Secondary Air Nozzles Research Articles

Heat transfer characteristics in a horizontal swirling fluidized bed

An innovative horizontal swirling fluidized bed (HSFB) with a rectangular baffle in the center of an air distributor and three layers of horizontal secondary air nozzles located at each corner of fluidized bed was developed. Experiments on heat transfer characteristics were conducted in a cold HSFB test model. Heat transfer coefficients between immersed tubes and bed materials in the HSBF were measured with the help of a fast response heat transfer probe. The influences of fluidization velocity, particle size of bed materials, measurement height, probe orientation, and secondary air injection, etc. on heat transfer coefficients were intensively investigated. Test results indicated that heat transfer coefficients increase with fluidization velocity, and reach their maximum values at 1.5–3 times of the minimum fluidization velocity. Heat transfer coefficients are variated along the circumference of the probe, and heat transfer coefficients on the leeward side of the probe are larger than that on the windward side of the probe. Heat transfer coefficients decrease with increasing of measurement height; heat transfer coefficients of the longitudinal probe are larger than that of the transverse probe. The proper secondary air injection and particle size of bed materials can generate a preferred hydrodynamics in the dense zone and enhance heat transfer in a HSFB.

Gas-Solids Flow Properties of a Square Circulating Fluidized Bed with Secondary Air Injection

Effects of secondary air (SA) injection or air-staging on the gas-solids flow properties in the riser of a circulating fluidized bed (CFB) were investigated. The experiments were carried out in a CFB cold model with a square cross-section of 0.25m×0.25m and a height of 6.07m. The axial pressure drop profile along the riser was reported. And the local solids holdup profile at the centerline and the diagonal line of some cross sections which influenced by the SA injection was measured with an optical fiber probe. Two SA arrangement modes, i.e. four SA nozzles located on four walls (Wall SA) and four corners (Corner SA) were conducted. Air-staging results in a denser bottom bed for both two SA modes. The Wall SA case has a higher solids holdup than the Corner SA case in most regions of lower bed except the corner region but was leaner in the vicinity of SA injection level. W-shaped solids concentration profile was found in the region immediately above the SA injection level for Wall SA case but not obvious for Corner SA case. Fractal dimension was analyzed for pressure drop fluctuations and local solids concentration signal. Air-staging led to smaller fractal dimension value than the case without SA injection. The SA jets affected the local gas-solids distribution and fluctuation in the region close to the SA injection. Radial and axial solids transfer should be considered for the fractal analysis.

Combustion characteristics of simulated gas fuel in a 30kg/h scale pyrolysis-melting incinerator

Combustion characteristics of gas fuel in a pyrolysis-melting incinerator having a 30 kg/h capacity were investigated. Pyrolyzed gas from waste was simulated by propane that was injected in the combustion chamber, and burnt through multi-staged combustion by distributing the combustion air to primary, secondary, and tertiary air nozzles. Temperatures and the concentrations of gas components in the combustion chamber were measured. Combustion performance was evaluated with respect to the temperature distribution and combustion gas concentrations of O2, CO and NOx. Using secondary air and/or tertiary air, the combustion performance was improved, and, in particular, NOx concentration decreased significantly following the tertiary air injection.

Raising the Reliability, Efficiency, and Ecological Safety of Operation of the BKZ-210-140F Boiler Transferred to Stage Firing of Kuznetsk Coal in a U-Shape Flame

The BKZ-210-140F boiler of the West-Siberian Cogeneration Plant was equipped initially with four uniflow tangentially oriented burners and tertiary air nozzles. In order to raise the efficiency of operation and lower harmful emissions the boiler was reconstructed. U-shape aerodynamics was organized in the furnace by mounting 8 burners, 8 secondary air nozzles, and 8 tertiary air nozzles on the front and rear walls of the furnace. The reconstruction ensured higher stability of ignition of pulverized coal without flame division and rated temperatures of the superheater metal, lowered the optimum excess air factor at the outlet from the superheater to 1.2 – 1.25, decreased the concentration of nitrogen oxides in the combustion products to 360 – 380 mg/m3, and increased the gross efficiency of the boiler to 91.5 – 91.7%.

The effect of swirling flow on elutriation in a vortexing fluidized bed

The elutriation of fine particles in a vortexing fluidized bed (VFB) was studied by a batch and binary system. The diameter of the coarse particles was 545 Μm, and the diameter of the fine particles for the elutriation test was 81, 97, 115, 137, 163, and 193 Μm, respectively. It was found that the swirling flow caused by the secondary air injection is the dominating factor to influence the elutriation rates. The effect of primary air velocity, swirling flow, injection angle of secondary air nozzle, and diameter of fine particles on the elutriation rate constant was also studied. The Taguchi experimental method and Regular analysis are used to identify the effects of various operating variables. A correlation was developed to estimate the specific elutriation rate constant (K∞*) in the vortexing fluidized bed. The specific elutriation rate constant (K∞*) was found to be a function of the primary air velocity, the diameter of fine particles, the secondary air velocity, and the height of secondary air injection.

