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1.
CO2 gasification of torrefied forest residues (birch and spruce branches) was investigated by means of a thermogravimetric analyser operated non-isothermally (400–1273 K) and isothermally (1123 K) under the kinetic regime, followed by kinetic analyses assuming different models. For the non-isothermal gasification, the distributed activation energy model (DAEM) with four or five pseudo-components was assumed. It is found that the severity level of torrefaction had great influences on gasification behaviour as well as devolatilization step. The activation energy of non-isothermal gasification step of three samples varied in the range of 260–290 kJ/mol. The char reactivity decreased with increased torrefaction temperature. For the isothermal gasification, the random pore model (RPM), shrinking core model (SCM), and homogeneous model (HM) were tested. The result has confirmed the trend of decrease in char reactivity with increased torrefaction temperature observed from the non-isothermal gasification. However, different trends in char reactivity due to different wood types were observed by the two methods of gasification.  相似文献   

2.
Pollutants from the combustion of solid biomass fuels   总被引:1,自引:0,他引:1  
This review considers the pollutants formed by the combustion of solid biomass fuels. The availability and potential use of solid biofuels is first discussed. This is followed by the methods used for characterisation of biomass and their classification. The various steps in the combustion mechanisms are given together with a compilation of the kinetic data. The chemical mechanisms for the formation of the pollutants: NOx, smoke and unburned hydrocarbons, SOx, Cl compounds, and particulate metal aerosols are outlined. Examples are given of emission levels of NOx and particulates from combustion in fixed bed combustion, fluidised bed combustion and pulverised biomass combustion and co-firing. Modelling methods for pollutants are outlined. The consequential issues arising from the wide scale use of biomass and future trends are then discussed.  相似文献   

3.
This review critically examines the state of the art of rate laws and kinetic constants for the gasification, with carbon dioxide and steam, and the combustion of chars produced from lignocellulosic fuels, including a brief outline about yields and composition of pyrolysis products. The analysis also gives space to the role played by various factors, such as heating rate, temperature and pressure of the pyrolysis stage, feedstock and content/composition of the inorganic matter, on char reactivity. Finally, directions for future research are suggested.  相似文献   

4.
A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period (<2% conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus-effectiveness factor relationship is obeyed over the particle's lifetime.  相似文献   

5.
In this paper the results of a complete set of devolatilization and combustion experiments performed with pulverized (∼500 μm) biomass in an entrained flow reactor under realistic combustion conditions are presented. The data obtained are used to derive the kinetic parameters that best fit the observed behaviors, according to a simple model of particle combustion (one-step devolatilization, apparent oxidation kinetics, thermally thin particles). The model is found to adequately reproduce the experimental trends regarding both volatile release and char oxidation rates for the range of particle sizes and combustion conditions explored. The experimental and numerical procedures, similar to those recently proposed for the combustion of pulverized coal [J. Ballester, S. Jiménez, Combust. Flame 142 (2005) 210-222], have been designed to derive the parameters required for the analysis of biomass combustion in practical pulverized fuel configurations and allow a reliable characterization of any finely pulverized biomass. Additionally, the results of a limited study on the release rate of nitrogen from the biomass particle along combustion are shown.  相似文献   

6.
Kinetic parameters, the pre-exponential factor and the activation energy, of two Thai lignite char of different ash contents and electrode carbon have been experimentally determined based on the Semenov's thermal ignition theory adapted to single particle ignition. Combustion rate constants obtained have been comparatively plotted with those from Field's correlation (1969). Results indicated that the reaction rate coefficient for lignite char approaches value estimated from Field's equation at temperature about 850°C.  相似文献   

7.
生物质流化床燃烧过程中的结渣特性   总被引:1,自引:1,他引:1  
在生物质燃料的热化学转化技术中,生物质流化床燃烧技术具有诸多独特的优势,但在实际的流化燃烧利用过程中,经常出现床料结渣的现象.文章对这一现象进行了概述,并从生物质燃料特性、运行条件、床料类型以及覆盖层的形成等方面分析了结渣的影响因素,探讨了由床料与燃料灰之间发生反应所引起的结渣的内在机理,最后提出了相应的防止措施.  相似文献   

