How partially-interlaced primary air works and improves recovery boiler operation

Transcription

1 How partially-interlaced primary air works and improves recovery boiler operation by Colin MacCallum, PEng, President, Boiler Island Air Systems Inc. First presented at the Western Canada BLRBAC meeting in Vancouver in March 2013 Keywords: BOILER; RECOVERY; DESIGN; COMBUSTION; AIR; PRIMARY; EFFICIENCY. ABSTRACT Many operational problems in the furnaces of kraft recovery boilers can be traced to the ineffectiveness of the primary air system. A number of recovery boilers have accumulated more than a combined total of 15 years with fully-opposed two-wall primary air jets and have all demonstrated improved operation. Further improvements have been demonstrated in a short trial with partially-interlaced primary air jets on a large unit. In two-wall primary-air mode, most of the primary air is shut off from two opposing walls and almost the entire primary air flow is introduced via the ports on the other two opposing active walls. The powerful, individual air jets from adjacent primary air ports in each active-wall register combine to form a slab of air that is strong enough in combination with the other slabs of air from the other registers to form a flat char bed and to provide combustion air to the entire char bed, without forming the flue-gas chimney that is created by conventional four-wall primary air systems. The improved gas mixing which is achieved results in better combustion, a hotter lower furnace, less carryover and higher thermal efficiency, among other benefits. The two-wall primary air jets and in particular partially-interlaced primary air jets interfere minimally with the secondary air jets and allow the secondary air system to operate in its optimal fashion. A short trial is sufficient to confirm that the system will improve the operation of a particular boiler. Application The two-wall primary air technique is readily implemented in all kraft recovery boilers in which the primary air is directed just a few degrees below the horizontal and can be applied to boilers with steeply-sloping ports if the ports are fitted with inserts or if they are rebuilt to direct the primary air jets a few degrees below the horizontal. Introduction In conversation with recovery boiler supervisors and operators it is apparent that many operating problems, including the following, are a concern to management: Safety issues jellyrolling of smelt Excessive carryover and/or fume High char beds and poor bed control High excess air and low thermal efficiency High TRS, SO2 and CO emissions. Often, these problems are believed to be expensive to correct but, in fact, all of the above problems can be traced to ineffective primary air. For example: Jellyrolling is due to low temperatures at the char bed Carryover is promoted by a central flue-gas chimney High char beds and poor bed control result from low liquor temperatures, but poor combustion at the char bed is also a related factor High excess air is the result of poor gas mixing TRS, SO2 and CO stem from low temperatures in the char bed. Conventional primary air has small air jets from all four furnace walls. Typically, each register has four or five ports, each about 50 mm wide and about 150 mm high. The jets combine a short distance from the port opening, as shown in Figure 1, to form a weak, low-momentum slab of air that extends across the full width of all the walls. Fig. 1 Primary jets combining When the slabs of air from the four walls move towards the centre of the furnace, unobstructed by a char bed, they push the flue gases into a central upward-flowing gas column. Alternatively, these jets, when deflected upwards by the char bed, strengthen the central upward-flowing gas column in the centre of the furnace. Small liquor droplets sprayed into this column are carried upwards into the furnace. The stronger the jets from the four walls, the more powerful is the central gas column and this interferes with the secondary air jets and further increases carryover. The foregoing conceptualization can be confirmed by CFD modelling. For example, CFD modelling 1 by Yen-Ming Chen, Dave Burton and Iryna Leontyuk of Alstom Power, shows the four-wall primary air jets being deflected upwards by the char bed and into the centre of the furnace. As illustrated in principle in Figure 2, there is a strong, central flue-gas chimney which persists above the tertiary elevation despite an excellent secondary air system comprising a 5+5 partially-interlaced pattern of jets. The modelling indicates strong interference between the primary air jets on adjacent walls in the corners; this interference reinforces the central gas chimney. Page 1 of 6

2 T S P Tangential Tertiary Partially interlaced S When a third elevation of air is added just above the primary air and then designated the secondary air, the flow to the uppermost, tertiary elevation is reduced to some 15% of the total flow, the secondary elevation receives some 50% and the flow to the primary air system is reduced to about 35%. Since the primary air flow is reduced, then fewer primary air ports are required to maintain the same primary air velocity as before. Similarly, when a fourth elevation of air is added, for example as an upper secondary elevation below the liquor guns, still more air is generally transferred from the primary elevation to the new secondary elevation. Again, still fewer primary airports are required to maintain the same primary air velocity as before. Throughout this procedure of adding elevations of overfire air, the total