Mechanism of wood combustion: 2. Weight loss analysis

The combustion of moisture-free wood cylinders was studied by measuring weight loss at 750 °C. The rate of weight loss increased for an initial period of ≈ 60s; thereafter it decreased asymptotically in an overall first-order reaction. Particles with protected ends showed a characteristic total burning time that was exponentially proportional to the initial diameter of the particle.     

Numerical investigation of turbulent non-premixed combustion of a wood pyrolysis gas

A fully compressible database of turbulent non-premixed flames of a wood pyrolysis gas is developed by means of direct numerical simulation (DNS). A reduced kinetic mechanism is used to model the combustion of a pyrolysis gas-air mixture. The instantaneous flame surface density evolution equation based on the concept of a displacement speed is examined. The normal component of the displacement speed is nearly constant with respect to curvature, while the curvature-related component tries to restore the flame front to a planar shape. The strain-rate term is mainly a source as the flame is mostly extended. The normal displacement is responsible for both positive and negative contributions to the flame area. The displacement/curvature term is primarily a sink, since it is dominated by its curvature component. Effects of strain and curvature are analyzed by considering their correlations with reaction rates. Reaction rates are enhanced with increased positive strain rates owing to an increase in the flame surface area and to a decrease in curvature. The analyzed results aid in the development of turbulent combustion models. Finally, a new model for a mean variance of the scalar dissipation rate, based on a scale similarity approach, is proposed and examined. A comparison with DNS results shows that the proposed model provides a significant improvement over existing models. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 15–34, May–June, 2007.     

Flue gas desulfurization under simulated oxyfiring fluidized bed combustion conditions: The influence of limestone attrition and fragmentation

Flue gas desulfurization by means of limestone injection under simulated fluidized bed oxyfiring conditions was investigated, with a particular focus on particle attrition and fragmentation phenomena. An experimental protocol was applied, based on the use of complementary techniques that had been previously developed for the characterization of attrition of sorbents in air-blown atmospheric fluidized bed combustors. The extent and pattern of limestone attrition by surface wear in the dense phase of a fluidized bed were assessed in bench scale fluidized bed experiments under simulated oxyfiring conditions. Sorbent samples generated during the oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a particle impactor. The experimental results were compared with those previously obtained with the same limestone under air-blown atmospheric fluidized bed combustion conditions. The profound differences in the attrition and fragmentation extents and patterns associated with oxyfiring as compared to air-blown atmospheric combustion and the role played by the different attrition/fragmentation paths were highlighted. In particular, it was noted that attrition could effectively enhance particle sulfation under oxyfiring conditions by continuously disclosing unconverted calcium to the sulfur-bearing atmosphere.     

煤与垃圾衍生燃料的流化床混烧试验研究

使用鼓泡流化床燃烧炉进行了煤与垃圾衍生燃料(RDF)的混烧实验研究,分析了燃烧尾气中O2、CO、CO2、H2O、NOx、CH4、SO2、HCl、HCN浓度的变化,并对各成分之间的浓度进行了相关性分析。研究结果表明在750~900℃的范围内,高温利于SO2的脱除;NO的浓度随床温的提高而增加,NO2的浓度与床温的关系不大;HCl的浓度随床温的升高而增加,说明高温不利于HCl的脱除;在空气量较低的情况下,CO、CH4、N2O、HCN等气体的浓度较高,相关分析表明,这些气体浓度之间有较强的正相关关系,说明这些气体适宜于还原气氛下生成;随着HCl气体浓度的增加,SO2浓度降低,说明HCl的存在可促进石灰石脱除SO2的反应,同时还证实煤与RDF混烧可使尾气中的SO2和HCl气体浓度均有所下降。     

焦炉废气含氧分析技术改进

炼焦行业当前普遍使用煤气燃烧废气中的含氧量,来衡量煤气燃烧是否充分。较为普遍的做法是实时检测机、焦两侧分烟道中的废气含氧量。这种废气含氧量不能真实地反映出各燃烧室燃烧废气的含氧量,因此不能有效提供所需最佳空气输入量数据。通过技术改进,利用一台氧化锆氧含量分析仪分时扫描多个蓄热室小烟道的燃烧废气含氧量,进而实现了最佳控制全炉所有燃烧室的燃烧状况。该技术的推广应用将有助于焦化企业节能降耗以及自动化操作程度的提高。     