8.
几种生物质颗粒燃料等温燃烧过程和动力学研究   总被引:1,自引:0,他引:1  
对4种华北地区生物质颗粒燃料进行等温燃烧失重试验.由试验数据曲线拟合得到了拟合方程,求解出活化能E、反应级数n、频率因子A.结果表明,生物质颗粒燃料在750~950℃温度下等温燃烧过程的反应级数为2~3.  相似文献   

9.
With respect to the use of densified biomass fuels in fully automatic heating systems for the residential sector a high quality of these fuels is required. Several European countries already have implemented standards for such fuels. In other countries such standards are in preparation or planned. Furthermore, in some countries also standards from associations are existing (e.g. from the Austrian Pellets Association). In addition to these national standards, European standards for solid biomass fuels are under development. For producers of densified biomass fuels, especially for pellet producers, it is therefore very important to produce high-quality fuels keeping the limiting values of the standards addressed. However, in this context it has to be considered that as a high fuel quality as is necessary for the combustion of densified biomass fuels in automatic small-scale furnaces is not necessary if these fuels are used in larger industrial furnaces as they are equipped with more sophisticated flue gas cleaning, combustion and process control systems. Two pellet qualities, one for industrial and one for small-scale consumers seem to be more meaningful.

Within the framework of the EU-ALTENER-project “An Integrated European Market for Densified Biomass Fuels (INDEBIF)” a questionnaire survey of European producers of densified biomass fuels was performed. In this connection the possibility was offered to the producers to participate in an analysis programme with their fuels. An overview was obtained of the qualities of densified biomass fuels offered in the European market, covering pellets and briquettes from Austria, Italy, Sweden, Spain, Norway and the Czech Republic.

The parameters analysed were the dimensions of the fuels, the bulk and the particle density, the water and the ash content, the gross and the net calorific value, the abrasion, the content of starch (as an indication for the use of biological binding agents), the concentrations of C, H, N, S, Cl, K as well as of the heavy metals Cd, Pb, Zn, Cr, Cu, As and Hg. These parameters have been chosen following the Austrian, German, Swiss and Swedish standards for densified biomass fuels.

The results showed that a majority of the participating producers produce fuels of high quality. However, wood pellets of some producers show a high abrasion, one of the most important quality parameters for pellets. An increased amount of fines often causes failures in the feeding systems used in the residential heating sector. In order to decrease abrasion, the addition of small amounts of biological binding agents (e.g. maize or rye) is possible. This kind of additive is most common in Austria.

Moreover, some producers obviously use not only chemically untreated raw materials or additives, which increase the content of pollutants. Such fuels cause problems regarding emissions, deposit formation and corrosion. Emission problems are expected due to increased contents of N, Cl, S as well as heavy metals. Increased concentrations of heavy metals additionally contaminate the ash, increased Cl concentrations raise the risk of corrosion. Moreover, an increased content of K has a negative effect on the ash melting behaviour and causes higher aerosol formation, which enhances deposit formation and particulate emissions.  相似文献   


10.
生物质燃烧动力学特性实验研究   总被引:10,自引:1,他引:9  
采用非等温热重分析法对农作物秸秆燃烧动力学特性进行了研究。提出了秸秆挥发分析出过程的特性参数,建立了反应动力学方程;测算了反映燃烧性能的燃烧特性指数和反映生物质秸秆燃烧放热特性的差热峰面积指标。结果表明:玉米秸秆和小麦秸秆的挥发分初析温度随升温速度的增加而降低,稻秆的挥发分初析温度随升温速度的增加而增加;3种样品的活化能随升温速度的增加而降低;差热峰面积、燃烧特性指数随升温速度的增加而增大。  相似文献   

