Thesis (M. Eng. Sc. ) -- University of Adelaide, Dept. of Civil Engineering, 1978.

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Waste-to-energy (WTE) is the environmentally preferred method of managing post-recycling wastes. In this process, municipal solid waste is combusted under controlled conditions to generate steam and electricity. Waste is by nature heterogeneous and has a substantially high composition of chlorine (0.47-0.72 wt%) as compared to other solid fuels used for power production. During combustion, chlorine is converted to hydrogen chloride and metal chlorides, which can accelerate the high temperature corrosion of boiler surfaces, especially superheater tubes. This corrosion can significantly affect plant efficiency and profitability by causing unplanned shutdowns or preemptively forcing operators to limit steam temperatures. The following work focuses on the role of chlorine compounds on boiler tube corrosion and investigates approaches for minimizing its effects. The corrosion behavior was studied by conducting laboratory furnace tests on alloys of current and future interest to the WTE industry. Test specimens were coupons machined from boiler tubes to a nominal area of 3.2 cm² (0.5 in²). An chemical environment was introduced in an electrical furnace that replicates the fireside of superheater tube. This included a mixed gas stream with O₂, CO₂, H₂O, HCl, SO₂, and N₂, and temperatures ranging between 400-550°C (752-1022°F). For some experiments, a salt layer was applied to the coupons with a loading of 4.0 ±10% mg/ cm² to understand the behavior of the effects of metal chlorides. Following each experiment, the corrosion rate was determined by taking the mass loss as specified in an American Standard Testing Method (ASTM) protocol, G1-09. Additional insights were obtained by characterizing the coupons via scanning electron microscopy (SEM) and elemental dispersive spectroscopy (EDS). Additionally, the corrosion scale and salt layer were characterized via powder X-ray diffraction (XRD). The addition of 800 ppm of hydrogen chloride (HCl) gas to a mixed gas oxidizing environment accelerated the corrosion rate of SA178A (Fe-0.1C) at 500°C (932°F) as determined by the change in the parabolic rate constant over a period of 72 hours, from 0.18 to 1.7 μm²/h (3.0 E-03 to 2.5 E-02 mil²/h). The findings from the EDS and XRD scale analyses were compared to other literature and thermodynamic calculations that showed that effect that HCl accelerates corrosion via an active oxidation mechanism. A parametric study was performed on the effect of hydrogen chloride on three alloys, SA178A, SA 213-T22 (2.5 Cr-1 Mo-Fe) and NSSER-4 (Fe-17Cr-13Ni). Varying the concentration from 400 ppm to 800 ppm at 500°C increased the mean mass loss by 17.5%, as compared to the 60% increase from 0 to 400 ppm. For each alloy, the mass loss increased sharply with temperature between 450, 500, and 500°C, with corresponding apparent activation energies of Ea NSSER- 4 53 kJ/mol, Ea SA213 T22 110 kJ/mol, and Ea SA178A 111 kJ/mol. The lower apparent activation energy for NSSER-4 demonstrates that effect of hydrogen chloride is mitigated with austenitic alloys versus carbon steel or low alloyed steel. In a comparative study between isothermal and temperature gradient tests, it was also shown that the corrosion of SA178A was not impacted by a temperature gradient up to 250 °C. Another important chlorine compound in WTE boilers are metal chlorides, which are readily contained in fly ash and boiler deposits. Using sodium chloride as a surrogate compound, the corrosion behavior under chloride salts was investigated by applying a salt layer (4.0 mg/cm²) on coupon surfaces. Corrosion under the chloride layer was much more severe than below the HCl-containing atmospheres alone. The mass loss for the commercial steels was increased by more than an order of magnitude. Based on SEM and XRD coupon and corrosion product characterization, this behavior was the result of a second active oxidation mechanism in which sodium chloride reacts with and depletes protective oxides such as chromium (II) oxide. The WTE furnace tests with the sodium chloride layer were executed for six different Ni-Cr coatings, including Inconel 625 (Ni-Cr-Mo), SW1600, SW1641 (Ni-Cr-Mo-B-Si) and Colmonoy 88 and SP 99 (Ni-Cr-B-W). The primary corrosion attack observed was pitting located under the original salt layer. Colmonoy 88, showed superior corrosion resistance with mass losses between 0.3-3.1 mg/cm2 between 450-550°C as compared to the Ni-Cr-Mo, and Ni-Cr-B-So coatings which has mass losses between 10-30 mg/cm². The enhanced corrosion performance of Colmonoy 88 and SP 99 was attributed to the alloying addition of tungsten, which had been previously shown in literature to also improve the pitting resistance for Ni-Cr in aqueous environments. The corrosion behavior under metal chlorides was compared with metal sulfates, which are also prominent in WTE fly ash and boiler deposits. The application of sulfate salts on coupon surfaces was shown to semi-protective on WTE boiler tube surfaces up to temperatures of 550°C. The mass loss for carbon steel and Fe-17Cr-13Ni (NSSER-4) below sodium sulfate was an order of magnitude lower than under sodium chloride. These results motivated experiments aimed at sulfating chloride boiler deposits by increases the sulfur/chlorine gas ratio (SO₂/HCl) in WTE fuel gas. The SO₂/HCl ratio was modified between 0.3 to 0.6 and 1.0 respectively. By increasing the SO₂/HCl ratio, the sodium chloride layer applied on the coupon surface was converted from a chloride rich salt to a sulfate rich and was shown to dramatically reduce the corrosion of tube alloys up to 500°C. The impact of sulfating the alloy was most prominent with alloys with high mass loss under the sodium chloride layer. Tests showed a reduction in the corrosion rates of SA213 T22 (37%), Inconel 625 (23%), and NSSER-4 (27%). At 550 °C, there was no trend with respect to increases of the ratio, which suggests that other corrosion reactions are faster than the rate of sulfation. Finally, the annualized cost factor was defined and proposed as a method for replacing current superheater alloys with alternative materials, such as those tested in this thesis. From this discussion it was calculated that the installation of a colmonoy 88 protected superheater can cost approximately 1.4 times the cost of an Inconel 625 cladded replacement, or as much as 4.3 times the cost of a T22 superheater tube and remain a cost effective option.

