低氧气氛下煤粉燃烧特性及动力学

采用热重分析仪研究了不同煤粉在烟气气氛下的燃烧特性,考虑了升温速率和氧气浓度对燃烧特性的影响。研究结果表明:低氧条件、高升温速率下,煤粉的TG、DTG曲线均向高温区靠近,燃烧速率变慢,燃尽时间增长;改变氧气浓度和升温速率对煤的着火温度影响不大,在一定氧浓度(5%~15%)范围内,高水分低阶煤采用烟气干燥输送的制粉系统,能满足其安全性要求;氧气浓度和升温速率主要对煤的燃烧阶段产生影响,随着升温速率的升高和氧浓度的降低,燃尽温度明显升高,且氧浓度对燃烧特征参数的影响大于升温速率的影响。此外,采用Coats-Redfern积分法对煤粉在程序升温过程中的燃烧反应做了相应的动力学分析。结果表明:氧气浓度和升温速率的变化均对活化能产生影响,且随着氧气浓度的降低,煤阶对活化能的影响逐渐减弱。     

氧煤在炉膛内的燃烧方式分析

采用高温预热氧化剂或提高射流速度的方式,可将氧煤燃烧和MILD燃烧相结合,氧煤燃烧可以得到较高CO_2浓度的烟气,约90%,便于实现燃煤过程的CO_2捕集和封存(CCS),而MILD燃烧模式下的氧浓度整体水平较低,从而降低了煤的化学反应速率。利用煤粉的反应动力学参数,建立了氧煤燃烧炉膛的辐射传热模型,同时对不同氧浓度下煤粉燃尽时间和炉膛温度分布进行了计算。结果表明:煤炭颗粒的燃尽时间随着氧浓度和燃烧温度的降低而延长,且在低氧浓度下(10%),氧浓度的变化对燃尽时间的影响程度增大,而燃烧温度的变化对燃尽率的影响不大。煤样固定碳含量高,则炉膛介质温度整体提高,随着氧浓度的降低,燃烧峰值降低且位置稍有延后,且温度分布趋于均匀化。     

CO_2效应对煤焦颗粒增氧燃烧影响的数值研究

采用考虑炭粒表面氧化还原反应以及气相CO-O_2反应的连续膜模型,在增氧燃烧中添加一定量的CO_2并采用Ar来调节煤焦颗粒表面温度,分析O_2浓度效应、CO_2化学效应及热效应对煤焦颗粒表面燃烧速率的影响.结果表明:当增氧燃烧系统中O_2体积分数从21%提高到26.5%时,煤焦颗粒表面温度升高154K,燃烧速率增大,且着火时间和燃尽时间均提前;在煤焦颗粒增氧燃烧过程中,O_2浓度效应对煤焦颗粒表面燃烧速率的影响占主导作用,其次是CO_2化学效应,而热效应的影响最低,三者影响的相对贡献率分别为56.4%、25.7%和17.9%.     

粒径和氧浓度对烟梗燃烧产物析出特性的影响

为探究粒径和氧浓度对烟梗燃烧产物析出特性的影响,文章采用热重-质谱联用技术对烟梗进行燃烧实验,重点研究了烟梗在4种粒径和3种氧浓度条件下燃烧时的挥发性有机化合物(VOCs)析出特性。研究结果表明:烟梗燃烧析出的VOCs主要包括乙炔、丙烯、乙腈、丙烯腈、苯、甲苯和氯甲烷,其中,乙炔和乙腈的相对累积量较大,其次是丙烯和氯甲烷;VOCs在纤维素和木质素燃烧阶段均有析出,而氯甲烷在半纤维素燃烧阶段也表现出明显的析出特性;粒径和氧浓度的改变对VOCs的析出有明显影响,随着烟梗粒径的减小,VOCs的析出峰值温度均向低温区移动,乙腈、丙烯腈和氯甲烷的相对累积量先减小后增大,而其余4种VOCs均减小;当烟梗粒径为140目时,VOCs在木质素燃烧阶段达到最大析出强度;随着氧浓度的增加,VOCs的析出峰值温度向低温区移动,VOCs的相对累积量逐渐增大,当氧浓度为21%时,VOCs在木质素燃烧阶段达到最大析出强度。     

富氧燃烧氛围下纯煤掺烧生物质的污染物排放模拟研究

利用ASEPN PLUS流程模拟软件对富氧燃烧氛围下纯煤掺烧生物质时污染物的排放特性进行了模拟分析,结果表明:在掺烧比例一定时,富氧燃烧氛围下烟气中NO_x和SO_x排放浓度远低于常规空气燃烧氛围下的值,而CO的排放浓度显著高于常规空气燃烧氛围的值,且随O_2浓度的提高,NO_x和SO_x的排放浓度逐渐增加,CO的排放浓度逐渐降低;在O_2浓度一定时,燃烧温度对CO的排放浓度影响很大,当温度超过1 200℃以后,CO的排放浓度急剧上升,NO_x的排放浓度随燃烧温度的升高而增加,SO_x的排放浓度受燃烧温度影响不大,随燃烧温度的升高略有增加。     