primary air flow is being reduced and primary ports are eliminated or partially blocked such that the primary air register pressure is maintained around mm w.g. and the port velocity is unchanged. That is, the primary air jets are still weak and do not reach the centre of the furnace; they are deflected by the char bed as before and create the same central flue-gas chimney as described. Fig. 2 CFD Model results in principle - Note restriction created by primary air persists above tertiary elevation The weak primary air jets create a hot zone around the periphery of the furnace but do not reach the centre of the char bed. So, with a low char bed, there is a large, lowtemperature zone of poor combustion on and above the centre of the char bed, especially in large furnaces. This zone of the bed, being cool, is a source of TRS, H 2S and CO. With a high char bed, then the bed is cut off by the secondary air jets. There is still a hot zone around the periphery of the char bed, but the slopes of the pile are relatively cool and the top is hot because the secondary jets are delivering combustion air to the top of the pile. Tavares, Tran and Reid 2, show that there is twice as much fume generated from a char bed with uneven temperatures. Excessive fume constitutes dead load and is to be avoided. Based on the above, logic tells us that all recovery boilers with conventional four-wall primary air suffer from this chimney, to a greater or lesser extent. The problem is more pronounced at higher loads and/or higher primary air flows. Primary Air and Multiple Overfire Air Elevations The simplest recovery boiler has only two elevations of combustion air: primary air, comprising 60-70% of the total air flow below the liquor guns and one elevation of secondary air comprising 30-40% of the air above the guns. It does not matter how well designed your secondary air system is the interference from the four-wall primary air jets affects the secondary system adversely. Since this flue-gas chimney persists in spite of welldesigned secondary air systems, the boiler designer should be striving for a primary air system that does not create the chimney in the first place. Challenge to Design a Better Primary Air System An effective primary air system will provide: combustion air to the entire char bed surface no interference with the secondary air jets, that is, with minimal chimney created a hotter lower furnace a simpler, less expensive system. In order to provide combustion air to the entire char bed, higher primary air pressures are required perhaps mm w.g. However, operators relate higher pressures to higher primary air flow and their experience has shown them that carryover and/or excess air then increase, often to unacceptable levels. In comparison with the proposed higher pressures at the primary air elevation, a modern two-wall secondary air system with a register pressure of mm w.g. is quite normal. Typically these powerful jets from opposing walls extend beyond the furnace centreline and create the jet pattern desired by the designer. The jets are strong enough to resist the upward flow of the furnace gases and thus remain relatively horizontal and do not of themselves create a central flue-gas chimney. Page 2 of 6

3 The high char bed observed in many conventional furnaces has a peripheral trench about a metre wide, carved out by the primary air jets which push the char away from the wall to form a rampart of char. The width of the trench is dependent on the strength of the jets. Some char is burned in the trench and some char is pushed away from the walls until the jets' energy is insufficient to widen the trench any farther. In order to move the char farther away from the wall and to have the air jets penetrate farther into the furnace, the jets must be larger that is, they must have a higher momentum, Now, momentum = (mass X velocity), so a higher momentum can be obtained by increasing either the mass of air in the jet or the velocity of the air jet or by increasing both the mass and the velocity. The higher the momentum of the jet, the farther it penetrates into the furnace and the greater is the jet's ability to affect its surroundings by entraining more and more furnace gases into the jet, as shown in principle in Figure 3. In practice, the primary air jets are angled downwards at about 5 degrees in some boilers and more steeply in others. With higher-momentum jets, the gas mixing is improved and thus combustion is improved. gas mixing and combustion. Thus, with strong two-wall primary air jets which provide better gas mixing and which deliver oxygen to the centre of the furnace to provide better combustion, the temperature at the char bed is increased and the reduction efficiency is higher in spite of the low char bed. Each primary-air register has four or five ports, each about 50 mm wide, so the combined width of, say, four ports is 200 mm. In comparison, a modern secondary air port is about 150 mm wide. The centreline of the slab of combustion air from each register is directed at an angle of some 5 degrees below the horizontal. The height of the slab increases with distance from the port opening in the normal fashion for a free jet and the lower side of the air slab impinges on the char bed about half a metre from the active wall. The slab is not deflected by the weak upward flow of flue gases from the char bed. The average velocity of gases from the char bed is around 0.25 m/s; the upward velocity is somewhat higher close to the ports because of the more intense combustion there, but the jets are more powerful close to the walls and