Diameter combustor

The PEBC at the Tidd Station operates at 1200 kPa (170 psi) and a bed temperature of 860°C (51). A pressure vessel, 13.4 m in diameter by 20.7 m high, houses the combustor and its ancikafies. Coals, which contain ash contents less than about 25%, are blended with dolomite and pumped to the combustor as a paste having a total water content of 20—25%. Coals, which contain ash contents higher than 25%, and dolomite are individually fed pneumatically via separate lock hoppers. Both coal and dolomite are cmshed to 3-mm top size before being fed to the unit. 

Length. Combustor length must be sufficient to provide for flame stabilization, combustion, and mixing with dilution air. The typical value of the length-to-diameter ratio for liners ranges from three to six. Ratios for casing range from two to four. 

In practice it has been found that the diameter of holes in the primary zone should be no larger than 0.1 of the liner diameter. Tubular lines with about 10 rings of eight holes each give good efficiency. As discussed before, swirl vanes with holes yield better combustor performance. 

Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

For a typical case, an axisymmetric jet with a mean velocity of 100 m/s flows through the cylindrical inlet of diameter D into a cylindrical combustion chamber of twice the diameter. An annular or central exit at the end of the combustion chamber is modeled to produce choked flow. Particles are injected from the inlet-combustor junction with a streamwise velocity of 50 m/s and zero radial velocity. If the number of particles is small (that is, for low-mass loadings), the effect of the particles on the flow can be neglected. Still the flow has an effect on the particles that depends on parameters such as the size and density of the particles. Such systems are called one-way coupled systems and are discussed next. 

Based on the initial studies of unforced flows described above, inflow was selectively perturbed to investigate if the amount of particle dispersion and the location of enhanced dispersion within the combustor can be shifted as desired. Calculations were performed in which an acoustic perturbation was imposed from the back wall of the combustor with an amplitude of 0.5% of the initial chamber pressure and a frequency of 1380 Hz, 690 Hz, or 145 Hz, the characteristic frequencies of the system under study. The vortex-shedding frequency was 1380 Hz, the first-merging frequency was 690 Hz, and 145 Hz was the quarter-wave mode of the inlet. In addition, simulations were also performed with forcing at a frequency unrelated to the system, 1000 Hz. The particle size chosen for these simulations was 15 pm in diameter (St = 0.97), since this size particles were found to be optimally dispersed in the unforced flow case. All other parameters remain unchanged from the unforced case discussed above. 

A water-cooled sampling probe of internal diameter 1 mm and external diameter 5 mm with a 3-meter long heated line was used to measure concentrations of unburned hydrocarbon (flame ionization detector, Analysis Automation, 520) and NOj (chemiluminescence analyzer, Thermal Environment Instruments, 42) at the combustor exit on a wet basis. The former, measured to a precision of the order of 1 ppm, was used to ensure complete consumption of fuel within the duct, and the latter with a precision of around 0.2 ppm was used to quantify the effect of oscillations on NOj. emissions. 

Axial and swirling air streams in the combustor issued from a circular chamber through a conical nozzle. The chamber was utilized both as an acoustic resonator and a settling chamber. It contained a honeycomb to straighten the how and two acoustic drivers to apply acoustic excitation to the jet. The nozzle exit diameter was 3.8 cm and the maximum Reynolds number based on this diameter and the exit velocity with and without air forcing was 4800 and 1400, respectively. The tests were performed with total air how rate of 85 1/min, and fuel how rate of 0.063 1/min. The swirl was applied with tangential air injection and the maximum swirl number tested was Ns = 0.30. 

Figure 24.3 shows typical single-sweep (raw data) measurements of spectral absorption coefficients obtained simultaneously by tuning two diode lasers independently at 10-kilohertz rates across H2O transitions near 1343 and 1392 nm over a 5-centimeter path through the combustion region x/d = 2) of the forced combustor (jet diameter d = 2.1 cm, 4> = 0.75). The product of the spectral absorption coefficient at frequency v ki, cm ) and path length L, cm) is given by ki,L = ln(/o//)j/, where / and Iq are the transmitted and... 

Temperature profiles were measured at several axial locations to locate the peak temperatures in the combustor. The axial distance between the nozzle and the temperature-measurement cross-section is denoted by Lf With one insert in place, the peak gas temperature immediately downstream of the insert was lowered but the high-temperature region was extended radially, i.e., the pattern factor was improved, as shown in Fig. 28.2. The peak temperatures at each axial location are shown as a function of the distance from the nozzle, or Lt/D, in Fig. 28.3. For the baseline case the highest temperature of 1418 K was found at 1.8 pipe diameters downstream of the nozzle. With one porous layer present, the peak gas temperature was about 200 K lower at Lt/D = 1.8 2.2 but increased by up to 120 K and 200 K at 0.5 and 3.2 pipe diameters downstream of the nozzle, respectively. The highest flame temperature was lowered but the high-temperature region was extended to upstream and downstream. 

The size distribution of the particulate matter in the 0.01-5 ym size range is analyzed on line using an electrical mobility analyzer and an optical particle counter. Samples of particles having aerodynamic diameters between 0.05 and 4 ym are classified according to size using the Caltech low pressure cascade impactor. A number of analytical procedures have been used to determine the composition distribution in these particles. A discrete mode of particles is observed between 0.03 and 0.1 ym. The major components of these particles are volatile elements and soot. The composition of the fine particles varies substantially with combustor operating conditions. 

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