Description of Pressure Fluctuations in a Circulating Fluidized Bed by Statistical Analysis

In this paper we evaluate different methods for statistically analyzing the variability in pressure fluctuations measured at three locations in an 80-mm-ID, 5-m-tall CFB model operated with natural rutile particles and air at ambient conditions. The methods evaluated are the Shannon entropy, Fischer information matrix together with kernel density estimation, and an estimation of the magnitude of the pressure amplitudes. The accuracy of the different methods is estimated by the bootstrap method. We illustrate how informative statistics from these methods can be used to quantify the effect of the process variables on fluidization at different bed locations. Depending on the interest of the experimenter, the method and statistic can be selected which explains fluidization operation most accurately

An Analysis of Pressure Fluctuations in a CFB of Heavy Minerals

Pressure fluctuations were measured at high frequencies in a CFB model operated with air at ambient conditions. The model comprises an 80-mm-ID, 5-m-tall riser with a blind-T exit, a cyclone, a 50-mm-ID standpipe, and an L-valve. Tests were conducted with particles of natural rutile (TiO2), a heavy mineral mined from coastal dunes. The particles fall into group B of Geldart’s classification. The solids inventory was kept at 25 kg. The superficial gas velocity ranged from 3 to 6 m/s. The solid circulation flux varied between 10 and 40 kg/m2.s. Profiles of solid concentrations in the riser are C-shaped. The amplitude of pressure fluctuations increases with increasing solids-circulation rate, and the increase appears to be linear. The amplitude does not correlate with solids concentration, however. The implication is that gas-solid interactions differ significantly at the bottom and the top of the riser, despite similar solids concentrations in these two zones. The analysis in the frequency domain shows that the power of signals resides in those of low frequencies (less than 2 Hz). The pressure fluctuations reflect white noise: there is no dominant frequency and no periodic component. Pressure waves move at 25–45 m/s up the riser, an order of magnitude greater than the superficial gas velocity

Oxy-Combustion of Different Coals in a Circulating Fluidized Bed

Combustion of three Polish and one South African bituminous coal particles in air versus O2/CO2 mixtures with oxygen concentrations in the range from 21% to 60% vol. was conducted at temperature of 850°C in a 12 kW bench-scale CFB combustor. Combustion in air was proceeded at ~50˚C higher centre temperatures and was slightly shorter in time compared to combustion in O2/CO2 mixture with 21% vol. O2. Larger heat capacity of CO2 compared to that of N2 also retards the ignition of volatiles in O2/CO2 mixtures with 21% O2. However, when the concentration of oxygen in O2/CO2 mixtures is larger than 30%, the ignition time decreases and surface and centre temperatures increase significantly with increasing O2 content

Pressure-drop predictions in a fixed-bed coal gasifier

In the Sasol Synfuels plant in Secunda, Sasol-Lurgi fixed-bed dry-bottom gasifiers are used for the conversion of low-grade bituminous coals to synthesis gas (syngas). The gasifiers are fed with lump coal having a particle size in the range from 5 to 100 mm. Operating experience shows that the average particle size and particle-size distribution (PSD) of feed coal, char and ash influence the pressure drop across the bed and the gas-flow distribution within the bed. These hydrodynamic phenomena are responsible for stable gasifier operation and for the quality and production rate of the syngas. The counter-current operation produces four characteristic zones in the gasifier, namely, drying, de-volatilization, reduction and combustion. The physical properties of the solids (i.e. average particle size, PSD, sphericity and density) are different in each of these zones. Similarly, the chemical composition of the syngas, its properties (temperature, density and viscosity) and superficial velocity vary along the height of the bed. The most popular equation used to estimate the pressure drop in packed beds is that proposed by Ergun. The Ergun equation gives good predictions for non-reacting, isothermal packed beds made of uniformly sized, spherical or nearly spherical particles. In the case of fixed-bed gasifiers, predictions by the Ergun equation based on the average or inlet values of bed and gas flow parameters are unsatisfactory because the bed structure and gas flow vary significantly in the different reaction zones. In this study, the Ergun equation is applied to each reaction zone separately. The total pressure drop across the bed is then calculated as the sum of pressure drops in all zones. It is shown that the total pressure drop obtained this way agrees better with the measured result. (C) 2010 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2010.09.02

