Multiclone dust collectors are commonly used to centrifugally separate coarse particles from the flue gases of biomass (i.e. wood waste) fired boilers. Centrifugal action within parallel arranged cyclones provides the force to separate the particles and accumulate them in the hopper of the dust collector. The particulate contained in the flue gas is a combination of ash, sand and char, with these constituents having a significant variation in specific gravity and size – char particles are light and coarse while the ash and sand particles are fine and heavy. Dust collectors perform more efficiently on coarse heavy particles, and they generally can collect most particles above 15 microns in size. A collection efficiency of 85% is achievable on most biomass fired boilers, because the particulate size analysis shows approximately 15% less than 10 microns in size.
For over sixty years, multiclone dust collectors have been used on biomass fired boilers for the purpose of collecting char particles (for fuel reinjection) and ash and sand particles (for elimination of erosion on ID Fans). Using simple and basic guidelines in the design, erection and operation of this equipment can assure reliability in achieving satisfactory collection results. The information below defines the evolution of the multiclone dust collector design, as well as the shortcomings that have historically resulted in poor collector performance/premature fan failure.
First Generation of Multiclone Dust Collectors (limited access design)
Figure 1 illustrates the first generation of multiclone tube dust collectors having a sloped top tube sheet. This incremental variation in outlet tube length facilitates balancing the flow among individual cyclones, which is a prerequisite to achieving optimum collection efficiency. The inlet zone of this type dust collector provides little resistance for the front row of tubes and considerable resistance to the rear tubes. To help balance the flow, the outlet tube length is varied so the net effect on individual tube draft loss is the same. As a general “rule of thumb”, the number of tubes in the collector is selected for a pressure drop of 2.5” to 3.5” w.g. This rule applies to all dust collectors regardless of tube size or casing design.
Second Generation of Multi-clone Dust Collectors (totally accessible design)
Figure 2 illustrates the second generation of multiclone dust collectors which has a tube sheet arrangement providing total accessibility. Figure 3 shows a photograph of a plugged collection tube which must be accessible for cleaning during a planned maintenance outage, which is not uncommon. It is caused by the gradual stratification of fly ash to cool surfaces during start-up, or from the accumulation of fly ash which intermittently collapses from upper surfaces and is induced into the nearest restricted passage. Second generation multiclone dust collectors provide access alleyways to all inter-compartments for internal area inspection and necessary cleaning and maintenance services.
Before EPA controls and regulations became so stringent, the primary purpose of the multiclone dust collector had been to serve as the air pollution control device. The basic design was even modified in several ways to increase collection efficiency as much as possible. Multiclone dust collectors were made with 6” diameter inlet tubes (which theoretically have a higher collector efficiency). Another variation used two assemblies operating in series. A third improvisation, known as “hopper evacuation”, consisted of evacuating five to ten percent of the gas from the hopper area. A final option, called "secondary shave-off", expounded on the principal of additional removal of concentrated materials from the periphery of the outlet tube. These proposed improvements worked in theory, but failed in practice because of fundamental material handling problems. For example, the secondary shave-off concept resulted in serious pluggage in the small slotted area of the outlet tube. The hopper evacuation concept proved to be a material handling nightmare. Using two multiclone dust collectors in series proved to be the most practical approach to boost dust collection efficiency. However, eliminating stack opacity became impossible, as it is made up of the fine particles (usually smaller than 1 micron) which are beyond the ability of the multiclone dust collector to collect.
Over time the role of the multiclone dust collector changed from being an air pollution control device to that of a pre-cleaner to protect centrifugal fans and other equipment from erosion and abrasion. Figure 4 illustrates what happens to an ID Fan rotor when the multiclone dust collector is not working properly. A secondary benefit of using the multiclone dust collector is the ability of the assembly to concentrate coarse char particles, which have high BTU value. The char can be re-injected into the boiler and may increase boiler efficiency by up to 2%. In many cases the dust collector is installed downstream of the air heater, where the number of tubes and the initial cost can be minimized. However, sometimes the boiler manufacturer will install it upstream of the air heater to add further protection to boiler auxiliary equipment, requiring a larger and more costly assembly since the gas volume is greater at elevated temperatures.
