Characteristics of NOx emission with flue gas dilution in air and fuel sides

Flue gas recirculation (FGR) is a method widely adopted to control NO_x in combustion system. The recirculated flue gas decreases flame temperature and reaction rate, resulting in the decrease in thermal NO production. Recently, it has been demonstrated that the recirculated flue gas in fuel stream, that is, the fuel induced recirculation (FIR), could enhance a much improved reduction in NO_x per unit mass of recirculated gas, as compared to the conventional FGR in air. In the present study, the effect of FGR/FIR methods on NO_x reduction in turbulent swirl flames by using N₂ and CO₂ as diluent gases to simulate flue gases. Results show that CO₂ dilution is more effective in NO reduction because of large temperature drop due to the larger specific heat of CO₂ compared to N₂ and FIR is more effective to reduce NO emission than FGR when the same recirculation ratio of dilution gas is used.     

Numerical investigation on NOX formation in oxygen-CH4 MILD combustion with air leakage

In this paper, we focus on formation of NO_X under oxy-fuel MILD combustion using JHC (jet-in hot co-flow) burner. The effect of the different oxygen dilution ratio in the co-flow is analyzed for MILD combustion formation and also for characterizing NO_X emission. RANS (Reynolds averaged Navier Stokes) with the modified k-ε equation and EDC (eddy dissipation concept) model were applied for 2D-axisymmetric computational domain using the Ansys Fluent. It is found that the effect of the oxygen concentration rather than the composition of dilution gas is more significant for the formation of the MILD combustion and as the oxygen concentration of the dilution gas increases under MILD combustion conditions, the region with D_a < 1 expressed as MILD regime tends to be decreased. The effect of a diluted oxygen concentration in the oxy-fuel combustion on NO_x emission under MILD condition is found to be much greater than the amount of air leakage into co-flow, and NO emissions are almost unaffected by the increase of N₂ concentration in case of MILD combustion condition within practical air leak ranges.

     

An experimental study on the combustion characteristics of a low nox burner using reburning technology

The combustion characteristics of a low NO_x burner using reburning technology have been experimentally studied. The reburn burner usually has three distinct reaction zones which include the primary combustion zone, the reburn zone and the burnout zone by provided secondary air. NO_x is mainly produced in a primary combustion zone and a certain portion of NO_x can be converted to nitrogen in the reburn zone. In the burnout zone, the unburned mixtures are completely oxidated by supplying secondary air. Liquefied Petroleum Gas (LPG) was used as main and reburn fuels. The experimental parameters investigated involve the main/ reburn fuel ratio, the primary/secondary air ratio, and the injection location of reburn fuel and secondary air. When the amount of reburn fuel reaches to the 20-30% of the total fuel used, the overall NO reduction of 50% is achieved. The secondary air is injected by two different ways including vertical and parallel injection. The injector of secondary air is located at the downstream region of furnace for a vertical-injection mode, which is also placed at the inlet primary-air injection region for a parallel-injection mode. In case of the vertical injection of the secondary air flow, the NO_x formation of stoichiometric condition at a primary combustion zone is nearly independent of the reburn conditions (locations, fuel/air ratios) while the NO_x emission of the fuel-lean condition is considerably influenced by the reburn conditions. In case of the parallel injection of the secondary air, the NO_x emission is sensitive to the air ratio rather than the fuel ratio and the reburning process often coupled with the multiple air-staging and fuel-staging combustion processes.     

Oxy-fuel combustion boiler for CO2 capturing: 50 kW-class model test and numerical simulation

A novel oxy-fuel burner was devised and integrated into a 50 kW-class furnace-type boiler system. A series of experiments was conducted to verify its feasibility for industrial applications. Additionally, numerical simulations were performed and the results validated against experimental data on the detailed physics inside the conventional-design combustion chamber. The oxy-fuel burner, with the help of gas radiation, could effectively heat the combustion chamber. The composition of the exhaust gas revealed that the sealing of the system is crucial to the achievement of high CO₂ concentrations and low NOx emissions.     

