Coal-fired Combustion System Research Articles

Torrefaction of pine wood in a continuous system and optimization of torrefaction conditions

Red pine wood particles were torrefied in a screw conveyor reactor system continuously having a capacity of 5 kg/hour. During torrefaction, operating conditions were very important. Changes in the reactor temperature and the reactor residence times had large influences on product yields. With increasing torrefaction temperature, the volatile matter and oxygen content of biomass decreased, while fixed carbon content and heating value greatly increased. Design-Expert software package was used for the design of experiments (DOE) and to carry out the statistical analysis according to the experimental results. The optimum biocoal yield of 56.59%, energy yield of 47.49%, higher heating value of 26 761.9 kJ/kg, hardgrove grindability index of 91.76, H/C ratio of 0.099, and O/C ratio of 0.312 were obtained at 299.71°C and 28.4 minutes (approximately 300°C and 30 minutes). The biocoal prepared in this work exhibited similar properties to selected Turkish lignites. It suggests that the biocoal can be used as a supplementary fuel for production of energy in coal-fired combustion systems, especially as an auxiliary fuel for existing power plants.

Mercury policy and regulations for coal-fired power plants

Mercury is a high-priority regulatory concern because of its persistence and bioaccumulation in the environment and evidence of its having serious adverse effects on the neurological development of children. Mercury is released into the atmosphere from both natural and anthropogenic sources. Coal-fired utilities are considered to be one of the largest anthropogenic mercury emission sources. The period since the late 1990s has been marked by increasing concern over mercury emissions from combustion systems to the extent that a number of national governments have either already implemented or are in the process of implementing, legislation aimed at enforcing tighter control over mercury emissions and a reduction in mercury consumption. This review examines the most important national and international policies and agreements for controlling mercury emissions from coal-fired combustion systems. To provide a global perspective, this study lists the countries with the largest estimated mercury emissions and regulatory efforts to reduce them.

Mercury control technologies for coal combustion and gasification systems

Development and testing of mercury control technologies have largely focused on coal-fired combustion systems, with less emphasis on advanced power systems. Mercury control is influenced by coal properties and chemistry, plant configuration, pollution control devices, flue gas conditions, and plant operations, which differ between combustion and gasification systems. Sorbents such as treated activated carbons have shown promising results in low-temperature environments; however, elevated temperature and reducing environments of many advanced systems remain challenging, requiring research and development to obtain acceptable mercury control levels. Concurrent pollutant/multipollutant control strategies that include CO 2 control are critically needed for both conventional and advanced power systems.

A technique to control mercury from flue gas: The Thief Process

The Thief Process is a mercury removal process that may be applicable to a broad range of pulverized coal-fired combustion systems. This is one of several sorbent injection technologies under development by the U.S. Department of Energy for capturing mercury from coal-fired electric utility boilers. A unique feature of the Thief Process involves the production of a thermally activated sorbent in situ at the power plant. The sorbent is obtained by inserting a lance, or thief, into the combustor, in or near the flame, and extracting a mixture of partially combusted coal and gas. The partially combusted coal or sorbent has adsorptive properties suitable for the removal of vapor-phase mercury at flue gas temperatures that are typical downstream of a power plant preheater. One proposed scenario, similar to activated carbon injection (ACI), involves injecting the extracted sorbent into the downstream ductwork between the air preheater and the particulate collection device of the power plant. Initial laboratory-scale and pilot-scale testing, using an eastern bituminous coal, focused on the concept validation. Subsequent pilot-scale testing, using a Powder River Basin (PRB) coal, focused on the process development and optimization. The results of the experimental studies, as well as an independent experimental assessment, are detailed. In addition, the results of a preliminary economic analysis that documents the costs and the potential economic advantages of the Thief Process for mercury control are discussed.

