume flow in R2 from Figure 7. with a statistical model. Furthermore, the model assumes that the parameters are varied within the investigated limits of the design space.

_{X}emission is due to the influence of the increased fuel input and corresponds to the findings from the analysis of the regression coefficients from Equation (5). The increase in the fuel input is directly reflected in the coal mass flow; the air ratio λ takes into account the incoming air flows and is anti-proportional to the coal mass flow according to Equation (4). The oscillation frequency in R2 and the interaction of the oscillation frequency with the volume flow in R2 reflect the investigated effect of air oscillation. Obviously, it is possible to exert a significant effect on NO

_{X}emissions by oscillating the air volume flow. The influence of the swirl number also has a significant effect in the evaluated data. This is consistent with the observation in Equation (5), where the swirl number has the third largest standardized regression coefficient.

#### 4.3. Test Campaign B

_{N}

^{3}/h from the annular gap R2 was investigated.

_{X}emissions on the level of the oscillated volume flow in R2 and the oscillation frequency at a constant swirl and a constant air ratio at the burner ${\mathsf{\lambda}}_{burner}$ is shown. The reference value is the NO

_{X}concentration at the respective non-oscillated state. The further parameter settings of the constant incoming flows can be seen on the side of the figure and in Table 3. The nitrogen oxide emissions at a volume flow of 65 m

_{N}

^{3}/h increase slightly due to the oscillation of 2.67 Hz. By increasing the volume flow entering the annular gap R2 to 164 m

_{N}

^{3}/h, the nitrogen oxide emissions in the non-oscillated state are significantly increased. On the other hand, the oscillation of this volume flow shows a significant reduction in nitrogen oxide emissions. For a volume flow of 180 m

_{N}

^{3}/h, the nitrogen oxide emissions in the non-oscillated state are somewhat higher than for a volume flow of 164 m

_{N}

^{3}/h, but a similar behavior in NO

_{X}reduction can be observed here regarding the oscillated state.

_{N}

^{3}/h in the oscillated state. Reference [19] emphasizes the careful coordination of the different flow fields of the fuel-related primary air streams and the fuel-remote secondary air streams in a pulverized coal burner. Accordingly, the increase in the nitrogen oxide concentration in the flue gas during the shift of the combustion air into the annular gap R2 is explained by a worse preheating of the coal mass flow. This leads to an unfavorable combustion behavior of the coal dust. At the same time, a swirl number of 0.3 is not enough to create a recirculation area which would guarantee better preheating of the fuel flow.

_{theo}≈ 0.5. However, at the prevailing temperatures the formation mechanisms are limited to the fuel N mechanism. According to Figure 1, an increase in oxygen concentration leads to the preferred reaction path from HCN via NH

_{i}to NO. Since there are no reducing conditions at an increased oxygen supply in the flame, the reaction of NH

_{i}with already formed NO to N

_{2}is inhibited. Although ${\mathsf{\lambda}}_{loc}$ < 1 applies to the local stoichiometries of all investigated test settings, ${\mathsf{\lambda}}_{loc}$ is significantly larger at the higher volume flows in R2. If one now compares the local stoichiometry with the TFN curve shown in Figure 2, the NO

_{X}concentration increases with increasing air ratio. In addition, the exhaust gas mixture contains more HCN and NH

_{3}at the local air ratios of 0.28 to 0.51. The concentration of HCN and NH

_{3}in the exhaust gas mixture increases with the local air ratios of 0.28 to 0.51. For both N-species, the residence time in the post-combustion chamber is enough to react to NO

_{X}at a total air ratio in the post-combustion chamber of 1.1 to 1.4 up to the measuring point of the nitrogen oxide concentration. From consideration of the oscillation effect of the two high oscillated volume flows of 164 and 180 m

_{N}

^{3}/h at 2.67 Hz, the nitrogen oxide emission can be reduced despite the high local air ratios. The interruption of the volume flow thus leads to alternating reducing and oxidizing conditions in the flame, resulting in a NO

_{X}reduction. Considering Figure 1, more NO is produced under oxidizing conditions. Under the reducing conditions the NO is reduced to N

_{2}. In parallel, NO molecules can be returned to the reaction path of the fuel-N mechanism via the NO-Recycle mechanism. These two mechanisms explain the lower nitrogen oxide emissions. In further considerations, it would be useful to measure the local concentrations of the N-species in order to obtain more conclusions about the formation mechanism under oscillating conditions. Related to the reference value of the nitrogen oxide concentration at 65 m

_{N}

^{3}/h and without oscillating addition of air, no reduction of the nitrogen oxide emissions can be observed at the selected test settings, but the results show that the NO

_{X}formation mechanisms can be substantially reduced by oscillation.

