Analysis of Scale Deposit in the Geothermal Power Plant

Gunung Salak is one of the potential geothermal area in Indonesia. The steam character is categorised as the water – dominated steam with low steam purity. During inspection, deposit was found in several equipment which are turbine, demister, scrubber and separator. Demister is an essential in the geothermal system as it keeps the steam quality above the saturated condition, by removing the water phase from the steam. The deposit was predicted to be form because of scaling problem due to the impurities of the steam. Oxygen is expected to be responsible of the scaling problem. This research is to analyze the effect of oxygen partial pressure to the formation of scaling through thermodynamic approach. The formation of iron oxide scaling is to be focused in this research as it is mostly found in the deposit alongside other compounds. Demister’s deposit was analyzed using ICP, AAS, XRD and SEM – EDX method. The actual composition found in the demister is then compared to the phase stability diagram results using a chemical reaction software. The deposit used is from the demister. The results of this study shows that Fe2O3 reaction can occur in demister current operational condition which are 164.3 ◦C in temperature and oxygen partial pressure, 3.307x10−5 bar


Introduction
One of potential geothermal area in Indonesia, Gunung Salak has contributed to fulfill national electric demand for over the years. During inspection, scaling deposition was found on the turbine's blade as well as on other surface of essential equipments which are separator, scrubber, demister and strainer [1]. The deposit was analyzed and the composition was determined. Iron oxide was one of the compound mainly found in the deposit of each components alongside others. Iron oxide was not initially assumed to be found in the deposit because the steam analysis from well showed no presence of iron [?]. However, the deposit's analysis results showed a presence of iron oxide. This had happened before in 2005 and caused de -rating of turbine. Presence of dissolved oxygen in steam was believed to be responsible of the iron oxide formation. Therefore, a further study on the effect of oxygen partial pressure is carried out in this research through thermodynamic analysis approach. The thermodynamic parameters used in this study are the equipments' system temperature and pressure. Below is the simplified scheme of the geothermal power plant system analysed in this study is given in Figure 1.  The steam flows from the well to the separator for separaring solid particles from the steam. Before that, the steam was washed using condensate water injected into the system. After going through separator steam flows to the scrubber for again the separation of solid particles from the steam. After that it goes to the demister to separate the water phase from the gas phase of the steam. Next it goes to its last solid separation defense, the strainer. Hence, steam that enters turbine is in a high purify and quality of steam (slightly above saturated condition). The presence of dissolved oxygen is believed to not only come from the well, but also from the water used in the steam wash system. Demister as mentioned before is essential to separate the water phase from the steam to keep it in a saturated phase when it enters the turbine. Figure 2 shows how demister works. When entering through the feed pipe (inlet) the steam uses its centifugal force and gravity to separate the water phase that is more densed than its gas phase. Then the steam will flow through the vane separator as shown in Figure 2b. Gas with lesser density than water can flow easily follows the vane profile. Meanwhile, water with greater density than gas cannot follows the vane profile and stuck at the intersections. It will then drops down due to gravity. The presence of deposit can reduce the centrifugal force of the steam and change the surface of the vane, causes it to lose the profile needed to separate water from the steam.
N. Mundhek in his research stated that scaling can be caused by the redox reaction between elements in the working fluid without interacting with base metal of the equipment, or redox reaction between elements in the working fluid and the base metal of the equipment, which is also known as corrosion [3,4]. Both scaling and corrosions formed deposits that will be deposited on the surface of the equipment. Nucleation can happen both inside the bulk fluid and on the surface of base material, called homogenous and heterogenous nucleation respectively [3]. Nucleation inside the bulk fluid will produce deposit as the product of the redox reaction and deposited on the equipment's surface as it is saturated [2,5]. Both of the nucleation process is affected by its saturation and thermodynamic condition.
In further discussion, this research will focus on the effect of oxygen partial pressure to formation of iron oxide scaling through thermodynamic approach. Therefore, it is assumed that the oxygen fraction through out the whole system is equal. The temperature is also assumed constant through out the system.

Experimental Procedures
During thermodynamic calculation, the compounds assumed to be found in the deposit was pre-assumed by referring to the compund mainly found in the deposit previously. Those are iron oxides. Other compounds were also found in the deposits but this study will focus on the formation of iron oxides. The data used in the simulation are concentration of the involved elements and pressure of the equipments. Demister deposit is analysed using ICP -OES, SEM -EDX and XRD. As a comparison result to ICP -OES, AAS is also used. ICP -OES is used to determine the composition and concentration of the elements found in the deposit. A standard solution of the elements that are observed, is needed to compare it for analysis. The elements that are observed using this method are iron (Fe) and chromium (Cr). This is determined by previous inspection report that shows the presence of Fe in the deposit [1] and assumption of the possibilities that Fe is from the corrosion of the demister material. SEM analysis is used to determine the morphology of the deposit, while EDX is used to determine the composition of each area of morphology, if different morphology is found, as well as the composition of the whole area. The advantage from this method is that any elements presence will be shown in the analysis result, without having to compare it to its standard element. While the XRD method is used to determine the compound that are found in the deposit.
These deposit analysis result is then compared to the thermodynamic analysis using a chemical reaction software. Using this software, a phase stability will be obtained from the simulation. From the diagram, the stable phase that is formed in demister's operational condition is determined, by calculating its oxygen partial pressure and match it to the diagram from the simulation. The input data needed to simulate this diagram are the elements and temperature of the system. The elements are O 2 , H 2 O(g) and Fe. The temperature is 164.3 • C. The diagram formed will be the Fe−O 2 −H 2 O(g) diagram in a 164.3 • C system.

