| This power station has operated residual oil fired 16.5 mw boilers for 13 years without the need for fireside physical cleaning. Design level steam production was maintained and there was no accumulation of ash deposits on tubes and air pre-heaters. Increased heat transfer, longer running time, less soot blowing, and less maintenance have reduced power generation costs. |
| Periodic fouling and physical cleaning of fossil fuel boilers are always a problem. The process takes time which is frequently needed for other projects . Many factors must be considered when a unit is to be scheduled for a physical cleaning. The cost of replacement energy can be determined in advance of a shutdown, as well as the cost of the cleaning program. However, another cost which should be considered is the gradual and progressive reduction in heat transfer as residues accumulate on heat exchange surfaces. As ash is deposited throughout the boiler it causes fouling of the superheater tubes, economizer tubes and air pre-heater surfaces. The net result is a lowering of heat transfer rates. Soot blowing will remove some of the deposits but that is not always enough to keep heat transfer rates at or near design level. As heat exchange surfaces get dirty, higher fuel firing is needed to maintain steam pressure. It is usually necessary to increase ID fan openings resulting in further losses of available heat. Consequently, there is a lowering of the effective use of the BTU input. Eventually, the boiler must be shut down for physical cleaning. |
| At an east coast municipal power station the annual outage during the summer of 1989 marked the start of the 13th year since there was a fireside cleaning of the 16.5 mw base loaded boilers. The units are inspected every year for fireside residue problems, lane blockage, and any other condition which would indicate that physical cleaning was needed. The last physical cleaning was in 1976. |
| The 16.5 mw boilers were built by Babcock and Wilcox and were installed in 1961 as side by side units. They were supplied with coal and gas burners and have the following specifications: |
| STEAM DRUM RESSURE | 1025 psi |
| STEAM FLOW | 165,000 LBS. PER HOUR |
| MAIN STEAM TEMPERATURE | 900°F |
| MAIN STEAM PRESSURE | 850 psi |
| LJUNGSTROM REGENERATIVE AIR PREHEATER |
| 3 GAS PASSES, EACH WITH A STEAM SOOT BLOWER |
| In the latter part of 1972 the boilers were converted to firing # 6 residual fuel oil. Each boiler was equipped with 4 return flow mechanical atomization burners. After the conversion the fuel oil, as fired, was: |
| BTU/LB | 18,581 | 18,698 | 18,154 |
| BTU/GAL | 148,150 | 148,670 | 150,025 |
| SULFUR | 1.75% | 2.03% | 2.90% |
| ASH | 0.05% | 0.04% | 0.09% |
| VANADIUM | 50 ppm | 144 ppm | 300 ppm |
| SODIUM | 58 ppm | 81 ppm | 100 ppm |
| The Oil Supply Was: |
| # 1 tank - 2 million gallon capacity, Useable level of 1.5 million gallons. | |
| # 2 tank - 500,000 gallon capacity, Useable level of 400,000 gallons. | |
| Hot water coil heating. | |
| Storage oil temperature of 125°F. | |
| No oil tank sludge treatment. | |
| Occasional draining of water from the Bottom of each tank. | |
| Tank rotation every 3 months. |
| Operating Information: |
| Fuel consumption per boiler at full Load - 32,980 gallons per day | |
| Fuel oil pressure of 600 psi, Recirculating system | |
| Fuel oil temperature at burners is 190 to 210°F, based on viscosity tests | |
| Boiler O2 range is 2% to 3% | |
| Carbon Dioxide level is 12% | |
| Air preheaters water washed every 6 months | |
| Strainers cleaned once a week |
| Soot blowing 3 times per day, each with: |
| 12 minutes warm up | |
| 5 - 6 minutes in each gas pass | |
| 15 minutes in the regenerative air preheater |
| It is a known fact that # 6 residual oil will produce vanadium deposits on the superheater tubes. Also, there will be other types of residues and deposits on the heat exchange surfaces. While soot blowing can keep some of the gas passes open, it does not remove tightly bound deposits. Therefore, a magnesium oxide front end fuel oil additive was used . From 1972 to 1975 many types of front end additives were tried. |
