不同升温速率脱脂餐厨垃圾燃烧特性及动力学研究

利用热重分析法和Coats-Redfern积分法,对不同升温速率下脱脂餐厨垃圾的燃烧特性进行研究,得出各试样的着火温度、燃尽温度及综合燃烧特性指数,并通过燃烧动力学分析得到各试样的活化能。实验结果表明,燃烧温度从室温升至1000℃时,脱脂餐厨垃圾燃烧过程可分为3个失重阶段:水分析出阶段、挥发分析出及燃烧阶段和固定碳燃烧阶段;升温速率对脱脂餐厨垃圾最大失重率以及燃烧特性指数等燃烧特性参数都有显著影响。根据Coats-Redfern积分法计算结果,脱脂餐厨垃圾在空气氛围下的燃烧反应不能单纯用一级反应来描述,低温和高温阶段的活化能分别为63.4~77.83 kJ/mol和78.63~94.58 kJ/mol。     

几种生物质的TG-DTG分析及其燃烧动力学特性研究

采用热重分析技术对木屑、麦秆、玉米秆和玉米芯4种生物质的燃烧特性进行了研究,考察了其着火、燃尽特性和综合燃烧特性,研究了升温速率对生物质燃烧特性的影响,同时在热天平上对其进行了动力学试验研究.研究表明:生物质燃烧过程大致可以分为3个阶段,即水分析出阶段、挥发分析出燃烧阶段、固定碳燃烧与燃尽阶段:生物质具有着火温度低、燃尽温度低、燃尽率高等优点;随着升温速率的提高,着火温度、各试样挥发分最大释放速率、燃尽温度均呈升高趋势,燃烧特性随升温速率的提高而变好.采用一级反应动力学模型和积分法对生物质燃烧动力学参数的研究表明,生物质具有较低的活化能,有利于点燃.     

粉末与颗粒状木质燃料燃烧动力学特性研究

为掌握粉末状与颗粒状木质燃料的动力学特性,选用同一木质成型燃料,通过热重分析仪和管式加热炉系统分别对其粉末状和颗粒状原料进行实验研究。采用双步双等法、等温法等进行动力学拟合推断,求解两者燃烧动力学三因子并进行比较。结果表明:由于燃料内部传热传质性质差异较大,粉末状与颗粒状木质燃料的最概然机理函数并不相同,前者燃烧全过程的燃烧机理基本一致,后者挥发分燃烧阶段与焦炭燃烧阶段的燃烧机理差异较大;粉末与颗粒状活化能值分别为92.33 kJ/mol和71.20 kJ/mol,两者相差21.13 kJ/mol;指前因子值分别为2.55×10~8 s~(-1)和78.55 s~(-1),前者远大于后者,两者相差7个数量级。     

挥发分对CFB锅炉燃烧的影响

煤的挥发分越高,煤的着火热越小,着火性能越强,着火温度越低,着火燃烧越快速、稳定;挥发分析出过程中煤炭颗粒会膨胀、破碎或爆裂成细小颗粒,同时挥发分的析出使煤炭颗粒内部形成孔隙从而增加了煤炭颗粒与氧的接触表面积,有利于提高焦炭的燃烧速度和燃烬度,可降低机械未完全燃烧损失,也可能造成扬析、夹带的颗粒增多;挥发分越高,煤碳化程度越低,煤中难燃的固定碳成分越少,挥发分析出的孔隙越多,焦炭燃烧反应速度越快,煤炭越容易燃烬。当挥发分波动幅度较大时,会影响炉膛燃烧份额比例分配。     

半焦燃烧分段反应动力学参数的计算

利用TG/DSC热重分析仪对半焦在空气气氛下不同升温速率进行热重实验,运用Flynn-Wall-Ozawa方法对半焦的燃烧动力学参数进行分析研究。结果表明:通过活化能的变化趋势可以把半焦的燃烧过程分为三段:水分和挥发分的析出、挥发分的燃烧、固定碳的燃烧。对半焦燃烧过程的燃烧动力学参数进行分段拟合,可以得到每段燃烧过程的表观活化能、表观指前因子、反应级数等动力学参数以及每个阶段的转换温度点,拟合曲线与实验数据的拟合程度很好。     

