MILD combustion for hydrogen and syngas at elevated pressures

As gas recirculation constitutes a fundamental condition for the realization of MILD combustion, it is necessary to determine gas recirculation ratio before designing MILD combustor. MILD combustion model with gas recir- culation was used in this simulation work to evaluate the effect of fuel type and pressure on threshold gas recir- culation ratio of MILD mode. Ignition delay time is also an important design parameter for gas turbine combustor, this parameter is kinetically studied to analyze the effect of pressure on MILD mixture ignition. Threshold gas re- circulation ratio of hydrogen MILD combustion changes slightly and is nearly equal to that of 10 MJ/Nm3 syngas in the pressure range of 1-19 atm, under the conditions of 298 K fresh reactant temperature and 1373 K exhaust gas temperature, indicating that MILD regime is fuel flexible. Ignition delay calculation results show that pres- sure has a negative effect on ignition delay time of 10 MJ/Nm3 syngas MILD mixture, because OH mole fraction in MILD mixture drops down as pressure increases, resulting in the delay of the oxidation process.     

A comprehensive study on NOx emission and fuel nitrogen conversion of solid biomass in bubbling fluidized beds under staged combustion

Despite the relatively low emissions in fluidized-bed combustion, NOx emission for biomass combustion is still a major concern because of increasingly stricter regulations. To realize NOx emission behavior in fluidized beds comprehensively, the effects of bed temperature, excess oxygen, staged combustion, and flue gas recirculation (FGR) are investigated in this study. In particular, three different types of operation are applied in staged combustion to find out the key parameter. The results indicate that NOx emissions increase with both bed temperature and excess oxygen, in which the influence of excess oxygen is greater than the other. Lowering bed temperature by water addition seems to be able to simultaneously reduce NOx emission and agglomerate formation, especially for fuels with high nitrogen content, but the pros and cons should be considered. The results in staged combustion infer that the residence time is much more critical than the stoichiometry in the bed. As for FGR, its impact appears to depend on the type of fuel. The correlation between NOx emission behavior and fuel characteristics is also scrutinized; it is concluded that the fuel-N conversion to NOx is essentially related to some features of fuels.     

天然气柔和燃烧锅炉结构参数影响模拟研究

在天然气锅炉中引入柔和燃烧技术将大大降低NOx排放,高速未燃气卷吸高温烟气回流并与之快速掺混再燃烧是柔和燃烧的重要特征,因此,开展天然气锅炉关键结构参数优化设计以组织流场形成柔和燃烧所需的高温低氧反应气氛非常必要。基于天然气锅炉的工况特征,设计了热负荷15kW的模型燃烧室,采用数值模拟手段详细研究了燃烧室高度、喷嘴孔径、喷嘴相对位置及烟气出口尺寸对燃烧室流场、组分场及关键参数——烟气回流比的影响规律,并最终确定了燃烧室结构优选方案,对天然气锅炉柔和燃烧机设计提供理论基础数据。     

Control of nitrogen oxide emissions from coal combustion

The paper gives a discussion of the regulatory framework currently being used to limit nitrogen oxide (NO_x) emissions, with emphasis being placed on EC and UN measures. It then follows with a discussion of the procedures being adopted at present to control or limit these emissions. The topics discussed include combustion modification measures, new combustion technologies, flue gas treatment, selective catalytic reduction, selective noncatalytic reduction and combined SO_x/NO_x processes.     