| To one degree or another, all of them were offered as a way to keep the residues on the boiler tubes friable for easy removal during the soot blowing cycle. The boilers started firing fuel oil in the latter part of 1972. Before the summer of 1973 it was obvious that the boilers were getting dirty. It was subsequently determined that irrespective of the front end additives being used and soot blowing 3 times a day, before the end of 5 months of operation the ID fans were running flat out; and also, there was a gradual drop in the steam temperature. When the boilers had to be shut down, the physical cleaning program required 7 men for 4 hours per boiler using 3,000 - 5,000 psi pressure water washing. |
| General criteria used to evaluate progressive loss in heat transfer are: |
| Reduction in superheater steam temperature | |
| Reduction in the air preheater gas inlet temperature | |
| Increase in the air preheater gas outlet temperature | |
| Increase in the air preheater outlet draft | |
| Increase in the ID fan damper opening |
| The above conditions were noted through 1973 and continued into 1975, even with full physical cleaning programs in the summer of 1973 and also in 1974 . In the early part of 1974 , and again in 1975, in addition to the regular summer outage cleaning, the boiler had to come down for air preheater cleaning and whatever additional cleaning could be done in the available time. Naturally, it was important to get the boilers up and running as quickly as possible. After the physical cleaning in February 1975 the air preheater outlet draft was - 8.3" H2O. By May 21,1975 the air preheater outlet draft was up to - 11.5" H2O. Generally, a boiler should be shut down when the air preheater draft reaches - 14.6" H2O. During the time between February and May 1975 there were 3 soot blowing cycles per day, and a front end fuel oil additive was used. Also, during this period, the ID fan damper opening gradually increased to a wide open position. The following temperatures will indicate the loss in heat transfer within a few months after a physical cleaning. The temperatures were taken at a steam load of 165,000 lbs. per hour. |
| 2/27/75* | 3/21/75 | 4/16/75 | 5/21/75 | |
| Air Preheater Gas Inlet | 710 | 580 | 688 | 730 |
| Air Preheater Gas outlet | 357 | 285 | 340 | 370 |
| Superheater Steam | 775 | 900 | 860 | 840 |
| *Prior to shutdown for cleaning |
| On may 22 , 1975, without a physical cleaning, a special fireside treatment program was started in both boilers. The magnesium oxide addition to the fuel oil to boiler # 1 was stopped and the soot blowing was reduced to once a day. The magnesium oxide addition was continued in boiler # 2 with full soot blowing cycles 3 times a day. The fireside product was added once a day to boiler # 1. Boiler # 2 was treated 3 times a day with 1/3 of the calculated daily amount needed. The amount of chemical added to the boiler was based on the fuel consumption. At the start of this program the ID fans were already up to the maximum opening, and the draft loss across the air preheaters was considered marginal but satisfactory. From May 1975 to the shutdown in July 1975 there was no further loss of superheater steam temperature in either boiler, and there was no further increase in the draft loss through the air preheaters. All other data indicated that both units were operating in a stable condition, without the expected drop in operating efficiency. |
| During the physical cleaning in July 1975 Boiler # 1 which had no front end fuel oil additive since may and only 1 soot blowing cycle per day cleaned up faster and easier than did Boiler # 2. Therefore, after the physical cleaning, both boilers were converted to operation without a front end fuel oil additive, with only 1 soot blowing cycle per day, and with the fireside product added after the soot blowing. These boilers normally operate 7 to 8 months per year. This fireside product allowed the units to stay on line until the next scheduled outage. During that time, the combustion side blowing was reduced to twice a week , but the air preheaters were still blown once each day. In July 1976 the boilers were again water washed. However, the cleaning program was much faster and only a firehose with city water pressure was used. That was the last time the last time the boilers had to be physically cleaned we had to physically clean. |