无烟煤和焦炭的NO_x生成规律研究

通过改变无烟煤和焦炭燃烧时的粒径、温度和氧浓度,采用管式炉进行焦炭和无烟煤的燃烧试验,并测量烟气中NO的体积分数。试验研究了铁矿石烧结时存在的粒径、温度、氧浓度对无烟煤和焦炭燃烧过程中NO_x释放和燃烧速率的影响。结果表明,焦炭的NO_x转化率会随着粒径增大而减小,而无烟煤的NO_x转化率则会随着粒径增大先增大后减小。焦炭和无烟煤的NO_x转化率都会随着温度升高而减小。在1000℃时,焦炭和无烟煤都会随着氧浓度的升高,NO_x转化率随之升高,但是在1100℃时,焦炭的NO_x转化率会随着氧浓度升高而减小。     

流化床O_2_CO_2燃烧_氧浓度对粒径的影响

为研究氧浓度对燃料粒径的影响,在15 kW循环流化床试验系统和0.15 MW循环流化床试验系统上进行不同氧气浓度、不同燃料、不同粒径的燃烧试验。试验结果表明,燃烧高挥发分的煤时,氧气浓度对燃料的热破碎有较大影响,进而影响燃料粒径。50%左右氧气浓度下时,燃烧高挥发分燃料,平均粒径要比在常规空气下燃烧增加约35%。低挥发分的神木半焦在燃烧过程中热破碎较弱,氧气浓度对热破碎特性影响不大,粒径和空气燃烧条件下无明显差异。     

炭黑非催化燃烧特性的固定床实验研究

含碳燃料在还原气氛下燃烧会生成炭黑，在动力设备的燃烧装置中，炭黑的后期氧化对污染控制是非常重要的。利用石英管固定床反应器对天然气扩散火焰中生成的炭黑在不同氧浓度下（20％、15％、10％和5％）的燃烧特性进行了研究，并选用了蜡烛炭黑、丁烷炭黑和煤焦作为对比。根据实验中得出的燃烧特性，与煤焦相比，炭黑的着火温度较低，但是炭黑的燃烧活化能相对更高。氧浓度对各试样着火温度影响不大，而却影响各试样燃烧过程。还进行了水蒸汽对天然气炭黑燃烧的影响研究，水蒸汽能引起炭黑燃烧速率的显著增大。图9参12     

发动机起动动态过程富氧燃烧排放及其失火特性研究

针对发动机选择性组分进气富氧燃烧,采取氧浓度21%～27%的富氧进气,研究点燃式发动机起动初始段瞬态过程燃烧与排放特性.试验表明:随着进气氧浓度的增加,CO、HC排放明显下降,NOx排放有所升高.在氧浓度23%～25%的低富氧程度时,对于CO、HC排放的影响相对显著,随着富氧程度的进一步增加,该影响作用明显减弱.尽管富氧起动带来一定程度的NOx排放增加,但是起动过程的NOx排放仍然处于较低水平.缸内燃烧探测可知,合适的富氧氛围有利于火焰传播和扩散,明显减少失火现象,失火率呈线性降低趋势,起动燃烧稳定性明显增强.     

氧浓度对生物质烘焙特性的影响

为探究烟气中氧浓度对生物质烘焙过程的影响,以麦秆为研究对象,在固定床反应器上进行温度及氧浓度下的烘焙实验。研究结果表明:随着氧浓度的增大,烘焙过程可分为3个阶段:氧浓度较低时(6%),主要为半纤维素的脱羧基和羰基化反应以及综纤维素的缓慢氧化;随着氧浓度的增加(6%),氧化反应加剧;当氧浓度10%时,氧化反应导致局部热量堆积,出现挥发分燃烧现象。烟气烘焙耐受氧浓度为6%,此时质量和能量产率分别保持在60%和70%之上。     

Biomass combustion in IEA countries

Bioenergy use and potential, biofuels and bioenergy systems including combustion equipment and technology development are reviewed in the IEA member countries participating in the combustion activity. Focus has been put on pollutant emissions, emission measurement and reduction techniques, energy and emissions from contaminated wood combustion and fuel reactivity and modelling of pyrolysis and combustion. The paper describes the status on these topics within the IEA cooperation agreements.     

