KRAFT PULPING IN CANADA

Transcription

1 KRAFT PULPING IN CANADA RECOVERY BOILER AND AUXILIARY OPERATIONS Presented to Alkaline Pulping Committee, Prince George, British Columbia September 26, 1983 CANADIAN PULP AND PAPER ASSOCIATION TECHNICAL SECTION ALKALINE PULPING COMMITTEE R. T. Boughner, P. Eng. Fort Frances, Ontario Kraft Production Manager June 20, 1983 Boise Cascade Canada Ltd

2 KRAFT PULPING IN CANADA RECOVERY BOILER & AUXILIARIES R. T. Boughner, P. Eng. Kraft Production Manager Boise Cascade Canada Ltd. Fort Frances, Ontario ABSTRACT This paper summarizes the results of a survey carried out by the Alkaline Pulping Committee in covering operating practices, design and maintenance. It examines the recovery unit and also key peripherals and represents over half of all Canadian kraft pulp mills. 1. INTRODUCTION This paper is the publication version of a project of the Alkaline Pulping Committee, a group of individuals with varying degrees of either technical or production backgrounds primarily on the fibre line side of kraft pulp manufacturing. Most of us have less experience with recovery boiler operations but recognize that with their physical size, capital cost, economic impact and technical complexity, they have major significance in our process. Historically and traditionally, there has been an understanding gap if not a communications gap between the Steam and Recovery chief and the Pulpmill people. This has not arisen from a lack of literature material but most technical papers, almost by definition, are written by experts. This paper, however, digests the results of questions asked and results interpreted from the Pulpmill perspective. It was not intended to pass judgement or make critical comparisons among differing operating practices. The purpose of this paper is to help Pulpmill people to appreciate and understand the important design and operating characteristics of recovery boilers. It attempts to cover why factors are as they are and to explain what is commonplace and what may be unique. The basic source of data used was the 1981 Recovery Boiler survey. Survey data are cited to support interpretation. Overall, at survey time there were 56 recovery boilers in operation at 46 Canadian kraft pulp mills. This summary includes

3 data on 40 recovery units at 27 mills and represents an 80% response to the questionnaires we issued. The common unit of capacity for recovery boiler systems is millions of pounds of dry solids per day in the fired liquor. One air-dry ton of full bleach softwood yields about 3200 pounds of solids, hardwood yields about 2800 pounds and linerboard 2500 pounds. The responding size range in our survey was from 0.50 to 4.50 million pounds per day. The age range spanned over 30 years from 1945 to The summary of general characteristics is outlined in Table I. Some of the key dimensions of six representative Canadian recovery boilers are shown in Table II.

4 Survey respondents were asked about their respective layout of facilities and operational responsibilities. In all cases the recovery boiler and power boilers are run by the Steam and Recovery Department. In half of the mills, the Pulpmill runs the evaporators and in all but one, the Pulpmill operates the recausticizing system. The feedwater system and the evaporators are virtually always located very near the recovery boiler. The only closely tied process which is often remote from the recovery boiler is the recaust system. This is generally in cases where it is under the Pulpmill jurisdiction. Among the responding mills, we found cases where the recovery boiler crew also looks after power boilers, recausticizing, evaporators, turbogenerators, effluent treatment, and feedwater treatment. In general, a three-man crew is essentially dedicated to the recovery boiler: an operator, a helper and a utility or spout man.

