Some aspects of the formation of nitric oxide during the combustion of biomass fuels in a laboratory furnace

The influence of bed-region stoichiometric ratio and fuel nitrogen content on the formation of gaseous species formed during grate combustion of biomass fuels is reported based on gas measurements made within the fuel bed. Three fuels were studied: two mixtures of pelletized bark and wood chips and one of pelletized straw. Experiments were performed in a vertical, cylindrical, laboratory-scale grate-furnace with 0.245 m i.d. and 1.8 m height. Primary air was supplied through a grate consisting of a steel plate with 340 holes of 3.7 mm diameter. Secondary air was supplied 0.66 m above the grate. Gas analysis was performed for O₂, CO₂, CO, H₂ and NO. Results were compared with values calculated using a computer program for thermochemical equilibrium conditions. The measured contents of O₂, CO₂, CO and H₂ show good agreement with calculated equilibrium conditions at all bed region stoichiometries. A higher formation of NO was found for the straw fuel (0.58% fuel nitrogen) than for the bark/wood chip fuels (≈0.25% fuel nitrogen). This is not in accordance with the thermochemical equilibrium calculations indicating that the formation of nitric oxide does not attain thermochemical equilibrium and that the nitrogen content of the fuel has an influence on the amount of NO that is formed. The fuel nitrogen conversion to NO ranged from 3 to 20% at reducing conditions and from 20 to 40% at bed region stoichiometries between 1.00 and 1.25.     

Particle-metal interactions during combustion of pulp and paper biomass in a fluidized bed combustor

We compare interactions between metals and solid particles during the classic fluidized bed combustion (FBC) and a new low-high-low temperature (LHL) combustion of selected biomass. The biomass was a mixture of bark and pine wood residues typically used by a paper mill as a source of energy. Experiments, conducted on a pilot scale, reveal a clear pattern of surface predominance of light metals (Ca, Na, K) and core predominance of heavy metals (Cd, Cr) within the LHL-generated particles. No such behavior was induced by the FBC. Metal migration is linked to the evolution of inorganic particles. A composite picture of the metal rearrangements in the particles was obtained by a combination of independent analytical techniques including electron probe microanalysis, field emission scanning electron microscopy, inductively coupled plasma spectrometry, and X-ray diffractometry. It is suggested that the combination of (1) the high-temperature region in the LHL and (2) changes in the surface free energy of the particles is the driving force for the metal-particle behavior. Important practical implications of the observed phenomena are proposed, including removal of hazardous submicron particulate and reduction in fouling/slagging during biomass combustion. These findings may contribute to redesigning of currently operating FBC units to generate nonhazardous, nonleachable, reusable particles where heavy metals are immobilized while environmental and technological problems reduced.     

Modeling Study of a New Circulating Fluidized Bi—Bed Boiler Combustion System

INTRODUCTIONThecirculatingfluidizedbed(CFB)boileriscurrentlybeingactivelydevelopedinChina.AnalysisofthecharacteristicsofCFBismadetoeffectivelydesign,operateandcontroltheCFB.Mechanismanalysisandaccumulatedexperiencewereusedtodevelopasetofgeneraldynami...     

Comparison of hazelnut and peanut shells combustion in bubbling fluidized-bed combustor

The combustion of peanut and hazelnut shells was studied in an atmospheric bubbling fluidized bed. The impact of the enrichment of air with oxygen and the flow rate of fluidizing gas on CO₂ and CO concentrations was analyzed. It was stated that in air enriched with oxygen up to 25% the mole ratios of CO₂ to CO were improved by 15–30%, depending on the flow rate used. For the peanut shell the combustion of volatiles with a hematite as an oxygen carrier was also studied. The effects were observed above ~ 450°C.     

Modeling and countermeasures of combustion characteristics of char particles in CFBC

INTRAODUCTIONAsahigh-efficiencyandcleancoalcombustiontechnology,circulatingfluidizedbed(CFB)combustiontechnologyachievesrapiddevelopmentinChinaforburningvariouslow--gradefuels.ThescalerupofCFBboilersbecomesakeypointconcernedbytheCFBboilerdesigners.At...     

