Atmospheric Fluidized Bed Combustion Research Articles

96/05199 Atmospheric fluidized bed combustion of high sulfur high volatile N.E. Region coals of India

96/05199 Atmospheric fluidized bed combustion of high sulfur high volatile N.E. Region coals of India

ACIDIC MINESPOIL RECLAMATION WITH AFBC BY-PRODUCT AND YARD-WASTE COMPOST

Economic and environmental incentives to reduce solid waste volumes have spurred interest in the development of beneficial uses for urban and industrial by-products. This project investigated the reclamation efficacy and impacts on soil and water quality of two such materials: atmospheric fluidized bed combustion (AFBC) by-product and yard-waste compost. Six 1-acre watersheds were constructed on acidic abandoned mined land spoil (pH range 3.5 to 4.5) Two watersheds each were then reclaimed with 8 in of borrow soil, 125 tons/acre of AFBC, or 125 tons/acre of AFBC and 50 tons/acre of compost, and planted with a grass-legume seed mix. Watersheds were instrumented to record hydrographs of storm-water runoff events, measure erosion, and collect samples of surface- and percolate-water flow. One year after reclamation the AFBC and AFBC+ compost treatments compared favorably with the traditional resoil reclamation practice. Spoil pH in the 0 to 4 in depth was increased to the range 6 to 8 which was similar to the resoil pH, and complete vegetative cover was successfully established on all watersheds. However, plant biomass production was approximately 2 times larger on the resoiled watersheds than on the amended spoil. Consequently, erosion was smallest on the resoiled watersheds. All three reclamation treatmentsmore » increased runoff water pH to >7 and decreased soluble Al. Concentrations of Ca and S were larger in runoff- and percolate-water samples from AFBC-treated watersheds than from the resoiled watersheds. Trace element concentrations in all water samples remained very low and showed almost no treatment effects.« less

Petrography and Chemistry of High-Carbon Fly Ash from the Shawnee Power Station, Kentucky

Abstract The Shawnee power station in western Kentucky consists of ten 150-MW units, eight of which bum low-sulfur ( 1 wt %) coal in an atmospheric fluidized-bed combustion unit and in a research unit. The eight low-sulfur coal units were sampled in a 1992 survey of Kentucky utilities. Little between-unit variation is seen in the ash-basis major oxide and minor element chemistry. The carbon content of the fly ashes varies from 5 to 25 wt %. Similarly, the isotropic and anisotropic coke in the fly ash varies from 6% to 42% (volume basis). Much of the anisotropic coke is a thin-walled macroporous variety, but there is a portion that is a thick-walled variety similar to a petroleum coke.

Reaction of CO2 with clean coal technology ash to reduce trace element mobility

The combustion of coal in power plants generates solids (e.g., fly ash, bottom ash) and flue gas (e.g., SO x , CO2). New Clean Air Act mandated reduction of SO x emissions from coal burning power plants. As a result, a variety of Clean Coal Technologies (CCT) are implemented to comply with these amendments. However, most of the CCT processes transfer environmentally sensitive elements (e.g., As, Cd, Pb, Se) from flue gas to CCT ash. The objective of this study was to determine the effect of a pressurized CO2 treatment on the chemistry of CCT ash. Three CCT ash samples, produced from lime injection, atmospheric fluidized bed combustion, and sodium carbonate injection processes were reacted under different CO2 pressure treatment conditions. Treated and untreated samples were subjected to various experiments including, X-ray diffraction (XRD) analysis, calcium carbonate solubility studies, and trace element extraction studies. Factors influencing the efficiency of a CO2 treatment for CCT ash samples include combustion process, moisture, CO2 concentration, and pressure. The CO2 pressure treatment resulted in the precipitation of calcite in CCT ash samples, and thus lowered the pH and the concentration of extractable trace elements (e.g., Cd, Pb, Cr, As, Se). Furthermore, we found that CO2 pressure treatment was more effective for lime injection and atmospheric fluidized bed combustion processed samples than for sodium carbonate injection processed samples.

