Sewage sludge combustion

In the current review paper, various issues related to the combustion of sewage sludge are discussed. After briefly explaining the formation and treatment of sewage sludge, current and future sludge production are discussed. Thereafter, the four sludge disposal methods which are currently used, i.e. recycling in agriculture, landfilling, dumping into sea and incineration, are examined, and the future trend presented showing the increasing role of sludge incineration. Thereafter, technologies for thermal processing of sewage sludge are presented. They are discussed in three groups, i.e. mono-combustion, co-combustion and alternative processes. Various mono-combustion incinerators, including multiple hearth, fluidized bed and smelting furnaces are briefly discussed, whereas for co-combustion, attention has been given to co-combustion with coals in pulverized and fluidized bed coal combustors, as well as co-incineration with municipal solid wastes in various furnaces. Where possible, data from large scale plants are presented. Currently being discussed in the sludge disposal cycles are the alternative thermal processes to sludge combustion. These include wet oxidation, pyrolysis, oil from sludge processes, and combinations of pyrolysis, combustion and gasification processes. Some of these alternative technologies are also briefly discussed. An important aspect during thermal processing of sewage sludge is its combustion mechanisms. Compared to coals, sewage sludge has very high contents of moisture and volatile matter which can affect the combustion process. The importance of the drying and devolatilization processes for sewage sludge combustion is thus examined. In a special case, the release and combustion of the volatiles during sludge combustion in fluidized bed combustors is analysed, and some information concerning the combustion of sludge char is presented. Another important issue of sludge combustion is the emissions of pollutants gases as well as the handling of solid by-products. Of concern include the heavy metals, mercury, dioxins and furans, acid gases, as well as NO_x and N₂O. These are also briefly discussed. A peculiar characteristic of sewage sludge is its high content of nitrogen, and attention has been given to see how this affects the N₂O and NO_x emissions. In a special case, emission performance of large scale combustors of sewage sludge is presented.     

Alcohol combustion chemistry

Alternative transportation fuels, preferably from renewable sources, include alcohols with up to five or even more carbon atoms. They are considered promising because they can be derived from biological matter via established and new processes. In addition, many of their physical-chemical properties are compatible with the requirements of modern engines, which make them attractive either as replacements for fossil fuels or as fuel additives. Indeed, alcohol fuels have been used since the early years of automobile production, particularly in Brazil, where ethanol has a long history of use as an automobile fuel. Recently, increasing attention has been paid to the use of non-petroleum-based fuels made from biological sources, including alcohols (predominantly ethanol), as important liquid biofuels. Today, the ethanol fuel that is offered in the market is mainly made from sugar cane or corn. Its production as a first-generation biofuel, especially in North America, has been associated with publicly discussed drawbacks, such as reduction in the food supply, need for fertilization, extensive water usage, and other ecological concerns. More environmentally friendly processes are being considered to produce alcohols from inedible plants or plant parts on wasteland. While biofuel production and its use (especially ethanol and biodiesel) in internal combustion engines have been the focus of several recent reviews, a dedicated overview and summary of research on alcohol combustion chemistry is still lacking. Besides ethanol, many linear and branched members of the alcohol family, from methanol to hexanols, have been studied, with a particular emphasis on butanols. These fuels and their combustion properties, including their ignition, flame propagation, and extinction characteristics, their pyrolysis and oxidation reactions, and their potential to produce pollutant emissions have been intensively investigated in dedicated experiments on the laboratory and the engine scale, also emphasizing advanced engine concepts. Research results addressing combustion reaction mechanisms have been reported based on results from pyrolysis and oxidation reactors, shock tubes, rapid compression machines, and research engines. This work is complemented by the development of detailed combustion models with the support of chemical kinetics and quantum chemistry. This paper seeks to provide an introduction to and overview of recent results on alcohol combustion by highlighting pertinent aspects of this rich and rapidly increasing body of information. As such, this paper provides an initial source of references and guidance regarding the present status of combustion experiments on alcohols and models of alcohol combustion.     