Development of an air fuel control system for a domestic wood pellet boiler

Wood pellets are a kind of solid biomass energy and a renewable energy source. Made by compressing sawdust, wood pellets have a higher energy density than split firewood and wood chips. In 2007, the new and renewable energy (NRE) portion was 2.4% with respect to total primary energy in Korea. The Korean government wants to increase the new and renewable energy (NRE) portion up to 6.1% by 2020 [1]. To achieve this target, the government has been establishing some policies, such as incentive policy, NRE mandatory use for public building and renewable portfolio standard (RPS) and so on. To supply wood pellets as fuel for the combustion chamber of a wood pellet boiler, most domestic wood pellet boilers put a constant volume by using an auger type fuel feed system. In an auger system as fuel feeding, there is the possibility of changing energy input due to the different density of wood pellets even in a constant volume flow rate of wood pellets. If fuel input rate is changed without any correction of air flow rate for combustion, the condition of combustion in a wood pellet boiler can be deteriorated. We have developed an air-fuel control system for a domestic wood pellet boiler by using flue gas oxygen concentration measurement and a PID controller. To measure O₂ concentration of flue gas, a wide band O₂ sensor was adopted. We changed fuel input from 100% to 50% by artificial manipulation to confirm the control system. The O₂ concentration in flue gas can be controlled to be 8.5% ± 1% without significant change of CO and NO_x concentration.     

Effect of various gas compositions on gas interchangeability and combustion characteristics for domestic appliances

In this study, an investigation into the gas interchangeability and combustion characteristics of various gas compositions for domestic appliances was performed. In order to suggest the appropriateness of gas interchangeability using the specific gravity (SG) and the Wobbe index (WI) values, combustion characteristics included incomplete combustion and flame lifting were measured and observed for the upper and lower limits using the gas-oven as a domestic partial-premixed type appliance and the condensing boiler as a domestic premixed type appliance. The flame was stable, and the CO and NO_x concentrations increased when the WI and SG values increased. Specifically, the behavior of the flame lifting changed between WI values of 52.0 MJ/Nm³ and 53.0 MJ/Nm³. The CO and NO_x concentrations of the reference gas were approximately 173 ppm and 74 ppm for the gas-oven and 175 ppm and 35 ppm for the boiler, respectively. Consequently, a WI value of 53.0 MJ/Nm³ can be considered the flame lifting limit as the lower limit for gas interchangeability. For the upper limit of gas interchangeability, the CO and NO_x concentrations varied significantly with the type of burner.     

Experimental analysis of emission reduction by the split injection strategy using close post injection with a double-row nozzle in heavy EGR conditions

As EURO-6 regulations will be enforced in 2014, simultaneous reduction of NO_x and PM emissions becomes an important issue in recent diesel engine research. New combustion concepts, such as LTDC and pHCCI, have been introduced to overcome the NO_x and PM trade-off relation. However, these novel combustion concepts are usually implemented with a high EGR rate and by advancing the main injection timing which cause high CO and THC emissions along with poor fuel consumption due to low combustion efficiency. Therefore, the split injection strategy, which was consisted of applying post injection close to the main injection, was carried out in this experiment. Specifically, two different nozzles — a 7-hole conventional nozzle and a 12-hole double-row nozzle — were evaluated to determine the effects of nozzle configurations on engine-out emissions. The result shows that CO emission was reduced by the close post injection strategy regardless of the nozzle configuration. However, THC and PM emissions were reduced only when the 12-hole double-row nozzle was used. Thus, the use of close post injection with the 12-hole double-row nozzle could increase the combustion efficiency in heavy EGR conditions.     

Experimental investigations on combustion characteristics of a swirl-stabilized premixed burner

Experimental investigations have been conducted to understand the combustion characteristics of a swirl-stabilized double-cone pre-mixed burner used for industrial gas turbines for power generation. NOx and CO emissions, extinction limit, combustion noise, pressure loss, and wall temperature distributions were measured for various operating conditions. Results show that NOx emissions are decreased with increasing air/fuel ratio or decreasing air load unless the air load is too small. CO emissions are also decreased with increasing air/fuel ratio, leading to a positive correlation between NOx and CO emissions. Flame extinction limit is reduced with increasing air flow rate as the flow residence time is reduced. Combustion noise has its peak amplitude at the frequencies of 150 or 300 Hz, which are considered to be the resonance frequencies of the longitudinal mode of the combustor. The noise level at the peak frequency is maximized when the flame is considered to be located near the burner exit. Pressure loss is decreased with the A/F ratio as the flame moves downstream out of the burner.