are not deflected by the upward-flowing gases, Two-wall primary air has almost all the primary air from the two active opposing walls, therefore both the air weight and the air-jet velocity through the ports on the active walls are virtually doubled. Thus the original register pressure of say, mm w.g. increases to mm w.g. These large principal jets have four times the momentum of the small, conventional jets and sweep low in to the centre of the furnace, entraining furnace gases on the way. Small scavenging jets are useful in the corners to sweep char out of the corners and into the path of the principal jets. By minimizing piling in the corners, they minimize burndown time prior to a shutdown. Figure 3 Large, high-momentum air jet and entrained gases Thus, with a higher air pressure in the primary air registers, the slabs of air from the two active walls have a much higher momentum, so they provide better gas mixing by entraining the furnace gases from above and below the slab of air. The jets are more powerful, so the char which is not burned is pushed away from the walls. With strong primary air jets from only two opposing active walls, in a fully-opposed arrangement, as shown in Figure 4, in which each arrow represents the combined air jet from a register with four or five air ports, the char is pushed towards the centre of the char bed to form a ridge of char across the furnace and parallel to the active walls. This mode of operation creates a relatively low, flat char bed. As they exit the ports, the jets have 21% by volume of oxygen; at the centre of the furnace, about 6 m from the port in a large furnace, the oxygen content of the large jet is about 3% but the residual velocity and momentum of the jet, which, at the furnace centreline, is composed mainly of nitrogen and entrained furnace gases, is still high enough to move the char to the centreline of the furnace; a ridge of char has been observed in a number of boilers operating with fully-opposed primary air, as shown in Figure 4. Ridge of char Scavenging jets Again, operators believe that a low char bed means poor operation and a low reduction efficiency, because a low char bed occurs when the liquor temperature is high, in which case much of the liquor is burned in suspension and carryover increases. However, the boiler can run well with a low char bed if the char is burned more rapidly by improving Fig. 4 Fully-opposed jets Partially-interlaced jets Page 3 of 6

4 When a partially-interlaced arrangement of jets is adopted, with each large jet opposed by a smaller jet, as shown in Figure 4, then the large jet can penetrate beyond the centreline of the furnace and create a pattern of jets that minimizes the upward velocity extremes and minimizes carryover. There is no ridge of char and the bed is flat. Another obvious benefit of the higher port air-mass, velocity and momentum is that the ports stay cleaner and the rodding frequency can be reduced. In a new or rebuilt boiler, a smaller number of somewhat larger primary air ports on a wider spacing can be used because the more powerful jets readily entrain the furnace gases from between adjacent ports. Two-wall primary air may sound revolutionary, but, before 1985 four-wall secondary air was standard and ineffective as discussed by MacCallum and Blackwell 3, 4. Nowadays, the secondary air systems comprise large ports and powerful air jets arranged in various patterns, such as fullyinterlaced, partially interlaced and stacked air. So - if we can arrange the secondary air in various patterns, then we can select the most promising of these arrangements for the primary air. Operating Experience with Two-wall Primary Air Variants of two-wall primary air have accumulated more than 15 years of operating experience with fully-opposed jets, as shown in Figure 5. All of these boilers have the primary air ports directed at about 5 degrees downwards, except the boiler at Mackenzie which was a B&W boiler with steeply sloping primary air ports. Fig. 5 Operating years with fully-opposed primary air Blackwell et al. 5 describe the trials with two-wall primary air at several mills, including the operation on the original CE boiler at Crestbrook Forest Industries, Skookumchuck, BC. Interestingly, a full set of port rodders had been ordered for one of the boilers at Northwood, but, when the rodders arrived, half were were installed on the active walls on each boiler. These boilers ran for 13 and 14 months with two-wall primary air until smelt and char entered the dampered inactive ports on one boiler and the mill reverted to four-wall operation on both boilers and ordered more port rodders. The Crofton boiler with its 11.5 m deep furnace, had forty air ports blocked with refractory on the sidewalls and operated with fully-opposed primary air jets from the front and rear walls for more than four years. The char bed was generally flat and low, with good temperature distribution. All the boilers indicated in Figure 5 experienced higher temperatures in the lower furnace between 50 and 160 C higher, than when they operated with conventional four-wall primary air. Generally, the primary air flow could be reduced and thus the thermal efficiency of the boilers was slightly higher. Advantages of Increased Temperatures in the Lower Furnace The high-momentum, powerful primary-air jets provide much better gas mixing than small, conventional jets, so combustion is more intense and the temperature at the char bed is higher. The benefits of high temperatures at the char bed are explained by Blackwell and King 6 and include: more capacity less odour