Effect of steam addition and distance between inlet nozzles on non-catalytic POX process under MILD combustion condition

Moderate or intensive low-oxygen dilution (MILD) combustion is a novel combustion technology with high efficiency and low emissions. Few studies have been performed on the application of this technology for partial oxidation processes. In this research, a Computational Fluid Dynamics study for the effect of different parameters on the natural gas partial oxidation under MILD combustion conditions has been carried out. The combustion chamber was in the form of cylinder with a diameter of 300 mm and a length of 1500 mm. The effect of parameters such as different kinetic mechanisms, adding ethane and propane to methane (shale gas feed), adding steam to the feed and distance (interval) between methane and oxygen nozzles were investigated. Results showed that addition of ethane and propane to methane increased the mole fraction of CO and C₂H₂ so that, in the case of mixed methane with ethane and propane compared to the case of pure methane, an increase of 18.75% and 12.93% was observed for CO and C₂H₂, respectively. In addition, with increasing the percentage of steam in the inlet feed at a constant flow rate, methane conversion increased so that it in the case of 30% inlet steam was 77.17%, which showed 11% promotion compared to the pure methane case. Also, increasing the distance between the fuel and oxidizer nozzles led to an increase in the maximum temperature in the combustion chamber.     

Morphological and statistical features of reaction zones in MILD and premixed combustion

Direct numerical simulation (DNS) results of turbulent MILD premixed and conventional (undiluted) premixed combustion have been investigated to shed light on the physical aspects of reaction zones and their morphology in MILD combustion. Results of a premixed case are used for comparative analyses. The analyses show that the regions with strong chemical activity in MILD combustion are distributed over a substantial portion of the computational domain unlike in the premixed case where these regions are confined to a small portion of the domain. Also, interactions of reaction zones are observed in MILD combustion with their spatial extent increasing with dilution level. These interactions give an appearance of distributed combustion for MILD conditions. The morphology of these reaction zones is investigated using the Minkowski functionals and shapefinders commonly employed in cosmology. Predominant sheet-like structures are observed for the premixed combustion case whereas a pancake-like structure is observed as the most probable shape for the MILD cases. Spatial and statistical analyses of various fluxes involved in a progress variable transport equation are conducted to study autoignitive or propagative characteristics of MILD reaction zones. The results suggest that there are local regions with autoignition, propagating-flames, and their coexistence for the conditions considered in this study. Typically, reaction dominated or ignition front and propagating-flame dominated regions are entangled for high dilution cases. Scalar gradient plays a strong role on whether reaction or propagating-flame dominated activities are favoured locally.     

Comprehensive numerical study of molecular diffusion effects and Eddy Dissipation Concept model in MILD combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion is a clean combustion technology with high thermal efficiency and low levels of emissions. In this paper, by employing Adelaide Jet-in-Hot-Co-flow (AJHC), several approaches are examined to increase the numerical solution accuracy. First, molecular diffusion effects are investigated in MILD combustion. Second, adjusting the Eddy Dissipation Concept (EDC) coefficients is comprehensively discussed, and finally, the reaction fraction coefficient and EDC formulation are investigated. The results show that the effect of enthalpy transport caused by molecular diffusion on the energy equation must be considered in the low oxygen concentration regions. Also, the maximum temperature in the MILD region can be kept constant by adjusting EDC coefficients. Furthermore, it is shown that applying the reaction fraction factor increases the accuracy of the numerical solution in the MILD region.     

Fast Ignition and Stable Combustion of Coarse Coal Particles in a Nonslagging Cyclone Combustor

A combustion set-up of an innovative nonalagging cyclone combustor called “Spouting-Cyclone Combustor(SCC)”，,with two-stage combustion,organized in orthogonal vortex flows,was established and the experimental studies on the fast ignition and stable combustion of coarse coal particles in this combustor were carried out.The flame temperature versus ignition time and the practical fast ignition the temperature fields in SCC were obtained.These results whow that it is possible to obtain highly efficient and clean combustion of unground coal particles by using this technology.     

Impact of nitrogen oxides (NO,NO2, N2O) on the formation of soot

The emission of both nitrogen oxides and soot from combustion processes is still a matter of concern. When a flue gas recirculation (FGR) technique is applied, the presence of a given nitrogen oxide in the recirculated mixture can affect the emissions of other pollutants, such as soot, and be used for its control in a combustion process. In this context, the present work is focused on the identification of the effect of the main nitrogen oxides (NO, NO₂ and N₂O) present in combustion systems on soot and main product gases formation from the pyrolysis of ethylene, at atmospheric pressure and in the 975–1475 K temperature range. The experimental results are examined to assess the effectiveness of each nitrogen oxide in suppressing or boosting soot formation, to achieve the possible nitrogen oxides reduction, and to identify the elementary steps involved in the nitrogen oxides and ethylene conversion as function of the different nitrogen oxides. This analysis is supported on model calculations.     

