Oxy-fuel combustion of solid fuels

Oxy-fuel combustion is suggested as one of the possible, promising technologies for capturing CO₂ from power plants. The concept of oxy-fuel combustion is removal of nitrogen from the oxidizer to carry out the combustion process in oxygen and, in most concepts, recycled flue gas to lower the flame temperature. The flue gas produced thus consists primarily of carbon dioxide and water. Much research on the different aspects of an oxy-fuel power plant has been performed during the last decade. Focus has mainly been on retrofits of existing pulverized-coal-fired power plant units. Green-field plants which provide additional options for improvement of process economics are however likewise investigated. Of particular interest is the change of the combustion process induced by the exchange of carbon dioxide and water vapor for nitrogen as diluent. This paper reviews the published knowledge on the oxy-fuel process and focuses particularly on the combustion fundamentals, i.e. flame temperatures and heat transfer, ignition and burnout, emissions, and fly ash characteristics. Knowledge is currently available regarding both an entire oxy-fuel power plant and the combustion fundamentals. However, several questions remain unanswered and more research and pilot plant testing of heat transfer profiles, emission levels, the optimum oxygen excess and inlet oxygen concentration levels, high and low-temperature fire-side corrosion, ash quality, plant operability, and models to predict NO_x and SO₃ formation is required.     

Pollutants from the combustion of solid biomass fuels

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.     

Gasification inhibition in chemical-looping combustion with solid fuels

Chemical-looping combustion (CLC) is a novel technology that can be used to meet growing demands on energy production without CO₂ emissions. The CLC process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N₂ from the air with CO₂ from the combustion giving combustion gases that consist almost entirely of CO₂ and H₂O. The technique reduces the energy penalty that normally arises from the separation of CO₂ from other flue gases, hence, CLC could make capture of CO₂ cheaper. For the application of CLC to solid fuels, the char remaining after devolatilization will react indirectly with the oxygen carrier via steam gasification. It has been suggested that H₂, and possibly CO, has an inhibiting effect on steam gasification in CLC. In this work experiments were conducted to investigate this effect. The experiments were conducted in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction periods. Two different oxygen carriers were used as well as an inert sand bed. During the reducing period varying concentrations of CO or H₂ were used together with steam while the oxidation was conducted with 10% O₂ in N₂. The temperature was constant at 970 °C for all experiments. The results show that CO does not directly inhibit the gasification whereas the partial pressure of H₂ had a significant influence on fuel conversion. The results also suggest that dissociative hydrogen adsorption is the predominant hydrogen inhibition mechanism under the laboratory conditions, thus explaining why char conversion is much faster in a bed of oxygen carrying material, compared to an inert sand bed.     

利用低温碳化技术生产清洁固体燃料

介绍了一种利用低温碳化技术开发出的低成本、高效率的劣质固体燃料(结合生物能、褐煤、煤、原煤)预处理技术。这种预处理过程在燃烧之前的小规模的还原性气氛中除去有害的气体污染物(如氮、硫、氯、汞)，从而提高燃料效率。它主要应用在小于50MW的小型电厂和热电联产机组，以及小于300MW的中型电厂。     