The EU waste hierarchy includes energy recovery facilities where waste is used as fuel in combined heat- and powerplants. When waste is incinerated can the thermal energy be used in for example, district heating networks and or electricity generation. The purpose of the degree project is to make a pre-study of Heat Management's system solution HISS, optimization of steam soot blowing at Mälarenergi's waste boiler. Waste is a heterogeneous fuel that typically contains a variety of substances such as alkali, chlorine and heavy metals that often contribute to fouling on heat transfer surfaces. These coatings reduce the efficiency of heat transferring surfaces such as superheaters. Cleaning by soot blowing using steam is done at regular intervals to maintain efficiency. Superheater cleaning on the Block 6 is done by retractable rotating soot blowers equipped with a nozzle from where steam is sprayed out at pressure of 25 bar. The blowers are inserted one at a time in a sequence, which means that they wait for the previous blower before the next one can enter. Steam is taken from the main process, which leads to reduction in load on the turbine and causes wear during the sweeping process due to the high impact force. This can lead to erosive damage and thinning of the material on the tubes, thus shortening the lifespan of the superheaters. The optimisation adjusts the soot sequence so that full steam pressure is used only in one direction of travel of the lance, this allows an overlapping soot sequence to be used and thus halves the time required for sooting. Analysis of shorter sooting time and reduced steam consumption based on production data and case studies has led to the following results. The energy consumption of the auxiliary power is reduced by 14.25 MWh per sweeping sequence and 7 115.5 MWh annually. The turbine can produce 2.51 MWh more electricity per sweeping as the time it runs at reduced load is reduced, totalling 1 254.39 MWh in one year. 24.03 tonnes of steam are saved per sweeping and 12 003 tonnes in one year. Payback time for the optimisation is 1.01 years based on an average spot price from Mälarenergi's budgeting from last year. Considering current electricity prices could the payback time be further reduced. The conclusion from the pre-study is that the optimisation as the investment is economically viable and has other positive benefits such as: steadier steam flow and reduced pressure surges. Wear and tear on superheaters are reduced as they are only sprayed with high pressure once instead of twice per sweeping sequence. / I EU:s avfallshierarki ingår energiåtervinningsanläggningar där avfall används som bränsle i förbränningsanläggningar. Vid förbränning utav avfall så kan den utvunna termiska energin användas exempelvis i fjärrvärmenät och- eller generering utav el. Examensarbetets syfte är att göra en förstudie på Heat managements systemlösning HISS, optimering utav ångsotning på Block 6 som är Mälarenergis avfallspanna. Avfall är ett heterogent bränsle som typiskt innehåller en mängd olika ämnen som alkali, klor och tungmetaller som ofta bidrar till att beläggningar bildas på värmeöverförningsytor. Dessa beläggningar försämrar ytors värmeöverförningsverkningsgrad på till exempel överhettare. Rengöring i form av ångsotning görs i jämna intervall för att hålla verkningsgraden uppe. Sotning av överhettare på Block 6 skes av en utdragbar, roterande sotlans utrustad med en dysa där ånga sprutas ut med 25 bars tryck. Lansarna körs in en åt gången i en sekvens vilket innebär att dem väntar på föregående lans innan nästa kan köras. Ångan som används tas från huvudprocessen vilket medför reducerad last på turbinen och dessutom uppstår slitage vid sotningen ty den höga anslagskraften. Detta kan leda till erosiva skador och förtunning utav godset på tuberna och således förkortas livslängden på överhettarna. Optimeringen justerar sot sekvensen så att fullt ångtryck används bara i ena färdriktningen av lansen, detta möjliggör att en överlappande sotsekvens kan användas och på så vis halveras tidsåtgången för sotningen. Analysen utav kortare sottidsåtgång samt minskad ångförbrukning baserat på produktionsdata hämtad från styrsystemet 800xA samt fallstudier har mynnat ut i följande resultat. Hjälpkraftens energianvändning minskas med 14,25 MWh per sotningssekvens och årligen 7 115,5 MWh. Turbinen kan producera 2,51 MWh mer el vid varje sotning då tiden som den körs med reducerad last blir kortare, totalt på ett år blir det 1 254,39 MWh. Vid varje sotning sparas 24,03 ton ånga och på ett år 12 003 ton. Återbetalningstiden för optimeringen blir 1,01 år baserat på ett medelspotpris från Mälarenergis budgetering från förra året. Med hänsyn till dagens elpriser förkortas återbetalningstiden ytterligare.   Förstudien rekommenderar optimeringen då dels är investeringen lönsam rent ekonomiskt, dels medför den andra positiva fördelar. Optimeringen med överlappande sotning skapar ett jämnare ångflöde, det reducerar tryckstötar som är skadliga. Slitage på överhettartuber minskas då dem bara besprutas med högt tryck en gång i stället för två gånger per sotningssekvens. Behovsstyrd sotning skulle kunna implementeras upp till 3 gånger per dygn utan att det skulle kosta mer mot dagsläget där man stora 1,5 gånger per dygn.