An investigation of water-gas transport processes in the gas-diffusion-layer of a PEM fuel cell by a multiphase multiple-relaxation-time lattice Boltzmann model

In order to study water-gas transport processes in the gas-diffusion-layer (GDL) of a proton exchange membrane (PEM) fuel cell system, a multiphase, multiple-relaxation-time lattice Boltzmann model is presented in this work. The model is based on the mean-field diffuse interface theory and can handle the multiphase flows with large density ratios and various viscosities. By using the standard bounce back boundary condition and an approximate average scheme for the non-slip and wetting boundary walls, respectively, detailed liquid-gas transportation in the GDL, in which exact boundary condition is difficult to be implemented, can be simulated. Unlike most of lattice Boltzmann methods based on the Bhatnagar–Gross–Krook collision operator, the present model shows a viscosity-independent velocity field, which is very important in simulating multiphase flows where various viscosities coexist. We validate our model by simulating a static droplet on a wetting wall and compare with theoretical predictions. Then, we simulate a water-gas flow in the GDL of a PEM fuel cell and investigate the saturation-dependent transport properties under different conditions. The results are shown to be qualitatively consistent with the previous numerical and theoretical works.     

Optimization of a 50 MW bubbling fluidized bed biomass combustion chamber by means of computational particle fluid dynamics

An efficient utilization of biomass fuels in power plants is often limited by the melting behavior of the biomass ash, which causes unplanned shutdowns of the plants. If the melting temperature of the ash is locally exceeded, deposits can form on the walls of the combustion chamber. In this paper, a bubbling fluidized bed combustion chamber with 50 MW biomass input is investigated that severely suffers deposit build-up in the freeboard during operation. The deposit layers affect the operation negatively in two ways: they act as an additional heat resistance in regions of heat extraction, and they can come off the wall and fall into the bed and negatively influence the fluidization behavior. To detect zones where ash melting can occur, the temperature distribution in the combustion chamber is calculated numerically using the commercial CPFD (computational particle fluid dynamics) code, Barracuda Version 15. Regions where the ash melting temperature is exceeded are compared with the fouling observed on the walls in the freeboard. The numerically predicted regions agree well with the observed location of the deposits on the walls. Next, the model is used to find an optimized operating point with fewer regions in which the ash melting temperature is exceeded. Therefore, three cases with different distributions of the inlet gas streams are simulated. The simulations show if the air inlet streams are moved from the freeboard to the necking area above the bed a more even temperature distribution is obtained over the combustion chamber. Hence, the areas where the ash melting temperatures are exceeded are reduced significantly and the formation of deposits in the optimized operational mode is much less likely.     

Numerical simulation of laminar premixed combustion in a porous burner

Premixed combustion in porous media differs substantially from combustion in free space. The interphase heat transfer between a gas mixture and a porous medium becomes dominant in the premixed combustion process. In this paper, the premixed combustion of CH₄/air mixture in a porous medium is numerically simulated with a laminar combustion model. Radiative heat transfer in solids and convective heat transfer between the gas and the solid is especially studied. A smaller detailed reaction mechanism is also used and the results can show good prediction for many combustion phenomena. Translated from Journal of Combustion Science and Technology, 2006, 12(1): 46–50 [译自: 燃烧科学与技术]     

生物质链条炉床层燃烧建模及一次风影响分析

基于多孔介质非热平衡的方法,考虑了床层高度的变化及颗粒内部温度梯度的影响,建立了一维非稳态燃烧模型来模拟炉排上移动床层的生物质燃烧。模拟计算结果与实验值对比分析表明,总体上数值计算结果与实验数据吻合较好。通过对不同一次风参数下床层燃烧的模拟结果分析得到,随着一次风风量的增加,床层剩余质量先减小后增大;在燃烧前期,床层出口气体温度上升速度减慢,挥发分析出速率降低,焦炭燃烧速率增大;在燃烧中期,床层出口气体温度先上升后下降,焦炭燃烧速率下降。一次风风温相比于一次风风量对床层燃烧影响较小,增大一次风风温可以提高挥发分析出速率,降低床层出口气体温度和床层剩余质量。     

Investigation of factors affecting channelling in fixed-bed solid fuel combustion using CFD

