天然气催化燃烧炉窑特性研究及烧制琉璃的应用

将天然气催化燃烧炉窑应用到琉璃烧制中。并在催化燃烧过程中始终保持2.0的过量空气系数,通过合理调节空气和天然气的流量值,使得炉窑内温度缓慢达到900℃左右。研究表明:炉窑内温度分布满足烧制要求,且当达到稳定催化燃烧时,排放烟气中甲烷和NO_x、CO等污染物含量均为1 mg/m~3左右,甲烷几乎完全燃烧且污染物达到近零排放,说明天然气催化燃烧炉窑适合琉璃烧制,且具有节能环保的特性。     

天然气热利用率及影响因素

用热力学理论和燃烧理论建立了天然气排烟温度与热利用率的计算模型,计算得出了天然气热利用率与排烟温度的关系曲线,可供实际工程参考.对影响天然气热利用率的因素(排烟温度、过剩空气系数和烟气冷凝温度)的主要影响因素(燃烧空气含湿量)进行了定量分析,提出了提高天然气热利用率的措施,可使天然气的热利用效率提高1000/~2000/.     

天然气加湿富氧燃烧特性分析及烟气置换NGH方案

天然气工业锅炉的污染物排放标准日趋严格,采用单一的控制污染物排放技术手段已难以满足现有需求.本文以提高燃烧热效率与控制污染物排放量为目标,基于富氧燃烧、加湿燃烧与低氮燃烧的组合技术路线,提出一套天然气富氧加湿燃烧及烟气置换开采天然气水合物(NGH)方案.对天然气预混水蒸气燃烧进行数值模拟,确立最佳水蒸气预混比为Rf=0.33.在此基础上,分析不同烟气再循环率及过量空气系数对天然气加湿燃烧特性及污染物排放特性的影响规律以得出最优工况.该工况经烟气循环一次后,直接置换开采NGH的能源投资回报值(EROI)为7.5,具有实际开采价值.     

天然气低氮氧化物燃烧研究进展与展望

相比于其他化石燃料,天然气燃烧所生成的污染物较少,但NOx排放量仍然较多.随着我国天然气消费量的逐年增加,降低NOx的排放量成为人们普遍关注的重要课题.文章简述了天然气燃烧过程中的NOx生成和还原机理,综述了几种不同低氮燃烧技术的减排效果及研究进展,对比分析了各项低氮燃烧技术的动力学原理及其优缺点,最后展望了天然气低氮燃烧的研究趋势.天然气燃烧生成的NOx主要是热力型NOx,针对热力型NOx的生成机理,减少NOx排放的有效手段包括降低燃烧温度、控制氧气浓度,缩短反应物在高温区的停留时间等.由此发展出了分级燃烧、贫燃预混燃烧、烟气再循环、MILD燃烧和催化燃烧等低氮燃烧技术.其中,分级燃烧、烟气再循环等技术已广泛应用在工程实践当中.这些技术都是通过营造不利于NOx生成的反应气氛来控制NOx排放,但同时这些措施也会引起燃烧不稳定、燃烧效率低等问题.因此,低氮燃烧、稳定燃烧以及燃料完全燃烧之间的协同研究将是今后的研究方向.MILD燃烧、催化燃烧等新兴技术具有优良的低氮燃烧特性,但其反应机理还有待进一步研究.降低MILD燃烧的形成条件、开发高活性和高稳定性的催化剂仍是目前的研究重点.     

氢气/空气预混合微尺度催化燃烧

通过耦合专用软件FLUENT和CHEMKIN并采用空间气相和表面催化详细化学反应机理,对氢气和空气的预混合气体在微型管道内的催化燃烧过程进行数值模拟,讨论了不同反应模型的燃烧特性以及预混合气体入口速度和管径对催化燃烧反应的影响.计算结果表明,表面催化反应对空间气相反应有抑制作用;入口速度和管径对氢气的催化燃烧过程有重要的影响,在入口速度较小时,燃烧主要是空间气相化学反应,随着入口速度的增大,燃烧过程同时存在着表面催化反应和空间气相反应两种控制因素,在入口速度较大时,燃烧主要是表面催化燃烧过程;随着管径的减小,微型管道内反应的最高温度降低.此结果为在微型动力系统中实现催化燃烧以及扩展燃烧极限提供了理论依据.     