increased thermal efficiency better recovery operation. Blackwell and King list many advantages of higher temperatures at the char bed, including the following: increased radiant heat transfer to the furnace walls, so the furnace exit gas temperature is decreased increased smelt reduction efficiency. reduced formation of TRS, SO 2 and SO 3 reduced stickiness of particles in economizer and electrostatic precipitator because of less bisulphate formation less corrosion in electrostatic precipitator and flues because of decreased SO 2 increased rate of liquor drying less char formed and increased rate of char burning with increased rate of liquor incineration, a coarser spray can be used, lowering the point in the furnace where maximum temperature occurs and reducing entrainment of liquor particles decreased risk of blackouts less carbon in smelt leaving the furnace reduces green liquor dregs lower caustic consumption in flue-gas scrubbers. Secondary Air Coverage The pattern of secondary jets affects the flue-gas flow and composition in the upper furnace. A major factor is the coverage of the furnace plan area. At first glance, secondary patterns with relatively few jets in plan view provide poor coverage of the furnace. However, the coverage is increased by the use of large, high- Page 4 of 6

5 momentum jets which entrain furnace gases from the region surrounding each jet see Figure 3. However, if the jets are too far apart, then there will be regions between the jets that are unaffected by the jets. For example, the two-high stacked-air arrangement at The Pas has, on each elevation, only four air jets, comprising a single jet close to each sidewall issuing from the rear wall and a pair of jets fairly close together on the boiler centreline, issuing from the front wall. The effectiveness of this arrangement is dependent on a high momentum of the air jets to provide sufficient gas mixing in the spaces between the air jets. At the Pas, the CO content of the flue gases is high up to 4000 ppm, so the gas mixing provided by the secondary air system is insufficient to burn the CO. The boiler operates with a low char bed, so much of the liquor is being burned in suspension and this may also contribute to the high CO. A two-wall, more effective primary air system with a minimal flue-gas chimney would allow more liquor droplets to be burned on the char bed. Thus, more of the CO would be burned before it reaches the secondary air elevation. Residual CO would be distributed more evenly across the plan area of the furnace and the weaker flue-gas chimney would allow the secondary air to be more effective. On the other hand, a fully-interlaced secondary air pattern provides better coverage of the furnace plan area. This pattern is, however, like the stacked-air pattern, loaddependent, which means that if the jets penetrate short of the opposite wall, then the flue gases from the char bed can bypass the ends of the jets. If the jets are too powerful, they can impinge on the opposite wall creating areas of stronger upflow at the wall. The fullest coverage of the furnace plan area is provided by a fully-interlaced arrangement of secondary air jets. As noted above, this pattern also minimizes the upward velocity extremes and minimizes carryover. However, the flue-gas chimney created by the primary air jets is not necessarily eliminated even by a good secondary air system referring again to Figure 1. It was not until several years later, however, that it was appreciated that partial-interlacing would also be advantageous at the primary elevation. The scavenging jets solved the problem of char piling in the corners. New patents were eventually obtained 8. Partially-interlaced Primary Air Ideally, furnaces would operate with no excess air, but, because gas mixing is imperfect, recovery furnaces are designed for 5-8% excess air, equivalent to about 1.3% O 2 by volume, dry, in the furnace. That is, there is lots of oxygen in the furnace to provide complete combustion. When combustion is incomplete, as indicated by a high CO content in the flue gases, then poor gas mixing is certainly part of the problem. Another contributing factor in the generation of CO is the trend towards lower and lower amounts of primary air, with no compensating improvements in the design of the primary air delivery arrangement. The strong, high-momentum primary air jets in a two-wall system penetrate well into the furnace, they entrain furnace gases into the jets and improve gas mixing. The improved mixing improves the combustion and thus the fireball in the furnace occurs lower down and the expensive heating surfaces of the furnace are better utilized. In the fall of 2011, a large Metso boiler operated for several days in a trial with a preliminary arrangement of partiallyinterlaced primary air and similar improvements were demonstrated. During the trial, only the primary air was adjusted neither the secondary air nor the tertiary air, nor the liquor-spraying systems were altered. The furnace was 11 m square and the char bed was flat and well controlled, as shown in Figure 6. The CO content of the flue gases in furnaces with fullyinterlaced and partially-interlaced secondary air is typically ppm. Why Partially-interlaced Primary Air? In 1988, Sandwell conducted a confidential study which was funded by The BC Science Council, 12 pulp companies, the BC Ministry of Environment and Sandwell. Using a 1/12th scale physical model, two-wall primary air was found to reduce the upward velocity extremes in the furnace. Partially-interlaced air jets at the secondary level eliminated the