生物质超临界水流化床气化制氢系统与方法研究

本文提出在生物质超临界水气化制氢过程中采用流化床反应器防止反应器结渣堵塞.首先,基于流化床内多相流动力学理论,通过分析获得了超临界水流化床的主要参数包括最小流化速度、终端速度、流化方式等的设计准则.应用此设计准则成功构建了一套超临界水流化床气化制氢系统,实验研究表明高浓度生物质浆料在长达5个小时的气化过程中反应器未见堵塞现象,并获得了与生物质在管流反应器中气化相似的气体产物.     

超临界水流化床内煤气化过程建模与仿真(1):数学模型及物理场分布规律

煤的超临界水气化是极具发展前景的高效洁净煤炭转化技术.本实验室发明新型超临界水流化床反应器,避免了管式反应器中出现的结焦堵塞现象,实现了高浓度煤浆的高效连续气化.本文对超临界水流化床反应器内的物理过程进行了数学建模,并对宽广的温度参数范围下反应器内温度场、流场、以及颗粒停留时间演变规律等进行数值模拟分析,揭示了反应器内部各物理场的特征,加深了对超临界水流化床内流动传热过程的认识,对反应器的运行与设计具有理论参考价值。     

超临界水流化床内煤气化过程建模与仿真(2):气化反应动力学模型及气化规律

本文成功建立了煤超临界水气化动力学模型,其中包括煤在超临界水中热解、液化、固相残碳及液化产物的蒸汽重整等均相和非相反应。该动力学模型能准确反应煤在超临界水中气化特征。在前述第一部分工作的基础上,耦合该气化动力学模型,对煤在超临界水流化床中气化过程进行了建模。通过该模型研究了宽温度参数范围下反应器内典型反应速率、反应组分分布演变规律,揭示了反应器内部化学反应特征与气化规律。研究加深了对超临界水流化床内煤气化过程的认识,对超临界水流化床反应器的优化、放大以及实际运行具有指导意义。     

超临界水流化床内两相流特性的数值模拟研究

本文基于欧拉双流体模型和颗粒动理学理论,对超临界水流化床反应器内气固两相流特性进行数值模拟,模拟获得了超临界水流化床反应器内颗粒分布特性和最小流化速度,分析了表观速度、压力、温度、初始床层高度对超临界水流化床流动特性的影响.本文的研究对超临界水流化床反应器结构的设计及操作条件的优化具有重要的指导意义.     

煤炭超临界水流化床制氢反应器内颗粒流动及传热特性的数值分析

煤炭超临界水气化是极具发展前景的高效洁净煤转化利用技术,深入研究反应器内流动和传热过程对反应器的设计及优化具有重要意义.本文基于欧拉-拉格朗日方法,耦合化学反应动力学模型,建立了煤炭超临界水流化床制氢反应器的三维瞬态CFD模型,研究了反应器内流动和传热特性、颗粒及产物组分的分布转化规律,得到了煤炭颗粒表面的对流和辐射换热系数,并从传热学的角度,揭示了颗粒粒径对煤炭转化速率的影响机理。     

球形颗粒在超临界水中自然对流传热特性研究

超临界水流化床反应器可以有效气化湿生物质并且高效产氢。超临界水流化床反应器工作的表观流速比较低,颗粒与流体之间的自然对流换热是主要的传热方式.本文通过Boussiesq模型和真实物性流体模型模拟研究了层流条件下单颗粒与超临界水在拟临界点附近的自然对流换热.由于物性变化的影响,颗粒表面附近呈现高速度梯度、高表面涡量和高温度梯度的现象。临界水导热系数和比热的变化对拟临界区的自然对流传热起主导作用,Grashof数和平均Nusselt数在对数坐标下为线性关系。     

超临界水流化床床床层与壁面间传热特性研究

本文基于颗粒动理学理论建立了流化床中的欧拉双流体模型,采用了单气泡沿壁面运动物理模型,从颗粒分布特性、温度分布特性和瞬时传热特性三个方面对超临界水流化床和气固流化床的床层与壁面间传热特性进行了对比研究。研究表明,相同条件下,相对于气固流化床,超临界水流化床中气泡的直径和上升速度都较小;超临界水流化床中床层与壁面间的传热系数与颗粒浓度呈反相关关系,而对于气固流化床则是正相关关系;并且不同于气固流化床,超临界水流化床床层与壁面间传热系数在颗粒浓度较低处对表观速度变化比较敏感。     

载镍褐煤残渣对甲苯催化分解性能的研究

本文以褐煤与生物质共气化为研究背景,以负载镍的褐煤残渣与生物质焦油(以甲苯为模型化合物)在共气化过程中的甲苯催化分解特性、煤焦气化特性以及协同作用为研究点,应用SEM、XRD表征方法,通过与Ni/Al_2O_3催化剂性能对比、共气化温度对比、水碳比影响等,表明经处理后的褐煤残渣与生物质焦油共气化,不仅产气率高,且具有生物质焦油催化分解作用。     

非预混燃烧模型模拟流化床生物质气化器中富氢气体的制备

对用流化床生物质气化器合成富氢气体,建立了基于非预混燃烧的模型对气化器中的生物质在空气-水蒸汽环境中的气化反应过程的模型,并采用流体力学软件 FLUENT 6.0对过程进行了模拟.通过模拟结果与实验结果的对比分析发现,水蒸汽与生物质的比、空氧比和生物质颗粒的粒径大小是决定产气中氢气含量的重要参数.同时,对气化器中氢气的分布进行了研究.     