5 2. GENERAL CHARACTERISTICS From 0.50 million pounds of solids per day in 1945, capacities have increased almost by a factor of ten to nearly 5.0 million in 1980 but the basic elements of recovery boiler design have changed very little over the past 40 years. The basic design concept of a steam generating heat recovery unit mounted atop a tall combustion chamber remains as developed about Although there are now five potential suppliers available, the European firms Gotaverken, Ahlstrom and Tampella have no installations in our country yet; there are only two domestic suppliers of recovery boilers in Canada: Babcock & Wilcox and Combustion Engineering. Individual differences between the two suppliers are a major part of the heritage of the industry. Some of the major differences are as follows: C-E uses tangent wall construction with 3" tubes on 3" centres; B&W uses membrane wall construction with 3" tubes and 4" centres with a membrane bar between the tubes. B&W uses splash-plate liquor nozzles which oscillate up and down while rotating axially to the left and right. The nozzles are sized so that in the range of 1.5 to 3.0 million, two guns are used: one on the front wall and one on the back. C-E uses atomizing nozzles which oscillate only up and down. On units over a 2.0 million rating, guns are mounted in all four walls: either two or three in each wall. During the past few years there has been a trend to stationary firing in which there is no oscillation of the guns. This is done to stabilize the combustion zone and to minimize carryover of liquor droplets. B&W has traditionally used three levels of combustion air injection and aims to develop good turbulence with the upper level. C-E up until recently used only two air levels and the upper is injected tangentially to develop a cyclonic effect. Since survey completion, to meet the needs of increased production, there has been much developmental work carried out on where to add the air and in what pattern. The objectives have been to maximize steam generation efficiency and solids throughput while satisfying TRS emission constraints. B&W uses concurrent flow of hot exhaust gases from the combustion chamber through the heat recovery equipment, with the coolest steam from the steam drum being heated by the hottest flue gas. C-E uses countercurrent flow so the final hottest steam is heated by the hottest flue gas.

6 Aside from the recent air and gas mixing work mentioned above, some other more equipment-dependent changes have occurred over the past 20 years in odour control technology. Black liquor oxidation was the preferred process for odour control in the 1960's. With such oxidized liquor, contact with flue gas is a much reduced problem. TRS emissions in the range of ppm can be achieved. All wet-bottom precipitator systems in our survey were installed in conjunction with oxidized liquor systems. Since 1970, the preferred method for odour control has been complete elimination of contact between liquor and flue gas. This so-called low-odour technology is what has been successful in reducing TRS emissions to the trace level, generally less than 3 ppm. Black liquor concentrators rather than direct contact evaporators are used exclusively in systems with dry-bottom precipitators.

7 3. CONCENTRATION OF BLACK LIQUOR Cascade evaporators represent the Combustion Engineering approach to direct contact evaporation of strong black liquor up to firing concentrations. Although only one has been installed in Canada since 1970, cascades are a significant factor in recovery system operations: there are 17 of them represented in our survey responses. There are several different possible arrangements. For small boilers up to about 0.6 million, a single-wheel unit is installed. On larger installations, a double-wheel unit is employed; either an over-and-under arrangement or a side-by-side layout. Cascades have a vertical downcomer gas entry from the economizer. The gas then flows through the rotor bars, entering on the falling side then exhausts from the back to the precipitator. Cyclone evaporators are the Babcock & Wilcox way of using the sensible heat in the furnace flue gas to bring the feed liquor up to firing concentrations. The adiabatic humidification principle is the same as for the C-E cascade. The major difference is that the cyclone sustains liquor movement by an external recirculation system rather than an internal rotor. Although there have been no cyclones installed in Canada since the late 1960's, there are still about 35 in operation. The only real variation in cyclone installations is in number and size. Very small units have only one but all others have two identical in parallel. They consist of a thin-walled vertical cylindrical shell with a height-to-diameter ratio of :1. The cyclone has a conical bottom with pump sump, a rectangular tangential gas inlet in the side near the bottom, and a concentric circular outlet on top. Concentrators using indirect steam heating for the evaporation of black liquor to 65-68% solids have been in use only since These have until recently been essentially long-tube evaporators with scaling inhibited by increased liquor velocities in the tubes. This is developed by external recirculation, internal multiple passes or some combination of the two. More recent concentrator installations since the time of our survey go even further in their attempts to retard scaling: these systems are capable of attaining up to 75% concentration. A number of different concentrator configurations have been employed in Canadian mills. From an energy economy standpoint, the best arrangement is a double-effect concentrator with the second effect vapours vented into the evaporator train. This has been done in several mills. In another, the second effect

8 vapours vent to a double-effect pre-evaporator. In still another, a retrofit double-effect concentrator is so remote from the evaporator train that vapour ducting was not practical; the concentrator has its own dedicated surface condenser. In general though, combined considerations of energy conservation and capital cost lead to the selection of a single-effect concentrator with the vapours venting into the evaporator train as far upstream as possible.