Synthetic fuels and combustion

As the supply of hydrocarbons for transportation fuels includes an increasing proportion of low hydrogen-to-carbon ratio sources, such as coal, the cost and waste of energy of converting these materials to the high hydrogen-to-carbon ratio fuels now required by land and air propulsion systems will increase. In the extreme, where coal is the major source of liquid fuel, elimination of restrictions on aromatics content (H/C ratio) could reduce refining energy cost by as much as 20% of the heat of combustion of the syncrude being processed. Refining costs are approximately proportional to refining energy consumption, and an energy saving of this magnitude would reduce the total cost of refined products by one-third. For a syncrude product cost of 30 $/bbl, this would be a cost saving of 25 ¢/gal. of product.Such a large conservation and economic driving force provides a powerful incentive for choice of power plants capable of burning fuels of low hydrogen-to-carbon ratio in a clean and environmentally acceptable manner.The main combustion problem is the increasing difficulty of avoiding the emission of soot, and the relative ability of power plants to completely burn out the soot formed in the early stages of combustion will be an important selection criterion.In automotive systems, the combustion problems appear much more easily solved for the Stirling cycle and the gas turbine because of the steady flow conditions and the potentially longer time that can be provided for soot burnout. The liquid injection Diesel and stratified charge engines are at a disadvantage in this regard and may not be able to compete successfully with the Otto cycle engine, for which aromatics offer an improvement in efficiency because of their high octane number. Improvement in the ability of aircraft engines to burn highly aromatic, wide boiling range fuels offers the possibility of advances in economics and fuel conservation in air transportation.Fortunately, there is every indication that combustion research and development can be expected to eliminate the need for high levels of hydrogenation and boiling range conversion in fuels manufacture. Much more research is needed in the chemistry of soot formation and burnout, and the mechanics of reactive flows involving high molecular weight liquids and vapors and soot, for the complex systems of practical interest. In development programs, highly aromatic fuels should be used even though the economic and conservation driving force for fuel specification changes might appear well into the future. While the examples and numbers used in this discussion are based on an extreme that is indeed well into the next century, the trend toward lower hydrogen-to-carbon ratio feed stocks is already underway, and it is timely to begin moving toward less energy intensive fuels manufacture in addition to working on more thermodynamically efficient propulsion machinery.     

Flameless combustion for hydrogen containing fuels

In this paper, flameless combustion was promoted to suppress thermal-NO_x formation in the hydrogen-high-containing fuel combustion. The PSRN model was used to model the flameless combustion in the air for four fuels: H₂/CH₄ 60/40% (by volume), H₂/CH₄ 40/60%, H₂/CH₄ 20/80% and pure hydrogen. The results show that the NO_x emissions below 30 ppmv while CO emissions are under 50 ppmv, which are coincident with the experimental data in the “clean flameless combustion” regime for all the four fuels. The simulation also reveals that CO decreases from 48 ppmv to nearly zero when the hydrogen composition varies from 40% to 100%, but the NO_x emission is not sensitive to the hydrogen composition. In the highly diluted case, the NO_x and CO emissions do not depend on the entrainment ratio.     

A combustion model for IC engine combustion simulations with multi-component fuels

Reduced chemical kinetic mechanisms for the oxidation of representative surrogate components of a typical multi-component automotive fuel have been developed and applied to model internal combustion engines. Starting from an existing reduced mechanism for primary reference fuel (PRF) oxidation, further improvement was made by including additional reactions and by optimizing reaction rate constants of selected reactions. Using a similar approach to that used to develop the reduced PRF mechanism, reduced mechanisms for the oxidation of n-tetradecane, toluene, cyclohexane, dimethyl ether (DME), ethanol, and methyl butanoate (MB) were built and combined with the PRF mechanism to form a multi-surrogate fuel chemistry (MultiChem) mechanism. The final version of the MultiChem mechanism consists of 113 species and 487 reactions. Validation of the present MultiChem mechanism was performed with ignition delay time measurements from shock tube tests and predictions by comprehensive mechanisms available in the literature.A combustion model was developed to simulate engine combustion with multi-component fuels using the present MultiChem mechanism, and the model was applied to simulate HCCI and DI engine combustion. The results show that the present multi-component combustion model gives reliable performance for combustion predictions, as well as computational efficiency improvements through the use of reduced mechanism for multi-dimensional CFD simulations.     

Flame chemiluminescence in premixed combustion of hydrogen-enriched fuels

The growing interest in the use of syngas, produced from gasification of a number of fuels such as coal, biomass or waste, has increased the need of developing new strategies of flame control and monitoring in order to ensure a quiet and efficient combustion in the current combustion systems, which had been optimized for conventional fuels.     