催化燃烧技术

<正> 1 前言在铂族金属或镍、钻、铁、锰、铬等的氧化物为氧化剂的空气气氛中,这些氧化物与可燃性气体一接触,在较低温度下,就会引起燃烧反应。所谓催化燃烧技术就是利用上述固体催化剂的接触氧化作用进行燃烧反应的一种方法。近年来,它作为一种谋求更好地抑制氮氧化物(NO_x)的生成和节约能源的新的燃烧方法逐渐应用开来。将燃烧器燃烧中所用到的一些概念,与燃料和空气与摧化剂相接触的方法联系考虑,可以把燃烧法分成扩散催化燃烧法和预混合催化燃烧法。     

不同稀释方式降低天然气发动机NO_x排放的研究

为了使天然气发动机在采用机内净化措施的情况下大幅度降低NOx排放,满足未来的排放法规,试验研究了3种稀释方式(空气稀释、当量燃烧+EGR稀释、空气与EGR双重稀释)对稀燃天然气发动机控制NOx排放潜力的影响,并考察了燃烧边界条件的改变对发动机燃烧及排放性能的影响。研究结果表明:通过燃烧边界条件优化,3种稀释方式均能使发动机NOx排放满足国-Ⅳ及国-Ⅴ排放标准,在同等NOx排放前提下,对其经济性而言,双重稀释方式优于空气稀释方式,空气稀释方式优于当量燃烧+EGR稀释方式。     

表面催化反应对小尺度空间内气相燃烧的影响

表面催化燃烧是微、小尺度空间内组织燃烧的重要手段.采用表面催化反应动力学方法,基于FLUENT软件自主开发了仿效CHEMKIN的催化反应动力学模型,实现了化学反应和流动的同步模拟.通过验证算例发现,该模型可获得与DETCHEM软件一致的计算结果,且与实验结果符合较好.采用该数值模型,对小尺度二维渐扩通道内Pt表面催化时CH4/空气燃烧进行研究.计算结果表明,不同当量比时表面催化反应都显著缩小了气相燃烧的熄火距离;气相燃烧火焰面增厚,火焰前沿处温度梯度变小.此外,表面催化对贫燃料燃烧的影响更显著.     

预混天然气催化燃烧特性

将含有钯、镧、锶、钴和锰的催化剂加载在鄞青石上进行预混天然气催化燃烧．实验表明,催化燃烧温度范围在600～900℃,过低的温度可能导致熄火,过高的温度可能使催化剂失效．在混合气体中,天然气体积分数为6％～12％,在催化剂中钯含量增至γ-Al2O3质量的0．21％及燃空比为8％时,实现了较好的催化燃烧（排放较少）．实验同时证实,在较短的催化载体内,催化燃烧效果较好,载体表面温度较低;在较长的载体内,载体表面温度较高,NOx排放增加．同时,分析了载体内传热传质特性．     

初始温度/压力对天然气层流燃烧速率的影响

在定容燃烧弹内利用高速纹影摄像法研究了不同初始温度和初始压力下不同当量比的天然气-空气混合气的火焰传播过程,并结合火焰传播照片分析了初始压力和初始温度对混合气层流燃烧速率的影响.研究表明,天然气-空气混合气的无拉伸火焰传播速率和无拉伸层流燃烧速率随当量比的增加先增大后减小,且最大值出现在化学当量比附近.火焰传播速率和层流燃烧速率均随着初始压力的升高而降低,随着初始温度的升高而提高.     

Increasing the efficiency of radiant burners by using polymer membranes

Gas-fired radiant burners are used to convert fuel chemical energy into radiation energy for various applications. The radiation output of a radiant burner largely depends on the temperature of the combustion flame. In fact, the radiation output and, thus, the radiant efficiency increase to a great extent with flame temperature. Oxygen-enriched combustion can increase the flame temperature without increasing fuel cost. However, it has not been widely applied because of the high cost of oxygen production. In the present work, oxygen-enriched combustion of natural gas in porous radiant burners was studied. The oxygen-enriched air was produced passively, using polymer membranes. The membranes were shown to be an effective means of obtaining an oxygen-enriched environment for gas combustion in the radiant burners. Two different porous radiant burners were used in this study. One is a reticulated ceramic burner and the other is a ceramic fibre burner. The experimental results showed that the radiation output and the radiant efficiency of these burners increased markedly with rising oxygen concentrations in the combustion air. Also investigated were the effects of oxygen enrichment on combustion mode, and flame stability on the porous media.     