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.     

湍流燃烧模型在燃烧室数值计算中的对比分析

对比分析了不同燃烧模型对某型回流式燃气轮机燃烧室流场的影响,建立了描述燃烧室流场的控制方程组,采用Realizablek-ε湍流模型,湍流燃烧模型分别为涡耗散模型(ED)、涡耗散概念模型(EDC)、简单概率密度模型(PDF)和稳态小火焰模型(SFM).对比分析了不同燃烧模型下燃烧室的温度场、速度分布以及NOx排放量,并...     

A review on plasma combustion of fuel in internal combustion engines

In recent years, new ways of improving the combustion efficiency of fuel during gas turbine operations have been developed. The most significant has been the application of plasma technology for the combustion of fuel in gas turbine operations. Plasma is formed when gas is exposed to either high temperature or high‐voltage electricity. This technology is very promising and has proven to enhance the performance of gas turbines and reduce toxic emissions. Recent studies have shown the use of different types of plasma applications in gas turbine operations such as plasma torch, filamentary discharge, and nanosecond pulse discharge, whose results show that plasma technology has great potential in improving flame stabilization, the fuel/air mixing ratio, and flash point values of these fuels. These findings and advances have further provided new opportunities in the development of efficient plasma discharges for practical uses in plasma combustion of fuel for gas turbine operations. This article is a comprehensive overview of the advances and blind spots in the knowledge of plasma combustion of fuel during internal combustion engine operations. This review also focuses on applications, methods, and experimental results in plasma combustion of fuel in gas turbines.     

The distinctive characteristics of combustion duration in hydrogen internal combustion engine

As a practical solution to reduce the emission pollution and energy crisis, the research and development of HICE has been processed in several decades. The focus of this paper is trying to explore the new features of the combustion duration in HICE not only by engine experiment, but also by analysis of the physical properties of hydrogen, especially the obvious difference from that of gasoline. Firstly, the laminar flame speed difference between hydrogen and gasoline was studied and discussed. Secondly, a distinctive rule of combustion duration in HICE was discovered by analyzing the experiment data. Finally, as a key reference point to the HICE operation, a new characteristic of the location of 50% mixture combust up was proposed and analyzed, this will be helpful for the calibration of optimum ignition timing.     

乙醇微型燃烧室燃烧特性的数值模拟研究

为了解决化石燃料储备不足与环境污染问题,生物质燃料作为石油替代能源得到大力提倡,如何合理地将化石燃料替换为生物质燃料且维持设备正常运行成为工程上亟待解决的问题。本文采用CFD软件研究了车载5 kW生物乙醇微型燃烧室的燃烧特性,对比分析了不同功率（0.5~5 kW）和出口温度（840~960 K）时的回流区长度与宽度、回流量、出口温度分布系数（OTDF）、出口NO体积分数等特征参数。结果表明：随着出口温度升高,回流区长度逐渐缩短,回流量减少,出口温度均匀性逐渐变差,出口NO体积分数明显增加;随着燃烧室功率增大,回流区长度变长,回流量增加,OTDF先增大后减小,NO体积分数随着功率的降低而显著升高,最大值出现在1 kW时,达到满负荷时的7倍。因此,为了实现稳定燃烧和减少污染物排放,该乙醇微型燃烧室应在较高的空燃比（即较低的出口温度）和功率下运行。     

An onboard hydrogen generator for hydrogen enhanced combustion with internal combustion engine

Hydrogen enhanced combustion (HEC) for internal combustion engine is known to be a simple mean for improving engine efficiency in fuel saving and cleaner exhaust. An onboard compact and high efficient methanol steam reformer is made and installed in the tailpipe of a vehicle to produce hydrogen continuously onboard by using the waste heat of the engine for heating up the reformer; this provides a practical device for the HEC to become a reality. This use of waste heat from engine enables an extremely high process efficiency of 113% to convert methanol (8.68 MJ) for 1.0 NM of hydrogen (9.83 MJ) and low cost of using hydrogen as an enhancer or as a fuel itself. The test results of HEC from the onboard hydrogen production are presented with 2 gasoline engine vehicles and 2 diesel engines; the results indicate a hike of engine efficiency in 15–25% fuel saving and a 40–50% pollutants reduction including 70% reduction of exhaust smoke. The use of hydrogen as an enhancer brings about 2–3 fold of net reductions in energy, carbon dioxide emission and fuel cost expense over the input of methanol feed for hydrogen production.     