Flow field in the freeboard of vortexing fluidized bed

AbstractA systematic investigation on the flow field in a vortexing fluidized bed cold model was reported. The gas velocity profiles within the freeboard with diameters of 0.19 m and 0.29 m were measured by using a five‐hole pitot tube. A new parameter, called vortex number, Vor defined as the ratio of tangential velocity to axial velocity of the swirling gas stream, was proposed for representing the swirl intensity. Vor is found to be increased with secondary air velocity, and decreased with primary air velocity and diameter of secondary air nozzles. It is also found that the profile of swirl flow is significantly affected by the arrangement of the secondary air nozzles. The effects of inserted length of secondary air nozzles and geometric structure of expansion section on the swirl flow are also studied. Based on the experimental data, a correlation is presented to estimate the vortex number. Vortex number is found to be a function of the geometric structure of exhaust tube, diameter of secondary air nozzle and tangential air flow rate.

3-Dimensional Simulation of Air Mixing in the MSW Incinerators

Abstract Combustion control strategies to minimize the pollutants emission from municipal solid waste (MSW) incinerators are formulated based on the improved mixing of air with the products of incomplete combustion and the subsequent increase in oxidative destruction. Secondary air injection into the combustion chamber plays the key role in this mixing process. However, design variables of the air jet into the combustion gas stream are not clearly identified, and the performance of mixing and reaction is not fully understood. Three-dimensional flow simulation was performed to study the mixing performance of a full-scale incinerator combustion chamber according to the secondary air nozzle configuration. A detailed flow was analyzed and the degree of mixing was quantitatively evaluated by introducing a statistical parameter based on the chemical species distribution. The gas residence time distribution was analyzed using the particle trajectory. The overall flow field was strongly influenced by the nozzle configuration. It was demonstrated that the degree of mixing could be improved by selecting larger inter-jet spacing and stronger jet velocity. The staggered arrangement of two opposite nozzle arrays was found to be more effective in terms of mixing and gas residence time distribution.

Limiting NO x and SO 2 emissions from an industrial-size fluidised-bed combustor

For the reported series of tests, the largest reductions in NO x emissions were achieved (together with maximum combustion efficiencies) by using a primary-to-secondary-air ratio of 30 70 for the fluidised bed (FB). Under such an operating condition, high freeboard-temperatures could be attained, e.g. up to 200° C above that of the bed, so promoting the combustion of unburnt carbon-fines in the freeboard zone, i.e. secondary combustion ensued. This tended to counteract the effect of the reduced rate of carbon combustion in the bed, i.e. as a result of the increased rate of oxidation occurring in the freeboard. Reductions in both the NO x and SO 2 emissions could be achieved by using such a two-stage combustion process, without affecting adversely the overall combustion efficiency. If a primary-to-secondary-air ratio of 70 30 (rather than 30 70 ) was employed, the rate of SO 2 emissions fell, but the temperature uplift in the freeboard zone was less pronounced and a reduction in combustion efficiency of 2% was observed. The rates of emissions of NO x and SO 2 could also be reduced by the adoption of wise choices for the design and operation procedure of the fluidised-bed combustor (FBC). For example, a 45% decrease in the rate of NO x emissions was achieved in one test merely by halving the bed's depth. The lower the available oxygen-concentration (including that provided by recycled flue-gases) within the bed, the smaller was this decrease. On the other hand, a doubling of the bed's depth led to a reduction in the SO 2 emission of approximately 4%, provided that the in-bed oxygen concentration was maintained sufficiently high, so as to promote sulphation of the ash and any limestone present in the bed. The location of the secondary-air nozzle (i.e. in the freeboard zone) also had significant influences on the emission rates of the NO x and SO 2. A 13% reduction in the rate of NO x emissions was achieved by increasing the height (above the sparge pipes) of the secondary-air's nozzle from 1·5 to 2·6 m. The larger this height, the greater the opportunity for char-NO x-reducing reactions (for which high-temperature conditions are preferred within the freeboard) to ensue, prior to the secondary-air injection. However, the rates of SO 2 emission fell by ∼ 10% when the height of the secondary-air's nozzle was reduced from 2·6 to 1·5 m above the sparge pipes. This was due presumably to the increased residence times of the ash and limestone sorbents (for facilitating sulphation) in the freeboard, under the oxidising condition (following secondary-air injection), which favours the complete sulphation of the sulphidated ash/lime to CaSO 4. In the presently reported series of tests, the rate of NO x emissions could be reduced by up to 83% merely by adjusting the bed's depth and the secondary-air's nozzle-height, although these alterations led simultaneously to a 14% increase in the rate of SO 2 emissions, and vice versa. Thus, the emissions of NO x and SO x could be controlled by the use of an appropriate design of FBC. The choice made, in practice, usually depends on which of these emission rates is critical with respect to complying with the environmental-pollution directives. Great care must be taken when comparing emission rates from the FBs, either of different sizes or incorporating alternative methods of air distribution, in order to take account of scaling effects. It is unwise, for instance, to use observations from a laboratory-size FBC to predict the quantitative behaviour of an industrial-size (i.e. large) FBC unit. This arises because of the variations in the oxygen concentration, resulting from the usually non-homogeneous, relatively less effective fluidisation achieved in the large bed; the rates of NO x emissions from the large FBC tend to be smaller (as a result of the existence of pockets of relatively low concentration oxygen in the bed) and simultaneously the rates of SO 2 emission are invariably higher. In the presently reported tests, under otherwise nominally similar conditions, an approximate halving of the rate of NO x emissions resulted when using the larger FBC (rather than the small one) together with a doubling of the SO 2-emission rates, even though the average percentages of oxygen present in the flue gases remained identical for both the small and large FBC units. The recommended bed-depth depends upon what emission one seeks to reduce. However, where feasible, for an industrial-size combustor, it is wise to employ a shallow bed (∼ 340 mm depth, when static) in order to incur relatively low operating costs.