9 4. COMBUSTION AIR AND AUXILIARY FUEL Combustion air for the recovery boiler is supplied by the forced draft fan and passes through the air heater to raise its temperature to degrees F before admission to the furnace. The typical installation has only one F.D. fan which supplies all levels of air admission and which is located at a high elevation in the building so it can draw in air preheated by radiation loss from the boiler. The fan is generally mounted in a bottom vertical discharge arrangement. These exceptions were also noted: a few installations with two F.D. fans, one for primary air and the other for the rest; some with the fans mounted on the ground floor with horizontal flow through the heater; some with flue gas to preheat the air in a regenerative tubular heater. Flow through the air heater is generally vertical downflow. Most heaters consist of several banks of finned steam coils mounted one above the other. Normal steam pressure is in the range of psig. On some systems, two steam pressures are used; the lower pressure for the first one or two banks and the higher pressure for the rest. There are three true purposes for auxiliary fuel: to initiate combustion before firing liquor, to stablize combustion during upset periods and to burn out the char bed for shutdown after liquor firing is stopped. The most common auxiliary fuels in use are natural gas and bunker "C" oil. C-E boilers normally have four auxiliary fuel burners, one in each corner. These are startup burners in the strictest sense because they can carry only 10-20% of maximum steam generation. Any need for more steam from the boiler requires installation of load-carrying burners at the secondary air registers level. B&W systems have four to eight auxiliary burners. Steaming capacity without liquor is 60-70% of maximum. Unlike a C-E, a B&W boiler is normally brought on line and brought up to 60-70% of maximum steaming rate before firing liquor. In spite of this capability, any recovery boiler regardless of make, should be equipped with a load-carrying burners if it is expected to routinely burn a supplementary fuel. Otherwise, the low level admission of auxiliary fuel will seriously hinder the chemical function of saltcake reduction.

10 5. BLACK LIQUOR PREPARATION AND FIRING Low odour recovery circuits have heavy black liquor storage tanks providing an average of ten hours of storage capacity. Liquor from storage is pumped to the mix tank system. When direct contact evaporators are used, a direct bleed-off of products goes to the mix tank. With a dry-bottom electrostatic precipitator located at the ground floor elevation, there are often two mix tanks; the first one receives the saltcake return from the recipitator. The saltcake mix tank is a well-agitated vessel of minutes retention time serving three or four functions. It is the point of addition for the purchased saltcake makeup to the recovery cycle. It provides the source of recycle liquor for sluicing accumulated chemical ash from the boiler and economizer hoppers. It houses the primary liquor heater, a circular run of pipe on the tank bottom with upward facing steam nozzles to heat the liquor from to degrees F. It also receives the saltcake catch returned from the precipitator. The mix tank is located on the second floor level to ensure a good positive head on the firing pumps. From the mix tank, the heavy black liquor is pumped through the secondary liquor heater. This provides the final heating to firing temperature which is degrees F for viscosity control in a B&W boiler and degrees F for improved atomization in a C-E. Since the time of surveying, there has been a trend starting to favour indirect heating of liquor in order to avoid the dilution and efficiency loss which results from direct heating. Combustion Engineering boilers have 4 to 14 liquor guns. Of the furnaces surveyed, each wall can have 0, 2, 3 or 4 guns; facing pairs of walls each have the same number of guns. The nozzles provide mechanical atomization which produces a conical sheet of coarse drops. The guns are oscillated vertically and the liquor drops flash dry as they fall freely through the hot gases to the hearth. This spray drying operation is best accomplished at a pressure of psig and a liquor concentration of 65-68% solids. Babcock & Wilcox units have normally two liquor guns ; one in the front wall and one in the back. Boilers smaller than 1.0 million rating have only one gun and the largest surveyed at 4.5 million rating has four, one in each wall. The nozzles utilize a splash plate to produce flat sheet of drops spraying across to hit the opposite wall. The guns are oscillated in two modes at once; vertical and rotational, the result is a spray painted band of drying liquor which builds up on all four walls. When the accumulation of char gets heavy enough, it breaks off under its

11 own weight and falls onto the hearth. The wall drying operation is best at about 35 psig and 60-65% solids. The quantity of chemical makeup to the black liquor system varies widely from mill to mill. It has to compensate for losses from the precipitator stack, brown stock washer carryover, evaporators discharges and a number of possible losses from the recausticizing system. The most prevalent form of makeup is saltcake or sodium sulphate. Whether black liquor oxidation is employed or not, sulphidity control is required. To raise it, elemental sulphur or sodium hydrosulphide is employed by our surveyed mills. To lower sulphidity, caustic soda is the most common sulphur-free form of sodium-rich additive.