Update for combustion properties of wood components

The combustion properties of various biomass and wood materials from various references and from our laboratory were reanalysed. The net heat of combustion for cellulosic materials was found to be 13.23 kJ/g times the ratio of stoichiometric oxygen mass to fuel mass, r_o, regardless of the material composition. Bomb calorimeter data for original, charred and volatilized material components provide gross heating values, while elemental analysis of the materials for carbon, hydrogen, oxygen and ash provide direct evaluation for r_o. We corrected these data as provided in various references by converting gross heating values to lower heating values and converting elemental compositions, char fractions and r_o to a moisture‐free and ash‐free basis. Some existing formulae were found to disagree with data from vegetation, charred wood with high ash content, and with volatiles from cellulose treated with the fire retardant NaOH. We also established various functional correlations of r_o with elemental compositions, or volatization fractions of untreated and treated materials, or material fractions for cellulose, lignin and extractives, or volatile fractions for tar, combustible gases and inert gases in pure nitrogen carrier gas. An interesting predictive result provides nearly constant heat of combustion while the volatile tar fraction is decreasing and combustible and inert gas fractions are increasing with time during the charring of Douglas‐fir wood. Published in 2002 John Wiley & Sons, Ltd.     

Investigation of combustion kinetics of treated and untreated waste wood samples with thermogravimetric analysis

In the present study, combustion mechanisms, thermal kinetics, and phases of combustion were investigated for waste wood samples (untreated pine and treated MDF, plywood and particleboard). Waste wood samples were combusted in air at 10, 20 and 30 °C/min heating rates in TGA. The results of TG analysis have shown that thermal decomposition of treated samples takes place at lower temperatures as compared to the untreated pine sample because of the catalyzing effects of the chemicals in the samples. Therefore, there were less flammable products, lower weight losses in the main oxidation region, and decrease in the maximum weight loss temperatures and formation of more char for treated samples as compared to untreated pine sample. Thermal kinetic constants for the samples were calculated by using Coats Redfern Method. In order to find out the mechanisms responsible for the oxidation of the waste wood samples, six solid-state mechanisms of Coats Redfern Method were tested. The results of the Coats Redfern Method have shown that diffusion mechanism is the effective mechanism for all regions of the pine samples. For the main oxidation region, D4 mechanism (Ginstling–Brounshtein equation) was found to be the main diffusion mechanism for pine samples. For the treated samples such as MDF, plywood and particleboard, first-order reaction (F1) was found as an effective mechanism. Treatment of wood with different additives and glues seemed to alter the thermal oxidation process and change the effective oxidation mechanisms.     

Exterior wood coatings. Part-1: Performance of semitransparent stains on preservative-treated wood

Pressure-treated lumber available for residential applications had been changed from chromated copper arsenate (CCA) treatment to Cu–amine formulations (since January 2004). This change created a need to investigate the performance of coatings on these new Cu-based preservatives as compared with CCA. This study evaluated performance of a number of semitransparent deck stains on CCA, alkaline copper quat (ACQ), copper azole (CA), and untreated wood samples during 3 years of natural weathering. Coatings were evaluated for their ability to reduce water uptake, color change, checking of wood, and mildew growth and to improve general appearance of preservative-treated wood samples. Overall, Cu–amine-treated wood had on average 50% higher water uptake than CCA-treated wood samples. Although solvent-based coatings had better water repellency performance on CCA-treated and untreated wood, there was no significant difference in water-repellent effectiveness (WRE) between water-based and solvent-based coatings on ACQ- or CA-treated wood. All preservative-treated woods had less color change and better visual ratings than those of untreated wood samples. Based on image analysis which was used to determine the total area of surface checking on samples after 3 years, preservative treatments had no significant effect on reducing checking of wood while coatings reduced surface checking by 30–40%.     

Mechanism of combustion of dinitroglycol

It is shown, on the basis of an analysis of published and new data on the combustion velocity of dinitroglycol and taking into account the temperature dependences of the density and heat capacity, that combustion of dinitroglycol at atmospheric pressure occurs in two characteristic states: from inflammation surfaces in condensed and gas phases.Sergiev Posad. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 32–36, March–April, 1993.     

