oxy-MILD Combustion Research Articles

Numerical simulations on Oxy-MILD combustion of pulverized coal in an industrial boiler

The purpose of this study is to analyze the possibility of combining two innovative combustion technologies in large-scale pulverized coal fired plants: Moderate or Intense Low Oxygen Dilution (MILD) and oxy-combustion. The combination of both technologies, namely Oxy-MILD combustion, is expected to bring synergetic effects: NOx reduction, CO2 capture possibility, fuel flexibility and uniformity of heat fluxes and species concentrations. In this work, the predictable advantages of adopting this technology, with respect to conventional boilers, are evaluated by means of CFD modeling, in terms of pollutant emissions and uniformity of heat fluxes. In a previous work, the developed CFD model was validated against available experimental data of MILD combustion in a pilot-scale furnace and it was demonstrated that the proposed model captures the combustion features with good accuracy. In order to identify the effective potential of Oxy-MILD combustion and its possible uses on an industrial scale, an application in the boiler is analyzed in the current work. Results show that the temperature and species concentration distributions reach an acceptable level of uniformity in the boiler; similarly, the wall heat flux profile is uniform along the boiler height, as in the fluidized bed technology. The CO2 concentration at the boiler exit, for an excess oxygen ratio of 1.1, is about 95.8%; this value is slightly increased for lower excess oxygen ratio, even though in this case, incomplete combustion occurs. Finally, lower NOx (70 mg/MJ) than other technological solutions are obtained owing to the prevention of thermal and prompt NOx, owing to the absence of N2 in the oxidizer, and to the re-burning mechanism, which is predominant especially when recycled NOx is considered.

NOx emissions for oxy-mild combustion of pulverized coal in high temperature pre-heated oxygen.

Oxy-MILD (Moderate or Intensive Low Oxygen Dilution) combustion is an attractive technology for increasing combustion efficiency and reducing the nitric oxides with respect to the conventional combustion in traditional boilers or furnaces. This technology is a combination of MILD combustion, which exploits the initial preheating of combustion air up to 800÷1300 °C and the simultaneous recirculation of the hot flue gases, and of Oxy-Combustion, which uses pure oxygen (greater than 95% purity) as oxidant instead of air. The combination of both technologies is expected to bring synergetic effects: NOx reduction, CO2 capture possibility, fuel flexibility and uniformity of heat fluxes and species concentrations. This study focuses, in particular, on the analysis, by a CFD approach, of the NOx emissions for Oxy-MILD combustion of pulverized coal in a new concept of industrial boiler. The boiler, fueled with a high-volatile coal, is equipped with five burners and three outlets of the combustion products, localized on the top wall. Each burner is made-up of a central pulverized coal jet surrounded by six preheated oxygen jets. This configuration allows the achievement of the MILD combustion in pure oxygen. Results show that the goal of NOx reduction with this combustion approach is achieved. A value of about 296 mg/Nm3@6%O2 at the boiler exit is obtained under Oxy-MILD conditions, which is significantly lower than 600-800 mg/Nm3@6%O2 obtained in a traditional boiler.

Experiment investigation of coal MILD-Oxy combustion integrated with flue gas recirculation at a 0.3 MWth furnace

This paper presents an experimental study of coal MILD-Oxy combustion at 0.3mw pilot-scaled facility in Huazhong University of Science and Technology (HUST), which contains an entirely flue gas recycle system. By increasing the jet velocity to 100m/s, the MILD combustion was achieved without high temperature preheating. Meanwhile the experiment was operated under Oxy-fuel atmosphere that was established by the flue gas recirculation. A swirl Oxy-fuel combustion was also tested as a comparison. The CO2 concentration in flue gas could sustain a high level during the combustion. The major conclusions include: The coal MILD-Oxy combustion can be achieve by high-velocity jet in some extent. But the full dilution and homogenization of heat and reactants can't be achieved under non-premixed combustion because of the separation of fuel and oxygen, which means the ideal MILD-Oxy combustion can't be achieved but only be approached. The reaction zone was enlarged to the vast area of furnace, in which the combustion temperature and reaction rate decreased. The carbon burnout rate decreased due to the reduction of residence time and reaction rate. The NOx emission decreased nearly 40% because of the lower flame temperature.