Detailed numerical simulation of syngas combustion under partially premixed combustion engine conditions

Two-dimensional detailed numerical simulation is performed to study syngas/air combustion under partially premixed combustion (PPC) engine conditions. Detailed chemical kinetics and transport properties are employed in the study. The fuel, a mixture of CO and H₂ with a 1:1 molar ratio, is introduced to the domain at two different instances of time, corresponding to the multiple injection strategy of fuel used in PPC engines. It is found that the ratio of the fuel mass between the second injection and the first injection affects the combustion and emission process greatly; there is a tradeoff between NO emission and CO emission when varying the fuel mass ratio. The ignition zone structures under various fuel mass ratios are examined. A premixed burn region and a diffusion burn region are identified. The premixed burn region ignites first, followed by the ignition of mixtures at the diffusion burn region, and finally a thin diffusion flame is formed to burn out the remaining fuel. NO is produced mainly in the premixed burn region, and later from the diffusion burn region in mixtures close to stoichiometry, whereas unburned CO emission is mainly from the diffusion burn region. An optimization of the fuel mass in the two regions can offer a better tradeoff between NO emission and CO emission. The effects of initial temperature and turbulence on the premixed burn and diffusion burn regions are investigated.     

Environmental impact of combustion

Large amounts of fuels, burned in various types of furnaces are one of the main sources of pollution in the environment. Flue gases pollute the atmosphere and the associated ashes pollute soil and water. To compare the influence of various fuels burned in different installations on the environment, one universal index would be helpful. Such a coefficient, which represents the harmfulness of combustion processes of a particular fuel in a particular installation, is defined in this paper. This coefficient takes into account the composition of the fuel, the thermal efficiency of the installation, the type and process efficiency of creation of harmful compounds in the combustion chamber, efficiency of cleaning devices, ability of emitter to propagate pollutants into the atmosphere, as well as the relative toxicity of various components. Finally methods of calculation of propagation of pollutants in the atmosphere are introduced. All these factors are essential for a reliable assessment and comparison of fuel and installations. An individual coefficient of harmfulness could be calculated for each compound present in the flue gases as well as a total coefficient for all compounds. Each compound created in the combustion processes is the source of many types of environmental impact. It could have significant influences on human health, plants and animals as well as for example on the corrosion process taking place in many different installations. We have to take into consideration that the result of a combustion process could appear in many different places, sometimes distant from the place of the process itself. All these results should be analyzed and introduced into the universal coefficient of “harmfulness” of specific compound. Economic parameters and methods are important and therefore are also introduced into the assessment method presented in the paper. In this way we are able to compare the values of extremely different phenomena such as a human health and corrosion.     

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.     

Hydrogen-fueled internal combustion engines

The threat posed by climate change and the striving for security of energy supply are issues high on the political agenda these days. Governments are putting strategic plans in motion to decrease primary energy use, take carbon out of fuels and facilitate modal shifts.     