     

Numerical study on no emission with flue gas dilution in air and fuel sides

Flue gas recirculation (FGR) is widely adopted to control NO emission in combustion systems. Recirculated flue gas decreases flame temperature and reaction rate, resulting in the decrease in thermal NO production. Recently, it has been demonstrated that the recirculated flue gas in fuel stream, that is, the fuel induced recirculation (FIR), could enhance much improved reduction in NO per unit mass of recirculated gas, as compared to conventional FGR in air. In the present study, the effect of dilution methods in air and fuel sides on NO reduction has been investigated numerically by using N3 and CO2 as diluent gases to simulate flue gases. Counterflow diffusion flames were studied in conjunction with the laminar flamelet model of turbulent flames. Results showed that CO2 dilution was more effective in NO reduction because of large temperature drop due to the larger specific heat of CO2 compared to N₂. Fuel dilution was more effective in reducing NO emission than air dilution when the same recirculation ratio of dilution gas was used by the increase in the nozzle exit velocity, thereby the stretch rate, with dilution gas added to fuel side.     

Combustion and emission characteristics of HCNG in a constant volume chamber

Finding an alternative fuel and reducing environmental pollution are the main goals for future internal combustion engines. Hydrogenmethane (HCNG) is now considered an alternative fuel due to its low emission and high burning rate. An experimental study was carried out to obtain fundamental data for the combustion and emission characteristics of pre-mixed hydrogen and methane in a constant volume chamber (CVC) with various fractions of hydrogen-methane blends. A pre-mixed chamber was used to obtain a good mixture of these gases. Exhaust emissions were measured using a Horiba exhaust gas analyzer for various fractions of hydrogen-methane blends. The results showed that the rapid combustion duration was shortened, and the rate of heat release elevated as the hydrogen fraction in the fuel blend was increased. Moreover, the maximum mean gas temperature and the maximum rate of pressure rise also increased. These phenomena were attributed to the burning velocity, which increased exponentially with the increased hydrogen fraction in the fuel blend. Exhaust HC and CO₂ concentrations decreased, while NO_X emission increased with an increase in the hydrogen fraction in the fuel blend. Our results could facilitate the application of hydrogen and methane as a fuel in the current fossil hydrocarbon-based economy and the strict emission regulations in internal combustion engines.     

Control of a pulse combustion reactor with thermoacoustic phenomena

We have developed a novel way of controlling parameters in reactors using flue gases from pulse combustion as a direct source of heat and as a means of transport of particulate materials synthesized in a slightly reductive environment or oxidative environment. The reactor is used for a spray pyrolysis synthesis of materials or the sintering of different ceramic powders. The reactor is heated directly, which means reduced energy losses, and the enhanced drying reported using pulse combustion is implemented for faster reaction. A slightly reductive atmosphere is maintained by combusting a stoichiometric fuel and air mixture and adding acetylene to the flue gas flow. Reaction conditions must be carefully controlled; this is achieved by influencing the characteristic times of the pulse combustion and changing the frequency of combustion and with it the temperature and flue gas composition in the reactor. The frequency is changed by nitrogen dilution of burning gas, influencing the mixing of the combustible mixture with hot flue gases and damping of frequencies with a secondary Helmholtz resonator. The frequency of pressure oscillations in the combustor should be the same as one of the harmonic frequencies of the reactor pipe to reach an acoustic resonance. In this work, the frequency of pulse combustion was altered in such a way that resonance was established with the reactor pipe and that a suitable reaction environment was obtained. With good control over all parameters, we were able to synthesize different Li-ion cathode materials, such as LiFePO₄ and Li(Ni_xMn_yCo_z)O_2.     