Recent advances in mercury removal technology at the National Energy Technology Laboratory

The in-house research effort on mercury studies at the National Energy Technology Laboratory is an integrated, multi-task approach, including experimentation at both the laboratory-scale and the pilot-scale, as well as a numerical modeling effort to aid in interpretation of pilot-scale results. In the laboratory-scale work, novel sorbents and techniques for the removal of mercury from flue gas are being investigated using a small packed-bed reactor. The reactor system is used to screen novel sorbents for their capability to remove mercury from gas streams. The capacities of these novel sorbents are determined as a function of gas composition and temperature and are compared to results with commercially available activated carbons. In the pilot-scale work, an existing pilot unit has been characterized with respect to the distribution and fate of hazardous air pollutants in flue gas, with an emphasis on mercury. The pilot unit is a 500-lb/h (227-kg/h) pulverized coal-fired (PCFC) combustion system that includes a furnace, air preheater, spray dryer, ductwork and a pulse-jet fabric filter. The investigations with this unit have entailed evaluation of various activated carbons and novel sorbents, as well as comparisons of various sampling techniques for the determination of total and speciated forms of mercury. The impact of various parameters (temperature, sorbent-to-mercury ratio, baghouse pressure drop) on the removal of mercury has been investigated. Additionally, a slipstream allows for the investigation of sorbent injection in a fly ash-free flue gas over a wide range of operating conditions, including residence time. A summary of the results for mercury measurement and control from the 500-lb/h (227-kg/h) PCFC combustion system will be presented, as well as the evaluation of methods for measurement of in-duct removals using the slipstream. Finally, a discussion of the numerical modeling effort will be given.

03/02209 Activated carbons produced from unburned carbon in fly ash and their application for mercury capture: Zhang, Y. et al. Preprints of Symposia — American Chemical Society, Division of Fuel Chemistry, 2003, 48, (1), 32–33

Introduction Following the current demand to develop technologies to utilize high carbon content fly ash from coal-fired utility combustors or gasifiers, a one-step activation protocol has been developed by the authors to produce activated carbons from unburned carbon in fly ash. Compared to the conventional two-step process that includes a devolatilization of the raw materials, followed by an activation step, unburned carbon only requires a one-step activation process, since it has already gone through a devolatilization process while in the combustor or gasifier. The produced activated carbons with a fine particle size are not only rich in micropores, but they also present a high content of mesopores, which leads to good mass transfer properties during the adsorption process. Mercury has been identified as a hazardous air pollutant of greatest potential public health concern by the Environmental Protection Agency (EPA), with coal-fired utility boilers being the largest source of anthropogenic mercury emissions. Based upon the EPA report, in 1997 it was estimated that 48 tons of mercury were emitted into atmosphere from coal-fired utilities, including 26 tons of elemental mercury and 20.5 tons of oxidized mercury, and where the state of Pennsylvania emitted around 5 tons. The injection of carbon adsorbent upstream of the electrostatic precipitator (ESP) or baghouse particulate collection devices is a promising technology to control mercury emissions from coal-fired combustion systems. A large excess of carbon adsorbent is normally required for injection upstream of the ESP or baghouse in order to obtain a high level of mercury capture. This is because the concentration of mercury in the flue gas is extremely low, (on the order of 1 ppb by volume), as well as the complexity of the flue gas composition and the poor selectivity of the carbon sorbent towards mercury. The typical carbon-tomercury mass ratios used in recent pilot-scale studies of mercury capture in power plant systems were between 1000:1 to 100,000:1. Therefore, the cost of the carbon sorbent plays an important role in the feasibility of the proposed technologies. Based on the low cost of fly ash carbon, its inherent porosity together with its fine particle size, the unburned carbon has been tested as a potential mercury sorbent candidate. However, its mercury capacity is lower than that of commercial carbons, probably due to its high ash content and low surface area. Accordingly, the present paper focuses on the activation of the unburned carbon after conventional demineralization process in order to increase the porosity of the unburned carbon. In addition, the prepared activated unburned carbon samples were tested for mercury adsorption using a fixed-bed with a simulated flue gas at 280F.