11.
Pre-treatments, such as torrefaction, can improve biomass fuels properties. Dedicated and coal co-firing plants, in which pulverised biomass and torrefied biomass can be used, are exposed to explosion hazards during handling, storage and transport from the mills to the boiler. Data on the explosion characteristics of biomass and torrefied biomass are scarce. This study presents explosion characteristics (maximum explosion pressure, deflagration index and minimum explosible concentration) of two torrefied wood samples and compares their reactivity to that of their corresponding untreated biomass materials and to a sample of Kellingley coal. Torrefied biomass samples showed higher reactivity, overpressures were around 9 bar (0.9 MPa, 1 bar = 105 Pa) for all biomass samples irrespective of size or sample composition. Derived laminar burning velocities ranged between 0.1–0.12 m s−1, and were therefore similar to that of coal (0.12 m s−1). The differences in explosion reactivity influence the design of explosion protection measures and can be used to introduce suitable modifications for safe operations with torrefied biomass.  相似文献   

12.
Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O2 to the burning solid and of the products CO and CO2 away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchar (for ). This linear dependence of Sh on dchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiff, is constant in the region of interest (). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for ), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation
  相似文献   

13.
Characterization and prediction of biomass pyrolysis products   总被引:2,自引:0,他引:2  
In this study some literature data on the pyrolysis characteristics of biomass under inert atmosphere were structured and analyzed, constituting a guide to the conversion behavior of a fuel particle within the temperature range of 200-1000 °C. Data is presented for both pyrolytic product distribution (yields of char, total liquids, water, total gas and individual gas species) and properties (elemental composition and heating value) showing clear dependencies on peak temperature. Empirical relationships are derived from the collected data, over a wide range of pyrolysis conditions and considering a variety of fuels, including relations between the yields of gas-phase volatiles and thermochemical properties of char, tar and gas. An empirical model for the stoichiometry of biomass pyrolysis is presented, where empirical parameters are introduced to close the conservation equations describing the process. The composition of pyrolytic volatiles is described by means of a relevant number of species: H2O, tar, CO2, CO, H2, CH4 and other light hydrocarbons. The model is here primarily used as a tool in the analysis of the general trends of biomass pyrolysis, enabling also to verify the consistency of the collected data. Comparison of model results with the literature data shows that the information on product properties is well correlated with the one on product distribution. The prediction capability of the model is briefly addressed, with the results showing that the yields of volatiles released from a specific biomass are predicted with a reasonable accuracy. Particle models of the type presented in this study can be useful as a submodel in comprehensive reactor models simulating pyrolysis, gasification or combustion processes.  相似文献   

14.
Fundamental pyrolysis and combustion behaviors for several types of biomass are tested by a thermo-gravimetric analyzer. The main compositions of cellulose and lignin contents for several types of biomass are analyzed chemically. Based on the main composition results obtained, the experimental results for the actual biomass samples are compared with those for the simulated biomass, which is made of the mixture of the cellulose with lignin chemical. The morphological changes before and after the reactions are also observed by a scanning electron microscope. The main compositions in the biomass consisted of cellulose and lignin. The cellulose content was more than lignin for the biomass samples selected in this study. The reaction for the actual biomass samples proceeded with the two stages. The first and second stage corresponded to devolatilization and char combustion during combustion, respectively. The first stage showed rapid mass decrease caused by cellulose decomposition. At the second stage, lignin decomposed for pyrolysis and its char burned for combustion. For the biomass with higher cellulose content, the pyrolysis rate became faster. While, the biomass with higher lignin content gave slower pyrolysis rate. The cellulose and lignin content in the biomasses was one of the important parameters to evaluate the pyrolysis characteristics. The combustion characteristics for the actual biomass depends on the char morphology produced.  相似文献   