Hydrogen assisted diesel combustion

Hydrogen assisted diesel combustion was investigated on a DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine, with a focus on exhaust emissions. Hydrogen was substituted for diesel fuel on an energy basis of 0%, 2.5%, 5%, 7.5%, 10% and 15% by aspiration of hydrogen into the engine's intake air. Four speed and load conditions were investigated (1800 rpm at 25% and 75% of maximum output and 3600 rpm at 25% and 75% of maximum output). A significant retarding of injection timing by the engine's electronic control unit (ECU) was observed during the increased aspiration of hydrogen. The retarding of injection timing resulted in significant NO_X emission reductions, however, the same emission reductions were achieved without aspirated hydrogen by manually retarding the injection timing. Subsequently, hydrogen assisted diesel combustion was examined, with the pilot and main injection timings locked, to study the effects caused directly by hydrogen addition. Hydrogen assisted diesel combustion resulted in a modest increase of NO_X emissions and a shift in NO/NO₂ ratio in which NO emissions decreased and NO₂ emissions increased, with NO₂ becoming the dominant NO_X component in some combustion modes. Computational fluid dynamics analysis (CFD) of the hydrogen assisted diesel combustion process captured this trend and reproduced the experimentally observed trends of hydrogen's effect on the composition of NO_X for some operating conditions. A model that explicitly accounts for turbulence–chemistry interactions using a transported probability density function (PDF) method was better able to reproduce the experimental trends, compared to a model that ignores the influence of turbulent fluctuations on mean chemical production rates, although the importance of the fluctuations is not as strong as has been reported in some other recent modeling studies. The CFD results confirm that temperature changes alone are not sufficient to explain the observed reduction in NO and increase in NO₂ with increasing H₂. The CFD results are consistent with the hypothesis that in-cylinder HO₂ levels increase with increasing hydrogen, and that the increase in HO₂ enhances the conversion of NO to NO₂. Increased aspiration of hydrogen resulted in PM, and HC emissions which were combustion mode dependent. Predominantly, CO and CO₂ decreased with the increase of hydrogen. The aspiration of hydrogen into the engine modestly decreased fuel economy due to reduced volumetric efficiency from the displacement of air in the cylinder by hydrogen.     

Fast reaction nonpremixed combustion

Recent measurements in turbulent, single-phase, reacting flows are reviewed. Attention is confined to nonpremixed flows at the fast reaction limit, which is defined as conditions where rates of reaction are fast enough to maintain local equilibrium. Reactant combinations considered include: hydrogen/air, carbon monoxide/air, hydrogen/fluorine, nitric oxide/ozone, acid/base (in liquids) and the dissociation of nitrogen tetroxide in warm air. Criteria for the fast-reaction limit, as well as the laminar flamelet concept, are discussed in some detail. Other aspects of measurements in these systems are also discussed, e.g. effects of initial and boundary conditions; types of averaging; and the interpretation of velocity, temperature and other scalar property measurements. Existing measurements in round free jets, plane free shear layers and wall boundary layers are summarized and discussed. Experimental difficulties in controlling hydrodynamic and reaction variables are substantial, even in these relatively simple flows; therefore, no existing data set is completely satisfactory for r definitive evaluation of methods of analyzing turbulent reaction processes at the fast-reaction limit. Recent measurements in round, free-jet, hydrogen/air diffusion flames, using optical diagnostics for structure measurements, come closest to this ideal; therefore, these results are discussed in detail and sources of tabulated data for use during model evaluation are cited.     

Plasma-assisted ignition and combustion

The use of a thermal equilibrium plasma for combustion control dates back more than a hundred years to the advent of internal combustion (IC) engines and spark ignition systems. The same principles are still applied today to achieve high efficiency in various applications. Recently, the potential use of nonequilibrium plasma for ignition and combustion control has garnered increasing interest due to the possibility of plasma-assisted approaches for ignition and flame stabilization. During the past decade, significant progress has been made toward understanding the mechanisms of plasma–chemistry interactions, energy redistribution and the nonequilibrium initiation of combustion. In addition, a wide variety of fuels have been examined using various types of discharge plasmas. Plasma application has been shown to provide additional combustion control, which is necessary for ultra-lean flames, high-speed flows, cold low-pressure conditions of high-altitude gas turbine engine (GTE) relight, detonation initiation in pulsed detonation engines (PDE) and distributed ignition control in homogeneous charge-compression ignition (HCCI) engines, among others. The present paper describes the current understanding of the nonequilibrium excitation of combustible mixtures by electrical discharges and plasma-assisted ignition and combustion.