The Formation of VOC, PAH and Dioxins During Incineration

The formation of polynuclear aromatic hydrocarbons (PAH) dioxins and furans which arise from incinerators and coal-fired combustion systems has been the focus of attention for many years. Some of these compounds are considered to be formed in the combustion region and other in the cooler post-combustion environment. The accurate measurements of trace emissions such as those of dioxins are difficult and expensive; consequently it is useful to examine these from a modelling and thermodynamic point of view in order to make design predictions and derive a full analytical specification from incomplete experimental data. The thermodynamic properties combined with the kinetic pathways have been used to examine the likely routes of formation of dioxins. These processes were modelled using the Sandia National Laboratories code PSR (Perfectly Stirred Reactor). The properties demonstrated that formation of these compounds occurred during the quenching process for the species which were thermodynamically favoured. Proper insight into the chemical features may help to improve incinerator installation so as to reduce, or eliminate, these emissions.

98/03070 Evaluating physical, chemical, and energetic properties of perennial grasses as biofuels

The suitability of energy crops for either energy conversion into fuels or energy release through combustion can be measured by several indices that reflect energy content, density, and ease of recovery. These properties ultimately determine both the suitability and diversity of potential end uses of feedstocks and their potential value as agroindustrial resources. Our analyses indicate that switchgrass should be a versatile bioenergy feedstock. Energy content of switchgrass is comparable to that of wood with significantly lower initial moisture content. Early analyses of ethanol recovery from enzymatic hydrolysis by the National Renewable Laboratory (NREL), indicate that switchgrass is very suitable substrate and produces high ethanol yield using current simultaneous saccharification and fermentation (SSF) technology. Contrary to earlier reports, more extensive analysis of ash and alkali content of switchgrass indicates that it typically has a relatively low alkali content and should have low slagging potential in coal-fired combustion systems. As an agrofiber source for pulping, switchgrass has a relatively high cellulose content, low ash content, and good fiber length to width ratios. It appears to be a promising substitute for hardwoods in the production of high quality paper. Harvesting and handling strategies can improve switchgrass suitability for industrial endpoints.

CFD simulations on a coal-coal reburning test facility

Fuel staging or fuel reburning is a possible primary measure for the reduction of NOx emissions from fossil-fired steam generators. It is intended to investigate if this primary measure can be applied efficiently to coal-fired combustion systems where pulverized coal is also used as a reburning fuel: coal reburning over a coal fire. This technology has to be tested accordingly. The company ENEL SpA is at present evaluating, together with ANSALDO Energia SpA, the option of demonstrating and extending the use of this technology in its coal-fired utility steam generators. Both companies have decided to erect a boiler simulation facility (5 MWth) at ENEL's experimental area Santa Gilla (Cagliari). This facility is intended to replicate the time/temperature profiles of real boilers. One tool to support the design of firing systems is CFD simulation. Consequently, CFD simulation was applied to perform a ‘proof of concept’. Most important for the design of firing systems for the furnaces of utility steam generators or test facilities is the distribution of temperature inside the combustion chamber and the furnace exit temperature—especially the radiation transport responsible for the amount of energy transferred from the gaseous environment of the hot furnace to the furnace walls. Subsequently, this determines the mean furnace exit temperature. This is why the accuracy of temperature calculations depends directly on the radiation transport model, the gas emissivity approximations, the particle emissivity and the radiative properties of the furnace enclosure. The results of several combustion simulations of the coal reburning test facility are shown and discussed. Estimates of the possible effect of different wall temperatures on the combustion process itself and on the gas temperature inside the furnace are presented. © 1997 by John Wiley & Sons, Ltd.