Comparison of the effects of multiple injection strategy on the emissions between moderate and heavy EGR rate conditions: part 1-pilot injections

As vehicle emissions regulations have become stricter, improving injection strategies and using high EGR rates has become important to reduce engine-out emissions. Previous research has found that, a multiple injection strategy with a pilot or post injection has the potential to improve the distribution of the air-fuel mixture or oxidation. Thus, it is important to determine the effect of multiple injection strategies on the engine-out emissions. Especially, since the latest diesel engines have used high EGR rates to reduce NO_x emission, the target of emissions is different from that of moderate EGR rate condition. Under heavy EGR rate condition, it is needed to improve combustion efficiency by reduction of CO and THC emissions. Therefore, in the first research as part 1, the effects of pilot injection on the engine-out emissions were systemically evaluated for two different EGR rate conditions (30% and 60%). The characteristics of the pilot injection were different between the two EGR rate conditions because the behavior of diesel combustion is significantly different when the EGR rate is changed. This research condition was investigated using varying injection parameters such as the timing and quantity of the pilot injection. The results show that each different pilot injection strategy was suggested between two EGR rate conditions to reduce engine-out emissions. Under moderate EGR rate condition, very earlier pilot SOI with large amount has the potential to reduce NO_x and PM emissions simultaneously. On the other hand, under heavy EGR rate condition, the closest pilot SOI to main SOI has the potential to reduce CO and THC emissions.     

Investigation of combustion and emission characteristics of n-hexane and n-hexadecane additives in diesel fuel

One of the most important basic requirements of diesel-powered vehicles that they have lower pollutant emissions and fuel consumption. In diesel engines, combustion and engine performance are influenced by the physical and chemical properties of the used fuel. Engine design studies are not enough to increase engine performance and reduce exhaust emissions alone. By adding fuel additives in diesel fuel, the physical and chemical properties of the fuel can be improved. Fuel additives affect engine performance, combustion and emissions positively by exerting catalyst effect during combustion. In this study, n-hexane and n-hexadecane were added in diesel fuel (D0) by volume of 4, 12 % and 20 %. With respect to D0 fuel, in DHD20 and DHX20 fuels engine torque increased by 1.60 % and 1.32 %, respectively, while the brake specific fuel consumption decreased by 3.12 % and 1.98 %, respectively. Maximum cylinder pressures and heat release rate values of the ingredient added fuels increased. It was seen that NO_x emissions increased while HC, CO and soot emissions decreased with increasing contribution ratio.

     

方箱炉旋流燃烧技术研究与应用

旋流燃烧可以提高火焰速度,从而强化传热、提高燃烧效率,采用多个同旋向喷嘴形成旋流燃烧器,或者通过喷口不同方向的倾斜排列形成两排燃烧器,可以实现很好的方箱炉旋流速度场。通过冷态流场数值模拟和试验分析,验证了旋流燃烧能加强炉子流场的均匀性,热态试验和工业应用结果表明,炉膛下部燃烧和传热得到强化,烟气出口烟温下降,方箱炉热效率显著提高。     

A study on characteristics and control strategies of cold start operation for improvement of harmful exhaust emissions in SI engines

Emission regulations for automobiles have become more stringent and the improvement of emission during cold start has been a major key issue to meet these regulations. Among many kinds of factors that affect cold start operation, ignition timing is crucial to improve emission characteristics due to the influence on exhaust gas temperature. Recent progress in variable valve timing allows optimized valve event strategies under various ranges of engine operating conditions including cold start. This study investigates effects of ignition and exhaust valve timing on exhaust gas temperature, combustion stability and emission characteristics through cold start bench tests of an SI engine. Experimental results show that exhaust valve timings and ignition timings significantly affect exhaust gas temperature and stability of engine operation under cold start condition. Exhaust valve timing also affects CO and NO_x emission due to changes in residual gas fraction of the combustion chamber. Ignition timing mainly affects exhaust gas temperature and HC emission. A control strategy, advanced exhaust valve timing and retarded ignition, is plausible in order to achieve reduction of exhaust emission while maintaining stability under cold start operation of SI engines.     

Numerical evaluation of NOx mechanisms in methane-air counterflow premixed flames