15.
The thermal degradation of three categories of raw and torrefied biomass [agri-residue: wheat straw, forest residue: sawdust, energy crop: miscanthus] was studied in a TGA-FTIR system with or without catalyst (CaO). The thermal degradation of biomass was carried out in the temperature range of 25–900 °C at a heating rate of 20 °C/min. The air flow rate was controlled based on the stoichiometric air requirement for complete combustion. The non-linear regression (NLREG) model was adopted to determine the kinetic parameters. The weight loss, heat flow, maximum weight loss temperature, and activation energy were observed to be dependent on the types of biomass and the process parameters. The maximum weight loss temperature was higher for torrefied biomass compared with raw biomass. The activation energy was higher in the case of torrefied biomass compared with raw biomass, and CaO helped to shrink the activation energy. The maximum weight loss temperature and activation energy were varied from 310 to 509 °C and 15–85 kJ/mol, respectively. The CaO supplement seems to have a positive impact on the thermal degradation process; thus, it may help in improving the thermal degradation process of torrefied biomass.  相似文献   

16.
A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.  相似文献   

17.
The use of the fluidized bed combustor (FBC) has increased. It began in the 20th century as coal combustion and gasification, which then developed into catalytic reactions. Only recently, the application field has been extended to the incineration of biomass and pre-treated waste, for either power generation or waste disposal. The success of fluidized bed combustion is due to high combustion efficiency, great flexibility when it comes to the heating value of the fuel and reduction in pollutants emitted with the flue gas.  相似文献   

18.
Thermogravimetric curves in air, measured for the different types of agricultural residues from olive trees (leaves, pruning and wood) at different heating rates (5, 10, 20, 40, 100 K/min), are subjected to kinetic evaluation by model-based and model-free methods. It is shown that the combustion process in the samples analyzed can be divided into three stages: water removal, roasting phase and char decomposition. At every stage, the activated energy varies with the mass conversion for the kinetic models considered. Its value was determined by the model-free methods, of which Flynn–Wall– Ozawa and Kissinger–Akahira–Sunose were the most appropriate for this purpose and resulted in similar values of activated energy. Once the activation energy was determined, the order of the reactions and the frequency factors of each stage were calculated by means of the Coats–Redfern model-based method in order to complete the determination of the kinetic triplet. From the results obtained, it was deduced that the most feasible reaction order was one.  相似文献   

19.
Experimental study on oxygen-enriched combustion of biomass micro fuel   总被引:1,自引:0,他引:1  
S.Y. Luo  B. Xiao  Z.Q. Hu  S.M. Liu  Y.W. Guan 《Energy》2009,34(11):1880-1884
The oxygen-enriched combustion of biomass micro fuel (BMF) was carried out respectively in the thermogravimetric analyzer and cyclone furnace to evaluate the effects of oxygen concentration on combustion performance. The experimental results show that with the increasing oxygen concentration, the volatile releasing temperature, ignition temperature and burnout temperature were decreasing. Oxygen-enriched atmosphere subtracts burning time and improves combustion activity of biomass micro fuel. Oxygen-enriched atmosphere improves the combustion temperature of BMF in cyclone furnace; while the improvement is weaken as oxygen concentration is above 40%.  相似文献   

20.
This study discusses the performance and exhaust emissions of a vehicle fueled with low content alcohol (ethanol and methanol) blends and pure gasoline. The vehicle tests were performed at wide-open throttle and at vehicle speeds of 40 km h−1, 60 km h−1, 80 km h−1 and 100 km h−1 by using an eddy current chassis dynamometer. The test results obtained with the use of alcohol-gasoline blends (5 and 10 percent alcohol by volume) were compared to pure gasoline test results. The test results indicated that when the vehicle was fueled with alcohol-gasoline blends, the peak wheel power and fuel consumption slightly increased. And also, in general, alcohol-gasoline blends provided higher combustion efficiency compared to pure gasoline use. In exhaust emission results, a stable trend was not seen, especially for CO emission. But, on average, alcohol-gasoline blends exhibited decreasing HC emissions. In 100 km h−1 vehicle speed test, the alcohol-gasoline blends provided lower vehicle performance and lower NOx emission values compared to pure gasoline. At all vehicle speeds, minimum CO2 emission was obtained when 5% methanol was added in gasoline. The low content alcohol blends did not reveal any starting problem, or irregular operation on the engine.  相似文献   

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