Channelling is an undesirable phenomenon in fixed-bed combustion. It is characterised by an uneven air distribution, and thus fuel conversion, throughout the fuel bed. To investigate factors that influence channelling, an unsteady, two-dimensional numerical model capable of predicting solid fuel combustion under fixed-bed conditions is presented. Biomass is the focus of this study, but the model can be readily applied to other solid fuels, such as coal and municipal solid waste. The model includes drying, pyrolysis, and heterogeneous char reactions, and incorporates bed shrinkage processes comprised of both continuous shrinkage and abrupt collapses. It is also capable of representing spatial non-uniformities which may occur throughout a bed, arising from irregular packing and non-homogeneous fuel composition. The overall model is validated by means of two different data sources: the first for ignition rates and the second for species profiles through a biomass fuel bed. The validated model is then applied to investigate factors affecting channelling in a randomly packed bed containing a high-porosity passage. The influence of flow resistance through the grate and bed height are compared with previous observations. Additional factors investigated include flue gas recirculation and initial moisture content of the fuel. Predictions show that increasing the flow resistance of the grate improves the gas distribution and reduces channelling because it inhibits flow from deviating towards the relatively porous passage. For deeper beds, however, the effectiveness of grate resistance is diminished because the gas then has more residence time within the bed to track towards the passage. Increasing the initial moisture content from 0% to 30% has a weak influence; nonetheless, wetter fuels show a propensity to amplify channelling. The impact of flue gas recirculation on channelling appears to be insignificant, although its benefits, such as reduced peak temperature, are apparent.     

Influence of operating conditions and the role of sulfur in the formation of aerosols from biomass combustion

The properties of the fine particles generated from burning biomass have been experimentally studied in a laboratory facility under a variety of combustion and postcombustion conditions; the parameters varied include combustion temperature and the concentrations of oxygen and SO₂ in the flue gases. SO₂ was added as a pure gas or generated in cofiring experiments. Fine particles are composed only of K, Cl, and S, in the form of potassium sulfate and chloride, except for the tests at 1450 °C, where phosphorus appeared also in significant amounts, although the species in which it was contained could not be determined exactly. From previous studies, K₂SO₄ is known to nucleate first when the gas cools, KCl condensing on these nuclei at lower temperatures. The chloride/sulfate ratio in fine particles is shown here to be greatly affected by the initial [SO₂] and [O₂] in the flue gases; this dependence can be adequately modeled if the conversion of SO₂ to SO₃ is assumed to be the only limiting step in the route to K₂SO₄ formation. Evidence for such a kinetic limitation is provided. Both the experimental results and theoretical considerations show that the presence of Cl in the submicron particles, associated with severe boiler corrosion, can be at least partly avoided with adequate combustion strategies (e.g., cofiring). The properties of coarse (>1 μm) particles have also been studied; both their chemical composition and size distribution are consistent with the break-up model of fly-ash formation.     

Assessment of a novel numerical model for combustion and in-flight heating of particles in an industrial furnace

The key factors for efficient in-flight particle heating in a combusting flow were investigated within this paper for the development of a novel boiler slag bead production furnace. A natural gas fired industrial burner with a thermal input of 1.2?MW was thus evaluated using Computational Fluid Dynamics (CFD). The steady laminar flamelet model (SFM) and a detailed chemical reaction mechanism, considering 25 reversible chemical reactions and 17 species were used to account for the steady-state gas phase combustion. Measurements of gas temperature and flow velocity within the furnace were found to be in good accordance with the numerical results. In the second step, sintered bauxite beads were injected into the furnace as an experimental material and heated up in flight. The particle heating characteristics were investigated using the Discrete Phase Model (DPM). The computational results of the particle laden flow raised the issue that convective heat transfer is a key factor for efficient particle heating. At the burner chamber outlet, the temperature of a particle which had been injected into the burner flame was 178?K higher compared to a particle, which trajectory led through zones with lower gas temperatures.     

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast?slow?fast?slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.     