基于燃料氧化分级的化学链燃烧冷态实验研究

基于燃料氧化分级的原理,设计并搭建了新型串行流化床化学链燃烧反应器,该反应器由空气反应器、气化反应器、还原反应器、空气旋风分离器、燃料旋风分离器、空气返料器、燃料返料器和隔离器组成一个循环回路。采用压力测量和气体成分检测的方法,研究了该系统内的压力分布、物料循环速率以及反应器间气体混合规律。结果表明:隔离器流化风量是影响系统压降以及颗粒循环的决定性因素;空气反应器与燃料反应器之间几乎不发生气体串混,隔离器气体隔离作用良好。     

Effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions

The effect of gasification reactions on biomass char conversion under pulverized fuel combustion conditions was studied by single particle experiments and modelling. Experiments of pine and beech wood char conversion were carried out in a single particle combustor under conditions of 1473-1723 K, 0.0-10.5% O₂, and 25-42% H₂O. A comprehensive progressive char conversion model, including heterogeneous reactions (char oxidation and char gasification with CO₂ and H₂O), homogeneous reactions (CO oxidation, water-gas shift reaction, and H₂ oxidation) in the particle boundary layer, particle shrinkage, and external and internal heat and mass transfer, was developed. The modelling results are in good agreement with both experimental char conversion time and particle size evolution in the presence of oxygen, while larger deviations are found for the gasification experiments. The modelling results show that the char oxidation is limited by mass transfer, while the char gasification is controlled by both mass transfer and gasification kinetics at the investigated conditions. A sensitivity analysis shows that the CO oxidation in the boundary layer and the gasification kinetics influence significantly the char conversion time, while the water-gas shift reaction and H₂ oxidation have only a small effect. Analysis of the sensitive parameters on the char conversion process under a typical pulverized biomass combustion condition (4% O₂, 13% CO₂, 13% H₂O), shows that the char gasification reactions contribute significantly to char conversion, especially for millimeter-sized biomass char particles at high temperatures.     

Continuous in situ measurements of alkali species in the gasification of biomass

This work describes the first on-line, in situ measurement of alkali species in biomass gasification using excimer laser induced fragmentation fluorescence (ELIF). Three pelletised biomass fuels were gasified under different operating conditions in a bubbling fluidized-bed reactor, using steam as the gasifying medium. Concentrations of potassium and sodium were measured in real time for several hours per measurement. Average concentrations obtained for potassium ranged from 140 to 350 ppb, and those for sodium ranged from 1.7 to 60 ppb. Also observed was a slow release of alkali after shutting off the fuel feed, which suggests that alkali species had been taken up in the bed material (olivine); this was confirmed by analysing the bed material before and after use. Consideration of the olivine structure/composition and possible interactions with alkali and other elements during gasification suggests that alkali-containing layers could have formed on the bed particles but appear not to form crystalline phases. This may allow for re-release of alkali while the bed is still hot.     

Numerical Simulation Study on Flow Laws and Heat Transfer of Gas Hydrate in the Spiral Flow Pipeline with Long Twisted Band

The natural gas hydrate plugging problems in the mixed pipeline are becoming more and more serious. The hydrate plugging has gradually become an important problem to ensure the safety of pipeline operation. The deposition and heat transfer characteristics of natural gas hydrate particles in the spiral flow pipeline have been studied. The DPM model (discrete phase model) was used to simulate the motion of solid particles, which was used to simulate the complex spiral flow characteristics of hydrate in the pipeline with a long twisted band. The deposition and heat transfer characteristics of gas hydrate particles in the spiral flow pipeline were studied. The velocity distribution, pressure drop distribution, heat transfer characteristics, and particle settling characteristics in the pipeline were investigated. The numerical results showed that compared with the straight flow without a long twisted band, two obvious eddies are formed in the flow field with a long twisted band, and the velocities are maximum at the center of the vortices. Along the direction of the pipeline, the two vortices move toward the pipe wall from near the twisted band, which can effectively carry the hydrate particles deposited on the wall. With the same Reynolds number, the twisted rate was greater, the spiral strength was weaker, the tangential velocity was smaller, and the pressure drop was smaller. Therefore, the pressure loss can be reduced as much as possible with effect of the spiral flow. In a straight light flow, the Nusselt number is in a parabolic shape with the opening downwards. At the center of the pipe, the Nusselt number gradually decreased toward the pipe wall at the maximum, and at the near wall, the attenuation gradient of the Nu number was large. For spiral flow, the curve presented by the Nusselt number was a trough at the center of the pipe and a peak at 1/2 of the pipe diameter. With the reduction of twist rate, the Nusselt number becomes larger. Therefore, the spiral flow can make the temperature distribution more even and prevent the large temperature difference, resulting in the mass formation of hydrate particles in the pipeline wall. Spiral flow has a good carrying effect. Under the same condition, the spiral flow carried hydrate particles at a distance about 3–4 times farther than that of the straight flow.     