| From July 1976 to July 1977 the superheater steam pressure held at 880°F to 890°F. An inspection in July 1977 confirmed that the boilers did not need to be water washed. The tubes were clean, the lanes were open, and the boilers looked good. The boilers operated at peak efficiency from start - up to shutdown. One purpose for using soot blowers is to remove residues in order to improve heat transfer. Another purpose is to keep the gas passes open. A drop in exit gas temperature after a blow would indicate an increase in heat transfer somewhere in the system. However, it is usually noted that within a few hours after a blow, and certainly before the next blow cycle, the exit gas temperature will be back to the higher level. The soot blowing in these boilers was now 1 - 2 times per week and was done only to be sure the blowers were functional. The tubes were kept clean and the gas passes were open as a result of the chemical action of the fireside product being used. This chemical action resulted in better heat transfer and more efficient boiler operation. The fact that the boilers did not need to be physically cleaned, and the fact that the ID fan damper openings remained the same from start - up to shutdown represent only a fraction of the actual benefit. In April 1978 Boiler # 1 was shut down for inspection . The boiler tubes and the air preheater looked good. Also, the cold end section was dry. Boiler # 2 was not inspected at that time since it was felt that the conditions would be essentially the same as in Boiler # 1. In august 1978 both boilers were maintaining a superheater steam temperature of 880°F. Prior to the use of the fireside chemical the blowers were never able to maintain this condition. Any loss in steam temperature means that a portion of the heat is not effectively used. The heat loss values shown in Table I can be used to estimate the benefits which could be obtained by preventing residue and ash accumulation on boiler tubes and other heat exchange surfaces.In order to review the performance of the boilers, the entropy, or the work which is done by the steam, can be determined. However, when one is looking for differences in the processes or for differences in the effects of the heat which is supplied to the system, the enthalpy, or the heat content of the steam, provides a more reliable method by which comparisons could be more easily made. The BTU values in Table I are not corrected for all of the boiler functions since we are only concerned with the differences in the heat content of the superheated steam at various temperatures. The steam pressure was maintained at 850 psi. The value calculations are based on a steam flow of 165,000 lbs. Per hour, at a fuel oil heating value of 148,948 BTU per gallon, and a fuel oil cost of $ 14. 95 per barrel. Table I provides an easy method to estimate the value of the loss of heat transfer. Figure II shows the enthalpy of the superheated Steam at various temperatures. The BTU value of superheated steam at 900 degrees F and 850 psi is 1454.00 per lb. of steam. The BTU value of superheated steam at 775°F and 850 psi is 1382. 01 per pound of steam. The Heat loss factor is 66.27 BTU per pound of steam. At a steam flow of 165,000 pounds per hour per boiler, the heat loss rate is 11.87835 x 106 BTU per hour of operation per boiler. Using an average fuel oil heating value of 148,948 BTU per gallon, the loss of heat transfer is equivalent to 79.7483 gallons or 1.8987 barrels of fuel oil per hour of operation per boiler. That is the amount of fuel oil which would be fired and for which there would be essentially no heat transfer to usable steam. There is a significant financial advantage when maximum heat transfer is maintained, and boiler downtime is kept at a minimum. |
| In all start - up programs the loss of heat transfer during the 1st and 2nd month of operation is not as high as during the 3rd and subsequent months. However, it is still a loss. In addition, increased id fan power consumption is an increase in station service. |
| In conclusion, the desired heat transfer level was maintained and, at the same time, operating and maintenance expenses were reduced. Those benefits resulted in a lower cost for the energy produced at this power station. |
| SH STEAM °F | ENTHALPY BTU | BTU/HR x 10 6 | $ LOSS/HR |
| 900 | 1454.00 | 239.9100 | 0.0000 |
| 890 | 1448.28 | 238.9662 | 2.2551 |
| 880 | 1442.56 | 238.0224 | 4.5102 |
| 870 | 1436.84 | 237.0786 | 6.7654 |
| 860 | 1431.12 | 236.1348 | 9.0205 |
| 850 | 1425.40 | 235.1910 | 11.2756 |
| 840 | 1419.68 | 234.2472 | 13.5308 |
| 830 | 1413.96 | 233.3034 | 15.7859 |
| 820 | 1408.24 | 232.3596 | 18.0411 |
| 810 | 1402.52 | 231.4158 | 20.2962 |
| 800 | 1396.80 | 230.4720 | 22.5513 |
| 790 | 1391.08 | 229.5282 | 24.8065 |
| 780 | 1385.36 | 228.5844 | 27.0616 |
| 775 | 1382.01 | 228.0316 | 28.3825 |
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