A mathematical model for exit gas composition in a 10 MW fluidized bed coal combustion power plant

A mathematical model has been developed for the oxygen mass balance for a 10 MW fluidized bed coal combustion power plant operated at Jamadoba (TISCO, India). Assuming the three phase theory of fluidization, the fluid bed is considered to consist of a number of equivalent stages in series. Within each stage, an exchange of gas takes place between the bubble, cloud-wake, and emulsion phases. An effective chemical reaction rate of char combustion has been derived considering the single film theory of char combustion for shrinking particles. The model has been used to predict the consumption of oxygen in the fluidized bed combustor, the outlet gas composition, variation of oxygen concentrations in different phases and also the variation of average oxygen concentration along the bed height. Model predictions were compared with plant data, and reasonable accuracy was obtained.     

Experimental investigation of the primary combustion zone during staged combustion of wood-chips in a commercial small-scale boiler

In this work the primary combustion zone of a modified, commercial, small-scale boiler was investigated during staged combustion of wood-chips. Experimental research on thermal conversion of biomass in fixed beds is necessary to supply reliable data for gas phase combustion model validation and optimization. Furthermore, scruting of pollutant emission formation and combustion efficiency enhancement can be conducted. Two different fuel moistures were used while the primary combustion zone of a small-scale boiler was investigated as a function of the primary air ratio. The combustible products leaving the fuel layer were analyzed under continuous operation by an extractive method. This approach is new in the field of small-scale biomass combustion research and considers the strong coupling between the products leaving the fuel bed and the heat fluxes emitted by the flame of the secondary combustion zone. Additionally, fine particulate matter emissions were quantified to study the effect of varying primary air ratio and different fuel moisture on particulate formation. Results show that the primary air ratio and the fuel moisture have a significant influence on the primary combustion products composition, on the fuel bed behavior and on fine particulate matter emissions. At low primary air ratios, tars constitute a significant part of the heating value of primary combustion products. The smallest amount of particulate emissions was found at low primary air ratio and low fuel moisture. Experimental data was validated with an elemental balance, which showed perfect accordance.     

Modeling of the non-uniform combustion in a scramjet engine

Due to the high-speed of the air stream, the scramjet combustion is neither uniform nor complete. An empirical fuel distribution model is developed to describe non-uniform combustion for scramjet engines. The combustor is subdivided into different regions by radius with different local equivalence ratios. The conservation equations of the regions for the combustion and exhaust expansion are computed independently. The results indicate that scramjet thrust is more related to the fuel equivalence ratio and combustion efficiency. If the combustion efficiency is 100% and the fuel equivalence ratio is constant, there is no obvious effect of different fuel distribution to the engine except for the exhaust parameter distribution. It is also revealed that the sum of isolator shock loss and combustor Rayleigh loss is nearly constant under the same isolate cross-sectional area. Lower isolator inlet Mach is benefit to the thrust performance and the best thrust performance is at the thermal choking boundary. When the isolator inlet Mach increases, the thrust decreases.     

Development and validation of a comprehensive two-zone model for combustion and emissions formation in a DI diesel engine

A two-zone model for the calculation of the closed cycle of a direct injection (DI) diesel engine is presented. The cylinder contents are taken to comprise a non-burning zone of air and another homogeneous zone in which fuel is continuously supplied from the injector holes during injection and burned with entrained air from the air zone. The growth of the fuel spray zone, consisting of a number of fuel–air conical jets equal to the injector nozzle holes, is carefully modelled by incorporating jet mixing to determine the amount of oxygen available for combustion. Application of the mass, energy and state equations in each one of the two zones yields local temperatures and cylinder pressure histories. For calculating the concentration of constituents in the exhaust gases, a chemical equilibrium scheme is adopted for the C–H–O system of the 11 species considered, together with chemical rate equations for the calculation of nitric oxide (NO). A model for the evaluation of soot formation and oxidation rates is incorporated. A comparison is made between the theoretical results from the computer program implementing the analysis, with experimental results from a vast experimental investigation conducted on a fully automated test bed, direct injection, standard ‘Hydra’, diesel engine located at the authors' laboratory, with very good results, following a multi-parametric study of the constants incorporated in the various sub-models. Pressure indicator diagrams and plots of temperature, NO, soot density and of other interesting quantities are presented as a function of crank angle, for various loads and injection timings, elucidating the physical mechanisms governing combustion and pollutants formation. Copyright © 2003 John Wiley & Sons, Ltd.     