12 6. COMBUSTION AND CHEMICAL RECOVERY The purpose of primary combustion air is to burn out the carbonaceous material from the char bed. It also controls the bed shape and height for best reduction. There has to be enough primary air to develop sufficient heat release to sustain the endothermic reduction reaction and melt the inorganic products. On the other hand, primary air flow has to be held low enough to maintain reducing conditions; the reason that not all the air can be added at that level. Primary air is added through numerous ports in all four furnace side walls near the hearth. A typical unit has about ports in each of the four walls. They are about three feet above the hearth in a B&W and four feet up in a C-E. The primary air is 40-55% of total in a B&W and 60-65% in a C-E. Air admission is at a relatively low pressure of 3-4 inches of water to minimize bed disturbance. In a C-E boiler, the remaining 35-40% of the total air is supplied as secondary at 25-30' above the hearth, injected at 5 inches of water through four specially designed registers, one in each wall. They are arranged tangentially to develop a good swirl to give intense mixing to burn off all volatiles and complete the combustion. In a B&W unit, there are two other levels of air injection; 20-40% goes in as secondary through ports in all four walls about 7-10 feet above the hearth and 15-30% as tertiary from two opposite walls about feet above the hearth. The secondary supply pressure is 5-8 inches of water and it provides the heat necessary to dry the char on the walls. The tertiary air is at 6-12 inches of water to develop the violent turbulence required to complete combustion and burn out the char particles. One of the most common operating problems associated with liquor firing is blackouts in which the char bed grow beyond its normal confines and partially blocks off air admission through the primary ports. Survey respondents were asked about blackout causes in their mills. The most commonly mentioned were liquor solids high in inorganics (45%), low solids liquor (30%), char bed falling over (30%), smelt ledges near the primary ports (30%) and defective nozzles (30%). Problems arising from poor liquor combustibility can be partially offset by increasing the temperature of the liquor (up to the limit of carryover) or of the combustion air. Although more recent than our survey results, modern continuous control systems for recovery boilers use thermocouples, bed monitoring cameras and continuous monitoring of carbon monoxide

13 and TRS compounds to predict and avert blackout conditions which are especially critical on heavily loaded units.

14 7. SMELT DISSOLVER SYSTEM The purpose of the smelt dissolver system is to dissolve the molten inorganic chemicals (roughly 70% sodium carbonate, 25-30% sodium sulphide and 1-5% sodium sulphate) which drain off from the furnace hearth. The dissolver must also safely dissipate the large quantity of heat released. Provision must be made for the resulting green liquor to be a suitable and consistent quality for subsequent causticizing. All surveyed recovery systems below a rating of 2.0 million employed a single circular dissolver tank. On larger units the two manufacturers differ: B&W uses a single oval-shaped tank with semi-circular ends with smelt spouts on the back side but C-E uses two identical circular tanks, and has two sets of spouts, one on each side of the boiler. The smelt is discharged to the dissolver through either 2, 3 or 4 water cooled spouts on each tank. The cooling water supply must be properly conditioned to avoid excessive corrosion. In spite of that, spouts have to be changed out regularly to maintain safe operation. Most mills surveyed (60%) change them every year. Other reported frequencies were 4-6 months (10%) 9 months (5%) and 24 months (15%). Corrosion of the dissolver tank itself has been a long-standing continuing concern, particularly for mills with significant chlorides or sulphide concentrations. Floor construction was mild steel in 40% of the responses but stainless steel was used in 35%, concrete in 20% and brick in 5%. Wall construction has also been subject to a lot of trial and error with mild steel (45%), stainless steel (30%), and brick and gunnite (5% each) being used. The remaining 15% used a mix of mild steel and stainless steel with stainless being used in the vapour phase and at the interface. Life expectancies for dissolver lining ranged from years. Good agitation is essential in the dissolver to avoid localized excess concentrations. A variety in design is evident. Side mounted horizontal shaft agitators were used in 85% of systems. The 15% using top mounted vertical shaft agitators all reported having had mechanical problems; although not fully conclusive, this design may be a configuration to avoid. When the hot smelt at 1550 degrees F runs into the dissolver, the stream tends to be very cohesive and run in slugs, particularly at sulphidities below 27% AA. In all cases, the stream must be shattered to avoid explosions at the liquor surface. This can be done in either one stage with steam shattering jets or in two