Characterizing fuels for atmospheric fluidized bed combustion

A complete methodology for characterizing coal combustion in atmospheric fluidized bed reactors is presented. The methodology comprises studies of fragmentation and particle size variations during combustion, necessary to allow an accurate determination of kinetic parameters and attrition rates. Samples of three different carbonaceous materials (a medium-ash lignite, a medium-ash anthracite and a graphite) were pyrolyzed in N 2 and partially burned in air in a bench-scale fluidized bed reactor (5 cm i.d.) at different operating conditions (0.68 mm < d 0 < 3.50 mm, 750°C < T b < 950°C and U = 0.13 m/s). The samples were retrieved from the sand-bed by means of a basket. The particle size distribution, apparent density and number of particles were evaluated by Image Analysis. Thus, the evolution of these parameters with burnoff was determined for different materials. Additionally, the sphericity factors were calculated. Combustion studies were carried out in batch experiments in the laboratory-scale, fluidized bed reactor at the same operating conditions. The reactor outlet concentrations of O 2, CO 2, and CO were monitored continuously. The results indicate that only anthracite particles experienced both primary (due to devolatilization) and secondary (during char combustion) fragmentation. Graphite particles underwent secondary fragmentation, whereas lignite particles did not significantly vary in number during combustion. Size and density variations during combustion suggest that graphite particles burn under regime II, intraparticle diffusion being the rate controlling step. On the other hand, anthracite and lignite particles developed an ash layer, which may control combustion. The attrition constants of the medium-ash materials (lignite and anthracite) were found to be very low (0.3 × 10 −7 and 0.8 × 10 −7, respectively), whereas that of graphite was much higher (17.6 × 10 −7), due mainly to peripheral percolation during combustion.

95/05036 Overall modeling. 1. Overall modeling of atmospheric fluidized bed combustion and experimental verification

95/05036 Overall modeling. 1. Overall modeling of atmospheric fluidized bed combustion and experimental verification

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

AbstractFor the design, simulation and optimization of sulphur retention in atmospheric fluidized bed coal combustors, a mathematical model is needed that would be able to predict the behaviour of the combustor in a wide range of operating conditions. In this work, a sensitivity analysis of the sulphur retention predictions of the different hypotheses, equations and parameters, which define the different submodels and phenomena occurring in the combustor, has been carried out. It has been found that the hypotheses related to the gas flow, devolatilization type and sulphur distribution in the pyrolysis products imply an important division among models. The greatest effect on sulphur retention predictions is exercised by the parameters defining the fines elutriation and sorbent sulphation capacity. However, those corresponding to the bed hydrodynamics (minimum fluidization velocity and bed expansion) do not have a significant effect on the sulphur retention predictions.The sulphur retentions obtained in the combustion of high sulphur lignites with eight different limestones were used for model validation. A good fit of the experimental sulphur retentions was found, without using any adjustable parameter. Finally, a simulation of the process was made. The great effect of the bed height, air velocity and the particle size distribution of the limestone must be pointed out, as well as the effect of its reactivity through the maximum conversion attainable by each particle size.

Wastage of low alloy steels in a fluidized bed environment

The wastage behaviour of four low alloy steels, suitable for use as evaporator tubing in industrial atmospheric fluidized bed combustors (AFBCs), was examined in a laboratory-scale test rig. Specimens exposed in the test apparatus experienced a high flux of impacts at low particle velocities similar to conditions in a FBC boiler. The influence of time, velocity and temperature on the wastage behaviour was examined and incubation times and velocity exponents were determined and their values discussed. Since high-temperature oxidation played an important role in this process, the short-term oxidation rate of each of the steels was measured. The mechanisms of material loss across the temperature range were discussed and the behaviour of the low alloy steels in the current work was compared with that of high alloy and stainless steels in earlier studies.

