Moderate Intense Low-oxygen Dilution Combustion Research Articles

Experimental study and mechanism analysis of the NOx emissions in the NH3 MILD combustion by a novel burner

NH3 can be used as an H2 energy carrier or an alternative carbon-free fuel because of its high volumetric energy density and easy liquefaction, which leads to convenient and economic transportation and storage within the global energy network. However, the combustion of NH3 is associated with some practical challenges, such as weak reactivity and high NOx emission. In this study, Moderate & Intense Low Oxygen Dilution (MILD) combustion of NH3 was conducted experimentally in a novel burner. The heated and diluted air was produced by burning a premixed lean CH4 mixture using a metal fiber burner installed at the bottom of the combustion chamber and NH3 was injected into the furnace by 20 nozzles oppositely installed on the sidewalls of the combustion chamber at 40 mm above its bottom. A total of 20 cases were investigated in the present work, including premixed combustion of CH4 and CH4/NH3, and MILD combustion of NH3, under various conditions. The Chemical reactor network (CRN) in the ANSYS Chemkin 17.0 and the HUST mechanism were used to simulate and analyze the NO formation mechanism under several experimental conditions. The experimental results showed that the NH3 MILD combustion significantly reduces the NOx emissions. The NOx reduction kinetics and the impact of NH3 flow rate and the equivalence ratio on NO emission in NH3 MILD combustion were analyzed in detail.

New-Concept Gas Turbine Burner Simulation in Moderate Intense Low-Oxygen Combustion Regime

In a trapped-vortex combustor (TVC) flame stabilization is achieved through intense internal exhaust gases recirculation, which is promoted by the adoption of cavities. Thanks to its peculiar features, a trapped-vortex burner produces low pressure drop and emissions and it is characterized by extended blow-out limits. The strong mixing of fresh reactants with flue gases due to internal recirculation represents the basis for the establishment of a distributed MILD, i.e. "Moderate Intense Low-Oxygen Dilution Combustion" regime, which is characterized by reduced temperature peaks, volumetric distributed reactions, low NOx emissions and no thermo-acoustic instabilities. Aim of the work is to study the possibility to obtain a MILD regime in our available trapped-vortex device, taking the advantage of the combined effect of TVC strong internal exhaust gases recirculation and of oxy-combustion external exhaust recirculation, attaining the benefits of CO2 capture at the same time. To this end a series of computational fluid dynamics simulations were conducted on our TVC device, in order to understand the influence on combustion of the main operating parameters, such as the equivalence ratio, the level of dilution, the injection temperature, the velocity, etc.. A preheating temperature and a range of oxygen concentrations that at the same time complies with a distributed reactions regime and an efficient combustion were identified for the premixed and non-premixed operating modes.

Study on characteristics and emission in pulverized coal air-staged MILD combusting

Moderate & Intense Low-oxygen Dilution (MILD) combustion is a useful measure to reduce NO x , CO emission significantly. It also works well to reduce NO x in air-stage combustion. So we take air-stage MILD combustion as our study theme direction. The study was carried on a 0.3 MW/m3 large-scale laboratory entrained furnace whose radius is 500 mm and length is 3000 mm. The study content includes: flow distribution of the entrained furnace; effect on the temperature distribution and NO x , CO emission of the pulverized coal entrained furnace for different air-stage coefficient and stage air location. The main conclusions are as follows: NO emission firstly decreases and then increases as air-stage coefficient increases, CO emission decreases continuely; it is benefit to decrease NO emission but CO emission increases when air-stage location is close to the outlet. NO emission reaches only 139.8 mg/Nm3 when air-stage coefficient is 0.7 and stage air location is 2600 mm (all the emission value in this test for the NO and CO is converting to O2 concentration ϕ (O2)=9%). It slightly increases the carbon content in fly ash using air-stage combustion and combustion stability does not change significantly.

Scalar gradient behaviour in MILD combustion

The results of three-dimensional Direct Numerical Simulation (DNS) of Moderate, Intense Low-oxygen Dilution (MILD) and conventional premixed turbulent combustion conducted using a skeletal mechanism including the effects of non-unity Lewis numbers and temperature dependent transport properties are analysed to investigate combustion characteristics using scalar gradient information. The DNS data is also used to synthesise laser induced fluorescence (LIF) signals of OH, CH2O, and CHO. These signals are analysed to verify if they can be used to study turbulent MILD combustion and it has been observed that at least two (OH and CH2O) LIF signals are required since the OH increase across the reaction zone is smaller in MILD combustion compared to premixed combustion. The scalar gradient PDFs conditioned on the reaction rate obtained from the DNS data and synthesised LIF signals suggests a strong gradient in the direction normal to the MILD reaction zone with moderate reaction rate implying flamelet combustion. However, the PDF of the normal gradient is as broad as for the tangential gradient when the reaction rate is high. This suggests a non-flamelet behaviour, which is due to interaction of reaction zones. The analysis of the conditional PDFs for the premixed case confirms the expected behaviour of scalar gradient in flamelet combustion. It has been shown that the LIF signals synthesised using 2D slices of DNS data also provide very similar insights. These results demonstrate that the so-called flameless combustion is not an idealised homogeneous reactive mixture but has common features of conventional combustion while containing distinctive characteristics.