Combustion and pollutant emissions characteristics of Camellia oleifera shells in a vortexing fluidized-bed combustor

Because of their saponin content, Camellia oleifera shells cannot be directly discarded. However, this eco-unfriendly agricultural waste is suitable for use as a fuel. A comparison between the combustion of crushed C. oleifera shell (CCS) and pelletized C. oleifera shells (PCS) was carried out in a pilot-scale fluidized-bed. Both directly combustion and flue gas recirculation (FGR) combustion modes were employed. The effects of particle size and the FGR ratio on the combustion behavior and pollutant emissions characteristics were also investigated. Results show that the combustion efficiencies for both CCS and PCS are higher than 99%. The combustion fraction in the bed region of CCS and PCS-6 are 55% and 85%, respectively, resulting in different combustion and pollutant emission characteristics. CCS has smaller CO and NO emissions and higher PCDD/Fs concentration of the fly ash compared with those of pelletized fuels. However, considering the material bridging in the feeding process of CCS, using pellets as the fuel is a better choice. The CO emissions increase while dioxin and furan emissions decrease with an increase in FGR ratio. The lowest NO emissions (about 150 ppm) of the three pelletized fuels combustion were achieved at an FGR ratio of 42%. In addition, all the pollutant emissions are lower than the minimum standards required for municipal solid waste (MSW) incinerators in Taiwan.     

Analysis of nitrogen oxide emissions from modern vehicles using hydrogen or other natural and synthetic fuels in combustion chamber

The paper presents a calculated analysis of the equilibrium emission of nitrogen oxides on the exhaust of carburetor and diesel internal combustion engines. The temperature of fuel oxidation is assumed to be 1,400 °C while the pressure for carburetor and diesel engines is assumed to be 60 atm and 80 atm respectively. The studies have been carried out for natural and synthetic fuels such as hydrogen, ethanol, methanol, petroleum, diesel fuel and methane at the excess air coefficient corresponding to the fuel oxidation temperature of 1,400 °C. In the paper, the method for calculating the equilibrium composition based on the equilibrium constant and mass conservation equations has been applied. It is shown that with an increase in pressure from 1 atm to 60 atm for carburetor engines and up to 80 atm for diesel engines, the reaction of nitrogen dioxide formation may shift towards an increase in NO₂. The formation of NO may be not affected by the increase in pressure by virtue of the fact that the reaction proceeds without changes in the amount. It has been determined that NO is the major atmospheric pollutant. However, it would be advisable to use more extensively the fuels characterized by the lowest output of nitrogen dioxide (methane and methanol), since nitrogen dioxide (NO₂) related to the 2nd hazard class is appeared to be the most dangerous to humans. It has been revealed that the reduction in oxidation temperature using hydrogen as a fuel for electrochemical current generators may allow reducing nitrogen oxide emissions by more than an order of magnitude as compared to the best results for ICE.     

Numerical study of further NOx emission reduction for coal MILD combustion by combining fuel‐rich/lean technology