Results and Discussion
ICP -OES shows that 919890 ppm of Fe and 71907 ppm of Cr is presence in the deposit. Meanwhile AAS shows that 2114 ppm of Fe and 45.23 ppm of Cr is presence in the deposit. Both results shows the presence of Fe and Cr but with different concentration. This is can be caused by the concentration analysis error that used the elements standard solution as a comparison. The EDX result of the overall area shows 20.56 wt.% Fe, 74.16 wt.% O, 3.86 wt.%C, 0.12 wt.%Cr and 1.3%S. While XRD shows that the deposit is made up of 49% Fe2.9 and 51% Fe 2 O 3 .   Figure 3 shows the different morpholgy found on the deposit. There is an evidence of chromium (Cr) and carbon (C) presence in the area with rectangular form. Meanwhile sulfide (S) is presence in the fibric area. As for Fe and O is presence in the overall area.
The output of simulation is the phase stability diagram of Fe−O 2 −H 2 O. The formation of Fe 2 O 3 has a reation equation: 2 Fe + 1.5 O 2 (g) −−→ Fe 2 O 3 and equilibrium when ∆G 0 = -168.069 kcal/mol. The log pO 2 (g) is -42. It is equal to 10-42 bar of oxygen partial pressure. Meanwhile, formation of Fe 3 O 4 has a reaction equation of 3 Fe+2 O 2 (g)−−2 O 2 (g)−− and equilibrium when ∆G 0 = -230.628 kcal/mol. The log pO 2 (g) is -57.616. It is equal to 10-57.616 bar of oxygen partial pressure. The calculation of oxygen partial pressure is determined to predict the reaction of oxides formation that took place in the system.
It is seen that the partial pressure of oxygen in demister operational condition is 3.31x10 −5 bar, with a 5.325x10 −6 fraction of oxygen. This results in log pO 2 (g) = -4.48. Meanwhile, Fe 2 O 3 is equilibrium when the partial oxygen is 10-42 bar. So, in a 6.21 bar system, the oxygen fraction to result in the equilibrium of Fe2O3 is 1.61 x 10 −43 . For the equilibrium of Fe 3 O 4 , the oxygen partial pressure 10-57.616 bar. This shows that Fe 3 O 4 is formed with a smaller amount of oxygen than the formation of Fe 2 O 3 . Thus, with the given operational condition, the phase that is likely to form is Fe 2 O 3 rather than Fe 3 O 4 .
Deposit analysis using various methods is shown in Table 1. XRD analysis shows that the iron oxides found in the deposit are Fe 2 O 3 and Fe 2.9 ). The difference between these results happen because thermodynamic analysis uses a split second approach. Meanwhile, this demister operates 24 hours non -stop for 2 years. Thus, it is a continuous system. According to an experiment conducted by Xiaozhe Zhang in 2016, Fe 2 O 3 may experience reduction of oxygen and form Fe 3 O 4 respective to time [3]. The time constant of transformation depends on the temperature and pressure of the working system [3]. This possibly what causes the transformation of Fe 2 O 3 to Fe 3 O 4 . To determine whether the oxidation process is only between the elements inside the fluid or a possible interaction with the equipment's base metal, a stoichiometric calculation is used from the reaction 2 Fe + 1.
From a stoichiometric calculation, Fe concentration in the steam, 0.105 ppm is only enough to form 0.0525 ppm of Fe 2 O 3 . While there is 51% Fe 2 O 3 which is equal to 510000 ppm. Thus, oxidation of demister's base material does occur in the system. It has dark reddish brown colour. This colour is identified as the colour of Fe 2 O 3 . It is important to differentiate the difference between Fe 2 O 3 and Fe 2 O 3 · H 2 O since there are still water phase present in the demister. This is to determine the reaction that occurs to the demister base material, whether it is caused by wet or dry steam. Since the colour is identified as the colour of Fe 2 O 3 , the analysis approach to study this reaction is using the Ellingham Diagram.
So, we got that formation of FeO and Fe 3 O 4 need much less oxygen than the formation of Fe 2 O 3 . Remember that the operational oxygen partial pressure is 3.31 x 10 −5 bar from the oxygen fraction 5.325x 10 −6 . Thus we got that with the operational oxygen partial pressure, the oxide that likely to form is Fe 2 O 3 . This shows that oxidation of demister base material could occur and form Fe 2 O 3 . Besides the iron oxide formation, evidence of Cr in the deposit shows that chromium oxides also form on the surface of the base material. Chromium can cause reduction reaction to Fe 2 O 3 . It will form chromium oxide, Cr 2 O 3 and reduces Fe 2 O 3 to Fe. But there are still iron oxides found in the deposit. This concluded that iron oxidation still occurs regardless of the chromium oxide formation to protect the base material. This could happen because of the diffusion of oxygen to the base material through initial deposit possibly from scaling.

Conclusion
From the data and samples analysis, it can be concluded that the demister operational temperature of 164.3 • C and oxygen partial pressure 3.307 x 10 −5 bar, can cause the reaction to form Fe 2 O 3 . This can be confirmed by Ellingham diagram calculation of iron oxide formation could occur to the iron of the demister base material. The sample analysis using various methods was used to analyze the sample of deposit validate the findings of iron oxides chromium oxide in the deposits which suggest that deposit is originated from the corrosion of the part not only resulted of deposition on carryover minerals in the steam.