Burning characteristics of high-ash bangladeshi peat

The combustion characteristics of high-ash Bangladeshi peat was investigated in an atmospheric fluidized-bed combustor at bed temperatures of 750, 800 and 850°C. The particle size was varied from 8.05 mm to 22.76 mm. Combustion of this variety of peat occurred at constant size and thus followed a shrinking core model. Volatile evolution is complete before the particles reached bed temperature and during char combustion the particle temperature exceeded the bed temperature. Combustion times can be represented by power law equations. Results indicated that the burning rate of peat char is a combination of diffusion and kinetic control mechanisms.

High-calcium coal combustion by-products: Engineering properties, ettringite formation, and potential application in solidification and stabilization of selenium and boron

Four high-calcium coal combustion by-products (two pulverized coal fly ashes (PCFA), a flue gas desulfurization (FGD) residue, and an atmospheric fluidized bed combustion (AFBC) fly ash), were tested for engineering properties and ability to immobilize boron and selenium. These data are needed to explore high-volume utilization in engineered structures or in solidification/stabilization (S/S) technology. Strengths of cured pastes (91 days), varied from as much as 27 MPa (3900 psi) for one of the PCFA specimens to 4.6 MPa (670 psi) for the FGD specimen. All of the coal by-product pastes developed more than the 0.34 MPa (50 psi) required for S/S applications. Ettringite formation is important to engineering properties and S/S mechanisms. XRD on plain specimens cured for 91 days indicated that the two PCFA pastes formed 5–6% ettringite, the FGD paste formed 22%, and the AFBC paste formed 32%. The hydrating PCFA pastes showed little expansion, the FGD paste contracted slightly, and the AFBC paste expanded by 2.9% over 91 days. Se and B were spiked into the mixing water as sodium selenite, selenate and borate, and for most pastes this had little effect on strength, workability, and expansion. Leaching of ground specimens (cured for 91 days) showed a generally positive correlation between the amount of ettringite formed and resistance to Se and B leaching. Se spiked as selenate was more readily leached than Se spiked as selenite. B showed a high level of fixation.

Utilization of highly improved fly ash for SO2 capture.

A fly ash of low calcium content (1.67 wt% CaO) was ground (to particle sizes ofunder 5 μm) and treated with slaked lime (Ca(OH)2) to produce an improved sorbent for high-temperature dry sorbent injection. Four samples were treated at 350 K for 8 hours with different Ca(OH)2 concentrations (6.6–28.6 wt%). The specific pore surface area and pore volume were extensively characterized before and after treatment. Sulfation tests were performed by exposing the treated samples to a 0.31 vol% SO2 containing atmosphere at 1123 K in a thermo-gravimetric analyzer (TGA). The TGA experimental conditions were carefully controlled to closely simulate those encountered in an atmospheric fluidized bed combustor (AFBC).The treated fly ash sorbents showed higher SO2 sorptivity compared to untreated sorbents. The sorption capacity was found to increase as the Ca(OH)2 concentration increased in the range of interest. Up to 92% CaO conversion could be achieved when the 28.6% Ca(OH)2 fly ash sorbent was sulfated for 1 hour. Treatment of fly ash with Ca(OH)2 enhanced the specific surface area (by about 5 to 8 times), which was principally responsible for the high SO2 capture.

Burning times of volatiles from Turkish coals during fluidized bed combustion

The burning times of the volatiles from twelve Turkish coals ranging from bituminous to lignite was experimentally investigated in a bench-scale atmospheric fluidized bed combustor. Devolatilization time was determined by visual observation of the plume-like volatiles flame. The time lapse between the appearance and disappearance of this flame was recorded as flame extinction time. The effects of particle size (1.0–11.2 mm) and bed temperature (650–920 °C) were investigated, and the effect of moisture was determined by measurements on oven-dried samples. Fragmentation was observed for a few coals after devolatilization was virtually complete, but the majority preserved their original particle shape. Flame extinction times were related to initial particle diameter by a power law; numerical constants for this relation were determined for each coal by regression analysis. Two parameters — volatile matter/fixed carbon ratio and volatile matter heating value — are proposed to characterize the effect of coal type on devolatilization time. The observed effects of particle size, bed temperature, moisture pore structure and coal type on flame extinction time are consistent with heat transfer to and in the coal particle as the rate-controlling step.

