Showing papers on "Coal published in 2000"

Biomass cofiring: the technology, the experience, the combustion consequences

Abstract: Cofiring, the practice of supplementing a base fuel with a dissimilar fuel, is an extension of fuel blending practices common to the solid fuels community. Recently, there has been considerable emphasis on cofiring biomass opportunity fuels with coal in pulverized coal (PC) and cyclone boilers owned and operated by electricity generating utilities in order to address such issues as potential portfolio standards, voluntary actions to reduce fossil CO2 emissions, customer service, and the generation of green power within the context of deregulation. Biomass fuels considered for cofiring include wood waste, short rotation woody crops, short rotation herbaceous crops (e.g., switchgrass), alfalfa stems, various types of manure, landfill gas and wastewater treatment gas. Of these, the solid biofuels such as sawdust, urban wood waste and switchgrass have received the most attention. The Electric Power Research Institute (EPRI), along with the Tennessee Valley Authority (TVA), GPU Genco, Northern Indiana Public Service Company (NIPSCO), Central and South West Utilities (CS this agreement was extensively supported by the energy efficiency and renewable energy (EERE) element of USDOE. European cofiring programs also have been extensive and include gasification-based cofiring in Lahti, Finland and straw cofiring in Denmark. Three general techniques comprise the cofiring technology family: blending the biomass and coal in the fuel handling system and feeding that blend to the boiler; preparing the biomass fuel separately from coal, and injecting it into the boiler without impacting the conventional coal delivery system; and gasifying the biomass with subsequent combustion of the producer gas in either a boiler or a combined cycle combustion turbine (CCCT) generating plant. Commercialization has proceeded on the direct combustion approaches to cofiring, beginning with engineering and economic studies, parametric testing and the construction of demonstration projects. The direct combustion cofiring techniques are now ready for commercial deployment. This paper reviews the key projects, and details some of the influences of cofiring on the combustion process.

Air pollution control by electrical discharges

Abstract: Air pollution caused by gas emission of pollutants produced from a wide range of sources including coal, oil and gas burning power plants, diesel engines, paper mills, steel and chemical production plants must be reduced drastically and urgently, as mandated by recent worldwide national legislation which recently are being reinforced increasingly by international agreements. Non-thermal plasmas in which the mean energy of the electrons is substantially higher than that of the gas offer advantages in reducing the energy required to remove the pollutants. The electrical energy supplied into the discharge is used preferentially to create energetic electrons which are then used to produce radicals by dissociation and ionization of the carrier gas in which the pollutants are present. These radicals are used to decompose the pollutants. There are two technologically promising techniques for generating non-thermal plasmas in atmospheric gas pressure containing the pollutants, namely electron beam irradiation and electrical discharge techniques. Both techniques are undergoing intensive and continuous development worldwide. This is done to reduce the energy requirement for pollutant removal, and therefore the associated cost, as well as to obtain a better understanding of the physical and chemical processes involved in reducing the pollutants. In the present paper only electrical discharge techniques are reviewed and emphasis is given to the more recent published work. The paper summarizes the chemical reactions responsible for the removal of the major polluting constituents of NO, NO/sub 2/ and SO/sub 2/ encountered in flue gases and exhaust emissions. The constructional features of the various types of electrical discharge reactors commonly employed in the removal of gas pollutants as well as pilot systems used in industrial plants are described briefly The results on the removal efficiency of the various pollutants including hydrocarbons and volatile compounds and their dependency on the type of discharge reactor, the type and the magnitude of the applied voltage (dc, ac and pulsed), the polarity of the voltage (de and pulsed), the effect of the pulse width, the initial concentration of the pollutants, the addition of ammonia, argon and other hydrocarbons, the gas flow rate, the residence time of the pollutants in the discharge reactor, the gas temperature and on the type of the gas constituents will be reviewed. The removal of pollutants using arc plasmas will be discussed. The specific energy density which is supplied in various forms of electrical discharges to reduce the pollutants is discussed. The energy required to remove the pollutants is expected to be one of the main considerations in selecting the technology to be used to remove the pollutants and therefore it is of prime importance.