内燃机代用燃料的发展分析

指出与说明了代用燃料的定义、分类、选择以及使用标准,概述了内燃机代用燃料的发展历史和研究现状,总结了国内外内燃机代用燃料的发展与应用动态。指出由于石油资源的日益减少,代用燃料是今后能源应用与研究的方向。     

涡流室式LPG柴油双燃料发动机燃烧模型

根据涡流室、主燃室中气体流动过程的质量和能量的交换关系 ,建立了涡流室式 LPG ( liquefiedpetrdeum gas) /柴油双燃料发动机准维燃烧模型的方程 ,提出了如何使两种不同性质燃料的燃烧在同一个燃烧过程中相互联系、相互作用的燃烧模型。根据模型提供的方程 ,对缸内燃烧过程和 NOx 的生成进行了模拟仿真 ,将其同发动机台架试验的结果进行了验证、分析和讨论。     

Non-equilibrium plasma assisted combustion of low heating value fuels

This paper describes the effects of non-equilibrium air plasma generated by a dielectric barrier discharge (DBD) on the combustion of low heating value fuels. The experimental results indicate that addition of a very small amount of energy to the air flow in the form of DBD significantly improves the flame stability. Moreover, main combustion characteristics such as flame propagation speed, combustion intensity and lean blow-off limits are also enhanced by the effect of plasma. Some active radicals such as excited O atom and excited N2 molecule are observed by spectrograph in the discharge area. Based on the results of numerical investigation we can conclude that these active radicals generated in discharge area can accelerate the production rate of active OH radical which plays a key role in the oxidation process of low heating value fuel, and thus the whole combustion process is accelerated.     

生物质流化床燃烧过程中的结渣特性

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.     

A general mathematical model of solid fuels pyrolysis

The aim of this work is to present and discuss a detailed kinetic model that describes the devolatilization process of solid fuels under pyrolysis conditions. The major reason for this interest in better understanding pyrolysis and combustion of coal, biomasses and solid fuels lies in the increasing concern for the environmental impact of large scale combustion processes. The common chemical and structural aspects of the different fuels are singled out and used as the starting point to define this mathematical model. The formation of light gases and liquid tars is the first step in the pyrolysis process. Particular attention is also devoted to the generality and flexibility of numerical and mathematical methods. Two major critical points are present inside this model: the first is related to the definition of the initial structure of the fuel and the second is constituted by the set of reference kinetic parameters of the different reactions. Several comparisons with experimental data are analysed and the molecular weight distributions of the tar from different coals evolved at different temperatures are also discussed.     

Detailed combustion chemical mechanism for surrogates of representative jet fuels

Surrogate fuels are useful for computational fluid dynamics (CFD) simulations. However, there are two main challenges in surrogate fuel: defining an appropriate surrogate and designing compact and reliable kinetic models. Many works of defining an appropriate surrogate have been done in our previous work. In this study, we focus on obtaining compact and reliable kinetic models for jet surrogate fuels. A detailed mechanism was developed by combining n-dodecane, 2,5-dimethylhexane and toluene sub-mechanisms with AramcoMech 2.0 C₀–C₄ mechanism to mimic the combustion properties of three representative jet fuels. Extensive validation of the component mechanisms in surrogate is performed using abundant experimental data sets. Thereafter, various combustion properties of three jet fuels like ignition delay times, species concentrations in flow reactor and laminar flame speeds are simulated by using this surrogate kinetic mechanism and previous surrogate formulations. The simulations are compared with other well-known surrogate models and a wide range of experimental data. The comparisons show that the developed mechanism is substantially improved prediction accuracy of most considered combustion properties for different jet fuels in a wide range of conditions.     

Detailed chemical kinetic models for the combustion of hydrocarbon fuels

The status of detailed chemical kinetic models for the intermediate to high-temperature oxidation, ignition, combustion of hydrocarbons is reviewed in conjunction with the experiments that validate them.

All classes of hydrocarbons are covered including linear and cyclic alkanes, alkenes, alkynes as well as aromatics.     