15 steps with steam and green liquor respectively. A lot of heat is released when the smelt dissolves in the liquor. The normal dissolver operating level is 50%: this provides sufficient volume for vapour release and explosion cushion. As the released vapour escapes through the vent, green liquor may be entrained in it. Weak wash showered mist eliminators installed in the stack were commonly used in the 1960's. Venturi scrubbers have been used more recently. To ensure uniform operation of the Recausticizing system, good uniform control of green liquor density is required. The most advanced automatic system reported in the survey was a twin bubble pipe arrangement to determine density in the dissolver. This, in turn, regulates the automatic control valve in the weak wash supply line. Many examples of a lesser degree of automation were reported but all relied on green liquor density to set the weak wash flow. More recent developments include the use of conductivity and temperature differentials coupled with two-stage major and minor dilution configurations. The range of green liquor concentrations was reported g/l TTA as Na 2 0 with a density range of The survey also asked about reduction: data reported ranged 85-99%.

16 8. COMBUSTION GASES HEAT RECOVERY The exhaust gases which leave the combustion zone at degrees are reduced about degrees F by radiation to the water-cooled wall tubes in the 4 to 5 seconds it takes to reach the top of the furnace chamber. The bull nose deflects the exhaust gas at degrees F over toward the front wall; the gas flow then sweeps back toward the heat recovery section at a perpendicular approach for optimum heat transfer. The bull nose also protects the superheater elements by sheltering the lower ends from direct char bed radiation. The screen tubes consist of 3-inch tubes on 24-inch side spacing and a 6-inch back spacing. This screen of water-cooled tubes serves to cool the gases to below the freezing point of the chemical ash entrainment. With this material already in the solid state as it leaves the screen section, the tendency to slag in the superheater is greatly reduced. The basic principle of soot blowing is the use of multiple jets of high pressure steam to clean the tube surfaces. Steam pressure varied from boiler to boiler and within each the quoted range was psig. Most mills were unable to report their soot blowing steam consumption; for those who did, it ranged from pounds per ton. The superheater's prime function is elevating the boiler's product steam temperature; the superheater completes the cooling of the flue gas to prevent slagging in the boiler section. Modern superheaters generally consist of panel-like assemblies of 2-inch tubes. Tangent assembly prevents ash from accumulating all around a tube and as a result makes slag easier to knock off. The platens are suspended on a side spacing of 12 to 14 inches. The generating section is made up of the upper steam drum and the lower mud drum. Connecting the two drums are vertical banks of tubes, bent to meet the drums radially and rolled in. There are normally no baffles in the generating section so the flue gas goes through in a single pass at feet per second. Temperature is reduced by convective absorption. The purpose of the economizer is to serve as a heat sink to improve the overall thermal efficiency of the recovery unit as a steam generator. It does this be pre-heating feedwater. The flue gas entering the economizer is about 750 degrees F. The discharge temperature is 600 degrees F for a conventional boiler arrangement where the gas goes on to a direct contact evaporator. For a low-odour operation where the flue gas goes direct to the

17 precipitator, the gas must be cooled to the degrees F range. In spite of the best efforts at continuous soot blowing, the sticky nature of the chemical ash from the furnace eventually fouls the heat transfer surfaces in the steam generating section. Thorough washing of the surfaces with water is required on an intermittent basis to restore good heat transfer and flue gas flow. The frequency of water washing requirement varies with the number of sootblowers and success of operating cycle strategy; it is primarily determined by the degree of overload. Among surveyed mills, those operating at or less than rated capacity required washing only twice a year: at the start of each major shutdown. At the opposite extreme, three very heavily overloaded units are washed every 8-10 weeks. The recent trend to lower pressure drop finned-tube economizer configurations has reduced wash frequencies on a small number of overloaded boilers.