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.     

催化肋片对微燃烧室内燃烧特性的影响

针对带有催化肋片的微燃烧室内部的氢氧预混合燃烧过程,利用CFD计算软件建立数值模型,在实验验证的基础上进行了模拟计算。结果表明:不同流速下,燃烧室内布置催化肋片能提高燃烧室外壁面平均温度以及燃烧效率。在低流速时催化肋片布置越靠近入口,表面催化反应对气相反应的抑制程度越大;肋片位置越靠近出口,燃烧室外壁面平均温度越高。在高流速下,催化肋片位置越靠近出口,表面催化反应对气相反应的促进作用越明显,使得燃烧室外壁面温度分布越均匀、燃烧室出口截面温度越高。     

A fibre‐mat catalytic burner for the heating system of PVC tiles

This study aims to develop the low‐temperature catalytic burner using Al₂O₃ fibre and Pt catalyst as a substrate and an active material, respectively, and then to apply it to the heating system of PVC tiles. Three types of fibre‐mat catalytic burners—natural diffusion catalytic burner, forced diffusion catalytic burner and premixed catalytic burner—were tested. For the natural diffusion catalytic burner, the combustion efficiency was significantly affected by the installation method. Its combustion efficiency was above 99.5% when it was placed vertically and upward, whereas the combustion efficiency rapidly deteriorated to less than 80% when it was installed in downward position. When the forced diffusion combustion mode was employed, the combustion efficiency of the fibre‐mat catalytic burner was successively improved over 99.5%. The optimal operation condition of the premixed fibre‐mat catalytic burner was obtained when the excess air ratio is 4.3 and the heat load is 3.0 kcal h^?1 cm^?2. The heat load of the premixed catalytic burner was found to be twice as much as those of the natural and forced diffusion catalytic burner, meaning that the size of the catalytic burner can be reduced to its half if the premixed catalytic burner is used. We also performed the field test of the fibre‐mat catalytic burner in the heating system of the PVC tiles. The combustion efficiency of the catalytic burners appeared to be satisfactory, above 99.5%, and the energy saving by replacing the existing electrical heaters with the fibre‐mat catalytic burners were shown to be 27.7%. Copyright © 2002 John Wiley & Sons, Ltd.     

天然气再燃降低NOx排放的实验研究

主燃区停留时间、再燃区停留时间和天然气输入热量比不但影响再燃还原NOx的效果,而且是与天然气喷入位置、燃尽风喷入位置、再燃技术经济性关系紧密的3个因素.利用一维再燃热态实验炉对此3个因素与再燃还原NOx效果的关系进行了实验研究,得到了NOx排放的基本趋势和规律,并着重分析了主燃区停留时间影响再燃还原NOx效果的原因.     

Development of a high efficiency radiation converter using a spiral heat exchanger

Heating by radiation is widely used for materials processing. Electrical radiant heaters are the most commonly used heaters. Electricity is expensive and the combustion of fossils fuels for electricity production emits CO₂. In order to convert the energy from the fuel to radiation energy directly and efficiently, our group has developed a compact, high efficiency, radiation converter using a spiral heat exchanger to recover the energy from high-temperature exhaust gas. The spiral heat exchanger has a weld-free construction to prevent cyclic thermal stress, and is constructed from inexpensive ferrite steel plates. The combustion chamber, equipped with a swirler to mix the gas fuel and air, can achieve stable combustion. The distribution of the surface temperature on the radiant tube was measured by a radiation thermometer, called a thermo viewer, and then the radiant energy emitted from the radiant tube was estimated. The efficiency of the spiral heat exchanger was measured from the temperature of the inlet air and exhaust gas. The heat exchanger achieved a high effectiveness, and heat loss from the exhaust gas was minimized. Consequently, a highly efficient radiation converter was produced to convert the fuel energy to radiation energy.     