The control of nitrogen oxides (NO_x) has been a major issue in designing combustion systems, since NO_x play a key role in ozone depletion and the generation of photochemical smog. The characteristics of NO_x emission can be essential information for the development of a clean combustor having suitable reduction methodologies. In the present study, NO_x emission characteristics were evaluated numerically, accounting for the effect of equivalence ratio, stretch rate, pressure, and initial temperature. In general, peak NO_x emission appeared near the equivalence ratio of unity case, and NO_x emission increased with pressure and initial temperature due to the temperature sensitivity in NO_x mechanism. NO_x decreased with stretch rate due to the decrease in residence time in high temperature region. Furthermore, the thermal and prompt mechanisms were evaluated with equivalence ratio for two calculation methods. The conventional methods ignore the interaction of coupled mechanism of thermal and prompt NO_x. The reaction path diagram was introduced to understand effective reaction pathways in various conditions. This paper was recommended for publication in revised form by Associate Editor Kyoung Doug Min Dr. Eun-Seong Cho received his B.S. and M.S. degrees in Mechanical Engineering from Hanyang University, Korea, in 1996 and 1998, respectively. He then received his Ph.D. degree from Seoul National University, Korea, in 2005. He was a principal engineer of KD Navien research center and currently a research associate at Delft University of Technology, The Netherlands. His research interests include eco-friendly clean combustion technology, new and renewable energy systems. Prof. Suk Ho Chung received his B.S. degree from Seoul National University, Korea, in 1976 and Ph.D. degree in Mechanical Engineering from Northwestern University, USA, in 1983. He is a Professor since 1984 in the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests cover combustion fundamentals, pollutant formation, laser diagnostics, and plasma-assisted combustion.     

NO2 chemiluminescent emission from flames influenced by acoustic noise

The effect of acoustic noise on combustion is investigated from the perspective of NO_x emissions. A robust, plug-in probe that exploits the natural emission signal from the combustion gases, and which can have practical relevance, is used. Acoustically pulsed flames are stabilized on aburner, and NO₂ chemiluminescence is measured with an intensified detector at various frequencies. The results indicate the NO₂ emission increases in noisy flames at certain frequencies more significantly than others. Noise at higher frequencies in the range 0.8≈1 kHz effects the nitrogen chemistry in stoichiometric flames (ϕ=1), but not that in lean flames (ϕ-0.7 and 0.8).     

Numerical analysis of flow characteristics of a swirl-stabilized premixed burner with different swirl vane configurations

This study numerically investigates the flow and combustion characteristics of a swirl-premixed burner with a curved vane swirler (i.e., curved type) to reduce the pressure loss of the burner nozzle with a conventional vane swirler (i.e., flat type). For the curved type swirler, the pressure loss is decreased by 35 % due to the inhibition of flow separation at the vane, and NO_X emission is reduced by 69 % because fuel-air mixing is enhanced at the flame front, leading to more uniform fuel distribution toward downstream. In addition, the characteristics of the jet penetration are analyzed because the jet behavior is crucial to fuel-air mixing. The jet trajectory in non-swirling flow can be reasonably predicted by a conventional formula, but it is difficult to predict the jet trajectory in swirling flow by this formula because of the effect of a radial pressure gradient formed by the swirling flow.

     

Effect of spark plug protrusion on the performance and emission characteristics of an engine fueled by hydrogen-natural gas blends

Mixtures of hydrogen and natural gas are promising for improving efficiency and reducing harmful emissions in spark ignition engines, since limits of flammability can be extended while stable combustion is secured. In this research, the combustion characteristics of long electrode spark plugs were evaluated in a hydrogen blended with natural gas (HCNG) engine. Decreases in the flame propagation distance through the use of spark plugs can lead to increased burning rates and further improvement of fuel economy in HCNG engines. An 11-liter heavy duty lean burn engine was employed and performance characteristics including emissions were assessed according to the spark timing of the minimum advance for best torque (MBT) for each operating condition. Retarded MBT spark advance timing with long electrode spark plugs due to increased burning speed supported increases in engine efficiency and reductions of nitrogen oxide (NO_x) emissions. The lower positions of initial flame kernels due to the use of long electrode spark plugs were preferable to improvements of cyclic variability due to reduced flame front quenching, and carbon monoxide (CO) emissions at the flammability limit were also improved.     

An experimental study on combustion processes and nox emission characteristics of the air-staged burner

The combustion processes and emission characteristics in air-staged burner have been experimentally studied. The light fuel oil doped with pyridine(C₅H₅N) is used to investigate the fuel NO_X emission characteristics. Experiments are carried out for a wide range of operating conditions of single-staged and multi-staged burner. The detailed discussions are made for the flame structure of the air-staged burner as well as effects of excess air ratios, staged air flow percentage, and spray conditions on flame pattern and NO_X emission characteristics.