chimney in the centre of the furnace and provided more or less plug flow upwards that is, the upward-velocity extremes were more or less eliminated. In 1992, MacCallum 7 described the development of partial interlacing and presented its advantages. Figure 6 Large Metso furnace operating with partially-interlaced primary air The partially-interlaced arrangement was preliminary in that the primary air from the sidewalls was not shut off as much as a the optimum arrangement would require, because an air pressure of mm w.g. was maintained in the sidewall ports to ensure that char did not enter any of the sidewall registers. The large jets from the active walls were created with windbox pressures of mm w.g. and the small jets were created by pressures of about 50 mm w.g. The primary air was 25% of the total combustion air and the Page 5 of 6

6 CO was around 20 ppm during most of the trial, with one spike to 50 ppm and another to 60 ppm. A significant advantage of the partially-interlaced arrangement is that the air jets can be introduced from the front and rear walls when the smelt spouts are on the front or rear walls, or from the sidewalls when the spouts are on one or both sidewalls. High-solids firing seems to be the ultimate in modernizing a recovery boiler. However, it is still difficult to operate with low O 2 because of the limitations of conventional four-wall primary air. Partially-interlaced primary air can be expected to improve this situation. Reasons to Adopt Partially-interlaced Primary Air The main advantages of partially-interlaced primary air are that the arrangement: improves gas mixing at the primary elevation and allows the boiler to operate with lower excess air, thus improving thermal efficiency increases the temperature in the lower furnace and improves reduction efficiency minimizes spiking of TRS and SO 2 utilizes the furnace heating surfaces more fully provides smooth flow of smelt improves char bed control allows good operation with a low char bed which minimizes burndown time prior to a shutdown minimizes carryover by eliminating the flue-gas chimney and minimizing the upward-velocity extremes reduces dregs eliminates quite a number of air ports and port rodders from the inactive walls can use fewer, larger ports on the two active walls in a new or rebuilt furnace allows the port-rodding frequency to be reduced on the active walls because the higher air velocity in these ports keeps the ports cleaner employs fewer primary air ports and associated equipment, so the capital cost of a new or rebuilt boiler is lower. By the simple expedient of blocking selected air ports on the inactive walls, the technology, described in the Canadian and US patents 8. can be applied to all recovery boilers where the primary air is directed slightly downwards, say 5 degrees below the horizontal. Even if the boiler in question runs like a watch, the technology can be expected to further improve the operation. Where a revamp of the secondary and tertiary air systems is being considered, before proceeding with the revamp, a short operating trial with partially-interlaced primary air should be conducted or, alternatively, the partially-interlaced primary air technology could be modelled with the existing system. The secondary and/or tertiary systems may not need to be modified when a more effective primary air system is employed. When a furnace rebuild or a new boiler is being considered, the capital cost savings in ports and ducting and associated equipment such as port rodders, are considerable. Operating Trials with Two-wall Primary Air Because BIAS has 15 years of experience with two-wall primary air systems, a long operating trial on another boiler is unnecessary - a couple of hours is plenty to demonstrate an improvement. A trial is easy to set up, the implementation of the technology extends to the blockage of selected primary air ports, the boiler is easy to run and the system improves the operation of any secondary air system and the overall operation of the entire recovery boiler. The operating cost savings and operational improvements are readily assessed. In today's world, cost savings are always welcome. References 1. Yen-Ming Chen, Dave Burton, Iryna Leontyuk, Alstom Power Inc,, Comparison of secondary air system arrangements in a recovery boiler using CFD modeling techniques, International Chemical Recovery Conference, Charleston, VA, USA, Alarick J. Tavares, Honghi Tran, and Timothy P. Reid, Effect of char bed temperature and temperature distribution on fume generation in a kraft recovery boiler, TAPPI Journal, September MacCallum, C., Blackwell, B.R., Modern kraft recovery boiler air systems, TAPPI International Chemical Recovery Conference, New Orleans, LA, USA, MacCallum, C., Blackwell, B.R., Critical review of kraft recovery boiler liquor-spray and air systems, Pulp & Paper Canada, 88: 10 (1987). 5. Blackwell, B.R., Hambleton, H. and Brown, E., Twowall primary air in kraft recovery boilers, CPPA, Whistler, BC, May Blackwell, B.R. and King, T., The importance of high temperatures in the lower furnace of kraft recovery boilers, Pulp & Paper Canada, 87: 1 (1986) 7. MacCallum, C., Towards a superior recovery boiler air system, International Chemical Recovery Conference, Seattle, WA, USA, US Patent 7,694,637 and Canadian Patent 2,429,838 Method and apparatus for a simplified primary air system for improving fluid flow and gas mixing in recovery boilers issued April 13, 2010 and February 17, 2009, respectively. Author: Colin MacCallum, PEng President, Boiler Island Air Systems Inc. maccallu@shaw.ca Tel: Page 6 of 6