Liquid fuels spray and combustion characteristics in a new micro gas turbine combustion chamber design

Experimental nozzle spray analysis of different nozzle sizes was performed to investigate the effect of the spray profile on combustion quality. Detailed numerical investigation analysis investigated the effect of discrete phase model (DPM) on liquid fuel atomization and combustion characteristics. Four injectors of 2.98, 5.95, 8.93, and 11.90 kg/h nominal capacities numbered from 1 to 4 were tested on new micro gas turbine (MGT) chamber designed especially for liquid biofuels. The fuel was tested in the range of 2.36 to 9.43 kg/h achieving stable turbine operation in the pressure range of 0.1 to 1 bar. Stable operation was achieved for injector number 2 in the range of 0.1 to 0.5 bar compared with 0.2 to 0.6 bar for injector number 3 and 0.5 to 1 bar for injector number 4, while the smallest injector number 1 was not operational above 0.1 bar. The experimental results produced favourable low CO emissions of 95 ppm, NO_x emission of 31 ppm, and average turbine inlet temperature (TIT) of 1316 K at maximum pressure. The numerical simulation with DPM using similar injector and operating conditions showed good agreement with the experimental results averaging CO emissions of 99 ppm and NO_x of 13 ppm at TIT of 1329 K.     

Free convection diffusion flames from burning solid fuels

A general analysis for free convection diffusion flames is presented for the burning of solid fuels having surface geometries which give rise to boundary layer flows. The combustion chemistry is treated at the level of a single global reaction. Thermal radiation from the flame and the burning fuel surface is included in the formulation of the problem as well as the possibility of radiation from an external source. Allowance is also made for the case in which the gaseous products of pyrolysis include an inert component in addition to the combustible fuel. Solutions are presented for a variety of cases which show the effects of varying the chemical reaction rate, the radiation parameters, the Lewis number and the inert component in the fuel. Extinction phenomena are discussed, and results are given which show how extinction limits are affected by ambient oxygen concentration, water droplet sprays and chemically active fire suppressants. A discussion of some of the numerical techniques developed to obtain and verify the solutions is included.     

几种生物质颗粒燃料等温燃烧过程和动力学研究

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.     

灰煤混合燃料的燃烧动力学特性研究

利用TGA/SDTA851e型热重分析仪,对煤及不同灰煤比的混合燃料进行了热失重实验,获得了其热失重特性曲线。采用单个扫描速率的Coats-Redfern法、多重扫描速率的FWO（Flynn-Wall-Ozawa）法和Starink法三种典型的热分析方法求取了各样品的动力学参数。结果表明：随着灰煤比的升高,样品燃烧反应平均过程的活化能增高;灰煤比由0升高到0.15时,样品的活化能、着火温度和燃烬温度变化较大;灰煤比从0.15升高到0.45时,活化能、着火温度和燃烬温度变化较小。同时,通过对比几种分析方法的计算结果,认为采用多重升温速率法求取活化能时要谨慎,建议采用单重升温速率法和多重升温速率法相结合来分析燃料的热解及燃烧机理。     

Combined solar energy and combustion of hydrogen-based fuels under MILD conditions

This work presents the stability and performance characteristics of a Hybrid Solar Receiver Combustor operating in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime. The device was operated at 12-kW_th in two different modes of operation, i.e. combustion-only (MILD) and mixed (combustion and solar introduced into the device simultaneously), using natural gas (NG), liquefied petroleum gas (LPG), hydrogen (H₂), NG/H₂ and LPG/H₂ blends. A 5-kW_el xenon-arc lamp was used to simulate the concentrated solar radiation introduced into the device. The influence of the mode of operation and fuel composition on the combustion stability, thermal efficiency, energy balance, pollutant emissions, heat losses and distribution of heat flux within the receiver are presented for a range of values of the heat extraction. It was found that MILD combustion can be successfully stabilised within the HSRC over a broad range of operating conditions and fuel type, and in mixed operations, with low CO (for carbon-based fuels) and NO_x emissions. The addition of H₂ and/or concentrated solar radiation to the MILD process was found to increase its stability limits. Mixed and combustion-only operations showed similar performance, regardless of the fuel type, providing further evidence that the fuel flow rate can be used dynamically to compensate for variability in the solar resource. Also, the heat extracted from the heat exchanger and the specific fuel consumption were found to increase and decrease, respectively, by adding H₂ to the system for both modes of operation, showing that hydrogen addition is beneficial. The numerical analysis revealed that the higher performance with H₂ is attributable to a higher radiative heat transfer rate than for NG and LPG under MILD conditions.