Spouted bed combustion of wood charcoal: performance comparison of three different designs

The combustion efficiency of a conventional spouted bed reactor with a central spout inlet is compared with two alternative reactors having different spout inlet arrangements. For one of the designs, air was introduced into a square duct column through a rectangular slit across the whole cross-section of the column, while in the other the air was introduced into a cylindrical column through a circular slit. Column cross-sectional area and air inlet area for all the reactors were approximately the same. The highest gas–solid contacting and lowest carbon monoxide (CO) emission was achieved in the design with circular slit air inlet while the lowest gas–solid contacting and highest CO emission was observed in the design with rectangular slit air inlet.     

Supercritical fluid extraction of wood pulp with analysis by capillary gas chromatography-mass spectrometry

A supercritical fluid (SF) extraction method for the analysis of extractable material in eight hardwood and softwood pulp samples was investigated. While the majority of the extractable material in a Soxhlet extract with methylene chloride was determined to be fatty acids, the presence of their corresponding methyl esters along with fatty acids was noted in the SF extract. Grinding the samples increased extractable fatty acid methyl esters (FAMEs) by 10-fold, and in-situ derivatizations resulted in higher FAME recoveries than derivatization after SFE. Liquid trapping enhanced recoveries of lower FAMEs when compared with tandem (solid/liquid) trapping. In-situ acetylations sometimes yielded acetylated glucoses during SFE. Large differences in FAME concentrations were seen for hardwood samples, but lesser differences were seen for the softwood pulp samples.     

Modeling of unsteady filtration gas combustion

Unsteady filtration gas combustion with various gas flow parameters is studied by mathematical modeling. Transition processes due to a sudden change in the calorific value of the gas mixture, gas flow velocity, and flow direction are considered. Trends and mechanisms of change in the structure of the filtration gas combustion wave and its propagation velocity are analyzed for various types of transition processes. It is found that with a sudden change in gas flow parameters, the flame can abruptly move large distances in the porous medium. Subsequently, at the new flame localization, a wave of filtration gas combustion forms which corresponds to the changed parameters of the gas flow. If in the porous medium, the amount of heat is insufficient, the transition process ends with quenching. As the gas flow direction changes, the combustion wave continues to propagate in the former direction for some time, which can lead to the spread of the high-temperature zone in devices based on the reverse process with a homogeneous gas-phase reaction.     

Combustion of silicon powders containing organic additives in nitrogen gas under pressure: 1. Effect of dopants on combustion phenomenology

Introduction of organic dopants (in amounts up to 5 wt %) to Si powders was found to intensify their combustion in nitrogen gas under pressure without significant influence on the combustion temperature [T _c = 2270 ± 50 K at P(N₂) = 70 atm]. The addition of organic compounds also suppressed the coagulation of Si particles, improved the extent of conversion, and promoted combustion of coarse Si powders. This was associated with the reaction of organic species R (formed upon thermal decomposition of dopants) with solid Si(s) giving gaseous Si(g), which intensifies the processes of mass transport involving organic species R as gas transport agents.      

Mathematical modelling of the combustion of a single wood particle

A mathematical model describing the thermal degradation of densified biomass particles is presented here. The model uses a novel discretisation scheme and combines intra-particle combustion processes with extra-particle transport processes, thereby including thermal and diffusional control mechanisms. The influence of structural changes on the physical–thermal properties of wood in its different stages is studied together with shrinkage of the particle during its degradation. The model is used to compare the predicted data with data on the mass loss dynamics and internal temperature of several particles from previous works and relevant literature, with good agreement.     

烟气分析法测试燃煤SO_2释出量的研究

为了有效控制SO2的排放量,有必要研究不同温度下燃煤排放SO2含量的测试方法。介绍了烟气分析法用于测试燃煤可释出SO2含量时的试验方法,其中包括间歇式烟气分析法和连续式烟气分析法。通过分析烟气分析法的精确度及重现性,分析该方法的优缺点,并验证其可行性。结果表明:烟气分析法的精确度和重现性不佳,但是可以及时反映燃煤排放SO2的真实情况。因此在完善测试装置后,烟气分析法作为燃煤SO2的测试方法值得进一步深入研究。     

Determination of combustion parameters for wood by means of spectroradiometers

A set of spectroradiometric equipment is described which makes it possible to study objects for long waves on time and space coordinates. Results are given for determining the linear combustion rate of wood, the temperature and concentration of soot in the flame, and also some other wood combustion parameters.State Institute of Applied Optics, 420075 Kazan. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 3, pp. 28–33, May–June, 1994.