NO and CO formation in an industrial gas-turbine combustion chamber using LES with the Eulerian sub-grid PDF method

The advances in computing power and numerical schemes allow Large Eddy Simulation (LES) to use more detailed turbulent combustion models as well as to be applied to real gas turbine combustors. In this work, we investigate the emissions formation in an industrial gas-turbine combustion chamber using LES with an Eulerian stochastic sub-grid pdf model with reduced chemistry. Sub-grid stresses are represented by a dynamic version of the Smagorinsky model and sub-grid species fluctuations are characterised by eight stochastic fields. The chemistry was represented by an ARM reduced GRI 3.0 mechanism with 15 reaction steps and 19 species. All calculations were carried out using a detailed block-structured mesh capturing all geometrical features of the Siemens SGT-100 burner operating at a pressure of 3 bar. The influence of the radiation heat losses was investigated and the impact of an alternative 4-step chemical mechanism was discussed. The results show good agreement with the experimental data. The NO formation rates were quantified with prompt NO dominating the thermal and _N2O${\mathrm{N}}_2\mathrm{O}$ formation paths.     

Performance analysis of a steady flamelet model for the use in small-scale biomass combustion under extreme air-staged conditions

Small-scale biomass boiler development is often based on empirical methods resulting in high efforts for experimental test runs using several prototypes. CFD simulations are able to reduce both, development time and efforts for tests and prototypes, supposing that the models reliability is high and its computational effort is low. Extreme air-staging with an initial gasification stage and a subsequent fuel gas burnout in a downstream gas-burner is a promising new method to reduce NO_X and PM emissions in small-scale biomass boilers. Gasification conditions in the first combustion stage lead to high accumulation of gaseous tars in the fuel gas contributing challenges for combustion simulation because common CFD models use 2 or 3-step global methane reaction schemes to describe combustion chemistry. In this work, the performance of a computationally inexpensive steady flamelet model (SFM) together with a detailed reaction mechanism (18 species, 42 reactions) was scrutinized. In order to evaluate the performance of the SFM, two furnace designs were examined, running under different load shifts and various excess air ratio. Comparative numerical simulations were performed with classical species transport models. The numerical simulations and the experiments for validation were carried out on a wood-chip boiler with a heat output of 40 kW. Results show that flue gas temperature, flame shape, main flue gas concentrations and NO_X can be quantitatively predicted. The SFM shows also reasonable good predictions for CO variation trends. With the present approach, calculation time can be reduced by 90% compared to commonly used models (EDC). The SFM provides sufficiently accurate results within 24 h using a standard processor consisting of six cores (mesh size 1.5 million elements). Thus, the presented model is a perfectly suitable method for applied science and industrial research.     

Overview of combustion and gasification of rice husk in fluidized bed reactors

Rice is cultivated in more than 75 countries in the world. The rice husk is the outer cover of the rice and on average it accounts for 20% of the paddy produced, on weight basis. The worldwide annual husk output is about 80 million tonnes with an annual energy potential of 1.2 × 10⁹ GJ corresponding to a heating value of 15 MJ/kg. India alone generates about 22 million tonnes of rice husk per year. If an efficient method is available, the husk can be converted to a useful form of energy to meet the thermal and mechanical energy requirements of the rice mills themselves. This paper provides an overview of previous works on combustion and gasification of rice husk in atmospheric bubbling fluidized bed reactors and summarizes the state of the art knowledge. As the high ash content, low bulk density, poor flow characteristics and low ash melting point makes the other types of reactors like grate furnaces and downdraft gasifiers either inefficient or unsuitable for rice husk conversion to energy, the fluidized bed reactor seems to be the promising choice. The overview shows that the reported results are from only small bench or lab scale units. Although a combustion efficiency of about 80% can normally be attained; the reported values in the literature, which are more than 95%, seem to be in higher order. Combustion intensity of about 530 kg/h/m² is reported. It is also technically feasible to gasify rice husk in a fluidized bed reactor to yield combustible producer gas, even with sufficient heating value for application in internal combustion engines. A combustible gas with heating value of 4-6 MJ/Nm³ at a rate of 2.8-4.6 MW_th/m² seems to be possible. Only very little information is available on the pollutant emissions in combustion and tar emissions from gasification. The major conclusion is that the results reported in the literature are limited and vary widely, emphasizing the need for further research to establish suitable and optimum operating conditions for commercial implementations.