Two different gas-solid separators are used in CFB boilers for different reasons. Cyclone or other impingement separators are used within the CFB boiler loop to trap hot solids and return them to the bed. This makes the residence time of fuel in the furnace long enough for complete combustion. This is called the primary particles collection. Electrostatic precipitators (ESP) or bag-house filters are used at the cold end stream of the boiler process to reduce fine particle emission to the atmosphere. This is called the secondary collection. These chapters are focused upon the primary separators, since they are a unique feature for CFB boilers.
The solids separator is a vital part of the CFB technology. The solids separator is primarily designed to provide an efficient separation of the entrained solids from the hot flue gas and return most of the unburned carbon and available calcined limestone for more efficient use. Sand and inert ash particles are also returned. These particles are needed to maintain the proper bed inventory and quality. The separator, located at the outlet of the combustion chamber, collects particles greater than 60 microns with 99.5% or higher efficiency. The solids captured in the separator are recirculated through a non-mechanical sealing device back to the combustion chamber.
Cyclones
Cyclones are commonly used for the separation of hot solids. It has a simple construction, since it has no moving parts, and a high efficiency. Cyclones are located in the hot loop of CFB boiler, and hot particles are entrapped and recycled to the furnace bed. Flue gas flows out from the top of the cyclones to the backpass. The efficiency of the cyclone can be improved by several factors:
• Higher entry velocity of the mixture of gas and solids • Larger size of solid
• Higher density of particles • Smaller radius of the cyclone • Lower viscosity
To evaluate the probability of particles captured, one cut-off size is defined as the size of particles that are likely to be collected with 50% efficiency by a given cyclone.
STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boiler Design
(
p g)
in c th V N L d ρ ρ π µ − = 2 9 (17)where, Nc is the effective number of turns made by the gas-solid stream in the separator, normally 5 is assumed.
Higher mixture inlet velocity to the cyclone helps to capture much finer particles and increase efficiency, but the pressure drop of gas through the cyclone increases. The aim of the design is to find the optimum velocity.
Mechanical design of the solids separator varies in both construction and shape. Based on customer preference, fuel fired, unit size and/or cycle condition the separator walls may be steam cooled, water cooled or of refractory construction. The conventional solids separator design is a refractory lined, uncooled cyclone. This type of cyclone is lined with a two-layer refractory (Figure 8). The inner refractory layer is abrasion resistant material to resist the erosive effects of high velocity ash particles. The outer refractory layer, against the metal shell, provides insulation to minimize heat loss and protect the carbon steel outer casing from overheating. The amount of refractory in this type of cyclone is very large and therefore high maintenance costs and availability problems are envisioned. Typically, a cooled separator design is preferred (Figure 8).
Solid Separator for Foster Wheeler CFB Mineral wool Refractory ca. 50 mm Membrane wall FEATURES • Square
• Integrated with furnace • No expansion joints • Membrane walls • Water or steam cooled • Normal insulation
Figure 8: Cyclone design from Foster Wheeler. [2]
U-Beams particle separators
Babcock & Wilcox Ltd (B & W) uses a primary particle separator that functions by impact force. Figure 9 shows the location of the U-Beams in the CFB process. B & W's primary solids collectors consists of 2 rows of U-Beams located within the furnace at the gas exit and 4 additional rows of U- Beams located immediately downstream of the in-furnace U-Beams. Solids collected by the front
STEAM BOILER TECHNOLOGY – Circulating Fluidized Bed Boiler Design
two rows discharge downward directly to the furnace along the rear wall, and return by gravity to the furnace through opening distributed across the width of the unit. Solids collected by 4 additional rows of U-Beams return to solid storage hopper. Figure 10 shows a schematic of the U-Beams. The U-Beams are made of stainless steel. Individual U-beams are in the form of channels, 152 mm wide by 178 mm deep. Two bolts through the water-cooled roof suspend each beam, protected by an enclosure. Dynamics (gas and solids)
stresses, static (dead load) stresses, design temperatures and material creep strength are used to design the U-Beams.
A pan at the lower ends of each U-Beam in alignment accommodates horizontal and vertical expansion. These pans also form a gas barrier at the bottom discharge end of the beams to prevent gas bypassing and improve particle collection.
The erosion is low due to the chromium oxide layer that forms on the stainless steel at the furnace operating temperatures. Lower gas velocity through the U-beam and design with all impact angles at 90 degrees is also favorable. Figure 11 shows the gas flow through U-Beams. [4]