1 OPERATIONAL AUDIT OF PULVERIZED COAL FIRED BOILERs AT AN UTILITY POWER PLANT- By K.K.Parthiban / Director / Venus Energy Audit System / India Introduction The plant availability and efficiency are improved when an audit is performed. An audit is a focused activity which brings out points for improvement. Power plant managers are generally preoccupied with routine activities. This is a fact for almost all power plants. Any external audit brings out the scope for improvement. The cost of an audit is nothing compared to the savings achieved by preventing an unforeseen shut down by a single observation. The cost of audit gets justified by a fractional efficiency improvement. This article is about the observations made by us in a power plant during an operational audit. About the boiler & water treatment This operational audit was conducted in 2 x 330 MW units at this power plant. A good point was that the deterioration of performance could be identified by comparison of performances of the 2 units. The availability of the boilers as such was informed to be good. During discussions with operation engineers, it was learnt that there was no perennial / unsolved problem in the boilers which affect boiler availability / performance. The boilers are once thorough boilers with condensate polishing system. The present treatment is purely volatile based. Both hydrazine and ammonia are volatile nature and pose no problem. The unit is copper free and hence there is no issue on the water side. To know water side issues usually drum is checked. In this case there is no drum. Only tube sampling and deposit analysis will give the efficiency of the internal water treatment system. Performance The two major issues related to PF boiler performance are the airheater leakage and the unburnt carbon. Air heater seal system decides the loss of heat via the leakage air. It is recommended to upgrade to better seals that are available in the market. Unburnt carbon is decided by the combination of the input grade of coal to the boiler & boiler operational practices. The two aspects are discussed separately below. Air heater leakage impact on efficiency The air heater leakage is estimated by using the CO 2 / O 2 values measured before and after the airheater. It was seen that the oxygen before the airheater is 3.27 % and at the outlet the oxygen was seen to be
2 6.83%. The % air leakage as per the formula was worked out to be 22.8%. The fuel fired at the time of audit was South African coal. The boiler efficiency was worked out to be 87.62% for an oxygen level of 6.83% in flue gas. The airheater leakage was generally considered as 10% for the airheater with proper seal (new). The corresponding oxygen level in flue gas will be 5.03%. The efficiency for this optimum oxygen level was worked out to be 88.27%. The efficiencies are compared below: For flue gas oxygen level of 6.83% For flue gas oxygen level of 5.03% The penalty due to high airheater leakage is 0.65%. This is to be weighed against the benefits of going for better sealing systems available in the market. Unburnt carbon impact on efficiency / Fuel consumption The unburnt carbon ranged between 6.5% and 8% at the time of audit. This was high. Unburnt carbon is dependent on the FC/VM ratio, ash content, Inherent moisture and the ash in coal. The South African coal FC/VM ratio was seen to be For this coal the impact of LOI% on boiler efficiency is discussed below. The efficiency was also calculated with 2.5% LOI with oxygen at 6.83%. The results are compared below with the case of 6.5 % LOI. For flue gas oxygen level of 6.83%- LOI of 6.5% For flue gas oxygen level of 6.83% LOI of 2.5% The gain available in achieving LOI of 2.5% is about 0.73%. A 4% reduction of LOI will get a benefit of 0.73%. Every 1% LOI reduction will result in efficiency improvement by 0.18%. Nowadays it has become necessity to reduce the LOI in the fly ash from ash disposal point of view.