CFD modeling and thermodynamic analysis of a concept of a MILD-OXY combustion large scale pulverized coal boiler

A concept of a large scale pulverized coal boiler working in the oxycombustion mode has been proposed. The combustion chamber has been designed using the Moderate and Intensive Low-oxygen Dilution (MILD) combustion technology. By screening CFD solutions of several geometric configurations the best one has been selected, for which the dependence of the oxygen excess ratio and recirculation rate of the flue gases analyzed onto the efficiency of the combustion chamber has been investigated. The results of the CFD simulations have been then embedded into a process model of the entire plant. The preliminary simulations show the possibility of efficiency increase of more than 3% points.

Flame Characteristics of CH4/H2 on a Jet-in-Hot-Coflow Burner Diluted by N2, CO2, and H2O

To enhance the knowledge to the combination of oxy-fuel combustion and MILD (moderate or intense low-oxygen dilution) combustion, namely oxy-MILD combustion, the present article examines the effects of different diluents (i.e., N2, CO2, and H2O) on the flame characteristics of CH4/H2 blended fuel in a JHC (jet-in-hot-coflow) burner, which can produce MILD combustion regime in the upstream and conventional combustion regime in the downstream. CFD modeling method considering RANS equations and detailed reaction mechanism (GRI-Mech 2.11) are used for such purpose. Results show that, in MILD combustion regime, peak temperature under oxy-fuel condition (diluted by CO2 and H2O) is reduced as compared to air-fuel condition (diluted by N2) from both physical and chemical effects of the diluents. However, in the conventional combustion regime, chemical effect of the diluents on suppressing the peak temperature is weakened, resulting in the predominance of its physical effect. As for the flame ignition behavior, th...

Numerical Investigation of Oxy-mild Combustion of Pulverized Coal in a Pilot Furnace

Abstract The Conventional coal-fired plants are large contributors to air pollution and greenhouse gas. The combustion generates pollutants such as oxides of sulphur, nitrogen, and carbon as well as fine organic and inorganic particulates. The new technologies able to reduce drastically the pollutant emissions and facilitate to use of coal in an environmentally more friendly way, are commonly known as clean coal technologies (CCT). In this context the CCS technologies play an important role to reduce the CO2 emissions. The only form with truly zero CO2 emissions in existence today is pre-combustion gas separation, namely, the combustion of fuel using oxygen instead of air. It is well known that burning pulverized coal in pure oxygen increases the flame temperatures, thus also increases NOx emissions. Therefore, to moderate the flame temperature and reduce NOx the oxygen is mixed with recycled flue gas (RFG). This approach to reduce CO2 emissions is often called oxy-firing or oxy-fuel combustion. The purified CO2 stream is then compressed and condensed to produce a manageable effluent of liquid CO2, which can be sequestered for storage (CCS) or for use in subsequent processes (CCR). MILD (Moderate or Intensive Low Dilution) or HiTAC (High Temperature Air Combustion) is an innovative combustion technology and probably the most important achievement of the combustion technology in recent years. In MILD combustion the reactions take place in almost the whole volume of the combustion chamber. This leads to temperature and species concentration fields uniform in the chamber. The fuel is oxidized in an environment that contains a substantial amount of inert gases (N2, CO2, H2O) and low oxygen concentrations. This is caused by an internal recirculation of combustion products generated by injecting preheated air jets into the combustion chamber with very high momentum, bringing the temperatures close to the combustion products temperature, reducing the NOx emissions. Because both technologies allow reductions of pollutant emissions, the aim of this work is to demonstrate the advantages of a combination of these two combustion technologies in order to analyze the temperature and specie concentrations field, the CO2 and NOx emissions by means of CFD. The goal is understand if it is possible to combine the MILD combustion and OXY one in order to reduce the NOx emissions, and capture the CO2.