玉米芯/煤粉恒温下混燃动力学特性

为能更接近实际情况下研究煤粉/玉米芯混燃动力学特性,改造热重分析设备,改变其只能逐步升温的方法,使其具备在指定温度下检测样品反应动力学的功能。研究不同掺混比、不同温度、不同煤种等对煤粉/玉米芯混燃动力学的影响规律。通过可失重额余量随时间的变化曲线,进行燃烧动力学影响因素分析。结果表明:煤粉与玉米芯在恒温条件下混燃不像逐步升温下那样会明显分为4个阶段,而是无明显的阶段性;玉米芯掺混比例越大,活化能越小,燃烧越剧烈,试样的平均失重速率越大,燃烬时刻提前。温度对混燃的影响存在一段作用不明显的温度区间,在此区间外,温度越高,活化能越小,试样失重速率越大,燃烬时刻越提前;不同煤种/玉米芯混燃,呈现出较大差异。玉米芯对含挥发分较少相对难燃的煤种促进燃烧和燃烬的效果较明显,对含挥发分较多相对易燃的煤种促进作用相对较弱。     

柠条燃烧特性及燃烧动力学研究

应用热分析仪对柠条生物质燃料的燃烧过程进行分析,研究颗粒度、升温速率和风量对燃烧特性与动力学参数的影响。结果表明:(1)颗粒度为0.16 mm试样在升温速率为20 K/min,风量为40 mL/min的工况下,着火温度为221.1℃,最大燃烧速率温度为336.2℃,燃尽温度为559.4℃,最大燃烧速率0.6 mg/min,平均燃烧速率为0.129mg/min,相对于10 K/min和30 K/min升温速率,20 K/min工况下的燃料动力学参数最优,活化能为39.094 kJ/mol,频率因子为2.175×10~7L/min;(2)升温速率的增大会使平均燃烧速率和燃烧特性指数增大,着火温度降低;风量对燃烧速率无影响,但较大风量不利于挥发分析出和燃烧稳定性;颗粒度对挥发分析出有显著影响,颗粒度较大时需较高升温速率和风量才可充分燃烧,而颗粒度较小时即使风量较小也能充分燃烧。     

城市污泥热解特性及热解过程中Pb、Cd迁移特性

以城市污泥为研究对象,考察了其在不同升温速率下的热解特性、热解反应动力学特征以及重金属Pb和Cd在热解过程中的迁移规律。通过热解失重曲线图得出,污泥的热解过程可分为水分析出阶段、挥发分析出阶段和半焦分解阶段。提高升温速率会导致污泥的失重量减少,挥发分最大失重速率增加。根据Coats-Redfern积分法计算结果得到,挥发分析出的第1阶段和第2阶段的反应级数分别为1和2,且不同升温速率下挥发分析出的第1阶段和第2阶段下的活化能总体上变化不大。通过管式炉热解实验可知,在热解温度为400、500、600和700℃范围内,重金属Pb和Cd的残留率随热解温度的升高均表现为先上升后下降的规律。Cd的残留率在热解温度为500和700℃时分别达到最高(41.64%)和最低(2.92%),而Pb的残留率随温度变化不大,均为93%以上。热解温度为400~500℃,Cd和Pb挥发较少。     

纤维素类草本能源植物燃烧动力学研究

利用热重分析仪进行了柳枝稷、芒草、芦竹和象草的燃烧试验,研究发现其燃烧过程可分为3个阶段,即水分析出、挥发分析出与燃烧、焦炭表面燃烧;其综合燃烧性能为芒草柳枝稷芦竹象草,4种能源草具有比玉米秆更好的综合燃烧特性.4种能源草在水分析出阶段存在1个吸热峰,在挥发分析出燃烧及焦炭燃烧阶段存在1个放热峰;芦竹和芒草先达到放热峰且放热量较大.动力学分析表明,挥发分的析出与燃烧较容易,所需活化能小于32,k J/mol,而焦炭表面燃烧较难,柳枝稷所需的活化能最大,高达108,k J/mol.     

生物质加压热重分析研究

对两种生物质木屑和松针进行了不同压力和升温速率下的热重分析试验,通过生物质热重失重率（TG）和失重速率（DTG）曲线,获得了相关热解特性参数,提出了生物质的挥发分综合释放特性指数D．并通过热分析数学方法求取了生物质热解动力学参数．试验结果表明,氮气气氛中,木屑与松针常压和增压下主要热解阶段可认为两段一级反应;热解压力的提高,将延迟生物质挥发分初析温度和DTG峰值温度,降低最大析出率和DTG峰值,生物质的挥发分综合释放特性指数D也减小,增加了生物质挥发分的析出难度,并改变了热解反应活化能和频率因子．同一压力下,提高热解升温速率,生物质综合特性指数D将增加．     

Experimental studies of ash film fractions based on measurement of cenospheres geometry in pulverized coal combustion