3 Review of the statistics of unburnt carbon in the past On request the best LOI achieved in unit 1 & 2 were received from the statistics department along with the coal analysis. The FC/VM ratio was 1.88 on 1 Feb The fly ash LOI in Unit 1 was at 3.22% and at unit 2, the same was 2.4%. Review of PG test report PG test report is an important document. Since the units were about 16 years old and persons have changed, the reports could not be located. In many plants we find that this not being referred by the operating engineers. The importance of this document goes to the fact that the OEM engineers try the best performance out of the boiler. A comparison with current parameters generally point out the issue that is to be addressed. Review of present unburnt carbon % in fly ash and Flue gas Combustion is about achieving less CO in flue gas and less LOI (loss on ignition -unburnt carbon in fly ash) in fly ash & bottom ash. CO levels were found to be below detectable limits. Being a tangential fired system, the VM burning is seen to be at its best. However the unburnt carbon ranges from 6 % to 9 %. The daily report from lab did not correlate to the characteristics of the coal fired. There was no consolidated data with relation to fuel fired. The operation / performance engineers are requested to appreciate the relationship between FC/VM to unburnt carbon. It is also to be noted that more the ash percent, lesser will be the LOI due to dilution by ash. Our observations which relate to improvement of combustion 1. Fineness of coal Fineness was being checked only when there was a fuel change. It was not monitored on a fixed time period. Fineness was not checked with respect to hours after the mill was serviced. Fineness was not being correlated to HGI of coal. Higher HGI is easier to grind. Currently used coals have 62 HGI which is more than the standard HGI 55 for which the mill performance is generally guaranteed. Fineness is checked by ASME method in vertical lines. However the boiler maker had given the
4 sampling points in horizontal lines. There would be error when sampling is done in horizontal lines. Rotating probe as per ISO 9931 should be used for the representative sample. Fineness should be compiled in a report form given above. When the past mill outlet fineness data was compared, it indicated that the mill D was a problem. Hence mill D was inspected. The rolls were seen to be in good condition. The throat clearance was least at the bowl end. However the classifier vanes were found plugged with foreign material. The top plate above the classifier was found eroded and developed big holes. Both these defects have an effect on fineness in the mill output. 2. Measurement of refuse / reject bulk density Mill rejects can be analyzed to check the grinding ability of the mill. At present there is no monitoring of mill refuse. In the past there had been a measurement of coal % in total refuse from all mills. This was discontinued. Samples of mill refuse were taken from the mill and analyzed for bulk density. The report showed that coal not being crushed by mill E. See report below. Mill E rejects bulk density showed 864 kg/m3 which was caused by presence of coal. The coal particles were separated manually. It was seen that 89% of the refuse was coal. This was informed to operating engineer in charge. Mill E needed internal inspection. We recommend that our report format must be used by the operating engineer in order to keep a watch on the performance of the mill. This can help to keep the mill power consumption under control. High fuel loss via rejects can be quickly identified, once the report is generated.
5 If a mill is unable to grind it would certainly reflect on the refuse. This report will give confidence to DCS operator. DCS operators tend to have a belief that a particular mill is not good. But it should be on facts & figures. Such essential data should be made public unit-wise & mill-wise. The data on the hours lapsed after the service should be correlated. 3. Testing and reporting unburnt carbon in ash The report on unburnt carbon in fly ash was somewhat simple. It was pointed out that the coal & its FC/VM ratio must appear in a consolidated report. Comparison with past performance would always help the plant engineers to identify any problem quickly. In addition the fly ash from ESP fields must be analyzed for % wt & combustibles in +75 µm and -75µm ash particles. The report above, which was prepared with the help from lab personnel. There are +75µm particles in ash to an extent of 13%. The unburnt % is 31% in this part of ash. The overall unburnt contribution by coarser ash is 40%. Reducing +75µm particles would improve the combustion. In fact, some of the -75µm particles are generated out of fission of the +75µm particles. The -75µm particles is to an extent of 87% and its LOI is 6.9%. It implies that the combustion needs to be improved by methods discussed below. A. The +75µm particles are to be reduced by adjusting classifier setting. It was found that the operating team was under the impression that the mill DP was to be controlled by the classifier setting. It is true that the mill DP can be brought down by opening the classifier at wide open condition. But it leads to compromise on efficiency. When the mill DP is brought