Current Generation of Multiclone Dust Collectors (totally accessible design with larger size tubes)
It is well documented that particles less than 10 microns in size do not contribute to the erosion process of centrifugal machinery. Total collection efficiency has become less of a primary design requirement, while the need to reliably collect coarse particles has become the area of greatest importance. As a result of this logic, the design philosophy has produced the most recent generation of multiclone dust collector. This design includes the features of total accessibility and the utilization of large diameter tubes. Earlier collectors utilized 6” or 9” diameter inlet tubes, while present day collectors use inlet tubes that are 14” and 24” in diameter. Many of the tube assembly components are made out of cast iron with a Brinell hardness of up to 400. Castings of greater hardness are seldom used because of the loss of all ductile properties. The larger diameter tubes provide more reliable collector operation because of the reduced possibility of tube pluggage and the promotion of balanced flow. Tube pluggage problems are caused by the gradual accumulation of fly ash on all internal surfaces, most significantly the annular guide vane area. Over time some pluggage cannot be prevented, but since the larger diameter inlet tube has greater inlet openings the possibility of total bridging is reduced. Figure 5 illustrates the detrimental effect of tube pluggage. Regardless of size, every multiclone dust collector must be inspected and cleaned on a yearly basis.
It has been observed for many years that larger multiclone dust collector modules are simply less efficient than smaller units. This phenomenon is explained by the fact that balance flow must occur if optimum efficiency is to be achieved. Since capacity is directly proportional to tube diameter, the use of a larger diameter tube results in the requirement for fewer tubes. For example, a 9” diameter inlet tube can handle up to 750 ACFM while a 14” diameter inlet tube can handle up to 2000 ACFM. Consequently, the diameter assists the designer in directing internal flow patterns for a more balanced flow condition.
Even when using larger diameter tubes, caution must be taken to install turning vanes in the outlet zone where upward spiral velocities from individual outlet tubes are extremely high. This flow must be guided to the exit area as shown in Figure 6.
Another important feature of the current generation of multiclone dust collectors is the design of the lower portion of the inlet tube of the upper hopper zone. Some manufacturers use the peripheral discharge concept which works on the principal of tangential discharge from three horizontally mounted slots at the base of the tube. This is shown in Figure 7. This same illustration shows the conical discharge, which is the alternate and preferred method of discharging the particles from the inlet tube. The upper hopper area is a very critical zone, since it is in this area that particles are drawn to the bottom hopper area for subsequent removal. Re-entrainment must be prevented in this area. It is imperative that this upper zone contain stagnant gases with no internal gas movement to stir up the particles and prevent their ultimate settling to the lower hopper area. The peripheral discharge device does in fact promote air currents, and discourages the gravitational settling of the particles. At a paper mill in New Hampshire, the power plant superintendent replaced the peripheral discharge devices with conical devices and increased the amount of dust removed from the hopper from seven to eleven truckloads per day. This case history should encourage the use of conical discharge devices at the bottom of the inlet tube.
Since gravitational force moves the particles to the lower hopper area, the obvious movement of gas in the upper hopper zone must be discouraged. A sectional plate between hoppers is almost always incorporated as a means of discouraging flow from one hopper zone to the other. However, it is also important to realize that flow must not occur from one tube to an adjacent tube in the upper hopper zone, since such flow would cause re-entrainment and offer resistance to gravitational settling.
In addition to the previously discussed factors, there are also precautionary measures which must be followed by the operator and erector. One measure is the provision of an air tight hopper and collector casing. Any leakage of ambient air through the rotary valve, double dump valve or field joints cannot be tolerated. If a rotary valve is used to feed the collected material from the dust collector hopper, it must be inspected on a regular basis to assure that an adequate seal is provided between the rotor and the casing. Double dump valves are generally preferred over rotary valves, since continuous operation does not wear operating parts and increase the possibility of air entrainment.
Any leakage between inner compartments cannot be tolerated if reliable collection efficiency is to be expected. Figure 8 is a photograph showing a hole in an outlet tube and a gap in the joint between the inlet tube and the tube sheet. Holes in outlet tubes or gaps in the upper flange of inlet tubes can allow leakage between inner compartments. The inlet section must be isolated from the outlet section, and both of these sections must be isolated from the hopper zone.
Using the simple criteria above for sizing and maintaining a multiclone dust collector will greatly contribute to successful system operation and equipment long life.