Comparative Reactivity of Two Different Ca(OH)2-Derived Oxides for SO2 Capture.

The sulfation capacity of sorbents derived from fine hydrated lime powder (termed H-CaO) and those derived from carbonated H-CaO (termed C-CaO) were investigated. All calcination, carbonation and sulfation experiments were conducted in a thermogravimetric analyzer (TGA) under conditions closely simulating those prevailing in an atmospheric fluidized bed combustor (AFBC). The effect of such parameters as calcination and sulfation temperatures and active gas concentration on the capacity of SO2 removal were studied. The change in porosity with progress of the sulfation reaction was investigated for both H-CaO and C-CaO sorbents.N2-adsorption/desorption isotherms showed that both H-CaO and C-CaO sorbents exhibited slit-shaped pores. The pore-size and specific surface area distribution curves indicated a higher reactivity of H-CaO calcines over that of C-CaO calcines at all reaction temperatures. The conversion of both H-CaO and C-CaO decreased with decreasing porosity and reached an ultimate value where the porosity approached zero. These ultimate conversions of both H-CaO and C-CaO were found to be 71 and 61%, respectively, at 1123 K and 0.31 vol% SO2 concentration.

The evolution of porosity and intrinsic reactivity of coke particles

Experiments have been performed with coke particles in the freeboard region of a 250 kW Atmospheric Fluidized Bed Combustor (AFBC) with particle sizes ranging from 90 μm to 360 μm in the gas temperature range from 1050 K to 1350 K. Partially burned particles were collected at various heights and residence times. The observed morphological development during combustion indicates clear effects of internal burning resulting in an increase of pore volume and internal surface area. A pore model has been developed to describe the evolution of pores and total surface area, and to calculate the intrinsic reactivity and burning rates. The model is based on a combination of a Thiele analysis and the random pore model.

Effect of formulation of steady-state heat balance for char particles on AFBC modelling

For the solution of population balances of char particles burning in the bed in modelling of an atmospheric fluidized-bed combustor (AFBC), a submodel for calculating the surface temperature of reacting char particles is included by solving the corresponding heat balance. This particle heat balance is strongly interrelated with other submodels through the kinetic term of heat generation at the reaction surface. Three possible formulations for calculating the particle temperature are analysed: 1. (a) without particle heat balance; 2. (b) heat balance around the particle with a log mean of oxygen concentration; 3. (c) heat balance around the particle at each height in the bed. These assumptions are introduced into a model for an AFBC without a plume of volatiles, with slow bubble regime in the bed and with plug flow of gas, using the shrinking unreacted-core model and considering char losses by carryover and overflow. The effect of incorporating these hypotheses in the model is analysed through the solution of the population balance. The results obtained indicate that the effect is appreciable on the carbon combustion efficiency and carbon loading in the bed.

Current status of captive breeding programmes: L. E. M. De Boer, in: Biotechnology and the conservation of genetic diversity. Proc. symposium, London, 1990, ed H.D.M. Moore & others, (Clarendon Press, for Zoological Society of London; Symposia of the Zoological Society of London, 64), 1992, pp 5–16

Due to the externalities associated with energy production and consumption, public policy is necessary to provide a stimulus for the development and diffusion of more environmentally sound energy technologies. Based on an in-depth history of technological development for four electric power technologies, this paper draws lessons for the design of future policies to promote innovation in energy technologies. The technologies examined are: wind turbines, solar photovoltaics, gas turbines, and atmospheric fluidized bed combustion. The analysis considers both supply-push and demand-pull approaches for stimulating technological change. It concludes that government activities to promote environmentally enhancing technological development must include both supply-push and demand-pull policies during the period spanning precommercialization, first commercial use, and lead adoption. Furthermore, this analysis identifies five industry sector characteristics that influence the level of government effort necessary to support commercialization: the size, strength, and risk of the private market niche; industry structure; firm financial capability; firm technological capability; and sources of innovation.

