THE USE OF THE ENAMELLED STEEL ON ROTARY AIR PREHEATERS AND GAS-GAS HEATERS: ADVANTAGES AND PROFITABILITY

Transcription

1 THE USE OF THE ENAMELLED STEEL ON ROTARY AIR PREHEATERS AND GAS-GAS HEATERS: ADVANTAGES AND PROFITABILITY THE CONTEXT Rotary Air prehetaers (AHs) are installed in all the power stations equipped with traditional boilers generating thermal energy from the combustion of fossil fuel. Typically the implementation of AHs is foreseen in Power Plant burning fuel oil and coal but also, usually in reduced size, in boilers dedicated to the production of steam for downstream utilities as desalination plant, refinery, cement, sugar and paper factories. The rotary Air preheaters installed on the power stations are machines that allow energy savings in a range between 10 and 15% with respect to thermal energy introduced into the boiler with the fuel. The combustion gases leaving the boiler have a residual temperature ranging from 300 to 350 C and therefore, without Ahs, they would leave the stack without exploiting their remaining potential energy content. The rotary Air preheater, installed at the exit of the boiler, has exactly the purpose of transferring the heat still contained in the exhaust gases conveyed to the chimney, to the cold combustion air in order to preheat it before the injection into the boiler. In this way all the calories of the fuel will be fully available for the heating and the evaporation of the boiler water without their consumption for the heating of the combustion air. To ensure the above mentioned energy savings, it is therefore necessary to induce the passage of the flue gases through the Air preheater. In case these gases would contain Sulfur Oxides (which are chemicals always produced burning fuels containing Sulfur), below a given temperature in the range of 130 C called dew point the gases will condensate forming Sulfuric Acid; such condensation on the metal surfaces will cause a severe corrosion of the heat exchange elements. The action of Sulfuric Acid on carbon steel is devastating and just after few hours in contact with the metallic surfaces, these will be already compromised. A possible way to limit this phenomenon is to discharge the flue gases at a temperature above the temperature of the Sulfuric Acid condensation (known as said above as dew point temperatures and proportional to the content of Sulfur Trioxide present in the flue gases themselves), jeopardizing however the energy saving and the efficiency of the boiler operation. In other words, the higher the % of Sulfur in the fuel and more SO 3 is formed and consequently a higher concentration of Sulfur Trioxide will trigger a higher dew point temperature of the flue gases. Thus, higher temperatures of the leaving gases would have to be kept to avoid catastrophic corrosions of the metal sheets contained in the AHs through which the flue gases are passing. 1

2 Following however such solution, that is to operate the AHs at a temperature of the cold zone higher than the dew point temperature of the gases, the energy saving will be undoubtedly reduced and compromised. From an economic standpoint, a full exploitation of the Air preheater, that is their operation at low temperatures (in the range of C), is always recommended although the inevitable corrosion which occur operating them at low temperatures may lead to a more frequent replacement of the heat exchange elements. According the above, during the last two decades the correct approach to this problem of the power stations and industrial utilities generating thermal energy through traditional boiler burning fossil fuel has been the following: - to operate the thermoelectric units with a low temperature of the exhaust combustion gases - intervening to protect the heat exchangers from the acid attack Avoid the frequent replacement of the entire set of AHs baskets operating at low temperatures, it means not only to save the direct cost due to the supply of new basket but also to avoid the others direct costs involved with the shutdown of the unit for the baskets/elements replacement as for example manpower, consumables for maintenance and the use of dedicated machineries (depending on the size of the unit, it can take from 20 to 40 days the entire substitution of the basket elements). It's worth noting however that, although during the outage for the replacement of the baskets the unit takes advantage to perform other maintenance activities, the restoration of the Air preheater usually constitutes the critical path of all maintenance activities of the boiler area. Therefore a time reduction of the AHs works as well as a seldom frequency of the basket replacement would have an undoubted positive impact on the maintenance costs. The mandatory increase of the operating life and consequently the delay of baskets replacement, has forced the technology to target a very well selected materials to be used in the manufacture of AHs baskets; in particular - for the hot AHs layers (those still operating above the dew point), carbon steel is used either for the baskets and for the exchange plates; this material is significantly corroded by Sulfuric Acid but the working temperature to which it is submitted, generally does not permit the Acid condensation and the corrosion is quite limited. - for the "intermediate" AHs layers (where they exist), either the carbon steel or the Corten steel are used (Corten is a steel similar to the carbon steel where small amount of Copper has been added: this element gives some resistance to the corrosion of the Sulfuric Acid); the choice between these two types of steel is often linked to the temperature profile of the Air preheater requested by the engineering of the exchanger. Accordingly, it is possible to find installations with intermediate layers made entirely of carbon steel (plates and baskets), completely in Corten steel (plates and baskets) or mixed, with baskets of carbon steel and plates of Corten. In any case it must be stressed that the best resistance of the Corten steels to the Sulfuric Acid compared with the carbon steel, does not solve the problem of corrosion but only attenuate it. 2