^{3}

_{N}/h, a longer flame and a more uniform distribution of the bright flame range over the entire length of the flame result. The higher volume flow in the tangentially introduced combustion air leads to a more stable, elongated flame pattern.

_{N}

^{3}/h show a higher flame brightness in the area near the flame root than in the burnout area of the flame. Here the volume flow of combustion air is significantly reduced (cf. Table 3), which leads to a shorter and bushier flame.

^{3}

_{N}/h, since both the relative flame fraction area and the relative stable flame fraction area stable are significantly lower for the two flames.

^{3}

_{N}/h, a more uniform burnout of the combustion gases along the flame length is possible, which is associated with a lower nitrogen oxide concentration in the flue gas (cf. Figure 3). At Definition there are no Type I/II flames due to the too low swirl number, which means that the results cannot be compared with the previous studies of [10]. However, the influence of the flame shape on the nitrogen oxide concentration in the flue gas can be clarified from the optical evaluations discussed above.

_{X}emissions due to oscillation of the volume flow in R2 at 2.67 Hz is clearly visible. For the two experiments a constant nitrogen oxide concentration of 620 mg/m

_{N}

^{3}for the oscillation-free and 350 mg/m

_{N}

^{3}N for the oscillating process control is achieved over the 60-min test period. Accordingly, the nitrogen oxides in the flue gas are reduced by 45%.

_{X}concentrations of the trials from 2018 are compared to the results from 2019. The settings of the parameters in Figure 14 are according to Table 4.

_{X}concentration by −43%.

## 5. Conclusions

_{X}emissions. The campaign consisted of a screening trial where the data was used to identify relevant influencing parameters using a statistical model. Based on these results a second trial was initiated to further investigate the optimal test settings and the reproducability of the test setup.

_{X}concentration of the investigated test area could be reduced by up to 25%.

_{N}

^{3}/h of air was added from the outer annular gap R2 at a oscillating frequency of 2.67 Hz. Here, the nitrogen oxide concentration in the flue gas could be reduced by 45% from 624 mg/m

^{3}

_{N}at 0 Hz to 342 mg/m

^{3}

_{N}at 2.67 Hz in relation to 6% by volume oxygen. This result was in line with the above-mentioned finding that the level of the oscillated air volume flow is essential for nitrogen oxide reduction in the flue gas. The results could be reproduced and lead to an average reduction potential of 43%.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**NO formation mechanisms [8].

**Figure 2.**TFN in dependency of the air ratio λ [11].

**Figure 3.**Behavior of stoichiometry in an oscillating combustion [4].

System | Parameter | Level | Unit | ||||
---|---|---|---|---|---|---|---|

1 | 2 | 3 | 4 | 5 | |||

Rotary kiln burner | Combustion air rotary kiln burner | 1200 | 1400 | - | - | - | m_{N}^{3}/h |