97/02172 Prediction and measurements of volatile metal partitioning in coal-fired combustion systems

Power generation using pressurized fluidized bed combustion (PFBC) of coal coupled with combined cycles is a proven high-energy efficiency technology which ensures low environmental impact. Due to the inherent characteristics of PFBC technology, both SOx and NOx emissions can be maintained at low levels. Several demonstration power plants have been constructed worldwide employing PFBC technology. However, PFBC installations are not without their problems, including bed agglomeration, fouling and erosion of the gas turbine blades. Further improvements in efficiency can be achieved through the partial gasification of coal in a carbonizer to produce a fuel gas which is then combusted in a topping combustor (second-generation PFBC systems).

Advanced Ceramic Materials for Use in High-Temperature Particulate Removal Systems

First-generation monolithic porous ceramic filter materials have experienced thermal fatigue, high-temperature creep, and a loss of material strength when operated for extended periods of time in advanced coal-fired combustion and gasification systems. Fiber-reinforced and advanced ceramic composites potentially provide a means to mitigate the degradation mechanisms encountered by monolithic filter matrices and ultimately extend operating life. In this paper we will review advancements which have recently been made during the development of the second-generation porous ceramic filter materials and provide insight into the performance of the filter elements during bench-scale qualification testing and operation in demonstration plant test facilities.

95/06759 Co-combustion of treated sludges in coal-fired combustion systems

95/06759 Co-combustion of treated sludges in coal-fired combustion systems

95/01765 Predictions of metals emissions and partitioning in coal-fired combustion systems

95/01765 Predictions of metals emissions and partitioning in coal-fired combustion systems

Predictions of metals emissions and partitioning in coal-fired combustion systems

The EPA is required by Title III of the 1990 Clean Air Act Amendments to regulate the emissions of 189 hazardous air pollutants (including trace metals) from industrial facilities. These facilities may be required to apply technology-based standards such as maximum achievable control technology (MACT) to their emitting processes. A new legislation will be implemented by specific rules and regulations promulgated by federal (EPA), state, and local agencies. Once rules are promulgated, facilities may have to conduct compliance tests and trial burns to demonstrate compliance with emission limits. The process of planning and performing such tests is formidable due to the planning time requirement and the high cost. This paper presents a computer modeling approach to predict metals behavior in coal-fired combustion systems. This treatment is not intended to be a substitute for actual compliance tests but can serve as a guide to help planners, engineers, and facility owners/operators predict the levels of metals emissions from their facilities, and to gain an insight into the impact of operating conditions on metals behavior. To test the predictive applicability of the modeling approach, predictions of the overall partitioning of metals in a pulverized bituminous coal-fired combustor were compared to measured data from full-scale facilities in the Netherlands. In general, the comparison between predicted and measured values is favorable. This indicates that using the modeling approach described in this paper can be effective in planning tests, optimizing operating conditions, and determining the effectiveness of air pollution control equipment prior to conducting actual compliance tests.

Technology Status of Particulate Removal from High-Temperature High-Pressure Combustion Gases

The adoption of coal-fired pressurized fluidized-bed combined-cycle combustion systems by utilities for electric power generation depends to a large extent on the development of an efficient and economic cleanup system between the combustor and gas turbine. To accomplish particulate removal from a dust-laden gas stream, a number of separation devices have been developed. These include conventional and augmented cyclones; porous metal, fabric, and ceramic membrane filters; fixed-, continuously moving-, and fluidized-bed granular filters; electrostatic precipitators; as well as several other novel separation devices. The results of tests of these devices are discussed in relation to pressurized fluidized-bed combustion applications.

Heat Transfer through Irradiated, Semi-transparent Layers at High Temperature

A model is proposed for predicting conduction and radiation heat transfer across an irradiated layer of semitransparent material at high temperature. The validity and accuracy of the model are established by comparing the predictions with those reported in the literature. Results of sample calculations are included for the purpose of determining the relative importance of radiation in comparison to the total heat transfer across the layer. The sample results show that treatment of the slag as an opaque substance could result in large underpredictions of the total heat transfer in comparison to the case where the material of the layer is semitransparent to radiation. The findings have practical implications to the design of components of coal-fired combustion systems.