In pulverized coal particle combustion, part of the ash forms the ash film and exerts an inhibitory influence on combustion by impeding the diffusion of oxygen to the encapsulated char core, while part of the ash diffuses toward the char core. Despite the considerable ash effects on combustion, the fraction of ash film still remains unclear. However, the research of the properties of cenospheres can be an appropriate choice for the fraction determination, being aware that the formation of cenospheres is based on the model of coal particles with the visco-plastic ash film and a solid core. The fraction of ash film X is the ratio of the measuring mass of ash film and the total ash in coal particle. In this paper, the Huangling bituminous coal with different sizes was burnt in a drop-tube furnace at 1273, 1473, and 1673 K with air as oxidizer. A scanning electron microscope (SEM) and cross-section analysis have been used to study the geometry of the collected cenospheres and the effects of combustion parameters on the fraction of ash film. The results show that the ash film fraction increases with increasing temperature and carbon conversion ratio but decreases with larger sizes of coal particles. The high fraction of ash film provides a reasonable explanation for the extinction event at the late burnout stage. The varied values of ash film fractions under different conditions during the dynamic combustion process are necessary for further development of kinetic models.     

Thermogravimetric kinetic analysis of Nannochloropsis oculata combustion in air atmosphere

The thermal behavior of Nannochloropsis oculata combustion in air atmosphere were investigated by performing experiments on STA PT1600 Thermal Analyzer at heating rates of 10°C/min, 40°C/min and 70°C/min and range of temperatures from room temperature to 1200°C. The kinetic parameters were evaluated by using Kissinger and Ozawa methods. The result showed that Nannochloropsis oculata combustion occurred in five stages. Started with initial devolatilization, the main thermal decomposition and combustion process, transition stage, the combustion of char and the last stage was the slow burning reaction of residual char. In line with increasing heating rate, the mass loss rate increased as well, but it delayed the thermal decomposition processes toward higher temperatures. The average activation energy at the main thermal decomposition stage and the stage of char combustion were approximately 251 kJ/mol and 178 kJ/mol, respectively.     

酒糟在流化床中燃烧特性的试验研究

利用X射线荧光光谱仪、灰熔点仪和热重分析仪等对酒糟的灰成分、灰熔点及着火温度进行了测试,并在小型流化床试验台上进行了酒糟燃烧试验,对不同含水率酒糟的着火特性、烧结特性和NOx排放质量浓度进行了研究.结果表明:酒糟灰中K2O和Na2O的含量很低,只有4.936%;酒糟灰的软化温度较高,高于1 290℃;干燥酒糟的挥发分较高,高于250℃就能够着火燃烧;当石英砂的平均粒径为300μm、流化床内的流化速度大于0.36m/s时,流化状况良好;在900℃下,酒糟在石英砂流化床中燃烧不会出现烧结现象;在不同的燃烧温度下,NOx排放质量浓度均较大,且随着燃烧温度的升高而增大;酒糟可以在石英砂流化床中燃烧,且燃尽效果良好.     

Thermo-oxidative characterization and kinetics of tar sands

In this research, non-isothermal kinetics and thermal analysis of Gerçü? tar sand sample is studied by DSC (differential scanning calorimeter) and TG/DTG (thermogravimetry). Experiments were performed using three different mesh size (20-35, 35-50 and >50) of sample. Differential scanning calorimeter (DSC) curves revealed three reaction regions in the temperature range of 20-600 °C. On the other hand, thermogravimetry (TG/DTG) curves of tar sand samples at different particle sizes demonstrated three stages of weight loss. Two different kinetic models (Coats & Redfern and Arrhenius) were used to determine the kinetic parameters of the samples and it was observed that the average activation energy values were between 17.5 and 26.6 kJ/mol, for reaction region-II and 126.2-160.1 kJ/mol for reaction region-III, respectively. In order to see the contribution of each region to the overall reactivity of the tar sand sample, weighted mean apparent activation energy of the samples are also determined.     

Investigation on the effects of fly ash particles on the thermal radiation in biomass fired boilers

Ash is produced in combustion of biomass. Some part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also can affect the thermal radiation. The effects of fly ash particles on the thermal radiation are considered in this investigation. By analyzing sampled data in an electrostatic filter, a realistic particle size distribution is found. Although the optical data on biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fe coal fly ash can be applied for the biomass fly ash. The Mie theory is used to predict scattering and absorption coefficients and phase function. The mean Planck scattering and absorption coefficients and phase function are predicted by averaging over the particle size distribution and Planck function, respectively. The effects of fly ash particles on thermal radiation are evaluated by a three-dimensional test case. It is assumed that the medium is a mixture of non-grey gases and different level of particle loading. Predicted results from the test case showed that the fly ash can be influential on the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important on the radiative heat source and radiative heat flux to the wall whereas their absorption effect is important and can increase the radiative heat source and wall heat fluxes.     