Mining subsidence and its effect on the environment: some differing examples

Abstract: The impact of mining subsidence on the environment can occasionally be very catastrophic, destroying property and even leading to the loss of life. Usually, however, such subsidence gives rise to varying degrees of structural damage that can range from slight to very severe. Different types of mineral deposits have been mined in different ways and this determines the nature of the associated subsidence. Some mining methods result in contemporaneous subsidence whereas, with others, subsidence may occur long after the mine workings have been abandoned. In the latter instance, it is more or less impossible to predict the effects or timing of subsidence. A number of different mineral deposits have been chosen to illustrate the different types of associated subsidence that result and the problems that arise. The examples provided are gold mining in the Johannesburg area; bord and pillar mining of coal in the Witbank Coalfield, South Africa; longwall mining of coal in the Ruhr district; mining of chalk and limestone in Suffolk and the West Midlands, respectively; and solution mining of salt in Cheshire. These mineral deposits have often been worked for more than 100 years and, therefore, a major problem results from abandoned mines, especially those at shallow depth, the presence of which is unrecorded. Abandoned mines at shallow depth can represent a serious problem in areas that are being developed or redeveloped. Abstraction of natural brine has given rise to subsidence with its own particular problems and cannot be predicted. Although such abstraction is now inconsequential in Cheshire, dereliction associated with past subsidence still remains.

Flow of Coal-Bed Methane to a Gallery

Abstract: Coal-seam methane reservoirs have a number of unique feature compared to conventional porous or fractured gas reservoirs. We propose a simplified mathematical model of methane movement in a coal seam taking into account the following features: a relatively regular cleat system, adsorptive methane storage, an extremely slow mechanism of methane release from the coal matrix into cleats and a significant change of permeability due to desorption. Parameters of the model have been combined into a few dimensionless complexes which are estimated to an order of magnitude. The simplicity of the model allows us to fully investigate the influence of each parameter on the production characteristics of the coal seam. We show that the reference time of methane release from the coal matrix into cleats – the parameter which is most poorly investigated – may have a critical influence on the overall methane production.

System for improving coalbed gas production

Abstract: A method of stimulating coalbed (5) methane production by injecting gas into a producer (2) and subsequently placing the producer (2) back on production is described. A decrease in water production may also result. The increase in gas production and decrease in water production may result from: the displacement of water from the producer (2) by gas; the establishment of a mobile gas saturation at an extended distance into the coalbed (5), extending outward from the producer (2); and the reduction in coalbed (5) methane partial pressure between the coal matrix (11) and the coal's cleat system.

Catalytic effects observed during the co-gasification of coal and switchgrass

Abstract: We are investigating catalytic gasification of coal char using biomass-derived potassium salts. Alkali metal salts, especially those containing potassium, are excellent promoters of gasification reactions but are generally considered too expensive for commercial use. Fast-growing biomass, which contains large quantities of potassium, may prove to be an excellent source of inexpensive gasification catalyst. A series of CO2-char gasification tests were performed in a thermogravimetric analyzer (TGA) to evaluate the catalytic activity of alkali-rich biomass-derived materials. Both switchgrass char and switchgrass ash displayed catalytic activity in mixtures with coal char produced from Illinois No. 6 coal. The results obtained with switchgrass ash were especially impressive, with an almost eight-fold increase in coal char gasification rate at 895°C in a 10:90 mixture of coal char and switchgrass ash. These results give encouragement that biomass could be the source of inexpensive, coal gasification catalysts.