6 down, coarser particles are transported to the furnace. When a mill is unable to deliver the desired output, operator will tend to open the classifier. B. The input coal size required for mill by design is 25 mm and below. This is to be corrected first. The larger size particles, particularly harder particle will lift off the roll and pass coal in to the rejects. Many experienced boiler / mill manufacturers recommend that the raw coal size to mill should be - 25 mm. Ideal size is 19 mm and below. A pre-crusher should be incorporated at the coal handling plant. C. Fineness can be improved by adjustment of roller pressure. D. Fineness can be improved by adjustment of the classifier vanes. The purpose of the classifier vanes is to improve the fineness. If the mill is unable to deliver, the coal mill DP will increase. Mill DP can be allowed to be more as long as the required PA flow is achieved. E. In order to bring uniformity in mill fineness from all mills, the setting of the mill classifier vanes must be made identical. We studied the optimum classifier position is 3 to 3.5. Keeping the classifier vane position at 2 or 1 increases coarser particles percentage in coal going to boiler. F. Operators were advised to switch to setting of the classifier vanes to 3 immediately. See the comparison of vane openings and angle for various settings. It can be seen that at position 1-2, the air will hit the vortex finder shell and create turbulence. G. Operators inform that the mill condition is not good and it calls for adjustment of the classifier vanes to permit coarse coal in order to meet the generation. This can be an illusion. It is advised to display the mill service date & due date for service at the mill bay itself. It can be as below. This can be displayed in the DCS hall as well. Type of service Type A Type B Type C Mill A Mill B Mill C Mill D Serviced Next Serviced Next Serviced Next Serviced on due on on due on on due on on Next due on
7 4. Settings of secondary air dampers The setting of secondary air dampers at field was reviewed. There were 5 dampers in closed condition. The indication at the DCS was wrong. These were informed to C&I staff and were corrected. Field signals need to be verified on regular basis. 5. Balancing of lines Each mill outlet piping length differs as per the layout to different corners. Accordingly there will be difference in PA flow. This will automatically create an unbalance in coal feeding to different corners. For this purpose fixed diaphragms were provided by the boiler maker. The status of the diaphragm dimensions was not available. Only visual inspections were made during type A & B service. Only in type C service the diaphragms are replaced. The balancing of lines in reality can be checked by collecting the samples for a fixed period of time. This measurement was not possible in the present layout for two reasons. One reason was that the sampling points were in the horizontal lines. Second reason was that the inaccessibility. 6. Tilting of burners The tilting of burners has to be in unison. This was not so. We marked the burner tilt angles at the locations & took photographs to show the plant engineers. The C& I team attended to this after it was informed. The work was partly attended. There could be mechanical problem. The burner nozzles may have undergone distortion. This had to be seen in the shutdown.
8 7. Rotating sampler The present sampler system available for coal line fineness measurement is fixed type. This is suitable for vertical line only. Rotating sampler as per ISO 9931 must be procured for representative measurement of the fineness. The same kit can be used for measuring the unbalance in coal feeding. The same sampler can be used for checking the unbalance in coal flow between the different lines. Such unbalance study may be done once in six months. Pitot tube shall also be procured for measuring the air velocity in the line. This will give an idea about the air flow unbalance. If the unbalance is very high, we should go for adjustable orifice plates in the future. Many plants are provided with adjustable orifice plates. This work is regularly done by performance monitoring cell. 8. Checking of the feeder speed The feeder speeds were locally checked by us and matched with indicated % at DCS. The variable speed drives are perfect. 9. Checking of fuel feed rate between feeders
9 The coal feeders available at this plant were drag chain feeders. Present day practice is to have belt weigh feeders in order to ensure that the feed rate between elevations does not vary. The dam plate was found to be different from original design. The taper top part was removed in one feeder. The above is the original design to set the feeder discharge capacity. The drag chain feeders have been modified from the original design. The original design had an arrangement for level control of the coal in the feeder. This was not there now. This could cause fuel variations between feeders and thus between elevations. It was advised to take a coal drop test and to set the bias suitable between the feeders so as to deliver the same amount of coal. The dam gate should be at same height for all feeders. The dam gate should be sloped equally in all feeders to have uniform feed rate with all fuel particles and fuel moisture levels. Generally SS plates do a good job. The slope should be at 30 deg to horizontal for better control. Bigger particles do not flow out easily at the gates. They roll