Composition of Flue Gases during Oxy-Combustion of Energy Crops in a Circulating Fluidized Bed

In recent years, global warming and climate change associated with emissions of CO2 from fossil fuel-fired power systems are a big worry for authorities in many countries worldwide. The utilization of biomass as an alternative, carbon-neutral fuel can reduce emissions of CO2 and other greenhouse gases. Furthermore, the coupling of oxy-combustion of biomass with CO2 capture is an option for carbon-negative power generation technology. In this study, emissions of NOx, SO2, and CO from the air- and oxy-combustion of three energy crops (Miscanthus giganteus, Sida hermaphrodita, and Salix viminalis) are presented and compared with emissions from other biomass fuels and reference coal. Combustion tests in air and O2/CO2 mixtures were conducted in a 12-kW bench-scale CFB combustor at 850 °C. Measurements of flue gas compositions were taken using an FTIR spectrometer. In all tested atmospheres, emissions of SO2, N2O, and CO for biomass were lower than those for the reference coal. The oxidation of volatile nitrogen compounds was behind high emissions of NOx from biomass burned in air and O2/CO2 mixtures. The lowest concentrations of NO were found in the 21% O2/70% CO2 mixture. Combustion in mixtures containing more oxygen (30% and 40% O2) led to a decrease in emissions of N2O and CO and an increase in emissions of NO and SO2

Composition of Flue Gases during Oxy-Combustion of Energy Crops in a Circulating Fluidized Bed

In recent years, global warming and climate change associated with emissions of CO2 from fossil fuel-fired power systems are a big worry for authorities in many countries worldwide. The utilization of biomass as an alternative, carbon-neutral fuel can reduce emissions of CO2 and other greenhouse gases. Furthermore, the coupling of oxy-combustion of biomass with CO2 capture is an option for carbon-negative power generation technology. In this study, emissions of NOx, SO2, and CO from the air- and oxy-combustion of three energy crops (Miscanthus giganteus, Sida hermaphrodita, and Salix viminalis) are presented and compared with emissions from other biomass fuels and reference coal. Combustion tests in air and O2/CO2 mixtures were conducted in a 12-kW bench-scale CFB combustor at 850 °C. Measurements of flue gas compositions were taken using an FTIR spectrometer. In all tested atmospheres, emissions of SO2, N2O, and CO for biomass were lower than those for the reference coal. The oxidation of volatile nitrogen compounds was behind high emissions of NOx from biomass burned in air and O2/CO2 mixtures. The lowest concentrations of NO were found in the 21% O2/70% CO2 mixture. Combustion in mixtures containing more oxygen (30% and 40% O2) led to a decrease in emissions of N2O and CO and an increase in emissions of NO and SO2

Pollutant Emissions during Oxy-Fuel Combustion of Biomass in a Bench Scale CFB Combustor

Nowadays oxy-fuel combustion of coal and biomass is the most promising option for the reduction of CO2 emissions from power plants. In this paper, emissions of NOx (NO, NO2, N2O and their precursors, such as NH3 and HCN), SO2 and CO during conventional and oxy-fuel combustion of three kinds of biomass (agro, woody and energy crop) and a reference coal are presented and discussed. Combustion tests were conducted at 850 °C in the laboratory-scale circulating fluidized bed (CFB) reactor in air and O2/CO2 atmospheres. A FTIR spectrometer was used to measure instantaneous concentrations of all pollutants in the flue gas. Emissions of SO2, N2O and CO for the combustion of biomass in all atmospheres were lower than those for the combustion of reference coal. It was found that oxidation of nitrogen species released with volatile matter was responsible for high emissions of NOx during combustion of biomass fuels in air and mixtures of O2 and CO2. The lowest NO emissions for tested fuels were detected in oxy-21 atmosphere (21% O2/70% CO2). Oxy-combustion of biomass in O2/CO2 mixtures at 30% and 40% O2 caused a decrease in emissions of N2O and CO while NO and SO2 emissions increased. The results of this study show that the tested biomass fuels are ideal renewable energy resources both in conventional and oxy-fuel conditions with a minor potential for environmental pollution