Comparison of the effects of EGR and lean burn on an SI engine fueled by hydrogen-enriched low calorific gas

A naturally aspirated spark ignition (SI) engine fueled by hydrogen-blended low calorific gas (LCG) was tested in both exhaust gas recirculation (EGR) and lean burn modes. The “dilution ratio” was introduced to compare their effects on engine performance and emissions under identical levels of dilution. LCG composed of 40% natural gas and 60% nitrogen was used as a main fuel, and hydrogen was blended with the LCG in volumes ranging from 0 to 20%. The engine test results demonstrated that EGR operations at stoichiometry showed a narrower dilution range, inferior combustion characteristics, lower brake thermal efficiency, faster nitrogen oxides (NO_x) suppression, and higher total hydrocarbon (THC) emissions for all hydrogen blending rates compared to lean burn. These trends were mainly due to the increased oxygen deficiency as a result of using EGR in LCG/air mixtures. Hydrogen enrichment of the LCG improved combustion stability and reduced THC emissions while increasing NO_x. In terms of efficiency, hydrogen addition induced a competition between combustion enhancement and increases in the cooling loss, so that the peak thermal efficiency occurred at 10% H₂ with excess air ratio of 1.5. The engine test results also indicated that a close-to-linear NO_x-efficiency relationship occurred for all hydrogen blending rates in both operations as long as stable combustion was achieved. NO_x versus combustion duration analysis showed that adding H₂ reduced combustion duration while maintaining the same level of NO_x. The methane fraction contained in the THC emissions decreased slightly with an increase in hydrogen enrichment at low EGR or excess air dilution ratios, but this tendency was diminished at higher dilution ratios because of the combined dilution effects from the inert gas in the LCG and the diluents (EGR or excess air).     

天然气-汽油双燃料发动机燃烧特性试验研究

为了探究天然气-汽油双燃料燃烧模式在现代发动机上的适用性及潜在优势,基于一台增压直喷发动机结合进气道喷射天然气和缸内喷射汽油,开展了不同负荷、过量空气系数和天然气替代率下天然气-汽油双燃料燃烧特性试验研究。结果表明,低负荷固定转矩工况下,随着天然气质量流量增加,发动机最高燃烧压力提高,燃烧相位提前,循环变动降低,且在稀燃条件下尤为明显。中等负荷固定转矩工况下的燃烧特性变化规律与低负荷工况相似,而在高天然气替代率、稀燃条件下有效热效率随天然气质量流量增加明显提高。高负荷节气门全开工况下,尽管发动机最大转矩有所下降,但爆震起点和强度得到有效抑制,燃烧相位也明显改善,因此可以通过增压来弥补发动机功率不足的问题。     

Internal combustion engines fueled by natural gas—hydrogen mixtures

In this study, a survey of research papers on utilization of natural gas–hydrogen mixtures in internal combustion engines is carried out. In general, HC, CO₂, and CO emissions decrease with increasing H₂, but NO_x emissions generally increase. If a catalytic converter is used, NO_x emission values can be decreased to extremely low levels. Consequently, equivalence zero emission vehicles (EZEV) standards may be reached. Efficiency values vary with H₂ amount, spark timing, compression ratio, equivalence ratio, etc. Under certain conditions, efficiency values can be increased. In terms of BSFC, emissions and BTE, a mixture of low hydrogen percentage is suitable for using.     

Combustion of methane in catalytic honeycomb monolith burners

The heat transfer efficiency, stability, and pollutant emissions characteristics of ultra‐lean methane–air combustion in some precious metal‐based honeycomb monoliths were investigated. The interpretation of the experimental results was assisted using numerical modelling of the gas‐phase combustion process. The thermal radiation output of the monoliths varied between 27 and 30 per cent of the thermal input, and this compared favourably with equivalent porous inert media burners. The minimum fuel concentrations for very‐low emission stable combustion were found to be significantly lower than for conventional gas‐phase combustion and were shown to vary with the nature and loading of the catalyst, as well as with flow rates. The palladium catalyst was found to have a larger window of mixture strengths and flow rates for stable operation than the platinum one. During all the runs under stable combustion conditions, only extremely small amounts of CO, NO_x and unburnt hydrocarbons were detected. Thus, the operating conditions verified ‘near‐zero’ pollutant emissions that only a catalytic combustion process can achieve at present. Temperature profiles inside the monoliths channels proved that the catalyst's role was not only to enable the ignition of fuel mixtures below flammability limits, but also to ensure the complete oxidation of the fuel to CO₂ via surface reactions in the steady state. The reaction zone inside the catalysts was found to end at about 10 mm from the monolith's entrance. The effect of monolith length was investigated and a reduction of 70 per cent in the original length was found possible. Copyright © 2000 John Wiley & Sons, Ltd.