105 research outputs found
Co-combustion of sewage sludge with wood/coal in a circulating fluidised bed boiler - A study of NO and N2O emissions
Reduction of emissions of NO and N2O from co-combustion of wet or dried sewage sludge with coal or wood is investigated. This is motivated by the high nitrogen content in sewage sludge that may give rise to high emissions. An advanced air-staging method for combustion in circulating fluidised bed is applied. It is shown that with fluidised bed combustion the emissions are low as long as the sludge fraction is not too high (say, less than 25%), and the conversion of fuel nitrogen to NO or N2O is only a few percent. However, air staging as such is not efficient for high volatile fuels, and any air supply method can be applied in such a case, in contrast to combustion of coal, when the air supply arrangement has a decisive influence
Axial Concentration Profiles and NO Flue Gas in a Pilot-Scale Bubbling Fluidized Bed Coal Combustor
Atmospheric bubbling fluidized bed coal combustion of a bituminous coal and anthracite with
particle diameters in the range 500-4000 Ãm was investigated in a pilot-plant facility. The
experiments were conducted at steady-state conditions using three excess air levels (10, 25, and
50%) and bed temperatures in the 750-900 °C range. Combustion air was staged, with primary
air accounting for 100, 80, and 60% of total combustion air. For both types of coal, high NO
concentrations were found inside the bed. In general, the NO concentration decreased monotonically
along the freeboard and toward the exit flue; however, during combustion with high air
staging and low to moderate excess air, a significant additional NO formation occurred near the
secondary air injection point. The results show that the bed temperature increase does not affect
the NO flue gas concentration significantly. There is a positive correlation between excess air
and the NO flue gas concentration. The air staging operation is very effective in lowering the
NO flue gas, but there is a limit for the first stage stoichiometry below which the NO flue gas
starts rising again. This effect could be related with the coal rank
Particle emissions from pellets stoves and modern and old-type wood stoves
The purpose of this work is to characterise particle emissions from pellets stoves and modern and old-type residential wood stoves. The mass concentration of particulate material in the hot flue gas was 19e82 mg/MJ, roughly the same for wood stoves and pellets stoves, but the old-type wood stoves tended to emit even higher quantities. Furthermore, during combustion of wood logs the considerably higher emission of organic gaseous carbon indicates an additional contribution to the emission from secondarily formed condensable organic particles.
The particle mass emitted was dominated by fine particles (<1 mm) in all cases. The fine particle maximum was displaced towards larger size during inefficient combustion, e.g. during the start-up phase. The number concentration of particles from wood stoves was 1.01014 #/MJ, from pellets 3.01013 #/MJ, and in all cases dominated by fine particles. Efficient combustion of pellets resulted mainly in inorganic particles, dominated by potassium sulphate and potassium chloride. Zinc, the second most frequent metal, and calcium showed the highest concentrations among the less volatile components.
The lowest concentration detected was for cadmium, about 105 times lower than for potassium
Particle emission from combustion of oat grain and its potential reduction by addition
Second-rate cereals, unsuitable for food, can be used as fuel for small-scale production of heat and hot water.
However, there are more problems related to cereals than to woody fuels. This work aims at characterising
the particle emission from residential combustion of oat grain and its potential reduction by addition of
limestone or kaolin with the fuel. Then, to a large extent, the potassium supplied by the fuel is expected to be
found in coarse particles, leaving the boiler as bottom ash, instead of being emitted to the air in the form of
submicron particles. Combustion experiments were performed on a residential boiler, using filter sampling
and low-pressure impactors to measure the mass and number concentrations and size distributions of the
emitted particles. The particles and the bottom ash were subsequently analysed for inorganic material. To
check the combustion conditions and basic emissions from combustion of cereals, the flue gas was analysed
with respect to gaseous O2, CO2, CO, NOx, TOC (total organic carbon), HCl and SO2. Furthermore,
thermodynamic equilibrium analysis was used to support the experimental data. Finally, it is concluded that
the particle emission can be lowered by supplying kaolin, while there was no effect of limestone.
© 2008 Elsevier B.V. All rights reserve
Sensitivity study of fluid dynamic effects on nitric oxide formation in CFB combustion of wood
The paper presents results from simulations by a 1.5D numerical model developed to study the formation of the NO and N2O emissions in a circulating fluidized bed combustor (CFBC) under different operating conditions and burning different fuels. A comprehensive kinetic scheme for the homogeneous chemistry and a single particle model for char combustion are used. Fluiddynamic factors, including gas mixing and release of volatiles, are investigated in the case of wood combustion under normal air staging conditions. The pattern of release of volatiles, the mixing of secondary air, and the lateral mixing of gas are observed to play significant roles in the formation of the relatively high NO emissions from combustion of wood. Comparisons are made with measurement data and also with coal combustion
Sensitivity study of fluid dynamic effects on nitric oxide formation in CFB combustion of wood
The paper presents results from simulations by a 1.5D numerical model developed to study the formation of the NO and N2O emissions in a circulating fluidized bed combustor (CFBC) under different operating conditions and burning different fuels. A comprehensive kinetic scheme for the homogeneous chemistry and a single particle model for char combustion are used. Fluiddynamic factors, including gas mixing and release of volatiles, are investigated in the case of wood combustion under normal air staging conditions. The pattern of release of volatiles, the mixing of secondary air, and the lateral mixing of gas are observed to play significant roles in the formation of the relatively high NO emissions from combustion of wood. Comparisons are made with measurement data and also with coal combustion
Properties of Particles in the Fly Ash of a Biofuel-Fired Circulating Fluidized Bed (CFB) Boiler
A quench/dilution probe was designed and used for sampling of particles and vapors in the flue gas upstream of the convective path in a circulating fluidized bed boiler. Downstream of the convective path, flue gas was sampled by a heated probe. The sampled gas was lead through low-pressure impactors for analysis of the mass size distribution of particles. The particles were analyzed for Cl, S, K, Na, Ca, Mg, P, Al, Fe, Ba, Mn, and Si. A series of tests was carried out to study minimization of problems originating from the alkali content in the fuel, including the effect of additives to the fuel, change of bed material, and co-combustion with sewage sludge. Particle concentration and composition were compared to deposit growth and composition on a deposit probe, which represents a superheater. Alkali chloride in the particles was compared to online measurements of gaseous alkali chlorides. The continuous mass deposition on the heat-transfer surface in the convective path was illustrated by calculation of the reduction of heat transfer by means of the temperature drop of the flue gas. Fine particles (1 μm) particles increased. These test cases also showed the lowest deposit growth on the superheater probe. The particles deposited in the convective path were mostly coarse ones. The most favorable case with respect to low deposits was co-combustion with sewage sludge
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