10 and try to take some time before leaving the dam gate. Sloped dam gates permit easy rolling of the bigger particles. The hurdle will now become easy. The present bunker at this plant was made of RCC. It was learnt that the liner plates have all gone. It is a must that the liners are placed inside. It shall be SS liners. The liners should extend to 5 meters high inside so that the arch tendency is removed. The present discharge opening of the bunker was about 1200 mm long. Not all the coal would get out from this opening. Only at the leading end the coal would come out. In remaining places, the coal would remain stagnant. We recommended that the coal inlet chute to any feeder should be as shown in the figure. In this arrangement the hydrostatic pressure from above forces the coal to loosen and go in to the feeder. Thus interruptions in coal feeding do not happen. Using coal of 19 mm and below will help in uniform feed rate as the flowability will be better with this size range. If necessary crusher has to be added in the coal handling system. The fuel feed rate variations between levels varied to a large extent. See table below. The lowest rpm was about 75% of the max feeder rpm. Such large variations do not allow good combustion. 10. Selection of elevation of burners for normal operation For longer residence time for combustion, one must aim at using the bottom level burners. This gives more residence time for the char to burn up. In the above table, we could see highest firing rate at the top level. 11. Air leakage in gas recirculation dampers The gas recirculation fans were found to be rotating in the reverse direction. The casing temperatures of the gas recirculation fans were found to be deg C. There could be air passing from furnace to airheater. This air has not taken part in combustion. 12. Fuel volatile matter (VM) and fixed carbon (FC)
11 The FC/VM on air dried basis provides an indication of carbon burn up possible. The current coals are Russian coal and South African coal. Russian coals have 32.9% VM and burn well. South African coals have 24.9% Volatile matter. The ash effect cannot be discounted. Fuels with less ash always show up higher unburnt, as the ash is not available to dilute the unburnt. In general power plant engineers focus simply on LOI and create a fuss. We need to worry how much heat was lost in the ash. 13. Feeder Bias allowable to operator It was observed that the feeder bias available to operator was to an extent of ± 20%. This needed to be reduced. Many plants operate at a bias of ± 5%. Again DCS operators may have opinion that the particular mill is not good. One wants to have more bias to have operational flexibility. It is a conflict between efficiency & flexibility. More the bias, more will be the PA flow. Automatically the efficiency will be lost. Problems related to fuel flow interruptions or any other factor must be addressed. 14. Primary air to coal ratio The optimum ratio of primary air to coal is 1.8. Several consulting companies have attempted to optimize this and proved the efficiency improvement. The primary air flow did not follow even the OEM recommendations. There was a revision in the air flowmeter range in Then there was change of fuel feeder drive from electromagnetic coupling to variable frequency drive. After these changes there had been considerable increase in PA flow. The logic of PA flow must be changed. Past records above show the PA flow was much less as compared to present day PA flow. Unit 2 was operating at lesser PA flow based on the data in Feb same time last year. The data were pulled out and explained to operating staff. 15. Flue gas analysis -grid measurement The flue gas sampling was done by us. The report is pictorially presented below. We observed
12 that there was no presence of CO in the flue gas. This was really great. The volatiles were burning fully. This can be due to the tangential firing system. However the unburnt present in ash was high. This now depends on the fineness, residence time, ash content and the FC/VM ratio. CONCLUSION There were several observations which had to be addressed in order to improve the combustion. The engineers from the plant were taken back by some of the observations we made. We explained the fact their routine job was not finding the faults. It is the job of some agency internally or externally assigned to identify the deviations and advise the O&M engineers periodically. Such an activity pays back. Some plants have sufficient QC engineers with a focus to inspect the O&M standards are adhered to. K.K.Parthiban / / /