18 9. STEAM GENERATION SYSTEM Feedwater from the deaerator storage tank is pumped by a 5 or 6 stage centrifugal pump into the economizer inlet header. The feedwater control mechanism is of the 3-element type: based on steam drum level, feedwater flow and steam flow. The economizer is made up of a lower inlet header and an upper outlet header with the two of them connected by a bank of tubes. To prevent gas side condenstion and corrosion, most systems run the feedwater temperature at 275 degrees F or more - the range is degrees F. One survey response reported two water passes but all others have only one vertical upflow pass. The heated feedwater from the top of the economizer is collected in a manifold and crosses over into the steam drum. The drum is equipped with internal cyclone separators to prevent water carryover to the superheater. There is a continuous blowdown valve on the steam drum which discharges about 2-3% of the feedwater feed to purge solids from the system. The actual steam generation itself occurs in the generating section which is a large bundle of tubes, generally 28 feet high, which connects the upper steam drum and lower mud drum. Baffling within the upper drum ensures that the downstream 30% of the tubes are reserved for downflow of colder incoming water and the upstream 70% are used for upflowing saturated boiling water. The circulation is by convection. The incoming makeup is from the economizer and the outgoing product passes into the superheater. The mud drum forms the lower header for the generating section. Water flows from the mud drum through the two vertical downcomers to the waterwalls and through a dedicated horizontal header to the screen tubes. The two feedwater downcomers, one from each end of the mud drum feed the main waterwall supply header under the furnace and into the upflow wall tubes on all four sides of the furnace. The front wall tubes bend to form the roof then feed right into the upper drum. The rear wall tubes form the bull nose then come up right into the steam drum between the superheater and generating section. The two side walls each have an upper collector header which then feeds into the steam drum. Saturated steam leaves the upper steam drum through the cyclone separators and though cross-over pipes to the primary superheater inlet header. The amount of superheating required varies from mill to mill but the amount of combustion gas cooling ahead of the generating section is quite constant. This means that the

19 screen tube area varies inversely with superheater area and is in fact determined by it. In a B&W boiler, the steam flow is concurrent with the flue gas. It enters right behind the screen tubes and exits next to the generating section. In a C-E, the flow is countercurrent; entering next to the generating section and leaving from next to the screen tubes. The C-E superheater is two-stage with an intermediate desuperheater or attemperator for close control of steam temperature. The B&W superheater has two banks of plantens in the primary and one in the secondary. The final steam pressures and temperatures vary from mill to mill. Our survey indicated a pressure range of 400 to 850 psig and a temperature range of 650 to 825 degrees F.

20 10. ELECTROSTATIC PRECIPITATOR After the exhaust gas leaves the economizer of a low-odour operation or the direct contact evaporator of a conventional boiler system, it has been cooled to the degrees F range. It is still heavily laden with saltcake carryover from the furnace at a concentration of grains per standard dry cubic foot in a conventional boiler and twice that in a low-odour unit. This corresponds to a loss of or pounds per ton respectively and recovery is imperative from both economic and environmental standpoints. Over the years, various forms of flue gas scrubbing have been tried on a stand-alone basis. There are still several recovery systems operating in Canada with only a scrubber on the flue gas; the newest was started up in With very few exceptions, electrostatic precipitation is the key to economic and environmentally acceptable purification of recovery boiler flue gas. There has been quite a history of design evolution but all electrostatic saltcake precipitators have these three general components in common: a casing complete with ducting and fan to confine and direct the gas flow, a system of high voltage ionization electrodes complete with a source of high voltage electrical current and finally a system of grounded collecting electrodes complete with a mechanism to effectively remove the collected material. The first casing material employed for saltcake precipitators was concrete-filled glazed tile followed later by hollow glazed tile. There were a number of tile-casing precipitator units in our survey (30%). These were among the older systems with startup ranging from Some had been later repaired by lining with stainless steel. Another 45% reported the use of stainless steel as a casing material; these were newer, ranging from The remaining 15% included one of aluminum which has stood up well so far and the rest were a combination of stainless and mild steels. The power supply systems for the earliest precipitator installations were based upon vacuum-type electronic tubes. Newer units now use silicone diode rectifiers instead. Most often, the combined transformer-rectifier set is installed together in one sealed housing. Automatic power controls provide for optimum performance at varying loads of volumetric gas flow and dust concentration. These canned T-R sets are most often mounted on the roof of the precipitator casing to minimize the length of cable runs. Modern control strategy is to modulate the