A simple scattering model for measuring particle mass fractions in multiphase flows

In this paper we present a simple theoretical model of how pulsed ultrasound is attenuated by the particles in a solid/liquid flow. The theoretical model is then used to predict the attenuation of sound, given the mass fraction, the density, and the size distribution of the solid particles. The model is verified experimentally for suspensions of 0-10% (by mass) Dolomite ((Ca,Mg)CO3) particles and water. The experimental results show that the attenuation of sound due to particles varies linearly with mass fraction, and that the proposed theoretical model can be used to predict this attenuation. In all experiments the transmitter and receiver array were clamped onto the pipe wall, thus providing a completely non-invasive and non-intrusive measurement technique.     

Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char

The effect of pyrolysis conditions on char reactivity has been studied using Raman spectroscopy. This paper reports on the relationship between the properties of biomass char and the gasification rate. The gasification kinetics of biomass char have been revealed by measuring the rate of weight loss during its reaction with CO₂ as a function of temperature. First-order kinetic rate constants are determined by fitting the weight loss data using a random pore model. The relationship between the char structure and CO₂ gasification reactivity was investigated in the range of 15–600 °C/min at a constant pyrolysis pressure (0.1 MPa), and 0.1–3.0 MPa at a constant heating rate (15 °C/min). The experimental results reveal that the reactivity of biomass char is determined by the pyrolysis condition. The CO₂ gasification rates in char generated at 0.1 MPa exhibited approximately twice the values as compared to those obtained at 3 MPa. This is because the uniformity of the carbonaceous structure increases with the pyrolysis pressure. The uniformity of carbonaceous structures would affect the CO₂ gasification reactivity, and the decreasing uniformity would lead to the progression of cavities on the char surface during the CO₂ gasification process. The gasification rate of biomass char increases with the heating rate at pyrolysis. This is due to the coarseness (surface morphology) of biomass char and rough texture, which increases with the heating rate.     

An analysis of the transient combustion and burnout time of carbon particles

The various coupled and transient processes controlling the gasification mechanism and burnout time of carbon particles were analyzed, with emphasis on the influence of the initial particle size for the size range that is relevant to the firing of pulverized solid fuels. The formulation recognizes the suppression of the envelop gas-phase CO flame because of the small particle size, and allows for the three surface reactions of C + O₂, C + CO₂, and C + H₂O, as well as radiation heat transfer because of the potential high temperature attainable by the carbon particle. Results show that while the particle temperature continuously increases during the combustion of sufficiently large particles, the gasification actually consists of three phases: namely an initial particle heating period, an activation period for the surface reactions, and a diffusion-controlled, d²-law gasification period characterized by perpetually maximized surface reaction rates in spite of the continuously decreasing particle size. Radiation heat transfer is shown to have the same magnitude as those of reaction heat release and conduction, and actively affects the particle gasification response. For smaller particles, activation of the surface reactions is either substantially delayed subsequent to the initial heating period, or is completely suppressed, which respectively leads to either long burnout times or incomplete particle gasification. Influences due to the ambient oxygen concentration and the presence of CO₂ and H₂O as the oxidizer were also studied. Comparisons with literature experimental data show adequate agreement.     

Investigation of the Flow Field in the Upper Part of a Cyclone with Laser and Phase Doppler Anemometry

The cyclone is a well known apparatus for separating particles out of a gas stream. With the modern laser diagnostic technologies of laser and phase Doppler anemometry (LDA and PDA), there is the potential to measure the flow and particle field inside the cyclone. The gas phase only measurements used micron‐sized oil seeding droplets, whereas the solid phase, chosen for the PDA particle size measurements, was limestone powder. To assess the possibility of measuring milled limestone particles with PDA, the measured size distribution was compared with those obtained by laser diffraction. The measurements inside the cyclone showed that the flow field in the upper part of the cyclone was different to that commonly thought. Therefore, the vertical height of the cyclone's vortex finder could be shortened without deterioration of the separation efficiency. The particles found in the hold‐up of the cyclone air flow were considerably larger than the average particle size in the feed pipe.