Combustion of particles in a large pulverized brown coal flame

A model of the ignition of a polydisperse cloud of brown coal particles, in a known gas environment, is presented and used to predict the behavior of the particles in a burner jet of a utility boiler. The model allows for drying, devolatilization, and char combustion of the particles. It is assumed that the volatiles burn in the free stream so that char combustion can occur during volatiles evolution, the diffusion of oxygen to the particle surface being inhibited due to the net outflow of volatiles. The model is used to calculate the behavior of a cloud of p.f. size particles along the centerline of a brown coal burner jet in which the gas temperature and composition have been measured. Rates of volatile release and char combustion are calculated and shown to be in agreement with measurements of volatile material in the flame. It is found that particles smaller than about 80 μm contribute most to the ignition of the jet and that they closely follow the local gas temperature. The unique character of brown coal of combustion, its high volatile evolution on rapid heating, the high activity of its char at low temperature, and the demonstrated ignition of its char without a jump in temperature make the overlap of devolatilization and char combustion more likely than with other coals. The mathematical formulation that allows this overlap gives oxygen consumption levels consistent with measurement. An analysis is made of the relative importance of radiation from the flame front to the particle, and entrainment of hot combustion gases into the jet. It is found that the radiation is of secondary importance compared to the effect of entrainment which is the controlling mechanism in the initial heating of the particles. Also, the significance of the assumption that the volatiles burn in the free stream is discussed.     

A model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis

A mathematical model for the combustion in air of a single entrained spherical coal particle, 30 μm in diameter, has been developed incorporating thermogravimetric analysis data of Whitwick coal. The model is based on a set of ordinary differential equations, describing the reaction rates and the mass and heat transport processes. The system of equations was solved numerically. The combustion mechanism of the particle was described by locating the reaction zone at the solid surface, where gas-phase combustion of volatiles and heterogeneous reaction between gaseous oxygen and the carbon and hydrogen in the solid occurred in parallel. The combustion process was chemical-reaction-rate-controlled, with the oxygen partial pressure at the surface almost that of the surrounding bulk gas. The simulation results using this model, with the kinetic parameters for devolatilization and combustion derived from the experimental thermogravimetric data, are consistent with previously reported combustion lifetimes of approximately 1 s, for particles of this size and rank. They are also consistent with the anticipation that higher ambient gas temperatures should result in shorter burn-out times. The use of thermogravimetric data in the modelling of the combustion of small particles of these low-rank coals is a potentially valuable method for characterization of feedstocks for pulverized coal-fired boilers. © 1998 John Wiley & Sons, Ltd.     

Kinetics of boron hydrolysis for hydrogen production

This study examines the kinetics of boron hydrolysis. The boron hydrolysis reaction is classified as a noncatalytic heterogeneous reaction. It was found the steam rapidly hydrolyzes the boron, forming an oxide ash layer that then also reacts with the steam, forms gaseous boric acid, and re-exposes the boron substrate to steam. Additionally, the thickness of the ash layer, once formed, was found to be relatively constant through the reaction. In order to facilitate kinetic analysis the reaction was divided into two stages: a fast oxidation stage and a second, slower stage. The corresponding kinetic parameter for the first stage activation energy is 25 kJ/mol. The shrinking core model was used to model the second stage of the hydrolysis reaction where it was found that diffusion of steam through the ash layer was the rate limiting step.     

Combustion characteristics of Turkish lignites at oxygen-enriched and oxy-fuel combustion conditions

Combustion and oxy-fuel combustion characteristics of two Turkish lignites (Orhaneli and Soma) were investigated by Thermogravimetric Analysis (TGA) method. Experiments were carried out under oxygen-enriched air and oxy-fuel combustion conditions with 21, 30, 40% oxygen concentrations. Three heating rates of 5, 10, and 20 °C/min were considered and the isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate activation energies. The uncertainty assessment in obtaining the activation energy values was also considered. The obtained results indicated that the combustion of volatiles at both air and oxy-fuel conditions were approximately identical. However, at air combustion conditions, the decomposition of CaCO₃ took place at temperatures above 700 °C. This decomposition process was independent of the oxygen concentration and took place when the temperature reached to a certain threshold. The decomposition of CaCO₃ did not accomplish in oxy-fuel conditions as far as the temperature was higher than 900 °C. Combustion in oxy-fuel conditions had higher activation energy values comparing to conventional combustion atmosphere. The activation energy values were approximately unchanged at the start of combustion regardless of oxygen concentration or combustion atmosphere at about 165 kJ/mol and 150 kJ/mol for Orhaneli and Soma lignites, respectively. The apparent activation energies were higher at elevated oxygen concentrations. The uncertainties values related to FWO method were lower than KAS and Friedman methods. The calculated average uncertainty values were found to be at the range of 5–15% for most of the cases.