3 - for the cold AHs layers (those that theoretically should work to the limit of the Acid temperature condensation or below it), Corten steels are used (either for baskets and for plates); these layers are the most affected by the corrosion and consequently are those requiring a more frequent replacement of the heat exchange elements (plates). Since many years, however, plant operators and maintenance personnel of the power stations prefer to replace the use of the elements made on Corten steel with enamelled plates which are totally inert against the attack of the Sulfuric Acid. PROBLEMS AND SOLUTIONS The use of enamelled plates is therefore the solution already proven against the acid corrosion and it is generally accepted by the thermoelectric and thermal generation industry as its use only brought benefits. Of course not all the enamels are equals and not all exhibit a same behavior when applied to the different AHs layers: their correct choice is therefore mandatory. However some typical parameters are characteristic for all the enamels because international regulations require minimum values for their acceptability in AHs and GGHs (Gas Gas Heaters) applications. In this regard the quality of the substrate steel has to be selected carefully as a correct enameling process requires to start from low-carbon (or very low-carbon) steel coils. Accordingly, the choice of suitable raw materials becomes a key issue. When compared to the surface of plate made of corten steel, the enameled surface of the plates is extremely more resistant to the Sulfuric Acid and additionally all the commercial profiles can be enameled as well as the heat exchange capability of the enamelled plates is practically equal to the one of the plate made of corten steel. The study of the metal-enamel interface especially investigated by Smaltiflex, allowed the development of enamels mechanically stronger and very light at the same time; today Smaltiflex produces enamels with hardness greater than that of corten or carbon steels but with densities similar to those of Aluminum. We can affirm that the corrosion of the Air preheaters is completely controlled by the use of enameled plates, whose application for the cold layers are increasing, becoming a standard solution by most of the power stations operators; also the installation of enameled elements in the intermediate layers is going up. In conclusion, the solution for the acid corrosion problem has been identified and its practical application is implemented since years. Nevertheless and unfortunately, the acid corrosion is not the only phenomenon affecting the Air pre-heaters and Gas-Gas Heaters exchangers during their operation. The flue gases are significant masses of fumes (depending on the boiler output they may exceed one million Nm 3 /h) containing variable percentage of ashes, which together with the unburned carbon particles always present, constitute a solid mass suspended on the combustion flue gases. 3

4 In their path, this mass is partially deposited inside the boiler (when impacting the superheater, reheater and economizer tubes), partially is deposited inside the Air preheaters (the sudden change in flow favors the phenomenon) and partially is able to pass through this equipment reaching directly the stack (unless it will be captured by the electrostatic filters/bag filters eventually installed). Part of the solid mass continuously crossing the AHs is progressively accumulated on the channels between the plates (whose profile can lead to a higher or lower tendency to this phenomenon) causing the increment of the boiler pressure losses ("delta P"). The boilers have very specific operating limits with respect to the maximum "delta P" allowed and above certain values the steam generators must drop off the thermal load or even be forced to the total shut-down. The plugging/clogging of the AHs is therefore a second problem that worries the power station operators even because such phenomenon happens regardless of the presence of the enameled or not enameled steel basket. By analyzing this phenomenon with more details it is easy to realize that the enamel has a roughness ranging from ⅓ to ½ to the one exhibited by the Corten (or others steels) and by itself this aspect is already enough to understand as the particles traversing the enamelled plates will tent to sliding off (because of the less grip available for their sticking), when compared to a not enameled metal surface. Furthermore, in presence of H 2 SO 4, the roughness of the Corten increases to double its initial value (stabilizing its values to about 80% of its max. excursion) while the roughness of an enamel surface increases by only 8% max. As the surface becomes rough, the greater the tendency to plugging/clogging ("stick phenomena") and more deposits will be accumulated between the channels of the plates, the more they will continue to grow. From the foregoing, it is clear that the simple use of the enameled plates, it is itself an advantage over the Corten, against the plugging phenomenon. It has to be remarked that in order to improve the sliding behavior of the enameled coating, Smaltiflex developed an high performance enamel ("Lotus Effect" type), mixing the standard enamel with nano-particles and applying special technologies on its formulation; as this product has shown a remarkable improvement on the sliding characteristics, it makes the plates treated with such enamel especially useful in presence of undesirable delta P problem. A third phenomenon typical of the AHs is that of the mechanical fatigue: these machines are continuously subjected to cyclical rotations (although at slow speed) with temperature changes systematically repeated in presence of a higher impacting particle load. Furthermore, in the horizontal-shaft machines, in addition to the above, repeated and cyclical mechanical micro-shocks take place due to the reversal of the baskets at each AH rotation. 4