Coal burner | Mass flow coal Z | 70 | 90 | - | - | - | kg/h |

Air flow Z | 70 | 90 | - | - | - | m_{N}^{3}/h | |

Air flow R1 | 80 | 90 | 100 | 110 | - | m_{N}^{3}/h | |

Oscillating frequency R1 | 0 | 1 | 2 | 2.67 | 3 | Hz | |

Air flow R2 | 50 (65) | 70 | 90 | 110 | - | m_{N}^{3}/h | |

Oscillating frequency R2 | 0 | 1 | 2 | 2.67 | 3 | Hz |

Abbr. | Parameter | Unit |
---|---|---|

a | Oscillating frequency R1 | Hz |

b | Oscillating frequency R2 | Hz |

c | Mass flow coal Z | kg/h |

d | Air flow Z | m_{N}^{3}/h |

e | Air flow R1 | m_{N}^{3}/h |

f | Air flow R2 | m_{N}^{3}/h |

g | Combustion air rotary kiln burner | m_{N}^{3}/h |

h | Swirl number S | - |

i | ${\mathsf{\lambda}}_{burner}$ | - |

ae | Interaction in R1 | Hz m_{N}^{3}/h |

bf | Interaction in R2 | Hz m_{N}^{3}/h |

System | Parameter | Trial | Unit | |||||
---|---|---|---|---|---|---|---|---|

1 | 2 | 3 | 4 | 5 | 6 | |||

Coal burner | Mass flow coal Z | 70 (constant) | kg/h | |||||

Air flow Z | 80 (constant) | m_{N}^{3}/h | ||||||

Swirl number S | 0.3 (constant) | |||||||

${\mathsf{\lambda}}_{loc}$ | 0.28 | 0.28 | 0.48 | 0.48 | 0.51 | 0.51 | - | |

${\mathsf{\lambda}}_{burner}$ | 1.11 | 1.11 | 1.11 | 1.11 | 1.12 | 1.12 | - | |

Combustion air | 420 | 420 | 321 | 321 | 313 | 313 | m_{N}^{3}/h | |

Air flow R2 | 65 | 65 | 164 | 164 | 180 | 180 | m_{N}^{3}/h | |

Oscillating frequency R2 | 0 | 2.67 | 0 | 2.67 | 0 | 2.67 | Hz |

System | Parameter | Trial | Unit | |
---|---|---|---|---|

1 | 2 | |||

Coal burner | Mass flow coal Z | 57 (constant) | kg/h | |

Air flow Z | 70 (constant) | m_{N}^{3}/h | ||

Swirl number S | 0.3 | - | ||

${\mathsf{\lambda}}_{loc}$ | 0.6 (constant) | - | ||

${\mathsf{\lambda}}_{burner}$ | 1.22 (constant) | - | ||

Combustion air coal burner | 258 (constant) | m_{N}^{3}/h | ||

Air flow R1 | 0 (constant) | m_{N}^{3}/h | ||

Oscillating frequency R2 | 0 | 2.67 | Hz | |

Air flow R2 | 180 (constant) | m_{N}^{3}/h |

Abbr. | Parameter | Unit | ${\mathit{\beta}}_{\mathit{j}}/\mathbf{Unit}$ | ${\mathit{\beta}}_{\mathit{j},\mathit{n}\mathit{o}\mathit{r}\mathit{m}}/\mathbf{mg}/{{\mathbf{m}}_{\mathbf{N}}}^{3}$ | |
---|---|---|---|---|---|

- | Interception y axis | - | 8485.27 | mg/m_{N}^{3} | 598.66 |

a | Oscillating frequency R1 | Hz | 55.62 | mg/(Hz m_{N}^{3}) | 65.94 |

b | Oscillating frequency R2 | Hz | 26.08 | mg/(Hz m_{N}^{3}) | 64.06 |

c | Mass flow coal Z | kg/h | −43.43 | (h mg)/(kg m_{N}^{3}) | −421.64 |

d | Air flow Z | m_{N}^{3}/h | 2.5 | (h mg)/m_{N}^{6} | 12.39 |

e | Air flow R1 | m_{N}^{3}/h | 1.78 | (h mg)/m_{N}^{6} | 20.71 |

f | Air flow R2 | m_{N}^{3}/h | 1.92 | (h mg)/m_{N}^{6} | 44.10 |

g | Combustion air rotary kiln burner | m_{N}^{3}/h | 0.04 | (h mg)/m_{N}^{6} | 3.98 |

h | Swirl number S | - | −2901.28 | mg/m_{N}^{3} | −118.61 |

i | ${\mathsf{\lambda}}_{burner}$ | - | −3035.93 | mg/m_{N}^{3} | −444.21 |

ae | Interaction in R1 | Hz m_{N}^{3}/h | −0.61 | (h mg)/(Hz m_{N}^{6}) | −66.35 |

bf | Interaction in R2 | Hz m_{N}^{3}/h | −0.79 | (h mg)/(Hz m_{N}^{6}) | −83.19 |

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Jolibois, N.; Aleksandrov, K.; Hauser, M.; Stapf, D.; Seifert, H.; Matthes, J.; Waibel, P.; Vogelbacher, M.; Keller, H.B.; Gehrmann, H.-J.
Analysis of Oscillating Combustion for NO_{x}−Reduction in Pulverized Fuel Boilers. *Inventions* **2021**, *6*, 9.
https://doi.org/10.3390/inventions6010009

**AMA Style**

Jolibois N, Aleksandrov K, Hauser M, Stapf D, Seifert H, Matthes J, Waibel P, Vogelbacher M, Keller HB, Gehrmann H-J.
Analysis of Oscillating Combustion for NO_{x}−Reduction in Pulverized Fuel Boilers. *Inventions*. 2021; 6(1):9.
https://doi.org/10.3390/inventions6010009

**Chicago/Turabian Style**

Jolibois, Nicklas, Krasimir Aleksandrov, Manuela Hauser, Dieter Stapf, Helmut Seifert, Jörg Matthes, Patrick Waibel, Markus Vogelbacher, Hubert B. Keller, and Hans-Joachim Gehrmann.
2021. "Analysis of Oscillating Combustion for NO_{x}−Reduction in Pulverized Fuel Boilers" *Inventions* 6, no. 1: 9.
https://doi.org/10.3390/inventions6010009