Wood ash dilemma-reduced quality due to poor combustion performance

Recycling of wood ash is based on the presumption that moderate concentrations of environmentally harmful elements are a part of the nutrient cycle and do not increase in net concentrations in the forest soil. It is assumed that the same quantities of harmful elements are harvested from the forest and recycled back. This principle does not apply to polycyclic aromatic hydrocarbons (PAHs) since these pollutants are formed during the combustion process, especially when the combustion performance is poor. Additionally, industrial combustors are adjusted in order to reduce NO_x-emissions, indirectly causing formation of PAHs. This study examined fly ash from combustion of pulverized wood for its elemental and PAH concentrations during a period of 9 weeks. The 16 EPA-PAH concentrations range between 40 and 300 mg kg⁻¹. Re-burning of the ash reduces the PAH concentrations to 0.24 mg kg⁻¹ and organic carbon concentration from 40% to 5%, enhancing its composition significantly. It is important to determine the amount and fate of PAHs spread on forest soils with wood ash to ensure the improvement of the health of the forest ecosystem. Maximized energy efficiency of industrial boilers is the key to reducing anthropogenic emissions of greenhouse gases and enabling a sustainable nutrient recycling system.     

Formation of dioxins and furans during combustion of treated wood

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F, dioxin) are produced in combustion of wood treated with copper-based preservatives. This review summarises and analyses the pertinent literature on the role of preservatives in the formation of dioxin in the low-temperature, vitiated environment that exists in the domestic combustion of wood, and in large-scale fires. Until recently, the role of preservatives was not thoroughly examined in the literature with respect to fires, as incineration attracted most of the research focus. However, latest studies have demonstrated that some current and emerging wood preservatives significantly increase dioxin formation during combustion in domestic stoves and in fires. The following pathways are identified: (i) copper, a common biocide that is chemically bound to the wood, is an important dioxin catalyst, (ii) preservative metals promote smouldering of wood char following cessation of flaming, providing the required temperature environment for dioxin formation, and (iii) chlorinated organics added as secondary preservative components yield dioxin precursors upon thermal decomposition. These conclusions indicate that it remains hazardous to dispose of preservative impregnated timber via domestic combustion even if arsenic is not present.     

Syngas production in hybrid filtration combustion

Rich and ultrarich combustion of butane inside porous media composed of aleatory wood pellets and alumina spheres is studied experimentally to evaluate the suitability of the concept for syngas production. Temperature, velocity, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 2.6. It is observed that hydrogen and carbon monoxide are dominant partial oxidation products for ultrarich hybrid combustion waves of butane and wood pellets. Syngas yield in hybrid filtration combustion is found to be essentially higher than for butane filtration combustion in an inert porous medium.     

Waste wood characterization and combustion behaviour in pilot lab scale

This work aims at testing the suitability of waste wood as feedstock in an industrial combustion unit. Firstly, an extensive characterization of properties relevant for combustion was performed on two samples of waste wood and one sample of natural wood for comparison. Both waste wood samples were found to contain significantly higher amounts of metals such as iron and titanium than natural wood. However, one sample was much richer than the other one in all types of impurities, which resulted in a particularly high amount of ash (about 10 w %), and the sample was also found to contain many fine particles. These observations seemed to be explained by an external pollution of the sample by soil. Then, experiments were carried out in a bench-scale fixed-bed combustion unit on the different samples under variable conditions of air flow. All samples could be successfully burnt. However, the waste wood sample polluted with soil appeared to have a lower conversion rate due to its heterogeneity of particle size. A separation between fines and the rest of the sample seemed to solve this issue. Regarding gas emissions, as expected, both waste wood samples gave rise to higher amounts of NO_x emissions than natural wood due to their content.     

Effect of raw material composition in woody biomass pellets on combustion characteristics

Char yield, char combustion time and char combustion rate of pellets made from different tree parts of Norway spruce and industrially made stem wood pellets of Norway spruce and Scots pine were studied. The pellets were incinerated in a laboratory scale oven at various temperatures, gas flows and oxygen concentrations.It was found that the combustion time for a single pellet mostly depended on the raw material composition and to a minor extent on the density. Pellets made of bark had up to a 50% longer char combustion time compared to that of stem wood pellets, due to differences in char yield. Industrially made stem wood pellets of pine and spruce sawdust were found to have small differences in combustion characteristics. The variations in combustion characteristics of pellets are discussed in relation to composition of raw material.     