The reductive decomposition of calcium sulphate—I. Kinetics of the apparent solid—solid reaction

The reductive decomposition of calcium sulphate by hydrogen is used for the regeneration of calcium-based atmospheric fluidized bed combustion (AFBC) SO 2 sorbents. The apparent solid—solid reaction between CaS and CaSO 4, one of the steps involved in the reaction mechanism of the reductive decomposition of CaSO 4, has been investigated in a thermobalance construction provided with a coupled system for the analysis of product gases. By creating suitable reaction conditions in the experimental set-up, it is possible to study this step separately. The apparent solid—solid reaction between CaS and CaSO 4 takes place when a mixture of these compounds is heated in an inert atmosphere to temperatures above 1100 K: ▪ The fractional production rate of CaO (or SO 2) appears to depend on the ratio in which CaSO 4 and CaS are present initially. A simple model is formulated to calculate the fractional production rate as a function of the fractional degree of production and the initial composition of the reactants. One of the principal assumptions of the model is the presence of a liquid phase formed by mixtures of CaS and CaSO 4. Fractional production rates calculated after determining the required model parameters (kinetic constants and volume plus composition of the liquid phase) show close agreement with experimentally obtained values.

Control of in-bed agglomeration by fuel blending in a pilot scale straw and wood fueled AFBC

Bed agglomeration in a pilot scale atmospheric fluidized bed combustor was controlled by blending wood into rice straw in concentrations of 50% wood or more and stoichiometrically holding reaction temperature at or below 800°C. At higher straw concentrations, agglomeration of the bed occurred over run times inversely proportional to the straw concentration in the blend. Bed agglomeration was preceded by a characteristic decline in bed pressure drop most likely related to combustion air channeling through the bed. Blend ash composition was not substantially affected until straw concentration decreased to 50% or below because of the four-fold higher ash concentration in the straw compared to wood. Blend ash base-to-acid ratio also was not substantially affected above 50% straw concentration, remaining essentially constant at 0.3, compared to the wood ash base-to-acid ratio of 1.1. Initial deformation temperature, as measured by standard cone test of the blend ash, increased from a minimum of 880°C for a 75% straw blend to 1120°C for a 10% straw concentration in the blend.

Modeling of lignite combustion in atmospheric fluidized bed combustors. 1. Selection of submodels and sensitivity analysis

The application of a specific model to lignite combustion in small and medium-sized combustors presents the initial difficulty of having to choose from the different submodels proposed in the literature, where some important discrepancies still exist. Therefore, in this work a sensitivity analysis of the combustion efficiency predictions of the different hypotheses, equations, and parameters defining the different submodels has been carried out. Thus, it has been found that the hypotheses related to the gas hydrodynamics exercise an important effect on the predictions

Modeling of lignite combustion in atmospheric fluidized bed combustors. 2. Model validation and simulation

In this work, different strategies to fit the experimental carbon combustion efficiencies obtained in the combustion of different lignites in a AFBC pilot plant with a 200-mm internal diameter have been analyzed with a model of the system. Different fitting methods used in literature bred on the crossflow factor, the elutriation constant, or the fuel reactivity, have been considered for this analysis. It has been found that a great majority of the high carbon combustion efficiencies obtained experimentally cannot be predicted with a model which considers perfect mixing of gas in the emulsion phase, even when an finite crossflow is considered