This paper numerically examines the feasibility of further reducing NO_x emission from a semi‐industrial scale coal MILD (moderate and intense low‐oxygen dilution) combustion furnace by adopting fuel‐rich/lean technology. The implementation is achieved by separating the original fuel jet into two parallel jets which will be used as rich and lean streams. An effort has been made to develop a 13‐step reaction mechanism and NO_x evolution UDFs (user defined functions) for better understanding the interactions between MILD combustion and fuel‐rich/lean technology. The experiment of the reference case (Combustion and Flame 156.9 (2009): 1771‐1784) is well reproduced by the present numerical simulation, indicating high reliability of developed models. The validity of the further reduction of NO_x emission is assessed by the comparison among inner‐fuel‐rich (IFR), outer‐fuel‐rich (OFR), and reference cases resulting from the adjustment of the fuel supply through the two fuel‐rich/lean jets. The results show that both IFR and OFR configurations succeed in achieving further reduction of NO_x emission as compared with the reference case, which stems from both thermal and fuel paths. Specifically, the decrease of thermal‐NO emission originates from the contraction of high‐temperature regions (>1800 K), where nearly 94% reduction occurs within the temperature range of 1800 K and 1950 K while only 6% within 1950 K and 2030 K despite their high temperature sensitivity. The reduction of the fuel‐NO emission is mainly attributed to the promoted NO reduction on char surface and neutralization with HCN and NH₃. Generally, the NO_x emission can be minimized by enlarging the equivalence ratio difference between rich and lean jets, and the OFR configuration exhibits a higher potential than the IFR counterparts. However, since a relatively high temperature (1623 K) secondary air was used in the experiment, the maximum NO_x reduction potential was limited to only 2.5%.     

Simulation of producer gas flameless combustion with fresh reactant diluted by hot flue gas

Flameless combustion is considered as a flexible and efficient combustion process for low heating value gas fuel. This paper presents numerical simulations of premixed flameless combustion using producer gas as a fuel. Different initial conditions of the premixed fresh reactant (air/fuel mixture) and dilution levels are taken into account for the investigation. The numerical simulations were investigated using a network of chemical reactor models with the detailed reaction mechanism of GRI‐Mech 3.0. A threshold dilution level for flameless combustion fuelled by producer gas was determined. The numerical results show that dilution of the fresh reactant with hot combustion products and initial fresh reactant temperature play important roles in flameless combustion formation and its auto‐ignition behaviour, rather than equivalence ratio of the fresh reactant. In the flameless combustion regime, temperature and chemical concentrations were reduced while chemical kinetics process was decelerated, resulting in delay of the auto‐ignition process.     

Air gasification of dried sewage sludge in a two-stage gasifier. Part 2: Calcined dolomite as a bed material and effect of moisture content of dried sewage sludge for the hydrogen production and tar removal

In order to produce a clean producer gas, the air gasification of dried sewage sludge was conducted in a two-stage gasifier that consisted of a bubbling fluidized bed and a tar-cracking zone. The kind and amount of bed materials, the kind of additives in the upper-reactor, and the moisture content in the sewage sludge were selected as operating variables in order to investigate their effects on the development of the producer gas characteristics. In our experiments, the gasification of a dried sewage sludge sample containing 30 wt.% of moisture with a combination of calcined dolomite as the bed material and activated carbon in the tar-cracking zone removed the most tar and produced the highest hydrogen concentration. The total tar removal efficiency and the H₂ content in the producer gas from the sample noted above reached 88.4% and 32.1 vol.%, respectively. The LHVs of all the producer gases were high with values above 7 MJ Nm⁻³.     

Use of scrap tires in cement production and their impact on nitrogen and sulfur oxides emissions

Cement production is characterized by extremely high energy consumption per unit of product. Energy costs and environmental standards encouraged cement manufacturers worldwide to evaluate to what extent conventional fuels can be replaced by alternative fuels, i.e., processed waste materials, such as scrap tires. The decisive factors promoting the use of cement kilns for the utilization of scrap tires are: the high incineration temperature, the large area of the furnace, the significant length of the kiln, the long period of time the fuel stays inside the kiln, and the alkaline environment inside the kiln. The use of scrap tires in cement kilns is one of the best technologies for a complete and safe destruction of these wastes, due to the fact that there is a simultaneous benefit of destroying wastes and getting the energy. Thus, the use of scrap tires as alternative fuel in cement kilns has energy and economic justifiability, and it is environmentally friendly. In this article, monitoring results of nitrogen and sulfur oxides emissions from cement kilns in a cement factory in Serbia are given, depending on the ratio of scrap tires in total fuel quantity. Research was carried out for 0 to 15% share of scrap tires in total heat production. Nitrogen and sulfur oxides emission measurements from cement kilns were done during a trial use of scrap tires as a secondary fuel in a cement factory. During nitrogen and sulfur oxides emissions monitoring from the cement kiln, coal and petroleum coke were used as primary fuel, and whole or shredded tires were used as secondary fuel. Experimental results have shown the encouraging results: in particular, clinker characteristics were unmodified, and stack emissions of NOx and SO₂ were, in the case of tires, slightly decreased, in some cases were incremented, but remaining always below the law imposed limits.     