Comparison of Particle Size Distributions and Elemental Partitioning from the Combustion of Pulverized Coal and Residual Fuel Oil

Abstract: U.S. Environmental Protection Agency (EPA) research examining the characteristics of primary PM generated by the combustion of fossil fuels is being conducted in efforts to help determine mechanisms controlling associated adverse health effects. Transition metals are of particular interest, due to the results of studies that have shown cardiopulmonary damage associated with exposure to these elements and their presence in coal and residual fuel oils. Further, elemental speciation may influence this toxicity, as some species are significantly more water-soluble, and potentially more bio-available, than others. This paper presents results of experimental efforts in which three coals and a residual fuel oil were combusted in three different systems simulating process and utility boilers. Particle size distributions (PSDs) were determined using atmospheric and low-pressure impaction as well as electrical mobility, time-of-flight, and light-scattering techniques. Size-classified PM samples from this study are also being utilized by colleagues for animal instillation experiments. Experimental results on the mass and compositions of particles between 0.03 and > 20 microns in aerodynamic diameter show that PM from the combustion of these fuels produces distinctive bimodal and trimodal PSDs, with a fine mode dominated by vaporization, nucleation, and growth processes. Depending on the fuel and combustion equipment, the coarse mode is composed primarily of unburned carbon char and associated inherent trace elements (fuel oil) and fragments of inorganic (largely calcium-alumino-silicate) fly ash including trace elements (coal). The three coals also produced a central mode between 0.8- and 2.0-micron aerodynamic diameter. However, the origins of these particles are less clear because vapor-to-particle growth processes are unlikely to produce particles this large. Possible mechanisms include the liberation of micron-scale mineral inclusions during char fragmentation and burnout and indicates that refractory transition metals can contribute to PM < 2.5 microns without passing through a vapor phase. When burned most efficiently, the residual fuel oil produces a PSD composed almost exclusively of an ultrafine mode (approximately 0.1 micron). The transition metals associated with these emissions are composed of water-soluble metal sulfates. In contrast, the transition metals associated with coal combustion are not significantly enriched in PM < 2.5 microns and are significantly less soluble, likely because of their association with the mineral constituents. These results may have implications regarding health effects associated with exposure to these particles.

Tar evolution profiles obtained from gasification of biomass and coal

Abstract: The tar content of the product gases from gasification of biomass is one of the major factors affecting the subsequent process stages. In this work, evolution profiles of the main tar constituents, ...

Coal combustion in O2/CO2 mixtures compared with air

Abstract: As part of CO2 abatement strategies for climate change, we are investigating coal combusion behaviour in various O2/CO2 mixtures and in air. The goal is to simulate conditions of coal combustion with flue gas recirculation in order to maximize the CO2 concentration in the flue gas prior to its recovery. A western Canadian sub-bituminous coal and a U.S. eastern bituminous coal were investigated. Thermal input was set at 0.21 MW with a flue gas oxygen concentration of 5 vol%. Experiments were done using various O2/CO2 mixtures and air. The oxygen concentration ranged from 21% to 42%. Up to 95% CO2 concentrations were achieved in the flue gas. This paper describes experimental results in terms of flame temperatures and pollutant emissions (NOx', SO2 and CO).

Ash Formation Mechanisms during pf Combustion in Reducing Conditions

Abstract: A range of pulverized coals were combusted in a laboratory drop-tube furnace at temperatures of 1573, 1723, and 1873 K under oxidizing and reducing conditions to determine the effect of combustion ...

Mercury capture by distinct fly ash carbon forms

Abstract: Carbon was separated from the fly ash from a Kentucky power plant using density gradient centrifugation and a lithium heterolpolytungstate high-density media. Relative concentrations of inertinite (up to 77% vol), isotropic carbon (up to 77% vol), and anisotropic carbon (up to 76% vol) were isolated from the original fly ash. Mercury concentration was lowest in the parent fly ash (which contains non-carbon components); followed by inertinite, isotropic coke, mixed isotropic−anisotropic coke fraction, and, with the highest concentration, the anisotropic coke concentrate. The latter order corresponds to the increase in BET surface area of the fly ash carbons. Previous studies have demonstrated the capture of mercury by fly ash carbon. This study confirms prior work demonstrating the varying role of carbon types in the capture, implying that variability in the carbon forms influences the amount of mercury retained on the fly ash.