21 power supply on feedback control from a continuous opacity measurement. The precipitator casing is filled with a network of vertical wires on a side spacing of 9-10 inches and a back spacing of about 10 inches. Each field of wires is energized by a transformer-rectifier set. The wires have to be supported from insulators in sealed compartments. The bottom ends of the wires must be prevented from swaying, otherwise a short circuit to the collecting electrodes will develop. There are two common anti-sway measures: the older bottom pendant weight and the newer bottom rigid frame. The wires themselves may be of two distinct types: a smooth monofilament or a specially designed type with uniformly spaced spikes or barbs to pormote a uniform corona discharge. To clear the inevitable gradual accumulation of saltcake, the electrode wires have to be shaken periodically, either pneumatically or mechanically. The collecting electrode system consists of a series of sheet metal plates mounted parallel to the gas flow, mid-way between the parallel rows of wires. These plates are grounded so they can be supported direct from the casing without concern for insulation. The plates have to be prevented from bowing and warping, either by corrugation or by attachment of stiffener strips. The energized saltcake particles are electrically attracted to the collecting plates and then have to be knocked off to the bottom by rapping. Both the emitter wires and collector plates have to be rapped to remove the accumulated saltcake. There were two distinctly different rapper types reported in our survey: the top-mounted pneumatic cylinder and the rotary shaft-driven swing hammer. More recent developments include electromagnetically induced mechanical striking. The very first electrostatic precipitators on kraft mill saltcake applications used the pyramidal bottom hoppers transferred from other industries. The results were disastrous due to the stickiness of the collected product. The next generation was a primitive version of a dry-bottom discharger. This was also of little success due to mechanical problems. None of these first and few of the second stages of evolution are still in operation. About 50% of current precipitators have wet-bottom collection either with a bottom-driven agitator system or a sloped flooded bottom flushed by liquor from flowbox and discharging into a collector trough. The major drawback of this wet-bottom type of operation is the tendency of the hot flue gases sweeping across the liquor surface to cause increased emissions of TRS.

22 This led to further upgrading of dry-bottom technology. The newest 50% of precipitators are of the dry-bottom design with a drag chain to remove the saltcake from the flat floor of the gas flow chamber. The next conveying step to the mix tank could be any number of designs: a sluice box, a narrow drag chain, a ribbon screw or a solid-flight auger. Normally, the precipitator exhaust gas goes to the induced draft fan. Of the survey reponses, 10% had the I.D. fan before the precipitator but 90% had it downstream. The I.D. fan is normally a double suction fan with one suction for each chamber. There is a single discharge to the flue gas stack. To control the furnace draft, the I.D. fan is on variable speed drive, either a steam turbine (50%) or an electric motor (50%). To keep the fan rotor free oo any accumulation of saltcake, there is nomally a system of high pressure steam blast jets to keep the blades clean. As a further precaution, many mills now have continuous vibration monitors installed to prevent self-destruction of the fan if an imbalance develops. During the past fifteen years there has been a definite trend in overall precipitator dimensions from short and wide to longer and narrower. The flue gas discharging from the precipitator is ready for discharge to atmosphere in most caes. There are a few examples of secondary dust collectors such a MoDo scrubber, Teller scrubber or another precipitator but these are few in number. The fan discharges the exhaust gas to the stack which is designed to release the gas feet above the ground level at feet per second for good plume dispersion.

23 11. GENERAL MAINTENANCE STRATEGY Maintenance service effectiveness is essential in all areas of the kraft pulp mills but in the case of a recovery boiler, it is even more critical because of the relatively long times required for shutting down and restarting. Our survey made a distinction between mills with one, two and three boilers to see if there was any major difference in approach to maintenance. The only impact was that mills with more than one recovery boiler tend to stagger their shutdowns and to continue operation at a reduced rate while one unit is down. All recovery boilers covered by our survey are shut down at least twice a year for major maintenance; two of them three times. Total downtime ranges for 4 to 12 days averaging 8.5 days. Only two boilers were reported to have suffered an explosion requiring extensive downtime for repair. Both were screen tube ruptures which caused an explosive water leak onto the smelt bed. Of the installations queried, 92% had pneumatic instrumentation and only 8% electronic. This is a reflection of the average age of the boilers in question since there is now a strong trend toward electronic instrumentation on new installations. Higher level controls are now starting to make strong advances but at survey time, only 10% had supervisory control system and 5% had full fledged computer control. Questions from the 1964 survey were repeated relating to centrifugal pumping of concentrated black liquor. Packing was still on 67% with mechanical seals on 33% but a strong trend toward seals is expected due to the favourable energy impact of keeping water out of the concentrated liquor.