基于连接权值算法的燃烧调整参数敏感性研究

为确认燃烧调整过程中影响燃气轮机运行状态的主导因素,首先将影响运行状态的天然气压力、天然气温度、压气机排气温度、压气机进口温度等14个参数作为输入变量,将表征燃气轮机运行状态的功率、燃烧室、加速度、NO_x质量浓度作为输出变量,建立粒子群算法优化的Elman神经网络模型,得到隐含层与输入层、输出层之间的连接权值;然后利用Olden方法处理神经网络的连接权值,获得各因素对燃气轮机运行状态影响显著性的量化值表达式,建立了燃烧调整过程中燃气轮机运行状态影响因素显著性分析的方法;最后结合燃气轮机运行数据进行计算分析。结果表明：燃气轮机的运行状态主要受排气温度、预混气压力及流量、值班气流量、压气机进口差压以及进气导流叶片开度5个因素的影响,并且燃烧调整过程中需要统筹调整输入参数,以保证燃气轮机燃烧始终处于稳定、低NO_x排放区域。     

The effect of air staged combustion on NOx emissions in dried lignite combustion

Experiments were carried out in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of BRXL lignite and its dried coals. The impact of moisture content, multiple air staging, pulverized coal fineness and burnout air position on NOx emissions under deep, middle and shallow air-staged combustion conditions. Moreover, the impact of blending coals on NOx emissions was investigated in this paper. The unburned carbon concentration in fly ash was also tested. Experimental results based on the combustion of BRXL lignite and its dried coals show that NOx emissions can be reduced drastically by air-staged combustion. NOx emissions reduce with the increase of the air that is staged and the distance between the burner and burnout air position. Dried coal of BRXL lignite emits a smaller amount of NOx than that of BRXL lignite. However, the dried degree of BRXL lignite is closely related to R₉₀ fineness. Dried coal with optimal moisture content yields least NOx emissions. When deep or middle staged combustion was adopted, the application of multi-staged combustion is conducive to NOx reduction. However, when shallow staged combustion was adopted, NOx emissions are higher in multi-staged combustion than that in single-staged combustion with M_S = 0.54. Thus, the existence of a certain concentration of O₂ in reduction zone would significantly reduce NOx emissions. The blending coals that dried coals of BRXL lignite were blended with bituminous coals emit a larger amount of NOx than that of the dried coal alone. NOx emissions decrease with the increase of the proportion of dried coal in the blending coal. Moreover, the unburned carbon concentration in fly ash of dried coal in staged combustion is lower than that of BRXL lignite in staged combustion. On the whole, the dried coal of BRXL lignite is conducive to NOx reduction in staged combustion.     

Emissions of organic compounds from the combustion of oats – a comparison with softwood pellets

Oats are a new biofuel possible to use in modified residential wood pellet combustion appliances. The emissions of organic compounds from five sequential combustion stages; initial smouldering, early flaming, late flaming, after-flame smouldering and final glowing, for incomplete burning of oats on a laboratory scale were determined by gas chromatography and compared to those of softwood pellets.High concentrations of 1,6-anhydroglucose and furan-related compounds were released from the initial smouldering of oats, while high concentrations of methoxyphenols were released during the initial smouldering of wood pellets. The results indicate that oats are a biofuel with relatively low emissions during combustion, almost as low as those from wood pellets. After-flame smouldering of oats released lower concentrations of methane, alkenes and aromatic hydrocarbons than the after-flame smouldering of wood pellets. The large differences in emissions from the various combustion stages should be considered when evaluating the environmental aspects and health effects of residential burning of oats and wood pellets.     

MILD combustion of hydrogenated biogas under several operating conditions in an opposed jet configuration

MILD combustion of biogas takes its importance firstly from the combustion process that diminishes significantly fuel consumption and reduces emissions and secondly from the use of biogas which is a renewable fuel. In this paper, the influence of several operating conditions (namely biogas composition, hydrogen enrichment and oxidizer dilution) is studied on flame structure and emissions. The investigation is conducted in MILD regime with a special focus on chemical effects of CO₂ in the oxidizer. Opposed jet diffusion combustion configuration is adopted. The combustion kinetics is described by the Gri 3.0 mechanism and the Chemkin code is used to solve the problem.It is found that oxygen reduction has a significant effect on flame temperature and emissions while less sensitivity corresponds to hydrogen enrichment in MILD combustion regime. Temperature and species are considerably reduced by oxygen decrease in the oxidizer and augmented by hydrogen addition to the fuel. The maximum values of temperature and species are not influenced by the composition of the biogas in MILD regime. Blending biogas with hydrogen can be used to sustain MILD combustion at very low oxygen concentration in the fuel.In MILD combustion regime, the chemical effect of CO₂ in the oxidizer stream reduces considerably the flame temperature and species production, except CO which is enhanced. For high amounts of CO₂ in the oxidizer, the chemical effect of CO₂ becomes negligible.