5 In both types of Air preheaters (vertical and/or horizontal), the phenomenon of the fatigue is always latent and once a crack is formed, due to the cyclical effect it will increase and propagate inside the thickness of the plate until it will reach its minimum resistant value beyond which the plates will break (with the disastrous consequences this entails). Considering that all the above happens in an aggressive environment (acid fumes), in addition to chemical corrosion, the phenomenon of the fatigue is also causing a structural corrosion, making the life of the heating elements increasingly short. Even in presence of standard enamel a crack could be induced by the fatigue phenomenon and sooner or later it will reach the base of the metal and at this point the corrosive attack of the Sulfuric Acid will destroy the plate from "inside". For cases like these, Smaltiflex developed high performance enamels ("Self Repair" type) which block the cracks once formed; again with the help of special nano-elements Smaltiflex is able to formulate enamels that can stop the progression of the crack absorbing its energy and diverting the propagation path. 5

6 THE COMPARISON The table below compares some typical properties and values of the most important parameters characterizing both Corten steel and "Smaltiflex" enamels. Weight Loss after Immersion in Sulfuric Acid Roughness Surface Density of the superficial layer Corten steel Smaltiflex enameled steels Remarks g/m 2 after 1 hour Sample totally destroyed after a few hours g/m 2 after 20 minutes g/m 2 after 30 minutes, with sample completely destroyed after less than 1 hour 1,2 μ 1,0 g/m 2 after 1 hour 1,7 g/m 2 after 24 hours 1,5 g/m 2 after 9 hours 1,8 g/m 2 after 24 hours 0,3 to 0,6 μ kg/m kg/m 3 10% w/w boiling H 2 SO 4 30% w/w boiling H 2 SO 4 depending on the technology used for the enamel application (wet or dry) Hardness (Vickers) Heat Transfer Coefficient "X" X" -1.2% irrelevant difference Application (*) CL - IL CL - IL - HL sometimes even more Service Life (**) "X" months "2X" to "4X" Plugging Phenomenon Mitigation Fatigue Phenomenon Mitigation No No Yes Yes using "Lotus Effect" enamel type using Self Repair enamel type (*) CL = Cold Layer IL = Intermediate Layer HL = Hot Layer (**) The absolute value depends from the Sulfur content in the fuel, the boiler excess of air, the AHs leakages, the way as the unit is operated (exhaust gases temperature, SCAPH in service or not, service factor, etc.) 6

7 The analysis of the table immediately highlights the convenience of the enamel over the Corten steel: 1. Incomparable resistance of the enamel to the Sulfuric Acid (3-4 thousand times the one of the Corten steel) 2. Lower roughness of the enamel compared with Corten steel (therefore less grip for incoming particles load) 3. The enamel is a material lighter than Corten steel (the thickness of the enamel, about 200 μ for each side of the plate, replaces the equivalent thickness of steel, 3 times heavier) 4. The enamel is a material harder than Corten steel (as evidenced by the impact tests systematically carried out during the enameled plates test) 5. The heat transfer of the enameled plate is practically equivalent to that of a Corten steel plate 6. The enamel is applicable above all the layers of Air preheaters, but it is essential to the cold layers, recommended for the intermediate layers and optional for hot layers (technically, for Corten too there are no contraindications for its installation in the intermediate layers but its cost is higher if compared to the carbon steel) 7. The service life is perhaps the most important characterizing parameter; doubling, tripling or even quadrupling (if not more) the period of operation without having to replace the baskets, it means "to touch with the hands" an evident saving that largely re-pays the major cost of the enamel compared with Corten (this major cost is in any case moderate). And this in addition to logistics arrangements and manpower needed for the baskets replacements that would be requested only one time if compared to two, three, four times required in a same period of time having installed Corten plates. A simulation for a traditional boiler 400 MWh burning heavy fuel oil has been run comparing corten and carbon steel basket with enameled steel type; the result was quite expected but the cost saving resulted in a period of time of 4 years has been quite astonishing, about 10.5 million US$. 8. The attenuation of the plugging is a unique feature (and additional to the others of standard enamels) of "Lotus Effect" engineered enamel, available on request from Smaltiflex. 7

8 9. The mitigation of the AHs fatigue failures is another unique feature (and additional to the others of standard enamels) of Self Repair engineered enamel, available on request from Smaltiflex. REMARK: Lotus-Effect (no-stick) and Self-Repair (anti-crack propagation) enamels can be supplied in a single product combining both characteristics 8