24 12. METALLURGY AND DESIGN Metallurgical selection has been of critical importance over the years as design evolution has led to steady increases in steam temperature and pressure. Materials presently in use include the following: Welded low-strength carbon steel tubing SA-178 A is used in a number of older economizers and generating sections and a very few superheaters. Maximum design temperature is 850 degrees F. Seamless low strength carbon steel tubing SA-192 is widely used throughout the newer units except in the hotter zones of the superheater. Its only difference from the former is the forming technique which improves its temperature rating to 950 degrees F. More recent superheater applications have utilized a molybdenum alloy SA-209 TIA with a design temperature of 975 deg F and two chromium-molybdenum alloys, SA-213 T11 and SA- 213 T22 with temperature ratings of 1050 degrees F and 1125 degrees F respectively. The first two materials have the same moly level but the first has no chrome. The third is higher than the second in both chrome and moly. The above mentioned alloys are used for water walls but in the interest of improved corrosion resistance in the firebox, composite tubes are starting to gain favour. The inner 85% of the wall thickness is SA-210 1A1 carbon steel for strength and the outer 15% is Type 304 austentic stainless steel for corrosion resistance. This is rapidly gaining favour for both new installations and rebuilds.

25 ACKNOWLEDGEMENT On behalf of the Alkaline Pulping Committee, I would like to thank the people at the following mills for their contributions to this project: Smooth Rock Falls, Marathon, Fort Frances, Newcastle, Crofton, LaTuque, Port Mellon, Cariboo, Pontiac, Chaleurs, Crestbrook, Red Rock, Quevillon, Dryden, Thunder Bay, Intercon, Irving, Terrace Bay, Thurso, Northwood, Prince George, Nackawic, Hinton, Tahsis and Kamloops. REFERENCE LIST 1. RICHARDSON, D. L. and MERRIAM, R. L., "A Study of Black Liquor Recovery Furnace Firing Conditions, Char Bed Characteristics and Performance",Arthur D. Little Inc., December HENDERSON, J. S., "Final Survey Results for Noncontact Recovery Boiler Electrostatice Precipitators, I. Precipitator Design Features", Tappi, December "Steam, Its Generation and Use", Chapters 26 and 28, Babcock and Wilcox, FRYLING, G. R., "Combustion Engineering", Chapter 27, Combustion Engineering, WHITNEY, R. P. ed., "Chemical Recovery in Alkaline Pulping Processes", Chapters 3 and 5, Technical Association of the Pulp and Paper Industry, MACDONALD, R. G. ed., "Pulp and Paper Manufacture, Volume I, The Pulping of Wood", Chapter 10, McGraw-Hill, "Cottrell Electrical Precipitators", Western Precipitation Corporation. 8. SMITH, E. L., "Modern Recovery Unit Design in the Sulphate Pulp Mill", Pulp and Paper Magazine of Canada, CARLAW, D. N., and GREEN, L. G., "The Air Smells Sweeter at Miramichi", Pulp and Paper Magazine of Canada, December, SMITH, E. L., "Kraft Mill Chemical Recover Units - The Third Generation", Paper Trade Journal, November 2, 1964.

26 11. "Summary of Replies to Recovery Furance Questionnaire - Alkaline Pulping Committee", Technical Section, Canadian Pulp and Paper Association, January, "C-E Chemical Recovery and Steam Generating Units", Combustion Engineering, Inc., BACKTEMAN, E. H. and MARR, R. Y., "The Art of Obtaining High Solids Black Liquor for Direct Firing", Pulp and Paper Magazine of Canada, December, 1971.

27 List of Diagrams

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44 CHAR SMELTER, HEAT RECOVERY STEAM GENERATOR AND BATCH RECAUSTICIZING Vintage ~1930