Characterisation of fuel bound nitrogen in the gasification process and the staged combustion of producer gas from the updraft gasification of softwood pellets

This paper presents experimental results derived from test runs performed with a laboratory-scale updraft fixed-bed gasifier coupled to a combustion chamber to produce data for the characterisation of fixed-bed gasifier operation and to investigate the release behaviour and the conversion of fuel-bound nitrogen during gasification and subsequent staged combustion of the producer gas using softwood pellets. Spatial temperature profiles and the composition of the producer gas of the gasifier have been measured for different air flow rates. In addition, the concentrations of relevant nitrogenous gas species including tars have been measured in the producer gas and at different positions in the combustion chamber. The air flow rate has a significant influence on the composition of the producer gas and the temperature profile of the packed bed of the gasifier. Results show that during updraft fixed bed gasification almost the entire fuel-bound nitrogen is released as N bound in tars from the packed bed and is then subsequently released as HCN, NO, NH₃ and N₂ as a result of tar cracking during combustion. This strong N-fixation in the tars was not expected and is of great relevance concerning NO_x formation during combustion of the producer gas.     

Mechanisms of NO formation in MILD combustion of CH4/H2 fuel blends

The mechanisms of formation and destruction of NO in MILD combustion of CH₄/H₂ fuels blends are investigated both experimentally and numerically. Experiments are carried out at a lab-scale furnace with the mass fraction of hydrogen in fuel ranging from 0% to 15%; furnace temperature, extracted heat and exhaust NO_x emissions are measured. Detailed chemical kinetics calculations utilizing computational fluid dynamics (CFD) and well-stirred reactor (WSR) are performed to better analyze and isolate the different mechanisms.     

Increasing the power output of hydrogen internal combustion engines by means of supercharging and exhaust gas recirculation

Spark ignition engines can be relatively easily converted to hydrogen using port fuel injection (PFI). However, because of the lower volumetric energy density of a hydrogen–air mixture and the occurrence of abnormal combustion phenomena such as backfire, hydrogen-fueled PFI engines suffer from a power deficit in comparison with gasoline engines. This paper reports measurements on a single-cylinder hydrogen engine equipped with a supercharger and an exhaust gas recirculation (EGR) system. Using EGR combined with supercharging and a three-way catalyst (TWC) is shown to significantly increase the power output while limiting tailpipe emissions of oxides of nitrogen (NO_x).     

烟气循环燃烧减排NO_x的影响因素研究

烟气循环燃烧是一种新型燃烧方式,将燃烧烟气返回用于燃烧,形成循环燃烧过程,可以减少NOx的排放,富集回收CO2。通过正交方法设计燃烧实验,得到影响NOx排放的关键因素。实验表明烟气循环比是影响烟气中NOx含量的重要因素,烟气全部循环可以减少NOx排放24%。循环比与排出的CO2量成正比,利用烟气循环将空气燃烧产生的烟气与空气混合共同助燃,可以富集CO2。     

低氮燃烧技术在瑞明电厂的实际应用

通过低氮燃烧技术在国产420 t/h燃煤锅炉上的实际应用实例,对NOx产生机理、低氮燃烧改造方案、改造后的效果分析、运行调节注意事项、改造后出现的问题及今后的完善措施进行了阐述.