Numerical study of oxy-fuel MILD (moderate or intense low-oxygen dilution combustion) combustion for CH4–H2 fuel

This paper demonstrates a numerical study on the combination of Oxy-Fuel and MILD (moderate or intense low-oxygen dilution combustion) combustions, i.e. OXY-MILD. The N2 of a hot oxidizer was replaced with CO2 and H2O in a MILD combustion test case. The study was conducted using a CFD analysis, a zero-dimensional well-stirred reactor analysis, and a reactors network analysis. In the CFD analysis, RANS equations with modified k−ε equations were solved for a 2D-axisymmetric computational domain. Results showed a decrease in temperature gradient, reaction rate, and Damköhler number under the OXY-MILD condition in comparison with the MILD one. It seems the higher the oxygen level in the preheated oxidizer, the more effective the removal of N2 from the hot oxidizer was on the uniformity and extending of the reaction zone. Under OXY-MILD condition, an increment of inlet oxygen level decreased concentrations of CO, NO, CH2O, and HCO and increased temperature in the reaction zone. Furthermore, the effect of fuel hydrogen content on the reaction zone was investigated. A reduction of fuel hydrogen content led to an increase in both the uniformity of temperature field and the area of the reaction zone, and a decrease in formations of NO and CO for the OXY-MILD combustion.

Numerical study of H2O addition effects on pulverized coal oxy-MILD combustion

This paper numerically investigates the effects of H2O addition on pulverized coal oxy-MILD (moderate or intense low oxygen dilution) combustion based on our previous study, in which only O2/CO2 was treated as oxidizer. While in the present study, the initial H2O mole fraction in oxidizer is varied at a constant oxygen level (30% by volume) to represent four typical oxy-fuel operations, i.e. ideal dry recycle (0% H2O+70% CO2), practical dry recycle (5% H2O+65% CO2), wet recycle (15% H2O+55% CO2) and oxy-steam (70% H2O+0% CO2). It is revealed that, with the addition of H2O, the internal recirculation rate (KV) as well as the coal ignition is improved, however, due to the difference in physical effects between CO2 and H2O, the temperature peak is likewise promoted. CO2 is preferred than H2O to moderate the flame temperature and realize uniform temperature distribution under MILD combustion. Higher H2 and lower CO formations are obtained by adding H2O because of the competing char gasification reactions between CO2 and H2O under in-furnace low oxygen condition of MILD combustion. H2O addition tends to suppress the volatiles-N conversion while accelerate the char-N conversion in the upper furnace, respectively, and reduce the final NO emission by enhanced H2 formation. Moreover, H2O addition is beneficial to increasing heat transfer in both radiative and convective forms.

Numerical study of combustion characteristics for pulverized coal under oxy-MILD operation

MILD (Moderate or Intense Low-oxygen Dilution) combustion is a novel technology to reduce NOX emission from combustion. With the development of this technology, it is expected to combine this technology with oxy-fuel combustion to jointly control the pollutants from fossil fuel combustion. The present paper investigates the combustion characteristics of coal under oxy-MILD operation with the aid of CFD simulations. A seven-step global reaction mechanism is used for modeling coal combustion involving gaseous volumetric reaction and particle surface reaction. The predicted results using the adopted models for the reference case agree well with the experimental results. It is revealed that, the combustion temperature level increases with the enhancement of initial oxygen concentration because of the reduced injection momentum and promoted heat release. The in-furnace CO formation increases but the final CO emission reduces when enhancing the oxygen concentration under oxy-MILD combustion. Moreover, oxy-MILD combustion shows a greater potential on NOX formation reduction than air-MILD combustion, and this potential is promoted at higher initial oxygen concentration.